Stato della ricerca e prospettive in collisioni ultrarelativistiche nucleo-nucleo

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Stato della ricerca e prospettive in collisioni
ultrarelativistiche nucleo-nucleo

• Federico Antinori
• INFN Padova

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Contenuto

• Introduzione (QGP, SPS)
• Risultati principali di RHIC
• Prospettiva hard probes (heavy flavour)
• non tratterò di FAIR (GSI, bassa energia)

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QCD phase diagram
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(Partial) Chiral Symmetry Restoration

• Confined quarks acquire an additional mass ( 350
  MeV) dynamically, through the confining effect of
  strong interactions
• M(proton) ? 938 MeV m(u)m(u)m(d) 10?15 MeV
• Deconfinement is expected to be accompanied by a
  restoration of the masses to the bare values
  they have in the Lagrangean
• As quarks become deconfined, the masses would go
  back to the bare values e.g.
• m(u,d) 350 MeV ? a few MeV
• m(s) 500 MeV ? 150 MeV

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Due predizioni storiche...
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Strangeness enhancement

• restoration of c symmetry -gt increased production
  of s
• mass of strange quark in QGP expected to go back
  to current value
• mS 150 MeV Tc
• copious production of ss pairs, mostly by gg
  fusion
• Rafelski Phys. Rep. 88 (1982) 331
• Rafelski-Müller P. R. Lett. 48 (1982) 1066
• deconfinement ? stronger effect
• for multi-strange
• can be built recombining uncorrelated
• s quarks produced in independent
• microscopic reactions
• strangeness enhancement increasing
• with strangeness content
• Koch, Müller Rafelski Phys. Rep. 142 (1986)
  167

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Charmonium suppression

• QGP signature proposed by Matsui and Satz, 1986
• In the plasma phase the interaction potential is
  expected to be screened beyond the Debye length
  lD (analogous to e.m. Debye screening)
• Charmonium (cc) and bottonium (bb) states with r
  gt lD will not bind their production will be
  suppressed

For T 200 MeV lD 0.1 0.2 fm
(very) rough estimate...
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Lera dellSPS...
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CERN heavy-ion programme

• 1994 to 2003
• typically 4 - 6 weeks per year Pb nuclei _at_ 158 A
  GeV/c
• (40 GeV in 1999, 158 GeV In in 2003, short 20,
  30, 80 GeV runs)
• 9 experiments
• WA97 (silicon pixel telescope spectrometer
  production of strange and multiply strange
  particles)
• WA98 (photon and hadron spectrometer photon and
  hadron production)
• NA44 (single arm spectrometer particle spectra,
  interferometry, particle correlations)
• NA45 (ee- spectrometer low mass lepton pairs)
• NA49 (large acceptance TPC particle spectra,
  strangeness production, interferometry,
  event-by-event studies)
• NA50 (dimuon spectrometer high mass lepton
  pairs, J/y production)
• NA52 (focussing spectrometer strangelet search,
  particle production)
• NA57 (silicon pixel telescope spectrometer
  production of strange and multiply strange
  particles)
• NA60 (dimuon spectrometer pixels dileptons and
  charm)

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A Pb-Pb collision at the SPS

• Busy events! (thousands of produced particles)
• High granularity detectors are employed (TPC, Si
  Pixels,...)

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Particle production

• Rapidity distribution for negative hadrons
  (mostly pions) for central Pb-Pb collisions (from
  NA49)
• 2500 hadrons per collision
• corresponding to an initial energy density of
  2 3 GeV/fm3

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Strangeness enhancement at the SPS

• Enhancement relative to p-Be

Enhancement is larger for particles of higher
strangeness content (QGP prediction!) up to
a factor 20 for W So far, no hadronic model
has reproduced these observations (try
harder!) Actually, the most reliable hadronic
models predicted an opposite behaviour of
enhancement vs strangeness
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J/y suppression at the SPS

• measured/expected J/y suppression vs estimated
  energy density
• anomalous suppression sets in at e 2.3 GeV/fm3
  (b 8 fm)
• effect seems to accelerate at e 3 GeV/fm3
  (b 3.6 fm)
• this pattern has been interpreted as successive
  melting of the cc and of the J/y

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What have we learned at the SPS?

• We create a strongly interacting fireball in a
  state of very large energy density, at which the
  very concept of individual, separated hadrons is
  not too meaningful
• At freeze-out (when the reinteractions between
  the produced particles cease) the system is
  expanding at ½ c
• The properties of this state are not explained in
  terms of those of a conventional system of
  strongly interacting hadrons we have found a new
  regime for strongly interacting system, which
  has to be investigated further
• This state exhibits many features of the QGP, in
  particular the expected effects of deconfinement
  (J/y suppression) and chiral simmetry restoration
  (strangeness enhancement) are there. It looks
  like the partonic degrees of freedom are active
• in order to better understand the properties of
  this state we need to go to higher energy ? RHIC,
  LHC
• conditions closer to ideal, harder probes

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The next step RHIC

• Relativistic Heavy Ion Collider (RHIC)
• at Brookhaven National Lab (USA - NY)
• Au-Au collision in STAR
• ?sNN 200 GeV, ? 10 from SPS
• higher energy densities, temperatures, expected
  QGP lifetimes, volumes
• QCD calculations more reliable ? improve our
  understanding!
• 10 units rapidity span better separation of
  central and fragmentation regions
• 4 dedicated, complementary experiments
• 2 larger (STAR hadronic signals, PHENIX
  leptonic, e.m. signals),
• 2 smaller (PHOBOS, BRAHMS)

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RHIC Headlines

• Rather low multiplicity
• Gluon saturation? (Colour Glass)
• Large Elliptic Flow
• v2
• High pT Suppression
• Rcp, RAA
• Suppression of Away-Side Jets
• Plus an New Structure in Away-Side?
• Valence Quark Counting Rules
• Recombination?

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Particle multiplicity

• Multiplicity per nucleon-nucleon collision
  increases
• Bjorken estimate of energy density ? ? 5.5
  GeV/fm3
• Extrapolation to LHC dNch/dy 2500

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Colour Glass?

• At small x, large A, saturation of gluon pdfs?

• would explain relatively low multiplicity

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Elliptic Flow

• Non-central collisions are azimuthally asymmetric
• The 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 -gt larger pressure
  gradient -gt larger momentum
• extreme ideal liquid (zero mean free path,
  hydrodynamic limit)

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Azimuthal Asymmetry

• at low pT azimuthal asymmetry as large as
  expected at hydro limit
• very far from ideal gas picture of plasma

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Rcp, RAA, RdAu

• Yield/collision in central collisions
• Yield/collision in peripheral collisions

Yield/collision in nucleus-nucleus
Yield/collision in proton-proton
Yield/collision in deuteron-nucleus
Yield/collision in proton-proton
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High pT suppression

• High pT particle production expected to scale
  with number of binary NN collisions if no medium
  effects
• Clearly does not work for more central collisions

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AA vs pp

• Differences can arise due to both initial-state
  and final-state effects

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Centrality Dependence
Au-Au (cold matter and medium effects)
dAu (only cold matter effects expected)
Preliminary Data
Final Data

• Opposite behaviour with centrality
• Looks like suppression in AA is due to medium
• Parton energy-loss in the medium? (Jet
  Quenching)

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Azimuthal Correlations (pp)

• In high energy collisions particles are
  correlated in azimuth due to jets
• e.g. at RHIC in proton-proton collisions from
  STAR
• trigger particle 4 lt pTlt 6
  GeV/c
• associated particles pT gt 2 GeV/c

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Azimuthal Correlations

• away-side jet still present in dAu
• but disappears in central AuAu
• away-side jet quenched?
• trigger bias on jets produced close to surface?

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Two peaks?

• Disappearance of away jet in central collisions
  at RHIC
• 4 lt pT(trig) lt 6 GeV/c
• pT(assoc) gt 2 GeV/c
• if pT(assoc) is lowered, peak turns up again, but
  with strange shape
• 0.15 lt pT(assoc) lt 4 GeV/c

F. Wang QM 2004
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• Similar results from PHENIX, too

PHENIX
STAR

• Low pT associated particles seem to peak
• 1 radian ( 60O)
• away from p

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Shock wave?

• Idea
• Casalderrey-Solana, Shuryak, Teaney
  hep-ph/0411315
• (see also Stocker Nucl.Phys. A750 (2005) 121)
• medium is dense
• away jet parton travels through medium with
  speed c
• faster than speed of sound in medium cs
• shock wave (sonic boom) is emitted at angle
• with cs2 0.2

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Baryon puzzle _at_ RHIC

• Central Au-Au as many p- (K-) as p (L) at pT
  1.5 ? 2.5 GeV

• ee- ?jet (SLD)
• very few baryons from fragmentation!

p
K
p
H.Huang _at_ SQM 2004
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Rcp

• strange particles come to rescue!

• if loss is partonic, shouldnt it affect p and p
  in the same way?

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Quark Recombination

• if hadrons are formed by recombination, features
  of the parton spectrum are shifted to higher pT
  in the hadron spectrum, in a different way
  for mesons and baryons
• ? constituent quark counting

S.Bass _at_ SQM04
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Where should recombination work?

• Proponents say _at_ pT between 1 and 4 GeV (6 GeV)
  for mesons (baryons)
• hydro below, fragmentation above (at RHIC energy)

fragmenting parton ph z p, zlt1
recombining partons p1p2ph
R.Fries _at_ QM04
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elliptic flow v2

• Recombination also offers an explanation for the
  v2 baryon puzzle...

scaled with n(quarks)
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A few hiccups...

• Strict recombination has a few theoretical
  problems...
• it violates 2nd law of thermodynamics
• reduction of the number of particles
  ? lower
  disorder ? entropy decreased
• it actually also violates the 1st...
• impossible to conserve energy and momentum
  simultaneously
• what happens to gluons?

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Picture emerging from RHIC

• System formed in AuAu collisions
• presumably partonic
• Strongly collective ( liquid behaviour)
• Very opaque medium (strong energy loss)
• Indications for hadronization by recombination
• How can we test this scenario?
• Need to confirm this picture with a different
  probe
• Heavy quarks

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Charm beauty ideal probes

• calculable in pQCD calibration measurement from
  pp
• rather solid ground
• caveat modification of initial state effects
  from pp to AA
• shadowing 30
• saturation?
• pA reference fundamental!
• produced essentially in initial impact
• probes of high density phase
• no extra production at hadronization
• probes of fragmentation
• e.g. independent string fragmentation vs
  recombination

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Probing the medium

• quenching vs colour charge
• heavy flavour from quark (CR 4/3) jets
• light flavour from (pT-dep) mix of quark and
  gluon (CR 3) jets
• quenching vs mass
• heavy flavour predicted to suffer less energy
  loss
• gluonstrahlung dead-cone effect
• beauty vs charm
• heavy flavour should provide a fundamental
  cross-check of quenching picture emerging from
  RHIC
• at LHC high stats and fully developed jets

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Heavy flavour production in AA

• binary scaling
• can be broken by
• initial state effects (modified PDFs)
• shadowing
• kT broadening
• gluon saturation (colour glass)
• (concentrated at lower pT)
• final state effects (modified fragmentation)
• parton energy loss
• violations of independent fragmentation (e.g.
  quark recombination)
• (at higher pT)

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Heavy flavour energy loss?

• Energy loss for heavy flavours is expected to be
  reduced
• i) Casimir factor
• light hadrons originate predominantly from gluon
  jets, heavy flavoured hadrons
  originate from heavy quark jets
• CR is 4/3 for quarks, 3 for gluons
• ii) dead-cone effect
• gluon radiation expected to be suppressed for q lt
  MQ/EQ
• Dokshitzer Karzeev, Phys. Lett. B519 (2001)
  199
• Armesto et al., Phys. Rev. D69 (2004) 114003

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)
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Large suppression at RHIC!

• yet, region above 3-4 GeV expected to be
  dominated by beauty...

• n.p. electrons as suppressed as expected for c
  only (no b)

Xin Dong_at_QM05

• disentangling c/b is a must!
• e.g. full reconstruction of D vertices

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ALICE
hlt0.9 B 0.4 T TOF TPC ITS with - Si
pixels - Si drifts - Si strips
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Track Impact Parameter

• expected d0 resolution (s)

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LHC is a Heavy Flavour Machine!

• cc and bb rates
• ALICE PPR (NTLO shadowing)

cc
bb
PbPb/pp
PbPb/pp
PbPb
PbPb
pp
pp
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• some prediction ...

beauty
charm
Armesto et al. Phys.Rev. D71 (2005) 054027
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D0 ? K-p

• expected ALICE performance
• S/B 10
• S/?(SB) 40 (1 month
  Pb-Pb running)

• ? similar performance in pp
• (wider primary vertex spread)

pT - differential
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RAA(D) in ALICE

• expected performance (1 month Pb-Pb, 9 months
  p-p)

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B ? e X

• Expected ALICE performance (1 month Pb-Pb)
• e identification from TRD and dE/dx in TPC
• impact parameter from ITS

S/(SB)
S per 107 central Pb-Pb events
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At LHC real jets!
2 GeV 20
GeV 100 GeV 200 GeV
Mini-Jets 100/event 1/event
100k/month

• Well visible event-by-event! e.g. 100 GeV jet
  underlying event
• e.g. study quenching with b-tagged jets!

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b tagging

• e.g. ATLAS u rejection (Ru) performance in Pb-Pb
• H ? bb, uu with MH 400 GeV

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Away side?

• Collective behaviour opposite to jet
• eg Mach cone

• Casalderrey-Solana, et al. hep-ph/0411315
• Stocker Nucl.Phys. A750 (2005) 121)

• What happens with big-fat-heavy-quark jets?
• e.g. modification of Mach cone? FA, E Shuryak
  J.Phys. G31 (2005) 19

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Machine commissioning scenario (as envisaged
today)

• T0 ( 1st of July 2007 as of today)
• One month to get the machine ready for beams (T0
  1 month)
• Three months to commission the machine with beams
  (T0 4 months)
• One month of rather stable operations,
  interleaved with machine development with 43 and
  156 bunches, with the possibility of collisions
  for physics during nights ( 20 shifts of 10
  hours each L 1to 3.5x1030cm2s1) (T0 5
  months)
• Shutdown (T0 8 to 9 months) today machine
  people talk about 3 to 4 months. It will depend
  on requirements by experiments. If T0 1st of
  July, start of shutdown will coincide with the
  Christmas holidays!

T0
first Pb-Pb collisions in 2008
Stable beams
Preparation
First collisions.
Shutdown 3 to 4 months?
July
Nov.
Feb.
Mar
Aug.
Sept.
Oct.
Dec.
Jan.
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Example B in pp

• B ? e X
• electron id in TRD and TPC, electron impact
  parameter from ITS

• Expected stats from first large pp sample in
  ALICE a few 107 evts

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Conclusioni

• Stato il campo è più vivo che mai
• SPS osservazione delle signatures storiche
• RHIC comportamento idrodinamico? forte
  attenuazione dei jet?
• Prospettive lLHC è una macchina ideale per AA
• altissima energia -gt deep deconfinement
• alte rate di hard probes
• un esperimento dedicato (ALICE), buone
  performance in altri due (ATLAS, CMS)

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Lattice QCD

• In lattice QCD, non-perturbative problems are
  treated by discretization on a space-time lattice

• zero baryon density, 3 flavours
• e changes rapidly around Tc
• Tc 170 MeV
• ? ec 0.6 GeV/fm3
• at T1.2 Tc e settles at about 80 of the
  Stefan-Boltzmann value for an ideal gas of q,q g
  (eSB)