# Presentation: 816302

**The BRAHMS Experiment at RHIC**
**- an overview -**

- Claus Jørgensen, Niels Bohr Institute
- for the BRAHMS Collaboration

- XLII International Winter Meeting on Nuclear Physics
- Bormio, January 25 – February 1, 2004

## What I’ll talk about in the next 35 min. 

- The BRAHMS Experiment
-   - The Relativistic HI collider
-   - global detectors
-   - spectrometers & PID
- Charged Particle Multiplicity
-   - Au+Au and d+Au @ sNN=200GeV

- Particle Spectra from
- Au+Au @ sNN=200GeV
-   - Kinetic freeze out
-   - Chemical freeze out
-   - Nuclear Stopping

- Relativistic HI
- collision evolution

## The BRAHMS Collaboration

**H. Bøggild****7 ****, C. Chasman****1****, C. H. Christensen****7****, P. Christiansen****7****, J.Cibor****4****, R.Debbe****1****,**
**E. Enger****12****, J. J. Gaardhøje****7****, ****M. Germinario****7**** ****, ****K. Grotowski****4**** , K. Hagel****8****, O. Hansen****7****, **
**A.K. Holme****12****, H. Ito****11****, E. Jacobsen****7****, A. Jipa****10****, J. I. Jordre****10****, F. Jundt****2,**** C.E.Jørgensen****7****, **
**R. Karabowicz****4**** , T. Keutgen****9****, E. J. Kim****5****, T. Kozik****3****, T.M.Larsen****12****, J. H. Lee****1****, Y. K.Lee****5****, **
**G. Løvhøjden****2****, Z. Majka****3****, A. Makeev****8****, B. McBreen****1****, M. Mikkelsen****12****, M. Murray****8****, **
**J. Natowitz****8****, B.S.Nielsen****7****, K. Olchanski****1****, D. Ouerdane****7****, R.Planeta****4****, F. Rami****2****, **
**D. Roehrich****9****,  B. H. Samset****12****, D. Sandberg****7****, S. J. Sanders****11****, R.A.Sheetz****1****, Z.Sosin****3****, **
**P. Staszel****7****, T.S. Tveter****12****, F.Videbæk****1****, R. Wada****8****, A.Wieloch****3 ****and I. S. Zgura****10**** **
**1****Brookhaven National Laboratory, USA, ****2****IReS and  Université Louis Pasteur, Strasbourg, France**
**3****Jagiellonian University, Cracow, Poland, ****4****Institute of Nuclear Physics, Cracow, Poland**
**5****Johns Hopkins University, Baltimore, USA,**** 6****New York University, USA**
**7****Niels Bohr Institute,  University of Copenhagen, Denmark**
**8****Texas A&M University, College Station. USA, ****9****University of Bergen, Norway **
**10****University of Bucharest, Ro****mania,**** ****11****University of Kansas, Lawrence,USA   **
**12**** University of Oslo Norway **
**56 physicists, 12 institutions...**

## The Relativistic Heavy Ion Collider

- Au+Au

- d+Au

- p+p

- Top energy:
- sNN=200GeV

## The BRAHMS Experiment

- Midrapidity
- Spectrometer

- Front Forward
- Spectrometer

- Back Forward
- Spectrometer

- Global
- Detectors

- Zero Degree
- Calorimeter

## Global Detectors

- Beam-Beam counters:
- for vertex determination.

- ZDC counters: at 18
- meters. Measure
- spectator neutrons.

## Charged Particle Multiplicity

**0-5%**
**5-10%**
**10-20%**
**20-30%**
**30-40%**
**40-50%**

- 0-5% central Au+Au:
- Total charged particle
- multiplicity: 4630370
- (PRL 88, 202301(2002))

## Limiting Fragmentation

- Shift the dNch/d distribution by the beam rapidity, and scale by Npart. Lines up with lower energy  limiting fragmentation

- Au+Au sNN=200GeV (0-5% and 30-40%)
- Au+Au sNN=200GeV (0-5%)
- Pb+Pb sNN=17GeV (9.4%)

- d+Au sNN=200GeV and sNN=19.4GeV
- lines up on the d-side. Not on the Au side!
-          Au+Au sNN=200GeV

## The BRAHMS Spectrometers

## PID I – Time-of-flight

- TIME-OF-FLIGHT

- Particle Separation: pmax (2 cut)=

## PID II - Cherenkov

## The BRAHMS Acceptance

- give unique rapidity coverage :
**B**road RAnge Hadron
- Magnetic Spectrometers

- The BRAHMS Acceptance

- Transverse momentum [GeV/c]

- Rapidity

## Particle Yields I 

- we get the final invariant yields over a broad range of phase-space

- Particle Yields I

**Ph.D. thesis by**
**Peter Christiansen**
**Djamel Ouerdane**

## Particle Yields II

- Top 5% central collisions

## Rapidity Densities

- Integrated
- multiplicities
- (Gaussian fit)
- N()    ~ 1780
- N(+)    ~ 1760
- N(K+)    ~   290
- N(K)    ~   240
- N(pbar)~     85

- Total number of
- +K+p > 4000
- (consistent with
- dNch/d measurement)

## Transverse Dynamics

- ,K,p spectra
- described by
- blast wave model
- Parametrization including:
-  Freeze out temperature
-  Flow velocity
-  System geometry
-    -size and flow profile
- (Schnedermann et al. PRC48(1993))

- 

- K

- p

- BRAHMS preliminary

## Transverse Dynamics

- Flow versus y

- Temperature vs y

- T115 Mev, T0.7c at y=0
-  Flow velocity decreases with rapidity.
-    Lower density  lower pressure  less flow
-  Temperature increases.
-    Lower density  faster freeze out  higher temperature

- BRAHMS preliminary

- BRAHMS preliminary

## Ratios – Chemical freeze out

- Freeze out at QCD
- phase boundary.
- 	Different sources
-  	(B,T) at different
-  	rapidities?

- Hadron Gas Statistical model
- (grand canonical ensemble)
- reproduces (all) particle ratios
-  Tch, B

## Bjorken vs. Landau

- BRAHMS

- Entropy ~ Pion production
- Expansion ~ isentropic
- Pion gas = 3D relativistic fluid
- [L.D. Landau, Izv. Akad. Nauk SSSR 17 (1953)  52
-  P.Carruthers, M.Duong-van, PRD 8  (1973)  859]

## Stopping or Transparancy?

## Net Proton Rapidity Distribution

**Net-proton rapidity densities (top 5% central collisions)**

- BRAHMS,
- submitted to PRL, nucl-ex/0312023

## Nuclear Stopping I

- Total E=25.72.1TeV

- Net-baryon after
- feed-down & neutron corrections

## Nuclear Stopping II

**Bjorken: “There exist a leading-baryon effect. That is, the original baryons of a projectile is found in fragments of comparable momentum (more precisely of rapitity within ~2-3 units of the rapidity of the source).”   (**PRD 27, 140 (1983)**)**

## Energy Balance

-  Fit , K and p distributions (dN/dy and mT vs y)
-      total energy of , K and p
-  Assume reasonable distribution
-    for particles we don’t detect (0,n,...)
-  Calculate the total energy...

## Conclusions

## Backup I

- High pT suppression in central
- Au+Au collisions.
- High pT enhancement
- in d+Au!
- The
- smoking
- gun of
- QGP?

## Backup II

- Parametrization:

- “soft”  + “hard”
- (what do we learn from that?)

- dN/d = Npart+Ncoll

## Backup III

- Thermal (Tch) and chemical (ni) equilibration at chemical freeze out
- Tch and ‘s (+ system size), gives ni's from _grand canonical ensemble._ In praticle ratios the system size cancels out.
- Conservation laws: Baryon Number, Strangeness, Isospin

## Backup IV

- Blast wave:

- T and T are
- anti-correlated:

- versus system size

- versus energy