# Presentation: 466368

## Photon and -Jet Reconstruction in the STAR Endcap EMC; Towards -Jet  Constraints on G  

***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
-  *γ**/**π**0* shower shape discrimination with the  ESMD shower max detector
-  status and outlook

- DNP08, 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****, **** ,**je**t  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 *f**q**/ f**q *(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 < 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

##  Dominant background to prompt γ production: π0(η)→γγ

********* ******- ******Jet: challenge of rare probes***

*** ****Significant ****G(x) constraints at  achievable ****L **dt requires **** (-jet) ID well below “original”  p**T **= 10 GeV/c plan.*
* *γ/π0 ≈ 1/10 at pT=10 GeV, but only 1/40 at pT=5 GeV
-  _how low in p__T__ 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: ****π****0****(****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:

## Photon Reconstruction for STAR (Spin) Physics

- 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. *

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

**Status of ********-jet analysis:**

##  - Jet Analysis and Detector Response 

**2.  cos(phi_gamma - phi_jet) < -0.8 **: g-jet opposites
**3.  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/ candidate
**7.  N_(5-strip clusler)^u > 3** : min # SMD strips u-plane
**8.  N_(5-strip cluster)^v > 3** : min # SMD strips v-plane
**9.  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 pure

- most bkgnd counts/issues/etc

- 7 GeV

## Maximum 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)

- _Maximum 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)

## Experimental Challenges: Shower Max. Det. Response

- 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!

***Experimental Challenges: Shower Max. Det. Response***

_**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 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

- _Example of 3-Gauss fit of DD shapes__ _

## Status of Isolated Photons in the Endcap EMC

- pre1=0 pre2=0

- pre1=0 pre2>0

- MC ****** -jet evts

- MC QCD bkgd

***Status 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]

**response**: data_peak [u+v]

- “Purity” of direct photons in data sample depends strongly on pre-shower response.

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

-  looks promising; BTW curves ordered reflecting inherent purity of sample

## Summary

***Summary***

***Summary and Outlook***

**Lots of good progress!  Positive steps include:**
_**Most essential features / dependences of 2006 data down to P**__**T**__**=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. p**T**)*
_**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