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Parton energy loss in heavy-ion collisions from RHIC to LHC. Andrea Dainese INFN Legnaro. Layout. The discovery of “jet quenching” at RHIC High- p T particle production in nucleus-nucleus collisions, according to perturbative QCD Calculating parton energy loss (BDMPS framework)
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Parton energy lossin heavy-ion collisionsfrom RHIC to LHC Andrea Dainese INFN Legnaro Padova, 09.05.2006 Andrea Dainese
Layout • The discovery of “jet quenching” at RHIC • High-pT particle production in nucleus-nucleus collisions, according to perturbative QCD • Calculating parton energy loss (BDMPS framework) • Light-flavour hadron suppression in the Parton Quenching Model at RHIC andLHC • Energy loss for heavy quarks • RHIC: electrons RAA vs data • LHC: testing E loss with heavy-to-light ratios • Conclusions Padova, 09.05.2006 Andrea Dainese
PHENIX p0 (0-10%) Discovery at RHIC:high-pT suppression • Nuclear modification factor of pT distributions: factor 5 suppression! compilation by D.d’Enterria Padova, 09.05.2006 Andrea Dainese
What about jets? pp 2 back-to-back jets Jets via di-hadron correlations: • trigger: highest-pT track, (4--6 GeV) • Df distribution: 2 GeV < pT < pTtrigger near side away side STAR Coll., PRL 90 (2003) 082302. slide courtesy of P.Jacobs Padova, 09.05.2006 Andrea Dainese
Discovery at RHIC:no away-side jet in central Au-Au • Quantified via ratio of integrals: ~1 in peripheral AA ~0 in central AA STAR Coll., PRL 90 (2003) 082302. Padova, 09.05.2006 Andrea Dainese
Final-state effect Is it final-state energy loss? • … or saturation of the parton densities in the initial nuclei? Control experiment, without medium: d-Au no high-pT suppression the away-side jet is there PHENIX Coll., PRL 91 (2003) 072303. STAR Coll., PRL 91 (2003) 072304. Padova, 09.05.2006 Andrea Dainese
figure by D. d’Enterria Final-state effect Is it final-state energy loss? • … or saturation of the parton densities in the initial nuclei? “Control experiment”, with medium-blind probe: g’s in Au-Au no suppression, but Ncoll scaling Padova, 09.05.2006 Andrea Dainese
h RAB Au 1 medium formed in the collision q q Au q h q ~2-4 GeV/c pT kLkT fgPb / fgp Q2 = 5 GeV2 Cronin enhancement h p pp h+X LHCRHICSPS RAA<1~1>1 q p h x High-pT particle prod. in AA collisions • A schematic view: • Ingredients: • pp baseline (pQCD) • initial-state effects: • PDF (anti)shadowing • kT broadening (Cronin) • final-state effects: • energy loss • in-medium hadronization (coalescence) Padova, 09.05.2006 Andrea Dainese
… Parton Energy Loss • Partons travel ~4 fm in the high-density medium • Bjorken (`82): energy loss due to elastic scattering Bjorken, FERMILAB-Pub-82/59-THY (1982). Padova, 09.05.2006 Andrea Dainese
path length L hard parton Parton QCD Energy Loss • Partons travel ~4 fm in the high colour-density medium • Successive calculations (`92 ): a QCD mechanism dominates, medium-inducedgluon radiation • Coherent wave-function gluon acummulates kT due to multiple inelastic scatterings in the medium it decoheres and is radiated kT Gyulassy, Pluemer, Wang, Baier, Dokshitzer, Mueller, Peigne’, Schiff, Levai, Vitev, Zhakarov, Salgado, Wiedemann, … Padova, 09.05.2006 Andrea Dainese
path length L w kT l Calculating Parton Energy Loss BDMPS-Z formalism STATIC MEDIUM transport coefficient Radiated-gluon energy distrib.: (BDMPS case) Casimir coupling factor: 4/3 for q, 3 for g sets the scale of the radiated energy related to constraint kT <w, controls shape at w << wc Baier, Dokshitzer, Mueller, Peigne‘, Schiff, NPB 483 (1997) 291. Zakharov, JTEPL 63 (1996) 952. Salgado, Wiedemann, PRD 68(2003) 014008. Padova, 09.05.2006 Andrea Dainese
Finite parton energy (qualitatively) • If E < wc (e.g. small pT parton with large L): • dependence on parton energy • : smaller sensitivity to density Calculating Parton Energy Loss gluons volume-density and interaction cross section Probe the medium Padova, 09.05.2006 Andrea Dainese
pT dN/dpT PYTHIA Fragmentation pT – DE pT Application: Parton Quenching Model • BDMPS + Glauber-based medium geometry and density profile + PYTHIA for parton generation and fragmentation • The procedure in short: • generate parton (q or g) with PYTHIA (or back-to-back pair) • calculate its L and average along the path • use quenching weights (from wdI/dw) to get energy loss • quench parton and then hadronize it (independent fragm.) Dainese, Loizides, Paic, EPJC 38 (2005) 461. Padova, 09.05.2006 Andrea Dainese
(Quark Matter 05) matches pT-indepence of suppression at high pT Model vs RHIC “light” data • Density ( ) “tuned” to match RAA in central Au-Au at 200 GeV Similar results by Eskola, Honkanen, Salgado, Wiedemann Dainese, Loizides, Paic, EPJC 38 (2005) 461. Padova, 09.05.2006 Andrea Dainese
pT > 4.5 GeV Model vs RHIC dataCentrality dependence of RAA • Centrality evolution according to Glauber-model collision geometry Dainese, Loizides, Paic, EPJC 38 (2005) 461. Padova, 09.05.2006 Andrea Dainese
STAR preliminary Charged hadrons Testing L-dependence? RAA for 63Cu-63Cu RAA Padova, 09.05.2006 Andrea Dainese
PHENIX p0 v2 v2 from E loss: Dainese, Loizides, Paic, EPJC 38 (2005) 461 Testing L-dependence? v2 at high pT from E loss • The azimuthal asymmetry --v2 or RAA(f)-- of high-pT particle yields in non-central collisions tests the path-length dependence of E loss (almond-shaped medium) Padova, 09.05.2006 Andrea Dainese
Extrapolation in c.m.s. energyIntermediate RHIC energy s = 62 GeV • Extrapolation in s: assuming Ngluons/volume (s)0.6 (EKRT saturation model) • First test: energy extrapolation works reasonably well EKRT: Eskola, Kajantie, Ruuskanen, Tuominen, NPB 570 (2000) 379. PHENIX Coll., JPG 31 (2005) S473. Padova, 09.05.2006 Andrea Dainese
Model prediction for LHC • Extrapolation to LHC according to saturation model gives: • Most partons are absorbed • Already at RHIC, only those from the surface can escape … surface emission dominates Padova, 09.05.2006 Andrea Dainese
Model vs RHIC dataDisappearence of the away-side jet near side away side STAR Coll., nucl-ex/0501016. Padova, 09.05.2006 Andrea Dainese
Observation of true di-jets in Au-Au STAR Padova, 09.05.2006 Andrea Dainese
Di-jets associated yields (IAA) Padova, 09.05.2006 Andrea Dainese
Tangential di-jets? Padova, 09.05.2006 Andrea Dainese
Putting the pieces together Padova, 09.05.2006 Andrea Dainese
? large RAA indep. of Limited sensitivity of RAA • Strong suppression requires very large density • Surface emission scenario • RAA determined by geometry rather than by density itself • Limited sensitivity to • Need more differential observables: • massive partons • study of jet shapes • … Eskola, Honkanen, Salgado, Wiedemann, NPA 747 (2005) 511. Padova, 09.05.2006 Andrea Dainese
Heavy Quarks Padova, 09.05.2006 Andrea Dainese
Gluonsstrahlung probability Dokshitzer Q Lower E loss for heavy quarks ? • In vacuum, gluon radiation suppressed at q < mQ/EQ “dead cone” effect • Dead cone implies lower energy loss(Dokshitzer-Kharzeev, 2001): • energy distribution wdI/dw of radiated gluons suppressed by angle-dependent factor • suppress high-w tail Dokshitzer, Khoze, Troyan, JPG 17 (1991) 1602. Dokshitzer and Kharzeev, PLB 519 (2001) 199. Padova, 09.05.2006 Andrea Dainese
m/E = 0.1 massive massless dead cone Lower E loss for heavy quarks ? • Detailed massive calculation shows that: Medium-induced gluon radiation fills the dead cone Still, kT-integrated radiation is suppressed, but only for quite large m/E (i.e. large m, low pT)* kT R = 105 *Warning: large uncertainties Armesto, Salgado, Wiedemann, PRD 69 (2004) 114003. Padova, 09.05.2006 Andrea Dainese
Model for heavy quarks • Initially only charm • Baseline at RHIC: PYTHIA, with EKS98 shadowing, tuned to match pT-shape of D cross section measured in d-Au by STAR (QM04) • Heavy-quark E loss using (m/E)-dependent QW • extracted from p0,hRAA (central) • Thermalize charms that lose all energy dN/dmTmT exp(-mT/T), T = 300 MeV baseline dN/dpT baseline RAA Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027. EKS98: Eskola, Kolhinen, Salgado, EPJC 9 (1999) 61. Padova, 09.05.2006 Andrea Dainese
other contributions become important larger uncertainties Charm RAA at RHIC (extracted from light-hadron data) effect of the mass thermalized component Small effect of mass for charm (~50% for D, ~30% for e) at low pT [large uncertainties!] Basically no effect in “safe” pT-region Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027. Padova, 09.05.2006 Andrea Dainese
Large uncertainty on b/c crossing point in pT: from scales/masses variation it changes from 3 to 9 GeV PHENIX hep-ex/0508034 Note: still, electron cross section in pp at 200 GeV underestimated at high pT Charm + beauty + DY electrons at RHIC(pQCD baseline revised, and its uncertainties) FONLL: electron spectrum may be ~50% c + ~50% b for 3 < pT < 8 GeV Drell-Yan component investigated as well: < 10% up to 10 GeV FONLL calculation: Cacciari, Nason, Vogt, PRL95 (2005) 122001 Drell-Yan from: Gavin et al., hep-ph/9502372 Comparison: Armesto, Cacciari, Dainese, Salgado, Wiedemann, hep-ph/0511257 Padova, 09.05.2006 Andrea Dainese
Theory predicts e RAA (~0.4)larger than p RAA (~0.2) e spectra in principle sensitive to mass hierarchy Perturbative uncertainty on the pp baseline comparable to model-intrinsic uncertainty in determination of Preliminary STAR data not conclusive for comparison with theory RAA of c and b electrons at RHIC • Due to larger mass of b quark electron RAA increased to ~0.4 • Mass uncertainty (in E loss) and scales uncertainty (in pQCD b/c crossing) were studied b c+b c Armesto, Cacciari, Dainese, Salgado, Wiedemann, hep-ph/0511257 Padova, 09.05.2006 Andrea Dainese
Heavy Quark Energy Loss at LHC • ~100 cc pairs and ~5 bb pairs per central Pb-Pb collision • Experiments will measure with good precision RAA for D and B, and for their decay leptons What can we learn from a comparative quenching study of massive and massless probes at the LHC? Padova, 09.05.2006 Andrea Dainese
Heavy Flavour RAA at LHC • Baseline: PYTHIA, with EKS98 shadowing, tuned to reproduce c and b pT distributions from NLO pQCD (MNR) • (m/E)-dep. E loss with • * EKRT Saturation model: Eskola, Kajantie, Ruuskanen, Tuominen, NPB 570 (2000) 379. Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027. MNR: Mangano, Nason, Ridolfi, NPB 373 (1992) 295. Padova, 09.05.2006 Andrea Dainese
mass effect For 10 < pT < 20 GeV, charm behaves like a m=0 quark, light-flv hadrons come mainly from gluons RD/h enhancement probes colour-charge dep. of E loss RB/h enhancement probes mass dep. of E loss Colour-charge and mass dep. of E loss with Heavy-to-Light ratios at LHC Heavy-to-light ratios: Compare g h , c D and b B Armesto, Dainese, Salgado, Wiedemann, PRD71 (2005) 054027 Padova, 09.05.2006 Andrea Dainese
Beauty-to-Charm ratio: RB/D • Compare c and b same colour charge • Mass effect Enhancement of factor ~2, independent of (for ) • Challenging from experimental point of view… Padova, 09.05.2006 Andrea Dainese
Summary • One of the most exciting discoveries at RHIC ! • We are dealing with energy loss in an extremely opaque medium, but … • theoretically, we are just starting to learn how to probe the medium; still large uncertainties (see also next talk) • experimentally, going to more differential observables • vs system size / azimuthal angle (path-length dependence) • vs quark mass -- c (b) SUPPRESSED at RHIC, but need to disentangle c and b to clarify picture • The LHC will be a `hard / heavy probes machine’ and quenching studies will play a central role • Example: Heavy-to-light ratios as probes of E loss… • … colour-charge dependence (RD/h) • … parton-mass dependence (RB/h, RB/D?) Padova, 09.05.2006 Andrea Dainese
EXTRA SLIDES Padova, 09.05.2006 Andrea Dainese
NA57 PLB623 (2005) 17 NA49, QM2005 E loss (Wang) Parton energy loss at SPS (s 17 GeV)?RCP(K) vs E loss calculations • Wang calculation: pQCD + Cronin + (anti-)shadowing + energy loss (gluon density scaled by dNch/dh) • Good agreement between data and both Wang and PQM calculations ( ) Padova, 09.05.2006 Andrea Dainese
Open points (2): the opacity problem • Can we really probe the medium? • Need to relate extracted to an energy density e • QCD estimate for ideal QGP: • A recent analysis* of RHIC data, similar to that presented, extracts energy density 5 larger than that estimated from produced transverse energy dET/dy (Bjorken estimate) • Opacity problem: the interaction of the hard parton with the medium is much stronger than expected (Baier) Baier, NPA 715 (2003) 209. * Eskola, Honkanen, Salgado, Wiedemann, NPA 747 (2005) 511. Padova, 09.05.2006 Andrea Dainese
Collisional energy loss revisited Mustafa and Thoma, Acta Phys.Hung. A22, 93 (2005) Earlier estimates: collisional (elastic) energy loss is negligible relative to radiative effects New estimate: Boltzmann transport + 1-D Bjorken expansion Q(p) = RAA RAA = 0.25-0.4 for reasonable pathlength, similar to radiative E-loss Can we really ignore collisional energy loss? Padova, 09.05.2006 Andrea Dainese
8 < pT(trig) < 15 GeV/c STAR Preliminary pT(assoc) > 5 GeV/c 1/Ntrig dN/dj Reappearance of di-jets:surface emission of tangential partons? Padova, 09.05.2006 Andrea Dainese
Medium Expansion • The density of scattering centres is time-dependent: a = 1.5, 1.0, 0.5, 0 • Dynamical scaling law: same spectrum obtained for equivalent static trasport coefficient • Calculations for a static medium apply to expanding systems Salgado and Wiedemann, PRL 89 (2002) 092303 Padova, 09.05.2006 Andrea Dainese
Static vs Bjorken Expansion Padova, 09.05.2006 Andrea Dainese
Static vs Bjorken: numbers Padova, 09.05.2006 Andrea Dainese
f = p/2 f = 0 Armesto, Cacciari, Dainese, Salgado, Wiedemann, hep-ph/0511257, PLB to appear v2 at high pT from E loss • The azimuthal asymmetry --v2 or RAA(f)-- of D and B mesons in non-central collisions tests: • at low/moderate pT: recombination scenario, v2 of c/b quarks, hence degree of thermalization of medium • at higher pT: path-length dependence of E loss (almond-shaped medium => v2~ 0.05--0.10) STAR results withdrawn; new analysis in progress PHENIX p0 v2 Cole @ Quark Matter 05 v2 from E loss: Dainese, Loizides, Paic, EPJC 38 (2005) 461 Padova, 09.05.2006 Andrea Dainese