# Presentation: 803491

## Recent Results from the BRAHMS Experiment at RHIC

**Fouad RAMI* for the BRAHMS Collaboration**
*** Institut de Recherches Subatomiques, Strasbourg**

** ****Introduction**
** ****The ****BRAHMS**** Experiment**
** ****Review of Main Physics Results**
**      ********Global ****Observables ****(****dN****ch****/d********)**
**      ********Baryon Stopping (anti-particle/particle)**
**      ********High p****t**** hadron spectra (Au+Au and d+Au)**
** ****Summary**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

##     Ultra-Relativistic Heavy Ion Collisions 

**Phase Diagram of Nuclear Matter**

**RHIC ******** Large program with 4 heavy-ion **
**          ****experiments (complementarity)**** **

**    ****Ultra-Relativistic Heavy Ion Collisions **

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

##   Relativistic Heavy Ion Collider @ BNL 

** ****Run 1: ****June - September 2000 **
**     ****First Physics Run**
**    ****Au+Au @ two energies**
**    ********S****NN**** = 56 and 130 GeV**

** ****Run 2: ****July 2001- January 2002 **
**    ****Au+Au @ ********S****NN**** = 200 GeV**
**     ****(maximal design energy)**
**    ****p+p (reference data)**

**PHOBOS**

**PHENIX**

**STAR**

**BRAHMS**

**  ****R****elativistic ****H****eavy ****I****on ****C****ollider @ BNL **

** ****RHIC accelerates all species from p to Au**

** ****Run 3: ****December 2002- May 2003 **
**     ****d+Au @ ********S****NN**** = 200 GeV**
**     ****p+p  @ ********S****NN **** = 200 GeV**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

##        I.G. Bearden7, D. Beavis1, C. Besliu10, Y. Blyakhman6,J. Bondorf7, J.Brzychczyk4, B. Budick6,  
H. Bøggild7, C. Chasman1, C. H.Christensen7, P. Christiansen7, J.Cibor4, R.Debbe1,  J. J. Gaardhøje7, 
M. Germinario7, K. Grotowski4, K. Hagel8, O. Hansen7, H. Heiselberg7, A. Holm7, A.K. Holme12, H. Ito11, E.Jacobsen7, A. Jipa10, J. I. Jordre10, F. Jundt2, C. E. Jørgensen7, T.Keutgen9, E. J. Kim5, T. Kozik3,
T.M.Larsen12, J. H. Lee1, Y. K.Lee5, G. Løvhøjden2, Z. Majka3, A. Makeev8, B. McBreen1, M. Murray8, J.Natowitz8, B.S.Nielsen7, K. Olchanski1, D. Ouerdane7, R. Planeta4, F. Rami2, D. Roehrich9,
 B. H. Samset12, S. J. Sanders11, D. Sandberg7, I. S. Sgura10, R.A.Sheetz1, Z.Sosin3, P. Staszel7,
T.S. Tveter12, F. Videbæk1, R. Wada8 and A. Wieloch3. 

1Brookhaven National Laboratory, USA                                                            2IReS and  Université Louis Pasteur, Strasbourg, France                                     3Jagiellonian University, Cracow, Poland                                      
      4Institute of Nuclear Physics, Cracow, Poland                                                      5Johns Hopkins University, Baltimore, USA                                                    6New York University, USA                                                            
   7Niels Bohr Institute, Blegdamsvej 17, University of Copenhagen, Denmark                                    
  8Texas A&M University, College Station. USA               
       9University of Bergen, Norway                                                      10University of Bucharest, Romania 
       11University of Kansas, Lawrence,USA                                                        12 University of Oslo Norway 

**H. Bøggild****7****, C. Chasman****1****, C. H.Christensen****7****, P. Christiansen****7****, J.Cibor****4****, R.Debbe****1****,  J. J. Gaardhøje****7****, **
**M. Germinario****7****, K. Grotowski****4****, K. Hagel****8****, O. Hansen****7****, H. Heiselberg****7****, A. Holm****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****, 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. 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****, S. J. Sanders****11****, D. Sandberg****7****, I. S. Sgura****10****, R.A.Sheetz****1,**** Z.Sosin****3****, P. Staszel****7****,**
**T.S. Tveter****12****, F. Videbæk****1, ****R. Wada****8**** and A. Wieloch****3.**** **
**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, Blegdamsvej 17, University of Copenhagen, Denmark                                    **
**  ****8****Texas A&M University, College Station. USA               **
**       ****9****University of Bergen, Norway                                                      ****10****University of Bucharest, Romania **
**       ****11****University of Kansas, Lawrence,USA                                                        ****12**** University of Oslo Norway**** **

**The BRAHMS Collaboration**

**~55 physicists from 12 institutions**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

## BROAD RANGE HADRON MAGNETIC SPECTROMETERS

**B****ROAD ****RA****NGE ****H****ADRON ****M****AGNETIC ****S****PECTROMETERS**

** ****Detailed measurements of momentum **
**    ****spectra and yields of charged**
**    ****hadrons as a function of y and p****t**

** ****Good Particle ID over wide**
**    ****range of y (0<|y|<4) and**
**    ****p****t**** (0.2<p****t****<4GeV/c) **

**NIM, A499 (2003) 437**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

**MRS**
******=0**

##     EVENT CHARACTERIZATION       COLLISION CENTRALITY

**Au+Au @ ********S****NN****=130GeV**

** ****Measured with Multiplicity**
**  ****Detectors (TMA and SiMA) **

**Central**** ******

****** ****Peripheral**

** ****Define Event Centrality Classes**
**    ******** ****Slices corresponding to different fractions of the cross section**

**Central**
**  ****b=0**

**Peripheral**
**  ****b large**

** ****For each Centrality Cut**
**   ******** ****Evaluate the corresponding number of participants ****N****part ****and**
**           ****number of inelastic NN collisions ****N****COLL ****(from Glauber Model)**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

##       dNch/d measurements at BRAHMS 

**0 - 5 %**

-  **5 -10%**

**10-20%**

**20-30%**

**30-40%**

**40-50%**

**TPM1**

**BBC**

**SiMA**

**TMA**

**Au+Au @ ********S****NN****=130GeV**

** ****Data from**** ******** ****detectors**
**     ******** ****Consistency**
** ****By combining all results**
**   ******** ****Cover wide ******** range **
**       ****-4.7 ******** ******** ******** 4.7**
**    “****Complete” distribution**
** ****By averaging**
**  ******** ****Reduce Systematic **
**     ****errors (high accuracy)**

** ******** ****= -ln (tan(********/2))**

**dN****ch****/d******

****

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

##      dNch/d distributions

**     ****dN****ch****/d******** distributions**

**BRAHMS, PLB523(2001)227**

** ********   ****0-5%**
******  ****30-40%**

**BRAHMS, PRL88(2002)202301**

**3860 ******** 300****  **

**4630 ******** 370****  **

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

******S****NN****=130GeV**

******S****NN****=200GeV**

**Very Large Charged Particle Multiplicities**

**N****ch****(-4.7****<********<****4.7)**** **

##   dNch/d at Mid-Rapidity      Energy Dependence

**  ****reproduce the measured multiplicities**

** ****Au+Au data much larger than pp**
**     ******** ****Not a simple superposition**
**  ****Medium effects ******** important role**
**                        ****in AA collisions  **

**  ****dN****ch****/d******** at Mid-Rapidity**** ****     ****Energy Dependence**

** ****Good agreement between **
**  ****BRAHMS**** and ****PHOBOS****  **

** ****Large increase from ****SPS**** to ****RHIC**
**   ****(almost a factor of 2)  **
**   ******** ****Higher energy densities**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

**Need to extend the comparisons**
**to other observables**** in order to**
**put more constraints on models**

##    BARYON STOPPING from p/p MEASUREMENTS

**  ****Energy of incoming nuclei so high that**
**  ****collisions among nucleons would not stop**
**  ****the nuclei ******** “pass through each other”****  **
**                    **

** ****How to measure the degree of **
**  ****Baryon Stopping (Transparency) ?  **

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

-    **BARYON STOPPING from ****p/p**** MEASUREMENTS**

** ̄**

## Anti-particle/particle Ratios in Au+Au @ SNN=200 GeV 

**Anti-particle/particle Ratios in Au+Au ****@**** ********S****NN****=200 GeV**

**At Mid-rapidity**
**     ********-****/********+**** = 1.01 ±0.04**
**     ****K****-****/K****+**** = 0.95 ±0.05**
**     ****pbar/p =  0.75 ±0.04**

-  **BRAHMS, PRL 90 (2003) 102301**

**20% centrality cut**

**High degree of Transparency but still a significant contribution **
**    ****from participant baryons over the entire rapidity range**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

**Moving away from the central region ******** the influence of the participant baryons becomes more and more important**

**   ********75% of measured protons are **
**     ****coming from pair production**

## p/p Ratio – Comparison to SPS data

** ****Analysis of the measured**
**   ****ratios within the Statistical**
**   ****Model (at Mid-Rapidity)**

** ̄****  **

**p/p Ratio – ****Comparison to SPS data**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

** ****Much higher ratios than**
**   ****at the ****SPS**

** ******** ********B ******** 25MeV at ********S****NN****=200GeV**** **
**         ****(240-270MeV at SPS)**

** ****A new Regime of low**
**   ****Net-Baryonic Densities**

##   High pt Suppression & Jet Quenching

** ****Particles with high p****t****’s (****above ~2GeV/c****) **
**  ****are primarly produced in hard scattering **
**  ****processes early in the collision**
**  ******** ****Probe of the dense and hot stage**

**Experimentally ******** depletion of the high p****t**** region in hadron spectra**

** ****In A-A, partons traverse the medium ****  **

** ****p+p experiments ******** Hard scattered **
**  ****partons fragment into ****jets of hadrons**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

** ****If QGP ******** partons will lose a large**
**               ****part of their energy**
**               ****(induced gluon radiation)**
**            ******** ****Suppression of jet production**
**                    ******** ****Jet Quenching**

**  ****High ****p****t**** Suppression & Jet Quenching**

##         Charged Hadron Spectra 

** ****Reference spectrum**
**    ******** ****p+pbar spectra (UA1)**** **

-         **Charged Hadron Spectra **

**BRAHMS, PRL91(2003)072305**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

** ****R****AA**** measured at ****SPS**** **

** ****Data do not show suppression **
** ****Enhancement (R****AA****>1)**
**   ****due to initial state multiple **
**    ****scattering (“Cronin effect”) **
**    ****Known in p+A collisions**

##    High pt Suppression in Au+Au

** ****Central Collisions **
**      ****R****AA ****< 1 ****at high p****t **

**   ****High p****t**** Suppression in Au+Au**

** ****Au+Au**** @ ********S****NN****=200 GeV**** **
**BRAHMS****, PRL91(2003)072305**

**At Mid-Rapidity (********=0****)**

** ****Peripheral Collisions **
**      ****R****AA**** ~ 1 ****at high p****t **

** ****Clear suppression effect**
**Consistent with Jet Quenching**

** ****No suppression (as expected)**

** ****Confirms first observations**
**   ****by ****PHENIX**** and ****STAR**** **

** ****BRAHMS**** can measure at**
**     ****more forward rapidities**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

**Central**

**Peripheral**

**Central/Peripheral**

##       High pt Suppression in Au+Au

-       **High p****t**** Suppression in Au+Au**

** ****Au+Au**** @ ********S****NN****=200 GeV**** **
**BRAHMS****, PRL91(2003)072305**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

**Central**

**Peripheral**

**Central/Peripheral**

##    High pt Suppression – Hydro-Jet Model Calculations

-    **High pt Suppression – Hydro-Jet Model Calculations**

** ****Use full 3-D hydro simulations**
**   ****to study the density effects**
**   ****on parton energy loss**

** ****Hydro ******** description of the soft**
**   ****Part of the produced matter**

** ****Hard part ******** use a pQDC model**
**  ****(PYTHIA) **

**Generation of momentum**
**spectrum jets**

****** ****Good agreement with BRAHMS**
**   ****data at both ********=0 and 2.2**
****** ****Similar effect at ********=0 and 2.2.**
**   ****Due to comparable time**
**   ****evolution of the parton **
**   ****density at ********=0 and 2.2**
**   ****in hydro.**

**Indirect evidence of the presence of hot thermalized**
**       ****matter in the region -2.2 < ******** < 2.2**** **

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

##       Comparison of Au+Au to d+Au

-       **Comparison of Au+Au to d+Au**

** ****Enhancement in d+Au**

**    ****Typical behaviour**
**     ****of ****Cronin effect **

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*

** ****Absence of suppression in d-Au**

** ********S****NN****=200 GeV**

** ******** ****Supports the Jet Quenching**
**     ****interpretation for central **
**     ****Au+Au collisions**
** ******** ****Excludes alternative**
**    ****interpretation in terms**
**    ****of initial state parton**
**    ****saturation effects**

**BRAHMS, PRL91(2003)072305**

##      SUMMARY

** ****Large hadron multiplicities**
**    ******** ****Almost a factor of 2 higher than at SPS (******** higher**** ********) **
**    ******** ****Much higher than in pp (******** medium effects)**

** ****Large Anti-proton/proton ratio**
**    ******** ****High degree of Transparency**
**    ******** ****Very low ********B**** ****(******** ****25 MeV**** at ********S****NN****=200 GeV****)**** **

**Three 1****st**** runs ******** Very interesting and new results**
**                    ****covering several physics subjects**** **

** ****Suppression of high p****t**** particles in central Au+Au collisions**
**   ******** ****observed at ********=0 and 2.2**
**   ******** ****Consistent with a Jet Quenching scenario**
**   ******** ****Supported by the absence of the suppression in d-Au data**
**   ******** ****But further more detailed studies are still required **
**      ******** ****Main focus of ****BRAHMS**** in the forthcoming runs**

*F.Rami, IReS Strasbourg                                                                                                                                                                                                        Kemer2003*