Particle Production in p + p Reactions at GeV

K. HagelCyclotron Institute

Texas A & M Universityfor the

BRAHMS Collaboration

200s

Outline• Description and characteristics of BRAHMS

• Particle spectra– Fits and fit parameters

• Rapidity densities

• Nuclear Stopping

• Limiting fragmentation

• High pt pQCD comparisons to data

• Strangeness

GlobalDetectors

Front Forward Spectrometer

Back ForwardSpectrometer

• Mid-rapidity Spectrometer– TPC, TOF, Cherenkov– 30o – 90o = 0 - 1.5

• Mid-rapidity Spectrometer– TPC, TOF, Cherenkov– 30o – 90o = 0 - 1.5

• Forward Spectromter– TPC, DC, TOF, Cherenkov, RICH

– 2.3o – 30o = 1.5 – 4

Particle Identification

1

L

TOFcpm

2

2222TIME-OF-FLIGHT

0<<1

(MRS)

1.5<<4

(FS)pmax

(2 cut)

TOFW (GeV/c)

TOFW2 (GeV/c)

TOF1 (GeV/c)

TOF2 (GeV/c)

K/ 2.0 2.5 3.0 4.5

K/p 3.5 4.0 5.5 7.5

RICH: Cherenkov light focusedon spherical mirror ring on image plane

Ring radius vs momentum gives PID / K separation 25 GeV/cProton ID up to 35 GeV/c

CHERENKOV

(2 settings)

Rotatable spectrometers give unique rapidity coverage :Broad RAnge Hadron Magnetic Spectrometers

The BRAHMS Acceptance

Tra

nsv

ers

e m

om

en

tum

[G

eV

/c]

Rapidity

Experimental Coverage

Fitting particle spectra• One method to extrapolate to parts of the spectrum

not measured.• Different functions might (or might not) be appropriate

for different spectra.• It is still an extrapolation that adds to systematic

error.• Fit used in this work is Levy Function

n

TTT

nTmm

A

dydp

Nd

p

)(

12

1 2

Where22

02

TT pmm and )2(

)2)(1(

2

1

0

nmnTnT

nn

dy

dNA

• Performed global fit using T = T0 + ay, n = n0 + by

200 GeV Pion Spectra

T0 = 0.058 GeV, n0 = 4.45 T0 = 0.056 GeV, n0 = 4.38

200 GeV Kaon Spectra

T0 = 0.127 GeV, n0 = 6.44 T0 = 0.125 GeV, n0 = 6.23

200 GeV Proton Spectra

T0 = 0.149 GeV, n0 = 8.36 T0 = 0.184 GeV, n0 = 14.58

62 GeV p+p spectra

dN/dy

Stopping

• Obtained from net baryon dN/dy– Gives information on initial distribution of

baryonic matter at the first moment of the collision.

• Net-Baryon = Net(p)+Net()+Net(Casade)+Net(n), where each part involves feed-down corrections.

• We have measured net proton dN/dy

• Simply dN/dyp – dNdypbar shown previously

net proton dN/dy

y ~ 1.26 (momgaus)

y ~ 1.20 (Hijing/B; remember dN/dy!)

Limiting Fragmentation

Net proton dN/dyLimiting Fragmentation

Nucl Phys. A661 (1999) 362.

Rapidity dependence of Mean pt

NLO pQCD comparisons to data at large rapidity

BRAHMS Phys. Rev. Lett. 98, 252001 (2007)

• Comparison of different fragmentation functions

– Modified KKP (Kniehl-Kramer-Potter) does better job than Kretzer (flavored FFs) on -, K+

• Difference driven by higher contributions from gluons fragmentating into pions– gg and gq processes dominate at mid rapidity (STAR PRL 91, 241803 (2003).

– Processes continue to dominate at larger rapidity.

– AKK (p+pbar)/2 (p~pbar) reproduces experimental p, but not pbar

Rapidity dependence of NLO pQCD comparison to data

• KKP describes data from mid-rapidity (PHENIX, 0) to large rapidity (BRAHMS, -; STAR 0)

Global fits to dataincluding BRAHMS large rapidity data

PRD 75, 114010 (2007)

• Charged separated fragmentation functions

• Fragmentation functions significantly constrained compared to previous “state of the art” when adding RHIC data into fits.

NLO pQCD comparisons of 62 GeV +, K+ data at large rapidity

• scale factor of μ=pT

• DSS also shown (dashed lines)• K- data suppressed order of

magnitude compared to K+ (valence quark effect).

• NLO pQCD using the recent DSS fragmentation functions give approximately same K-, K- yield (?) Related to fragmentation or PDFs?

- KKP+ KKP

K/

K/ comparison to Au + Au

• Larger K/ for Au+Au– Radial flow

– Absence of cannonical K suppression

K/ vs rapidity

• Increasing K+/K- suppression with increasing rapidity

Strangeness enhancement• p+p evolution with

pbar/p– cannonical K

suppression – larger for K-

• Larger values for Au+Au – strangeness effects turing on– More energy

available.

What Can we say about LHC Physics

• Net proton dN/dy– Use lower energy limiting fragmentation

data

LHC p+p stopping prediction

• Merge limiting fragmentation plots

• Add LHC beam rapidity to them

• Fit with momGaus

y ~ 2• CMS will

measure to 2.2• Stopping with

energy (subtract from incoming energy)

Summary• Particle production

– dN/dy– Net proton dN/dy -> Stopping

• Limiting Fragmentation– dN/dy– Net proton dN/dy

• High pt pQCD calculations• Strangeness enhancement• Prediction for LHC