experimental review of high pt phenomena at mid-rapidity at rhic workshop on crititical examination...
TRANSCRIPT
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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
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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
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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:
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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.