Parity Violating Electron Scattering in Resonance region (Res-Parity)

1
Parity Violating Electron Scattering in Resonance
region (Res-Parity)
(P05-xxx) P. Bosted, Hall A meeting, June
2005

• Physics Overview
• Motivation
• Experiment
• Count rates and Errors
• Expected Results
• Request
• Summary Easy experiment, never done before,
  relevant to wider community

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PARITY VIOLATING ASYMMETRY
Electron can scatter off of proton by exchanging
either a virtual photon or a Z0
e e
g
e e
Z0

P
P

• The cross section in terms of electromagnetic,
  weak and interference contribution
• Asymmetry due to interference between Z0 and g

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Deep Inelastic asymmetry
In the Standard Model and assuming quark degrees
of freedom, at LO
In the valence region, for a proton target
1-0.75x
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Resonance region asymmetry

• For a resonance ARL can be written in terms of
  response functions

• Isospin symmetry relates weak and EM vector
  current
• Enchanced d,s quark contributions
• Sensitive to axial hadronic current also

Details have so far been worked out only for
N??(1232)
sensitive to axial vector transition form factor
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A Simple Model

• sin2qW 0.25 -gtaxial current suppressed
• Isospin symmetry
• Negligible strange and charm form factors

Assume
DEUTERON
PROTON

r(W) depends on (I0)/(I1)
r(W)

• Different dependencies in the resonant and DIS
  cases
• Resonant case the current is expressed through
  the square of the sum over parton charges
• DIS case the sum of the square gives the current

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QUARK-HADRON DUALITY

• In QCD can be understood from an OPE of moments
  of structure functions
• Duality is described in OPE as higher twist (HT)
  effects being small or cancelling

For spin-averaged structure function, duality
works remarkably well to low values of Q2
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DUALITY for the gamma-Z interference tensor ?
Leading order criteria
Simple Model
Duality is satisfied if on average sn/sp 2/3
PROTON

• Average of the D(1232) and the elastic peak tend
  to equal the DIS curve
• Higher resonances oscillate around the DIS curve

DIS model
Resonance model
DATA NEEDED!
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PHYSICS MOTIVATION

• Provide the first measurements of the parity
  violating asymmetries over the full resonance
  region for proton, deuteron, and carbon.
• Explore both global and local quark-hadron
  duality with the previously un-studied
  combination of structure functions
• Sensitive to down and strange quark currents
• Sensitive to axial hadronic current

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PHYSICS MOTIVATION

• The results are of practical importance

• Modeling neutrino cross sections needed for
  neutrino oscillation experiments.
• Understanding backgrounds and radiative
  corrections for other PV experiments (E05-007 in
  near future)
• Understanding the role of higher twist effects
• If duality is verified small higher
• twist
  effects

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NEUTRINO OSCILLATION

• Major world-wide program to study neutrino mass,
  mixing
• Interpretation needs neutrino cross sections in
  few GeV region on various nuclei (proton, oxygen,
  steel, )
• Neutrino beams not monochromatic, flux hard to
  measure direct cross section measurements
  problematic
• Rely on models. Res-Parity will constrain models
  (especially d,s quark currents, axial hadronic
  current, and nuclear dependence is EMC effect
  same for u and d quarks?).

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BACKGROUND IN E158

• SLAC E158 measured PV in Moller scattering, with
  large systematic error from low Q2 ep background
• Res-PV will constrain models used to estimate the
  background
• Result is improved confidence in the important
  E158 limits on physics beyond the Standard Model

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Experimental Setup
Target density fluctuation other false asym
monitored by the Luminosity Monitor
C, 25-cm LD2, LH2 targets (highest cooling power)
Pol e- beam, 4.8 GeV, 80 uA, 80, D Pb/Pb 1.2
Electrons detected by the two HRS independently
Beam intensity asymmetry controlled by parity DAQ
Fast counting DAQ handle up to 1MHz rate with
103 pion rej.
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KINEMATICS AND RATES
for LD2 target
x Y Q2 E W2 p/e MHz
dA/A 0.175 0.50 0.6 2.8 3.9 0.6 0.8
4.5 0.245 0.39 0.7 3.2 3.1 0.2 0.9
4.0 0.359 0.29 0.8 3.6 2.3 0.0 1.0
3.8 0.575 0.19 0.9 4.0 1.5 0.0 1.2
3.2

• Rates similar to E05-007.
• Pion/electron ratio smaller
• Run low E settings in one HRS, high E in other

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New Instruments and/or Upgrades

• Compton polarimeter will use green laser (in
  progress) expect to achieve D Pb/Pb 1.1 for
  electron analysis method
• 25-cm long racetrack-shaped LH2/LD2 cell, 2.5
  gm/cm2 C target (as used in Hall C)
• FADC-based and scaler-based fast counting DAQs,
  both being developed by the E05-007 (PVDIS)
  collaboration.

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PROJECTED STATISTICAL ERRORS

• Relative error of 5 to 10 per bin
• Local duality (3 resonance regions) tested at 4
  level
• Global duality (whole region) tested at 2 level
• Ratio of proton/deuteron (d/u) and C/deuteron
  (EMC effect) tested to 3 level

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SYSTEMATIC ERRORS
Source dA/A
Beam Polarization Kinematic determination of Q2 Detector and Electronic Linearity Electromagnetic radiative corrections Beam asymmetry Pion contamination Pair symmetric background Target purity and density fluctuations 0.012 0.010 0.005 0.005 0.005 0.005 0.002 0.002
Total 0.020
The ratio of asymmetries on hydrogen or C to
deuterium is almost free of experimental
systematic errors, allowing a very precise
comparison with theory
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REQUEST
E Target P HRS-L/R time
4.8 GeV LH2 4.0, 3.2 GeV 2 days 4.8 GeV LH2 3.6, 2.8 GeV 2 days 4.8 GeV LD2 4.0, 3.2 GeV 2 days 4.8 GeV LD2 3.6, 3.8 GeV 2 days 4.8 GeV C 4.0, 3.2 GeV 2 days 4.8 GeV C 3.6, 2.8 GeV 2 dyas
Checkout 4 hours Pass Change from E05-007 8 hours Polarization measurements 8 hours e asymmetry 4 hours
Total 13 days
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REQUEST (continued)

• Electronics same as E05-007
• Compton polarimeter as for E05-007
• High beam polarization, moderately good beam
  stability and charge asymmetry (less stringent
  than Happex or G0)

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COLLABORATION

• Experience in PV (E158, Happex, G0)
• Augments E05-007 (DIS-PV more
• people to develop needed equipment

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SUMMARY

• FIRST weak current measurements in full resonance
  region. Surprises possible.
• Measure Ap, Ad, and AC for M lt W lt 2.2 GeV and
  ltQ2gt 0.8 GeV2
• Relatively easy (for PV) experiment using same
  equipment as E05-007
• Emphasizes d-quark contributions, sensitive to
  strangeness and axial hadronic current
• New regime for study of duality, higher twist
  effects, and EMC effect
• Needed to constrain models, which in turn are
  used for neutrino oscillation studies,
  backgrounds to experiments like SLAC E158,
  radiative corrections for DIS-parity experiments.

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BACKUP SLIDES
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DAQ Comparison of two options

• FADC-based
• Is what we eventually need (12 GeV program)
• Full event sampling for detailed off-line
  analysis
• If there is a highly-rated experiment, 2 years is
  possible.

• Scaler-based
• Similar to previous SLAC, and current Hall C
  scalers
• Mostly work for the electronics group, easy to
  do
• Still, need extra man-power and cost
• Specialized
• Only scaler info is recorded (on-line PID
  critical).

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FADC-based Fast Counting DAQ

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Scaler Electronics-based Fast Counting DAQ
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RELATION TO PR05-007

• Complementary lower W and Q2
• Study HT lower Q2 near W2 GeV, effects bigger
• Both proton and deuteron used
• Information needed for precision DIS-Parity to
  accurately calculate radiative corrections and
  constrain HT.

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COLLABORATION
W. Boeglin, P Markowitz Florida International
University, Miami, FL C Keppel Hampton
University, Hampton VA G. Niculescu, I
Niculescu James Madison University, Harrionburg,
VA P. E. Bosted (spokesperson), V. Dharmawardane
(co-spokesperson), R. Ent, D. Gaskell, J. Gomez.
M. Jones, D. Mack, R. Michaels, J. Roche, B.
Wojtsekhowski Jefferson Lab, Newport News, VA T.
Forest, N. Simicevic, S. Wells Louisiana Tech
University, Ruston, LO K. Kumar, K.
Paschke University of Massachusetts, Amherst, MA
F. R. Wesselmann Norfolk state university,
Norfolk, VA Yongguang Liang, A. Opper Ohio state
university, Athens, OH P. Decowski Smith
College, Northampton, MA R. Holmes, P.
Souder University of Syracuse, Syracuse, NY S.
Connell, M. Dalton University of Witwatersrand,
Johannesburg, South Africa R. Asaturyan, H.
Mkrtchyan (co-spokesperson), T. Navasardyan, V.
Tadevosyan Yerevan Physics Institute, Yerven,
Armenia
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Pion Background

• p/e ratio ranges 0.005 to 1.2 average about 0.2
• p signal 20x smaller than electron signal net
  contamination average is 1

• Pion asymmetry measured in 4th layer lead glass
  (lead between 3rd and 4th layers to fill electron
  signal). Expect lt 1ppm based on SLAC experiment

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Kinematic Determination of Q2

• dA/A proportional to dQ2/Q2
• From standard HMS uncertainties of q and 0.1 in
  E, central Q2 determined to 0.8
• Uncertainties in target, beam, collimater and
  quadrupole positions increase uncertainty in
  measured Q2 to 1
• Will be checked using normal counting mode at low
  beam current

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RADIATIVE CORRECTIONS
Un-radiated to radiated spin averaged cross
section
Determined by the x, Q2 dependence of F2

• The ratio of radiated to un-radiated ed parity
  violating asymmetry (Rp ) is close to unity ?
  Shape and magnitude of Rp determined by the
  probablity for an electron to radiate a hard
  photon
• Radiative corrections for Ap will be determined
  by an iterative fit to the data of this proposal
• systematic error in Ap lt 1

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SFs for the interference tensor
ARL in terms of structure functions for the
interference tensor of EM and weak currents
Proportional to the weak
interaction
W1
W2
W3
Well measured

• Can be use to study parity violating part of the
  weak neutral current
• Depending on the isospin of the final excited
  state the interference cross section is expected
  to show a resonance structure

ARL can be used to test duality for a linear
combination of W1,2,3
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OTHER PV EXPPERIMENTS

• Most electron PV experiments have focused on
  elastic channels
• SAMPLE at BATES, HAPPEX and G0 at Jlab, A4
  at Mainz
• probe the strange quark form factors of the
  nucleon
• Qweak at Jlab
• searching for physics beyond the standard
  Model
• G0 plan to study N-D transition
• No approved experiment to study the full
  resonance region!
• Essential in understanding the background
  in other PV
• experiments
• Largest systematic error of E158
• Understand the role of HT effects