Measurement of hard probes in heavy ion collisions
Measurement of hard probes in heavy ion collisions at LHC with ALICE: Jets and photons Gustavo Conesa Balbastre 1/25
Introduction Heavy-Ion collisions at ultrarrelativistic energies: Search of QGP q RHIC: Au-Au, √s=200 Ge. V. q Since year 2000 until now LHC: Pb-Pb, √s=5. 5 Te. V. 1 st HI run hopefully end 2009 Several probes to study the medium: q q q Quarkonia Strangeness Elliptic flow Photons Jets … Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 2
Why jets and photons: Jet-quenching Partons traversing the medium of high colored density are modified. Energy loss through gluon emission High p. T jets are our tomographical probe of QGP. Constant energy loss E ~ 20 Ge. V. Nucleus A Jet reconstruction in AA is not feasible for energies smaller than 50 Ge. V and in general energy measurement is not very precise. Jet Direct photon tagged-jets are an useful observable. : E Ejet Back-to-back in azimuth The measurement: • • Particle species spectra S ( p. Th) RAA = SNN/(Norm x Spp) Fragmentation function • FF ( z = p. Th/Ejet ) • RFF = FFNN/(Norm x FFpp) } parton Nucleus B QGP prompt Medium coefficient transport Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 3
Direct photon sources Direct EM probes convey unperturbed information and their production probe the medium q q q Tag medium-modified jets: Prompt photons from 2 ->2 hard process (E > 10 Ge. V) Fragmentation photons (E < Ejet) Medium modified production: Medium produced photon: q q Prompt q γ g LO g γ Fragmentation g q q Jet Bremsstrahlung/jet coversion induced by medium Pb γ g Isolated photons Bremsstrahlung and jet conversion (E < Ejet) Thermal photons (E < 10 Ge. V) γ q g NLO q High z isolated photons QGP Pb Thermal radiation Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 4
Photon sources Photons are produced during all stages of the collision. Challenge: q q Disentangle the different sources. Neutral mesons decay. q Pre-equilibrium: Prompt photons But decay photons provide a first choice probe of medium effects Identify real photons (EM calorimetry, trigger) and e+e- from virtual and converted photons (tracking and PID, trigger) Equilibrium: Thermal-Bremsstrahlungjet conversion photons Freeze-out Decay photons Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 5
Estimates with a thermal model Turbide, Gale, Jeon, and Moore PRC (2004) • Photons abundantly produced. • Jet bremsstrahlung & fragmentation correlated with hadrons. • Jet-plasma & thermal, uncorrelated. • At LHC p. QCD photons dominant for p. T > 20 Ge. V In contrast to RHIC, at LHC sources of non prompt direct photons are dominant up to high p. T Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 6
RHIC : PHENIX RAA Hadron suppression of factor 5 at high p. T. q Run 2: No direct suppression (PRL 94, 232301 (2005)). Run 4 (QM 06): High p. T direct suppression q q q Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop Leading jet particle suppression Isospin (PDF) effect Fragmentation photon suppression? Something else? 7
RHIC : PHENIX RAA PQM model J. Nagle HP 2008 ~13 RAA is not a very good discriminator to calculate transport coefficient! Need to study what is really quenched, the parton-jet Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 8
Jet fragmentation function with quenching L. Cunqueiro HP 2008 Suppresion of leading particles at low (as observed with RAA) Increase of soft particles at high Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 9
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How can we measure direct photons and jets in ALICE? Currently under construction at LNF E > 10 Ge. V DE/E < 3 % sx =[3, 50] mm Tracking System resolution Dp/p = 2%, =1. 1º E > 10 Ge. V DE/E < 1. 5%, sx =[0. 5, 2. 5] mm Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 13
How many direct photons and jets? 10 k/year … but / 0 = 0, 01 -0, 1 for p. T > 10 Ge. V/c We need a good / 0 PID Large sample of direct LO -jet Balbastre @ Strings for p. T < 30 Ge. V/c in PHOS Gustavo and p. TConesa < and Strong Interactions Workshop 50 Ge. V/c in EMCal … 14
Jets measurement in ALICE Jets are measured in : q Tracking system, charged particles q Calorimeters, neutral particles Only EMCal, PHOS too small =110 degrees , -0. 7< <0. 7 Not available first runs, and maybe an small fraction first year Several jet reconstruction algorithms considered: q Complete azimuth coverage, -0. 9< <0. 9 Cone, k. T, Deterministic Annealing, Fast. Jet … Jet energy resolution: q q Only tracking system: E/E 0. 45 Tracking system + EMCal: E/E 0. 3 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 15
Thanks to Joern P. Jets: RCP( ) for 125 Ge. V jets Central to Peripheral Pb-Pb collisions Nuclear modification will be observed with great accuracy Gustavo Conesa. Tracking Balbastre @ Strings combining the Central System and EMCal and Strong Interactions Workshop 16
- 0 discrimination in the calorimeters Three regions of analysis increasing p. T well separated clusters invariant mass analysis merged clusters not spherical shower shape analysis Opening angle << 1 cell all 0’s at this energy are in jets isolation cut < 10 Ge. V/c in EMCal < 30 Ge. V/c in PHOS 10 - 30 Ge. V/c in EMCal 30 - 100 Ge. V/c in PHOS > 30 Ge. V/c only method in EMCal IP TPC R Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop Isolated if: • no particle in cone with p. T > p. T thres • or p. T sum in cone, Sp. T < Sp. Tthres candidate PHOS/EMCal 17
PYTHIA -jet (signal) and jet-jet ( 0 –hadron background) events simulated and reconstructed in ALICE, full material budget. PHOS identified spectrum pp and Pb. Pb annual statistics ALICE-INT-2005 -014 G. Conesa et al. , NIM A 580 (2007) 1446 Y. Mao, Poster QM 2008, ALICE-INT-2007 -021 2 PHOS modules IC: R =0. 3, S (p. T)=2 Ge. V/c IC: R =0. 2, p. T>2 Ge. V/c Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 18
Direct photon identification in EMCal: Preliminary Isolation Cut : Prompt photon / jet clusters Ratio isolated clusters in -jet / isolated clusters in jet-jet pp @ √ 14 Te. V Pb. Pb @ √ 5. 5 Te. V, qhat=0 Pb. Pb @ √ 5. 5 Te. V, qhat = 50 Prompt photons signal larger than jet-jet clusters background for p. TGustavo larger than around 15 Ge. V/c for pp and Conesa Balbastre @ Strings and Strong Interactions Workshop quenched Pb. Pb events 19
Why -jet correlations? Ø Ø Ø p 0 Jet ^ Medium effects redistribute ( q. L) the parton energy, Eparton, inside the hadron jet (multiplicity, k. T). Hadron redistribution can be best measured in Fragmentation Function. . . If we know Eparton. Prompt the HI environment limits the precision on the energy of the reconstructed jet/parton: Measure Eprompt Eparton Ø Study medium modification in fragmentation function (RAA of FF) from isolated -jet and isolated -hadron correlations. Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 20
Tagging jet with photon Strategy Search identified prompt photon (PHOS or EMCal) with largest p. T (E > 20 Ge. V). Search leading particle or jet core : - leading 180º With Standard jet algorithms, only high p. T, not enough statistics. Construct jet with particles around the leading or jet core inside a cone of size R Monte. Carlo studies: q q min Pythia pp events to generate gamma and jet signal Hijing to generate Pb. Pb background Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop max EMCal R Reconstruct the jet in TPC and EMCal (if available): (event by event): Leading Jet core TPC IP EMCal/PHOS 21
Reconstructed jet selection 40 Ge. V jets, photon in PHOS, leading particle is seed, R=0. 3 ALICE-INT-2005 -014 G. Conesa et al. , NIM A 585(2008) 28 p-p collisions, p. T, part > 0. 5 Ge. V/c TPC alone Pb-Pb collisions, pp. T, T, part > 0. 5 2 Ge. V/c part> TPC alone TPC+EMCAL Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 22
-tagged Fragmentation Function. F and RFF photon in PHOS, jets in TPC+EMCal ALICE-INT-2005 -014 G. Conesa et al. , NIM A 585(2008) 28 Pb. Pb fragmentation function UE background removed statistically Systematic errors due to jet(p 0)-jet background No quenching in simulation If signal is quenched Sensitive to medium modifications at low z if larger than Gustavo Conesa Balbastre @ Strings ~5% in both configurations. and Strong Interactions Workshop 23
Tagging jets with photons in EMCal G. Bourdaud preliminary Final Fragmentation function and Nuclear Modification Factor Realistic spectrum simulated: q q Pb. Pb/pp E -jet > 30 Ge. V Jet core is the jet seed 1/10 of a year statistic. range usable is 0. 5 < < 3. 2 No quenching Bkg not substracted Bkg substracted Pb. Pb/pp With quenching Deviation from 1 used to calculate systematical errors Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 24
Summary Heavy Ion collisions at LHC will produce large amounts of direct photons and jets. q q RHIC has measured jets and photons and observed the quenching effect: q q q Probe properties of QGP with jets and photons studying the jet-quenching effect. Calculate the coefficient transport. Hadrons are suppressed Direct Photons are suppressed at high p. T, not understood completely. First measurements of jets at RHIC: no quenching observed … measurement biased. ALICE is prepared to measure direct photons and jets q q q Jets can be reconstructed with the tracking system alone or together with the calorimeter. Quenching effects can be observed. Identification of direct prompt photons feasible with the calorimeters with the Isolation cut technique. Photon-tagged jet algorithms can be used also to study the quenching effect for the lower jet energies where standard jet reconstruction is not possible due to large background. Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 25
Back-up Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 26
-hadron correlation in ALICE EMCal/PHOS w Strategy following François Arleo studies (event by event): Search identified prompt photon (PHOS or EMCal) with largest p. T (E > 20 Ge. V). Search for all charged hadrons neutral 0 (EMCal or PHOS): (TPC) 90º< - hadron < 280º p. T hadron > 2 Ge. V/c TPC+ITS hadron IP or PHOS/EMCal Y. Mao, Poster QM 2008, ALICE-INT-2007 -021 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 27
How can we distinguish different direct photon sources? Prompt : RAA = 1, v 2=0 (not considering isospin effects) Fragmentation: RAA<1, v 2>0 Thermal, Bremsstrahlung, Jet Conversion: RAA>1, v 2<0 (v 2>0 for thermal) q Unambiguous signal of medium production Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 28
RHIC : PHENIX v 2 Gale QM 2008 v 2: small! Consistent with zero (within errors) Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 29
Other approach: photon conversions Identify photons converting in the ITS and TPC Study performed by Ana Marin (GSI) beampipe, q. Clean photon identification q. Provide directional information q. Non vertex background (important source of systematic errors in measurement of direct photons) can be rejected. Independent measurement of the same quantities, with different systematics compared to PHOS/EMCAL. Increase level of confidence in the results Counting annual statistics for p. T > 20 Ge. V/c (very rough stimations) Loss of efficiency at high p. T under investigation Needs to be improved ! pp @ √s=14 Te. V Pb. Pb @ √s=5. 5 A Te. V EMCal 20000 PHOS 3000 Tracking 4000 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 30
Azimuthal correlation: Direct converted– charged particles Study performed by Ana Marin (GSI) detected in Central Barrel Isolation Cut: R=0. 2, p. T>0. 7 Ge. V Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 31
Jets: Resolution M. Estienne. | | < 0. 3 TPC+EMCal TPC only For a jet of R=0. 4: limit for a jet to be totally included in the calorimeter Conesa Balbastre limit at which the leading. Gustavo (here center) of@ Strings and Strong Interactions Workshop the jet is still in the detector acceptance 100 Ge. V Jets Full simulation R=0. 4 Jet reconstruction resolution from 45% to 30% 32
Particle identification with the calorimeters Different particles produce showers of different shapes. 7 parameters used to define the shower topology : q Shower ellipse axis l 0 , l 1; lateral dispersion; core energy; sphericity; maximal deposited energy; multiplicity. l 0 PHOS can also identify particles with TOF (slow nucleons) and CPV (charged vs neutral) Bayesian approach used to give to the measured cluster an identification probability EMCal 0 70 Ge. V 0 l 2 0 PHOS 50 Ge. V l 1 0 l 2 0 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 0 l 2 0 33
Fragmentation function ALICE-INT-2005 -014 z = p. T, jet particle /E g Any neutral signal in PHOS G. Conesa et al. , NIM A 585(2008) 28 Prompt identified in PHOS Background If signal is quenched HIC background Pb-Pb collisions Signal PHOS : E > 20 Ge. V/c; TPC+EMCal detect jet particles, R=0. 3 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop 34
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