# Presentation: 004085

## Three-Particle Azimuthal Correlations from STAR

- Jason Glyndwr Ulery
- Purdue University
- for the STAR Collaboration
- 2nd International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions
- 12 June 2006

## Motivation

- Jets are expected to be modified by the medium we create.
- We can therefore use jets to probe the medium.
- 3-particle correlations can give additional information.
- Can be used to look for mach cone.

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

## Conical Flow vs Deflected Jets

## Analysis Procedure

- Trigger particle selected with transverse momentum  3<pT<4 GeV/c.
- Look at Δ=Assoc-Trigger for all pairs of associated particles with 1<pT<2 GeV/c.
- Plot Δ1 vs Δ2 for each pair of associated particles.

- Δ

**Δ********1**

**Trigger**

**Δ********2**

## Change in the Raw Signal From QM

- Mesh present in the QM signal.
- This has been corrected using a single particle  acceptance correction.
- Increased statistics.

- QM 2005

- 1/NTrigger (dN/d(Δφ))2

- STAR Preliminary

## Analysis Procedure

- What we get contains (Jet+Bkgd) x (Jet+Bkgd).
- To get Jet x Jet we must subtract Bkgd x Bkgd and Jet x Bkgd (and Bkgd x Jet.)

- Δ1

- Δ

## Jet x Background (Hard-Soft)

- Top plot is 2-particle correlation.
- Red is Jet + Background
- Black is Background and open black is scaled background (such that Red - Open Black is zero around ±1.)
- Jet x Background term is created by folding 2-particle jet signal with 2-particle background.
- (Red – Open Black) x Open Black + Open Black x (Red – Open Black)

- Δ

- Δ1

- Δ2

## Background x Background (Soft-Soft)

- Term is constructed using a random trigger particle angle and looking at the pairs of associated particles in background events.
- Contains correlations between associated particles that are not associated with a trigger particle.

- Δ1

- Δ2

## Flow

- Previous term contains flow terms between the two associated particles but there are additional flow contributions that include trigger particle flow times associated particle flow.
- Top plot contains terms of v2Trigger*v2Associated.
- Bottom plot contains terms of v2*v2*v4 with v4 = 1.15*v22.
- v2 is taken as average of reaction plane and 4-particle measurements.

- Δ1

- Δ1

- Δ2

- Δ2

## Soft-Soft Including Flow

- These three background x background terms are summed and then scaled such that the projection of the final signal to either Δ axis will be zero in the range of |Δ±1|<0.2

- +

- +

- =

## Final Signal

- Final signal is constructed by subtracting the background from the raw signal.

**_**

- _

**=**

- Raw          –        Jet x Bkgd       –      Bkgd x Bkgd

- (Hard-Soft)

- (Soft-Soft)

## Change from QM

- Mesh pattern removed.
- Increased statistics by about a factor of 1.5.
- Analysis improvements.
- Qualitatively similar structure.
- Not much signal at 1 radian from π, but interesting structure maybe present beyond 1 along the off diagonal in both.

- φ2=φ2-φtrig

- Au+Au 0-10%

- Δ1

- Δ2

## Quantitatively...

- Large systematic fluctuation due to mesh.
- Mesh effect did not completely cancel out in differences between different areas; this was not taken into account in the systematics.

- φ2=φ2-φtrig

- Au+Au 0-10%

- Δ1

- Δ2

- away – deflected =
-    0.3 ± 0.3 (stat) ± 0.4 (syst)
- away – cone =
-    2.6 ± 0.3 (stat) ± 0.8 (syst)

- away – deflected =
-    -0.04 ± 0.06 (stat) ± 0.09 (syst)
- away – cone =
-    0.13± 0.06 (stat) ± 0.05 (syst)

## Signal Comparison

- Signal Comparison

*pp*

- d+Au

- Au+Au
- 50-80%

- Au+Au
- 30-50%

- Au+Au
- 10-30%

- Au+Au
- 0-10%

## Flow Systematic

- Flow is varied between the reaction plane result and the 4 particle cumulant result.
- Result is robust with the variation in v2.

- Reaction Plane v2

- 4 Particle Cumulant v2

## Background Normalization Systematic

- Normalization around Δ=±1 assuming zero yield at minimum.
- Default uses a normalization range of 0.35.
- Normalization range of 0.70 used to check systematic.
- Result is robust with respect to normalization range.

- Default

- Wide

## Signal Strengths

- Δ2

-   Evaluate signals by calculating average signals in the boxes.
-   Near Side, Away Side, Cone, and Deflected.

## Cone Position

- Optimum placement of boxes can be determined from varying the placement and from fits.
- Fit fails for
-     0-10% Au+Au.
- 1.3 radians from π was chosen.

- Radians from π

- Average Cone Signal

- Au+Au Central 0-12% Triggered

**Average Signal in 0.7x0.7 Squares**

- Stat. error only

## Signals

- Signals taken from average in 0.7x0.7 squares.
- Open symbols are ZDC triggered and are shifted to the left.
- Large source of systematic errors comes from uncertainty in flow.
- Another significant source is the width of the fitting region for background normalization.
- Signal consistent with mach cone at about π±1.3 in more central Au+Au collisions.

## Summary and Conclusions

- Analysis improvements in 3-particle correlations.
- Improved statistics.
- Elongation along the diagonal seen in *pp*, d+Au and peripheral Au+Au.
- Signal along the off-diagonal at about π±1.3 consistent with mach cone and/or Cherenkov radiation.

## Backups

## V4 Systematic

- V4 Systematic

- V4 is taken as 1.15*v22
- This is varied by ±20%.
- Has no significant effect on the result.

- v4*1.2

- v4*0.8

## Trigger Flow

- Trigger particle flow effect may be different for 3 particle correlations than for 2 particle correlations because of the additional associated particle, ie v2(trig+assoc)≠v2(trig)
- Take difference between v2(3.5 GeV/c) and v2(5.0 GeV/c) of 20% as an estimate.
- No significant effect on the result.

- Default

- Trigger Particle Flow*1.2

## Soft-Soft Systematics

- Soft-soft term is constructed from minbias data.
- The underlying soft-soft bkgd in triggered events may be different due to trigger bias.
- This systematic was checked using triggered event multiplicity distribution.
- No significant effect.

- Default

- Result from this check

## V2 of Central Trigger Data

- Currently do not have flow measurements for central trigger data.
- Default has same v2 as minbias data with the same centrality (from TPC multiplicity cut).
- This effect is checked using 5% most central v2 for the central data corresponding to the minbias top 5% and v2 values from 10-5% for the rest of the data.
- No significant change.

- Default

- This check