Photon and -Jet Reconstruction in the STAR Endcap EMC; Towards -Jet Constraints on G W. W. Jacobs...

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Photon and Photon and -Jet Reconstruction in the -Jet Reconstruction in the STAR Endcap EMC; Towards STAR Endcap EMC; Towards -Jet -Jet Constraints on Constraints on G G W. W. Jacobs Indiana University for the Collaboration STAR STAR motivation with focus on Endcap (EEMC) issues and challenges (briefly) simulation, data and analysis techniques γ/π 0 shower shape discrimination with the ESMD shower max detector status and outlook DNP08, 24 October 2008, Oakland CA

Transcript of Photon and -Jet Reconstruction in the STAR Endcap EMC; Towards -Jet Constraints on G W. W. Jacobs...

Page 1: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

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

Page 2: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

-- 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

Page 3: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

STAR Endcap EMC: Component OverviewSTAR Endcap EMC: Component Overview

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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

Page 4: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

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- - 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:

Page 5: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

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.

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Photon Reconstruction for STAR (Spin) PhysicsPhoton Reconstruction for STAR (Spin) Physics

Status of Status of -jet analysis:-jet analysis:

Page 6: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

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

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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

Page 7: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

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

Page 8: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

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)

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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

Page 9: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

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!

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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

Page 10: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

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

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Example of 3-Gauss fit of DD shapes

Page 11: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

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.

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present “filtered” QCD bkgnd reject vs. -jet eff for diff preshower cond’s

looks promising; BTW curves ordered reflecting inherent purity of sample

Page 12: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

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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!

Page 13: Photon and  -Jet Reconstruction in the STAR Endcap EMC; Towards  -Jet Constraints on  G W. W. Jacobs Indiana University for the CollaborationSTAR

Backup Slides

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