Picture two children playing on swings in a playground. One is a daredevil, launching themselves high off the ground in big arcs. The other daydreams, swinging gently.
Now picture the children holding either end of a long spring. Tension in the spring now accelerates the daydreaming child forwards and backwards to follow their friend, whose swings are slowed and shortened.
This is the principle behind a groundbreaking new technological demonstration reported today in Nature by the BASE collaboration – an international particle-physics collaboration based at CERN’s antimatter factory. The energetic child represents a single proton oscillating inside the magnetic and electric fields of a Penning trap. The daydreamer represents a laser-cooled cloud of beryllium ions inside a second trap. The spring represents a unique innovation by the BASE collaboration: a superconducting resonant electric circuit that transfers energy from the proton to the ions, just as the spring transfers energy from one swing to the other. Smaller swings mean a lower temperature proton and greater precision in experimental studies.
“This is an important milestone in precision Penning trap spectroscopy,” says BASE deputy spokesperson Christian Smorra of RIKEN and the University of Mainz, where the demonstration was performed. “With optimised procedures we should be able to reach particle temperatures of the order of 20 to 50 mK, ideally in cooling times of the order of 10 seconds. Previous methods allowed us to reach 100 mK in 10 hours.”
The speedy new two-trap cooling procedure promises a huge increase in the statistics that are available to experimenters. It is also a game-changing development for the study of BASE’s main particle of interest: the antiproton. Conventional cooling techniques are difficult to apply to antimatter because it is highly challenging to put matter and antimatter in the same trap. Applying the new technique should allow a significant improvement on BASE’s already world-leading measurements of fundamental properties of antiprotons. Such measurements have the potential to shed light on one of the biggest unanswered questions in fundamental physics: the unexplained surfeit of matter over antimatter in the universe.
“Our vision is to continually improve the precision of our matter–antimatter comparisons to develop a better understanding of the cosmological matter–antimatter asymmetry,” says BASE spokesperson Stefan Ulmer of RIKEN. “The newly developed technique will become a key method in these experiments, which aim to measure fundamental antimatter constants at the sub-parts-per-trillion level.”
For more details check out the full report in CERN Courier magazine.
Today, the LHCb experiment at CERN is presenting a new discovery at the European Physical Society Conference on High Energy Physics (EPS-HEP). The new particle discovered by LHCb, labelled as Tcc+, is a tetraquark – an exotic hadron containing two quarks and two antiquarks. It is the longest-lived exotic matter particle ever discovered, and the first to contain two heavy quarks and two light antiquarks.
Quarks are the fundamental building blocks from which matter is constructed. They combine to form hadrons, namely baryons, such as the proton and the neutron, which consist of three quarks, and mesons, which are formed as quark-antiquark pairs. In recent years a number of so-called exotic hadrons – particles with four or five quarks, instead of the conventional two or three - have been found. Today’s discovery is of a particularly unique exotic hadron, an exotic exotic hadron if you like.
The new particle contains two charm quarks and an up and a down antiquark. Several tetraquarks have been discovered in recent years (including one with two charm quarks and two charm antiquarks), but this is the first one that contains two charm quarks, without charm antiquarks to balance them. Physicists call this “open charm” (in this case, “double open charm”). Particles containing a charm quark and a charm antiquark have “hidden charm” – the charm quantum number for the whole particle adds up to zero, just like a positive and a negative electrical charge would do. Here the charm quantum number adds up to two, so it has twice the charm!
The quark content of Tcc+ has other interesting features besides being open charm. It is the first particle to be found that belongs to a class of tetraquarks with two heavy quarks and two light antiquarks. Such particles decay by transforming into a pair of mesons, each formed by one of the heavy quarks and one of the light antiquarks. According to some theoretical predictions, the mass of tetraquarks of this type should be very close to the sum of masses of the two mesons. Such proximity in mass makes the decay “difficult”, resulting in a longer lifetime of the particle, and indeed Tcc+ is the longest-lived exotic hadron found to date.
The discovery paves the way for a search for heavier particles of the same type, with one or two charm quarks replaced by bottom quarks. The particle with two bottom quarks is especially interesting: according to calculations, its mass should be smaller than the sum of the masses of any pair of B mesons. This would make the decay not only unlikely, but actually forbidden: the particle would not be able to decay via the strong interaction and would have to do so via the weak interaction instead, which would make its lifetime several orders of magnitude longer than any previously observed exotic hadron.
The new Tcc+ tetraquark is an enticing target for further study. The particles that it decays into are all comparatively easy to detect and, in combination with the small amount of the available energy in the decay, this leads to an excellent precision on its mass and allows the study of the quantum numbers of this fascinating particle. This, in turn, can provide a stringent test for existing theoretical models and could even potentially allow previously unreachable effects to be probed.
As a carrier of the electroweak force, the W boson plays a crucial role in testing the Standard Model of particle physics. Though discovered nearly four decades ago, the W boson continues to provide physicists with new avenues for exploration.
ATLAS researchers analysed the full LHC Run-2 dataset, recorded by the detector between 2015 and 2018, to observe the WWW process with a statistical significance of 8.2 standard deviations – well above the 5 standard-deviation threshold needed to declare observation. This result follows an earlier observation by the CMS collaboration of inclusive three weak boson production.
Achieving this level of precision was no mean feat. Physicists analysed around 20 billion collision events recorded and pre-filtered by the ATLAS experiment in their search for just a few hundred events expected from the WWW process.
As one of the heaviest known elementary particles, the W boson is able to decay in several different ways. The ATLAS physicists focused their search on the four WWW decay modes that have the best discovery potential due to their reduced number of background events. In three of these modes, two W bosons decay into charged leptons (electrons or muons), carrying the same positive or negative charge, and neutrinos, while the third W boson decays into a pair of light quarks. In the fourth decay mode, all three W bosons decay into a charged lepton and a neutrino.
To pick out the WWW signal from the large number of background events, researchers used a machine-learning technique called Boosted Decision Trees (BDTs). BDTs can be trained to identify specific signals in the ATLAS detector, spotting small – but key – differences between the predicted event properties. The improved separation between signal and background provided by the BDTs – along with the massive dataset provided by Run 2 of the LHC – enhanced the precision of the overall measurement and enabled the first observation of WWW production.
This exciting measurement also allows physicists to look for hints of new interactions that might exist beyond the current energy reach of the LHC. In particular, physicists can use the WWW production process to study the quartic gauge boson coupling – where two W bosons scatter off each other – a key property of the Standard Model. New particles could alter the quartic gauge boson coupling through quantum effects, modifying the WWW production cross section. The continued study of WWW and other electroweak processes offers an enticing road ahead.
The Monopole and Exotics Detector at the Large Hadron Collider (MoEDAL) does what it says on the tin. It searches for magnetic monopoles – hypothetical particles with either a “north” or a “south” magnetic charge instead of both – and other exotic theoretical particles. These searches have so far come up empty-handed, but they have delivered crucial information to help guide future searches. Now, in a first for an experiment at a particle collider, MoEDAL has searched for magnetic monopoles produced through a process called the Schwinger mechanism.
Nobel Prize winner Julian Schwinger showed that pairs of particles with electrical charge can be spontaneously created in a strong electric ﬁeld. Similarly, pairs of magnetic monopoles could be spontaneously created in a strong magnetic field. Compared to other means of producing magnetic monopoles, this process, known as the Schwinger mechanism, has advantages, including that the monopoles should be created at a greater rate, thus increasing the chances of spotting them.
The MoEDAL team usually looks for magnetic monopoles by exposing the experiment’s “magnetic monopole trappers”, which consist of 800 kg of aluminium blocks, to proton–proton collisions produced at the Large Hadron Collider (LHC). To search for Schwinger magnetic monopoles, however, the team exposed the blocks to lead–lead collisions produced by the LHC in November 2018, just before the collider was shut down for maintenance.
Lead–lead collisions at the LHC generate extremely strong magnetic fields, and the November 2018 run generated a maximum magnetic ﬁeld that was more than ten thousand times stronger than the strongest magnetic fields in the cosmos, which are found on the surfaces of fast-spinning neutron stars called magnetars, and ten million times stronger than the field strength required to create Schwinger monopoles. Therefore, these collisions could have produced such monopoles.
After exposing the blocks to the lead–lead collisions, the MoEDAL researchers used a device called a SQUID magnetometer to scan the blocks for any trapped magnetic charges belonging to Schwinger monopoles. The researchers found no signs of such monopoles in the blocks, but the lead–lead collision data allowed them to rule out the existence of Schwinger monopoles that have masses up to 75 GeV/c2, where c is the speed of light, for magnetic charges ranging from 1 to 3 base units of magnetic charge.
“A unique feature of the Schwinger monopoles is that they are not point-like, they have a finite size,” explains MoEDAL spokesperson James Pinfold. “Our mass bound is the ﬁrst lower mass limit for finite-size monopoles from a collider search, and it’s tighter than previous similar mass bounds, such as that obtained from neutron-star data.”
The MoEDAL team will continue its searches during the next run of the LHC, which will start in 2022 and deliver more proton–proton and lead–lead collision data for analysis.
Machine learning is everywhere. For example, it’s how Spotify gives you suggestions of what to listen to next or how Siri answers your questions. And it’s used in particle physics too, from theoretical calculations to data analysis. Now a team including researchers from CERN and Google has come up with a new method to speed up deep neural networks – a form of machine-learning algorithms – for selecting proton–proton collisions at the Large Hadron Collider (LHC) for further analysis. The technique, described in a paper just published in Nature Machine Intelligence, could also be used beyond particle physics.
The particle detectors around the LHC ring use an electronic hardware “trigger” system to select potentially interesting particle collisions for further analysis. With the current rate of proton–proton collisions at the LHC, up to 1 billion collisions per second, the software currently in use on the detectors’ trigger systems chooses whether or not to select a collision in the required time, which is a mere microsecond. But with the collision rate set to increase by a factor of 5 to 7 with the future upgraded LHC, the HL-LHC, researchers are exploring alternative software, including machine-learning algorithms, that could make this choice faster.
Enter the new study by CERN researchers and co-workers, which builds on previous work that introduced a software tool to deploy deep neural networks on a type of hardware, called field-programmable gate arrays (FPGAs), that can be programmed to perform different tasks, including selecting particle collisions of interest. The CERN researchers and their colleagues developed a technique that reduces the size of a deep neural network by a factor of 50 and achieves a network processing time of tens of nanoseconds – well below the time available to choose whether to save or discard a collision.
“The technique boils down to compressing the deep neural network by reducing the numerical precision of the parameters that describe it,” says co-author of the study and CERN researcher Vladimir Loncar. “This is done during the training, or learning, of the network, allowing the network to adapt to the change. In this way, you can reduce the network size and processing time, without a loss in network performance.”
In addition, the technique can find which numerical precision is best to use given certain hardware constraints, such as the amount of available hardware resources.
If that wasn’t enough, the technique has the advantage that it is easy to use for non-experts, and it can be used on FPGAs in particle detectors and in other devices that require networks with fast processing times and small sizes.
Looking forward, the researchers want to use their technique to design a new kind of trigger system for spotting collisions that would normally be discarded by a conventional trigger system but that could hide new phenomena. “The ultimate goal is to be able to capture collisions that could point to new physics beyond the Standard Model of particle physics,” says another co-author of the study and CERN researcher Thea Aarrestad.
Zoom into an online particle physics conference, and the chances are you’ll hear the term muon anomaly. This is a longstanding tension with the Standard Model of particle physics, seen in the magnetism of a heavier cousin of the electron called a muon, that has recently been strengthened by measurements made at Fermilab in the US.
In a paper accepted for publication in Physical Review Letters, a trio of theorists including Andreas Crivellin of CERN shows that a class of new unknown particles that could account for the muon anomaly, known as leptoquarks, also affects the transformation, or “decay”, of the Higgs boson into muons.
Leptoquarks are hypothetical particles that connect quarks and leptons, the two types of particles that make up matter at the most fundamental level. They are a popular explanation for the muon anomaly and other anomalies seen in certain decays of particles called B mesons.
In their new study, Crivellin and his colleagues explored how two kinds of leptoquarks that could explain the muon anomaly would affect the rare decay of the Higgs boson into muons, of which the ATLAS and CMS experiments recently obtained the first indications.
They found that one of the two kinds of leptoquarks increases the rate at which this Higgs decay takes place, while the other one decreases it.
“The current measurements of the Higgs decay to muons are not sufficient to see this increase or decrease, and the muon anomaly has yet to be confirmed,” says Crivellin. “But if future measurements, at the LHC or future colliders, display such a change, and the muon anomaly is confirmed, it will be possible to pick out which of the two kinds of leptoquarks would be more likely to explain the muon anomaly.”
Quarks are among the elementary particles of the Standard Model of Particle Physics. Besides up and down quarks, which are the basic building blocks of ordinary matter in the Universe, four other quark flavours exist and are also abundantly produced in collisions at particle accelerators like the CERN Large Hadron Collider. Quarks are not observed in isolation due to a fundamental aspect of the strong interaction, known as colour charge confinement. Confinement requires particles that carry the charge of the strong interaction, called colour, to form states that are colour-neutral. This in turn forces quarks to undergo a process of hadronisation, i.e. to form hadrons, which are composite particles mostly made of a quark and an antiquark (mesons) or of three quarks (baryons). The only exception is the heaviest quark, the top, which decays before it has time to hadronise.
At particle accelerators, quarks with a large mass, such as the charm quark, are produced only in the initial interactions between the colliding particles. Depending on the type of beam used, these can be electron-positron, electron-proton or proton-proton collisions (as at the LHC). The subsequent hadronisation of charm quarks into mesons (D0, D+, Ds) or baryons (Λc, Ξc, …) occurs on a long space-time scale and was considered to be universal - that is, independent of the species of the colliding particles - until the recent findings by the ALICE collaboration.
The large data samples collected during Run 2 of the LHC allowed ALICE to count the vast majority of charm quarks produced in the proton-proton collisions by reconstructing the decays of all charm meson species and of the most abundant charm baryons (Λc and Ξc). The charm quarks were found to form baryons almost 40% of the time, which is four times more often than what was expected based on measurements previously made at colliders with electron beams (e+e- and ep in the figure below).
These measurements show that the process of colour-charge confinement and hadron formation is still a poorly understood aspect of the strong interaction. Current theoretical explanations of baryon enhancement include the combination of multiple quarks produced in proton-proton collisions and new mechanisms in the neutralisation of the colour charge. Additional measurements during the next run of the LHC will allow these theories to be scrutinised and further our knowledge of the strong interaction.
The LHCb collaboration has measured a difference in mass between two particles of 0.00000000000000000000000000000000000001 grams – or, in scientific notation, 10-38 g. The result, reported in a paper just submitted for publication in the journal Physical Review Letters and presented today at a CERN seminar, marks a milestone in the study of how a particle known as a D0 meson changes from matter into antimatter and back.
The D0 meson is one of only four particles in the Standard Model of particle physics that can turn, or “oscillate”, into their antimatter particles, which are identical to their matter counterparts in most ways. The other three are the K0 meson and two types of B mesons.
Mesons are part of the large class of particles made up of fundamental particles called quarks, and contain one quark and one antimatter quark. The D0 meson consists of a charm quark and an up antiquark, while its antiparticle, the anti-D0, consists of a charm antiquark and an up quark.
In the strange world of quantum physics, just as Schrödinger's notorious cat can be dead and alive at the same time, the D0 particle can be itself and its antiparticle at once. This quantum “superposition” results in two particles, each with their own mass – a lighter and a heavier D meson (known technically as D1 and D2). It is this superposition that allows the D0 to oscillate into its antiparticle and back.
The D0 particles are produced in proton–proton collisions at the Large Hadron Collider (LHC), and they travel on average only a few millimetres before transforming, or “decaying”, into other particles. By comparing the D0 particles that decay after travelling a short distance with those that travel a little further, the LHCb collaboration has measured the key quantity that controls the speed of the D0 oscillation into anti-D0 – the difference in mass between the heavier and lighter D particles.
The result, 10-38 g, crosses the “five sigma” level of statistical significance that is required to claim an observation in particle physics.
“To put this incredibly small mass difference in context, it is still a small number even when compared with the mass of the D0 particle – the same as the mass of a snowball compared to the mass of the entire Mont Blanc, the highest peak in Europe, standing at over 4800 metres,” says LHCb spokesperson Chris Parkes. “And it’s a big step in the study of the oscillatory behaviour of the D0 particles.”
With the tiny mass difference now observed, a new phase of particle exploration can begin. Researchers can make further measurements of the D0 decays to obtain a more precise mass difference and look for the effect on the D0 oscillation of unknown particles not predicted by the Standard Model.
Such new particles could increase the average speed of the oscillation or the difference between the speed of the matter-to-antimatter oscillation and that of the antimatter-to-matter oscillation. If observed, such a difference could shed light on why the universe is made up entirely of matter, even though matter and antimatter should have been created in equal amounts during the Big Bang.LHCb spokesperson Chris Parkes explains the new result. (Video: CERN)
Read more on the LHCb website.
It’s a first at the Large Hadron Collider (LHC), or indeed at any particle collider: the FASER collaboration has detected the first candidate particle interactions for neutrinos produced in LHC collisions. The result, described in a paper posted online, paves the way for studies of high-energy neutrinos at current and future colliders.
Neutrinos are the most abundant fundamental particles that have mass in the universe, and they have been detected from many sources. Yet, no neutrino produced at a particle collider has ever been directly detected, even though colliders produce them in abundance. Studying such collider neutrinos could shed new light on the still enigmatic nature of these fundamental particles, not least because collider neutrinos are produced at high energies, at which their weak interactions with matter have been little studied.
The FASER experiment’s FASERν detector and the newly approved SND@LHC detector have both been designed to catch and study collider neutrinos, and they are expected to be installed at the LHC over the course of 2021 and to begin taking data when the collider starts up again in 2022. However, the FASER collaboration was in for an early treat when it took four weeks’ worth of proton–proton collision data with a smaller pilot version of FASERν shortly before the LHC was shut down for maintenance and upgrades at the end of 2018.
After analysing the pilot detector data and estimating a background of particle events that could mimic the signal from neutrino interactions, the FASER team found several candidate events for collider neutrinos. The result has a statistical significance of 2.7 standard deviations, a little below the 3 standard deviations required to claim evidence of a particle or process in particle physics.
“The goal of the pilot detector was to demonstrate the feasibility of neutrino measurements in the experimental environment of the LHC,” says FASER co-spokesperson Jamie Boyd. “So we are very excited that this small detector, which is only about 1% of the final detector, allowed us to see the first candidate events for neutrino interactions at a collider.”
The team expects to observe about 20 000 collider neutrino interactions with the full-fledged FASERν detector in the next LHC run, from 2022 to 2024.Two candidate events for neutrinos produced in LHC collisions and interacting in the FASERν pilot detector. The neutrinos enter the detector from the left, and interact with the detector material to produce a number of charged particles. The different lines in each event show tracks from these charged particles, originating from the neutrino interaction point. (Image: FASER/CERN)