Two-particle Correlations in pp and Pb-Pb Collisions with ALICE

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wo-particle Correlations in pp and Pb-Pb Collisions with ALICE Xiangrong Zhu, Ruina Dang (for the ALICE Collaboration) Institute Of Particle Physics, Central China Normal University he 9th Chinese

1 wo-particle Correlations in pp and Pb-Pb Collisions with ALICE Xiangrong Zhu, Ruina Dang (for the ALICE Collaboration) Institute Of Particle Physics, Central China Normal University he 9th Chinese Physical Society Conference on High Energy Physics April, 2014, Wuhan Many thanks to Jan Fiete Grosse-Oetringhaus and Nicolas Arbor Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

2 Outline Motivation ALICE detector wo-hadron correlations Modification of charged hadrons yield in Pb-Pb Isolated photon-hadron correlations Imbalance parameter x E extraction Summary Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

Motivation Hard scattered partons generated in the early stage lose their energy when propagating through the hot and dense QCD medium, Quark-Gluon-Plasma (QGP) ( jet quenching ).

3 Motivation Hard scattered partons generated in the early stage lose their energy when propagating through the hot and dense QCD medium, Quark-Gluon-Plasma (QGP) ( jet quenching ). Charged hadrons Full jets Nuclear modification factor of hadron and jet yield: Parton energy loss leads to hadron and jet yield suppression. Modification of parton fragmentation in comparison to pp collisions? Provide crucial insight into the nuclear medium effects of energy loss in the QCD matter. wo-particle azimuthal correlations: sensitive to parton fragmentation, energy loss in the QGP a powerful tool to investigate the properties of QGP. wo-hadron correlations photon-charged hadron correlations: the golden channel to understand the QGP. photons do not interact strongly with QGP. direct photon is dominated by quark-gluon Compton scatting and quark anti-quark annihilation, which is balanced with the the recoil jet (parton). Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

4 ALICE detector IS (Inner racking System) six cylindrical layers of silicon detectors, η < 0.9 and φ = 2π localize the primary vertex reconstruct the secondary vertices of hyperons and D and B mesons track and identify particles down to p 0 M ev /c PC (ime Projection Chamber) a cylindrical gas detector, η < 0.9 and φ = 2π charged particle momentum (0.15 < p < 0 GeV /c) particle identification (de/dx resolution better than %) two-track separation (resolution in relative momentum below 5 MeV/c) VZERO Centrality determination rigger Xiangrong Zhu, Ruina Dang (CCNU) EMCal (ElectroMagnetic Calorimeter) a lead-scintillator sampling calorimeter, η < 0.7 and φ = 5π 9 high energy jets high-p neutral pions and photons wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

5 Datasets wo-hadron correlations Pb-Pb at s NN = 2.76 ev : 15 M events from 20 data (Lint 1.8 µb 1 ) pp reference at s = 2.76 ev : 55 M events from 2011 data (Lint 1.0 nb 1 ) Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

Correlation function A particle at one p region ( trigger particle ) correlated with particles from another p region ( associated particles ) where p,assoc< p,trig.

for two-track acceptance in bins of centrality and vertex as C( φ, η) = N mixed 1 d 2 Npair same ( N trig d φd η )/N same 1 d 2 Npair mixed ( N trig d φd η ) (2) Per-trigger

6 Correlation function A particle at one p region ( trigger particle ) correlated with particles from another p region ( associated particles ) where p,assoc< p,trig. Per-trigger yield as a function of φ and η ( φ = φ trig φ assoc, η = η trig η assoc): Y = 1 d 2 N pair N trig d φd η Correlation function is obtained by event mixing correction for two-track acceptance in bins of centrality and vertex as C( φ, η) = N mixed 1 d 2 Npair same ( N trig d φd η )/N same 1 d 2 Npair mixed ( N trig d φd η ) (2) Per-trigger yields corrected for tracking efficiency and contamination (no influence on shapes) (1) Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

7 Correlations at high p (8.0 < p,trig < 15.0 GeV/c) Per-trigger yield as a function of φ at high p Jet-like region (small collective effects): 8.0 < p,trig < 15.0 GeV/c 3.0 < p,assoc < p,trig Different background subtraction method: Zero Yield At Minimum (ZYAM) with constant fitting in region φ π/2 < 0.4 elliptic flow from ALICE v 2 η gap (1.0 < η < 2.0) Per-trigger yield in two different region: φ < 0.7 for Near-side φ π < 0.7 for Away-side ) 1 dn assoc /d ϕ (rad 1/N trig a) not background subtracted b) zoomed 8 < p < 15 GeV/c t,trig 4 < p < 6 GeV/c t,assoc s NN = 2.76 ev c) background subtracted Pb Pb 0 5% centrality Pb Pb 60 90% centrality pp ϕ (rad) PRL (2012) Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

8 I I Modification of assoicated hadrons yield at high p,assoc Modification factor Y = 1 dn assoc N trig d φ Central events: Near-side enhancement ( 1.2): Change of the fragmentation function? Change of the quark vs gluon jet ratio? Bias on the parton pt spectrum? Away-side suppressed ( 0.6): Energy loss in medium Peripheral events: Consistent with unity Collective contribution small at high p d φ IAA = Y P bp b Y pp and I CP = Y P central bp b AA (0 5% / 60 90%) Near side s NN = 2.76 ev 8 GeV/c < p < 15 GeV/c t,trig p < p η < 1.0 t,assoc t,trig Y peripheral P bp b Away side 0 5% Pb Pb/pp 60 90% Pb Pb/pp Flat bkg Flat bkg v 2 bkg v 2 bkg η gap η gap ALICE a) p (GeV/c) p (GeV/c) t,assoc t,assoc CP Near side s NN = 2.76 ev 8 GeV/c < p < 15 GeV/c t,trig p < p η < 1.0 t,assoc t,trig Away side Flat bkg v 2 bkg η gap ALICE PRL (2012) 0.5 b) p (GeV/c) p (GeV/c) t,assoc t,assoc Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

9 Modification of assoicated hadrons yield going to low p,assoc Alternative subtraction approach Signal yield (Y S ) from a cone with radius R = 0.2 Background yield (Y B ) estimated in two R = 0.2 cones at large η Gap > 1.1 Subtract background Y B from peak region yield Y S Avoid flow modulation by using same φ region Calculate I AA as I AA = (Y S 0.5 Y B )/N trig P b P b (Y S 0.5 Y B )/N trig pp On Near-side, 20-50% enhancement in central Pb-Pb, compared to pp Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

10 Data sets Photon-hadron correlations pp at s = 7 ev : M EMCal triggered events from 2011 data (Lint 500 nb 1 ) Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 / 33

11 Measurement observables EMCal trigger Use EMCal trigger capabilities to enrich high-p rigger threshold 5.0 GeV/c photons statistics Imbalance parameter: φ = φ γ trig φh± assoc approximate p p x E distribution describes fragmentation function for 0.2 < x E < 0.8 Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

12 Isolation analysis Isolated photon Most of direct photons are isolated, while most of decay photons are not (jet) Isolation parameters: a). cone radius R = ( φ) 2 + ( η) 2, b). p threshold Isolation technique Isolation efficiency iso total /N = N thresh Isolation R=0.4, p <0.5 GeV/c γ Pythia (γ jet) s=7 ev π 0 pp data s=7 ev 26/07/ No particles, including track and cluster, with p > 0.5 GeV/c in cone R = p ALI PERF (GeV/c) 80% of direct photons are isolated. About % of π 0 pass the isolation criteria (main background). Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

13 x E of isolated cluster and π 0 Underlying events contribution is subtracted x E from isolated cluster-h ± correlation x E from isolated π 0 -h ± correlation dn dx E 1 N trig 1 pp, s = 7 ev Isolated clusters trig 8 GeV/c < p < 12 GeV/c trig 1 12 GeV/c < p < 16 GeV/c (x ) trig 2 16 GeV/c < p < 25 GeV/c (x ) dn dx E 1 N trig 1 pp, s = 7 ev Isolated π 0 trig 8 GeV/c < p trig 12 GeV/c < p trig 16 GeV/c < p < 12 GeV/c 1 < 16 GeV/c (x ) 2 < 25 GeV/c (x ) /07/ x E ALI PERF Result is a mix of isolated photons and background Need to know photon purity (see backup) x E ALI PREL Background 95% π 0 decay photons Use π 0 to evaluate background 8.0 < p trig < 12.0 GeV/c 12.0 < p trig < 16.0 GeV/c 16.0 < p trig < 25.0 GeV/c Nucl. Phys. A 904 (2013) 697c QM 2012 Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

14 x E of isolated photon Sum up p bins ( p = 1.0 GeV/c) 25 GeV /c D(x γ iso E ) = ( 1 cluster iso D i(xe ) 1 pi D i(x π 0 iso E )) p i=8 i p i 25 GeV /c i=8 D i(x UE E ) (3) dn dx E 1 N trig pp, s = 7 ev Isolated photons trig 8 GeV/c < p < 25 GeV/c 1 Fit (exponential) Fit function: Ae Bx Inverse slope : 7.8 +/ 0.9 B = 7.8 ± Exponential shape [ ] x E ALI PREL Baseline for the study of medium modified parton fragmentation in Pb-Pb collisions Nucl. Phys. A 904 (2013) 697c QM 2012 Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

15 E x E of isolated π 0 Isolated π 0 : E π 0 samples a large fraction of E parton < z π 0 >= 0.8 (Pythia + cuts) Fit slope parameter of isolated π 0 Compare x E slopes with fragmentation function dn dx E 1 N trig 1 1 pp, s = 7 ev Isolated π 0 trig 8 GeV/c < p < 12 GeV/c trig 1 12 GeV/c < p < 16 GeV/c (x ) trig 2 16 GeV/c < p < 25 GeV/c (x ) Negative x slope pp, s = 7eV thres Isolated π 0 (R=0.4, p = 0.5 GeV/c) π ± DSS NL0 quark π ± DSS NL0 gluon x E range [ ] <z> = ALI PREL x E 4 <z> = trig p (GeV/c) ALI PREL < p trig < 12.0 GeV/c 12.0 < p trig < 16.0 GeV/c 16.0 < p trig < 25.0 GeV/c Nucl. Phys. A 904 (2013) 697c QM 2012 Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

Summary wo-hadron correlations: Modification of charged hadrons yield in Pb-Pb at high p : Constraint for models: enhancement in near-side yield, but suppression at away-side.

16 Summary wo-hadron correlations: Modification of charged hadrons yield in Pb-Pb at high p : Constraint for models: enhancement in near-side yield, but suppression at away-side. Photon-hadron correlations: Establish global shape of fragmentation function through the measurement of isolated photon-hadron correlations with isolated photon trigger at 8.0 < p < 25.0 GeV/c in pp collisions at s = 7 ev. Extract isolated π 0 slope parameter to study fragmentation bias in using isolated π 0 -hadron correlations. Correlations measured in pp collisions will serve as a reference for future correlation measurements in Pb-Pb collisions. Work is ongoing. Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

17 Backup Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

18 wo-hadron correlations Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

19 Event and track selection Data sets PbPb at s NN = 2.76 ev : 15 M events from 20 data taking period in 0-90% centrality class pp reference: 55 M events from 2011 low energy run Centrality selection: VZERO (2.8 < η < 5.1 and 3.7 < η < 1.7) racking PC tracks constrained to the primary vertex optimal azimuth (φ) acceptance = uniformity for angular correlations Minimize twotrack cluster merging effects in the PC wo step correction procedure 2-track acceptance correction using mixed events shape Single particle efficiency and contamination corrections yield Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

20 Single particle corrections racking efficiency rack contamination Npair corrected ( η, φ, p,trig, p,assoc, C) = Nraw raw ( η, φ, p,trig, p,assoc, C) C trkeff (p,assoc, C) C trkeff (p,trig, C) C cont (p,assoc ) C correlatedcont ( η, φ, p,trig, p,assoc ) Ntrig corrected (p,trig, C) = Ntrig raw (p,trig, C) C trkeff (p,trig ) C cont (p,trig ) Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

21 Systematic uncertainties Sources η range of flow subtraction rack selection Vertex range Influence of resonances and conversions wo-track effect Wing (increase at large η) correction wo different fit procedures Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

22 Photon-hadron correlations Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

23 Experimental aspects Background from decay Relative contributions of the quark-gluon Compton, q q annihilation and fragmentation subprocesses in NLO isolated photon production. R. Ichou et al. arxiv: [hep-ph] Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

25(d xx d zz) 2 + d 2 xz (dxx: cluster position in x direction weighted by the cell energy) Photon

24 Cluster in EMCal ElectroMagnetic Shower Shower shape: long axis λ 2 0, short axis λ 2 1 λ 2 0(1) = 0.5(dxx + dzz) ± 0.25(d xx d zz) 2 + d 2 xz (dxx: cluster position in x direction weighted by the cell energy) Photon identification with shower shape photon: 0.1 < λ 2 0 < 0.27 π 0 : λ 2 0 > 0.5 Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

25 Shower shape parameters η = i cell φ = i cell w i cell = max(0, ln E i cell E clester ), w total = d 2 ηφ = i cell λ 2 0 = 0.5(d φφ + d ηη) + w i cell η i cell w total (4) w i cell φ i cell w total (5) i cell w i cell (6) w i cell η i cell φ i cell w total η φ (7) 0.25(d φφ d ηη) 2 + d 2 φη (8) λ 2 1 = 0.5(d φφ + d ηη) 0.25(d φφ d ηη) 2 + d 2 φη (9) Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

26 π 0 identification Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

27 Imbalance parameter extraction strategy Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

28 Isolation photon purity estimation Shower shape method Isolated clusters sample = isolated photons + background. Binned likelihood fit of the shower shape distribution: combined signal (MC) and background (data) shower shape to fit data. Entries < E < 25 GeV/c thres Isolation : R = 0.4, p < 0.5 GeV/c pp s = 7 ev Combined fit Background 25/07/ ALI PERF λ 0 Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

$Isolated π 0 fraction Isolated π 0 : E π 0 samples a large fraction of E parton < z π 0 >= 0.$

29 Isolated π 0 fraction Isolated π 0 : E π 0 samples a large fraction of E parton < z π 0 >= 0.8 (Pythia + cuts) Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

Systematic uncertainties Main systematic uncertainties are : Shower shape MC / Data Likelihood fit parameters (binning, range) Background template composition (signal

30 Systematic uncertainties Main systematic uncertainties are : Shower shape MC / Data Likelihood fit parameters (binning, range) Background template composition (signal contamination, shower shape) Underlying event subtraction Detectors effects correction Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

31 Isolated π 0 slopes: KKP Negative x slope E pp, s = 7eV thres Isolated π 0 (R=0.4, p = 0.5 GeV/c) π ± KKP NL0 quark π ± KKP NL0 gluon range [ ] x E <z> = <z> = trig p (GeV/c) ALI PREL Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

32 thres Isolation photon analysis in Pb-Pb Photon identification in PbPb Collision: Started to separate isolated photons from background, estimate isolation Photon purity Need to understand the more complex background (flow) Entries < E < 25 GeV/c Isolation : R = 0.3, p 0 20% Pb Pb Combined fit Background < 3 GeV/c s NN = 2.76 ev 25/07/ Centrality 0-20% Head Region ( φ - π < π/5 rad) ZOWW, NLO, µ = 0.5 p 0 = 1.48, 1.68, 1.88 GeV/fm I AA 0 γ 5 < p < 7 GeV/c γ 7 < p < 9 GeV/c ALI PERF λ 0 0 γ 9 < p < 12 GeV/c z γ 12 < p < 15 GeV/c z PHINEX Phys. Rev. C 80, (2009) Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

33 Medium mofified fragmentation function Xiangrong Zhu, Ruina Dang (CCNU) wo-particle Correlations in pp and Pb-Pb with ALICE April 21, / 33

Prospective of gamma hadron correlation. study in CMS experiment

Prospective of gamma hadron correlation. study in CMS experiment Yeonju Go (Korea University) for the CMS collaboration 5-6 Dec. 2014 HIM meeting Contents Physics Motivation Direct gamma-hadron correlation