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The physics program of ALICE

The physics program of ALICE. Enrico Scomparin INFN-Torino (Italy). Frascati, September 18-19 2008. String & Strong Interaction Workshop. Topics. Ultrarelativistic heavy-ion physics: a field With a solid background (>20 years at CERN and BNL) At the eve of a quantum jump.

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The physics program of ALICE

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  1. The physics program of ALICE Enrico Scomparin INFN-Torino (Italy) Frascati, September 18-19 2008 String & Strong Interaction Workshop

  2. Topics Ultrarelativistic heavy-ion physics: a field With a solid background (>20 years at CERN and BNL) At the eve of a quantum jump Heavy-ions at the LHC Why ? Short introduction to the physics topics Heavy-ions at the LHC Where ? CERN (and LHC) as a heavy-ion accelerator Heavy-ions at the LHC How ? ALICE, a dedicated heavy-ion collider experiment Heavy-ions at the LHC What ? A general view on probes and observables Some specific physics performance studies Not only heavy-ions! First (pp) physics

  3. Why heavy ions (at the LHC) ? • Study the phase diagram of strongly interacting matter • First ideas are almost 30 years old..... “Experimental hadronic spectrum and quark liberation” Cabibbo and Parisi Phys. Lett. 59B, 67 (1975) Basic conclusions already there

  4. Phase diagram of strongly interacting matter ...to nowadays understanding Cross over Map the QCD phase diagram in still unexplored regions 1st order phase transition • Two “directions” for forthcoming machines • Study of the B=0 transition (LHC) • Study of the critical point (FAIR)

  5. QGP @ LHC: hotter, larger, longer • To cover the high temperature • region of the QCD phase diagram, one • has to increases of the collision • Unfortunately, the energy density  • increases slowly with s • Keeping a fixed 0 (formation time) • one has e (t=1 fm/c) ~ dN/dy ~ ln(Ös) (caveat: formation time are expected to decrease) ..and the temperature increases slowly when increasing e   T4 (Stefan-Boltzmann) Significant increase in T, dimension and lifetime of the QGP phase is anyway expected at the LHC!

  6. CERN accelerator complex • The acceleration of a heavy-ion beam poses several technical • problems that do not exist (or are less severe) for a Z=1 beam • At CERN, when first light ion beams were accelerated, it was possible • to use essentially the same hardware, with minor improvements • Already in 1994, when Pb ions • were first accelerated at CERN, a • significant upgrade of the • accelerator complex had to take • place • Today, to prepare the advent of • heavy-ions in the LHC, dedicated • studies have been performed, and • new hardware put in place

  7. Problems in ion colliders • Heavy-ion colliders are affected by further problems with respect to • hadron colliders, connected with the lifetime of the colliding beams Maximum luminosity much lower than for p-p (1034 cm-2 s-1) Electron Capture by Pair Production(ECPP) (prop. to luminosity) E.m. interaction between ions (high cross section) Particle is lost Electron capture e+e-pair production Quenching risk Luminosity limited to ~ 51026 cm-2 s-1

  8. Quenches can be destructive • A very large magnetic energy is stored in the LHC dipoles Ecoil= ½ L I2 with L = 0.12 H I = 11.5 kA • This gives Ecoil = 7.8106 J • The magnet weighs 26 tons  the magnetic stored • energy is equivalent to the kinetic energy of • 26 tons travelling at 88 km/h • When there is a quench all this energy has to be dissipated quickly • Security systems developed for this occurrence • Fast quench propagation to the whole mass of the coil Total magnetic energy stored in the LHC = 11 GJ ! Beam energy density is also very high

  9. Eskola, Kolhinen, Vogt hep-ph/0104124 New features of the LHC energy regime (1) • High density (saturated) parton distributions determine particle production At LHC we probe a continuous Bjorken-x range down to x<10-5 Strong nuclear gluon shadowing expected Access a region of gluon saturation

  10. New features of the LHC energy regime (2) • Hard processes contribute significantly to the total A-A cross section Example: open charm cross section • Hard strongly interacting probes can be • used to study the early stages of the • collisions (their attenuation can be • calculated) •  Jets •  Heavy quarkonia At LHC beauty channel opens up for study

  11. New features of the LHC energy regime (3) • Weakly interacting hard probes become available Direct photons, but also Z0 and W± Information about nuclear parton distribution at very high Q2 Can be used as a medium-blind reference that scales with the number of binary collisions

  12. y z x New features of the LHC energy regime (4) • Parton dynamics dominate the fireball expansion • QGP lifetime significantly larger than the time needed for the • thermalisation of the system • Collective features of the observed hadronic final state more directly • related to the early stages Renewed interest in soft probes (e.g. anisotropic flow)

  13. HI @LHC: which observables ? Even if some observables are specific to LHC, many (most) of them have already been investigated at previous facilities (SPS, RHIC) but continue to be extremely interesting at ALICE • Soft processes: • High cross section • Decouple late  indirect signals for QGP EM probes (real and virtual photons): insensitive to the hadronization phase • Hard processes: • Low cross section • Probe the whole evolution of the collision Try to measure everything !

  14. A HI experiment at LHC • General purpose (contrarily to SPS and, to a certain extent, to RHIC) • In contrast to experiments mainly devoted to study (hard) p-p physics • at the LHC, an experiment focussed on heavy-ion physics should have: 1) The capability of coping with the high multiplicity generated in HI collisions (2000 charged particles for unit rapidity) • Not really necessary for selected hard probes such as , • but important to access the bulk of particle production, i.e. • to study soft observables (e.g. flow) • Implies, in particular • High granularity • Large bandwidth for DAQ 2) The possibility of pushing down as much as possible its pT reach • Implies a not too high B field for momentum measurement •  Competing requirement with accuracy for hard probes

  15. Zero-degree calorimeters Central barrel Muon arm

  16. Size: 16 x 26 meters Weight: 10,000 tons TOF TRD HMPID ITS PMD Muon Arm PHOS Added since 1997: • V0/T0/ACORDE • TRD(’99) • EMCAL (’06) TPC ALICE Set-up

  17. Building the experiment

  18. Complete - fully installed & commissioned ITS, TPC, TOF, HMPID, MUONS, PMD, V0, T0, FMD, ZDC, ACORDE, TRIGGER, DAQ Partially completed TRD (20%) to be completed by 2009 PHOS (40%) to be completed by 2010 EMCAL (0%) to be completed by 2010/11 At start-up full hadron and muon capabilities Partial electron and photon capabilities Start-up Configuration

  19. First collisions (not pp!) After the “big” media event (September 10), one 450 GeV beam circulated for some time in the machine during the night on September 12 p-Si interaction in the SPD √s ~ 30 GeV, no black hole was produced......

  20. The next step.....first pp collisions

  21. ALICE Tracking Performance dNch/dy=6000 drop due to proton absorption TPC acceptance = 90% Momentum resolution ~ 5% @ 100 GeV • Robust, redundant tracking from < 100 MeV/c to > 100 GeV/c • Very little dependence on dN/dy up to dN/dy ≈ 8000 p/p < 5% at 100 GeV with careful control of systematics

  22. Particle Identification in ALICE • ‘stable’ hadrons (π, K, p): 0.1<p<5 GeV/c; (π,p with ~ 80 % purity to ~ 60 GeV/c) •  dE/dx in silicon (ITS) and gas (TPC) + time-of-flight (TOF) + Cherenkov (RICH) • decay topologies (K0, K+, K-, Λ, D) •  K and L decays beyond 10 GeV/c • leptons (e,μ ), photons, π0 •  electrons TRD: p > 1 GeV/c, muons: p > 5 GeV/c, π0 in PHOS: 1<p<80 GeV/c • excellent particle ID up to ~ 50 to 60 GeV/c

  23. Hard probes • Heavy quark production (charm, beauty) • Heavy quarkonia ( and  family) • Jets (see next presentation by Gustavo)

  24. Charm production in nuclear collisions • Hard primary production in parton processes (pQCD) • Binary scaling for hard process yield: • long lifetime of charm quarks allows them to live through the thermalization phase of the QGP and be affected by its presence • Secondary (thermal) c-cbar production in the QGP • mc (≈1.2 GeV) only 10%-50% higher than predicted temperature of QGP at the LHC (500-800 MeV) • Thermal yield expected much smaller than hard primary production • can be observed if the pQCD production in A-A is precisely understood Medium effects investigated through the ratio

  25. c antishadowing A D shadowing SPS RHIC u D LHC c A Binary scaling break-up • Initial state effects • PDFs in nucleus  PDFs in nucleon • Anti-shadowing and shadowing • kT broadening (Cronin effect) • Parton saturation (Color Glass Condensate) • Final state effects (due to the medium) • Energy loss • Mainly by gluon radiation • In medium hadronization • Recombination vs. fragmentation Present also in pA (dA) collisions Concentrated at lower pT Only in AA collisions Dominant at higher pT

  26. BDMPS formalism for radiative energy loss  Baier et al., Nucl. Phys. B483 (1997) 291 Final state effects: energy loss distance travelled in the medium average energy loss transport coefficient of the medium Casimir coupling factor Obtain information on the energy density! • Energy loss for heavy flavours expected to be reduced by: • Casimir factor • light hadrons originate predominantly from gluon jets, • heavy flavoured hadrons originate from heavy quark jets • CR = 4/3 for quark-gluon coupling, 3 for gluon-gluon coupling • Dead-cone effect • gluon radiation expected to be suppressed for q < MQ/EQ

  27. RHIC results: RAA STAR • Similar high-pT suppression for charm and light hadrons • Even using high densities (q=14 Gev2/fm)and taking into account • collisional energy loss models tend to overestimate the data • More recent approaches including Qq resonances in plasma work better • Understanding is still not satisfactory • Difficulties in reproducing both RAA and v2

  28. muon arm central barrel Charm at ALICE (1) • Large cross-section • Much more abundant production with respect to SPS and RHIC • Small x • unexplored small-x region can be probed with charm at low pT and/or forward rapidity • down to x~10-4 at y=0 and x~10-6 in the muon arm

  29. charm 12% Charm at ALICE (2) • p-p collisions • Test of pQCD in a new energy and x regime • Reference for Pb-Pb (necessary for RAA) • p-Pb collisions • Probe nuclear PDFs at LHC energy • Disentangle initial and final state effects • Pb-Pb collisions • Probe the medium formed in the collision • WARNING: pp, pPb and PbPb will have different s values • Need to extrapolate from 14 TeV to 5.5 TeV to compute RAA • Small (≈ 10%) theoretical uncertainty on the ratio of results at 14 and 5.5 TeV

  30. primary vertex decay length = L q track impact parameter decay vertex Charmed mesons and baryons • Weakly decaying charm states • Mean proper length ≈ 100 mm • Main selection tool: displaced-vertex • Tracks from open charm decays are typically displaced from primary vertex by ≈100 mm • Need for high precision vertex detector (resolution on track impact parameter ≈ tens of microns)

  31. Charm production at the LHC • ALICE baseline for charm cross-section and pT spectra: • NLO pQCD calculations, Mangano, Nason, Ridolfi, NPB373 (1992) 295 • Theoretical uncertainty = factor 2-3 • Average between cross-sections obtained with MRSTHO and CTEQ5M sets of PDF • ≈ 20% difference in scc between MRST HO and CTEQ5M • Binary scaling + shadowing (EKS98) to extrapolate to p-Pb and Pb-Pb

  32. Acceptance for heavy flavours • ALICE channels: • electronic (|h|<0.9) • muonic (-4<h<-2.5) • hadronic (|h|<0.9) • ALICE coverage: • low-pT region • central and forward rapidity regions • Precise vertexing in the central region to identify D (ct ~ 100-300 mm) and B (ct ~ 500 mm) decays

  33. D mesons in central barrel • No dedicated trigger in the central barrel  extract the signal from minimum bias events • Large combinatorial background (benchmark study with dNch/dy = 6000 in central Pb-Pb!) • SELECTION STRATEGY: invariant-mass analysis of fully-reconstructed topologies originating from displaced vertices • build pairs/triplets/quadruplets of tracks with correct combination of charge signsandlarge impact parameters • particle identification to tag the decay products • calculate the vertex (DCA point) of the tracks • good pointing of reconstructed D momentum to the primary vertex D0  K-+, D+ → K-p+p+ Main channels under study

  34. rf: 50 mm z: 425 mm central Pb–Pb PIXEL CELL < 60 mm (rf) for pt > 1 GeV/c Two layers: r = 4 cm r = 7 cm Track impact parameter in Pb-Pb • Resolution on track impact parameter mainly provided by the 2 layers of Silicon Pixel Detectors • Interaction point (primary vertex) • x and y coordinates known with high precision from beam position given by LHC (sbeam=15 mm) • z coordinate measured from cluster correlation on the two layers of SPD

  35. D0 K-p+ : selection of candidates

  36. D0 K-p+: Results (I) central Pb-Pb With dNch/dy = 3000 in Pb-Pb, S/B larger by  4 and significance larger by  2

  37. D0 K-p+: Results (II) inner bars: stat. errors outer bars: stat.  pt-dep. syst. not shown: 9% (Pb-Pb), 5% (pp, p-Pb) normalization errors 1 year at nominal luminosity (107 central Pb-Pb events, 109 pp events) + 1 year with 1 month of p-Pb running (108 p-Pb events) • Down to pt ~ 0 in pp and p-Pb (1 GeV/c in Pb-Pb) • important to go to low pT for cross-section measurement

  38. D0 K-p+ : Results (III) • 1 year at nominal luminosity • 1 month  107 central Pb-Pb events • 10 months  109 pp events ‘High’ pT (6–15 GeV/c) Only parton energy loss Low pT (< 6–7 GeV/c) Also nuclear shadowing

  39. mass effect Moving to heavier quarks: beauty Heavy-to-light ratios: Compare g  h , c  D and b  B For 10 < pT < 20 GeV, charm behaves like a m=0 quark, light-flv hadrons come mainly from gluons RD/h enhancement probes color-charge dep. of E loss RB/h enhancement probes mass dep. of E loss

  40. rec. track e Primary Vertex B d0 X Measuring beauty via electrons • Expected yields of charm (b.r.: 10%) and beauty (b.r.: 11% +10% as bce) decay electrons: • b quark has ct  500 mm decay electrons d0 ~ few-100 mm Let’s consider the main background sources.... transverse plane

  41. Detection strategy • Electron PID: reject most of the hadrons • Impact parameter cut: reduce charm and bkg electrons • Subtract (small) residual background

  42. fraction of misidentified pions Electron identification • Combined info from TRD (trans. rad.) and TPC (dE/dx) • TRD rejects 99% of the p and ALL heavier hadrons (pt > 1 GeV/c) • TPC further rejects residual pions (up to 99% at low p) • About 20% of electrons rejected TRD TPC

  43. Effect of electron identification vs pT Beauty signal dominates! At least at high pT....

  44. Cross section of electrons from B inner bars: stat. errors outer bars: stat.  pt-dep. syst. errors not shown: norm. error (5% pp, 9% Pb-Pb) 1 year at nominal luminosity (107 central Pb-Pb events, 109 pp events)

  45. Measuring beauty in the muon channel • Possibility of evaluating bb at forward rapidities dbb /dy, bb • Different systematics with respect to central rapidity measurement Single muon spectra Two different “channels” Muon pairs (dimuons) Low mass (m < 5 GeV) High mass (5< m < 20 GeV) Dimuon channel  kinematic correlation of the pair strongly related to parent BB

  46. Signal spectra (dimuons) Central Pb-Pb (0-5%) 1 month of data taking pT>1.5 GeV/c Before background subtraction After background subtraction Mixed event technique will be used to remove comb. background

  47. Signal spectra (single ) Central Pb-Pb (0-5%) 1 month of data taking Signal dominates at large pT

  48. Heavy quarkonium in AA • Two main effects of a deconfined medium on the quarkonium potential • Confinement term (kr)disappears • Coulomb term screened by high color density D  Debye screening length (a screening mass mD is also defined (=1/D)) • Maximum distance for the quark in the pair to be still bound • Goes approximately as 1/T

  49. Screening length and temperature

  50. (3S) (2S) b(2P) c(1P) (2S) b(1P) J/ (1S) J/  Sequential suppression • Every resonance has a typicaldissociation threshold Digal et al., Phys.Rev. D64(2001)094015 • Usually one measures the more strongly bound state (e.g. J/) • Higher-mass resonances often have a non-zero branching ratio towards • more tightly bound states

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