# Presentation: 413868

## Global Photon Summary

**Sung-Won Lee**
**Texas A&M Univ/Fermilab**
**(for the CDF/D0/ZEUS/H1/E706 Experiments)**
**14th Topical Conference on Hadron Collider Physics**
**University of Karlsruhe, Germany, 2002**** **

## Outline

**Physics Motivations for Prompt Photon Production**
**Production Mechanisms for Prompt Photon**
**Measurement Techniques for Isolated Photons**
**Experimental Results**
**Part  I.  Prompt Photon Productions at Tevatron**
**Summary of ****CDF/DØ**** Run 1 Photons and kT issue**
**Part II.  Prompt Photon Productions at HERA**** **
**Summary of ****ZEUS/H1**** Photons and kT issue**
**Summary and Outlook**

## Why High Energy Photons? 

**Photons have a point-like coupling to the hard interaction, **
**     ****allowing for ****prompt probes and precision tests of perturbative 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 that of a 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, we can search a new physics with photons**
**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

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

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

**Hadron collider is a broad-band quark and gluon collider**

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

**QCD Compton**
**scattering**

**Photon**

**Recoil Jets**

## 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(k****T****), NLO**** etc...**

**direct**

**resolved**

## 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**
**There are a number of different technique to distinguish photons from ********0 **** backgrounds. **
**Conversion Probability:**** allow ********’****s to convert in preshower detector**
**Shower Profile:**** 2 ********’****s from ********0****  will produce EM showers with broader**
**                                   ****lateral and smaller longitudinal profiles **
**Reconstruction:**** requires good EM/angular resolution (fixed target)**** **

## Prompt Photons

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

**Probing QCD**

** ****CDF/D0 Background Subtraction Methods**
** ****Summary of CDF/D0 Run 1 Photon Results**
** ****New puzzles from Tevatron photons**
** ****CDF/D0 Run 2a Photon Results, so far...**

**Proton**

**Proton**

## Signal and Background

**Signal**
**Background**

**Signal and Background**

****

****

****

******0**

**Preshower**
**Detector**

**Shower Maximum**
**Detector**

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

**Experimental Techniques in Run 1**

** ****CDF/D0 uses two techniques for**
**   ****determination of photon signal;**
**   ****1. EM Shower width**
**   ****2. Conversion Probability**
** ****CDF measured transverse profile ****   at start of shower (Preshower****   Detector) and at shower   ****   maximum (best below 35 GeV****)**
**  ****DØ measured longitudinal  ****   shower development at start****   of shower**

## Photon Purity Estimators

**CDF**

**DØ**

**D0 model longitudinal E 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)**

## 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**

**  ****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.**

## DØ Prompt Photons at 630 GeV

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

**(****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 with NLO pQCD**

## Comparison of Photons at 1.8 TeV and 0.63 TeV

**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**

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

**xT=2pT/********s**

** ****Deviations from predictions of NLO QCD **** **
** ****steeper  slope at low  p****T**
** ****normalization problem at high p****T**** at 1800 GeV**

**CDF**

**CDF**

## CDF Results consistent those from DØ/UA2

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

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

**pdfs don’t appear to be the answer**

## What’s Happening at Low pT?

**What’s Happening at Low pT?**

**One possibility is the uncompleted description of initial state parton shower in NLO QCD calculation with possible kT recol 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**

## Fixed Target Photon Production

**Even larger deviations from QCD observed in fixed target (E706)**
**Again, Gaussian smearing ****(~1.2 GeV)**** can account for the data.**
**Theoretical uncertainties are too large to use prompt photons for determination of gluon distribution.**

- <footer>

## Direct Photons and Parton kT

**Tevatron exp. highlighted 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**

## TeVatron Diphoton Productions

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

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

**We need NLO to model the data at large p****T****, but NLO is divergent at  p****T**** = 0**
**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**

**Run 2**

**gg**

**qq**

## CDF Photon + Heavy Quark

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

**charm/bottom = 2.4 ******** 1.2**

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

**The data agree in shape 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)**

## Run II CDF Photons

**Data from Aug 8 – Apr 5 (15 pb****-1****)**
**Inclusive photon sample**
** ****- cal/tracking Iso, HAD/EM cuts**
** ****- result is 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**

## Prompt Photons

**Probing QCD**

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

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

## Prompt Photon Measurement at HERA

**Example of direct process prompt photon at ZEUS**
**Clearly identified in calorimeter and well isolated**
**ZEUS BCAL has good granularity to separate high E****T**** 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

****

******0**

****

******0**

- 

- 

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

**Width of photon candidate in Z direction**

**Fraction of cluster E in highest E CAL cell**

**  ****Structure indicating ********, ********0****  and a multi-******** tail modelled as eta-mesons**
**  ****Cuts at 0.65 to remove most eta-mesons**
**  ****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**

## 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**
**  ****Modelling kT: Vary ‘intrinsic’ contribution, k0, in PYTHIA parton shower model**
**  ****Fit pT distribution using series of k0 values**
**  ****Determine <kT>intr  from fit at detector level with extra k0 points**
**  ****Use PYTHIA again at parton level to incorporate parton shower effects**

## A Consistent Picture of kT

**There may be an interesting connection between Tevatron and HERA, and new CDF 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 effective kT of**
**  ****parton in proton.**
** ****Lower energies: expect a value ~**
**  ****0.5 GeV corresponding to size of**
**  ****proton.**
** ****Higher energies: higher values**
**  ****obtained – initial state parton**
**  ****showers?**
** ****Different exp. Use different**
**  ****methods, but the trend is evident.**
** ****ZEUS consistent with this trend.**

## Prompt Photon Production in DIS at HERA

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

**First Observation of Prompt Photon in DIS at HREA**

**Inclusive Photon**

**Photon + Jet**

**5.94 ± 0.61 (+0.19,–0.26)**

**0.90 ± 0.15 (+0.19,–0.08)**

**Total measured cross section**

## 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**
**   ****is low in normalization(30%)**
**  ****PYTHIA indicates effect of**
**   ****MI at large rapidity; would**
**   ****reduce NLO prediction**

**H1, ICHEP02, 1007**

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

## ZEUS Photon vs. H1 Photons

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

## Summary and Outlook

**Photon results from Hadron Collision consistent with NLO QCD generally.**
**Recent Run 1 measurements of inclusive photon production at Tevatron experiments indicate discrepancies with NLO QCD.  ****kT smearing effects in simple Gaussian model works fine, though for gluon distribution studies one need more fundamental approaches. Improved theoretical predictions are being developed.**
**From ZEUS prompt photon results, there are indications that our current understanding of the photon structure is lacking; It’s the time to review the current parametrisation of the photon parton densities.**
**Photon analyses at Tevatron/HERA are well underway and all photon measurements will benefit significantly from more events.**
**Again, high luminosity photon data set should provide experimental guidance to a better theoretical modelling of possible initial-state soft gluon radiation effect**