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HotQuarks 2006 conference “How important are next-to-leading order models in predicting strange particle spectra in p+p collisions at STAR ?”. Mark Heinz (for the STAR collaboration) Yale University. Outline. Status of Models STAR vs Leading Order (PYTHIA) STAR vs Next-to-Leading Order (NLO)

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mark heinz for the star collaboration yale university

HotQuarks 2006 conference“How important are next-to-leading order models in predicting strange particle spectra in p+p collisions at STAR ?”

Mark Heinz (for the STAR collaboration)

Yale University

  • Status of Models
    • STAR vs Leading Order (PYTHIA)
    • STAR vs Next-to-Leading Order (NLO)
  • Quark vs Gluon fragmentation
  • Strange Baryon production in perturbative QCD (pQCD)

Hotquarks 2006, Sardinia, Italy

leading order vs next to leading pqcd

NLO (several theorists)

  • 2-2 and 2-3 processes
  • Parametrized Parton Distribution function using Deep inelastic Scattering (e+p) data
  • Parameterized flavor separated Fragmentation Functions from e+e- data
Leading-order vs Next-to-Leading pQCD
  • 2-2 processes only
  • Leading-Log approx-imation of higher order processes: Initial and final state radiation, Multiple scattering
  • Lund Symmetric String fragmentation


Hotquarks 2006, Sardinia, Italy

intro pythia leading order pqcd
Intro PYTHIA: Leading order pQCD
  • Parton showers based on Lund String Model
  • Universal fragmentation function for all collision systems
  • Strangeness and Di-Quark production is suppressed by model parameter

z = fractional long. momentum of hadron/parton

a, b = tunable parameter

Strange suppression: P(s)/P(u) = 0.3

Diquark suppression: P(qq)/P(q) = 0.1

Strange Diquark suppression:

[P(us)/P(ud)]/[P(s)/P(d)] = 0.4

Hotquarks 2006, Sardinia, Italy

p t spectra for strange particles
pT-spectra for strange particles
  • PYTHIA Version 6.326 used
    • Incorporates parameter tunes from CDF (tune A)
    • New multiple scattering and shower algorithms
  • Tune:
    • MSEL=1 (inelastic collisions)
    • K-Factor = 3 (higher order corrections)

Is a Flavour dependant K-factor necessary ?

Hotquarks 2006, Sardinia, Italy

what about other particles
What about other particles ?

Non-strange mesons and baryons

Strange Resonances

All this is published or submitted STAR data !

Pi/proton to 6.5 GeV: submitted PLB, nucl-ex/0601033

Phi/K* : PLB 612 (2005), PRC 71(2005)

Sigma*(1385): submitted PRL, nucl-ex/0604019







Hotquarks 2006, Sardinia, Italy

k factor in lo pqcd
2 Definitions:

Kobs= exp / LO

Kth= NLO / LO

In PYTHIA K-factor changes relative x-section of underlying parton processes


Eskola et al Nucl. Phys A 713 (2003)

K-factor in LO pQCD

PYTHIA 500’000 p+p events

No events

  • K-factor=3 has been observed previously for charged hadrons at s=200 GeV




Hotquarks 2006, Sardinia, Italy

nlo for non strange particles


Van Leeuwen, nucl-ex/0412023

NLO for non-strange particles
  • Albino, Kramer and Kniehl (AKK) use latest OPAL data to calculate light flavor (u,d,s) separated fragmentation functions for the first time.
  • Baryons show a large improvement with AKK FF.
  • EPOS achieves good agreement with data
  • Inclusive charged hadrons have been well described for the last 10 years by Fragmentation functions (FF) from Kretzer, KKP and others.

Largest uncertainty comes from flavor dependance of FF

Hotquarks 2006, Sardinia, Italy

nlo for strange particles
NLO for strange particles
  • First NLO calculations K0s and Lambda at 200 GeV were obtained privately from W.Vogelsang (BNL)
  • In 2005 calculations at NLO by Albino, Kniehl & Kramer (AKK) for K0s and Lambda produced better agreement by constraining gluon FF.
  • Normalization of Gluon Fragmentation function is constrained using STAR data

Largest uncertainty comes from Gluon FF

 important contribution in p+p

Hotquarks 2006, Sardinia, Italy

quark vs gluon fragmentation



Quark vs Gluon fragmentation
  • FF: Collider data available from 3-jet events from ALEPH and OPAL
  • PDF: DIS data from ie. ZEUS and H1
  • In both cases the gluon processes are least known

Gluon Fragmentation func.

Recent 3-jet data from OPAL


Gluon Distribution func.

Evolution of parameterizations


ALEPH (52 GeV)

OPAL (52 GeV)

AKK, Nucl.Phys.B725(2005)

How can we experimentally help constrain the Gluon FF ?

Hotquarks 2006, Sardinia, Italy

m t scaling of identified particles

Preliminary DATA

Gluon jet

Quark jet


mT scaling of identified particles

Arbitrarily scaled mT-spectra data and PYTHIA simulation agree well

  • 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.

Hotquarks 2006, Sardinia, Italy

baryon meson anomalies
Baryon-meson “anomalies”
  • PYTHIA also underpredicts the Baryon/meson ratio for higher energies at UA1, s= 630 GeV
  • PYTHIA cannot describe Baryon/Meson ratio at intermediate pT even with tuned K-factors. In addition di-quark probabilities need to be tuned.
  • Gluon Jets will produce a larger Baryon/Meson ratio than quark-jets in the region of interest

Hotquarks 2006, Sardinia, Italy

  • STAR data vs PYTHIA
    • PYTHIA version 6.3 describes STAR data for strange particles and resonances well if a K-factor =3 is used
    • Pions and protons agree best with K=1
  • Baryon enhancement
    • Strange baryon to meson ratio at intermediate pT cannot be reproduced with PYTHIA and K-factor tune.
    • The Diquark suppression parameter in the Lund fragmentation function needs to be adjusted to achieve agreement with data.
  • NLO calculations
    • Recent calculations by Albino et al. (AKK) using new flavor separated fragmentations functions reproduce STAR strangeness data nicely
    • STAR Lambda data constrains gluon fragmentation function
  • Mt-Scaling
    • Scaled mT–spectra of mesons and baryons exhibit different shapes observed in p+p data and PYTHIA model calculation.
    • This behavior is consistent with dominant particle production from gluon jets with respect to quark jets.

Hotquarks 2006, Sardinia, Italy


Hotquarks 2006, Sardinia, Italy

charged multiplicity distribution

STAR data

PYTHIA 6.3, K=3

Charged multiplicity distribution
  • Pythia + Simulated Trigger and detector acceptance.
  • Probability of high multiplicity events is very sensitive to NLO corrections

STAR Preliminary

STAR data


Hotquarks 2006, Sardinia, Italy

pythia p t vs n ch
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

Hotquarks 2006, Sardinia, Italy

ratios vs p t gluon vs quark jet


Ratios vs pT (gluon vs quark jet)

STAR (Phys Lett. B submitted)

  • 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 preliminary


Hotquarks 2006, Sardinia, Italy

consistency with data at 630 gev

Consistency with data at 630 GeV
  • How well does the constrained fragmentation function extrapolate to other energies?


UA1 (630GeV)

UA1 (630GeV)

STAR (200GeV)

STAR (200GeV)

Albino,Kniehl,Kramer et al. ,hep-ph/0510173

NLO Lines are for μ=2*pT, pT, pT/2

Hotquarks 2006, Sardinia, Italy