ANISOTROPIC FLOW MEASUREMENTS IN ALICE

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ANISOTROPIC FLOWMEASUREMENTS IN ALICE

• Sudhir Raniwala
• for the ALICE collaboration

Department of Physics University of Rajasthan
Jaipur
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PROLOGUE

• ALICE Letter of Intent in 1993 motivated by
• results from relativistic heavy ion collisions
  from AGS and
• results from ultra-relativistic heavy ion
  collisions at SPS
• ALICE Technical Proposal 1995
• summary of proposed signals listed in chapter 1
• open charm production to study parton kinematics
• prompt photons for thermal radiation
• high pT hadrons for energy loss in plasma
• J/? production to probe deconfinement
• Strangeness production and chiral symmetry
  restoration
• multiplicity fluctuations and critical phenomena
• particle ratios for probing evolution
• line shape parameters for probing evolution
• interferometry to determine freeze-out radius
• Flow was not a consideration for the experiment,
  or its design

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INTRODUCTION

• Jean-Yves Ollitraults suggestion (1992)
• Finite impact parameter collisions gt anisotropic
  spatial density unequal pressure gradients
  (assuming thermalisation) produces an anisotropic
  momentum distribution of particles. The strength
  of the anisotropy, and its systematic dependence
  on various parameters, provides information on
  the equation of state.
• Flow at the SPS energies was first reported by
  WA93 experiment (PL B403 (1997) 390). Measurement
  included contribution from non-flow ?
• Discovery of flow at higher energies the
  essence
• realisation of existence of a reaction plane in
  AA collisions at high energies
• One of the first results published at RHIC were
  on observation of flow.

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RESULTS FROM SPS AND RHIC
WA98 Collaboration, EPJ C41 (2005) 287

• SPS
• Consistent results(WA98, NA49, CERES), v2 v2
  (pT, y, part-id, b)

STAR Collaboration PR C72 (2005) 014904

• RHIC
• Consistent results
• pT dependence shows 3 regions
• Low pT gt hydrodynamics
• Intermediate pT gt quark coalescence
• High pT gt hard scattering, parton energy loss

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IDEAL MEASUREMENTS

• v2 v2 (pT, y, particle-id) for each event? gt
  complete event shape.
• Measure v2 v2 (s, pT, y, particle-id, b), an
  average event shape (just need azimuthal angle)
• Different techniques with relative merits
• Reaction Plane Method
• Cumulants
• Lee Yang Zeroes
• General rule more is the multiplicity and flow,
  better is the determination
• What values do we expect at LHC?

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SCALING OF MULTIPLICITY
Charged multiplicity is parametrized as a
function of Npart
where
? 0.288 GBW parameter d 0.79
0.02 fit parameter N0 0.47
overall normalization
The model perfectly fits RHIC data, and can be
easily extrapolated to LHC energy
Result for PbPb, expect dN/d? 2000 for central
PbPb at LHC.
Armesto,Salgado,Wiedemann PRL 94 (2005) 022002
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EXPECTED v2 at LHC
Heinz, Kolb, Sollfrank
Hirano et al , nucl-th/0701075v2
Hirano, to check reference.
N.Borghini and U.A.Weidemann, J.Phy.G 35 (2008)
023001
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FLOW MEASUREMENTS IN ALICE

• TPC (-0.9 lt ? lt 0.9)
• ITS SPD(-2 lt ? lt 2 (layer 1))
• FMD(-3.4 lt ? lt -1.7,
• 1.7 lt ? lt 5.0)
• PMD (2.3 lt ? lt 3.5)
• ZDC (beam rapidity)
• And everything else, once the reaction plane is
  known ?
• Measure in the range -3.4 lt ? lt 5.0
• Fairly good particle identification over large
  range. Details in poster by Naomi (P-127)

• Feasibility of measurements
• Full simulation(ALIROOT)
• Independent simulations (a) generator level (b)
  fast simulation folding detector response
• Variety of combination of multiplicity and v1
  (for ZDC) v2 (for others)

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Designed for dN/d? 8000 Maximum occupancy
throughout the volume varies between 15 -
40 Momentum resolution 1-2 for particles with
momentum 100 MeV/c to 1 GeV/c Particles up to
100 GeV/c will be tracked with a 10 resolution
(combined tracking including ITS and TRD)
-0.9 lt ? lt 0.9
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RECONSTRUCTION ABILITY TWO PARTICLE NON-FLOW
CORRELATIONS
Poster by Naomi van der Kolk (P-127)
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Layer 1 ?? ? lt 2.0 Layer 2 ?? ? lt 1.4

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

T. Virgili
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FLOW USING THE FMD
C.Nygaard (Masters Thesis)
Can reconstruct v2 to better than 10 accuracy.
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Directed Flow and Zero Degree Calorimeter

• Reaction Plane best determined by measuring
  spectator neutrons
• Good check on estimated second order event plane.
  
• Estimate independent (or less dependent) of
  eccentricity arguments
• Minimal non-flow effects because of large
  rapidity gap
• Estimate independent of details of evolution of
  hot and dense matter
• Estimate will yield sign of elliptic flow
• Need to measure this away from mid-rapidity
• Experience from RHIC suggests values of 20
• Can ALICE do it?
• Poster Nora de Marco (P-190)

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First order Event Plane Estimate
1st-order Event Plane Resolution v/s Centrality
50000 Pb-Pb minimum bias events at 2.76 TeVA
have been simulated by means of HIJING
generator. A fast simulation takes into account
nuclear fragmentation, Fermi momentum, beam
parameters, detector smearing and directed flow
of spectator neutrons.
full event plane resolution (2 arms) sub-event
plane resolution (1 arm) The reconstructed
resolution is same (Poster P-190, Nora de Marco)
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FLOW USING THE PMD
WA98 Collaboration, EPJ C41 (2005) 287
Measure pT integrated flow for photons (low pT
threshold, deduce for neutral pions?)

• Determine the event plane accurate resolution
• For 5 flow and multiplicity 2400 in PMD (1.8 lt?
  lt 2.6)
• No Scattering
  Moderate Scattering
• Expected Resolution 0.831
  0.808
• Estimated Resolution 0.836 /- 0.002
  0.812 /- 0.003

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ANISOTROPIC EMISSION OF J/? IN FORWARD RAPIDITY
USING EVENT PLANE FROM PMD

• Away from midrapidity
• Glauber absorption isotropic and comover
  absorption anisotropic
• gtv2 (J/? ) - 0.025 (Heiselberg and Mattielo
  PRC60 (1999) 44902)
• J/? from dimuon channel
• measured in DiMuon Spectrometer( 4.0 lt ? lt
  -2.5)
• Data taking rate of PMD and DiMuon Spectrometer
  is same
• Hijing parametrisation, folding detector response
• Luminosity 5. 1026 (Results for 106 seconds of
  running)
• Different centralities

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INVARIANT MASS SPECTRA OF LIKE SIGN AND UNLIKE
SIGN MUONS
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BACKGROUND SPECTRA IN 12 AZIMUTHAL BINS
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Flow in unlike sign pairs
Signal in 12 bins
Wider mass region gt lower S/B gt lower flow v2
v2meas (1S/B)/(S/B)
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RESULTS DETERMINE REQUIRED SIGNIIFICANCE
Significance depends upon large number of
factors Significance a vNevts Significance 0.7
/ (v2 f) To measure 5 flow with 10 accuracy
requires a significance of 140 for a given S/B.
To probe 2.5 flow with similar accuracy will
require 4 times as many events.
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EPILOGUE

• Though none of the ALICE sub-detectors were
  initially designed keeping flow in mind, ALICE is
  now ready to measure flow and every other signal
  with respect to the reaction plane.

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BACK UP SLIDES
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CONCLUSIONS
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E.Simili
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ALICE Zero Degree Calorimeter
The 2 neutron zero degree calorimeters ZNs have
full acceptance for spectator neutrons (?ZN gt
8.7).
GEANT-based simulation
The 2x2 ZN segmentation gives a rough
localization of the spectator neutrons spot on
the front face of the calorimeter. This
localisation provides the first order event plane
(spectator neutrons)
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RECONSTRUCTION ABILITY
E.Simili
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FLUCTUATIONS
A
ALICE PPR
Large multiplicity environment enables
determination of fluctuations..
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