Beta-decay Asymmetry Research Articles

Abstracts from the 131st Meeting of the Tennessee Academy of Science November 2021

Adam Holley grew up in Raleigh, North Carolina, but attended college in Pennsylvania, where he earned a B.S. in physics and mathematics from Haverford College. Uncertain about whether theoretical or experimental physics was right for him, he temporized by teaching high school physics in New York City, where he had the good fortune to collaborate on the development and teaching of a three-year research course for high school students. That experience ultimately helped him decide to become an experimentalist. This he did at North Carolina State University, joining a group of scientists developing the United States' first ultracold neutron (UCN) source at Los Alamos National Lab. He earned his Ph.D. as part of the associated UCNA experiment that for the first time used UCN to measure the neutron beta decay asymmetry. During his subsequent postdoc at Indiana University, Dr. Holley got involved in another beta decay experiment called UCNτ. He continued his involvement with that effort when he joined the faculty at Tennessee Tech, where he leverages unique aspects of UCN physics to involve undergraduates in the work. The Tortoise and the Hare: A Race for Answers to Big Questions about the Universe The production of energetic exotic particles has long been a primary method for discovering the rules that underlie what we observe about the universe. There are, however, a number of gaps in this understanding, which the study of more familiar systems, such as neutrons, atoms, or molecules, can help elucidate. As the overall precision of these experimental efforts increases, there are a growing number of tantalizing anomalies, any of which could point the way to a shift in our understanding of how the universe formed, what makes it up, and whether ours is the only universe or is one of many universes. The race to discover new fundamental physical behavior currently features a diverse array of high-precision approaches, spanning an enormous range of energies. “Ultracold” neutrons (UCN), with twenty orders of magnitude smaller energies than particles produced at the Large Hadron Collider, provide one example of how low-energy measurements can facilitate the requirement for high precision. A classic example of a UCN-based experiment is determination of the free neutron lifetime, an empirical observable involved in a number of potentially interesting anomalies. An experiment called UCNτ that operates at Los Alamos National Laboratory has set the standard for precise determinations of this quantity. Using UCNτ as an example of the physics, engineering, and computational challenges high-precision work entails, this talk will describe the ongoing race to answer some long-standing questions about the universe.

Disagreement between measurements of the neutron lifetime by the ultracold neutron storage method and the beam technique

Recent measurements of the neutron lifetime performed using a gravitational trap of ultracold neutrons (UCNs) (Konstantinov Petersburg Nuclear Physics Institute, Russia) and a magnetic UCN trap (LANL, USA) have confirmed PNPI’s result of 2005. The results of experiments with stored UCNs agree with each other but differ from those of the beam experiment (NIST, USA) by 3.5σ (corresponding to 1% in the decay probability). This disagreement is currently being discussed in the literature as a ‘neutron anomaly’. We analyze possible reasons for that disagreement and test the experimental data for the neutron lifetime and beta-decay asymmetry within the Standard Model. The test is only passed for neutron lifetime values that are obtained using the UCN storage method.

Development of a polarized 31Mg+ beam as a spin-1/2 probe for BNMR

A 28 keV beam of 31Mg+ ions was extracted from a uranium carbide, proton-beam-irradiated target coupled to a laser ion source. The ion beam was nuclear-spin polarized by collinear optical pumping on the \(^{2}\textit {S}_{1/2}\)–\(^{2}\textit {P}_{1/2}\) transition at 280 nm. The polarization was preserved by an extended 1 mT guide field as the beam was transported via electrostatic bends into a 2.5 T longitudinal magnetic field. There the beam was implanted into a single crystal MgO target and the beta decay asymmetry was measured. Both hyperfine ground states were optically pumped with a single frequency light source, using segmentation of the beam energy, which boosted the polarization by approximately 50 % compared to pumping a single ground state. The total decay asymmetry of 0.06 and beam intensity were sufficient to provide a useful spin-1/2 beam for future BNMR experiments. A variant of the method was used previously to optically pump the full Doppler-broadened absorption profile of a beam of 11Be+ with a single-frequency light source.

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

We report on a new measurement of neutron beta-decay asymmetry. From the result A(0) = -0.1189(7), we derive the ratio of the axial vector to the vector coupling constant lambda = g(A)/g(V) = -1.2739(19). When included in the world average for the neutron lifetime tau = 885.7(7) s, this gives the first element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix V(ud). With this value and the Particle Data Group values for V(us) and V(ub), we find a deviation from the unitarity condition for the first row of the CKM matrix of Delta = 0.0083(28), which is 3.0 times the stated error.

The measurement of the beta asymmetry in the decay of polarized neutrons

The beta-decay asymmetry parameter A was measured for the free neutron with high precision. A time projection chamber (TPC) and plastic scintillators were used as a track and energy detector for the decay electrons. Thus the decay volume was well defined. Electron tracks with different angle θ between the electron momentum and the neutron spin could be selected and compared with the expected cos θ distribution. A deviation from this distribution was observed and remains unexplained. Using the electron track detector only to delimit the fiducial volume and adopting the theoretically predicted cos θ distribution a value of A 0 = −0.1160±0.0015 was determined including corrections for recoil and weak magnetism. To our best knowledge this result is not influenced by the observed deviation from the cos θ distribution of the asymmetry. From this result the ratio g A g V = −1.266±0.004 can be deduced within the Standard Model. Using the recent neutron lifetime value leads to g V = 1.4172 (34) · 10 −62 J m 3. This value is in good agreement with g V from ft(0 +→0 +) and μ decay.

Fragment polarization of43Ti in the46Ti + C collision

Spin polarization of43Ti produced through the projectile fragmentation process in46Ti on C collisions at 116 MeV/nucleon has been observed as a function of fragment momentum by means of beta-decay asymmetry. From the momentum dependence of the fragment polarization, it was concluded that negative angle deflection of the43Ti fragment due to the nuclear attractive potential is the main reaction path in the case of light targets.

Neutron beta decay and the right-handed currenr problem

The renormalization of the axial-vector coupling constant λ = g A / g V , extracted from the neutron lifetime ( λ τ ) and from the neutron beta decay asymmetry ( λ c ) is considered carefully In the standard model of the electroweak interaction λ τ should be equal to λ c . According to recent experimental data on neutron beta decay, λ τ and λ c seem to differ from each other. This fact is explained in the framework of the manifestly left-right symmetric model SU(2) L × SU(2) R × U(1). The possibility of the existence of the right-handed current in the neutron beta decay is pointed out. A joint analysis of neutron beta decay and muon decay is performed, the right-handed current parameters are estimated. The axial-vector constant renormalization ( λ N ) by right-handed currents is evaluated.

The beta-decay asymmetry of the neutron

The beta-decay asymmetry of the free neutron is measured with polarized neutrons and a long solenoidal beta-spectrometer with 4π electron-detection solid angle. The asymmetry parameter corrected for recoil and weak magnetism isA0=−0.1146±0.0019, implyinggA/gv=1.262±0.005 for the ratio of the axial-vector to the vector weak coupling constants.

The superconducting neutron decay spectrometer PERKEO

The superconducting neutron beta-decay spectrometer PERKEO is described. PERKEO is installed at the High Flux Reactor of the Institut Laue-Langevin where it measures beta-decay spectra from a beam of polarized or unpolarized neutrons. It has 4π detection and measurements are made with high count rates at low background. PERKEO has already been used to measure the neutron lifetime and the neutron beta-decay asymmetry. It was also employed in a search for a short-lived axion emitted in neutron capture on protons.

Angular correlations in the decay of the free neutron

The sensitivity of a 2×2 π solid-angle neutron beta-decay spectrometer to various beta-decay correlation coefficients is calculated. A spectrometer similar to one (PERKEO) recently used to measure the neutron beta-decay asymmetry, but incorporating both electron and proton detection, would provide a practical method for measuring the neutrino asymmetry and the electron-neutrino correlation.

Beta-Decay Asymmetry of the Neutron andgAgV

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Activation reactionC12(n→,p)B12as a spin analyzer for fast neutrons

It is shown that /sup 12/B nuclei produced in the /sup 12 /C(/b n/ oarr ,/b p/)/sup 12/B reaction are polarized. The polarization of /sup 12/B is measured through its beta-decay asymmetry. The analyzing `efficiency' epsiv =/b P//sub N///b P //sub n/ (/sup 12/B polarization versus that of the incident neutron) for longitudinally polarized neutrons of average energy /b E/ macr /sub n/=18 MeV is: | epsiv |=0.11+/-0.02.

Axial and magnetic tests for nuclear Dirac wave functions

A currently popular Dirac model of nuclear wave functions is examined to determine whether its ratio of "small" to "large" components agrees with recent data on the asymmetry in polarized $A=12$ nuclear beta decay, as well as with $A=15$ magnetic moments. The magnetic moments agree well, and no clear inconsistency is found with the beta-decay asymmetry, but future experiments on the latter might reveal a problem since theory and experiment disagree by about 1 standard deviation.NUCLEAR STRUCTURE $^{15}\mathrm{O}$, $^{15}\mathrm{N}$; calculated magnetic moments. Relativistic single-particle wave functions, exchange current. $A=12$; calculated axial dipole matrix element. Relativistic treatment, Cohen-Kurath configurations.

Beta-Decay Asymmetry and Nuclear Magnetic Moment of Argon-35

The asymmetry in beta emission from polarized ${\mathrm{Ar}}^{35}$ nuclei and the nuclear magnetic moment of ${\mathrm{Ar}}^{35}$ have been determined by an atomic-beam method used previously for similar measurements on ${\mathrm{Ne}}^{19}$. The results are: for the asymmetry parameter, $A=+0.16\ifmmode\pm\else\textpm\fi{}0.04$; and for the nuclear moment, $\ensuremath{\mu}=+0.632\ifmmode\pm\else\textpm\fi{}0.002$ nm. The experimental value of $A$ is used to obtain the Gamow-Teller beta-decay matrix element: $〈\ensuremath{\sigma}〉=0.10\ifmmode\pm\else\textpm\fi{}0.05$. This result is consistent with the measured $\mathrm{ft}$ value and electron-neutrino angular correlation coefficient in the decay of ${\mathrm{Ar}}^{35}$. The results are also consistent with a rotational model of ${\mathrm{Ar}}^{35}$ using a deformation parameter close to that of ${\mathrm{Cl}}^{35}$.

Beta-Decay Asymmetry and Nuclear Magnetic Moment of Neon-19

The asymmetry parameter and the nuclear magnetic moment of Ne/sup 19/ were determined to be -0.057 surface proces 0.005 and -1.886 surface proces 0.001 nm, respectively, by observing the reversal of beta-decay asymmetry that occurs when poiarized Ne/sup 19/ nuclei undergo resonance reorientation. The procedure and assumptions used are discussed. (D.C.W.)

Beta decay asymmetry of Li 8, Ag 108 and Ag 110 nuclei produced in the capture of polarized thermal neutrons

A study is made of the beta decay asymmetry of Li 8, Ag 108 and Ag 110 nuclei produced in the capture of polarized thermal neutrons. The capture of thermal neutrons by Li 7 nuclei is shown to occur mostly with the production of a spin 2 − compound nucleus. The spin 1 − compound states of Ag 108 and Ag 110 are found to play a notable role in the capture of thermal neutrons by Ag 107 and Ag 109 nuclei. An estimate is made of the relaxation time of Ag 108 nuclear moments in AgCl at 6°K.

Paschen-Back Effect as a Means of Detecting Muonium

Although formation of muonium (the bound system of a ${\ensuremath{\mu}}^{+}$ meson and an electron) can be expected in suitable materials, the precession test for it has been negative. Since this test is sensitive to slight perturbations it is proposed to substitute for it a more positive and insensitive test which uses a strong magnetic field in the direction of the initial muon orientation rather than a weak field at right angles. In this way the depolarizing effect of the muonium hyperfine coupling is quenched and the beta-decay asymmetry will be increased. The amount of increase determines the fraction of muons forming muonium. For a field of five kilogauss the quenching should be 90% complete.