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Ramiro DebbePhysics Department

Brookhaven National Laboratory

RECENT RESULTS FROM BRAHMS

CIPANP 2003 New York May 2003

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Introduction

We report comprehensive results of our analysis of the 200 GeV Au-Au data.

This work concentrates on the 5% most central events.

This work is based on the PhD theses of two NBI students:

Djamel Ouerdane and Peter Christiansen

All results presented today are PRELIMINARY.

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Introduction cont.

BRAHMS is one of the five RHIC experiments. BRAHMS is designed to study the bulk properties of the collisions. We measure charged particle production in a wide rapidity range and transverse momenta extending up to ~5 GeV/c.

These measurements are done with two conventional spectrometers with small acceptance.

We put special emphasis on our Particle Identification capabilities with a combination of Time of Flight and Čerenkov detectors.

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BRAHMS Coverage

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Particle Identification

C1 π threshold

H1 & C1

⎟⎟⎠

⎞⎜⎜⎝

⎛−

β= 1

1pm

222

Mid-rapidity Spectrometer Forward Spectrometer

TOFW

RICH

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Mean Transverse momentum

K

P

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Net Charge as function of y

The net charge is fitted to :

A cosh(y/b)

D. Kharzeev PLB378(96)238

2=7.4/6

A = 8.5 0.3

b = 2.4 0.1

Total number of protons in y range: -3, 3

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K+p/pion

Y = 0

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Anti-proton/pion at y=0

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K+P/ at y=3

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Anti-proton/pion at y~3

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RHIC data is “far” from the “resonance gas” ; the contribution of rescattering to the shape of distributions is small. Transverse flow is the dominant feature

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Thermal source with transverse flow

A change to a Boltzman distrib. Integration over y and phi produce the following function:

Changing to a Boltzman distrib. And integrating over y and produces the following function to fit the measured spectra. (βT = tanh)

Distributions of particles are assimilated to the counting of particles traversing a hyper-surface beyond which the system decouples

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2/NDF = 6.7

βT = 0.53

T = 138. MeV

Transverse flow at y=0

Fits + - K+ P Pbar. Errors are statistical

• pions

• kaons

• proton

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2/NDF = 6.6

βT = 0.57

T = 115 MeV

Transverse flow at y~1

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2/NDF = 56/30

βT = 0.47

T = 135. MeV

Transverse flow at y~2

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Transverse flow at y~3

2/NDF = 1.2

βT = 0.42

T = 140 MeV

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Transverse flow at y=3

The presence of transverse flow in the system is most evident in the shape of the proton distribution.

Even though a fit to a thermal source with transverse flow does not fail, the comparison to the distribution at y=0 supports the assertion that transverse flow is much reduced at y=3.

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Summary

� BRAHMS measurements in a range of up to 2 units of rapidity around 0, are consistent with a thermalized system that exhibits strong transverse flow (β≥0.5).

� Hadronization at RHIC happens when the system is dilute. Secondary interaction (pion wind ) are not the main contributors to the shape of the distributions.

� Transverse flow is reduced at y~3.