# Presentation: 118070

## Why does high-pt suppression persist at forward rapidity?

- I.G. Bearden  ( for the BRAHMS collaboration)
- Niels Bohr Institute,  Københavns Universitet

- Quark Matter '08 Jaipur India

- 5. Februar 2008

## Outline

- Introduction
- Experiment
- RAA
- Results
- 200GeV Au+Au
- 62GeV Cu+Cu & Au+Au
- Model Comparisons
- Summary

- Quark Matter '08 Jaipur India

- 5. Februar 2008

## The BRAHMS Experiment

- Two small solid angle
- spectrometers (FS and MRS)

- provide excellent PID over
- broad range in y-pT

- Quark Matter '08 Jaipur India

- 5. Februar 2008

## Nuclear Modification

- Quantified by:

-  Cronin Enhancement
-  Shadowing/Saturation
-  Jet-quenching

- - yield relative to that from
- N+N collisions, scaled for
- the nuclear geometry (Nbin)

- Quark Matter '08 Jaipur India

- 5. Februar 2008

## Charged hadron pT spectra 200GeV Au+Au

- Nb: filled are +, open -, drawn lines are BRAHMS p+p data

- Quark Matter '08 Jaipur India

- 5. Februar 2008

## RAA for charged hadrons

- at high pT, the charged hadron yields produced in most central 0-10% Au+Au collisions are suppressed compared to binary scaled p+p yields
- Little change  in RAA as function of eta

- BRAHMS Preliminary

- Quark Matter '08 Jaipur India

- 5. Februar 2008

##  RAA @ 62.4 GeV 

- ~3.1

- ~0

- Au+Au

- Cu+Cu

- BRAHMS Preliminary

- BRAHMS Preliminary

- Quark Matter '08 Jaipur India

- 5. Februar 2008

**Notes:**
- results from mid rapidity for 2 centrality ranges and Cu+Cu and Au+Au collision
- thus we can look for difference in suppression/enhancement vs the size and geometry
- linear scale – focus in high pT part
- central CuCu and AuAu are similar: balanse of suppression and enhancment
- peripheral CuCu and AuAu are also similar: strong enhancement
- where are these resutls compared to SPS and RHIC top energy?

## Caveat emptor!

- Quark Matter '08 Jaipur India

- 5. Februar 2008

## RAuCu0- 10% & 40-60%

- Quark Matter '08 Jaipur India

- 5. Februar 2008

**Notes:**
- using CuCu in the denimonator instead of p+p
- Both pT spectra are thus scaled with Ncoll
- 0-10 and 40-60 shown.
- focus on high pT part
- both reach RAA=1 at high pT
- We can see the overlap geometry below, indicating that AuAu volume and Rtheta us much larges than CuCu

## RAuCu with same Ncoll at Midrapidity

- Quark Matter '08 Jaipur India

- 5. Februar 2008

**Notes:**
- lets find centralities with same Ncoll and compare spectra
- thus no scaling of spectra is needed
- now the low pT part show approximate scaling: Low pT is related to Npart which is also the same for both systems.
- but the high pT particles in AuAu is much less suppress than in CuCu, reaching values of more than 1.5
-  below is geometic overlap image which shows there are still differences.
- there are two of them that are the same: CuCu 10-20 and AuAu 30-45

## RAuCu mid vs. forward 62.4GeV (10%)

- pT

- RAuCu

- Midrapidity
- Forward (3.1)

- Quark Matter '08 Jaipur India

- 5. Februar 2008

## RAuCu with same ‘radius’

- Quark Matter '08 Jaipur India

- 5. Februar 2008

**Notes:**
So, what do we make of this?  These two sources should be the same, they have the same Nart & Nbin, yet there appear to be more particles at high-pt in Au+Au than in 
Cu+Cu. Why the hell should this happen, and what might it tell us?

## RAA for identified particles (200GeV)

- the charged pion yields are suppressed by a factor of ~ 2-3 as compared with binary scaled p+p pion yields.
- RAA for pions is  independent of rapidity
- the proton and antiproton yields in central Au+Au at 200 GeV do not show suppression, baryon meson difference remains

- Quark Matter '08 Jaipur India

- 5. Februar 2008

## RAA at 62.4 GeV, y=0

- BRAHMS Preliminary

- BRAHMS Preliminary

- Quark Matter '08 Jaipur India

- 5. Februar 2008

**Notes:**
- present what we see: pions kaon proton, bith charges, in 0-10% CuCu and AuAu at midrap.
- present the momentum range. Not too high! Due to PID resolution and statistics.
- the mesons behanve as the charged hadron for both system. They also dominate the particle production at this rap.
- the protons show strong enhancement. which is something that is also seen at 200GeV

## RAA, y>3, 62GeV, top 10% central

- Quark Matter '08 Jaipur India

- 5. Februar 2008

**Notes:**
- Again the same at forward rapidity
- pions show a clear dependence on charge.
- positive are suppressed, like the charged hadrons. No supp or Enhansement of negatives.
- kaons are unchanged
- protons are suppressed, like the charged hadrons. These dominate the charge particles. Most also come from the fragementation of the colliding nuclei. which we see on the next slide

## Hydro+Jet: Interplay between Soft and Hard

**Interesting region**
******Intermediate *****p******T***
**(2<*****p******T*****<3.5 GeV/*****c*****)****‏**
**Pion ********hard****,**** ****Proton**** ********soft**

- PT, cross~
- 1.8 GeV/c for 
- 2.7 GeV/c for K
- 3.7 GeV/c for p

**Hydro+Jet: Interplay between Soft and Hard**

- Calculation based on two models with CGC as initial conditions:
- 3D hydrodynamic model (Hydro)‏
- dynamics of high pT partons (Jet)‏
- pT,cross is the point for Yieldsoft=Yieldhard
- pT,cross moves toward high pT with mass of particles because of the effects of radial  flow.

- y=0

- Quark Matter '08 Jaipur India

- 5. Februar 2008

## Model comparison I

- pT spectra for charged hadrons  in 200 GeV Au+Au reasonably well described by the hydro+jet model calculation at η=3.2

- Quark Matter '08 Jaipur India

- 5. Februar 2008

## Model comparison II

- opacity **n = L/l**
**	****L****	**– effective length of matter
**	****l** 	– mean free path
- less jet quenching +
- stronger initial effects
- at higher rapidities
- maintain rapidity independent NMF

- Barnafoldi et al. hep-ph 0609023

- Quark Matter '08 Jaipur India

- 5. Februar 2008

## Is RAA an appropriate measure?

- Renk, Eskola, PRC 75, 054910 (2007)

- Wicks,Horowitz,Djordjevic,Gyulassy
- NPA 784, 426 (2007)

- Two rather different
- Models both reproduce
- RAA.
- RAA only tells us that we only
- ‘see’ particles from near
- Hot&dense surface?

- Quark Matter '08 Jaipur India

- 5. Februar 2008

## Summary

- At 200GeV, intermediate pT constant behavior vs. y
- For 62.4GeV, pronounced changes from mid to forward rapidity.
- + suppressed for 62.4 y=3.4, p “suppressed” but not partonically
- RAA flat for same mean radius (<R>). Why?
- Hydro+jet looks good, but...
- RAA not a good model descriminator

- Quark Matter '08 Jaipur India

- 5. Februar 2008