CERN Accelerating science

LHC experiments present latests results from heavy-ion runs (old)

by Panos Charitos

The 7th edition of the Hard Probes International Conference series was hosted by McGill University in Montréal, Québec, Canada, from 29 June to 3 July.

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 amount of 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. 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 unraveling the fascinating complexity of the hottest and densest state of matter ever created. The conference brought together experts from all over the world for an intense five days of fruitful presentations and discussions.

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

he LHCb collaboration presented for the first time its plan and potential in heavy ion physics. 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 while the very good mass-resolution of the LHCb allowed to study Y-production in the forward region and the sequential suppression of the Y(2s) and Y(3s) states. 

LHCb has participated so far only in the proton-lead run of 2013, but not in the lead-lead runs of 2010 and 2011. The reason has been that the detector occupancy in the forward region for central lead-lead collisions is too large. However, detector occupancy is not a problem for proton-lead collisions, neither for more peripheral lead-lead collisions. Since LHCb is fully instrumented in the forward region, it can study nucleon-nucleon collisions in a unique kinematic range.  

ALICE Results in Hard Probes 2015

ALICE, presented a wealth of scientific results. Equipped with a central barrel with high-resolution tracking systems, several detectors for identifying particles from low to high momentum, electromagnetic calorimeters, and with a muon spectrometer at forward rapidity, the ALICE detector is ideally suited for studies involving hard probes.

ALICE results indicate that partons interact strongly with the medium quarks and gluons, losing energy, as evidenced by the suppression of the yield of high momentum charged particles, p0, heavy-flavour hadrons, and jets over a broad momentum range in central Pb-Pb collisions with respect to the yields measured in proton-proton and proton-Pb collisions.

ALICE also presented several important new results from the p-Pb run. 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. 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. These results are important to set constraints to models that explains the observed structure in terms of initial or final state effects.

Following the Phase I upgrade the detector is now ready to exploit the physics potential of Run II of the LHC.

CMS results presented at Hard Probes.

CMS has delivered a comprehensive set of differential RAA measurements, quantifying the jet quenching effect for hard probes of various flavours. Profound suppression of strongly interacting probes in 2.76 TeV lead-lead collisions is established with high precision for inclusive charged hadrons, spanning the wide range of transverse momenta from soft-sector all the way 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 complete reliance on Glauber model. Most recent CMS updated result for the Z-boson nuclear modification factor confirms the previously reported RAA of unity for non-interacting probes.

CMS also presented new PbPb results for U, 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. In addition to RAA 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. In addition, 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. 

Finally, peripheral PbPb collisions were 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 dijet are found at low transverse momenta (1-2GeV/c), and their angular distributions appear broadened in Δη and Δφ  dimensions. 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. 

New results from ATLAS

Finally, ATLAS presented a number of measurements from PbPb data including a measurement of high-pΤ photons and W mesons, high-precision measurement of jet and charged particle RΑΑ, studies of jet fragmentation, path length dependence of jet quenching, neighboring jet production and studies of flow and correlations of low pΤ particles. The pPb results included the measurement of Z boson, J/ψ, Ψ(2), 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 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 with respect to the interaction plane.

ATLAS has performed a similar analysis for the dijets; the dijet asymmetry was studied for different angles that the leading-jet makes with respect to the interaction-plane. These measurements allow to study the path-length dependence of the dijet asymmetry.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. This measurement together with the original measurement of the azimuthal dependence of inclusive jet yields and the measurement of neighboring 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-pT. The RAA 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 behavior together with the RAA at high-pT could provide significant constrains on the energy loss modeling.

One of the striking features seen in pPb collisions is the centrality dependence of the nuclear modification factor of inclusive jets, RpPb, 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.


The LHC experiments now looking forward to Run 2, when Pb nuclei will be collided at energy larger by a factor of 2 with respect to Run 1, which translate in a significantly higher production cross-section at high momentum for hard probes. Therefore new exciting results are expected. Stay tuned! 


Read more:

Click here to read more about the results presented by ALICE, ATLAS, CMS and LHCb.

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.


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