The MuCap experiment is a high-precision measurement of the rate for the basic electroweak process of muon capture,
μ
−+p→n+
ν
μ
. The experimental approach is based on an active target consisting of ...a time projection chamber (TPC) operating with pure hydrogen gas. The hydrogen has to be kept extremely pure and at a stable pressure. A Circulating Hydrogen Ultra-high Purification System was designed at the Petersburg Nuclear Physics Institute (PNPI) to continuously clean the hydrogen from impurities. The system is based on an adsorption cryo pump to stimulate the hydrogen flow and on a cold adsorbent for the hydrogen cleaning. It was installed at the Paul Scherrer Institute (PSI) in 2004 and performed reliably during three experiment runs. During several months long operating periods the system maintained the hydrogen purity in the detector on the level of 20
ppb for moisture, which is the main contaminant, and of better than 7 and 5
ppb for nitrogen and oxygen, respectively. The pressure inside the TPC was stabilized to within 0.024% of 10
bar at a hydrogen flow rate of three standard liters per minute.
The MUON Detector (MD) of LHCb is one of the largest instruments of this kind worldwide, and one of the most irradiated. It has performed exceptionally well during the RUN1 and RUN2 of the LHC at an ...instantaneous luminosity of 4\(\times\)10\(^{32}\) cm\(^{-2}\)s\(^{-1}\), with tracking inefficiencies at the level of 1\(\%\) and 2.6\(\%\), respectively. Looking forward for the future LHCb Upgrade 2 (U2) planned in 2031 and aiming in running the detector at increased luminosity by a factor \(\sim\)50, and at the same time keeping a very high (\(\sim\)99\(\%\)) detection efficiency, an option with reuse significant part of the present Multi-Wire Proportional Chambers (MWPC) in a new Muon System is presented. In addition, the first idea of new Front End Electronics (FEE) and an existing test setup applicable for designing both: new MWPCs with a higher granularity of the cathode readout pads and new FEE are described.
Background: The rate \lambda_pp\mu\ characterizes the formation of pp\mu\ molecules in collisions of muonic p\mu\ atoms with hydrogen. In measurements of the basic weak muon capture reaction on the ...proton to determine the pseudoscalar coupling g_P, capture occurs from both atomic and molecular states. Thus knowledge of \lambda_pp\mu\ is required for a correct interpretation of these experiments. Purpose: Recently the MuCap experiment has measured the capture rate \Lambda_S from the singlet p\mu\ atom, employing a low density active target to suppress pp\mu\ formation (PRL 110, 12504 (2013)). Nevertheless, given the unprecedented precision of this experiment, the existing experimental knowledge in \lambda_pp\mu\ had to be improved. Method: The MuCap experiment derived the weak capture rate from the muon disappearance rate in ultra-pure hydrogen. By doping the hydrogen with 20 ppm of argon, a competing process to pp\mu\ formation was introduced, which allowed the extraction of \lambda_pp\mu\ from the observed time distribution of decay electrons. Results: The pp\mu\ formation rate was measured as \lambda_pp\mu = (2.01 +- 0.06(stat) +- 0.03(sys)) 10^6 s^-1. This result updates the \lambda_pp\mu\ value used in the above mentioned MuCap publication. Conclusions: The 2.5x higher precision compared to earlier experiments and the fact that the measurement was performed at nearly identical conditions to the main data taking, reduces the uncertainty induced by \lambda_pp\mu\ to a minor contribution to the overall uncertainty of \Lambda_S and g_P, as determined in MuCap. Our final value for \lambda_pp\mu\ shifts \Lambda_S and g_P by less than one tenth of their respective uncertainties compared to our results published earlier.
The MuCap experiment at the Paul Scherrer Institute performed a high-precision measurement of the rate of the basic electroweak process of nuclear muon capture by the proton, \(\mu^- + p \rightarrow ...n + \nu_\mu\). The experimental approach was based on the use of a time projection chamber (TPC) that operated in pure hydrogen gas at a pressure of 10 bar and functioned as an active muon stopping target. The TPC detected the tracks of individual muon arrivals in three dimensions, while the trajectories of outgoing decay (Michel) electrons were measured by two surrounding wire chambers and a plastic scintillation hodoscope. The muon and electron detectors together enabled a precise measurement of the \(\mu p\) atom's lifetime, from which the nuclear muon capture rate was deduced. The TPC was also used to monitor the purity of the hydrogen gas by detecting the nuclear recoils that follow muon capture by elemental impurities. This paper describes the TPC design and performance in detail.
The MuCap experiment at the Paul Scherrer Institute has measured the rate L_S of muon capture from the singlet state of the muonic hydrogen atom to a precision of 1%. A muon beam was stopped in a ...time projection chamber filled with 10-bar, ultra-pure hydrogen gas. Cylindrical wire chambers and a segmented scintillator barrel detected electrons from muon decay. L_S is determined from the difference between the mu- disappearance rate in hydrogen and the free muon decay rate. The result is based on the analysis of 1.2 10^10 mu- decays, from which we extract the capture rate L_S = (714.9 +- 5.4(stat) +- 5.1(syst)) s^-1 and derive the proton's pseudoscalar coupling g_P(q^2_0 = -0.88 m^2_mu) = 8.06 +- 0.55.
We propose to measure the rate \Rd\ for muon capture on the deuteron to better than 1.5% precision. This process is the simplest weak interaction process on a nucleus that can both be calculated and ...measured to a high degree of precision. The measurement will provide a benchmark result, far more precise than any current experimental information on weak interaction processes in the two-nucleon system. Moreover, it can impact our understanding of fundamental reactions of astrophysical interest, like solar pp fusion and the \(\nu+d\) reactions observed by the Sudbury Neutrino Observatory. Recent effective field theory calculations have demonstrated, that all these reactions are related by one axial two-body current term, parameterized by a single low-energy constant. Muon capture on the deuteron is a clean and accurate way to determine this constant. Once it is known, the above mentioned astrophysical, as well as other important two-nucleon reactions, will be determined in a model independent way at the same precision as the measured muon capture reaction.
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