# Presentation: 016837

## First STAR results on π0 production over an extended pT range from 200 GeV Au+Au collisions

- Guoji Lin
*for the STAR Collaboration*
- Yale University

- Jaipur, India, February 4 – 10, 2008

## Outline

- Motivation
- STAR detectors & analysis method
- π0 spectra result
- Systematic errors
- Spectra
- High pT suppression
- Future analysis prospect
- Summary

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## Motivation

- Measuring π0 spectra is fundamental:
-  Compare π0 spectra with charged π.
-  Additional evidence for the suppression of high pT particles and jet quenching.
-  Preparation for the future direct photon analysis in heavy ion collisions.
-  Triggered data can be used to reach high pT – test STAR EMC capability.

- STAR gamma-jet correlation: A. M. Hamed’s talk in Session XV

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## STAR Detectors

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## Analysis Methods 

- π0 is reconstructed using two methods:
- 1. Pairing two photons measured by the BEMC.
- 2. Pairing one conversion photon with the other one from the BEMC.
- Invariant mass distribution after background subtraction is fitted to extract the yield.
- Raw yield is corrected by efficiency calculated from π0 embedding data.

- Data sets: year 4 Au+Au 200 GeV collisions.
- 11 M minbias events; 17 M central triggered events.
- 2.4 M high tower triggered events (ET > 3 GeV/sin(θ)).

- Advantage of using EMC-TPC photon combination:
-  Clean sample of conversion photon + large sample of EMC photons.
-  Gives clear π0 signal from below 1 GeV/c to intermediate pT and in most central events.

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## π0 Signals

- π0 Signals

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## Geometry Correction

- Geometry description is crucial for the photon conversion probability calculation.

-  The comparison between data and simulation indicates that materials are underestimated in SVT and SSD area.
-  Final conversion probability is corrected by normalizing the number of conversions in TPC gas.

- * This issue has been known by collaborators for a while. So far it only affects this analysis.

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## Systematic Uncertainty

| π0 spectra | MB Emc-Tpc | HT Emc-Tpc | HT Emc-Emc |
| --- | --- | --- | --- |
| photon cuts | 20% | 10-20% | 10-20% |
| Energy scale (+/- 4%) | 20% | 20-30% | 20-30% |
| Yield extraction | 10% | 5% | 10% |
| Geometry effect | 10% | 10% | -- |
|  |  |  |  |
| Acceptance (correction) | 12% | Small | Small |
| Efficiency (stat.) | 50% pT<1 GeV/c | 10% | 10% |

- Dominant uncertainties on π0 spectrum:
-  Photon cuts
-  EMC energy scaling
-  Low pT efficiency

- * Systematic errors are bin by bin. Shown here are typical values.

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## π0 Spectra (I)

-  STAR π0 spectra are measured with different methods over a large pT range.
-  π0 spectra from conversion photon method and from EMC photon pairs are consistent.
-  π0 spectra follows the same trend of the charged π spectra.

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## π0 Spectra (II)

-  STAR π0 spectra is compared with PHENIX results.
-  Within error bars results from two experiments are consistent.

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## Nuclear Modification Factor -- Rcp

-  π0 shows similar suppression at high pT as charged π.
-  Additional evidence for jet quenching.

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## Nuclear Modification Factor -- RAA

- STAR pp π0: AIP Conf. Proc. 915:343-346, 2007
- STAR dAu π0: Eur. Phys. J. C43, 311 (2005)

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## ρ0/π0 Ratio

- 5 < pT < 10 GeV/c
- p+p:
- ρ0/π0 = 1.68±0.27
- Au+Au:
- ρ0/π0 = 1.79±0.30

- Statistical errors only.

- STAR ρ0 results: P. Fachini’s talk in Session V

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## Prospect in Run7

- Run7 Au+Au 200 GeV data:
-  Higher tower trigger threshold with ET>5.5 GeV.
-  Full BEMC coverage.
-  605.7 (ub)-1 rare triggered events and 75.8 M mb-vpd events.
- For π0 analysis we can reach 12 GeV/c for EMC-TPC method and 15 GeV/c for EMC-EMC method.

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## Photon Converter

- We have proposed to install a photon converter of about 0.1 radiation length at r~45 cm inside the inner field cage. Test run in run 9 working with DAQ 1000.
-  Simple, well defined geometry.
-  Higher photon resolution with well known conversion point.
-  Benefit a lot of interesting analysis: π0, η, direct photon HBT...

- QM2008 Session I - High pT physics

- Feb. 5, 2008

## Summary

- First STAR π0 spectra over an extended pT range in 200 GeV Au+Au collisions are measured.
- Results from different methods and different data samples are consistent.
- The π0 spectra and suppression at high pT are consistent with charged π and π0 result from PHENIX.
- It serves as reference for the background study in direct photon analysis in heavy ion collisions.
- Future analysis is promising with large run7 data sample and possible converter configuration.

- QM2008 Session I - High pT physics

- Feb. 5, 2008