350 likes | 354 Vues
News from alice. ALICE Experiment News from pp collisions Physics Results from PbPb. Pasquale Di Nezza. LNF, Mar 2011. ALICE. Technologies: 18 Tracking: 7 PID: 6 Calo. : 5 Trigger, N ch :11. Collaboration: > 1000 Members > 100 Institutes > 30 countries. Detector:
E N D
News from alice • ALICE Experiment • News from pp collisions • Physics Results from PbPb Pasquale Di Nezza LNF, Mar 2011
ALICE Technologies:18 Tracking: 7 PID: 6 Calo. : 5 Trigger, Nch:11 Collaboration: > 1000 Members>100 Institutes > 30 countries Detector: Size: 16 x 26 meters Weight: 10,000 tons 2
Ready for Data EMCal, the LNF baby!
pp Physics in ALICE • Core Business is Heavy Ions • Physics with pp • collect ‘comparison data’ for heavy ion program • many signals measured ‘relative’ to pp • comprehensive study of MB@LHC • tuning of Monte Carlo (background to BSM) • soft & semi-hard QCD • very complementary to other LHC expts • address specific issues of QCD • very high multiplicity pp events • dNch/dh comparable to HI => mini-plasma ?
Data Samples LHC performed exceedingly well: - increasing pp L by 105 in 2010 (L > 2x1032) - delivering > 8 mb-1 Pb in 4 weeks (L > 2x1025, ~ 1/20 Lmax)
Physics exploitation of ALICE started for good ! submitted to EPJC 28 Nov 2009 Recorded all of 280 events • It took: • 20 years to built ALICE • 40 minutes to take the first data • 1 hour to get the prel. result (±10%) • 2 days for the final result • and 3 days to agree on the Authorlist 6
BE enhancement vs qINV unlike sign p+p- high Nch, kT high multiplicity high momentum (Phojet) baseline low multiplicity low momentum kT Bose Einstein Correlations • QM enhancement of identical Bosons at small momentum difference • ‘enhancement’ rel. to phase-space and any non-BE correlations (‘baseline’) • non-BE correlations important at high √(s) (‘minijets’) • less so at RHIC, but definitely at FNAL/LHC ! Important Message: At LHC, even MB events show signs of pQCD ! We may not always be able to rely on common wisdom & analysis techniques which were correct at lower energies
Source Radius vs pair momentum Using different Baselines Source Radius vs Multiplicity HBT @ 900 GeV Results: • Radius increases with Nch, comparable to ISR, RHIC, TeV • much smaller <kT> dependence than at FNAL dependence usually interpreted as sign of ‘flow’ in heavy ions sign. systematic uncertainty from ‘baseline’ shape • neglecting non-BE correlations (‘flat baseline’) can cause kT dependence (at high √ s)!
|h| < 1 L/K0SRatio 900 GeV |y| < 0.75 • very good agreement between STAR (200 GeV) and ALICE (900 GeV) • very different from CDF (630/1800) and UA1 (630) for pT > 1.5 GeV - UA1(630) and CDF(630) don’t agree either … to be further investigated (different triggers, acceptance, feed-down correction ?)
string junction M M M B B B p/p Ratio • Can one stop a proton ‘on its track’ at LHC ? • where does the conserved baryon number reappear after the pp collision ? • fragmentation function f(z) of baryon number • Di-quark qq: z2 => a = -1, small Δy • single q: √z =>a = 0.5, medium Δy • no valence q:a = ??; large Δy ?? Veneziano: a ≈ 0.5 others: a ≈ 1(pQCD estimates, s(p-pbar annihilation), ‘odderon’) a ≈ 1 => f(y) = constant, pbar/p approaches 1 very slowly (< 0.95 at LHC) d u za ~ e –aΔ y =e –(1-α)Δ y (Δy »1) α = intercept of relevant Regge trajectory Δy = ybeam – ybaryon = ‘rapidity loss’ u B.Z. Kopeliovich, Sov. J. Nucl. Phys. 45, 1078 (1987); G.C. Rossi and G. Veneziano, Nucl. Phys. B123, (1977) 507 u d SJ u
p/p vs pT p/p vs √s 900 GeV |y|<0.5 QGSM aSJ = 0.9 enhanced stopping MC’s HIJING/B, Perugia-soft 7 TeV |y|<0.5 0.9 TeV: p/p = 0.957±0.006(stat) ±0.014(syst) 7 TeV: p/p = 0.990±0.006(stat) ±0.014(syst) p/p Ratio @ LHC standard Pythia Results: • you can’t stop a proton on its track (at least not at LHC) • ‘string junction’ picture: aSJ ≈ 0.5 (G.V. was right !) • little room for any additional diagrams which transport baryon number over large rapidity gaps
Much more to come... S*→ L p K*→ K p X → L p W → L K < 2% of statistics at 7 TeV
Charm at 7 TeV D0→ K p p p D0→ K p D+→ Kpp Lc→ pKp Ds+→ Φπ→ KKp D*→ D0p
J/Y→ m+m- , e+e- at 7 TeV <pT2> vs √ s <pT> vs √ s J/Y → m+m- Prel. Results: • agreement ALICE/LHCb • <pT>&<pT>2 follow √s trend • energy dependence ~ pQCD
Events A Single Event • Properties of average events instead of average event properties
Anti-Nuclei ~ 2 M Pb-Pb Min Bias events
Role of LHC after RHIC/SPS • Search for the ‘QGP’ is essentially over • Discovery of QGP is well under way (with fantastic results & surprises at RHIC) • Measuring QGP parameters has just begun • 1) Quantitative differences • significantly different state of QGP in terms of energy density, lifetime, volume • large rate for ‘hard probes’ : jets, heavy quark states (b,c,U,J/Y ),… • 2) Test & validate the HI ‘Standard Model’ (< 10 years old !)QGP = very strongly interacting (almost) perfect liquid • Test predictions/extrapolations from RHIC to LHC • examples: flow (‘soft’) Quarkonia suppression (‘hard’) • 3) ‘Precision’ measurements of QGP parameters • Quantitative and systematic study of the new state of matter • Equation-of-Statef(e,p,T), viscosityh (flow), transport coefficientq (jet quenching), Debye screening mass (Quarkonia suppression), … • Confront data with Theory and Models: • standard tools: Lattice QCD, pQCD, Thermo- and Hydrodynamics, … • new tools: AdS/CFT (‘duality’), Classical QFT (‘Colour Glass Condensate’) • 4) Surprises ? • we are dealing with QCD in the strong coupling limit ! > 10 year program where are we after 3 weeks ? ^
pQCD shadowing saturation pp extrapolations Who gets it right and why ? • Multiplicity and Energy density e: • dNch/dh (@5%) ~ 1600 ± 76 (syst) 8.3 per participant pair • somewhat on high side of expectations • growth with √s faster in AA than pp (√s dependent ‘nuclear amplification’) • Energy density ≈ 3 x RHIC (fixed t) • lower limit, likely t0(LHC) < t0(RHIC)
Saturation Models HIJING DPMJET Who gets it right and why ? • dNch/dh as a function of centrality (norm. to ‘overlap volume’ ~ Nparticipants) • soft process dNch/dh~ number of scattered nucleons (strings, participants, …) • ‘nuclear amplification’ should be energy independent • (very) hard processes dNch/dh~ number of nucleon-nucleon collisions • getting more important with √s & with centrality • DPMJET MC • gets it right for the wrong reason • HIJING MC • strong centr. dependentgluon shadowing • Others • saturation models: Color Glass Condensate,‘geometrical scaling’ fromHERA/ photonuclear react. Important constraint for models sensitive to details of saturation
Bits and Pieces ... • Centrality determination with Glauber fits: • very tight correlation of several centrality measures (different acceptance/detectors) • 1/√N is a small number at LHC ! • very robust results fitting centrality between 30% and 90% of total cross section • Glauber fit Nch ~ f * Npart + (1-f) * Ncoll visible better than Nch ~ (Npart)a • NpartGlauber fit = Npart from pure nuclear geometry (slicing in impact paramet b) ZDC – V0 Trigger scintillators V0A: 2.8 < h < 5.1 V0C: -2.5 < h < -3.7 TPC tracks – V0
DE Fragmentation function f(z) Jet Quenching • Jet quenching:jet E -> jet E’ (=E-DE) + soft gluons (DE) modified jet fragmentation function via matter induced gluon radiation/scattering => QGP properties • how much energy is lost ? (measures e.g. q^) • very difficult question, may depend on jet cone R, pt-cutoff, .. • how is it lost ? (e.g. multiple soft or few hard gluons ?) • look at soft part of f(z), pt < 2-5 GeV • ‘response of QGP’ (shock waves, Mach cones ??) • properties of bulk matter around jet, pt ~ 1 GeV Atlas • Both Atlas and CMS see very striking effects • in the dijet-imbalance for central events ! • Atlas paper: http://arxiv.org/abs/1011.6182 • Cacciari et al: http://arxiv.org/abs/1101.2878
Jets in ALICE (TPC) we see qualitatively a similar effect quantitative analysis is ongoing small acceptance (statistics), => need full 2010 data try to include low pt (study pt-cut off dependence of imbalance) 10-20% peripheral 168 GeV 192 GeV Dh Df 0-10% central 102 GeV 47 GeV Df Dh Charged Jets bin size: 0.1x0.1
Data driven Interpolation 900 GeV & 7 TeV or using NLO for change in shape 7 TeV * NLO (2.76 TeV)/NLO(7 TeV) Including CDF data 0.9 TeV * NLO (2.76 TeV)/NLO(0.9 TeV) Jet Quenching as seen by pt spectra • Suppression of high pt particles ( ~ leading jet fragments) • Minimum RAA ~ 1.5 – 2 x smaller than at RHIC • Rising with pt ! (ambiguous at RHIC !) • accuracy limited by pp reference => need pp at 2.76 TeV ! RAA = 1 for (very) hard QCD processes in absence of nuclear modifications
Associated pTt Δf Trigger High pT Particle Correlations ‘near’ side ‘away’ side UE Trigger Particle Trigger Particle: highest pT particle in event (pTt) Associate Particle: all the others (pTa)
q q Star@RHIC pT,trig 8-15 GeV Jet Quenching seen by High pT Correlations • classic ‘jet quenching signal’ • away side correlation in central Pb-Pb washed out up to pT,trig > 10 GeV PT associated 2 – 6 GeV ‘near’ side ‘away’ side Df Df Df
pp Pb PbPb peripheral pp 7 TeV CMS pp 7 TeV ‘near side ridge’ PbPb central PbPb central Jet Quenching (?) seen via Multiparticle Correlations • ‘Autocorrelation’: d2Nch/dDhdDf (signal)/d2Nch/dDhdDf (mixed events) • ‘near side ridge’: • - striking effect, not really understood • response of QGP to jet quenching ? • initial state gluon radiation ? • ???
Z Reaction plane Y X f Pz Py Px Nch yield Testing the HI ‘Standard Model’ • Elliptic Flow: one of the most anticipated answers from LHC • experimental observation: particles are distributed with azimuthally anisotropy around the scattering plane • Are we sure Hydro interpretation is correct ? Elliptic Flow v2asinterpreted by Hydrodynamics Pressure gradient converts spatial anisotropy → momentum anisotropy → particle yield anisotropy
LHC ? Testing the HI ‘Standard Model’ • Hydro seems to work very well for first time at RHIC • LHC prediction: modest rise (Depending on EoS, viscosity, speed of sound, dNch/dh, ..) • (‘better than ideal is impossible’) • experimental trend & scaling predicts large increase of flow • (‘RHIC = Hydro is just a chance coincidence’) BNL Press release, April 18, 2005: Data = ideal Hydro "Perfect" Liquid New state of matter more remarkable than predicted – raising many new questions (scaled) Flow
STAR at RHIC ALICE +30% RHIC ALICE First Elliptic Flow Measurement at LHC • v2 as function of pt • practically no change with energy ! • extends towards larger centrality/higher pt ? • v2 integrated over pt • 30% increase from RHIC • <pt> increases with √s • pQCD powerlaw tail ? • Hydro predicts increased ‘radial flow’ • very characteristic pt and mass dependence; to be confirmed !
Testing the HI ‘Standard Model’ • Hydro passed the first test ! • many more tests of Hydro and the HI-SM to come…. LHC ! CERN Press release, November 26, 2010: ‘confirms that the much hotter plasma produced at the LHC behaves as a very low viscosity liquid (a perfect fluid)..’ Disclaimer: very rough guesstimate, assuming geometry not to change between RHIC and LHC
Charm in Pb-Pb • ‘Jet quenching’ with heavy quarks: • Energy loss depends on • color charge (quark/gluon) • mass (light/heavy quarks)
Conclusions and Perspectives 2010 has been a memorable year for ALICE ! • Excellent performance of the detector, data analysis has smoothly and quickly delivered the first physics results • 13 physics papers published so far, many more in the pipeline • 2011 LHC run about to start • pp (short) run at √s=2.76 TeV Essential as a reference for heavy-ion results • Long pp run at √s = 7 TeV, 1 month of PbPb at √s=2.76 TeV/N