Photon and ï§ -Jet Reconstruction in the STAR Endcap EMC; Towards ï§ -Jet...

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Transcript of Photon and ï§ -Jet Reconstruction in the STAR Endcap EMC; Towards ï§ -Jet...

  • Photon and -Jet Reconstruction in the STAR Endcap EMC; Towards -Jet Constraints on G motivation with focus on Endcap (EEMC) issues and challenges (briefly) simulation, data and analysis techniques /0 shower shape discrimination with the ESMD shower max detector status and outlookDNP08, 24 October 2008, Oakland CA

  • - Jet Coincidence Measurements: Why? Direct dominated (~ 90% of yield) by QCD Compton process: q+g q+, with large LO gluon spin sensitivity For -jet coincidences, pT, ,jet x1, x2 and the angle * can be determined event-by-event. One uses high-x quarks (where most polarized) to probe low-x gluons (where they are abundant) Select kinematics to optimize G(x) sensitivity: high xq high fq/ fq (large quark polarization); Inclusive cannot compete statistically with incl. jet ALL but -jet conic. meas. a golden channel backward * large aLL (cross section also peaks here!) above: very asymmetric collisions s boosted into STAR Endcap EMC *

  • STAR Endcap EMC: Component Overview* Pb/Scint sampling e.m. calorimeter Covers 1.09 < < 2 over full azimuth 720 projective towers (~ 22 0) 2 preshower layers, postshower layer, and shower max. detector (SMD) L0 trigger- high tower, jet patches Scintillating strip SMD, 288 strips each per u and v planes WLS fiber - 16-anode MAPMTs 30o sectors w/ no gaps ~ 1 mm peak resolutionFully installed and operating since 2005

  • Dominant background to prompt production: 0()* - Jet: challenge of rare probes Significant G(x) constraints at achievable L dt requires (-jet) ID well below original pT = 10 GeV/c plan. /0 1/10 at pT=10 GeV, but only 1/40 at pT=5 GeV how low in pT can analysis be pushed while retaining high efficiency and purity? - need clever algos for /0 separation and overall bkgnd reduction (e.g, use shower max, preshower along w/ full detector response). charged particle vetoing from tracking with the STAR TPC (time projection chamber) gives out near middle ( ~ 1.5) of the Endcap tower response from initial analyses shows strong dependent bkgnd yields in both data and simulations; we hope/need to suppress these via cuts on the full detector response!30o sector tower reponse vs. preshower condition:

  • Main goal: use realistic MC simulations to discriminate efficiently & effectively between direct & QCD background evts, compare to 2006 dataSoftware tools:isolation cuts remove events where accompanied by jet fragmentsSMD response ensure energy dist. in SMD consistent with single showerpre- / post-shower exploit differing conversion efficiencies / discriminate against hadronic showersaway-side jet require back-to-back to reduce background, pT matchingcomplete detector response -> LDAData samples:MC and SMD data-driven MC of -jet events for 5 < pT < 35 GeV/cSimilarly MC and modified MC for QCD background events initial set 3 < pT < 65 GeV/c filtered set 3 < pT < 65 GeV/c pp_long polarized data from 2006 run use only events from L2_gamma trigger for nowNote: different pT samples combined with proper weighting, normd to 3.1 pb-1 Emphasis to date has been on Endcap photons + barrel (fully recon.) jets Two approaches: 1) di-jet jetfinder approach w/ selection of gamma-like and recoil jets for addtl analysis and 2) gamma tree and jet tree approach, which combined produce gamma-jet candidates for additional cuts and algo analysis. *Photon Reconstruction for STAR (Spin) PhysicsStatus of -jet analysis:

  • 1. N_events : 3 di-jet evts (by jet-finder) 2. cos(phi_gamma - phi_jet) < -0.8 : g-jet opposites3. R_{3x3cluster} > 0.9 : 3x3 cluster/total jet energy. 4. R_EM^jet < 0.9 : neutral E fraction cut on away jet 5. N_ch=0 : no chrg tracks assoc w/ candidate 6. N_bTow = 0 : no barrel towers assoc. w/ candidate7. N_(5-strip clusler)^u > 3 : min # SMD strips u-plane 8. N_(5-strip cluster)^v > 3 : min # SMD strips v-plane9. gamma-algo fail : failed tower SMD uv match, etc. 10. Tow:SMD match : tower SMD uv match bad, etc. initial jetfinder (di-jet) type analysis: a sequence of cuts select gamma and away side jets: early candidate response in the various Endcap detector layers subsequent investigations of influence of converting materials, assoc bkgnds,etc. suggest analysis vs. preshower conditions important ! - Jet Analysis and Detector Response *cut effects Cuts effectively select: jets opposite in phi gamma: large neutral fraction, recoil jet: lower neutral (e.g., with charged particles) select gammas in Endcap; jets in Barrel region other detector match/response details

  • MC Simulations vs 2006 pp Data MC vs. data and preshower condition: w/ in Endcap, jet in Barrel EMC di-jet analysis conditions with isolation (3x3 tower patch)/(r=0.7) > 0.90 data =black; MC -jet=red; MC QCD bkgnd=green similar but with isolation (3x3 tower patch)/(r=0.7) > 0.98 Overall good agreement of data and MC; similarly for pre, post specta, etc. highly selected/most puremost bkgnd counts/issues/etc *7 GeV

  • Maximum Sided Residual:Look at transverse shower profile in Shower Maximum Det. (SMD) and e trans profile => expect single peak (response composed of narrow+wide Gaussians w/ common centroid in each SMD (u.v) plane)0 expect double peak structure: main peak and peaklet (e.g., as for an asymmetric 0 decay)Fit main peak & compute residual=(data fit) on each side of main peak => pick maximum residual (0s should have more residual than s)

    /0 Discrimination in Endcap SMD*

  • How to make MC more realistic:Compile library of shower shapes from data (no test beam so, data in situ)

    In MC, replace all shower shapes (25 strips=central +/- 12 strips) with appropriate shapes from library after proper energy scaling, translation in SMD plane and superposition on underlying event Data-driven MC

    Consistency check: data-driven MC in better agreement with data!

    *Experimental Challenges: Shower Max. Det. ResponseDo we understand SMD response shape? find simple MC width too narrow separately, know from 0 finding algos, that MC doesnt reproduce strip fluctuations (extra spikey behavoir) that appear to drive low inv mass bkgnd further study reveals strong dependence on presh conditions (material effects), and other details! photon data_1

  • Data Driven Shower Max. Det. Response Library library shapes/replacement initially binned by: preshower response (pre1, pre2) photon energy at present use average shape over: SMD plane (U and V) Sector configuration (plane ordering) other effects wrt detector , , etc.

    Separated photons from etas ()S/B ~ 3:1 in range: use standard 0 finder with L2-gamma trigger try to make event selection w/o biasing shape turn off split, also lower floors, etc. but require minimum 20-strip peak separation soft peak isolation: 70% energy in central 5 strips0.45

  • pre1=0 pre2=0pre1=0 pre2>0MC -jet evtsMC QCD bkgdStatus of Isolated Photons in the Endcap EMC0
  • *SummarySummary and Outlook

    Lots of good progress! Positive steps include:Most essential features / dependences of 2006 data down to PT=7 GeV well reproduced by simulations (filtered MC sample in particular helped clarify)Significant investment of time and effort to generate new data-driven MC samples good reproduction of SMD response essential for all photon / meson /hadron discriminationMachinery in place ( and jet trees) to allow more detailed analysis, and including overall detector response, etc. eventually to fold into a more sophisticated algorithm optimization (e.g., Linear Discriminate Analysis) but still more to be done (re: direct photon purity and efficiency vs. pT)Optimization of isolation cuts (and vs. what theorists calculate). Charged particle veto (added isolation) highly desirable, but not easily implemented over much of Endcap. Sided-residual technique is powerful, but requires judicious choices of fitting function, fit range, range (# of strips) used for residual, boundary between signal and bkgd, etc. more tweaking needed here (also expanded shape library)Engage full detector response in advanced analysis! Anxious to look at run 8 data w/ reduced material near IR!

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