# Presentation: 023369

## Direct observation of dijets in central Au+Au collisions at RHIC 

**Direct observation of dijets in central Au+Au collisions at RHIC **

**Dan Magestro, The Ohio State University**
**for the STAR Collaboration**

- Quark Matter 2005

**Jets and azimuthal correlations**
**Raising the p****T**** scale**
**Dijet tomography of QCD matter**
**Modified picture of jet suppression**

**??**

- August 5, 2005

## The key theoretical question

**What is the nature of high-p****T**** suppression?**

- How is energy loss reflected in away-side suppression?
- What is the **relationship** between jet “associated” yields and inclusive spectra?
- Is there really a surface bias? Is there **near-side** energy loss?
- Can data distinguish between **different energy loss scenarios**? (e.g. collisional vs. radiative)
- What is the **path length dependence** of energy loss?
- .....

- Drees *et al*, PRC 71 (2005) 034909

## Evolution of  correlations at RHIC

- “Trigger-associated” technique valuable for tagging jets in high-multiplicity environment (vs. jet-cone algorithms)
- Probes the jet’s interaction with the QCD medium
- Provides stringent test of energy-loss models

_****__** **__**correlations**_

- Associated

- Associated

## Probing fragmentation region w/ pT

**Probing fragmentation region w/ p****T******

**Successful RHIC Run 4 (2004) ****→ ****~10x more central, minbias Au+Au**
- STAR event reconstruction ongoing
**Raising p****T**** thresholds allows to:**
- Extend into fragmentation region
- Significantly reduce uncorrelated background, relative v2 contribution
- Look for leading away-side hadrons

**How does ******** correlation evolve with increasing p****T****?**
- Does away-side jet “punch through”?
- Is the fragmentation modified? If so, how?
- Does away-side suppression disappear at very high pT?

**~pQCD**

- ??

##  Emergence of dijets with increasing pT(trig)

**Au+Au, 0-5%**

****** ****correlations (not background subtracted)**

**Hint of narrow back-to-back peak for higher p****T****(trig)**
- Higher pT(trig) reflects higher-Q2 hard scattering

##  Emergence of dijets w/ increasing pT(assoc)

****** ****correlations (not background subtracted)**

**Narrow peak emerges cleanly above vanishing background**

**p****T****(assoc) > 2 GeV/c**

**p****T****(assoc) > 3 GeV/c**

**p****T****(assoc) > 4 GeV/c**

**p****T****(assoc) > 5 GeV/c**

**p****T****(assoc) > 6 GeV/c**

**p****T****(assoc) > 7 GeV/c**

**p****T****(assoc) > 8 GeV/c**

## Centrality dependence of near, away-side yield

**Centrality dependence of near, away-side yield**

**Near-side yields consistent within errors**
**Away-side yields decrease monotically with increasing N****Part**
- Suppression pattern similar for two pT(assoc) ranges!

- Fit scaledby x2

- 8 < pT(trig) < 15 GeV/c

## Initial physics thought on dijets 

**Are we seeing “tangential” dijets?**

- Red lines determined from constraining ratio of b+c to total to measured RAA

*C. Loizides: Poster #78 (Thur-Fri)*

**a**

**b**

**c**

- Hard scattering takes place at surface, with neither parton traversing matter
- Dainese *et al*: use inclusive suppression to estimate probability for parton(s) to escape

**From fit to R****AA****, probability is vanishingly small for both partons to escape**

- L1

- L2

- L1

- L2

- L1

- L2

- A. Dainese *et al*, EJP **38** (2005) 461

## Hadron-triggered fragmentation functions

**Xin-Nian Wang: Modified di-hadron fragmentation function**
- Quantify energy loss in terms of away-side particle production per trigger particle

- Without knowledge of **parton energy**, Dhh(zT) is next best thing

- zT = pT(assoc)/pT(trig)

- pT(assoc)

## Hadron-triggered fragmentation functions

**Away-side D(z****T****) suppressed, but shape unchanged**

- ~0.54

- ~0.25

**Scaling**
_**factors**_

- 8 < pT(trig) < 15 GeV/c

## Dijet assoc. yields (IAA) vs. RAA

**Dijet assoc. yields (I****AA****) vs. R****AA**

- STAR, Phys. Rev. Lett. 91 (2003) 072304

**Near-side yields consistent with unity**
**Away-side associated yields similar to R****AA**** values**

## Confronting predictions - 1

**Confronting predictions - 1**

**Majumder *****et al:***** **_**Near-side energy loss**_** affects trigger bias**
- Trigger carries smaller <z> of higher-E parton → more energy for associated production, IAA >> 1 as pT(assoc) increases
**X-N Wang: **_**Away-side fragmentation**_** altered relative to p+p**
- Drops steeply below zT~0.4, roughly constant above zT~0.5

- Majumder, Wang, Wang, nucl-th/0412061

- X-N Wang, PLB 595 (2004) 165

**Majumder *****et al*****: Near-side energy loss affects trigger bias**
- Trigger carries smaller <z> of higher-E parton → more energy for associated production, IAA >> 1 as pT(assoc) increases
**X-N Wang: Away-side fragmentation altered relative to p+p**
- Drops steeply below zT~0.4, roughly constant above zT~0.5

**No evidence for near-side E-loss; away-side data more suppressed**

## Similarity of away-side widths

**Similarity of away-side widths**

**Widths similar**
- No evidence for away-side  broadening that is observed at lower pT

- 8 < pT(trig) < 15 GeV/c

**4 < pT(trig) < 6 GeV/c**
**pT(assoc) > 1.5 GeV/c**

*J. Ulery, STAR (Parallel 3c)*

## Confronting predictions - 2

**Vitev: Away-side broadening due to gluon radiation, to high p****T**
- Broadening due (entirely?!) to radiated gluons

- Vitev, hep-ph/0501225

- A. Dainese *et al*, EJP **38** (2005) 461

## Combining the observations

**Combining the observations**

- 

**Dijets more suppressed from d+Au to central collisions**

**Away-side fragmentation pattern unchanged**

- 

- 

**Away-side widths similar for central, noncentral**

## Theoretical basis for noninteracting dijets

**vacuum fragmentation**

**radiated gluon(s)**
**fragmentation**

**parent parton**
**fragmentation**

**Theoretical basis for noninteracting dijets**

- PRC 71 (2005) 014903

**E-loss models contain noninteracting term**
- Wang, Wang: Modified fragmentation functions

- Salgado, Wiedemann: Discrete term in quenching probability

- PRL 89 (2002) 092303

**no medium-induced energy loss**

**medium-induced energy loss**

**First observation of **_**unmodified**_** fragmentation?**

## The beginnings of dijet tomography

- 8 < pT(trig) < 15 GeV/c

- 6 < pT(trig) < 8 GeV/c

- preliminary

- preliminary

**What will they teach us about energy loss?**
- What is the **nature** of jet quenching?
- What are the properties of induced **radiation**?
- Does **collisional energy loss** need to be reconsidered?

**These are the first precision measurements of dijets in the fragmentation regime in Au+Au collisions at RHIC**
- Dijet suppression driven by **geometry** (system, system size)
- Near-side jet fragmentation in Au+Au **consistent** with p+p
- Away-side fragmentation **suppressed**; shape, peak widths **unchanged**
**Away-side I****AA**** quantitatively consistent with R****AA**

**What will they teach us about the medium?**
- Mechanisms leading to partonic equilibration?
- Determine significant **upper bound** on the gluon density?
- Idea: Constrain degrees of freedom?  Müller, Rajagopal, hep-ph/0502174

## Potential back-up slides 

## An upper bound on energy density?

**M****üller and Rajagopal: From entropy and jet quenching to deconfinement  **hep-ph/0502174
- The number of degrees of freedom can be determined independently of direct measurements of temperature

- Energy density:

- Entropy:

- Degrees of freedom:

- A lower bound on (T) is constrained by an upper bound on energy density

## Reminder: charge ordering in b2b paper 

**Charge ordering showed jetlike nature of near-side correlation**
- Comparison to Pythia showed agreement

**STAR, PRL 90 (2003) 082302**

## Dijets: Like vs. unlike sign (first look)

**Strong near-side charge ordering; away-sides consistent**
**Comparison to Pythia ****→ generic information about parton?**

**3 < p****T****(assoc) < 4**

**4 < p****T****(assoc) < 5**

**5 < p****T****(assoc) < 6**

**p****T****(assoc) > 6**

- preliminary

- 8 < pT(trig) < 12 GeV/c

## Hadron-triggered F.F. vs. Pythia

**Pythia agrees in magnitude with d+Au on away-side**

- 0.54

- 0.25

**Scaling**
_**factors**_

- preliminary

##  correlations at 62 GeV

**High-p****T**** rate reduced ****→**** far fewer pairs in dataset**

- Away-side peak emerges in central events, _**not**_ present for similar trig/assoc thresholds at 200 GeV !
- Weak near-side peak due to harder fragmentation

- preliminary

##  correlations: a solid foundation

****** ****correlations: a solid foundation**