Transcript LNS Lunch Seminar, MIT, September 11, 2007 Test of two-photon exchange in elastic scattering: A precision measurement with BLAST@DORIS Douglas Hasell, Michael Kohl,

LNS Lunch Seminar, MIT, September 11, 2007
Test of two-photon exchange in elastic scattering:
A precision measurement with BLAST@DORIS
Douglas Hasell, Michael Kohl, Richard Milner (MIT)
John Arrington (Argonne)
Overview
• Introduction
• Motivation from previous data
• Description of the proposed experiment
• Summary
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Nucleon Elastic Form Factors …
 Fundamental quantities
 Defined in context of single-photon exchange
 Describe internal structure of the nucleons
 Related to spatial distribution of charge and magnetism
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Rigorous tests of nucleon models
Determined by quark structure of the nucleon
Ultimately calculable by Lattice-QCD
Input to nuclear structure and
parity violation experiments
50 years of ever increasing activity
 Tremendous progress in experiment and theory
over last decade
 New techniques / polarization experiments
 Unexpected results
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(Hadronic) Structure and (EW) Interaction
Structure
Interaction
Factorization!
|Form factor|2 =
Probe
Object
s(structured object)
s(pointlike object)
→ Interference!
→ Utilize spin dependence of electromagnetic
interaction to achieve high precision
Born Approximation
Inelastic
Elastic
Structure
Electroweak probe
Lepton scattering
Hadronic
object
Interaction
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Nucleon Elastic Form Factors
General definition of the nucleon form factor
Sachs Form Factors
In One-photon exchange approximation above form factors are
observables of elastic electron-nucleon scattering
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Rosenbluth Separation
GE2
tGM2
θ=180o
θ=0o
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GpE and GpM from Unpolarized Data
charge and magnetization density
Dipole form factor
within 5% for Q2 < 10 (GeV/c)2
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Nucleon Form Factors and Polarization
Double polarization in elastic/quasielastic ep or en scattering:
Recoil polarization or (vector) polarized target
1,2H(e,e’p), 1,2H(e,e’p),
2H(e,e’n), 2H(e,e’n)
Polarized cross section
Double spin asymmetry = spin correlation
Asymmetry ratio (“Super ratio”)
independent of polarization or analyzing power
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Recoil Polarization Technique
 Pioneered at MIT-Bates
 Pursued at Hall A and MAMI A1
 In preparation for Hall C
Focal-plane polarimeter
Secondary scattering of polarized
proton from unpolarized analyzer
V. Punjabi et al.,
Phys. Rev. C71 (2005) 05520
Spin transfer formalism
To account for spin precession
through spectrometer
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Proton Form Factor Ratio
Jefferson
Lab
Dramatic discrepancy!
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All Rosenbluth data from SLAC and Jlab in agreement
Dramatic discrepancy between Rosenbluth and recoil polarization technique
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Multi-photon exchange considered best candidate
Proton Form Factor Ratio
F. Iachello et al., PLB43 (1973) 191
F. Iachello, nucl-th/0312074
mpGpE/GpM
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Iachello 1973:
Drop of the ratio already
suggested by VMD
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Q2/(GeV/c)2
A.V. Belitsky et al., PRL91 (2003) 092003
G. Miller and M. Frank, PRC65 (2002) 065205
S. Brodsky et al., PRD69 (2004) 076001
Quark angular momentum
Helicity non-conservation
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New Measurements at High Q2
 Extension proposed at Jefferson Lab
 Hall C PR01-109/PR04-108 recoil
polarization to run in fall 2007
 Hall C PR05-017 Super-Rosenbluth
Q2 = 0.9 - 6.6 (GeV/c)2 now completed
M.K. Jones et al., PRC74 (2006) 035201
Polarized Target:
Independent verification crucial
Polarized internal target / low Q2: BLAST
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Two-Photon Exchange
Two-photon exchange theoretically suggested
Interference of
one- and two-photon amplitudes
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P.G. Blunden, W. Melnitchouk, and J.A. Tjon, PRC72 (2005) 034612, PRL91 (2003) 142304
P.A.M. Guichon and M. Vanderhaeghen, PRL91 (2003) 142303
M.P. Rekalo and E. Tomasi-Gustafsson, EPJA22 (2004) 331
Y.C. Chen et al., PRL93 (2004) 122301
A.V. Afanasev and N.P. Merenkov, PRD70 (2004) 073002
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Two-Photon Exchange
Two-photon exchange theoretically suggested
Interference of
one- and two-photon
amplitudes
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P.G. Blunden, W. Melnitchouk, and J.A. Tjon, PRC72 (2005) 034612, PRL91 (2003) 142304
P.A.M. Guichon and M. Vanderhaeghen, PRL91 (2003) 142303
M.P. Rekalo and E. Tomasi-Gustafsson, EPJA22 (2004) 331
Y.C. Chen et al., PRL93 (2004) 122301
A.V. Afanasev and N.P. Merenkov, PRD70 (2004) 073002
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Experiments to Verify 2g Exchange
Precision comparison of positron-proton and electron-proton
elastic scattering over a sizable ε range at Q2 ~ 2-3 (GeV/c)2
J. Arrington, PRC 69 (2004) 032201(R)
SLAC data
At low ε : <Q2> ~ 0.01 to 0.8 (GeV/c)2
At high ε : <Q2> ~ 1-5 (GeV/c)2
Θ=180o
Θ=0o
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Experiments to Verify 2g Exchange
At low ε : <Q2> ~ 0.01 to 0.8 (GeV/c)2
At high ε : <Q2> ~ 1-5 (GeV/c)2
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Experiments to Verify 2g Exchange
Interference of
one- and two-photon amplitudes
Experiment proposals to verify hypothesis:
e+/e- ratio:
CLAS/PR04-116
Novosibirsk/VEPP-3
BLAST@DORIS/DESY
secondary e+/e- beam
storage ring / internal target
storage ring / internal target
SSA:
e-dependence:
PR05-15 (Hall A)
PR04-119 (polarized), PR05-017 (unpolarized)
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e+p/e-p Cross Section Ratio
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e+p/e-p Cross Section Ratio
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Proton Form-Factor Ratio
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Node of GE from positron-proton scattering at Q2 = 2.6 (GeV/c)2
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Proposed Experiment
• Electrons/positrons (100mA) in multi-GeV storage ring
DORIS at DESY, Hamburg, Germany
• Unpolarized internal hydrogen target (buffer system)
3x1015 at/cm2 @ 100 mA → L = 2x1033 / (cm2s)
• Redundant monitoring of luminosity
pressure, temperature, flow, current measurements
small-angle elastic scattering at high epsilon / low Q2
• Large acceptance detector for e-p in coincidence
BLAST detector from MIT-Bates available
• Measure ratio of positron-proton to electron-proton
unpolarized elastic scattering to 1% stat.+sys.
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DORIS parameters
Positron energy:
4.45 GeV
RF frequency
500 MHz
Initial positron beam current (5 bunches):
120 mA
Circumference:
289.2 m
Number of buckets:
482
Number of bunches:
1 (for tests), 2 and 5
Bunch separation (minimum):
964 nsec (for tests),
480 nsec and 192 nsec
Horizontal positron beam emittance:
404 pi nmrad
Coupling factor:
3%
Vertical positron beam emittance:
12 pi nmrad
Positron beam energy spread (rms):
0.11%
Curvature radius of bending magnets:
12.1849 m
Magnetic field of bending magnets:
1.2182 T
Critical photon energy from bending magnets:
16.04 keV
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Lifetime versus target thickness
DORIS
qm = 1.3 mrad
wm = 0.8%
SHR
qm = 1.0 mrad
wm = 0.13%
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BLAST at MIT-Bates
Bates Large Acceptance Spectrometer Toroid
Symmetric, large acceptance, general purpose detector
Detection of e±, p±, p, d, n
Longitudinally polarized electrons in SHR
850 MeV, 200 mA, Pe = 65%
Highly polarized internal gas target of pure H and D
(Atomic Beam Source)
6 x 1013 atoms/cm2, L = 6 x 1031/(cm2s), PH/D = 80%
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The BLAST Detector
Left-right symmetric
Large acceptance:
0.1 < Q2/(GeV/c)2 < 0.8
20o < q < 80o, -15o <  < 15o
COILS
BEAM
DRIFT CHAMBERS
TARGET
COILS
Bmax = 3.8 kG
DRIFT CHAMBERS
Tracking, PID (charge)
dp/p=3%, dq = 0.5o
CERENKOV
COUNTERS
CERENKOV COUNTERS
e/p separation
SCINTILLATORS
Trigger, ToF, PID (p/p)
NEUTRON COUNTERS
Neutron tracking (ToF)
BEAM
NEUTRON COUNTERS
SCINTILLATORS
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The BLAST Detector
Bates
MIT
UNH
ASU
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BLAST Scientific Highlights
Proton mGE/GM
C.B. Crawford et al.,
PRL98 (2007) 052301
Deuteron Structure
Neutron GnE and GnM
Analysis in progress
PRL in preparation
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BLAST Event Reconstruction
850 MeV energy
Electron
Left
ep-elastic
Electron
Right
 Advantages of magnetic field:
- suppression of background
- 2-3% momentum resolution
 σθ = 0.5o and σφ = 0.5o
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Acceptance for ep-elastic with BLAST
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Lowest epsilon ~0.4 only for E < 2.3 GeV
At epsilon = 0.4, require E>2 GeV to maintain Q2 > 2 (GeV/c)2
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Count Rate Estimate
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Sufficient counts at all angles for all energies < 4.5 GeV
At Q2 = 2.6 (GeV/c)2 beam energies 2.3-4.5 GeV for Rosenbluth sepn.
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Count Rate Estimate
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Sufficient counts at all angles for E < 4.5 GeV
epsilon = 0.4 achievable only for E < 2.3 GeV
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Projected Results for BLAST@DORIS
1000 hours each
for e+ and eLumi = 2 x 1033 cm-2 s-1
Control of Systematics
BLAST @ DORIS
Luminosity monitors
10o
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Change BLAST polarity once a day
Change between electrons and positrons once a day
Left-right symmetry
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Control of Systematics
i = e+ or ej= pos/neg polarity
Geometric proton efficiency:
Ratio in single
polarity j
Geometric lepton
efficiency:
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Control of Systematics
Super ratio:
Cycle of four states ij
Repeat cycle many times
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Change between electrons and positrons every other day
Change BLAST polarity every other day
Left-right symmetry
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Summary
• Significant effect theoretically predicted
• Convinced from feasibility of proposed experiment
• Contacted DESY, briefly presented to PRC in May 2007
• Submitted letter of intent in June 2007
• Presented to DOE at LNS review in July 2007
• Next steps:
Review of DORIS future program in September 2007
Preparation of full proposal
Envision running of one month per year for several years
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