Dijet A LL Dijet cross section 2006 Dijet A LL Preparation for Tai Sakuma MIT

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1 Dijet A LL 2005 Dijet cross section 2006 Dijet A LL Preparation for 2008 Tai Sakuma MIT

2 Motivation for dijet A LL is to constrain ΔG G with initial event kinematics The initial state variables can be written in terms of the final state variables

3 Dijet A LL is the first correlation analysis from STAR to constrain ΔG Inclusive Analyses Inclusive Jet A LL (p T ) First Polarized Proton Collisions at RHIC Inclusive π 0 A LL (p T ) Inclusive π ± A LL (p T ) Correlation Analyses Dijet A LL Photon-jet / ~

4 Data Selection and Yields RHIC 2005/06 Run Longitudinally polarized pp collisions 200 GeV Midpoint Cone Algorithm hep-ex/ Cone Radius: 0.4(2005), 0.7(2006) Event Selection Require Vertex Reconstruction Cut on vertex: vertex z < 60 (for 2005 cross section) BBC timebin (for 2006 A LL ) Remove Detector Edge: 0.2 < η < 0.8 (2005), -0.7 < η < 0.9 (2006) Choose back to back events (Δϕ > 2.0) pt > 5 GeV, M(jj) > 20 GeV

5 2005 dijet cross section measurement

6 Steps taken to determine cross section Data selections Event selection run list, vertex cut, Jet finding midpoint cone algorithm, R = 0.4, Kinematical cuts invariant mass cut eta cut measured dijet distributions Unfolding cross section with Pythia sample MC production Vertex correction Data - MC comparison Binning Iterative bin-by-bin unfolding true dijet distributions Corrections Vertex efficiency, Luminosity dijet cross sections

7 Steps taken to determine cross section Data selections Event selection run list, vertex cut, Jet finding midpoint cone algorithm, R = 0.4, Kinematical cuts invariant mass cut eta cut Mainly, I ll talk about this part for this presentation measured dijet distributions Unfolding cross section with Pythia sample MC production Vertex correction Data - MC comparison Binning Iterative bin-by-bin unfolding Luminosity true dijet distributions Corrections Vertex efficiency, dijet cross sections

8 Timeline summary of the dijet MC production ~ October 2006 Used Pythia sample generated for the inclusive analysis October 2006 Realized that we needed more events for the dijet study especially low pt events (pt < 15 GeV) November 2006 Requested MC productions with asymmetric cuts on cos(theta) May 2007 MC production jobs were submitted The jobs failed June 2007 Found a bug in Pythia Released a patch for the bug July 2007 The patch was applied to startsim The MC production jobs were resubmitted August 2007 The MC production is complete

9 MC production for dijet study: Events are successfully generated in the regions of the phase space from which most dijet events come from old All events Dijet events

10 MC production for dijet study: Events are successfully generated in the regions of the phase space from which most dijet events come from new All events Dijet events

11 Problem was a bug in pythia a patch for the bug Patch How to incorporate the patch into starsim The patch is included from pythia Pythia update notes Correction in PYKLIM for the case when cuts on pthat (CKIN(3), CKIN(4)) and asymmetric cuts on cos(thetahat) (CKIN(27), CKIN(28)) conspire to close one of the two regions with cos(thetahat) > 0 and < 0, but not both.

12 Vertex correction

13 Data - pythia comparison with new pythia sample BJP2

14 What is the true invariant mass distribution of dijets?

15 Detector resolution

16 Binning and Migration Bin width is forty percent of lower bin edge 20.0 is a bin boundary

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28 Steps taken to determine cross section Data selections Event selection run list, vertex cut, Jet finding midpoint cone algorithm, R = 0.4, Kinematical cuts invariant mass cut eta cut measured dijet distributions Unfolding cross section with Pythia sample MC production Vertex correction Data - MC comparison Binning Iterative bin-by-bin unfolding true dijet distributions Corrections Vertex efficiency, Luminosity dijet cross sections

29 2006 dijet A LL measurement

30 Size of statistical errors on 2006 dijet A LL BJP1

31 for 2008

32 Evolution of STAR Dijet program First cross section measurement west barrel - west barrel dijets First A LL measurement barrel - barrel dijets, barrel - endcap dijets May provide better constraint on Δ G than inclusive jets Wide acceptance including endcap - endcap dijets

33 Dijets from different η range are sensitive to different x range

34 A LL prediction by Pythia with the weight for 20pb -1 and P = 65%

35 End