LNS Laboratory for Nuclear Science 1 There is a large discrepancy - - PowerPoint PPT Presentation
Polarization Observables using Positron Beams Axel Schmidt MIT JPos17, September 12, 2017 LNS Laboratory for Nuclear Science 1 There is a large discrepancy in proton form factor data. 2 Unpolarized d/d 1 . 5 G E /G M 1 0 . 5
Polarization Observables using Positron Beams
Axel Schmidt
MIT JPos17, September 12, 2017
Laboratory for Nuclear Science
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There is a large discrepancy in proton form factor data.
0.5 1 1.5 2 1 2 3 4 5 6 7 8 9 Unpolarized dσ/dΩ Polarization asymmetries µGE/GM Q2 [GeV/c]2
2
Two-photon exchange might be the cause.
?
GE(Q2), GM(Q2) − → GE(Q2), GM(Q2), δ ˜ GE(Q2, ǫ), δ ˜ GM(Q2, ǫ), ˜ F3(Q2, ǫ), ˜ F4(Q2, ǫ), ˜ F5(Q2, ǫ), ˜ F6(Q2, ǫ)
3
Two-photon exchange might be the cause.
?
GE(Q2), GM(Q2) − → GE(Q2), GM(Q2), δ ˜ GE(Q2, ǫ), δ ˜ GM(Q2, ǫ), ˜ F3(Q2, ǫ)
4
Two-photon exchange might be the cause.
?
GE(Q2), GM(Q2) − → GE(Q2), GM(Q2), δ ˜ GE(Q2, ǫ), δ ˜ GM(Q2, ǫ), ˜ F3(Q2, ǫ) σe+p σe−p = 1 − 4GMRe
GM + ǫν M2 ˜ F3
τ GERe
GE + ν M2 ˜ F3
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Recent σe+p/σe−p measurements were not a slam dunk.
TPE is there. . . . but it’s small. Higher Q2?
Afanasev, Blunden, Hasell, and Raue, Prog. Nucl. Part. Phys. (2017)
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The experimental goal should be to validate theory from multiple angles.
A precise experimental determination of TPE will be a challenge. We need to validate theories that allow interpolation/extrapolation. Constraints should come from multiple channels.
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Constraining TPE using Polarization
1 Polarization transfer with e+
Systematically clean Statistics prohibitive
2 Beam-normal single-spin asymmetry
Really statistics prohibitive
3 Target-normal single-spin asymmetry
Feasible
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Polarization transfer is a better way to measure the proton form factor ratio.
Measurements are performed at one kinematic setting. Radiative corrections are small. Measure a ratio rather than a cross section.
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What polarization is transfered to the proton?
e e'
Pt = −hPe
τ GEGM G 2
M+ ǫ τ G 2 E
Pl = hPe √ 1 − ǫ2
G 2
M
G 2
M+ ǫ τ G 2 E
Pt/Pl =
2ǫ GE GM
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What polarization is transfered to the proton?
e e'
Pt = −hPe
τ GEGM G 2
M+ ǫ τ G 2 E
Pl = hPe √ 1 − ǫ2
G 2
M
G 2
M+ ǫ τ G 2 E
Pt/Pl =
2ǫ GE GM
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Polarization can be measured with a focal plane polarimeter.
Dipole
Drift Chambers Trigger Scintillators Rescatterer Drift Chambers
Focal Plane Polarimeter 12
The FPP converts transverse polarization into an azimuthal distribution.
1 2 3 4 5 6 0.08 − 0.06 − 0.04 − 0.02 − 0.00 0.02 0.04 0.06 0.081 2 3 4 5 6 )
)/(f
(f 0.10 − 0.05 − 0.00 0.05 0.10 FPP1 > = 0.153 ε < 1 2 3 4 5 6 )
)/(f
(f 0.10 − 0.05 − 0.00 0.05 0.10 > = 0.638 ε < (rad)
fppϕ 1 2 3 4 5 6 )
)/(f
(f 0.10 − 0.05 − 0.00 0.05 0.10 > = 0.790 ε < 1 2 3 4 5 6 0.10 0.05 0.00 0.05 0.10 FPP2 1 2 3 4 5 6 0.10 0.05 0.00 0.05 0.10 (rad)
fppϕ 1 2 3 4 5 6 0.10 0.05 0.00 0.05 0.10
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History of PT measurements
0.2 0.4 0.6 0.8 1 1.2 1 2 3 4 5 6 7 8 9
µpGEp GMp
Q2 [GeV2] MIT-Bates (1999) Hall A (2001) Mainz (2001)
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History of PT measurements
0.2 0.4 0.6 0.8 1 1.2 1 2 3 4 5 6 7 8 9
µpGEp GMp
Q2 [GeV2] Hall C (2006) BLAST (2007)
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History of PT measurements
0.2 0.4 0.6 0.8 1 1.2 1 2 3 4 5 6 7 8 9
µpGEp GMp
Q2 [GeV2] Hall A (2006) Hall C (2006) Hall A (2007) Hall A (2010) Hall A (2011)
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History of PT measurements
0.2 0.4 0.6 0.8 1 1.2 1 2 3 4 5 6 7 8 9
µpGEp GMp
Q2 [GeV2] GEp-I (Hall A) GEp-II (Hall A) GEp-III (Hall C) GEp-2γ (Hall C)
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Polarization transfer is sensitive to TPE.
Pt Pl =
τ(1 + ǫ) GE GM × [1 + . . .
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Polarization transfer is sensitive to TPE.
Pt Pl =
τ(1 + ǫ) GE GM × [1 + . . . + Re
GM GM
GE Re
GE + ν m2 ˜ F3
GM Re
GM + ǫν (1 + ǫ)m2 ˜ F3
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Polarization transfer is sensitive to TPE.
Pt Pl =
τ(1 + ǫ) GE GM × [1 + . . . + Re
GM GM
GE Re
GE + ν m2 ˜ F3
GM Re
GM + ǫν (1 + ǫ)m2 ˜ F3
Different dependence from σ(e+p)/σ(e−p)!
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Without TPE, GE
GM should be constant with ǫ.
GE GM =
2ǫ Pt Pl × [1 + . . .? Any ǫ dependence is a signature of TPE.
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The GEp-2γ experiment looked for TPE.
40 days, data taken in 2007–08, Hall C Q2 = 2.5 GeV2/c2 Meziane et al., PRL 106, 132501 (2011)
arXiv:1707.08587v1 [nucl-ex] (2017)
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What do positrons get you?
Largest systematics in PT: Proton polarimetry Spin precession in spectrometer fields Alignment of the polarimeter (Pl ↔ Pt)
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What do positrons get you?
Largest systematics in PT: Proton polarimetry Spin precession in spectrometer fields Alignment of the polarimeter (Pl ↔ Pt) By taking the ratio: (Pt(e+)/Pl(e+))/(Pt(e−)/Pl(e−)) Proton polarimetry offsets cancel. Point-to-point biases eliminated ǫ-dependence at fixed Q2 is a signature. Statistics limited measurements!
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What do positrons get you?
Largest systematics in PT: Proton polarimetry Spin precession in spectrometer fields Alignment of the polarimeter (Pl ↔ Pt) By taking the ratio: (Pt(e+)/Pl(e+))/(Pt(e−)/Pl(e−)) Proton polarimetry offsets cancel. Point-to-point biases eliminated ǫ-dependence at fixed Q2 is a signature. Statistics limited measurements! Positrons can’t help you get the form factors (biases have the same sign).
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Figure-of-merit
F.o.M. ∝ APe
dΩΩLTε A: polarimeter analyzing power − → same Pe: beam polarization ≈ 80% − → ≈ 60% L: luminosity ≈ 80 µA − →≈ 100 nA T: run time ??? ε: polarimeter efficiency − → same Factor 38 increase in uncertainty!
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Imagined set-up
SHMS HMS Big Cal Big Cal II e+/e– beam
BigCal from GEp-III, GEp-2γ
Protons in SHMS/HMS Non-magnetic lepton detector (BigCal) SHMS for low-ǫ, in parallel with other kinematics in HMS
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Imagined set-up
SHMS HMS Big Cal Big Cal II e+/e– beam
BigCal from GEp-III, GEp-2γ
Protons in SHMS/HMS Non-magnetic lepton detector (BigCal) SHMS for low-ǫ, in parallel with other kinematics in HMS
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Kinematics
1 2 3 4 5 0.2 0.4 0.6 0.8 1 Q2 [GeV2] ǫ Bates Mainz Hall A Hall C
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Q2 = 1.15 GeV2
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 0.2 0.4 0.6 0.8 1 20 days of e+ 20 days of e− 16+16 4+4 40 days total µpGE/GM ǫ Hall A (Gayou et al.) Hall C (MacLachlan et al.) GEp-I, Hall A (Punjabi et al.) Projected 30
Q2 = 1.15 GeV2
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 0.2 0.4 0.6 0.8 1 45 days of e+ 45 days of e− 36+36 4+4 90 days total µpGE/GM ǫ Hall A (Gayou et al.) Hall C (MacLachlan et al.) GEp-I, Hall A (Punjabi et al.) Projected 31
Q2 = 1.15 GeV2
0.6 0.8 1 1.2 1.4 0.2 0.4 0.6 0.8 1 90 days 72 days 18 days 90 days total e+/e− ǫ Projected: Q2 = 1.15 GeV2
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To summarize:
TPE can show up in polarization transfer. e+/e− is a clean way to measure it.
Systematics are on the proton side. Non-magnetic lepton detection
Getting enough stats is the hard part.
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Single-spin transverse asymmetries are sensitive to the imaginary part of TPE.
Target-normal: An =
√τ
M + ǫ τ G 2 E
×
GE + ν M2 ˜ F3
GM + 2ǫν M2(1 + ǫ) ˜ F3
Beam Normal: Bn = 4mM
Q2 G 2
M + ǫ τ G 2 E
F3 + ν M2(1 + τ) ˜ F5
F4 + ν M2(1 + τ) ˜ F5
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Transverse asymmetries do not violate parity.
e e'
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Transverse asymmetries do not violate parity.
e e' e e'
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Transverse asymmetries do not violate parity.
e e' e e'
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Transverse asymmetries do not violate parity.
Beam-normal Target-normal Suppressed by me/Q ≈ 10−4–10−6 False asym. in PV Previously measured by:
SAMPLE G0 Mainz A4 HAPPEX/PREX QWeak (prelim)
≈ 10−3 Previously measured
1970’s, looking for T-violation HERMES (including with e+)
3He, Hall A
F.o.M = P
dΩΩLT
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Previous beam-normal asymmetry data
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.2 0.4 0.6 0.8 1 Q2 [GeV2] ǫ SAMPLE Mainz A4 G0 HAPPEX QWeak
39
Low-ǫ beam-normal asymmetry data
−200 −150 −100 −50 50 100 150 200 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 An [ppm] Q2 [GeV2] SAMPLE G0 G0 (2H) Mainz A4 Mainz A4 (2H)
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High-ǫ beam-normal asymmetry data
−20 −15 −10 −5 5 10 15 20 0.05 0.1 0.15 0.2 0.25 0.3 An [ppm] Q2 [GeV2] Mainz A4 G0 HAPPEX (1H) HAPPEX (4He) HAPPEX (12C) HAPPEX (208Pb) QWeak
41
Challenges with beam-normal asymmetries and positrons
Making transversely polarized beam Need high luminosity Resolve ppm asymmetries Positrons don’t help with systematics
Beam polarimetry False asymmetries
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Target-Normal Asymmetry Estimate
Assumptions L = 1035 cm−2s−1
Limited by target ≈ 100 nA
12% target polarization (including NH3 dilution factor) Both Hall A HRSs at 17◦ 50% live time
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Target-Normal Asymmetry Estimate
−4 −3 −2 −1 1 2 3 4 0.2 0.4 0.6 0.8 1 1.2 5 days 25 days 30 days Projected e+ on 1 H Hall A 3 He An × 103 Q2 [GeV2]
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Target-Normal Asymmetry Estimate
e+ measurement is feasible Adequate statistics Problems are systematic
Luminosity e+/e− switching time Target polarization Target flip time Positrons do not help.
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Summary
Polarization transfer is clean, but statistics limited. Beam-normal asymmetries are statistics limited. Target-normal asymmetries might be feasible
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Summary
Polarization transfer is clean, but statistics limited. Beam-normal asymmetries are statistics limited. Target-normal asymmetries might be feasible Getting TPE data in multiple channels is important for validating theory!
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