# Presentation: 618209

## Jets in PHENIX

- Jiangyong Jia, Columbia Univerisity

- Hot Quark Matter, Taos Valley, NM

## Hard-scattering and Jet fragmentation

- Partons scatters with large Q**2** – hard-scattering
- Outgoing partons fragment into sprays of hadrons –Jets
- Properties that we want to measure
- The spread of the hadrons around the jet axis and
- 	relative orientation of the two jets – jT, kT.
- The multiplicity of hadrons – fragmentation function D**q********h**(z)

- Leading hadron

- Q**2**

## jT and kT

- jT = Momentum perpendicular to jet axis: jT= pT sin
- <jT> is related to the non-perturbative QCD.
- Typical value is 500 MeV/c, very weakly depends on pT and s.

- Jets are not exactly back-to-back in transverse direction
- kT = Intrinsic + radiative transverse momentum of the initial partons.

## Same jet correlation

******tq**** trigger-parton**
******aq**** associated-parton**
******ta**** trigger-associated**

## Far side jet correlation

-  		        ,**qq** is the angle between the jets.

**Projected to azimuth plane**

## jT, kT RMS values

-  1D RMS value:

-  Pout is directly related to the angular width:

**(for Gauss statistics)**

-  Comparing with Jan’s formula

**(QM2004)**

## Comparison using Pythia simulation

-  Trigger pt>5 GeV/c,  change associated pT

- Jan’s formula

- This formula

## kT Broadening in dAu

- Presence of cold medium can broaden the jet kT

- p+p

- p+A

## 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

-  the sensitivity on broadening decreases as pT increases.

## Fragmentation function  Conditional yield

- Direct jet reconstruction.

- e**+**e**-**

## Two particle azimuth correlation method

- In ideal acceptance, real pair distribution is

## 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**

## Normalization for 2D and 1D CY

- 2D CY

## Test the correction with Pythia simulation

- Generate 1 M triggered events and 1 M minimum bias events.
- Mixed distribution is obtained by mixing trigger with minbias event.
- Requiring trigger always has ||<0.35.
- If we don’t constrain associated particle, we would get full yield.

- Compare three correlations.
- No cut on associated particle  full jet yield (near side)
- Near side jet has a gauss shape in — the integral of the gauss.
- Cut | |<0.7 on associated particle  full yield in ||<0.7(away side)
- Far side jet has a very broad shape in 
- PHENIX acceptance cut  measured yield in near and away side.

## CY, with no constrain on associated particle

- Trigger pt > 5 GeV/c, associated 1<pT <1.5 GeV/c
- Trigger ||<0.35, associated no eta cut.
- This gives the true conditional yield for the near side :0.717

- True conditional yield

## CY, with associated particle in ||<0.7

- CY, with associated particle in ||<0.7

- Trigger pt > 5 GeV/c, 1<pTasso <1.5 GeV/c
- Trigger ||<0.35, and associated particle: ||<0.7
- This gives the true conditional yield with in ||<0.7 for the far side: 0.92

- True conditional yield

**Because the away side correlation is very wide in ********. **
**We just want the yield in ****|********|<0.7****, which is the range sampled by PHENIX.**

## Conditional yield in PHENIX acceptance

- Trigger pt > 5 GeV/c, 1<pTasso <1.5 GeV/c
- Trigger ||<0.35, and associated ||<0.35.
- Azimuth acceptance cut on both particles..
- This gives the Measured conditional yield for same side and way side
- 0.279(near), 0.233(far).

- FG

**MIX**

- Raw conditional yield

## Corrected CY compared with true CY

- Corrected CY compared with true CY

- Near side

- Far side

## The ratio between true and corrected

- The agreement is good.
- This implies that our correction and extrapolation is valid.

## Summary

- Discuss the general formula for           and
- Some difference from previously used formula, especially for kTz and low associated pT region.
- The sensitivity on kT dies out as the trigger pT increases.
- Discuss the how to extract the conditional yield using two particle correlation method and event mixing. The correction factor is derived for limited detector acceptance(can be trivially generalized to other detectors).
- Verified with Pythia simulation