# Presentation: 886928

##  BRAHMS role and results in the RHIC program 

- F. Videbœk
- Physics Department
- Brookhaven National Laboratory

## Overview

- A brief history
- Particle Production in HI
- Multiplicities
- Identified particle in rapidity
-  Limiting Fragmentation, thermal aspects
- High-pt suppression
- Energy , rapidity dependence
- d-Au and the CGC
- RdAu for charged and identified hadrons
-  pp
- Baseline reference and relevance to pQCD
- Transverse Spin
- Final Words

- Kirkfest, BNL

- March27, 2006

## BRAHMS proposed as LOI3 –in 1990
Approved, but not funded until 1997
Construction completed in 2001
Took First data in 2000

- Approved, but not funded until 1997
- Construction completed in 2001
- Took First data in 2000

**HI data at 200, 130 and 62.4 GeV**
**Au.Au, CuCu**
**Important Reference data from**
** ****d.Au and pp (200 GeV)**
** ****pp spin data**
**And to  come 62.4 GeV pp**

- Kirkfest, BNL

- March27, 2006

## Slide 4

- Kirkfest, BNL

- March27, 2006

## Mapping Space-time Evolution

**Formation of Hot Matter, QGP**
**  ****-**Identifying and Characterizing the Hot Matter
**  **- How does the system extend/develop? Transverse and longitudinal dynamics
-    - Strong constraints for theoretical modeling/interpretation
-  Initial Conditions/Partonic Dynamics: High-pT vs. y
-  Collective Hydro-dynamics: Flow (radial and elliptic) vs. y
-  Thermodynamic and freeze-out properties: Temperatures,  Particle composition vs y
-  Baryon Transport: Net-baryon vs y
-  Bulk Properties: multiplicity, dN/dy

- Kirkfest, BNL

- March27, 2006

## Charged Particle Multiplicity

- _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

- Kirkfest, BNL

- March27, 2006

**Notes:**
Even at high rapidities the energy densities are high

## Limiting Fragmentation

- Shift the dNch/d distribution by the beam rapidity, and scale by Npart. Lines up with lower energy  limiting fragmentation. Collision view in rest frame of projectile nucleus.

- Au+Au sNN=200GeV (0-5% and 30-40%)
- Au+Au sNN=130GeV (0-5%)
- Pb+Pb sNN=17GeV (9.4%)

-  **PRL 88 (2002)**

- “Crucial Observation” for universal QCD

- Kirkfest, BNL

- March27, 2006

**Notes:**
This longitudinal scaling has been the subject of many papers by Phobos.

## Kaons are convenient to test mT dependence

**Kaons are convenient to test m****T ****dependence**

- Longitudinal Scaling in all available rapidity ranges
- <pt> driving Hydro

- Longitudinal scaling of inverse slopes.

- Kirkfest, BNL

- March27, 2006

## Baryon Transport: How much energy available from the collision?

**Baryon Transport: How much energy available from the collision?**

- AGS->RHIC : Stopping -> Transparency
- Rapidity Loss <dy>: 2±0.4: not linearly increase with ybeam
- Energy loss <dE> per nucleon: 73±6 GeV
- Available energy for excitation: ~3/4 of total energy
- The pp rapidity loss is consistent with <dy> ~ 1

**Au+Au 200 GeV 0-5% Central**

**Net-proton in p+p**

-  PRL 93 102301 (2004)

- Kirkfest, BNL

- March27, 2006

## Longitudinal Distributions 

**Longitudinal Distributions**** **

**Even though the dn/d******** are pion distributions at all energies are Gaussian.**

- Kirkfest, BNL

- March27, 2006

**Notes:**
First time wider than prediction.

## <Npart> Scaling in Baryon/meson

- Scale with <Npart>, independent of system
- Decrease with rapidity and increase with <Npart>

- Kirkfest, BNL

- March27, 2006

**Notes:**
Lessons from Cu-Au comparisons.

## Anti-particle to particle ratios

**At 62 GeV****: ********-****/********+**** = 1.0, K****-****/K****+**** = 0.84, pbar/p =  0.45,**
** ****At |y|<1 matter********antimatter**

** ****pbar/p verus K****-****/K****+**** : good statistical model description with ********B****= ********B****(y) with  T~170MeV**
**But this describes also energy depencency at y=0 ******** ****only ********B ****controls the state of matter**

**BRAHMS PRELIMINARY**

- Kirkfest, BNL

- March27, 2006

**Notes:**
STAR and NA47 measures pbar/p versus 
  -/+

## Slide 13

- Kirkfest, BNL

- March27, 2006

## Rapidity Dependent High-pT Measurement

-  Partons are expected to loose energy in the dense matter
- Different rapidities provide different densities of the medium: Sensitive to the dynamics
- Largest medium effect  at mid-rapidity (“Scale” to multiplicity)?
-      :increasing “Dissipative Viscous Hadronic Corona”?
- Rapidity dependent high-pT suppression factors: provide information on dynamical medium effect

- at fixed time

- y~0

- high-y

**Rapidity Dependent High-p****T**** Measurement**

- Kirkfest, BNL

- March27, 2006

## Nuclear Modification RAuAu at ~0 and 1

**Nuclear Modification RAuAu at ********~0 and 1**

**pp reference is based on ISR collider data**** ****extrapolated to acceptance.**

- Kirkfest, BNL

- March27, 2006

## Nuclear Modification Factors for ,K,p at y~3.1

- Suppression for pions and Kaons: RAuAu: pi<K<p
- RAuAu ≠ Rcp (<Ncoll>,<Npart> for 40-60% ~ 70,56)

- Kirkfest, BNL

- March27, 2006

## BRAHMS results in the context of CGC

- “BRAHMS ability to work at high rapidity (~4) opens a window  to study the presence of a Color Glass Condensate in the initial conditions of d-A collision”  (Beam Use proposal for RUN-3)

- Kirkfest, BNL

- March27, 2006

## BRAHMS d+Au results as function of rapidity and centrality

- BRAHMS, PRL 93, 242303

**R****dAu****=**** **

**Y****dAu**

**N****coll****Y****pp**

- Normalized ratio of previously measured dN/d

- Calculated from spectra

- Rcp ratios are constructed in wide  bins and with data from same run period

- The data are as you know well described by CGC models, and has given rise to other interpretations and additional measuremens.

- Kirkfest, BNL

- March27, 2006

##  Identified Particle RdAu

- RdAu of identified particle consistent with charged
- The protons may exhibit less suppression (Cronin?)

- Brahms preliminary

- pT  (GeV/c)

- Kirkfest, BNL

- March27, 2006

## pp at forward : more then  reference spectra 

- BRAHMS Preliminary

- pp at forward : more then  reference spectra** **

- NLO pQCD describes data at forward rapidity at 200 GeV
-      (NLO pQCD Calculations done by W. Vogelsang)
- Similar agreement as with o at y~0 and y~3-4

- pT (GeV/c)

**BRAHMS Preliminary**

- Kirkfest, BNL

- March27, 2006

## BRAHMS and Transverse Spin

- Early  (naive) QCD predicted this effect to be small
- Non-zero Single Transverse Spin Asymmetry (SSA/ An) requires
- Spin Flip Amplitude and phase difference in intrinsic states
- Such studies may clarify  properties of transverse quark structure of the nucleon
**Sivers effect ****[Phys Rev D41 (1990) 83; 43 (1991) 261]**
-       Flavor dependent correlation between the proton spin , momentum  and transverse momentum  of the un-polarized partons inside the proton.
**Collins effect ****[Nucl Phys B396 (1993) 161]**
-      Correlation between the quark spin, momentum and transverse momentum  of the pion.

- An = (+ - -) / (+ + -)
- Where the spin cross section is determined with the spin direction defined by kb x kpi

- Kirkfest, BNL

- March27, 2006

## Summary of pT dependence

- 0.15-0.30 in xF
- AN(pi-) 0.078+-0.002 low pT
- AN (pi-) 0.045+-0.003 higher pT
- AN (pi+) 0.066+-0.002 low pT

- Systematic errors from online polarization is (presently estimated at ~20% [scale] )

- Kirkfest, BNL

- March27, 2006

## BRAHMS White Paper: NPA A757 (2005) 1-27 

- Kirkfest, BNL

- March27, 2006

**Notes:**
The formulation of the white papers was an outgrow of proposal by STAR/PHENIX to summarize the first years findings, but brought up
and discussed extemsively in the spokepeople meetings, instituted by Tom (Kirk)

## Final Words

- The RHIC program made enormous progress during the years going from the first surveys, the discoveries, and now precision measurements in heavy ion and pp reactions leading to fundamental understanding of matter and QCD.
- Questions about strongly interacting matter:
-  ”How does matter behave at very high temperature and/or density?”
- Jet-quenching suppression in AA, not d-A
- Au-Au, Cu-Cu, pp. Bulk properties energy dependence
- ”What is the nature of gluonic matter? and how does it appear inside of strongly interacting particles?”
- - d Au at high rapidities (low-x)
- “What is the spin structure of the nucleon ?”
- Single Spin Asymmetries at large xF

- These and other important questions have been raised and answered during the tenure of Tom . You can can be proud these accomplishments were brought into the public domain, and I sincerely hope you will enjoy  years to come in Colorado.

- Kirkfest, BNL

- March27, 2006