Strange%20Particle%20Correlation%20Studies%20with%20the%20STAR%20detector

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Strange Particle Correlation Studies with the
STAR detector Thomas J. Humanic and Helen Caines,
Ohio State University, OH for the STAR
Strangeness and HBT Physics Working Group and the
STAR Collaboration
K0s-K0s
L-L
Theory
Theory
ABSTRACT We present preliminary analysis of the
K0s-K0s , L-Proton and L-L correlations as
measured by the STAR detector at RHIC. We also
discuss some of the unique physics questions and
experimental challenges that are probed by these
more unconventional correlation studies.

• Advantages
• No coulomb repulsion problems
• Less 2 track resolution problems
• Few distortions from resonance decays
• Could use K-K0s mixing as background to remove
  even these resonances
• K0s is not a strangeness eigenstate - unique
  interference term that may provide additional
  space-time information

The shape of the L-L correlation is harder to
predict for a given freeze-out radius as the
scattering length (a) and effective range (Reff)
are not well known. The plot opposite shows
predictions for a 6 fm source assuming different
values of a and Reff
S. Pratt
L-p
Shown here is a set of simulations showing how
the STAR resolution will affect the K0sK0s
measurement.
Theory
Another possible interesting feature of the L-L
correlation is the possibility of detecting the
presence of a L-L resonance. Shown opposite is
are predictions for the correlation assuming
differing radii and widths of the assumed
resonance.
L-p correlations may be more sensitive to large
source sizes than p-p correlations as the L-p
system does not suffer from coulomb repulsion
affects which mask large sources in charged
particle correlations.
Previous measurements
If, however, the resonance is quasi-bound (the
H-di-baryon) the observed affect becomes harder
to distinguish and it is unlikely that the H0
will be discovered this way (but you never know!)
The line is not a fit to the data but an
indication of the shape of the correlation
function for a 6 fm source.
Shown are p-p and L-p correlations for source
sizes 4 (? ), 6 (), 10 ( ? ) fm. Note here k is
used. k pL pp /2 as opposed to q pL
pp
Previous measurements
The line is not a fit to the data but an
indication of the shape of the correlation
function for a 2 fm source, assuming Fermi-Dirac
statistics only
For unlike particle correlations it is important
to form the q (k) in terms of the relative
momentum measured in the center-of-mass frame of
the two particles.
STAR results
We currently reconstruct 1.3 K0s/event. Although
the mass, and hence momentum resolution is not as
good as for the L it is still perfectly
reasonable for correlation studies. As shown
above.
One may also extract information about the time
difference of the freeze-out of different
particle species. By conserving the sign of the
relative momentum vector one may be able to
determine which particle was emitted first. If
strange - non-strange baryon correlations can get
sufficiently detailed (the few percent level) we
may be able to resolve the question of whether
strange particles are emitted simultaneously with
non-strange baryons or not.
STAR results
As can be seen in the plot below we are
statistics limited and hence no firm comment can
currently be made about the strength of the L-L
correlation. Clearly, however, we have the
coverage and an excellent momentum resolution for
the L.
(See STAR poster by Adam Kisiel on K-p
correlations for more details)
c2/ndf 25.21/24 l 0.7 0.5 R 6.5 2.3
STAR results
While the L-p correlation seems ideal due to the
lack of merging etc. there is a problem in
obtaining low-q pairs. The mean pt for L ?1 GeV/c
while the high pt cut off for the proton is also
? 1 GeV/c, due to the need to identify the proton
via dE/dx. Hence obtaining low q is not as simple
as one might assume.
We currently reconstruct 0.4 L/event. Therefore
it is predicted that 10 Million events are
needed to produce a reasonable correlation with
the present reconstruction efficiencies
With the inclusion of the SVT in the next run,
and the higher energy, it is expect that the
yield will increase and hence the required number
of events will decrease
A radius of 6.6 fm is obtained from a fit to the
data. This is large compared to that obtained for
charged kaons ( 4fm). However again we feel that
the fit is statistics limited, indicated by the
large error on the L parameter.
STAR Preliminary
More statistics from the next run will allow to
make a firm statement about the K0s radius.