The new Mainz Energy-Recovering Superconducting Accelerator (MESA) is under construction. It is planned to carry out at the accelerator a P2 experiment which aims for a high precision determination ...of the weak mixing angle sin
2
θ
W
to a precision of 0.15% at a four-momentum transfer of 4.5 × 10
−3
GeV
2
. The planned time for collecting statistics is 11000 h using a 150 μA polarized electron beam impinging on a 60 cm liquid hydrogen target. For continuous online measurement of polarization, a Hydro-Møller Polarimeter is provided in the design of the P2 experiment. One of the main parts of this polarimeter is
3
He/
4
He dilution refrigerator (DR), which allows to maintain an ultra-low temperature (<250 mK) in a continuous mode. In this article a description of the design, construction and recent results of the horizontal dilution refrigerator for Hydro-Møller Polarimeter @ MESA P2 experiment are given.
A precise measurement of the differential cross sections dσ/dΩ and the linearly polarized photon beam asymmetry Σ_{3} for Compton scattering on the proton below pion threshold has been performed with ...a tagged photon beam and almost 4π detector at the Mainz Microtron. The incident photons were produced by the recently upgraded Glasgow-Mainz photon tagging facility and impinged on a cryogenic liquid hydrogen target, with the scattered photons detected in the Crystal Ball/TAPS setup. Using the highest statistics Compton scattering data ever measured on the proton along with two effective field theories (both covariant baryon and heavy-baryon) and one fixed-t dispersion relation model, constraining the fits with the Baldin sum rule, we have obtained the proton electric and magnetic polarizabilities with unprecedented precision: α_{E1}=10.99±0.16±0.47±0.17±0.34, β_{M1}=3.14±0.21±0.24±0.20±0.35; in units of 10^{-4} fm^{3} where the errors are statistical, systematic, spin polarizability dependent, and model dependent.
We report a measurement of the spin polarization of the recoiling neutron in deuterium photodisintegration, utilizing a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured ...photon energy range of 300-700 MeV provides the first measurement of recoil neutron polarization at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarization in a narrow structure centered around E_{γ}∼570 MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behavior could be related to the excitation of the d^{*}(2380) hexaquark.
We report a first measurement of the double-polarisation observable, Cx′, in π+ photoproduction off the proton. The Cx′ double-polarisation observable represents the transfer of polarisation from a ...circularly polarised photon beam to the recoiling neutron. The MAMI circularly polarised photon beam impinged on a liquid deuterium target cell, with reaction products detected in the Crystal Ball calorimeter. Ancillary apparatus surrounding the target provided tracking, particle identification and determination of recoil nucleon polarisation. The Cx′ observable is determined for photon energies 800-1400 MeV, providing new constraints on models aiming to elucidate the spectrum and properties of nucleon resonances. This is the first determination of any polarisation observable from the beam-recoil group of observables for this reaction. Inclusion of the new data in the database of the SAID partial wave analysis shifted the solution to a new global minima which, not only gives better agreement with the current data, but also improves the description of a range of other single and double polarisation observables for charged pion photoproduction.
Photoproduction of π0π±-pairs from quasifree nucleons bound in the deuteron has been investigated to study the helicity dependence of this reaction. Measurements with a liquid deuterium target were ...used to extract the unpolarized cross sections for reactions on protons and neutrons. A deuterated, longitudinally polarized solid-butanol target, together with a circularly polarized photon beam, determined the double polarization observable E. From these results the spin-dependent cross sections σ1/2 and σ3/2, corresponding to the anti-parallel and parallel spin configurations of the beam photon and target nucleon, have been derived. The measurements were performed at the Mainz MAMI accelerator with tagged, circularly-polarized photon beams produced via bremsstrahlung from longitudinally polarized electron beams. The reaction products were detected with an almost 4π solid-angle covering calorimeter composed of the Crystal Ball and TAPS detectors, supplemented by plastic scintillation detectors for charged particle identification. The results are sensitive to sequential decays of nucleon resonances via intermediate states and also to the decay of nucleon resonances by emission of charged ρ mesons, and are compared to recent model results.
We report the first large-acceptance measurement of the beam–spin asymmetry for deuteron photodisintegration (γ→d→pn) in the photon energy range 420<Eγ<620MeV. The measurement provides important new ...constraints on the mechanisms of photodisintegration above the Δ resonance and on the photocoupling of the recently discovered d⁎(2380) hexaquark.
A first measurement of the polarisation transfer from a circularly-polarised photon to the final state neutron (Cx′n) in deuterium photodisintegration has been carried out. This quantity is ...determined over the photon energy range 370 – 700 MeV and for neutron centre-of-mass breakup angles ∼45−120∘. The polarisation of the final state neutrons was determined by an ancillary large-acceptance nucleon polarimeter, surrounding a cryogenic liquid deuterium target within the Crystal Ball detector at MAMI. The polarimeter characterised (n,p) charge exchange of the ejected neutrons to determine their polarisation. The new Cx′n data are also compared to a theoretical model based on nucleonic and nucleon resonance degrees of freedom constrained by the current world-database of deuterium photodisintegration measurements. Structures in Cx′n observed in the region of the d⁎(2380) could not be explained by conventional models of deuteron photodisintegration.
The quasifree
photon beam asymmetry,
, has been measured at photon energies,
, from 390 to 610 MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the first time. The data were ...collected in the A2 hall of the MAMI electron beam facility with the Crystal Ball and TAPS calorimeters covering pion center-of-mass angles from 49
to 148
. In this kinematic region, polarization observables are sensitive to contributions from the
and
(1440) resonances. The extracted values of
have been compared to predictions based on partial-wave analyses (PWAs) of the existing pion photoproduction database. Our comparison includes the SAID, MAID and Bonn-Gatchina analyses; while a revised SAID fit, including the new
measurements, has also been performed. In addition, isospin symmetry is examined as a way to predict
photoproduction observables, based on fits to published data in the channels
,
and
.
The quasifree
γ
→
d
→
π
0
n
(
p
)
photon beam asymmetry,
Σ
, has been measured at photon energies,
E
γ
, from 390 to 610 MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the ...first time. The data were collected in the A2 hall of the MAMI electron beam facility with the Crystal Ball and TAPS calorimeters covering pion center-of-mass angles from 49
∘
to 148
∘
. In this kinematic region, polarization observables are sensitive to contributions from the
Δ
(
1232
)
and
N
(1440) resonances. The extracted values of
Σ
have been compared to predictions based on partial-wave analyses (PWAs) of the existing pion photoproduction database. Our comparison includes the SAID, MAID and Bonn–Gatchina analyses; while a revised SAID fit, including the new
Σ
measurements, has also been performed. In addition, isospin symmetry is examined as a way to predict
π
0
n
photoproduction observables, based on fits to published data in the channels
π
0
p
,
π
+
n
and
π
-
p
.
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