arXiv subjects

Xinshuai Yan

Publications and source records attributed to Xinshuai Yan.

5 recordsLinked to original sources

arXiv2020

LIght scalars with lepton number to solve the $(g-2)_e$ anomaly

Scalars that carry lepton number can help mediate would-be lepton-number-violating processes, such as neutrinoless double $β$ decay or lepton-scattering-mediated nucleon-antinucleon conversion. Here we show that such new scalars can also solve the anomaly in precision determinations of the fine-structure constant $α$ from atom interferometry and from the electron's anomalous magnetic moment, $a_e \equiv (g-2)_e/2$, by reducing $|a_e|$. Study of the phenomenological constraints on these solutions favor a doubly-charged scalar with mass below the GeV scale. Significant constraints arise from the measurement of the parity-violating asymmetry in Møller scattering, and we consider the implications of the next-generation MOLLER experiment at Jefferson Laboratory and of an improved $a_e$ measurement.

hep-ph↗

arXiv2019

Processes that break baryon number by two units and the Majorana nature of the neutrino

We employ the simplest possible models of scalar-fermion interactions that are consistent with the gauge symmetries of the Standard Model and permit no proton decay to analyze the connections possible among processes that break baryon number by two units. In this context we show how the observation of $n$-${\bar n}$ oscillations and of a pattern of particular nucleon-antinucleon conversion processes --- all accessible through e-d scattering --- namely, selecting from $e^- p \to e^+ {\bar p}$, $e^- p \to {\bar n} {\bar ν}$, $e^- n \to {\bar p} {\bar ν}$, and $e^- n \to e^- {\bar n} $ would reveal that the decay $π^- π^- \to e^- e^-$ must occur also. This latter process is the leading contribution to neutrinoless double beta decay in nuclei mediated by new short-distance physics, in contrast to that mediated by light Majorana neutrino exchange. The inferred existence of $π^- π^- \to e^- e^-$ would also reveal the Majorana nature of the neutrino, though the absence of this inference would not preclude it.

hep-ph↗

arXiv2017

Phenomenology of neutron-antineutron conversion

We consider the possibility of neutron-antineutron ($n-\bar n$) conversion, in which the change of a neutron into an antineutron is mediated by an external source, as can occur in a scattering process. We develop the connections between $n-{\bar n}$ conversion and $n-{\bar n}$ oscillation, in which a neutron spontaneously tranforms into an antineutron, noting that if $n-{\bar n}$ oscillation occurs in a theory with B-L violation, then $n-{\bar n}$ conversion can occur also. We show how an experimental limit on $n-{\bar n}$ conversion could connect concretely to a limit on $n-{\bar n}$ oscillation, and vice versa, using effective field theory techniques and baryon matrix elements computed in the M.I.T. bag model.

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↗