# Presentation: 670352

## Anomalous centrality variation of minijet angular correlations in Au-Au collisions at 62 and 200 GeV from STAR

**Anomalous centrality variation of minijet angular correlations in Au-Au collisions at 62 and 200 GeV from STAR**

- Michael Daugherity
- University of Texas at Austin
- for the STAR Collaboration

## Overview

- These are sensitive to ***mini*****jets**, elliptic flow, resonances, HBT, etc. allowing a novel comparison of correlation amplitudes and ranges
- Each correlation source has a **unique distribution **on relative (η,φ) making decomposition possible
- We observe a surprising trend in ***minijet correlations***

**Overview**

**minijet: ****Same-side jet-like correlations with *****no trigger particle***

- M. Daugherity, STAR Collaboration

##  Correlation Measure

- We use the correlation definition

-    and measure pair density ρ
- Covariance = object - uncorrelated reference *(mixed-event pairs)*
- The denominator provides ***per-particle**** *normalization:
-  Correlations are directly comparable ***regardless of event multiplicity***
-  This is done with ***all possible pairs***
*** ******No trigger particle ***is specified.

- M. Daugherity, STAR Collaboration

## Proton+Proton Components

**STAR Preliminary**

**Proton+Proton Components**

**Longitudinal Fragmentation** (“strings”) 1D Gaussian, US pairs
**HBT** peak at origin, LS pairs

**Minijets** 2D Gaussian at origin,
- away-side peak actually cos(φΔ)

**J of Phys: Conf  27 (2005) 98**

**These structures *****define***** minijet correlations **

- M. Daugherity, STAR Collaboration

***axial *****correlations**

**η****Δ**

**φ****Δ**

## Fit Function (in 5 Easy Pieces)

**Fit Function (in 5 Easy Pieces)**

**Proton-Proton fit function**

**=**

**+**

**STAR Preliminary**

**η****Δ**

**φ****Δ**

**η****Δ**

**φ****Δ**

**η****Δ**

**φ****Δ**

- M. Daugherity, STAR Collaboration

## 200 GeV Data

**proton-proton**

- note: 38-46% not shown

**We see the evolution of correlation structures from peripheral to central Au+Au **

**η****Δ**

**φ****Δ**

**η****Δ**

**φ****Δ**

- Analyzed 1.2M minbias 200 GeV Au+Au events, and 13M 62 GeV minbias events (not shown) Included all tracks with pT > 0.15 GeV/c, |η| < 1, full φ

**STAR Preliminary**

- M. Daugherity, STAR Collaboration

## 200 GeV Model

**200 GeV Model**

**η****Δ**

**φ****Δ**

**η****Δ**

**φ****Δ**

**Fit model**

**STAR Preliminary**

- M. Daugherity, STAR Collaboration

## 200 GeV Residual

**200 GeV Residual**

**Fit residual = data - model**

- We have a good fit with the ***simplest possible*** fit function.  Other than adding the cos(2φΔ) quadrupole term, no other modification was necessary.
- Residuals at 62 GeV are comparable.

**η****Δ**

**φ****Δ**

**η****Δ**

**φ****Δ**

**STAR Preliminary**

- M. Daugherity, STAR Collaboration

## Quadrupole Component

**Quadrupole Component**

**Data**

**cos(2φ****Δ****) component**

- Instead of removing a *background*, we can **make a measurement**

-  62 and 200 have the **same shape**
-  *Substantial *amp. change* *with energy

**200 GeV**
**62 GeV**

**η****Δ**

**φ****Δ**

**η****Δ**

**φ****Δ**

- M. Daugherity, STAR Collaboration

**Amplitudes**

## Minijet Same-Side Peak

**Minijet Same-Side Peak**

**Observations**
-  Amplitude and η widths start small and experience a ***sharp transition***
-  Transition occurs at ~55% centrality at 200 GeV, is more central (~40%) for 62
-  φ width has a *very* *different centrality dependence*

- STAR Preliminary

- X-axis shows mean participant path-length

**200 GeV**
**62 GeV**

- Statistical and fitting errors shown
- Systematic error is 9% of correlation amplitude

- STAR Preliminary

- STAR Preliminary

**Peak Amplitude**

**Peak ****η**** Width**

**Peak ****φ**** Width**

- M. Daugherity, STAR Collaboration

## Binary Scaling

**Binary Scaling**

*** ******Deviations from binary scaling represent new physics unique to heavy ion collisions ***

- minijet amplitude assuming binary scaling in Kharzeev and Nardi model

***Large increase ***in amplitude and η width are unexpected

**200 GeV**
**62 GeV**

**small increase before transition**

**constant widths**

- STAR Preliminary

- STAR Preliminary

- STAR Preliminary

**Peak Amplitude**

**Peak ****η**** Width**

**Peak ****φ**** Width**

- M. Daugherity, STAR Collaboration

## HIJING Minijets

- The observed minijets correlation is actually ***far greater*** than predicted by HIJING (factor of 4)

**HIJING 1.382 default parameters, 200 GeV, quench off**
- Quench on causes slight amplitude *decrease*

**200 GeV**
**62 GeV**

- STAR Preliminary

- STAR Preliminary

- STAR Preliminary

**Peak Amplitude**

**Peak ****η**** Width**

**Peak ****φ**** Width**

- M. Daugherity, STAR Collaboration

## Consistency Check

- Does interaction between same-side peak and cos(φΔ) terms cause the transition?

***The results are consistent***
- Cancellation in fit terms does not cause the amplitude increases.

_**Two-stage fit:**_

**STAR Preliminary**

- M. Daugherity, STAR Collaboration

## Transition

**Transition**

*Does the transition from narrow to broad **η**Δ** occur quickly or slowly?*

**data - fit (except same-side peak) **

**Shape changes little from peripheral to the transition**

***The transition occurs quickly***

**83-94%**

**55-65%**

**η****Δ**** width**

- STAR Preliminary

- STAR Preliminary

- M. Daugherity, STAR Collaboration

## Scaling

- What is the best way to compare different energies?  Does the transition scale?

***Minijet parameters seem to scale with system density***

- Peripheral bins are compressed.

- M. Daugherity, STAR Collaboration

## Yield Estimates

- M. Daugherity, STAR Collaboration

*See also T. Trainor, arXiv:0710.4504, accepted to J Mod Phys E *

## Summary

- We measure 2D angular autocorrelations on (η,φ) to study minijets in Au+Au collisions and make two primary observations:
**Transition**
-  Minijet correlations follow binary scaling in peripheral Au+Au collisions and deviate at a ***sharp transition point***
*** ***The transition points for 62 and 200 GeV occur at about the ***same value of transverse particle density***
**Yield**
-  Beyond the transition point the peak amplitude and η width ***increase dramatically***
*** ***The same-side correlations include a ***large number ***of hadrons, estimated at ***up to ~1/3 of the particle yield ***in central Au+Au collisions

- M. Daugherity, STAR Collaboration

## Backup Slides

## Yt-Yt Correlations

**Yt-Yt Correlations**

- Corresponding to excess minijet amplitude and width, we see a sudden onset of localized growth on *y*t < 2.5 → pt < 0.8

**proton-proton**

- By looking in yt-yt space we can find where these new correlations are in momentum

- M. Daugherity, STAR Collaboration

## Away-side

**η****Δ**

**φ****Δ**

-  Momentum conservation for an (other-wise) independent-particle many-body system produces a cos(φΔ) component whose amplitude is constant for our measure.
-  Instead, the amplitude exactly follows the minijet peak amplitude

**We have found the away-side jet!**
- Instead of suppression we can now study momentum transport

**STAR Preliminary**

- M. Daugherity, STAR Collaboration

## Away-side

- Since φ is periodic, any AS Gaussian at φΔ=π will also need to copied to φΔ= -π and φΔ= 3π.
- I tried replacing cosine term with Gaussians at odd multiples of π, and letting ROOT freely fit the amplitude and width.   The result was indistinguishable from a cosine.

-  The data *demand* a cos(φΔ) **shape**.
-  A model with away-side Gaussians is statistically indistinguishable, though it has one extra parameter.

- M. Daugherity, STAR Collaboration

## Longitudinal Fragmentation

**Longitudinal Fragmentation**

- But from studying the **charge-dependence**, we know that we are seeing two distinct processes.
- longitudinal fragmentation: soft hadronization process, charge-*dependent, *energy*-independent, *monotonically decreasing with centrality.
- A second process that is charge-*independent, *energy*-dependent, *non-monotonic with centrality.  Seems to have constant mean at nu ~ 3.5, width decreases with energy
- *The other charge-dependent results are interesting, but that’s another story.

- We follow the 1D Gaussian along ηΔ which is seen in p-p collisions and described by the Lund String Model

**62**
**200**

**amplitude**

**width **

- M. Daugherity, STAR Collaboration

## Finer Centrality Bins

- As a test, I tried offsetting and doubling the number of centrality bins around the step for 200 GeV.
- Here the red points are the “fine bins” test:

- Fine bins test, 200 GeV, all parameters

- Let’s zoom in on the minijet parameters:

- Results are consistent and support a sudden turn-on

- M. Daugherity, STAR Collaboration

## Why minijets?

- We *hypothesize* that the transition is due to particles associated with minijets rather than a new physical process
- The ΦΔ width doesn’t change at the transition
- The pT correlation amplitude trend doesn’t change, the widths change slightly

- M. Daugherity, STAR Collaboration

## What about the sharp peak? 

- Many different processes compete near φΔ = ηΔ = 0:
-  e+e- pairs from γ conversion (big)
-  HBT / Coulomb correlations (big)
-  resonance decay (small, <10% of peak)
-  tracking inefficiencies: splitting, merging, crossing, etc. (small)
-  high pt jets (very small for minbias)
-  plus other structures that we want

- Four general approaches:
- kill these structures with cuts
- use simulations to model the big contributors
- ignore the bins near the origin
- fit a sharp exponential

- I tried them all and got the sensible result:
- No matter how you handle the bins near the origin the large-scale structure doesn’t change
*Or, *what happens at the origin, stays at the origin...

- M. Daugherity, STAR Collaboration