1. Prompt Photons in Photoproduction and Deep Inelastic Scattering at HERA
Eric Brownson

2. Outline Introduction to ep scattering
HERA I, luminosity upgrade to HERA II and ZEUS
Kinematics
HERA I MC generation and usage
Verification via Deeply Virtual Compton Scattering
Prompt photons with jet in photoproduction
Event Sample and Cuts
Cross Sections
HERA II MC generation and usage
New Tools Developed
Prompt Photons in Deep Inelastic Scattering
Cross Section

3. Lepton Proton Collisions
Particle Scattering
A smaller wavelength means greater resolution
Wavelength of probe: l = h/Q
Q: Probe’s momentum
HERA Collisions
Ee=27.5 GeV , Ep=920 GeV
HERA provides ep collisions with CMS Energy ~ 320 GeV
Provides γor W/Z as probes
Deep Inelastic Scattering (DIS): Q2 < 40,000 GeV2
Currently possible to probe to .001 fm (Proton is 1 fm)
Introduce the basics of what goes on to separate high energy from other branches of Physics. Mention that cross section should be though of as a probability.
***Make sure to include relationship between Q and photon’s momentum in notes.

4. Quark-Parton Model Hadrons: particles that interact strongly
Bound states of structure-less particles (quarks)
Quark-parton model
Quark properties: mass, electric charge, spin
Quarks treated as point-like, non-interacting
Introduce the quarks and lead into gluons and QCD

5. Quark-Parton Model Proton contains only valence quarks
Partons considered point-like particles
Structure functions describing individual particles’ momenta distribution depend only on xBJ
XBJ is the fraction of the proton’s momentum carried by the valence parton
No Q dependence (Bjorken scaling)
fi(x) Parton density functions (PDF’s)
Must be experimentally determined

6. QCD and Colored Gluons Problems with Quark-Parton Model
Statistics for fermion D++
D++ comprised of 3 u quarks
Violation of exclusion principle under QPM
Valence quarks don’t carry proton’s entire momentum
By experiment
Single quarks never observed
Quantum Chromodynamics: Theory with color quantum number
D++ quark composition: uRuBuG
Mediator of color force  gluon
Gluons carry roughly half proton momentum
Gluons carry color  Observed particles “colorless”
Isolated quarks not observed  Confinement
Application of QCD to transition into next slide

7. Deep Inelastic Scattering
Photon has high 4-momentum
Photon is highly virtual
Deep Inelastic Scattering (Q2>1)
Scattered e- observed
Cross section has a (1/Q4) dependence
Only proton PDFs contribute

8. Photoproduction Direct: Resolved:
Photon carries very little 4-momentum
Photon is almost real
Photoproduction (Q2~0)
Scattered e- not seen (escapes down beampipe)
Direct Photoproduction: Photon couples to a parton
Resolved Photoproduction: Photon fluctuates into partonic state
xg = fraction of exchange photon’s momentum involved in the collision

9. Prompt Photons Background: ISR FSR Radiative Prompt Photon:
g is produced in the hard scatter
 Carries information about the struck parton
No Hadronisation correction
Sensitive to both quark and gluon densities
Non-Prompt Background:
Initial State Radiation (ISR): Photon is radiated from e- before interaction
Final State Radiation (FSR): Photon is radiated from e- after interaction
Radiative events: Photon is radiated after the interaction
Neutral mesons: Photon originates from a decay of a hadron
FSR
Radiative

10. Perturbative QCD for Prompt Photons in PHP
Arrange diagrams by increasing powers of as
LO:
A = A0 + A1as + A2as2 + …
Compton Process aem2
NLO:
Resolved initial photon asaem
Resolved final photon asaem
asaem2
asaem2
NNLO:
incomplete
+ …
Double resolved as2aem2
Box diagram as2aem2

11. Jets and Hadronization
Struck Parton  Jet
Colored parton produced in the interaction  “Parton Level”
Colorless hadrons form via hadronization  “Hadron Level”
Collimated “spray” of particles  “Jets”
Particle showers observed as energy deposits in detectors  “Detector Level”

12. Prompt Photons + Jet in Photoproduction
Presence of a jet:
More sensitivity to underlying partonic processes
Introduces some hadronisation
Smaller hadronisation correction than dijets
Photoproduction (Q2 ≈ 0):
Exchange photon is real
Large cross section
No additional Pt given to the g+jet system by e±
The g+jet will be back to back  Well separated
No ISR/FSR background
Resolved contribution:
g hadronic structure
Constrain gluon distribution
NLO calculation available

13. NLO QCD Prompt g with Jet in PHP Calculations
K.Krawczyk & A.Zembrzuski (KZ):
GRV parametrisation:
photon structure function
proton structure function
fragmentation function
Fontanaz, Guillet & Heinrich (FGH):
MRST proton structure function
AFG photon structure function
BFG fragmentation function
A.Lipatov & N.Zotov (LZ):
Kt-factorization approach
Unintegrated quark/gluon densities using Kimber-Martin-Ryskin prescription
I want to keep this one the same. I will re-read the papers before hand to be able to verbally highlight any differences. This way at least in print I treat the theories on equal footing.
ZEUS:
FGH used MRST01, AFG02
H1:
FGH used MRST02 AFG??

14. Prompt Photons in DIS Presence of a jet (Not Used):
More sensitivity to underlying partonic processes
Decrease s by a factor of ~6.5
Deep Inelastic Scattering (Q2 » 0):
Exchange photon is highly virtual
Only proton PDFs contribute
Q2 energy scale
Smaller cross section than PHP
ISR/FSR background contribution e±
ISR
FSR

15. H1 & ZEUS are general purpose detectors
HERA Description
920 GeV Protons
27.5 GeV e- or e+
CMS Energy 320 GeV
Equivalent to 50 TeV fixed target
220 bunches
Not all filled
96 ns crossing time
Currents:
~90mA protons
~40mA positrons
Instantaneous Luminosity:
1.8x1031cm-2s-1
ZEUS
Obligatory aerial photo of the HERA ring with a description of what it has.
Updated values for Currents and Luminosity are needed!
DESY
Hamburg, Germany
H1 & ZEUS are general purpose detectors

16. HERA Luminosity
Total HERA I Integrated Luminosity from ’93  ’00: ~193 pb-1
Total HERA II Integrated Luminosity from ’02  ’07 : ~561 pb-1
HERA II
HERA I
Obligatory plot of the HERA Luminosity
Should probably include the actual rate increase that the upgrade provided…
99-00 run has HERA 94.95
ZEUS on-tape  73.4
Events  * 106
The First half of ’02 has  ~78 pb^-1 by itself
Running Period
Years
e-p
e+p
HERA I
300
10.3
71.0
320
17.1
94.9
HERA Upgrade!
HERA II
290.9
270.2

17. ZEUS Detector Proton (920 GeV) Electron (27.5 GeV)
Things to highlight;
CTD:
BCAL

18. Central Tracking Detectorp
View Along Beam Pipe
Side View
Cylindrical Drift Chamber inside 1.43 T Solenoid
Region of good acceptance: < h < 1.75
Good track resolution
Measures event vertex
Vertex Resolution
Transverse (x-y): 1mm
Longitudinal (z): 4mm
Mention that the +Z axis is along the Proton Beam
FTD:
-3 planar drift chambers
-Polar Angles deg
RTD:
-1 planar drift chamber
-Measure scattered e in low Q2 events
CTD:
-Radius: 86cm, Polar Angle: deg
-9 superlayers, each with 8 sense wire layers, 576 drift cells, 4608 sense wires and field wires
-Gas mixture of Ar:CO2:C2H6 (90:8:2)
-Signal wire: gold plated Mb Pot wire: gold plated Tungsten
-Signal wires (+) and potential wires (Ground)
-Charged particle ionizes atoms and electrons travel to signal wire. Time of arrival gives track info
R= p/qB
R=mv/qB relativistic
Pseudorapidity

19. Uranium-Scintillator Calorimeter
h = 1.1 q = 36.7o
h = 0.0 q = 90.0o
h = q = 129.1o
h = 3.0 q = 5.7o
h = q = 174.3o
Electromagnetic (EMC) Cells
Hadronic (HAC) Cells
Depleted Uranium and Scintillator
99.8% Solid Angle Coverage Near Hermetic
Energy Resolution (single particle test beam)
Electromagnetic:
Hadronic:
Compensating
Equal signal from hadrons and g / electron particles of same energy
Measures energy and position of final state particles
Detailed description of the Cal.

20. Barrel Preshower Detector
As a particle moves from the interaction point it passes through dead material in front of the BCAL
This leads to energy loss before measurement
BCAL Presampler measurement
Measured energy is proportional to the number of particles, not the energy of the individual particles  Neutral meson separation

21. ZEUS Trigger First Level Second Level Third Level
107 Hz Crossing Rate, 105 Hz Background Rate, 10 Hz Physics Rate
First Level
Dedicated custom hardware
Pipelined without deadtime
Global and regional energy sums
Isolated m and e+ recognition
Track quality information
Second Level
Commodity Transputers
Calorimeter timing cuts
E - pz cuts
Energy, momentum conservation
Vertex information
Simple physics filters
Third Level
Commodity processor farm
Full event info available
Refined jet and electron finding
Advanced physics filters
Description of the Trigger
DST Bit 79
 FLT cuts on; BCAL EMC energy, RCAL EMC energy, Total EMC energy, Total Cal energy, E_t in Cal, Require at least 1 good track
 SLT cuts on; Z_VTX from the CTD, (E-Pz), Box cut, (Pz/E)
 TLT cuts on; An “Electron” candidate must be found in ( |eta|<2 ) with ( Et > 3.5 GeV )

22. Kinematic Variables Center of Mass Energy of ep system squared
s = (p+k)2 ~ 4EpEe
Center of Mass Energy of gp system squared
W2 = (q+p)2
Photon Virtuality (4-momentum transfer squared at electron vertex)
q2 = -Q2 = (k-k’)2
Fraction of Proton’s Momentum carried by struck quark
x = Q2/(2p·q)
Fraction of e’s energy transferred to Proton in Proton’s rest frame
y = (p·q)/(p·k)
Variables are related
Q2 = sxy
Basic high energy variables, e.g. s, w^2, Q^2, x, y, etc …

23. Kinematic Reconstruction
Four measured quantities: EH, gH, E‘e, qe
Three reconstruction methods used: Double angle, Electron method, Jaquet-Blondel
Methods have different resolutions over different kinematic regions
Photoproduction scattered electron escapes down beampipe  Jaquet-Blondel gH qe
Variable
Double angle method (gH,qe)
Electron method (E’e,qe)
Jaquet-Blondel (EH,gH) Q2 x y
Main point: Many different ways to reconstruct the important variables
Note: although I use all three, I don’t necessarily use all three for each variable, but overall, I use them all.

24. Model Events: PYTHIA Parton Level Hadron Level Model Detector Level
Matrix element + Parton shower
Hadron Level Model
Fragmentation Model
Lund String (Next Slide)
Detector Level
Detector simulation based on GEANT
Parton Level
Hadron Level
Detector Simulation
Factorization: Long range interactions below certain scale absorbed into proton’s structure
PDFs

25. Lund String Fragmentation

26. CAL Islands & Energy Flow Objects
CAL cells have a finite size  Particles transverse multiple cells
Reconstruct particles by clustering cells with highest energy neighbor  “CAL Islands”
Can be thought of as number of local maxima
CAL Islands are clustered via probability functions of angular separation
Tracks are extrapolated to an impact point with the CAL
CAL Islands and tracks are clustered if the track’s extrapolated path is within 20cm of the CAL island  “Energy Flow Objects”

27. Jet Finding: Longitudinally Invariant KT Algorithm
In ep: kT is transverse momentum with respect to beamline
For every object i and every pair of objects i, j compute
di = E2T,i (distance to beamline in momentum space)
dij = min{E2T,i,E2T,j}[Dh2 + Df2] (distance between objects)
Calculate min{ di , dij } for all objects
If (dij/R2) is the smallest, combine objects i and j into a new object
If di is the smallest, then object i is a jet
Advantages:
No ambiguities (no seed required and no overlapping jets)
kT distributions can be predicted by QCD
The d^2_ij definition looks funny…
Mention that the parameter ‘R’ is similar to the cone size from the cone algorithm.

28. Isolated g Identification
Photons
Isolated e.m. calorimeter shower
No associated track
Neutral mesons decay into photons
Produce wider shower in the calorimeter (width)
Shower shape variables
Deposit energy at different rates
Barrel Preshower Detector (# of particles)
Barrel Preshower Detector
Barrel EMC Calorimeter
Barrel HAC Calorimeter g p0

29. Two methods can be used:
g Finders
Two methods can be used:
ELEC5
Modified e- finder
10 most energetic EMC cells
Energy in cones around it
Calculate & cut on a probibility
Extrapolate tracks to CAL for rejection
Standard method & relatively simple
Cone based isolation
KTCLUS
Modified KTCLUS jet finder
Run KT Algorithm on EFOs
Only jets that are trackless, electromagnetic & small number of CAL islands
Inherent track handling & kT-Cluster based isolation
New use as a g finder
ELEC5:
10 most energetic EMC cells
- If with 12 deg only highest energy one is considered
Calculated the energy in cones around it
- EMC inner = Energy with 0.25 rad
- EMC outer = Energy in annulus of cones of radii 0.25 and 0.4
- HAC1 inner = HAC1 energy in cone of 0.3 rad
- HAC2 inner = HAC2 energy in cone of 0.3 rad
Quality factor includes
- Energy weighted radius of emc energy within a 0.25 rad cone
- EMC outer / EMC inner
- HAC1 inner / (EMC inner + HAC1 inner)
- HAC2 inner / (EMC inner + HAC2 inner)
Cut on
- N_cells <= 35
- Quality factor
- E_e > 2 GeV
- EMC to total > 0.9

30. Deeply Virtual Compton Scattering
Provides a clean photon sample
DVCS Sample:
Only one track per event (e±)
Two isolated EM deposits (g,e±)
No hadronic activity
Data sample
Other cuts as in the paper:
“Measurement of Deeply Virtual Compton Scattering at HERA” Physics Letters B537 (2003) 46-62 g
Detection of DVCS photons:
BPRE
BCAL emc
BCAL hac

31. HERA I: DVCS Data vs. GenDVCS
Energy deposited in BPRE in mips
GenDVCS MC
Minimum ionizing particle units (mips)
Counts the number of charged particles passing through it
Improved agreement with additional dead material in MC
Probability for a photon to convert (ge+e-) well reproduced
Validates the modeling of photons by GenDVCS MC

32. HERA I: Prompt g with Jet in PHP
HERA I, Data, 77.1 pb-1
Photoproduction Sample:
0.2 ≤ YJB ≤ 0.8
Q2 <1 GeV2
2 or more jets from the Kt algorithm:
Photon candidate:
EEMC/ETotal ≥ 0.9
-0.7 ≤ hg ≤ 1.1 (BCAL region)
5.0 ≤ Etg ≤ 16.0 GeV
No associated track or within 1.0 in h,j
Low multiplicity: # of energy flow objects
Associated jet:
EEMC/ETotal ≤ 0.9
-1.6 ≤ hjet ≤ 2.4
6.0 ≤ Etjet ≤ 17.0 GeV
(Note: Etg cut ≠ Etjet cut)
Jet g

33. HERA I: PHP Prompt g + Jet LO MC
Data will contain prompt g with jet & inclusive Dijet events
Two PYTHIA MC sample must be generated
Signal MC: PYTHIA Prompt photon with jet subprocess only
Background MC: Inclusive PYTHIA Dijet MC without the Prompt photon subprocess
The relative amount of each is unknown
Add the Signal and Background MC according to a fit to BPRE distribution (g response confirmed  DVCS photons)
Require agreement in lowest BPRE bin
Use fractions from fit for analysis

34. Prompt g + Jet Data vs. MC
Fit sum of prompt g MC & background MC to Barrel Preshower Detector (BPRE)
Require agreement in lowest BPRE bin
Determine relative amounts
Large fraction of events with < 1 MIP  high purity
Excess of MC events in 1-3 mip range
No cut used
Tail well described
Examine other variables
DET = ET,Total – ET,(g + jet)
Dr = Distance (in hf) from g to any track
Both are well reproduced by the sum of MCs
Description verified via comparison between DVCS Data and GenDVCS MC

35. Identification of Jet & g by emc fraction
Fraction of energy in BEMC for g & jet
For Jet: EEMC/ETotal ≤ 0.9
For g: EEMC/ETotal ≥ 0.9
Well reconstructed
Minor shift towards lower Eemcg/Etotalg
Prevents double counting,
jet & g also identified by: tracking distinction and number of energy flow objects
Unique Assignments

36. Prompt g with Jet Data vs. MC
ETg and ETjet well described
hg and hjet shifted forward in data
Indication that higher order diagrams contribute

37. Prompt g with Jet Data vs. MC: xg
xg = fraction of exchange photon’s momentum involved in the collision
PYTHIA has deficiency in Resolved PHP
Indication that higher order diagrams contribute

38. Differential Cross Section & Correction Factors
N is the number of events in bin of size DY for the integrated luminosity L
Number of reconstructed PYTHIA events C=
Number of hadron level prompt photon with jet PYTHIA events
Only small deviations from bin to bin
High-hg and Low-xg are ~3x higher
Decrease in efficiency in forward region
Isolation from proton remnant

39. Prompt g + Jet in PHP HERWIG 13.5 pb PYTHIA 20.0 pb KZ 23.3 pb FGHLZ pb

40. Prompt g + Jet in PHP Data vs. LO MC
I use one global fit of dijet background to prompt g signal MC
Published ZEUS fits background ratio in each bin ETg, ETjet, hg and hjet
Agree well except for: high-hg and hjet
high-hg, has low efficiency
hjet, depends on photon in an indirect way
Data above the MC predictions
Particularly at low ET
High order diagrams
ZEUS 99-00, This Thesis
ZEUS 99-00, Published
PYTHIA 6.3
HERWIG 6.5
ETg (GeV) hg
ETjet (GeV)
hjet
Results published: Eur.Phys.J.C49: ,2007

41. Prompt g + Jet in PHP Data vs. NLO Calculation
ZEUS H1
Published ZEUS Result
Published H1 Result
Published H1 Result
ETg (GeV)
ETg (GeV) hg
Published ZEUS Result
LO, HERWIG & PYTHIA:
Underestimate measured cross section, particularly at Low-ETg
NLO, KZ & FGH:
Improved agreement with measured cross section
NLO, LZ:
Improves description for Low Etg & low hg hg
Eur.Phys.J.C49: ,2007

42. Prompt g + Jet in PHP Data vs. NLO Calculation
ZEUS H1
Published ZEUS Result
Published H1 Result
Published H1 Result
ETjet (GeV)
ETjet (GeV)
hjet
Published ZEUS Result
LO, HERWIG & PYTHIA:
Underestimate measured cross section, particularly at high hjet
NLO, KZ & FGH:
Improved agreement with measured cross section
NLO, LZ:
Improves description of high hjet
hjet
Eur.Phys.J.C49: ,2007

43. Prompt g + Jet in PHP: xg Published ZEUS Result
ZEUS 99-00, This Thesis
ZEUS 99-00, Published
PYTHIA 6.3
HERWIG 6.5
Published ZEUS Result
Agreement with published ZEUS data
Published ZEUS fits background ratio in each bin
KZ & FGH:
Improvement compared to LO MC, particularly at high Xg (Direct contribution)
LZ:
Improvement for low Xg (Resolved contribution)
Eur.Phys.J.C49: ,2007

44. Prompt Photons in DIS Same as photoproduction:
Isolated photon
Confirm description via DVCS study
Hadronic activity
Separation from background (neutral hadrons)
Additional selection and tools:
ELEC5 photon finder also used
Shower shapes
No jet requirement
Scattered electron
Large contribution from ISR & FSR from e±
ARIADNE for background simulation e±

45. Photon Finding by Shower Shapes
Combine signal (PYTHIA) & background (ARIADNE) MC to describe fmax & <dz> p0
Energy in most energetic BEMC cell
Total energy of cluster
fmax=
Energy weighted width in z direction
fmax
<dz> (cells)

46. HERA II: DVCS data
Start with cuts as the HERA I, 99-00, DVCS data study
Only one track per event (e±)
Two isolated EM deposits (g,e±)
No hadronic activity
New Requirements:
More restrictive hg requirements
-0.7 ≤ hg ≤ 0.9
Both ELEC5 and KT algorithms used
BPRE, fmax and <dz> studied
<dz> ≤ 0.65 cells
Provides a comparison between:
KTCLUS Vs. ELEC5
BPRE vs. fmax vs. <dz>

47. HERA II: DVCS Data vs. GenDVCS (ELEC5)
g selected with ELEC5 photon finder
ETg and hg not well reproduced
Confirm agreement in each region of ETg and hg (Next slide)
Number of cells in g, BPRE, fmax and <dz> are well reproduced

48. HERA II: DVCS Data vs. GenDVCS hg regions (ELEC5)
BCAL g g
Dead material
(Exaggerated)
No disagreement for ranges of ETg (Not shown)
Central regions show more compact photons
Less dead material
Shorter distances
Interaction point

49. HERA II: DVCS Data vs. GenDVCS (KTCLUS)
g selected with KTCLUS photon finder
Agrees with what was seen for ELEC5 selection
Number of cells in g, BPRE, fmax and <dz> are well reproduced
<dz> Is slightly broader than ELEC5

50. HERA II DVCS: ELEC5 vs. KTCLUS
Select photons found by ELEC5 and KTCLUS
Compare ELEC5 and KTCLUS reconstruction
KT clusters more cells into photon
ETg is similar
fmax and <dz> show a slight broadening for KT
Good agreement between ELEC5 and KTCLUS reconstruction of HERA II DVCS photons

51. HERA II: Prompt Photons in DISg
HERA II, Data,109 pb-1
No jet requirement
Deep Inelastic Scattering Sample:
Scattered e- in RCAL with Ee-> 10 GeV
Q2 > 35 GeV2
35 < E-PZ < 65 GeV
At least 2 tracks
Photon candidate from Kt algorithm:
EEMC/ETotal ≥ 0.9
-0.7 ≤ hg ≤ 0.9 (BCAL region)
5.0 ≤ Etg ≤ 20.0 GeV
No associated track or within 0.2 in h,j
Photon candidate from ELEC5 algorithm:
ET (GeV) j e- h g

52. KT & ELEC5: ETg Reconstruction
HERA II, PYTHIA, prompt photons (No jet requirement)
KTCLUS
ELEC5
Both show good resolution
Both usable as photon finders in DIS

53. HERA II, Prompt g in DIS: KTCLUS vs. ELEC5 Track Isolation
HERA II prompt photons (No jet requirement)
04-05 Data Vs. PYTHIA (Prompt g MC) & ARIADNE (Inclusive Dijet MC)
KTCLUS
ELEC5
Figures shown are normalized for Dr > 1.1
Similar treatment for prompt photons (PYTHIA)
Different treatment of neutral mesons (ARIADNE)
KTCLUS works better
Neutral mesons originating from jets get clustered with jet
Rejected by proximity of tracks

54. HERA II: Fit Dijet Background & Prompt g Signal MCs
HERA II prompt photon
(No jet requirement)
Require Dr > 0.2
Fit ratio of PYTHIA (Prompt g MC) to ARIADNE (Inclusive Dijet MC)
Perform c2 fit independently for each variable for each photon finder (Next slide)
Vary c2 per degree of freedom by 1.0 for uncertainty on fit
Calculate the cross section with each independent value for comparison (Next slide)
ELEC5
KTCLUS
ELEC5
KTCLUS
ELEC5
KTCLUS

55. HERA II: Prompt g in DIS cross section
HERA II prompt photon in DIS, cross section
(No jet requirement)
Large uncertainty on fit
fmax & <dz> are consistent
BPRE predicts more prompt photon PYTHIA than fmax & <dz>
% signal MC for KT changes little with increasing Dr
KT and ELEC5 start to converge for Dr>1.1
Distribution
% signal MC
% background MC
s (pb)
fmax
11.9
88.1
1.75
5.8±1.0
<dz>
15.7
84.3
0.77
6.9±1.3
BPRE
52.2
47.8
2.86
30.3
69.7
0.73
8.58±2.6
34.7
65.3
0.72
9.74±3.2
73.5
26.5
2.17
32.8
67.2
0.75
7.31± .35
40.1
59.9
0.52
8.26± .38
64.0
36.0
1.72
47.4
52.6
0.59
9.02± .44
54.4
45.6
0.62
9.92± .48
79.3
20.7
1.49
ELEC5
Dr > 0.2
KTCLUS
Previous had ~20% errors
- Used lower E_t^gamma range
- Show chi^2 equation for rand error on fit.
- poor eta^gamma description
ELEC5
Dr > 1.1
KTCLUS

56. Summary Photoproduction of prompt photons with jets:
Measured for 77.1 pb-1of data:
s(e±p  e± + prompt g + jet + X) = (stat.) (syst.) pb
First ZEUS use of a preshower detector to identify prompt g
PYTHIA & HERWIG MC underestimate the data by ~ x2
Particularly in low-Et & resolved PHP regions, largest ratios of NLO/LO
KZ, FGH & LZ NLO calculations underestimate data by 15-35%, mostly in the forward jet region & low-Et regions
Indication that higher order diagrams are needed
KZ & FGH best model direct photoproduction region while LZ best models resolved photoproduction region.
Cause: Gluon & photon PDFs contribute to resolved PHP
Results published: Eur.Phys.J.C49: ,2007
First HERA-II measurement of prompt photons in DIS
New tools developed and tested which provide more effective extraction of prompt g signal
Final HERA-II BPRE calibration (done)
Combinations with shower shape tools to provide ultimate g selector (Next)

57. The End.
Now for backup slides …

58. Jet Finding: Cone Algorithm
Maximize total ET of hadrons in cone of fixed size
Procedure:
Construct seeds (starting positions for cone)
Move cone around until ET is maximized
Determine the merging of overlapping cones
Issues:
Overlapping
Seed, Energy threshold
Infrared unsafe, s  ∞ as seed threshold  0
For the Jet:

59. Minimum ETJet>Minimum ETg in Prompt g with jet in PHP
5.0 ≤ Etg ≤ 16.0 GeV
6.0 ≤ Etjet ≤ 17.0 GeV
Improved agreement between data & all Theories
7.0 ≤ Etg ≤ 16.0 GeV
6.0 ≤ Etjet ≤ 17.0 GeV