Experimental Review of high pt

phenomena at mid-rapidity at

RHICWorkshop on Crititical Examination

of RHIC ParadigmsUT-Austin, Texas

14-17th April, 2010

Ahmed Hamed (Texas A&M University)

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Machine and Experiments

As in typical high energy physics experiments, detectors design is driven by the delivered

luminosity and DAQ.

At high pt

Spectra CorrelationsWealth of beautiful data

Contents:Contents:

The Road Behind “Spectra and Correlations”

Consulting the theory

Table of Contents and Disclaimer

Disclaimer: The road behind is personal view, so biases are expected.

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The Road Ahead

Baseline Initial-State Effects The RHIC Paradigm of Jet Suppression

Challenges to the Paradigm

Summary

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How many bodies are required before we have a problem?

In eighteenth-century Newtonian mechanics, the three-body problem was insoluble.

With the birth of relativity and QED, the two- and one-body problems became insoluble.

And within modern QFT, the problem of zero bodies (vacuum) is insoluble.

So, if we are out after exact solutions, no bodies at all is already too many!

R. D. Mattuck

G. E. Brown:History carries the answer

e-6pT

Power law

At mid-rapidity

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What are high-pt particles?

Power law spectra pT > 2 GeV/c Rare processes

p+p

P

P

High-pt particle“biased event”

What is the functional form of pt and s?

High pt particles:

P

P

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Hard Scattering:oHigh-pT particles are produced from the

hard scattering processes.

xP xP

High momentum

transfer

Why high-pt particles are interesting?

Direct access for the underlying scale

Rates are calculable via pQCD “Factorization”

p+p

xT~2Pt/s

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Methods of high pt particles?

Full jet reconstruction

Associated yield w/ high-pt particle ,

Fragmentation Function, IAA

Det

ecto

r

etcp

e

,,

,

Par

ticle

P

P

xP xP

Direct access for the underlying scale

Inclusive single high-pt particle spectra- Leading particle, RAA

“Golden channel” direct

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Baseline p+p “Spectra”

show good agreement with NLO pQCD predictions 0 and direct spectra

Phys. Rev. Lett. 98, (2007) 012002

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Baseline p+p “full jet”

STARSTAR

Jet cross section at mid-rapidity is

Consistent with world data

Well described by NLO pQCD

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Control experiment “d+Au”-Spectra

STARSTARSTARSTAR

show good agreement with NLO pQCD predictions 0 and direct spectra

arXiv:0912.3838

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Au+Au-Spectra

PRL. 96, 202301 (2006)

Direct photons follow binary scaling

0 and yields in central collisions are suppressed by factor of 5 compared to their yields in p+p

No suppression for charged hadrons in d+Au relative to p+p

PRL. 91 072304 (2003)

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Au+Au-Correlations

?

Suppression of the back-to-back correlation in central Au+Au

STAR, PRL 91 (2003) 072304 4<pT <6 GeV, pT > 2GeVtrg assoc

Surface-bias

Central Au+Au

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Theory Support Experiment

LPM-effect based approaches: BDMPS & AMY Opacity expansion: GLV & ASW

Medium enhanced higher twist effects Medium modified MLLA

assumes that factorization holds and extract medium parameters.

Extracted parameters indicate a medium formation with much higher energy density than that of CNM.

Phenomenological studies for RHIC data

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RHIC Paradigm of jet suppression

From qualitative to quantitative description, the challenges start!

Hadron suppression in central Au+Au is final-state effect

Suppressions took place at the partonic level

Hadron emission is surface-biased

Conjecture

Perform similar measurements for different particles, and at different kinematics regime.

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Challenges to the Paradigm-I

Heavy quarks suppress as much as light quark

PRL 98 (2007)192301

STARSTAR

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Challenges to the Paradigm-II

No sign for the color factor effect.

arXiv: 0804.4760

STAR preliminary

STARSTAR

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Challenges to the Paradigm-III

Tangential emission ?

8<pT <15 GeV , pT > 6GeVtrg assoc

Non interacting jets.

h

h

STARSTAR

8 < pT(trig) < 15 GeV/c

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Picture Evolution

Surface emission

h

h

Tangential emissionor Punch-throughh

No color factor effect

Re-appearance of away-side jet with no broadening

Suppression at the hadronic level

Different hadrons (0,) suppress to similar level

Suppression at the partonic level

Direct follows binary scaling

Away-side suppressed in central AuAu but not in vacuum

Final state effect and surface bias

Different partons (light, heavy) suppress to similar level

Final state effect and tangential emission

Do we really probe the medium?

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The Theory!

Medium

q

E

Lform << form >> Static Medium Dynamic Medium

Scattering power of the medium q̂ q2=2/

Independent successive scattering centers1/ <<

Relative phase:dErad/dz CR s E <q

2>dErad/dz CR s ln(E) <q

2>

dErad/dz L2 dErad/dz L

P( E) F( E, L, CR, f )

Conjecture of energy loss functional form

“independent variables”

The parton propagation is “time-ordered” and time-oredered perturbation theory is the natural framework to calculate the

radiation amplitude.Different models successfully describe the data with very different

medium parameters ( q~3-19 GeV2/fm).

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Hard Scattering in nuclear medium

Dvac

c/h(z)

p+p or peripheral

Au+Au

Hard Scattering in vacuum-QCD

.

ab

h

c

Central Au+Au

Gluon radiation is induced

by multiple scattering

Hard Scattering in the medium med

c/hD (z)

Hadron productions in nuclear medium can differ significantly from vacuum. Phys. Rev. Lett. 40, 1624

(1978).onPDFs do not spoil the requirements for factorization up to NLO according to DIS off nuclei and Drell-Yan process on

nuclear target, nPDF is universal.oWhat about the FFs in vacuum and in medium?

Seeking fragmentation free measurements!

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LO channels of dir

The dir-jet coincidence measurement is “golden probe” to study the parton energy loss in the medium created at RHIC.

The Road Ahead

Full jet reconstruction in minimum bias data

Uniform spatial distribution of the hard scattering vertex

h

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Parton Initial Energy Dependence of Energy LossSTARSTAR

STARSTAR

Different theoretical calculations describe the data within the current uncertainty

No significant dependence on the parton initial energy

0-10% 0-10%

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0-10%

STARSTAR

Path-length Dependence of Energy Loss

STAR measurement does not show path-length

dependence. PHENIX measurement

show path-length dependence.

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0 v2 (pt) and RAA() show statistically significant dependence on the path length particularly at pt <

6GeV

Path-length Dependence of Energy Loss

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Full Jet Reconstructions “Spectra”

First results of full jet reconstruction in heavy ion collisions

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Full Jet Reconstructions “Spectra”

Full energy NOT recovered, jet broadened

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Full Jet Reconstructions “Correlation”

No centrality dependent broadening observed within sensitivity

Significant suppression of recoil

Jets indicates broadening: Energy shifts to larger

cone radii (>0.4) or some Jets

“absorbed” in the limit

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Summary

1. Baseline and initial state effects seek improvements, eRHIC

3. More reliable theoretical calculations are required.

2. Measure the energy loss dependence on the independent variables separately

PHENIX, STAR: direct photons v2 at high-ptPHENIX, STAR: direct photons-h correlations

in- and out-of-planeSTAR, 0-h in- and out-of-plane.

5. Extend the kinematics range to lower zt

6. Full jet reconstruction is another promising tool for the parton energy loss

4. More differential observables are needed

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“No amount of experimentation can ever prove me right; a single experiment can prove me wrong”

If a model fit data, it is really great!

Albert Einstein

RHIC enjoys a plenty of beautiful data for very difficult problem in which vacuum means too many bodies, so careful and very critical

interpretations are required.

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Backup slides

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Challenges to the Paradigm-IV

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v2 of electromagnetically interacting particles.

Path length dependence of parton energy loss.

Why v2 of direct photons?

V2<0

V2>0

V2=0

x

yPhys.Rev. Lett. 90,132301

(2003)JETP Lett. 80,

1(2004)

Jet-medium photons

v2 < 0

v2 = 0

v2 > 0

Compton scattering Frag. photons

Production mechanisms of photons

v2 = 0 No preferred direction w.r.t. reaction plane

v2 < 0 Particles preferred to traverse through the longer path “out-of-plane”

v2 > 0 Particles preferred to traverse through the shorter path “in-plane”

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Previous measurements of direct photons v2 at RHIC Phys. Rev. Lett.

96.032302 (2006)

This measurements implied that

v2 of direct photons is ~ 0

PHENIX BBC: 3.1 < | | < 3.9

Measured 0 and inclusive : || < 0.35

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New Results : Direct -Triggered IAA vs. centrality

Ahmed M. Hamed QM2009 Knoxville, Tennessee

Theory describes the centrality dependence of the direct photon-triggered IAA

within the current uncertainties.The pT dependence (also in other centralities) does not support the volume vs.

surface emission picture within the measured range of pT associated.