# Presentation: 233021

## , , (and  measurements in ALICE

## “The virtue of  and  Measurements”

**  ****initial**** ****parton distributions: ****Intrinsic k****T,  ****k****T**** Broadening,**** Shadowing, ****Anti-shadowing,**** Saturation, ...**
** ****some ****final state**** effects (parton/hadron rescatterings): ****Thermal,**** ****Jet/Parton Radiation or ****fragmentation after Eloss****,...**

**“****The virtue of ************ and ******** Measurements”**

**To properly deduce the properties of the produced QCD matter we must separately distinguish ****initial state**** effects from ****final state**** effects.**

** **Experimental virtues (calorimeter measurement):
-  Measure  and  in same detector
-  Identified particles to very high pT
-  ’s abundantly produced

**********’****s produced in ****final state**** (fragmentation or recombination): **
** ****initial**** ****parton distributions: as for ******** **
** ****large ****final state**** effects: Rescattering (low p****T****), ****Absorption,**** ****k****T**** Broadening,**** **_**Jet/Parton Energy Loss**_** ,...**

## Centrality Dependence of High pT Suppression

**Centrality Dependence of High p****T**** Suppression**

- PHENIX nucl-ex/0503003

-  Trend of  yield with centrality (impact parameter) follows expected binary collision scaling behavior (=constant) and agrees with pQCD (=1).
-  **Direct ******** are not suppressed - strong evidence that hadron suppression is due to final state, i.e. parton energy loss.**

## Direct Photons and  Production for Central Au+Au

**Direct Photons and ************ Production for Central Au+Au**

- PHENIX nucl-ex/0503003

-  High pT  yield consistent with binary scaled pQCD in contrast to factor of 5 suppression of  & hadron yields.
-  **Direct ******** are not suppressed - strong evidence that hadron suppression is due to final state, i.e. parton energy loss.**
** **Errors don’t preclude a thermal  contribution at low pT

## Centrality Dependence RAA for 

**  ******** ****suppression increases with increasing nuclear overlap volume.**
**  ******** ****suppression same as ************ - behaviour agrees with “meson-like” expectation**

**PHENIX preliminary**

**PHENIX preliminary**

## inclusive jets

- 10 Hz @ 50 GeV

- few x 104/year for ET>150 GeV

- EFS = 250 GeV
- (PHOS 80 GeV)

**Extend measurements to much higher p****T**

- From Peter Jacobs

##  Photons (all in fact): Continum Spectrum with Many Sources

- Turbide, Rapp, Gale

- E

- Rate

- Hadron Gas Thermal *T*f

- QGP Thermal *T*i

- Pre-Equilibrium

- Jet Re-interaction

- pQCD Prompt

- Final-state photons are the sum of emissions from the entire history of a nuclear collision.

- +Weak+EM decay ’s () = Bkgd

## Resolution: PHOS vs EMCal - PHOS wins

- Due to much better energy resolution, lower occupancies,  and lower tower thresholds PHOS will be the preferred detector for low pT photon measurements.

- PHOS: 3.5%/sqrt(E)

- EMCal: 12%/sqrt(E) or worse?

## Tower granularity 0 separation - PHOS wins 

**Tower granularity ********0**** separation - PHOS wins**** **

- 0 opening angle

-  shower shape discrimination

- Heather Gray, LBNL/Cape Town

- 

- 

- rejection for pT<~30 GeV/c

- At high pT merging of showers of photon pairs from  decays become indistinguishable from single photons - become a background.
-  Use EMCal vs PHOS comparisons to determine (PHOS rate limited)
-  Other “tricks” like tagging conversion rate
-  As pT increases ’s dominate

- preliminary

## So...Why Large EMCal in ALICE?

**Measure neutral energy of jets**
**Improves jet energy resolution,  ~30% -> ~15%**
**PHOS is too small to contain jet**
**Extend PHOS measurements**** ********,********0****, and e****+/-**** to higher p****T**** **
**Increase acceptance for****  ******** ****(EMCal/PHOS) + jet(TPC)**
**Allow back-to-back coincidences**** **
** ******** ****(PHOS) + jet (TPC+EMCal) **
** ******** ****(EMCal) + leading**** ********0 ****(PHOS)**
** ******** ****(EMCal/PHOS)**** + e****+/-**** ****from D,B decay (heavy q jet)     (PHOS/EMCal)**

## ALICE EMCal is large enough for jet measurements

- PROBLEM: Underlying event
- non-jet energy:
- Radius  	Cone Energy (GeV)
-   0.7	    750
-   0.5	    380
-   0.3	    140
-   0.1	      15

- Jet Energy fraction within cone radius

- Use “small” jet-cone, R~0.3

## Direct /0  Ratio

- Ratio ******/********0** provides figure of merit for direct gamma measurement:
-  ******/********0 ****~ 10% easy**
**             ****~ 3-5% limit**
- 0 suppression by factor of ~5-10 (jet quenching) would allow direct photon measurement down to a few GeV/c
- Expectations for 0 suppression at high pT at LHC?

## Empirical Investigation of Energy Loss

- Observe  0 spectra in p+p have power-law form:

- If we interpret suppression as a pT shift from Energy Loss:

- where

- then

- The Energy Loss, or pT Shift Sloss, can then be extracted from RAA as

## Suppression / Energy Loss vs Centrality

- Gyulassy, Vitev, Wang:
- 1D expansion, simple geometric scaling
-  Large gluon density dNg/dy~1000, Large energy density.
-  If behaviour extrapolates to LHC, expect even larger pT independent suppression

## Further Illumination with Direct Photons

- High pT direct  produced in hard q-g Compton-like scatterings. Sensitive to PDFs, (especially gluons).

- In the HI case,  from initial hard scattering escape.  Can be used to tag recoiling jet to study jet energy loss (“Jet Quenching”), which may produce additional Brems. ’s.

- Additional thermal ’s produced in scatterings in QGP or hadronic phase of collision.

- If ”Gluon Saturation” occurs, initial  (and 0) will be suppressed.
- With parton Eloss, the fragmentation contribution could also be suppressed.

## Investigate  yields vs Reaction Plane (Pathlength)

-  Measure Reaction Plane using BBCs
-  Measure Yields vs. orientation w.r.t. RP 
-  Expect largest E-loss or suppression at  = 90 in “ long” direction
-  Interplay between flow (v2) and Supression? Both give expectation of in-plane enhancement.

- Long path, large Eloss

##  and inclusive v2 analysis

********** ****and inclusive ********v****2**** analysis**

-   show large v2 to high pT  - effect of suppression? (more later)
- Inclusive  show similar large v2
- Use  measurement as input to MC to predict decay v2 - if no direct  signal, or v2(direct ) = v2(bgd ), then should have v2(bgd ) = v2(incl )
- Use Direct  measurement R=Total/Decay then
-                v2(direct )  = ( R*v2(incl ) - v2(bgd ) ) / (R-1)
- Or, assume v2(direct ) = 0 to extract R. Gives agreement with measurement of R=0 in this pT range.  (Proof of principle at this point)

- pT (GeV/c)

**PHENIX preliminary**

## ALICE studies using  Measurements 

- Measure direct  in p+p:
- Extract fragmentation contribution by analysis with/without isolation cuts
- Measure direct  in A+A:
- Search for effects of gluon saturation
- Thermal region (PHOS):
- Other techniques -  HBT, conversions
- Measure direct  vs Reaction Plane in A+A:
- q+g Compton isotropic, Thermal flows, Fragmentation enhanced or suppressed with pathlength?
-  Measure +jet :
- The  measures pT of the recoiling parton - measure modified recoil Fragmentation Function to extract parton energy loss directly.