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EXPLORATORY HEAVY-QUARK JET TOMOGRAPHY OF QUARK-GLUON PLASMA AT THE LHC

Final Report Summary - EXPLOREQQJETINHI@LHC (Explotary heavy-quark jet tomography of quark gluon plasma at the LHC)

Heavy quarks are important probes of QCD production and hadronization in both fundamental 'pp' collisions and ultra-relativistic heavy-ion collisions (HIC). Because the heavy quark production time is smaller or comparable to the formation time of a quark-gluon plasma heavy quarks experience the full collision history. Thus, heavy-flavour observables probe both production and propagation through the medium.

Before the start of the LHC, no measurement existed separately for charm and bottom in HIC. The only measurement from RHIC was for inclusive non-photonic electrons coming from both B {\it and} D meson decays. The result in itself was surprising, showing that at high 'p_T' the suppression reaches the same level as the ones of charged hadrons, coming from gluon and light-quark fragmentation. This is against all theoretical believes and calculations of the mass dependence of the energy loss, which predict the heavy-quark products to be less suppressed than those of the light partons. It was clear the importance and need for a separate charm and beauty measurement.

The main objective of the project was the identification of beauty quarks fragmentation products for the first time in HIC and gaining, for the first time, unambiguously, information on the mass dependence of energy loss. The proposed measurement, via tagging b-jets when the 'B' meson decays in the semileptonic decay channel ('\mu+X') and their shape modification, could not be done using the first year data, due to the low luminosity delivered by the LHC machine and hence low available statistics. However, the objective was achieved by measuring beauty via 'J/\psi \rightarrow \mu^+\mu^-' coming from 'B \rightarrow J/\psi+X' decay. The measurement is complementary to the proposed one, since it gives access to quark low-p'_T' region, where the jet reconstruction is not feasible. In parallel, the infrastructure for the full 'b-jet' measurement, to be performed with the second year 'PbPb' data at the LHC was implemented. All the technical details, pertaining to muon reconstruction, identifying events with muons (triggering) as well as implementation of algorithms for tagging the 'b-jet' were addressed.

Using the 2011 PbPb run, with 20 times more data available compared to the first year, a more detailed analysis of the 'B' mesons was possible (more bins in 'p_T' for example). The preliminary results, show that indeed, in central HIC at low-'p_T', beauty quarks energy loss is smaller than that of charm quarks, which is smaller than that of light quarks. However, at higher-'p_T', there seem to be no difference. These are unexpected results, unique for HIC, and they open a new era in the understanding the energy-loss mechanisms and heavy-quark production in the high-density QCD medium created in central HIC. The next step is going to higher-'p_T', with fully reconstructed 'b-jets', where preliminary results show that there is no difference between heavy and light partons.

Another important aspect of any measurement in HIC is the reference. Any effect due to the medium created in HIC is calculated with respect to the same measurement in 'pp' collision, or, with respect to a process that is not expected to be modified by the heavy-ion medium, an {\it in situ} reference. This unmodified {\it in situ} reference were direct photons for previous heavy-ion experiments, alone, or accompanied by a jet, case in which is a reference for any jet measurement. At the LHC, for the first time in HIC, another reference became energetically possible: the 'Z \rightarrow \mu^+\mu^-' boson. The first published paper from the CMS LHC HI, was on the 'Z' production in HIC and the fellow had a leading role in the analysis. This was the first step for the measurement, when the LHC luminosity will allow, of the 'Z+jet' reference for any jet measurement, including the 'b-jet' proposed.

A summary of the work performed during the 2-year long fellowship is given bellow, with emphasis on the main effort and results.

The fellow had a leading role in the first measurement of beauty energy loss in HIC via non-prompt 'J/\psi' identification. The result, first of its kind, is the only available reference used by the theorists at present, for HIC. Established the feasibility and implemented the algorithms for measuring b-tagged jets in heavy-ion environment.

Together with Nicolas Rhone (Ecole polytechnique student) during his 3 months of stage in LLR studied different methods for identifying and reconstructing heavy-quark tagged-jets in heavy-ion collisions. The results, the first within the CMS heavy-ion community, established the feasibility of such measurements in PbPb environment. The fellow also had a leading role in the team to produce the first paper from the CMS HI group, on the first measurement of the Z boson in AA collisions, the first step toward a 'Z+jet' measurement, the reference channel for any jet measurement in HIC. The fellow was one of the coordinators of the internal document on the first muon physics results in heavy-ion collisions.

The fellow was in charge of the HI muon reconstruction software and responsible for the (di)muon triggers for the 2011 PbPb run. She created and implemented new muon trigger paths to achieve a factor of approximately 9 reduction in the High Level Trigger (HLT) output, needed for 2011 run. Developed a hybrid muon reconstruction algorithm to achieve an increase of 10-20\% single muon reconstruction efficiency which translates in an increase of 20-30\% for dimuons; using this approach, all muons detected by the muon stations and lost due to reconstruction inefficiency in the silicon tracker are recovered at high-p'_T'. The same logic and approach was extended and applied to improve the reconstruction of tracks inside jets. The concept can be applied to any object built independent of the silicon tracker (e.g. calorimetric cluster) that can be used to define the region in which the iterative steps to be applied.