Prompt Photons in Photoproduction at HERA

1. Prompt Photons in Photoproduction at HERA Preliminary Examination Eric Brownson University of Wisconsin Dec. 13, 2004

2. Outline • High Energy Physics • HERA and ZEUS • Kinematics • Prompt Photon Events • Event Sample and Cuts • MC Generation and Usage • Related Results • Summary and Plan for the Future

3. Structure Of The Proton • Studied via Probe Exchange • Wavelength of probe: l = h/Q • h: Planck’s Constant • Q: Related to the Probe’s Momentum • A smaller wavelength means greater resolution • HERA Collisions • Provides g or W/z as probes • HERA provides ep collisions with CMS Energy ~ 300 GeV • Deep Inelastic Scattering (DIS): Q2 < 40,000 GeV2 • Currently possible to probe to .001 fm (Proton is 1 fm)

4. Quark Parton Model • As people investigated particle creation and decays they found that particles could be grouped together. • The symmetries of those groups lead to the Quark Parton Model • Protons consist of point-like non interacting constituents i.e. Quarks • 3 Families of Quarks • The have Mass, Charge, and Spin

5. Quarks, Gluons & QCD • Quantum Chromodynamics (QCD) • QCD describes the “Strong” interaction • The Strong Force is mediated by the exchange of Gluons • Quarks are colored objects that interact via Gluon exchange • Individual Quarks have color, but only exist in colorless combinations • Parton Distribution Function (PDF) give the probability of finding a given Parton with a given momentum within the Proton

6. Jets and Hadronization When a Parton is Kicked out of the Proton you get a Jet… • Colored Partons produced in the interaction • Partons undergo hadronization to form colorless hadrons (Fragmentation) • Colorless collimated “spray” of hadrons called a “Jet” • Particle shower in calorimeter  observe deposited energy

7. Photoproduction Direct: Resolved: • Photon is almost real • Photon carries very little 4-momentum (Q2 ~ 0) • Most ep events are Photoproduction • Cross section has a (1/Q4) dependence • Direct: The Photon couples directly with a Parton • Resolved: The Photon fluctuates into a Partonic State

8. Prompt Photons Prompt: • Prompt Vs. Radiative • g is produced at the initial interaction point • g is radiated after the initial scattering takes place • It has information about the parton it hit • It Does NOT Undergo Hadronization Radiative:

9. HERA Description 820/920 GeV Protons 27.5 GeV e- or e+ CMS Energy 300/318 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 DESY Hamburg, Germany

10. HERA Luminosity • Total Integrated Luminosity from ’92  ’00: ~193 pb-1 • Total From ’02  ’04 : ~84 pb-1

11. ZEUS Detector

12. Central Tracking Detector e p View Along Beam Pipe Side View • Cylindrical Drift Chamber inside 1.43 T Solenoid • Measures event vertex • Vertex Resolution • Transverse (x-y): 1mm • Longitudinal (z): 4mm

13. Uranium-Scintillator Calorimeter h = 1.1 q = 36.7o h = 0.0 q = 90.0o h = -0.75 q = 129.1o h = 3.0 q = 5.7o h = -3.0 q = 174.3o Hadronic (HAC) Cells Electromagnetic (EMC) Cells Pseudorapidity Depleted Uranium and Scintillator 99.8% Solid Angle Coverage Energy Resolution (single particle test beam) • Electromagnetic: • Hadronic: Measures energy and position of final state particles

14. Barrel Presampler In Front of the Barrel Calorimeter We Have… The Barrel Presampler As an Electron or a Photon enters the BCal it first passes through a thin Scintillating strip • 416 Channels, one in front of each EMC/HAC tower • Each channel has 2, 5mm thick plates of scintillator • Enables us to evaluate the photon signal independent of it’s energy • The energy deposit is proportional to the number of photons that hit it

15. 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 Longitudinal momentum Vertex information Simple physics filters Third Level Commodity processor farm Full event info available Refined jet and electron finding Advanced physics filters ZEUS Trigger 107 Hz Crossing Rate, 105 Hz Background Rate, 10 Hz Physics Rate

16. 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

17. Measured Quantities: Eh, pz, pT2 Kinematic Reconstruction e’ e P

18. Other than g, What else do we want to see? In each process there is either a quark or a gluon kicked out of the proton by the scatter The Quark or Gluon will then Hadronize and form a jet…

19. 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 a stable position is found • Decide whether or not to merge overlapping cones • Advantages: • Lorentz invariant along z axis • Conceptually simple For the Jet:

20. 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

21. Prompt Photon Event

22. Photoproduction Observables Xgmeas: Fraction of the Photon’s momentum involved in the collision • Direct Photoproduction: Xg ~ 1 • Resolved Photoproduction: Xg < 1 Xpmeas: Fraction of the Proton’s momentum involved in the collision Momentum Imbalances of the Photon relative to the jet: Y g X jet

23. Model Events: PYTHIA Generator • Parton Level • LO Matrix Element + Parton Shower • Hadron Level Model • Fragmentation Model • Detector Level • Detector simulationbased on GEANT Parton Level Hadron Level Detector Simulation Factorization: Long range interactions below certain scale absorbed into proton’s structure

24. Related ZEUS Results ZEUS-Note 00-26 ZEUS-Note 01-09 ZEUS-Note 04-??

25. Related H1 Results DESY-04-118

26. Event Vertex • |Zvertex| < 55 cm •  Excludes Bam Gas Background • Also needed to accurately reconstruct the Event • Pt, Et, h, etc… • All need an accurate vertex What a nice Vertex… XY and Z

27. Tracking ?? • We want more than 3 Fitted Tracks from the Vertex •  Remove Wide angle Bremsstrahlung Events • We want the ratio of vertex fitted to fitted tracks to be less than 10 •  Beam Gas Vertex Fitted Tracks 1 Vertex Tracks Ratio 2

28. Ymeas • 0.2 < Ymeas < 0.8 • Lower Cut, • Proton Gas Background • Cosmic Events Upper Cut, DIS Events (Includes Events where the Photon is actually a misidentified Electron) Y meas 1

29. Xgmeas Fraction of the Photon’s momentum involved in the collision X_Gamma 1 • Direct Photoproduction: Xg ~ 1 • Resolved Photoproduction: Xg < 1

30. The Photon & Jet Et of Gamma 1 Eta of Gamma 1 Photon & Hadronic Jet found with the Kt Jet Finder The Photon: Eemc/Etot > 0.9 -0.74 < hg < 1.1 ETg > 5 GeV The Hadronic Jet: Eemc/Etot < 0.9 -1.6 < hg < 2.4 ETjet > 6 GeV Et of Jet 1 Eta of Jet 1

31. Electromagnetic and Hadronic Jets E_emc to E_tot of Gamma 1 • Electrons and Photons Deposit most of their energy in the EMC Section • Hadronic Jets Deposit most of their energy in the HAC Section E_emc to E_tot of Jet 1

32. Background & Neutral Mesons Background: BPRE Mips Figure 29 from the paper I got from Sascha Solution: Barrel Presampler Can be extended to the Forward and Rear Calorimeter With the FPRE and RPRE

33. Background & Neutral Mesons Background: Delta_z 1 Solution: F Max 1 Energy in the most energetic cell Total energy of the cluster

34. Summary

35. Plans for the Future

36. Plans for the Future • Change name: • Move to a second or third world country e.g. • Canada, Mexico, Florida, Russia • Set up some small scam to take power • CRUSH ALL THOSE THAT OPPOSE ME! • Laugh in an evil maniacal fashion • HA! HA! HA!….