Two photon exchange: What to measure next Jan C. Bernauer ACFI - - PowerPoint PPT Presentation

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Two photon exchange: What to measure next Jan C. Bernauer ACFI - - PowerPoint PPT Presentation

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Two photon exchange: What to measure next Jan C. Bernauer ACFI workshop The Electroweak Box September 2017 1 Phenomenology 2 Rosenbluth Polarization Litt 70 Gayou 01 Bartel 73 Punjabi 05 Andivahis 94 Jones

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Two photon exchange: What to measure next

Jan C. Bernauer ACFI workshop ”The Electroweak Box” – September 2017

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Phenomenology

0.5 1 1.5 2 1 2 3 4 5 6 7 OLYMPUS VEPP-3 JLAB µGE/GM Q2 [(GeV/c)2] Rosenbluth Litt ’70 Bartel ’73 Andivahis ’94 Walker ’94 Christy ’04 Qattan ’05 Polarization Gayou ’01 Punjabi ’05 Jones ’06 Puckett ’10 Paolone ’10 Puckett ’12

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Phenomenology

0.5 1 1.5 2 1 2 3 4 5 6 7 OLYMPUS VEPP-3 JLAB µGE/GM Q2 [(GeV/c)2] Rosenbluth Litt ’70 Bartel ’73 Andivahis ’94 Walker ’94 Christy ’04 Qattan ’05 Polarization Gayou ’01 Punjabi ’05 Jones ’06 Puckett ’10 Paolone ’10 Puckett ’12 Fits Bernauer ’13 Fit Rosenbluth Fit all + phen. TPE

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Direct measurement: Three modern experiments

CLAS

e− to γ to e+/−

  • beam
  • Phys. Rev. C 95,

065201 (2017) PRL 114, 062003

VEPP-3

1.6/1 GeV beam no field

  • Phys. Rev. Lett. 114,

062005 (2015)

OL MPUS

DORIS @ DESY 2 GeV beam

  • Phys. Rev. Lett. 118,

092501 (2017)

1 2 3 4 5 0.2 0.4 0.6 0.8 1 Q2 [(GeV/c)2] ǫ Kinematic Reach of Two-Photon Experiments CLAS OLYMPUS VEPP-3 Run I VEPP-3 Run II 1 2 3 4 5 0.2 0.4 0.6 0.8 1

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OLYMPUS results (B. Henderson et al., Phys. Rev. Lett. 118, 092501

(2017))

0.97 0.98 0.99 1 1.01 1.02 1.03 1.04 1.05 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 R2γ

ǫ

Main spectrometer 12◦ telescopes Correlated uncertainty Blunden N only Blunden N + ∆ Bernauer Tomalak 2.0 1.5 1.0 0.5 0.0 Q2 [(GeV/c)2]

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OLYMPUS results re-binned

0.97 0.98 0.99 1 1.01 1.02 1.03 1.04 1.05 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 R2γ

ǫ

Main spectrometer 12◦ telescopes Correlated uncertainty Blunden N only Blunden N + ∆ Bernauer Tomalak 2.0 1.5 1.0 0.5 0.0 Q2 [(GeV/c)2]

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Difference of data to prediction: Blunden’s hadronic calculation

−0.05 −0.04 −0.03 −0.02 −0.01 0.01 0.02 0.03 0.5 1 1.5 2 Rmeas.

2γ

− Rpred.

2γ

Q2 [(GeV/c)2] OLYMPUS VEPP-3 CEBAF

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Difference of data to prediction: Bernauer et al. phenomenological prediction

−0.05 −0.04 −0.03 −0.02 −0.01 0.01 0.02 0.03 0.5 1 1.5 2 Rmeas.

2γ

− Rpred.

2γ

Q2 [(GeV/c)2] OLYMPUS VEPP-3 CEBAF

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χ2 of the world data set

VEPP-3 CLAS OLYMPUS World

χ2 nd.f. χ2 nd.f.

N.

χ2 nd.f.

N.

χ2 nd.f.

No hard TPE 7.97 0.84 0.43σ 0.65 0.75σ 1.53 Blunden 4.01 0.70 1.23σ 0.73 2.14σ 1.088 Bernauer 1.95 0.58

  • 0.40σ

0.49 0.45σ 0.679 CLAS and OLYMPUS have too large errors Vepp-3 rules out no hard TPE Blunden et al get slope right, but large normalization shifts. Probability for worse shift in same direction: < 0.4% Phenomenological fit clearly preferred by all three experiments

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My view on this

For the measured values, good agreement with phenomenological extraction.

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My view on this

For the measured values, good agreement with phenomenological extraction. But not in good agreement with theory.

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My view on this

For the measured values, good agreement with phenomenological extraction. But not in good agreement with theory. Not clear how to calculate at higher Q2 − →Can not extract GE and GM from Rosenbluth exps! Not clear if TPE is full effect − →Can not trust polarization based exps on GE/GM?

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My view on this

For the measured values, good agreement with phenomenological extraction. But not in good agreement with theory. Not clear how to calculate at higher Q2 − →Can not extract GE and GM from Rosenbluth exps! Not clear if TPE is full effect − →Can not trust polarization based exps on GE/GM? Need new measurements at relevant kinematics

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Phenomenology

0.5 1 1.5 2 1 2 3 4 5 6 7 OLYMPUS VEPP-3 JLAB µGE/GM Q2 [(GeV/c)2] Rosenbluth Litt ’70 Bartel ’73 Andivahis ’94 Walker ’94 Christy ’04 Qattan ’05 Polarization Gayou ’01 Punjabi ’05 Jones ’06 Puckett ’10 Paolone ’10 Puckett ’12 Fits Bernauer ’13 Fit Rosenbluth Fit all + phen. TPE

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Effect size

We assume a correction to the cross section: dσ → dσ (1 + δTPE) How does δTPE depend on ǫ, Q2?

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Effect size

We assume a correction to the cross section: dσ → dσ (1 + δTPE) How does δTPE depend on ǫ, Q2? From linearity of Rosenbluth: δTPE = (1 − ǫ)f(Q2) Effect on GE/GM seems to be linear in Q2

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Effect size

We assume a correction to the cross section: dσ → dσ (1 + δTPE) How does δTPE depend on ǫ, Q2? From linearity of Rosenbluth: δTPE = (1 − ǫ)f(Q2) Effect on GE/GM seems to be linear in Q2 However: dσred → dσred

  • 1 + (1 − ǫ) × f(Q2)
  • = ǫG2

E + τG2 M

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Effect size

We assume a correction to the cross section: dσ → dσ (1 + δTPE) How does δTPE depend on ǫ, Q2? From linearity of Rosenbluth: δTPE = (1 − ǫ)f(Q2) Effect on GE/GM seems to be linear in Q2 However: dσred → dσred

  • 1 + (1 − ǫ) × f(Q2)
  • = ǫG2

E + τG2 M

= ⇒ GE GM ∼ 1 − ατf(Q2) We can only expect weak dependence on Q2 = ⇒Logarithmic dependence in Mainz fit, many calculations

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Constructing a figure of merit

Use Mainz fit as benchmark of effect size to reconcile FF measurements. Signal is larger for smaller ǫ, larger Q2, but then σ is smaller → larger uncertainty

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Constructing a figure of merit

Use Mainz fit as benchmark of effect size to reconcile FF measurements. Signal is larger for smaller ǫ, larger Q2, but then σ is smaller → larger uncertainty FOM is the deviation of R2γ from unity, measured in units of uncertainty: FOM =

  • R2γ − 1
  • ∆2

stat + ∆2 syst

Statistical error: ∆stat =

  • 2

σ×L×t×A

Systematical error: ∆syst = 1%

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Possible locations for experiments at high Q2

Positron beams are scarce

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Possible locations for experiments at high Q2

Positron beams are scarce In the relevant energy range, almost non-existent

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Possible locations for experiments at high Q2

Positron beams are scarce In the relevant energy range, almost non-existent Jefferson Lab Has detectors, but no beam (yet)

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Possible locations for experiments at high Q2

Positron beams are scarce In the relevant energy range, almost non-existent Jefferson Lab Has detectors, but no beam (yet) DESY Has no detectors, but beam However: small time window: PETRA 3 will run with electrons only!

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DESY

DESY might have a test beam facility with positron/electron beams. Current: 60 nA (single bunch, maybe can do more?) Short window of opportunity: PETRA 3 might stop positron running. Target: Borrow from Mainz? Detector: Borrow something developed for Panda? Calorimeter? Assume 10 msr

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DESY @ 15 days per species

2 4 6 8 10 0.2 0.4 0.6 0.8 1 Q2 [(GeV/c)2] FOM ǫ FOM for DESY @ 15 day/species beamtime 2 4 6 8 10 12 Beam energy [GeV] 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

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DESY @ 30 days per species

2 4 6 8 10 0.2 0.4 0.6 0.8 1 Q2 [(GeV/c)2] FOM ǫ FOM for DESY @ 30 day/species beamtime 2 4 6 8 10 12 Beam energy [GeV] 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

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DESY projected errors (15 days per species)

0.98 1 1.02 1.04 1.06 1.08 1.1 1.12 1.14 0.2 0.4 0.6 0.8 1 R2γ ǫ Ebeam = 2.85 GeV

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Jefferson Lab

Assume 1µA positron/electron beam on 10 cm target = ⇒L = 2.6 · 1036/(cm2s) Acceptance: 6 msr

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JLab @ 5 days per species

2 4 6 8 10 0.2 0.4 0.6 0.8 1 Q2 [(GeV/c)2] FOM ǫ FOM for HRS style detector @ 5 days/species beamtime 2 4 6 8 10 12 Beam energy [GeV] 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

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JLab @ 1 day per species

2 4 6 8 10 0.2 0.4 0.6 0.8 1 Q2 [(GeV/c)2] FOM ǫ FOM for HRS style detector @ 1 day/species beamtime 2 4 6 8 10 12 Beam energy [GeV] 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

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JLab BigBite

96 msr! But limited momentum acceptance. Limits angle > 70 − 90◦

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JLab BigBite @ 1 day per species

2 4 6 8 10 0.2 0.4 0.6 0.8 1 Q2 [(GeV/c)2] FOM ǫ FOM for BigBite detector @ 1 day/species beamtime 2 4 6 8 10 12 Beam energy [GeV] 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

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Hall A

10 cm target two spectrometers, 6.7 msr BigBite, 96 msr runtime with 100% efficiency Ebeam 3.1 3.55 4.01 Angles 30/70/110 52.7/70/110 42.55/70/110 Q2 1.79/3.99/4.75 3.99/4.75/5.56 3.99/5.55/6.4 ǫ 0.822/0.32/0.1 0.49/0.3/0.09 0.6/0.28/0.08 Time 1 day 2 days 3 days

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Hall A projected errors

0.98 1 1.02 1.04 1.06 1.08 1.1 1.12 1.14 1.16 1.18 0.2 0.4 0.6 0.8 1 R2γ ǫ Ebeam = 3.1 GeV Ebeam = 3.55 GeV Ebeam = 4.01 GeV

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Hall C

10 cm target for HMS, SHMS HMS: 6 msr (e−), SHMS 4 msr (proton) runtime with 100% efficiency Ebeam 3.1 3.55 4.01 Angles 79.7/7.64 (120) 70/9.95 (100) 18/16.57 (65) Q2 4.25/4.84 4.76/5.43 1.3/5.35 ǫ 0.244/0.06 0.302/0.122 0.935/0.33 Time 3 days 2 days 1 days

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Hall C projected errors

0.98 1 1.02 1.04 1.06 1.08 1.1 1.12 1.14 1.16 1.18 0.2 0.4 0.6 0.8 1 R2γ ǫ Ebeam = 3.1 GeV Ebeam = 3.55 GeV Ebeam = 4.01 GeV

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What about the proton radius?

re [fm] rm [fm] (ours) McKinley/Feshbach 0.879 0.777 Borisyuk/Kobuskin 0.876 0.803 Arrington/Sick 0.875 0.769 Blunden et al. 0.875 0.799 more to come! Probably not important for electric radius. Very important for magnetic radius! = ⇒ Measure at low Q2 too!

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MUSE: The missing piece

rE [fm] ep µp Spectroscopy 0.8758 ± 0.077 0.84087 ± 0.00039 Scattering 0.8770 ± 0.060 ???? Measure radius with muon-proton scattering

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MUSE - Muon Scattering Experiment at PSI

World’s most powerful low-energy e/π/µ-beam: Direct comparison of ep and µp! Beam of e+/π+/µ+ or e−/π−/µ− on liquid H2 target Species separated by ToF , charge by magnet Absolute cross sections for ep and µp Charge reversal: test TPE Momenta 115-210 MeV/c ⇒ Rosenbluth GE,GM

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Experiment layout

Secondary beam = ⇒ track beam particles Low flux (5 MHz)= ⇒ large acceptance Mixed beam = ⇒ identify particles in trigger

  • R. Gilman et al., arXiv:1303.2160 [nucl-ex]

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MUSE projected errors (e±only)

0.95 0.96 0.97 0.98 0.99 1 1.01 1.02 0.2 0.4 0.6 0.8 1 R2γ ǫ Ebeam = 115 MeV Ebeam = 153 MeV Ebeam = 210 MeV Tomalak @ Ebeam = 153 MeV

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MUSE projected errors (e±only)

0.95 0.96 0.97 0.98 0.99 1 1.01 1.02 0.2 0.4 0.6 0.8 1 R2γ ǫ Ebeam = 115 MeV Ebeam = 153 MeV Ebeam = 210 MeV Tomalak @ Ebeam = 153 MeV

Can test ǫ behavior important for electric radius Maybe test theory Cannot test ǫ behavior important for magnetic radius Low-ǫ experiment at PSI not feasible.

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How to get a good result: Systematic errors I

Many systematics cancel if measured with same apparatus But: How same is same? Have to reverse field? Efficiency, dead time stable? Same beam energy / same beam angle?

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How to get a good result: Systematic errors I

Many systematics cancel if measured with same apparatus But: How same is same? Have to reverse field? Efficiency, dead time stable? Same beam energy / same beam angle? Switch beam species often. If possible, multiple times a day!

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Systematic errors II

Need beam-species-relative luminosity Easier than absolute luminosity Harder than same-species-relative luminosity

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Systematic errors II

Need beam-species-relative luminosity Easier than absolute luminosity Harder than same-species-relative luminosity Moeller/Bhabha not ideal Need essentially absolute cross section for both processes (including radiative effects) Super forward elastic lepton-proton High rates, but same process, so easier theory Look at random coincidences

  • nly works if beam is bunched

see: arxiv:1708.04616

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Systematic errors II

Need beam-species-relative luminosity Easier than absolute luminosity Harder than same-species-relative luminosity Moeller/Bhabha not ideal Need essentially absolute cross section for both processes (including radiative effects) Super forward elastic lepton-proton High rates, but same process, so easier theory Look at random coincidences

  • nly works if beam is bunched

see: arxiv:1708.04616 This is the trickiest part!

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Conclusion

New measurements crucial for understanding form factors at large Q2 Also crucial for magnetic radius Effect in GE/GM grows ~linearly → weak Q2 dependence of TPE Ideal program for large Q2 Pilot experiment at DESY Full study at JLAB Some low-Q2 data will come from MUSE. Probably not enough for magnetic radius. MUSE will also have pion data. Interesting?

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