One of the most interesting discoveries of the past decade is that of an unconventional hadron, the X(3872), by the Belle experiment (Belle 2003). Its decay to J/ψπ+π– indicates that it is charmonium-like but its narrow width and mass above the threshold for decay to open charm do not fit any of the spectrum of predicted cc states. Several experiments have since confirmed this observation, in different production mechanisms and decay modes. In parallel to these experimental investigations, many theoretical interpretations have been put forward but the fundamental question remains open of whether the X(3872) is a quark–antiquark meson or a more exotic state.
When any new resonance is observed it is mandatory to determine its quantum numbers. The observation of the decay X(3872)→J/ψγ fixed the charge conjugation: C = +1. However, angular analyses left two possibilities for JPC: 1++ and 2–+ (CDF 2007). Exotic models where the X(3872) is a DD* molecule or a tetraquark state predict JPC = 1++.
The LHCb collaboration has now reported an analysis of the decay chain B+ → X(3872)K+ → J/ψπ+π–K+, with J/ψ → μ+μ–, where they use all five angular variables to maximize the separation power between the hypotheses of 1++ and 2–+. The analysis uses the data sample of 1.0 fb–1 that LHCb collected during 2011, which contains 313 ± 26 B+ → X(3872)K+ decays. As figure 1 shows, the outcome of the multidimensional likelihood fit prefers JPC = 1++ with more than 8σ significance. Compared with previous analyses, the measurement benefits from larger statistics but importantly also makes use of the full angular information, which improves the ability to use correlations between angular variables to separate the two hypotheses (figure 2 shows an example).
This result rules out explanations of the X(3872) as the ηc2(11D2) state. Instead, it favours more exotic interpretations. However, distinguishing between molecular and tetraquark models will require studies of complementary decay modes. The 2.0 fb–1 data sample that LHCb accumulated during 2012, as well as the larger samples that will be recorded in future LHC runs, will allow the collaboration to keep on the trail of these and other puzzles in heavy-flavour spectroscopy.
The historic academic building of Utrecht University provided the setting for the 5th International Workshop on Heavy Quark Production in Heavy-Ion Collisions, offering a unique atmosphere for a lively discussion and interpretation of the current measurements on open and hidden heavy flavour in high-energy heavy-ion collisions. Held on 14–17 November, the workshop attracted some 70 researchers from around the world, a third of the participants being theorists and more than 20% female researchers. The topics for discussion covered recent results, upgrades and future experiments at CERN’s LHC, Brookhaven’s Relativistic Heavy-Ion Collider (RHIC) and the Facility for Antiproton and Ion Research (FAIR) at Darmstadt, as well as theoretical developments. There was a particular focus on the exchange of information and ideas between the experiments on open heavy-flavour reconstruction.
Open and hidden heavy flavour
Representatives from all of the major collaborations nicely summarized recent experimental results and prospects for future measurements. In particular, with the advent of the LHC, an unprecedented wealth of data on the production of heavy quarks and quarkonium in nuclear collisions has become available. One of the more spectacular effects observed at RHIC is the quenching of the transverse momentum (pT) spectra of light hadrons, related to the energy loss of quarks inside the hot quark–gluon plasma (QGP) phase produced in lead–lead (PbPb) collisions. This has now been studied in detail for the first time by the ALICE, ATLAS and CMS collaborations in the heavy-quark sector.
Among the highlights presented at the workshop, the ALICE collaboration reported a strong suppression (up to a factor around 5) of the production of D mesons in PbPb collisions at a centre-of-mass energy, √sNN, of 2.76 TeV, compared with proton–proton data at the same energy. The CMS experiment has also found a sizeable suppression of the yield of J/ψs coming from the decay of B hadrons. When this effect is compared with the one measured by the same experiments for light hadrons, interesting hints of a hierarchy of suppression are seen, with the beauty hadrons being less suppressed than the charmed hadrons and the latter less suppressed than light hadrons. Such an observation may be connected to the so called dead-cone effect, a reduction of small-angle gluon radiation for heavy compared with light quarks, predicted by QCD and related to the energy density reached in the medium.
In the quarkonium sector, the ALICE and CMS collaborations showed new and intriguing results on J/ψ and Υ production, respectively. A suppression of charmonium states had been previously observed at CERN’s Super Proton Synchrotron (SPS) and at RHIC and was explained as an effect of the screening of the binding colour force in a QGP. With data from the LHC, accurate results on the bottomonium states have proved for the first time – beyond any doubt – that the less-strongly bound Υ(2S) and Υ(3S) are up to five times more strongly suppressed in a QGP with respect to the tightly bound Υ(1S) state, an observation that is expected in a colour-screening scenario. On the contrary, the ALICE collaboration sees a smaller suppression-effect for the J/ψ with respect to RHIC and the SPS, despite the larger energy density reached in nuclear collisions at the LHC. An interesting hypothesis relates this observation to a recombination of cc pairs, which are produced with high multiplicity in each PbPb collision, in the later stages when the system cools down and crosses the transition temperature between the QGP and the ordinary hadronic world.
Theoretical developments
The talks on theory provided quite a comprehensive overview of the vigorous research efforts towards a theoretical understanding of heavy-quark probes in heavy-ion collisions. The experimental findings on open heavy-flavour suppression and elliptic flow have led to many theoretical investigations of heavy-quark diffusion in the strongly coupled QGP. Most models use a relativistic Fokker-Planck-Langevin approach, with drag and diffusion coefficients taken from various microscopic models for the heavy-quark interactions with the hot and dense medium. The microscopic models include estimates from perturbative QCD for elastic- and/or radiative-scattering processes, T-matrix calculations using in-medium lattice potentials (from both the free and the internal thermodynamic potentials) and collision terms in full transport simulations, including 2 ↔ 2 and 2 ↔ 3 processes in perturbative QCD.
First studies of the influence of the hadronic phase on the modifications of the open-heavy-flavour medium were presented at the workshop. Estimates of the viscosity to entropy-density ratio, η/s, from the corresponding partonic and hadronic heavy-quark transport coefficients, lead to values that are not too far from the conjectured anti-de Sitter/conformal field theory lower bound of 1/4π in the phase-transition region, showing the characteristic minimum around the critical temperature, Tc. Results from a direct calculation of the heavy-quark transport coefficients via the maximum-entropy method applied to lattice-QCD correlation functions were also reported.
In the field of heavy quarkonia, the notion of a possible regeneration of heavy quarkonia via qq recombination in the medium in addition to the dissociation/melting processes leading to their suppression in the QGP has in recent years led to detailed studies on the bound-state properties of heavy quarkonia in the hot medium. Here, the models range from the evaluation of static qq potentials in hard-thermal-loop resummed thermal-QCD to a generalization of systematic nonrelativistic QCD and heavy-quark effective theory studies, generalizing from the vacuum to thermal field theory.
These theoretical studies have already led to major progress in understanding the possible microscopic mechanisms behind the coupling of heavy-quark degrees of freedom with the hot and dense medium created in heavy-ion collisions. In future, it might be possible to gain an even better quantitative understanding of fundamental quantities such as the transport coefficients of the QGP (for example η/s) and the dissociation temperatures of heavy quarkonia, which could provide a thermometer for the QGP formed in heavy-ion collisions. Whatever happens, the workshop has provided an excellent framework to discuss this exciting theoretical work and trigger some fruitful ideas for its future development.
The observed signals for the QGP are expected to be even stronger in PbPb collisions at √sNN = 5.1 TeV (foreseen in 2015) and allow the properties of the QGP to be characterized further. Proton–lead data are urgently needed to measure the contribution from the effects in cold nuclear matter, such as nuclear shadowing and Cronin enhancement. The experimental teams at the LHC and at RHIC are working on upgrades of the inner tracking systems of their detectors, aiming for an improved resolution in impact parameter, which will make the measurement of open beauty in heavy-ion collisions feasible in the near future.
• The organizers would like to thank the Lawrence Berkeley National Laboratory and the Foundation for Fundamental Research on Matter (FOM) for financial support.
SPIN 2012, the 20th International Symposium on Spin Physics, took place at the Joint Institute for Nuclear Research (JINR) in Dubna on 17–22 September. Around 300 participants attended from JINR and institutes in 22 countries (mainly Germany, Italy, Japan, Russia and the US). It consisted of a traditional mix of plenary and parallel sessions. Presentations covered the spin structure of hadrons, spin effects in reactions with lepton and hadron beams, spin physics beyond the Standard Model and future experiments, as well as the techniques of polarized beams and targets, and the application of spin phenomena in medicine and technology.
The symposium began with a focus on work at Dubna, starting with the unveiling of a monument to Vladimir Veksler, who invented the principle of phase stability (independently from Edwin McMillan in the US) and founded the 10 GeV Synchro-phasotron in Dubna in 1955. Talks followed about the future projects to be carried out at JINR’s newest facility, the Nuclotron-based Ion Collider fAcility (NICA). The complex will include an upgraded superconducting synchrotron, Nuclotron-M, with an area for fixed-target experiments, as well as a collider with two intersections for polarized protons (at 12 GeV per beam) or deuterons and nuclei (5 A GeV per beam). It will provide opportunities for a range of polarization studies to complement global data and will particularly help to solve the puzzles of spin effects that have been awaiting solutions since the 1970s. The spin community at the symposium supported the plans for these unique capabilities, and JINR’s director, Victor Matveev, announced that the project is ready to invite international nominations for leading positions in the spin programme at NICA.
The experimental landscape
In the US, Jefferson Lab’s programme of experiments on generalized parton distributions (GPDs) will be implemented with upgraded detectors and an increase in the energy of the Continuous Electron Beam Accelerator Facility from 6 GeV up to 12 GeV. The laboratory is also considering the construction of a new synchrotron to accelerate protons and nuclei up to 250 GeV before collision with 12 GeV electrons. In a similar way, a new 10–30 GeV electron accelerator is being proposed at Brookhaven National Laboratory to provide collisions between electrons and polarized protons and ions, including polarized 3He nuclei, at the Relativistic Heavy-Ion Collider (RHIC). The aim will be to investigate the spin structure of the proton and the neutron.
At CERN, the COMPASS-II project has been approved, firstly to study Drell-Yan muon-pair production in collisions of pions with polarized nucleons, to investigate the nucleon’s parton distribution functions (PDFs). A second aim is to study GPDs via the deeply virtual Compton-scattering processes of exclusive photon and meson production. The latter processes will provide the possibility for measuring the contribution of the orbital angular momenta of quarks and gluons to the nucleon spin. The Institute of High Energy Physics (IHEP), Protvino, has a programme at the U-70 accelerator for obtaining polarized proton and antiproton beams from Λ decay for spin studies at the SPASCHARM facility, which is currently under construction.
The participants heard with interest the plans to construct dedicated facilities for determining the electric dipole moment (EDM) of the proton and nuclei, with proposals by the Storage Ring EDM collaboration at Brookhaven and the JEDI collaboration at Jülich. The dipole moment of fundamental particles violates both parity and time-reversal invariance. Its detection would indicate the violation of the Standard Model and would, in particular, make it possible to approach the problem of understanding the baryon asymmetry of the universe. The proposed experiments would reduce the measurement limit on the deuteron EDM down to 10–29 e cm.
Classical experiments studying the nucleon spin structure at high energies use both lepton scattering on polarized nucleons (e.g. in HERMES at DESY, COMPASS and at Jefferson Lab) and collisions of polarized hadrons (at RHIC, IHEP and JINR). A unified description of these different high-energy processes is becoming possible within the context of QCD, the theory of strong interactions. Related properties, such as factorization, local quark–hadron duality and asymptotic freedom, allow the calculation of the characteristics of the process within the framework of perturbation theory. At the same time, PDFs, correlation and fragmentation functions are not calculable in perturbative QCD, but being universal they should be either parameterized and determined using various processes or calculated within some model approaches. A number of talks at the symposium were devoted to the development and application of such models.
Theory confronts experiment
Experiments involving spin have brought about the demise of more theories than any other single physical parameter. Modern theoretical descriptions of spin-dependent PDFs, especially those including the internal transverse-parton motion, were discussed at the symposium. In this case, the number of PDFs increases and the picture that is related to them loses – to a considerable degree – the simplicity of a parton model with its probabilistic interpretation. One of the difficulties here concerns how the PDFs evolve with a change in the wavelength of the probe particle. A new approach to solving this problem was outlined and demonstrated for the so-called Sivers asymmetry measured in data from the HERMES and COMPASS experiments (figure 1).
The helicity distributions of the quarks in a nucleon are the most thoroughly studied so far. The results of the most accurate measurements by COMPASS, HERMES and the CLAS experiment at Jefferson Lab were presented by the collaborations. The present-day experimental data are sufficiently precise to include them in QCD analysis. Two new alternative methods for the QCD analysis of deep-inelastic scattering (DIS) and semi-inclusive DIS (SIDIS) data allow a positive polarization of strange quarks to be excluded with a high probability. As for the gluon polarization, the results of its direct measurement by the COMPASS experiment, which are confirmed by the PHENIX and STAR experiments at RHIC, also agree with QCD analysis. The low value of gluon polarization indicates that its contribution to nucleon spin is not enough to resolve the so-called nucleon-spin crisis. Hopes to overcome this crisis are now connected to the possible contributions of the orbital angular momenta of quarks and gluons, to be measured from GPDs. There were talks on different theoretical aspects of GPDs, as well as experimental aspects of their measurement, in the context of the HERMES, CLAS and COMPASS experiments.
Other important spin distribution functions manifest themselves in the lepton DIS off transversely polarized nucleons. The processes in which the polarization of only one particle (initial or final) is known are especially interesting. However, although relatively simple from the point of view of the experiment, they are complicated from the theoretical point of view (such complementarities frequently occur). These single-spin asymmetries are related to T-odd effects, i.e. they seemingly break invariance with respect to time reversal. However, it is a case of “effective breaking” – that is, it is not related to a true non-invariance of a fundamental interaction (here, the strong interaction, described by QCD) with respect to time reversal but to its simulation by the effects of re-scattering in the final or initial states. The single asymmetries have been studied by theorists for more than 20 years. These studies have received a fresh impetus in recent years in connection with new experimental data on single-spin asymmetries in the semi-inclusive electroproduction of hadrons off longitudinally and transversely polarized and unpolarized nucleons.
Reports from the COMPASS collaborations on transverse-momentum-dependent (TMD) asymmetries were one of the highlights of the symposium. The experiment is studying as many as 14 different TMD asymmetries. Two of them, the Collins and Sivers asymmetries (figure 2) – which are responsible for the left–right asymmetries of hadrons in the fragmentation of transversely polarized quarks and quark distributions in transversely polarized nucleons – are now definitely established in the global analysis of all of the available data, although other TMD effects require further study. The results of studies of the transverse structure of the proton at Jefferson Lab were also presented at the symposium.
The PHENIX and STAR collaborations have new data on the single-spin asymmetries of pions and η-mesons produced in proton–proton collisions at 200 GeV per beam at RHIC, with one of the beams polarized and the other unpolarized. They observe amazingly large asymmetries in the forward rapidity region of the fragmenting polarized or unpolarized protons, with a fall to zero in the central rapidity region. A similar effect was observed earlier at Protvino and at Fermilab, but at lower energies, thus confirming energy independence (figure 3). In addition, there is no fall with rising transverse momentum in the values of the asymmetry measured at RHIC. The particular mechanism for these asymmetries remains a puzzle so far.
So although single-spin asymmetries on the whole are described by existing theory, developments continue. The T-odd distribution functions involved lose the key property of universality and become “effective”, that is, dependent on the process in which they are observed. In particular, the most fundamental QCD prediction is the change of sign of the Sivers PDF determined from SIDIS processes and from Drell-Yan pair-production on a transversely polarized target. This prediction is to be checked by the COMPASS-II experiment as well as at RHIC, NICA and in the PANDA and PAX experiments at the Facility for Antiproton and Ion Research.
New data from Jefferson Lab on measurements of the ratio of the proton’s electric and magnetic form factors performed by the technique of recoil polarization gave rise to significant interest and discussions at the symposium. The previous measurements from Jefferson Lab showed that this ratio is not constant, as had been suggested for a long time, but decreases linearly with increasing momentum transfer, Q2 – the so-called “form factor crisis”. New data from the GEp(III) experiment indicate a flattening of this ratio in the region of Q2 = 6–8 GeV2. The question of whether this behaviour is a result of an incomplete calculation of radiative corrections – in particular, two-photon exchange – remains open.
The symposium enjoyed hearing the first results related to spin physics from experiments at CERN’s LHC. In particular, many discussions focused on the role of spin in investigating the recently discovered particle with a mass of 125 GeV, which could be the Higgs boson, as well as in studies of the polarization of W and Z bosons, and in heavy-quark physics. A number of talks were dedicated to the opportunities for theory related to searches for the Z’ and other exotics at the LHC and the future electron–positron International Linear Collider.
On the technical side there was confirmation of the method of obtaining the proton-beam polarization at the COSY facility in Jülich by spin filtration in the polarized gas target. This method can also be used for polarization of an antiproton beam, which will be important for measurements of different spin distributions in the nucleon via Drell-Yan muon-pair production in polarized proton–antiproton collisions in the PANDA and PAX experiments. There were also discussions on sources of polarized particles, the physics of polarized-beam acceleration, polarimeters and polarized-target techniques. In addition, there were reports on applications of hyperpolarized 3He and 19F in different fields of physics, applied science and medicine.
The main results of the symposium were summarized in an excellent concluding talk by Franco Bradamante from Trieste. The proceedings will be published in special volumes of Physics of Elementary Particles and Atomic Nuclei. The International Committee on Spin Physics, which met during the symposium, emphasized the excellent organization and success of the meeting in Dubna and decided that the 21st Symposium of Spin Physics will take place in Beijing in September 2014.
In November the Baryon Oscillation Spectroscopic Survey (BOSS) released its second major result of 2012, using 48,000 quasars with redshifts (z) up to 3.5 as backlights to map intergalactic hydrogen gas in the early universe for the first time, as far back as 11,500 million years ago.
As the light from each quasar passes through clouds of gas on its way to Earth, its spectrum accumulates a thicket of hydrogen absorption lines, the “Lyman-alpha forest”, whose redshifts and prominence reveal the varying density of the gas along the line of sight. BOSS collected enough close-together quasars to map the distribution of the gas in 3D over a wide expanse of sky.
The largest component of the third Sloan Digital Sky Survey, BOSS measures baryon acoustic oscillations (BAO) – recurring peaks of matter density that are most evident in net-like strands of galaxies. Initially imprinted in the cosmic microwave background radiation, BAO provide a ruler for measuring the universe’s expansion history and probing the nature of dark energy.
In March 2012, BOSS released its first results on more than 350,000 galaxies up to z = 0.7, or 7000 million years ago. However, only quasars are bright enough to probe the gravity-dominated early universe when expansion was slowing, well before the transition to the present, where dark energy dominates and expansion is accelerating. When complete, BOSS will have surveyed 1.5 million galaxies and 160,000 quasars.
To resolve the nature of dark energy will need even greater precision. The BigBOSS collaboration, which, like BOSS, is led by scientists at Lawrence Berkeley National Laboratory (LBNL), proposes to modify the 4-m Mayall Telescope to survey 24 million galaxies to z = 1.7, plus two million quasars to z = 3.5. The Gordon and Betty Moore Foundation recently awarded a grant of $2.1 million to help fund the spectrograph and corrector optics, two key BigBOSS technologies.
Events with a single jet of particles in the final state have traditionally been studied in the context of searches for supersymmetry, for large extra spatial dimensions and for candidates for dark matter. Having searched for new phenomena in monojet final states in the 2011 data, the ATLAS collaboration turned its attention to data collected in 2012, with the first results presented at the Hadron Collider Physics (HCP) symposium in Kyoto in November.
Models with large extra spatial dimensions aim to provide a solution to the mass-hierarchy problem (related to the large difference between the electroweak unification scale at around 102 GeV and the Planck scale around 1019 GeV) by postulating the presence of n extra dimensions, such that the Planck scale in 4+n dimensions becomes naturally close to the electroweak scale. In these models, gravitons (the particles hypothesized as mediators of the gravitational interaction) are produced in association with a jet of hadrons; the extremely weakly interacting gravitons would escape detection, leading to a monojet signature in the final state.
Dark-matter particles could also give rise to monojet events. According to the current understanding of cosmology, non-baryonic non-luminous matter contributes about 23% of the total mass-energy budget of the universe but the exact nature of this dark matter remains unknown. A commonly accepted hypothesis is that it consists of weakly interacting massive particles (WIMPs) acting through gravitational or weak interactions. At the LHC, WIMPs could be produced in pairs that would pass through the experimental devices undetected. Such events could be identified by the presence of an energetic jet from initial-state radiation, leading again to a monojet signature. The LHC experiments have a unique sensitivity for dark-matter candidates with masses below 4 GeV and are therefore complementary to other searches for dark matter.
The study presented at HCP uses 10 fb–1 of proton–proton data collected during 2012, at a centre-of-mass energy of 8 TeV. As with the earlier analysis, the results are still in good agreement with the predictions of the Standard Model (figure 2). The new results have been translated into updated exclusion limits on the presence of large extra spatial dimensions and the production of WIMPs, as well as new limits on the production of gravitinos (the supersymmetric partners of gravitons) that result in the best lower bound to date on the mass of the gravitino.
It has taken decades of hunting but finally the first evidence for one of the rarest particle decays ever seen in nature, the decay of a Bs (composed of a beauty antiquark and a strange quark) into two muons, has been uncovered by the LHCb collaboration.
In the Standard Model, the decay Bs → μμ is calculated to occur only three times in every 1000 million Bs decays. While the Standard Model has been incredibly successful, it leaves many unanswered questions concerning, for example, the origin of the matter–antimatter asymmetry and the essence of dark matter. Extended theories, such as supersymmetry, may resolve some of these issues. These theories allow for new particles and phenomena that can affect measurable quantities. The branching fraction B(Bs → μμ), for example, can be enhanced or reduced with respect to the Standard Model prediction, so the measurement has the potential to reveal hints of new physics. The LHCb experiment is particularly suited for such an indirect search for the effects of new physics, complementary to direct searches for new particles.
The LHCb collaboration performed the search for Bs → μμ (and B0 → μμ) by analysing 1.0 fb–1 of proton–proton collisions at 7 TeV in the centre of mass (from 2011) and 1.1 fb–1 at 8 TeV (2012). The signal selection starts with the search for pairs of oppositely charged muons that make a vertex that is displaced from the proton–proton interaction vertex (see figure 1). The signal and background are then separated using simultaneously the invariant mass of the two muons as well as kinematic and topological information combined in a multivariate analysis classifier. The particular classifier used is a boosted decision-tree (BDT) algorithm, which is calibrated with data for both signal and background events. The latter are dominated by random combinations of two muons from two different B mesons; this contribution is carefully determined from data.
The number of B0 → μμ candidates that LHCb observes is consistent with the background expectation, giving an upper limit of B(B0 → μμ) < 9.4 × 10–10 at 95% confidence level. This is the world’s most stringent upper limit from a single experiment on this branching fraction. However, for Bs → μμ, LHCb sees an excess of candidates with respect to the background expectation (figure 2). A maximum-likelihood fit gives a branching fraction of B(Bs → μμ) = 3.2 +1.5–1.2 × 10–9. The probability that the background could produce an excess of this size or larger is 5.3 × 10–4, corresponding to a signal significance of 3.5σ.
The measurement of Bs → μμ is close to the Standard Model prediction, albeit with a large uncertainty. This eagerly awaited result was presented at the Hadron Collider Physics Symposium in Kyoto and at a CERN seminar, and is now published. While it does not provide evidence for supersymmetry, it does constrain the parameter space for this and other models of new physics, and is a step further in understanding the universe.
The CMS collaboration has published its first result on proton–lead (pPb) collisions (CMS collaboration 2012), related to the observation of a phenomenon that was seen first in nucleus–nucleus collisions but also detected by CMS in 2010 in the first LHC proton–proton (pp) collisions at a centre-of-mass energy of 7 TeV (V Khachatryan et al. CMS collaboration 2010). The effect is a correlation between pairs of particles formed in high-multiplicity collisions – that is, collisions producing a high number of particles – which manifests as a ridge-like structure.
About once in every 100,000 pp collisions with the highest produced particle multiplicity, CMS observed an enhancement of particle pairs with small relative azimuthal angle Δφ (figure 1a). Such correlations had not been observed before in pp collisions but they were reminiscent of effects seen in nucleus–nucleus collisions first at Brookhaven’s Relativistic Heavy-Ion Collider (RHIC) and later in collisions of lead–lead nuclei (PbPb) at the LHC (figure 1b shows peripheral PbPb collisions from CMS).
Nucleus–nucleus collisions produce a hot, dense medium similar to the quark–gluon plasma (QGP) thought to have existed in the first microseconds after the Big Bang. The long-range correlations in PbPb collisions are interpreted as a result of a hydrodynamic expansion of this medium and are used to determine its fluid properties. Remarkably, this matter is found to have low frictional resistance (shear viscosity/entropy density ratio), behaving as a (nearly) perfect liquid. Because a QGP medium was not expected in the small pp system, the CMS results led to a large variety of theoretical models, which attempted to explain the origin of these ridge-like correlations (Wei Li 2012).
In September 2012, the LHC provided a short pilot run of pPb collisions at a centre-of-mass energy of 5 TeV per nucleus, for just a few hours. CMS collected two million pPb collisions (figure 2) – and now the first correlation analysis of these data has revealed strong long-range correlations, most easily visible as the ridge-like structure highlighted in figure 1c. As was the case for the pp data, the most common simulations of pPb collisions do not show ridge-like correlations, thus indicating a new, still unexplained phenomenon. Surprisingly, the effect in pPb collisions is much stronger than in pp collisions. In fact, it is similar to that seen in PbPb collisions.
The 2013 pPb run should yield at least a 30,000-fold increase in the pPb data sample at the same collision energy. Combined with the surprisingly large magnitude of the observed correlations, this will enable detailed studies and open a new testing ground for basic questions in the physics of strongly interacting systems and the nature of the initial state of nuclear collisions.
More than 20 years ago, the CMS and ATLAS experiments at the LHC embarked on a long road into the unknown and, rather like Christopher Columbus, the two collaborations reached a new land last summer. But did they discover what they expected – the long awaited Higgs boson of the Standard Model – or have they found the first hint of a new unknown world? The only way to find out is to measure the characteristics of the new particle to establish if it is compatible with the expectations of the Standard Model.
The decay of the new boson to two Z bosons and subsequently to four leptons (figure 1) is an especially powerful tool. This decay channel produces four well measured tracks of particles in a low-background environment and contains a rich set of information that no other channel can provide. The CMS collaboration has exploited this information first to boost the significance of signal observed last summer and then to go even further. By using the decay kinematics – understanding how the masses and angles of all of the particles in the process are correlated – they have attempted to determine if the new particle is the Standard Model Higgs boson or a gateway to a new world.
Using the full event information, the analysis assigns to each event the probability that it is a genuine Higgs boson, a more exotic particle or is just background. From these probabilities, it is possible to say how likely one model is compared with another. Figure 2 shows the expected likelihood for a genuine scalar Higgs boson (pink) and a pseudo-scalar boson (blue). The two hypotheses differ in the parity of the particle; in effect, the pseudo-scalar boson has a reversed mirror image. The green arrow on the plot is the measurement showing that the probability of a pure pseudo-scalar boson is small, indicating that this option is largely disfavoured by the data. This observation makes it possible to rule out a set of possible extensions of the Standard Model. A similar test of the hypothesis of a spin-2 particle has also been performed but it requires more data for a conclusive result. These are just the first steps into this new world. Further studies of the new boson will be possible in future as more data become available.
There was a keen sense of anticipation and excitement throughout the ATLAS collaboration as 2012 dawned. The LHC had performed superbly over the previous two years, delivering 5 fb–1 of proton–proton collision data at a centre-of-mass energy of 7 TeV in 2011, thereby allowing ATLAS to embark on a thorough exploration of a new energy regime. This work culminated with the first hints of a potential Higgs-like particle at a mass of about 126 GeV being reported by both the ATLAS and CMS collaborations at the CERN Council meeting in December 2011. With the promise of a much larger data sample at the increased collision energy of 8 TeV in 2012, everyone looked forward to seeing what the new data might bring.
The period leading up to the first collisions in early April 2012 saw intensive activity on the ATLAS detector itself, with the installation of additional sets of chambers to improve the coverage of the muon spectrometer, as well as the regular winter maintenance and consolidation work – essential for making sure that the detector was ready for the long year of data-taking ahead. With the promise of high-luminosity data with up to 40 simultaneous proton–proton collisions (“pile-up”) per bunch crossing – some 2–3 times more than seen in 2011 – experts from the groups responsible for the trigger, offline reconstruction and physics objects worked intensively to ensure that the online and offline software and selections were ready to cope with the influx of data. Careful optimization ensured that the performance of selections for electrons, τ leptons and missing transverse momentum, for example, were made stable against high levels of pile-up, while still keeping within the limits of the computing resources and maintaining – or even exceeding – the efficiencies and purities obtained in the 2011 data.
Meanwhile, the physics-analysis teams worked to finalize their analyses of the 2011 data for presentation at the winter/spring conferences and subsequent publication, while at the same time preparing for analysis of the new data. Members of the Higgs group focused attention on the two high mass-resolution channels H→γγ and H→ZZ(*)→4 leptons (figure 1), where the Higgs signal would appear as a narrow peak above a smoothly varying background. These channels had shown hints in the 2011 data and had the greatest potential to deliver early results in 2012. Using data samples from 2011 and a Monte Carlo simulation of the anticipated new data at 8 TeV, the analyses were re-optimized to maximize sensitivity in the mass region of 120–130 GeV, taking full advantage of the new object-reconstruction algorithms and selections.
The race to Australia
Once data-taking began in early April, the first priority was to calibrate and verify the performance of the detector, trigger and reconstruction, comparing the results with the new 8 TeV Monte Carlo simulation. The modelling of pile-up was particularly important and was checked using a dedicated low-luminosity run of the LHC, where events were recorded with only a single interaction per bunch crossing. Having established the basic conditions for physics analysis, attention then turned to preparations for the International Conference on High-Energy Physics (ICHEP) taking place on 5–11 July in Melbourne, where the particle-physics community and the world’s media would be eagerly awaiting the latest results from the new data.
As ICHEP drew nearer, the LHC began to deliver the goods, with up to 1 fb–1 of data per week
As ICHEP drew nearer, the LHC began to deliver the goods, with up to 1 fb–1 of data per week. Each new run was recorded, calibrated and processed through the Tier-0 centre of the Worldwide LHC Computing Grid at CERN, before being thoroughly checked and validated by the ATLAS data-quality group and delivered to the physics-analysis teams on a regular weekly schedule. At the same time, the worldwide computing Grid resources available to ATLAS worked round the clock to prepare the corresponding Monte Carlo simulation samples at the new collision energy of 8 TeV. At first, the analysers in the Higgs group restricted their attention to control regions in data, aiming to prove to themselves and the rest of the collaboration that the new data were thoroughly understood. After a series of review meetings, with a few weeks remaining before ICHEP, the go-ahead was given to “un-blind” the data taken so far – a moment of great excitement and not a little anxiety.
At first only hints were visible but as more data were added week by week and combined with the results from an improved analysis of the 2011 data, it rapidly became clear that there was a significant signal in both the γγ and 4-lepton channels. The last few weeks before ICHEP were particularly intense, with exhaustive cross-checks of the results and many discussions on exactly how to present and interpret what was being seen. With the full 5.8 fb–1 sample from LHC data-taking up until 18 June included, ATLAS had signals with significances of 4.5σ in the γγ channel and 3.4σ in 4 leptons, leading to the reporting of the observation of a new particle with a combined significance of 5.0σ at the special seminar at CERN on 4 July and at the ICHEP conference.
Similar signals were seen by CMS and both collaborations submitted papers reporting the discovery of this new Higgs-like resonance at the end of July. As well as the γγ and 4-lepton results reported at ICHEP, the paper by ATLAS also included the analysis of the H→WW(*)→lνlν channel, which revealed a broad excess with a significance of 2.8σ around 125 GeV. The combination of these three channels together with the 2011 data analysis from several other channels established the existence of this new particle at the 5.9σ level (figure 2), ushering in a new era in particle physics.
Searching for the unexpected
As well as following up on the hints of the Higgs seen in the 2011 data, the ATLAS collaboration has continued to conduct intensive searches across the full range of physics scenarios beyond the Standard Model, including those that involve supersymmetry (SUSY) and non-SUSY extensions of the Standard Model. More than 20 papers have been published or submitted on SUSY searches with the complete 2011 data set, with a similar number published on other searches beyond the Standard Model. One particular highlight is the search for the dark matter that is postulated to exist from astronomical observations but which has never been seen in the laboratory. By searching for “unbalanced” events, in which a single photon or jet of particles is produced recoiling against a pair of “invisible” undetected particles, limits can be set on the interaction cross-sections of the dark-matter candidates known as weakly interacting massive particles (WIMPs) with ordinary matter. Using the full 2011 data set, ATLAS was able to set limits on such WIMP-nucleon cross-sections for WIMPs of mass up to around 1 TeV; these limits are complementary to those achieved by direct-detection and gamma-ray observation experiments.
Another highlight is the search for new particles that decay into pairs of top (t) and antitop (t) quarks, giving rise to resonances in the tt– invariant mass spectrum. The complete 2011 data set gives access to invariant masses well beyond 1 TeV, where the t and t tend to decay in “boosted” topologies with two sets of back-to-back collimated decay products. By reconstructing each top decay as a single “fat” jet and exploiting recently developed techniques to search for distinct objects within the “substructure” of these jets, ATLAS was able to set limits on the production of resonances from the decay of Z’ bosons or Kaluza-Klein gluons in the tera-electron-volt range, even though high levels of pile-up added noise to the jet substructure. Such techniques will become even more important in extending these searches to higher masses with the full 2012 data sample.
The search for SUSY continued apace in 2012, with new results from 8 TeV data presented at both the SUSY 2012 conference in August and the Hadron Collider Physics Symposium in November. By looking for events with several jets and large missing transverse energy, limits on the strong production of squarks and gluinos were pushed beyond 1.5 TeV for equal-mass squarks and gluinos in the framework of minimal supergravity grand unification (mSUGRA) and the constrained minimal supersymmetric extension of the Standard Model (CMSSM). The lack of evidence for “generic” SUSY signatures with masses close to the electroweak and top-quark mass scales – together with the discovery of a light Higgs-like object around 126 GeV – has led to much theoretical interest in scenarios where only the third generation of SUSY particles (top and bottom squarks, stau lepton) are relatively light. ATLAS performed a series of dedicated searches for the direct production of bottom and top squarks. The latter in particular give rise to final states that are similar to top-pair production, so searches become particularly challenging if the masses of the top squark and quark are similar. Data from 2012 were used to fill much of the “gap” around the mass of the top quark (figure 3).
Precision measurements
The ATLAS search programme described above relies on a thorough understanding of the Standard Model physics-processes that form the background to any search, but are also interesting to study in their own right. Fully exploiting the large statistics of the 2011 and 2012 data samples requires an understanding of the efficiencies, energy scales and resolutions for physics objects such as electrons, muons, τ leptons, jets and b-jets to the level of a few per cent or better, which in turn requires a dedicated effort that continued throughout 2012. This effort paid off in a large number of precise measurements involving the production of combinations of W and Z bosons, photons and jets, including those with heavy flavour. In many cases, these results challenge the current precision of QCD-based Monte Carlo calculations and provide important input for improving the ability to describe physics at LHC energy scales. Studies of high-rate jet production and soft QCD processes have also continued, with measurements of event shapes, energy flow and the underlying event contributing to knowledge of the backgrounds that underlie all physics processes at the LHC. The measurements of WW, WZ, ZZ, Wγ and Zγ production have allowed stringent constraints to be placed on anomalous couplings of these bosons at high energies, in addition to being an essential ingredient in understanding the backgrounds to Higgs searches.
The large top-quark samples available in the data from 2011 and now 2012 have opened up a new era in the study of the heaviest known fundamental particle. The cross-sections for the production of both tt– pairs and single top quarks have been measured precisely at both 7 TeV and 8 TeV; evidence for the associated production of a W boson and a top quark has also been observed. Limits have been set on the associated production of tt pairs together with W and Z particles, and even Higgs bosons, and these studies will be extended with the full 2012 data set. The asymmetry in tt production has also been measured with the full 7 TeV data set – although, unlike at the Tevatron at Fermilab, no hints of anomalies have been seen. The polarizations of top quarks and W bosons produced in their decays have been measured and spin correlations between decaying t and t quarks observed. Furthermore, ATLAS has begun to characterize the top-quark production processes in detail, looking at kinematic distributions and the production of associated jets – key ingredients in increasing the precision of top-quark measurements, as well as in evaluating top-quark backgrounds in searches for physics beyond the Standard Model.
In addition, ATLAS has continued to exploit the large samples of B hadrons produced at the LHC, in particular those from dimuon final states, which can be recorded even at the highest LHC luminosities. Highlights include the detailed study of CP violation in the decay Bs→J/ψφ, which was found to be in perfect agreement with the expectation from the Standard Model, and the precise measurement of the Λb mass and lifetime.
In late 2011, ATLAS recorded around 20 times more lead–lead collisions than in 2010, allowing the studies of the hot, dense medium produced in such collisions to be expanded to include photons and Z bosons, as well as jets. A new technique was developed to subtract the “underlying event” background in lead–lead collisions, enabling precise measurements of jet energies and the identification of electrons and photons in the electromagnetic calorimeter. Bosons emerge from the nuclear collision region “unscathed”, opening the door to using the energy balance in photon-jet and Z-jet events to study the energy loss suffered by jets. In addition, ATLAS has pursued a broad heavy-ion physics programme, which includes the study of correlations and flow, charged-particle multiplicities and suppression, as well as heavy-flavour production. The collaboration looks forward eagerly to the proton–lead physics run scheduled for early 2013.
What is next?
At the time of writing, ATLAS is on track to record more than 20 fb–1 of proton–proton collision data in 2012 and studies of these data by the various teams are in full swing across the whole range of search and measurement analysis. Building on the discovery announced in July, the next task for the Higgs analysis group is to learn more about the new particle, comparing its properties with those expected for the Standard Model Higgs boson and various alternatives. A first step was presented in September, where the July analyses were interpreted in terms of limits on the coupling strength of the new particle to gauge bosons, leptons and quarks, albeit with limited precision at this stage. It is also important to see if the particle decays directly to fermions, by searching for the decays H→ττ and H→bb.
These analyses are extremely challenging because of the high backgrounds and low invariant-mass resolution but first results using 13 fb–1 of 8 TeV data were presented at the Hadron Collider Physics Symposium in November. These results are not yet conclusive; the full 2012 data sample is needed to make any definite statements. At that point, it should also be possible to probe the spin and CP-properties of the new particle and improve the precision on the couplings, bringing the picture of this fascinating new object into sharper focus. At the same time, first results from searches beyond the Standard Model with the complete 2012 data set should be available, further increasing the sensitivity across the full spectrum of new physics models. The analysis of this data set will continue throughout the 2013–2014 shutdown, setting the stage for the start of the 13–14 TeV LHC physics programme in 2015 with an upgraded ATLAS detector.
• This article has only scratched the surface of the ATLAS physics programme in 2012. For more details of the more than 200 papers and 400 preliminary results, please see https://twiki.cern.ch/twiki/bin/view/AtlasPublic.
Some 400 theorists and experimentalists from all around the world convened in Munich on 8–12 October to discuss developments in the theory of strong interactions. They were attending the tenth conference on “Quark Confinement and the Hadron Spectrum” (ConfX) at the Garching Research Campus, hosted by the Physics Department of the Technical University of Munich (TUM), with support from the Excellence Cluster “Origin and Structure of the Universe”. Topics included areas at the boundaries of the field, such as theories beyond the Standard Model with a strongly coupled sector and QCD approaches to nuclear physics and astrophysics.
Inaugurated in 1994 in Como, Italy, this series of conferences has established itself as an important forum in the field, bringing together people working in strong interactions on approaches that range from lattice QCD to perturbative QCD, models of the QCD vacuum to phenomenology and experiments, the mechanism of confinement to deconfinement and heavy-ion physics, and from effective field theories to physics beyond the Standard Model. Taking place at a particularly important time for particle physics, with the observation of a Higgs-like particle at CERN, the tenth conference provided a valuable opportunity not only to reconsider what was done on past occasions but also to discuss the perspectives for strongly coupled theories.
The scientific focus of ConfX was spread across seven main scientific sessions: vacuum structure and confinement; light quarks; heavy quarks; deconfinement; QCD and new physics; nuclear and astroparticle physics; and strongly coupled theories. These subjects are relevant for the physics of B factories (Belle and BaBar), tau-charm experiments (BESIII), LHC experiments (LHCb, CMS, ATLAS), heavy-ion experiments (RHIC, ALICE), future experiments at FAIR-GSI (Panda, CMB) and in general for many low-energy experiments (such as at Jefferson Lab, COSY, MAMI) and some parts of experimental astrophysics.
It is impossible to summarize here the wealth of results presented at the meeting, the intensity of the discussions and the flow of information. What follows is just a brief selection.
The first plenary session began with recent progress in the theoretical calculations of double parton-scattering at the LHC presented by Aneesh Manohar of the University of California, San Diego. The application of soft collinear effective theory to many collider physics processes was then introduced by Thomas Becher of Bern University and followed by a review of quarkonium production by Kuang-Ta Chao of Peking University. In particular, J/ψ production has now finally been calculated at next-to-leading order in nonrelativistic QCD (NRQCD) and the extraction of colour-octet matrix elements from a combined fit to collider data has become possible for the first time. The current picture hints at the universality of the NRQCD matrix elements and a proof of the NRQCD factorization in the fragmentation approach seems to be close. Predictions for the production of Υ and other quarkonia states at the LHC experiments are now available. The progress in theory together with the new LHC data should soon allow the resolution of the long-standing puzzles about the J/ψ polarization and the production mechanism of quarkonium, both at hadron colliders and at B factories.
Heavy ions and more
The study of quarkonium production and suppression at finite temperature in heavy-ion collisions as a probe of quark–gluon plasma was reviewed in the context of a new effective field-theory approach (potential NRQCD at finite temperature). Here the shift in paradigm from the typical phenomenological description is apparent, the quarkonium dissociation being caused by the emergence of a large imaginary part in the quark–antiquark potential rather than by a Debye screening phenomenon as reported by Jacopo Ghiglieri of McGill University. The effective field-theory approach allows a systematic calculation of the thermal modifications in the energy and width of the Υ(1S) as produced at the LHC in heavy-ion collisions.
There has been great progress in developing the capabilities of the lattice approach to calculate the properties of heavy and light quarks, and also in connection to chiral effective field theories, as Peter Lepage of Cornell University, Laurent Lellouch of the Centre de Physique Théorique, Marseilles, and Zoltan Fodor of the University of Wuppertal reported.
The interest and relevance of light scalars, as well as the long-standing controversy dating back to the 1950s about their existence and nature, has been resolved in recent years by means of better data and more powerful theoretical techniques that include effective Lagrangians and dispersion theory, as José Pelaez of the Complutense University of Madrid argued.
Highlights in strong physics beyond the Standard Model presented at the conference include: composite dynamics as put in context by Francesco Sannino of the Centre for Cosmology and Particle Physics Phenomenology, Odense, at the time of the Higgs discovery; gauge gravity duality; holographic QCD explained by Shigeki Sugimoto from Tokyo University; and applications of anti-deSitter/conformal field theory correspondence to heavy-ion collisions contrasted to proton–proton physics at the LHC now and in the future, including the outstanding LHC results, presented by Günther Dissertori of ETH Zurich. This session culminated in a heated discussion about future strongly coupled scenarios, led by Antonio Pich of Valencia University, in which different views of scenarios beyond the Standard Model were discussed but remained unreconciled among the panel members Estia Eichten of Fermilab, Emanuel Katz of Boston University, Juan Maldacena of the Institute of Advanced Study, Princeton, and Stefan Pokorski of the University of Warsaw.
The conference featured a total of 250 talks
The plenary session on Wednesday morning was dedicated to the impact of QCD on nuclear and astroparticle physics. Opening the session, Ulrich Wiedner of Ruhr University Bochum presented a comprehensive review of the highlights and future of low-energy experiments in hadron physics. An effective field theory and lattice description of a variety of nuclear bound states and reactions, as well as a review of the low-energy interaction of strange and charm hadrons with nucleons and nuclei, were presented by Evgeny Epelbaum, also of Bochum, and William Detmold at Massachusetts Institute of Technology. Charles Horowitz of Indiana University spoke about multimessenger observations of neutron-rich matter, describing the Lead Radius Experiment (PREX) at Jefferson Lab, which measures the neutron density of 208Pb using parity-violating electron scattering. This has important implications for neutron-rich matter and neutron stars. He also described X-ray observations of radii of neutron stars, which are possibly model dependent, and their implications for the equation of state. Gravitational-wave observations of merging neutron stars and r-mode oscillations were discussed in terms of the equation of state, mechanical properties and bulk and shear viscosities of neutron-rich matter. This prepared the ground for the roundtable discussion on “What can compact stars really tell us about dense QCD matter”, chaired by Andreas Schmitt of the Vienna University of Technology.
On Thursday morning, Pich gave an overview of the perturbative determination of αs in which he presented the final value of 0.1187 ± 0.0007 and discussed the impact of the different type of αs extractions on the final result.
A number of low-energy precision measurements are sensitive to new physics either because the Standard Model prediction for the measured quantity is precisely known – for example, the anomalous magnetic moment of the muon (g-2) – or because the Standard Model “background” is small, as in the case of electric dipole moments (EDMs). Timothy Chupp of the University of Michigan presented several studies that are under way to probe physics beyond the Standard Model, including g-2 and EDMs. He also described the prospects for the precision measurement of the Cabibbo-Kobayashi-Maskawa matrix element, Vud, from neutron decay, i.e. the neutron lifetime and measurement of the axial-vector coupling constant (gA), as well as couplings beyond the Standard Model accessible from neutron decay. The discussion culminated in the roundtable “Resolving physics beyond the Standard Model at low energy” led by Susan Gardner of the University of Kentucky.
The final plenary session on Friday afternoon started with a talk by Mikko Laine of the University of Bern, in which he drew analogies and relationships between hot QCD and cosmology. John Harris of Yale University went on to review the latest heavy-ion data from Brookhaven’s Relativistic Heavy-Ion Collider (RHIC) and the LHC. In particular, the data show how the “soup” of quark–gluon plasma flows easily, with extremely low viscosity – suggesting a near-perfect liquid of quarks and gluons. However, it appears opaque to energetic partons at RHIC and less so to the extremely energetic parton probes available in collisions at the LHC. This review was followed by presentations on the theoretical challenges and perspectives in the exploration of the hot QCD matter, including recent highlights in lattice calculations at finite temperature and finite density as presented by Peter Petreczky of Brookhaven National Laboratory. The session culminated with a roundtable about “Quark Gluon Plasma: what is it and how do we find it out?” chaired by Berndt Mueller of Duke University.
Yiota Foka of GSI and CERN reported on the International Particle Physics Outreach Group, which has developed an educational activity that brings LHC data into the classroom. Each year since 2005, thousands of high-school students in many countries go to nearby universities or research centres for one day to unravel the mysteries of particle physics and to be “scientists for a day”. In 2012, 10,000 students from 130 institutions in 31 countries took part in the popular event over a four-week period.
The conference featured a plenary session and seven sessions running in parallel on the subjects of the seven topical sections, with a total of 250 parallel talks. The sections on vacuum structure and confinement and on deconfinement constituted almost two conferences in themselves, with a total of 54 talks in 17.5 hours and 57 talks in 24 hours, respectively. The conference as a whole ended with a visionary talk by Chris Quigg of Fermilab on “Beyond Confinement”. The extraordinary scientific discussion and exchange that characterized the conference has served as a trigger for a document “Strongly Coupled Physics: challenges, scenarios and perspectives” that is currently in preparation in collaboration with the section conveners.
During the poster session, participants could also enjoy tasting cheese and a variety of wine from all of the countries represented. A ride down the gigantic slide belonging to the Mathematics Department complemented the lively scientific discussions. An evening session on the “Colourful world of quark and gluons” given by Gerhard Ecker, “The shaping of QCD”, and Thomas Mannel, “The many facets of QCD”, attracted the public from Garching city and from the many campus research institutes, as well as conference participants. Tours of Munich, glimpses of Bavarian culture at the famous Hofbräuhaus and a social dinner at the Hofbräukeller complemented the opportunity to discover the local campus facilities (the TUM Institute of Advanced studies and the TUM engineering, mathematics and physics departments).
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