# Presentation: 646256

## Isolated Photon  Cross Section at DØ

**Ashish Kumar  **
**State Univ of New York at Buffalo**
**On behalf of the D****Ø**** Collaboration**

## Outline

** ****D******** expt at Fermilab Tevatron**
** ****Motivation **
** ****Analysis strategy**
** ****Cross section results**
** ****Comparison with theory**
** ****Summary**

- new

**hep-ex/0511045**
**Accepted by Phys.Lett. B**

**Tevatron pp-collider**

- -

**Run II (March 2001********) √s = 1.96 TeV**
-    36x36 bunches colliding per 396 ns
-    2-3 interactions/crossing

**Excellent Tevatron performance!**
**  ********L dt** : 27 pb-1 /week
-    **Delivered ********1.4 fb****-1**** **
-    Goal : 8 fb-1 by 2009

**record high**

**~1.2 fb****-1**** on tape  **
**(10x Run I )**

## The DØ Detector

- Inner tracker (silicon mictrostrips
-    and scintillating fibers) inside 2T
-    superconducting solenoid : |η|2.5
-    ******precise vertexing and tracking**
- Wire tracking and scintillating muon
-    system: |η|2
- Three-Level trigger  50Hz

**The D****Ø**** Detector**

- Liquid Ar sampling & U absorber
- Hermetic with full coverage (|η|4.2)
- 4 EM Layers : shower-max EM3
**Fine transverse segmentation**
-    x  = 0.1x0.1 (0.05x0.05 in EM3)
-  Good energy resolution

**x**

**y**

**Z**

- 

****

## Motivation

**Direct photons** emerge unaltered from the hard interaction
- **direct probe** of the hard scattering
-    dynamics
**clean probe** without complication from fragmentation & systematics associated with jet identification and measurement

-  **Precision test of pQCD**
-  **Direct information on gluon density**
-     in the proton : gluon involved at LO in
-     contrast to DIS & DY processes
-  Test of soft gluon resummation,
-     models of gluon radiation,..
-  Understanding the QCD production
-     mechanisms of photons is prerequisite
-     to searches for new physics.

- Compton

- Annihilation

**Dominant**

## Direct Photon Production

- Primarily produced by qg  q for pT  150 GeV
- precision test of QCD over much
**  ****wider p****T******** range** than Run I .
- probe G(x,Q2) with large Q2 & in
-    wide range: **0.02******** x****T**** ********0.25**

- Extremely challenging!
- (jets)/() 103  **severe background** from jet fragmenting into a leading 0 (or ), particularly at small pT

- Small background from electroweak processes (mainly W) at high pT

- signal

- background

**Bremsstrahlung**

## Photon Identification

**Photon Identification**

- Require :
-  ******95%** of energy in EM layers
-  **Isolation **:
-  **Veto track(s)** around EM cluster
-  Shower profile compatible with photon

- Reconstruct EM objects from energy clusters in calorimeter by cone algorithm

**Signal**

**Background**

- Suppress most of the jet background except when single 0 or  carries most of the jet’s energy : significant amount due to large jet cross section

## Event Selection

-  Single high pT EM triggers
-  Vertex: |z| 50 cm,  3 tracks
-  pT  23 GeV
-  || 0.9
-  Small missing ET (ET/pT  0.7) to
-     suppress Ws(e) and cosmic events.

- Main background : Highly em-jets with energetic 0, , Ks0, .  Can be reduced but not entirely removed.

**Photon**

**Recoil jet**

- Selection efficiencies estimated with fully simulated direct+jet events
- corrections derived from comparison of Ze+e- data/MC events.

-  **photon selection eff. **
**(-geom. accp. ~84%)****     **

## Background Suppression

- Design a neural network (NN)

**Cluster**
**width**

- NN trained to discriminate between direct photons and em-jets.

- NN test on Ze+e- events

- NNoutput0.5 eff ~94%

## Photon Purity

**Photon Purity**

- Photon purity determined from fitting NN output in data to predicted NN outputs for signal and background.
- statistical uncertainty dominated by
-    MC statistics (em-jet) at low pT and
-    data statistics at high pT.
- systematic  uncertainty from fitting
-    and fragmentation model in Pythia.

- Data well described by the sum of MC signal + background samples, especially for events with NNoutput0.5.

- After NN selection : 2.7x106 photon candidates : 17 pT bins

## Isolated Photon Cross Section

- ||0.9

- 2.7x106  candidates : 23pT 300 GeV

- Correction for finite detector resolution. pT corrected for shift in energy scale.

- Results shown with statistical  systematic uncertainties.

- Theory : NLO pQCD calculation from JETPHOX (P. Aurenche et. al.) using CTEQ6.1M PDFs & BFG FFs.

- NLO calculation by Vogelsang et. Al. based on small- cone approx. and using GRV FFs agree within 4%.

- Theoretical predictions consistent with measured cross-section.

## Data vs Theory

- Good agreement within uncertainties, in the whole pT range.

- Shape diff. at low pT : interpretation difficult due to large theoretical scale uncertainty and exp. syst. uncertainty.

- NNLO calculations should reduce scale dependence. Calculations enhanced for soft-gluon contributions should provide better descriptions of data at low pT.

-  Uncertainty from choice of PDFs  (MRST2004/Alekhin2004)  7%.
-  Variation in calculations for 50% change in isolation requirement and hadronic fraction in the cone 3%

- Measurement uncertainties
- Statistical : 0.1% - 13.2%
- Systematic : 13% - 25%
-       -- mainly from purity estimation

## Summary

- Direct photon production is an ideal testing ground for QCD predictions and constraining PDFs.

- D**** has measured inclusive cross section of isolated photons in central region (||0.9) and in the widest pT domain ever covered (23  pT  300 GeV). Results from the NLO pQCD agree with the measurement within uncertainties.

- Exciting work in progress with ~1 fb-1 data.
- Also on other fronts : , +jet,  +heavy flavor jet ..
- So stay tuned!

- Stack of disks with D data will soon eclipse Eiffel Tower.

## Backup

## Photon Energy Scale

- Photons lose noticeably less energy in the material upstream of  calorimeter than electrons (used for energy calibration)
- systematic over-correction in the energy scale for photons which would yield shift in the measured cross section.
- need to correct pT

- Use +jet and em-jet simulated events to determine shift between true and reconstructed pT 1.9% at 20 GeV, 1% at 40 GeV and 0.3% above 70 GeV.

- Neutral mesons component yield photons of smaller energy  additional shift of the measured pT.

## Systematic Uncertainties

- Luminosity : 6.5%
- Vertex determination : 3.6 - 5.0%
- Energy calibration : 9.6 – 5.5%
- Fragmentation model : 1.0 – 7.3%
- Photon conversions : 3%
- Photon purity fit : 6 – 13%

- Statistical uncertainties on determination of
- Geometric Acceptance : 1.5%
- Trigger efficiency : 11 – 1%
- Selection efficiency : 5.4 – 3.8%
- Unsmearing : 1.5%

## Gluon distribution uncertainties 

**Gluon distribution uncertainties **

- Most uncertain of the PDFs. The plot shows current uncertainty of the gluon distribution (due to experimental inputs only) estimated by CTEQ6.
-  15% for x0.25 and increases rapidly for larger x.
- at small x, the theoretical uncertainty (not included here) should increase widening the error band