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Selected topics in Heavy Ion PhysicsPrimorsko 2014Peter HristovLecture 4: dileptons, quarkonia, melting2Based on the lectures ofF.Antinori, E.Scomparin

3PbPb: Global characteristics@LHC: higher temperature, bigger volume, longer lifetime

T = 30451 MeV ~ 1.4 x TRHIC Lifetime: +20% wrt RHIC ~ 10 fm/c

Volume ~ 2 x RHIC (R3 ~ 300 fm3)

Energy density~ 3xRHIC~ 10 GeVfm3PRL 105, 252301 (2010)The dilepton invariant mass spectrum4The lepton (e+e-, +-) pairs provide important information on the early stages of the collisionDileptons do not interact strongly, once produced can cross the system without significant re-interactions (not altered by later stages)Several resonances can be easily accessed through the dilepton spectrum

low s version

high s versionHeavy quarkonium states5Quarkonium: bound state of q-qbar pair with mass smaller than 2mD(mB)Several quarkonium states exists, distinguished by their quantum numbers (JPC) qq

Charmonium (c-cbar) familyBottomonium (b-bbar) familyQuarkonia: probes of the QGP6Ideal properties of a QGP probeCreated early in the history of the collision and sensitive to the short-lived QGP phaseProduction in elementary NN collisions under control (accessible reference)Interaction with cold nuclear matter under controlNot (or slightly) sensitive to the final-state hadronic phaseHigh sensitivity to the properties of the QGP phase

VACUUMHADRONICMATTERQGPColor Screening7At T=0 the binding between the quarks can be described by the Cornell potential

The QGP contains deconfined color charges => color screeningThe confinement contribution disappears The high color density induces a screening of the coulombian term of the potentialqqqq

confinement termCoulombian term due togluon exchangeResonance melting8Screening stronger at high TD => maximum size of a bound state, decreases when T increasesDifferent states, different sizesResonance melting=> QGP thermometer

Screening ofstrong interactionsin QGP

Feed-down and suppression pattern9Feed-down process: charmonium (bottomonium) ground state resonances can be produced through decay of larger mass quarkonia => Effect : ~30-40% for J/, ~50% for (1S)Due to different dissociation temperature for each resonance, one should observe steps in the suppression pattern of measured J/ or (1S)

J/(3S)b(2P)(2S)b(1P)(1S)(2S)c(1P)J/Digal et al., Phys.Rev. D64(2001)094015Ideally, one could vary Tby studying the same system (e.g. Pb-Pb) at various sby studying the same system for various centrality classes

Quarkonium regeneration10Statistical approach:Charmonium fully melted in QGPCharmonium produced, together with all other hadrons, at chemical freeze-out, according to statistical weightKinetic recombination:Continuous dissociation/regeneration over QGP lifetimeContrary to the suppression scenarii described before, these approaches may lead to a J/ enhancement

At sufficiently high energy, the cc pair multiplicity becomes largeEffect of cold nuclear matter11There is suppression of the J/ already in pA! This effect can mask a genuine QGP signal. Needs to be calibrated and factorized outCommonly known as Cold Nuclear Matter Effects (CNM)Effective quantities are used for their parameterization (, abs, )

NA50, pA 450 GeV

SPS: the anomalous J/ suppression12In semi-central and central Pb-Pb collisions there is suppression beyond CNM => anomalous J/ suppressionMaximum suppression ~ 30%. Could be consistent with suppression of J/ from c and (2S) decays (sequential suppression)

After correction for EKS98 shadowingDrell-Yan usedas a reference here!In-In 158 GeV (NA60)Pb-Pb 158 GeV (NA50)AnomaloussuppressionB. Alessandro et al., EPJC39 (2005) 335R. Arnaldi et al., Nucl. Phys. A (2009) 345

J/ @ RHIC 13Using RAA, no cold nuclear effects taken into accountQualitatively, very similar behavior at SPS and RHIC!Do we see (as at SPS) suppression of (2S) and c?Or does (re)generation counterbalance a larger suppression at RHIC?PHENIX: comparison between RAA at central and forward rapidity

J/ @ RHIC: forward vs central y14Stronger suppression at forward rapiditiesNot expected if suppression increases with energy density (which should be larger at central rapidity)Are we seeing a hint of (re)generation, since there are more pairs at y=0?Comparisons with theoretical models tend to confirm this interpretation, but the LHC results should clarify the situation

Quarkonia @ LHC: Advantages 15Higher charmonium states are accessible => study of regenerationPossibility for detailed study (for the first time) of bottomonium suppression

(3S)b(2P)(2S)b(1P)(1S)

Massr0J/, ALICE vs PHENIX16Even at the LHC, NO rise of J/ yield for central events, but.Compare with PHENIXStronger centrality dependence at lower energySystematically larger RAA values for central events in ALICEFirst possible evidence for (re)combination

pT dependence of the suppressionLarge pT: compare CMS with STARSmall pT: compare ALICE with models17At high pT no regeneration expected: more suppression at LHC energiesAt small pT ~ 50% of the J/ should come from regeneration

Initial temperature: melting of heavy resonances ()18T > 1.5Tc ~ 300 MeV PRL 109 (2012) 2223

CMS: in PbPb collisionsSurprisingly large (/)PbPb / (/)pp ratio confirmed: new pp reference, 20 times larger, now negligible uncertaintynon-prompt component subtracted very suppressed at high pT (more than ) RAA() = 0.13 0.05Much less at lower pT RAA() = 0.67 0.19

Talk by Moon, HIN-12-007to be submitted soon

40100%2040%020%(Centrality-integrated RAA)RGdC@QM2014Could some regeneration models favour lower pT? 1919

ALICE:J/y and RAA J/y RAA shows strong pT dependenceContrary to RHICSuggests contribution from (re)combinationp-Pb (RpA x RAp) (anti)shadowing expectation

(1S) RAARAA with LHCb reference about 50% smaller wrt interpolated referenceMore suppression in data than in transport models (Emerick et al., suppression + regeneration)

ALICEPHENIX

arXiv:1311.0214, accepted by PLBarXiv:1405.4493RpA x RAp J. Castillo (Wed AM) J. Book (Tue AM)J/y RAA vs pTfinalfinalPb-PbJFGO@QM2014U RAA vs NpartJ/y RAA/RpA vs pTRAA2020Recombination there where there are lots of charm: low pT, central, mid-rapiditytransport model: "kinetic rate-equation approach in an evolving QGP and includes both suppression and regeneration effects. CNM effects are taken into account by means of an effective absorption cross section varied between 0 and 2mb, resulting in a band for the predicted RAA . " (Model: EPJA48(2012))Conclusions on quarkonia21Very strong sensitivity of quarkonium states to the medium created in heavy-ion collisionsTwo main mechanisms at play in AA collisionsSuppression by color screening/partonic dissociationRe-generation (for charmonium only!) at high scan qualitatively explain the main features of the results Cold nuclear matter effects are an important issue (almost not covered here and in these lectures): interesting physics in itself and necessary for precision studies => study pA at the LHC

Hard probes, jets, energy losses22Based on the lectures of M. van Leeuwen, P. Jacobs, G. Salamand the talks of R. Grannier de Cassagnac,B.ColeSoft QCD matter and hard probesHard-scatterings produce quasi-free partons Initial-state production known from pQCD Probe medium through energy lossHeavy-ion collisions produceQCD matterDominated by soft partons p ~ T ~ 100-300 MeVHard Probes: sensitive to medium density, transport properties2323Hard processes in QCD: ReminderHard process: scale Q >> LQCDHard scattering High-pT parton(photon) Q ~ pTHeavy flavour production m >> LQCDCross section calculation can be split into Hard part: perturbative matrix elementSoft part: parton density (PDF), fragmentation (FF)Soft parts PDF, FF are universal: independent of hard processQM interference between hard and soft suppressed (by Q2/L2 Higher Twist) Factorization

parton densitymatrix elementFF24

Parton density distributionLow Q2: valence structureValence quarks (p = uud)x ~ 1/3Soft gluonsQ2 evolution (gluons)Gluon content of proton risesquickly with Q225pQCD illustrated

CDF, PRD75, 092006jet spectrum ~ parton spectrum

fragmentation26

Fragmentation and parton showerslarge Q2Q ~ mH ~ LQCDmFAnalytical calculations: Fragmentation Function D(z, m) z=ph/EjetOnly longitudinal dynamicsHigh-energy

parton(from hard scattering)HadronsMC event generatorsimplement parton showersLongitudinal and transverse dynamics27Nuclear modification factor RAA (again)p+pAu+AupT1/Nbin d2N/d2pTEnergy lossShifts spectrum to leftAbsorptionDownward shiftMeasured RAA is a ratio of yields at a given pTThe physical mechanism is energy loss; shift of yield to lower pT

28Components of a realistic Eloss modelEnergy loss is a distributionGeometry: density profile; path length distributionEnergy loss is partonic, not hadronicFull modeling: medium modified showerSimple ansatz for leading hadrons: energy loss followed by fragmentationQuark/gluon differences29Medium-induced radiation

propagating partonradiatedgluonLandau-Pomeranchuk-Migdal effect: the quantum interference between successive scatterings leads to suppressionFormation time importantRadiation sees length ~tf at onceEnergy loss depends on density:

and nature of scattering centers(scattering cross section)Transport coefficientCR: color factor (q, g) : medium densityL: path lengthm: parton mass (dead cone eff)E: parton energy

Path-length dependence Lnn=1: elasticn=2: radiative (LPM regime)n=3: AdS/CFT (strongly coupled)Energy loss30Energy loss theory: four formalismsHard Thermal Loops (AMY)Dynamical (HTL) mediumSingle gluon spectrum: BDMPS-Z like path integralNo vacuum radiationMultiple soft scattering (BDMPS-Z, ASW-MS)Static scattering centersGaussian approximation for momentum kicksFull LPM interference and vacuum radiationOpacity expansion ((D)GLV, ASW-SH)Static scattering centers, Yukawa potential Expansion in opacity L/l (N=1, interference between two centers default)Interference with vacuum radiationHigher Twist (Guo, Wang, Majumder)Medium characterised by higher twist matrix elementsRadiation kernel similar to GLVVacuum radiation in DGLAP evolution

Multiple gluon emissionFokker-Planckrate equationsPoisson ansatz(independent emission)DGLAPevolutionSee also: arXiv:1106.110631Energy loss distributions

Radiated gluon distributionMain theory uncertainty:Large angle radiation

Multiple gluon emission:Poisson Ansatz

Energy loss probability distributionBroad distributionSignificant contributions at DE=0, DE=E32Energy loss formalismsPlenty of room for interesting and relevant theory work!Large differences between formalisms understoodLarge angle cut-offLength dependence (interference effects)Mostly technical issues; can be overcomeUse path-integral formalismMonte Carlo: exact E, p conservationFull 23 NLO matrix elementsInclude interferenceNext step: interference in multiple gluon emission33

`known from e+e-knownpQCDxPDFextractParton spectrumFragmentation (function)Energy loss distributionThis is where the information about the medium isP(DE) combines geometry with the intrinsic process Unavoidable for many observablesNotes:This is the simplest ansatz most calculation to date use it (except some MCs)Jet, g-jet measurements fix E, removing one of the convolutionsEnergy loss: a simplified approach34Jets and parton energy loss

Motivation: understand parton energy loss by tracking the gluon radiationQualitatively two scenarios:In-cone radiation: RAA = 1, change of fragmentationOut-of-cone radiation: RAA < 135Jets at LHC

ALICEhjTransverse energy map of 1 eventClear peaks: jets of fragments from high-energy quarks and gluonsAnd a lot of uncorrelated soft background36Jet reconstruction algorithmsTwo categories of jet algorithms:Sequential recombinationDefine distance measurekT/Durham (LEP): dij=2min(E2i,E2j)(1-cosij)kT: dij = min(p2ti, p2tj)R2ij/R2, R2ij = (yi yj)2 + (i j)2Cambrige/Aachen: dij = RijAnti-kT: dij = 1/min(p2ti, p2tj)R2ij/R2Cluster closest (until given threshold)ConeDraw Cone radius R around the hardest particle (collinear unsafe)Sum the momenta and use it as new seed directionIterate until stable h,jjet = particlesFor a complete discussion, see: http://www.lpthe.jussieu.fr/~salam/teaching/PhD-courses.htmlSum particles inside jet Different prescriptions exist, most natural: E-scheme, sum 4-vectorsJet is an object defined by jet algorithmIf parameters are right, may approximate parton37Infrared & Collinear safe algorithms38ktsequential recombinationhierarchical in relative ptCatani et al 91Ellis, Soper 93NlnNCambridge/Aachensequential recombinationhierarchical in angleDokshitzer et al 97Wengler, Wobish 98NlnNanti-ktsequential recombinationgives perfectly conical hard jetsMC, Salam, Soyez 08(Delsart)N3/2SISConeSeedless iterative cone with split-mergegives economical jetsSalam, Soyez 07N2lnNPbPb jet background

Cacciari et alBackground density vs multiplicityh-j space filled with jetsMany background jetsBackground contributes up to ~180 GeV per unit areaStatistical fluctuations remain after subtractionSubtract background:

Jet finding illustration39Jet energy asymmetry

Centrality

ATLAS, arXiv:1011.6182 (PRL)Jet-energy asymmetryLarge asymmetry seen for central eventsHowever:Only measures reconstructed di-jets (dont see lost jets)Not corrected for fluctuations from detector+backgroundBoth jets are intereracting No simple observable

Suggests large energy loss: many GeV~ compatible with expectations from RHIC+theory40Jets @ LHC41Pb-Pb event with high energy asymmetry

the second jet looses a lot of energy, but is in the expected direction!

CMS: arXiv:1102.1957Jet broadening: R dependence

Ratio of spectra with different RLarger jet cone:catch more radiation Jet broadeningATLAS, A. Angerami, QM2012However, R = 0.5 still has RAA < 1 Hard to see/measure the radiated energy42Where is the energy of the missing jet?43

Z = Epart/Ejet, PbPb to ppIncrease @ low Z:soft radiationDecrease @ intermediate Z:energy loss in QGPM. Rybar@QM2012

-hadron correlations

The missing particles with highenergy are found as many particleswith low energy|-|