Direct photon and light neutral meson production in the era of precision physics at the LHC
DDirect photon and light neutral mesonproduction in the era of precision physicsat the LHC
Meike Charlotte Danisch on behalf of the ALICE Collaboration
Physikalisches Institut, Universit¨at Heidelberg, Heidelberg, Germany [email protected]
Abstract.
In these proceedings we present the latest results from AL-ICE on direct photon and light neutral meson production in p–Pb andPb–Pb collisions. The direct photon excess ratio R γ in different chargedparticle multiplicity classes of p–Pb collisions at √ s NN = 5.02 TeV isshown. In addition, we present the direct photon elliptic flow coefficient v in central and semicentral events of Pb–Pb collisions at √ s NN = 2.76 TeV.An outlook on ongoing and future measurements is given. Keywords: direct photons, electromagnetic probes, heavy-ion collisions,quark-gluon plasma
Direct photons, being defined as photons not originating from hadron decays,are a valuable tool to investigate the space-time evolution of the medium createdin heavy-ion collisions. The measured elliptic flow coefficient of photons reflectsthe momentum anisotropy of the source, convoluted with the photon emissionrate, integrated over time. Direct photons can also contribute to the effort oftesting bulk effects in high multiplicity collisions of small systems.There are different sources of photons in heavy-ion collisions. Prompt photonsare created in initial hard scatterings. In addition, we expect thermal photonsfrom the QGP and the hadron gas phases, which are sensitive to the mediumtemperature [ ]. There will also be photons from hadron decays, which are thevast majority of all photons. The latter originate mostly from decays of theneutral mesons π and η into two photons. Therefore we need to measure theirspectra precisely in order to be able to obtain the excess of direct photons overthis decay photon backgound. Due to the different shapes of transverse momen-tum ( p T ) distributions of prompt and thermal photons (power-law and close-to-exponential respectively) they can be distinguisted on a statistical basis. Promptphotons dominate direct photons at high p T (cid:38) c and thermal photons atlow p T (cid:46) c . Therefore, a direct photon excess at low p T can be inter-preted as thermal photon signal. So far it was assumed that thermal photons arerelevant only in AA collisions but after collective effects have been observed alsoin high-multiplicity pp and p–A collisions (see for example [ ]) we can questionthis assumption. a r X i v : . [ h e p - e x ] M a y Meike Charlotte Danisch on behalf of the ALICE Collaboration R γ in p–Pb collisions at √ s NN = 5.02 TeV Observation of a direct photon signal implies that the ratio of inclusive ( γ inc )over decay photons ( γ dec ) is larger than one. In order to eliminate parts ofthe uncertainty, we define the double ratio as R γ = γ inc γ dec ≡ γ inc π , p / γ dec π , p , where π , p stands for the parametrisation of the measured π spectrum. In the anal-ysis presented here, inclusive photons are measured with three different tech-niques: PCM, EMCal and PHOS [ ]. The EMCal is a sampling calorimetercomposed of alternating layers of lead and plastic scintillators. It is placed ata radius of R = 4 . ∆ϕ = 100 ◦ inazimuthal angle and | η | < . × . et al. Shen = 5.02 TeV NN s = 5.02 TeV NN s γ R = 5.02 TeV NN s = 5.02 TeV NN s ) c (GeV/ T p = 5.02 TeV NN s ALI−PREL−306631
Fig. 1.
Direct photon R γ , labeled by thepercentile of the multiplicity distribution. The Photon Spectrometer PHOS is ahomogeneous calorimeter made fromPbWO crystals. The acceptance of ∆ϕ = 60 ◦ and | η | < .
125 is smallerthan the one of EMCal but PHOS hasa finer granularity with a cell size ofabout 2 . × . at R = 4 . R < . | η | < . | η | < . p T .In addition to the inclusive photons, wemeasure π and η mesons via their de-cay to two photons, performing an in-variant mass analysis of photon pairs.For this purpose, the photon samplesare taken from the same methods as mentioned above, as well as from one ad-ditional hybrid method (PCM-EMC), where one photon is detected with PCMand one with EMCal. The decay photon spectra are then obtained using a MonteCarlo simulation including all relevant hadron decays, based on the measuredneutral meson spectra and m T scaling. For each of the four methods, the R γ iscalculated. In case of the PCM-EMC method, inclusive photons are taken fromPCM. After checking that the results from all methods are consistent, they werecombined. The analysis was performed in event multiplicity classes of data fromp–Pb collisions recorded in 2013. The results are shown in Fig. 1. The dottedblue, red and purple lines, starting at p T = 3 GeV/ c show results of different irect photons and neutral mesons measured with ALICE 3 pQCD calculations. They are all well compatible with the measured points athigh p T . For 0-20% and 0-100% samples a green band is drawn in addition,which shows a prediction from a hydrodynamic model [ ] including thermalphoton emission at low p T . The current data are not sensitive to the predictedvery small thermal photon signal. v in √ s NN = 2.76 TeV Pb–Pb collisions The inclusive photon v is obtained using the scalar product method [ ]. Refer-ence particles are measured in the V0 scintillation detectors placed in a differentpseudorapidity region (2 . < η < . − . < η < − .
7) [ ]. Results fromthe independent methods PCM and PHOS are combined after they were foundto be consistent. In the analysis presented here, Pb–Pb collisions were analyzedin two centrality classes, 0-20% and 20-40%. For PCM, 13 . × events wereavailable and 18 . × for PHOS. The direct photon v γ, dir2 can be calculated ) c (GeV/ T p γ v NN s , dir γ v , ALICE simulation , dec γ v et al. , hydro, Paquet , dir γ v et al. , hydro, Chatterjee , dir γ v et al. , PHSD, Linnyk , dir γ v Boxes indicate total uncertainties
ALI−PUB−158400 ) c (GeV/ T p γ v NN s
20 40% Pb Pb, , ALICE , dir γ v , ALICE simulation , dec γ v et al. , hydro, Paquet , dir γ v et al. , hydro, Chatterjee , dir γ v et al. , PHSD, Linnyk , dir γ v Boxes indicate total uncertainties
ALI−PUB−158404
Fig. 2.
Direct and decay photon v in central (left) and semicentral (right) collisionscompared with hydrodynamic and cascade model predictions [ ]. by subtracting the v γ, dec2 of decay photons from the v γ, inc2 of inclusive photonsusing the following formula: v γ, dir2 = v γ, inc2 R γ − v γ, dec2 R γ − , where the R γ measured withPCM and PHOS [ ] was used. v γ, dec2 is obtained from a MC simulation includingall relevant hadron decays. The simulation is based on hadron spectra and v measurements and uses KE T scaling [ ] when necessary. The results for decayphotons and direct photons are shown in Fig. 2. At low p T , where thermal pho-tons should dominate, we measure a positive v γ, dir2 which is close to v γ, dec2 . Thisindicates an already developed momentum anisotropy of the medium at directphoton production times. At higher p T , where the prompt photon contribution Meike Charlotte Danisch on behalf of the ALICE Collaboration increases, the v γ, dir2 decreases. In more peripheral events, the thermal photon v , and therefore also the direct photon v at low p T , are expected to be largerthan in central events because of the more pronounced initial spatial anisotropyof the medium. Because the direct photon signal is smaller in more peripheralevents [ ] the uncertainties are larger in this case and therefore we cannot yetmake a conclusive statement on how the direct photon v changes with central-ity. Calculations from different theoretical models are illustrated by the dashedlines. They tend to underestimate the v with respect to the measured values. In summary, ALICE has measured the direct photon elliptic flow coefficient v inPb–Pb collisions at √ s NN = 2.76 TeV. It was found to be consistent with the cur-rent knowledge of the space-time-evolution and photon emission rates but smalleruncertainties will be needed to confirm or exclude given model predictions. Thedirect photon R γ in high multiplicity p–Pb collisions at √ s NN = 5.02 TeV is notyet sensitive to the predicted very small thermal photon signal. − × ) c (GeV/ T p − − − − − − − − − −
10 110 - ) c ( G e V / y d T p d T p N d ev N π = 5.02 TeV NN s Pb-Pb at × : 0-10 % π × : 10-20 % π × : 20-40 % π × : 40-60 % π × : 60-80 % π TCM fits to Pb-Pb
ALICE Preliminary = 5.02 TeV s pp at π TCM fit to pp
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Fig. 3.