# Presentation: 940408

## High-pt probes of  the quark-gluon plasma: STAR/PHENIX results at RHIC

- Craig Ogilvie, Iowa State University

- What happens to a high-pt parton as it travels through a quark-gluon plasma (QGP)?
- What does this tell us about the QGP?
- Not much time on how QGP responds to the hard parton.

## QCD Phase Diagram

- Within sQGP, momentum transfers ~ T ~ few 100 MeV/c
- 	=> coupling large
- Non-peturbatively interacting plasma of quarks and gluons
- 	far from an ideal gas

## One way to probe the sQGP

- Craig Ogilvie

- _hard-scattered_
- _parton during Au+Au_

- Hadron distribution changed
- 	- singles spectra
- 	-2-particle correlations
- 	- jet-structure
- Information on the plasma?

- _hard-scattered parton: _
- calc. with perturbative QCD

- high pt

- jet of hadrons

- p

- p

## Hard-scattering as a calibrated probe

- Large scale that makes perturbative QCD applicable:
- high momentum transfer Q2
- Assume factorization between
- perturbative hard part 
- universal, non-perturbative
- parton distribution functions (fA, fB)
- fragmentation (Dh/c) functions
- from e++e-, p+p....

- Sep 22, 2008

- Craig Ogilvie

## √s=200 GeV, p+p => x    NLO QCD agrees well with data

- Craig Ogilvie

- √s=200 GeV, p+p => x    NLO QCD agrees well with data

- D. d’Enterria
- nucl-ex/0611012

## Partons lose energy as they travel through QGP

- Craig Ogilvie

- 0 spectra at √s=200 GeV
- Phys.Rev.C76:034904,2007
- PHENIX

- p+p cross-section scaled by # of nucleon collisions in Au+Au
- Fewer high-pt 0 in Au+Au
- Energy-lost by parton => info on opacity, density of QGP

**E****loss**

## Au+Au 0 RAA

- Craig Ogilvie

- Au+Au 0 RAA

- Suppressed

- Enhanced

## Elliptic asymmetry at high-pt

- Sep 22, 2008

- Craig Ogilvie

- Asymmetry, v2, dN/d=A(1+2v2*cos(2))

## Particles correlated with high-pt trigger

- Craig Ogilvie

- A+A

- Correlation survives high-multiplicity environment of A+A

- p+p, PHENIX
- PRD, 74 072202 (06).

- STAR PRL **97** (2006) 162301

-  = 1-2

## Suppression of far-side hadrons

- Sep 22, 2008

- Craig Ogilvie

- Far-side yield per trigger
- sensitive to relative energy lost by both partons
- => Alphabet of observables, D(zT), IAA, JAA, ...

## Use measurements to learn about QGP

- Wiedermann: models of gluon radiation, transport parameter
- Note
- Hard-scattering takes place throughout collision volume
- Data and models average over wide range pathlengths...
- Medium expands rapidly
- Parton travels through a medium whose density decreases

- Sep 22, 2008

- Craig Ogilvie

- Scattering power of the QCD medium:

## Comparison data + models (e.g. PQM)

- Craig Ogilvie

- But no model uncertainty yet

- PRC 77, 064907 (2008)

## Strong energy-loss

- Craig Ogilvie

- Experimentalist’s reaction:
- 1) Reduce averaging over path-length
- 2) Other observables to check understanding of energy-loss

- Large <q>
- => high momentum transfer => strong QGP coupling

## Change average over path-length: RAA versus reaction plane

- Change average over path-length: RAA versus reaction plane

- Sep 22, 2008

- Craig Ogilvie

- Model T. Renk, vary Eloss
- PHENIX prelim centrality 20-30%
- 0 6<pt<7 GeV/c

- Overlap zone is elliptical:
- More energy-lost as parton travels through long-direction of ellipse
- RAA smaller out-of-plane

- This and other models fail, yet reproduce RAA vs pt
- Need stronger variation of Eloss for different paths, or
- Sharper early spatial distribution of energy density, or
- More rapid variation of q with , or ......

## Test for other mechanisms of energy-loss:

- Sep 22, 2008

- Craig Ogilvie

- S.Wicks, W. Horowitz, M. Djordjevic M. Gyulassy (**WHDG**) nucl-th/0512076

- Radiation
- due to scattering

- Elastic collision,
- energy re-distribution

- Many calcs on relative importance
- Probe via high-pt heavy-quarks
-  smaller energy loss after elastic collision
-  gluon radiation also reduced
- interference during radiation
- dead-cone effect

## Heavy-Quark Energy-loss

- Sep 22, 2008

- Craig Ogilvie

- Semi-leptonic decays of charm
- beauty mesons, R. Averbeck

- Strong suppression of high-pt charm

- PRL**98**, 172301 (2007)

## Radiative+collisional energy-loss models struggle

- Sep 22, 2008

- Craig Ogilvie

- STAR Phys. Rev. Lett. **98** (2007) 192301

## Strong resonance interaction in-medium

- Sep 22, 2008

- Craig Ogilvie

- Heavy-quarks may form resonances in sQGP near Tc

- H. van Hees et al, PRL 100, 192301 (2008)

## Separation charm/beautyPHENIX VTX, STAR HFT 

- Craig Ogilvie

- Separation charm/beautyPHENIX VTX, STAR HFT

- silicon pixel+strip detectors
- Tracks extrapolate back to collision
- Displaced vertices
- => charm (D), beauty (B)
- Requires ~  50 m precision

- Great hardware opportunity for post-docs

- _[Next Steps](slide23.xml)_

- _[Conclusion](slide27.xml)_

## Medium Response: Low-pt far-side

- Sep 22, 2008

- Craig Ogilvie

- STAR arXiv:0805.0622v1, PHENIX PRL 98, 232302 (2007)

- Trigger: 3 < pT < 4 GeV/c  Assoc: 1 < pT < 2 GeV/c,

**Growing evidence for conical medium response**
- 	STAR: 3-particle correlations
- 	PHENIX :angle of conical emission independent of pt
- 		=> not bremstrahlung

## No clean separation between medium-response and fragmentation?

- Sep 22, 2008

- Craig Ogilvie

- Shocked medium contributes
- to fragmentation
- e.g. coalescence of protons from
- shower+medium partons
- => Additional high-pt protons
- R. Hwa

## Protons

- Fragmentation to protons enhanced by combining with shocked medium?
- and/or
- As parton propagates in medium it can change flavor => energy-loss comparable for gluons and quarks,

- Sep 22, 2008

- Craig Ogilvie

- More protons than pions at high-pt, even out to 10 GeV/c

- Liu, W. and R.J. Fries,
*PRC 2008. ****77 054902***

- _[Conclusion](slide27.xml)_

## Next Steps (I/III): Excitation Function

- Does q return to perturbative QCD at LHC?
- Evaluate q at SPS (in progress)
- Low-E RHIC, onset of strong opacity?

- Sep 22, 2008

- Craig Ogilvie

- Pion gas

- perturbative interacting QGP

- Cold nuclear matter

- RHIC

- LHC?

- SPS?

- _[Conclusion](slide27.xml)_

## sNN Dependence: pT Dependence of 0 RAA in Cu+Cu

- 62.4, 200 GeV:
- Suppression consistent with parton energy loss for *p*T > 3 GeV/*c*
- 22.4 GeV:
- No suppression
- Enhancement consistent with calculation that describes Cronin enhancement in p+A
- Parton energy loss starts to prevail over Cronin enhancement between 22.4 and 62.4 GeV

- PHENIX, arXiv:0801.4555 [nucl-ex]

- _[Conclusion](slide27.xml)_

- Sep 22, 2008

- Craig Ogilvie

## Next Steps (II/III) Measure E directly

- Sep 22, 2008

- Craig Ogilvie

- Direct  to tag energy
- RHIC-II, LHC

- _[Conclusion](slide27.xml)_

## Next Steps (III/III): Fragmentation within jet

- Jets with pt-cut off
- Higher-pt => LHC, RHIC-II

- Sep 22, 2008

- Craig Ogilvie

## Conclusion

- Energy-loss as high-pt parton travels through QGP
- Opaqueness parameter, momentum transferred/length
-         larger than expected pQCD, => strongly coupled QGP
- Puzzles
- RAA vs , modeled Eloss too flat => stronger spatial variation?
- Large heavy-flavor Eloss => quasi-resonances near Tc?
- Proton RAA closer to 1 => shocked medium recombining?
- Next steps
- Excitation function of          , LHC, SPS, low-E RHIC
- Eloss via -h and reconstructed jets

- Sep 22, 2008

- Craig Ogilvie

## Backup

- Craig Ogilvie

## Protons in jets

- For each proton trigger, number of mesons starts to decrease
- Additional source of protons, e.g. from medium response

- Sep 22, 2008

- Craig Ogilvie

- proton trigger particle
- with N mesons on near side

## Ratio of (Au+Au)/(scaled p+p spectra)

- Craig Ogilvie

- Mesons suppressed  5 → energy-lost in QGP
- scale with parton flux

- Drop possibly due to
- isospin difference
- p+p and A+A

## RAA 0, , , J/,  Mesons and Direct 

- Same suppression pattern for 0 and :
- parton energy loss and fragmentation in the vacuum
*R*AA for ‘s larger than 0 *R*AA for 2 < *p*T < 5 GeV/*c*

## Fate of Gluons

- Craig Ogilvie

- Nathan Grau, Hua Pei

- Do radiated gluons
- produce broader jet?

- Jets broader in Cu+Cu than p+p
- Fragmentation of induced gluon radiation?

## Far-side Production of Particles

- Craig Ogilvie

- Observation of particles produced
- ~1 radian away from back-to-back!

- Fit with 2 Gaussians, each D radians away from 
- D scales with system size
-      => emission consistent with medium’s response to jet

- PHENIX preliminary
- nucl-ex/0611019
- 1<pt,ass<2.5<pt,trig<4 GeV/c

## Response of medium to passage of high-pt parton

- Craig Ogilvie

- Near-side, generation of ridge => strength large  (STAR talk)
- Far-side: does super-sonic parton generate a mach-cone ?

- hep-ph/0410067;
- H.Stocker...
- Jorge Casalderry-Solana

## Conical? flow –  QM08: B. Cole

- Cone? angle does not change appreciably as a function of pT of trigger or associated hadron.
- Or centrality, or angle *wrt* reaction plane

**Beware: PHENIX measurement from 2 particle, STAR 3 particle**

## Baseline p+p 0 spectra compared to pQCD

- Craig Ogilvie

- Baseline p+p 0 spectra compared to pQCD

- parton distribution functions, for partons a and b
- measured in DIS, universality

- perturbative cross-section (NLO)
- requires hard scale
- factorization between pdf and cross section

- fragmentation function
- measured in e+e-

- Phys. Rev. Lett. 91, 241803 (2003)

## Fragmentation within jet: J. Putschke

- Sep 22, 2008

- Craig Ogilvie

## Strongly Interacting QGP

- Sep 22, 2008

- Craig Ogilvie

- S. Gupta QM08

## LHC predictions: Xin-Nian Wang

- Sep 22, 2008

- Craig Ogilvie

## Quantitative Comparison: Stefan Bass

- Jet Energy-Loss in a 3D Hydrodynamic Medium #

- define local transport coefficient along trajectory  for all three approaches and compare initial maximum value q0:

- for ASW, use Baier formula:
- For AMY use:

| q0 [GeV2/fm] | ASW | HT | AMY |
| --- | --- | --- | --- |
| T | 10 | 2.3 | 5.5 |
|  | 20 | 4.5 | X |
| s |  | 3.4 | X |

- (all values quoted for a gluon jet)

- different medium scaling can affect q by a factor of 2
- need higher precision data and theory advances to provide guidance for *proper* medium scaling