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ANISOTROPIC FLOWMEASUREMENTS IN ALICE
Sudhir Raniwala for the ALICE collaboration
Department of Physics University of Rajasthan
Jaipur
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 2
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/ψ 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……
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 3
INTRODUCTION
• Jean-Yves Ollitrault’s suggestion (1992): – Finite impact parameter collisions => 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.
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 4
RESULTS FROM SPS AND RHIC
• SPS•Consistent results(WA98, NA49, CERES), v2 = v2 (pT, y, part-id, b)
•RHIC•~Consistent results• pT dependence shows 3 regions•Low pT => hydrodynamics•Intermediate pT => quark coalescence•High pT => hard scattering, parton energy loss
ST
AR
Col
labo
rati
on
PR
C72
(20
05)
0149
04
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98 C
olla
bora
tion
, E
PJ
C41
(20
05)
287
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 5
IDEAL MEASUREMENTS
• v2 = v2 (pT, y, particle-id) for each event => 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?
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 6
SCALING OF MULTIPLICITYSCALING OF MULTIPLICITY
The model perfectly fits RHIC data, and can be easily extrapolated to LHC energy
Armesto,Salgado,Wiedemann – PRL 94 (2005) 022002
Charged multiplicity is parametrized as a function of Npart
3
1
0
0~
1
part
AAch
part
NsNd
dN
N
λ = 0.288 GBW parameter
δ = 0.79 ± 0.02 fit parameter
N0 = 0.47 overall normalization
where :
Result for PbPb, expect dN/dη ~ 2000 for central PbPb at LHC.
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 7
Hirano, to check reference….
EXPECTED v2 at LHC N
.Bor
ghin
i and
U.A
.Wei
dem
ann,
J.
Phy
.G 3
5 (2
008)
023
001
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ollf
rank
Hir
ano
et a
l ,
nucl
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0701
075v
2
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 8
FLOW MEASUREMENTS IN ALICE
• TPC (-0.9 < η < 0.9) • ITS: SPD(-2 < η < 2 (layer 1))• FMD(-3.4 < η < -1.7, 1.7 < η < 5.0)• PMD (2.3 < η < 3.5)• ZDC (beam rapidity)• And everything else, once the
reaction plane is known • Measure in the range -3.4 < η < 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)
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 9
Designed for dN/dη ~ 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 < η < 0.9
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 10
RECONSTRUCTION ABILITY &TWO PARTICLE NON-FLOW CORRELATIONS
Poster by Naomi van der Kolk (P-127)
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 11
• 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
Layer 1 ׀η 2.0 > ׀Layer 2 ׀η 1.4> ׀
T. Virgili
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 12
Can reconstruct v2 to better than 10% accuracy.
η coverage
η < 5.03η < 3.68η < 2.29η < -1.70η < -2.01
FLOW USING THE FMD
C.Nygaard (Master’s Thesis)
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 13
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)
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 14
First order Event Plane Estimate
full event plane resolution (2 arms)sub-event plane resolution (1 arm)
The reconstructed resolution is same(Poster P-190, Nora de Marco)
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.
1st-order Event Plane Resolution v/s Centrality
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 15
FLOW USING THE PMD
• Determine the event plane: accurate resolution• For 5% flow and multiplicity 2400 in PMD (1.8 <η < 2.6)
» No Scattering Moderate Scattering – Expected Resolution 0.831 0.808 – Estimated Resolution 0.836 +/- 0.002 0.812 +/- 0.003
, coverage 2.3-3.5, 2Distance from IP 361.5 cm
Cell cross-section 0.22 cm2
Cell depth 0.5 cm
Total no. of cells 221184
Measure pT integrated flow for photons (low pT threshold, deduce for neutral pions?) W
A98
Col
labo
rati
on,
EP
J C
41 (
2005
) 28
7
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 16
ANISOTROPIC EMISSION OF J/Ψ IN FORWARD RAPIDITY USING EVENT
PLANE FROM PMD
•Away from midrapidity•Glauber absorption isotropic and comover absorption anisotropic
=>v2 (J/ψ ) ~ - 0.025 (Heiselberg and Mattielo PRC60 (1999) 44902) •J/ψ from dimuon channel
•measured in DiMuon Spectrometer( – 4.0 < η < -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
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 17
INVARIANT MASS SPECTRA OF LIKE SIGN AND UNLIKE SIGN MUONS
Nμ Signal
(x 103)
Bkg
(x 103)
S/B Significance
5.0 167.4 487.5 0.343 207
5.8 176.1 650.6 0.271 194
7.0 173.8 879.2 0.200 169
7.8 192.6 1494.1 0.129 149
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 18
BACKGROUND SPECTRA IN 12 AZIMUTHAL BINS
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 19
Wider mass region => lower S/B => lower flowv2 = v2
meas (1+S/B)/(S/B)
Flow in unlike sign pairs Signal in 12 bins
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 20
Significance depends upon large number of factors
Significance α √N evts
Significance ~ 0.7 / (v 2 * 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.
RESULTS DETERMINE“REQUIRED SIGNIIFICANCE”
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 21
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.
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 25
ALICE Zero Degree Calorimeter
ZP
ZN
ZP
ZN
ZN
The 2 neutron zero degree calorimeters ZNs have full acceptance for spectator neutrons (ZN > 8.7).
The 2x2 ZN segmentation gives a rough localization of the spectator neutron’s spot on the front face of the calorimeter. This localisation provides the first order event plane (spectator neutrons)
GEANT-based simulation
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 26
RECONSTRUCTION ABILITY
ESD
E.Simili
QUARK MATTER 2008, February 4-10, Jaipur, India Sudhir Raniwala 27
FLUCTUATIONS
Large multiplicity environment enables determination of fluctuations…..
2.
2statrealdyn
A
ALICE PPR
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