# Presentation: 040740

## High PT Hadron Spectra at Large Rapidity

- Ramiro Debbe
- for the **BRAHMS** collaboration
- Physics Department

**High P****T**** Hadron Spectra at Large Rapidity**

- QM2004 13-Jan Oakland CA.

## Motivation

-  = Q2R02(x)

- Transverse “size” of the dipole ~ 1/Q

**“****Geometric Scaling”** at HERA (**K. Golec-Biernat PRL 86 2001**)
- R0  “saturation radius” ~x
- defines a scale: for values of Q2 such that for 1/Q≥R0
- the cross section becomes a constant.

- QM2004 13-Jan Oakland CA.

## Motivation cont.

- Further theoretical work postulates the existence of a new regime: the Color Glass Condensate.
- Use of nuclei with large A facilitates the study of saturation:
**Q****2****s ****~ A****1/3****/x******
**x****2**** = (m****T****/√s ) e****-y**** **
- Measurements at large rapidities in p-A systems “probe” small x of A:

- QM2004 13-Jan Oakland CA.

## Predictions from theory

- I. Vitev nucl-th/0302002 v2

- D. Kharzeev et al. **Phys.Rev.D68:094013,2003**

**CGC at y=0**

**Y=0**

**Y=3**

**Y=-3**

- Very high energy

**As y grows**

- QM2004 13-Jan Oakland CA.

## The data at forward rapidities was collected with FFS at 4o ( ~3) and 12o (~2)

**INEL**
**2.47<********<5.29**

- The data at forward rapidities was collected with FFS at 4o (**** ~3) and 12o (******~2****)**

- QM2004 13-Jan Oakland CA.

## Spectra from p-p collisions

- All spectra shown on this presentation have been corrected for acceptance and tracking efficiencies.
- Fits to power law.
- Pythia run with our trigger cond. and momentum resolution

- Minimum biased (80%±3% of NSD p-p cross section)

- QM2004 13-Jan Oakland CA.

## Spectra from d-Au collisions

- Minimum biased events corresponding to 91%±3% of the d-Au inelastic cross section.

- QM2004 13-Jan Oakland CA.

## Nuclear modification factor RdAu

- The behavior of the many-body systems we study (A-A or d-A) can be “calibrated”  with a “simple system” like p-p .
- Borrowing from “Cronin effect” studies, we compare particle production in d-A to the scaled production in p-p:
**R****dAu ****= (d****2****N****dAu****/dp****T****d********) / (N****coll ****d****2****N****pp****/dp****T****d******** )**
- With Ncoll being the estimated number of binary collisions as d “goes through” the Au nuclei.

- QM2004 13-Jan Oakland CA.

## RdAu at different rapidities

- All ratios extracted from minimum biased data samples

- Number of binary collisions in minimum biased events is estimated:
**N****coll ****= 7.2±0.3 **
- Statistical errors dominant over the systematic ones at =2 and 3
- Systematic error (not shown) ~15%
- The values for =0 were published in:
**PRL 91 072305 (2003)**

- QM2004 13-Jan Oakland CA.

## Centrality dependence

- To reach further understanding of the effects seen in the ratio **R****dAu** as function of rapidity, we studied the  **centrality dependence** of particle yields.
- We infer centrality from the measured  charged particle multiplicity in         -2<<2

- QM2004 13-Jan Oakland CA.

## Rcp definition

- We select three data samples:
- “Central”: centrality 0-20%
- “Semi-central”: centrality 30-50%
- “Peripheral”: centrality 60-80%
- Then we calculate the ratios Central/Peripheral and Semi-central/Peripheral as function of pT at different rapidities.
- Many corrections and systematics cancel out.

- QM2004 13-Jan Oakland CA.

## Centrality dependence

- All numerators and denominator are scaled by the appropriate estimated number of binary collisions (HIJING + BRAHMS GEANT)
- The ratios are corrected for trigger inefficiency.
- All other corrections (acceptance, tracking efficiency.. ) cancel out.

- QM2004 13-Jan Oakland CA.

## Discussion 

**BRAHMS** has measured a clear modification of the Cronin peak as we detect charged particles at pseudo-rapidities ranging from 0 to 3.
- We also found that particle yields at all values of pT are more suppressed in central events at high rapidity.
- Both results are consistent with a description of the Au wave function **evolving in ln(1/x)** (rapidity) into a saturated non-linear medium.

- QM2004 13-Jan Oakland CA.

## Positive and negative hadrons from d-Au at ~3

**“****h****+****>h****- ****clear evidence of Valence quark fragmentation “**
**M. Gyulassy ****BNL 12.4.03 **

**Positive and negative hadrons from d-Au at ********~3**

- The results at ~3 have been attributed to **beam**** ****fragmentation** as the dominant process for >3.
**RIKEN High P****T**** Physics at RHIC Dec 2-6, 2003**
- Valence quarks fragment by “string breaking” with a 1/x “wee” parton distribution.  The ratio of particle yields:
- (dNdAu/d )/ (dNpp/d ) drops from a maximum at -Au  to a minimum on the deuteron fragmentation side following a triangular distribution.

- QM2004 13-Jan Oakland CA.

## Summary and outlook

**Compared to previous results at ********=0, we have measured considerably smaller values of R****dAu**** at pseudo-rapidities 1, 2.2 and 3.2**
**Yields of charged particles per binary collision are reduced  in data samples of higher centrality.**
-  We will put “final touches” to the present analysis and submit it for publication.
-  We continue the analysis of Run III data with the full complement of our detectors. That analysis will reveal details of the “suppression” for different particles species (see Z. Yin’s presentation this afternoon)
- Looby the RHIC community for additional d-Au running.

- QM2004 13-Jan Oakland CA.

## BRAHMS collaboration

**I. Arsene, I.G. Bearden, D. Beavis, C. Besliu, B. Budick, H. Bøggild, C. Chasman, **
**C. H. Christensen, P. Christiansen, J. Cibor,  R. Debbe, E. Enger J. J. Gaardhøje, **
**M. Germinario, ****K. Hagel, O. Hansen, A. Holme,  A.K. Holme, H. Ito,  A. Jipa, **
**J.I.  Jørdre, F. Jundt,  C.E.Jørgensen, R. Karabowicz,  E.J. Kim, T. Kozik, **
**T.M. Larsen, J.H. Lee, Y. K.Lee, S. Linda, G. Løvhøjden, R. Lystad, Z. Majka, **
**A. Makeev, M. Mikelsen, ****M. Murray****,**** J. Natowitz, B. Neumann, B.S. Nielsen, **
**K. Olchanski, D. Ouerdane, R.Planeta, F. Rami, C. Ristea, O. Ristea, D.Roehrich,**
**B. H. Samset, D. Sandberg, S. J. Sanders, R.A.Sheetz, P. Staszel, T.S. Tveter, **
**F.Videbæk, R. Wada,Z. Yin  and I. S. Zgura**
**Brookhaven, Strasbourg, Krakov, Johns Hopkins, NYU, Niels Bohr.**
** ****Texas A&M, Bergen, Bucharest, ****Kansas****,**** Oslo**

- QM2004 13-Jan Oakland CA.