9 research outputs found
Measuring the weak mixing angle with the P2 experiment at MESA
The P2 experiment in Mainz aims to measure the weak mixing angle in electron-
proton scattering to a precision of 0.13 %. In order to suppress uncertainties
due to proton structure and contributions from box graphs, both a low average
momentum transfer of GeV and a low beam energy
of 155 MeV are chosen. In order to collect the enormous statistics required for
this measurement, the new Mainz Energy Recovery Superconducting Accelerator
(MESA) is being constructed. These proceedings describe the motivation for the
measurement, the experimental and accelerator challenges and how we plan to
tackle them.Comment: Proceedings of the PhiPsi15, Sep. 23-26, 2015, Hefei, Chin
Quasi-elastic polarization-transfer measurements on the deuteron in anti-parallel kinematics
We present measurements of the polarization-transfer components in the
H reaction, covering a previously unexplored kinematic
region with large positive (anti-parallel) missing momentum, , up
to 220 MeV, and . These measurements, performed
at the Mainz Microtron (MAMI), were motivated by theoretical calculations which
predict small final-state interaction (FSI) effects in these kinematics, making
them favorable for searching for medium modifications of bound nucleons in
nuclei. We find in this kinematic region that the measured
polarization-transfer components and and their ratio agree with the
theoretical calculations, which use free-proton form factors. Using this, we
establish upper limits on possible medium effects that modify the bound
proton's form factor ratio at the level of a few percent. We also
compare the measured polarization-transfer components and their ratio for H
to those of a free (moving) proton. We find that the universal behavior of
H, He and C in the double ratio
is maintained in the positive
missing-momentum region
The P2 experiment
This article describes the future P2 parity-violating electron scattering
facility at the upcoming MESA accelerator in Mainz. The physics program of the
facility comprises indirect, high precision search for physics beyond the
Standard Model, measurement of the neutron distribution in nuclear physics,
single-spin asymmetries stemming from two-photon exchange and a possible future
extension to the measurement of hadronic parity violation. The first
measurement of the P2 experiment aims for a high precision determination of the
weak mixing angle to a precision of 0.14% at a four-momentum transfer of Q^2 =
4.5 10^{-3} GeV^2. The accuracy is comparable to existing measurements at the Z
pole. It comprises a sensitive test of the standard model up to a mass scale of
50 TeV, extendable to 70 TeV. This requires a measurement of the parity
violating cross section asymmetry -39.94 10^{-9} in the elastic electron-proton
scattering with a total accuracy of 0.56 10^-9 (1.4 %) in 10,000 h of 150
\micro A polarized electron beam impinging on a 60 cm liquid H_2 target
allowing for an extraction of the weak charge of the proton which is directly
connected to the weak mixing angle. Contributions from gamma Z-box graphs
become small at the small beam energy of 155 MeV. The size of the asymmetry is
the smallest asymmetry ever measured in electron scattering with an
unprecedented goal for the accuracy. We report here on the conceptual design of
the P2 spectrometer, its Cherenkov detectors, the integrating read-out
electronics as well as the ultra-thin, fast tracking detectors. There has been
substantial theory work done in preparation of the determination of the weak
mixing angle. The further physics program in particle and nuclear physics is
described as well.Comment: Invited EPJ A Manuscript, many figures, large file siz