features similar to above signal: correlations are broader and not

as strong

ty

ty

Soft and Semi-hard Components in p-p Collisions at sNN= 200 GeV

R. Jeff Porter, Brookhaven National Laboratory & University of Washington (STAR Collaboration) Abstract:

m

p

m

p tt

2

1lnty

(GeV/c)tpty

transverse rapidity

}1)()(

),({

21

21 xx

xxN

A

AN

two-particle correlations

evaluated from same event pairs

evaluated from sibling (mixed-event) pairs

,

t

Δ 1 2 Δ 1 2

p η, φ

φ φ φ , η η η

x

x

where

To examine correlation structure at larger pt, we change variables to transverse rapidity yt. This variable compresses data at larger pt and reduces density variation on the transverse momentum variable.

0.5

5.04.03.0

1.0

2.0

pt (GeV/c)

yt,1 x yt,2 correlations(STAR Preliminary data)

ty

t,2y

t,1y

soft-component autocorrelations

HBT local charge conservation about ~ 0

-conversion to e+epairs

STAR Preliminary data

Pythia 6.2 Minbias

charge conservation broader than data

No HBT

Low-yt structure: local charge conservation near ~ 0 for US pairs

LS pairs have a strong small-angle peak consistent with HBT

Results from Pythia show no HBT signal but do reveal a charge conservation structure similar to data

Correlations on yt x yt provide a transparent cut space on which one can separate correlations due to soft-string fragmentations from hard scattering

ty

ty

0.8<pt1,pt2<2.3 GeV/c

“jet-finding” selection:e.g., trigger particle pt > 4GeV/c

jet-like features:small-angle peak plus broad -independent peak on at

Correlations from semi-hard scattering emerge above pt ~ 0.5 GeV/c Autocorrelations on at moderate pt have features in common with those found with “jet-finding” selections

2-particle correlations are part of a continuum: jets, jet-remnants and mini-jets

In this analysis we forgo reliance on trigger-particle selections and map out these correlations over the entire pt range.

semi-hard-component autocorrelations

- Two-particle correlations on yt x yt (and Nch) provide a transparent cut space to study soft and semi-hard scattering components

- Low-pt correlations reveal local charge conservation and HBT signals - At higher pt (> 0.5 GeV/c), structures asso-ciated with jet correlations show a continuum of features resulting from semi-hard scattering

- These p-p minijet correlation studies provide an essential reference for A-A collisions

correlation dependence on Nch

Signal pushes out to larger ytxyt with increasing Nch while correlation strength at moderate yt grows.

Δ Δ

Δ η Δ

Δ Δns ns ns

η

Δ Δas cdas cd

2 2ΔΑ 1 η 1

Ν CΑ 2 σ 2 σ

2 21 π 1 η

C C C2 σ 2 σ

exp

exp exp

away-side structure about

small charge-dependent band about

model function description

2 2

ψ η φσ σ σ Δ Δη φε σ σ

Δφ Δη

ty

ΔφΔηty

STAR Preliminary data model fit

/ 2χ 1.09

Autocorrelations reveal simple structures easily modeled by standard functions. Correlations at moderate pt are well fit by gaussians and share features (near-side peak or “jet-cone”) with those obtained from Pythia. Data structures are sharper and more eccentric than Pythia but converge with Pythia at pair pt ~ 6 GeV/c. The integrated correlation strengths from Pythia grow with total pt while those for data remain flat up to pt ~ 6 GeV/c.

t t

d

d

N

p p t t

d

d

N

y y

ΔΑΝ

Α

ΔΑΝ

Α

ΔΑΝ

Α

ΔΑΝ

Α

ΔΑΝ

Α

ΔΑΝ

Α

,ty

,ty

ψσ ε C

,ty,ty

near-side (small-angle) peak structure

ΔφΔηns σσ2πCC

Histogram KeyLS=Like-sign pairs US=Unlike-sign pairsCD=LS – US CI=LS + US

Summary