ANISOTROPIC FLOW MEASUREMENTS IN ALICE Sudhir Raniwala for the ALICE collaboration Department of Physics University of Rajasthan Jaipur

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……


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.


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

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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?


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

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dN

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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.


Hirano, to check reference….

EXPECTED v2 at LHC N

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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)


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


RECONSTRUCTION ABILITY &TWO PARTICLE NON-FLOW CORRELATIONS

Poster by Naomi van der Kolk (P-127)


• 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


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)


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)


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


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

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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


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


BACKGROUND SPECTRA IN 12 AZIMUTHAL BINS


Wider mass region => lower S/B => lower flowv2 = v2

meas (1+S/B)/(S/B)

Flow in unlike sign pairs Signal in 12 bins


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”


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.


BACK UP SLIDES


CONCLUSIONS


E.Simili


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


RECONSTRUCTION ABILITY

ESD

E.Simili


FLUCTUATIONS

Large multiplicity environment enables determination of fluctuations…..

2.

2statrealdyn

A

ALICE PPR



Documents

ANISOTROPIC FLOW MEASUREMENTS IN ALICE Sudhir Raniwala for the ALICE collaboration Department of Physics University of Rajasthan Jaipur