Trijets in Neutral Current Deep Inelastic Scattering

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Prompt Photons in Photoproduction at HERA
Preliminary Examination
Eric Brownson University of Wisconsin Jan. 13,
2005
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Outline

• Introduction
• HERA and ZEUS
• Kinematics
• Prompt Photon Events
• Related Experimental Results
• MC Generation and Usage
• Event Sample and Cuts
• Summary and Plan for the Future

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Structure Of The Proton

• Studied via Probe Exchange
• Wavelength of probe l h/Q
• h Plancks Constant
• Q Related to the Probes Momentum
• A smaller wavelength means greater resolution
• HERA Collisions
• Ee27.5 GeV , Ep920 GeV
• HERA provides ep collisions with CMS Energy
  300 GeV
• Provides g or W/Z as probes
• Deep Inelastic Scattering (DIS) Q2 lt 40,000
  GeV2
• Probe to .001 fm (Proton is 1 fm)

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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

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Quark-Parton Model

• Proton contains only valence quarks
• Partons considered point-like particles
• Structure functions describing individual
  particles momenta distribution depend only on
  xBj
• No Q2 dependence (Bjorken scaling)
• fi(x) ? Parton density functions (PDFs)
• Must be experimentally determined

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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
• Sum rule for F2
• If QPM correct
• Value of integral shown to be 0.5 by experiment
• Quarks carry roughly half proton momentum
• Single quarks never observed
• Quantum Chromodynamics gluons with color quantum
  number
• D quark composition uRuBuG
• Mediator of strong force ? gluon
• Gluons carry roughly half proton momentum
• Observed particles colorless ? color
  conservation
• Isolated quarks not observed ? Confinement

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Photoproduction
Direct
Resolved

• Photon is almost real
• Photon carries very little 4-momentum
• Photoproduction (Q2 0)
• DIS (Q2 gtgt 0)
• Most ep events are Photoproduction
• Cross section has a (1/Q4) dependence
• Direct Photoproduction Photon couples to a
  parton
• Resolved Photoproduction Photon fluctuates into
  Partonic State

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Jets and Hadronization
Struck Parton ? Jet

• Colored Partons produced in the interaction ?
  Parton Level
• Colorless Hadrons form via hadronization ?
  Hadron Level (Fragmentation)
• Collimated spray of particles ? Jets
• Particle showers observed as energy deposits in
  detectors ? Detector Level

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Jets in Resolved Direct Photoproduction
For direct and resolved either a quark or a gluon
strikes out of the proton This struck quark or
gluon will hadronize and form a jet

• Direct Photoproduction
• ?Virtual photon scatters off of one of the quarks
• This leads to a sensitivity to the quark
  distribution
• Resolved Photoproduction
• ?A quark or gluon from the resolved virtual
  photon strikes a gluon from the proton
• ?This leads to a sensitivity to
• the gluon distribution

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Prompt Photons
Prompt

• Prompt
• g is produced at the initial interaction point
• ? Carries information about the struck parton
• No Hadronization
• Background
• Radiative Events, The photon is radiated after
  the interaction
• Neutral Mesons, The photon originates from a
  decay of s hadron

Radiative
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Photoproduction of Prompt g Jet

• Presence of a jet ?
• Allows the underlying QCD process in the gp
  interaction to be identified more clearly
• Presence of the prompt photon ?
• Provide information about the underlying parton
  process that is relatively free of hadronization
  uncertainties
• We have NLO calculations for the process
• Closely tied to ltKTgt of the partons

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Photoproduction of Prompt g Jet

• When we combine prompt photons and Jets
• We get knowledge about the hard scatter from
  jets
• We get the clean measurement of the Photon
• No non-perturbative hadronization of the photon
  as opposed to dijets
• Transverse energy measurements of
  electromagnetic showers have smaller experimental
  uncertainties than for jets
• More straightforward handle on quark and gluon
  distributions
• In particular the h distribution of the prompt
  photon is sensitive to the quark density of the
  proton
• We have NLO calculations for the interaction

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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

H1
ZEUS
DESY Hamburg, Germany
H1 ZEUS are general purpose detectors
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HERA Luminosity

• Total Integrated Luminosity from 92 ? 00 193
  pb-1
• Total From 02 ? 04 84 pb-1
• Plan for
• 05-07
• 0.5 fb-1

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ZEUS Detector
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Central Tracking Detector
e
p
Side View
View Along Beam Pipe

• Cylindrical Drift Chamber inside 1.43 T Solenoid
• Measures event vertex
• Vertex Resolution
• Transverse (x-y) 1mm
• Longitudinal (z) 4mm

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Uranium-Scintillator Calorimeter
h 0.0 q 90.0o
h 1.1 q 36.7o
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

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Barrel Presampler

• As a a particle moves from the interaction point
  it passes through dead material in front of the
  BCAL
• This leads to energy loss and particle decay
  before measurement
• BCAL Presampler measurement
• 416 Channels, one in front of each EMC/HAC tower
• Each channel has 2X5mm thick plates of
  scintillator
• Measured energy is proportional to the number of
  photons, not the energy of the individual photons

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Online Event SelectionZEUS Trigger

• 10 MHz crossing rate, 100 kHz Background rate,
  10Hz physics rate
• First level Use data subset 10 MHz ? 500 Hz
• Dedicated custom hardware
• Pipelined without deadtime
• Global and regional energy sums
• Isolated m and e recognition
• Track and vertex information
• Second level Use all data 500 Hz ? 100 Hz
• Calorimeter timing cuts
• E pz lt 55 GeV
• Energy, momentum conservation
• Vertex information
• Simple physics filters
• Commodity transputers
• Third level Use full reconstruction information
  
• 100 Hz ? lt 10 Hz
• Processor farm
• Full event information
• Refined jet and electron finding

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Kinematic Variables

• Center of Mass Energy of ep system squared
• s (pk)2 4EpEe
• Center of Mass Energy of gp system squared
• W2 (qp)2
• Photon Virtuality (4-momentum transfer squared at
  electron vertex)
• q2 -Q2 (k-k)2
• Fraction of Protons Momentum carried by struck
  quark
• x Q2/(2pq)
• Fraction of es energy transferred to Proton in
  Protons rest frame
• y (pq)/(pk)
• Variables are related
• Q2 sxy

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Kinematic Reconstruction
Escapes down beam pipe

• Measured
  Quantities Eh, pz, pT2

Photoproduction Topology
Luminosity Detectors
e
e
P
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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
• Issues
• Overlapping
• Seed Energy threshold
• Infrared unsafe s? 8 as seed threshold ? 0

For the Jet
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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 minE2T,i,E2T,jDh2 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

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Model Events PYTHIA Generator

• Parton Level
• LO Matrix Element Parton Shower
• Hadron Level Model
• Fragmentation Model
• Lund String (Next Slide)
• Detector Level
• Detector simulationbased on GEANT

Parton Level
Hadron Level
Detector Simulation
Factorization Long range interactions below
certain scale absorbed into protons structure
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Lund String Fragmentation

• color "string" stretched between q and q moving
  apart
• confinement with linearly increasing potential
  (1GeV/fm)
• string breaks to form 2 color singlet strings,
  and so on., until   only on-mass-shell hadrons.

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Photoproduction Observables Xgmeas
Xgmeas Fraction of the Photons momentum
involved in the collision

• Direct Photoproduction Xg 1
• Resolved Photoproduction Xg lt 1

Xpmeas Fraction of the Protons momentum
involved in the collision
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Photoproduction Observables P-

• Momentum Imbalances of the photon relative to the
  jet
• Linked to the intrinsic momentum of the struck
  parton ltKTgt

Xgmeasgt0.9
T
Y
g
X
jet
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Prompt Photon Event
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H1 Prompt Photons in Photoproduction hg
Measurement of Prompt Photon Cross Sections in
Photoproduction at HERA DESY Preprint 04-118

• NLO calculation describes data better
• Factor of just under 2 for the two cases
• Same qualitative shape

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H1 Prompt Photons in Photoproduction ETg
Measurement of Prompt Photon Cross Sections in
Photoproduction at HERA DESY Preprint 04-118

• Factor of just under 2 for the two cases
• Same qualitative shape
• Large errors associated with g Jet

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Previous ZEUS Prompt Photons in Photoproduction
Study of the effective transverse momentum of
partons in the proton using prompt photons in
photoproduction at HERA Physics Letters B 511
(2001) 19-32

• Photons with negative rapidity are of interest
• The errors are statistical

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Previous ZEUS Prompt Photons in Photoproduction
Study of the effective transverse momentum of
partons in the proton using prompt photons in
photoproduction at HERA Physics Letters B 511
(2001) 19-32
ltKTgt (GeV)
Xgmeasgt0.9
T

• ? Gives K0 1.42 GeV
• The errors are largely statistical

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New ZEUS Prompt g Jet Photoproduction Analysis

• Trigger Cuts
• FLT
• EMC Energies gt Threshold
• Total Cal. Energy gt Threshold
• At least one good Track
• SLT
• Zvtx lt 60 cm
• E-Pz gt Threshold
• ET(Box) gt 8.0 GeV
• TLT
• Limit on the Number of Bad Tracks
• At least one electron from the elec5 electron
  finder with,
• ETggt4.0 GeV , -3.0lthglt1.5

• Offline Cuts
• Zvtx lt 55 cm
• No Scattered electron
• ? Selects Photoproduction Events
• 0.2 lt YJB lt 0.8
• ?Lower Remove Beam Gas
• ?Upper Remove DIS Events
• Photon Jet
• Hadronic Jet
• If Two Hadronic Jets are found the One with
  higher ET is used

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Photoproduction Prompt g Jet Event Vertex

• Zvertex lt 55 cm
• Excludes Beam Gas Background
• Needed to accurately reconstruct the event
• PT, ET, h, etc

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Photoproduction Prompt g Jet Ymeas

• 0.2 lt Ymeas lt 0.8
• Y gt 0.2,
• Eliminate proton gas background
• Eliminate cosmic events
• Y lt 0.8,
• ?Eliminate DIS events
• i.e. Events where the photon is actually a
  misidentified electron

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Photoproduction Prompt g Jet Energy Deposit
Ratio

• Separating the photon and hadronic Jet
• Photons deposit almost all of their energy in
  the EMC section of the CAL
• Hadronic jets deposit more of their energy in
  the HAC section of the CAL than photons

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Background Neutral Mesons
Background
Solution Barrel Presampler

• BPRE signal lt 7 (mips)
• Reject events where the photon interacted with
  dead material
• Reject events with more photons

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Photoproduction Prompt g Jet ETg, hg
Photon Hadronic Jet found with the Kt Jet
Finder Photon cuts Eemc/Etot gt 0.9 -0.74 lt hg lt
1.1 ETg gt 5 GeV Hadronic jet cuts Eemc/Etot lt
0.9 -1.6 lt hg lt 2.4 ETjet gt 6 GeV
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Photoproduction Prompt g Jet ETjet, hjet
Photon Hadronic Jet found with the Kt Jet
Finder Photon cuts Eemc/Etot gt 0.9 -0.74 lt hg lt
1.1 ETg gt 5 GeV Hadronic jet cuts Eemc/Etot lt
0.9 -1.6 lt hg lt 2.4 ETjet gt 6 GeV If 2 jets were
found the one with the highest ET was used
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Summary Plan

• Summary
• Photoproduction of prompt photon jet provides
  a clean well described sample with sensitivity to
  quark gluon distributions
• They provide a means of measuring ltKTgt for
  partons
• BPRE provides a means of background separation
  independent of the photon energy (Commissioned in
  98)
• Plan
• Analyze new high luminosity sample
• Examine disagreement with model for forward jets
• Systematic error study
• Ymeas Lower cuts on jet and photon ET are of
  particular importance
• Calculate ltKTgt for partons