Osservabili%20

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Osservabili soft in ALICE

• Quark Matter Italia
• Roma, 22-24 Aprile 2009

Luciano Ramello Università del Piemonte
Orientale I.N.F.N. for the ALICE Collaboration
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Indice

• Lesperimento ALICE a LHC
• Osservabili soft in ALICE
• Molteplicità di particelle cariche
• Flusso direzionato (v1) ed ellittico (v2)
• Spettri in pT, rapporti h?/h?, B/M, ...
• Risonanze
• Correlazioni HBT
• Fluttuazioni

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Lesperimento ALICE
Configurazione per le prime collisioni p-p e
Pb-Pb a LHC
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ALICE dedicated to H.I.

• General purpose (as opposed to all SPS and some
  RHIC expts)

• In contrast to LHC experiments mainly devoted to
  (hard) p-p physics
• ALICE is focussed on heavy-ion physics
  therefore has

1) Capability of coping with the high
multiplicity generated in H.I. collisions
(2000 charged particles per unit rapidity,
designed for 6000)

• Not strictly necessary for selected hard probes
  (such as ????),
• but important to access the bulk of particle
  production, i.e.
• to study soft observables

• Implies, in particular
• High granularity
• Large bandwidth for Data Acquisition system

2) Possibility of pushing down as much as
possible its pT reach

• Implies a not too high B field for momentum
  measurement
• (note Conflicting requirement with accuracy
  for hard probes!)

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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)

ALICE
TPC
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ALICE configuration at start-up

• 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
• HLT (High Level Trigger) (50)
• At start-up full hadron and muon capabilities,
• Partial electron and photon capabilities

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ALICE Detector Installation Goal 2009
2008/09 Shutdown used for additional installation
and repairs
Complete ITS, TPC, TOF, HMPID, FMD, T0, V0,
ZDC, Muon arm, Acorde PMD, Trigger,
DAQ Partial installation PHOS(3/5) 6-8/18
TRD 2-4/6 EMCAL 50 HLT
ALICE Status
P. Kuijer QM 2009
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Number of TRD EMCAL modules depends on access
conditions during LHC power test !
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Alice tracking performance

• Central barrel tracking ITS TPC TRD
• Robust, redundant tracking from lt 100 MeV/c to
  gt 100 GeV/c
• Very little dependence on dNch/dy up to dNch/dy
  8000

• ?p/p lt 5 at 100 GeV with careful control of
  systematics

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Alice particle identification

• stable hadrons (p, K, p) 0.1ltplt5 GeV/c (p, p
  with 80 purity to 60 GeV/c)
• ? dE/dx in silicon (ITS) and gas (TPC)
  time-of-flight (TOF) Cherenkov (HMPID)
• decay topologies (K0, K, K-, ?, cascades, D)
• ? K and ? decays beyond 10 GeV/c
• leptons (e, µ ), photons, p0
• ? electrons TRD p gt 1 GeV/c, muons p gt 5 GeV/c,
  p0 in PHOS 1ltplt80 GeV/c

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Cosmic event with SPD trigger

• Probably a muon interaction in the magnets iron
• about 350 tracks reconstructed in TPC

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Cosmic event with ACORDE trigger
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p beam in ALICE

• LHC pilot beam at 450 GeV p-Si collision in the
  SPD (Sept. 12, 2008)

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Breve introduzionecosa ci aspettiamo di
imparare studiando le osservabili soft a LHC?

• Ovvero dati i risultati ottenuti a RHIC cè
  ancora qualcosa da scoprire a LHC?

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Reminder space-time evolution

• Thermal freeze-out
• Elastic interactions cease
• Particle dynamics (momentum spectra) fixed
• Tfo (RHIC) 110-130 MeV
• Chemical freeze-out
• Inelastic interactions cease
• Particle abundances (chemical composition) are
  fixed (except maybe resonances)
• Tch (RHIC) 170 MeV
• Thermalization time
• System reaches local equilibrium
• teq (RHIC) 0.6 fm/c

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Introduction/1

• Initial conditions measuring dNch/d? will be
  crucial for evaluating energy/entropy density
  confirm (or reject) the saturation model (Lacey,
  QM 2009)
• Thermo/hydrodynamics of QCD at T 400 MeV (LHC)
  a new regime should emerge (Wiedemann, QM 2009)
• ? consequences for v2, HBT correlations, pT
  spectra
• Chemical composition will statistical models
  still work at LHC energy? Measuring hadron yields
  ( their ratios) will give the answer (limiting
  temperature of 160 MeV?)

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Introduction/2

• Nature of phase transition at LHC (high T, low
  ?B) a crossover transition is expected it
  should be verified by studying fluctuations of
  conserved quantities (Mohanty, QM 2009), while
  other experimental programmes (NA61/SHINE at
  CERN, the RHIC Critical Point Search, CBM at GSI)
  will explore other regions of the phase diagram
• Lesson from RHIC know your reference, in our
  case we will have collected pp data before Pb-Pb
  ones, but it will be necessary later to collect
  p-A (d-A) data as well, to understand cold
  nuclear matter effects

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Molteplicità di particelle cariche
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Charged multiplicity at the LHC
increasing ?s decreasing x

• Extrapolation of dNch/d?max vs ?s
• Fit to dN/d? ? ln s (limiting fragmentation)
• or Saturation model (dN/d? ? ?s? with ?0.288)?
• Clearly distinguishable with the first 10k LHC
  events

Saturation model ?Armesto Salgado Wiedemann, PRL
94 (2005) 022002
Central collisions
Models prior to RHIC
Extrapolation of dN/dh ? ln s
5500
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dNch/d? PHOBOS extrapolation
W. Busza QM 2009
AuAu Data from PHOBOS, Nucl. Phys. A757 (2005)
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PHOBOS extrapolations to LHC energy, Wit Busza,
J. Phys. G35, 044040 (2008) Total Nch (PbPb
vsNN 5.5 TeV) 15 000 1 000 Mid-rap. dNch/d?
_at_ Npart386 (PbPb vsNN 5.5 TeV) 1 200
100 Total Nch for inelastic pp _at_ vs 14 TeV
(10 TeV) 60 10 (56 9)
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Total particle production
PHOBOS Nucl. Phys. A 757 28 (2005) E178
PRL34(1975)836 Wit Busza Acta. Phys. Pol.
B35(2004)2873
Similarity of total particle production in ee-,
pp, pA, KA, pA and AA collisions
What is the mechanism that makes the total
particle production insensitive to the
intermediate state?
W. Busza QM 2009
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Charged multiplicity in ALICE
Reconstructed dNch/d? with tracklets in
SPD (generated dNch/d? 3000)
ALICE PRELIMINARY
T. Virgili
Wide rapidity coverage provided by ITS (SPD),
TPC in the central region FMD in the forward
region
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dNch/d? in pp SPD

• dNch/d? from tracklets (vertex
• 2 SPD layers)
• larger ? and pT acceptance
• less stringent calibration/alignment needs

ALICE PRELIMINARY
F. Prino M. Nicassio QM 2009

• Several careful corrections needed to go from
  Reconstructed to Corrected dNch/d?
• background
• algorithm detector efficiency
• geometrical acceptance

• and also
• vertexing efficiency
• Min. Bias trigger efficiency

ALICE PRELIMINARY
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dNch/d? in pp FMD
The FMD will complement the dNch/d?
measurement for -3.4lt?lt-1.7 and
1.7lt?lt5 Background correction is most crucial
since gt50 of particles crossing FMD are
secondaries
ALICE PRELIMINARY
ALICE PRELIMINARY
H.H. Dalsgaard
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Flusso direzionato (v1) ed ellittico (v2)
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Anisotropic flow
Azimuthal asymmetry in coordinate space
(transverse plane)
Kolb Heinz
produces azimuthal asymmetry in momentum space
The amount of observed flow depends on centrality
and on the spatial eccentricity ?
v1 directed flow v2 elliptic flow
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Flow and reaction plane
Produced particles angular distribution in the
transverse plane (xy) ?
Reaction plane (xz)

• The Fourier expansion is referred to the reaction
  plane angle (?RP) which must be located in the
  experiments reference frame
• The reaction plane angle can be evaluated at any
  particular order of the expansion, using produced
  particles or even spectators (e.g. bounce off
  of spectator neutrons, which can provide both the
  first order reaction plane and the amount of
  directed flow v1)

Figures courtesy F. Prino
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Directed flow in ALICE

• v1 can be measured in ALICE via spectator
  neutrons (?gt8.7), namely by their centroids as
  obtained by the two zero-degree ZN calorimeters

7.04 cm

• For a range of plausible v1 values (10 20 30)
  at LHC, the first order event plane resolution
  obtained by combining both ZNs is quite adequate
• In addition, this measurement provides the sign
  of v2

2.76 TeVA Pb-Pb minimum bias events HIJING
simulation
N. Demarco
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Elliptic flow vs dependence
From the observed v2 dependence on vs
one expects v2(0) _at_ LHC/ALICE 0.08
Large signal ? easy measurement, but.. beware of
non-flow contributions (jets...)!
v2 (elliptic flow) is supposed to scale as
eccentricity ? (more on this later) from
hydrodynamics calculations, it appears that the
contribution to v2/? by the QGP phase (rather
than from the cascade) is much larger at LHC
with respect to lower energies
T. Hirano, U. Heinz, D. Kharzeev, R. Lacey, Y.
Nara, QM 2008
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Elliptic flow and eccentricity

• The experimental definition of eccentricity is
  tricky from geometry (Glauber) or taking into
  account event-by-event fluctuations in the actual
  number of participants?
• the Participant Plane (PP) must be distinguished
  from the Reaction Plane (RP)

A. Poskanzer QM 2009
momentum space
coordinate space

• v2 fluctuations (in events of the same centrality
  class) are closely related to fluctuations of
  ?part - which can be computed via Glauber
  MonteCarlo

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Elliptic flow different methods

• Problem measurement of v2 is affected by both
  non-flow and fluctuations
• Different methods to extract v2 have been
  developed, based on event plane, 2-particle or
  many-particle correlations they give different
  results

published
corrected to PP
STAR, J. Adams et al., PRC 72, 014904 (2005)
but they agree on mean v2 in Participant Plane!
A. Poskanzer QM 2009
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Elliptic flow one result?

• A v2 for theorists (corrected to Reaction
  Plane)

reaction plane
corrected to RP
Glauber
CGC
Voloshin, Poskanzer, Tang, and Wang, Phys. Lett.
B 659, 537 (2008)
Still, some dependence on the assumption
(Glauber vs. CGC) about fluctuations
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What about viscosity?

• Relativistic viscous hydrodynamics is making nice
  progress (see e.g. D. Teaney, R. Snellings P.
  Romatschke at QM 2009), 2nd order theory (with
  zero bulk viscosity) is under control
• Shear viscosity (?) reduces v2 comparing
  hydrodynamics calculations with RHIC data, with
  high confidence one can conclude ?/s lt 0.5 6/4?

M. Luzum and P. Romatschke, 2008 (erratum 2009)
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More on data vs. hydrodynamics

• Parametrization of v2/? deviation from ideal
  hydro
• v2/? h / (1B/(1/S dN/dy)) h / (1Kn/K0)
• h ideal hydro limit for v2/?
• 1/S dN/dy inversely proportional to Knudsen
  number Kn ?/L (mean free path/system size)

• The B parameter scales with ?/s but is also
  sensitive to the EoS
• STAR data are well described using a CGC e with
  soft EoS and ?/s 2/4p or Glauber e with hard
  EoS and ?/s 4/4p
• The shift of the peak of v2 as a function of pT
  may be an additional independent way of
  extracting ?/s

R. Snellings, QM 2009
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Elliptic flow in ALICE
ALICE Pb-Pb simulation the v2 flow signal (for 3
different extrapolations) is clearly disentangled
from the non-flow contribution over a wide range
of centrality (charged multiplicity).
v2/e0.33
hydro
LDL
v2/e0.22
Centrality class
dNch/dh 2000
Relativistic hydrodynamics prediction v2/?
constant J. Y. Ollitrault P.R. D 46
(1992) Low Density Limit prediction v2/?
const. (1/S)dNch/dy Heiselberg, Levy P.R. C 59
(1999) Poskanzer, Voloshin P.L. B 474 (2000)
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Flow event plane other methods
event plane resolution (TPC) vs. v2 for 1000
charged tracks
Reaction Plane Cumulants Lee Yang Zeroes
v2 vs pT for ?v2 ?0.0625
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Flow with inner ITS layers

• SPD alone useful for low multiplicity events,
  and has higher acceptance
• wrt TPC (low pT threshold 15 MeV)
• Simulations suggest
• flow for negative and positively charged
  particles separately
• coarse pT binning may be possible

Layer 1 ?? ? lt 2.0 Layer 2 ?? ? lt 1.4
In addition, FMD and PMD will measure flow at
forward rapidities
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Spettri in pT e rapporti

• Composizione chimica,
• meccanismi di adronizzazione

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Statistical hadronization

• Statistical models assume that the hadronic
  system at freezeout can be described as a Hadron
  Resonance gas in chemical and thermal
  equilibrium, whose composition (ratios of
  particle species) follows statistical mechanics
  and depends on two parameters chemical freezeout
  temperature Tch and baryonic chemical potential
  ?B
• Other model parameters such as fireball volume,
  ?I3, ?S are constrained from initial state
• These models have been remarkably successful in
  describing hadron yields ratios up to RHIC - see
  e.g. A. Andronic et al., Nucl. Phys. A772 (2006)
  167

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Statistical hadronization at LHC

• Temperature Tch increases rapidly at low vs, then
  reaches about 160 MeV at 7-8 GeV and stays
  constant chemical potential ?B decreases
  continuously with increasing vs (A. Andronic et
  al., arXiv0711.0974 hep-ph)

TLHC 1614 MeV
mBLHC0.8(1.2,-0.6) MeV
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Particle spectra in ALICE

• Transverse momentum ranges for particle
  identification in ALICE (central barrel)

Expected charged hadrons yields for 107 Pb-Pb
central collisions (TPC PID on statistical basis)
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Antiparticle/particle ratios
Phys. Rev. C 77, 061901(R) (2008) Phys. Rev. C
71, 021901(R) (2005)
ALICE PRELIMINARY
Systematic error on asymmetry lt1.5 for a 10
material budget uncertainty
W. Busza QM 2009
P. Christakoglou, M. Oldenburg
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Identified hadrons spectra (TPC)
ALICEcosmics real data
ALICE simulation
A. Kalweit
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Strange particle spectra
? optimized, pT-dependent selection cuts
?
?
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How to fit spectra?
Crucial to obtain the total yield (extrapolation
down to pT0 7-15 of the yield is not measured
directly) The proposed functional forms include

• Exponential in mT (Boltzmann)
• Levy
• Tsallis blast-wave (power law in mT)

B. Hippolyte, H. Ricaud
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Baryon/meson ratios vs. pT/1
Sarah Blyth (STAR) QM 2006
High baryon to meson ratio (1) at intermediate
pT discovered at RHIC in AuAu reactions,
inconsistent with pQCD predictions

• Baryon to meson ratio should be sensitive to
• hadronization mechanism (quark count)
• radial flow of medium (hadron mass)
• High B/M ratios may be explained by quark
  coalescence, but not uniquely

AuAu 200 GeV
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Baryon/meson ratios vs. pT/2
-
The ratio (??)/K0 at mid-rapidity in pp
collisions at RHIC energy is flat and below
unity, while at higher energies (UA1, CDF) it
rises above unity.
-
The maximum value of (??)/K0 at mid-rapidity in
pp collisions is not reproduced by PYTHIA. EPOS
(with the mini-plasma option) predicts a strong
increase of this ratio at LHC energy.
B. Hippolyte, H. Ricaud
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Risonanze
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Why resonances?

• Resonance Life-time fm/c
• r(770) 1.3
• ?(1232) 1.7
• f0(980) 2.6
• K(892) 4.0
• S(1385) 5.7
• L(1520) 13
• ?(783) 23
• (1020) 45

• Decay time comparable with (or even shorter
  than) QGP lifetime
• Rescattering and regeneration between chemical
  and kinetic freeze-out will affect the final
  yields
• Comparing leptonic and hadronic final states
  provides further insight

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Resonances at RHIC
P. Fachini QM 2009
Mass shift 45 MeV/c2 observed
M. Naglis QM 2009
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Resonances in ALICE
Invariant mass reconstruction, background
subtracted (like-sign method) mass
resolutions 1.5 - 3 MeV/c2 and pT stat. limits
from 8 (r) to 15 GeV/c (f, K)
r0(770) pp- 106 central Pb-Pb
K(892)0 K p 15000 central Pb-Pb
Mass resolution 2-3 MeV
Invariant mass (GeV/c2)
Generated reconstructed f for 107 central Pb-Pb
Mass resolution 1.2 MeV
f (1020) KK-
ALICE PRELIMINARY
A. Badalà, A. Pulvirenti
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Correlazioni HBT (Hanbury-Brown Twiss, a.k.a.
Femtoscopy)
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No more RHIC HBT puzzle?
RHIC HBT PUZZLE 1st order phase transition
flow spectra OK ideal hydro (no viscosity)
HBT radii NOT OK ?01.0 fm/c
Solution Early acceleration (t lt 1 fm/c) Shear
viscosity EoS (crossover) Initial energy profile
S. Pratt, QM 2009
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Another solution to the HBT puzzle
1) semi-hard EoS
W. Florkowski WPCF 2008
2) Single freezeout modeled by THERMINATOR 3)
Initial conditions 2-D gaussian in transverse
plane, from Glauber MC
?Spectra, v2 and HBT radii well reproduced
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Femtoscopy in ALICE/1
Projected 3-D two-pion correlation function C2
for LHC PbPb collisions, for b8 fm centrality
and pT bin 0-200 MeV/c
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Femtoscopy in ALICE/2
Anti-merging cut Tracks that would be merged in
the TPC if they were in the same event have to
be removed from the background (mixed event
sample)
Anti-splitting cut here the effect of the cut on
identical K correlations is shown
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Fluttuazioni
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QCD phase transition / Critical Point
Lattice and other QCD based models ?B 0 ?
Cross-over transition TC 170-195 MeV ?B gt 160
MeV ? QCD critical point
B. Mohanty QM 2009
Experiments See distinct signatures that
relevant d.o.f. are quark and gluons Tinitial(dire
ct photons) gt TC(Lattice) - PHENIX direct photon
arXiv0804.4168 No signature of QCD critical
point established, possible hints at SPS (see
C.Hoehnes talk)
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Phase transition at ?B 0
See also Spontaneous strong Parity violation at
RHIC B. Voloshin QM 2009
B. Mohanty QM 2009

• Fluctuations at deconfinement transition visible
  through
• Second moment of event-by-event distributions of
  multiplicity,
• mean pT, mean ET after removing non-dynamical
  fluctuations
• Fluctuations in particle ratios sensitive to
  particle numbers at chemical FO (not kinetic FO),
  volume effects should cancel
• Fluctuations in conserved quantities net
  charge, net baryon number, net strangeness

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Fluctuation results (QM 2009)
Observable Experiment (Beam energy in GeV) Conclusions
K/? and p/? NA49(6.3 - 17.3) arXiv0808.1237 M.I. Gorenstein et al, arXiv0811.3089 p/??fluctuations similar results from UrQMD K/? higher than UrQMD at lower energy HSD transport gives similar energy dependence
K/? and p/? STAR(19.6 - 200 GeV) arXiv 0901.1795 G. Westfall - WWND09 K/? Statistical hadronisation model (?qgt1) agrees. HSD transport model similar results p/??fluctuations similar to default UrQMD
Observable Experiment (Beam energy in GeV) Conclusions
Net-charge STAR (19.6 - 200 GeV) pp, CuCu, AuAu arXiv0807.3269 Lie between charge conservation effects and resonance gas model.
Net-charge NA49 (6 - 17 GeV) PRC 70,064903 (2004) Consistent with charge conservation
Net-charge PHENIX (130 GeV) PRL 89, 082301 (2002) Similar to RQMD calculations.
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EbyE in ALICE/1
Particle ratios ?
HIJING PbPb events _at_ vsNN 5.5 TeV
C. Zampolli
Temperature fluctuations
generated
reconstructed
protons from a single HIJING central event T
31913 MeV
300 central events
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EbyE in ALICE/2
Balance functions
P. Christakoglou WPCF 2008 (Cracow)
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Future explorations
C. Höhne QM 2009
complete scan of the QCD phase diagram with
modern, 2nd generation experiments on the horizon!

• RHIC beam energy scan
• - evolution of medium properties
• - turn-off of established signatures
• - search for CP and PT
• NA61 at SPS (2007 acc. by SPSC)
• - search for CP and PT in energy-system size
  scan
• both essentially limited to high yield
  observables
• - RHIC energy dependent
• FAIR and NICA
• - new accelerator projects
• - FAIR high intensities! ? rare probes!

30A GeV
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Protone-protone
Alcuni esempi delle potenzialità di ALICE con i
dati p-p del primo giorno / primo mese (Ecm 10
TeV)
65
Charged multiplicity
After unfolding
ALICE PRELIMINARY
66
pT spectra ? and anti-?
Tracking in ITSTPC Topological PID (decay
vertex) Gamma conversions partly removed No
correction for absorption
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Resonances
With 100K - a few 100K events with PID, f and
K spectra feasible up to 3-4 GeV With 1M a few
M events f reconstruction feasible without PID,
being studied for K
Example of first day ? analysis with TPC only,
no PID pp 10 TeV
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EbyE particle ratios
M. Fragkiadakis M. Vassiliou
Simulated pp data at 10 TeV ITS, TPC and TOF PID
combined PID Effic. Contamination K/p and
p/p Event-by-Event particle ratios extracted ?
verified that (as expected) dynamical
fluctuations in pp (PYTHIA) are zero
ALICE PRELIMINARY
69
Conclusions

• ALICE detectors are installed, apart from full
  e.m. calorimetry (due for completion in 2011)
• All installed detectors are being commissioned
  with cosmic muons calibration and alignment are
  well under way
• Analysis procedures have been developed and
  tested on the Grid, both for PbPb and pp first
  phyiscs
• ALICE is well equipped, both detector-wise and
  analysis-wise, to produce a wealth of soft
  physics results with first pp and PbPb
  collisions

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ALICE References

• ALICE Physics Performance Report
• Volume 1 F. Carminati et al., J. Phys. G. Nucl.
  Part. Phys. 30 (2004) 1517
• Volume 2 B. Alessando et al., J. Phys. G. Nucl.
  Part. Phys. 32 (2006) 1295
• ALICE Detector technical paper
• K. Aamodt et al., The ALICE Experiment at the
  CERN LHC, 2008 JINST 3 S08002.
• http//aliceinfo.cern.ch/

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Many thanks to

• ALICE collaborators, in particular for their
  recent presentations/contributions
• Bruno Alessandro
• Panos Christakoglou
• Angela Badalà
• Boris Hippolyte
• Christian Kuhn
• Francesco Prino
• Alberto Pulvirenti
• Enrico Scomparin