arXiv subjects

David McKeen

Publications and source records attributed to David McKeen.

81 records · Page 5Linked to original sources

arXiv2012

Signatures of sub-GeV dark matter beams at neutrino experiments

We study the high-luminosity fixed-target neutrino experiments at MiniBooNE, MINOS and T2K and analyze their sensitivity to light stable states, focusing on MeV--GeV scale dark matter. Thermal relic dark matter scenarios in the sub-GeV mass range require the presence of light mediators, whose coupling to the Standard Model facilitates annihilation in the early universe and allows for the correct thermal relic abundance. The mediators in turn provide a production channel for dark matter at colliders or fixed targets, and as a consequence the neutrino beams generated at fixed targets may contain an additional beam of light dark matter. The signatures of this beam include elastic scattering off electrons or nucleons in the (near-)detector, which closely mimics the neutral current scattering of neutrinos. We determine the event rate at modern fixed target facilities and the ensuing sensitivity to sub-GeV dark matter.

hep-ph↗

arXiv2012

The Pion-Photon Transition Form Factor and New Physics in the Tau Sector

Recent measurement of the $γγ^\ast$ form factor of the neutral pion in the high $Q^2$ region disagrees with {\em a priori} predictions of QCD-based calculations. We comment on existing explanations, and analyze a possibility that this discrepancy is not due to poorly understood QCD effects, but is a result of some new physics beyond the standard model (SM). We show that such physics would necessarily involve a new neutral light state with mass close to the mass of $π^0$, and with stronger than $π^0$ couplings to heavier SM flavors such as $c$, $τ$, and $b$. It is found that only the coupling to the $τ$ lepton can survive the existing constraints and lead to the observed rise of the pion form factor relative to $Q^{-2}$ at high $Q^2$. We perform numerical fits to data and determine the allowed range of masses and couplings for such new particles. This range of masses and couplings could also reduce or eliminate the tension between the $e^+e^-$ and $τ$ decay determinations of the hadronic vacuum polarization. Dedicated experimental analysis of $τ$ pair production in association with such new states should provide a conclusive test of the new physics hypothesis as an explanation to the pion form factor rise. We also comment on the calculations of the pion form factor in the chiral quark model, and point out a possible dynamical origin of the quark mass scale inferred from the form factor measurement.

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arXiv2011

New Parity-Violating Muonic Forces

The recent discrepancy between proton charge radius measurements extracted from electron-proton versus muon-proton systems is suggestive of a new force that differentiates between lepton species. We identify a class of models with gauged right-handed muon number, which contains new vector and scalar force carriers at the 100 MeV scale or lighter, that is consistent with observations. Such forces would lead to an enhancement by several orders-of-magnitude of the parity-violating asymmetries in the scattering of low-energy muons on nuclei. The relatively large size of such asymmetries, O(10^{-4}), opens up the possibility for new tests of parity violation in neutral currents with existing low-energy muon beams.

hep-ph↗

arXiv2010

Muon Capture Constraints on Sterile Neutrino Properties

We show that ordinary and radiative muon capture impose stringent constraints on sterile neutrino properties. In particular, we consider a sterile neutrino with a mass between 40 to $80 {\rm MeV}$ that has a large mixing with the muon neutrino and decays predominantly into a photon and light neutrinos due to a large transition magnetic moment. Such a model was suggested as a possible resolution to the puzzle presented by the results of the LSND, KARMEN, and MiniBooNE experiments. We find that the scenario with the radiative decay to massless neutrinos is ruled out by measurements of the radiative muon capture rates at TRIUMF in the relevant mass range by a factor of a few in the squared mixing angle. These constraints are complementary to those imposed by the process of electromagnetic upscattering and de-excitation of beam neutrinos inside the neutrino detectors induced by a large transition magnetic moment. The latter provide stringent constraints on the size of the transitional magnetic moment between muon, electron neutrinos and $N$. We also show that further extension of the model with another massive neutrino in the final state of the radiative decay may be used to bypass the constraints derived in this work.

hep-ph↗

arXiv2010

Forward-Backward Asymmetry of Top Quark Pair Production

We adopt a Markov Chain Monte Carlo method to examine various new physics models which can generate the forward-backward asymmetry in top quark pair production observed at the Tevatron by the CDF Collaboration. We study the following new physics models: (1) exotic gluon $G^\prime$, (2) extra $Z^\prime$ boson with flavor-conserving interaction, (3) extra $Z^\prime$ with flavor-violating $u$-$t$-$Z^\prime$ interaction, (4) extra $W^\prime$ with flavor-violating $d$-$t$-$W^\prime$ interaction, and (5) extra scalars $S$ and $S^\pm$ with flavor-violating $u$-$t$-$S$ and $d$-$t$-$S^\pm$ interactions. After combining the forward-backward asymmetry with the measurement of the top pair production cross section and the $t\bar{t}$ invariant mass distribution at the Tevatron, we find that an axial vector exotic gluon $G^\prime$ of mass about $1 {\rm TeV}$ or $2 {\rm TeV}$ or a $W^\prime$ of mass about $2 {\rm TeV}$ offer an improvement over the Standard Model. The other models considered do not fit the data significantly better than the Standard Model. We also emphasize a few points which have been long ignored in the literature for new physics searches: (1) heavy resonance width effects, (2) renormalization scale dependence, and (3) NLO corrections to the $t\bar{t}$ invariant mass spectrum. We argue that these three effects are crucial to test or exclude new physics effects in the top quark pair asymmetry.

hep-ph↗

arXiv2009

Contributions to the Muon's Anomalous Magnetic Moment from a Hidden Sector

The measurement of the anomalous magnetic moment of the muon provides a stringent test of the standard model and of any physics that lies beyond it. There is currently a deviation of $3.1σ$ between the standard model prediction for the muon's anomalous magnetic moment and its experimental value. We calculate the contribution to the anomalous magnetic moment in theories where the muon couples to a particle in a hidden sector (that is, uncharged under the standard model) and a connector (which has nontrivial standard model gauge and hidden sector quantum numbers).

hep-ph↗

arXiv2009

WIMPless Dark Matter and Meson Decays with Missing Energy

WIMPless dark matter offers an attractive framework in which dark matter can be very light. We investigate the implications of such scenarios on invisible decays of bottomonium states for dark matter with a mass less than around $5 {\rm GeV}$. We relate these decays to measurements of nucleon-dark matter elastic scattering. We also investigate the effect that a coupling to $s$ quarks has on flavor changing $b\to s$ processes involving missing energy.

hep-ph↗

arXiv2009

Constraining Light Bosons with Radiative $Υ(1S)$ Decays

Light bosons can be found in large classes of theories beyond the Standard Model. These light bosons may not be ruled out by current experiments and, indeed, may even provide an explanation for some anomalous experimental results. The radiative decays of quarkonium ($c \bar c, b \bar b$) states offer a promising opportunity to investigate such light bosons. Specifically, we investigate the reach that current CLEO data can have on models with light scalar and pseudoscalar bosons.

hep-ph↗

arXiv2007

Masses and Mixings in a Grand Unified Toy Model

The generation of the fermion mass hierarchy in the standard model of particle physics is a long-standing puzzle. The recent discoveries from neutrino physics suggests that the mixing in the lepton sector is large compared to the quark mixings. To understand this asymmetry between the quark and lepton mixings is an important aim for particle physics. In this regard, two promising approaches from the theoretical side are grand unified theories and family symmetries. In this note we try to understand certain general features of grand unified theories with Abelian family symmetries by taking the simplest SU(5) grand unified theory as a prototype. We construct an SU(5) toy model with $U(1)_F \otimes Z'_2\otimes Z''_2 \otimes Z'''_2$ family symmetry that, in a natural way, duplicates the observed mass hierarchy and mixing matrices to lowest approximation. The system for generating the mass hierarchy is through a Froggatt-Nielsen type mechanism. One idea that we use in the model is that the quark and charged lepton sectors are hierarchical with small mixing angles while the light neutrino sector is democratic with larger mixing angles. We also discuss some of the difficulties in incorporating finer details into the model without making further assumptions or adding a large scalar sector.

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