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22nd Winter Workshop on Nuclear Dynamics
“Can STAR p+p data help constrain fragmentation functions for strange hadrons”
Mark Heinz (for the STAR collaboration)
Yale University
[email protected] WW San Diego, March 20062
Outline
Perturbative QCD Methods Factorization Fragmentation functions
Next-to-Leading order calculations (NLO) Calculations by KKP, Kretzer, AKK,
Vogelsang Leading order MC (Pythia)
PT-spectra <pT> systematics Gluon vs Quark jets
[email protected] WW San Diego, March 20063
Factorization Ansatz
c
chbbaa
abcdba
T
hpp
z
Dcdab
td
dQxfQxfdxdxK
pdyd
d
0
/222
)(ˆ
),(),(
LO parton processes
NLO parton processes
Fragmentation Function (non-pert.)
BKK, Phys Rev D (1995)
Pions
Parton Distribution Function(non-pert.)
RHIC
K-Factor
[email protected] WW San Diego, March 20064
Universality of Fragmentation func.
Suggested by Paper by Kniehl,Kramer & Poetter (2001)
Experimental data from different collisions systems have been fit with the same fragmentation function
KKP, Nucl.Phys.B597(2001)
Fragmentation function for inclusive hadrons
KKP, Nucl.Phys.B582(2000)
s=91.2 GeVOPAL,ALEPH
uds-quark
c-quark
b-quark
all
[email protected] WW San Diego, March 20065
The gluon FF
Collider data available from 3-jet events from ALEPH and OPAL
Statistics still very limited
Dotted: DelphiDashed: BKKFull: KKP (new)Mf=52 + 80 GeV
AKK, Nucl.Phys.B725(2005)
Curves scaled by 1/100
[email protected] WW San Diego, March 20066
NLO for non-strange particles Inclusive charged hadrons have been well described for the last 10 years by Fragmentation functions
(FF) from Kretzer, KKP and others. Baryons have been notoriously difficult to fit, due to limited knowledge of FF. Albino, Kramer and Kniehl (AKK) use latest OPAL data to calculate light flavor (u,d,s) separated
fragmentation functions for the first time.
Van Leeuwen, nucl-ex/0412023 STAR Preliminary
STAR 200 GeV p+p data for identified non-strange mesons and baryons agrees well with NLO calculation by AKK.
[email protected] WW San Diego, March 20067
NLO for strange particles First NLO predictions for RHIC energies K0s and Lambda were obtained by
W.Vogelsang (RIKEN) In 2005 calculations at NLO by Albino, Kniehl & Kramer (AKK) for K0s and Lambda
produced better agreement. Shape of GluonLambda FF was constrained by Gluon Proton FF Magnitude was constrained by STAR data
[email protected] WW San Diego, March 20068
Consistency with data at 630 GeV
How well does the constrained fragmentation function extrapolate to other energies?
K0s
NLO Lines are for μ=2*pT, pT, pT/2
UA1 (630GeV)
STAR (200GeV)
UA1 (630GeV)
STAR (200GeV)
Albino,Kniehl,Kramer et al. ,hep-ph/0510173
[email protected] WW San Diego, March 20069
Leading order pQCD (PYTHIA)
Parton showers based on Lund String Model JETSET was used to successfully describe e+e- collisions Flavor dependence introduced by strange quark suppression
factor Baryon production governed mainly by di-quark probabilities K-Factor accounts for NLO perturbative processes
“Lund Symmetric fragmentation function”
z = fractional momentum of parton/hadron a, b = tunable parameter
[email protected] WW San Diego, March 200610
pT-spectra for strange particles
PYTHIA Version 6.3 used (January 2005) Incorporates parameter tunes from CDF New multiple scattering and shower algorithms
Necessary tune: K-Factor, which accounts for NLO processes in hard cross-section
[email protected] WW San Diego, March 200611
What about non-strange particles ?
Comparison to published STAR data Good agreement for pions with K=1 and proton with K>1 However only lower pT region measured
[email protected] WW San Diego, March 200612
What about strange resonances ?
Published STAR data for , K* Preliminary STAR data for * (baryon resonance) K-factor = 3 fits all resonances very nicely
STAR Preliminary
PYTHIA 6.3 PYTHIA 6.3PYTHIA 6.3PYTHIA 6.3,K=3 PYTHIA 6.3,K=3PYTHIA 6.3,K=3
[email protected] WW San Diego, March 200613
K-factor in LO pQCD
How is the K-factor defined? 2 Definitions:
Kobs= exp / LO
Kth= NLO / LO Flavor dependence of K-
factor, ie. NLO contributions ?
For h- it decreases for collision energy, ie. contribution of NLO processes is smaller at higher energies
STAR
Eskola et al Nucl. Phys A 713 (2003)
[email protected] WW San Diego, March 200614
<pT> systematics in p+p
Perturbative QCD models are ideal to look at Mini-jet phenomenology High multiplicity p+p events more mini-jets Higher pT final states higher
<pT> of hadrons
XN.Wang et al (Phys Rev D45, 1992)
Nch
Njet=2
dNch/d
<p
T>
[email protected] WW San Diego, March 200615
Charged multiplicity distribution
Pythia + Simulated Trigger and detector acceptance. Probability of high multiplicity events is very sensitive to NLO corrections
STAR Preliminary
STAR data
PYTHIA 6.3PYTHIA 6.3, K=3
STAR data
[email protected] WW San Diego, March 200616
PYTHIA <pT> vs Nch
More sensitive observable to implementation of multiple scattering algorithm This phenomenology has also been previously attributed to mini-jets
Higher K-factor, more NLO contributions, are required to account for increase of <pT> with charged multiplicity
[email protected] WW San Diego, March 200617
Gluon vs Quark jets
Extensive studies of jet properties have been done in e+e- data
Gluon jets produce higher particle multiplicity
Quark jets fragment produce a harder pT-spectrum
The ratio of anti-particle to particle production should be sensitive to quark-vs-gluon jet
[email protected] WW San Diego, March 200618
Ratios vs pT (gluon vs quark jet)
Gluons have equal probability of fragmenting into particles or antiparticles, Quarks fragment predominantly into particles
At higher pT (higher z) we are probing the quark-jet dominated region.
STAR (Phys Lett. B submitted)
p+pSTAR preliminary
d+Au
[email protected] WW San Diego, March 200619
mT - scaling
mT-scaling first studied with ISR data.
In the Color Glass Condensate (CGC) picture mT-scaling would be indicative of evidence of gluon saturation.
No absolute scaling. Species are scaled with arbitrary prefactors to overlap in low pt region
STAR data reveals an interesting feature of baryon vs meson splitting above 2 GeV in mT
STAR preliminary
[email protected] WW San Diego, March 200620
mT scaling in PYTHIA
Gluon jets produce meson vs baryon “splitting”, Quark jets produce mass splitting in mT. This confirms that our p+p events are gluon jet dominated.
PYTHIA Preliminary DATA
Arbitrarily scaled mT-spectra data and PYTHIA simulation agree well
Gluon jet Quark jet
[email protected] WW San Diego, March 200621
Baryon-meson “anomalies” Baryon production at intermediate pT is interesting since fragmentation by itself
cannot describe data Strange baryon/meson ratio is under-predicted by PYTHIA at 200 and 630 GeV Magnitude of UA1 ratio is similar to central Au+Au in STAR.
PYTHIA 6.3
[email protected] WW San Diego, March 200622
Summary
Most recent NLO calculations by AKK using constrained fragmentation functions reproduce STAR and UA1 strangeness data nicely
Latest version of the PYTHIA model (6.3) describes strange particle and resonance data well if a LO K-factor=3 is used
Increase of <pT> of strange hadrons with Nch due to mini-jets & multiple scattering is successfully modeled in PYTHIA 6.3 with K-factor 3
In p+p collisions the anti-baryon/baryon ratio vs pT does not yet show any clear quark vs gluon jet signature due to limited statistics. In d+Au however a significant drop of the ratio is observed.
Perturbative QCD models are unable to reproduce the large baryon/meson ratio at intermediate pT
mT scaling of identified particles may be a useful tool for investigating quark vs gluon jets phenomena