# Presentation: 752395

## Elastic, Inelastic and Path Length Fluctuations inJet Tomography

- Simon Wicks
- Hard Probes 2006
*Work done with William Horowitz, Magdalena Djordjevic and Miklos Gyulassy*
*arXiv: nucl-th/0512076 and in preparation*

*Re: The single electron puzzle at RHIC*

## Introduction – The single electron puzzle

- Pion suppression predictions seem to work well ...

- DGLV predictions: **Djordjevic**, Gyulassy, Vogt, Wicks
- Phys Lett B 632:81-86 (2006)

- GLV predictions for pions compared to PHENIX data
**Vitev** prediction – plot from T. Isobe nucl-ex/0605016

** ... ****but (entropy constrained) pQCD radiative only FALSIFIED by e****-**** data.**

## Possible Solutions

- Fit the data with ‘free’ parameters, explain the magnitudes later ...
*Improve understanding of pQCD production spectra **(but still cannot match data with entropy bounded radiative energy loss alone)*
- Use novel non-perturbative techniques to explain extra charm / bottom quenching
- Fully explore all perturbative energy loss techniques.
- Q: Can pQCD explain the pion and electron suppressions at RHIC, or do we have to resort to solution III above?

## What have we swept under the rug?

- Elastic energy loss:
- - Same order of magnitude as radiative energy loss for the kinematic region of interest.

- M. **Mustafa**, Phys. Rev. **C72**:014905 (2005)                          SW, W. Horowitz, M. Djordjevic M. Gyulassy, nucl-th/0512076

## Elastic energy loss

- Finite time effects on elastic energy loss **SMALL**
- 	(see poster by A.Adil, nucl-th/0606010, talk and nucl-th/0603066 by M.Djordjevic, nucl-th/0604040 by X-N Wang)
- Interference between collisional and radiative energy losses left as open question. See later ...

- Thoma-Gyulassy (TG) vs Braaten-Thoma (BT) as estimate of uncertainty of leading log approximation
*Romatschke-Strickland infinite time / Djordjevic finite time calculations lie ~ TG for bottom, ~BT for charm (note unphysical energy gain at low p**t** for bottom in BT)*

## Path length fluctuations

- Our model: NOT fragile, NOT surface emission (nor is DGLV radiative alone).
*	**See parallel talk by W. Horowitz on Wednesday.*
- Various uses of fixed lengths: L=5-6fm.
- 	*We find a hierarchy of fixed lengths fit the full geometrical calculations.*

- No *a priori* justification for any fixed length without doing the full numerically intensive calculation.
- (if a fixed length can fit the full geometry at all)
**V important for gluons and consistency of electron and pion predictions.**

## WHDG extended theory

|  | ~Ideal | Our model |
| --- | --- | --- |
| Production | All orders pQCD | NLO pQCD (FONLL for LHC spectra)
(large uncertainty in magnitude, small uncertainty in the power law of the tail) |
| Geometry | Propagate through evolving hydro simulation |  Realistic Woods-Saxon nuclear density
 Jets produced ~ TAA
 Propagate through Bjorken expanding ρpart |
| αs | Running | Fixed αs=0.3
(large uncertainty as energy loss strongly dependent on αs) |
| Energy loss mechanism | Collisional and radiative in same theoretical framework | Incoherent addition of
DGLV radiative and leading log TG / BT collisional |

- NOTE: Using realistic dNg/dy=1000

## Results

- RHIC
- RHIC theoretical uncertainties
- LHC

## Results – RHIC

- Gluons, u,d ->Pions

- Charm, bottom ->Electrons

- Note: kt smearing, EMC ... neglected here.

- Good agreement with pions, improved agreement with e- than for (entropy constrained) radiative only.

## Results – RHIC - Uncertainty

- Sources of uncertainty:
- Fixed αs=0.3 approximation
*Shape** of spectra, fragmentation ...**(does not affect R**AA** significantly)*
- Magnitude of spectra:ie c / b ratio in contribution to e-

## Results – RHIC – Uncertainty in αs

- αs =0.4 improves e- fit, over predicts pion quenching

- ... need a fully running coupling calculation to reduce theoretical uncertainty.

## Results – RHIC – uncertainty in c/b ratio

- Uncertainty in charm spectrum and c / b contribution to e-
- A quick estimate of uncertainty – shift entire charm cross-section up / down.

- Armesto, Cacciari, Dainese, Salgado, Wiedermann
**hep-ph/0511257**

## Results – LHC (1)

- Energy loss for high pT jets at LHC ‘dominated’ by radiative ...

- ... but collisional energy loss is still a **~25% effect**.

## Results – LHC (2)

- Partonic RQ(pT) (for one sample value of dNg/dy = 2900):

## Results – LHC (3)

- Pion RAA(pT) (for two sample values of dNg/dy = 1750, 2900):

## Conclusion

- Important effects to include:
- Elastic energy loss
- Path length fluctuations – ‘Geometry’
**For stronger conclusions, we need:**
-  Theory:
- Elastic and inelastic in the same theoretical framework
- Include effect of fully running coupling
- Theory or experiment
- Better understanding of **charm** and bottom productioneg measure D mesons directly
- Further applications:
- V2
- Multi-particle correlations
- More LHC work

## (end of talk)(go England!!)