# Presentation: 840847

##  STAR’s Measurement of Energy and System Size Dependence of K/ Fluctuations at RHIC

*Zubayer Ahammed for the STAR Collaboration*
*                 **VECC, Kolkata, India*

- Outline:

- Motivation
- Measures
- STAR measurement
- Comparison with SPS
- Comparison with  models
- Summary

- Quark Matter,2008

## Motivation

- QCD predicts QGP phase transition at sufficient high temperature/ energy density.
- The nature of this phase transition can be reflected in the increased/reduced fluctuations in the global observables.
- Kapusta and Mekjian [J.I. Kapusta and A. Mekjien, Phys. Rev. D33(1986)1304]
- suggested that  fluctuations in the Kaon to Pion ratio can reflect the nature of  phase transition.

- Quark Matter,2008

## 

Strangeness production is related to de-confinement.

Excitation function of  <K+>/<+> shows “horn” around  √sNN = 7 GeV.

The excitation function of <K->/<-> is monotonic. 

- Strangeness production is related to de-confinement.
- Excitation function of  <K+>/<+> shows “horn” around  √sNN = 7 GeV.
- The excitation function of <K->/<-> is monotonic.

- ___  __at SPS_

- C. Blume (NA49), hep-ph/0505137

- Quark Matter,2008

## K/ Fluctuations measurement

- NA49 measured the fluctuations of K/ as a function of incident energy for central Pb+Pb collisions from √sNN = 6 to 17 GeV using the observable σdyn.
- Measure the K/ ratio event-by-event.
- K = K+ + K-
-  = + + -
- Produce histogram of the K/ ratio.
- Extract the width of K/ histogram to get σreal
- Do the same for mixed event to get σmixed

- _K/____ Fluctuations measurement_

- Quark Matter,2008

## Define the dynamical fluctuations in terms of σdyn

-      Divide by the mean and multiply by 100
-         to get %

- _K/____ Fluctuations at SPS_

- C. Blume (NA49), hep-ph/0505137

- Quark Matter,2008

## Study Au+Au collisions at √sNN =  62,130 and 200 GeV.
Study Cu+Cu collisions at √sNN=62 and 200 GeV.
Extract the number of K+ + K- and + + - event-by-event using energy loss and curvature in the STAR TPC
Select kaons and pions with 0.2 < pt < 0.6 GeV/c and 
       |η| < 1.0  
kaons: Nσ,K < 2, Nσ, > 2
pions: Nσ, < 2, Nσ,K > 2
electrons: Nσ,e < 1

- Study Cu+Cu collisions at √sNN=62 and 200 GeV.
- Extract the number of K+ + K- and + + - event-by-event using energy loss and curvature in the STAR TPC
- Select kaons and pions with 0.2 < pt < 0.6 GeV/c and
-        |η| < 1.0
- kaons: Nσ,K < 2, Nσ, > 2
- pions: Nσ, < 2, Nσ,K > 2
- electrons: Nσ,e < 1

- _K/____ Fluctuations in STAR_

- Quark Matter,2008

## STAR’s K/Histograms

- K/for data and mixed events.

*STAR Preliminary*

*STAR Preliminary*

- Cu+Cu 62GeV

- Cu+Cu 200 GeV

- Data

- Mixed

- 0-10%

- Data

- Mixed

- 0-10%

- Quark Matter,2008

## Measured dyn

- Similar at 62 and 200   GeV  for Cu+Cu.
- Similar at 62,130 and 200 GeV for Au+Au.
- Similar for Cu+Cu and Au+Au  systems.

- Consistent with NA49.

- Quark Matter,2008

## Comparison with Statistical Hadronization Model

- Consistent with Torrieri with q (1.6)
-    fitted at higher energy.
- Inconsistent with Torrieri with q=1
-       (equilibrium scenario)

- Girgio Torreiri;Nucl-th/0702062(2007)

*Au+Au(0-5%)*
*Cu+Cu(0-10%)*
*NA49(0-6%)*

- Quark Matter,2008

## The use of σdyn is problematic at smaller multiplicities because it involves the event-by-event fluctuation of a ratio.
A better measure is dyn,K.

- A better measure is dyn,K.

- dyn introduced to study net charge fluctuations. (PRC **68**,  044905 [2003])
- dyn,K is insensitive to efficiency.
- dyn,K properly deals with small multiplicities.

- _Another Measure, ___dyn

- Quark Matter,2008

## a +  b/ (dN/d)

- Independent of energy  for Cu+Cu

- a +  b/ (dN/d)

- a +  b/ (dN/d)

- 1/N centrality dependence

- Independent of energy for Au+Au .
- 1/N centrality dependence.

- _Measured ___dyn

- STAR Preliminary

- STAR Preliminary

- Cu+Cu

- Au+Au

**sys. error for Au+Au~17.6%**

- error: stat.+sys

- error: stat. only

- Quark Matter,2008

## System size dependence

- Independent of system size.

- STAR Preliminary

- sys. error for Au+Au~17.6%

- Cu+Cu: stat.+sys.

- Au+Au: stat.

- Quark Matter,2008

## Scaled with dN/d

- Au+Au: stat.

- Cu+Cu: stat.+sys.

- Quark Matter,2008

## Comparison with SPS results

- NA49 errors: only stat. error
- shown here
- Consistent with NA49
- within errors.

- 2dyn = dyn

**sys. error for Au+Au~17.6%**

- Cu+Cu: stat.+sys.

- Au+Au: stat.

- Quark Matter,2008

## For Charges Separately

-  in data and Hijing .
- Higher in summed charge than same
- and opposite sign  in data and Hijing

- _For Charges Separately_

- (C)

*STAR Preliminary*
- Cu+Cu 62 GeV

- (A)

*STAR Preliminary*
- Cu+Cu 200 GeV

- (B)

- Error: stat.+sys.

- Error: stat.+sys.

- Quark Matter,2008

## Comparison with models

- AMPT, A Multi Phase
-    Transport Model

- Hijing + hadronic evolution(
-     multiple re-scattering)

- In agreement  with
- AMPT  predictions.

- sys. error for Au+Au~17.6%

*STAR Preliminary*

- Au+Au: stat.

- Cu+Cu: stat.+sys.

- Quark Matter,2008

## Comparison with Scaled dyn

- Cu+Cu: stat.+sys.

- Quark Matter,2008

## Summary

- We have measured K/ fluctuations on an event-by-event basis using the measured  dyn and dyn,K.
- The measured fluctuations is found to be independent of energy.
- Measured fluctuation is similar for Cu+Cu and Au+Au system.
- Measured fluctuation strength is consistent with SPS results.
- SH model (Torreiri) with fitted q is consistent at higher energy however the Equilibrium scenario (q=1) under-predicts at all energies.
- Hijing model over-predicts the measured K/ fluctuations.
-  AMPT results (Hijing + re-scattering) is in better agreement with data.

- Quark Matter,2008