# Presentation: 618158

## Measuring jet properties from  — h correlation in dAu and pp

-  Introduction
-  study of jet shape
-  study of fragmentation function
-  conclusion

- Jiangyong Jia Columbia University, Nevis Labs
- For the PHENIX Collaborations

## Hard-scattering and Jet fragmentation

- Properties of di-jet system
**The spread of the hadrons around the jet axis and **
**	****relative orientation of the two jets – ****jT, kT****, ****Pout**
**The multiplicity of hadrons – fragmentation function ****D(z)**
- They can be accessed with two particle correlation in .
- We present the result for leading  — h correlation from dAu and pp collisions

##  — h azimuthal distribution

-  — h azimuthal distribution

**0.4-1 GeV/c**

**1-2 GeV/c**

**2-3 GeV/c**

**3-5 GeV/c**

- dAu

- Trigger pion 5-10 GeV/c

- PHENIX identify  at pT> 4.7 GeV/c

- Jet shape jT, kT is given by the near side and far side width
- Jet multiplicity is given by the area under the peak

- pp

**PHENIX preliminary**

## Jet shape

- Near side correlation

- j**T** is fragmentation momentum

- k**T** is parton initial momentum

- Far side correlation

- RMS of jT kT can be extracted from the jet width
- More direct way is via the projection of assoc to trigger : Pout
- 	Pout = p**T,assoc** sin(****)

## Relation jT and kT to Pout

## Pout distribution

**pp**

- Far side Pout is broader than near side due to kT

- Radiative tails

- Pout distribution carries more information than just RMS of jT, kT.

**PHENIX preliminary**

## Pout dAu central vs pp

- No sensitivity to broadening?

**PHENIX preliminary**

**PHENIX preliminary**

## Jet multiplicity  Conditional yield

- Study any modification of jet multiplicity in dAu
- Multiple scattering, modification of PDF, etc..

- Fragmentation function

- Correlation method give per trigger yield

## Jet Yield

- 1/N**trig** dN**h**/dx**E **
- x**E** from 0.1-0.7
- Near side yield is related to di-hadron fragmentation function.
- Far side is closer to single fragmentation function.
- No difference between dAu and pp within errors

## Scale dependence

- Study the CY as function of trigger pT (thus the Q2)

- PHENIX preliminary

- PHENIX preliminary

**PHENIX preliminary**

## Jet multiplicity: centrality dependence

- 1/N**trig** dN**h**/dp**T **
- p**T **from 0.4-5 GeV/c.
- as function of centrality and compare with pp
- Modification of jet multiplicity in dAu

**PHENIX preliminary**

## Conclusion

- The di-jet decay kinematics are studied with  — h correlation
- Pout distribution are very similar between dAu and pp, indicating no significant broadening in cold nuclear medium
- CY distribution are also similar between dAu and pp, indicating no significant increase in jet multiplicity in dAu relative to pp.

##  identification

- RICH detects  at pT> 4.7 GeV/c
- EMCal is used to reject high pT background from conversion
- Remaining background is less than 5%.

- Turn on around 6 GeV/c

##  Rcp

- Up to 16 GeV/c

## Two particle azimuth correlation method

- In ideal acceptance, real pair distribution is

- Pair acceptance function can be determined from event mixing technique

- Real/mix gives the acceptance corrected CY (modulo constant background  ).

- Real distribution is modulated by pair acceptance function **Acc****(********)**.

- Fit with a double gauss + constant to extract the Near side and far side yield.

## Pair acceptance function ACC in PHENIX

- Triangle results from convoluting two flat distribution

- effi is 100% at , 
-  Average is 25%

- Shape from overlapping four triangles:
**west1-west2, east1-east2, west1-east2, east1-west2**

## The corrections

- The mathematical framework is detailed in nucl-ex/0409024
- The normalization factor can be determined experimentally.

- Correct near side jet yield to full jet yield ||<
- Correct far side jet yield to yield in ||<0.7
- Procedure has been verified with MC simulation

- The near side peaks in both  and .
- Far side is very broad in (almost flat in |<1)

## Normalization for 2D and 1D CY

- 2D CY

## Is kT in dAu sensitive to broadening?

**hep-ph/0310274**

**I.Vitev**

- Seems radiation contribution dominate over the broadening

- 10% difference between
-  dAu and pp for 4.5 GeV trigger

- **far** not very sensitive
- to additional broadening

- Radiation contribution is even stronger at higher pT

- -h correlation

- Pythia

-  the sensitivity on broadening decreases as pT increases.

## kT Broadening in dAu

- Presence of cold medium can broaden the jet kT

- C~ 0.2-0.4

- 0.8-1.6 (GeV/c)2
- in central collisions

-  the sensitivity on broadening decreases as pT increases.

- However, radiation contribution seems dominate over broadening

- pythia

- Small difference between dAu and pp

- hep-ph/0310274

**I.Vitev**

## Jet shape

- Near side correlation

- jets have intrinsic pT

-   j**T** is fragmentation momentum
-   Pout is projection of assoc to trigger
- 	 Pout = p**T,assoc** sin(****)

- Far side correlation