# Presentation: 852893

## Modification of the jet properties at the Relativistic Heavy Ion Collider 

- Jan Rak
- Department of Physics and Astronomy
- Iowa State University

**Notes:**
Find Iowa logo
I added the subtitle Search for “Standard Model” of HI physics at RHIC
I will argue, that at the RHIC machine opened the  new era of HI physics which forces us to revise the fundaments .... 

## QCD in Heavy Ion collisions

- Two particles azimuthal correlations in pp, dAu and AuAu
- nuclear modification of  jet properties:
- Intrisic momentum kT
- Jet transverse fragmentation momentum jT
- Parton distribution function
- Fragmentation function

## Hard scattering 

- Hard scattering

## Jet Fragmentation (width of the jet cone)

- Partons have to materialize (fragment) in colorless world

- jT and kT are 2D vectors. We measure the mean value of its projection into the transverse plane |jTy| and |kTy| .

- |jTy|  is an important jet parameter. It’s constant value independent on fragment’s pT is characteristic of jet fragmentation (jT-scaling).
- |kTy|  (intrinsic + NLO radiative corrections)*** ***carries the information on the parton interaction with QCD medium.

## Fragmentation Function (distribution of parton momentum among fragments)

- In Principle

- jet

- Fragmentation function

## xE in pp collisions 

**PHENIX preliminary**

**1/********x****E******** ******** -4  to –5**

## z extracted from pp data

- We measured xE and

- xTtrigg=2.pTtrigg/s

- Only one unknown variable z   iterative solution

## pp and dAu correlation functions

-  3.0<pT<6.0

-  1.0<pT<1.5

- Fit = const + Gauss(0)+Gauss()

- p+p  h+-

- d+Au

- 5.0<pTtrigg <16.0 GeV/c

- Near-side peak

- Fixed correlation:
- both pTtrigg and pTassoc are in the same range
- Assorted correlation:
- pTtrigg and pTassoc different

- Away side peak

- d+Au h

-  1.0<pT<1.5

-  1.5<pT<2.0

**Jet function assumed to be Gaussian**

## N, A , |jTy|, |kTy| relations

- Knowing N and A it is straightforward to extract |jTy| and ztrigg |kTy|
- In the high-pT limit  (pT >>|jTy| and pT >>|kTy|** **)

## N ,A |jTy|, |kTy| in  pp data

**PHENIX preliminary**

-  PHENIX preliminary
- |jTy| = 35911 MeV/c
- |kTy| = 96449 MeV/c

- Both |jTy| and |kTy| in
- very good agreement  with previous  measurements: *PLB97  (1980)163 *
*PRD 59 (1999) 074007*

******A**

## Di-jet fragmentation

- pTq

- pTassoc

- pTq

- z>z

- z<z

- Q2  > Q2

- pTtrig

## “wave function collapse”

- For fixed pT correlation the parton distribution function is almost unaffected by the condition of having fragment on the opposite side.

## kT-smearing

- Associated parton distribution  fa = kTft and the final formula for invariant cross section is

- Correlation fcn between b2b parotns

- gaussian for kt>0 or delta function for kT=0

## Di-jet mean z

- plateau

- Phase space effect

- Deviation from power law

- inclusive

- conditional

## Simulation

- Sorry, I can show only simulation – the final data      Phys. Rev. C.

## PHENIX/CCOR ptrigg slopes

- trigg

## Associated yields – complementary method of |kTy| extraction   

- Associated yields – complementary method of |kTy| extraction

- STAR |kTy|=1.2 GeV/c

- PHENIX |kTy|=0.9 GeV/c

- We can describe all pTassoc distribution with unique value of |kTy|=0.9 GeV/c, but not STAR data.

## AuAu |jTy| and z |kTy| from CF

- (2.5pTtrigg4.0)(1.0pTassoc2.5)

## AuAu associated yields

- Note pT is rather low;  associated particle yields increase with centrality

- (2.5pTtrigg4.0)(1.0pTassoc2.5) GeV/c

## pT distributions on near and away side

- _Near side:_
**Overall**
**enhancement**
**from pp to AA **

_**Away side:**_
- energy from
- initial parton seems
- to be converted
- to lower pT  particles
- reminiscent of energy
- loss predictions

- Apparent modification of the fragmentation function ?

## Summary and conclusions 

- Jet production and fragmentation:
- Good agreement of the jet properties in pp collisions with other lower s experiments
- dAu jT and kT consistent with pp
- In AuAu significant broadening of “effective”  kT - with centrality
- Yield of away side associated particles is suppressed at pT>2GeV/c and shows rising trend with Npart below 2GeV/c. Remnant of high-pT jets  - hint of jet-quenching balance ?