11/1/06William Horowitz 1 Jet Quenching at RHIC and the LHC William Horowitz Columbia University...

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11/1/06 William Horowitz

Jet Quenching at RHIC and the LHC

William HorowitzColumbia UniversityNovember 1, 2006

With many thanks to Simon Wicks, Azfar Adil, Magdalena Djordjevic, and Miklos

Gyulassy.

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Outline

• What a difference the LHC makes!– HUGE disagreement over LHC

predictions– Why there’s a difference– Why we’re right (hopefully robustly)

• P0, P0, P0…

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LHC Predictions

WH, S. Wicks, M. Gyulassy, M. Djordjevic, in preparation

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K. J. Eskola, H. Honkanen, C. A. Salgado, and U. A. Wiedemann, Nucl. Phys. A747:511:529 (2005)

A. Dainese, C. Loizides, G. Paic, Eur. Phys. J. C38:461-474 (2005)

BDMPS-Based Predictions

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Suppression of BDMPS– LHC predictions require an extrapolation from

RHIC• Their pQCD-based controlling parameter qhat must

be nonperturbatively large to fit RHIC data-pQCD gives qhat = c 3/4, where c ~ 2; they require c ~ 8-20 for RHIC

-Needed because radiative only energy loss (and Pg0 > 1?); R = (1/2) qhat L3


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BDMPS Extrapolation to the LHC

• Importance of medium density– qhat ~ scatterers

– EKRT used => LHC ~ 7 RHIC

– qhat goes from 14 at RHIC to 100 at the LHC!

– Almost all the energy loss is in (-1) part of P()

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11/1/06 William HorowitzWH, S. Wicks, M. Gyulassy, M. Djordjevic, in preparation

Asymptopia at the LHCAsymptotic pocket formulae:Erad/E 3 Log(E/2L)/EEel/E 2 Log((E T)1/2/mg)/E

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LHC Production Spectra

• Much flatter power law and asymptotic jet energies allows for easy interpretation of LHC predictions

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LHC Conclusions• LHC appears to reach jet

asymptopia where pocket formulae hold

• Lack of fragility means pions will make a good, independent probe of the density

• With current predictions, the momentum dependence of RAA at LHC should distinguish between BDMPS and GLV type loss models

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A Quick Update on Fragility

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Jets as a Tomographic Probe

• Requires:– Theoretical understanding of

underlying physics (esp. quenching mechanisms)

– Mapping from the controlling parameter of the theory to the medium density

– Sensitivity in the model + data for the measurement used (FRAGILITY???)

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Recall the BDMPS-based Plots



The lack of sensitivity needs to be more closely examined because (a) unrealistic geometry (hard cylinders) and no expansion and (b) no expansion shown against older data (whose error bars have subsequently shrunk

(a) (b)

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Our Jets Probe the Volume and are Sensitive to the Medium


S. Wicks, WH, M. Gyulassy, and M. Djordjevic, nucl-th/0512076

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BDMPS with Realistic Geometryis Not Fragile!

T. Renk and K. J. Eskola, hep-ph/0610059

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Heavy Quark Puzzle

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Before the e- RAA, the picture looked pretty good:

– Null Control: RAA()~1

– Consistency: RAA()~RAA()

– GLV Prediction: Theory~Data for reasonable fixed L~5 fm and dNg/dy~dN/dy

Y. Akiba for the PHENIX collaboration, hep-ex/0510008

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But with Hints of Trouble:

• Theory v2 too small

• Fragile Probe?

A. Drees, H. Feng, and J. Jia, Phys. Rev. C71:034909 (2005)(first by E. Shuryak, Phys. Rev. C66:027902 (2002))


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What Can Heavies Teach Us?

• Provide a unique test of our understanding of energy loss– Mass => Dead Cone => Reduction in E

loss

Bottom Quark =

(Gratuitous Pop Culture Reference)

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Entropy-constrained radiative-dominated loss FALSIFIED by e- RAA

Problem: Qualitatively, RAA~ e- RAA

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Inherent Uncertainties in Production Spectra

M. Djordjevic, M. Gyulassy, R. Vogt, S. Wicks, Phys. Lett. B632:81-86 (2006)

How large is bottom’s role?

– Vertex detectors could de-convolute the e- contributions

N. Armesto, M. Cacciari, A. Dainese, C. A. Salgado, U. A. Wiedemann, hep-ph-0511257

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The BDMPS-Z-WS Approach

• Increase to 14 to push curve down

• Fragility in the model allows for consistency with pions

N. Armesto, M. Cacciari, A. Dainese, C. A. Salgado, U. A. Wiedemann, hep-ph-0511257

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What Does Mean?

We believe it’s nonperturbative:– = .5 => dNg/dy ~ 13,000

R. Baier, Nucl. Phys. A715:209-218 (2003)

“Proportionality constant ~ 4-5 times larger than perturbative estimate”


“Large numerical value of not yet understood”

U. A. Wiedemann, SQM 2006

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Is this Plausible?Renk says No

T. Renk and K. J. Eskola, hep-ph/0610059

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Our Results

• Inclusion of elastic decreases the discrepancy

• Direct c and b measurements required to truly rule out approaches


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LHC Predictions for Heavies


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Conclusions III

– Elastic loss cannot be neglected when considering pQCD jet quenching• Coherence and correlation effects between elastic

and inelastic processes that occur in a finite time over multiple collisions must be sorted out

• Fixed must be allowed to run; the size of the irreducible error due to integration over low, nonperturbative momenta, where > .5, needs to be determined

– Large uncertainties in ratio of charm to bottom contribution to non-photonic electrons• Direct measurement of D spectra would help

separate the different charm and bottom jet dynamics

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Backup Slides

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Our Extended Theory

• Convolve Elastic with Inelastic energy loss fluctuations

• Include path length fluctuations in diffuse nuclear geometry with 1+1D Bjorken expansion

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Significance of Nuclear Profile

• Simpler densities create a surface biasHard Cylinder Hard Sphere Woods-Saxon

Illustrative Only! Toy model for purely geometric radiative loss from Drees, Feng, Jia, Phys. Rev. C.71:034909

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– Null Control: RAA()~1

– Consistency: RAA()~RAA()

Y. Akiba for the PHENIX collaboration, hep-ex/0510008

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Elastic Can’t be Neglected!

M. Mustafa, Phys. Rev. C72:014905 (2005) S. Wicks, WH, M. Gyulassy, and M. Djordjevic, nucl-th/0512076

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Length Definitions– Define a mapping from the line integral

through the realistic medium to the theoretical block

– where

– Then

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Geometry Can’t be Neglected!

• P(L) is a wide distribution– Flavor

independent

• Flavor dependent fixed length approximations LQ’s not a priori obvious



Comparison to Vitev

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Increasing dNg/dy

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Our Extended Theory

• Convolve Elastic with Inelastic energy loss fluctuations

• Include path length fluctuations in diffuse nuclear geometry with 1+1D Bjorken expansion

• Separate calculations with BT and TG collisional formulae provide a measure of the elastic theoretical uncertainty

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• Qhat = c eps^3/4 \propto rho (density of scattering centers)

• pQCD=> c~=2• To fit RHIC, c ~ 8-20• Extend to LHC, everything crushed

to nothing

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BDMPS RHIC ’s



Huge qhat needed!

(a) (b)

Note: fragility due to lack of Bjorken expansion

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11/1/06William Horowitz 1 Jet Quenching at RHIC and the LHC William Horowitz Columbia University...

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