arXiv subjects

William J. Marciano

Publications and source records attributed to William J. Marciano.

At least 19 recordsLinked to original sources

arXiv2025

Status of the Proton EDM Experiment (pEDM)

The Proton EDM Experiment (pEDM) is the first direct search for the proton electric dipole moment (EDM) with the aim of being the first experiment to probe the Standard Model (SM) prediction of any particle EDM. Phase-I of pEDM will achieve $10^{-29} e\cdot$cm, improving current indirect limits by four orders of magnitude. This will establish a new standard of precision in nucleon EDM searches and offer a unique sensitivity to better understand the Strong CP problem. The experiment is ideally positioned to explore physics beyond the Standard Model (BSM), with sensitivity to axionic dark matter via the signal of an oscillating proton EDM and across a wide mass range of BSM models from $\mathcal{O}(1\text{GeV})$ to $\mathcal{O}(10^3\text{TeV})$. Utilizing the frozen-spin technique in a highly symmetric storage ring that leverages existing infrastructure at Brookhaven National Laboratory (BNL), pEDM builds upon the technological foundation and experimental expertise of the highly successful Muon $g$$-$$2$ Experiments. With significant R\&D and prototyping already underway, pEDM is preparing a conceptual design report (CDR) to offer a cost-effective, high-impact path to discovering new sources of CP violation and advancing our understanding of fundamental physics. It will play a vital role in complementing the physics goals of the next-generation collider while simultaneously contributing to sustaining particle physics research and training early-career researchers during gaps between major collider operations.

hep-ex↗

arXiv2023

Searching for new physics effects in future $W$ mass and $\sin^2θ_W (Q^2)$ determinations

We investigate the phenomenology of the dark $Z$ boson, $Z_d$, which is associated with a new Abelian gauge symmetry and couples to the standard model particles via kinetic mixing $\varepsilon$ and mass mixing $\varepsilon_Z^{}$. We examine two cases: (i) $Z_d$ is lighter than the $Z$ boson, and (ii) $Z_d$ is heavier than that. In the first case, it is known that $Z_d$ causes a deviation in the weak mixing angle at low energies from the standard model prediction. We study the prediction in the model and compare it with the latest experimental data. In the second case, the $Z$-$Z_d$ mixing enhances the $W$ boson mass. We investigate the effect of $Z_d$ on various electroweak observables including the $W$ boson mass using the $S$, $T$, and $U$ parameters. We point out an interesting feature: in the limit $\varepsilon \to 0$, the equation $S = - U$ holds independently of the mass of $Z_d$ and the size of $\varepsilon_Z^{}$, while $|S|\gg |U|$ in many new physics models. We find that the dark $Z$ boson with a mass of $O(100)~\mathrm{GeV}$ with a relatively large mass mixing can reproduce the CDF result within $2σ$ while avoiding all other experimental constraints. Such dark $Z$ bosons are expected to be tested at future high-energy colliders.

hep-ph↗

arXiv2022

Electron Mass Singularities in Semileptonic Kaon Decays

We show that recent improvements in the $O(α)$ long-distance quantum electrodynamics (QED) corrections to the radiative inclusive $K_{e3}$ decay rate using the Sirlin representation are free from infrared divergences and collinear electron mass singularities in the limit $m_e\rightarrow0$, as predicted by the Kinoshita-Lee-Nauenberg theorem. We also verify that in massless QED with the simultaneous dimensional regularization of QED photon infrared divergences and electron mass singularities leads to the same result for the inclusive rate in the limit of four space-time dimensions. The equivalence of the two approaches results in part from an interesting interplay between a small chirality-breaking effect in the massless electron limit and the generalization of space-time algebra and phase space integrals to $d>4$ dimensions. Our finding supports the small theoretical uncertainty claimed for $K_{e3}$ radiative inclusive rates and reaffirms its utility in precision unitarity tests of the quark mixing matrix.

hep-ph↗

arXiv2022

The storage ring proton EDM experiment

We describe a proposal to search for an intrinsic electric dipole moment (EDM) of the proton with a sensitivity of \targetsens, based on the vertical rotation of the polarization of a stored proton beam. The New Physics reach is of order $10^~3$TeV mass scale. Observation of the proton EDM provides the best probe of CP-violation in the Higgs sector, at a level of sensitivity that may be inaccessible to electron-EDM experiments. The improvement in the sensitivity to $θ_{QCD}$, a parameter crucial in axion and axion dark matter physics, is about three orders of magnitude.

hep-ph↗

arXiv2022

Electric dipole moments and the search for new physics

Static electric dipole moments of nondegenerate systems probe mass scales for physics beyond the Standard Model well beyond those reached directly at high energy colliders. Discrimination between different physics models, however, requires complementary searches in atomic-molecular-and-optical, nuclear and particle physics. In this report, we discuss the current status and prospects in the near future for a compelling suite of such experiments, along with developments needed in the encompassing theoretical framework.

hep-ph↗

arXiv2021

Update on $|V_{us}|$ and $|V_{us}/V_{ud}|$ from semileptonic kaon and pion decays

We discuss the recent progress in the study of semileptonic kaon and pion decays, including new experimental results, improved electroweak radiative corrections, form factor calculations and isospin-breaking effects. As a result, we obtain $|V_{us}|=0.22309(40)(39)(3)$ from kaon semileptonic decays and $|V_{us}/V_{ud}|=0.22908(66)(41)(40)(2)(1)$ from the ratio between the kaon and pion semileptonic decay rates. We report an apparent violation of the top-row Cabibbo-Kobayashi-Maskawa matrix unitarity at a $3.2\sim 5.6σ$ level, and a discrepancy at a $2.2σ$ level between the value of $|V_{us}/V_{ud}|$ determined from the vector and axial charged weak interactions. Prospects for future improvements in those comparative precision tests involving $|V_{ud}|$, $|V_{us}|$ and their implications for physics beyond the Standard Model are described.

hep-ph↗

arXiv2020

The muon $g$-2 and $Δα$ connection

The discrepancy between the Standard Model theory and experimental measurement of the muon magnetic moment anomaly, $a_μ=\left(g_μ-2\right)/2$, is connected to precision electroweak (EW) predictions via their common dependence on hadronic vacuum polarization effects. The same data for the total $e^+e^- \rightarrow \text{hadrons}$ cross section, $σ_{\rm had}(s)$, are used as input into dispersion relations to estimate the hadronic vacuum polarization contributions, $a_μ^{\rm had,\,VP}$, as well as the five-flavor hadronic contribution to the running QED coupling at the $Z$-pole, $Δα_{\rm had}^{(5)}(M_{Z}^2)$, which enters natural relations and global EW fits. The EW fit prediction of $Δα_{\rm had}^{(5)}(M_{Z}^2) = 0.02722(41)$ agrees well with $Δα_{\rm had}^{(5)}(M_{Z}^2) = 0.02761(11)$ obtained from the dispersion relation approach, but exhibits a smaller central value suggestive of a larger discrepancy $Δa_μ=a_μ^{\rm exp} - a_μ^{\rm SM}$ than currently expected. Postulating that the $Δa_μ$ difference may be due to missing $σ_{\rm had}(s)$ contributions, implications for $M_W$, $\sin^2 \! θ^{\rm lep}_{\rm eff}$ and $M_H$ obtained from global EW fits are investigated. Shifts in $σ_{\rm had}(s)$ needed to bridge $Δa_μ$ are found to be excluded above $\sqrt{s} \gtrsim 0.7$ GeV at the 95\%CL. Moreover, prospects for $Δa_μ$ originating below that energy are deemed improbable given the required increases in the cross section. Such hypothetical changes to the hadronic data are also found to affect other related observables, such as the electron anomaly, $a_e^{\rm SM}$, the ratio $R_{e/μ} = (m_μ/m_e)^2 (a_{e}^{\rm had,\,LO\,VP}/a_μ^{\rm had,\,LO\,VP})$ and the running of the weak mixing angle at low energies, although the consequences of these are currently less constraining.

hep-ph↗

arXiv2020

Pion Beta Decay and CKM Unitarity

Pion beta decay, $π^+\to π^0 e^+ν(γ)$, provides a theoretically clean $\pm 0.3\%$ determination of the CKM matrix element $V_{\text{ud}}$. That value falls short of super-allowed nuclear beta decays where an order of magnitude better precision already exists. We advocate a new strategy for utilizing pion beta decay, based on its role in determining ${V_{\text{us}} / V_{\text{ud}}}$ via the ratio $R_V=Γ\left(K\to πl ν(γ)\right)/Γ\left(π^+\to π^0 e^+ν(γ)\right)$. $R_V$ measures $ { f_+^K(0) |V_{\text{us}}| / f_+^π(0) |V_{\text{ud}}| }$ and is insensitive to the Fermi constant and some radiative corrections. Its dependence on the ratio of form factors may also prove useful for lattice gauge theory calculations. Employing a lattice based value ${ f_+^K(0) / f_+^π(0) } = 0.970(2)$, we find $V_{\text{us}}/V_{\text{ud}}=0.22918(88)$ compared to $V_{\text{us}}/V_{\text{ud}}=0.23131(45)$ obtained from $R_A=Γ(K\to μν(γ))/Γ(π\to μν(γ))$. Those vector and axial-vector based $V_{\text{us}}/V_{\text{ud}}$ values exhibit a 2.2$σ$ discrepancy. That tension may be relieved by a shift in the lattice $ { f_+^K(0) / f_+^π(0) } $ towards the consistency range 0.961(4), changes in experimental input or new physics. Other implications of $R_V$ and $R_A$ are also discussed.

hep-ph↗

arXiv2019

Radiative Corrections to Neutron and Nuclear Beta Decays Revisited

The universal radiative corrections common to neutron and super-allowed nuclear beta decays (also known as ``inner'' corrections) are revisited in light of a recent dispersion relation study that found $+2.467(22)\%$, i.e.~about $2.4σ$ larger than the previous evaluation. For comparison, we consider several alternative computational methods. All employ an updated perturbative QCD four-loop Bjorken sum rule (BjSR) defined QCD coupling supplemented with a nucleon form factor based Born amplitude to estimate axial-vector induced hadronic contributions. In addition, we now include hadronic contributions from low $Q^2$ loop effects based on duality considerations and vector meson resonance interpolators. Our primary result, $2.426(32)\%$ corresponds to an average of a Light Front Holomorphic QCD approach and a three resonance interpolator fit. It reduces the dispersion relation discrepancy to approximately $1.1σ$ and thereby provides a consistency check. Consequences of our new radiative correction estimate, along with that of the dispersion relation result, for CKM unitarity are discussed. The neutron lifetime-$g_A$ connection is updated and shown to suggest a shorter neutron lifetime $< 879$ s. We also find an improved bound on exotic, non-Standard Model, neutron decays or oscillations of the type conjectured as solutions to the neutron lifetime problem, $\text{BR}(n\to \text{exotics}) < 0.16 \%$.

hep-ph↗

arXiv2018

A Tale of Two Anomalies

A recent improved determination of the fine structure constant, $α= 1/137.035999046(27)$, leads to a $\sim 2.4 σ$ negative discrepancy between the measured electron anomalous magnetic moment and the Standard Model prediction. That situation is to be compared with the muon anomalous magnetic moment where a positive $\sim 3.7 σ$ discrepancy has existed for some time. A single scalar solution to both anomalies is shown to be possible if the two-loop electron Barr-Zee diagrams dominate the scalar one-loop electron anomaly effect and the scalar couplings to the electron and two photons are relatively large. We also briefly discuss the implications of that scenario.

hep-ph↗

arXiv2018

Nucleon Axial Radius and Muonic Hydrogen - A New Analysis and Review

Weak capture in muonic hydrogen ($μ$H) as a probe of the chiral properties and nucleon structure predictions of Quantum Chromodynamics (QCD) is reviewed. A recent determination of the axial-vector charge radius squared, $r_A^2(z\; {\rm exp.}) = 0.46(22)\;{\rm fm}^2$, from a model independent $z$ expansion analysis of neutrino-nucleon scattering data is employed in conjunction with the MuCap measurement of the singlet muonic hydrogen capture rate, $Λ_{\rm singlet}^{\rm MuCap} = 715.6(7.4)\;{\rm s}^{-1}$, to update the induced pseudoscalar nucleon coupling: $\bar{g}_P^{\rm MuCap} = 8.23(83)$ derived from experiment, and $\bar{g}_P^{\rm theory} = 8.25(25)$ predicted by chiral perturbation theory. Accounting for correlated errors this implies $\bar{g}_P^{\rm theory}/\bar{g}_P^{\rm MuCap}= 1.00(8)$, confirming theory at the 8% level. If instead, the predicted expression for $\bar{g}_P^{\rm theory}$ is employed as input, then the capture rate alone determines $r_A^2(μ{\rm H})=0.46(24)\, {\rm fm}^2$, or together with the independent $z$ expansion neutrino scattering results, a weighted average $r_A^2({\rm ave.}) = 0.46(16)\, {\rm fm}^2$. Sources of theoretical uncertainty are critically examined and potential experimental improvements are described that can reduce the capture rate error by about a factor of 3. Muonic hydrogen can thus provide a precise and independent $r_A^2$ value which may be compared with other determinations, such as ongoing lattice gauge theory calculations. The importance of an improved $r_A^2$ determination for phenomenology is illustrated by considering the impact on critical neutrino-nucleus cross sections at neutrino oscillation experiments.

hep-ph↗

arXiv2018

Neutron Lifetime and Axial Coupling Connection

Experimental studies of neutron decay, $n\to pe\barν$, exhibit two anomalies. The first is a 8.6(2.1)s, roughly $4σ$ difference between the average beam measured neutron lifetime, $τ_n^\text{beam}=888.0(2.0)$s, and the more precise average trapped ultra cold neutron determination, $τ_n^\text{trap}=879.4(6)$s. The second is a $5σ$ difference between the pre2002 average axial coupling, $g_A$, as measured in neutron decay asymmetries $g_A^\text{pre2002}=1.2637(21)$, and the more recent, post2002, average $g_A^\text{post2002}=1.2755(11)$, where, following the UCNA collaboration division, experiments are classified by the date of their most recent result. In this study, we correlate those $τ_n$ and $g_A$ values using a (slightly) updated relation $τ_n(1+3g_A^2)=5172.0(1.1)$s. Consistency with that relation and better precision suggest $τ_n^\text{favored}=879.4(6)$s and $g_A^\text{favored}=1.2755(11)$ as preferred values for those parameters. Comparisons of $g_A^\text{favored}$ with recent lattice QCD and muonic hydrogen capture results are made. A general constraint on exotic neutron decay branching ratios, $<0.27\%$, is discussed and applied to a recently proposed solution to the neutron lifetime puzzle.

hep-ph↗

arXiv2017

Top-quark loops and the muon anomalous magnetic moment

The current status of electroweak radiative corrections to the muon anomalous magnetic moment is discussed. Asymptotic expansions for some important electroweak two loop top quark triangle diagrams are illustrated and extended to higher order. Results are compared with the more general integral representation solution for generic fermion triangle loops coupled to pseudoscalar and scalar bosons of arbitrary mass. Excellent agreement is found for a broader than expected range of mass parameters.

hep-ph↗

arXiv2016

Implications of a Light "Dark Higgs" Solution to the $g_μ-2$ Discrepancy

A light scalar $ϕ$ with mass $\lesssim 1$ GeV and muonic coupling $\mathcal{O}(10^{-3})$ would explain the 3.5 $σ$ discrepancy between the Standard Model (SM) muon $g-2$ prediction and experiment. Such a scalar can be associated with a light remnant of the Higgs mechanism in the "dark" sector. We suggest $ϕ\to l^+l^-$ bump hunting in $μ\to eν\barνϕ$, $μ^-p\toν_μnϕ$ (muon capture), and $K^\pm\to μ^\pmνϕ$ decays as direct probes of this scenario. In a general setup, a potentially observable muon electric dipole moment $\lesssim 10^{-23}\ e \cdot\textrm{cm}$ and lepton flavor violating decays $τ\toμ(e) ϕ$ or $μ\to e ϕ$ can also arise. Depending on parameters, a deviation in BR($H\toμ^+μ^-$) from SM expectations, due to Higgs coupling misalignment, can result. We illustrate how the requisite interactions can be mediated by weak scale vector-like leptons that typically lie within the reach of future LHC measurements.

hep-ph↗

arXiv2015

Low Q^2 Weak Mixing Angle Measurements and Rare Higgs Decays

A weighted average weak mixing angle theta_W derived from relatively low Q^2 experiments is compared with the Standard Model prediction obtained from precision measurements. The approximate 1.8 sigma discrepancy is fit with an intermediate mass (~ 10-35 GeV) "dark" Z boson Z_d, corresponding to a U(1)_d gauge symmetry of hidden dark matter, which couples to our world via kinetic and Z-Z_d mass mixing. Constraints on such a scenario are obtained from precision electroweak bounds and searches for the rare Higgs decays H -> Z Z_d -> 4 charged leptons at the LHC. The sensitivity of future anticipated low Q^2 measurements of sin^2 theta_W(Q^2) to intermediate mass Z_d is also illustrated. This dark Z scenario can provide interesting concomitant signals in low energy parity violating measurements and rare Higgs decays at the LHC, over the next few years.

hep-ph↗

arXiv2015

Running in the Dark Sector

The "dark photon" $γ_d$ of a gauged $U(1)_d$ can become practically invisible if it primarily decays into light states from a dark sector. We point out that, in such scenarios, the running of the $U(1)_d$ "fine structure constant" $α_d$, with momentum transfer $q^2$, can be significant and potentially measurable. The $γ_d$ kinetic mixing parameter $\varepsilon^2$ is also expected to run with $q^2$, through its dependence on $α_d$. We show how the combined running of $\varepsilon^2 α_d$ may provide a probe of the spectrum of dark particles and, for $α_d\gtrsim {\rm few}\times 0.1$, substantially modify predictions for "beam dump" or other intense source experiments. These features are demonstrated in simple models that contain light dark matter and a scalar that breaks $U(1)_d$. We also discuss theoretic considerations, regarding the $U(1)_d$ model in the ultraviolet regime, that may suggest the infrared upper bound $α_d \lesssim 0.1$.

hep-ph↗

arXiv2014

Michel decay spectrum for a muon bound to a nucleus

The spectrum of electrons from muons decaying in an atomic bound state is significantly modified by their interaction with the nucleus. Somewhat unexpectedly, its first measurement, at the Canadian laboratory TRIUMF, differed from basic theory. We show, using a combination of techniques developed in atomic, nuclear, and high-energy physics, that radiative corrections eliminate the discrepancy. In addition to solving that outstanding problem, our more precise predictions are potentially useful for interpreting future high-statistics muon experiments that aim to search for exotic interactions at $10^{-16}$ sensitivity.

hep-ph↗

arXiv2014

Muon g-2, Rare Kaon Decays, and Parity Violation from Dark Bosons

The muon g-2 discrepancy between theory and experiment may be explained by a light vector boson Z_d that couples to the electromagnetic current via kinetic mixing with the photon. We illustrate how the existing electron g-2, pion Dalitz decay, and other direct production data disfavor that explanation if the Z_d mainly decays into e+e-, mu+mu-. Implications of a dominant invisible Z_d decay channel, such as light dark matter, along with the resulting strong bounds from the rare K -> pi + 'missing energy' decay are examined. The K decay constraints may be relaxed if destructive interference effects due to Z-Z_d mass mixing are included. In that scenario, we show that accommodating the muon g-2 data through relaxation of K decay constraints leads to interesting signals for dark parity violation. As an illustration, we examine the alteration of the weak mixing angle running at low Q^2, which can be potentially observable in polarized electron scattering or atomic physics experiments.

hep-ph↗