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Medium information from harmonic flow & jet quenching in relativistic HI collisions

Medium information from harmonic flow & jet quenching in relativistic HI collisions. Subrata Pal Tata Institute of Fundamental Research, Mumbai, India. Outline A brief history of jet quenching in HI collision AMPT model updated with jets & new PDF

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Medium information from harmonic flow & jet quenching in relativistic HI collisions

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  1. Medium information from harmonic flow & jet quenching in relativistic HI collisions Subrata Pal Tata Institute of Fundamental Research, Mumbai, India • Outline • A brief history of jet quenching in HI collision • AMPT model updated with jets & new PDF • AMPT predictions versus RHIC & LHC data: • Particle yield, Anisotropic flow, Jet quenching • Conclusions

  2. Jet quenching schematic view of jet production Leading hadron + pT hadrons N N -pT hadrons Balancing hadron PHENIX: PRL 88 (2002) 022301. STAR: PRL 91 (2003) 172302. • High pT > 2 GeV back-to-back partons (jets) produced from initial hard NN collision • Fragmentation of the jets produce a cluster of high pT hadrons (also called jets!) • p+p or d+A: no QGP formed, both the back-to-back jets survive • Central A+A: If QGP formed, one or both jets in the dense partonic medium suffer energy loss– jet quenching RAA(pT) =1: Particle yield in A+A collision is simply superposition of independent p+p collisions. RAA(pT) 1 at high pT : Deviation from this simple superposition concept (viz final-state effects). High pT hadron yield in central Au+Au suppressed compared to p+p collision in-medium jet energy loss

  3. Jet formalism Inclusive hadron distribution – calculable in pQCD p+p colln: zc = ph/pq Parton distribution fns Perturbative cross section Fragmentation fn Jet quenching: pQCD energy loss ΔEof partons by gluon bremsstrahlung ΔE A+A colln: zc=ph/(pq-ΔE) Gyulassy, Levai, Vitev NPB 594 (2001) 371 Jet quenching also sensitive to: Initial spatial parton distribution; Nuclear shadowing of PDF; Collective flow sQGP + large soft particle production  nonperturbative physics systematic model study reqd.

  4. ΔE AMultiPhase Transport model (AMPT) Inclusive hadron distribution – calculable in pQCD Energy loss Lin, Ko, Li, Zhang, SP, PRC72 (2005) 064901 • Initial particle distribution obtained from updated HIJING 2.0 model • Strings from HIJING converted to valence (anti)quarks – String Melting • Partons scatter in ZPC model with elastic scattering cross section: • Phase-space coalescence of freeze-out partons produce the hadrons • Hadrons evolve with (in)-elastic scatterings via ART transport model σ≈ 9s2/μ2

  5. Hard jets & its energy loss in AMPT Momentum distributionof hard partons from LO pQCD in p+p collision GRV94 Gaussian NLO SP, Pratt, PLB 574 (2003) 14 Total energy loss by a jet of energy E via gluon radiation SP, PRC 80 (2009) 041901(R) Parton density Number of gluons emittedfrom ΔE is related to entropy increase ΔS T = ε(r,τ)/3ρ(r,τ) From parton cascade σ ≈ 9s2/2μ2 Radiated gluons scatter in medium with Parton hadron duality: Ng → Nπ

  6. AMPT with updated HIJING 2.0 Deng,Wang, Xu, PLB 701 (2011) 133 • GRV parametrization of parton distribution function • c.m. energy dependence in 2-component HIJING 2.0 p0 = 2 GeV/c PDF in nucleus: Impact parameter dependent shadowing sq = 0.1 (fixed) from deep-inelastic-scattering data off nuclear targets. sgfitted to centrality dependence of measured dNch/dy in A+A collision.

  7. dNch/dy in HIC at RHIC & LHC (0-5)% Centrality Parameters in AMPT In string fragmentation function, Default HIJING: a=0.9, b=0.5 GeV-2. s=0.33, =3.226 fm-1   = 1.5 mb HIJING: dNch/dη||η|0.5 = 705 (RHIC) = 1775 (LHC) • Parton scatterings lead to 15% decrease in dNch/dy at RHIC & LHC!! • Hadron scattering insensitive to dN/dη. AMPT hadron yield ratios at LHC SP, Bleicher, PLB 709 (2012) 82

  8. Centrality dependence of dNch/dη Au+Au collisions at RHIC: Measured charged hadron multiplicity density per participant pair constrains gluon shadowing parameter sg= 0.10 - 0.17 Pb+Pb collisions at LHC: Stronger centrality dependence in ALICE data due to large minijet production (at small x) gives a stringent constraint on sg≈0.17 [5] HIJING2 (w/o FSI) fits with sg=0.20 - 0.23 Deng,Wang, Xu, PLB 701 (2011) 133 ALICE Collab, PRL 106 (2011) 032301

  9. z y x 2 3 4 Anisotropic flow vn in HI collision vn are Fourier coeff in φ distribution of particles relative to the Reaction Plane Elliptic flow v2: initial spatial ellipticity converted to final momentum anisotropy by interaction among particles Ollitrault, PRD 46 (1992) 229 RPcannot be measured directly. Estimated by 2 Origin of triangular flow v3: fluctuations in the position of participant nucleons 2, 4 particle azimuthal correlations: Alver & Roland, PRC 81 (2010) 054905 Event Plane angle: nonflow part Odd harmonics = 0 Borghini et al, PRC 64 (2001) 054901 Odd harmonics ≠ 0

  10. Anisotropic flow @ RHIC SP, Bhalerao, in prep. Ma, Wang, PRL 106 (2011) 162301 AMPT b=0 • v2 > v3 > v4 in AMPT (b0) describes RHIC data • Initial spatial asymmetry and matter flow in AMPT consistent with RHIC

  11. Anisotropic flow @ LHC 0-5% centrality 30-40% centrality ALICE, PRL 107 (2011) 032301 • <vn> slightly larger at LHC due to large <pT> • Magnitude, pattern of vn(pT) at RHIC & LHC similar! At LHC larger densitybut faster expansion • Av.v2,v3,v4 described by AMPT over large centrality RHIC

  12. Charged particle spectra @ RHIC & LHC STAR Collab, PRL 91 (2003) 0172302 ALICE Collab, PLB 696 (2011) 30 p+p, Au+Au @ RHIC AMPT spectra agrees with STAR data up to high pTfor both peripheral & central collisions p+p @ LHC AMPT spectra is less steep at high pT compared to the ALICE data obtained from interpolation between s = 0.9 and 7 TeV • Pb+Pb @ LHC • Peripheral collision: AMPT spectra agrees with ALICE data. • 0-5% central collision: Enhanced energy loss in AMPT even withelasticparton-parton scattering. SP, Bleicher, PLB 709 (2012) 82

  13. 4 <pTtrig<6 GeV/c Trig + x Momentum conservation away -x Jet quenching and lost jet remnants Dijet asymmetry ratio Au+Au @ 200 GeV at b=0 fm Zhang et al, PRL 98 (2006) Overall momentum imbalance: projection of all track pT on pT,trig Trigger SP, Pratt, PLB 574 (2003) 14 0-30% Pb+Pb @ 2.76 TeV dN/dpx= Trigger Away side Trigger side  Trigger jet is surface biased Unbalanced jet Balanced jet Away side:energy lost by high pT partons is converted to soft particles with |px| < 600 GeV/c Low pT, momentum balanced in an event Out-of-cone low pT particles balance the event In-cone large mom. imbalance at high pT

  14. pT(trig)  pT(assoc) Dihadron azimuthal angle correlation SP, PRC 80 (2009) 041901(R) Data: PHENIX, PRC 78 (2008) 014901 = 3-4 GeV/c Background v2 subtracted p+p in pQCD agrees with data Default:away-side hadrons peak → small rescatterings and dNg/dy String melting Low pT:away-side jet traverse into dense medium, scatter & thermalize → broad distribution around Δφ = π High pT:Jets produced near surface → “conical flow” peaks at Δφ = π ± 1 Data p+p: peaks at near side & away-side (Δφ≈π) Au+Au: away side peak shifts to Δφ≈π± 1.2 SM with σ = 10 mb explains Au+Au data

  15. Flow contribution to dihadron correlation PHENIX, PRC 78 (2008) 014901 Ma & Wang, PRL 106 (2011) 162301 AMPT : On subtraction of harmonic flows vn (n=2-6), away side peaks suppressed  v3 contributes most to double peak PHENIX data: Au+Au: away side peaks at Δφ≈π±1.2  only v2 contribution subtracted

  16. Jet quenching data in HI Collision Medium effects at high pT quantified by nuclear modification factor: SPS energy: rising/flat RAA≈ 1 with pT little/no jet energy loss RAA(LHC) < RAA(RHIC) at pT < 6 GeV/c: Enhanced energy loss as the medium is denser at LHC than at RHIC Rise of RAA at pT > 7 GeV/c due to slow fall with increasing pT of primary jet spectra (pQCD physics)

  17. Jet quenching at RHIC & LHC Au+Au @ 200 GeV • RAAin AMPT describes RHIC data  Model parton energy loss consistent with that of the evolving medium (RAA)ch > (RAA)0 Larger (anti)baryon yield than 0from parton coalescence Pb+Pb @ 2076 GeV • Stronger suppression in AMPT for 0-5% centrality due to significant jet energy loss in a dense matter: •   c: [ c(LHC)≈ 2c(RHIC) ] • Recombination of hard partons in AMPT not enough to enhance RAA. • Medium at LHC less opaque?? • Smaller RAA in AMPT leaves no room for model study of energy loss by gluon radiation?? ALICE: PLB 696 (2011) 30

  18. Is large E-loss in models generic at LHC? Horowitz, Gyulassy, NPA 872(2011) 265 Further experimental results and theoretical investigations required • Control d+Pbexpt required to get sg. • Decrease in saturation parameter sg enhance RAA but destroys dNch/dy fit. • Understanding the medium opacity. • Coupling of geometry to flow. • Approximations in E-loss equation. Suggestions:

  19. Modification of jet-medium coupling s pQCD screening mass (T dependent) Parton elastic scattering cross section Estimate initial Ti at RHIC & LHC: Fixed values used in AMPT at RHIC & LHC: Scaling reln (~10% viscous entropy production) s = 0.33, =3.22 fm-1   = 1.5 mb Gyulassy, Matsui, PRD 29 (1984) 419 SP, Pratt, PLB 578 (2004) 310 QGP with massless gas of light q, q-bar (0-5)% exptdNch/dyy≈0≈687 (RHIC), 1601(LHC) i = 1fm/c ≈ 320 MeV  ≈ 1.4 mb(RHIC) ≈436 MeV ≈0.76 mb(LHC) Ti s= 0.33 Quenching at LHC suggest thermal suppression of QCD coupling s by ~30% Alternative: =3.22 fm-1 isconstant at RHIC & LHC To get  = 0.76 mb at LHC requires s = 0.24

  20. Summary • dNch/dy at RHIC and LHC quite sensitive to parton scattering • dNch/dy versus Centrality data at LHC strongly constrains sg = 0.17 • AMPT fluctuations and matter flow agree with measured v2 > v3 > v4 • AMPT jet quenching consistent with RHICdata at all centralities • Quenching data at LHC suggests thermal decrease in s by ~30% • Lost jet energy & its medium excitations re-appear as soft particles

  21. RESERVE SLIDES

  22. AMPT comparison with p+p collision data Measured charged hadron spectra well described in AMPT model up to high pT pT in AMPT underpredict data  Hadron production by recombination is not efficient in p+p collision

  23. Elliptic flow (v2) in AMPT model Charged hadrons Lin, Ko, PRC 65 (2002) 034904 Lin, Ko, Li, Zhang, SP, PRC72 (2005) 064901 • String melting enhances parton density • v2well explained with string melting & parton recombination • v2 sensitive to parton elastic σqq→qq (= 6 mb best fit) → dense partonic medium required to explain v2

  24. Origin of v2, v3 in AMPT Alver & Roland, PRC 81 (2010) 054905 <v2>  2: Final elliptic flow proportional to initial eccentricity <v3>  3: Final triangular flow proportional to initial triangularity

  25. h spectra in pp and RAA @ LHC • charged particle spectra • in pp at √s = 0.9, 2.76, 7 TeV H. Appelshäuser, Talk at Quark Matter 2011

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