09/12/2009Paola Pagano Università di Salerno & INFN 1 Elliptic flow of meson in Pb-Pb Paola Pagano.

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09/12/2009 Paola Pagano Università di Salerno & 1 Elliptic flow of meson in Pb-Pb Paola Pagano

Transcript of 09/12/2009Paola Pagano Università di Salerno & INFN 1 Elliptic flow of meson in Pb-Pb Paola Pagano.

Page 1: 09/12/2009Paola Pagano Università di Salerno & INFN 1 Elliptic flow of meson in Pb-Pb Paola Pagano.

09/12/2009 Paola Pagano Università di Salerno & INFN

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Elliptic flow of meson in Pb-

Pb

Paola Pagano

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Elliptic flow

x

z

y

participants

spectators

spectators

The flow is a collective motion of the particles produced in the ultrarelativistic nuclear reaction. A non central collision produces an anisotropic flowin the transverse plane

The anisotropic transverse flow, i.e. elliptic flow,

springs from the initial azimuthal asymmetry of the interaction region

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Elliptic flow

b=0

It has origin from the anisotropic nature of the overlap region in the plane x-y with impact parameter b>0

The secondary interaction of particles converts the initial spatial anisotropy in an observed momentum anisotropy in the final state

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Elliptic Flow

....2cos2)cos(212)( 2221

0

2

vvN

d

dN

Elliptic flow coefficient

x

y

2

Poskanzer and Voloshin, Phys. Rev. C58, 1998

To measure the elliptic flow, we

need first to estimate the

reaction plane. Then the

coefficients of the Fourier

expansion of the azimuthal

distribution of particles with

respect to the reaction plane

are evaluated.

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Elliptic flow measurement: recent results at RHIC

2007

To describe the observed scaling, two different models have been

used:

•Quark recombination models

• Hadronic transport models.

In the context of both models, the

observed v2 values seem to

indicate that elliptic flow is built up prior to hadronization, in the

partonic phase.

A scaling of the elliptic flow parameter v2 with the number of quarks is observed

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meson’s elliptic flow measurement :

recent results at RHIC (I)

v2 of meson increases with the eccentricity of the central region of overlap (i.e. the reduction of the centrality)

2007

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meson’s elliptic flow measurement:

recent results at RHIC (II)

-mesons are produced

via coalescence of

seemingly thermalized

quarks

in central Au+Au

collisions.

This observation implies that

hot and dense matter with partonic collectivity

has been formed at RHIC

NQ-scaling fit

phi mesonNQ = 2.3 ± 0.4

2007

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How can we perform this measurement in ALICE?

Mass invariant method

1) Mass invariant histogram for phi meson

2) Estimation of Signal and Background: fit with Breit-Wigner + polynomial

3) Estimation of reaction plane angle for each event:

wi e i for the l’ith particle of a given event

)2cos(

)2sin(tan

2

1 12

ii

ii

w

w

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4) Estimate of the opening angle for all K+K-

pairs for each event

5) Histogram of v2(minv)= <cos2(KK-2)>

6) Fit of the last histogram with the function:v2(minv) = a(minv)v2S+[1-a(minv)]v2B(minv)

where v2(minv) = <cos2(KK-2)>

v2S is the v2 we are looking for

v2B is the v2 of combinatorial background

a(minv) = N(minv) / NK+ K_ (minv)

How can we perform this measurement in ALICE?

Mass invariant method (cont’d)

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Status of the analysis• The analysis tools are ready.• We are still producing a sufficient number of events Estimation: approximately 106

events.• Two Productions: - one official, managed by the collaboration, for the

production of the background, now I’m generating the signal - one private, containing only the generated particles

without recostruction-> ongoing on the Salerno computer cluster

• The time and space needed to perform this analysis, make essential the use of the world computing Grid facility.

• Here we show some preliminary results for the estimation of: - reaction plane angle - v2 for all particles

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Reaction Plane Angle estimation: checking the algorithm

Reconstructed Particles

Generated Particles

)2cos(

)2sin(tan

2

1 12

ii

ii

w

w

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v2 estimation: checking the generator

After the determination of reaction plane angle Ψ2 , it’s possible to build a global distribution dN/d(φ-Ψ2) using events with same multiplicity and centrality. This distribution can be interpolated with the function:

dN/dφ = cost.(1/2π) + 2 v2 cos2(φ-

Ψ2)v2: 0.02794 ± 0.0011

Generated Particles

φ-Ψ2 (degree)

v2: 0.03163 ± 0.0011

Reconstructed Particles

φ-Ψ2 (degree)

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The End

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Back up

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Metodo della matrice di sfericità trasversa (I)

• Diagonalizzazione della matrice di sfericità trasversa

2212

1211

sincossin

cossincos

SS

SS

iiiii

iiiii

02212

1211

SS

SS

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Metodo della matrice di sfericità trasversa (II)

2

4)( 212

222112211

2,1

SSSSS

Il rapporto è proporzionale a v2

Dagli autovettori si ricava l’angolo del piano di reazione 2 :

21

21

111

1221 2

S

SATan