# Presentation: 658980

## Thomas Henry
Texas A&M University

for the

STAR      Collaboration

- Thomas Henry
- Texas A&M University
- for the
- STAR      Collaboration

- Jet Reconstruction at STARin p+p and d+Au collisions

- Thomas Henry - Texas A&M - STAR Collaboration

## Outline

- Introduction
- Inclusive jet studies
- Dijet studies
- Dihadron studies

- Thomas Henry - Texas A&M - STAR Collaboration

## What is a Jet?

- Jets arise from hard partonic collisions
- High pt partons fragment into a collimated spray of particles.

- Thomas Henry
- STAR Collaboration

- parton

- parton

- hadrons

- In general, the cone is not bisected by the thrust axis.

- Jet Thrust Axis: **E****t**

- Hadron **p****t**

- Leading Hadron

**J****t**

- Cone Radius

-  transverse
- momentum p1

- p2

- z= **p****t****/**p1

- Thomas Henry - Texas A&M - STAR Collaboration

## Why is Jet Reconstruction Important?

- Observables from fully reconstructed jets compare directly with pQCD theory
- Reconstructed **E****t** approximates parton **p****1**
- Reduces fragmentation function ambiguities

- Thomas Henry
- STAR Collaboration

- parton

- parton

- hadrons

- Jet Thrust Axis: **E****t**

- Hadron **p****t**

- Leading Hadron

**J****t**

- Cone Radius

- p1

- p2

- z= **p****t****/**p1

- Thomas Henry - Texas A&M - STAR Collaboration

## Jets in p+p, d+Au, and Au+Au

- p+p leads to Jt and intrinsic kt
- d+Au leads to intrinsic kt + nuclear kt
- Au+Au jets cannot be fully reconstructed due to huge multiplicity
- Di- (and multi-) hadron correlations necessary
- p+p and d+Au  jet reconstruction calibrates these correlations

- Thomas Henry - Texas A&M - STAR Collaboration

## Hard parton fragmentation products are strongly correlated in momentum
Jets reconstructed by exploiting hadron momenta correlations
Cone algorithms:  capture spray of particles with geometric cone.  Two strategies:
Center the cone on the seed particle
More robust for high multiplicity
Optimize the direction of the cone
Cone direction optimized for maximum energy
Kt algorithm:  exploit relative pt
Add hadrons with progressively larger relative momenta
Hadrons below pt threshold not used

- Jets reconstructed by exploiting hadron momenta correlations
- Cone algorithms:  capture spray of particles with geometric cone.  Two strategies:
- Center the cone on the seed particle
- More robust for high multiplicity
- Optimize the direction of the cone
- Cone direction optimized for maximum energy
- Kt algorithm:  exploit relative pt
- Add hadrons with progressively larger relative momenta
- Hadrons below pt threshold not used

- Jet Reconstruction Algorithms

- Thomas Henry - Texas A&M - STAR Collaboration

## The STAR Detector

- The complete  coverage and large  coverage of the TPC (with particle ID) and EMC make STAR excellent at reconstructing jets.

- EM Calorimeter

- Thomas Henry - Texas A&M - STAR Collaboration

## The STAR EMC

- Neutral energy (which includes 0 decay photons) is measured with the STAR Electromagnetic Calorimeter
- For 2003 RHIC run, Barrel EMC included:
**2******** coverage in ********; 0 < ******** < 1**
- 2400 Towers ( x  = 0.05 x 0.05)
- See posters by D. Arkhipkin, M.M. de Moura
- Read out in Minimum Bias Events
- Also used as trigger to select events likely to contain jets
**“****High tower” trigger with E****T**** > 2.5,4.5 GeV**
- Other trigger topologies available but not used in 2003.

- Thomas Henry - Texas A&M - STAR Collaboration

## From Observed Energy to Jet Energy

- Measure both charged tracks and neutral energy
- Correct for double counting of charged energy
- Charged particle tracking efficiency ~.9
- EMC geometric acceptance ~.94
- Long lived neutrals (n, KL, ...) lost
- Soft particles may fall outside jet cone
- Total correction factors from Pythia
- 1/~0.8 for minbias
- 1/~0.86 for high tower

- Need verification

- Thomas Henry - Texas A&M - STAR Collaboration

## Jet Et

**J****t**

**p****t**

**E****t**

- Jet **E****t**

- p+p collisions; Rcone=.7
- MinBias
- High tower triggered
- Corrected <**E****t**> = 11.3 ± 0.7sys GeV

- p1

- Observed (uncorr.) Jet** E****t **(GeV)

- p+p High Tower
- p+p MinBias

- Raw Per Event Yield

- STAR
- Preliminary

- Thomas Henry - Texas A&M - STAR Collaboration

## Toward a Jet Fragmentation Function

- Pythia-GEANT is pythia run through the STAR detector simulator
- Agree at low z
- High z deviations under study

- z= **p****t****/**p1

- p1

**p****t**/(Uncorrected) **E****t **(~z)

- slope ~ 8
- p+p MinBias
- pythia-GEANT

- Raw Per Event Yield

- STAR
- Preliminary

- Thomas Henry - Texas A&M - STAR Collaboration

## Jet Jt

- Jet **J****t**

- STAR
- Preliminary

- pp MinBias
- pythia-GEANT

**J****t **in GeV/c

- Relative 1/**J****t**dN/d**J****t**

- pt > 0.6

- pt > 2.0

- Thomas Henry
- STAR Collaboration

- Primarily sensitive to jet axis direction
- Low pt kinematic limit: “Seagull Effect”
- RMS **J****t**
- 490 ± 50sys MeV/c, pt>0.6
- 615 ± 60sys MeV/c, pt>2.0

**Agree within 4%**

- Thomas Henry - Texas A&M - STAR Collaboration

## in leading order
Gluon radiation broadens 
 is a probe of kt

- in leading order
- Gluon radiation broadens 
-  is a probe of kt

- DiJet  Distribution

- Thomas Henry - Texas A&M - STAR Collaboration

## DiJet Reconstruction

- Trigger Jet
- Reconstructed from EMC and TPC
- Includes high tower trigger
- Determines energy scale and first thrust axis
- 0.2 <  < 0.65
**E****t** > 5 GeV

- Thomas Henry
- STAR Collaboration

- parton

- parton

- Trigger Jet

- High tower
- trigger

- Away Jet
- Charged particles only
- Determines second          thrust axis
- -0.5 <  < 0.5
**E****t** > 4 GeV

- Away Jet

- Thomas Henry - Texas A&M - STAR Collaboration

## p+p DiJet  Distribution

- p+p DiJet  Distribution

- Dijet high tower corrected <**E****t**>= 13.0±0.7Sys GeV
**k****t****=2.3±0.4 ±      GeV/c**

- s (GeV)

- L. Apanasevich et al.,
- Phys. Rev. D59, 074007

- STAR
- Preliminary

**0.67**
**1.11**

- 0

- 

- 

- Raw Per Event Yield

**p+p**

- =0.23±0.02±

- STAR Preliminary

**0.03**
**0.05**

- 0

- 1.610-3

- (radians)

- Thomas Henry - Texas A&M - STAR Collaboration

## Nuclear kt in d+Au

- Nuclear kt in d+Au

- ktobs2 = ktintrin2 + ktnucl2
- d+Au vs p+p:  **Nuclear k****t**** = 2.8±1.2±1.0 GeV/c**

******=0.22 ± 0.02 ± .06**
******=0.31 ± 0.05 ± .06**

- 0

- 

- 

- Raw Per Event Yield

- R=.35 Cdf
**p+p**
**d+Au**

- STAR Preliminary

- (radians)

- 0

- 1.4 10-3

- Thomas Henry - Texas A&M - STAR Collaboration

## Dihadron Correlations

****

- p1

- p2

- Au+Au: jet reconstruction fails
- Resort to dihadron correlations:
- extract Jt and kt<z>
- Strategy:
- Calibrate d+Au dihadron with d+Au jets
- Compare Au+Au dihadron with d+Au dihadron

- Thomas Henry - Texas A&M - STAR Collaboration

## High Tower – Charged Hadron Correlation Functions(variation with trigger energy)

- See poster by S. Chattopadhyay

- High Tower – Charged Hadron Correlation Functions(variation with trigger energy)

- Will measure this to very high Ettrig in Au+Au collisions during current RHIC run

- Preliminary results:
- Jt = 500 ± 40 ± 150 MeV/c
- kt= 1.9 ± 0.2 ± 0.3 GeV/c /<z>
- <z> needed for trigger hadron

- 4.5 < Ettrig < 6.5 GeV/c
- 6.5 < Ettrig < 8.5 GeV/c
- ptassoc > 2.0 GeV/c

- Thomas Henry - Texas A&M - STAR Collaboration

## DiHadrons and DiJets

- Jt
- p+p jets:              615 ± 60sys MeV/c
- d+Au dihadrons:	  500 ± 40 ± 150 MeV/c
****** ****Consistent between inclusive jets and dihadrons**
- kt
- p+p dijets:         intrinsic kt = 2.3 ± 0.4          GeV/c
- d+Au dijets:        nuclear kt = 2.8 ± 1.2 ± 1.0 GeV/c
- d+Au dihadrons:     total kt = (1.9 ± 0.2 ± 0.3)/**<z>** GeV/c
- Uncertainties are conservative

- +0.67
- -1.11

- Thomas Henry - Texas A&M - STAR Collaboration

## Fully reconstructed jets in p+p and d+Au at RHIC
p+p:  Jt and intrinsic kt
d+Au:  nuclear kt
Pythia provides a good description of Jt

Future:
Jet and dijet cross sections in p+p and d+Au
Rd+Au for jets/partons

- p+p:  Jt and intrinsic kt
- d+Au:  nuclear kt
- Pythia provides a good description of Jt
- Future:
- Jet and dijet cross sections in p+p and d+Au
- Rd+Au for jets/partons

- Conclusions

- Thomas Henry - Texas A&M - STAR Collaboration

## Slide 21

## STAR-PHENIX Comparison

- Jt=615±60Sys MeV/c
- kt=2.3±0.4±     GeV/c

- s (GeV)

- <pt>pair (GeV/c)

- J. Rak, PHENIX,
- arXiv:nucl-ex/0306031 v3

- s (GeV)

- L. Apanasevich et.al.,
- Phys. Rev. D59 074007

- STAR
- Preliminary

- STAR

**0.67**
**1.11**

- PHENIX

- Jt=661±28 MeV/c
- kt=(1.3±0.06)/<z> GeV/c

- Thomas Henry - Texas A&M - STAR Collaboration

## Difficulty in d+Au due to High Multiplicity 

**E****t**

- Difficulty in d+Au due to High Multiplicity

- d+Au jet signals at radius .7 are swamped by false jets
- Smaller radius reduces measured **E****t**
- < **E****t**(R=.35)> = .88*< **E****t**(R=.7)> for high tower triggered events

**E****t**(R=.35)/ **E****t**(R=.7) for identical jets

**E****t** (Uncorrected) in GeV

- p+p : R= .35Cdf, .35, .7

- 0

- 1

- 7

- 25

- Log scale

- Log raw per event yield

- STAR Preliminary

- STAR Preliminary

- p+p
- Pythia-
- GEANT

- Cone Radius

- Thomas Henry - Texas A&M - STAR Collaboration

## Correction for Double Counting of Charged Hadron Energy

- Charged particles can leave some energy in the EMC
- Either can remove 20% of Track E or 0.3 GeV per Track from the EMC hits
- 20% comes from Monte Carlo
- .3 GeV = MIP
- The two approaches are consistent, and reduce the energy as expected

- Uncorrected Jet Energy in GeV

- Raw Per Event Yield

- No subtraction
- 0.2 x track E
- 0.3 GeV/track

- d+Au

- STAR Preliminary

- .009

- Thomas Henry - Texas A&M - STAR Collaboration