Light neutral meson production in the era of precision physics at the LHC
LLight neutral meson production in the era ofprecision physics at the LHC
Mike Sas ∗† University of Utrecht & Nikhef, NetherlandsE-mail: [email protected]
The production of light neutral mesons in different collision systems is interesting for a variety ofreasons: In nucleus-nucleus (AA) collisions the measurements provide important information onthe energy loss of partons traversing the Quark-Gluon Plasma (QGP) which is formed in heavy-ion collisions at the LHC. In proton–proton (pp) collisions, neutral mesons allow us to test withhigh precision the predictions of perturbative QCD and other model calculations, and also serveas a reference for pA and AA collisions. In pA collisions, cold nuclear matter effects are studied.In the ALICE experiment, which is dedicated to the study of the QGP, neutral mesons can bedetected via their decay to two photons. The latter can be reconstructed using the two calorimetersEMCal and PHOS or via conversions in the detector material.Combining state-of-the-art reconstruction techniques with the large data sample delivered by theLHC in Run 2 gives us the opportunity to enhance the precision of our measurements. In theseproceedings, an overview of neutral meson production in pp, p–Pb and Pb–Pb collisions at LHCenergies, as measured with the ALICE detector is presented.
European Physical Society Conference on High Energy Physics - EPS-HEP2019 -10-17 July, 2019Ghent, Belgium ∗ Speaker. † for the ALICE Collaboration c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] A ug ight neutral meson production in the era of precision physics at the LHC Mike Sas
1. Introduction
In the field of heavy-ion physics we are faced with interesting questions such as: what are the dif-ferent particle production mechanisms across different system sizes? can we find the onset of theQGP in heavy-ion collisions? and is there a QGP droplet formed in small collision systems? [1]In proton–proton collisions the particle production mechanism should be dominated by the frag-mentation of high momentum partons into jet-like structures. In collisions of heavy nuclei such asPb–Pb the production of particles is expected to be dominated by the hadronisation of the QGP.Studying the particle production mechanisms is thus key to understand the physics governing bothsmall and large systems.Identified hadron spectra are a good probe to study both the production mechanisms in pp colli-sions [2], as well as the parton energy loss in heavy-ion collisions. Among these identified hadronsare the neutral pion ( π ) and η meson, which are very abundant and have large branching ratiosinto two photons, making them suitable probes to study particle production. In addition, measuringneutral mesons grants the possibility of extracting the direct photon yield which come as an excessyield above the photons from hadronic decays, which probes the temperature of the QGP [3].We present the invariant yield of neutral mesons in pp at √ s = √ s NN = .
02 TeV, with the ALICE detector. The measurements are compared to eventgenerators and model calculations.
2. Method
The neutral mesons are measured using the ALICE detector [4] via the two photon decay channel.The photons are reconstructing using the photon conversion method (PCM), and with the calorime-ters PHOS and EMCal. With PCM, the photons that convert in the detector material to an electronand positron pair are reconstructed using the ITS and TPC detectors. With a conversion probabilityof 8%, this method is limited in statistical precision but it profits greatly from the high momentumresolution of the ALICE central barrel.The calorimeters are situated outside the inner detectors and are able to measure the photons byabsorbing their full energy in their calorimeter towers. The PHOS calorimeter consists of leadtungstate crystals with a cell size of 2.2 cm × × The neutral mesons are reconstructed as follows. First, the photons are reconstructed and the in-variant mass of every photon pair is calculated. The neutral meson yield is situated on top of acombinatorial background. Second, the meson raw yield is obtained by integrating the invariant1 ight neutral meson production in the era of precision physics at the LHC
Mike Sas mass distributions around their corresponding mass, after subtracting the combinatorial and remain-ing background. Third, the raw yield is corrected for efficiency, acceptance, and feed-down fromsecondaries. At last, the different reconstruction methods are combined into a single measurement. ) c - ( pb G e V p d σ d E = 5 TeV s pp, ALICE preliminary γγ → π Datanorm. unc. 2.4%TCM fitLevy-Tsallis fitPYTHIA 8.2, Monash 2013 T p = 0.5 µ T p = µ T p = 2 µ NLO, PDF:CT10 - FF:DSS14 T C M f i t N L O , D a t a ) c (GeV/ T p T C M f i t P y t h i a , D a t a ALI-PREL-146126 ) c (GeV/ T p / G e V ) c ( y d T p d T p N d ev . N π − − − − − − − − − − = 5.02 TeV NN s p Pb, NSD, ALICE π EPOS3VISHNUDPMJetHIJING γ CGC MVNLO: EPPS16, DSS14NLO: nCTEQ, DSS14Tsallis fit × η EPOS3VISHNUDPMJetHIJINGNLO: nCTEQ, AESSSTsallis fit
ALI−PUB−143300
Figure 1:
The production of neutral pions in pp collisions at √ s = √ s NN = .
02 TeV (right) [5].
3. Results
Figure 1 (left) shows the invariant yield of neutral pions as function of transverse momentum in ppcollisions at √ s = . < p T <
30 GeV/ c , and is compared to the PYTHIA event generator [6] and NLO pQCDpredictions, which both over-predict the π production. This spectrum is used also as a referencefor the yield of neutral pions in p–Pb and Pb–Pb collisions. In the future, more differential studieswould be able to disentangle if this difference comes from either jet production or the underlyingevent. 2 ight neutral meson production in the era of precision physics at the LHC Mike Sas
In p–Pb collisions the spectra of π and η mesons are measured for minimum-bias (MB) collisionsusing the PCM and calorimeters PHOS and EMCal [5]. The invariant yield of neutral pions asfunction of transverse momentum in p–Pb collisions at √ s NN = .
02 TeV is shown in Fig. 1 (right).It covers 0 . < p T <
20 GeV/ c , and is compared to various event simulators [7] and theoreticalpredictions, which are mostly consistent with the measurement due to the relatively large uncer-tainties. Figure 2 shows the invariant yield of π (left) and η meson(right) for different collisioncentralities, where the V0A detector is used as centrality estimation. A clear ordering is observed;more central p–Pb collisions produce more neutral mesons. To study the cold nuclear matter ef-fects the Q pA = dN pA / d p T < T pA > d σ pp / d p T is calculated. The result is shown in Fig. 3, with π (left) and η meson (right). Within uncertainties both the mesons show a similar centrality dependence. Periph-eral p–Pb collisions shows a rather flat Q pA as function of p T , while central p–Pb shows a clearenhancement at p T = c . ) c (GeV/ T p − × / G e V ) c ( y d T p d T p N d ev N π − − − − − − − − − − NN s V0A p-Pb, ALICE preliminary γγ → π ALI−PREL−307333 ) c (GeV/ T p − × / G e V ) c ( y d T p d T p N d ev N π − − − − − − − − − −
101 = 5.02 TeV NN s V0A p-Pb, ALICE preliminary γγ → η ALI−PREL−307354
Figure 2:
The production of neutral pions (left) and η mesons (right), for different collision centralities, inp–Pb collisions at √ s NN = .
02 TeV.
The invariant yield of neutral pions as function of transverse momentum in Pb–Pb collisions at √ s NN = .
02 TeV is shown in Figure 3.3 (left), and is measured with the PHOS calorimeter. Itcovers 0 . < p T <
30 GeV/ c for central and semi-central collisions, and 0 . < p T <
20 GeV/ c for peripheral collisions. Furthermore, the invariant yield is compared to the hydrodynamic SHMmodel predictions [8], which describes the yield at lower p T relatively well, but under-predict theproduction at higher p T . Figure 3.3 (right) shows the nuclear modification R AA = dN AA / d p T < T AA > d σ pp / d p T as function of p T . A clear suppression is observed, where central Pb–Pb collisions show moresuppression than peripheral ones. 3 ight neutral meson production in the era of precision physics at the LHC Mike Sas ) c (GeV/ T p p A Q ALICE preliminary = 5.02 TeV NN s V0A, p-Pb, γγ → π ALI−PREL−307468 ) c (GeV/ T p p A Q ALICE preliminary = 5.02 TeV NN s V0A, p-Pb, γγ → η ALI−PREL−307483
Figure 3:
The Q pA of neutral pions (left) and η mesons (right) in p–Pb collisions at √ s NN = .
02 TeV. − × ) c (GeV/ T p − − − − − − − − − −
10 110 - ) c ( G e V / y d T p d T p π N d ev N π = 5.02 TeV NN s Pb-Pb at × × × × NEQ : 0-10 % × EQ : 0-10 % × NEQ : 20-40 % × EQ : 20-40 % × NEQ : 60-80 % × EQ : 60-80 %
ALICE Preliminary
ALI-PREL-148472 ) c (GeV/ T p AA R = 5.02 TeV NN s Pb-Pb at : 0-10 % π : 20-40 % π : 60-80 % π ALICE Preliminary
ALI-PREL-148480
Figure 4:
The invariant yield of neutral pions (left) and the nuclear modification factor R AA of neutral pions(right) in Pb–Pb collisions at √ s NN = .
02 TeV. ight neutral meson production in the era of precision physics at the LHC Mike Sas
4. Conclusion
The neutral meson invariant yield in pp, p–Pb , and Pb–Pb collisions has been measured with theALICE detector. In all these collision systems the measurements are compared to event simula-tions and model calculations, enabling us to learn about particle production mechanisms. For p–Pband Pb–Pb collisions, the invariant yield is calculated for different collision centralities, therebyprobing possible medium effects. In addition, using the neutral meson invariant yield in pp colli-sions, the nuclear modification factors Q pA and R AA are calculated for p–Pb and Pb–Pb collisions,respectively.For future measurements, it is planned to utilise all the available photon reconstruction methods andcombine all neutral meson measurements into a single precise spectrum, increasing the precisionas well as the reach in p T . Furthermore, more differential measurements will help to disentanglethe different particle production mechanisms in both pp, p–Pb , and Pb–Pb collisions. References [1] W. Busza, K. Rajagopal and W. Schee,
Heavy Ion Collisions: The Big Picture, and the Big Questions , Annual Review of Nuclear and Particle Science (2018) [ ].[2] M. Stratmann, R. Sassot and Z. P., Inclusive Hadron Production in the CERN-LHC Era , Phys.Rev.D (2010) [ ].[3] C. Shen, U. W. Heinz, J.-F. Paquet and C. Gale, Thermal photons as a quark-gluon plasmathermometer reexamined , Phys. Rev.
C89 (2014) 044910 [ ].[4] ALICE collaboration, K. Aamodt et al.,
The ALICE experiment at the CERN LHC , JINST (2008)S08002.[5] ALICE collaboration, S. Acharya et al., Neutral pion and η production in p-Pb collisions at √ s NN =5.02 TeV , Eur. Phys. J. C (2018) 624 [ ].[6] T. Sjostrand et al., An Introduction to PYTHIA 8.2 , .[7] S. Roesler, R. Engel and J. Ranft, The Monte Carlo event generator DPMJET-III , in
Advanced MonteCarlo for radiation physics, particle transport simulation and applications. Proceedings, Conference,MC2000, Lisbon, Portugal, October 23-26, 2000 , pp. 1033–1038, 2000, hep-ph/0012252 , DOI.[8] V. Begun and W. Florkowski,
Bose-Einstein condensation of pions in heavy-ion collisions at the CERNLarge Hadron Collider (LHC) energies , Phys. Rev. C (2015) [ ].].