RHIC-II 29-Apr-2005

Ramiro Debbe Physics Department

Forward Light Hadrons

RHIC-II Forward and pA Workshop

RHIC-II 29-Apr-2005

Outline of the presentation

• Brief description of the BRAHMS setup.

• Charged particle production as function of rapidity and centrality.

• Particle identification:

a) RdAu for anti-protons and - at =3.2

b) Composition of h+ at =3.2

• Results from p+p collisions.

• Summary

RHIC-II 29-Apr-2005

The BRAHMS Detector

MRS

FFS

BFS

Geometrical Acc.: 0.8 msr

p/p = 0.0018p [GeV/c]

RHIC-II 29-Apr-2005

Event characterizationThe centrality of the collision for the results that will be presented is defined as fractions of the total multiplicity measured with the TMA in -2<<2

The centrality of Au-Au collisions can also be defined with the ZDC and BB or TMA correlations.

Our triggers are defined with the ZDC and BB, p+p and d+Au collisions were triggered with the INEL detectors.

TMA

INEL

ZDC ZDCBB BB

RHIC-II 29-Apr-2005

I have a personal preference for the ~direct connection between gluon distributions in the colliding ions and the distribution of produced particles.

For d+Au collisions, multiplicity densities show “cascading” in Au side and dAu = pp at d frag. side. Already seen at lower energies.

RHIC-II 29-Apr-2005

Energy and momentum conservation

xL = xa - xb =(2MT/√s)sinh y

ka + kb = k, xaxb = MT2/s

A solution to this system is:

xa = (MT/√s) ey

xb = (MT/√s) e-y

where y is the rapidity of the (xL,, k) system

From Guzey, Strikman, and Vogelsang.

Most of the data collected at 4 degrees would have x2~0.01

pT

x2

The reach “into” the target wave function

RHIC-II 29-Apr-2005

Acceptance and d+Au and p+p Spectra

PRL 93 242303(nucl-ex/0403005)

RHIC-II 29-Apr-2005

Modification of pt spectra as function of rapidity

In both p+p and d+Au systems the distributions get softer at higher rapidities, beyond a trivial thermal behavior.

RHIC-II 29-Apr-2005

RdAu ratios

CroninCronin like like enhancement enhancement at at =0.=0.

Clear Clear suppression suppression as as changes changes up to 3.2up to 3.2

Same ratio Same ratio made with made with dn/ddn/d follows the follows the low plow pTT R RdAudAu

RdA d2Nd+Au/dpTd

<Ncoll> d2Nppinel/dpTd

where < < Ncoll> = 7.2> = 7.2±0.3

RHIC-II 29-Apr-2005

Rcp ratios At At =0 the =0 the central events central events have the ratio have the ratio systematically systematically above that of above that of semi-central semi-central events. We see events. We see a reversal of a reversal of behavior as we behavior as we study events at study events at =3.2=3.2

Rcp

1/<Ncoll central> NABcentral(pT,

1/<Ncoll periph> NAB

periph(pT,)

RHIC-II 29-Apr-2005

d+Au particle abundances

The ratios proton/pion from dAu collisions measured at mid-rapidity are consistent with “fragmentation in vacuum”

RHIC-II 29-Apr-2005

Using ratios to obtain the RdAu of identified negative particles.

RHIC-II 29-Apr-2005

Use of theUse of the BRAHMSBRAHMS RICHRICH to identify particlesto identify particles

We make use of already produced spectra and RdAu factors and use the RICH at high momentum to bypass the long process of invariant yield extraction.

17 GeV/c

Above the selected momentum threshold the RICH is ~94% efficient.

RHIC-II 29-Apr-2005

RdAu for anti-protons and pions (min bias)

Preliminaryd Au

p p

RHIC-II 29-Apr-2005

Difference between h+ and h- in dAu at ~3

But pQCD calculations that do well reproducing 0 would have at most a 10% difference not the factors ~3 we measure.

Our measurement is showing that the difference is driven by the protons.

• proton

kaon

RHIC-II 29-Apr-2005

BRAHMS results from p+p collisions

The ratio of particle to anti-particle measured in p+p collisions is remarkably similar to the one we measured in Au+Au collisions.

RHIC-II 29-Apr-2005

Comparison of NLO pQCD and measurement at = 0

Work done by Marco van Leeuwen STAR Calculations by W. Vogelsang

RHIC-II 29-Apr-2005

NLO pQCD calc. From W. Vogelsang

Frag. Function: modified KKP is an attempt to reproduce h-

Measured h- at 4 degrees and a NLO pQCD calculation

BRAHMS data

RHIC-II 29-Apr-2005

Spectra for identified particles at =2.2

RHIC-II 29-Apr-2005

Comparison of particle ratios measured in p+p collisions and simulated with PYTHIA

For protons we find a remarkable difference that may indicate other processes besides parton fragmentation.

We measure a small excess of kaons and see an emerging trend that suffers from low statistics at high pt.

=3.2

=3.2

RHIC-II 29-Apr-2005

SummaryInclusive charged particle production in dAu compared to scaled pp shows a suppression that depends on rapidity and the centrality of the collisions.

First results that include particle identification in dAu collisions show particle abundances consistent with “fragmentation in the vacuum”.

Different “Cronin effect” of baryons and mesons is also seen at forward rapidities.

NLO pQCD describes pp inclusive pt distributions up to the highest rapidities measured.

We see that PYTHIA fails to describe the baryons (protons) at forward rapidity.