Photon and -Jet Reconstruction in the STAR Endcap EMC; Towards -Jet Constraints on G W. W. Jacobs...
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Transcript of Photon and -Jet Reconstruction in the STAR Endcap EMC; Towards -Jet Constraints on G W. W. Jacobs...
Photon and Photon and -Jet Reconstruction in -Jet Reconstruction in the STAR Endcap EMC; Towards the STAR Endcap EMC; Towards --
JetJet Constraints on Constraints on GG
W. W. JacobsIndiana University
for the CollaborationSTARSTAR
motivation with focus on Endcap (EEMC)
issues and challenges (briefly)
simulation, data and analysis techniques
γ/π0 shower shape discrimination with the ESMD shower max detector
status and outlookDNP08, 24 October 2008, Oakland CA
-- Jet Coincidence Measurements: Why?Jet Coincidence Measurements: Why?
p
p
direct photon
jet
q
g
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);
LO pQCD
g q → q
*
Inclusive cannot compete statistically with incl. jet ALL … but -jet conic. meas. a “golden channel”
LLba
baLL a
ff
ffA ˆ
backward * large aLL (cross section also peaks here!)
above: very asymmetric collisions ’s boosted into STAR Endcap EMC 2
STAR Endcap EMC: Component OverviewSTAR Endcap EMC: Component Overview
3
Pb/Scint sampling e.m. calorimeter Covers 1.09 < η < 2 over full azimuth 720 projective towers (~ 22 Χ0) 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 MAPMT’s 30o sectors w/ no gaps ~ 1 mm peak resolution
Fully installed and operating since 2005
4
- - Jet: challenge of rare probesJet: challenge of rare probes Significant G(x) constraints at achievable L dt requires (-jet) ID well below “original” pT = 10 GeV/c plan.
γ/π0 ≈ 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 γ/π0 separation and overall bkgnd reduction (e.g, use shower max, preshower along w/ full detector response).
Dominant background to prompt γ production: π0(η)→γγ
charged particle vetoing from tracking with the STAR TPC (time projection chamber) “gives out” near middle (η ~ 1.5) of the Endcap
tower response from initial analyses shows strong η 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 data
Software tools:
isolation cuts – remove events where “” accompanied by jet fragments
SMD response – ensure energy dist. in SMD consistent with single shower
pre- / post-shower – exploit differing conversion efficiencies / discriminate against hadronic showers
away-side jet – require back-to-back to reduce background, pT matching
complete detector response -> “LDA”
Data samples:
MC and SMD “data-driven” MC of -jet events for 5 < pT < 35 GeV/c
Similarly 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 now
Note: different pT samples combined with proper weighting, norm’d 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 addt’l analysis and 2) “gamma tree” and “jet tree” approach, which combined produce gamma-jet candidates for additional cuts and algo analysis.
5
Photon Reconstruction for STAR (Spin) PhysicsPhoton Reconstruction for STAR (Spin) Physics
Status of Status of -jet analysis:-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 Jet Analysis and Detector Response
6
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 DataMC Simulations vs 2006 pp Data
pre1=0 pre2=0 pre1=0 pre2>0 0<pre1< 4 MeV 4<pre1< 10 MeV
momentum transfer pT
coun
ts
momentum transfer pT
coun
ts 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.
high
ly s
elec
ted/
mos
t pu
rem
ost bkgnd counts/issues/etc
7
7 GeV
γγ//ππ00 Discrimination in Endcap SMD Discrimination in Endcap SMDMaximum Sided Residual:
Look at transverse shower profile in Shower Maximum Det. (SMD) γ and e trans profile => expect “single peak” (response composed of
narrow+wide Gaussians w/ common centroid in each SMD (u.v) plane) π0→γγ expect “double peak” structure: main peak and peaklet (e.g., as
for an asymmetric π0 decay) Fit main peak & compute residual=(data – fit) on each side of main peak
=> pick maximum residual (π0’s should have more residual than γ’s)
8
Initial checks were w/ Pythia “slices” & STAR 2006 geometry:
u+
v y
ield
Calc. (evt-by-evt) the “distance” to γ/π0 separation curve … then sliding cut on this (projected) “distance” plot maps out signal efficiency vs. rejection efficiency
Direct γ’s (9-11 Gev) QCD bkgrd (25-35 GeV)
u+
v y
ield
u+v residual
π0’sγ’s
u+v residual Signal eff.B
akg
rd r
ejec
tio
n
How to make MC more realistic:
Compile library of shower shapes from “data” (no test beam … so, data in situ)
In MC, replace all γ 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!
9
Experimental Challenges: Shower Max. Det. Experimental Challenges: Shower Max. Det. ResponseResponse Comparison of Shower Shapes
0.001
0.010
0.100
1.000
0 5 10 15 20 25
Strip Number
He
igh
t Will's photons
MC photons
Pibero's photons
Do we understand SMD response shape? find simple MC width too narrow
separately, know from π0 finding algo’s, that MC doesn’t 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 LibraryData 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 “η”, Φ, etc.
Separated photons from etas (η→γγ)
S/B ~ 3:1 in range: use standard π0 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 strips
0.45<mγγ<0.65 GeV
10
Example of 3-Gauss fit of DD shapes
pre1=0 pre2=0
pre1=0 pre2>0
MC -jet evts
MC QCD bkgd
Status of Isolated Photons in the Endcap EMCStatus of Isolated Photons in the Endcap EMC
0<pre1< 4 MeV
4<pre1< 10 MeV
2006 pp data
residual: max( data_tail – fit_tail)[u+v]
resp
on
se:
data
_pea
k [u
+v]
“Purity” of direct photons in data sample depends strongly on pre-shower response.
11
present “filtered” QCD bkgnd reject vs. -jet eff for diff preshower cond’s
looks promising; BTW curves ordered reflecting inherent purity of sample
12
Summary Summary and OutlookSummary and OutlookLots 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 discrimination
Machinery 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!
Backup Slides
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