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-
Accessing Nucleon Polarizabilities with Compton Scattering
13th European Research Conference on
Electromagnetic Interactions with Nucleons and Nuclei
Philippe Martel Paphos, Cyprus - 30 October 2019
Institute for Nuclear Physics
Johannes Gutenberg University of Mainz
2A
-
Nucleon Polarizabilities
π+π+
π+π+
π+π+
u
d
uπ+
π+
π+π+
π+π+
+++++++
−−−−−−−
++++
−−−−
E
Electric polarizability αE1
π+π+
π+π+
π+π+
u
d
u
π+π+
π+π+
π+π+
How could we measure this?
π+π+
u
d
u
π+π+
N N N N N N N
S S S S S S S S
NN
NN
SS
SS
H
Paramagnetic
Diamagnetic
Magnetic polarizability βM1
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 2/16
-
Nucleon Polarizabilities
π+π+
π+π+
π+π+
u
d
uπ+
π+
π+π+
π+π+
+++++++
−−−−−−−
++++
−−−−
E
Electric polarizability αE1
π+π+
π+π+
π+π+
u
d
u
π+π+
π+π+
π+π+
How could we measure this?
π+π+
u
d
u
π+π+
N N N N N N N
S S S S S S S S
NN
NN
SS
SS
H
Paramagnetic
Diamagnetic
Magnetic polarizability βM1
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 2/16
-
Nucleon Polarizabilities
π+π+
π+π+
π+π+
u
d
uπ+
π+
π+π+
π+π+
+++++++
−−−−−−−
++++
−−−−
E
Electric polarizability αE1
π+π+
π+π+
π+π+
u
d
u
π+π+
π+π+
π+π+
How could we measure this?
π+π+
u
d
u
π+π+
N N N N N N N
S S S S S S S S
NN
NN
SS
SS
H
Paramagnetic
Diamagnetic
Magnetic polarizability βM1
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 2/16
-
Nucleon Polarizabilities
π+π+
π+π+
π+π+
u
d
uπ+
π+
π+π+
π+π+
+++++++
−−−−−−−
++++
−−−−
E
Electric polarizability αE1
π+π+
π+π+
π+π+
u
d
u
π+π+
π+π+
π+π+
How could we measure this?
π+π+
u
d
u
π+π+
N N N N N N N
S S S S S S S S
NN
NN
SS
SS
H
Paramagnetic
Diamagnetic
Magnetic polarizability βM1
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 2/16
-
Compton Scattering
π+π+
π+π+
π+π+
u
d
uπ+
π+
π+π+
π+π+
+++++++
−−−−−−−
++++
−−−−
E
Photon
Proton
ScatteredPhoton
Recoil Proton
π+π+
u
d
u
π+π+
N N N N N N N
S S S S S S S S
NN
NN
SS
SS
H
Paramagnetic
Diamagnetic
Neutral Pion
Recoil Proton
DecayPhotons
Recoil Proton
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 3/16
-
Compton Scattering
π+π+
π+π+
π+π+
u
d
uπ+
π+
π+π+
π+π+
+++++++
−−−−−−−
++++
−−−−
E
Photon
Proton
ScatteredPhoton
Recoil Proton
π+π+
u
d
u
π+π+
N N N N N N N
S S S S S S S S
NN
NN
SS
SS
H
Paramagnetic
Diamagnetic
Neutral Pion
Recoil Proton
DecayPhotons
Recoil Proton
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 3/16
-
Compton Scattering
π+π+
π+π+
π+π+
u
d
uπ+
π+
π+π+
π+π+
+++++++
−−−−−−−
++++
−−−−
E
Photon
Proton
ScatteredPhoton
Recoil Proton
π+π+
u
d
u
π+π+
N N N N N N N
S S S S S S S S
NN
NN
SS
SS
H
Paramagnetic
Diamagnetic
Neutral Pion
Recoil Proton
DecayPhotons
Recoil Proton
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 3/16
-
Compton Scattering
π+π+
π+π+
π+π+
u
d
uπ+
π+
π+π+
π+π+
+++++++
−−−−−−−
++++
−−−−
E
Photon
Proton
ScatteredPhoton
Recoil Proton
π+π+
u
d
u
π+π+
N N N N N N N
S S S S S S S S
NN
NN
SS
SS
H
Paramagnetic
Diamagnetic
Neutral Pion
Recoil Proton
DecayPhotons
Recoil Proton
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 3/16
-
Compton Scattering
π+π+
π+π+
π+π+
u
d
uπ+
π+
π+π+
π+π+
+++++++
−−−−−−−
++++
−−−−
E
Photon
Proton
ScatteredPhoton
Recoil Proton
π+π+
u
d
u
π+π+
N N N N N N N
S S S S S S S S
NN
NN
SS
SS
H
Paramagnetic
Diamagnetic
Neutral Pion
Recoil Proton
DecayPhotons
Recoil Proton
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 3/16
-
Compton Scattering Equations
Zeroth Order - Mass and Electric Charge
H(0)eff =
~π2
2m+ eφ (where ~π = ~p − e ~A)
First Order - Anomalous Magnetic Moment
H(1)eff = −
e(1 + κ)
2m~σ · ~H − e(1 + 2κ)
8m2~σ ·[~E × ~π − ~π × ~E
]
Second Order - Electric and Magnetic Polarizabilities
H(2)eff = −4π
[1
2αE1 ~E
2 +1
2βM1 ~H
2
]
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 4/16
-
Compton Scattering Equations
Zeroth Order - Mass and Electric Charge
H(0)eff =
~π2
2m+ eφ (where ~π = ~p − e ~A)
First Order - Anomalous Magnetic Moment
H(1)eff = −
e(1 + κ)
2m~σ · ~H − e(1 + 2κ)
8m2~σ ·[~E × ~π − ~π × ~E
]
Second Order - Electric and Magnetic Polarizabilities
H(2)eff = −4π
[1
2αE1 ~E
2 +1
2βM1 ~H
2
]
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 4/16
-
Compton Scattering Equations
Third Order - Spin Polarizabilities
H(3)eff = −4π
[1
2γE1E1~σ · (~E × ~̇E) +
1
2γM1M1~σ · ( ~H × ~̇H)− γM1E2EijσiHj + γE1M2HijσiEj
]
Presently Known Values
γ0 = −γE1E1 − γE1M2 − γM1E2 − γM1M1 = (−1.0± 0.08)× 10−4 fm4 J. Ahrens et al. (GDH/A2), PRL 87, 022003 (2001)H. Dutz et al. (GDH), PRL 91, 192001 (2003)
γπ = −γE1E1 − γE1M2 + γM1E2 + γM1M1 = (8.0± 1.8)× 10−4 fm4 M. Camen et al. (A2), PRC 65, 032202 (2002)
γE1M2 = −γE1E1 −1
2γ0 −
1
2γπ γM1E2 = −γM1M1 −
1
2γ0 +
1
2γπ
This leaves us with two unknown and two known (with error) terms.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 5/16
-
Compton Scattering Equations
Third Order - Spin Polarizabilities
H(3)eff = −4π
[1
2γE1E1~σ · (~E × ~̇E) +
1
2γM1M1~σ · ( ~H × ~̇H)− γM1E2EijσiHj + γE1M2HijσiEj
]
Presently Known Values
γ0 = −γE1E1 − γE1M2 − γM1E2 − γM1M1 = (−1.0± 0.08)× 10−4 fm4 J. Ahrens et al. (GDH/A2), PRL 87, 022003 (2001)H. Dutz et al. (GDH), PRL 91, 192001 (2003)
γπ = −γE1E1 − γE1M2 + γM1E2 + γM1M1 = (8.0± 1.8)× 10−4 fm4 M. Camen et al. (A2), PRC 65, 032202 (2002)
γE1M2 = −γE1E1 −1
2γ0 −
1
2γπ γM1E2 = −γM1M1 −
1
2γ0 +
1
2γπ
This leaves us with two unknown and two known (with error) terms.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 5/16
-
Compton Scattering Equations
Third Order - Spin Polarizabilities
H(3)eff = −4π
[1
2γE1E1~σ · (~E × ~̇E) +
1
2γM1M1~σ · ( ~H × ~̇H)− γM1E2EijσiHj + γE1M2HijσiEj
]
Presently Known Values
γ0 = −γE1E1 − γE1M2 − γM1E2 − γM1M1 = (−1.0± 0.08)× 10−4 fm4 J. Ahrens et al. (GDH/A2), PRL 87, 022003 (2001)H. Dutz et al. (GDH), PRL 91, 192001 (2003)
γπ = −γE1E1 − γE1M2 + γM1E2 + γM1M1 = (8.0± 1.8)× 10−4 fm4 M. Camen et al. (A2), PRC 65, 032202 (2002)
γE1M2 = −γE1E1 −1
2γ0 −
1
2γπ γM1E2 = −γM1M1 −
1
2γ0 +
1
2γπ
This leaves us with two unknown and two known (with error) terms.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 5/16
-
Compton Scattering Equations
Third Order - Spin Polarizabilities
H(3)eff = −4π
[1
2γE1E1~σ · (~E × ~̇E) +
1
2γM1M1~σ · ( ~H × ~̇H)− γM1E2EijσiHj + γE1M2HijσiEj
]
Presently Known Values
γ0 = −γE1E1 − γE1M2 − γM1E2 − γM1M1 = (−1.0± 0.08)× 10−4 fm4 J. Ahrens et al. (GDH/A2), PRL 87, 022003 (2001)H. Dutz et al. (GDH), PRL 91, 192001 (2003)
γπ = −γE1E1 − γE1M2 + γM1E2 + γM1M1 = (8.0± 1.8)× 10−4 fm4 M. Camen et al. (A2), PRC 65, 032202 (2002)
γE1M2 = −γE1E1 −1
2γ0 −
1
2γπ γM1E2 = −γM1M1 −
1
2γ0 +
1
2γπ
This leaves us with two unknown and two known (with error) terms.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 5/16
-
Mainz Microtron (MAMI) e− Beam
Halle A Halle B
SpeH/A1
(ExH3)
(ExH4)
TagH/A2
SFTU
Halle C2
Halle C1
VLHO
VLHW
ExHX
ExH MESA 2
ExH MESA 4
ExH MESA 3
Racetrack Microtron (RTM)
• Linac sends e− beam into dipole• Dipoles return the beam beam back
into the linac at increasing radii
• ‘Kicker’ magnet ejects the beam
180 MeV − 1.6 GeV (15 MeV steps)
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 6/16
-
Mainz Microtron (MAMI) e− Beam
Halle A Halle B
SpeH/A1
(ExH3)
(ExH4)
TagH/A2
SFTU
Halle C2
Halle C1
VLHO
VLHW
ExHX
ExH MESA 2
ExH MESA 4
ExH MESA 3
Racetrack Microtron (RTM)
• Linac sends e− beam into dipole• Dipoles return the beam beam back
into the linac at increasing radii
• ‘Kicker’ magnet ejects the beam
180 MeV − 1.6 GeV (15 MeV steps)
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 6/16
-
Mainz Microtron (MAMI) e− Beam
Halle A Halle B
SpeH/A1
(ExH3)
(ExH4)
TagH/A2
SFTU
Halle C2
Halle C1
VLHO
VLHW
ExHX
ExH MESA 2
ExH MESA 4
ExH MESA 3
Racetrack Microtron (RTM)
• Linac sends e− beam into dipole• Dipoles return the beam beam back
into the linac at increasing radii
• ‘Kicker’ magnet ejects the beam
180 MeV − 1.6 GeV (15 MeV steps)Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 6/16
-
Polarized Photon Beam
A high energy electron can produce Bremsstrahlung
(‘braking radiation’) photons when slowed down by a
material.
• Longitudinally polarized electrons producecircularly polarized photons (helicity transfer).
• Diamond radiator produces linearly polarizedphotons (coherent Bremsstrahlung).
• Residual electron paths bent in a spectrometermagnet.
• Detector array determines the e− energy, and‘tags’ the photon energy by energy conservation.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 7/16
-
Polarized Photon Beam
A high energy electron can produce Bremsstrahlung
(‘braking radiation’) photons when slowed down by a
material.
• Longitudinally polarized electrons producecircularly polarized photons (helicity transfer).
• Diamond radiator produces linearly polarizedphotons (coherent Bremsstrahlung).
• Residual electron paths bent in a spectrometermagnet.
• Detector array determines the e− energy, and‘tags’ the photon energy by energy conservation.
(MeV)γE0 50 100 150 200 250 300 350 400 450
e/P γ
P
0
0.2
0.4
0.6
0.8
1
Pγ = Pe4EγEe − E 2γ
4E 2e − 4EγEe + 3E 2γ
• Pe ≈ 80%• Helicity flipped every second
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 7/16
-
Polarized Photon Beam
A high energy electron can produce Bremsstrahlung
(‘braking radiation’) photons when slowed down by a
material.
• Longitudinally polarized electrons producecircularly polarized photons (helicity transfer).
• Diamond radiator produces linearly polarizedphotons (coherent Bremsstrahlung).
• Residual electron paths bent in a spectrometermagnet.
• Detector array determines the e− energy, and‘tags’ the photon energy by energy conservation.
energy [MeV]γIncident 100 150 200 250 300 350 400 450 500 550 600
Nor
mal
ised
Enh
ance
men
t
1
1.5
2
2.5
3
• Coherent edge is tunable• Polarization plane can be flipped
(usually every hour)
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 7/16
-
Polarized Photon Beam
A high energy electron can produce Bremsstrahlung
(‘braking radiation’) photons when slowed down by a
material.
• Longitudinally polarized electrons producecircularly polarized photons (helicity transfer).
• Diamond radiator produces linearly polarizedphotons (coherent Bremsstrahlung).
• Residual electron paths bent in a spectrometermagnet.
• Detector array determines the e− energy, and‘tags’ the photon energy by energy conservation.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 7/16
-
Polarized Photon Beam
A high energy electron can produce Bremsstrahlung
(‘braking radiation’) photons when slowed down by a
material.
• Longitudinally polarized electrons producecircularly polarized photons (helicity transfer).
• Diamond radiator produces linearly polarizedphotons (coherent Bremsstrahlung).
• Residual electron paths bent in a spectrometermagnet.
• Detector array determines the e− energy, and‘tags’ the photon energy by energy conservation.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 7/16
-
Targets
Unpolarized protons
• Simple - LH2
Polarized protons
• Frozen Spin Target -Butanol (C4H9OH)
• Dynamic NuclearPolarization (DNP)
• PmaxT > 90%, τ > 1000 hLow energy polarized protons
• Rejecting backgrounds• 70 MeV detection threshold• Limited to top right of plot
Spacer
PoS(PSTP2015)005
Mainz Active Polarized Proton Target M. Biroth
1. Introduction
At the MAMI electron accelerator in Mainz, Germany, the A2 Collaboration investigates thespin-polarizabilities of the proton by scattering experiments with spin-polarized energy-taggedphotons. Due to the excellent temperature stability of the Mainz Frozen Spin Target a large de-gree of proton polarization with high relaxation times can be achieved.
At the core of the frozen spin target for the Crystal Ball detector at MAMI is a roughly 2 mlong, horizontal 3He/4He dilution refrigerator that was built in cooperation with the Joint Institutefor Nuclear Research (JINR) Dubna. The cryostat has a separator working at 3 K and an evaporatorworking at 1.2 K in the pre-cooling stages. At the target position the cryostat provides a very lowoperation temperature of 25 mK.
The forming of highly polarized target nuclei is a two step process: in the initial step a high de-gree of nucleon polarization is achieved through a microwave pumping process, known as DynamicNucleon Polarization (DNP). This requires placing the target material in a highly uniform magneticfield of typically 2.5 Tesla and passing microwave radiation at a frequency near 70 GHz through it.The use of the microwaves leads to a moderate increase of the base temperature of the cryostatfrom 0.02 K to around 0.2 K. In a second step, the microwaves are switched off. Consequently,the temperature of the target material drops and the relaxation time of the nucleons increases tosomewhere in the order of several thousand hours, although the field is reduced to a holding fieldof only 0.68 T for the longitudinal polarization and 0.5 T for the transverse polarization. Then ameasurement period of up to approximately one week in the frozen spin mode is possible.
The dynamically polarized, frozen spin target at MAMI was constructed for use inside theCrystal Ball detector with beams of tagged photons. When being polarized the cryostat is movedoutside of the Crystal Ball. Thin superconducting holding coils were installed on the thermal radia-tion protection shields of the refrigerator to maintain the target polarization during the experiments.Details of the frozen spin target at MAMI can be found, e.g. in Ref. [1].
T = 4 K T = 25 mK
Outer Vacuum Seal Inner Vacuum Seal
Light Guide Tube, Vacuum Inside
3He-4He-Mixture
Target HeadSiPM Detector Board
Cryostat
Figure 1: Schematics of the active polarized proton target. The target is immersed in a liquid 3He/4Hemixture with a temperature of T ∼ 25 mK at the target head. This design includes wavelength-shiftingmaterial to transport light from the scintillators in the target head to the glass tube which is read out at thewarm side by SiPMs.
An active polarized proton target is being developed to identify the reactions below the pionthreshold by detecting recoil protons inside the Mainz-Dubna dilution cryostat [2]. Polarizableplastic scintillator disks are stacked in a target head made of wavelength-shifting material or
2
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 8/16
-
Targets
Unpolarized protons
• Simple - LH2Polarized protons
• Frozen Spin Target -Butanol (C4H9OH)
• Dynamic NuclearPolarization (DNP)
• PmaxT > 90%, τ > 1000 h
Low energy polarized protons
• Rejecting backgrounds• 70 MeV detection threshold• Limited to top right of plot
Spacer
PoS(PSTP2015)005
Mainz Active Polarized Proton Target M. Biroth
1. Introduction
At the MAMI electron accelerator in Mainz, Germany, the A2 Collaboration investigates thespin-polarizabilities of the proton by scattering experiments with spin-polarized energy-taggedphotons. Due to the excellent temperature stability of the Mainz Frozen Spin Target a large de-gree of proton polarization with high relaxation times can be achieved.
At the core of the frozen spin target for the Crystal Ball detector at MAMI is a roughly 2 mlong, horizontal 3He/4He dilution refrigerator that was built in cooperation with the Joint Institutefor Nuclear Research (JINR) Dubna. The cryostat has a separator working at 3 K and an evaporatorworking at 1.2 K in the pre-cooling stages. At the target position the cryostat provides a very lowoperation temperature of 25 mK.
The forming of highly polarized target nuclei is a two step process: in the initial step a high de-gree of nucleon polarization is achieved through a microwave pumping process, known as DynamicNucleon Polarization (DNP). This requires placing the target material in a highly uniform magneticfield of typically 2.5 Tesla and passing microwave radiation at a frequency near 70 GHz through it.The use of the microwaves leads to a moderate increase of the base temperature of the cryostatfrom 0.02 K to around 0.2 K. In a second step, the microwaves are switched off. Consequently,the temperature of the target material drops and the relaxation time of the nucleons increases tosomewhere in the order of several thousand hours, although the field is reduced to a holding fieldof only 0.68 T for the longitudinal polarization and 0.5 T for the transverse polarization. Then ameasurement period of up to approximately one week in the frozen spin mode is possible.
The dynamically polarized, frozen spin target at MAMI was constructed for use inside theCrystal Ball detector with beams of tagged photons. When being polarized the cryostat is movedoutside of the Crystal Ball. Thin superconducting holding coils were installed on the thermal radia-tion protection shields of the refrigerator to maintain the target polarization during the experiments.Details of the frozen spin target at MAMI can be found, e.g. in Ref. [1].
T = 4 K T = 25 mK
Outer Vacuum Seal Inner Vacuum Seal
Light Guide Tube, Vacuum Inside
3He-4He-Mixture
Target HeadSiPM Detector Board
Cryostat
Figure 1: Schematics of the active polarized proton target. The target is immersed in a liquid 3He/4Hemixture with a temperature of T ∼ 25 mK at the target head. This design includes wavelength-shiftingmaterial to transport light from the scintillators in the target head to the glass tube which is read out at thewarm side by SiPMs.
An active polarized proton target is being developed to identify the reactions below the pionthreshold by detecting recoil protons inside the Mainz-Dubna dilution cryostat [2]. Polarizableplastic scintillator disks are stacked in a target head made of wavelength-shifting material or
2
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 8/16
-
Targets
Unpolarized protons
• Simple - LH2Polarized protons
• Frozen Spin Target -Butanol (C4H9OH)
• Dynamic NuclearPolarization (DNP)
• PmaxT > 90%, τ > 1000 hLow energy polarized protons
• Rejecting backgrounds• 70 MeV detection threshold• Limited to top right of plot
Spacer
0
20
40
60
80
100
120
140
160
180
200
Compton Kinematics - Recoil Energy (MeV)
Beam Energy (MeV)0 50 100 150 200 250 300 350 400 450
Co
mp
ton
An
gle
(d
eg)
0
20
40
60
80
100
120
140
160
180Compton Kinematics - Recoil Energy (MeV)
PoS(PSTP2015)005
Mainz Active Polarized Proton Target M. Biroth
1. Introduction
At the MAMI electron accelerator in Mainz, Germany, the A2 Collaboration investigates thespin-polarizabilities of the proton by scattering experiments with spin-polarized energy-taggedphotons. Due to the excellent temperature stability of the Mainz Frozen Spin Target a large de-gree of proton polarization with high relaxation times can be achieved.
At the core of the frozen spin target for the Crystal Ball detector at MAMI is a roughly 2 mlong, horizontal 3He/4He dilution refrigerator that was built in cooperation with the Joint Institutefor Nuclear Research (JINR) Dubna. The cryostat has a separator working at 3 K and an evaporatorworking at 1.2 K in the pre-cooling stages. At the target position the cryostat provides a very lowoperation temperature of 25 mK.
The forming of highly polarized target nuclei is a two step process: in the initial step a high de-gree of nucleon polarization is achieved through a microwave pumping process, known as DynamicNucleon Polarization (DNP). This requires placing the target material in a highly uniform magneticfield of typically 2.5 Tesla and passing microwave radiation at a frequency near 70 GHz through it.The use of the microwaves leads to a moderate increase of the base temperature of the cryostatfrom 0.02 K to around 0.2 K. In a second step, the microwaves are switched off. Consequently,the temperature of the target material drops and the relaxation time of the nucleons increases tosomewhere in the order of several thousand hours, although the field is reduced to a holding fieldof only 0.68 T for the longitudinal polarization and 0.5 T for the transverse polarization. Then ameasurement period of up to approximately one week in the frozen spin mode is possible.
The dynamically polarized, frozen spin target at MAMI was constructed for use inside theCrystal Ball detector with beams of tagged photons. When being polarized the cryostat is movedoutside of the Crystal Ball. Thin superconducting holding coils were installed on the thermal radia-tion protection shields of the refrigerator to maintain the target polarization during the experiments.Details of the frozen spin target at MAMI can be found, e.g. in Ref. [1].
T = 4 K T = 25 mK
Outer Vacuum Seal Inner Vacuum Seal
Light Guide Tube, Vacuum Inside
3He-4He-Mixture
Target HeadSiPM Detector Board
Cryostat
Figure 1: Schematics of the active polarized proton target. The target is immersed in a liquid 3He/4Hemixture with a temperature of T ∼ 25 mK at the target head. This design includes wavelength-shiftingmaterial to transport light from the scintillators in the target head to the glass tube which is read out at thewarm side by SiPMs.
An active polarized proton target is being developed to identify the reactions below the pionthreshold by detecting recoil protons inside the Mainz-Dubna dilution cryostat [2]. Polarizableplastic scintillator disks are stacked in a target head made of wavelength-shifting material or
2
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 8/16
-
Targets
Unpolarized protons
• Simple - LH2Polarized protons
• Frozen Spin Target -Butanol (C4H9OH)
• Dynamic NuclearPolarization (DNP)
• PmaxT > 90%, τ > 1000 hLow energy polarized protons
• Rejecting backgrounds• 70 MeV detection threshold• Limited to top right of plot
Spacer
0
20
40
60
80
100
120
140
160
180
200
Compton Kinematics - Recoil Energy (MeV)
Beam Energy (MeV)0 50 100 150 200 250 300 350 400 450
Co
mp
ton
An
gle
(d
eg)
0
20
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180Compton Kinematics - Recoil Energy (MeV)
PoS(PSTP2015)005
Mainz Active Polarized Proton Target M. Biroth
1. Introduction
At the MAMI electron accelerator in Mainz, Germany, the A2 Collaboration investigates thespin-polarizabilities of the proton by scattering experiments with spin-polarized energy-taggedphotons. Due to the excellent temperature stability of the Mainz Frozen Spin Target a large de-gree of proton polarization with high relaxation times can be achieved.
At the core of the frozen spin target for the Crystal Ball detector at MAMI is a roughly 2 mlong, horizontal 3He/4He dilution refrigerator that was built in cooperation with the Joint Institutefor Nuclear Research (JINR) Dubna. The cryostat has a separator working at 3 K and an evaporatorworking at 1.2 K in the pre-cooling stages. At the target position the cryostat provides a very lowoperation temperature of 25 mK.
The forming of highly polarized target nuclei is a two step process: in the initial step a high de-gree of nucleon polarization is achieved through a microwave pumping process, known as DynamicNucleon Polarization (DNP). This requires placing the target material in a highly uniform magneticfield of typically 2.5 Tesla and passing microwave radiation at a frequency near 70 GHz through it.The use of the microwaves leads to a moderate increase of the base temperature of the cryostatfrom 0.02 K to around 0.2 K. In a second step, the microwaves are switched off. Consequently,the temperature of the target material drops and the relaxation time of the nucleons increases tosomewhere in the order of several thousand hours, although the field is reduced to a holding fieldof only 0.68 T for the longitudinal polarization and 0.5 T for the transverse polarization. Then ameasurement period of up to approximately one week in the frozen spin mode is possible.
The dynamically polarized, frozen spin target at MAMI was constructed for use inside theCrystal Ball detector with beams of tagged photons. When being polarized the cryostat is movedoutside of the Crystal Ball. Thin superconducting holding coils were installed on the thermal radia-tion protection shields of the refrigerator to maintain the target polarization during the experiments.Details of the frozen spin target at MAMI can be found, e.g. in Ref. [1].
T = 4 K T = 25 mK
Outer Vacuum Seal Inner Vacuum Seal
Light Guide Tube, Vacuum Inside
3He-4He-Mixture
Target HeadSiPM Detector Board
Cryostat
Figure 1: Schematics of the active polarized proton target. The target is immersed in a liquid 3He/4Hemixture with a temperature of T ∼ 25 mK at the target head. This design includes wavelength-shiftingmaterial to transport light from the scintillators in the target head to the glass tube which is read out at thewarm side by SiPMs.
An active polarized proton target is being developed to identify the reactions below the pionthreshold by detecting recoil protons inside the Mainz-Dubna dilution cryostat [2]. Polarizableplastic scintillator disks are stacked in a target head made of wavelength-shifting material or
2
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 8/16
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Targets
Unpolarized protons
• Simple - LH2Polarized protons
• Frozen Spin Target -Butanol (C4H9OH)
• Dynamic NuclearPolarization (DNP)
• PmaxT > 90%, τ > 1000 hLow energy polarized protons
• Polarizable scintillators• Light guide to detectors• PmaxT > 50%, τ > 70 h
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0
20
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60
80
100
120
140
160
180Compton Kinematics - Recoil Energy (MeV)
PoS(PSTP2015)005
Mainz Active Polarized Proton Target M. Biroth
1. Introduction
At the MAMI electron accelerator in Mainz, Germany, the A2 Collaboration investigates thespin-polarizabilities of the proton by scattering experiments with spin-polarized energy-taggedphotons. Due to the excellent temperature stability of the Mainz Frozen Spin Target a large de-gree of proton polarization with high relaxation times can be achieved.
At the core of the frozen spin target for the Crystal Ball detector at MAMI is a roughly 2 mlong, horizontal 3He/4He dilution refrigerator that was built in cooperation with the Joint Institutefor Nuclear Research (JINR) Dubna. The cryostat has a separator working at 3 K and an evaporatorworking at 1.2 K in the pre-cooling stages. At the target position the cryostat provides a very lowoperation temperature of 25 mK.
The forming of highly polarized target nuclei is a two step process: in the initial step a high de-gree of nucleon polarization is achieved through a microwave pumping process, known as DynamicNucleon Polarization (DNP). This requires placing the target material in a highly uniform magneticfield of typically 2.5 Tesla and passing microwave radiation at a frequency near 70 GHz through it.The use of the microwaves leads to a moderate increase of the base temperature of the cryostatfrom 0.02 K to around 0.2 K. In a second step, the microwaves are switched off. Consequently,the temperature of the target material drops and the relaxation time of the nucleons increases tosomewhere in the order of several thousand hours, although the field is reduced to a holding fieldof only 0.68 T for the longitudinal polarization and 0.5 T for the transverse polarization. Then ameasurement period of up to approximately one week in the frozen spin mode is possible.
The dynamically polarized, frozen spin target at MAMI was constructed for use inside theCrystal Ball detector with beams of tagged photons. When being polarized the cryostat is movedoutside of the Crystal Ball. Thin superconducting holding coils were installed on the thermal radia-tion protection shields of the refrigerator to maintain the target polarization during the experiments.Details of the frozen spin target at MAMI can be found, e.g. in Ref. [1].
T = 4 K T = 25 mK
Outer Vacuum Seal Inner Vacuum Seal
Light Guide Tube, Vacuum Inside
3He-4He-Mixture
Target HeadSiPM Detector Board
Cryostat
Figure 1: Schematics of the active polarized proton target. The target is immersed in a liquid 3He/4Hemixture with a temperature of T ∼ 25 mK at the target head. This design includes wavelength-shiftingmaterial to transport light from the scintillators in the target head to the glass tube which is read out at thewarm side by SiPMs.
An active polarized proton target is being developed to identify the reactions below the pionthreshold by detecting recoil protons inside the Mainz-Dubna dilution cryostat [2]. Polarizableplastic scintillator disks are stacked in a target head made of wavelength-shifting material or
2
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 8/16
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Detectors
CB
NaI
PID
MWPC
Target
TAPS
BaF2
PbWO4
Crystal Ball (CB)
• 672 NaI Crystals• 24 Particle Identification Detector
(PID) Paddles
• 2 Multiwire Proportional Chambers(MWPCs)
Two Arms Photon Spectrometer (TAPS)
• 366 BaF2 and 72 PbWO4 Crystals• 384 Veto Paddles
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 9/16
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Σ2x - Circularly polarized photons, transversely polarized protons
Σ2x =NR+x − NL+xNR+x + N
L+x
PM et al. (A2)
PRL 114, 112501 (2015)
Eγ=273-303 MeV - Fix γE1E1
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.6−
0.4−
0.2−
0
0.2
0.4
0.62xΣ = 4.9M1M1
γ = 3.9
M1M1γ
= 2.9M1M1
γ = 1.9
M1M1γ
= 0.9M1M1
γ
Eγ=273-303 MeV - Fix γM1M1
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.6−
0.4−
0.2−
0
0.2
0.4
0.62xΣ = -2.3E1E1
γ = -3.3
E1E1γ
= -4.3E1E1
γ = -5.3
E1E1γ
= -6.3E1E1
γ
Fix one (γE1E1/M1M1), vary other. Band from γ0, γπ, αE1, and βM1 errors.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 10/16
-
Σ2x - Circularly polarized photons, transversely polarized protons
Σ2x =NR+x − NL+xNR+x + N
L+x
PM et al. (A2)
PRL 114, 112501 (2015)
Eγ=273-303 MeV - Fix γE1E1
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.6−
0.4−
0.2−
0
0.2
0.4
0.62xΣ = 4.9M1M1
γ = 3.9
M1M1γ
= 2.9M1M1
γ = 1.9
M1M1γ
= 0.9M1M1
γ
Eγ=273-303 MeV - Fix γM1M1
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.6−
0.4−
0.2−
0
0.2
0.4
0.62xΣ = -2.3E1E1
γ = -3.3
E1E1γ
= -4.3E1E1
γ = -5.3
E1E1γ
= -6.3E1E1
γ
Fix one (γE1E1/M1M1), vary other. Band from γ0, γπ, αE1, and βM1 errors.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 10/16
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Σ2z - Circularly polarized photons, longitudinally polarized protons
Σ2z =NR+z − NL+zNR+z + N
L+z
D. Paudyal et al. (A2)
arXiv:1909.02032 (2019)
Fix one (γE1E1/M1M1), vary other. Band from γ0, γπ, αE1, and βM1 errors.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 11/16
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Σ2z - Circularly polarized photons, longitudinally polarized protons
Σ2z =NR+z − NL+zNR+z + N
L+z
D. Paudyal et al. (A2)
arXiv:1909.02032 (2019)
Eγ=265-285 MeV - Fix γE1E1
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0
0.2
0.4
0.6
0.8
12zΣ
= 4.9M1M1
γ = 3.9
M1M1γ
= 2.9M1M1
γ = 1.9
M1M1γ
= 0.9M1M1
γ
Eγ=265-285 MeV - Fix γM1M1
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0
0.2
0.4
0.6
0.8
12zΣ
= -2.3E1E1
γ = -3.3
E1E1γ
= -4.3E1E1
γ = -5.3
E1E1γ
= -6.3E1E1
γ
Fix one (γE1E1/M1M1), vary other. Band from γ0, γπ, αE1, and βM1 errors.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 11/16
-
Σ2z - Circularly polarized photons, longitudinally polarized protons
Σ2z =NR+z − NL+zNR+z + N
L+z
D. Paudyal et al. (A2)
arXiv:1909.02032 (2019)
Eγ=285-305 MeV - Fix γE1E1
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0
0.2
0.4
0.6
0.8
12zΣ
= 4.9M1M1
γ = 3.9
M1M1γ
= 2.9M1M1
γ = 1.9
M1M1γ
= 0.9M1M1
γ
Eγ=285-305 MeV - Fix γM1M1
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0
0.2
0.4
0.6
0.8
12zΣ
= -2.3E1E1
γ = -3.3
E1E1γ
= -4.3E1E1
γ = -5.3
E1E1γ
= -6.3E1E1
γ
Fix one (γE1E1/M1M1), vary other. Band from γ0, γπ, αE1, and βM1 errors.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 11/16
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Σ3 - Linearly polarized photons, unpolarized protons
Σ3 =N‖ − N⊥N‖ + N⊥
C. Collicott et al. (A2)
Draft in progress (2019)
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 12/16
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Σ3 - Linearly polarized photons, unpolarized protons
Σ3 =N‖ − N⊥N‖ + N⊥
C. Collicott et al. (A2)
Draft in progress (2019)
Eγ=267-287 MeV - Fix γE1E1
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.1−
0
0.1
0.2
0.3
0.4
0.53Σ
= 4.9M1M1
γ = 3.9
M1M1γ
= 2.9M1M1
γ = 1.9
M1M1γ
= 0.9M1M1
γ
Eγ=267-287 MeV - Fix γM1M1
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.1−
0
0.1
0.2
0.3
0.4
0.53Σ
= -2.3E1E1
γ = -3.3
E1E1γ
= -4.3E1E1
γ = -5.3
E1E1γ
= -6.3E1E1
γ
Fix one (γE1E1/M1M1), vary other. Band from γ0, γπ, αE1, and βM1 errors.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 12/16
-
Σ3 - Linearly polarized photons, unpolarized protons
Σ3 =N‖ − N⊥N‖ + N⊥
C. Collicott et al. (A2)
Draft in progress (2019)
Eγ=287-307 MeV - Fix γE1E1
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.1−
0
0.1
0.2
0.3
0.4
0.53Σ
= 4.9M1M1
γ = 3.9
M1M1γ
= 2.9M1M1
γ = 1.9
M1M1γ
= 0.9M1M1
γ
Eγ=287-307 MeV - Fix γM1M1
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.1−
0
0.1
0.2
0.3
0.4
0.53Σ
= -2.3E1E1
γ = -3.3
E1E1γ
= -4.3E1E1
γ = -5.3
E1E1γ
= -6.3E1E1
γ
Fix one (γE1E1/M1M1), vary other. Band from γ0, γπ, αE1, and βM1 errors.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 12/16
-
Σ3 - Linearly polarized photons, unpolarized protons
Σ3 =N‖ − N⊥N‖ + N⊥
C. Collicott et al. (A2)
Draft in progress (2019)
Eγ=267-287 MeV
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.1−
0
0.1
0.2
0.3
0.4
0.53Σ
MAMI3ΣFit
MAMI
HDPV
PTχB
Eγ=287-307 MeV
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.1−
0
0.1
0.2
0.3
0.4
0.53Σ
MAMI3ΣFit
MAMI
HDPV
PTχB
Fit all observables with dispersion (HDPV) or chiral perturbation (BχPT) theories.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 12/16
-
Σ3 - Linearly polarized photons, unpolarized protons
Σ3 =N‖ − N⊥N‖ + N⊥
C. Collicott et al. (A2)
Draft in progress (2019)
Eγ=267-287 MeV
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.1−
0
0.1
0.2
0.3
0.4
0.53Σ
LEGS3Σ Fit
MAMI3ΣFit
MAMI LEGS
HDPV HDPV
PTχB PTχB
Eγ=287-307 MeV
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.1−
0
0.1
0.2
0.3
0.4
0.53Σ
LEGS3Σ Fit
MAMI3ΣFit
MAMI LEGS
HDPV HDPV
PTχB PTχB
Already performed fits in previous paper with older Σ3 data from LEGS.
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 12/16
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Σ2z - Circularly polarized photons, longitudinally polarized protons
Σ2z =NR+z − NL+zNR+z + N
L+z
C. Collicott et al. (A2)
Draft in progress (2019)
Σ2z - Eγ=265-285 MeV
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
00.10.20.30.40.50.60.70.80.9
12zΣ
LEGS3Σ Fit
MAMI3ΣFit
MAMI
HDPV HDPV
PTχB PTχB
Σ2z - Eγ=285-305 MeV
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
00.10.20.30.40.50.60.70.80.9
12zΣ
LEGS3Σ Fit
MAMI3ΣFit
MAMI
HDPV HDPV
PTχB PTχB
Large systematic errors at back angles permit these BχPT fits (model dependencies?).
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 13/16
-
Σ2x - Circularly polarized photons, transversely polarized protons
Σ2x =NR+x − NL+xNR+x + N
L+x
C. Collicott et al. (A2)
Draft in progress (2019)
Σ2x - Eγ=273-303 MeV
0 20 40 60 80 100 120 140 160 180 (deg)labθCompton
0.6−
0.4−
0.2−
0
0.2
0.4
0.62xΣ LEGS3Σ Fit
MAMI3ΣFit
MAMI
HDPV HDPV
PTχB PTχB
Best opportunity to improve - Run again late 2020/early 2021
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 14/16
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What about the Neutron?
The situation is even worse for the neutron (difficult with an unstable target)
• Low-energy neutron scattering• Elastic Compton scattering from deuterium• Quasi-free Compton scattering from deuterium• Compton scattering from heavier nuclei
• Elastic Compton scattering from 3He - Shukla, Nogga, and Phillips, NPA 819, 98 (2009)
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 15/16
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What about the Neutron?
The situation is even worse for the neutron (difficult with an unstable target)
• Low-energy neutron scattering• Elastic Compton scattering from deuterium• Quasi-free Compton scattering from deuterium• Compton scattering from heavier nuclei• Elastic Compton scattering from 3He - Shukla, Nogga, and Phillips, NPA 819, 98 (2009)
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 15/16
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What about the Neutron?
The situation is even worse for the neutron (difficult with an unstable target)
• Low-energy neutron scattering• Elastic Compton scattering from deuterium• Quasi-free Compton scattering from deuterium• Compton scattering from heavier nuclei• Elastic Compton scattering from 3He - Shukla, Nogga, and Phillips, NPA 819, 98 (2009)
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 15/16
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Conclusions
Regarding the proton
• Scalar polarizabilities → See E. Mornacchi’s talk next• Spin polarizabilities have been individually extracted for the first time• Analyses finished: one published, one submitted, one being written• More data on tape from which Σ3 can be extracted → LEGS vs MAMI• First test of an active polarized target has taken place → Will improve the extraction
(model dependence, static vs dynamic polarizabilities)
• Another run with the transverse butanol target to optimize what we have
Regarding the neutron
• Active helium target in development• Ran with liquid 4He target this past summer• Active polarized deuterated target for neutron spin polarizabilities
Philippe Martel - Accessing Nucleon Polarizabilities with Compton Scattering 16/16
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