In a paper published today in Physical Review Letters, Valerie Domcke of CERN and Camilo Garcia-Cely of DESY report on a new technique to search for gravitational waves – the ripples in the fabric of spacetime that were first detected by the LIGO and Virgo collaborations in 2015 and earned Rainer Weiss, Barry Barish and Kip Thorne the Nobel Prize in Physics in 2017.
Domcke and Garcia-Cely’s technique is based on the conversion of gravitational waves of high frequency (ranging from megahertz to gigahertz) into radio waves. This conversion takes place in the presence of magnetic fields and distorts the relic radiation from the early universe known as cosmic microwave background, which permeates the universe.
The research duo shows that this distortion, deduced from cosmic microwave background data obtained with radio telescopes, can be used to search for high-frequency gravitational waves generated by cosmic sources such as sources from the dark ages or even further back in our cosmic history. The dark ages are the period between the time when hydrogen atoms formed and the moment when the first stars lit up the cosmos.
“The odds that these high-frequency gravitational waves convert into radio waves are tiny, but we counterbalance these odds by using an enormous detector, the cosmos,” explains Domcke. “The cosmic microwave background provides an upper bound on the amplitude of the high-frequency gravitational waves that convert into radio waves. These high-frequency waves are beyond the reach of the laser interferometers LIGO, Virgo and KAGRA.”
Domcke and Garcia-Cely derived two such upper bounds, using cosmic microwave background measurements from two radio telescopes: the balloon-borne ARCADE 2 instrument and the EDGES telescope located at the Murchison Radio-Astronomy Observatory in Western Australia. The researchers found that, for the weakest possible cosmic magnetic ﬁelds, determined from current astronomical data, the EDGES measurements result in a maximum amplitude of one part in 1012 for a gravitational wave with a frequency of around 78 MHz, whereas the ARCADE 2 measurements yield a maximum amplitude of one part in 1014 at a frequency of 3−30 GHz. For the strongest possible cosmic magnetic fields, these bounds are tighter – one part in 1021 (EDGES) and one part in 1024 (ARCADE 2) – and are about seven orders of magnitude more stringent than current bounds derived from existing laboratory-based experiments.
Domcke and Garcia-Cely say that data from next-generation radio telescopes such as the Square Kilometre Array, as well as improved data analysis, should tighten these bounds further and could perhaps even detect gravitational waves from the dark ages and earlier cosmic times.
At the most fundamental level, matter is made up of two types of particles: leptons, such as the electron, and quarks, which combine to form protons, neutrons and other composite particles. Under the Standard Model of particle physics, both leptons and quarks fall into three generations of increasing mass. Otherwise, the two kinds of particles are distinct. But some theories that extend the Standard Model predict the existence of new particles called leptoquarks that would unify quarks and leptons by interacting with both.
In a new paper, the CMS collaboration reports the results of its latest search for leptoquarks that would interact with third-generation quarks and leptons (the top and bottom quarks, the tau lepton and the tau neutrino). Such third-generation leptoquarks are a possible explanation for an array of tensions with the Standard Model (or “anomalies”), which have been seen in certain transformations of particles called B mesons but have yet to be confirmed. There is therefore an additional reason for hunting down these hypothetical particles.
The CMS team looked for third-generation leptoquarks in a data sample of proton–proton collisions that were produced by the Large Hadron Collider (LHC) at an energy of 13 TeV and were recorded by the CMS experiment between 2016 and 2018. Specifically, the team looked for pairs of leptoquarks that transform into a top or bottom quark and a tau lepton or tau neutrino, as well as for single leptoquarks that are produced together with a tau neutrino and transform into a top quark and a tau lepton.
The CMS researchers didn’t find any indication that such leptoquarks were produced in the collisions. However, they were able to set lower bounds on their mass: they found that such leptoquarks would need to be at least 0.98–1.73 TeV in mass, depending on their intrinsic spin and the strength of their interaction with a quark and a lepton. These bounds are some of the tightest yet on third-generation leptoquarks, and they allow part of the leptoquark-mass range that could explain the B-meson anomalies to be excluded.
The search for leptoquarks continues.
By: Corinne Pralavorio
16 DECEMBER, 2020
In the seventh part of the LHC Physics at Ten series, we look at the surprising phenomena of the Standard Model at high energiesAn event display of the highest-mass dijet event, (Event 4144227629, Run 305777) recorded in 2016 by the ATLAS Experiment. Without the signature of the experiment (Image: CERN)
"Robust” is what scientists working on the Large Hadron Collider (LHC) like to use to describe the Standard Model. By stubbornly probing it for weaknesses over the past 10 years, they have run up against the extreme solidity of this theory, which describes particles and forces. However, particle physicists are well aware that this model, finalised in the 1970s, has a few shortcomings. They are therefore searching for a wider theory that could resolve certain mysteries, and are banking on the LHC to help them find it. But apart from the triumphant discovery of the Higgs boson, no other new fundamental particle has been discovered, nor any extraordinary phenomenon that might lead to a more comprehensive theory.
Has all this deterred them from their quest? “Quite the opposite,” smiles Nadjieh Jafari, co-leader of the top-quark group at the CMS experiment. “There are many different territories for us to explore with the LHC: it’s an exciting period.” By venturing to the highest energies ever reached, physicists are observing many phenomena that were previously out of their reach.“We are measuring the behaviour of nature at new energies,” says Jonathan Butterworth, a physicist with the ATLAS experiment. “Even though they fit with the Standard Model, these phenomena are totally new to us.” New energies bring new phenomena
The physicists at ATLAS are interested, for example, in the high-energy transverse jets of quarks and gluons. These jets can contain massive particles such as the W and Z bosons, the messenger particles of the weak force. “These new observations open up fields of research on the structure of such jets, to help us understand the strong interaction, as well as the electroweak interaction when a W or Z boson is emitted,” says Butterworth. The image above shows an ATLAS experiment event with two such jets (yellow and green cones).
The experiments are therefore examining every square centimetre of this new territory, looking for processes that have been predicted but are either extremely rare, have never been observed before, or even better, are completely unexpected. These experiments include ATLAS and CMS, which observe the fusion and diffusion of electroweak bosons – very rare interactions. These events produce W and Z bosons, which either fuse together to produce another particle (fusion) or bounce away from each other (diffusion). “It’s as if the LHC had become a collider of weak bosons; these phenomena are completely new at these energies,” says Paolo Azzurri, co-leader of the Standard Model group at CMS.On the left, a CMS event display of a candidate event in which two W bosons and one Z boson are produced. On the right, an ATLAS event display of a candidate event in which two Z bosons are produced. (Image: CMS and ATLAS, CERN)
Another observation in this region, which is around 50 times rarer than the production of the Higgs boson, is the simultaneous production of three weak bosons. This phenomenon is seen only once in approximately every 100 billion proton collisions. These interactions also provide a new tool with which to probe the Standard Model and the weak interaction carried by the W and Z bosons. “The programme of boson fusion and diffusion started recently,” says Andrew Pilkington, a physicist with the ATLAS experiment. “There is still a long way to go before we can move from observation to the precision measurements that could allow us to detect deviations.”The promise of virtual particles
Physicists measure the frequency of these phenomena (their cross section) as precisely as possible and compare it with theoretical predictions. Any difference could indicate the presence of new particles. If unknown particles exist, they may be too massive to be produced at the LHC, but their quantum behaviour could help spot them.“In quantum field theory, anything that isn’t forbidden can happen,” explains Claude Duhr, a theoretical physicist at CERN. “Particles that are too massive to be produced in reality may appear and disappear fleetingly during an interaction.” These particles are known as virtual particles: they are involved in the interaction, but they are not directly detected. “We can deduce their presence because they have an impact on the interaction. For example, we could observe an excess of events during an interaction, which would indicate the presence of virtual particles,” continues Duhr. This is why it is necessary to measure interactions very precisely, in order to be able to compare the results with the theoretical predictions.
However, one big difficulty is obtaining precise theoretical predictions. Due to the virtual particles, there are not just one but many ways in which the particles can be produced during a proton collision. Physicists have to take into account not only the direct processes (leading order or LO), in which these particles are directly produced without any contribution from virtual particles, but also the processes that result from the appearance of a virtual particle (next-to-leading order or NLO) or even two virtual particles (next-to-next-to-leading order or NNLO) and so on.
These processes with the appearance of virtual particles occur more frequently when the strong interaction is involved (which is the case for proton collisions) and when the energy level is high. It is crucial to take them into account for certain interactions, such as the production of the Higgs boson. However, these “perturbative” theoretical calculations are very complex and have required physicists to develop new mathematical tools, spurred on by the results from the LHC experiments. “It took four people four years to calculate the production of the Higgs boson at the next-to-next-to-leading order NNLO,” explains Duhr, a specialist in this field. And physicists are studying numerous interactions at the LHC, which pushes theorists to carry out many perturbative calculations to allow a comparison with the theory.
To make things even more challenging, the theoretical predictions also rely on solid knowledge of the proton. Paradoxically, the proton, which makes up all the matter around us, is a complex system and its structure is poorly understood. Its three quarks are bound by the strong force, which acts through the exchange of gluons, the messenger particles of the strong interaction. Determining the distribution of a given proton energy among the components of the proton (which we also refer to as partons) is anything but simple. This information is important to understand the initial conditions or, in other words, the energy available during the collision. “The huge amounts of data from the LHC have allowed us to considerably improve our understanding of the structure of the proton,” says Giorgio Passaleva, a physicist with the LHCb experiment.The top quark: a massive effect Event recorded by the CMS experiment in 2016 in which four top quarks were produced simultaneously (Image: CMS/CERN)
Among the many studies of the Standard Model, those relating to the top quark are particularly special. The top quark is the heaviest of the quarks and is almost 90 000 heavier than the lightest, the up quark. It has a very strong coupling with the Higgs boson, which is to be expected since the mechanism associated with this boson that gives elementary particles their mass. As the top quark is also sensitive to the strong, weak and electromagnetic forces, it can be produced by a myriad of processes. It is therefore an ideal candidate for exploring the new energy territories made accessible by the LHC. Florencia Canelli, co-leader of the top-quark physics group at the CMS experiment, started working on the topic at Fermilab in the US in 1998, three years after the laboratory discovered the quark. Pioneering studies were carried out at the Tevatron to define the top quark’s characteristics but, for the past 10 years, the LHC has provided an excellent observation ground for this particle. In the space of just a few years, ATLAS and CMS have been able to measure the mass of the top quark with excellent precision.“With the LHC, we have access to unexplored regions and huge amounts of data, which allow us to gain a more complete and precise understanding of the top quark. These measurements also allow us to constrain new physics processes,” explains Florencia Canelli, CMS physicist. Event recorded in 2018 by the ATLAS experiment in which four top quarks are produced. (Image: ATLAS/CERN)
The high energies at the LHC also provide an opportunity to study the production of top quarks with massive particles such as the W and Z bosons. “Or the simultaneous production of four top quarks, a quite extraordinary phenomenon,” confirms her colleague Nadjieh Jafari, who has been working on the subject since 2008 and is now co-leader of the CMS top-quark analysis group.
The study of the top quark is one of the main focuses of the search for physics beyond the Standard Model. It is thought that unknown particles with a higher mass could decay into top quarks. “The top quark opens a door to theories beyond the Standard Model. Many predict new particles would decay into top quarks or into the same final states as those of the top quark,” confirms Francesco Spano, co-leader of the ATLAS top-quark analysis group.
The study of the interactions involving this special particle is far from complete. Wolfgang Wagner, a physicist with the ATLAS experiment, displays a table indicating the different processes producing the top quark at the LHC and the analyses carried out for each of them. 19 of the 48 boxes in his table are marked with a cross, indicating that the process in question has been studied. “Ten years ago, we were just starting the study of the production of top-antitop pairs, the most accessible of the processes. Today, we have exceeded the precision of the theory for this process, but we still have many other processes to examine,” he explains.Strange assemblies Illustration of the possible layout of quarks in a pentaquark particle, such as those discovered at LHCb. (Image: Daniel Dominguez/CERN)
In its exploration of these new energy territories, LHCb has unearthed exotic assemblies of quarks in which four or even five quarks are bound by the strong interaction. According to the model of hadrons, there are two categories of composite particles: mesons, composed of pairs containing a quark and an antiquark, and baryons, such as protons, containing three quarks. In the quark model proposed in 1964, Murray Gell-Mann and George Zweig also predicted the possible existence of exotic hadrons such as tetraquarks and pentaquarks.
In 2010, LHCb spotted its first tetraquark, followed by several others over the course of the last 10 years. In 2015, the experiment created a stir by announcing the first discovery of a pentaquark. In 2019, a second pentaquark was identified. “These exotic systems are so extreme and strange that they have aroused the interest of theoretical physicists,” explains Giovanni Passaleva, a physicist and former LHCb spokesperson. In fact, the appearance of these exotic hadrons has inspired new research in order to understand their internal mechanisms.
“The study of these exotic assemblies is another tool for testing the hadron model and quantum chromodynamics, the theory of the strong interaction,” adds Tatsuya Nakada, the first spokesperson of LHCb.The experimental data on exotic hadrons will allow physicists to improve their understanding of quantum chromodynamics at low energies, which describes the bound states of quarks.
LHCb physicists are pursuing their examination of this small corner of the Standard Model, just like all the other thousands of LHC scientists studying the new areas opened up by the LHC. Even though the number of events produced by the LHC is already phenomenal, large quantities of data are still required to understand these new phenomena in detail. The Standard Model is robust, so scientists need patience and precision to find its limits.“We explore nature by getting close to the conditions at the very beginning of time, on the smallest scales ever achieved, and we look for deviations from our expectations. It’s in these minuscule regions of space and time that we will be able to detect the limits of the Standard Model,” concludes Francesco Spano, ATLAS physicist. In the seventh part of the “LHC Physics at Ten” series, we look at the surprising phenomena of the Standard Model at high energies
In a paper published today in Nature, the ALICE collaboration describes a technique that opens a door to high-precision studies at the Large Hadron Collider (LHC) of the dynamics of the strong force between hadrons.
Hadrons are composite particles made of two or three quarks bound together by the strong interaction, which is mediated by gluons. This interaction also acts between hadrons, binding nucleons (protons and neutrons) together inside atomic nuclei. One of the biggest challenges in nuclear physics today is understanding the strong interaction between hadrons with different quark content from first principles, that is, starting from the strong interaction between the hadrons’ constituent quarks and gluons.
Calculations known as lattice quantum chromodynamics (QCD) can be used to determine the interaction from first principles, but these calculations provide reliable predictions only for hadrons containing heavy quarks, such as hyperons, which have one or more strange quarks. In the past, these interactions were studied by colliding hadrons together in scattering experiments, but these experiments are difﬁcult to perform with unstable (i.e. rapidly decaying) hadrons such as hyperons. This difficulty has so far prevented a meaningful comparison between measurements and theory for hadron–hadron interactions involving hyperons.
Enter the new study from the collaboration behind ALICE, one of the main experiments at the LHC. The study shows how a technique based on measuring the momentum difference between hadrons produced in proton–proton collisions at the LHC can be used to reveal the dynamics of the strong interaction between hyperons and nucleons, potentially for any pair of hadrons. The technique is called femtoscopy because it allows the investigation of spatial scales close to 1 femtometre (10−15 metres) – about the size of a hadron and the spatial range of the strong-force action.
This method has previously allowed the ALICE team to study interactions involving the Lambda (Λ) and Sigma (Σ) hyperons, which contain one strange quark plus two light quarks, as well as the Xi (Ξ) hyperon, which is composed of two strange quarks plus one light quark. In the new study, the team used the technique to uncover with high precision the interaction between a proton and the rarest of the hyperons, the Omega (Ω) hyperon, which contains three strange quarks.
“The precise determination of the strong interaction for all types of hyperons was unexpected,” says ALICE physicist Laura Fabbietti, professor at the Technical University of Munich. “This can be explained by three factors: the fact that the LHC can produce hadrons with strange quarks in abundance, the ability of the femtoscopy technique to probe the short-range nature of the strong interaction, and the excellent capabilities of the ALICE detector to identify particles and measure their momenta.”
“Our new measurement allows for a comparison with predictions from lattice QCD calculations and provides a solid testbed for further theoretical work,” says ALICE spokesperson Luciano Musa. “Data from the next LHC runs should give us access to any hadron pair.”
“ALICE has opened a new avenue for nuclear physics at the LHC – one that involves all types of quarks,” concludes Musa.
- Outside drone footage: https://videos.cern.ch/record/2027842
- Views inside the detector: https://videos.cern.ch/record/1987362
- ALICE experiment: https://home.cern/science/experiments/alice
- Recreating Big Bang matter on Earth: https://home.cern/news/series/lhc-physics-ten/recreating-big-bang-matter-earth
By: Corinne Pralavorio
8 DECEMBER, 2020
Our sixth story in the LHC Physics at Ten series looks at the precision measurements of the Standard Model made at the Large Hadron ColliderBs0→ μ+μ- decay candidate event recorded in 2016 (Image: CERN)
At the start of 2010, the particle physics community was abuzz with hopes and excitement. Just a few weeks later, the experiments at the Large Hadron Collider (LHC) would venture beyond the energy frontier, where physicists hoped to find exotic particles that would pave the way for a more complete theory of the infinitely small: to physics beyond the Standard Model.
The Standard Model of particles and forces was developed in the second half of the 20th century to explain the discovery of a host of new particles, and to describe – within the framework of a single theory – their behaviour and the forces that link them. This model has been hugely successful and accurately summarises the various phenomena that have been observed. However, it leaves a number of questions unanswered, about subjects such as the nature of dark matter or the absence of antimatter in the universe.
Indeed, when the huge collider was on the verge of starting up, theories about “Beyond the Standard Model” physics were igniting passionate debates and the acronym BSM was cropping up in everyone’s presentations. Claude Duhr had just defended his thesis at the Catholic University of Leuven (Belgium) and was wondering which direction to take. “I had a choice of focusing on precision calculations of the Standard Model or studying physics beyond the Standard Model. Lots of my colleagues couldn’t see any future in precision calculations and advised me to pursue research into BSM theories,” he recalls, ten years later. But Claude Duhr’s hunch was right.The Standard Model of particles and forces describes three of the four forces of nature that act on 12 particles of matter through the exchange of messenger particles. (Image: Daniel Dominguez/CERN)
Two years later, ATLAS and CMS discovered the Higgs boson, confirming the validity of the Brout-Englert-Higgs mechanism. A fabulous discovery that has left physicists hungry for more. But the Higgs was a giant tree hiding a meadow full of well-known flowers. No exotic plants were to be found in these high-energy plains. No unknown particle has made its presence felt by producing a bump on the physicists’ charts since. Month after month, the Standard Model has revealed itself to be more solid than ever.On the road to precision
But our explorers weren’t discouraged. As no hitherto unknown particle had emerged, they would study, with ever more precision, known phenomena at all new energies. They scrutinised every blade of grass and every flower in this beautiful meadow, looking for a curiosity, an anomaly that would lead them to something new.“To begin with, scientists were looking for spectacular phenomena that have now mostly been ruled out. The approach now is to carry out precision measurements,” explains Paolo Azzurri, co-leader of the Standard Model group in the CMS experiment.
Over the years, the LHC has therefore been used for increasingly precise studies, which represent a real challenge for a hadron collider (we’ll see later why this is the case). This was the road that Claude Duhr, who today is a theorist at CERN, ultimately decided to take. “More and more of the work of theorists focused on precision calculations to test the Standard Model as thoroughly as possible,” he explains. In the CERN Courier article in March 2020 on the same theme as this series of features, Michelangelo Mangano, a theorist at CERN, reminded readers that 1600 of the 2700 articles on the LHC in peer-reviewed publications report measurements of Standard Model particles.
The Standard Model of particles and forces describes three of the four forces of nature that act through the exchange of messenger particles, known as bosons. The strong force, which binds quarks in protons and neutrons, is carried by gluons. The electromagnetic force is transmitted by photons, and the weak force, which is responsible for radioactive decay, is carried by the W and Z bosons. There are also 12 particles of matter, grouped into two families: quarks, like those that form protons and neutrons, which feel the strong and weak forces; and leptons, such as electrons, on which the electromagnetic and weak forces act. Each of the two families comprises six particles (see table above).
In reality, the Standard Model is built on two quantum theories : the electroweak theory, which describes the electromagnetic and the weak forces, and quantum chromodynamics, which describes the strong force. So, here we have the basics.Determining the free parameters A candidate event for a W boson decaying into one muon and one neutrino recorded by the ATLAS experiment in 2011. Such events were used for the measurement of the W boson’s mass. (Image: ATLAS/CERN) (Image: CERN)
One advantage of the Standard Model is that it is predictive: it predicts all possible interactions between particles with a precise probability (which physicists call the “cross section”). However, it doesn’t predict the masses of the fundamental particles: these are among the parameters measured by the experiments. Moreover, these masses vary greatly: for example, the mass of the heaviest quark, the top quark, is almost 90 000 greater than the up quark, the lightest.
In total, there are 19 free parameters (aside from the parameters relating to neutrinos). Measuring them precisely is crucial to be able to calculate the interaction cross sections and test the consistency of the Standard Model. Although the Standard Model doesn’t predict their values, it ties some parameters together. And, as the measurements still have a degree of uncertainty, “if the measured mass of the W boson changes while the measured mass of the top quark remains unchanged, then the predicted mass of the Higgs should also change,” explains Andrew Pilkington, a physicist with the ATLAS experiment. “By measuring all of these parameters independently, we test the relationships predicted by the Standard Model and impose constraints on physics beyond the Standard Model.
One of the success stories of the LHC is how it has improved the measurements of these free parameters, starting, of course, by determining the mass of the Higgs boson. ATLAS has also increased the precision of the mass of the W boson. “This was a remarkable achievement that no one had anticipated,” says Jonathan Butterworth, a physicist with the ATLAS experiment who was co-leader of the Standard Model group in 2010.
Using a hadron collider to make precise measurements is far from easy. The LHC collides composite particles, protons, formed of three quarks that interact via gluons. The starting energy is not known – we don’t know which components of the proton are colliding – and the background (all the simultaneous minor interactions that interfere with the result we’re looking for) is very significant. But, armed with ten years’ worth of simulations of their proposed detector and drawing on the outstanding work of hundreds of physicists to understand and reconstruct the events (physics jargon for the collisions and the particles emerging from them), the LHC experiments managed to deliver precise results after just two years.A collision event from 2016 in which a top quark is produced in association with a Z boson at CMS. (Image: CMS/CERN)
The LHC has also chalked up one of the best measurements of the mass of the top quark, which was discovered in 1995 at the Tevatron collider in the United States. “The value combining the results of the Tevatron was already very precise,” remarks Nadjieh Jafari, co-leader of the top-quark physics group of the CMS experiment. “But at the LHC we are able to measure the mass of the top quark using additional channels of production of top quarks, and for some we got equal or better accuracy.”
Other free parameters enter into the calculation of interactions. The precise measurement of the electroweak mixing angle is one of the key results from the LHC experiments. This result serves to constrain the masses of the W and Z bosons.Beauty particles and flavour physics
The LHCb experiment specialises in the study of B hadrons, particles that contain a bottom quark or its antiparticle, and has developed expertise in measuring the parameters that can be used to determine the probability that a quark will transform into another via the weak interaction. These transformation processes were first described by Nicola Cabibbo, Makoto Kobayashi and Toshihide Maskawa, and can be calculated using a matrix that bears their initials. The CKM matrix is made up of four free parameters – like the masses of particles – that are measured in experiments. Measurements via different processes can be used to test the robustness of the Standard Model. The structure of the CKM matrix can be represented graphically by triangles, with the parameters represented by the lengths of the sides and the angles. For example, LHCb has obtained the best measurement of one of these angles, γ. This work is linked to work on the phenomenon of charge-parity (CP) violation, which is at the origin of a difference in behaviour between matter and antimatter. The experiment has also obtained excellent results relating to CP violation, including proof of the phenomenon occurring with particles containing a charm quark, whereas before it had been observed only with particles containing a strange or a bottom quark.
But B mesons have opened up an even wider field of study for LHCb.“The LHCb programme has evolved not only to confirm CP violation with B mesons, but also to understand the phenomena of flavour physics in general,” explains Tatsuya Nakada, a pioneer of LHCb and its first spokesperson. “The study of these phenomena is an extremely useful way of measuring the coherence of the Standard Model.”
The smallest of the LHC’s main experiments has become a gold standard in the field of flavour physics. A great success considering that its capabilities were considered to be limited to begin with. “The start-up of LHCb wasn’t easy,” recalls Giovanni Passaleva, another former spokesperson of the experiment. “Our objectives were considered far too ambitious for a hadron collider with so much background. The B factories (the BaBar experiment in the United States and the Belle experiment in Japan – Ed.) already covered the research we were planning. We were worried, but today we are proud and happy.”LHCb experiment has become a gold standard in the field of flavour physics, achieving crucial results studies of the weak interaction, and in the field of CP violation. (Image: Maximilien Brice/CERN)
Among other phenomena, the experiment is interested in decays that the Standard Model predicts to be very rare. The comparison of their measurements with the predictions allows it to test the robustness of the Standard Model. “If you discover a deviation, you might have come across a sign of new physics,” explains Tatsuya Nakada. LHCb and CMS have thus measured the cross section of the decay of the B0s meson into two muons, a process that, according to the theory, is produced in only three of every billion decays of this meson (the image at the top shows such an event, recorded by LHCb in 2016. The two muon tracks from the B0s decay are seen as two green tracks running through the whole detector). LHCb has studied other very rare interactions of B mesons. The results are in agreement with the Standard Model but the possibilities for new measurements are far from being exhausted, covering many other phenomena.
“Precision is a fantastic tool for understanding the world of particles,” says Gian Giudice, head of CERN’s Theory department. “The LHC has moved from discovery to precision and there is lots to learn.”
Next up: The surprises of the Standard Model at high energiesOur sixth story in the LHC Physics at Ten series looks at the precision measurements of the Standard Model made at the Large Hadron Collider
By: Ana Lopes
13 NOVEMBER, 2020
Our fifth story in the LHC Physics at Ten series looks at how the LHC has recreated and greatly advanced our knowledge of the state of matter that is believed to have existed shortly after the Big BangFirst collisions of Pb+Pb seen by the ALICE experiment on 09.11.2010. (Image: CERN)
The Large Hadron Collider (LHC) at CERN usually collides protons together. It is these proton–proton collisions that led to the discovery of the Higgs boson in 2012. But the world’s biggest accelerator was also designed to smash together heavy ions, primarily the nuclei of lead atoms, and it does so every year for about one month. And for at least two good reasons. First, heavy-ion collisions at the LHC recreate in laboratory conditions the plasma of quarks and gluons that is thought to have existed shortly after the Big Bang. Second, the collisions can be used to test and study, at the highest manmade temperatures and densities, fundamental predictions of quantum chromodynamics, the theory of the strong force that binds quarks and gluons together into protons and neutrons and ultimately all atomic nuclei.
The LHC wasn’t the first machine to recreate Big Bang matter: back in 2000, experiments at the Super Proton Synchrotron at CERN found compelling evidence of the quark–gluon plasma. About five years later, experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in the US started an era of detailed investigation of the quark–gluon plasma. However, in the 10 years since it achieved collisions at higher energies than its predecessors, the LHC has taken studies of the quark–gluon plasma to incredible new heights. By producing a hotter, denser and longer-lived quark–gluon plasma as well as a larger number and assortment of particles with which to probe its properties and effects, the LHC has allowed physicists to study the quark–gluon plasma with an unprecedented level of detail. What’s more, the machine has delivered some surprising results along the way, stimulating new theoretical studies of this state of matter.“In the ultimate textbook about the theory of the strong interaction, the chapter on the quark–gluon plasma will be filled with figures of LHC data,” says ALICE experiment spokesperson Luciano Musa.
“These figures excel in data precision and kinematic reach, and they are the first to inform us about how quark–gluon plasma-like properties emerge gradually as one transitions from proton–proton to heavy-ion collisions.”Illustration of the history of the universe. About one microsecond (μs) from the Big Bang, protons formed from the quark–gluon plasma. (Image: BICEP2 Collaboration/CERN/NASA)Heavy collision course
When heavy nuclei smash into one another in the LHC, the hundreds of protons and neutrons that make up the nuclei release a large fraction of their energy into a tiny volume, creating a fireball of quarks and gluons. These tiny bits of quark–gluon plasma only exist for fleeting moments, with the individual quarks and gluons, collectively known as partons, quickly forming composite particles and antiparticles that fly out in all directions. By studying the zoo of particles produced in the collisions – before, during and after the plasma is created – researchers can study the plasma from the moment it is produced to the moment it cools down and gives way to a state in which composite particles called hadrons can form. However, the plasma cannot be observed directly. Its presence and properties are deduced from the experimental signatures it leaves on the particles that are produced in the collisions and their comparison with theoretical models.
Such studies can be divided into two distinct categories. The first kind of study investigates the thousands of particles that emerge from a heavy-ion collision collectively, providing information about the global, macroscopic properties of the quark-gluon plasma. The second kind focuses on various types of particle with large mass or momentum, which are produced more rarely and offer a window into the inner, microscopic workings of the medium.
At the LHC, these studies are conducted by the collaborations behind all four main LHC experiments: ALICE, ATLAS, CMS and LHCb. Although ALICE was initially specifically designed to investigate the quark–gluon plasma, the other three experiments have also since joined this investigation.Global properties
The LHC has delivered data that has enabled researchers to derive with higher precision than previously achieved several global properties of the medium.“The LHC can “hear” much more precisely the quark–gluon plasma,” says CERN theorist and quark–gluon plasma specialist Urs Wiedemann.
“If we listen to two different musical instruments with closed eyes, we can distinguish between the instruments even when they are playing the same note. The reason is that a note comes with a set of overtones that give the instrument a unique distinct sound. This is but one example of how simple but powerful overtones are in identifying material properties. Heavy-ion physicists have learnt how to make use of “overtones” in their study of the quark–gluon plasma. The initial stage of a heavy-ion collision produces ripples in the plasma that travel through the medium and excite overtones. Such overtones can be measured by analysing the collective flow of particles that fly out of the plasma and reach the detectors. While previous measurements had revealed only first indications of these overtones, the LHC experiments have mapped them out in detail. Combined with other strides in precision, these data have been used by theorists to characterise the plasma’s properties, such as its temperature, energy density and frictional resistance, which is smaller than that of any other known fluid,” explains Wiedemann.
These findings have then been supported in multiple ways. For instance, the ALICE collaboration estimated the temperature of the plasma by studying photons that are emitted by the hot fireball. The estimated temperature, about 300 MeV (1 MeV is about 1010 kelvin), is above the predicted temperature necessary for the plasma to be created (about 160 MeV), and is about 40% higher than the one obtained by the RHIC collider.
Another example is the estimation of the energy density of the plasma in the initial stage of the collisions. ALICE and CMS obtained a value in the range 12–14 GeV per cubic femtometre (1 femtometre is 10-15 metres), about 2–3 times higher than that determined by RHIC, and again above the predicted energy density needed for the plasma to form (about 1 GeV/fm3).Particle trajectories and energy deposition in the ALICE detector during the last lead–lead collisions of the second LHC run. (Image: CERN)Inner workings
The LHC has supplied not just more particles but also more varied types of particle with which to probe the quark–gluon plasma.“The LHC has given us access to a very broad palette of probes,” says ALICE physics coordinator Andrea Dainese.
“Together with state-of-the-art particle detectors that cover more area around the collision points as well as sophisticated methods of identifying and tracking particles, this broad palette has offered unprecedented insight into the inner workings and effects of the quark–gluon plasma.”
To give a few examples, soon after the LHC started, ATLAS and CMS made the first direct observation of the phenomenon of jet quenching, in which jets of particles formed in the collisions lose energy as they cross the quark–gluon plasma medium. The collaborations found a striking imbalance in the energies of pairs of jets, with one jet almost completely absorbed by the medium.
Another example concerns heavy quarks. Such particles are excellent probes of the quark–gluon plasma because they are produced in the initial stages of a heavy-ion collision and therefore experience the entire evolution of the plasma. The ALICE collaboration has more recently shown that heavy quarks “feel” the shape and size of the quark–gluon plasma, indicating that even the heaviest quarks move with the medium, which is mostly made of light quarks and gluons.
The LHC experiments, in particular ALICE and CMS, have also significantly improved our understanding of the hierarchical “melting” in the plasma of bound states of a heavy quark and its antiquark, called quarkonia. The more weakly bound the states are, the more easily they will melt, and as a result the less abundant they will be. CMS was the first to observe this so-called hierarchical suppression for bottomonium states, which consist of a bottom quark and its antiquark. And ALICE revealed that, while the most common form of charmonium states, which are composed of a charm quark and its antiquark, is highly suppressed due to the effect of the plasma, it is also regenerated by the recombination of charm quarks and antiquarks. This recombination phenomenon, observed for the first time at the LHC, provides an important testing ground for theoretical models and phenomenology, which forms a link between the theoretical models and experimental data.Surprises in smaller systems
The LHC data have also revealed unexpected results. For example, the ALICE collaboration showed that the enhanced production of strange hadrons (particles containing at least one strange quark), which is traditionally viewed as a signature of the quark-gluon plasma, arises gradually in proton–proton and proton–lead collisions as the number of particles produced in the collisions, or “multiplicity”, increases.
Another case in point is the gradual onset of a flow-like feature with the shape of a ridge with increasing multiplicity, which was first observed by CMS in proton–proton and proton–lead collisions. This result was further supported by ALICE and ATLAS observations of the emergence of double-ridge features in proton–lead collisions.As the number of particles produced in proton–proton collisions increases (blue lines), the more particles containing at least one strange quark are measured (orange to red squares in the graph). (Image: CERN)“The discovery of heavy-ion-like behaviour in proton–proton and proton–nucleus collisions at the LHC is a game-changer,” says Wiedemann.
“The LHC data have killed the long-held view that proton–proton collisions produce free-streaming sets of particles while heavy-ion collisions produce a fully developed quark–gluon plasma. And they tell us that in the small proton–proton collision systems there are more physical mechanisms at work than traditionally thought. The new challenge is to understand, within the theory of the strong force, how quark–gluon plasma-like properties emerge gradually with the size of the collision system.”
These are just examples of how 10 years of the LHC have greatly advanced physicists’ knowledge of the quark–gluon plasma and thus of the early universe. And with data from the machine’s second run still being analysed and more data to come from the next run and the High-Luminosity LHC, the LHC’s successor, an even more detailed understanding of this unique state of matter is bound to emerge, perhaps with new surprises in the mix.
“The coming decade at the LHC offers many opportunities for further exploration of the quark–gluon plasma,” says Musa. “The expected tenfold increase in the number of lead–lead collisions should both increase the precision of measurements of known probes of the medium and give us access to new probes. In addition, we plan to explore collisions between lighter nuclei, which could cast further light on the nature of the medium.”
LHC at 10: the physics legacy
An overview of experimental results from ultra-relativistic heavy-ion collisions at the CERN LHC: bulk properties and dynamical evolution
An overview of experimental results from ultra-relativistic heavy-ion collisions at the CERN LHC: hard probes
Don't miss the next articles of our series, which will cover the Standard Model and more.Our fifth story in the LHC Physics at Ten series looks at how the LHC has recreated and greatly advanced our knowledge of the state of matter that is believed to have existed shortly after the Big Bang
The BASE collaboration at CERN has bagged more than one first in antimatter research. For example, it made the first ever more precise measurement for antimatter than for matter, it kept antimatter stored for a record time of more than a year, and it conducted the first laboratory-based search for an interaction between antimatter and a candidate particle for dark matter called the axion. Now, the BASE team is developing a device that could take antimatter research to new heights – a transportable antiproton trap to carry antimatter produced at CERN’s Antimatter Decelerator (AD) to another facility at CERN or elsewhere, for higher-precision antimatter measurements. These measurements could uncover differences between matter and antimatter.
The Big Bang should have created equal amounts of matter and antimatter, yet the present-day universe is made almost entirely of matter, so something must have happened to create the imbalance. The Standard Model of particle physics predicts a certain difference between matter and antimatter, but this difference is insufficient to explain the imbalance, prompting researchers to look for other, as-yet-unseen differences between the two forms of matter. This is exactly what the teams behind BASE and other experiments located at CERN’s AD hall are trying to do.
BASE, in particular, investigates the properties of antiprotons, the antiparticles of protons. It first takes antiprotons produced at the AD – the only place in the world where antiprotons are created daily– and then stores them in a device called a Penning trap, which holds the particles in place with a combination of electric and magnetic fields. Next, BASE feeds the antiprotons one by one into a multi-Penning-trap set-up to measure two frequencies, from which the properties of antiprotons such as their magnetic moment can be deduced and then compared with that of protons. These frequencies are the cyclotron frequency, which describes a charged particle’s oscillation in a magnetic field, and the Larmor frequency, which describes the so-called precessional motion in the trap of the intrinsic spin of the particle.
The BASE team has been making ever more precise measurements of these frequencies, but the precision is ultimately limited by external disturbances to the set-up’s magnetic field. “The AD hall is not the calmest of the magnetic environments,” says BASE spokesperson Stefan Ulmer. “To get an idea, my office at CERN is 200 times calmer than the AD hall” he says, smiling.
Hence the BASE team’s proposal of making a transportable antiproton trap to take antiprotons produced at the AD to a measurement laboratory with a calmer magnetic environment. The device, called BASE-STEP and led by BASE deputy spokesperson Christian Smorra, will consist of a Penning-trap system inside the bore of a superconducting magnet that can withstand transport-related forces. In addition, it will have a liquid-helium cooling system, which allows it to be transported for several hours without the need of electrical power to keep it cool. The Penning-trap system will feature a first trap to receive and release the antiprotons produced at the AD, and a second trap to store the antiprotons.
The overall device will be 1.9 metres long, 0.8 metres wide, 1.6 metres high and at most 1000 kg in weight. “These compact dimensions and weight mean that we could in principle load the trap into a small truck or van and transport it from the AD hall to another facility located at CERN or elsewhere, to further our understanding of antimatter,” says Smorra, who received a European Research Council Starting Grant to conduct the project.
The BASE team has started to develop the device’s first components and expects to complete it in 2022, pending decisions and approvals. Stay tuned for more developments.
Read more about BASE-STEP in this Experimental Physics newsletter article.
See also this story on making antimatter transportable for nuclear-physics research.
Scientists at CERN’s Large Hadron Collider (LHC) are finding novel ways to search for new particles. Elusive, long-lived particles could be decaying into other particles away from the LHC collision point – leaving an unusual signature in a detector. The ATLAS collaboration has broadened its extensive search programme to look for these unconventional collision events. In the process, they’ve drastically improved the limits on new massive long-lived particles decaying into particles called leptons.
Long-lived particles are a feature of the Standard Model, although for relatively low-mass particles only. Massive long-lived particles can occur in theories of new physics beyond the Standard Model. A theory that encompasses new long-lived particles in some of its manifestations is supersymmetry (SUSY). SUSY predicts that each particle of the Standard Model has a “superpartner” particle, which differs from its corresponding particle in a quantum property known as spin. In its new study, the ATLAS collaboration looked for the superpartners of the electron, muon and tau lepton, called “sleptons” (“selectron”, “smuon” and “stau”, respectively).
The typical search for new physics with ATLAS data is oriented towards new particles that would decay instantaneously, the way heavy Standard Model particles do and also most new physics particles are expected to do. For their new search, ATLAS physicists had to develop new methods of identifying particles in order to increase the likelihood of discovering long-lived particles.
Because the particles created by the decay of a long-lived particle would appear away from the collision point, unusual background sources can arise: photons misidentified as electrons, muons that are mismeasured, and poorly measured cosmic-ray muons. Cosmic-ray muons come from high-energy particles colliding with Earth’s atmosphere and can traverse the more than 90 metres of rock above the ATLAS detector, as well as the detector itself. Since they do not necessarily pass through the detector near the collision point, they can appear as if originating from a long-lived particle decay. ATLAS physicists have developed techniques not only for reducing but also for estimating the effects of these background sources.
The ATLAS collaboration found no evidence of long-lived particles in its search, but it was able to set limits on the mass and lifetime of long-lived sleptons decaying to Standard Model leptons inside the detector. For the slepton lifetime that the new search is most sensitive to (around 0.1 nanoseconds, corresponding to a flight length of about 30 centimetres), the researchers excluded selectrons and smuons up to a mass of around 700 GeV, and staus up to around 350 GeV. The previous best limits on these long-lived particles were around 90 GeV and came from the experiments on the Large Electron–Positron Collider (LEP) – CERN’s predecessor to the LHC – more than 20 years ago. The new result was able not only to meet LEP's best limits but also to surpass them.
Read more on the ATLAS website.
The CMS collaboration has seen evidence of top quarks in collisions between heavy nuclei at the Large Hadron Collider (LHC).
This isn’t the first time this special particle – the heaviest known elementary particle – has “made an appearance” at particle colliders. The top quark was first observed in proton–antiproton collisions at the Tevatron collider 25 years ago, and has since been spotted and studied in proton–proton and proton–nucleus collisions at the LHC. But the new finding, described in a paper just accepted for publication in Physical Review Letters, is sure to excite experimentalists and theorists alike, for analysis of top quarks in heavy-nuclei collisions offers a new and unique way to study the quark–gluon plasma that forms in these collisions and is thought to have existed in the early moments of the universe. In addition, such analysis could cast new light on the arrangement of quarks and gluons inside heavy nuclei.
There isn’t exactly a shortage of particles, or “probes”, with which to investigate the quark–gluon plasma. The LHC experiments have long been using several types of particle to study the properties of this extreme state of matter, in which quarks and gluons are not confined within composite particles but instead roam like particles in a liquid with small frictional resistance. But all of the existing probes provide time-averaged information about the plasma. By contrast, the top quark, owing to the particular way in which it transforms, or “decays” into other particles, can provide snapshots of the plasma at different times of its lifetime.
"Faster-moving top quarks provide later-time snapshots. By assembling snapshots taken with top quarks at a range of different speeds, we hope that it will eventually be possible to create a movie of the quark–gluon plasma’s evolution,” explains CERN-based researcher Guilherme Milhano, who co-authored a theoretical study on probing the quark–gluon plasma with top quarks. “The new CMS result represents the very first step down that road.”
The CMS collaboration saw evidence of top quarks in a large data sample from lead–lead collisions at an energy of 5.02 TeV. The team searched for collisions producing a top quark and a top antiquark. These quarks decay very quickly into a W boson and a bottom quark, which in turn also decay very rapidly into other particles. The CMS physicists looked for the particular case in which the final decay products are charged leptons (electrons or their heavier cousins muons) and “jets” of multiple particles originating from bottom quarks.
After isolating and counting these top–antitop collision events, CMS estimated the probability for lead–lead collisions to produce top–antitop pairs via charged leptons and bottom quarks. The result has a statistical significance of about four standard deviations, so it doesn’t yet cross the threshold of five standard deviations that is required to claim observation of top-quark production. But it represents significant evidence of the process – there’s only a 0.003% chance that the result is a statistical fluke. What’s more, the result is consistent with theoretical predictions, as well as with extrapolations from previous measurements of the probability in proton–proton collisions at the same collision energy.
“Our result demonstrates the capability of the CMS experiment to perform top-quark studies in the complex environment of heavy-nuclei collisions,” says CMS physicist Georgios Krintiras, a postdoctoral researcher at the University of Kansas, “and it’s the ﬁrst stepping stone in using the top quark as a new and powerful probe of the quark–gluon plasma.”
Read more on the CMS website.
The observed excess of matter over antimatter in the Universe is an enduring puzzle in physics. The imbalance implies a difference in the behaviour of matter and antimatter particles. This difference, or “asymmetry”, is known as CP violation and is a fundamental part of the Standard Model of particle physics. But the amount of CP violation predicted by the model and observed so far in experiments is too small to explain the cosmic imbalance, suggesting the existence of as-yet-unknown sources and manifestations of CP violation beyond the Standard Model.
At the nineteenth beauty conference last month and at a seminar today at CERN, the LHCb collaboration reported the first observation of so-called time-dependent matter–antimatter asymmetry in particles known as Bs0 mesons, which contain a beauty antiquark and a strange quark.
CP violation was first observed more than five decades ago in particles called K0 mesons, and has since been observed in other types of particle – including in B0 mesons in 2001 by experiments at the SLAC laboratory in the US and the KEK laboratory in Japan, and recently by the LHCb collaboration in D0 mesons. The effect can manifest itself in two forms: time-integrated and time-dependent. In the time-integrated form, the number of transformations, or “decays”, of a matter particle into certain particles differs from that of the corresponding antimatter particle. In the time-dependent form, the violation varies with the particle’s lifetime due to the spontaneous oscillation of the particle into its antiparticle and back.
The new LHCb study provides the first observation of time-dependent CP violation in Bs0 mesons, in their decays into charged K mesons. The result, obtained by combining data collected during the first and second runs of the Large Hadron Collider, has a statistical significance of 6.7 standard deviations, which is beyond the threshold of 5 standard deviations used by particle physicists to claim an observation.
“The Bs0 mesons oscillate between particle and antiparticle three thousand billion times per second, but the excellent resolution of our detector made it possible to observe the effect of these oscillations. Our observation of time-dependent CP violation in Bs0 mesons represents a further milestone in the study of the differences between matter and antimatter,” says LHCb spokesperson Chris Parkes, “adding to our previous observation of time-integrated CP violation in these mesons.”
The next steps will be to compare the measurement with other measurements of CP violation and with predictions from the Standard Model and beyond. It’s only after researchers make these comparisons that they will be able to tell whether or not the new measurement hides any surprises that might help to explain the matter–antimatter imbalance in the universe.
Read more on the LHCb website.