# Presentation: 266469

## Direct Photon Production

**Sung-Won Lee**
**Texas A&M University**
_**CTEQ Summer School on QCD Analysis and Phenomenology**__** **_
_**@ Sant Feliu de Guixols, Catalonia, Spain, 2003**_** **

## Physics Motivations for Prompt Photon Productions

2. Production Mechanisms for Prompt Photons

3. Measurement Techniques for Isolated Photons

4. Experimental Results

Part  I.  Prompt Photon Production at Tevatron
     Summary of CDF/DØ Run I/II Photon Results and kT issues

Part II.  Prompt Photon Productions at HERA 
     Summary of ZEUS/H1 Photon Results and kT issues

Part III.  Prompt Photon Productions at LEPII 
     Summary of LEPII Photon Results

5. Summary and Outlook

**2. **_**Production Mechanisms for Prompt Photons**_
**3. **_**Measurement Techniques for Isolated Photons**_
**4. **_**Experimental Results**_
_**Part  I.  Prompt Photon Production at Tevatron**_
**    ******** ****Summary of CDF/DØ Run I/II Photon Results and kT issues**
_**Part II.  Prompt Photon Productions at HERA**_** **
**    ******** ****Summary of ZEUS/H1 Photon Results and kT issues**
_**Part III.  Prompt Photon Productions at LEPII**_** **
**    ******** ****Summary of LEPII Photon Results**
**5. **_**Summary and Outlook**_

**Outline**

## Preamble

**Photons have a point-like coupling to the hard interaction, allowing for ****prompt probes and precision tests of perturbative QCD**
**Experimentally, the 4-vector of the photon can be measured more precisely than that of a jet, again pointing to precision tests of pQCD**
**In this lecture, I will focus on photon production in hadron-hadron (@ Fermilab), lepton-hadron (@ DESY), lepton-lepton (@ LEPII) collisions **
**The dominance of the gluon-Compton scattering diagram allows the ****potential for measuring the gluon distribution in the proton**** (although some theoretical complications are currently making that a bit difficult)**
**I will discuss about some puzzles in direct photon production**
**e.g.  effective parton kT, NLO resummation etc...**
_**Important to understand QCD of direct (prompt) photon production in order to reliably search for new physics and Higgs**_**  (see next slide)**

## Prompt Photon Motivation [I]

**As long as 20 years ago, direct(prompt) photon measurements were promoted as a way to:**
**Avoid all the systematics**** ****associated with jet ID and measurement**
**     ******** ****photons are simple, well measured EM objects**
**     ******** ****emerge directly from the hard scattering without fragmentation**
**Hoped-for sensitivity to the gluon content of the nucleon**
**     ******** “****QCD Compton process”**
**In the meantime... **
**     ****Jet measurements have become much better understood**
**Lower photon cross sections and ease of triggering on EM objects lead to photon data being at much lower E****T**** than typical jet measurements**
**Turn out to be susceptible to QCD effects at the few GeV level  **
_**Photons **__**have not been a simple test of QCD and have not given input to parton distributions**__**, and they continue to challenge our ability to calculate within QCD**_

## We can search for new physics with photons in the final state
Why photon? 

**Why photon?**

**Photon Signatures of New Physics**** **

**  **_**SUSY (N**__**2 **__**-> gN**__**1**__**, light gravitinos)**_
**  ****Large Extra Dimensions**
**  ****Excited leptons**
**  ****Technicolour**
**  ****New dynamics**
**  ****Higgs: V+Higgs -> V+gg**
**  ****W/Z+g production**

_**SUSY Models**_
** ****Minimal SUSY extension**
**   ****of  SM  ****(MSSM)**
** ****Minimal Super-Gravity**
**   ****(mSUGRA)**
** ****Gauge Mediated SUSY**
**   ****Breaking  ****(GMSB)**

**High P****T**** physics with photons and MET**

** ****Radiative decays of new ptl Predicted by SUSY, Technicolor,**
**  ****LED and other EWSB produce High E photons in the final state.**
** ****Anomalous multi-boson coupling lead to hadronic**
** ****production of photons in association with other gauge boson ****(WWgg, ZZgg)**

_**Empirically interesting !!**_

_**SM Higgs**_
_**H **__****__** **__****__**  is a discovery channel at LHC**_

## Published CDF Photon Analyses

| Photon + Dijet  | Dijet Properties (QCD) | PRD 57 (98) |
| --- | --- | --- |
| Photon + Muon | Intrinsic Charm (QCD) | PRL 77 (96), PRD 60 (99) |
| Photon + jet + X | Jet eta disribution (QCD) | PRD 57 (98) |
| Photon + X | Cross Section (QCD)
Angular distribution(QCD) | PRL 73 (94), PRD 48(93),
PRL 68 (92), PRL 71 (93) |
| Diphoton + X | eeggMet event (Search) | PRL 81 (98), PRD 59 (99) |
| Diphoton | Cross Section (QCD) | PRL 70 (93) |
| Diphoton + W/Z | Higgs/sGoldstino(Search) |  |
| Photon + Lepton+X | Signature-based (Search) |  |
| Photon + Z | Anomalous Couplings | PRL 74 (95) |
| Photon + W | Anomalous Couplings | PRL 74 (95) |
| Photon + b-jet + X | Techni-Omega (Search) | PRL 83 (99) |
| Photon + b | SUSY |  |
| Photon + D* | W -> photon + D* | PRD 58 (98) |
| Photon + track | W -> Photon + pion | PRL 76 (96), PRL 69 (92)
PRL 58 (98) |

## Published D0/ZEUS Photon Analyses

| Photon + X  | Cross Section (QCD) | PRL 77 (96) |
| --- | --- | --- |
| Diphoton + Met | SUSY (Search) | PRL 78 (97), PRL 80 (98) |
| Diphoton | Dirac Monopoles (Search) | PRL 81 (98) |
| Diphoton | LED (Search) | PRL 86 (01) |
| Diphoton + Dijet | Vhiggs (Search) | PRL 82 (99) |
| Many | Sleuth (Search) | PRL 86 (01), PRD 64 (01) |
| Z + Photon | Anomalous Couplings | PRL 75 (95), PRD 56 (97) |
| W + Photon | Anomalous Couplings | PRL 78 (97), PRD 58 (98) |
| b’ | Heavy Quark (Search) | PRL 78 (97) |
| Photon+Dijet + Met | SUSY (Search) | PRL 82 (99) |
| Photon + Z0 | Anomalous Couplings | PRL 78 (97) |

| Photon + Jet | First Observation (QCD) | PL B 413 (97) |
| --- | --- | --- |
| Inclusive Photon | Cross Section (QCD) | PL B 472 (00) |
| Photon + Jet | Effective Parton kT(QCD) | PL B 511 (01) |
| DIS Photon (+Jets) | Cross Section (QCD) | DIS 2002 |

## Why High Energy Photons? 

** ****Photons have a point-like coupling to the hard interaction, **
**        ****allowing for ****direct probes and precision tests of perturbative QCD**
**Experimentally, the 4-vector of the photon can be measured more precisely than that of a jet, again pointing to precision tests of QCD**
**As long as 20 years ago, prompt photon measurements were promoted as a way to:**
**avoid all the systematics associated with jet ID and measurement**
**Photon can be measured more precisely than jet. **
**emerge directly from the hard scattering without fragmentation**
**allows the potential for measuring the gluon distribution in the proton**
**Photons ****have not been a simple test of QCD and have not given input to parton distributions****, and they continue to challenge our ability to calculate within QCD**
**In addition, can search for new physics with photons in the final states:**
**Higgs:  ****H ******** ********  is a discovery channel at LHC**
**Recent SUSY Models:  ****Supergravity Model (mSUGRA), GMSB Model**
**Technicolor: ****Photon + dijet signatures, Diphoton resonances**
**Large Extra Dimensions, etc...**

## Prompt Photon Production at Tevatron (I)

**Production Mechanisms**

**Leading order structure has the photon recoiling against a jet**

_**The lowest order is**__** **__****__**s**_

** ****Large PT data test pQCD and**
**  ****help constrain fragmentation**
**  ****functions and PDFs.**
** ****The LO processes for prompt**
**   ****photon are relatively simple.**
** ****Prompt photon production is**
**  **_**sensitive to the g distribution.**_** **
** ****The lowest order is ********s****:**
**   ****LO structure has the photon**
**   ****recoiling against a jet**
** ****Next order(HO) is ********s********s ****: **
**   ****photon is balanced by 1 or 2**
**   ****jets in the final state**

** ****Complicated by**
****** ****parton distributions: a hadron collider is a broad-band quark and gluon collider**
****** ****both the initial and final states can be colored and can radiate gluons**
****** ****underlying event from proton remnants**

## Prompt Photon Production at the Tevatron

**Photon is balanced**
**by 1 or 2 jets in final state**

**Prompt photon production**
** ****is ****sensitive to the gluon**** distribution in proton**

**Experimentally we find that at about one third of the photon events have a second jet of significant E****T**

**Both the initial and final states can be colored and can radiate gluons**

**QCD Compton**
**scattering**

**Recoil Jets**

_**Simplest process**_** ********pp ******** ******** + jet**

## Typical Direct Photon Candidate Event

**Photon**

**Recoil Jet**

## Prompt Photon Production at HERA

***  ****prompt photon is ****produced**
**   ****directly**** in the hard scattering**

**Photoproduction**

**DIS**

**  ****Prompt photons can be produced in**
**   ****direct and resolved interactions.  **
**  ****In photoproduction, **_**only one LO**_
_**   **__**direct process: “Direct Compton”**_
**  ****HERA kinematics ****favor gluon from**
**   ****proton and quark from incoming**
**   ****photon ****(see resolved process)**

**  ****In DIS, prompt photons emitted by**
**   ****the direct process ****with no resolved**
**   ****contribution**

**  ****Sensitive to quark densities in**
**   ****photon at HERA**
**  ****The clean signature of the prompt**
**   ****photon can provide**** **_**a good means to**_
_**   **__**test QCD; photon structure, intrinsic**_
_**   **__**parton momentum(kT), NLO**_** ****etc...**

**direct**

**resolved**

**Prompt Photon Production at HERA**** **

**NLO QCD Predictions**

_**NLO QCD...**_
** ****photon pseudorapidity**
**   ****distribution is sensitive to **
**   ****the photon structure**
** ****Quark**
**   ******** ****backward region **
**        ****(eta < 2)**
** ****Gluon**
**   ******** ****forward region (eta > 2)**
** ****NLO QCD Calculation**
**   ******** ****Gordon and Vogelsang**

## Typical HEP Detector System

**Typical HEP Detector System**

**Hadronic **
**layers**

**Tracking system**** **
**Magnetized volume**

**Calorimeter**
**Induces shower**
**in dense material**

**Innermost **
**tracking layers**
**use silicon**

**Muon detector**

**Interaction**
**point**

**Absorber material**

**Bend angle ******** momentum**

**Electron**

**Experimental signature **
**of a quark or gluon**

**Muon**

**Jet **

**“****Missing transverse energy”**

**Signature of a non-interacting (or weakly**
**interacting) particle like a neutrino**

**EM layers**
**fine sampling**

## For example, CMS detector at LHC

_**Photon candidates**_**: isolated electromagnetic showers in the calorimeter, with no charged tracks pointing at them**

_**Identification of Photon Signal**_

## Photon Identification

**Usually jet contains one or more ********0**** mesons which decay to photons**
**we are really interested in **_**direct**_** photons (from the hard scattering)**
**but what we usually have to settle for is **_**isolated**_** photons (a reasonable approximation)**
**Isolation: require less than e.g. 2 GeV within e.g. ********R = 0.4 cone**
**This rejects most of the jet background, but leaves those cases where a single ********0**** or ******** meson carries most of the jet’s energy**
**This happens perhaps 10****–3**** of the time, but since the jet cross section is 10****3**** times larger than the isolated photon cross section, we are still left with a signal to background of order 1:1**

_**1. Conversion Probability:**_**  ********’****s to convert in a preshower detector**
_**2. Shower Profile:**_**  ****2 ********’****s from ********0****  will produce EM showers with broader**
**                               ****lateral and smaller longitudinal profiles**** **
_**3. Reconstruction:**_**  ****requires good EM/angular resolution (fixed target)**

**There are a number of different technique to distinguish photons from ********0 **** backgrounds. (see below)**

## Prompt Photons

**        ****at Tevatron**
** **

**Probing QCD**

**CDF/DØ**** ****Background Subtraction Methods**
**Summary of CDF/DØ Run 1 Photon Results**
**New puzzles from Tevatron photons**
**Run II Photon Results, so far...**

**Proton**

**Proton**

## The Fermilab Tevatron Collider

**The Fermilab Tevatron Collider**

**1992-95 **
**	****Run 1: 100 pb****-1**
**     ****1.8 TeV****  **
**	****Major detector upgrades**
**2001-04 ******** now  (2003)**** **
**	****Run 2a: 2 fb****-1****, 1.96 TeV****  **
**	****Run 2b Upgrade: short shutdown to install new silicon and others**
**2004-07(?)  ****Run 2b: ~ 15 fb****-1**

**CDF**

**DØ**

## Run 1  Run 2

**subprocess ********s**

**Number**
**of**
**Events**

**Run 2**

**Run 1**

**Increased reach **
**for discovery physics**
**at highest masses**

**Huge statistics **
**for precision physics**
**at low mass scales**

**Formerly rare processes**
**become high statistics**
**processes**

**The TeVatron is a broad-band quark and gluon collider**

**Extend the third orthogonal axis:**
**the breadth of our capabilities**

## Identification of Photon Signals

_**Signals**_
_**Backgrounds**_

****

****

****

******0**

_**Photon candidates**_**: isolated electromagnetic showers in the calorimeter, with no charged tracks pointing at them**

**Identification of Photon Signals**

**Central Calorimeter**

**Shower Maximum Detector**

**Preshower Detector**

_**CES has better separation, CPR better at high Et (Et>35)**_

** ****CDF/DØ uses two techniques for**
**   ****determination of photon signal;**
**   **_**1. EM Shower width**_
**   **_**2. Conversion Probability**_
** ****CDF measures the transverse **
**   ****profile at start of shower **
**   ****(preshower detector) and **
**   ****at shower maximum**
**  ****DØ measured longitudinal  **
**   ****shower development at start****   of shower**

|  | - | -Jet | Jet-jet |
| --- | --- | --- | --- |
|  CES | 24±6% | 28±8% | 48±7% |
|  CPR | 29±23% | 40±28% | 30±23% |

## Photon Purity Estimators

**CDF**

**DØ**

**DØ model longitudinal  energy depositions of photon’s and jets and perform a statistical comparison to data using the discriminant variable to determine the photon purity****.**

**Each E****T**** bin fitted**
** ****as sum of:**
**(a) = photons**
**(b) = bgd w/o tracks**
**(c) = bgd w/ tracks**

**For every photon, using the conversion and profile info.,  ****CDF find the fraction of candidates with this info****. ****(extracted signals statistically)**

**E1: E in the 1st  calo section**

## Standard Photon ID

Central Photon Cuts: 

Adjust transverse quantities to vertex
Number of 3-D track 
E fraction b/w HAD and EM Cal.
Calorimeter Isolation
Track Isolation
Two topological shower quantities
     1. EM Shower width
     2. EM Shower cluster energy

Additional Selections: 
Cosmic-ray
Tevatron Beam-Halo events

_**Standard Photon ID**_
_**Central Photon Cuts:**__ _
**Adjust transverse quantities to vertex**
**Number of 3-D track **
**E fraction b/w HAD and EM Cal.**
**Calorimeter Isolation**
**Track Isolation**
**Two topological shower quantities**
**     ****1. EM Shower width**
**     ****2. EM Shower cluster energy**
_**Additional Selections:**__ _
**Cosmic-ray**
**Tevatron Beam-Halo events**

_**Photon Triggers/Dataset**_
_**Many Triggers: all are running,**_
_** **__**including L2 (central/plug photons)**__ _
**Inclusive Photon: Et>25, w/   ISO**
**Ultra(Super) Photon: Et>50(70)**
**Diphoton: Et>12, w/   ISO**
**Diphoton: Et>18, w/o ISO**
**Triphoton: Et>10, w/o ISO**
**Photon: Et>16 + Muon**
**Photon: Et>16 + 2 jets (W/Z+   )**
**Photon: Et>10 + SVT track**
**Many studies started:**** ****Backgrounds,**
**calibration, fake rates, simulation...**** **
**Large samples are being collected**
**and tested.**

****

## CDF Photon Cross Sections at 1.8 TeV

**CDF,  PRD 65 (2002) 112003**

**(****Data-Theory)/Theory**

**Comparison of Run 1a and 1b**
**cross sections to CTEQ2M**

**Possible excess at low pT,**
**What’s going on at low ****E****T**** ****?**

**  ****CDF data from Run 1b agrees with that from 1a and probe both low Et and  high Et region in more detail.   Results show agreement with NLO, but shape at low p****T**** is suggestive. ****What causes the apparent shape at low pT?**

## DØ Photon Cross Sections at 1.8 TeV

**DØ, PRL 84 (2000) 2786**

**(****Data-Theory)/Theory**

**Central**

**Forward**

**Central**

**Forward**

**  ****The measured cross sections is in good agreement with NLO for Et > 36GeV**
**  ****The differences between the data and NLO for Et < 36 GeV suggests that a**
**    ****more complete theoretical understanding of the processes is needed.**

**why?**

## DØ Prompt Photons at 630 GeV

**(****Data-Theory)/Theory**

**DØ**

***s = 630 *GeV

**DØ, PRL 87 (2001) 251805**

**Measurement is higher than NLO at low Et in the central region but agrees at all other Et and in the forward region.**

**DØ**

**  ****Low xT deviations are not significant due to experimental uncertainties**
**  ****Good overall agreement w/ NLO QCD**

** ****At the end of Run 1, CDF and DØ both took data at lower CM energy,**** ********s = 630**
** ****DØ measured the photon x-sec at 630 and compared to 1800 photon x-sec.**

## Comparison of Photons at 1.8 TeV and 0.63 TeV

**CTEQ5M PDF**
**QCD scale = pT**

**xT=2pT/********s**

**PRD 65, 112003 (2002)**
** ****- ****Inclusive photon cross section at the different**
**   ********s ****compared to NLO QCD predictions**
** ****- ****A comparison of the 1.8 TeV and 0.63 TeV data**
**    ****to a NLO QCD as a function of pT and xT**

_**Deviations from NLO QCD predictions:**_
******steeper slope at low pT**
******normalization problem at high pT at 1.8 TeV**

**Photon pT and xT**** **

## CDF Results consistent those from DØ/UA2

**CDF Results consistent those from DØ/UA2**

_**pdf’s don’t appear to be the answer**_

**A comparison of the 1.8 TeV and**
** ****0.63 TeV cross sections to NLO**
**QCD using different PDFs;**
**        ****CTEQ5M (Solid)**
**        ****CTEQ5HJ, MRST99**
_**Many combinations of PDF and**_
_**scales have been tried and none**_
_**has been found that match the**_
_**shape of data**_

**CDF data agree well with**
**   ****the corresponding ****D0**
**   ****and UA2 measurements.**
** ****CDF and DØ data differ **
**   ****in  normalization by****~20%,**
**   ****consistent w/ systematic**
**   ****uncertainties...**

## Theory Wiggle Room

**Vogelsang et al. have investigated “tweaking” the renormalization, factorization and fragmentation scales separately, and can generate shape differences**

**Can add some shape to the prediction but hard to get good agreement with data...**
**And it is ****“NOT NATURAL”**** to change the scales so arbitrarily**

**Why is theory larger than data at high pT?**
_**Could soft gluon radiation from initial state by spoiling isolation cut?**_
****** ****One of puzzle at the moment...**

## What’s Happening at Low pT?

**What’s Happening at Low pT?**

**One possibility is an incomplete description of the initial state parton shower in NLO QCD calculation with possible kT recoil effect. **
**(see k****T**** Effects in Direct-Photon Production, PRD59 (1999) 074007)**

**kT denotes the magnitude of ****the effective transverse momentum**** of the colliding partons; Gaussian smearing of the transverse momenta by a few GeV can model the rise of cross section at low E****T **

**4 ****GeV kT Correction**

**Parton shower**
**(Baer and Reno)**

**3 ****GeV kT Correction**

**CDF**

**CDF**

**kT is expected to modify; **
**1****. ****back-to-back alignment of final state objects, **
**2. magnitude and shape of high pT inclusive x-sections**

## Fixed Target Photon Production

_**Fixed target experiment (E706) sees the largest disagreement with NLO**_
**Again, Gaussian smearing ****(~1.2 GeV)**** can account for the data.**
_**Theoretical uncertainties are too large to use prompt photons to determine the gluon distribution.**_

- <footer>

## Direct Photons and Parton kT

**The Tevatron exp. highlight serious**
**      ****limitations of current QCD description of prompt photon production**
**One offered explanation is that the partons in the proton may have a considerably higher kT ****due to soft gluon radiation at low pT**

**  ****<k****T****> ****increase as approximately**
**    ****logarithmic with ********s**
**  ****1 GeV for fixed target**
**  ****2.5 GeV at ********s = 630 GeV**
**  ****3~4 GeV for TeVatron at ********s = 1.8**

**Gaussian smearing of kT gives good agreement with Tevatron photon data**

**Comparison of photon XT for different photon experiments**

**expect ~2-2.5 GeV per parton**

## Why would you need to do this?

**Why would you need to do this?**

**Study the number and pT of Initial state gluons w/ PYTHIA MC**

**In PYTHIA, find that additional gluons****add an extra 2.5–5 GeV of p****T**** to the system**
**(2-3 GeV at 630 GeV – not shown)  **

**10 ****GeV ******** 2.6 GeV “k****T****”**

**50 ****GeV ******** 5 GeV “k****T****”**

**pT recoil values of 2-4 GeV may cause smearing of the measured direct photon falling pT spectrum.**
**...****but only matters if the effect is not completely included in the model considered, typically NLO QCD**

## Resummation

**Redictive power of Gaussian smearing is small**** **
**e.g. what happens at LHC?  At forward rapidities?**
**The “right way” to do this should be resummation of soft gluons**
**this works nicely for W/Z p****T, ****at the cost of introducing parameters**

**Threshold**
**resummation**

**Fixed Order**

**Laenen, Sterman, Vogelsang, **
**hep-ph/0002078**

**Threshold + recoil**
**resummation:**
**looks promising**

**Fink and Owens**
**hep-ph/0105276**

**DØ**

**Agreement with data is pretty**
**good now (Theory has improved!!)**

**Calculation using pT resummation of initial state gluons are on the horison..**

## Fink and Owens Resummed Calculations

**hep-ph/0105276**

**E706 data**

**DØ data**

**Agreement with data is pretty good**

_**Does require 2 or 4 non-perturbative parameters to be set**_

**Calculation using pT resummation of initial state gluons are on the horison..**

## Is It just the PDF?

**New PDF’s from Walter Giele can describe the observed photon cross section at the Tevatron without any k****T****, and predict the “deficit”**

**CDF (central)**

**DØ (forward)**

**Blue = Giele/Keller sets**
**Green = MRS99 set**
**Orange = CTEQ5M and L**

**Not all of Walter’s PDF sets****have this feature: it depends ****on what data are input**

## TeVatron Diphoton Productions

**Rate is very small: few hundred events in Run I (p****T**** > 12 GeV)**
**But interesting because**
**final state kinematics can be completely reconstructed **
**    ****(mass, p****T**** and opening angle of ******** system)**
**background to H ******** ******** at LHC**
**NLO calculations available**

## Diphoton Production at the Tevatron

**Diphoton production is interesting both for QCD tests and searches for new phenomena, but rate is very small (few hundred events in Run I)**
**The final state kinematics can be completely reconstructed  (mass, p****T**** and opening angle of ******** system)**

**p****T**** ~ 3 GeV**

**Need a resummation approach (RESBOS) or parton shower MC (PYTHIA) or ad hoc few-GeV k****T**** smearing**

**DØ**

**Diphoton mass reach for Run 2 extends out to nearly 600 GeV**

**gg**

**qq**

**Run 2 Prospects**

## Latest NLO Diphoton Calculation

**Binoth, Guillet, Pilon and Werlen, ****hep-ph/0012191**

**Shoulder at 30 GeV in calculation is a real NLO effect ****(contribution opens up with both photons on same side of the event)**

## Photon + Jet Angular distributions

**Back to back****in parton-parton****center of mass**

**boosted into lab frame**

**Photon + Jet Angular distributions**

_**The dominant process producing photons**_

_**Should be quite different from dijet production:**_

**Can we test this?**

_**Excellent agreement between OCD and Data...**_

**Unique test of pQCD**

## Photons as a probe of quark charge

**Inclusive heavy flavor production “sees” the quark color charge:**
**While photons “see” the electric charge:**

_**Photon vertex sensitive to electric charge.**_

**In semileptonic decays of heavy quarks, bottom  quark fraction is enhanced by harder fragmentation and sequential decays of the charm**

**Back to a photon sample, use the classic “pT relative” technique to separate final state charm and bottom. **

## CDF Photon + Heavy Quark

**The 1****st**** measurement of Heavy flavor contents of associated photon+******** events**
**The events are due to Compton Scattering process**** cg->c********->******

**2.9 (PYTHIA)  3.2 (NLO QCD)**

**The shape of the data agree with theory predictions, but ****fall below the theory in normalization by 2 standard deviations****.**

**A significant fraction of the events contain a final-state b quark. The ratio of c to b is in ****good agreement with QCD**

**PRD 65, 012003 (2002)**

**charm/bottom = 2.4 ******** 1.2**

## Run II CDF Photons

**Data from Aug 8 – Apr 5 (15 pb****-1****)**
**Inclusive photon sample**
** ****- cal/tracking Iso, HAD/EM cuts**
** ****- results are similar to Run 1B**
**Inclusive diphoton sample**
** ****- require 2 photons**
** ****- same requirement as single photon**
** **
**Diphoton is an interesting QCD measurement **
**but is also a great place to look for new**
**physics**

**trigger**

**offline**

## Run 2 Missing ET + di-EM Candidate

| EM1 | EM2 |
| --- | --- |
| ET = 27.4 GeV
 = 0.52
 = 3.78
Loose match with a low-pT track | ET = 26.0 GeV
 = 1.54
 = 5.86
No track match |
| MET = 34.3 GeV; M(diEM) = 53 GeV |  |

_****__**+**__**MET is**__** a signature of gauge-mediated SUSY-breaking**_

## Prompt Photons

**Probing QCD**

**Background Subtraction Methods**
**Summary of ZEUS Prompt Photon Results**
**ZEUS Determination of Parton kT**
**New H1/ZEUS Photon Results – Preliminary**

**Prompt Photons**
**              ****at HERA**
** **

## DESY HERA Collider

**First electron(positron)-proton collider in**
**the world, Hamburg, Germany**
**27.5 GeV electron + 920 GeV proton**** **
**Circumference: 6.336 km**
**4 exp: ZEUS, H1, HERMES and HERA-B **
**Data ~****130 pb****-1****/expt****. 2006~****1 fb****-1****/expt.**
**      ****ZEUS **
**Collaboration**

**e****+****p: **_**1994-2000**_** and e****-****p: **_**1998-1999**_

***s = 300 – 320 *GeV

## Prompt Photon Measurement at HERA

**Example of prompt photon production in the direct process at ZEUS**
**Clearly identified in calorimeter and well isolated**
**ZEUS BCAL has good granularity to separate high ET photon ****from neutral pion and eta meson backgrounds**
**Potentially significant backgrounds from jet fragments in dijet**
**Isolation cuts**** and ****Shower shape cuts**** are required to remove these**

## Identification of Photon Signal at ZEUS/H1

**Topological shower shape quantities are used to separate 2 nearby photons**

****

******0**

- 

****

******0**

- 

**1. Width of photon candidate in Z **
**2. Fraction of total photon energy**
**    ****in most energetic calorimeter cell**
** **
**1. Mean transverse shower radius**
**2. Shower hot core fraction**

**H1**

**H1**

**Signal extracted statistically by comparing events with  fmax > 0.75 and  fmax < 0.75**

## ZEUS  Inclusive Photon Cross Sections

**ZEUS, PLB 472 (2000) 175**

** **d/dET**all theoretical models describe the shape of the data well**
**                       ****PYTHIA does fairly well, HERWIG is a little low in magnitude**
-  d/d**generally described by LO and NLO over forward rapidities, **
**                      ****but ****there is a possible discrepancy in the rear region**** **
**  ****Given the discrepancies also seen in HERA dijet, there would appear**
**   ****a need to review the present theoretical modelling of the photon parton**
**   ****structure**

**Photoproduction**

## ZEUS Photon + Jet Cross Sections

_**ZEUS, PLB 472 (2000) 175**_

**  ****X********fraction of incoming photon energy taking part in the hard interaction**
** ****Clear peak near 1.0 : corresponding to Direct Compton process**
** ****There is a resolved contribution in x, observation are consistent with MC **
** ****Both the measured and theoretical distributions were found to be of a**
**   ****similar shape to those of inclusive photon production, but less strong**
**   ****discrepancy**

**Photoproduction**

## HERA kT - Experimental Motivation

**Comparison of photon XT for different photon experiments**
**Differing various exp have reported excesses at lower xT values compared to NLO predictions**

** ****TeVatron exp. Highlighted serious **
**   ****limitations of current pQCD description of**
**   ****high pT prompt photon production;**
**   ******** ****CDF/D0, E706 and other fixed target exp.**
** ****One offered explanation is that the partons**
**   ****in the proton may have a considerably **
**   ****higher kT (due to soft gluon rad. at low pT)**
** ****kT increase as approximately log with ********s**
** ****1 GeV for fixed target**
** ****2.5 GeV at ********s = 630 GeV**
**3~4 GeV for TeVatron at ********s = 1.8 **
** ****Gaussian smearing of kT gives good **
**  ****aggrement with TeVatron photon data**
** ****Can we see same intrinsic parton kT**
**   ****effect from HERA prompt photon data?**
****** ****Well, the answer is ...**** **** **

## ZEUS Determination of Parton <kT>

**Procedure to evaluate <kT>**
_**<kT>=1.69 ± 0.18 (+0.18,–0.20) [GeV]**_

**  ****Select a highly direct-enhanced sample to minimize effects of photon structure**
**  ****Modeling kT: Vary ‘intrinsic’ contribution, k0, in PYTHIA parton shower model**
**  ****Fit pT distribution using series of k0 values**
**  ****Determine <kT>intr from a fit at the detector level with extra k0 points**
**  ****Use PYTHIA again at parton level to incorporate parton shower effects**

**Photoproduction**

## A Consistent Picture of kT

**There may be an interesting connection between the Tevatron and HERA**
_**The new CDF/DØ Run2 measurement could add additional info to help interpret the kT effects and test theoretical models...**_

**W = invariant mass of photon + jet final state**** **

** ****Many experiments have made**
**   ****measurement of the effective **
**   ****parton kT in the proton**
**  ****Lower energies: expect a value **
**   ****~ 0.5 GeV corresponding to size **
**   ****of the proton**
**  ****Higher energies: higher values**
**   ****obtained – initial state parton**
**   ****showers?**
**  ****Different exp. use different**
**   ****methods, but the trend is evident**
**  ****ZEUS result consistent with **
**    ****this trend**

**it is an interesting puzzle**

## H1  Inclusive Photon Cross Sections

**  ****NLO describes the H1 data**
**   ****quite well, but is above the**
**   ****data in the forward region.**
**  ****PYTHIA, shape is OK, but**
**   ****low in normalization(30%)**
**  ****PYTHIA indicates effect of**
**   ****MI at large rapidity; would**
**   ****reduce NLO prediction**

**  ****NLO pQCD calculation**
**    ****Fontannaz , Guillet, Heinrich**
**    ****AFG/MRST2**
**  ****PYTHIA**
**   ****GRV(LO), MI, ISR/FSR**

**Photoproduction**

## ZEUS Photon vs. H1 Photons

**The H1 data are compared to the results of the ZEUS at**** ***********s = 300 GeV**** *
**The data are consistent, but the H1 data are somewhat lower at small rapidity, where the ZEUS results appear to exceed the NLO.**

**Photoproduction**

## Prompt Photon Production in DIS at ZEUS

**Reasonable agreement between the ZEUS data and NLO QCD calculations **
**(by Kramer and Spiesberger)**

**Photon + Jet**

**First Observation of prompt **
**photon in DIS at HERA**

**   ****Total measured x-sections**
** ****Inclusive photon**
**    ****5.95 ± 0.61 (+0.19,–0.26)  pb**
** ****Photon + Jet**
**    ****0.90 ± 0.15 (+0.19,–0.08)  pb**

## Prompt Photon Production at LEP

**Single-resolved**

**Double-resolved**

**PYTHIA badly normalized, why? ; **
**good agreement of differential cross-sections with NLO QCD calculations.**

## Summary and Outlook

**Prompt photon production in Hadronic collisions provides many unique tests of pQCD; ****generally agreement between QCD model and data****.**
**Recent Run 1 measurements of inclusive photon production at the Tevatron experiments indicate discrepancies with NLO QCD.  ****kT smearing effects in a simple Gaussian model works fine, though for gluon distribution studies one needs more fundamental approaches. Improved theoretical predictions are being developed. **_**(Theory is being pushed to higher order)**_
**From ZEUS prompt photon results, there are indications that ****our current understanding of the photon structure is lacking****; It is time to review the current parametrization of the photon parton densities.**
**Prompt photon analyses at the Tevatron/HERA are well underway and high luminosity photon data should ****provide experimental guidance to a better theoretical modeling**** of prompt photon production.**
**It is important to understand QCD photon production in order to reliably search for new physics with photons in the final states.**