The discovery of the Higgs boson was a landmark in the history of physics. It explained something fundamental: how elementary particles that have mass get their masses. But it also marked something no less fundamental: the beginning of an era of measuring in detail the particle’s properties and finding out what they might reveal about the nature of the universe.

One such property is the particle’s mass, which at 125 GeV is surprisingly small. Many theories have been put forward to explain this small mass, but none has so far been confirmed with data. In a paper just published in Physical Review Letters, Raffaele Tito D’Agnolo of CEA and Daniele Teresi of CERN propose a new theory to explain both the lightness of the Higgs boson and another fundamental physics puzzle.

In broad brushes, the duo’s theory works like this. In its early moments, the universe is a collection of many universes each with a different value of the Higgs mass, and in some of these universes the Higgs boson is light. In this multiverse model, universes in which the Higgs boson is heavy collapse in a big crunch in a very short time, whereas universes in which the boson is light survive this collapse. Our present-day universe would be one of these surviving light-Higgs universes.

What’s more, the model, which includes two new particles in addition to the known particles predicted by the Standard Model, can also explain the puzzling symmetry properties of the strong force, which binds quarks together into protons and neutrons, and protons and neutrons into atomic nuclei.

Although the theory of the strong force, known as quantum chromodynamics, predicts a possible breakdown in strong interactions of a fundamental symmetry called CP symmetry, such a breakdown is not observed in experiments. One of the new particles in D’Agnolo and Teresi’s model can solve this so-called strong CP problem, making strong interactions CP symmetric. Moreover, the same new particle could also account for the dark matter that is thought to make up most of the matter in the universe.

The jury is of course out on whether the new model, or any of the many other models that have been proposed to explain the Higgs boson mass or the strong CP problem, will fly.

“Each model comes with perks and limitations,” says Teresi. “Our model stands out because it is simple, generic and it solves these two seemingly unrelated puzzles at once. And it predicts distinctive features in data from experiments that aim to search for dark matter or for an electric dipole moment in the neutron and other hadrons.”

Other recent theoretical proposals to explain the Higgs mass include the relaxion field model, a new phenomenon in quantum cosmology, and the selfish Higss model, to mention a few. Older, more traditional theories are based on a Higgs boson that would be a composite particle or on a new type of symmetry called supersymmetry. Only time and data will tell which – if any – of the models will succeed.

While the Large Hadron Collider (LHC) is well known for smashing protons together, it is actually the quarks and gluons inside the protons – collectively known as partons – that are really interacting. Thus, in order to predict the rate of a process occurring in the LHC – such as the production of a Higgs boson or a yet-unknown particle – physicists have to understand how partons behave within the proton. This behaviour is described in Parton Distribution Functions (PDFs), which describe what fraction of a proton’s momentum is taken by its constituent quarks and gluons.

Knowledge of PDFs has traditionally come from lepton–proton colliders, such as HERA at DESY. These machines use point-like particles, such as electrons, to directly probe the partons within the proton. Their research revealed that, in addition to the well-known up and down quarks that are inside a proton, there is also a sea of other quark–antiquark pairs in the proton. This sea is theoretically made of all types of quarks, bound together by gluons. Now, studies of the LHC’s proton–proton collisions are providing a detailed look into PDFs, in particular the proton’s gluon and quark-type composition.

The ATLAS Collaboration has just released a new paper combining LHC and HERA data to determine PDFs. The result uses ATLAS data from several different Standard Model processes, including the production of W and Z bosons, pairs of top quarks and hadronic jets (collimated sprays of particles). The strange quark’s contribution to PDFs was expected to be lower than that of lighter quarks. The new paper confirms a previous ATLAS result, which found that the strange quark is not substantially suppressed at small proton momentum fractions and extends this result to show how suppression kicks in at higher momentum fractions.

Several experiments and theoretical groups around the world are working to understand PDFs, as variance in these results could impact high-energy searches for physics beyond the Standard Model.

Achieving high-accuracy PDFs is needed if physicists are to find evidence for new-physics processes – which is where the ATLAS analysis contributes most powerfully.  The ATLAS Collaboration is able to assess the correlations of the systematic uncertainties between their datasets and account for them – an ability put to great effect in their new PDF result. Such knowledge was not previously available outside ATLAS, making this result a new “vademecum” for global PDF groups.

Read the full article on the ATLAS website.

Additional links

• CERN Preprint: CERN-EP-2021-239
• arXiv: 2112.11266
• Figures: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2020-32
• Lepton photon talk: https://indico.cern.ch/event/949705/contributions/4556026/ 

In a paper published today in the journal Nature, the BASE collaboration at CERN reports the most precise comparison yet between protons and antiprotons, the antimatter counterparts of protons.

Analysing proton and antiproton measurements taken over a year and a half at CERN’s antimatter factory, a unique facility for antimatter production and analyses, the BASE team measured the electric charge-to-mass ratios of the proton and the antiproton with record precision. The results found these are identical to within an experimental uncertainty of 16 parts per trillion.

“This result represents the most precise direct test of a fundamental symmetry between matter and antimatter, performed with particles made of three quarks, known as baryons, and their antiparticles,” says BASE spokesperson Stefan Ulmer.

According to the Standard Model, which represents physicists’ current best theory of particles and their interactions, matter and antimatter particles can differ, for example in the way they transform into other particles, but most of their properties, including their masses, should be identical. Finding any slight difference between the masses of protons and antiprotons, or between the ratios of their electric charge and mass, would break a fundamental symmetry of the Standard Model, called CPT symmetry, and point to new physics phenomena beyond the Model.

Such a difference could also shed light on why the universe is made up almost entirely of matter, even though equal amounts of antimatter should have been created in the Big Bang. The differences between matter and antimatter particles that are consistent with the Standard Model are smaller by orders of magnitude to be able to explain this observed cosmic imbalance.

To make their proton and antiproton measurements, the BASE team confined antiprotons and negatively charged hydrogen ions[1], which are negatively charged proxies for protons, in a state-of-the-art particle trap called a Penning trap. In this device, a particle follows a cyclical trajectory with a frequency, close to the cyclotron frequency, that scales with the trap’s magnetic-field strength and the particle's charge-to-mass ratio.

Alternately feeding antiprotons and negatively charged hydrogen ions one at a time into the trap, the BASE team measured, under the same conditions, the cyclotron frequencies of these two kinds of particle, allowing their charge-to-mass ratios to be compared.

Performed over four campaigns between December 2017 and May 2019, these measurements resulted in more than 24000 cyclotron-frequency comparisons, each lasting 260 seconds, between the charge-to-mass ratios of antiprotons and negatively charged hydrogen ions. From these comparisons, and after accounting for the difference between a proton and a negatively charged hydrogen ion, the BASE researchers found that the charge-to-mass ratios of protons and antiprotons are equal to within 16 parts per trillion.

“This result is four times more precise than the previous best comparison between these ratios, and the charge-to-mass ratio is now the most precisely measured property of the antiproton.” says Stefan Ulmer. “To reach this precision, we made considerable upgrades to the experiment and carried out the measurements when the antimatter factory was closed down, using our reservoir of antiprotons, which can store antiprotons for years.” Making cyclotron-frequency measurements when the antimatter factory is not in operation is ideal, because the measurements are not affected by disturbances to the experiment’s magnetic field.

In addition to comparing protons and antiprotons with an unprecedented precision, the BASE team used their measurements to place stringent limits on models beyond the Standard Model that violate CPT symmetry, as well as to test a fundamental physics law known as the weak equivalence principle.

According to this principle, different bodies in the same gravitational field undergo the same acceleration in the absence of friction forces. Because the BASE experiment is placed on the surface of the Earth, its proton and antiproton cyclotron-frequency measurements were made in the gravitational field on the Earth’s surface. Any difference between the gravitational interaction of protons and antiprotons would result in a difference between the proton and antiproton cyclotron frequencies.

Sampling the varying gravitational field of the Earth as the planet orbits around the Sun, the BASE scientists found no such difference and set a maximum value on this differential measurement of three parts in 100.

“This limit is comparable to the initial precision goals of experiments that aim to drop antihydrogen in the Earth’s gravitational field,” says Ulmer. “BASE did not directly drop antimatter in the Earth’s gravitational field, but our measurement of the influence of gravity on a baryonic antimatter particle is conceptually very similar, indicating no anomalous interaction between antimatter and gravity at the achieved level of uncertainty.”

Videos:

Video about BASE: https://videos.cern.ch/record/2289533

Video about the Antimatter Factory : https://videos.cern.ch/record/2312142

Photos:

BASE experiment: https://cds.cern.ch/record/2748765

BASE penning trap: https://cds.cern.ch/record/2748764

[1] A hydrogen atom that has captured an extra electron.

The atomic nucleus is a tough nut to crack. The strong interaction between the protons and neutrons that make it up depends on many quantities, and these particles, collectively known as nucleons, are subject to not only two-body forces but also three-body ones. These and other features make the theoretical modelling of atomic nuclei a challenging endeavour.

In the past few decades, however, ab initio theoretical calculations, which attempt to describe nuclei from first principles, have started to change our understanding of nuclei. These calculations require fewer assumptions than traditional nuclear models, and they have a stronger predictive power. That said, because so far they can only be used to predict the properties of nuclei up to a certain atomic mass, they cannot always be compared with so-called DFT calculations, which are also fundamental and powerful and have been around for longer. Such a comparison is essential to build a nuclear model that is applicable across the board.

In a paper just published in Physical Review Letters, an international team at CERN’s ISOLDE facility shows how a unique combination of high-quality experimental data and several ab initio and DFT nuclear-physics calculations has resulted in an excellent agreement between the different calculations, as well as between the data and the calculations.

“Our study demonstrates that precision nuclear theory from first principles is no longer a dream,” says Stephan Malbrunot of CERN, the first author of the paper. “In our work, the calculations agree with each other, as well as with our ISOLDE data on nickel nuclei, to within a small theoretical uncertainty.”

Using a suite of experimental methods at ISOLDE, including a technique to detect the light emitted by short-lived atoms when laser light is shone on them, Malbrunot and colleagues determined the (charge) radii of a range of short-lived nickel nuclei, which have the same number of protons, 28, but a different number of neutrons. These 28 protons fill a complete shell within the nucleus, resulting in nuclei that are more strongly bound and stable than their nuclear neighbours. Such “magic” nuclei are excellent test cases for nuclear theories, and in terms of their radius, nickel nuclei are the last unexplored magic nuclei that have a mass within the mass region at which both ab initio and DFT calculations can be made.

Comparing the ISOLDE radii data with three ab initio calculations and one DFT calculation, the researchers found that the calculations agree with the data, as well as with each other, to within a theoretical uncertainty of one part in a hundred.

“An agreement at this level of precision demonstrates that it will eventually become possible to build a model that is applicable across the whole chart of nuclei,” says Malbrunot.

The Higgs boson doesn’t stick around for long. Once it is created in particle collisions, the famed particle lives for a mere less than a trillionth of a billionth of a second or, more precisely, 1.6 x 10-22 seconds. According to theory, that is, for so far experiments have only been able to set bounds on the value of the particle’s lifetime or to determine this property with a large uncertainty. Until now. In a new study, the CMS collaboration reports a value for the particle’s lifetime that has a small enough uncertainty to confirm that the Higgs boson does have such a short lifetime.

Measuring the Higgs boson’s lifetime is high on the wish list of particle physicists, because an experimental value of the lifetime would allow them not only to better understand the nature of the particle but also to find out whether or not the value matches the value predicted by the Standard Model of particle physics. A deviation from the prediction could point to new particles or forces not predicted by the Model, including new particles into which the Higgs boson would decay.

But it isn’t easy to measure the Higgs boson’s lifetime. For one, the predicted lifetime is too short to be measured directly. A possible solution entails measuring a related property called the mass width, which is inversely proportional to the lifetime and represents the small range of possible masses around the particle’s nominal mass of 125 GeV. But this isn’t easy either, as the predicted mass width of the Higgs boson is too small to be easily measured by experiments.

Quantum physics to the rescue. In addition to being produced with a mass equal or close to its nominal value, a short-lived particle such as the Higgs boson can also be produced with a much larger mass than the nominal value, although the odds of this happening are lower. This effect – and in fact the mass width of the particle as well – is a manifestation of a quantum quirk known as Heisenberg’s uncertainty principle, and a comparison between the production rate of these large-mass, or “off-shell”, Higgs bosons with that of the nominal or close to nominal, or “on-shell”, Higgs bosons can be used to extract the Higgs boson’s mass width and therefore its lifetime.

This is the method employed by the CMS team in their new study. By analysing data collected by the CMS experiment during the second run of the Large Hadron Collider (LHC), specifically data on Higgs bosons transforming into two Z bosons, which themselves transform into four charged leptons or two charged leptons plus two neutrinos, the CMS researchers have obtained the first-ever evidence for the production of off-shell Higgs bosons. From this result, which has only a 1 in 1000 chance of being a statistical fluke, the CMS team obtained a Higgs boson’s lifetime of 2.1 x 10-22 seconds, with an upper/lower uncertainty of (+2.3/-0.9) x 10-22 seconds. This value, the most precise yet, aligns well with the Standard Model prediction and confirms that the particle does indeed have a tiny lifespan.

“Our result demonstrates that off-shell Higgs-boson production offers an excellent way to measure the Higgs boson’s lifetime,” says CMS physicist Pascal Vanlaer. “And it sets a milestone in the study of the properties of this unique particle. The precision of the measurement is expected to improve in the coming years with data from the next LHC runs and new analysis ideas.”

_____

Read more on the CMS website.

The Higgs boson doesn’t stick around for long. Once it is created in particle collisions, the famed particle lives for a mere less than a trillionth of a billionth of a second or, more precisely, 1.6 x 10-22 seconds. According to theory, that is, for so far experiments have only been able to set bounds on the value of the particle’s lifetime or to determine this property with a large uncertainty. Until now. In a new study, the CMS collaboration reports a value for the particle’s lifetime that has a small enough uncertainty to confirm that the Higgs boson does have such a short lifetime.

Measuring the Higgs boson’s lifetime is high on the wish list of particle physicists, because an experimental value of the lifetime would allow them not only to better understand the nature of the particle but also to find out whether or not the value matches the value predicted by the Standard Model of particle physics. A deviation from the prediction could point to new particles or forces not predicted by the Model, including new particles into which the Higgs boson would decay.

But it isn’t easy to measure the Higgs boson’s lifetime. For one, the predicted lifetime is too short to be measured directly. A possible solution entails measuring a related property called the mass width, which is inversely proportional to the lifetime and represents the small range of possible masses around the particle’s nominal mass of 125 GeV. But this isn’t easy either, as the predicted mass width of the Higgs boson is too small to be easily measured by experiments.

Quantum physics to the rescue. In addition to being produced with a mass equal or close to its nominal value, a short-lived particle such as the Higgs boson can also be produced with a much larger mass than the nominal value, although the odds of this happening are lower. This effect – and in fact the mass width of the particle as well – is a manifestation of a quantum quirk known as Heisenberg’s uncertainty principle, and a comparison between the production rate of these large-mass, or “off-shell”, Higgs bosons with that of the nominal or close to nominal, or “on-shell”, Higgs bosons can be used to extract the Higgs boson’s mass width and therefore its lifetime.

This is the method employed by the CMS team in their new study. By analysing data collected by the CMS experiment during the second run of the Large Hadron Collider (LHC), specifically data on Higgs bosons transforming into two Z bosons, which themselves transform into four charged leptons or two charged leptons plus two neutrinos, the CMS researchers have obtained the first-ever evidence for the production of off-shell Higgs bosons. From this result, which has only a 1 in 1000 chance of being a statistical fluke, the CMS team obtained a Higgs boson’s lifetime of 2.1 x 10-22 seconds, with an upper/lower uncertainty of (+2.3/-0.9) x 10-22 seconds. This value, the most precise yet, aligns well with the Standard Model prediction and confirms that the particle does indeed have a tiny lifespan.

“Our result demonstrates that off-shell Higgs-boson production offers an excellent way to measure the Higgs boson’s lifetime,” says CMS physicist Pascal Vanlaer. “And it sets a milestone in the study of the properties of this unique particle. The precision of the measurement is expected to improve in the coming years with data from the next LHC runs and new analysis ideas.”

_____

Read more on the CMS website.

Alternating from spheres to rugby balls is no longer the sole preserve of mercury isotopes, an international team at CERN’s ISOLDE facility reports in a paper published in Physical Review Letters.

Isotopes are forms of a chemical element that have the same number of protons in their atomic nuclei but a different number of neutrons.

Atomic nuclei are usually spherical or nearly spherical. For a given element, though, when the number of neutrons changes, a gradual change in nuclear shape, or even a sudden one, can occur. However, 50 years ago, an experiment at ISOLDE revealed that the nuclei of mercury isotopes actually alternate dramatically in shape, from a sphere to a pronounced rugby ball, as single neutrons are removed from, or added to, the nucleus.

The finding remains one of the most remarkable discoveries in nuclear physics in the past five decades, and scientists have wondered ever since whether elements other than mercury also display this unusual ‘shape-staggering’ phenomenon.

The new study conducted at ISOLDE, the very same facility at which the phenomenon was discovered, has now delivered an answer to this question. By using ISOLDE’s ultrasensitive Resonance Ionisation Laser Ion Source, the team behind the study has now shown that bismuth isotopes also display shape staggering.

Specifically, examining bismuth nuclei produced at a challenging low rate of less than one atom per second, the team found that the nucleus of bismuth-188, which has 83 protons and 105 neutrons, has a much larger radius than those of its closest nuclear neighbours, bismuth-189, with one more neutron, and bismuth-187, with one fewer neutron. Interestingly, such a sharp increase in radius, which reveals a change from a sphere to a pronounced rugby ball, occurs at the same number of neutrons, 105, as that at which shape staggering starts in the mercury isotopes.

“We had no indication from theory or experiment that bismuth nuclei would also exhibit shape staggering,” says Bruce Marsh of CERN and co-author of the study. “Such light bismuth nuclei are remarkably difficult to make and study, and our best nuclear physics theories lack the power to predict the shape of these and other complex nuclei.”

If this experimental result wasn’t enough, the team gathered a unique collaboration of a dozen atomic-theory groups from five continents to extract nuclear properties from the ISOLDE measurements. At the same time, the researchers performed state-of-the-art nuclear theoretical calculations, paving the way to understanding the shape-staggering phenomenon.

“We can’t tell whether or not we’ll find another instance of shape staggering, but one thing is clear, this behaviour is no longer unique to mercury isotopes,” concludes Marsh.

Alternating from spheres to rugby balls is no longer the sole preserve of mercury isotopes, an international team at CERN’s ISOLDE facility reports in a paper published in Physical Review Letters.

Isotopes are forms of a chemical element that have the same number of protons in their atomic nuclei but a different number of neutrons.

Atomic nuclei are usually spherical or nearly spherical. For a given element, though, when the number of neutrons changes, a gradual change in nuclear shape, or even a sudden one, can occur. However, 50 years ago, an experiment at ISOLDE revealed that the nuclei of mercury isotopes actually alternate dramatically in shape, from a sphere to a pronounced rugby ball, as single neutrons are removed from, or added to, the nucleus.

The finding remains one of the most remarkable discoveries in nuclear physics in the past five decades, and scientists have wondered ever since whether elements other than mercury also display this unusual ‘shape-staggering’ phenomenon.

The new study conducted at ISOLDE, the very same facility at which the phenomenon was discovered, has now delivered an answer to this question. By using ISOLDE’s ultrasensitive Resonance Ionisation Laser Ion Source, the team behind the study has now shown that bismuth isotopes also display shape staggering.

Specifically, examining bismuth nuclei produced at a challenging low rate of less than one atom per second, the team found that the nucleus of bismuth-188, which has 83 protons and 105 neutrons, has a much larger radius than those of its closest nuclear neighbours, bismuth-189, with one more neutron, and bismuth-187, with one fewer neutron. Interestingly, such a sharp increase in radius, which reveals a change from a sphere to a pronounced rugby ball, occurs at the same number of neutrons, 105, as that at which shape staggering starts in the mercury isotopes.

“We had no indication from theory or experiment that bismuth nuclei would also exhibit shape staggering,” says Bruce Marsh of CERN and co-author of the study. “Such light bismuth nuclei are remarkably difficult to make and study, and our best nuclear physics theories lack the power to predict the shape of these and other complex nuclei.”

If this experimental result wasn’t enough, the team gathered a unique collaboration of a dozen atomic-theory groups from five continents to extract nuclear properties from the ISOLDE measurements. At the same time, the researchers performed state-of-the-art nuclear theoretical calculations, paving the way to understanding the shape-staggering phenomenon.

“We can’t tell whether or not we’ll find another instance of shape staggering, but one thing is clear, this behaviour is no longer unique to mercury isotopes,” concludes Marsh.

In an article recently published in Physical Review Letters, the ALICE collaboration has used a method known as femtoscopy to study the residual interaction between two-quark and three-quark particles. Through this measurement, an interaction between the ɸ (phi) meson (strange-antistrange quarks) and a proton (two up quarks and one down quark) has been observed for the first time.

Since the ɸ meson is not electrically charged, an interaction between the proton and the ɸ cannot be of electromagnetic origin and can only be attributed to the residual strong interaction. The strong interaction is what holds quarks together inside hadrons (such as the proton and the ɸ meson), while the residual strong interaction is the force that acts between hadrons. This is the interaction that holds protons and neutrons together in the form of atomic nuclei.

Unlike the residual strong interaction between protons and neutrons, which can be studied in stable bound states like the nuclei, the interaction between unstable hadrons produced in particle collisions is very difficult to observe. It was found to be possible in the LHC using an approach known as femtoscopy. Hadrons in the LHC collisions are produced very close to each other, at distances of about 10^-15 m (a unit known as a femtometer, hence the name femtoscopy). This scale matches the range of the residual strong force, giving the hadrons a brief chance to interact before flying away. As a result, pairs of hadrons that experience an attractive interaction will move slightly closer to each other, while, for a repulsive interaction, the opposite occurs. Both effects can be clearly observed through detailed analysis of the measured relative velocities of the particles.

The knowledge of the p-ɸ (proton-ɸ meson) interaction is of twofold interest in nuclear physics. First, this interaction is an anchor point for searches for the partial restoration of chiral symmetry. The left- and right-handed (chiral) symmetry that characterises the strong interaction is found to be broken in Nature, and this effect is responsible for the much larger mass of hadrons, such as the proton and the neutron, with respect to the masses of the quarks that make them up. Hence, chiral symmetry is linked to the origin of mass itself! A possible way of searching for the restoration of chiral symmetry and shedding light on the mechanism that generates mass is to study modifications of the properties of ɸ mesons within dense nuclear matter formed in collisions at the LHC. However, for this purpose, it is essential that the simple two-body p-ɸ interaction in vacuum is first understood.

The second point of interest is that, due to its strange-antistrange quark content, the ɸ meson is regarded as a possible vehicle of the interaction among baryons (hadrons consisting of three quarks) that contain one or more strange quarks, called hyperons (Y). Depending on the strength of this interaction, hyperons may form the core of neutron stars, which are among the densest and least understood astrophysical objects. Direct measurement of the Y-ɸ interaction strength, although feasible, has not yet been carried out, but already today this quantity can be estimated on the basis of the p-ɸ findings via fundamental symmetries. Therefore, measuring p-ɸ interaction provides indirect access to the Y-Y interaction in neutron stars.

The moderate interaction strength measured by ALICE provides a quantitative reference for further studies of the ɸ properties within the nuclear medium and also translates into a negligible interaction among hyperons in neutron stars. More accurate measurements will follow during the upcoming LHC Runs 3 and 4, allowing the precision of the extracted parameters to be significantly improved and also making it possible to pin down the Y-ɸ interaction directly.

In a recently published article in Physical Review Letters, the ALICE collaboration has used a method called femtoscopy to study the residual interaction between two-quark and three-quark particles. Through this measurement, an interaction between the ɸ (phi) meson (strange-antistrange quarks) and a proton (two up and one down quarks) was unveiled for the first time.

Since the ɸ meson is not electrically charged, an interaction between the proton and the ɸ cannot be of electromagnetic origin and can only be attributed to the residual strong interaction. The strong interaction is what holds together quarks inside hadrons (like the proton and the ɸ meson), while the residual strong interaction is the force that acts between hadrons. This is the interaction that holds protons and neutrons together in the form of atomic nuclei.

But unlike the residual strong interaction between protons and neutrons, that can be studied in stable bound states like the nuclei, the interaction between unstable hadrons produced in particle collisions is very difficult to observe. It was found to be possible in the LHC using an approach called femtoscopy. Hadrons in the LHC collisions are produced very close to each other, at distances of about 10-15 m (femtometre, hence the name femtoscopy). This scale matches the range of the residual strong force, giving them a brief chance to interact before flying away. As a result, pairs of hadrons that experience an attractive interaction will move slightly closer to each other, while for a repulsive interaction, the contrary occurs. Both effects can be clearly observed through detailed analysis of the measured relative velocities of the particles.

The knowledge of the p-ɸ (proton-ɸ meson) interaction is of twofold interest in nuclear physics. First, this interaction is an anchor point for searches of the partial restoration of chiral symmetry. The left- and right-handed (chiral) symmetry that characterizes the strong interaction is found to be broken in Nature and this effect is responsible for the much larger mass of hadrons, like the proton and neutron, with respect to the masses of the quarks that constitute them. Hence, chiral symmetry is connected to the origin of mass itself! A possible way to search for restoration of chiral symmetry and shed light on the mechanism that generates mass is by studying modifications of the properties of ɸ mesons within dense nuclear matter formed in collisions at the LHC. However, for this purpose, it is essential that the simple two body p-ɸ interaction in vacuum is understood first.

The second point of interest is that, due to its strange-antistrange quark content, the ɸ meson is regarded as a possible vehicle of the interaction among baryons (hadrons consisting of three quarks) that contain one or more strange quarks, called hyperons (Y). Depending on the strength of this interaction, hyperons may form the core of neutron stars, among the densest and least understood astrophysical objects. Direct measurement of the Y-ɸ interaction strength although feasible has not yet been carried out, but already today this quantity can be related to the p-ɸ findings via fundamental symmetries. Therefore, measuring p-ɸ  interaction provides indirect access to the Y-Y interaction in neutron stars.

The moderate interaction strength measured by ALICE provides a quantitative reference for further studies of the ɸ properties within the nuclear medium and also translates into a negligible interaction among hyperons in neutron stars. More accurate measurements will follow during the upcoming LHC Run 3 and Run 4 allowing to significantly improve the precision of the extracted parameters and also to pin down the Y-ɸ interaction directly.

On 18 and 19 October, ALICE held a workshop on ALICE 3, the next-generation heavy-ion experiment for Run 5 of the LHC and beyond. This new heavy-ion programme will address some of the physics questions that are not accessible with Runs 3 and 4. First discussed within the ALICE collaboration and at the heavy-ion town hall meeting in 2018, an expression of interest on the follow-up to ALICE was drawn up and submitted as input to the European Strategy for Particle Physics update. At the beginning of 2020, dedicated working groups were then set up in order to work out the physics case, the physics performance and a detector concept. The ALICE 3 workshop programme involved a mix of presentations on ALICE 3 physics performance studies and invited presentations outlining the landscape of theory and experiment over the next decade. The workshop took place in a hybrid format and was attended by more than 300 participants, both at CERN and via Zoom.

A major goal of ALICE 3 is to quantitatively understand the connection between heavy quark transport and hadronisation, e.g. by measuring beauty meson and baryon production and azimuthal asymmetries as well as azimuthal correlations between charm and anti-charm mesons and the production of multi-charm baryons. Another key area is the determination of the temperature and how particles flow in the quark–gluon plasma (QGP) in the early stage of the collision through measurements of real and virtual photon emission. Virtual photon emission is also sensitive to chiral symmetry restoration and, in particular, the mixing of the rho and a1 mesons at high temperatures. ALICE 3 would provide unique access to these topics and open up new possibilities in other areas too.

In order to achieve the required performance, a novel detector concept is being proposed, with an ultra-light tracker based on silicon pixel sensors, covering the pseudo-rapidity range of -4 to +4 and installed within a superconducting magnet system. A high-resolution vertex detector, retractably mounted inside the beam pipe, provides the ultimate pointing resolution. The tracking is complemented by particle identification over the full acceptance, realised with different technologies, including Si-based time-of-flight sensors. Further specialised detectors extend the physics reach in various areas.

The workshop was the first ALICE event with significant in-person participation since the start of the COVID-19 restrictions. It also marked the start of discussions of ALICE 3 with the community at large and of the review process with the LHC Experiments Committee (LHCC).

On 18 and 19 October, ALICE held a workshop on ALICE 3, the next-generation heavy-ion experiment for Run 5 of the LHC and beyond. This new heavy-ion programme will address some of the physics questions that are not accessible with Runs 3 and 4. First discussed within the ALICE collaboration and at the heavy-ion town hall meeting in 2018, an expression of interest on the follow-up to ALICE was drawn up and submitted as input to the European Strategy for Particle Physics update. At the beginning of 2020, dedicated working groups were then set up in order to work out the physics case, the physics performance and a detector concept. The ALICE 3 workshop programme involved a mix of presentations on ALICE 3 physics performance studies and invited presentations outlining the landscape of theory and experiment over the next decade. The workshop took place in a hybrid format and was attended by more than 300 participants, both at CERN and via Zoom.

A major goal of ALICE 3 is to quantitatively understand the connection between heavy quark transport and hadronisation, e.g. by measuring beauty meson and baryon production and azimuthal asymmetries as well as azimuthal correlations between charm and anti-charm mesons and the production of multi-charm baryons. Another key area is the determination of the temperature and how particles flow in the quark–gluon plasma (QGP) in the early stage of the collision through measurements of real and virtual photon emission. Virtual photon emission is also sensitive to chiral symmetry restoration and, in particular, the mixing of the rho and a1 mesons at high temperatures. ALICE 3 would provide unique access to these topics and open up new possibilities in other areas too.

In order to achieve the required performance, a novel detector concept is being proposed, with an ultra-light tracker based on silicon pixel sensors, covering the pseudo-rapidity range of -4 to +4 and installed within a superconducting magnet system. A high-resolution vertex detector, retractably mounted inside the beam pipe, provides the ultimate pointing resolution. The tracking is complemented by particle identification over the full acceptance, realised with different technologies, including Si-based time-of-flight sensors. Further specialised detectors extend the physics reach in various areas.

The workshop was the first ALICE event with significant in-person participation since the start of the COVID-19 restrictions. It also marked the start of discussions of ALICE 3 with the community at large and of the review process with the LHC Experiments Committee (LHCC).

It’s a triple treat. By sifting through data from particle collisions at the Large Hadron Collider (LHC), the CMS collaboration has seen not one, not two but three J/ψ particles emerging from a single collision between two protons. In addition to being a first for particle physics, the observation opens a new window into how quarks and gluons are distributed inside the proton.

The J/ψ particle is a special particle. It was the first particle containing a charm quark to be discovered, winning Burton Richter and Samuel Ting a Nobel prize in physics and helping to establish the quark model of composite particles called hadrons.

Experiments including ATLAS, CMS and LHCb at the LHC have previously seen one or two J/ψ particles coming out of a single particle collision, but never before have they seen the simultaneous production of three J/ψ particles – until the new CMS analysis.

The trick? Analysing the vast amount of high-energy proton–proton collisions collected by the CMS detector during the second run of the LHC, and looking for the transformation of the J/ψ particles into pairs of muons, the heavier cousins of the electrons.

From this analysis, the CMS team identified five instances of single proton–proton collision events in which three J/ψ particles were produced simultaneously. The result has a statistical significance of more than five standard deviations – the threshold used to claim the observation of a particle or process in particle physics.

These three-J/ψ events are very rare. To get an idea, one-J/ψ events and two-J/ψ events are about 3.7 million and 1800 times more common, respectively. “But they are well worth investigating,” says CMS physicist Stefanos Leontsinis, “A larger sample of three-J/ψ events, which the LHC should be able to collect in the future, should allow us to improve our understanding of the internal structure of protons at small scales.”

_____

Read more on the CMS website.

It’s a triple treat. By sifting through data from particle collisions at the Large Hadron Collider (LHC), the CMS collaboration has seen not one, not two but three J/ψ particles emerging from a single collision between two protons. In addition to being a first for particle physics, the observation opens a new window into how quarks and gluons are distributed inside the proton.

The J/ψ particle is a special particle. It was the first particle containing a charm quark to be discovered, winning Burton Richter and Samuel Ting a Nobel prize in physics and helping to establish the quark model of composite particles called hadrons.

Experiments including ATLAS, CMS and LHCb at the LHC have previously seen one or two J/ψ particles coming out of a single particle collision, but never before have they seen the simultaneous production of three J/ψ particles – until the new CMS analysis.

The trick? Analysing the vast amount of high-energy proton–proton collisions collected by the CMS detector during the second run of the LHC, and looking for the transformation of the J/ψ particles into pairs of muons, the heavier cousins of the electrons.

From this analysis, the CMS team identified five instances of single proton–proton collision events in which three J/ψ particles were produced simultaneously. The result has a statistical significance of more than five standard deviations – the threshold used to claim the observation of a particle or process in particle physics.

These three-J/ψ events are very rare. To get an idea, one-J/ψ events and two-J/ψ events are about 3.7 million and 1800 times more common, respectively. “But they are well worth investigating,” says CMS physicist Stefanos Leontsinis, “A larger sample of three-J/ψ events, which the LHC should be able to collect in the future, should allow us to improve our understanding of the internal structure of protons at small scales.”

_____

Read more on the CMS website.