Particle Production in p + p Reactions at GeV

K. HagelCyclotron Institute

Texas A & M Universityfor the

BRAHMS Collaboration

200s

p + p collisions at high energy

• Baseline measurement for Heavy Ion Reactions– Jet suppression and the sQGP

• Information on baryon transport

• Constrain pQCD models– Information on fragmentation functions

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

• LHC “prediction”

• Summary

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

– Has characteristics of an exponential at low pT and evolves toward a power law at high pT

n

T

TT nT

mm

nmnTnT

nn

dy

dN

dydp

Nd

p

)(1

)2(

)2)(1(

2

1

2

1 0

0

2

Where22

02

TT pmm • Performed global fit using T = T0 + ay + a2y2, n = n0 + by + b2y2

• 6 parameter fit + dN/dy for each rapidity bin

200 GeV Pion Spectra

x10-10

x10-12

200 GeV Kaon Spectra

x10-5

x10-7

200 GeV Proton Spectra

x10-9

x10-11

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(Cascade)+Net(n), where each part involves feed-down corrections.

• At GeV the correction from net proton to net baryon is expected to independent of rapidity

• We have measured and will show net proton dN/dy

• Simply dN/dyp – dN/dypbar shown previously

200s

net proton dN/dy

y(200) ~ 1.45 (fit) y (Hijing/BB) ~

1.20 y(62) ~ 1.00 (fit)

Limiting Fragmentation

Longitudinal scaling

• Proper scaling for p + p collisions is dN/dxF

• If dN/dxF = const, dN/dy ~ coshy

Batisita and Covolan, PRD59 (1999) 054006

Net proton dN/dyLimiting Fragmentation

Nucl Phys. A661 (1999) 362.

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 fragmenting into pions– gg and gq processes dominate at mid rapidity (STAR PRL 91, 241803 (2003).

– Processes continue to dominate at larger rapidity.

– AKK (p +p)/2 (where p ~p) reproduces experimental p, but notp

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

DSS, 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.

+

-

K+K-

Updated AKK FFs with charge separated data from BRAHMS

• These fits indicate that, at large rapidity, fragmentation from valence u and d quarks contributes strongly to p and pbar asymmetry.

p -pp +pAKK2008

hep-ph:0803.2768

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

• scale factor of μ=pT

• DSS (De Florian, Sassot, Stratmann) 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?

• Beam remnants that are not addressed in FFs

- KKP+ KKP

K/

• Decreasing K/ (+ and -) for larger rapidity.

• More pronounced for K-/-

K/ comparison to Au + Au

• Larger K/ for Au+Au– Radial flow

– Absence of canonical K suppression in Au + Au

K/ vs rapidity

• Increasing K+/K- suppression with increasing rapidity

Strangeness enhancement?• p+p evolution with

pbar/p– canonical K

suppression – larger for K-

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

available in heavy ion collision.

What Can we say about LHC Physics

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

data– Shift to LHC beam rapidity to have

predicted distribution

LHC p+p stopping prediction

• Merge limiting fragmentation plots

• Add LHC beam rapidity to them

• Fit with Function

y ~ 2• Experiments

at LHC will measure ,K,p to y=1

Summary• Particle production

– dN/dy– Net proton dN/dy

• Stopping• Variation in y with beam energy

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

• Comparison to pQCD calculations– Constraints on Fragmentation Functions

• K/ ratios and canonical K suppression• Net proton dN/dy “Prediction” for LHC