# Presentation: 652458

## Probe the QGP via dihadron correlations: Jet quenching and Medium-response

- Jiangyong Jia
- Stony Brook University

- Medium

- Jet

- ?

## Particle production mechanisms

- 5

- 3

- RAA=A+A/p+p

- pT

- 1

- Jet

- Flow+Recombination

- Production mechanisms
- Jet fragmentation (>5 GeV/c) and Flow+Recombination

- 0.2

- High pT: surface emission

-  factor of 5

## Particle production mechanisms

- 5

- 3

- RAA=A+A/p+p

- pT

- 1

- Jet

- Flow+Recombination

- Production mechanisms
- Jet fragmentation (>5 GeV/c) and Flow+Recombination

- 0.2

- High pT: surface emission

- Low pT: bulk emission
- Locally thermalized QGP -> flow -> recombination

**T********T****thermal****+m<v>****2**** **

-  factor of 5

## Jet contribution via dihadron correlation

- 5

- 3

- RAA=A+A/p+p

- pT

- 1

- Jet

- Flow+Recombination

- 0.2

- High pT:  Jet quenching and Jet tomography.
- Low pT:   Dissipation of lost energy to medium
- How the energy of the 80% jet redistributed to low pT?
- pT scan:  Evolution of jet fragmentation and medium
- 		        response

## Azimuth correlation at high pT

- d+Au

- Au+Au 0-5%

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

- pT(assoc)>6 GeV

- Per-trigger yield

- Observed jet are those “do not” suffer energy loss.
- Near-side: surface emission
- Away-side: tangential emission (factor of 5 suppression)

## Azimuth correlation at high pT

- d+Au

- Au+Au 0-5%

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

- pT(assoc)>6 GeV

- Per-trigger yield

- Observed jet are those “do not” suffer energy loss.
- Near-side: surface emission
- Away-side: tangential emission (factor of 5 suppression)

- Direct proof that high pT hadrons are from vacuum fragmentation.

- But they do not directly constrains the energy loss processes.
- Calculations model dependent:
- 	qhat 1~10 GeV2/fm

## Average energy loss vs. absorption?

**pT**

**Yield**

- 	RAA alone can’t distinguish the two cases.  But,
- Case I:  Suppression factor depends on the spectra shape,
- 		Flatter spectra -> larger shift.
- Case II: Suppression factor almost independent of the spectra shape

- p+p

- A+A

## Consider only absorption term

- Due to longer pathlength for away-side jet,
- it always leads to IAA<RAA. Need shift term!

## Consider only Energy shift

- Per-trigger spectra

- Away spectra flatter than single spectra

## Consider only Energy shift

-    Flatter spectra and longer pathlength) compensated by bigger
-      fractional energy loss --> IAA ~ RAA
-    For -jet, pure absorption: IAA=RAA pure energy shift: IAA>RAA.
-    By combing IAA and RAA, one gain some sensitivity on energy loss

- nucl-ex/0410003

- 50% bigger

- Per-trigger spectra

- Away spectra flatter than single spectra

## Low pT: medium response to jet

- Near-side:  elongated structure in , enhancement in yield.
- Jet + medium-induced component (ridge)
- Away-side:  strongly modified shape and yield
- Suppressed jet (head region) + medium-induced component (Shoulder region)

- STAR

- PHENIX

## Away-side particle composition

- Similar shape for asso Baryon and asso Meson

- Jet frag.<Bayron/meson< bulk medium.

- W.Holtzmann

- Recombine into correlated pairs?

## Away-side energy dependence

- p+p

- CM<0.7

- Same pT cut, similar  window
- Head200 GeV          Head17.2 GeV
- Shoulder200 GeV  2.5x Shoulder17.2 GeV
- 		At SPS
- Smaller jet quenching          -> Less suppressed Head
- Smaller medium component -> Smaller Shoulder

- ||<0.35

## pT Scan: Competition between Jet and medium

- Suppression in HR, enhancement in SR.
- Jet shape become similar between pp and AuAu at high pT.

- Dip grows

- Jet emerges

- arXiv:0705.3238 [nucl-ex]

## Away-side modification pattern vs pT 

- Shoulder Region: Response of the medium

- Away-side modification pattern vs pT

- jet

- medium

## 1<pTA,B < 4 -> RHS<1   -> Shoulder region dominant!
pTA or B >5   -> RHS>1   -> Head region dominant!
pTA or B < 1  -> RHS~1

- 1<pTA,B < 4 -> RHS<1   -> Shoulder region dominant!
- pTA or B >5   -> RHS>1   -> Head region dominant!
- pTA or B < 1  -> RHS~1

- arXiv:0705.3238 [nucl-ex]

- pt,1 pt,2>5

- 1<pt,1 pt,2<4

- Competition between “Head” and “shoulder”.
- Suppression and enhancement

- pT Scan: Competition between Jet and medium

## Jet spectra slope at low pT 

- Near-side: flat with Npart (>100), increase with pTA.
- Dominated by jet fragmentation

- arXiv:0705.3238 [nucl-ex]

**2<p****T****A****<3**

**3<p****T****A****<4**

**4<p****T****A****<5**

- Mean-pT at intermediate pT (1<pTB< 5)

- Shoulder region: flat with Npart (>100), independent of pTA !
- Dominated by medium.
- Universal slope ~0.44 GeV/c, reflects property of the medium?

## Low trigger pT, decrease with Npart
Onset of jet quenching, soft contribution dominates (feed in from shoulder)

High trigger pT, flat with Npart
Soft contribution dies out, Jet dominate.

- Onset of jet quenching, soft contribution dominates (feed in from shoulder)
- High trigger pT, flat with Npart
- Soft contribution dies out, Jet dominate.

**STAR Preliminary**

- ||<0.4

- dn/d

- Fuqiang

- Jet spectra slope at head region

- Jet and medium dominates different pT

## Parton-medium interaction

- 1) Radiative energy loss                           -> High pT suppression

- 2) Radiated energy converted into flow      -> Low pT enhancement

- 4) Or propagating partons get deflected

- 3) Radiated energy propagate                   -> Gluon feedback at low pT

## Radiation contribution

- C. Salgado, U. Wiedemann,
- hep-ph/0310079

- I. Vitev, gluon feedback

- Can be large angle  => But for hard jets, radiation almost collinear

- Can explain multiplicity

- _ _

- Modifications decrease with increasing trigger pT (flattening)

- Modification limited to pTA,B  4 GeV/c, similar to the away-side Shoulder.

- STAR: This is due to the Ridge.

- Near-side yield modification: IAA

- J. Putschke

## Low pT : dilution effect

- IAA reflects modification on Pairs **√** and  Triggers **x**

## Are low pT particles from jets?

- Near-side jet pairs with one particle in 5-10 GeV/c.
- Most of these pairs comes from jet fragmentation. When normalized by 5-10 GeV/c trigger, Iaa ~1. But when normalized by low pT triggers, Iaa <1.
- Origins:
- 	1) low pT triggers from soft processes such as Thermal-T recombination.
- 	2) low pT triggers are jet-induced but don’t have high pT partners:
-                    fragmentation of radiated partons

## Near side JAA

- At high pT, both hadrons comes from same jet!
- JAA represent the suppression on the jet (>pt1+pt2). Since Jet suppression is constant at high pT, JAA should approach the constant RAA level at high pT!
- Low pT Jaa is factor of 4-5 of the high pT limit? (almost no suppression)
- These pairs are remnants of quenched jets (not from surface!)

## Both important at pT<4, softer than jet, similar PID chemistry 
(see Jana, Anne’s talk).
Mechanisms for Ridge and cone should play a role on both sides. 
They do not suffer surface bias.

- (see Jana, Anne’s talk).
- Mechanisms for Ridge and cone should play a role on both sides.
- They do not suffer surface bias.

- 

- 

-       Sources of pairs
-  Fragmentation contribution   from survived jets
-  Medium-induced contribution from quenched jets

- high pT pairs are rare

- Connection between Ridge and Cone

## Summary

- Jet correlation @ high pT provide constraints
- 	on the jet quenching and geometrical bias
- Jet correlation @ low pT shows complex evolution due to competition between Jet quenching and medium response on both near- and away-side.
- Models should describe the full pT dependence  (shape and yield).

## Backup

## Recombination is important

- Medium are boosted by shock wave, which then recombine into hadrons? => jet frag<Bayron/meson

- Cooper-Fryer

**Notes:**
The issue with near side

## Outline

- Single particle production mechanisms
- High pT: jet fragmentation and surface bias.
- Low pT: medium response
- Away-side properties.
- Away-side Jet and medium competition
- Near-side properties.
- Near-side Pair suppression
- Connection between near- and away-side medium response
- Summary

## Should I worry about non-flow in correlation?

- PHENIX: event plane measured at 3<|<4, tracks in ||<0.35

- Embed PYTHIA dijet into HIJING event to estimate the non-flow due to jets
- HIJING event is weighed with measured v2(pt,****,b)
- PYTHIA has 10 GeV dijet
- Dijet->Biased Event plane->Fake v2 for trigger of the embedded jets.
- Use away-side pp jet to approximate the ridge

- Ridge

- 

- Hijing+flow

## Should I worry about non-flow in correlation?

- PHENIX: event plane measured at 3<|<4, tracks in ||<0.35

- Embed PYTHIA dijet into HIJING event to estimate the non-flow due to jets
- HIJING event is weighed with measured v2(pt,****,b)
- PYTHIA has 10 GeV dijet
- Dijet->Biased Event plane->Fake v2 for trigger of the embedded jets.
- Use away-side pp jet to approximate the ridge

- Fake v2

**nucl-ex/0609009**

- Ridge

- 

- Hijing+flow

##  What v2 to use in correlation?

- C() = (1+2<v2tv2a>cos2) + J()

- Non-flow due to jet is small with BBC Event plane
- Other Non-flow and v2 fluctuations contribute to    C(), so should be included in the two source model.
- If minijets are important, then it should be much longer range in ,  or many minijets emitted in a correlated way?