# Presentation: 808993

## Strange particle production at RHIC...

***Anthony Timmins for the STAR Collaboration***
*Hot Quarks*
*26th August 2008*

**STAR**

## Contents

- Strangeness as a QGP signature...
- Strangeness production in light systems...
- Model comparisons at RHIC energies...
- Empirical scalings...
- Discussion...
- Summary...

## Strangeness as a QGP signature

- Original Predictions...
- Strange quark’s dynamical mass drops in quark gluon plasma (QGP)
- s+s production cross section increases
- s+s in QGP would reach thermal expectations on a small time scale...
- QGP saturation time s ~3 fm/c
- Hadron gas saturation times (*Phys. Rep. 142 (1986) 167*)
- ~30 fm/c for Kaons (K)
- ~200 fm/c Lambdas () and Xis ()
- Heavy-ion collision lifetime at RHIC < 10 fm/c (HBT)...

*Phys. Rev. Lett. 48 (1982) 1066 *

- Strangeness saturation times in QGP for various ***S*...

## Strangeness as a QGP signature

**hadronization**

**Prior to collision**

**Initial state**

**QGP and expansion**

**hadronic phase**

- ...one naively expects final state strange hadron yields to also reach thermal expectations...

- If strangeness reaches thermal expectations in a QGP...

## Strangeness as a QGP signature

*J. Phys. G32 (2006) S105*

*Phys. Rev. Lett. 98 (2007) 062301 *

- In heavy-ion collisions, we address this in two ways experimentally...
- Measure enhancement factor *E*=**strangeness yields per participant** relative to pp
- Compare to Canonical (thermal) model predictions...
- These assume volume  *<N**part**>*
- Perform a thermal fit on measured particle ratios and extract the **strangeness saturation factor ***********s***
- Deviation of strangeness production from thermal model expectations...
***s**=*1 strangeness fully equilibrated
***s**=*0 strangeness fully suppressed

**Notes:**
For mid-rapidity yields (dN/dy) at y~0

Canonical predictions reproduce various aspects of data for Au+Au 200 GeV

Rising per participant yields with rising <Npart>
Hierarchy with respect to strangeness content...

s rises with <Npart> and ~1 in central Au+Au 200 GeV

Good “thermal” fits to p+p- and e+e- show elementary QCD is statistical by nature...

## Strangeness production in light systems...

- AGS and SPS energies showed something interesting at lower energies for **lighter systems...**
- Kaons per *<N**part**>* higher in Si+Al, Si+Au compared to Au+Au with similar *<N**part**>* at ~5 GeV
- K+/ higher in lighter systems compared to expected Pb+Pb values at 17.3 GeV
- What happens at RHIC energies?
- Cu+Cu collisions can help us investigate...
- What will strangeness measurements tell us about the current theory?

*Phys. Rev. C 60 (1999) 044904*

* **Nucl. Phys. A 715 (2003) 474 *

**Notes:**
3) Subject of my the work,,,

## Strangeness production in light systems...

- Mid-rapidity Cu+Cu 200 GeV yields higher at given* <N**part**>*, contrary to Canonical predictions...

*STAR Preliminary*

## Strangeness production in light systems...

- Evidence for extra multi-strange particle production in central Cu+Cu compared to mid-central Au+Au...
** particle show **above unity enhancement** at 200 GeV
- Also contrary to naive interpretation of the Canonical model
-  E=1 for particles with closed strangeness...
- Differences between Cu+Cu Au+Au not observed within errors...

## Further model comparisons at RHIC...

**EPOS** (*Phys. Rev. Lett. 98 (2007) 152301*)
**Core** hadron production
- High density region follows statistical emission...
- Expands from initial state and freezes out at critical energy density ***C*=0.22 GeV/fm3 to produce hadrons
- Upon hadronization, strangeness oversaturated with ***s*=1.3
**Corona** hadron production
- Superposition of p+p collisions
- Uses successful p+p/d+Au EPOS generator which treat hadron production dynamically
- Strangeness therefore undersaturated with    s<1

- Participant density in central Cu+Cu 200 GeV collisions from MC Glauber

*STAR Preliminary*

**Notes:**
Two component approach
Freeze out energy close to lattice QCD values...
Gamma_s =1.3 chosen to fit data..

## Further model comparisons at RHIC...

**AMPT** (*Phys.Rev. C72 (2005) 064901*):
- HIJING based generator
- Soft hadron production is governed by nucleon excitation and string breaking..
- Hard hadron production from (mini) jets whose cross sections are governed by pQCD..
- Mini-jet partons and newly produced hadrons allowed to rescatter...
- AMPT results reduce to HIJING without such mechanisms...
- Describes various aspects of Au+Au 200 GeV data well...

- Central Au+Au 200 GeV collisions..

## Further model comparisons at RHIC...

**Further model comparisons at RHIC...**

- Both models reproduce key **qualitative** features of the data...

*STAR Preliminary*

## Further model comparisons at RHIC...

**Further model comparisons at RHIC...**

*STAR Preliminary*

- Predictions for multi-strange particles follow similar trends..
- EPOS does a better job for** **’s
- Predicts large enhancements for ...

## Empirical scalings...

**Empirical scalings...**

- , , K0S yields show similar systematics for Cu+Cu and Au+Au at 200 GeV
- Bulk strangeness production must follow suit...
- Approximate total strangeness production at* y*~0
*<N**part**>* scaling clearly fails for data and theory...
- No surprise there given the above definition..

*STAR Preliminary*

## Empirical scalings...

- Test to see if strangeness scales with number of binary collisions *<N**bin**>*...
- Hard processes not the dominant contribution to strangeness production ?
-  Kharzeev-Nardi hypothesis:
- Soft production scales with *<N**part**>*
- Hard production scales with *<N**bin**>*
***x ***is energy dependent relative weight of each process
- Fails for strangeness at RHIC...

*STAR Preliminary*

## Empirical scalings...

- Test to see if strangeness scales with **participants** that **undergo** more **than** **one** collision = *<N**part>1**>*...
- Data moves** most** towards a common trend compared to other scalings...
- EPOS lies on common trend
- AMPT nearly lies on common trend..

*STAR Preliminary*

## Discussion...

- Why does *<N**part>1**>* work best in the models?
- AMPT: **String Phenomenology**
- Valence partons in these participants get more pT kicks
- When intra-nucleon strings decay, strangeness production is higher compared to participants with just one collision
- EPOS: **Core Size**
- Core production major source of strangeness
- Core size scales with *<N**part>1**>* as these participants sit in high density region...
- In both models, these features lead to..
- Rising per participant yields for a given system
- Higher per participant yields in Cu+Cu at given <Npart>
- Both are reproduced by data...

*STAR Preliminary*

*STAR Preliminary*

**Notes:**
2) No surprise as these participants sit in the central region of the collision zone....

## Discussion...

- Key question: What do the current comparisons tell us about strangeness production as a QGP signature?
**EPOS**
- Core aims to represent deconfined matter with differing hadron production scheme compared p+p
- If strangeness equilibrates/oversaturates in QGP, not inconceivable that *********s ***will be >=1 upon hadronisation..
- Are there are other measurements that support core/corona approach?
**AMPT**
- Offers different and more dynamical understanding of particle production in heavy-ion collisions...
- Strangeness yield systematics explained via varying levels nucleon excitation E.g. number of participants with more than one collision
- In conflict with original hypothesis?
- Soft hadron production from independently decaying strings rather than deconfined coupled medium..
- Can these mechanisms also explain *********s*** ~1 in heavy-ion collisions?
- Both models require further comparisons with data to improve                     distinguishing power...

## Summary...

- Strangeness production in light and heavy systems continues to challenge our understanding of QCD in heavy-ion collisions...
- Collision geometry key factor in mid-rapidity strangeness production at RHIC
- Strangeness per *<N**part**>* increases with *<N**part**> *for given system
- Strangeness production higher in Cu+Cu collisions at given *<N**part**>*
- Both EPOS and AMPT models reproduce these aspects albeit with differing hadron production mechanisms..
- Fraction of nucleons that undergo more one collision appears key underlying parameter in each case...

## Backup..

- Kaon to Pion ratios at SPS

## Backup..