Jet Reconstruction at STAR in pp and dAu

Jet Reconstruction at STAR in p+p and d+Au collisions Thomas Henry Texas A&M University for the STAR 01/16/2004 Collaboration Thomas Henry STAR Collaboration 1

Outline n n Introduction Inclusive jet studies Dihadron studies 01/16/2004 Thomas Henry STAR Collaboration 2

What is a Jet? Co n Ra e diu s Jets arise from hard partonic collisions n High pt partons fragment into a collimated spray of particles. z= pt/p 1 t p n o r ad Jet H st u Thr s: E i x A t n dro a H g n Leadi e s r e v s p 1 n a m u tr t n e mom hadrons 2 parton p n 01/16/2004 parton Jt In general, the cone is not bisected by the thrust axis. Thomas Henry STAR Collaboration 3

Why is Jet Reconstruction Important? Observables from fully reconstructed jets compare directly with p. QCD theory n n Co n Ra e diu s z= pt/p 1 t p n o r d a Reconstructed Et approximates parton p 1 Reduces fragmentation function ambiguities Jet H st u Thr s: E i x A p 1 hadrons parton 2 parton 01/16/2004 t n dro a H g n Leadi Jt p n Thomas Henry STAR Collaboration 4

Jets in p+p, d+Au, and Au+Au n n n 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 n n 01/16/2004 Di- (and multi-) hadron correlations necessary p+p and d+Au jet reconstruction calibrates these correlations Thomas Henry STAR Collaboration 5

Jet Reconstruction Algorithms n Hard parton fragmentation products are strongly correlated in momentum n n Jets reconstructed by exploiting hadron momenta correlations Cone algorithms: capture spray of particles with geometric cone. Two strategies: n Center the cone on the seed particle n n Optimize the direction of the cone n n More robust for high multiplicity Cone direction optimized for maximum energy Kt algorithm: exploit relative pt n n Add hadrons with progressively larger relative momenta Hadrons below pt threshold not used 01/16/2004 Thomas Henry STAR Collaboration 6

The STAR Detector n The complete f coverage and large h coverage of the TPC (with particle ID) and EMC make STAR excellent at reconstructing jets. EM Calorimeter 01/16/2004 Thomas Henry STAR Collaboration 7

The STAR EMC n Neutral energy (which includes p 0 decay photons) is measured with the STAR Electromagnetic Calorimeter n For 2003 RHIC run, Barrel EMC included: n 2 p coverage in f; 0 < h < 1 n n 2400 Towers (Dh x Df = 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 n n “High tower” trigger with ET > 2. 5, 4. 5 Ge. V Other trigger topologies available but not used in 2003. 01/16/2004 Thomas Henry STAR Collaboration 8

From Observed Energy to Jet Energy n Measure both charged tracks and neutral energy n n Correct for double counting of charged energy Charged particle tracking efficiency ~. 9 EMC geometric acceptance ~. 94 Long lived neutrals (n, KL, …) lost n Soft particles may fall outside jet cone n Total correction factors from Pythia n n 01/16/2004 1/~0. 8 for minbias 1/~0. 86 for high tower Need verification Thomas Henry STAR Collaboration 9

Jet Et p+p High Tower STAR Preliminary Raw Per Event Yield s = 200 Ge. V n n n p+p collisions; Rcone=. 7 Min. Bias High tower triggered n p+p Min. Bias pt Et Observed (uncorr. ) Jet Et (Ge. V) 01/16/2004 Corrected <Et> = 11. 3 ± 0. 7 sys Ge. V Thomas Henry STAR Collaboration p 1 Et Jt 10

Toward a Jet Fragmentation Function STAR Preliminary n Raw Per Event Yield s = 200 Ge. V n n slope ~ 8 Pythia-GEANT is pythia run through the STAR detector simulator Agree at low z High z deviations under study p+p Min. Bias pt pythia-GEANT pt/(Uncorrected) Et (~z) 01/16/2004 Thomas Henry STAR Collaboration z= pt/p 1 Et Jt 11

Relative 1/Jtd. N/d. Jt Jet Jt pt > 0. 6 STAR Preliminary s = 200 Ge. V pt > 2. 0 n n n Primarily sensitive to jet axis direction Low pt kinematic limit: “Seagull Effect” RMS Jt n n pp Min. Bias pt pythia-GEANT Jt in Ge. V/c Et Jt Agree within 4% 01/16/2004 490 ± 50 sys Me. V/c, pt>0. 6 615 ± 60 sys Me. V/c, pt>2. 0 Thomas Henry STAR Collaboration 12

Di. Jet Df Distribution n p 1 2 n Df = p in leading order Gluon radiation broadens Df Df is a probe of kt p n Df kt = <kt 2> (per parton) = <Et> sin s. Df 01/16/2004 Thomas Henry STAR Collaboration 13

Di. Jet Reconstruction n Trigger Jet n n n Reconstructed from EMC and TPC Includes high tower trigger Determines energy scale and first thrust axis 0. 2 < h < 0. 65 Et > 5 Ge. V Trigger Jet High tower trigger parton Away Jet n n Charged particles only Determines second thrust axis -0. 5 < h < 0. 5 Et > 4 Ge. V 01/16/2004 parton Away Jet Thomas Henry STAR Collaboration 14

p+p Di. Jet Df Distribution STAR Preliminary s = 200 Ge. V p+p 0 0 01/16/2004 n n 2 kt=<pt>pair (Ge. V/c) Raw Per Event Yield 1. 6* 10 -3 0. 03 Dijet high tower corrected <Et>= 13. 0± 0. 7 Sys Ge. V kt=2. 3± 0. 4 ± 0. 67 1. 11 Ge. V/c L. Apanasevich et al. , Phys. Rev. D 59, 074007 Df=0. 23± 0. 02± 0. 05 p Df (radians) 2 p Thomas Henry STAR Collaboration STAR Preliminary s (Ge. V) 15

1. 4 * 10 -3 Nuclear kt in d+Au STAR Preliminary Raw Per Event Yield R=. 35 Cdf s = 200 Ge. V p+p Df=0. 22 ± 0. 02 ±. 06 d+Au Df=0. 31 ± 0. 05 ±. 06 0 0 p Df (radians) n ktobs 2 = ktintrin 2 + ktnucl 2 n d+Au vs p+p: Nuclear kt = 2. 8± 1. 2± 1. 0 Ge. V/c 01/16/2004 Thomas Henry STAR Collaboration 2 p 16

Dihadron Correlations n n extract Jt and kt*<z> Strategy: n n Calibrate d+Au dihadron with d+Au jets Compare Au+Au dihadron with d+Au dihadron Df 01/16/2004 Thomas Henry STAR Collaboration p 1 2 n Au+Au: jet reconstruction fails Resort to dihadron correlations: p n 17

High Tower – Charged Hadron Correlation Functions (variation with trigger energy) See poster by S. Chattopadhyay 4. 5 < Ettrig < 6. 5 Ge. V/c 6. 5 < Ettrig < 8. 5 Ge. V/c ptassoc > 2. 0 Ge. V/c Preliminary results: Jt = 500 ± 40 ± 150 Me. V/c kt= 1. 9 ± 0. 2 ± 0. 3 Ge. V/c /<z> needed for trigger hadron Will measure this to very high Ettrig in Au+Au collisions during current RHIC run 01/16/2004 Thomas Henry STAR Collaboration 18

Di. Hadrons and Di. Jets n Jt n n kt n n p+p jets: 615 ± 60 sys Me. V/c d+Au dihadrons: 500 ± 40 ± 150 Me. V/c Consistent between inclusive jets and dihadrons +0. 67 p+p dijets: intrinsic kt = 2. 3 ± 0. 4 -1. 11 Ge. V/c d+Au dijets: nuclear kt = 2. 8 ± 1. 2 ± 1. 0 Ge. V/c d+Au dihadrons: total kt = (1. 9 ± 0. 2 ± 0. 3)/<z> Ge. V/c Uncertainties are conservative 01/16/2004 Thomas Henry STAR Collaboration 19

Conclusions n 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 n Future: n n n Jet and dijet cross sections in p+p and d+Au Rd+Au for jets/partons 01/16/2004 Thomas Henry STAR Collaboration 20

01/16/2004 Thomas Henry STAR Collaboration 21

STAR L. Apanasevich et. al. , Phys. Rev. D 59 074007 STAR Preliminary PHENIX <pt>pair (Ge. V/c) 2 kt=<pt>pair (Ge. V/c) STAR-PHENIX Comparison J. Rak, PHENIX, ar. Xiv: nucl-ex/0306031 v 3 s (Ge. V) n n Jt=615± 60 Sys Me. V/c kt=2. 3± 0. 4± 0. 67 1. 11 Ge. V/c 01/16/2004 n n Jt=661± 28 Me. V/c kt=(1. 3± 0. 06)/<z> Ge. V/c Thomas Henry STAR Collaboration 22

Difficulty in d+Au due to High Multiplicity n Et < Et(R=. 35)> =. 88*< Et(R=. 7)> for high tower triggered events p+p : R=. 35 Cdf, . 35, . 7 STAR Preliminary p+p s = 200 Ge. V Log raw per event yield n d+Au jet signals at radius. 7 are swamped by false jets Smaller radius reduces measured Et Log scale n Co n Ra e diu s Pythia. GEANT 0 1 Et(R=. 35)/ Et(R=. 7) for identical jets 01/16/2004 s = 200 Ge. V STAR Preliminary 7 25 Et (Uncorrected) in Ge. V Thomas Henry STAR Collaboration 23

Correction for Double Counting of Charged Hadron Energy n Charged particles. 009 can leave some energy in the EMC Either can remove 20% of Track E or 0. 3 Ge. V per Track from the EMC hits n n n 20% comes from Monte Carlo. 3 Ge. V = MIP The two approaches are consistent, and reduce the energy as expected 01/16/2004 d+Au STAR Preliminary s = 200 Ge. V Raw Per Event Yield n No subtraction 0. 2 x track E 0. 3 Ge. V/track Uncorrected Jet Energy in Ge. V Thomas Henry STAR Collaboration 24