# Presentation: 667093

## Highlights from the BRAHMS experiment at RHIC

- Dipali Pal
- University of Kansas
- BRAHMS Collaboration

- Global
- Detectors

- Front Forward
- Spectrometer

- Back Forward
- Spectrometer

## Particle Identification

- TIME-OF-FLIGHT

|  | 0<<1
(MRS) |  | 1.5<<4
(FS) |  |
| --- | --- | --- | --- | --- |
| pmax
(2 cut) | TOFW (GeV/c) | TOFW2 (GeV/c) | TOF1 (GeV/c) | TOF2 (GeV/c) |
| K/ | 2.0 | 2.5 | 3.0 | 4.5 |
| K/p | 3.5 | 4.0 | 5.5 | 7.5 |

- Ring radius vs momentum gives PID
-  / K separation 25 GeV/c
- Proton ID up to  35 GeV/c

- (2 settings)

**RICH**

## BRAHMS Acceptance 

## Reaction Systems Studied

| Reaction System | Energy
(GeV) |  |
| --- | --- | --- |
| 197Au+197Au | 200 | Create hot dense system at highest energy at RHIC with formation Quark Gluon Plasma |
| 197Au+197Au | 63, 130 | Energy scan |
| 63Cu + 63Cu | 200, 63 | Mass, energy scan |
| d +197Au
 | 200 | Control experiment to investigate initial/final state and cold nuclear medium effects. |
| p + p | 63, 200 | Baseline measurement. |

## Results

- Multiplicity distributions
-      - Energy density and stopping
-       - Anti-particle to particle ratios: Chemical freeze out
- Nuclear modification factors, RAA and RdA
-       - Particle production mechanisms
- Flow
-       - Collision geometry and particle production
- Vector meson production:   KK
-      - Medium effects and strangeness production
- Summary

## Particle production

- _Energy density:_ Bjorken 1983
**e****BJ**** = 3/2 ********(****<****E****t****>****/ ********R****2********0****) d****N****ch****/d******** **
-  assuming formation time **t****0****=**1fm/c:
-  >5.0 GeV/fm3 for AuAu @ 200 GeV
-  >4.4 GeV/fm3 for AuAu @ 130 GeV
-  >3.7 GeV/fm3 for AuAu @ 62.4 GeV

- At all RHIC energies, initial energy density exceeds critical energy density for
- phase transition from hadronic matter to QGP.

- BRAHMS

## Particle production and energy loss

- Total E=25.72.1TeV
- 73GeV per nucleon

- @ 200 GeV

- ~73% of beam energy is deposited and available for particle production.

## Nuclear stopping @ 62 GeV Au+Au

- Rapidity loss

- < y > = 2.14 +/- 0.22

- Gaussian fitting function:

- dN/dy vs y

- BRAHMS Preliiminary

## Stopping systematics

*f(y**CM**) = A + B (1 – exp(y**CM**))*

-  All points are fitted with

-   Linear fit to SPS and RHIC data

-  Linear fit to AGS and SPS data

-  Rapid increase in rapidity shifts from AGS to SPS
-  Slow increase from SPS to RHIC
-  Expected rapidity loss at LHC:  2.1 < y < 3.4

- (Preliminary)

## Particle Spectra in Au+Au @ 200 GeV : Top 5% centrality

- Particle Spectra in Au+Au @ 200 GeV : Top 5% centrality

## Rapidity Densities

- N()    ~ 1780   N(+)  ~1760
- N(K+)    ~   290   N(K) ~  240
- N(pbar) ~     85

- At y ~ 0, dN/dy is
-        ~ 300 (300) for + (-)
-        ~   47   (44) for K+ (K)
-        ~   27   (20) for p (pbar)

- N() >> N(K) > N(p)
- N(+) = N()

- N(K+) > N(K)  and
- N(p) > N(pbar)  systematically

- All distributions can be fitted with a
- Gaussian except protons
-  Fragmentation component

## Particle spectra in Au+Au @ 62.4 GeV 0 – 10% centrality

- Particle spectra in Au+Au @ 62.4 GeV 0 – 10% centrality

## Anti-particle/particle ratio vs y

| Mid-rapidity ratios |  |  |
| --- | --- | --- |
|  | 200 GeV | 63 GeV |
| -/+ | 1.0 | 1.0 |
| K-/K+ | 0.95 | 0.84 |
| pbar/p | 0.75 | 0.45 |

- Approximate balance between particles and anti-particles at y~0 for 200 GeV
- Pair creation mechanism
- Kaon and proton ratios are lower at all y for 63 GeV
- K and p ratios are falling at y>1
-  Influence of net-baryon rich medium at forward rapidities

## Anti particle/particle ratios (II)

- Statistical model with chemical and thermal equilibrium

## Anti particle/particle ratios (III)

- Good statistical model description with B= B(y) with  T~170MeV
- Also describes energy dependency at y=0
- only B controls the state of matter

## Summary on Multiplicity distribution and particle ratios

- Energy density at RHIC exceeds critical energy density for QGP formation.
- ~73% of beam energy is deposited and available for particle production.
-  Rapid increase in rapidity loss from AGS to SPS, then slow increase from SPS to RHIC.
- Anti-particle to particle ratios are consistent with chemical freeze out

## Nuclear modification factorRun4 Au + Au @ 200 GeV: Charged particle spectra

- Nuclear modification factorRun4 Au + Au @ 200 GeV: Charged particle spectra

- 10%

- 40-60%

- p+p

- No significant change in RAA with rapidity

## Nuclear modification factorIdentified particles: Au+Au @ 200 GeV

- Nuclear modification factorIdentified particles: Au+Au @ 200 GeV

-    RAA for mesons show suppression at all y
-    Protons/anti-protons show enhancement at intermediate pTfor all y.
-    RCP shows different behaviors of baryons and mesons

- BRAHMS Preliminary

## Au+Au Centrality Comparison at 62 and 200 GeV

**62 GeV Au + Au**

**200 GeV Au + Au**

- Brahms preliminary

- Less suppression (more enhancement) at 62 GeV than at 200 GeV.

## Comparison of Au+Au and Cu+Cu @ 62 GeV

- Suppression increases with increasing colliding ion mass

**62 GeV Au + Au**

**62 GeV Cu + Cu**

## Nuclear modification factor d+Au at all rapidities

- I. Arsene *et al.,* BRAHMS PRL 93 (2004) 242303.

- Unidentified hadrons
- Systematic decrease of RdA with increasing 

## Summary on Nuclear Modification Factors

- Charged hadron RAA’s do not change with .
- Mesons are suppressed and baryons are enhanced
- Suppression increases with collision energy
- Suppression increases with higher colliding  ion mass
- No suppression in RdA
-    - Cold nuclear matter
-    - Cronin enhancement

## v2 vs pT (10 – 50%): BRAHMS Preliminary 

- v2 vs pT (10 – 50%): BRAHMS Preliminary

- ----   STAR 0 – 50% hadrons: PRC 72, 014904 (2005)

- Very little change in v2 with  for all three particles

##   KK reconstruction: Invariant mass spectrum 

-   KK reconstruction: Invariant mass spectrum

- M2 = (p1 + p2)2
-      = p12 + p22 + 2p1p2
-      = m12 + m22 + 2(E1E2 + p1**.**p2)
-   Measured (same event)
-   Combinatorial Bkg
-       - Mixed event
-       - Normalization
-   Subtracted = Measured – CB
- Fitted with Relativistic Breit Wigner + Linear function from acceptance effects
- Centroid and width are consistent with PDG values:
-  M= 1.019456 GeV/c2 ,  = 4.26 MeV/c2

- N = 120 +/- 35
- Extracted by integrating  the signal within 1.013 < M < 1.031 GeV/c2

##   KK mT spectrum: dN/dy and T at y~1

-   KK mT spectrum: dN/dy and T at y~1

**   **

- dN/dy = 2.09 +/- 1.00 +/- 0.25
- T = 354 +/- 109 +/- 35 MeV

- Exponential fitting function:

- BRAHMS

##  meson yields at different energies

- Yield increases from AGS to RHIC
- Different trends between AGS and RHIC
- No significant difference between y~0 STAR and y~1 BRAHMS results.

## Summary on v2 and  meson measurements @ 200 GeV Au + Au collisions

- Flow
-     - No significant change in v2 vs pT with increasing rapidities
- First results on KK at y~1
-     - No change in line shape (centroid and width) from PDG
-      - Yield per participant pair shows different trends in AGS and RHIC
-      - Increase in yield per participant from AGS to RHIC
-      - Yield is consistent with STAR

## BRAHMS Collaboration @ RHIC

## Backups

## The BRAHMS Acceptance

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

- Transverse momentum [GeV/c]

- Rapidity

##   KK invariant mass spectrum from MC 

- Effect of acceptance on the spectral shape

## Ratios of N to yields within M < 1.013 GeV 

- Data and MC behave similarly.

## PID

- Fig shows positively charged
- Particles
- Similar result for negatively
- charged particles
- /K separation within
- 0.4 < p (GeV/c) < 2