Isolated photon-hadron correlations in proton-proton collisions at {\sqrt(s)} = 7 TeV with the ALICE experiment
aa r X i v : . [ h e p - e x ] N ov Isolated photon-hadron correlations in proton-proton collisions at √ s = Nicolas Arbor (for the ALICE Collaboration)
LPSC, Universit´e Joseph Fourier Grenoble 1, CNRS / IN2P3, Institut Polytechnique de Grenoble, 53 rue des Martyrs,Grenoble, France
Abstract
At high p T , direct photons produced in Compton and annihilation QCD leading order processesare associated to a jet in the opposite direction. Such processes are tagged experimentally byidentifying leading isolated photons and their correlated associated hadrons in the opposite az-imuthal direction. The jet fragmentation can be estimated from the hadrons and the photon viathe imbalance parameter x E = − ~ p hT .~ p γ T / | ~ p γ T | . We present the results extracted from gamma-hadron correlations measured by the ALICE experiment in pp collisions at √ s = x E distributions of isolatedphoton-hadron and isolated π -hadron correlations.High- p T partons and photons produced in hard processes are often used as probes to study thestrongly interacting medium created in heavy-ion collisions. As the production of these energeticparticles occurs slightly before the creation of the medium, high- p T partons can traverse themedium before they hadronize in a jet of hadrons. The modification of the parton fragmentationresulting from the energy loss (via collisional or radiative mechanisms) along the medium pathcan be used to infer medium properties. At the Large Hadron Collider (LHC), medium e ff ectswere first studied using back-to-back dijets or dihadron correlations [1][2]. However correlatingtwo probes that both interact with the medium induces a surface bias in which the sampled hardscatterings are likely to occur at, and to be oriented tangential to, the surface of the medium [3].In contrast to partons, direct photons are not colored objects and hence escape almost unmodifiedfrom the medium. At leading order (LO), their production in pp collisions is dominated by qg Compton scattering and q ¯ q annihilation. Photons emitted from such processes may be used toestimate the initial transverse momentum of the recoil parton. These tomographic studies of themedium using parton energy loss in heavy-ion collisions requires first detailed measurement ofdirect photon-hadron correlations in pp collisions.The measurement presented here consists in selecting direct photons in coincidence withcharged hadrons to access the away-side parton fragmentation. The fragmentation functionshould be given to a good approximation by the x E distribution where, x E = − ~ p hT .~ p γ T | ~ p γ T | = − | p hT | cos ∆Φ | p γ T | ,with ∆Φ corresponding to the azimuthal angle between photons and hadrons. Figure 1 illustratesthe x E distribution computed from a Diphox γ -jet production [4] and compared with the DSS Preprint submitted to Nuclear Physics A October 29, 2018 uark and gluon fragmentation functions [5]. It shows that even if the x E variable is not an exactmeasurement of the fragmentation function because of higher-order e ff ects, the x E distributionfollows the fragmentation behaviour over a large range (mainly the quark fragmentation due tothe dominating contribution of the Compton scattering cross-section). E x E d x d N t r i g N -3 -2 -1 > 3 GeV/c hT = 20 GeV/c, p trigT p -jet) γ Diphox ( E x ) gluon D(z,Q ) quark D(z,Q = 7 TeVspp, T = p µ DSS NLO, λ E n t r i es < 0.5 GeV/c thresT Isolation : R = 0.4, p < 25 GeV/c T
16 < E = 7 TeVspp Combined fitBackground
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Figure 1: Left: γ -hadron x E distribution compared to DSS quark and gluon fragmentation functions (arbitrary scaled) ;Right: Isolated clusters shower shape long axis distribution fitted with a two-component binned likelihood. This analysis uses approximately 10 million events from pp collisions at √ s = L int ≈ nb − ) recorded by the ALICE detector in 2011 [6]. Photons are detected in the EMCal elec-tromagnetic calorimeter which is a Pb-Scintillator sampling electromagnetic calorimeter thatcovers ∆Φ = ◦ in the azimuthal angle and | η | < . ∆ η × ∆Φ = × / cthreshold, to achieve the measurement of high- p T photons up to 25 GeV / c with enough rate.Photon candidates are reconstructed from energy clusters deposited in the p T range from 8 to 25GeV / c. On the opposite side, charged hadrons are detected with the ALICE tracking system inthe central barrel which consists of a silicon detector (Inner Tracking System) and a time projec-tion chamber (TPC). The ITS consists of six layers equipped with silicon pixel detectors (SPD),silicon drift detectors (SDD) and silicon strip detectors (SSD). The two innermost layers covera pseudo-rapidity range of | η | < | η | < .
4, respectively. The TPC is a cylindrical driftdetector with an acceptance of | η | < . ff ects, such as tracking e ffi ciency and energy resolution, to avoid biasing the fragmentationmeasurement both on the trigger (photon) and on the away (hadrons) sides.The single clusters distribution is dominated by a large background coming from electromag-netic decays of neutral mesons (mostly π ). This background can be reduced by about 80% byimposing isolation criteria on the photon candidates, based on the fact that neutral mesons areproduced inside a jet and are surrounded by hadronic activity. In this analysis, isolation criteriaare based on the absence of particles with a p T > . / c in a cone of radius R = p ∆ η + ∆Φ = p T > / c. Background contributions coming from isolated meson decays that carry a large fractionof the parent parton need to be subtracted. The shape of the electromagnetic shower producedinside the calorimeter may be used both to reject decay photons and to estimate the remainingcontamination in the isolated particles sample. Photons interacting with the calorimeter producean electromagnetic shower that deposits energy in several cells. Those cells are grouped forminga cluster using a simple algorithm that aggregates all cells with common edges if they have morethan 50 MeV. The energy redistribution in the cells has a shape that can be used to distinguishsingle photons from other particles and in particular from π decay photons. The cluster shape isquantified using the long axis of the ellipsoidal parametrization of the energy deposit matrix : λ = . × ( d ηη + d φφ ) + q . × ( d ηη − d φφ ) + d ηφ ,with d ii corresponding to energy distribution in the direction i . As shown in Fig.1 (right), thelong axis length, λ , is fitted by the signal and background distributions using a two-componentbinned likelihood. The signal component is obtained from γ -jet events generated with PYTHIAand propagated through the detectors with GEANT3. The background component shape (mainly π ) is extracted from data by taking the shower shape distribution of events which have failed theisolation criteria. Typical purity values extracted from fit results in the 8 to 25 GeV / c p T rangego from about 5 to 70%. E x0 0.2 0.4 0.6 0.8 1 E d x d N t r i g N -4 -3 -2 -1 π Isolated < 12 GeV/c trigT -1 < 16 GeV/c (x10 trigT
12 GeV/c < p ) -2 < 25 GeV/c (x10 trigT
16 GeV/c < p = 7 TeVspp,
ALI−PREL−34317ALI−PREL−34317ALI−PREL−34317 (GeV/c) trigT p8 10 12 14 16 18 20 22 24 26 s l ope E N ega t i v e x = 7TeVspp, = 0.5 GeV/c) thresT (R=0.4, p π Isolated DSS NL0 quark ± π DSS NL0 gluon ± π range [0.2-0.8] E x
Figure 2: Left: x E distributions of π -hadron correlations for 3 p T bins ; Right: slopes extracted from exponential fit ofisolated π -hadron correlations (5 p T bins) and compared to DSS quark-gluons fragmentation functions. The x E distribution is obtained by selecting hadrons within 2 π/ < ∆Φ < π/
3. To subtractthe contamination coming from neutral meson decays, we have used x E distribution of isolated π -hadron correlations and scaled this distribution with respect to the purity estimate describedabove. The isolated π -hadron correlations results are shown for three p T bins of the triggered π in Fig.2 (left). Isolated photon-hadron correlations are then corrected for the soft backgroundcoming from correlations between photon and hadrons not originating from the hard scatteringsuch as initial / final-state-radiations or multiple parton interactions. This background has beenestimated using two di ff erent underlying event regions π/ < ∆Φ < π/ π/ < ∆Φ < π/
3. The resulting x E distribution of isolated photon-hadron correlations has been extractedfor a p T range from 8 to 25 GeV / c. An exponential slope can then be obtained by fitting thefinal x E distribution (Fig.3). In addition to the isolated photon-hadron correlations analysis, wehave studied the possibility of using the isolated π as a trigger particle. The isolation selectsmainly π which carry a large fraction of the total jet energy. Thus, it increases significantlythe partonic momentum fraction < z > = p π T / p partonT carried by the π compared to the expectedvalue < z > ≈ . π -hadron x E distributions and DSS fragmentation functions (fig.2 right) shows that isolated π -hadroncorrelations still su ff er from the fact that the π is itself a parton fragment with p π T < p partonT .Isolated photon-hadron correlations are one of the most promising channels to study theparton energy loss. The detailed analysis done by the ALICE experiment on the 2011 pp data isan important step toward a better comprehension of the medium modified fragmentation function.The slope parameter, and the x E distribution itself, form an essential baseline for the on-goingPb-Pb collisions analysis. E x0 0.2 0.4 0.6 0.8 1 E d x d N t r i g N -2 -1 Isolated photons < 25 GeV/c trigT
ALI−PREL−34327ALI−PREL−34327ALI−PREL−34327
Figure 3: x E distribution of isolated γ -hadron correlations fitted by an exponential in the x E range [0.2-0.8]. References [1] ATLAS Collaboration, Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at √ s NN = √ s NN = + p and Au + Au collisions at √ S NN =
200 GeV, Phys. Rev. C 80 (2009)[4] T. Binoth, J. P. Guillet, E. Pilon and M. Werlen, A full next-to-leading order study of direct photon pair productionin hadronic collisions, Eur. Phys. J. C16 (2000), 311-330, [hep-ph /9911340].[5] DSS, D. de Florian, R. Sassot and M. Stratmann, Phys. Rev. D75 (2007) ; Phys. Rev. D76 (2007)[6] K. Aamodt et al. (ALICE Collaboration), JINST 3 (2008), S08002