Characterising the QCD Plasma with the ALICE Experiment

Transcription

1 Characterising the QCD Plasma with the ALICE Experiment Federico Antinori INFN, Padova, Italy and CERN, Geneva, Switzerland

2 The Standard Model and QCD strong interaction: binds quarks into hadrons binds nucleons into nuclei described by QCD: interaction between particles carrying colour charge (quarks, gluons) mediated by strong force carriers (gluons) beauty very successful theory! F Antinori - NTU - Singapore - 26 March 2013 jet production particle production at high pt heavy flavour production but with outstanding puzzles 2

3 Two puzzles in QCD: i) hadron masses beauty F Antinori - NTU - Singapore - 26 March 2013 A proton is made of two u and one d quarks The sum of their masses is around 12 MeV... but the proton mass is 938 MeV! how is the extra mass generated? 3

4 Two puzzles in QCD: ii) confinement beauty Nobody ever detected an isolated quark Quarks seem to be permanently confined within protons, neutrons, pions and other hadrons. It looks like one half of the fundamental fermions are not directly observable can we access experimentally the physics of confinement? F Antinori - NTU - Singapore - 26 March

5 Lattice QCD l in lattice QCD, non-perturbative problems are treated by discretization on a space-time lattice à cure ultraviolet divergences zero baryon density, 3 flavours ε changes rapidly around T c T c = 170 MeV: 3 flavours; (q-q)=0 ε c = 0.6 GeV/fm 3 at T~1.2 T c ε settles at about 80% of the Stefan-Boltzmann value for an ideal gas of q,q g (ε SB ) F Antinori - NTU - Singapore - 26 March

6 QCD phase diagram an artist s view ~ 10 µs after Big Bang Tc ~ 170 MeV Temperature Early Universe Hadron gas Nuclear matter LHC RHIC Quark-Gluon Plasma SPS AGS Neutron Star Baryon density ρ ~ 5-10 nuclear ε c ~ 1 GeV/fm 3 F Antinori - NTU - Singapore - 26 March

7 Big Bang and deconfinement we think that during the first instants of the life of the Universe, quarks and gluons were not trapped inside hadrons à quark epoch but could move freely, in a deconfined state: the Quark-Gluon Plasma until ~ 10 µs à T ~ 170 MeV (~ C) quarks and gluons recombine à hadron epoch F Antinori - NTU - Singapore - 26 March

8 How to learn more? how does matter behave in such extreme conditions? what were the properties of the Quark-Gluon Plasma? Quantum Chromo-Dynamics (the theory of Strong Interaction) does not allow us to calculate them from first principles even with the most powerful telescopes, it is only possible to look back in time ~ 400,000 y after the Big Bang is it possible to reproduce such conditions in the laboratory? about 2000 billion degrees? F Antinori - NTU - Singapore - 26 March

9 Nucleus-nucleus collisions! How do we test this theory in the lab? How can we compress/heat matter to such cosmic energy densities? By colliding two heavy nuclei at ultrarelativistic energies we recreate, for a short time span (about s, or a few fm/c) the conditions for deconfinement F Antinori - NTU - Singapore - 26 March

10 as the system expands and cools down it will undergo a phase transition from QGP to hadrons again, like at the beginning of the life of the Universe: we end up with confined matter again QGP lifetime ~ a few fm/c The properties of the medium must be inferred from the properties of the hadronic final state F Antinori - NTU - Singapore - 26 March

11 LHC as a HI accelerator Fully ionised 208 Pb nucleus accelerated in LHC (configuration magnetically identical to that for pp), e.g. (2011 numbers): ppb = Z pp = TeV = 287 TeV s PbPb = 574 TeV (!) the relevant figure is s per nucleon-nucleon collision: s NN s 2EPb Z 82 = = spp = s A A 208 NN pp = 2.76 TeV of course, real life is more complicated ion collimation sensitivity of LHC instrumentation injection chain F Antinori - NTU - Singapore - 26 March

12 For 2011 Pb-Pb run: ~ ions/bunch 358 bunches 200 ns basic spacing β* = 1 m L ~ cm -2 s -1 à ~ 4000 Hz interaction rate Pb nuclei in the LHC à one dedicated AA experiment: ALICE and AA capability in ATLAS and CMS F Antinori - NTU - Singapore - 26 March

13 Hard Probes! à large cross-sections for hard probes of the QCD medium e.g. heavy quarks: Pb! Pb! D! c! c! F Antinori - NTU - Singapore - 26 March D!

14 The ALICE Collabora/on Sweden ~ 1300 Members from both NP and HEP communities 35 Countries 132 Institutes ~ 160 MCHF capital cost (+ free magnet) Serbia Turkey Spain South Korea South Africa Slovakia Thailand Russia United Kingdom Ukraine Switzerland Brazil Chile China Cuba Armenia Croatia United States Czech Republic Denmark France Germany India Egypt Finland Greece Hungary Significant Asian Participation! China India Japan Pakistan South Korea Thailand Pakistan Romania Poland Peru Norway Netherlands Mexico Japan Italy F Antinori - NTU - Singapore - 26 March

15 : two main parts: barrel ( η <0.9), B = 0.5 Tesla muon spectrometer, -4<η<-2.5 The ALICE Experiment High precision reconstruction: low material tracking high resol. vertexing hadron and lepton ID Triggers: minimum-bias (MB) or centrality, in Pb-Pb single and di-muon EMCAL, high-mult., UPC Collisions systems (so far) : Pb-Pb, pp, p-pb, Pb-p F Antinori - NTU - Singapore - 26 March

16 Geometry of a Pb-Pb collision b central collisions small impact parameter b high number of participants à high multiplicity peripheral collisions large impact parameter b low number of participants à low multiplicity for example: sum of the amplitudes in the ALICE V0 scintillators reproduced by Glauber model fit (red): random relative position of nuclei in transverse plane Woods-Saxon distribution inside nucleus deviation at very low amplitude expected due to non-nuclear (electromagnetic) processes peripheral central F Antinori - NTU - Singapore - 26 March

17 Azimuthal asymmetry in the transverse momentum distribution of produced particles why is it important? non-central collisions are asymmetric in azimuth azimuth = angle in the plane of the screen Reaction plane Flow Flow Out-of-plane Y In-plane X transfer of this asymmetry to momentum space provides a measure of the strength of collective phenomena Large mean free path particles stream out isotropically, no memory of the asymmetry extreme: ideal gas (infinite mean free path) Small mean free path larger density gradient -> larger pressure gradient -> larger momentum extreme: ideal liquid (zero mean free path, hydrodynamic limit) à quantified by second coefficient (v 2 ) of Fourier expansion of azimuthal distribution F Antinori - NTU - Singapore - 26 March

18 Measurement of v 2 to quantify the asymmetry: à Fourier expansion of the angular distribution: + 2v cos( ϕ) + 2v cos(2ϕ ) in the central detector region (~ 90º) à v 1 ~ 0 à asymmetry quantified with v 2 we find that v 2 is almost as large as expected by hydrodynamics for fluid with very little viscosity (almost perfect liquid) F Antinori - NTU - Singapore - 26 March

19 Higher harmonics a beautiful phenomenon initial state geometrical asymmetries final state momentum asymmetries wonderful tool to study response of medium to initial fluctuations à infer medium properties F Antinori - NTU - Singapore - 26 March

20 Identified particles different particles have different mass à info on collective expansion p T distributions can be predicted assuming expansion is hydrodynamical (i.e.: one common velocity field) à OK for π and K, but p seem to misbehave (less yield, flatter spectrum) F Antinori - NTU - Singapore - 26 March

21 Azimuthal asymmetry of identified particles comparison of identified particles v 2 (p T ) with hydrodynamic prediction à again, protons are off à what s going on with protons? pp annihilation? stay tuned F Antinori - NTU - Singapore - 26 March

22 High p T suppression production of particles at high transverse momentum (high p T, say above 2-3 GeV) is expected to scale like the number of binary nucleon-nucleon collisions: dn dp T AA = N coll dn dp T can be modified by nuclear effects pp e.g.: particles can lose energy when traversing the QCD plasma fireball ( jet quenching ) à suppression of particle production at high p T e.g.: 7 participants 12 binary collisions (N coll ) define a nuclear modification factor R AA R = AA N dn dp coll T AA dn dp T pp in the absence of nuclear effects à R AA = 1 F Antinori - NTU - Singapore - 26 March

23 Strong quenching! Pb-Pb significantly below scaled pp for central collisions (filled points) RAA: intriguing modulation: minimum around 6-7 GeV (R AA ~ 0.14) clear increase at higher p T à not yet understood F Antinori - NTU - Singapore - 26 March

24 Strong angular dependence significant effect, even at 20 GeV and beyond! à sensitivity to path length dependence of energy loss F Antinori - NTU - Singapore - 26 March

25 Baryon/Meson effect à sensitivity to hadronisation in medium F Antinori - NTU - Singapore - 26 March

26 Charmonium suppression l QGP signature proposed by Matsui and Satz, 1986 l l In the plasma phase the interaction potential is expected to be screened beyond the Debye length l D (analogous to e.m. Debye screening): Charmonium (cc) and bottonium (bb) states with r > λ D will not bind; their production will be suppressed F Antinori - NTU - Singapore - 26 March

27 J/ψ suppression at SPS and RHIC substantial suppression of J/ψ production observed at low energies (SPS & RHIC) ~ similar levels of suppression F Antinori - NTU - Singapore - 26 March

28 J/ψ suppression at LHC LHC: 2.5 < y < 4, p T > 0 (ALICE) à less suppression than RHIC: 1.2 < y < 2.2, p T > 0 (PHENIX) à centrality dependence is much weaker! what s going on? à c-cbar coalescence? F Antinori - NTU - Singapore - 26 March

29 J/ψ R AA : p T dependence consistent with coalescence models F Antinori - NTU - Singapore - 26 March

30 J/ψ flow? some hint for a modulation? F Antinori - NTU - Singapore - 26 March

31 Heavy Flavours a very promising tool: probe the system with heavy quarks: c (charm) and b (beauty) these are produced in pairs in the initial impact between the two nuclei they propagate through the quark and gluon soup c and finally emerge carrying out information on the system properties c c F Antinori - NTU - Singapore - 26 March 2013 c 31

32 Theoretically... ΔE α s C R 2 qˆ L average energy loss distance travelled in the medium Casimir coupling factor transport coefficient of the medium R.Baier et al., Nucl. Phys. B483 (1997) 291 ( BDMPS ) Energy loss for heavy flavours is expected to be reduced: i) Casimir factor light hadrons originate from a mixture of gluon and quark jets, heavy flavoured hadrons originate from quark jets C R is 4/3 for quarks, 3 for gluons ii) dead-cone effect gluon radiation expected to be suppressed for θ < M Q /E Q [Dokshitzer & Karzeev, Phys. Lett. B519 (2001) 199] [Armesto et al., Phys. Rev. D69 (2004) ] F Antinori - NTU - Singapore - 26 March

33 Reconstructed D mesons! F Antinori - NTU - Singapore - 26 March

34 Heavy Flavours R AA + indication of less suppression for beauty? p T < 8 GeV/c: hint of less suppression than for π? p T > 8 GeV/c same suppression as for π F Antinori - NTU - Singapore - 26 March

35 The D s HF in-medium hadronisation! a hint of strangeness enhancement? more stats needed! F Antinori - NTU - Singapore - 26 March

36 D meson v 2 Hint of non-zero v 2 consistent with strong coupling of c to medium theory must describe simultaneously v 2 and R AA F Antinori - NTU - Singapore - 26 March

37 p-pb collisions in the LHC! tricky, but can be done 2-in-1 design à identical bending field in two beams à locks the relation between the two beam momenta: p (Pb) = Z p(proton) è different speeds for the two beams! adjust length of closed orbits! to compensate different speeds different RF freq for two beams at injection and ramps short low lumi pilot run (a few hours) on 12/9/2012 first run in Jan-Feb 2013! à ~ 30/nb F Antinori - NTU - Singapore - 26 March

38 Gluon shadowing different parton distribution functions in protons and nuclei x = fraction of nucleon momentum carried by gluon a priori, large uncertainty à measure p-pb collisions!!! [K J Eskola et al: JHEP04(2009)065] F Antinori - NTU - Singapore - 26 March

39 Control experiment: R ppb measurement of nuclear modifications in initial state R pa ~ 1 for p T > 3 GeV/c à confirms quenching is due to QCD medium F Antinori - NTU - Singapore - 26 March

40 The Ridge 2 < p T,trig < 4 GeV/c 1 < p T,assoc < 2 GeV/c 20% highest multiplicity 1/N trig d 2 N assoc /dδηdδϕ (zoomed) Near-side jet (Δϕ ~ 0, Δη ~ 0) Away-side jet (Δϕ ~ π, elongated in Δη) Near-side ridge (Δϕ ~ 0, elongated in Δη) Δϕ (rad) in addition to near side peak and away-side recoil there s an additional near side ridge in p-pb first observed by CMS [PLB718 (2013) 795] PLB719 (2013) 29 F Antinori - NTU - Singapore - 26 March

41 The Double Ridge Can we separate the jet and ridge components? no ridge seen in % and similar to pp à what remains if we subtract %? 0-20% % PLB719 (2013) 29 = the ridge is doubled! first observed by ALICE, then confirmed by ATLAS à the origin of this structure is unknown! (and currently under vigorous investigation stay tuned ) a similar structure observed in Pb-Pb is attributed to hydrodynamic flow! F Antinori - NTU - Singapore - 26 March

42 The Way Ahead Long Shutdown 1 continue analysis of the Run 1 sample (only basic measurements so far ) installation of additional TRD and EMCAL modules Run 2 1/nb à high statistics measurements (quarkonia, heavy flavours, jets, ) ALICE upgrades (ongoing) new Inner Tracking System (monolithic pixels!) GEM upgrade for TPC new, high speed DAQ/Trigger 2018 Long Shutdown 2 install upgrades Run 3 ALICE v 2.0 à x 100 gain for heavy flavours, quarkonia, low mass dileptons, F Antinori - NTU - Singapore - 26 March

43 ALICE keeps growing Number of par,cipa,ng ins,tutes in ALICE Total Full Members Associate Members new institutes just from Asia in last 12 months!i but lots of work ahead: many opportunities for new collaborators! à interested? come and talk to me!!! F Antinori - NTU - Singapore - 26 March

44 Conclusion the LHC has ushered in a new era for ultrarelativistic AA collisions abundance of hard probes state-of-the-art collider detectors (ALICE, + AA capabilities in ATLAS, CMS) rich harvest of results from first analyses of Run 1 data azimuthal asymmetry à close to hydrodynamic limit! quenching at high p T (absent in initial state) angular dependence of quenching à sensitivity to path length suppression of quarkonia (reappearance at low p T?) suppression of heavy flavour production à parton mass ordering? angular dependence of heavy flavour and quarkonia suppression? à models of QCD matter still catching up with experiment! discovery of double ridge in p-pb collisions, origin still unknown! the future looks bright! complete analysis of Run 1 à Run 2 à upgrades à Run 3 à final goal: measure fundamental properties of primordial QCD plasma! F Antinori - NTU - Singapore - 26 March