Jobs – Page 100 of 527

The hitchhiker’s guide to weak decays

Most travellers know that it is essential to have a good travel guide when setting out into unexplored territory. A book where one can learn what previous travellers discovered about these surroundings, with both global information on the language, history and traditions of the land to be explored, and practical details on how to overcome day-to-day difficulties. Andrzej Buras’s recent book, Gauge Theory of Weak Decays, is the ideal guide for both new physicists and seasoned travellers, and experimentalists and theoreticians alike, who wish to start a new expedition into the fascinating world of weak meson decays, in pursuit of new physics.

The physics of weak decays is one of the most active and interesting frontiers in particle physics, from both the theoretical and the experimental points of view. Major steps in the construction of the Standard Model (SM) have been made possible only thanks to key observations in weak decays. The most famous example is probably the suppression of flavour-changing neutral currents in kaon decays, which led Glashow, Iliopoulos and Maiani to postulate, in 1970, the existence of the charm quark, well before its direct discovery. But there are many other examples, such as the heaviness of the top quark, inferred from the large matter-to-antimatter oscillation frequency of neutral B mesons, again well before its discovery. In the post-Higgs-discovery era, weak decays are a privileged observatory in which to search for signals of physics beyond the SM. The recent “B-physics anomalies”, reported by LHCb and other experiments, could indeed be the first hint of new physics. The strategic role that weak decays play in the search for new physics is further reinforced by the absence on the horizon, at least in the near future, of a collider with a centre-of-mass energy exceeding that of the LHC, while the LHC and other facilities still have a large margin of improvement in precision measurements.

As Buras describes with clarity, signals of new physics in the weak decays of K, D, and B mesons, and other rare low-energy processes, could manifest themselves as deviations from the precise predictions of the corresponding decay rates that we are able to derive within the SM. In the absence of a reference beyond-the-SM theory, it is not clear where, and at which level of precision, these deviations could show up. But general quantum field theory arguments suggest that weak decays are particularly sensitive probes of new physics, as they can often be predicted with high accuracy within the SM.

The two necessary ingredients for a journey in the realm of weak decays are therefore precise SM predictions on the one hand, and a broad-spectrum investigation of beyond-the-SM sensitivity on the other. These are precisely the two ingredients of Buras’s book. In the first part, he guides the reader though all the steps necessary to arrive to the most up-to-date predictions for rare decays. This part of the book offers different paths to different readers: students are guided, in a very pedagogical way, from tree-level calculations to high-precision multi-loop calculations. Experienced readers can directly find up-to-date phenomenological expressions that summarise the present knowledge on virtually any process of current experimental interest. This part of the book can also be viewed as a well-thought-out summary of the history of precise SM calculations for weak decays, written by one of its most relevant protagonists.

Beyond the Standard Model

The second part of the book is devoted to physics beyond the SM. Here the style is quite different: less pedagogical and more encyclopaedic. Employing a pragmatic approach, which is well motivated to discuss low-energy processes, extensions of the SM are classified according to properties of hypothetical new heavy particles, from Z′ bosons to leptoquarks, and from charged Higgs bosons to “vector-like” fermions. This allows Buras to analyse the impact of such models on rare processes in a systematic way, with great attention paid to correlations between observables.

To my knowledge, this book is the first comprehensive monograph of this type, covering far more than just the general aspects of SM physics, as may be found in many other texts on quantum field theory. The uniqueness of this book lies in its precious details on a wide variety of interesting rare processes. It is a key reference that was previously missing, and promises to be extremely useful in the coming decades.

LHCb observes four new tetraquarks

The LHCb collaboration has added four new exotic particles to the growing list of hadrons discovered so far at the LHC. In a paper posted to the arXiv preprint server yesterday the collaboration reports the observation of two tetraquarks with a new quark content (cc̄us̄): a narrow one, Z_cs(4000)⁺, and a broader one Z_cs(4220)⁺. Two other new tetraquarks, X(4685) and X(4630), with a quark content cc̄ss̄, were also observed. The results, which emerged thanks to adding the statistical power from LHC Run 2 to previous datasets, follow four tetraquarks discovered by the collaboration in 2016 and provide grist for the mill of theorists seeking to explain the nature of tetraquark binding mechanisms.

Dalitz plot showing eight tetraquarks — **Mountain ridges** A Dalitz plot of 24 thousand B⁺→J/ψφK⁺ decays observed by the LHCb collaboration. Vertical and horizontal bands reveal the presence of intermediate particles during the decay. Credit: LHCb collaboration

The new exotic states were observed in an almost pure sample of 24 thousand B⁺→J/ψφK⁺ decays, which, as a three-body decay, may be visualised using a Dalitz plot (see “Mountain ridges” figure). Horizontal and vertical bands indicate the temporary production of tetraquark resonances which subsequently decay to a J/ψ meson and a K⁺ meson or a J/ψ meson and a φ meson, respectively. The most prominent vertical bands correspond to the cc̄ss̄ tetraquarks X(4140), X(4274), X(4500) and X(4700) which were first observed in June 2016. The collaboration has now resolved two new horizontal bands corresponding to the cc̄us̄ states Z_cs(4000)⁺ and Z_cs(4220)⁺, and two additional vertical bands corresponding to the cc̄ss̄ states X(4685) and X(4630).

These states may have very different inner structures
Liming Zhang

The results have already triggered theoretical head scratching. In November, the BESIII collaboration at the Beijing Electron–Positron Collider II reported the discovery of the first candidate for a charged hidden-charm tetraquark with strangeness, tentatively dubbed Z_cs(3985)^– (CERN Courier January/February 2021 p12). It is unclear whether the new Z_cs(4000)⁺ tetraquark can be identified with this state, say physicists. Though their masses are consistent, the width of the BESIII particle is ten times smaller. “These states may have very different inner structures,” says lead analyst Liming Zhang of the LHCb collaboration. “The one seen by BESIII is a narrow and longer-lived particle, and is easier to understand with a nuclear-like hadronic molecular picture, where two hadrons interact via a residual strong force. The one we observed is much broader, which would make it more natural to interpret as a compact multiquark candidate.”

59 hadrons

The new observations take the tally of new hadronic states discovered at the LHC – which includes several pentaquarks as well as rare and excited mesons and baryons – to 59 (see “Diagram of discovery” figure). Though quantum chromodynamics naturally allows the existence of states beyond conventional two- and three-quark mesons and baryons, the detailed mechanisms responsible for binding multi-quark states are still largely mysterious. Tetraquarks, for example, could be tightly bound pairs of diquarks or loosely bound meson-meson molecules – or even both, depending on the production process.

Who would have guessed we’d find so many exotic hadrons?
Patrick Koppenburg

“Who would have guessed we’d find so many exotic hadrons?” says former LHCb physics coordinator Patrick Koppenburg, who put the plot together. “I hope that they bring us to a better modelling of the strong interaction, which is very much needed to understand, for instance, the anomalies we see in B-meson decays.”

Alexei Onuchin 1934–2021

Alexei Onuchin, one of the pioneers of experiments at colliding beams, passed away on 9 January in Novosibirsk, Russia.

Onuchin was born in 1934 in a small village in the Gorky (Nizhny Novgorod) region. After graduating from high school with honours, he decided to try his hand at science and in 1953 he entered the physics department of Moscow State University. In 1959 he graduated with honours and was invited by Gersh Budker to work at the newly organised Institute of Nuclear Physics at Novosibirsk (INP).

At INP, Onuchin enjoyed many important roles. He took part in experiments at the world’s first electron–electron collider (VEP-1), actively worked on the preparation of a detector for the electron–positron collider VEPP-2, supervised the construction of the MD-1 detector for the VEPP-4 collider, was one of the leaders of the KEDR detector experiment at the VEPP-4M collider and was a great enthusiast of the detector project for the proposed Super Charm-Tau Factory. He was also an organiser and for many years the leader of the Budker INP group working at the BaBar experiment at SLAC.

During his career, Alexei made a great contribution to the development of experimental techniques in particle physics. It was this that determined the high level of experiments carried out at Budker INP and other laboratories. These include the development and production of multiwire proportional chambers for the MD-1, various counters based on Cherenkov radiation and the creation of a calorimeter based on liquid krypton, among many others.

Cherenkov counters held a special place in Alexei’s heart from the very beginning of his career as a student in the laboratory of Nobel-laureate Pavel Cherenkov. Starting from pioneering water-threshold counters in the experiment at VEPP–2, he later developed the MD–1 Cherenkov counters filled with ethylene pressurised to 25 bar, and finally suggested the aerogel counters with wavelength shifters (ASHIPH) now operating in the KEDR detector. For this work, in 2008 Alexei Onuchin was awarded the Cherenkov Prize of the Russian Academy of Sciences.

Alexei was a great teacher. Among his former students are professors, group leaders and members of the Russian Academy of Sciences. He was also a caring father and loving husband, who raised a large family with four children, five grandchildren and three great-grandchildren. He will always be remembered by his family, friends and colleagues.

Lifting the veil on supernova 1987A

The dusty core of SN1987A — **Missing link** A hot “blob” (inset) revealed by X-rays in the dusty core of SN1987A could be the location of the missing neutron star formed by the collapse of its progenitor star. The red area shows dust and cold gas observed at radio wavelengths by ALMA; the green and blue hues, recorded by Hubble and Chandra, show the collision between the expanding shock wave and material around the supernova. Credit: ALMA; NRAO/AUI/NSF; NASA/ESA

On 23 February 1987 astronomers around the world saw an extremely bright supernova, now called SN1987A. It was the closest supernova observed for over 300 years and was visible to the naked eye. The event was quickly confirmed to be the result of the collapse of “Sanduleak –69 202”, a blue supergiant star in the Large Magellanic Cloud. As the first nearby supernova in the era of modern astronomy, SN1987A remains one of the most monitored objects in the sky. Apart from confirming several important theories, such as radioactive decay being the source of the observed optical emission, the supernova also raised a number of questions that remain unanswered. The most important is: where is the remnant of the progenitor star?

Despite several false detection claims in the past, evidence is mounting that Sanduleak –69 202 collapsed into a neutron star

Despite several false detection claims in the past, evidence is mounting that Sanduleak –69 202 collapsed into a neutron star that is becoming more visible as the dust around it starts to settle. A new analysis by researchers in Italy and Japan based on high-energy X-ray data from the Chandra and NuSTAR space telescopes adds the latest support to this idea.

Even before the optical light from SN1987A was detected, several neutrino detectors around the world saw a burst of neutrinos. The brightest one was observed by Japan’s Kamiokande II detector, which detected a total of 12 antineutrinos approximately three hours before the first optical light reached Earth. The detection of antineutrinos seemed to confirm theoretical predictions for a star the size of Sanduleak –69 202: namely that it should collapse into a neutron star, and emit large numbers of neutrinos while doing so. The optical light arrives later because it is only produced when the shock waves from the collapse reach the surface of the star.

Since the newly formed neutron star would be expected to emit large amounts of energy at various wavelengths, one might assume it would be relatively easy to detect. However, no signs were found in follow-up searches over the past three decades, leading to much speculation about the fate of this star and its surrounding medium.

The first signs of the stellar remnants of SN1987A came from radio observations by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile in 2019. A group led by Phil Cigan from Cardiff University in the UK used ALMA data at various frequencies to study the core of SN1987A. Close to the centre, they found a bright “blob” structure, the emission from which appeared to be compatible with radio emission from particles accelerated by a neutron star, also called a pulsar wind nebula. Although the researchers could not exclude local heating from ⁴⁴Ti produced during the supernova as the source, the results provided the first hint that the blob houses a young neutron star.

Wind power

Inspired by the ALMA results, Emanuele Greco from the University of Palermo and coworkers started to study the same region using X-ray data from Chandra and NuSTAR taken during 2012, 2013 and 2014. They found that the detected soft X-ray emission (0.5–8 keV) was compatible with thermal emission produced in the remnant shock waves of the supernova event with the circumstellar medium. However, at higher energies (10–20 keV) the emission was clearly non-thermal in nature. Describing their findings in a preprint posted in January, the group studied the two possible sources for such emission: synchrotron emission from a pulsar wind nebula and synchrotron emission produced in shock waves in the region. Whereas models for both ideas fit the spectral data, the pulsar wind nebula is favoured because the shock emission would not be expected to look like this for such a young remnant.

It appears that after 34 years of searching we will finally understand what happened in SN1987A

The reason why this neutron star has escaped previous observations in optical or soft X-ray energies is likely absorption by cold dust emitted during the supernova, which appears to still absorb a large part of the synchrotron emission observed in X-rays, especially at lower energies. But the dust is expected to start to heat up during the coming decades, thereby becoming transparent to lower energy emission. Greco and colleagues predict that, if the emission is indeed induced by a neutron star, it will become visible in the soft X-ray regime by 2030 with Chandra.

Although astronomers have just two observational hints that Sanduleak –69 202 did, as it should according to theory, collapse into a neutron star, it appears that after 34 years of searching we will finally understand what happened in SN1987A.

Deep learning tailors supersymmetry searches

CMS charginos neutralinos — **Fig. 1.** Observed and expected exclusion limits for a model of electroweak supersymmetry with slepton-mediated chargino and neutralino decays. Results using neural networks (NN) are compared with the expected limits based on the search-region (SR) approach, and the previous CMS analysis (green). Credit: CMS

Supersymmetry is a popular extension of the Standard Model (SM) that has the potential to resolve several open questions in particle physics. As a result of a postulated new symmetry between fermions and bosons, the theory predicts a “superpartner” for each SM particle. The lightest of these new particles could be what makes up dark matter, while additional new superpartners could resolve the question of why the Higgs boson has a relatively low mass. Many searches for supersymmetry have already been performed by the ATLAS and CMS collaborations, but most have focused on strongly interacting superpartners that could be very heavy. It is possible, however, that electroweak production of supersymmetric particles is the dominant or only source of superpartners accessible at the LHC.

Supersymmetric events are expected to have an imbalance in transverse momentum

The unprecedented data volume of LHC Run 2 provides a unique opportunity to search for rare processes such as electroweak production of supersymmetric particles. A recent result from the CMS collaboration uses the Run-2 dataset to search for the superpartners of the electroweak bosons, called charginos and neutralinos. Events with three or more charged leptons, or two leptons of the same charge, were analysed. Such events are relatively rare in the SM, and, if they exist, charginos and neutralinos are predicted to create an excess of events with these topologies. Supersymmetric events are also expected to have an apparent imbalance in transverse momentum, because the lightest supersymmetric particle should evade detection. Correlations between the multiple leptons in the events, and between the leptons and the momentum imbalance, can be used to define a set of discriminating variables sensitive to chargino and neutralino production. These variables are used to assign the selected events into several search regions that address different possible signals of the production and decay of supersymmetric particles. Making such a multivariate binning optimal in every corner of phase-space, and for any possible manifestation of supersymmetry, is a challenging task.

Parametric machine learning

Events with three electrons and/or muons provide the bulk of the sensitivity by striking the best balance between signal purity and yields. A novel search approach is used that aims at better capturing the complexity of the events than is possible using predetermined search regions: parametric machine learning. The aim is to achieve the maximum sensitivity for any parameter choice nature might have made, as supersymmetry is not one model, but a class of models. Variations in the masses of the superpartners can substantially modify the observable signatures. Parametric neural networks were trained to find charginos and neutralinos with the unknown mass parameters added as input variables to the training. The network can evaluate the data at fixed values of the mass parameters, effectively performing a dedicated search for a signal with given masses in the data (figure 1).

The parametric neural network, together with a new optimised event binning of the other event categories, makes this analysis the most powerful search for charginos and neutralinos carried out by the CMS collaboration so far. The neural network alone results in a sensitivity boost that ranges from 30% to more than 100%. Substantial improvements occur for models where the decay of the charginos and neutralinos are mediated by the superpartners of leptons. The improvements become even larger when the mass splitting between sleptons and the chargino is relatively small. The data show no evidence for electroweak superpartner production, and chargino masses up to 1450 GeV, compared to 1150 GeV in earlier CMS searches for this scenario, are excluded at 95% confidence.

Farewell Daya Bay, hello JUNO

In October 2007, neutrino physicists broke ground 55 km north-east of Hong Kong to build the Daya Bay Reactor Neutrino Experiment. Comprising eight 20-tonne liquid-scintillator detectors sited within 2 km of the Daya Bay nuclear plant, its aim was to look for the disappearance of electron antineutrinos as a function of distance to the reactor. This would constitute evidence for mixing between the electron and the third neutrino mass eigenstate, as described by the parameter θ₁₃. Back then, θ₁₃ was the least well known angle in the Pontecorvo–Maki–Nakagawa–Sakata matrix, which quantifies lepton mixing, with only an upper limit available. Today, it is the best known angle by some margin, and the knowledge that it is nonzero has opened the door to measuring leptonic CP violation at long-baseline accelerator-neutrino experiments.

Daya Bay was one of a trio of experiments located in close proximity to nuclear reactors, along with RENO in South Korea and Double Chooz in France, which were responsible for this seminal measurement. Double Chooz published the first hint that θ₁₃ was nonzero in 2011, before Daya Bay and RENO established this conclusively the following spring. The experiments also failed to dispel the reactor–antineutrino anomaly, whereby observed neutrino fluxes are a few percent lower than calculations predict. This has triggered a slew of new experiments located mere metres from nuclear-reactor cores, in search of evidence for oscillations involving additional, sterile light neutrinos. As the Daya Bay experiment’s detectors are dismantled, after almost a decade of data taking, the three collaborations can reflect on the rare privilege of having pencilled the value of a previously unknown parameter into the Standard- Model Lagrangian.

Particle physics is fundamental and influential, and deserves to be supported
Yi-Fang Wang

Founding Daya Bay co-spokesperson Yi-Fang Wang says the experiment has had a transformative effect on Chinese particle physics, emboldening the country to explore major projects such as a circular electron–positron collider. “One important lesson we learnt from Daya Bay is that we should just go ahead and do it if it is a good project, rather than waiting until everything is ready. We convinced our government that we could do a great job, that world-class jobs need to be international, and that particle physics is fundamental and influential, and deserves to be supported.”

JUNO

The experiment has also paved the way for China to build a successor, the Jiangmen Underground Neutrino Observatory (JUNO), for which Wang is now spokesperson. JUNO will tackle the neutrino mass hierarchy – the question of whether the third neutrino mass eigenstate is the most or least massive of the three. An evolution of Daya Bay, the new experiment will also measure a deficit of electron antineutrinos, but at a distance of 53 km, seeking to resolve fast and shallow oscillations that are expected to differ depending on the neutrino mass hierarchy. Excavation of a cavern for the 20 kilotonne liquid-scintillator detector 700 m beneath the Dashi hill in Guangdong was completed at the end of 2020. The construction of a concrete water pool is the next step.

The next steps in reactor-neutrino physics will involve an extraordinary miniaturisation
Thierry Lasserre

The detector concept that the three experiments used to uncover θ₁₃ was designed by the Double Chooz collaboration. Thierry Lasserre, one of the experiment’s two founders, recalls that it was difficult, 20 years ago, to convince the community that the measurement was possible at reactors. “It should not be forgotten that significant experimental efforts were also undertaken in Angra dos Reis, Braidwood, Diablo Canyon, Krasnoyarsk and Kashiwazaki,” he says. “Reactor neutrino detectors can now be used safely, routinely and remotely, and some of them can even be deployed on the surface, which will be a great advantage for non-proliferation applications.” The next steps in reactor-neutrino physics, he explains, will now involve an extraordinary miniaturisation to cryogenic detectors as small as 10 grams, which take advantage of the much larger cross section of coherent neutrino scattering.

A day with particles

Outreach must continue, even in a pandemic: if visitors can’t come to the lab, we need to find ways to bring the lab to them. Few outreach initiatives do this as charmingly as “A day with particles” — a short independent film by three ATLAS physicists at Charles University in Prague. Mixing hand-drawn animations, deft sound design and a brisk script targeted at viewers with no knowledge of physics, the 30 minute film follows a day in the life of postdoc Vojtech Pleskot. In its latest pitstop in a worldwide tour of indie film festivals, it won “BEST of FEST (Top Geek)” last week at GeekFestToronto.

We want to break stereotypes about scientists
Vojtech Pleskot

“We just want to show that scientists are absolutely normal people, and that no one needs to fear them,” says Pleskot, who wrote and directed the film alongside producer Martin Rybar and animator Daniel Scheirich. Modest and self-effacing when I interviewed them, the three physicists produced the film with no funding and no prior expertise, beating off competition from well funded projects to win the Canadian award. Even within the vibrant but specialist niche of high-energy-physics geekery, competition included “The world of thinking”, featuring interviews with Ed Witten, Freeman Dyson and others, and a professionally produced film dramatising a love letter from Richard Feynman to his late wife Arline, who passed away while he was working on the Manhattan Project. But Pleskot, Rybar and Scheirich won the judges over with their idiosyncratic distillation of life at the rock-face of discovery. The trio place their film in the context of growing scepticism of science and scientists. “We want to break stereotypes about scientists,” adds Pleskot.

Not every stereotype is broken, and there is room to quibble about some of the details, but grassroots projects such as A Day With Particles boast a quirky authenticity which is difficult to capture through institutional planning, and is well placed to connect emotionally with non-physicists. The film is beautifully paced. Wide-eyed enthusiasm for physics cuts to an adorable glimpse of Pleskot’s two “cute little particles” having breakfast. A rapid hop from Democritus to Rutherford to the LHC cuts to tracking shots of Pleskot making his way through the streets of Prague to the university. The realities of phone conferences, failed grid jobs and being late for lab demonstrations are interwoven with a grad student dancing to discuss her analysis, conversations on free-diving with turtles and the stories of beloved professors recalling life in the communist era. Life as a physicist is good. And life as a physicist is really like this, they insist. “I hope that science communicators will share it far and wide,” says Connie Potter (CERN and ATLAS), who commissioned the film for the 2020 edition of ICHEP, and who was also recognised by the Toronto festival for her indefatigable “indie spirit” in promoting it.

A Day With Particles will next be considered at the World of Film International Festival in Glasgow in June, where it has been selected as a semi-finalist.

CMS targets Higgs-boson pair production

**Fig. 1.** The weighted diphoton invariant mass distribution for the selected γγbb events. The red line includes the observed HH signal contribution, while the blue line represents the contribution from background processes only. Credit: CERN

The Higgs boson discovered in 2012 by the ATLAS and CMS experiments is the pinnacle of the scientific results so far at the LHC. Measurements of its couplings to W and Z bosons and to heavy fermions have provided a strong indication that the mechanism of electroweak symmetry breaking is similar to that proposed by Brout, Englert and Higgs (BEH) more than 50 years ago. In this model, the BEH field exists throughout space with a non-zero field strength corresponding to the minimum of the BEH potential. The measurement of the shape of the BEH potential has become one of the main goals of experimental particle physics. It governs not only the nature of the electroweak phase transition in the early universe, when the BEH field gained its non-zero “vacuum expectation value” (VEV), but also the question of whether deeper minima than the present vacuum exist.

The measurement of the production of Higgs-boson pairs gives a direct way to measure λ

Interactions with the BEH VEV give mass not only to the W and Z bosons and the fermions, but also to the Higgs boson itself. If the mass of the Higgs boson is well known, the Standard Model (SM) can therefore predict the Higgs self-coupling, λ – the key unknown parameter in the shape of the BEH potential of the SM. The measurement of the production of Higgs-boson pairs (HH) gives a direct way to measure λ. Higgs-boson pair production is not yet established experimentally, as it is a thousand times less frequent than the production of a single Higgs boson. However, the presence of physics beyond the SM can substantially enhance the HH production rate. The search for HH production at the LHC is therefore an important test of the SM.

Best constraint

A recent result by the CMS collaboration describes a search for HH production in final states with two photons and two b-jets (figure 1). The large data sample collected during LHC Run 2 excludes a HH production rate larger than 7.7 times that predicted by the SM. CMS has set the best constraint to date on the ratio of the measured λ parameter to the SM prediction, κ_λ = 0.6^+6.3_–1.8.

The sensitivity of the analysis has been improved by about a factor four over the previous result that used the data collected in 2016, benefitting equally from the increase in luminosity and from a wealth of innovative analysis techniques. The electromagnetic calorimeter of the CMS experiment allows the measurement of H → γγ candidates with excellent resolution (about 1–2%). Advanced machine-learning techniques, including deep neural networks, were introduced to significantly improve the mass resolution of H → bb, from 15% down to 11%. The analysis combines information from the invariant mass of the HH system, reflecting the underlying physics processes, and a multivariate classifier exploring the kinematic properties as well as the identification of photons and b-jets.

Events were categorised to enhance the sensitivity to Higgs production via gluon fusion as well as, for the first time, vector-boson fusion. The latter constrains the quartic coupling between two vector bosons and two Higgs bosons, such as WWHH, which is an extremely rare interaction in the SM. In addition, dedicated categories from a previous analysis were added to account for the associated production of top quarks and a single Higgs boson, and to provide a simultaneous constraint on the top-quark Yukawa coupling and λ. Several hypotheses predicting new physics were also constrained. The results are an encouraging step forwards in the quest to measure the BEH potential and to further interrogate the SM.

Higgs boson gets SMEFT treatment

**Fig. 1.** Allowed ranges for the coupling coefficients of new EFT interactions in a so- called rotated Warsaw basis, including only contributions from the interference between SM and new-physics diagrams (purple), or also including pure new-physics terms that appear at higher order (orange). The SM prediction for all these coefficients is zero. Credit: CERN

The growing LHC dataset eight years after the discovery of the Higgs boson allows the experiments to study its properties more and more precisely, searching for hints of physics beyond the Standard Model (SM). New phenomena might occur at energy scales beyond the reach of the LHC, pointing to the existence of so-far undiscovered particles with masses too heavy to be directly produced in 13 TeV proton–proton collisions. Without knowing the exact nature of the new physics, LHC data can be analysed to systematically constrain new types of interactions in the framework of an effective field theory (EFT). One historical EFT example is Fermi’s effective interaction model for nuclear beta decay, which is valid as long as the probed energy scale is well below the mass of the W boson. The move to constrain EFTs rather than signal strengths for couplings marks a new, more comprehensive phase in SM tests at the LHC.

The move to constrain EFTs marks a new, more comprehensive phase in SM tests at the LHC

Almost all types of new physics would give rise to new interactions with SM particles, with different models leaving different EFT footprints. As the underlying dynamics is not known and effects can be subtle, it is important to combine as many measurements as possible across the full spectrum of the LHC research programme.

A new ATLAS analysis presented at the Higgs 2020 conference, held online from 26 to 30 October, takes a first step in this direction. The analysis combines measurements of production cross-sections and kinematic variables of Higgs-boson events in several decay channels (diphoton, four-lepton and di-b-quark decays) to constrain new phenomena within the so-called SMEFT framework. The combination of measurements allows multiple new interactions involving the Higgs boson to be constrained simultaneously. This approach requires fewer hypotheses on the other unconstrained interactions than studying the EFT terms one measurement at a time. The results are therefore more generic and easier to interpret in a broader context.

Predicted to vanish

Figure 1 shows the allowed ranges for the coupling coefficients of new EFT interactions to which the ATLAS combined Higgs analysis is sensitive. The coefficient c⁽³⁾_Hq, for example, describes the strength of an effective four-particle interaction between two quarks, a gauge boson and the Higgs boson. The SM predicts all these coefficients to vanish, as their corresponding interactions are not present. Significant positive or negative deviations would indicate new physics. For instance, a non-vanishing value of c⁽³⁾_Hq would cause deviations from the SM in the ZH and WH cross-sections at high transverse momentum of the Higgs boson, which are not observed in the measured channels.

All measurements are compatible with the SM, indicating that if new physics is present it either has a mass scale larger than 1 TeV (the reference scale for which these results are reported) – or it manifests itself in interactions to which the available measurements are not yet sensitive. In the meantime, thanks to the design of the analysis, the results can be added to wider EFT interpretations that combine measurements from different physics processes (e.g. electroweak- boson or top-quark production) studied by ATLAS and other experiments, providing a consistent and increasingly detailed mapping of the allowed new physics extensions of the SM.

Quark-matter fireballs hashed out in Protvino

The XXXII international workshop of the Logunov Institute for High-Energy Physics of the NRC Kurchatov Institute in Protvino, near Moscow, brought more than 300 physicists together online from 9 to 13 November to discuss “hot problems in hot and cold quark matter”. The focus of the workshop was chiral theories and lattice simulations, which allow estimates beyond perturbation theory for studying the strongly coupled quark–gluon plasma (sQGP) – the hot and/or dense plasma of quarks and gluons that is created in heavy-ion collisions, and which may exist inside neutron stars.

Participants considered the QCD phase diagram (pictured) as a function of temperature, magnetic field (B), baryon and isospin chemical potentials (μ_B and μ_I), and varying quark masses. The crossover line (yellow strip), which marks a transition between hadronic matter and sQGP, has long attracted great interest. Vladimir Skokov (Brookhaven) employed recent progress in the Lee–Yang approach to phase transitions to derive from first principles that μ_B > 400 MeV at the critical end point (a possible termination of the first-order phase-transition boundary). Discussions of the phase diagram also included a decrease in the pseudocritical temperature with B, the possibility of a first-order phase transition at μ_B = 0 as B tends to infinity, the existence and location of a superconducting phase, the disagreement between measured and predicted collective flows of direct photons in heavy-ion collisions, and the diamagnetic and paramagnetic natures of the pion gas and deconfined matter, respectively. Evgeny Zabrodin (Oslo) explained that the rotating fireballs of strongly interacting matter that are produced in heavy-ion collisions are not only superfluids but also supervortical liquids.

Gravitational-wave astrophysics

Impressive work was also shared at the intersection of heavy-ion collisions and gravitational-wave astrophysics on the subject of the equation of state (EoS) of neutron-star cores. The EoS is the relationship between pressure and density, and can indicate whether hadronic or quark matter is inside. Theoretical bounds on the EoS come from chiral effective theories, perturbative QCD, and the bound on the speed of sound c_s < 1/3. The quantities that can be extracted from experimental data are the mass–radius relation and the relationship between the tidal deformabilities of merging neutron stars and the peak frequency of the emitted gravitational waves. Several speakers observed that tidal deformabilities, which are measured in the inspiral phase, and the peak gravitational- wave frequency, which is measured in the post-merger phase, may together reveal the state of a neutron-star interior. Mergers observed since 2017 may already be able to shed light on the existence of a deconfined phase inside these ultra-compact objects.

Mariana Araújo offered a solution to the longstanding quarkonium polarisation puzzle

The Protvino workshop also revealed the enduring importance of studying heavy-quark physics. Since heavy quarks can be considered as approximately statically coloured sources, studies of quarkonia production are a step towards understanding hadron formation and the confinement mechanism. Peter Petreczky (Brookhaven) concluded from a lattice study of Bethe–Salpeter amplitudes that the potential model fails to describe bottomonium in terms of screened potential at high temperatures, with further investigations clearly needed in this field. Carlos Lourenço (CERN) showed that the lowering of quarkonia binding energies in the sQGP leads to nontrivial measured suppression patterns. Eric Braaten (Ohio) showed that the decrease with multiplicity of the ratio of the prompt production rates of X(3872) and Ψ(2S) in proton–proton collisions can be explained by the scattering of co-moving pions off X(3872) if it is a weakly bound charm-meson molecule. With equally impressively scrupulousness, Mariana Araújo (Innsbruck) offered a solution to the longstanding “quarkonium polarisation puzzle” by making use of a model-independent fitting procedure and taking into account correlations between cross sections and polarisations.

The next “hot problems” workshop will be held in November.

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