# Presentation: 290619

## Photon Cross Sections at Ecm=2TeV

- Photon Cross Sections at Ecm=2TeV

- Markus Wobisch, Fermilab

**for the CDF and D****Ø**** Collaborations**

**March 18-25, 2006**
**La Thuile, Vallee d’Aoste, Italy**

**QCD and High Energy**
**Hadronic Interactions**

## Motivation

- Calibration of electromagnetic energies well-understood
-           (in contrast to energies of hadronic jets)
- Large statistics
- Direct photons are substantial background
-      to many physics processes
- QCD production dominates:
-           Understanding the QCD production mechanism is
-           a prerequisite to searches for new physics & Higgs

- Direct photons come unaltered from the hard subprocess
-           direct probe of the hard scattering dynamics

**Notes:**
... in hadron collisions

## Fermilab Tevatron – Run II

**DØ**

**CDF**

**Chicago**

-  Ecm: 1.8  1.96 TeV
-  more Bunches 6  36
-  Bunch Crossing 3500  396ns
-  Run II started in March 2001
-  Peak Luminosity:
-                  1.7E32 cm-2 sec-1
-  Delivered: >1.5 fb-1
-                  (Run I : 0.26 fb-1)
-  Goal: 8 fb-1 before 2009

**Tevatron**

**Main Injector**
**& Recycler**

**Booster**

**p-bar source**

**Notes:**
1.5fb-1 delivered to each experiment – but presented analyses are using less

## Run II Detectors

- Multi-Purpose Detectors:
-  Tracking
-  Calorimeter
-  Muon System

## Probe the PDFs at 0.005<x<0.3
      where quarks are constrained 
         from HERA data
      sensitive to gluon density 
Test of NLO pQCD, 
     soft gluon resummation,
     models of gluon radiation

- Prompt Photon Production

- Probe the PDFs at 0.005<x<0.3
-       where quarks are constrained
-          from HERA data
-       sensitive to gluon density
- Test of NLO pQCD,
-      soft gluon resummation,
-      models of gluon radiation

**Notes:**
Figure shows sum of quark and anti-quark subprocesses

## Isolated Photon Production

- new DØ result: hep-ex/0511045 - submitted to Phys. Lett. B.

-   At least one isolated shower in the
-    electromagnetic (EM) calorimeter
-     - no associated track
-     - pseudorapidity < 0.9
-     - pT > 23 GeV
-   Small missing transverse energy:
-             MET/pT < 0.7
-     to reject cosmics and electrons from Ws
-  Main background:
-   jets with large EM fraction
-   - can be reduced but not entirely removed
-  Neural Net is trained to discriminate
-    between photons and EM jets
-    and tested on Z  ee
-  Photon purity: from fit of NN output
-    for MC signal and EM jets to data

**Notes:**
10% of background rejected by MET cut are Ws

## Integrated Luminosity: 326pb-1

2.7 million photon candidates

Analytical unfolding for 
    detector resolution

Inclusive Isolated Photon 
    Cross Section Results shown with stat. + syst. Uncertainties

Theory: NLO pQCD from JETPHOX with CTEQ6.1 PDFs
    and  μr = μf = μF  = pT

- Isolated Photon Cross Section

- Integrated Luminosity: 326pb-1
- 2.7 million photon candidates
- Analytical unfolding for
-     detector resolution
- Inclusive Isolated Photon
-     Cross Section Results shown with stat. + syst. Uncertainties
- Theory: NLO pQCD from JETPHOX with CTEQ6.1 PDFs
-     and  μr = μf = μF  = pT****

- |y|<0.9

**p****T******** (GeV)**

- L = 326 pb-1

## Ratio: data/theory (JETPHOX)
     good agreement over the
     whole range 23<pT<300GeV

A NLO pQCD calculation
    by Gordon and Vogelsang 
    agrees within 7%

Interpretation is limited by 
    - theoretical scale uncertainty
    - experimental syst. uncertainty

- Isolated Photon Cross Section

- Ratio: data/theory (JETPHOX)
-      good agreement over the
-      whole range 23<pT<300GeV
- A NLO pQCD calculation
-     by Gordon and Vogelsang
-     agrees within 7%
- Interpretation is limited by
-     - theoretical scale uncertainty
-     - experimental syst. uncertainty

-  PDF sensitivity requires:
-   Reduced exp. uncertainties – dominated by purity uncertainty
-   Improved theory (resummation / NNLO)

- L = 326 pb-1

**p****T******** (GeV)**

## Leading Order Diagram: 
dominant at 
high Di-Photon mass

- Di-Photon Production

- Leading Order Diagram:
- dominant at
- high Di-Photon mass

- Di-Photon final state: one of main discovery channels for Higgs at the LHC
- Possible signatures of new physics, such as large Extra Dimensions

**Notes:**
Left diagram is dominant at high masses (but not only there) 
 Higgs background at LHC 
 unique test at Tevatron (ppbar)

## Di-Photon Cross Section

-  **DIPHOX**:
-   - NLO prompt di-photons
-   - NLO fragmentation (1 or 2 ****)
-   - NNLO gg**** diagram
-  **ResBos**:
-  - NLO prompt di-photons
-  - LO fragmentation contribution
-  - Resummed initial state gluon
-     radiation (important for qT)
-  **PYTHIA** (increased by factor 2)

- CDF Collab., Phys. Rev. Lett. 95, 022003, 2005.

-  Integrated luminosity about 207pb-1
-  Two isolated showers in the EM calorimeter w/o associated track
-  Pseudorapidity < 0.9
-  Photon pT> 13 & 14 GeV  (avoid IR-sensitive regions in pQCD)
-  889 Di-photon candidate events - tot. eff: 15.2% - purity about 50%

**M******** (GeV/c****2****)**

## NLO fragmentation contribution
  - only in DIPHOX 
     at high qT, low , low mass

Resummed initial-state gluon radiation – only in ResBos  at low qT

Systematic uncertainties
Selection efficiencies (11%)
Background subtraction (20-30%)
Luminosity (6.5%)

- Di-Photon Cross Section

- NLO fragmentation contribution
-   - only in DIPHOX
-      at high qT, low ****, low mass
- Resummed initial-state gluon radiation – only in ResBos  at low qT
- Systematic uncertainties
- Selection efficiencies (11%)
- Background subtraction (20-30%)
- Luminosity (6.5%)

**************(rad)**

## outgoing quark flavor = incoming quark
dominated by:  HF + g  photon + HF
constrain heavy flavor PDFs
           (so far in CTEQ/MRST PDF fits: 
           generated from gluon-splitting / 
           no experimental constraints)

early Run II CDF preliminary results 
integrated Lumi = 67pb-1
good description of data by PYTHIA(CTEQ5L) 

new Tevatron analyses are in progress

- Outlook: Photon plus HF Jets

- outgoing quark flavor = incoming quark
- dominated by:  HF + g  photon + HF
- constrain heavy flavor PDFs
-            (so far in CTEQ/MRST PDF fits:
-            generated from gluon-splitting /
-            no experimental constraints)
- early Run II CDF preliminary results
- integrated Lumi = 67pb-1
- good description of data by PYTHIA(CTEQ5L)
- new Tevatron analyses are in progress

**photon ET (GeV)**

****+c

****+b

## Photon Production is an ideal testing ground for QCD predictions 
     & add. source for constraining PDFs

Discovery Channel for Higgs and
    Large Extra Dimensions, ...

CDF and DØ data for Isolated Photon and Di-Photon Cross Sections are 
    well-described by pQCD calculations

Current work in progress:
     Di-Photon, Photon + Jet,
     Photon + heavy flavor jet

-  Summary and Outlook

- Photon Production is an ideal testing ground for QCD predictions
-      & add. source for constraining PDFs
- Discovery Channel for Higgs and
-     Large Extra Dimensions, ...
- CDF and DØ data for Isolated Photon and Di-Photon Cross Sections are
-     well-described by pQCD calculations
- Current work in progress:
-      Di-Photon, Photon + Jet,
-      Photon + heavy flavor jet

- L=760pb-1

- New preliminary DØ result:
- Missing ET in Di-Photon events
-  limits for GMSB SUSY particles

**Limits (95% CL):**
**lightest neutralino > 120 GeV**
**lightest chargino   > 220 GeV**

- Missing ET (GeV)

**Notes:**
Gauge-mediated supersymmetry breaking