Transcript Seminar at University of South Carolina, Columbia, SC, Oct. 30, 2009 The OLYMPUS Experiment at DESY to Determine the Effect of Two-Photon Exchange.

Seminar at University of South Carolina, Columbia, SC, Oct. 30, 2009
The OLYMPUS Experiment at DESY to
Determine the Effect of Two-Photon
Exchange in Elastic Lepton Scattering
Michael Kohl <[email protected]>
Hampton University, VA 23668 and
Jefferson Lab, VA 23606, USA
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
 OLYMPUS
2
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





Rigorous tests of nucleon models
Determined by quark structure of the nucleon
Role of quark angular momentum
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
3
(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
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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
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Form Factors from Rosenbluth Method
Determine
|GE|, |GM|,
|GE/GM|
GE 2
tGM2
θ=180o
θ=0o
In One-photon exchange approximation above form factors are
observables of elastic electron-nucleon scattering
6
GpE and GpM from Unpolarized Data
7
GpE and GpM from Unpolarized Data
charge and magnetization density (Breit fr.)
Dipole form factor
within 10% for Q2 < 10 (GeV/c)2
8
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
9
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
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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
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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
12
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
13
Proton Form Factor Ratio
F. Iachello et al., PLB43 (1973) 191
F. Iachello, nucl-th/0312074
mpGpE/GpM
1
Iachello 1973:
Drop of the ratio already
suggested by VMD
0
0
2
4
6
8
10
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 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
 SANE /Hall C E05-017: Polarized Target
Q2 = 5 – 6 (GeV/c)2
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
 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
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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)
16
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≈5.75 (GeV/c)2
M.K. Jones et al., PRC74 (2006) 035201
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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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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
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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; modifications ~α3
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
 GE/GM from 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
assuming δGE, δGM = const.
 with Y2γ = const.  Y2γ = 0?
 Wait for Super-Rosenbluth
results E05-017 (non-linearity)
 Wait for e+/e- comparisons
OLYMPUS, VEPP-3, CLAS
Lepton-proton elastic scattering
2
+…
+
~α
~α2
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Experiments to Verify 2 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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Two-photon exchange
Elastic electron-proton to
positron-proton ratio (P. Blunden)
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Two-photon exchange
Elastic electron-proton to
positron-proton ratio (P. Blunden)
BLAST @ 2.0 GeV
Q2 = 0.6–2.2 (GeV/c)2
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Two-photon exchange
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OLYMPUS
pOsitron-proton and
eLectron-proton elastic scattering to test the
hYpothesis of
MultiPhoton exchange
Using
DoriS
2008 – Full proposal
2009/10 – Transfer of BLAST
2011/12 – OLYMPUS Running
29
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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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)
p/p=3%, q = 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
32
Identification of Elastic Events
Charge +/Coplanarity
BLAST
1H(e,e’p)
Kinematics
Timing
e-
left,
p+
right
E=850 MeV
e’
e- right, p+ left
p,d
 Advantages of magnetic field:
 suppression of background
 2-3% momentum resolution
 σθ = 0.5o and σφ = 0.5o
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Proton Form Factor Ratio
*
p
p
mpG E/G M
C.B. Crawford et al.,
PRL 98 (2007) 052301
Impact of BLAST data
combined with cross sections
on separation of GpE and GpM
Errors factor ~2 smaller
Reduced correlation
Deviation from dipole at low Q2!
*Ph.D. work of C. Crawford (MIT) and A. Sindile (UNH)
34
Neutron Electric Form Factor
*
n
G E
E. Geis, M.K., V. Ziskin et al.,
PRL
101 (2008) 042501
*Ph.D. work of V. Ziskin (MIT) and E. Geis
(ASU)
35
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)
• 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.
• Redundant monitoring of luminosity
(pressure, temperature, flow, current measurements)
small-angle elastic scattering at high epsilon / low Q2
Luminosity Monitors: Telescopes
2 tGEM telescopes, 3 tracking planes
3.9 msr, 10o, R=160 cm, dR=10 cm
Forward telescopes
10o
Forward Elastic Luminosity Monitor
•
•
•
Forward angle electron/positron telescopes or trackers
with good angular and vertex resolution
Coincidence with proton in BLAST
High rate capability
GEM technology
MIT protoype:
Telescope of 3 Triple GEM prototypes
(10 x 10 cm2) using TechEtch foils
F. Simon et al.,
Nucl. Instr. and Meth. A 598 (2009) 432
Control of Systematics
i = e+ or ej= pos/neg polarity
Geometric proton efficiency:
Ratio in single
polarity j
Geometric lepton
efficiency:
Control of Systematics
Super ratio:
Cycle of four states ij
Repeat cycle many times
•
•
•
Change between electrons and positrons every other day
Change BLAST polarity every other day
Left-right symmetry
Projected Results for OLYMPUS
1000
500hours
hourseach
each
forfor
e+e+
and
e eand
33 cm-2s-2
-1
Lumi=2x10
Lumi=2x1033
cm s-1
Projected Results for OLYMPUS
500 hours each
for e+ and eLumi=2x1033 cm-2s-1
42
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
43
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
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
46
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
47
Backup slides
48
Nucleon Form Factors: Last Ten Years
J. Arrington
PANIC08
Magenta:
underway
or approved
49
Extensions with Jlab 12 GeV Upgrade
J. Arrington
PANIC08
~8 GeV2
50
•
BLUE = CDR or PAC30 approved, GREEN = new ideas under development
OLYMPUS Collaboration
• 57 collaborators from 16 institutions
• The OLYMPUS collaboration is built from
- the core of the BLAST collaboration
- key technical expertise from HERMES
- strong hadron physics groups in Europe
- key DESY staff
• 12 FTEs of engineering are available to design, construct,
and install the experiment.
• In 2010-12, 13.6 Physicist FTEs and 14 graduate students are
committed to OLYMPUS.
• The collaboration is providing in-kind contributions to the
removal of ARGUS and the modifications to DORIS.
Richard Milner
DESY
September 15, 2009
51
Costs
• > $ 5 million of existing equipment is provided from the U.S.
• $ 1.221 million is requested from DOE for the tracking
upgrade, the target, and shipping to DESY.
• $125 k is requested from NSF for the luminosity monitor.
• $ 330 k is requested by Univ. of Bonn and Mainz from BMBF
for electronics and DAQ.
• The total operating cost is estimated at $ 900 k over the
lifetime of OLYMPUS => $ 6 k per Physicist Ph.D. per year
over three years.
Richard Milner
DESY
September 15, 2009
52