Current values of the neutron lifetime, determined by two entirely distinct measurement techniques of comparable precision, differ to a statistically significant degree, a result which has become known as the neutron lifetime anomaly. In a previous publication we have shown that the discrepancy can be resolved by taking account of electron transfer charge exchange reactions between residual gases and final state protons stored in a quasi-Penning trap. In this article we analyze unique experimental data obtained during the course of the first published neutron lifetime measurement that used a proton trap. These data employed trapping times greater by a factor of a thousand than became conventional in later experiments. The data show that significant event losses occur, probably due to residual gas other than molecular hydrogen or helium. Additionally, the molecular ion H 2 + produced by charge exchange in H 2 undergoes secondary molecular reactions, producing the molecular ion H 3 + and the ion HeH + which is also produced by secondary reactions in helium. These ions could result in event losses depending on the energy and time-of-flight acceptance windows. Energy losses are evaluated and ionic compositions are quantitively assessed as functions of trapping time and residual gas density to account for an energy spectrum obtained using a silicon surface barrier detector. The spectrum is strongly influenced by charge exchange, secondary molecular reactions and backscattering in the detector dead layer.

Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, λ = gA/gV, through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter a with a precision of δa/a = 10−3 and the Fierz interference term b to δb = 3 × 10−3 in unpolarized free neutron beta decay. These results, along with a more precise measurement of the neutron lifetime, aim to deliver an independent determination of the ratio λ with a precision of δλ/λ = 0.03% that will allow an evaluation of Vud and sensitively test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long asymmetric spectrometer that guides the decay electron and proton to two large area silicon detectors in order to precisely determine the electron energy and an estimation of the proton momentum from the proton time of flight. The Nab spectrometer is being commissioned at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Lab. We present an overview of the Nab experiment and recent updates on the spectrometer, analysis, and systematic effects.

We describe an apparatus used to measure the electron-antineutrino angular correlation coefficient in free neutron decay. The apparatus employs a novel measurement technique in which the angular correlation is converted into a proton time-of-flight asymmetry that is counted directly, avoiding the need for proton spectroscopy. Details of the method, apparatus, detectors, data acquisition, and data reduction scheme are presented, along with a discussion of the important systematic effects.

The standard model predicts that, in addition to a proton, an electron, and an antineutrino, a continuous spectrum of photons is emitted in the β decay of the free neutron. We report on the RDK II experiment which measured the photon spectrum using two different detector arrays. An annular array of bismuth germanium oxide scintillators detected photons from 14 to 782 keV. The spectral shape was consistent with theory, and we determined a branching ratio of 0.00335±0.00005[stat]±0.00015[syst]. A second detector array of large area avalanche photodiodes directly detected photons from 0.4 to 14 keV. For this array, the spectral shape was consistent with theory, and the branching ratio was determined to be 0.00582±0.00023[stat]±0.00062[syst]. We report the first precision test of the shape of the photon energy spectrum from neutron radiative decay and a substantially improved determination of the branching ratio over a broad range of photon energies.

This review is concerned with a detailed analysis of some of the technical problems which arise in the application of the Penning trap method to the experimental study of neutron β -decay,a technique which was first successfully tested on the low-flux swimming-pool reactor LIDO (capture flux =3· 10^6cm^(-2)s^(-1)) at AERE Harwell in the 1970's. It does not discuss the scientific merits or demerits of these studies. Of particular importance are the trapping and release of neutron decay protons, and the influence of magnetic mirror effects and radial drifting on the trapped particles. Since these have energies < 1 keV they must be accelerated to energies of order 20-30 keV following release, at which point they are recorded in a silicon surface barrier detector. However serious difficulties were encountered in the post-release acceleration process with vacuum breakdown in the presence of crossed electric and magnetic fields.

Precision measurements in neutron beta decay serve to determine the coupling constants of beta decay and allow for several stringent tests of the standard model. This paper discusses the design and the expected performance of the Nab spectrometer.

As part of an experiment to measure the spectrum of photons emitted in beta-decay of the free neutron, we developed and operated a detector consisting of 12 bismuth germanate (BGO) crystals coupled to avalanche photodiodes (APDs). The detector was operated near liquid nitrogen temperature in the bore of a superconducting magnet and registered photons with energies from 5 keV to 1000 keV. To enlarge the detection range, we also directly detected soft X-rays with energies between 0.2 keV and 20 keV with three large area APDs. The construction and operation of the detector is presented, as well as information on operation of APDs at cryogenic temperatures.

The theory of quantum electrodynamics predicts that beta decay of the neutron into a proton, electron, and antineutrino should be accompanied by a continuous spectrum of soft photons. We recently reported the first observation of this radiative decay mode of the neutron, measured by recording photons in coincidence with both the electron and proton emitted in neutron decay. The experiment was performed on the NG‐6 Fundamental Physics Beam Line at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). A new experiment is under development to measure both the branching ratio and energy spectrum for radiative decay with a relative standard uncertainty of a few percent. We briefly review the fundamental neutron physics program at the NCNR and describe the new radiative decay experiment.

This document presents the motivation, experimental method, manpower and schedule for the Nab experiment at the Fundamental Neutron Physics Beamline at the SNS. Thanks to its highly precise theoretical treatment within the framework of the standard model and high sensitivity to departures from the basic V −A description, neutron beta decay offers an attractive platform for searches for signals of new physics. The Nab experiment will precisely measure beta decays of the unpolarized neutron, with the goal to determine the electron–neutrino correlation with relative precision of 10−3, and the Fierz interference term, a distortion of the beta spectrum never before measured in neutron decay, with an uncertainty of ∼ 3× 10−3. These results will lead to a new precise determination of the ratio λ = GA/GV and to significant reductions in the allowed limits for both rightand left-handed scalar and tensor currents. Alternatively, the experiment will detect a nonzero signal consistent with certain realizations of supersymmetry. An optimized asymmetric magnetic and electrostatic spectrometer has been designed to achieve the required narrow momentum response function, and thus accomplish the physics goals of the experiment. Detailed breakdown of equipment cost, schedule of activities and distribution of collaborator effort are appended in separate spreadsheets. ∗Experiment Manager †Co-Spokesmen ‡On-site Manager Nab experiment at SNS/FnPB Proposal update and funding request 1. Physics motivation Neutron β decay, n → peν̄e, is one of the basic processes in nuclear physics. Its experimental study provides the most sensitive means to evaluate the ratio of axial-vector to vector coupling constants λ = GA/GV . The precise value of λ is important in many applications of the theory of weak interactions, especially in astrophysics; e.g., a star’s neutrino production is proportional to λ. More precise measurements of neutron β-decay parameters are also important in the search for new physics. Measurement of the neutron decay rate Γ, or lifetime τn = 1/Γ, allows a determination of Vud, the u-d Cabibbo-Kobayashi-Maskawa (CKM) matrix element, independent of nuclear models, because Γ is proportional to |Vud|, as seen in the leading order expression: Γ = 1 τn = fmec 4 2π3~7 ( |GV | + 3|GA| ) ∝ |GV | ( 1 + 3|λ| ) = |Vud| |gV | GF (1 + 3|λ|) , (1) where f = 1.71482(15) is a phase space factor, me is the electron mass, gV,A the vector and axial-vector weak nucleon form factors at zero momentum transfer, respectively, and GF is the fundamental Fermi weak coupling constant. While the conservation of vector current (CVC) fixes gV at unity, two unknowns, Vud and λ, remain as variables in the above expression for Γ. Hence, an independent measurement of λ is necessary in order to determine Vud from the neutron lifetime. Several neutron decay parameters can be used to measure λ; they are discussed below. Precise knowledge of Vud helps greatly in establishing the extent to which the three-generation CKM matrix is unitary. CKM unitarity, in turn, provides an independent cross-check of the presence of certain processes and particles not included in the Standard Model (SM) of elementary particles and interactions, i.e., an independent constraint on new physics. Currently, the most accurate value of the CKM matrix element Vud is obtained from measurements of 0 → 0 nuclear β-decays, the so-called superallowed Fermi transitions [1]. However, the procedure of the extraction of Vud involves calculations of radiative and nuclear structure corrections for the Fermi transition in nuclei. Despite the fact that these calculations have been done with high precision (see [2, 8] and references therein), it is impossible to verify the values of these nuclear corrections from independent experiments, and, as discussed below, questions concerning these corrections have been raised. A problem with CKM matrix unitarity at the 2−3σ level persisted for over two decades. For example, the 2002 Review of Particle Properties [3] reported values of CKM matrix elements that yield for the first row ∆ ≡ 1− |Vud| − |Vus| − |Vub| = (32± 14)× 10−4 . (2) The situation changed drastically in 2003 and 2004 when a series of experiments at Brookhaven, Fermilab and CERN reported revised values of Kl3 decay branching ratios, leading to an upward adjustment, by about 2.5σ, of the CKM matrix element Vus [4, 5, 6]. Skipping the details of this revolutionary development, we note that a revised CKM unitarity check yields [7, 1] ∆ = (1± 10)× 10−4 . (3)

The Nab collaboration will perform a precise measurement of a, the electron–neutrino correlation parameter, and b, the Fierz interference term in neutron beta decay, in the Fundamental Neutron Physics Beamline at the SNS, using a novel electric/magnetic field spectrometer and detector design. The experiment is aiming at the 10-3 accuracy level in Δa/a, and will provide an independent measurement of λ=GA/GV, the ratio of axial-vector to vector coupling constants of the nucleon. Nab also plans to perform the first ever measurement of b in neutron decay, which will provide an independent limit on the tensor weak coupling.

The familiar neutron decay into a proton, electron, and antineutrino can be accompanied by photons with sufficient energy to be detected. We recently reported the first observation of the radiative beta decay branch for the free neutron with photons of energy 15–340 keV. We performed the experiment in the bore of a superconducting magnet where electron, proton, and photon signals were measured. A bar of bismuth germanate scintillating crystal coupled to an avalanche photodiode served as the photon detector that operated in the cryogenic, high magnetic field environment. The branching ratio for this energy region was measured and is consistent with the theoretical calculation. An experiment is under way to measure the branching ratio with an improved precision of 1% relative standard uncertainty and to measure the photon energy spectrum. In this paper, the apparatus modifications to reduce the systematic uncertainties will be described. Central to these improvements is the development of a 12-element detector based on the original photon detector design that will improve the statistical sensitivity. During data acquisition, a detailed calibration program will be performed to improve the systematic uncertainties. The development of these modifications is currently under way, and the second run of the experiment commenced in July 2008.

The aCORN experiment is designed to make a precision (<1%) measurement of the electron–antineutrino angular correlation (a-coefficient) in neutron beta decay. It uses a new method proposed in 1996 by Yerozolimsky and Mostovoy. Electrons and recoil protons from neutron decay in a cold beam are detected in coincidence. The momenta of the particles are selected so that the protons form two kinematically distinct time-of-flight groups as a function of electron energy. The count rate asymmetry in these two groups is proportional to the a-coefficient. Precision spectroscopy of the protons is not required. The apparatus is currently under construction. It will be integrated and tested at the Indiana University Cyclotron Facility (IUCF) and then moved to the NIST Center for Neutron Research for the initial physics run.

Submitted for the APR06 Meeting of The American Physical Society Monte Carlo Simulation and Photon Detector Development for the Radiative Decay Experiment R.L. COOPER, T.E. CHUPP, U. Michigan, K.J. COAKLEY, M.S. DEWEY, T.R. GENTILE, H.P. MUMM, J.S. NICO, A.K. THOMPSON, NIST, B.M. FISHER, I. KREMSKY, F.E. WIETFELDT, Tulane U., E.J. BEISE, K.G. KIRILUK, U. Maryland, J. BYRNE, U. Sussex — We have recently observed the radiative decay mode of the neutron, where an electron and photon are observed in coincidence, followed by a delayed proton. A false signal can be obtained if bremsstrahlung from the electron detector reaches the photon detector. We estimate the contribution from this process to be small. The potential false signal was experimentally addressed by measuring the dependence of the radiative decay process on the available phase space of decay and comparing it to prediction. Phase space was controlled by varying the voltage on an electrostatic mirror to reflect decay protons. We discuss the simulation techniques used to study the systematic effects in the experiment. We also discuss the performance of the photon detector, which operates in a high magnetic field and at cryogenic temperatures. Additionally we present the design of a 12-element scintillation detector that will allow a precision measurement of the radiative decay spectrum. Robert Cooper University of Michigan Date submitted: 13 Jan 2006 Electronic form version 1.4

Submitted for the DNP06 Meeting of The American Physical Society An Experiment for a Precision Measurement of the Radiative Decay Spectrum of the Neutron R.L. COOPER, T.E. CHUPP, U. Michigan, K.J. COAKLEY, M.S. DEWEY, T.R. GENTILE, H.P. MUMM, J.S. NICO, A.K. THOMPSON, NIST, B.M. FISHER, I. KREMSKY, F.E. WIETFELDT, Tulane U., E.J. BEISE, K.G. KIRILUK, U. Maryland, J. BYRNE, U. Sussex — We have recently observed the radiative decay mode of the free neutron, in which a photon accompanies the usual beta decay products. Monte Carlo methods were used in the analysis of this observation, and these are being applied to optimize the apparatus for a precision measurement of the photon spectrum. The goal is to substantially increase the number of detected radiative decay events while better understanding the systematic effects. Increased statistical sensitivity is expected with a 12-element scintillation detector that is currently being constructed and modeled. These 12 independent channels for photon detection will allow a more thorough examination of our sources of background. Monte Carlo methods address subtle design issues regarding the charged particle detector and neutron transport. Direct photon detection with an avalanche photodiode as a potential photon detector will also be discussed.

Beta decay of the neutron into a proton, electron, and electron antineutrino is occasionally accompanied by the emission of a photon. Despite decades of detailed experimental studies of neutron beta-decay, this rare branch of a fundamental weak decay has never been observed. An experiment to study the radiative beta-decay of the neutron is currently being developed for the NG-6 fundamental physics endstation at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). The experiment will make use of the existing apparatus for the NIST proton-trap lifetime experiment, which can provide substantial background reduction by providing an electron-proton coincidence trigger. Tests and design of a detector for gamma-rays in the 10 keV to 200 keV range are under development. The need for a large solid-angle gamma-ray detector that can operate in a strong magnetic field and at low temperature has led us to consider scintillating crystals in conjunction with avalanche photodiodes. The motivation and experimental technique will be discussed.

The angular correlation between the beta electron and antineutrino in nuclear beta decay is characterized by the dimensionless parameter a. The value of a for free neutron decay, when combined with other neutron decay parameters, can be used to determine the weak vector and axial vector coupling constants gV and gA and test the validity and self-consistency of the Electroweak Standard Model. Previous experiments that measured a in neutron decay relied on precise proton spectroscopy and were limited by systematic effects at about the 5% level. We present a new approach to measuring a for which systematic uncertainties promise to be much smaller.

Results of the first experiment to search for the radiative decay mode of the free neutron are reported. The γ-spectrum was studied in the energy region from 35 keV to 100 keV in six Cs(Tl) scintillators, each set at an angle of 35° to, and shielded from, a central plastic scintillator electron detector. Triple coincidences were recorded with recoil protons detected in a micro-channel plate. A limit for the branching ratio BR < 6.9 × 10(-3) (90 % confidence level) was obtained, which is greater that the theoretical prediction by not more than a few tenths of a percent.

. The apparatus described here, a SPECT, will be used for a measurement of the neutrino-electron angular correlation coefficient a in the decay of free neutrons. The idea of the a SPECT spectrometer is to measure the integrated proton energy spectrum very accurately using an energy filter by electrostatic retardation and magnetic adiabatic collimation. The main ideas of the spectrometer are presented, followed by an explanation of the adiabatic transmission function. Details of the superconducting coil and of the electrode system are given, as well as a discussion of the most important systematic effects: magnetic field and electrostatic potential inhomogeneities, deviation from adiabatic motion, scattering in the residual gas, background, Doppler effect, edge effect, and detector efficiency. Using this spectrometer, the parameter a is planned to be measured with an absolute experimental uncertainty of δ a ≈ 3 . 10 -4 , from which the axial vector to vector coupling constant ratio λ can be determined with an accuracy of δλ ≈ 0.001.

The coefficient a 0 has been derived from a measurement of the integral spectrum of recoil protons stored in a quasi-Penning trap with inhomogeneous magnetic field and adiabatic focusing onto an electro-static mirror of potential variable in 10 V steps between 0 V and 850 V. Correction for incomplete transfer of energy from transverse to longitudinal degrees of freedom, and the violation of the adiabatic conditions on reflection at the mirror, is carried out by alternately measuring the spectrum at trapping times of 1 ms and 2 ms.The results a 0 = -0.1054± 0.0055 and |λ| = 1.271 ± 0.018 are comparable in precision with existing measurements of a 0 .