CORDIS - EU research results

The charm and the beauty of LHC

Final Report Summary - HFQGP (The charm and the beauty of LHC)

Executive Summary
June 2012- September 2012: co-convener of pA task force
prepare the proton-lead run, organize and steer the first analyses of the data.
3 articles prepared with the pilot run (Sep. 2012):
- Phys. Rev. Lett. 110, 032301 (2013) “Pseudorapidity density of charged particles in p-Pb collisions at sqrt(s) = 5.02 TeV”
- Phys. Rev. Lett. 110, 082302 (2013) “Transverse Momentum Distribution and Nuclear Modification Factor of Charged Particles in p-Pb Collisions at sqrt(s) = 5.02 TeV”
- Phys. Lett. B 719 (2013), “Long-range angular correlations on the near and away side in p--Pb collisions at sqrt(s) = 5.02 TeV”
September 2012 - March 2013: maternity leave
March 2013 - September 2013: centrality determination for the pA task force
determine centrality in the p-Pb data, analysis of pseudorapidity density and nuclear modification factor as a function of centrality
1 presentation at International Conference on the Initial Stages in High Energy Nuclear Collisions, Illa da Toxa (Galicia-Spain), September 8-14 2013
3 articles
- “Participants and Spectators of the heavy-ion fireball” (CERN Courier, May 2013)
- “ALICE Measurements in p-Pb Collisions: Charged Particle Multiplicity, Centrality Determination and implications for Binary Scaling” (to be published on Nuclear Physics A)
- “Centrality dependence of particle production in p-Pb collisions at sqrt(s) = 5.02 TeV measured by ALICE at the LHC and implications for particle production mechanism”, in preparation

In June 2012 I started working with a Marie Curie fellowship at INFN-Padova, on a project on heavy flavor correlations with ALICE. The idea is to gain a more coherent picture of the fundamental properties of nuclear matter analyzing the suppression of hard partons due to energy loss in the medium and its hierarchy with parton color-charge and mass, as well as the heavy-flavor hadro-chemistry which allows to assess the initial temperature and the degree of thermal equilibration of the partons.

In the heavy-ion run at the beginning of 2013, just before the LHC shutdown, the ALICE experiment had the opportunity to collect for the first time a data sample with the asymmetric p-Pb collisions, and a short pilot run was scheduled in September 2012. The p-Pb data is crucial for understanding the complexity of the Pb-Pb interactions in many levels and is a necessary complement for the baseline p-p data. In fact, the data from the p-Pb collisions will represent an ultimate benchmark for the already published results from Pb-Pb collisions. It would allow to decouple the cold nuclear matter effects and thus will shed light to our quest for the quark-gluon plasma. However, the importance of p-Pb collisions has soon been recognized not only as a reference benchmark for the understanding of Pb-Pb data. Indeed several measurements of particle production in the low and intermediate momentum region clearly show that p-Pb collisions can not be explained by an incoherent superposition of p-p collisions but rather indicate the presence of collective effects.

In order to cope with this run and to organize and steer the first analysis of the data, the ALICE management decided to create a special task force, and I was appointed as one of the two coordinators. The experience from the past proved the efficiency of such an initiative - both the first proton--proton physics task force in 2009 and first lead--lead physics task force in 2011 brought many high-quality results in just few weeks after the data taking. Our collaboration gained a great experience since the first data taking in 2009. Many analysis techniques and tools were established and widely used over the years. Nevertheless, the first proton--lead run represented a challenge in many aspects. Special care was taken in order to define and implement proper trigger, event and centrality selections, careful preparation of both sub-detector hardware configuration as well as the software and analysis tools. All these was discussed and implemented within the task force. The task force had also the mandate to identify and steer the early analyses of the data. The range of the topics which we have covered so far includes not only the basic measurements, such as multiplicity density and the spectrum of the charged particles, but also more advanced analyses, such as the charm and J/psi spectra and charged-particle correlations.

As convener of the Event Characterization working group, I dedicated particular attention to the centrality determination. Since nuclei are extended objects, the volume of the interacting region depends on the impact parameter of the collision, defined as the distance between the centers of the two colliding nuclei in a plane transverse to the beam axis. It is customary in the field of heavy-ion physics to introduce the concept of the centrality of the collision, which is directly related to the impact parameter. It is usually defined as a percentage of the total nuclear interaction cross section by integrating the impact parameter distribution and in ALICE, the centrality is expressed as the percentile of the hadronic cross section corresponding to a particle multiplicity above a given threshold. Therefore the centrality provides a geometrical scale to study the underlying collision dynamics and compare to “baseline” data from elementary proton collisions. While in Pb-Pb collisions, the centrality can be measured from the charged particle multiplicity produced around mid-rapidity, in p-Pb collisions, the asymmetric nature of the collision system as well as the lower particle multiplicities generate a bias in the determination of the centrality, which depends on the kinematic range used for the event characterization.
In proton-nucleus collisions traditionally the centrality has been determined from the number of low momentum particles coming from the nucleus fragmenting as knock-out and evaporation nucleons. From previous proton-nucleus experimental results and from simulations, it is expected that the number of charged particles produced in a rapidity region corresponding to the Pb-going direction should be roughly proportional to the number of participating nucleons in the Pb nucleus. However, an “underlying” event correlation with specific physics processes has been observed. This correlation introduces a bias which is related to how the collisions are categorized into different centrality classes using the different detectors. We have studied this bias using various detectors, at mid-rapidity and well as those which are well separated in rapidity from the central arm and from the muon arm acceptance, eg the zero-degree calorimeter.

The centrality measurement in p-Pb has therefore developed from a crucial tool to establish a baseline for geometry-dependent measurements, to an area which provides interesting effects to study but also new challenges to solve, both theoretically and experimentally. This has been widely recognized in the heavy-ion community, so that various major conferences dedicated special session to this topic (Strangeness in Quark Matter 2013, IS 2013, Hard Probes 2013, and a one-day workshop at CERN). We are currently preparing a publication to present the centrality dependence of particle production in p-Pb collisions at a center-of-mass energy of 5.02 TeV with ALICE, with a special emphasis on the centrality determination and discussion on the bias arising in the selection of the event sample and its impact on the physics results.