The 7th edition of the Hard Probes International Conference series was hosted by McGill University in Montréal, Québec, Canada, in the Summer of 2015 (June 29 - July 3). During the last two decades, high-energy nuclear physics has seen a tremendous progress; thanks to the combined efforts of CERN SPS, BNL RHIC and the LHC, we now have an ample evidence that the long sought-after Quark-Gluon Plasma (QGP) is indeed being created in relativistic heavy-ion collisions. Experimental and theoretical studies of this matter under extreme conditions have already given us the first glimpse of what our Universe was like right after the Big Bang. However, much about this fascinating state of matter remains yet to be explored.

In this regard, the International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions has made a critical contribution in understanding the pivotal role of jets and other high energy probes in unravelling the fascinating complexity of the hottest and densest state of matter ever created.  A selection of highlights presented by the four major LHC experiments are given below. The experiments are now looking forward to the Heavy Ion period of Run 2, when Pb nuclei will be collided at energy larger by almost a factor of 2 with respect to Run 1, which will translate in a significantly higher production cross-section at high momentum for hard probes. Exciting new results are expected, so stay tuned! 

LHCb is entering the regime of ultra-relativistic heavy-ion physics

During the conference, the LHCb collaboration presented for the first time its plan and potential in heavy ion PbPb collisions, while it had previously only analysed pPb data. During the proton-lead run of 2013, LHCb measured prompt and non-prompt J/ψ production in the forward region in proton-lead collisions and investigate cold-nuclear matter effects. The excellent vertex resolution of the LHCb detector allowed to distinguish prompt J/ψ events from J/ψ events of a B-meson decay. The very good mass-resolution of LHCb allowed also to study Y-production in the forward region and the sequential suppression of the Y(2s) and Y(3s) states. The results obtained for the nuclear modification factor of quarkonium production as function of rapidity are summarized in Figure 1.

Figure 1: Suppression in the forward region is smaller for Y(1s) than for J/ψ while a possible enhancement in the backward region can be observed for Y(1s) which might be due to anti-shadowing effects. The overall agreement with predictions of models (EPS09 NLO) is good.

Another interesting result obtained from the proton-lead run of 2013 is in relation to two-particle angular correlations. The correlations were measured as a function of relative pseudorapidity, Δη, and relative azimuthal angle, Δφ, for events in different classes of event activity and for different bins of particle transverse momentum, p_T. In high-activity events a long-range correlation on the near side is observed in the pseudorapidity range 2.0 < η < 4.9. This has been the first measurement of the so-called ‘near side ridge’ in proton-lead collisions in the forward region and extends previous observations in the central region. The correlation increases with growing event activity and is more pronounced in the direction of the lead beam. 

Figure 2: The two-particle correlation functions for events recorded in the proton-lead configuration (left) and lead-proton configuration (right) for the high event-activity classes and prompt charged particles in the pT-range of 1- 2 GeV/c. (The near-side peak around Δη = Δφ = 0 is truncated.)

ALICE results 

ALICE measured a significant suppression of the yield of high momentum charged particles, p⁰, heavy-flavour hadrons, heavy-flavour decay leptons in central Pb-Pb collisions with respect to the yields measured in proton-proton and proton-Pb collisions. These results indicate that partons interact strongly with the medium quarks and gluons, losing energy. A significant suppression was measured also for jets, suggesting that the energy lost is dissipated outside the jet area. The study of jets at large azimuth with respect to a high momentum particle represented a step forward towards probing the microscopic structure of the medium and its homogeneity (arXiv: 1506.03984). The rate of single large-angle scatterings (Molière-like) was measured to be not significant, though still with limited statistical precision. No evidence was found, within uncertainties, for an additional medium-induced acoplanarity, as indicated by the similar width of data and PYTHIA Δφ correlations shown in the left panel of Figure 3. Concerning the jet properties, no indication for a significant angular broadening of the recoiling jet structure was found. The absence of a modification of the relative yields of high momentum pion, proton, and kaons suggests that the jet hadrochemical composition remains unaltered (arxiv: 1506.07287). This is further supported by the fact that no evidence, within uncertainties, was found for an enhancement of the relative yield of Λ baryons with respect to K⁰_s mesons inside the jet cone, contrasting, as shown in the right panel of Figure 3, the dramatic increase characterizing the “bulk” of medium particles, effect that is usually ascribed to the hydrodynamic expansion of the medium. Therefore, despite the significant interaction of partons with the QGP, the emerging jets preserve, up to a certain extent, properties similar to those of jets in pp collisions.

Figure 3: difference of the azimuthal correlation distributions of hadron-jet pairs obtained with a low (8<p_Τ<9 GeV/c) and high (20<pT<50 GeV/c) p_Τ selection on the “trigger” hadron (left panel). The data points (black) are compared to a reference distribution (red) obtained by embedding pp events simulated with PYTHIA in real Pb-Pb events. The continuous lines represent the result of a fit done with a function composed of a constant and an exponential with parameter σ.  Right panel: Λ/K⁰s ratio measured as a function of p_Τ with particles in jets (red and blue points) and with inclusive particles (black points) in central Pb-Pb collisions. 

The final results of the measurement of D meson nuclear modification factor (R_AA) at high momentum as a function of the collision centrality were also presented (see also arxiv: 1506.06604). The comparison, shown in Figure 4 (left panel), with the preliminary measurement of R_AA of J/ψ from B-meson decay, performed by CMS, provides an indication that in-medium energy loss is larger for charm than for the more massive beauty quark. In the same figure, it is shown that the R_AA of high-momentum D mesons is similar to that of pions, which mainly come from the fragmentation of gluons and light quarks. The observed trends are described by models including a dependence of in-medium energy loss from the parton species and quark mass.

Figure 4: comparison of the centrality dependence of the R_ΑΑ of D mesons, charged pions, and non-prompt J/ψ from B meson decay, the latter measured by CMS (left panel). Right panel: inclusive J/ψ R_ΑΑ as a function of centrality in three different transverse momentum ranges.

The smaller suppression of J/ψ production in central heavy-ion collisions at the higher energies of the LHC compared to observations from RHIC has been one of the most exciting results for heavy-ion physics at the LHC. ALICE performed further studies for characterizing the suppression trend as a function of the collision centrality for different J/ψ momenta (arxiv 1506.08804). The results confirm that the reduced suppression concerns mainly low momentum J/ψ, as shown in the right panel of Figure 4, in agreement with what expected by models including charmonia formation via recombination of charm quarks not produced in the same hard scattering process. This investigation brought also the unexpected observation of an excess (R_AA~7) of J/ψ production at transverse momenta smaller than 0.3 GeV/c in peripheral collisions. Further studies are ongoing to understand the nature of this excess that could originate from J/ψ photo-production, a process well known to be effective in ultraperipheral heavy-ion collisions with an impact parameter larger than the nucleus diameter.

ALICE presented also several important new results from the study of p-Pb collisions. A complete diagnosis of J/ψ production was carried out, investigating its dependence on momentum, rapidity, and event multiplicity. The data indicate that “cold nuclear matter” effects are stronger in central than in peripheral p-Pb collisions and set stringent constraints to theoretical predictions (arxiv 1506.08808). ALICE observed also a suppression of ψ(2s) production that increases with the collision centrality, resembling what seen also by PHENIX in d-Au collisions at RHIC, as shown in the left panel of Figure 5. The suppression in minimum bias data is described by a model including interactions of quarkonia with comoving particles in the system. This “final state” effect could account also for the dependence of the suppression on the event multiplicity.

Since the first tantalizing observation of a double-ridge structure in the angular correlation distribution of particles produced in p-Pb events with high multiplicity, resembling the elliptic flow (v₂) correlation typical of peripheral Pb-Pb collisions, an effort was done by all the experiments for investigating its nature. ALICE presented the result of a new analysis in which muons reconstructed in the dedicated forward spectrometer are correlated with particles reconstructed in the central barrel, thus separated by a large rapidity gap (arxiv 1506.0832). Positive v₂ values were measured also for muons with transverse momentum larger than 2 GeV/c, which predominantly come from decays of heavy-flavour particles. Slightly larger v₂ values were measured in the case in which the muon follows the direction of the Pb nucleus than in the case in which it goes in the direction of the proton nucleus, as indicated by their ratio, displayed in the right panel of Figure 5. These results set important constraints to models in which a double-ridge structure in the angular correlation originates from effects related to the initial stage (e.g. from saturation of gluon nuclear parton distribution at low Bjorken x) or final stage (e.g. from a hydrodynamic evolution of the system) of the collision. 

Figure 5: comparison of the centrality trend of ψ(2s) nuclear modification factor measured by ALICE in p-Pb collisions at the LHC and by PHENIX in d-Au collisions at RHIC (left). The notation QpPb_mult is adopted by ALICE to recall the potential biases intrinsic to the determination of the collision centrality in the p-Pb system. Right panel: ratio of v₂ values measured for muon with 2.5<η_lab<4 in the Pb-going direction and in the p-going direction as a function of p_T. The nucleon-nucleon center of mass system is shifted by Δψ=0.465 in the p-beam direction.

CMS results

CMS has delivered a comprehensive set of differential R_AA measurements, quantifying the jet quenching effect for hard probes of various flavors. Profound suppression of strongly interacting probes in 2.76 TeV PbPb collisions is established with high precision for inclusive charged hadrons, spanning the wide range of transverse momenta from soft-sector all the way up to 100 GeV/c, and inclusive fully reconstructed jets, that remain quenched even at 300 GeV/c. It was also found that b-tagged jets in PbPb collisions are suppressed at a similar level to the inclusive (predominantly light) jets in the entire kinematic region studied. First hints of the flavour dependence of energy loss were provided by CMS measurement of displaced J/ψ (from b decays), which shows smaller suppression level compared to light-flavour hadrons below 20 GeV/c. Direct measurements of colourless probes, such as direct photons and W and Z bosons, give a great advantage to CMS nuclear modification studies, providing experimental check of the binary collision scaling for hard processes removing reliance on the Glauber model. The most recent CMS updated result for the Z-boson nuclear modification factor confirms the previously reported R_AA of unity for non-interacting probes.

CMS also presented new PbPb Upsilon results, following significant improvements made to the muon reconstruction. Additional and more drastic improvements in the uncertainties of the nuclear modification factor measurements were afforded by a factor of 20 increase in the integrated luminosity of the pp reference. The new 2013 pp data set, corresponding now to 5.4 pb^-1 integrated luminosity, allowed more differential studies of Y(1S) and Y(2S) states as a function of collision centrality, Upsilon rapidity and transverse momenta. Strong centrality dependence (Figure.5, left) of Upsilon suppression reported by early CMS measurement is now established with much greater precision. For centrality-integrated (minimum bias) PbPb collisions the Y(1S) are suppressed by about a factor of 2, while the Y(2S) state is diminished by an order of magnitude (the Y(3S) state remains unobserved in PbPb collisions, being suppressed by more than a factor of 7 at 95% confidence level). These suppression levels appear to have no pronounced transverse momenta (Figure.6, right) or rapidity dependence, giving new quantitative input for the theoretical calculations of the melting phenomena.

Figure 6: Collision centrality (left) and transverse momentum (right) dependence of nuclear modification factors for Y(1S) and Y(2S) bottomonia states from 2.76 TeV PbPb collisions at LHC. 

In addition to R_AA studies, CMS continues to advance the understanding of jet quenching phenomenon with the suite of correlation studies for fully reconstructed jets. After pioneering the dijet momenta asymmetry measurements in 2011, CMS has attempted to recover the details of the energy loss patterns by measuring event-wide momentum balance about the dijet axis, comparing separately samples of balanced (similar momenta) and unbalanced dijets in pp and PbPb collisions. Formed “missing p_T” distributions were studied differentially as function of transverse momenta of all charged particles in the event as well as the radial distance from the dijet axis, reaching out relative radial distance of R = 1.8. Extending the missing-p_T measurements to large angles is a crucial first step for constraining the energy loss mechanisms and quantifying the interaction strength within the QGP. High-precision measurements of the background-subtracted jet structures within the jet cone of R = 0.3 performed by CMS for PbPb and pp collisions at the same energy, have shown significant medium-induced modification of both p_T and angular distributions in jet shapes and fragmentation functions studies.  However, these in-cone changes only account for a fraction of the dijet momentum imbalance. 

Figure 7: Total yield excess observed in the PbPb data with respect to the reference measured in pp collisions is shown as a function of associated particle pT in four different centrality intervals for leading and subleading jets.

A new jet-track correlation analysis, separating the jet modifications up to large angles from the long-range correlations of the bulk medium of PbPb collisions, has been presented. The resulting jet-track correlations for back-to-back dijets with leading jet p_T > 120 GeV/c and subleading jet p_T > 50 GeV/c extend to |Dh,Dj| = 1.5, and are found to be modified compared to the reference measurement in pp collisions.

Peripheral PbPb data are found to have similar jet-track correlation structures as dijets from pp events. For central PbPb data, more charged particles associated with each side of the dijet are found at low transverse momenta (1-2GeV/c), and their angular distributions appear broadened in Δη and Δφ dimensions. The excess of the soft correlated yields is larger on the subleading side of the dijet, which experiences more quenching (summarized in Figure 7). Integrals of the PbPb to pp yield differences are shown for different collision centrality bins, from 50-100% most peripheral events (left) to 0-10% most central PbPb collisions (right). The excess yield and its broadening (not shown) are found to have pronounced centrality dependence, but diminish quickly with increasing particle transverse momentum. At highest transverse momenta studied (up to 8 GeV/c), the correlated yield observed in PbPb collisions becomes similar, if slightly below, to that of the pp reference.

The results summarized here represent a small fraction of the CMS results presented at Hard Probes 2015. The latest CMS heavy-ion results can be found here. 

ATLAS results

The ATLAS results from PbPb data included a measurement of high-p_Τ photons and W mesons, high-precision measurement of jet and charged particle R_ΑΑ, studies of jet fragmentation, path length dependence of jet quenching, neighbouring jet production and studies of flow and correlations of low p_Τ particles. The pPb results included the measurement of Z boson, J/ψ, Ψ(2S), the measurement of jet production and jet fragmentation. Further, the measurement of the correlation between jet production and the underlying event in pp collisions was presented which should improve the understanding of the interplay between soft processes and hard processes in pPb. Here are few selected highlights of the presented results.

A strong jet quenching at LHC was seen shortly after the first heavy ion collisions in 2010 in the measurement of large dijet asymmetry. Following this observation, a group of measurements aims to better understand the in-medium path length dependence of jet quenching. The first measurements of the azimuthal dependence of inclusive jet yields showed significant variation of inclusive jet suppression as a function of relative azimuthal angle, ΔΦ, with respect to the interaction plane.

The jet yields were observed to vary by as much as 20% between the directions pointing towards and out of the interaction plane. Motivated by this measurement, ATLAS has performed a similar analysis for the dijets. The dijet asymmetry, A_J, was studied for different angles that the leading-jet makes with respect to the interaction-plane angle. These measurements effectively study the path-length dependence of the dijet asymmetry by requiring the jet pair to traverse different lengths of the medium. The dependence of the A_J on the interaction-plane angle was quantified by calculating the second Fourier coefficient of the A_J azimuthal distribution, termed c₂. As shown in Figure 5, the measured c₂ signal is quite small (≤ 2%), however, it is consistently negative, indicating a slightly larger asymmetry when the dijet pair is oriented out-of-plane than in-plane.

Figure 8:  The c2, the second Fourier coefficient of the azimuthal distribution of dijet asymmetry, shown as a function of centrality. The vertical gray bands indicate systematic uncertainties and the vertical bars indicate statistical errors. The shaded bands indicate the statistical and systematic uncertainties added in quadrature. The three panels correspond to three jet-radius parameters, as indicated in the plots. Lines show fits to the constant function. 

This measurement was further extended by repeating the analysis when constraining the shape of the collision geometry by selecting events based on the magnitude of the second-order flow harmonic quantified by the magnitude of the q₂ vector. Within a given centrality interval, events with large q₂ (i.e. events with more elliptic geometry, show an increase in the c₂ for the 20-30% and 30-40% centrality bins). This measurement together with the original measurement of the azimuthal dependence of inclusive jet yields and the measurement of neighbouring jet suppression can provide significant constrains on the energy loss mechanism of hard scattered partons and its in-medium path length dependence.

ATLAS, has also delivered precise measurements of the nuclear modification factor (a measurement used for studying jet quenching) of inclusive charged particles at high-p_T that is shown together in Figure 9. The R_AA was also measured as function of the pseudorapidity being consistent with flat pseudorapidity dependence over the whole transverse momentum range in all centrality bins. This striking behaviour together with the R_AA at high-p_T should also provide significant constrains on the energy loss modelling.

Figure 9: The RΑΑ of charged particles measured as a function of pT in the centrality interval 0-5%. The ATLAS and CMS results correspond to the pseudorapidity range |η| < 1, the ALICE results to |η| <  0.8. Statistical uncertainties are shown with vertical bars and systematic uncertainties with brackets.

One of the striking features seen in pPb collisions is the centrality dependence of the nuclear modification factor of inclusive jets, R_pPb, which shows a suppression of the jet yield in central events and an enhancement in peripheral events. In pPb collisions, the centrality is estimated in forward calorimeters in the direction of the lead beam. The effects seen in RpPb of jets imply that the factorization between hard and soft processes is violated at an unexpected level in pPb collisions.

To improve the understanding of soft-hard correlations ATLAS measured the relationship between jet production and the underlying event in a pseudorapidity separated region in 2.76 TeV pp collisions. In that study, the underlying event was characterized through measurements of the average sum of the transverse energy at large negative pseudorapidity, <ΣE_T>, which were reported as a function of hard scattering kinematic variables. The hard scattering was characterized by the average transverse momentum and pseudorapidity of the two highest transverse momentum jets in the event. It was found that the <ΣE_T> is anticorrelated with the dijet p_T, decreasing by 25% as p_T varies from 50 to 500 GeV.

A result is shown in Figure 10. The measurement was repeated as a function of Bjorken-x of the target and projectile nucleons, where target is defined to be heading towards the negative pseudorapidity. It was found that the average level of transverse energy production is sensitive predominantly to the Bjorken-x of the target proton. These results measured in pp collisions provide useful context for understanding the results from p-Pb collisions implying that jets formed during these collisions are not due to trivial anticorrelation in individual nucleon-nucleon collisions (i.e. effects from the energy conservation).

Figure 10: Normalized total transverse energy at large negative pseudorapidity, <ΣE_T>/<ΣE_T>^ref, characterizing the underlying event evaluated as a function of dijet pT for different dijet pseudorapidity selections. The shaded bands represent the total systematic and statistical uncertainties in quadrature while the vertical error bars represent statistical uncertainties only.

You can read more about the ATLAS results here.  

The author would like to thank Andrea Rossi (ALICE), Martin Spousta (ATLAS), Olga Evdokimov & Camelia Mironov (CMS), Burkhard Schmidt (LHCb) for kindly contributing to this article.