NoMoS: AnR × Bdrift momentum spectrometer for beta decay studies

Abstract

The beta decay of the free neutron provides several probes to test the Standard Model of particle physics as well as to search for extensions thereof. Hence, multiple experiments investigating the decay have already been performed, are under way or are being prepared. These measure the mean lifetime, angular correlation coefficients or various spectra of the charged decay products (proton and electron). NoMoS, the neutron decay products momentum spectrometer, presents a novel method of momentum spectroscopy: it utilizes the R × B drift effect to disperse charged particles dependent on their momentum in an uniformly curved magnetic field. This spectrometer is designed to precisely measure momentum spectra and angular correlation coefficients in free neutron beta decay to test the Standard Model and to search for new physics beyond. With NoMoS, we aim to measure inter alia the electron-antineutrino correlation coefficient a and the Fierz interference term b with an ultimate precision of ∆a/a < 0.3% and ∆b < 10 −3 respectively. In this paper, we present the measurement principles, discuss measurement uncertainties and systematics, and give a status update. * Neutron Beta Decay in the Standard ModelThe beta decay of the free neutron is well described within the V-A theory of the SM. Fermi's Golden Rule for the neutron's decay rate yields the following insightful correlation between the mean lifetime τ n , the weak axial-vector coupling constant g A and V ud [3]: arXiv:1906.04511v1 [physics.ins-det]

First results for the pLGAD sensor for low-penetrating particles

“…Since the multiplication layer can be placed at any thickness within the sensor, future production runs will include a collection region of thickness > 0 and a thinner passivation or a metallic conduction layer. These steps will be taken as our driving application for the sensor was the NoMoS measurement concept [3], a new method of momentum-spectroscopy for the charged decay products from neutron beta decay, which requires position-resolved proton detection on a largearea sensor, at a reasonable cost. Any kind of lowpenetrating particles can be detected using a pL-GAD sensor, like soft X-rays, charged ions, alpha particles and with a thin conversion layer, also neutrons.…”

“…For the NoMoS (Neutron decay prOducts MOmentum Spectrometer) measurement concept [3], the benchmark is a proton with 15 keV. In this case, the maximum depth in which energy is deposited is roughly 300 nm, assuming a sensor with a thin metal passivation layer of 4 nm aluminium oxide and an incidence angle of the impinging proton of 0°(simulated 500,000 protons using IMSIL [4,5]).…”

“…It can easily be adapted to the needs of different experiments, e.g. for the case of low-energy protons in NoMoS [3], aSPECT [1], or Nab [2]. A pLGAD sensor can be used especially but not exclusively for low-energy spectroscopy experiments with high energy resolution.…”

Decay microscope for trapped neon isotopes

“…Observing table II, accumulation of 10 6 recoil ion events in the decay of 19 Ne already improves on the state of the art, and reduces the uncertainty in the prediction of A, which is well-motivated considering the discrepancies in other determinations of A, as well as deviations from the conserved-vector-current hypothesis as discussed in [97]. To complement the highly precise determination of V ud from the body of 0 + → 0 + decay data [53], and considering ongoing correlation measurement efforts with nuclear mirror [35,38,[100][101][102][103][104][105], and neutron [63,64,[106][107][108][109] decays, roughly 10 7 detected events are desired, especially in light of the emergence of a significant (4σ) deviation from CKM unitarity obtained in a recent analysis [110].…”

“…Due to the already monumental work on superallowed Fermi transitions over the last decades, which is still ongoing, it is difficult to imagine that beta-spectrum measurements of Fermi transitions will be competitive in b F extraction. On the other hand, there is a strong potential for improvement for the GT case, and it is the focus of several new betaspectrum campaigns utilizing nuclear [55][56][57][58][59][60][61], and neutron [62][63][64] decays, and aiming at σ(b GT ) 10 −3 (see table II). Correspondingly, theoretical corrections to the beta-spectrum shape are now at the level of a few 10 −4 [65].…”