# Archive: P2: Accomplishments of the second funding period

• Subproject A: Accelerator concept
The accelerator concept for the Mainz energy-recovering superconducting accelerator MESA has been further developed and the realization of parts (like the electron source) has begun. MESA is a major infrastructure initiative within the PRISMA cluster of excellence. The necessary R&D as well as invest funds are primarily funded by PRISMA. In 2015, an extension building of the MESA underground areas in the framework of the German “Forschungsbau”-program evaluated by the German science council was granted: the so-called ”Centrum für Fundamentale Physik, CFP”. This new building, in conjunction with the already existing halls, allows for an optimal arrangement of accelerator and the experiments. The civil construction will be completed in autumn 2020. The accelerator design is fixed and all components will be delivered ahead of the availability of the building. The delay caused by the civil construction gives us ample opportunity to test the components beforehand which will be an advantage concerning rapid installation and commissioning. In particular the most demanding components, the SRF-cryo modules, will be delivered by end of 2017 and will be tested in the HIM experimental hall, which is prepared for the 2K operation of superconducting cavities. A liquid Helium supply and a sub-atmospheric compressor system operating between 16 and 1100 mbar is already installed.
• Subproject B: Extraction of the weak charge of the proton
In order to predict the achievable precision in the measurement of the weak mixing angle, error propagation calculations were performed using Monte Carlo methods. All quantities that have an impact on the extracted value of $${\sin}^2\theta_W$$ were varied simultanously. The results of a full Monte Carlo simulation of the experimental setup (see following section) were used as an input to these calculations. In this way, a systematic scan of different setups varying target position, angular acceptance etc. have been tested to find the optimal configuration. It turned out that using almost the full MESA energy of E = 145 MeV and a polar scattering angle range of $$25^o \leq \theta 45^o$$ provides the best results. The achievable precision is $$\Delta{\sin}^2\theta_W = 3.2 \cdot 10^{-4}$$ which corresponds to a 0.13% measurement.
• Subproject C: Experimental set-up: solenoid spectrometer and detector
Experiment design
A full Monte Carlo simulation using GEANT4 has been implemented that includes processes in the target and tracks the particles on their way into the detector volume. Interactions of scattered particles with shieldings is also included as well as the response of the detectors to the impact of particles (see next section). The investigations showed that a solenoid magnetic field gives the best result. A superconducting solenoid magnet with an inner diameter of about 2.4 m and a maximum field strength of 0.8 T will be used. Currently, CAD drawings are being developed for the mechanical design. A cooperation with Silviu Covrig from Jefferson Lab has been initiated to investigate the design of a 60 cm long liquid hydrogen target.
Detector development
The concept of a P2 detector is based on single modules that consist of fused silica bars together with photomultiplier tubes. In the final design, 82 wedge-shaped modules that consists of the material Spectrosil-2000 from Heraeus, each with a length of 65 cm, will be used. Numerous beam tests at MAMI were performed, testing the response of the detectors both to electrons and photons at different energies in the X1 beam line and the A2 tagger facility. Simultaneously, the GEANT 4 simulations were further developed that provide the detector response for a wide range of impact scattering angles and impact energies for different particles. The beam tests allowed for a thorough validation of the GEANT4 detector response. It turned out that a setup with long fused silica bars and no light guide is the best solution.
Experiment electronics
A prototype of a new PMT base was developed which allows to change remotely the signal gain by about two orders of magnitude. This feature is necessary later in the experiment where there are two operating modes: A high current mode for data taking (integrating mode) and a low current mode for particle tracking (single events). This prototype was tested in the laboratory and with the MAMI beam. A low noise, high resolution ADC will be used to digitize the signals from the PMT tubes of the fused silica bars. These ADCs are developed jointly for the P2 experiment and the MOLLER experiment at Jefferson Lab. A cooperation with Michael Gericke from the University of Manitoba has been initiated. A prototype which was used for the QWeak experiment was tested in a test beam the MAMI facility.
• Subproject D: Suppression of false asymmetries
Testing helicity correlated beam parameters and monitors at MAMI for P2
We have carried out several beamtimes in our dedicated beam line at the 180 MeV stage of MAMI to measure the characteristics of electronics components for the MESA beam monitors and stabilization systems. Tests with polarized beam with helicity switching rates up to 1kHz have been performed, measuring beam position, angle and intensity, correlated with the beam helicity. An FPGA-based fully digital feedback system was successfully employed for the stabilization of beam position and angle. It showed better performance than achieved with analog systems at MAMI for the A4 parity violation experiment. From the acquired data we have estimated the systematic uncertainties for the P2 experiment due to uncertainties in helicity correlations of beam parameters. The solenoid tests have not yet taken place as all other components of this complex test setup had to be implemented and tested first, but is still an option for future tests.
• Subproject E: Strategy for polarization measurements
The Double Scattering Polarimeter (DSP)
This polarimeter employs double elastic (Mott-)scattering to measure the effective analyzing power of a polarimeter. The polarimeter is installed in a separate laboratory which also presently houses the source foreseen for MESA. Recently an additional Wien filter has been built and was integrated in the beam-line between source and polarimeter. By changing between unpolarized and polarized beams and by using the Wien filter to provide different spin orientations, double scattering measurements can be carried out in five different ways. This yields five equations with only four unknowns (2 effective analyzing powers, depolarization factor of the first target and beam polarization) It is therefore possible to extract the analyzing power in five independent ways, which provides an importance consistency check. First measurements have been done but they do not show a sufficient degree of consistency yet. This may be explained for instance by drifts of the beam polarization during the measurement. We will investigate the reasons for the behavior in the second part of the funding period.
R&D for Hydro-Møller at MESA
We have decided that it is necessary to have an in house fabrication of the 0.3 K atomic trap, since a long term reliable operation of the complex technology requires permanent on site competences. The conceptual design of the cryostat has been completed and the distribution of tasks between outside vendors and our own machine shop has been defined. Most parts have passed the design phase and are already in fabrication. The remaining parts, mostly the extremely delicate components of the high flow rate 0.3 K system are under final design, a process in which we take advantage of advice from several cryogenic specialists, for instance from Dubna, JLAB and CERN. First heat exchanger components have been made and the remaining parts will become available by the end of the funding period. The refrigerator will be tested in stages, the first stages will also have been tested by the end of the period.
• F: Theory
Photonic higher-order corrections do not contribute to the weak charge of the proton; however, the emission of unobserved photons will lead to a shift of the effective $$Q^2$$, thereby affecting the extraction of the weak charge from the measured polarization asymmetry. Our investigations have shown that these kinematic effects are relevant, leading to a correction of the asymmetry in the order of several percent which depends strongly on the energy and scattering angle. Higher-order corrections are expected to be much smaller. We have developped a Monte Carlo event generator, interfaced with the detector simulation software, including radiative effects at first and second order. This will allow us to keep QED radiative effects affecting the data analysis under full control.
A novel class of radiative corrections with an exchange of two photons and parity violation in the hadronic system has been studied. These corrections had previously not received much attention. They can be considered as a shift of the axial part of the $$\gamma Z$$-box correction. We could show that the effect can contribute with a small shift to the proton weak charge, $$\delta Q_W^p=(−0.17\pm 0.25)×10^{-3}$$, i.e. well inside the one permille precision goal of the MESA measurement.
We have shown that pion production in parity-violating electron scattering can be used as a promising way to better constrain strangeness contributions for elastic scattering. An updated model based on currently available data was used to obtain a new prediction for the vector part of the dispersive $$\gamma Z$$-box correction, $${\rm Re} ☐^V_{\gamma Z} (E = 0.155 {\rm GeV}) = (1.1 \pm 0.2) \times 10^{-3}$$ This confirms that the precision of measurements of the weak mixing angle at MESA will not be limited by this kind of theory uncertainty.
Tracking detector A tracking detector is necessary to determine the average momentum transfer Q$$^2$$ of the scattered electrons detected by the integrating detectors. Due to the high rates and low momenta of electrons, the tracker will be based on fast, thin, high-voltage monolithic active pixel sensors (HV-MAPS). We have developed a four plane tracker geometry based on the requirements of track finding and momentum reconstruction. Currently a detailed mechanical design including a gaseous helium cooling system is being worked out and simulated, mechanical prototyping has started. The HV-MAPS sensor is being developed in close collaboration with the Mu3e experiment. Recently, the first large area (2 by 1 cm$$^2$$) sensor has been received from the foundry and is under test. Its predecessor, the MuPix7 chip, demonstrated all essential capabilities on a small chip. Conceptual design and component selection and evaluation for the tracker data acquisition has started.