# Presentation: 661193

## Forward Light Hadrons

- 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:
-    RdAu for anti-protons and - at =3.2
-    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]

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## Event characterization

**The 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**.

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

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## pT

- Energy and momentum conservation
-        xL = xa - xb =(2MT/√s)sinh y
-           **k****a**** + k****b**** = k**,    xaxb = MT2/s
- A solution to this system is:
-     **x****a**** = (M****T****/√s) e****y**
**      ****x****b**** = (M****T****/√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 x****2****~0.01**

**p****T**

- x2

**The reach “into” the target wave function**

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## Acceptance and d+Au and p+p Spectra 

-  PRL 93 242303
- (nucl-ex/0403005)

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## 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.

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## RdAu ratios

- Cronin like enhancement at =0.
- Clear suppression as  changes up to 3.2
- Same ratio made with dn/d follows the low pT RdAu

- RdA 

-  d2Nd+Au/dpTd

- <Ncoll> d2Nppinel/dpTd

- where * <* Ncoll*> = 7.2*±0.3

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## Rcp ratios 

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

***R******cp***** ******
****

- 1/<Ncoll central>   NABcentral(pT,

- 1/<Ncoll periph>    NABperiph(pT,)

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## d+Au particle abundances 

**d+Au particle abundances**

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

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## Using ratios to obtain the RdAu of identified negative particles.

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## Use of the BRAHMS RICH to  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.**

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## RdAu for anti-protons and pions (min bias)

- Preliminary

- d Au

- p p

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

**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

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## BRAHMS results from p+p collisions 

**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.

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## Comparison of NLO pQCD and measurement at  = 0

**Comparison of NLO pQCD and measurement at ******** = 0**

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

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## Measured h- at 4 degrees and a NLO pQCD calculation

**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

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## Spectra for identified particles at =2.2

**Spectra for identified particles at ********=2.2**

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## 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

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## Summary

- Inclusive 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.

- RHIC-II 29-Apr-2005