Bulletin of the American Physical Society
46th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 60, Number 7
Monday–Friday, June 8–12, 2015; Columbus, Ohio
Session G2: Interface with Particle and Nuclear Physics |
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Sponsoring Units: GPMFC Chair: Zheng-Tian Lu, Argonne National Laboratory Room: Union ABC |
Wednesday, June 10, 2015 8:00AM - 8:30AM |
G2.00001: Searching for Physics Beyond the Standard Model with the World's Largest Penning Trap Invited Speaker: B. Lee Roberts Measurements of the magnetic moments of the electron and muon were intertwined with the development of the modern physics of the 20th century, where $\vec \mu = g(Qe\hbar/2m)\vec s$ with $Q = \pm 1$ and $e > 0$. The $g$-value consists of a Dirac piece and the anomaly, $g =2(1+ a)$ or $a = (g -2)/2$. For point-like particles, $a$ arises from radiative corrections. The simplest correction, $a = \alpha/2\pi \simeq 0.00116\cdots$, was first obtained by Schwinger. This result was also found to describe the muon's magnetic moment, indicating that the muon behaved like a heavy electron in a magnetic field. Loops containing all virtual particles that interact with the muon, including as yet undiscovered ones, can contribute to the muon anomaly. The relative contribution from heavier particles to the muon and electron anomalies scales as $(m_\mu/m_e)^2 \simeq 43,000$, giving the muon a distinct advantage in the search for effects from New Physics. E821 at the Brookhaven AGS obtained a relative precision of $\pm 0.54$ ppm, half the magnitude of the contributions from the $Z$ and $W$ gauge bosons. This result differs from the Standard-Model prediction by $> 3 \sigma$. To clarify whether this difference is a harbinger of New Physics beyond the Standard Model, E989 is being mounted at Fermilab with a design precision of 140 ppb. The 700 ton, 14 m diameter storage ring magnet will be shimmed to a point-to-point magnetic dipole field uniformity of $\pm 25$ ppm over the 1.137 m$^3$ volume where the muon beam is stored, with the azimuthal averaged uniformity $\leq 1$ ppm. Vertical focusing in the storage ring is provided by electrostatic quadrupoles placed with four-fold symmetry around the 44.7 m circumference of the storage ring. In this talk I will discuss the Standard-Model theory and the motivation for this new experiment, along with the experimental technique and outlook. [Preview Abstract] |
Wednesday, June 10, 2015 8:30AM - 9:00AM |
G2.00002: New Results from a Search for the Electric Dipole Moment (EDM) of $^{99}$Hg Invited Speaker: Blayne Heckel The measurement of a nonzero EDM of an atom or elementary particle, at current levels of experimental sensitivity, would imply CP violation beyond the CKM matrix of the Standard Model. Additional sources of CP violation have been proposed to help explain the matter-antimatter asymmetry observed in our universe and the magnitude of $\Theta_{QCD}$, the strength of CP-violation in the strong interaction, remains unknown. We have recently completed a set of measurements on the EDM of $^{199}$Hg, sensitive to both new sources of CP violation and $\Theta_{QCD}$.The experiment compares the phase accumulated by precessing Hg atom spins in vapor cells with electric fields parallel and anti-parallel to a common magnetic field. The statistical sensitivity of new measurements represents a factor of 3 to 4 improvement over previous results. A description of the EDM experiment and the data, along with the current state of the systematic error analysis, will be presented. [Preview Abstract] |
Wednesday, June 10, 2015 9:00AM - 9:30AM |
G2.00003: Single atom tagging and the quest for Majorana Neutrinos Invited Speaker: Giorgio Gratta Elementary spin 1/2 particles (fermions) are generally described by a 4-component Dirac wavefunction. However Nature only needs to work this way for charged particles, where particles and antiparticles are distinguished by the charge state. A simpler 2-component Majorana wavefunction can be used to describe \textit{neutral} spin 1/2 particles, in which case the particle-antiparticle and spin symmetries are related to each other. And indeed, Majorana particles have recently emerged in the condensed matter of topological materials. Within the Standard Model of elementary particle physics the neutrino is the only possible candidate for a Majorana particle. Dirac and Majorana behavior is only discernable for particles of finite mass, since in the massless case two of the Dirac states are impossible to reach. The recent discovery of finite neutrino masses has opened the question of whether neutrinos are elementary Majorana particles. In the affirmative case a new nuclear decay, the neutrinoless double-beta decay, is possible, albeit with a half-life that becomes infinite as the mass goes to zero. Present searches for neutrinoless double-beta decay have given negative results, with 90{\%} CL half-lives in excess of 10$^{25}$yrs. The next generation of experiments will use tons of a specific isotope and search for a few nuclear decays in years of data. The challenge is, of course, to distinguish such decays from the unavoidable background due to trace amounts of natural radioactivity. In the nEXO project we will use tons of the isotope $^{136}$Xe , liquefied, in a Time Projection Chamber. In addition to more conventional (and essential) methods to suppress backgrounds, the nEXO collaboration is developing several techniques to recover and spectroscopically identify single atoms of the decay daughter, $^{136}$Ba, of the double-beta decay of $^{136}$Xe. These techniques can take advantage of ultrasensitive detection methods of atomic physics for a second phase of the nEXO program, with goals of improving the sensitivity to half-lives above 10$^{28}$yrs, corresponding to neutrino masses well below 10meV. I will describe the general status of the field and the R{\&}D in progress to detect a few atoms of Ba produced in a year in tons of Xe. [Preview Abstract] |
Wednesday, June 10, 2015 9:30AM - 10:00AM |
G2.00004: Direct high-precision measurement of the magnetic moment of the proton Invited Speaker: Wolfgang Quint The challenge to measure the properties of the proton with great precision inspires very different branches of physics. The magnetic moment of the proton is a fundamental property of this particle. So far it has only been measured indirectly, by analyzing the spectrum of an atomic hydrogen maser in a magnetic field. Here we report the direct high-precision measurement of the magnetic moment of a single proton using the double Penning-trap technique. We drive proton-spin quantum jumps by a radio-frequency field in a Penning trap with a homogeneous magnetic field. The induced spin transitions are detected in a second trap with a strong superimposed magnetic inhomogeneity. This enables the measurement of the spin-flip probability as a function of the drive frequency. In each measurement the proton's cyclotron frequency is used to determine the magnetic field of the trap. From the normalized resonance curve, we extract the particle's magnetic moment in terms of the nuclear magneton: $\mu_{\mathrm{p}} \quad =$ 2.792 847 350 (9) $\mu_{\mathrm{N}}$. This measurement outperforms previous Penning-trap measurements in terms of precision by a factor of about 760. It improves the precision of the forty year-old indirect measurement by D. Kleppner et al., in which significant theoretical bound-state corrections were required to obtain $\mu_{\mathrm{p}}$, by a factor of 3. By application of this method to the antiproton magnetic moment, the fractional precision of the recently reported value can be improved by a factor of at least 1,000. Combined with the present result, this will provide a stringent test of matter/antimatter symmetry with baryons. [Preview Abstract] |
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