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Susan Gardner

Publications and source records attributed to Susan Gardner.

At least 55 records · Page 3Linked to original sources
arXiv2017

A New Paradigm for Hadronic Parity Nonconservation and its Experimental Implications

For decades the primary experimental goal in studies of hadronic parity nonconservation (PNC) has been the isolation of the isovector weak nucleon-nucleon interaction, expected to be dominated by long-range pion exchange and enhanced by the neutral current. In meson-exchange descriptions this interaction together with an isoscalar interaction generated by rho and omega exchange dominate most observables. Consequently these two amplitudes have been used to compare and check the consistency of the field's experiments. Yet to date, despite sensitive searches like that performed with 18F, no evidence for isovector hadronic PNC has been found. Here we argue, based on recent large-Nc treatments and new global analyses, that the emphasis on isovector hadronic PNC was misplaced. Large-Nc provides an alternative and theoretically better motivated simplification of effective field theories (EFTs) of hadronic PNC, separating the five low-energy constants (LECs) into two of leading order (LO), and three others that are NNLO. This scheme pivots the isospin coordinates we have traditionally used, placing one dominant axis in the isoscalar plane, and a second along the isotensor direction. We show that this large-Nc LEC hierarchy accurately describes all existing data on hadronic PNC, and we discuss opportunities to further test the predicted large-Nc hierarchy of LECs, illustrating the kind of analyses experimentalists can use to better constrain the LO theory and to determine the size of NNLO corrections.

nucl-th↗
arXiv2017

Particle Physics Models for the 17 MeV Anomaly in Beryllium Nuclear Decays

The 6.8$σ$ anomaly in excited 8Be nuclear decays via internal pair creation is fit well by a new particle interpretation. In a previous analysis, we showed that a 17 MeV protophobic gauge boson provides a particle physics explanation of the anomaly consistent with all existing constraints. Here we begin with a review of the physics of internal pair creation in 8Be decays and the characteristics of the observed anomaly. To develop its particle interpretation, we provide an effective operator analysis for excited 8Be decays to particles with a variety of spins and parities and show that these considerations exclude simple models with scalar particles. We discuss the required couplings for a gauge boson to give the observed signal, highlighting the significant dependence on the precise mass of the boson and isospin mixing and breaking effects. We present anomaly-free extensions of the Standard Model that contain protophobic gauge bosons with the desired couplings to explain the 8Be anomaly. In the first model, the new force carrier is a U(1)B gauge boson that kinetically mixes with the photon; in the second model, it is a U(1)(B-L) gauge boson with a similar kinetic mixing. In both cases, the models predict relatively large charged lepton couplings ~ 0.001 that can resolve the discrepancy in the muon anomalous magnetic moment and are amenable to many experimental probes. The models also contain vectorlike leptons at the weak scale that may be accessible to near future LHC searches.

hep-ph↗
arXiv2016

Project X: Physics Opportunities

Part 2 of "Project X: Accelerator Reference Design, Physics Opportunities, Broader Impacts". In this Part, we outline the particle-physics program that can be achieved with Project X, a staged superconducting linac for intensity-frontier particle physics. Topics include neutrino physics, kaon physics, muon physics, electric dipole moments, neutron-antineutron oscillations, new light particles, hadron structure, hadron spectroscopy, and lattice-QCD calculations. Part 1 is available as arXiv:1306.5022 [physics.acc-ph] and Part 3 is available as arXiv:1306.5024 [physics.acc-ph].

hep-ex↗
arXiv2016

Dark Sectors 2016 Workshop: Community Report

This report, based on the Dark Sectors workshop at SLAC in April 2016, summarizes the scientific importance of searches for dark sector dark matter and forces at masses beneath the weak-scale, the status of this broad international field, the important milestones motivating future exploration, and promising experimental opportunities to reach these milestones over the next 5-10 years.

hep-ph↗
arXiv2016

Protophobic Fifth Force Interpretation of the Observed Anomaly in $^8$Be Nuclear Transitions

Recently a 6.8$σ$ anomaly has been reported in the opening angle and invariant mass distributions of $e^+ e^-$ pairs produced in $^8\text{Be}$ nuclear transitions. The data are explained by a 17 MeV vector gauge boson $X$ that is produced in the decay of an excited state to the ground state, $^8\text{Be}^* \to {}^8\text{Be} \, X$, and then decays through $X \to e^+ e^-$. The $X$ boson mediates a fifth force with a characteristic range of 12 fm and has milli-charged couplings to up and down quarks and electrons, and a proton coupling that is suppressed relative to neutrons. The protophobic $X$ boson may also alleviate the current 3.6$σ$ discrepancy between the predicted and measured values of the muon's anomalous magnetic moment.

hep-ph↗
arXiv2016

CPT, CP, and C transformations of fermions, and their consequences, in theories with B-L violation

We consider the transformation properties of fermions under the discrete symmetries CPT, CP, and C in the presence of B-L violation. We thus generalize the analysis of the known properties of Majorana neutrinos, probed via neutrinoless double beta decay, to include the case of Dirac fermions with B-L violation, which can be probed via neutron-antineutron oscillations. We show that the resulting CPT phase has implications for the interplay of neutron-antineutron oscillations with external fields and sources and consider the differences in the Majorana dynamics of neutrinos and neutrons in the context of theories with self-conjugate isospin I=0 and I=1/2 fields.

hep-ph↗
arXiv2016

Trapping Penguins with Entangled B Mesons

The first direct observation of time-reversal (T) violation in the $B\bar{B}$ system has been reported by the BaBar collaboration, employing the method of Ba$\tilde {\rm n}\!$uls and Bernabéu. Given this, we generalize their analysis of the time-dependent T-violating asymmetry ($A_{T}$) to consider different choices of CP tags for which the dominant amplitudes have the same weak phase. As one application, we find that it is possible to measure departures from the universality of $\sin(2β)$ directly. If $\sin(2β)$ is universal, as in the Standard Model, the method permits the direct determination of penguin effects in these channels. Our method, although no longer a strict test of T, can yield tests of the $\sin(2β)$ universality, or, alternatively, of penguin effects, of much improved precision even with existing data sets.

hep-ph↗
arXiv2014

An overview of neV probes of PeV scale physics --- and of what's in between

Low-energy experiments which would identify departures from the Standard Model (SM) rely either on the unexpected observation of symmetry breaking, such as of CP or B, or on an observed significant deviation from a precise SM prediction. We discuss examples of each search strategy, and show that low-energy experiments can open windows on physics far beyond accessible collider energies. We consider how the use of a frequentist analysis framework can redress the impact of theoretical uncertainties in such searches --- and how lattice QCD can help control them.

hep-ph↗
arXiv2013

The Stellar Number Density Distribution in the Local Solar Neighborhood is North-South Asymmetric

We study the number density distribution of a sample of K and M dwarf stars, matched North and South of the Galactic plane within a distance of 2 kpc from the sun, using observations from the Ninth Data Release of the Sloan Digital Sky Survey. We determine distances using the photometric parallax method, and in this context systematic effects exist which could potentially impact the determination of the number density profile with height from the Galactic plane --- and ultimately affect a number density North-South asymmetry. They include: (i) the calibration of the various photometric parallax relations, (ii) the ability to separate dwarfs from giants in our sample, (iii) the role of stellar population differences such as age and metallicity, (iv) the ability to determine the offset of the sun from the Galactic plane, and (v) the correction for reddening from dust in the Galactic plane, though our stars are at high Galactic latitudes. We find the various analyzed systematic effects to have a negligible impact on our observed asymmetry, and using a new and larger sample of stars we confirm and refine the earlier discovery of Widrow et al. of a significant Galactic North-South asymmetry in the stellar number density distribution.

astro-ph.GA↗
arXiv2013

Beta Decays and Non-Standard Interactions in the LHC Era

We consider the role of precision measurements of beta decays and light meson semi-leptonic decays in probing physics beyond the Standard Model in the LHC era. We describe all low-energy charged-current processes within and beyond the Standard Model using an effective field theory framework. We first discuss the theoretical hadronic input which in these precision tests plays a crucial role in setting the baseline for new physics searches. We then review the current and upcoming constraints on the various non-standard operators from the study of decay rates, spectra, and correlations in a broad array of light-quark systems. We finally discuss the interplay with LHC searches, both within models and in an effective theory approach. Our discussion illustrates the independent yet complementary nature of precision beta decay measurements as probes of new physics, showing them to be of continuing importance throughout the LHC era.

hep-ph↗
arXiv2013

Dark Matter Studies Entrain Nuclear Physics

We review theoretically well-motivated dark-matter candidates, and pathways to their discovery, in the light of recent results from collider physics, astrophysics, and cosmology. Taken in aggregate, these encourage broader thinking in regards to possible dark-matter candidates --- dark-matter need not be made of "WIMPs," i.e., elementary particles with weak-scale masses and interactions. Facilities dedicated to nuclear physics are well-poised to investigate certain non-WIMP models. In parallel to this, developments in observational cosmology permit probes of the relativistic energy density at early epochs and thus provide new ways to constrain dark-matter models, provided nuclear physics inputs are sufficiently well-known. The emerging confluence of accelerator, astrophysical, and cosmological constraints permit searches for dark-matter candidates in a greater range of masses and interaction strengths than heretofore possible.

hep-ph↗
arXiv2013

A T-odd Momentum Correlation in Radiative β-Decay

We consider neutron radiative β-decay, n\to p e^- \barν_eγ, and compute the T-odd momentum correlation in the decay rate characterized by the kinematical variable ξ= {\bf l}_ν\cdot ({\bf l}_e\times{\bf k}) arising from electromagnetic final-state interactions in the Standard Model. Our expression for the corresponding T-odd asymmetry A_ξ^{SM} is exact in O(α) up to terms of recoil order, and we evaluate it numerically under various kinematic conditions. Noting the universality of the V-A law in the absence of recoil order terms, we retain the parametric dependence on masses and coupling constants throughout, so that our results serve as a template for the computation of A_ξ^{SM} in allowed nuclear radiative β-decays and hyperon radiative β-decays as well. We consider the pattern of A_ξ^{SM} which appears in nuclear decays and show that A_ξ^{SM} can be suppressed in particular cases, facilitating searches for new sources of CP-violation in such processes.

hep-ph↗
arXiv2013

Radiative Beta Decay for Studies of CP Violation

A triple-product correlation in the radiative β-decay rate of neutrons or of nuclei, characterized by the kinematic variable ξ\equiv (\vec{l}_ν\times\vec{l}_e)\cdot\vec{k}, where, e.g., n(p) \to p(p') + e^-(l_e) + \barν_e(l_ν) + γ(k), can be generated by the pseudo-Chern-Simons term found by Harvey, Hill, and Hill as a consequence of the baryon vector current anomaly and SU(2)_{L} x U(1)_Y gauge invariance at low energies. The correlation probes the imaginary part of its coupling constant, so that its observation at anticipated levels of sensitivity would reflect the presence of sources of CP violation beyond the standard model. We compute the size of the asymmetry in n\to p e^- \barν_e γdecay in chiral effective theory, compare it with the computed background from standard-model final-state interactions, and consider the new physics scenarios which would be limited by its experimental study.

hep-ph↗
arXiv2012

Galactoseismology: Discovery of Vertical Waves in the Galactic Disk

We present evidence for a Galactic North-South asymmetry in the number density and bulk velocity of solar neighborhood stars. The number density profile, which is derived from main-sequence stars in the Sloan Digital Sky Survey, shows a (North - South)/(North + South) deficit at |z| ~ 400 pc and an excess at |z| ~ 800 pc. The bulk velocity profile, which is derived from the Sloan Extension for Galactic Understanding and Exploration, shows a gradual trend across the Galactic midplane as well as smaller-scale features. We speculate that the North-South asymmetry, which has the appearance of a wavelike perturbation, is intrinsic to the disk. We explore the physics of this phenomenon through an analysis of the linearized Boltzmann and Poisson equations and through one-dimensional simulations. The perturbation may be excited by the passage of a satellite galaxy or dark matter subhalo through the Galactic disk, in which case we are witnessing a recent disk-heating event.

astro-ph.GA↗
arXiv2011

Pair Production Constraints on Superluminal Neutrinos Revisited

We revisit the pair creation constraint on superluminal neutrinos considered by Cohen and Glashow in order to clarify which types of superluminal models are constrained. We show that a model in which the superluminal neutrino is effectively light-like can evade the Cohen-Glashow constraint.

hep-ph↗
arXiv2011

Superluminal Neutrinos without Revolution

The velocity anomaly recently reported by the OPERA collaboration appears strikingly at odds with the theory of special relativity. I offer a reinterpretation which removes this conflict, to wit that neutrinos yield a truer measurement of Einstein's limiting speed, and that light and indeed all other matter are retarded by additional interactions with the dark universe. I discuss existing experimental constraints and show that such a notion, considered cosmologically, can be subsumed in the dark-energy equation of state in an expanding Friedman-Robertson-Walker (FRW) universe. Planned measurements of the temporal variation in redshift have the potential to distinguish the possibilities.

hep-ph↗
arXiv2011

Tracing Remnants of the Baryon Vector Current Anomaly in Neutron Radiative Beta Decay

Harvey, Hill, and Hill have found that unexpected interactions involving the nucleon, photon, and weak gauge bosons at low energies emerge from gauging the axial anomaly of QCD under the full electroweak symmetry of the Standard Model (SM). In this contribution we consider how such interactions can be isolated through a triple-product momentum correlation in neutron radiative beta-decay. The correlation is both parity- and naively time-reversal-odd, so that it vanishes in the Standard Model save for effects induced by final-state interactions (FSI). Nevertheless, the correlation can be generated by sources of CP violation beyond the Standard Model, and such couplings, being spin-independent, are not constrained by the nonobservation of the neutron electric dipole moment (EDM). We consider the sorts of limits on its strength which can be determined at existing and anticipated facilities, as well as the size of induced correlations from known FSI. We also briefly consider the possibility of nuclear beta-decay studies as well as the prospects for muon-induced reaction studies.

hep-ph↗