Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session DA: Precision Weak Interaction Studies at Low Energy |
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Sponsoring Units: DNP Chair: Elie Korkmaz, University of Northern British Columbia Room: Gaylord Opryland Tennessee C |
Friday, October 27, 2006 2:00PM - 2:36PM |
DA.00001: Observation of the Radiative Decay Mode of the Free Neutron Invited Speaker: Despite decades of detailed study, the radiative decay mode of the neutron has never been definitively observed. We report observation of this process, in which a photon is emitted along with the proton, electron and antineutrino. Photons with energies between 15 keV and 340 keV were detected by a scintillating crystal coupled to an avalanche photodiode and were distinguished from uncorrelated background photons by coincidence with both the decay electron and proton. Correlated background from external bremsstrahlung generated in the electron detector has been estimated to be minor, due to the physical separation and limited line-of-sight between the particle and photon detectors. Measurement of the dependence of the radiative decay rate on the available phase space of the decay is consistent with the expected behavior for radiative decay. The energy spectrum of the radiated photons, which differs from the uncorrelated background spectrum, is also consistent with the calculated spectrum. The measured branching ratio is consistent with theoretical predictions. We discuss the results of this first experiment, and the design of an improved apparatus to perform a precision measurement of the branching ratio and spectrum. \newline \newline In collaboration with M.S. Dewey, H.P. Mumm, J.S. Nico, A.K. Thompson, NIST-Gaithersburg; B.M. Fisher, I. Kremsky, F.E. Wietfeldt, Tulane University; T.E. Chupp, R.L. Cooper, University of Michigan; E.J. Beise, K.G. Kiriluk, University of Maryland; J. Byrne, University of Sussex; and K.J. Coakley, NIST-Boulder. [Preview Abstract] |
Friday, October 27, 2006 2:36PM - 3:12PM |
DA.00002: New results in rare pion and muon decays Invited Speaker: The PIBETA experiment, a program of precise measurements of rare pion and muon decays at PSI, completed in 2004 an experimental study of the pion and muon radiative decays, $\pi^+ \to e^+ \nu \gamma$ and $\mu^+ \to e^+ \nu \bar{\nu} \gamma$, respectively. The pion radiative decay data have enabled us to evaluate the branching ratio with better than 2\% accuracy, with broad phase space coverage. Consequently, we have evaluated $F_A$, the axial form factor of the pion, with $\sim 2.5$\% accuracy, a more than five-fold improvement over previous data, as well as improved six-fold the accuracy of $F_V$, the pion vector form factor, previously poorly determined. The latter result provides one of the most direct confirmation of the validity of the CVC hypothesis in the pion sector. We've also measured, for the first time ever, the momentum dependence of the pion form factors. All our results are in excellent agreements with chiral perturbation theory. Equally important is a new stringent upper limit on the long-debated tensor form factor for the pion. Our study of the muon radiative decay has resulted in a fourteen-fold improvement over previous data in the accuracy of this decay's branching ratio for a large phase space region ($E_\gamma > 10$ MeV, $\theta_{e-\gamma} > 30^\circ$). Our 2\% result is in excellent agreement with theoretical predictions. Focusing on a narrower range of phase space, we were able to improve significantly the upper limit on the Michel paramter $\bar{\eta}$, reducing the world average upper limit by a factor of 2.5. $\bar{\eta}$ is sensitive to non-(V-A) admixtures in the weak lagrangian. The new results, their implications, and prospects for future improvements are discussed in detail. [Preview Abstract] |
Friday, October 27, 2006 3:12PM - 3:48PM |
DA.00003: Significance of the TRIUMF Weak Interaction Symmetry Test Invited Speaker: The TRIUMF Weak Interaction Symmetry Test (TWIST) experiment uses unique features of muons to do a model independent search for new physics, and to provide constraints on properties of the weak interaction that are complementary to those coming from collider experiments, nuclear decay measurements, and astrophysics inputs. TWIST has provided the world's best measurements of 3 out of 4 parameters describing the distribution of positrons in polarized muon decay, and even more precise results are expected. This talk will discuss the physics output of TWIST, and give a brief update on the status of the experiment. [Preview Abstract] |
Friday, October 27, 2006 3:48PM - 4:24PM |
DA.00004: Weak Interactions with Neutral Atom Traps: new observables using beta-decay daughter momenta Invited Speaker: We use modern atomic physics techniques to trap localized samples of atoms with polarization known to high accuracy. The low-energy daughter nuclei escape the trap, and their detection permits a variety of new observables. We have placed the best general limits on first generation scalar interactions by measuring the $\beta$-$\nu$ correlation in $^{38{\rm m}}$K decay [Gorelov, PRL 94 (2005) 142501], and we have also made a 3\% measurement of the $\nu$ spin asymmetry in $^{37}$K decay [Melconian DNP 2005]. We plan upgrades of both. Here we concentrate on measurements of the daughter nucleus momentum by time-of-flight with respect to the atomic shakeoff electrons, a technique demonstrated by LBL researchers [Scielzo, Nucl.Phys.A 746 (2004) 677c]. The spin asymmetry of daughter nuclei in singles in a pure Gamow-Teller decay vanishes in the standard model [Treiman, Phys. Rev. 110 (1958) 448], so it is a very sensitive probe for new interactions. We have measured the daughter spin asymmetry in $^{80}$Rb decay, achieving statistical accuracy that would complement the best existing limits on tensor interactions in beta decay. The same observable in $^{37}$K decay would be sensitive to right-handed currents with statistics competitive with $\mu$ decay experiments. We also plan a search for the admixture of keV-mass $\nu$'s with the electron $\nu$ in the electron capture decay of $^{131}$Cs. Our goal is sensitivity to $<$10$^{-5}$ admixtures at mass $<$30 keV. Such a $\nu$ would be a warm dark matter candidiate and would have other astrophysics implications. [Preview Abstract] |
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