Transcript 3rd Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan October 13-17, 2009, Waikoloa, Big.

3rd Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
October 13-17, 2009, Waikoloa, Big Island, Hawaii, USA
Experimental Probes of
Two-Photon Exchange*
Michael Kohl <[email protected]>
Hampton University, VA 23668 and
Jefferson Lab, VA 23606, USA
* Supported
by NSF grant PHY-0855473
Outline
 Form factors in the context of one-photon exchange (OPE)
 The limit of OPE or:
 What is GEp ?
 What is the structure of lepton scattering?
 Two-photon exchange (TPE): New observables
 Current and future experiments to probe TPE
2
(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
~|α|2 (α=1/137)
Hadronic
object
Interaction
3
Form Factors in OPE
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
4
Form Factors from Rosenbluth Method
 Determine
GE 2
|GE/GM|
tGM2
θ=180o
θ=0o
In One-photon exchange approximation above form factors are
observables of elastic electron-nucleon scattering
5
Nucleon Form Factors and Polarization
Double polarization in elastic ep scattering:
Recoil polarization or (vector) polarized target
1H(e,e’p),
1H(e,e’p)
Polarized cross section
Double spin asymmetry = spin correlation
Asymmetry ratio (“Super ratio”)
independent of polarization or analyzing power
6
Proton Form Factor Ratio
Jefferson Lab 2000–



All Rosenbluth data from SLAC and
Jlab in agreement
Dramatic discrepancy between
Rosenbluth and recoil polarization
technique
Multi-photon exchange considered
best candidate
Dramatic discrepancy!
>800 citations
7
Proton Form Factor Ratio
Jefferson Lab 2000–



All Rosenbluth data from SLAC and
Jlab in agreement
Dramatic discrepancy between
Rosenbluth and recoil polarization
technique
Multi-photon exchange considered
best candidate
Dramatic discrepancy!
>800 citations
8
Two-Photon Exchange: A Lot of Theory
Two-photon exchange theoretically suggested
Interference of one- and two-photon amplitudes
 P.A.M. Guichon and M. Vanderhaeghen, PRL91 (2003) 142303;
M.P. Rekalo and E. Tomasi-Gustafsson, EPJA22 (2004) 331:
Formalism … TPE effect could be large
 P.G. Blunden, W. Melnitchouk, and J.A. Tjon,
PRC72 (2005) 034612, PRL91 (2003) 142304: Nucl. Theory … elastic ≈ half, Delta opposite
 A.V. Afanasev and N.P. Merenkov,
PRD70 (2004) 073002: Large logarithms in normal beam asymmetry
 Y.C. Chen et al., PRL93 (2004) 122301: Partonic calculation (GPD), TPE large at high Q2
 A.V. Afanasev, S.J. Brodsky, C.E. Carlson, Y.C. Chen, M. Vanderhaeghen,
PRD72 (2005) 013008: high Q2, small effect on asym., larger on x-sec., TPE on R small
 M. Gorchtein, PLB644 (2007) 322: Fwd. angle, dispersion ansatz, TPE sizable
 Y.C. Chen, C.W. Kao, S.N. Yang, PLB652 (2007) 269: Model-independent TPE large
 D. Borisyuk, A. Kobushkin, PRC74 (2006) 065203; 78 (2008) 025208: TPE effect sizable
 Yu. M. Bystritskiy, E.A. Kuraev, E. Tomasi-Gustafsson, PRC75 (2007) 015207:
Importance of higher-order radiative effects, TPE effect rather small!
 M. Kuhn, H. Weigel, EPJA38 (2008) 295: TPE in Skyrme Model
 D.Y. Chen et al., PRC78 (2008) 045208: TPE for timelike form factors
 M. Gorchtein, C.J. Horowitz, PRL102 (2009) 091806: gamma-Z box
 D. Borisyuk, A. Kobushkin, PRD79 (2009) 034001: pQCD, sizable
 N. Kivel, M. Vanderhaeghen, PRL103 (2009) 092004: pQCD, sizable
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Elastic ep Scattering Beyond OPE
k’
s=1/2 lepton
k
p’
s=1/2 proton
Kinematical invariants :
p
Next-to Born approximation:
(me = 0)
The T-matrix still factorizes, however a new response term F3 is generated by TPE
Born-amplitudes are modified in presence of TPE
New amplitudes are complex!
Observables involving real part of TPE
Pl 
~

G M2 
 (G M )
2
(1   )(1   )

Y2 
1  2
ds red 
GM
1 

~
ds red
~
 (G M )
 (GE ) 
R 2
R
2
/ GM  1 
2
 2R
 2 1   Y2

t
GM
tG M
t
~
~
 (GE )  GE (Q2 )   (GE (Q2 ,  ))
E04-019
(Two-gamma)
e+/e- x-section ratio
CLAS,VEPP3,OLYMPUS
Rosenbluth non-linearity
E05-017
~
~
 (GM )  GM (Q2 )   (GM (Q2 ,  ))
~
R  GE / G M
t (1  t )(1   )  ( F3 (Q2 ,  ))
Y2  0 
1 
GM
Born Approximation
Beyond Born Approximation
P.A.M. Guichon and M.Vanderhaeghen, Phys.Rev.Lett. 91, 142303 (2003)
M.P. Rekalo and E. Tomasi-Gustafsson, E.P.J. A 22, 331 (2004)
Slide idea:
L. Pentchev
Some remarks
~
 Presence of TPE modifies GE and GM, AND generates new structure F3
 Measurement of one type of observable (double polarization or Rosenbluth
cross sections is insufficient to separately determine both GE/GM AND Y2γ.
 Without positrons, it is possible to use double polarization observables AND
Rosenbluth cross sections as functions of Q2 and ε to extract both GE/GM
and Y2γ(Q2, ε) ASSUMING that TPE is the accepted picture.
 Any change in the ε dependence of Pl or Pt/Pl is an indicator of non-zero Y2γ,
however its absence is no disproof, as Y2γ can also be ε-independent. Small.
 Any non-linear ε dependence of cross section is an indicator of non-zero Y2γ.
Absence is no disproof, as Y2γ can also be ε-independent. Small effect.
 RB plots ARE very linear in ε
 Pt/Pl constant vs. ε


Y2γ constant vs. ε ?
(1–2εR/(1+ε)) Y2γ constant
Y2γ =
0 ?
 Positrons are needed to definitively establish TPE.
The Y2γ terms change sign with the charge of the lepton, so the ONLY
definitive test of the picture is to compare observables probed with e+ and e-
E04-019 (Two-gamma)
GE/GM from Pt/Pl constant vs. ε


(1–2εR/(1+ε)) Y2γ constant
with Y2γ = const.  Y2γ = 0?
M. Meziane, BF-05 (Wed.)
OLYMPUS
pOsitron-proton and
eLectron-proton elastic scattering to test the
hYpothesis of
MultiPhoton exchange
Using
DoriS
2008 – Full proposal
2009 – Funding approval
2010/11 – Transfer of BLAST
2012 – OLYMPUS Running
14
OLYMPUS: BLAST@DESY/DORIS
500 hours each
for e+ and eLumi=2x1033 cm-2s-1
15
e+/e- cross section ratio to verify TPE
VEPP3
CLAS
Experiment proposals to verify TPE hypothesis:
e+/e- ratio:
CLAS/PR04-116
Novosibirsk/VEPP-3
OLYMPUS@DESY
secondary e+/e- beam
– 2011/12
storage ring / intern. target – 2009
storage ring / intern. target – 2012
16
New Proton Measurements at High Q2
High-Q2 measurements at Jefferson Lab
 Hall C E05-017: Super-Rosenbluth
Q2 = 0.9 – 6.6 (GeV/c)2
Completed in summer 2007
 GEp-III /Hall C: E04-108/E04-019
Q2 = 2.5, 5.2, 6.8, 8.5 (GeV/c)2
Completed in spring 2008
BF-04 (Wed.)
BF-05 (Wed.)
 SANE /Hall C E05-017: Polarized Target
Q2 = 5 – 6 (GeV/c)2
LJ-06 (Sat.)
Completed in spring 2009
Proposed experiments
 PAC32: PR12-07-109 /Hall A (GEp-IV)
L. Pentchev, C.F. Perdrisat, E. Cisbani,
V. Punjabi, B. Wojtskhowski, M. Khandaker et al.
Q2=13,15 (GeV/c)2: Approved
CF-06 (Thu.)
 PAC32: PR12-07-108 /Hall A (high-Q2 x-sec.)
S. Gilad, B. Moffit, B. Wojtsekhowski, J. Arrington et al.
Q2 =7-17.5 (GeV/c)2: Approved
 PAC34: PR12-09-001 /Hall C (GEp-V)
E.J. Brash, M. Jones, C.F. Perdrisat, V. Punjabi et al.
Q2=6,10.5,13 (GeV/c)2: Conditionally approved
17
Imaginary part of TPE: SSA’s
spin of beam OR target
NORMAL to scattering plane
on-shell intermediate state (MX = W)
E.g. target normal spin asymmetry
Beam: PVES at Bates, MAMI and Jlab; Target: PR05-015, PR08-005
BF-06 (Wed.)
BF-07 (Wed.)
Transverse Beam Asymmetry
Plot: Courtesy of J. Mammei
Summary
The limits of OPE have been reached with available today’s precision
 Nucleon elastic form factors, particularly GEp under doubt
The TPE hypothesis is suited to remove form factor discrepancy,
however calculations of TPE are model-dependent
Experimental probes: Real part of TPE: Y2γ – Imaginary part: SSA’s
Need both positron and electron beams for a definitive test of TPE
OLYMPUS, CLAS, VEPP-3
ε dependence of polarization transfer, ε-nonlinearity of cross sections
transverse beam symmetries
Improved precision and extension of “standard” methods to high Q2
A comprehensive and rich program underway and/or proposed
is expected to be conclusive within a few years
Broader Impact: gamma-Z box in PVES; TPE effects in DIS
20
Interpreting Electron Scattering …
“[…] most of what we know and everything we believe
about hadron structure [… is based on electron scattering]”
(W. Turchinetz)
“The electromagnetic probe is well understood, hence …”
(a common phrase in many articles)
We have made big investments in lepton scattering facilities
to explore hadron structure
The elastic form factors characterize the simplest process
in nuclear physics, namely elastic scattering
(straightforward, one should think)
We have to understand the elastic form factors before
we can claim to have understood anything else
21
Backup slides
22
GpE and GpM from Unpolarized Data
23
GpE and GpM from Unpolarized Data
charge and magnetization density (Breit fr.)
Dipole form factor
within 10% for Q2 < 10 (GeV/c)2
24
Recoil Polarization Technique
 Pioneered at MIT-Bates
 Pursued in Halls A and C, and MAMI A1
 In preparation for Jlab @ 12 GeV
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
25
Polarized Targets
BLAST Internal Target:
Atomic Beam Source
UVA / “SLAC”-Target:
Dynamic Nuclear Polarization
Limited luminosity for
polarized
hydrogen/deuterium
targets,
Very precise at low to
moderately high Q2
from W. Meyer, SPIN2008
26
Nucleon Form Factors: Last Ten Years
J. Arrington
PANIC08
Magenta:
underway
or approved
27
Extensions with Jlab 12 GeV Upgrade
J. Arrington
PANIC08
~8 GeV2
28
•
BLUE = CDR or PAC30 approved, GREEN = new ideas under development
Two-Photon Exchange: Exp. Evidence
Two-photon exchange theoretically suggested
TPE can explain form factor discrepancy
J. Arrington, W. Melnitchouk, J.A. Tjon,
Phys. Rev. C 76 (2007) 035205
Rosenbluth data with
two-photon exchange
correction
Polarization transfer data
29
Polarized Target Experiments at High Q2
Polarized Target:
Independent verification of recoil
polarization result is crucial
Polarized internal target / low Q2: BLAST
Q2<0.65 (GeV/c)2 not high enough to
see deviation from scaling
RSS /Hall C: Q2 ≈ 1.5 (GeV/c)2
SANE/Hall C: completed March 2009
BigCal electron detector
Recoil protons in HMS parasitically
Extract GE/GM to <5% at Q2≈5-6 (GeV/c)2
M.K. Jones et al., PRC74 (2006) 035201
30
New Proton Measurements at High Q2
Extension to higher Q2 at Jefferson Lab
 GEp-III /Hall C: PR04-108/PR04-019
Completed in spring 2008
 Sign change of GE/GM observed
(preliminary, C. Perdrisat @ PANIC08)
 Or maybe not (preliminary, CIPANP09)
31
Imaginary part of TPE: SSA’s
spin of beam OR target
NORMAL to scattering plane
on-shell intermediate state (MX = W)
lepton
hadron
Beam: PVES at Bates, MAMI and Jlab; Target: PR05-015, PR08-005
BF-06 (Wed.)
BF-07 (Wed.)
Target normal spin asymmetry
2
general formula, of order e
involves the imaginary part of two-photon exchange
amplitudes