The Quark-Gluon Plasma and the ALICE Experiment

Transcription

1 The Quark-Gluon Plasma and the ALICE Experiment David Evans The University of Birmingham IoP Nuclear Physics Conference 7 th April 2009 David Evans IoP Nuclear Physics Conference

2 Outline of Talk Introduction to UR heavy-ion physics & the Quark-Gluon Plasma Review of major Heavy-Ion Results The ALICE Detector First proton-proton physics at ALICE Programme for 2009/2010 Summary David Evans IoP Nuclear Physics Conference

3 Aims of Ultra-Relativistic Heavy Ion Physics Study strongly interacting matter at extreme energy densities over large volumes and long time-scales. Study the role of chiral symmetry in the generation of mass in hadrons (accounts for over 98% of mass of nuclear matter). Study the nature of quark confinement. Study the QCD phase transition from nuclear matter to a deconfined state of quarks and gluons - The Quark-Gluon Plasma. I.e. study the equation of state of nuclear matter. Study the physics of the Quark-Gluon Plasma (QCD under extreme conditions). David Evans IoP Nuclear Physics Conference

4 The Quark-Gluon Plasma Normal hadronic matter Quarks confined in baryons (e.g. nucleons) or mesons (e.g. pions) Under extreme conditions of temperature and/or density nuclear matter melts into a plasma of free quarks and gluons. David Evans IoP Nuclear Physics Conference

5 Phases of Strongly Interacting Lattice QCD, m B = 0 Matter Both statistical and lattice QCD predict that nuclear matter will undergo a phase transition at a temperature of, T ~ 170 MeV and energy density, ~ 1 GeV/fm 3. David Evans IoP Nuclear Physics Conference

6 Phases of Strongly Interacting Matter colour David Evans IoP Nuclear Physics Conference

7 How to Make a QGP Collide ultra-relativistic heavy ions David Evans IoP Nuclear Physics Conference

8 Observables Jets Open charm, beauty David Evans IoP Nuclear Physics Conference

9 Key Results from SPS & RHIC SPS ( ) QGP signatures seen e.g. Enhancement of strangeness (up to a factor of ~20). Suppression of J/ (c c-bar meson). Discovery of new state of matter announced Feb RHIC (2000-present) results from SPS exps confirmed, plus Elliptic flow: collective flow of final state hadrons wrt to the reaction plane medium behaves like an ideal liquid rather than a gas as expected. Jet quenching: suppression of high p T hadrons wrt yield expected from superposition of nucleon-nucleon collisions. David Evans IoP Nuclear Physics Conference

10 Statistical Hadronisation at RHIC 10/51 Chemical freeze-out; Two parameters: T ch and m B; T ch = (160±2) MeV m B ; = (20±4) MeV A. Andronic et al., arxiv:nucl-th/ v3 David Evans IoP Nuclear Physics Conference

11 Phases of Strongly Interacting Matter colour David Evans IoP Nuclear Physics Conference

12 Evidence of collective behaviour Equal energy density contours P p T v 2 E d 3 N d 3 p 1 d 2 N 2 p T dp T dy 1 n 1 2v n cos n r Fourier coefficient Angle of reaction plane v 2 cos2 V 1 = directed flow. V 2 = elliptic flow. David Evans IoP Nuclear Physics Conference

13 Measuring elliptic flow Plot yield as a function of angle with respect to the reaction plane. Effect strongest in peripheral collisions where spatial asymmetry is largest. ± 6% variation v 2 cos suppressed zero Data fits ideal hydro-dynamical models i.e. zero viscosity David Evans IoP Nuclear Physics Conference

14 Flow continued Eccentricity: Z y Y X Flow: x It s been predicted (Phys Lett B474 (2000) 27.) in the low density limit, elliptic flow (v 2 ) and the density of scattering centres. Ie. v 2 / (1/S) dn ch /d where S is the area of overlap. v2/ should saturate at large particle densities (hydro-limit). David Evans IoP Nuclear Physics Conference

15 Is the QGP an ideal fluid? L H C Hydro limit RHIC data runs out at predicted hydro limit. Will data trend continue or does it flatten? David Evans IoP Nuclear Physics Conference

16 Jets in heavy ion collisions Studying deconfinement with jets Fragmentation key QCD prediction: jets are quenched X.-N. Wang and M. Gyulassy, Phys. Rev. Lett. 68 (1992) 1480 heavy nucleus radiated gluons quark di-quark soft beam jet jet p TOT p L p T Interaction at the quark (parton) level Models of jet suppression Various approaches; main points: DE med is independent of parton energy. DE med depends on length of medium, L. The same interaction at the hadron level DE med gives access to gluon density dn g /dy or transport coefficient Multiple soft scattering: Weidemann et al. Opacity expansion: Gyulassy et al. Twist expansion: Wang et al. David Evans IoP Nuclear Physics Conference Leads to a deficit of high p T hadrons compared to p+p collisions (no medium). q ˆ k T 2

17 Suppression of high p T hadrons Central collisions Nuclear modification factor R AA (p T ) d 2 N AA /dp T d T AA d 2 NN /dp T d Scale factor no. binary collisions T AA N binary p+p reference pp inelastic Divide P T Spectra of AA by pp (with scaling factor) David Evans IoP Peripheral Nuclear Physics collisions Conference

18 Use R AA to determine the medium density Eskola, Honkanen, Salgado, Wiedemann (2004) Need fully reconstructed, higher energy jets to determine medium density and process of energy loss. Hence need to go to higher energy collisions. The medium is dense (30-50 x normal matter), but R AA provides limited sensitivity. David Evans IoP Nuclear Physics Conference

19 Why Heavy LHC? hotter - bigger -longer lived Central collisions SPS RHIC LHC s 1/2 (GeV) dn ch /dy x10 3 (GeV/fm 3 ) V f (fm 3 ) x10 3 2x10 4 t QGP (fm/c) < ALICE is the only experiment able to study almost all observables (many on an event-by-event basis). David Evans IoP Nuclear Physics Conference

20 New Measurements from the 20/51 LHC Large production cross-sections for large mass particles high p T and jets hard probes. Some measured for first time - ( bb ), Z 0, jets Jets up to 200 GeV Factor > 1000 in high P T X 2000 Pion Production All observables in a single, high precision detector ALICE Many on event-by-event basis David Evans IoP Nuclear Physics Conference

21 Size: 16 x 26 metres Weight: 10,000 tonnes Detectors: 18 The ALICE Experiment David Evans IoP Nuclear Physics Conference UK-built Trigger Electronics

22 First interactions 12 th Circulating beam 2: stray particle causing an interaction in the ITS September 08 ITS tracks on reconstructed tracks, common vertex SPD/SSD, Sunday, 15.6 Dump on TED David Evans IoP Nuclear Physics Conference

23 First proton-proton physics Excellent tracking and PID make ALICE ideal for studying the global event characteristics in proton-proton interactions. Eg such a big leap in energy, we don t even know the average number of charged particles produced per p-p collision. Early measurements from ALICE will eliminate and constrain many theoretical models in the first week! UK playing leading role in preparations for first physics e.g. trigger corrections See Sparsh Navin (parallel sessions) and other p-p physics e.g. see Zoe Matthews (Parallel sessions). Of course, p-p data also needed as comparison to Pb-Pb data. David Evans IoP Nuclear Physics Conference

24 ALICE Physics Timeline Proton-proton (from Oct/Nov 2009) Start with 900 GeV p-p Multiplicity & p T distributions, Strangeness yields (exceed stats of UA5 etc.) Charged particle multiplicity. Detector calibrations (at 10 TeV) Multiplicity & p T distributions (p T 50 GeV) Strangeness yields, flow etc. 70M events 8 days 10 TeV p-p 20k events - 3 minutes 70M events 8 days Heavy flavour physics, jets 10 7 events 8 months David Evans IoP Nuclear Physics Conference

25 Heavy-ion physics with ALICE Pb-Pb (Summer weeks): 1/20 of nominal luminosity Ldt = cm -2 s -1 x 10 6 s Alignment calibration available from pp Global event properties (10 5 events): Multiplicity, rapidity density Elliptic flow Source characteristic (10 6 events): Particle spectra, resonances Differential flow interferometry High pt and heavy flavours (10 7 events): Jet quenching Quarkonia production An hour A day 2 weeks David Evans IoP Nuclear Physics Conference

26 Summary Evidence for QGP formation at CERN SPS and RHIC energies. ALICE will be able to study the physics of quark matter in detail. almost all known observables from early to late stages of QGP ALICE to study pp physics in its own right ALICE is ready for first physics We look forward to lots of exciting physics from this Autumn. David Evans IoP Nuclear Physics Conference

27 Extra Slides David Evans IoP Nuclear Physics Conference

28 Use R AA to determine the medium density Models are able to describe the data cold matter density David Evans IoP Nuclear Physics Conference

29 Tracking Good impact parameter resolution. central Pb Pb TPC High efficiency even at low p T. central Pb Pb pp Dp T /p T ~ 0.7% Dp T /p T ~ few % at 100 GeV/c central Pb Pb pp David Evans IoP Nuclear Physics Conference

30 Particle Identification de/dx in silicon (ITS) and gas (TPC) (inc relativistic rise) + Time-of-Flight (TOF) + Cerenkov (RICH) decay topology (K 0, K +, K -, ) K and decays up to at least 10 GeV leptons (e, m), photons, 0, electrons in TRD: p > 1 GeV, muons: p > 5 GeV, 0 in PHOS: 1 < p < 80 GeV Alice uses ~ all known techniques! David Evans IoP Nuclear Physics Conference

31 First (p-p) Physics at ALICE ALICE unique features: Excellent tracking and impact parameter resolution. Acceptance at low p T (~0.2 GeV) Relatively low field (0.5 T) Low material budget (total ~8% int. length) Excellent PID capabilities de/dx (TPC/ITS), TRD, TOF, HMPID PHOS, (EMCAL) Cost limited in luminosity for pp (L max ~ 3x10 30 cm -2 s -1 ) David Evans IoP Nuclear Physics Conference