The physics programme of the alice experiment at the lhc

Hajmi498 b.

The physics programme of the ALICE experiment at the LHC


Nucleus-nucleus (and pp) collisions at the LHC

Why heavy ion collisions: the QCD phase diagram

Lattice QCD calculations

New conditions created at the LHC

The ALICE experiment

Tracking: the major challenge for ALICE

Tracking efficiency

Combined momentum resolution

Track impact parameter resolution

ALICE: an ideal soft particle tracker

ALICE Particle Identification

PHOton Spectrometer: PHOS

  • High granularity detector:

    • 17920 lead-tungstate crystals (PbWO4), 5 modules (5664)
    • crystal size: 22  22  180 mm3
    • depth in radiation length: 20
  • Distance to IP: 4.4 m

  • Acceptance:

    • pseudo-rapidity [-0.12,0.12]
    • azimuthal angle 100o
  • Energy resolution ~ 3% / √ E

  • Dynamic range from ~ 100 MeV to ~ 100 GeV

  • Timing resolution of ~ 1.5 ns / √ E

  • Trigger capability at first level

  • Charged Particle Veto, CPV

    • multi-wire particle gas chamber


  • EM Sampling Calorimeter (STAR Design)

  • Pb-scintillator linear response

    • -0.7 <  < 0.7

    •  <  < 

  • Energy resolution ~15%/√E

ALICE Physics Goals

  • Event characterization in the new energy domain (for PbPb but also for pp)

    • multiplicity, η distributions, centrality
  • Bulk properties of the hot and dense medium, dynamics of hadronization

    • chemical composition, hadron ratios and spectra, dilepton continuum, direct photons
  • Expansion dynamics, space-time structure

    • radial and anisotropic flow, momentum (HBT) correlations
  • Deconfinement:

    • charmonium and bottomonium spectroscopy
  • Energy loss of partons in quark gluon plasma:

    • jet quenching, high pt spectra
    • open charm and open beauty
  • Chiral symmetry restoration:

    • neutral to charged ratios
    • resonance decays
  • Fluctuation phenomena, critical behavior:

    • event-by-event particle composition and spectra

Event characterization in ALICE

ZDC and centrality determination

ZDC + ZEM for centrality determination

Centrality classes in Npart

Multiplicity measurements in ALICE

Highlights on physics topics

Kinetic freezout at RHIC

Ratios of hadron spectra: RCP

Coalescence: possible mechanism at intermediate pT

  • The

    in vacuo fragmentation

    of a high momentum quark to produce hadrons competes with the

    in medium recombination

    of lower momentum quarks to produce hadrons
  • Example: creation of a 6 GeV/c or p

    • Fragmentation: Dq→h(z)
      • produces a 6 GeV/c  from a 10 GeV/c quark
    • Recombination:
      • produces a 6 GeV/c  from two 3 GeV/c quarks
      • produces a 6 GeV/c proton from three 2 GeV/c quarks

What to expect at LHC energies

Ratios of hadron spectra: RAA

Centrality Dependence of RAA

  • - Dramatically different and opposite centrality evolution of Au+Au experiment from d+Au control.

  • - High pT hadron suppression is clearly a final state effect (partonic energy loss in a dense medium) and not an initial state effect (gluon saturation).

  • - R dAu > 1 at intermediate pT shows a Cronin enhancement (multiple scattering)


Highlights on physics topics

An enhancement in the production of rare strange hadrons was among the first signatures proposed for QGP observation in relativistic heavy ion collisions

  • An enhancement in the production of rare strange hadrons was among the first signatures proposed for QGP observation in relativistic heavy ion collisions

    • J.Rafelski and B. Müller, Phys. Rev. Lett 48 (1982) 1066; Phys. Rev. Lett. 56 (1986) 2334.

  • If deconfinement has occurred, the reactions leading to strangeness production are partonic: they have lower thresholds and higher cross sections, especially if the strange quark mass reduces owing to the associated partial restoration of the chiral simmetry.

  • Strangeness enhancement of single-strange particle observed in nucleus-nucleus collisions already at rather low energies

  • Multi-strange baryon enhancement observed at SPS (and then at RHIC)

Anisotropic transverse flow

  • In non-central nucleus-nucleus collisions, at t=0:

    • geometrical anisotropy (almond shape)
    • momentum distribution isotropic
  • Interactions among constituents generate a pressure gradient which transforms the initial spatial anisotropy into a momentum- space anisotropy (that is observed as an azimuthal anisotropy of the outgoing particles)

Elliptic flow coefficient v2

  • Experimentally the anisotropic transverse flow (also called elliptic flow) is measured by taking the second Fourier component of the particle azimuthal distributions relative to the reaction plane:

v2 versus centrality at RHIC

  • Observed elliptic flow depends on:

    • Initial eccentricity (decreasing with increasing centrality)
    • Amount of rescatterings (increasing with increasing centrality)

v2 versus pT at RHIC

  • At low transverse momenta the elliptic flow is well described

  • by

    hydrodynamical models incorporating a softening of the Equation

  • of State

    due to quark and gluon degrees of freedom
  • Deviations at high pT

    • Hydrodynamics not applicable because high pT partons have not undergone sufficient re-scatterings to come to thermal equilibrium
    • Parton energy loss in the opaque medium is a source of anisotropy

Hadron v2 and quark recombination

Elliptic flow at the LHC

  • Multiplicity larger than at RHIC

    • by a factor 1.5-2
  • v2/ expected larger than at RHIC

    • Few predictions:
      • Teaney, Shuryak, Phys.Rev.Lett. 83 (1999) 4951.

      • Kolb, Sollfrank, Heinz, Phys.Rev. C62 (2000) 054909.

      • Bhalerao et al., Phys.Lett. B627 (2005) 49

  • Large flow values 5-10% are expected

Experimental methods to estimate vn

    • Event plane method (Poskanzer and Voloshin, Phys. Rev. C58 (1998) 1671.)
      • Calculate an estimator of the reaction plane (EVENT PLANE) from the anisotropy of particle azimuthal distributions

      • Correlate azimuth of each particle with the event plane calculated with all the other particles

      • WEAK POINT: assumes that the only azimuthal correlation between particles is due to their correlation to the reaction plane (i.e. to flow)

      • BUT other sources of correlation (NON-FLOW) are in due to momentum conservation, resonance decays, jets + detector granularity  SYSTEMATIC UNCERTAINTY

    • Two particle correlations (S. Wang et al, Phys. Rev. C44 (1991) 1091.)
      • No need for event plane determination

      • Calculate two-particle correlations for all possible pairs of particles

      • WEAK POINT: same bias from non-flow correlations as in event-plane method

    • “Cumulants” method (Borghini et al, Phys Rev C 63 (2001) 054906.)
      • Extract vn from multi-particle azimuthal correlations

      • Based on the fact that flow correlates ALL particles in the event while non-flow effects typically induce FEW-particle correlations

      • DRAWBACK: larger statistical error and more sensitivity to fluctuation effects

    • Lee-Yang zeroes method (Bhalerao et al, Nucl. Phys. A727 (2003) 373.)
      • Extension of cumulants method to infinite order

Event plane method

  • Event plane resolution depends on:

    • Amount of anisotropy (v2)
    • Number of used tracks

Highlights on physics topics

  • Heavy flavours and quarkonia

Heavy Flavour physics in ALICE: motivations

  • Energy loss of Heavy Quarks (HQ) in hot and high density medium

    formed in AA central collisions.
  • Brownian motion


    coalescence of low pT HQ

    in the quark gluon plasma (QGP).
  • Dissociation (and regeneration) of quarkonia in hot QGP

  • Heavy flavour physics in

    pp collisions

    : small x physics, pQCD, HQ fragmentation functions, gluon shadowing, quarkonia production mechanism.

c and b production in pp at the LHC

  • Important test of pQCD in a new energy domain (14 TeV)

Heavy quarks as a probe of small-x gluons

  • Probe unexplored small-x region with HQs at low pt and/or forward y

    • down to x~10-4 with charm already at y=0

Hard primary production in parton processes (pQCD)

  • Hard primary production in parton processes (pQCD)

    • Binary scaling for hard process yield:
  • Baseline predictions for charm / beauty:

  • NLO (MNR code) in pp + binary scaling (shadowing included for PDFs in the Pb)

  • 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

Initial state effects

  • Initial state effects

    • PDFs in nucleus different from 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

Lower E loss for heavy quarks?

  • In vacuum, gluon radiation suppressed at  < mQ/EQ

  •  “dead cone” effect

  • Dead cone implies lower energy loss (Dokshitzer-Kharzeev, 2001)

  • Detailed calculation confirms this qualitative feature, although effect is small and uncertainties significant (Armesto-Salgado-Wiedemann, 2003)

  • Exploit abundant massive probes at LHC & study the effect by measuring the nuclear modification factor for D and B

Heavy-flavours in ALICE

  • ALICE can study several channels:

    • hadronic (|η|<0.9)
    • electronic (|η|<0.9)
    • muonic (2.5 < η < 4)
  • ALICE coverage:

    • low-pT region (down to
    • pt ~ 0 for charm)
    • central and forward rapidity regions
  • High precision vertexing in the central region to identify D (c ~ 100-300 μm) and B (c ~ 500 μm) decays

D0 K-+: results (I)

Open charm in pp (D0 → K) Sensitivity to NLO pQCD parameters

Open Beauty from single electrons


  • Electron Identification (TRD+TPC): reject most of the hadrons

Charm and Beauty Energy Loss : RAA

Heavy-to-light ratios in ALICE

Charmonia in AA collisions: from SPS and RHIC to LHC

J/ measurement in NA50 at the SPS

  • Aim of NA50: study the production of J/ in Pb-Pb collisions

  • Experimental technique:

    • absorb all charged particles produced in the collision except muons
    • detect J/ by reconstructing the decays

      J/   (B.R.  5.9 %)

  • The

    measured dimuon spectrum

  • fitted to a source cocktail in order to

  • extract the J/, ’ and Drell-Yan

  • contributions

J/ anomalous suppression in NA50

J/ survival probability

Comments on J/ suppression at SPS and RHIC

Charmonia in AA collisions at LHC

  • Melting of ’ and c at SPS and RHIC, and melting of J/ at LHC?

  • Magic cancellation between J/ suppression and J/ regeneration?

Bottomonia in AA collisions at LHC

Quarkonia→e+e- (in PbPb with ALICE)


→ 

(in PbPb with ALICE)

Study of quarkonia suppression with ALICE

Highlights on physics topics

  • Jet physics

Jet studies with Heavy Ions at RHIC

Leading particle versus jet reconstruction

Jet rates at the LHC

Jet energy domain

ALICE detectors for jet identification

  • Measurement of Jet Energy

    • In the present configuration ALICE measures only charged particles with its Central Tracking Detectors
    • (and electromagnetic energy in the PHOS)
    • The proposed Large EM Calorimeter (EMCal) would provide a significant performance improvement
      • ET measured with reduced bias and improved resolution
      • Better definition of the fragmentation function: pt/ET
      • Larger pt reach for the study of the fragmentation of the jet recoiling from a photon and photon-photon correlations
      • Excellent high pt electrons identification for the study of heavy quark jets
      • Improved high ET jet trigger
  • Measurement of Jet Structure

    is very important
    • Requires good momentum analysis from ~ 1 GeV/c to ~ 100 GeV/c
    • ALICE excels in this domain

Jet reconstruction in ALICE

  • Background energy in a cone of size R is ~R2 (and background fluctuations ~R).

Background for jet structure observables: the hump-back plateau

Energy resolution (for ideal calorimetry)

Photon-tagged jets

  •  energy provides independent measurement of jet energy

  • Drawback: low rate !!

  • But... especially interesting in the intermediate range (tens of GeV) where jets are not identified

  • Direct photons are not perturbed by the medium

  • Parton in-medium-modification through the fragmentation function and study of the nuclear modification factor RFF


  • ALICE is well suited to measure

    global event properties


    identified hadron spectra

    on a wide momentum range (with very low pT cut-off) in Pb-Pb and pp collisions.
  • Robust and efficient tracking for particles with momentum in the range 0.1 – 100 GeV/c

  • Unique particle identification capabilities, for stable particles up to 50 GeV/c, for unstable up to 20 GeV/c

  • The nature of the bulk and the influence of hard processes on its properties will be studied via

    chemical composition, collective expansion, momentum correlations


    event-by-event fluctuations

  • Charm and beauty production

    will be studied in the pT range 0-20 GeV/c and in the pseudo-rapidity ranges |η|<0.9 and 2.5< η <4.0
  • High statistics of


    is expected in the muon and electronic channel
  • Upsilon family

    will be studied for the first time in AA collisions
  • ALICE will

    reconstruct jets

    in heavy ion collisions → study the properties of the dense created medium
  • Furthermore, ALICE will identify also

    prompt and thermal photons

    → characterize initial stages of collision region


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