# Presentation: 501065

## Trigger Particle Correlations at STAR

- Jason Glyndwr Ulery
- 21 June 2005
- Purdue University
- For The STAR Collaboration
- RHIC and AGS Annual Users Meeting

## Outline

- Motivation
- Charged Hadron-Hadron Correlations
- Near Side Δη-Δφ Correlations
- Away Side Δφ Correlations Systematics
- Correlations with Identified Pions, Protons, Ks and Lambda
- Summary

## Motivation I

- To create and study QGP
- A state of deconfined, thermalized quarks and gluons predicted by QCD at high energy density.

- F. Karsch, hep-lat/0401031.

**Lattice ****Q****C****D**** prediction:**

- Jets
- Can be calculated from pQCD thus are well calibrated in vacuum.
- Are expected to be modified by the medium and therefore can give information on the properties of the medium

## Motivation II

- A parton traverses and interacts with the medium, causing modification to its fragmentation
- Study this modification by looking at jets.
- Thermalization can be addressed.
- Additionally look at identified trigger particle jets to study the baryon/meson puzzle.

- STAR, nucl-ex/0501016

## Correlation Method

- A trigger particle is chosen within a particular pT range.
- Look at angular distributions
- 	Δη=ηassoc-ηtrigger
- 	Δφ=φassoc-φtrigger
- Mixed events are modified by v2
- average of reaction plane and 4 particles cumulant
-  Then normalized to the valleys of the Δφ distributions around |Δφ|=1
- Systematic uncertainties primarily from
- V2
- Background normalization

- AuAu Year 4  4<PTTrig<6
- 0.15<PTAssoc<4.0  20-10%

- STAR Preliminary

- Δφ

- 1/NTrig dNch/dΔφ

- Raw Signal

- ⁭⁭⁭

- Signal – Flat Mixed Events

- ↓

- V2

- ↑

- Signal – Mixed Events and V2

## d+Au and pp Comparison

- Excellent match-up of d+Au and *pp* trigger particle correlations.

- Similarity between d+Au and *pp *trigger particle correlation function and associated particle spectra.
- This implies that jet suppression is not an initial nuclear effect.
- d+Au can be used as a reference.

- 4<PTTrig<6 GeV/c
- 0.15<PTAssoc<4 GeV/c

- STAR Preliminary

- STAR Preliminary

- Δφ

- 1/NTrig dNch/dΔφ

- 4<PTTrig<6 GeV/c

## Au+Au, pp and d+Au

- Higher associated pT
- Suppressed away side yield.
- Inclusive associated pT
- Enhanced yield on both sides.
- Broadened away side width.

**0.15<P****T****Assoc<4.0 GeV/c**

**2.0<P****T****Assoc<P****T****Trig GeV/c**

**4.0<P****T****Trig<6.0 GeV/c**

## Δη and Δφ Correlations

- Δη and Δφ Correlations

- Differences present in the near side structure.
- Δη for particles  |Δφ|<1
- Au+Au stays above the *pp* on Δη peak tails.

- STAR nucl-ex/0501016

- Near

- Away

- Trigger

- 4<PTTrig<6 GeV/c

## Near Side Δη - Δφ Correlations

**3<P****T****Trig<6 GeV/c**
**2<P****T****Assoc<P****T****Trig GeV/c**

- Au+Au 5-0%

- d+Au 100-40%

- Au+Au seems to sit on a pedestal in Δη.

- D. Magestro (STAR), Hard Probes 2004

- STAR Preliminary

- STAR Preliminary

## Near Side Widths and Yield

- Significant broadening for low pT triggers for near side Δη.
- Same Gaussian Δη peak yield for all collision types.
- Underlying reasons not fully understood.

- D. Magestro, Hard Probes 2004

- STAR Preliminary

## Away Side

- Away side has a much softer spectra in Central Au+Au than *pp*.

-   Mean pT on the away side approaches inclusive in central collisions  Partial thermalization

- STAR, nucl-ex/0501016

- STAR, nucl-ex/0501016

## Δφ Number and PT Correlations

- Number correlations
- pT weighted correlations
- Mean pT by taking the ratio of these two.
- More robust b/c some systematic errors cancel

- 4<PTTrig<6 GeV/c
- 0.15<PTAssoc<4 GeV/c

## Novel Feature in Mean PT

- Dip in mean pt at the center of the away side in central Au+Au.
- Stronger for lower trigger pT
- Middle of away side jet appears more thermalized with the medium.
- Conical Flow?
- 	Stoecker, nucl-th/0406018.Casalderra, Shuryak, Teaney, hep-ph/0411315.

## Centrality & PT Dependence

- Peripheral Au+Au similar to *pp*.
- Away side broadens and flattens in more central collisions.
- Away side enhanced at low pT and suppressed at high pT.
*pp away side narrows with p**T*
*Au+Au away side broadens with p**T*
*conical flow?*

- STAR Preliminary

**0.15<P****T****Assoc<0.5**

**0.5<P****T****Assoc<1.0**

**1.0<P****T****Assoc<1.5**

**1.5<P****T****Assoc<2.0**

- 4.0<PTTrig<6.0

- 0.15<PTAssoc<4.0

- Δφ

## Unidentified Correlation Summary

- d+Au and *pp are very similar*
*Near side *Δη and Δφ studied
- Δη pedestal in Au+Au
- Δη Gaussian yield consistent between collisions
- Away side Δφ studied
- Broader in Δφ
- Softer mean pT, partial thermalization
- novel dip in mean pT distribution

## Baryon/Meson Puzzle

- Greatly enhanced baryon over meson ratio in central Au+Au relative to *pp*.
- Ratio nicely described by coalescence/ recombination.
**But** coalescence  no angular correlations
- Identified trigger particle jets can give insight into this puzzle.

- PHENIX, PRL 91, 172301 (2003)

- M. Lamont (STAR), QM2004

- ?

## Particle Identification in TPC

- Particle identification from TPC dE/dx distribution after the relativistic rise.
- Fit with Gaussians fixing kaon yield from measured π+/k0s.
- Select high dE/dx particles as pions and low dE/dx particles as protons/anti-protons.
- NSigma π = (dE/dx-predicted π dE/dx) /resolution

- NSigma π

- Log dE/dx

- Year 2 AuAu Central

- STAR Preliminary

- e
- π
- K
- p

- Log pT

- 75% Proton **←**

**→** 95% π+

## d+Au Identified Trigger

- Correlation functions for all four trigger particles are similar.

- STAR Preliminary

- STAR Preliminary

- 3<PTTrig<4

- 4<PTTrig<6

- 0.15<PTAssoc<3.0

- 2.0<PTAssoc<3.0 GeV/c

- 2.0<PTAssoc<4.0 GeV/c

- Δφ

- Δφ

-  75% Proton
-  75% Anti-Proton
-  95% π+
-  95% π-

- 0.15<PTAssoc<4.0

## Au+Au Identified Trigger

**3<P****T****Trig<4** GeV/c Au+Au Central Year 2
- Trigger particle v2 is obtained by
- v2baryon/v2meson=3/2
- v2baryon*yieldbaryon +  v2meson*yieldmeson = v2charged*yieldcharged
- Correlation functions look similar between different trigger particle species.

- STAR Preliminary

- Δφ

- 0.15<PTAssoc<3.0 GeV/c

- 2.0<PTAssoc<3.0 GeV/c

- Δφ

## Identified Trigger Particle Spectra

- d+Au MB

-   Spectra for 3<PTTrig<4 GeV/c
-   No significant differences seen between trigger particle types.

- STAR Preliminary

- STAR Preliminary

- Near Side

- Away Side

-  75% Proton
-  75% Anti-Proton
-  95% π+
-  95% π-

- Au+Au Central

## Strange Identified Trigger

- Λ,Λbar and Ks as the trigger particle.
- Correlations are fit to 2 Gaussians  + flow + constant.

- Y. Guo (STAR), RIKEN Jet Correlations Workshop

-                                         Δφ

- Λ+h Correlations for Au+Au 200 GeV/c

## Strange Trigger Particle Spectra

- Near side spectra.
- 3<PTTrig<6 GeV/c
- Spectra shape is fairly independent of trigger particle species.

- Y. Guo (STAR), RIKEN Jet Correlations Workshop

- 5-0%

- 50-10%

## Identified Trigger Summary

- Leading π+, π-, p, pbar, Λ, Λbar, and Ks studied.
- No significant differences between correlation functions with leading baryons and mesons despite significant increase of baryon/meson ratio in central collisions.
- Jet fragmentation appears to play important role at intermediate pT.

## Conclusion and Summary

- Two particle angular correlation systematics were studied on both the near and away side.
- Near side shows Δη as Gaussian + pedestal
- Away side
- softer pT and partial thermalization
- broadens with multiplicity and associated pT
- dip at π in <pT> as a function of Δφ
- Identified trigger particle correlations were examined.
- Little difference between different trigger particle species seen.