Is the unitarity of the quark-mixing CKM matrix violated in neutron beta-decay?

Final results for the neutron -asymmetry parameter A0 from the UCNA experiment

The UCNA experiment was designed to measure the neutron β-asymmetry parameter A0 using polarized ultracold neutrons (UCN). UCN produced via downscattering in solid deuterium were polarized via transport through a 7 T magnetic field, and then directed to a 1 T solenoidal electron spectrometer, where the decay electrons were detected in electron detector packages located on the two ends of the spectrometer. A value for A0 was then extracted from the asymmetry in the numbers of counts in the two detector packages. We summarize all of the results from the UCNA experiment, obtained during run periods in 2007, 2008–2009, 2010, and 2011–2013, which ultimately culminated in a 0.67% precision result for A0.

Measurement of the Weak Axial-Vector Coupling Constant in the Decay of Free Neutrons Using a Pulsed Cold Neutron Beam

We present a precision measurement of the axial-vector coupling constant g_{A} in the decay of polarized free neutrons. For the first time, a pulsed cold neutron beam was used for this purpose. By this method, leading sources of systematic uncertainty are suppressed. From the electron spectra we obtain λ=g_{A}/g_{V}=-1.27641(45)_{stat}(33)_{sys}, which confirms recent measurements with improved precision. This corresponds to a value of the parity violating beta asymmetry parameter of A_{0}=-0.11985(17)_{stat}(12)_{sys}. We discuss implications on the Cabibbo-Kobayashi-Maskawa matrix element V_{ud} and derive a limit on left-handed tensor interaction.

Dark decay channel analysis (n → χ + e+ e−) with the PERKEO II experiment

Discrepancies from beam and bottle type experiments measuring the neutron lifetime are on the 4σ level. In recent publications Fornal and Grinstein proposed that the puzzle could be solved if the neutron would decay on the one percent level via a dark decay mode [1], one possible branch being n → χ + e+e−. With data from the Perkeo II experiment we set limits on the branching fraction and exclude a one percent contribution for 96% of the allowed mass range for the dark matter particle. With this publication, we give a detailed description of the experiment and some selected details of the analysis.

Happy birthday, ultra-cold neutron!∗

What is driving the accelerated expansion of the universe and do we have an alternative for Einstein's cosmological constant? What is dark matter made of? Do extra dimensions of space and time exist? Is there a preferred frame in the universe? To which extent is left-handedness a preferred symmetry in nature? What's the origin of the baryon asymmetry in the universe? These fundamental and open questions are addressed by precision experiments using ultra-cold neutrons. This year, we celebrate the 50th anniversary of their first production, followed by first pioneering experiments. Actually, ultra-cold neutrons were discovered twice in the same year – once in the eastern and once in the western world [1, 2]. For five decades now research projects with ultra-cold neutrons have contributed to the determination of the force constants of nature's fundamental interactions, and several technological breakthroughs in precision allow to address the open questions by putting them to experimental test. To mark the event and tribute to this fabulous object, we present a birthday song for ultra-cold neutrons with acoustic resonant transitions [3], which are based solely on properties of ultra-cold neutrons, the inertial and gravitational mass of the neutron m, Planck's constant h, and the local gravity g. We make use of a musical intonation system that bears no relation to basic notation and basic musical theory as applied and used elsewhere [4] but addresses two fundamental problems of music theory, the problem of reference for the concert pitch and the problem of intonation.

Neutron Lifetime and Axial Coupling Connection

Experimental studies of neutron decay, n→peν[over ¯], exhibit two anomalies. The first is a 8.6(2.1) s, roughly 4σ difference between the average beam measured neutron lifetime, τ_{n}^{beam}=888.0(2.0)  s, and the more precise average trapped ultracold neutron determination, τ_{n}^{trap}=879.4(6)  s. The second is a 5σ difference between the pre2002 average axial coupling, g_{A}, as measured in neutron decay asymmetries g_{A}^{pre2002}=1.2637(21), and the more recent, post2002, average g_{A}^{post2002}=1.2755(11), where, following the UCNA Collaboration division, experiments are classified by the date of their most recent result. In this Letter, we correlate those τ_{n} and g_{A} values using a (slightly) updated relation τ_{n}(1+3g_{A}^{2})=5172.0(1.1)  s. Consistency with that relation and better precision suggest τ_{n}^{favored}=879.4(6)  s and g_{A}^{favored}=1.2755(11) as preferred values for those parameters. Comparisons of g_{A}^{favored} with recent lattice QCD and muonic hydrogen capture results are made. A general constraint on exotic neutron decay branching ratios, <0.27%, is discussed and applied to a recently proposed solution to the neutron lifetime puzzle.

Determination of the weak axial vector coupling λ = gA/gV from a measurement of the β-asymmetry parameter A in neutron beta decay

We report on a new measurement of the neutron β-asymmetry parameter A with the instrument Perkeo II. The main enhancements are the high neutron polarization of P = 99.7(1)% from a novel arrangement of supermirror polarizers and reduced background from improvements in beam line and shielding. The leading corrections were thus reduced by a factor of 4, pushing them below the level of statistical error and resulting in a significant reduction of systematic uncertainty compared to our previous experiments. We derive the β-asymmetry parameter A0 = -0.11972(45)(stat)((-44)(+32))(sys) = -0.11972((-65)(+53)) and the ratio of the axial vector to the vector coupling constant λ = gA/gV = -1.2761(12)(stat)((-12)(+9))(sys) = -1.2761((-17)(+14)).

× drift momentum spectrometer with high resolution and large phase space acceptance

We propose a new type of momentum spectrometer, which uses the R×B drift effect to disperse the charged particles in a uniformly curved magnetic field, and measures the particles with large phase space acceptance and high resolution. This kind of R×B spectrometer is designed for the momentum analyses of the decay electrons and protons in the PERC (Proton and Electron Radiation Channel) beam station, which provides a strong magnetic field to guide the charged particles in the instrument. Instead of eliminating the guiding field, the R×B spectrometer evolves the field gradually to the analysing field, and the charged particles can be adiabatically transported during the dispersion and detection. The drifts of the particles have similar properties as their dispersion in the normal magnetic spectrometer. Besides, the R×B spectrometer is especially ideal for the measurements of particles with low momenta and large incident angles. We present a design of the R×B spectrometer, which can be used in PERC. For the particles with solid angle smaller than 88 msr, the maximum aberration is below 10-4. The resolution of the momentum spectra can reach 14.4 keV/c, if the particle position measurements have a resolution of 1 mm.

Superthermal source of ultracold neutrons for fundamental physics experiments

Ultracold neutrons (UCNs) play an important role for precise measurements of the properties of the neutron and its interactions. During the past 25 years, a neutron turbine coupled to a liquid deuterium cold neutron source at a high-flux reactor has defined the state of the art for UCN production, despite a long history of efforts towards a new generation of UCN sources. This Letter reports a world-best UCN density available for users, achieved with a new source based on conversion of cold neutrons in superfluid helium. A conversion volume of 5 liters provides at least 274,000 UCN in a single accumulation run. Cyclically repeated operation of the source has been demonstrated, as well.

Determination of the axial-vector weak coupling constant with ultracold neutrons

A precise measurement of the neutron decay β asymmetry A₀ has been carried out using polarized ultracold neutrons from the pulsed spallation ultracold neutron source at the Los Alamos Neutron Science Center. Combining data obtained in 2008 and 2009, we report A₀ = -0.119 66±0.000 89{-0.001 40}{+0.001 23}, from which we determine the ratio of the axial-vector to vector weak coupling of the nucleon g{A}/g{V}=-1.275 90{-0.004 45}{+0.004 09}.

Application of Diamond Nanoparticles in Low-Energy Neutron Physics

Diamond, with its exceptionally high optical nuclear potential and low absorption cross-section, is a unique material for a series of applications in VCN (very cold neutron) physics and techniques. In particular, powder of diamond nanoparticles provides the best reflector for neutrons in the complete VCN energy range. It allowed also the first observation of quasi-specular reflection of cold neutrons (CN) from disordered medium. Effective critical velocity for such a quasi-specular reflection is higher than that for the best super-mirror. Nano-diamonds survive in high radiation fluxes; therefore they could be used, under certain conditions, in the vicinity of intense neutron sources.

Test of the conserved vector current hypothesis in T=1/2 mirror transitions and new determination of |V ud|

The V{ud} element of the Cabibbo-Kobayashi-Maskawa quark mixing matrix has traditionally been determined from the analysis of data in nuclear superallowed 0;{+}-->0;{+} transitions, neutron decay, and pion beta decay. After providing a new test of the conserved vector current hypothesis, we present here a new independent determination of |V{ud}| from a set of five T=1/2 nuclear mirror transitions. The extracted value, |V{ud}|=0.9719+/-0.0017, is at 1.2 combined standard deviations from the value obtained from superallowed 0;{+}-->0;{+} transitions and has a precision comparable to the value obtained from neutron decay experiments.

First measurement of the neutron beta asymmetry with ultracold neutrons

We report the first measurement of an angular correlation parameter in neutron beta decay using polarized ultracold neutrons (UCN). We utilize UCN with energies below about 200 neV, which we guide and store for approximately 30 s in a Cu decay volume. The interaction of the neutron magnetic dipole moment with a static 7 T field external to the decay volume provides a 420 neV potential energy barrier to the spin state parallel to the field, polarizing the UCN before they pass through an adiabatic fast passage spin flipper and enter a decay volume, situated within a 1 T field in a 2x2pi solenoidal spectrometer. We determine a value for the beta-asymmetry parameter A_{0}=-0.1138+/-0.0046+/-0.0021.

The mass of 22Mg

Mass measurements with a relative precision of better than 1.5 x 10(-8) were performed on 22Mg and its reaction partners 21Na and 22Na with the ISOLTRAP Penning trap mass spectrometer at CERN, yielding the mass excesses D(22Mg)=-399.92(27) keV, D(21Na)=-2184.71(21) keV, and D(22Na)=-5181.56(16) keV. The importance of these results is twofold. First, a comparative half-life (Ft value) has been obtained for the superallowed beta decay of 22Mg to further test the conserved-vector-current hypothesis. Second, the resonance energy for the 21Na proton capture reaction has been independently determined, allowing direct comparisons of observable gamma radiation in nova explosions with the yield expected from models.

Direct mass measurements on the superallowed emitter 74Rb and its daughter 74Kr: isospin-symmetry-breaking correction for standard-model tests

The decay energy of the superallowed beta decay 74Rb(beta+)74Kr was determined by direct Penning trap mass measurements on both the mother and the daughter nuclide using the time-of-flight resonance technique and was found to be Q=10 416.8(4.5) keV. The exotic nuclide 74Rb, with a half-life of only 65 ms, is the shortest-lived nuclide on which a high-precision mass measurement in a Penning trap has been carried out. Together with existing data for the partial half-life as well as theoretical corrections, the decay energy yields a comparative half-life of Ft=3084(15) s for this decay, in agreement with the mean value for the series of the lighter nuclides from 10C to 54Co. Assuming conserved vector current, this result allows for an experimental determination of the isospin-symmetry-breaking correction deltaC.

Measurement of the neutron lifetime using a proton trap

We report a new measurement of the neutron decay lifetime by the absolute counting of in-beam neutrons and their decay protons. Protons were confined in a quasi-Penning trap and counted with a silicon detector. The neutron beam fluence was measured by capture in a thin 6LiF foil detector with known absolute efficiency. The combination of these simultaneous measurements gives the neutron lifetime: tau(n)=(886.8+/-1.2[stat]+/-3.2[syst]) s. The systematic uncertainty is dominated by uncertainties in the mass of the 6LiF deposit and the 6Li(n,t) cross section. This is the most precise measurement of the neutron lifetime to date using an in-beam method.