Lambda Polarization Analysis

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Lambda Polarization Analysis
Prabhat R. Pujahari IIT Bombay
VECC Kolkota 24th to 26th April 06
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Outline
1.Introduction 2.Lambda polarization 3.Depolariz
ation effect in Au-Au 4.Analysis
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Introduction
One of the first proposed signatures to the
production of a QGP In relativistic
Nucleus-Nucleus collisions was to study the
change in the polarization properties of ?0
hyperons as compared to that observed in P-P
collisions The polarization effect was first
observed in 1976 at FNAL in 300 Gev/c proton on
Beryllium collision. We study the polarization
effect of lambda hyperon produced in Au-Au
collision by comparing the polarization in P-P
collision. Based on a recombination model of
Lambda production mechanism in Au-Au collision,
a vanishing polarization of the Lambda has been
proposed as a possible signature of QGP In
the case of relativistic nucleus-nucleus
collisions, the expectation is that, ?0 coming
from the zone where the critical density for
QGP Formation has been achieved. These plasma
created ?0 should show zero polarization.
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Lambda Polarization
Transverse ?0 polarization can occur when the
spin of the produced ?0 has its preferred
direction perpendicular to the production plane.


The polarization can be determined by a study
of the angular distribution of the decay product
of the lambda .
The production plane is defined by the vector n
n (pbeam X pLambda)/ pbeam X
pLambda
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Lambda Polarization
The Transverse Lambda polarization is known to be
dependent on PT and XF Where, XF
is a scaling variable introduced by Feynman in
the inclusive hadron interaction at large
energies and it is expressed as
XF PL/PL(max)
PL is
longitudinal momentum.
PL(max) is maximum allowed PL which is
Sqrt(s)/2.
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Dependence of the polarization on PT (left) and
XF for PT gt 1 GeV/c (right). The predictions
drawn are from the Thomas Precession model.
With respect to the PT -dependence the
predictions are given for the four XF values
corresponding with the data. For the XF
dependence the model is calculated at PT 1
GeV/c.
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Analysis
The main properties of the P-P measurements are
as follows a) Lambdas are produced with a
negative transverse polarization with respect to
the production plane. b) For the
kinematic range PT lt 1.5 Gev/c the polarization
increases linearly with PT and XF. c)
For the kinematic range PT gt 1.5 Gev/c the
polarization saturates as a function of PT
, but it is still linearly increasing as a
function of XF.
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k XF vs PT distribution for the analyzed lambda
sample obtained with the STAR-TPC
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The Berlin model prediction for the transverse
polarization as a function of XF for PT gt 1 GeV/c
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STAR-FTPC
2.5 lt ? lt 4.0
XF vs PT distribution for the analyzed lambda
sample obtained with the STAR-FTPC
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Thank You
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Depolarization in Nucleus-Nucleus collision
The production plane is completely undefined
and hence polarization vanishes . The
production plane is defined by the vector n
n (pbeam X pLambda)/ pbeam X pLambda