Particle Production in p + p

s

 K. Hagel Cyclotron Institute Texas A & M University for the BRAHMS Collaboration

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

Global Detectors Front Forward Spectrometer Back Forward Spectrometer

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

– 2.3

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

Particle Identification

TIME-OF-FLIGHT

p max (2  cut) K/  m 2  p 2   c 2 TOF 2 L 2  1   0<  <1 (MRS) TOFW (GeV/c) TOFW2 (GeV/c) 2.0

2.5

1.5<  <4 TOF1 (GeV/c) 3.0

(FS) TOF2 (GeV/c) 4.5

K/p 3.5

4.0

5.5

7.5

CHERENKOV

RICH: Cherenkov light focused on spherical mirror  ring on image plane (2 settings) Ring radius vs momentum  / K separation 25 GeV/c Proton ID up to 35 GeV/c gives PID

The BRAHMS Acceptance

Rapidity Rotatable spectrometers give unique rapidity coverage :

B

road RA nge H adron M agnetic S pectrometers

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 1 2 

p T

Where

m d

2

N dydp T

2

T

 

A m

0 2  1  (

m T



nT m

) 

p

2

T

and

n A

 1 2 

dN dy nT

 (

n nT

  1 )(

n m

0  (

n

2 )  2 )  • Performed global fit using T = T 0 + ay, n = n 0 + by

200 GeV Pion Spectra T 0 = 0.058 GeV, n 0 = 4.45

T 0 = 0.056 GeV, n 0 = 4.38

200 GeV Kaon Spectra

T 0 = 0.127 GeV, n 0 = 6.44

T 0 = 0.125 GeV, n 0 = 6.23

200 GeV Proton Spectra

T 0 = 0.149 GeV, n 0 = 8.36

T 0 = 0.184 GeV, n 0 = 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(

L

)+

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

• We have measured net proton dN/dy • Simply dN/dy p – dNdy pbar shown previously

net proton dN/dy

• •  y ~ 1.26 (momgaus)  y ~ 1.20 (Hijing/B; remember dN/dy!)

Limiting Fragmentation

Net proton dN/dy Limiting 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 data including 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 

-



+ KKP KKP

• • scale factor of μ=p T 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?

K/



K/



comparison to Au + Au

• Larger K/  for Au+Au – Radial flow – Absence of cannonical K suppression

K/



vs rapidity

• Increasing K + /K with suppression 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