430 likes | 548 Views
Review of Heavy-Ion Results from the CERN LHC. David Evans The University of Birmingham. Strong and Electroweak Matter 2012 Swansea, UK, 10 th – 13 th July 2012. Outline of Talk. Introduction The LHC Experiments Selection of results from ALICE, ATLAS, & CMS Particle spectra & ratios
E N D
Review of Heavy-Ion Results from the CERN LHC David Evans The University of Birmingham Strong and Electroweak Matter 2012 Swansea, UK, 10th – 13th July 2012
Outline of Talk • Introduction • The LHC Experiments • Selection of results from ALICE, ATLAS, & CMS • Particle spectra & ratios • Flow • Jet and high pT suppression (RAA) • Heavy Flavour • Summary
Aims of Heavy-Ion Programme • Study strongly interacting matter at extreme energy densities over large volumes and long time-scales (study of QCD at its natural scale, QCD ~ 200 MeV). • Study the QCD phase transition from nuclear matter to a deconfined state of quarks and gluons - The Quark-Gluon Plasma. • Study the physics of the Quark-Gluon Plasma (QCD under extreme conditions).
Phases of Strongly Interacting Matter Both statistical and lattice QCD predict that nuclear matter will undergo a phase transition, in to a deconfined state of quarks and gluons – a quark-gluon plasma, at a temperature of, T ~ 170 MeV and energy density, ~ 1 GeV/fm3. Lattice QCD, mB = 0 Heavy-Ion collisions far exceed these temperatures & densities at the LHC. colour
Heavy Ion Collisions Create QGP by colliding ultra-relativistic heavy ions pre-equilibration QGP hadronisation freeze out Colliders: AGS, SPS, RHIC, LHC SNN (GeV) = 5.4 19 200 2760 (5500)
Observables Jets Open charm, beauty 6
LHC Detectors ALICE ATLAS CMS 7
Centrality Selection – Glauber Model • Assume particle production is factorises into number of participants Npartnumber of collisions Ncoll • AssumeNegative Binomial Distribution for both f*Npart + (1-f) Ncoll • Relation of percentile classes to Glauber values (<Npart>, etc) purely geometrical by slicing in impact parameter Energy in ZDC (a.u.)
Charged Particle Multiplicity Phys. Rev. Lett. 105, 252301 (2010) Phys. Rev. Lett. 106, 032301 (2011) dNch/d (Pb-Pb) Theory dNch/d versus s Rises with s faster than pp Fits power law: Pb-Pb ~ s0.15 dNch/d = 1584 4 (stat.) 76 (syst.) Larger than most model predictions – good agreement between LHC experiments Bjorken formula estimates energy density, > 15 GeV/fm3 (3 x RHIC)
Bose–Einstein correlations(HBT) medium size: 2x larger than at RHIC RoutRsideRlong ~ 300 fm3 medium life time: 40% longer than at RHIC ~ 10-11 fm/c • QM enhancement of identical Bosons at small momentum difference • enhancement of e.g. like-sign pions at low momentum difference qinv=|p1-p2|, ALICE, Phys. Lett. B 696 (2011) 328.
Particle spectra LHC results agree well between experiments Very significant changes in slope compared to RHIC Good agree between experiments e.g. ALICE/ATLAS in this case
Identified Particle spectra RHIC Hydro Prediction RHIC Blast Wave Fit Very significant changes in slope compared to RHIC Most dramatically for protons Very strong radial flow, b≈ 0.66 even larger than predicted by most recent hydro
Strangeness Enhancement Multi-strange baryons enhanced with respect to pp. baryons enhanced more than s.
Thermal Models Thermal model: A.Andronic et al., Phys. Lett. B673:142-145, 2009 Low Tch suggested by proton spectra excluded by , If exclude p, data agree with thermal GC model prediction Tch = 164 MeV Thermal models can not fit all data simultaneously
'Baryon anomaly': L/K0 x 3 Ratio at Maximum RHIC L/K0 • Recent EPOS model calculation describes the data well: (K. Werner, arXiv: 1204.1394) • (not decribed by coalescence model) Baryon/Meson ratio still strongly enhanced x 3 compared to pp at pT = 3 GeV/c - Enhancement slightly larger than at RHIC 200 GeV - Maximum shift very little in pT compared to RHIC despite large change in underlying spectra !
Equal energy density contours y z x P v2 Fourier coefficient Angle of reaction plane Does QGP have elliptic Flow? Study angular dependence of emitted particles pT f V1 = directed flow. V2 = elliptic flow. 16
Elliptic Flow • v2 shape is identical to RHIC • v 2 ~30% larger than RHIC: larger <pT> • p,K reproduced by hydro calculation • p ok for semi-peripheral but is a remaining challenge for central collisions ALICE, Phys. Rev. Lett. 105 (2010) 252302. Hydro-calcs.: Shen, Heinz, Huovinen and Song, arXiv:1105.3226.
Elliptic Flow – Higher Orders Superposition of flow harmonics, including higher orders, is sufficient to describe Ridge and Cone effects without implying medium response. ATLAS ATLAS Phys. Rev. Lett. 107, 032301 (2011) Higher-order coefficients (n>2) do not show strong centrality dependence – consistent with fluctuations in initial state geometry being driving force.
Flow vs Centrality Flow strongest for semi-peripheral collisions (central collisions too symmetric).
V2 at High PT V2 drops to zero at high PT
Full Harmonic Spectrum Measure multi Vn - Over-constrain Hydro Models to get /s, initial conditions etc. CMS
Elliptic Flow for Identified Particles • intermediate pT: dep. on particle species • high pT: v2 and v3 small: hydrodinamic flow negligible • Indication for non zero D meson v2 (3σ in 2 < pT < 6 GeV/c) • Comparable with charged hadrons elliptic flow
Studying deconfinement with jets key QCD prediction: jets are quenched X.-N. Wang and M. Gyulassy, Phys. Rev. Lett. 68 (1992) 1480 quark di-quark radiated gluons heavy nucleus Jets in heavy ion collisions Free quarks not allowed when quark, anti-quark pairs are produced they fly apart producing back-to-back ‘jets’ of particles Fragmentation soft beam jet pTOT pT pL jet Interaction at the quark (parton) level The same interaction at the hadron level Quarks remain free in QGP and lose energy while travelling through 23
Charged Jets in Pb-Pb 10-20% peripheral 168 GeV 192 GeV Dh Df bin size: 0.1x0.1 0-10% central 102 GeV 47 GeV Df Dh Large suppression of jets seen in central collisions Back-to-back high-energy jets seen in peripheral collisions 25
Jet Suppression Significant imbalance between leading and sub-leading jets for central collisions
Jet Suppression Also seen by CMS
Where is Jet Energy? Another way to study this effect is to look at high pT suppression. Jet energy unbalanced inside cone, but unbalanced in opp direction outside cone - Overall balance is achieved.
Nuclear Modification Factor Divide pT spectra in Pb-Pb by p-p (with suitable normalisation factor) • particle interaction with medium • stronger suppression at LHC • ~factor 7 at 7 GeV/c Leading Hadron quark-gluon plasma
RAA vs centrality • Suppression increases with centrality. • Minimum remains around 6-7 GeV/c
RAA for s, Ws & Zs No suppression as expected
Strange RAA for different particles • For hadrons containing heavy quarks, smaller suppression expected • Enhancement of /K clearly seen • At high pT (>8-10 GeV/c) RAA universality for light hadrons
Rcp of Jets Jets suppressed by factor 2 in central w.r.t. peripheral collisions. Suppression flat with pT. Rcp of muons from heavy flavour decays about 0.4 for central collisions. Again, flat for 0.4 < PT < 14 GeV
Charm RAA for Heavy Flavour ● Hc,b → muons ● D mesons (charm) ● B → J/ψ (beauty) CMS arXiv:1201.5069 • Charm and beauty: no evidence of mass effects yet • Maybe a hint of hierarchy w.r.t pions!
RAA J/: Nuclear Modification Factor ALICE pT > 0 PHENIX pT > 0 almost no centrality dependence above Npart ~ 100 Unlike at RHIC Inclusive J/ RAA = 0.497 0.006 (stat.) 0.078 (syst.) Rather small suppression and less than at RHIC (forward rapidity, PT > 0)
RAA J/ Greater suppression seen at LHC in mid-rapidity, at pT > 6.5 GeV And centrality dependent. At low-pt, ALICE RAA ≥ 0.5, no centrality dependence → behaviour clearly different to the one observed in PHENIX where a larger suppression and a strong centrality dependence is seen At high-pt, larger suppression seen both by ALICE and CMS → behaviour is similar to the one observed at low energy
ϒ Suppression NΥ(2S)/NΥ(1S)|pp = 0.56 ± 0.13 ± 0.01 NΥ(2S)/NΥ(1S)|PbPb = 0.12 ± 0.03 ± 0.01 NΥ(3S)/NΥ(1S)|pp = 0.21 ± 0.11 ± 0.02 NΥ(3S)/NΥ(1S)|PbPb < 0.07 Ratios not corrected for acceptance and efficiency
ϒ(1S) and ϒ (2S) RAA – ϒ(1S) RAA in 7 centrality bins – first results on ϒ(2S) RAA – clear suppression of ϒ(2S) – ϒ(1S) suppression consistent with excited state suppression (~50% feed down)
Summary • We have started to piece together the standard model of heavy-ion collisions • We have measured many of the global features • Phase transition temperature - Agrees with theory Tc ~ 164 MeV • Energy density - Over 10x critical energy density, > 15 GeV/fm3 • size & lifetime of system ~ 5000fm3, ~ 10-11 fm/c • Started to measure dynamical features • Very strong radial flow, b≈ 0.66 • Strong elliptical flow – ideal liquid • Strong suppression of jets and high pT particles • Little evidence of quark mass dependence on energy loss (unlike predictions – although maybe a hint at this stage) • J/ suppressed less than RHIC at low PT - ϒ’ and ϒ’’ suppressed. • We are at the early stages of a 10 year programme and have just scratched the surface. • lots of work and exciting physics to look forward to. • Thank you for listening
Chiral Magnetic Effect ('strong parity violation') ? B - + RHIC ? RHIC B • Same charge correlations positive • Opposite charge correlations negative • RHIC ≈ LHC • somewhat unexpected • should decrease with Nch • may decrease with √s RHIC : (++), (+-) different sign and magnitude LHC: (++),(+-) same sign, similar magnitude + Local Parity Violation in strong magnetic Field ? -
Particle Identification ITS Silicon Drift/Strip dE/dx Ω ΛΚ TPC dE/dx • dE/dx:5% resolution • Time-of flight,resolution: ~85ps (Pb-Pb)~110 ps (p-p) • TPC dE/dx: separate p from K up to 1.1 GeV • Time of flight: separate K from up to ~ 2.2GeV TOF