STAR

Strangeness Production in Heavy-Ion Collisions at STAR

Anthony Timmins for the STAR Collaboration

Introduction.. Bulk strangeness production… Mid-pT spectra…

Hard processes and strangeness… Summary…

STAR

2/13

Introduction Strangeness production helps us

explore many processes…

– low q2

Strangeness enhancement Flow Recombination

– high q2

Jet quenching Flavor changing

New STAR data in this talk

– Cu+Cu 62 GeV

– High-pT p+p 200 GeV

– High-pT Au+Au 200 GeV

STAR Preliminary

STAR

3/13

Strangeness enhancement key measurement at 200 GeV

Old idea (Phys. Rev. Lett. 48 (1982) 1066)

– s+s production cross section increases in QGP

– s+s in QGP reaches thermal expectations on small time scale…

s ~3 fm/c (QGP) s ~20-200 fm/c (hadron gas)

Can it be understood via phase space restrictions?

– Canonical Formalism Strangeness in p+p suppressed

relative to A+A? (Phys. Lett. B486 (2000) 61)

Bulk strangeness productionAs a QGP signature…

STAR

4/13

Bulk strangeness production Enhancement at 200 GeV… New d+Au 200 GeV data (not shown)

– See Xianglei Zhu’s talk later in session

Cu+Cu and Au+Au and enhancements (E) approximately described by simple function:

Predicts…

– Rise in E with <Npart>

– Higher E for central Cu+Cu

– Above 1 E for d+Au 200 GeV

Fraction of participants that undergo multiple collisions

STAR Preliminary

K0S B = 2.10

B = 2.45

B = 5.05

B = 13.1

Multiple Collisions

Single Collisions

f=1/4

STAR

5/13

Bulk strangeness production dN/dy at 62 GeV…

STAR Preliminary

No p+p 62 GeV reference data yet

– Can still investigate dN/dy per <Npart>

and K0Sper participant

yields can be described by

– C f(Npart) + D

Reproduces qualitative features of yields per <Npart>

Turning to multi-strange…

Cu+Cu K0S and : Stat errors only

Au+Au K0S and : Stat+Sys errors

New Cu+Cu 62 GeV data

Cu+Cu and : Stat errors only

Au+Au and : Stat errors only

STAR

6/13

Bulk strangeness productionWhat about the ?

Enhanced in heavy-ion collisions at 62 and 200 GeV…

has closed strangeness

– Canonical Formalism predicts no enhancement…

– Sits between Kaon and Lambda values at 200 GeV...

Also can be described by:

62 GeV B = 1.2

200 GeV B = 2.9

arXiv:0810.4979v1

QuickTime™ and a decompressor

are needed to see this picture.

STAR

7/13

Bulk strangeness productionWhat does this scaling mean? Proposed by F. Becattini and J. Manninen to be indicative of

core-corona effects (arXiv:0811.3766v1)

– Participants with one collision = corona

– Participants with multiple collisions = core

Strangeness equilibrated in core

– B particle density

Strangeness p+p like in corona

Alternative core-core model EPOS approximately reproduces scaling

– As does AMPT (string+rescattering)….

STAR

8/13

Mid-pT spectraBaryon/meson ratios at Au+Au 200 GeV…

Strange baryon/meson anomalies established at STAR

– Quark coalescence? Mid-pT baryon production

preferable to meson production

Coalescence models reproduce qualitative features of ratios

– p/ /K0

S

/

Predict turnover…

– Fail to get pT position right…

Sarah Blyth, Quark Matter 2006

STAR

9/13

Mid-pT spectraBaryon/meson ratios at 62 GeV…

New Cu+Cu 62 GeV data…

High strange baryon/meson ratios observed for both systems– Coalescence occuring at lower energy?

STAR Preliminary

STAR

10/13

Mid-pT spectraComparisons to EPOS…

EPOS Core

– Flow induced, gives higher mass particles pT kick at mid-pT

EPOS Corona

– Dominates production at high-pT

62 GeV

200 GeV

STAR Preliminary

Energy =

Are high baryon/meson ratios unique to coalescence?

STAR

11/13

Hard processes and strangenessRAA of identifed particles…

New high-pT p+p and Au+Au 200 GeV data…

– p+p shown Yichun Xu’s talk in Ridge Phenomena and Jet Flavor Conversion

Parton flavor conversion prediction (Phys. Rev. C77 (2008) 054902)

– Hard scattered partons interact with medium, convert flavour

– Boosts kaon RAArelative to pion RAA

Data consistent with conversion scenario…

STAR preliminary

pT (GeV/c)

STAR

12/13

Hard processes and strangenessRAA vs. system size… Measure integrated RAA for pT > 5.5 GeV/c

– <pT> ~ 6.2 GeV/c

Ratio

– Above one for all centralities

– Strangeness enhancement at high-pT!

In particular, RAA (K) > RAA () for Au+Au where <Npart> ~ 20

– Does parton flavor conversion happen in small systems?

– Can we dismiss the Cronin effect?

– Do we have to push higher in pT for parton conversion searches?

New high-pT p+p and Au+Au 200 GeV data…

STAR

13/13

Summary Bulk strangeness production

– Enhancement for many particles appears to depend on fraction of participants that undergo multiple collisions

Expected in core-corona scenario Also reproduced by AMPT

Mid-pT spectra– Coalescence typically describes trends– Core-corona+flow can also explain strange high baryon/meson ratios

Hard processes and strangeness– Hint of jet conversions in central Au+Au

Kaon RAA higher than pion RAA – Also applies to small systems

Can jet conversions happen there also? Could other effects contribute?

STAR

14/13

Hard processes and strangenessFlavor changing…

Prediction: hard scattered partons interact with medium, convert flavour– Boosts Kaon RAArelative to non-conversion scenario

Kaon RAA factor of two higher at RHIC– Excellent PID for K0

S at STAR…– LHC predicts modest increase

STAR

15/13

Mid-pT spectraQuark coalescence in AMPT…

Phys. Rev. C 78 (2008) 034907

Use particle ratios and constituent quark scaling

– Extract shape of strange and up/down quark spectra

Use shapes as input for AMPT’s quark distributions

– Run in coalescence mode…

Good description of , and spectra in central Au+Au 200 GeV collisions

– Shows quark spectra shapes can be extracted accurately…