Light neutral meson production in heavy ion collisions with ALICE in the era of precision physics at the LHC
NNuclear Physics A 00 (2020) 1–4
NuclearPhysics A / locate / procedia XXVIIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions(Quark Matter 2019)
Light neutral meson production in heavy ion collisions withALICE in the era of precision physics at the LHC
Mike Sas ([email protected]), for the ALICE Collaboration
University of Utrecht & Nikhef, Netherlands
Abstract
The production of light neutral mesons in AA collisions probes the physics of the Quark-Gluon Plasma (QGP), whichis formed in heavy-ion collisions at the LHC. More specifically, the centrality dependent neutral meson spectra in AAcollisions compared to its spectra in minimum-bias pp collisions, scaled with the number of hard collisions, providesinformation on the energy loss of partons traversing the QGP. The measurement allows to test with high precision thepredictions of theoretical model calculations. In addition, the decay of the π and η mesons are the dominant back-grounds for all direct photon measurements. Therefore, pushing the limits of the precision of neutral meson productionis key to learning about the temperature and space-time evolution of the QGP.In the ALICE experiment neutral mesons can be detected via their decay into two photons. The latter can be re-constructed using the two calorimeters EMCal and PHOS or via conversions in the detector material. The excellentmomentum resolution of the conversion photons down to very low p T and the high reconstruction e ffi ciency and trigger-ing capability of calorimeters at high p T , allow us to measure the p T dependent invariant yield of light neutral mesonsover a wide kinematic range.Combining state-of-the-art reconstruction techniques with the high statistics delivered by the LHC in Run 2 gives us theopportunity to enhance the precision of our measurements. In these proceedings, new ALICE run 2 preliminary resultsfor neutral meson production in pp and Pb–Pb collisions at LHC energies are presented. Keywords:
ALICE, neutral mesons, neutral pion, eta meson, nuclear modification
1. Introduction
In the field of heavy-ion physics we are faced with fundamental questions: What are the di ff erent particleproduction mechanisms across di ff erent system sizes? Can we find the onset of the QGP in heavy-ioncollisions? Is there a QGP droplet formed in small collision systems [1]? In proton–proton collisions theparticle production mechanism at high p T ( (cid:38) / c ) is expected to be dominated by the fragmentation ofhigh momentum partons in jet-like structures. In collisions of heavy nuclei such as Pb–Pb , the productionof particles is expected to be dominated by the hadronisation of the QGP for low p T ( (cid:46) / c ), whilemodification of the hadron production at higher p T will also be influenced by parton-QGP interactions. a r X i v : . [ nu c l - e x ] A ug / Nuclear Physics A 00 (2020) 1–4
Studying the particle production mechanisms is thus key to understand the physics governing both smalland large systems.Identified hadron spectra are a good probe to study both the production mechanisms in pp collisions [2],as well as the parton energy loss in high-energy heavy-ion collisions. Among these identified hadrons arethe neutral pion ( π ) and η meson, which are abundant and have large branching ratios into two photons,making them suitable probes to study details of particle production. In addition, measuring neutral mesonsgrants the possibility of extracting the direct photon signal that is seen as an excess yield above the photonsfrom hadronic decays, and is probing e.g. the temperature of the QGP [3].We present the invariant yield of the π and η mesons in pp at √ s = √ s NN = .
02 TeV, as measured with the ALICE detector.
2. Method
The photons are reconstructed with the photon conversion method (PCM) and the calorimeters PHOS andEMCal. With PCM, the photons that convert in the detector material to an electron-positron pair are recon-structed using the ITS and TPC detectors. With a conversion probability of 8%, this method is limited instatistical precision but it profits greatly from the high momentum resolution of the ALICE central barrel.The calorimeters are situated outside the inner detectors and are able to measure the photons by absorbingtheir full energy in their calorimeter towers. The PHOS calorimeter consists of lead tungstate crystals witha cell size of 2.2 cm × × The neutral mesons are reconstructed as follows. First, the photons are reconstructed and the invariant massof every photon pair is calculated. Second, the meson raw yield is obtained by integrating the invariant massdistributions around its corresponding mass, after subtracting the combinatorial and remaining background.Third, the raw yield is corrected for e ffi ciency, acceptance, and feed-down from secondaries. Analyses wereperformed for various combinations of photons (PCM-PCM, PCM-PHOS, etc.) and then combined to asingle result. Combining these di ff erent photon reconstruction techniques allows us to reduce the statisticaland systematic uncertainties of the neutral meson spectra.
3. Results
Figure 1 shows the invariant yield of neutral pions (left) and η mesons (right) as function of transversemomentum in pp collisions at √ s = √ s NN = η/π ratio for pp collisions at √ s = √ s NN = .
02 TeV (right). Theseresults are compared to several model calculations. In pp collisions, m T scaling [4] does not describe thedata at low p T , while PYTHIA [5] only approximately reproduces the ratio. In Pb–Pb collisions, a hint of anenhancement of η/π ratio around 3 GeV / c with respect to HIJING and theoretical calculations can be seen,which could be attributed to radial flow e ff ects. Figure 3 shows the nuclear modification factor R AA = ( dN AA / d p T ) / ( < T AA > d σ pp / d p T ) of neutral pions(left) and η mesons (right) in Pb–Pb collisions at √ s NN = R AA issimilar for both mesons, despite the di ff erent quark content. Furthermore, it is the first time that the η mesonis measured in such a large range of p T and to such high precision, which is crucial in understanding thebackground present in direct photon and di-lepton measurements. Nuclear Physics A 00 (2020) 1–4 ALI-PREL-337526 ALI-PREL-337537
Fig. 1. The production of π ’s (left) and η (right) in pp and Pb–Pb collisions at √ s = ALI-PREL-337475 ALI-PREL-337564
Fig. 2. The η/π ratio for pp collisions at √ s = √ s NN = .
02 TeV (right). / Nuclear Physics A 00 (2020) 1–4
ALI-PREL-337593 ALI-PREL-337601
Fig. 3. The nuclear modification factor R AA of neutral pions (left) and η mesons (right) in Pb–Pb collisions at √ s NN =
4. Conclusion
The neutral meson invariant yield in pp and Pb–Pb collisions has been measured with the ALICE detector,utilising all the available photon reconstruction methods and combining the neutral meson measurements,improving the precision and extending the p T range.The η/π ratio was measured in pp and AA collisions. In pp it shows the universal behavior, independentof collision energy. At high p T the ratio is by construction reproduced with m T scaling, and PYTHIAonly approximately reproduces the p T dependence. In AA collisions the modifications to the η/π ratio arecharacteristic for the presence of radial flow. In addition, the nuclear modification factors R AA of the π and η mesons are calculated for Pb–Pb collisions, using the respective invariant yield in pp collisions, and arefound to be similar despite the di ff erent quark content. References [1] W. Busza, K. Rajagopal and W. Schee,
Heavy Ion Collisions: The Big Picture, and the Big Questions , Annual Review of Nuclearand Particle Science (2018) [ arXiv:1802.04801 [hep-ph] ].[2] M. Stratmann, R. Sassot and Z. P., Inclusive Hadron Production in the CERN-LHC Era , Phys.Rev.D (2010)[ arXiv:1008.0540v1 [hep-ph] ].[3] C. Shen, U. W. Heinz, J.-F. Paquet and C. Gale, Thermal photons as a quark-gluon plasma thermometer reexamined , Phys. Rev.
C89 (2014) 044910 [ arXiv:1308.2440 [nucl-th] ].[4] M. Bourquin and G. J.,
A Simple Phenomenological Description of HadronProduction , Nucl. Phys.
B114 (1976) 334.[5] T. Sjostrand et al.,
An Introduction to PYTHIA 8.2 , arXiv:1410.3012 [hep-ph]arXiv:1410.3012 [hep-ph]