# Presentation: 665438

## PHENIX High-pT Results

*Hard Probes of Nuclear Collisions at RHIC*

## Outline

- Physics Motivation
- Why High-pT is important to Heavy-Ion Collisions
- Measuring High-pT production
- The PHENIX detector
- Calibrated measurements
- Results
- Single-particle spectra
- Inclusive RAA for Au+Au and Cu+Cu
- Differential measurements
- RAA with respect to the Reaction Plane
- Jets/di-hadron correlations
- Summary

## The Case for Relativistic Heavy-Ion Collisions

- Seeking the Quark Gluon Plasma (QGP)
- Existed early in the universe
- Expected to be created in RHIC collisions
- Lattice QCD predicts a transition to the QGP
- T ~ 170 MeV
-  ~ 1.0 GeV/fm3
- For QGP, strong interaction plays critical role
- Deconfinement of quarks and gluons
- Need to study properties and dynamics of this strongly interacting system

- F. Karsch, Prog. Theor. Phys. Suppl. 153, 106 (2004)

**QCD “Phase Diagram”**

## Why high-pT?

- Domain of hard scattering processes
- Partonic collisions are characterized by large Q2 (~pT2)
- Cross-sections are factorizable
- p+p
- Fragmentation into QCD-vacuum
- A+A
- Occurs early in reaction phase
- Probe for evolution of later hot
- and dense phase

## High-pT as hard probe of the medium

- Hard-scattering cross-section is large in high energy collisions
- Initial yields and pT distributions can be predicted from p+p measurements + pQCD + collision geometry + cold (initial) nuclear effects
- Critical to perform measurements in reference p+p and p(d)+A systems
- Observed deviations from reference measurements can be attributed to the medium
- Need observables that can shed insight even with large BG (dN/dy ~ 700)

- vs.

## 0, and ) important sources of information
They are identified particles
Test different aspects of the medium
Highest pT reach of all measured particles

**d+Au**

**Au+Au**

**Cu+Cu**

**200 GeV**

**130 GeV**

**62.4 GeV**

**22.4 GeV**

**Reference**

**sQGP ?**

**Comp.**

- Summary of high-pT data in PHENIX

********

****

**Current run**

- 0, and ) important sources of information
- They are identified particles
- Test different aspects of the medium
- Highest pT reach of all measured particles

## High-pT measurement with the PHENIX Detector

- Tracking and Matching (DC, PC)
- charged hadrons

- EMCAL (PbSc + PbGl)
- ,0,Ks

- EMCAL --- measure energy deposit of electrons and gammas
- DC --- obtain momentum of charged particles

- PC --- measure hit position of charged particles

- h+

- h-

## Do we observe hard-scattering?

- Single-particle spectrum and QCD predictions

**PHENIX Collab.**
**PRL91, 241803**
**hep-ex/0304038**

- p+p **** 0+X

**Peripheral spectra agree well with p+p **
**(data & pQCD) scaled by *****N******coll***

**Central data exhibits suppression!**

## 0 measurement in Au+Au and Cu+Cu

- _RHIC Run4 Au+Au data_
- Sampled 241 b-1
- 1972 x 241 b-1 ~ 1 pb-1 p+p equivalent
- 1.5 B events recorded

- _RHIC Run5 Cu+Cu data_
- Sampled 3 nb-1
- 632 x 3 nb-1 ~ 1 pb-1 p+p equivalent
- 2.2 B events sampled
- 59 M minimum bias events
- 1.9 M high-pT triggered

## 0 RAA in       = 200 GeV Au+Au

- 0 RAA in       = 200 GeV Au+Au

- Strong suppression (RAA ~ 0.2) is seen even at high pT
- Constant for pT>4GeV/*c* for all centralities

- Nuclear Modification Factor

## 0 RAA in sNN = 200 GeV Cu+Cu

- RAA ~ 0.4 for central collisions
- Similar suppression pattern as Au+Au
- Constant for pT>4 GeV/c
- Strength of suppression increases with centrality

**Cu+Cu**

## Au+Au vs. Cu+Cu

- _System size dependence_
- Centrality (in % bins) dependence differs between AuAu and CuCu
- For cent bins with ~ same number of participants, suppression pattern is identical!

- _Integrated R__AA_
- Similar qualitative Npart dependence
- Vitev: Fair agreement with AuAu, but misses trend in CuCu
- Fixed to AuAu, no surface effects
- Pantuev: geometrical model including “corona”
- Surface jet bias, ellipticity included

## Modeling RAA(pT) of Au+Au and Cu+Cu

- RAA for central Au+Au
- Suppression strong and nearly constant up to 20 GeV/c
- Consistent with *dN**g**/dy* ~ 1200
- RAA for central Cu+Cu
- Also suppressed to high-pT
- Consistent with *dN**g**/dy* ~ 370

## Direct Photons

- Prompt photons don’t interact strongly – ideal hard scattering probe
- Run 4 **∫**Ldt extends measurement to 20 GeV/c
- And RAA... ?

**high p****T**

**Direct Photon Spectra vs Centrality**

## Photons shine, hadrons don’t

- Mesons suppressed ≈ 5 → energy-lost in QGP
- Same suppression pattern for both 0, 
- scale with parton flux

- Drop possibly due to isospin difference  between p+p and A+A, other investigations underway

**First direct ******** R****AA**** using p+p data as reference!**

## 0 RAA 62.4 GeV : NEW!!

- “Old”: p+p reference from fit to ISR data
- “New”: p+p reference measured at RHIC!
- Suppression in 62.4 GeV is now more similar to that for 200 GeV

## Measuring RAA as fcn of angle wrt React Plane

- 0 RAA(,pT) plotted in polar coords (folded to 2)

- 0-10 % Centrality

- 30-40 % Centrality

- 50-60 % Centrality

- Au+Au sNN = 200 GeV

## 0 RAA vs. “L dL”

- Angular and centrality dependence described by single curve!! Implies energy loss is dominant at high-pT

**???**

## Azimuthal Correlations (Concept)

- Tag (high-pT) particle: “trigger” particle
- Calculate  for “associated” particles in event
- Expect to observe correlations at
-  = 0 : “near side”
-  =  : “away side”

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

## Azimuthal Correlations and Jet Suppression

- Azimuthal correlation functions show ~ complete absence of “away-side” jet
- pT,trig: 4-6 or 3-4
- pT,assoc: 2 - pT,trig

- Further evidence for surface emission only (?)
- That is, “partner” in hard scatter is absorbed in the dense medium
- Question: where has the energy gone?
- (Partial) Answer: need to widen “search” in pT

**Notes:**
Make sure to find out the pt ranges – this is the difference from the results that will follow.
Intro explanation of technique/consequences: the awayside jet disappears, but the energy has to go somewhere... but where????

## Studying the far-side production of particles

*D* scales with collision energy and system size!
- Emission consistent with strong medium response to energy deposited by the traversing medium

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

- Fit away-side peak with double Gaussian, each centered at
****** ****= ******** ****±**** *****D***

- PHENIX, nucl-ex/0611019

- 1<pt,assoc<2.5<pt,trig<4 GeV/c

## pT Evolution of the Jet Shape

- Evolves from flat  concave  convex
- Suppression in HR, Enhancement in SR
- Reappearance of peak is not due to merging of side peaks. High pT is consistent with jet fragmentation.

- Au+Au & p+p s = 200 GeV

- arXiv:0705.3238 [nucl-ex]

- Per-trigger yield vs. *** *for various trigger and partner *p**T** *(*p**A**T** ** p**B**T*), arranged by increasing pair momentum (*p**A**T** * + *p**B**T**)*

- Establish the punch-through jet component (“Head Region”)
- and the medium-induced component (“Shoulder Region”)

**Notes:**
Need to carefully explain what is in this plot.  Emphasize, maybe with an arrow, the progression from flat/concave/convex

## Yield modification (IAA)

- HR is suppressed due to jet quenching, SR is enhanced,
- reflecting the response of medium to deposited energy.

- Analogous to RAA, but describes jet yield

- HR exhibits early onset of suppression, relative to p+p
- H+S (entire away side) exhibits overall enhancement

## Future directions: -h correlations

- Normal dijet (o-h):
- Trigger bias: E**** < Ejet,
- Possible surface bias
- Direct  tagged jet:
- No fragmentation: E**** ~ Ejet
- No strong interaction, sensitive
-    to the whole medium
- Inclusive -h Yield
- Trigger photon is a mixture of
-    direct and hadron-decay photon
- Need to subtract decay part

**Proposed in hep-ph/9605213**
**~10 years ago**

**hot dense medium**

- 

- 

## -h: the method

*Reasonable agreement between data and Pythia*
- Pythia 6.1, kT = 2.5 GeV

**PHENIX Preliminary**

## Direct -h Yield in p+p and Au+Au

- First yield measurement in Au+Au!
*Hint* of away side modification in Au+Au
- Au+Au data systematically lower than p+p
- We have the tools, now we need the statistics ...

- Black: Au+Au   Red: p+p

## Summary

- High-pT particle production gives access to hard-scattering processes
- Important observables:
- Spectra with high-pT reach
- Suppression seen in RAA(pT) at high-pT
- Similar patterns in AuAu vs. CuCu for same *N**part*
- Hints for corona/surface effects
- Evidence for energy loss as dominant effect
- Azimuthal Correlations provide path to jet studies of medium
- PHENIX physics program has steady progressed beyond “discovery” stage, have begun addressing next level of questions:
- How are jets affected by passage through the medium created in RHIC collisions?
- How does the medium respond to the impact of a high-pT probe?
- Future directions involve mapping out properties of the plasma
- Results shown here only scratch the surface of the methods for studying the medium in PHENIX!

*Extras*

## Mass dependence at high pT

-  same quark content as 0
- Factor 4 heavier
- Same *R**AA* as 0
- Suppression does not depend on hadron-mass

- nucl-ex/0601037

## Measurements wrt the R.P. in PHENIX

- Reaction Plane determination
- Use the Beam-Beam Counters @ 3<||<4, azimuthally symmetric
- Measure charged particle multiplicity as function of 
- Event-by-event determination
- 2 independent measurements from N and S counters – estimate of resolution
- Relative Yields as a function of 
- Measure 0 dN/dpT in 6 ****** bins over [0,/2]. (corrected for resolution)
- Multiply the ratio
- With previously measured ***R******AA***** ******** *****R******AA******(************)***

- Reaction Plane

## Quantifying Evolution: Ratio of Head/Shoulder

- RHS is symmetric wrt pAT, pBT.
- Non-monotonic evolution with trigger and partner pT
- 1) RHS~1 for pTA,B < 1
- 2) RHS<1 for 1<pTA,B < 4
- 3) RHS>1 for pTA,B >5

- RHS characterizes the evolution of the two regions. It qualitatively shows the transition between Shoulder-dominated (concave) region and Head-dominated (convex) region in pAT and pBT.

**Notes:**
Move to backup

## Truncated mean-pT for 1<pTB< 5, vs. Npart

- Nearside: flat with Npart (>100), increase with pTA.
**S** region: flat with Npart (>100) and pTA.
**H** region: smooth decrease with Npart up to 200.

- Intermediate pT: shows a very different centrality and pTA dependence of the <pT> between HR and SR.
- HR behavior consistent with jet quenching
- SR however, shows an universal partner spectra shape, reflecting the property of response of medium to lost energy.

**Notes:**
Move to backup