# Presentation: 821974

## Title

**experiment at RHIC**
**Zbigniew Majka***** (****for the**** ******BRAHMS****** ****Collaboration****)***
*Department of Hot Matter Physics, M. Smoluchowski Institute of Physics*
*Jagiellonian University,  Kraków, Poland*

## Plan

***What was built and why?***
***What has been accomplished?***
***What is still to come?***

## BRAHMS Experimental setup

***BRAHMS Experimental setup***

***Measurements:**
**p, K, ******** identified over wide range of rapidity****:**
**                                 **** ****0 < |y| < 4 **
**                           **** *****0.2 < pt < ~ 3GeV/c ***
***         ******central and fragmentation regi******on vs centrality***
***                                ******(******with high precision******)******.***

**Notes:**
BRAHMS

## * Results address:

**                                    ****Baryon ****s****topping.**
**                      ****Reaction ****m****echanism and dynamics**
**                              ****Chemical equilibrium**
**                                   ****Thermalization**
**                            ****Baryo-chemical potential**
**                                  ****Particle production**
**                            ****Particle to antiparticle ratios**
**                                   ****Mini-jet production**

**Notes:**
Merge these two pages into one crisp page!! Has been shown to many times and not too relevant for result presentation

## Tracking and PID

- Full Forward Spectrometer (2.3o <  < 15o )
- _High-momentum mode_
- sweeping D1,D2
- tracking and momentum determination by T2-T5, D3,D4
- PID: RICH (/K/p) separation < 25 GeV/c
- Tof-H2 ( /K < 5, K/p < 8.5 GeV/c with 4 cut)
- _Low-momentum mode_
- tracking and momentum determination by T1-T2, D2
- PID: C1 (/K) separation < 9 GeV/c
- Tof-H1 ( /K < 3.3, K/p < 5.7 GeV/c with 4 cut
- Front Forward Spectrometer (15o <  < 30o )
- Same as Low-momentum Mode
- Momentum resolution ******(dp/p) **~ 1%
***Mid-Rapidity Spectrometer***
- Tracking and Momentum determination,
-   MTP1,MTP2 and D5.
- PID TOFW ( /K < 2.2, K/p < 3.7 GeV/c with 4 cut).

**Tracking and PID**

## BRAHMS acceptance – run # 2

## Centrality determination – vertex distribution

**    **

-    **SiMA   **** ****(2.0< |******** |<2.0)**
** ****  ****PTMA**** **** (2.2< |********| <2.2****)**
**   ****B****BC ****(3 < |******** | < 4)**

***Centrality determination – vertex distribution***

**Beam–Beam **

## MRS Hadron identification - TOF

**m****2****=p****2****( t****2 ****/ L****2 ****-1)**

## FFS Hadron identification 

## FFS Hadron identification - C1

**Cos******** ****=**** ****1/n********  ****=>**** ********th****=1/n**

## BFS Hadron identification - RICH

## sNN = 130 GeV

******s****NN ****= 130 GeV**

***BRAHMS:****** PRL 87****** ******(2001)****** ******112305***

***BRAHMS:****** P******L B523 (2001) 227***

***Rapidity dependence of antiparticle-to-particle ratio***
***Pseudorapidity densities of charged particles***
***Comparison with models***

## sNN = 200 GeV       Pseudorapidity densities of charged particles 

***BRAHMS:****** PRL 8******8 ******(200******2******)****** 202******30******1***

## N_part

- =4.5

- =3.0

- =0

**p-p**

## Pseudorapidity densities - comparison

**** ******4630 charged particles produced for 0-5% central***
**** ******14% increase over 130GeV***
**** ******50% increase over p +************p (UA5) ***
***=> significant medium effects***

**Bjorken energy density****:             ********BJ**** = (1/********R****2********0****) ****<p ****t ****>dN ****ch****/d******
** ****(where************ ****0**** =1 fm/c, <p ****t ****>  =  0.5 GeV/c)*****            *********** 5 GeV/fm****3**
-                                                  (QGPcritical  1 GeV/fm3)

## Saturation of  excitation in fragmentation region

***Data s******hift******ed****** to ******the ******beam frame of reference***

***BRAHMS:****** PRL 8******8 ******(200******2******)****** 202******30******1***

## Antiparticle-to-particle ratio: - vs rapidity

***Antiparticle-to-particle ratio:****** ***- ***vs rapidity***

**130 GeV**

***- centrality and p******t ******dependence***

## Correlation –strange meson and baryon antiparticle-to-particle ratios

***Correlation –******strange meson and baryon ******antiparticle-to-particle ratios***

**BRAHMS****:**** submitted to PRL****, ****nucl-****ex/0207006**

## Summary from BRAHMS:

**The h****ighest particle multiplicity ****observed ****in nuclear collisions**
**                                      ****( ****N****ch ******** 4600****)**** **
**Large(st) energy density**
**                                      ****(******** ******** 5 ****•**** ********crit**** ****)**
**High reaction transparency**
** ****                                     ****(p-/p+**** ******** ****0.75**** at y=0****)**
** **
**Decoupling ****between**** central and ’fragmentation’ regions**
**Onset of boost invariant plateau**
**Preliminary:**
**Partonic (color) description appears necessary**
**Central region dominated by matter-antimatter balance: **
**                                     ****Qualitatively new physics regime**
**Tranverse flow at y=0 not significantly different from SPS?**
**Moderate transverse source sizes **