Searcharxiv⌕ Search

arXiv subjects

Patrick J. Fox

Publications and source records attributed to Patrick J. Fox.

At least 37 records · Page 2Linked to original sources
arXiv2022

DIMUS: Super-Compact Dimuonium Spectroscopy Collider at Fermilab

While dimuonium $(μ^+μ^-)$ has not yet been observed, it is of utmost fundamental interest. By virtue of the larger mass, dimuonium has greater sensitivity to beyond the standard model effects than its cousins positronium or muonium, both discovered long ago, while not suffering from large QCD uncertainties. Dimuonium atoms can be created in $e^+e^-$ collisions with large longitudinal momentum, allowing them to decay a small distance away from the beam crossing point and avoid prompt backgrounds. We envision a unique cost-effective and fast-timeline opportunity for copious production of $(μ^+μ^-)$ atoms at the production threshold via a modest modification of Fermilab's existing FAST/NML facility to arrange collisions of 408 MeV electrons and positrons at a 75$^{\rm o}$ angle. This compact 23 m circumference collider (DIMUS) will allow for precision tests of QED and open the door for searches for new physics coupled to the muon. Fermilab's FAST/NML is perfectly suited for DIMUS as there are existing SRF accelerators and infrastructure, capable of producing high energy, high current electron and positron beams, sufficient for $O(10^{32})\mathrm{cm}^2\mathrm{s}^{-1}$ luminosity and $\sim$0.5 million dimuons per year. The expansion will require installation of a second SRF cryomodule, positron production and accumulation system, fast injection/extraction kickers and two small circumference intersecting rings. An approximately meter-sized detector with several layers of modern pixelated silicon detector and crystal-based electromagnetic calorimeters will ensure observation of the decays of dimuonium to electron-positron pairs in presence of the Bhabba scattering background. An expansion of the system to would extend the physics program of DIMUS to include precision studies of rare processes with muons, pions, and $η$ mesons produced in $e^{+}e^{-}$ collisions.

hep-ex↗
arXiv2021

Beyond 4D Tracking: Using Cluster Shapes for Track Seeding

Tracking is one of the most time consuming aspects of event reconstruction at the Large Hadron Collider (LHC) and its high-luminosity upgrade (HL-LHC). Innovative detector technologies extend tracking to four-dimensions by including timing in the pattern recognition and parameter estimation. However, present and future hardware already have additional information that is largely unused by existing track seeding algorithms. The shape of clusters provides an additional dimension for track seeding that can significantly reduce the combinatorial challenge of track finding. We use neural networks to show that cluster shapes can reduce significantly the rate of fake combinatorical backgrounds while preserving a high efficiency. We demonstrate this using the information in cluster singlets, doublets and triplets. Numerical results are presented with simulations from the TrackML challenge.

physics.ins-det↗
arXiv2021

Characterizing Heavy Neutral Fermions via their Decays

Many extensions of the Standard Model of particle physics contain new electrically-neutral fermions. Should one of these particles be discovered, questions will naturally arise regarding its nature. For instance: is it a self-conjugate particle (i.e., is it a Dirac or a Majorana fermion)?, does it interact via the Standard Model force carriers or something else? One set of well-motivated particles in this class are Heavy Neutral Leptons (HNLs), Standard Model gauge-singlet fermions that mix with the neutrinos and may be produced in meson decays. We demonstrate that measuring the three body decays of the HNL (or phenomenologically similar heavy fermions) can help determine whether they are Majorana or Dirac fermions. We also investigate the ability to distinguish among different models for the physics responsible for the HNL decay. We compare the reach assuming full and partial event reconstruction, and propose experimental analyses. Should a new fermion be discovered, studying its three body decays provides a powerful diagnostic tool of its nature.

hep-ph↗
arXiv2021

Three-Body Decays of Heavy Dirac and Majorana Fermions

Nonzero neutrino masses imply the existence of degrees of freedom and interactions beyond those in the Standard Model. A powerful indicator of what these might be is the nature of the massive neutrinos: Dirac fermions versus Majorana fermions. While addressing the nature of neutrinos is often associated with searches for lepton-number violation, there are several other features that distinguish Majorana from Dirac fermions. Here, we compute in great detail the kinematics of the daughters of the decays into charged-leptons and neutrinos of hypothetical heavy neutral leptons at rest. We allow for the decay to be mediated by the most general four-fermion interaction Lagrangian. We demonstrate, for example, that when the daughter charged-leptons have the same flavor or the detector is insensitive to their charges, polarized Majorana-fermion decays have zero forward/backward asymmetry in the direction of the outgoing neutrino (relative to the parent spin), whereas Dirac-fermion decays can have large asymmetries. Going beyond studying forward/backward asymmetries, we also explore the fully-differential width of the three-body decays. It contains a wealth of information not only about the nature of the new fermions but also the nature of the interactions behind their decays.

hep-ph↗
arXiv2021

Neutrino Masses from Low Scale Partial Compositeness

We consider a class of models in which the neutrinos acquire Majorana masses through mixing with singlet neutrinos that emerge as composite states of a strongly coupled hidden sector. In this framework, the light neutrinos are partially composite particles that obtain their masses through the inverse seesaw mechanism. We focus on the scenario in which the strong dynamics is approximately conformal in the ultraviolet, and the compositeness scale lies at or below the weak scale. The small parameters in the Lagrangian necessary to realize the observed neutrino masses can naturally arise as a consequence of the scaling dimensions of operators in the conformal field theory. We show that this class of models has interesting implications for a wide variety of experiments, including colliders and beam dumps, searches for lepton flavor violation and neutrinoless double beta decay, and cosmological observations. At colliders and beam dumps, this scenario can give rise to striking signals involving multiple displaced vertices. The exchange of hidden sector states can lead to observable rates for flavor violating processes such as $μ\rightarrow e γ$ and $μ\rightarrow e$ conversion. If the compositeness scale lies at or below a hundred MeV, the rate for neutrinoless double beta decay is suppressed by form factors and may be reduced by an order of magnitude or more. The late decays of relic singlet neutrinos can give rise to spectral distortions in the cosmic microwave background that are large enough to be observed in future experiments.

hep-ph↗
arXiv2020

Chiral Abelian gauge theories with few fermions

We construct chiral theories with the smallest number $n_χ$ of Weyl fermions that form an anomaly-free set under various Abelian gauge groups. For the $U(1)$ group, where $n_χ= 5$, we show that the general solution to the anomaly equations is a set of charges given by cubic polynomials in three integer parameters. For the $U(1) \times U(1)$ gauge group we find $n_χ= 6$, and derive the general solution to the anomaly equations, in terms of 6 parameters. For $U(1) \times U(1) \times U(1)$ we show that $n_χ= 8$, and present some families of solutions. These chiral gauge theories have potential applications to dark matter models, right-handed neutrino interactions, and other extensions of the Standard Model. As an example, we present a simple dark sector with a natural mass hierarchy between three dark matter components.

hep-ph↗
arXiv2019

Searches for Decays of New Particles in the DUNE Multi-Purpose Near Detector

One proposed component of the upcoming Deep Underground Neutrino Experiment (DUNE) near detector complex is a multi-purpose, magnetized, gaseous argon time projection chamber: the Multi-Purpose Detector (MPD). We explore the new-physics potential of the MPD, focusing on scenarios in which the MPD is significantly more sensitive to new physics than a liquid argon detector, specifically searches for semi-long-lived particles that are produced in/near the beam target and decay in the MPD. The specific physics possibilities studied are searches for dark vector bosons mixing kinetically with the Standard Model hypercharge group, leptophilic vector bosons, dark scalars mixing with the Standard Model Higgs boson, and heavy neutral leptons that mix with the Standard Model neutrinos. We demonstrate that the MPD can extend existing bounds in most of these scenarios. We illustrate how the ability of the MPD to measure the momentum and charge of the final state particles leads to these bounds.

hep-ph↗
arXiv2019

A New Mask for An Old Suspect: Testing the Sensitivity of the Galactic Center Excess to the Point Source Mask

The Fermi-LAT collaboration has recently released a new point source catalog, referred to as 4FGL. For the first time, we perform a template fit using information from this new catalog and find that the Galactic center excess is still present. On the other hand, we find that a wavelet-based search for point sources is highly sensitive to the use of the 4FGL catalog: no excess of bright regions on small angular scales is apparent when we mask out 4FGL point sources. We postulate that the 4FGL catalog contains the large majority of bright point sources that have previously been suggested to account for the excess in gamma rays detected at the Galactic center in Fermi-LAT data. Furthermore, after identifying which bright sources have no known counterpart, we place constraints on the luminosity function necessary for point sources to explain the smooth emission seen in the template fit.

astro-ph.HE↗
arXiv2019

Luminous Signals of Inelastic Dark Matter in Large Detectors

We study luminous dark matter signals in models with inelastic scattering. Dark matter $χ_1$ that scatters inelastically off elements in the Earth is kicked into an excited state $χ_2$ that can subsequently decay into a monoenergetic photon inside a detector. The photon signal exhibits large sidereal-daily modulation due to the daily rotation of the Earth and anisotropies in the problem: the dark matter wind comes from the direction of Cygnus due to the Sun's motion relative to the galaxy, and the rock overburden is anisotropic, as is the dark matter scattering angle. This allows outstanding separation of signal from backgrounds. We investigate the sensitivity of two classes of large underground detectors to this modulating photon line signal: large liquid scintillator neutrino experiments, including Borexino and JUNO, and the proposed large gaseous scintillator directional detection experiment CYGNUS. Borexino's (JUNO's) sensitivity exceeds the bounds from xenon experiments on inelastic nuclear recoil for mass splittings $δ\gtrsim 240 (180)$ keV, and is the only probe of inelastic dark matter for ${350 \text{ keV} \lesssim δ\lesssim 600 \text{ keV}}$. CYGNUS's sensitivity is at least comparable to xenon experiments with $\sim 10 \; {\rm m}^3$ volume detector for $δ\lesssim 150$ keV, and could be substantially better with larger volumes and improved background rejection. Such improvements lead to the unusual situation that the inelastic signal becomes the superior way to search for dark matter even if the elastic and inelastic scattering cross sections are comparable.

hep-ph↗
arXiv2019

General solution to the U(1) anomaly equations

The anomaly cancellation equations for the $U(1)$ gauge group can be written as a cubic equation in $n-1$ integer variables, where $n$ is the number of Weyl fermions carrying the $U(1)$ charge. We solve this Diophantine cubic equation by providing a parametrization of the charges in terms of $n-2$ integers, and prove that this is the most general solution.

hep-th↗
arXiv2019

Bi$\boldsymbolν$o phenomenology at the LHC

We study the LHC constraints on an $R$-symmetric SUSY model, where the neutrino masses are generated through higher dimensional operators involving the pseudo-Dirac bino, named bi$ν$o. We consider a particle spectrum where the squarks are heavier than the lightest neutralino, which is a pure bi$ν$o. The bi$ν$o is produced through squark decays and it subsequently decays to a combination of jets and leptons, with or without missing energy, via its mixing with the Standard Model neutrinos. We recast the most recent LHC searches for jets+missing energy with $\sqrt{s}=13~$TeV and $\mathcal{L}=36~{\rm fb}^{-1}$ of data to determine the constraints on the squark and bi$ν$o masses in this model. We find that squarks as light as 350~GeV are allowed if the bi$ν$o is lighter than 150~GeV and squarks heavier than 950~GeV are allowed for any bi$ν$o mass. We also present forecasts for the LHC with $\sqrt{s}=13$~TeV and $\mathcal{L}=300~{\rm fb}^{-1}$ and show that squarks up to 1150~GeV can be probed.

hep-ph↗
arXiv2018

Dark Tridents at Off-Axis Liquid Argon Neutrino Detectors

We present dark tridents, a new channel for exploring dark sectors in short-baseline neutrino experiments. Dark tridents are clean, distinct events where, like neutrino tridents, the scattering of a very weakly coupled particle leads to the production of a lepton--antilepton pair. Dark trident production occurs in models where long-lived dark-sector particles are produced along with the neutrinos in a beam-dump environment and interact with neutrino detectors downstream, producing an on-shell boson which decays into a pair of charged leptons. We focus on a simple model where the dark matter particle interacts with the standard model exclusively through a dark photon, and concentrate on the region of parameter space where the dark photon mass is smaller than twice that of the dark matter particle and hence decays exclusively into standard-model particles. We compute event rates and discuss search strategies for dark tridents from dark matter at the current and upcoming liquid argon detectors aligned with the Booster beam at Fermilab -- MicroBooNE, SBND, and ICARUS -- assuming the dark sector particles are produced off-axis in the higher energy NuMI beam. We find that MicroBooNE has already recorded enough data to be competitive with existing bounds on this dark sector model, and that new regions of parameter space will be probed with future data and experiments.

hep-ph↗
arXiv2018

Analyzing the Gamma-ray Sky with Wavelets

We analyze the gamma-ray sky at energies of 0.5 to 50 GeV using the undecimated wavelet transform on the sphere. Focusing on the inner $60^{\circ} \times 60^{\circ}$ of the sky, we identify and characterize four separate residuals beyond the expected Milky Way diffuse emission. We detect the \textit{Fermi} Bubbles, finding compelling evidence that they are diffuse in nature and contain very little small-scale structure. We detect the "cocoon" inside the Southern Bubble, and we also identify its northern counterpart above 2 GeV. The Northern Cocoon lies along the same axis but is $\sim 30 \%$ dimmer than the southern one. We characterize the Galactic center excess, which we find extends up to $20^{\circ}$ in $|b|$. At latitudes $|b| \leq 5^{\circ}$ we find evidence for power in small angular scales that could be the result of point-source contributions, but for $|b| \geq 5^{\circ}$ the Galactic center excess is dominantly diffuse in its nature. Our findings show that either the Galactic center excess and {\it Fermi} Bubbles connect smoothly or that the Bubbles brighten significantly below $15^\circ$ in latitude. We find that the Galactic center excess appears off-center by a few degrees towards negative $\ell$. Additionally, we find and characterize two emissions along the Galactic disk centered at $\ell \simeq +25^{\circ}$ and $-20^{\circ}$. These emissions are significantly more elongated along the Galactic disk than the Galactic center excess.

astro-ph.HE↗
arXiv2018

Top-philic $Z'$ Forces at the LHC

Despite extensive searches for an additional neutral massive gauge boson at the LHC, a $Z^\prime$ at the weak scale could still be present if its couplings to the first two generations of quarks are suppressed, in which case the production in hadron colliders relies on tree-level processes in association with heavy flavors or one-loop processes in association with a jet. We consider the low-energy effective theory of a top-philic $Z'$ and present possible UV completions. We clarify theoretical subtleties in evaluating the production of a top-philic $Z'$ at the LHC and examine carefully the treatment of an anomalous $Z'$ current in the low-energy effective theory. Recipes for properly computing the production rate in the $Z'+j$ channel are given. We discuss constraints from colliders and low-energy probes of new physics. As an application, we apply these considerations to models that use a weak-scale $Z'$ to explain possible violations of lepton universality in $B$ meson decays, and show that the future running of a high luminosity LHC can potentially cover much of the remaining parameter space favored by this particular interpretation of the $B$ physics anomaly.

hep-ph↗
arXiv2017

Light Signals from a Lighter Higgs

With the Higgs search program already quite mature, there is the exciting possibility of discovering a new particle with rates near that of the SM Higgs. We consider models with a signal in $γγ$ below the SM Higgs mass, taking the recent $2.9\, σ$ (local) CMS excess at 95 GeV as a target. We discuss singlet models with additional vectorlike matter, but argue that a Type-I two Higgs doublet model provides a more economical scenario. In such a setup, going into regions of moderate-to-strong fermiophobia, the enhanced $γγ$ branching ratio allows signals from $VH$+VBF production to yield $σ\times BR_{γγ} $ comparable to total SM rates. Light $H$ production can be dominated via rare top decays $t \rightarrow b H^+ \rightarrow b W^{*} H$, which provides an alternate explanation of the excess. We consider this in the context of other Higgs anomalies, namely the LEP Higgs excess near the same mass, and excesses in $t\bar{t}h$ searches at Tevatron and LHC. We find that with $140\, \mathrm{GeV} < m_{H^+} < 160\, \mathrm{GeV}$, $\tan β\sim 5$ and a coupling to gauge bosons of $\sin^2 δ\sim 0.1$, such a scenario can produce all the excesses simultanously, where $tth$ arise from contamination from the rare top decays, as previously proposed. An implication of the Type-I scenario is that any $γγ$ excess should be associated with additional elements that could reduce background, including $b$-jets, forward jets or signs of vector boson production.

hep-ph↗
arXiv2017

Higgs-photon resonances

We study models that produce a Higgs boson plus photon ($h^0 γ$) resonance at the LHC. When the resonance is a $Z'$ boson, decays to $h^0 γ$ occur at one loop. If the $Z'$ boson couples at tree-level to quarks, then the $h^0 γ$ branching fraction is typically of order $10^{-5}$ or smaller. Nevertheless, there are models that would allow the observation of $Z' \to h^0 γ$ at $\sqrt{s} = 13$ TeV with a cross section times branching fraction larger than 1 fb for a $Z'$ mass in the 200--450 GeV range, and larger than 0.1 fb for a mass up to 800 GeV. The 1-loop decay of the $Z'$ into lepton pairs competes with $h^0 γ$, even if the $Z'$ couplings to leptons vanish at tree level. We also present a model in which a $Z'$ boson decays into a Higgs boson and a pair of collimated photons, mimicking an $h^0 γ$ resonance. In this model, the $h^0 γ$ resonance search would be the discovery mode for a $Z'$ as heavy as 2 TeV. When the resonance is a scalar, although decay to $h^0 γ$ is forbidden by angular momentum conservation, the $h^0$ plus collimated photons channel is allowed. We comment on prospects of observing an $h^0 γ$ resonance through different Higgs decays, on constraints from related searches, and on models where $h^0$ is replaced by a nonstandard Higgs boson.

hep-ph↗
arXiv2017

The Inelastic Frontier: Discovering Dark Matter at High Recoil Energy

There exist well motivated models of particle dark matter which predominantly scatter inelastically off nuclei in direct detection experiments. This inelastic transition causes the DM to up-scatter in terrestrial experiments into an excited state up to 550 keV heavier than the DM itself. An inelastic transition of this size is highly suppressed by both kinematics and nuclear form factors. We extend previous studies of inelastic DM to determine the present bounds on the scattering cross section, and the prospects for improvements in sensitivity. Three scenarios provide illustrative examples: nearly pure Higgsino DM; magnetic inelastic DM; and inelastic models with dark photon exchange. We determine the elastic scattering rate as well as verify that exothermic transitions are negligible. Presently, the strongest bounds on the cross section are from xenon at LUX-PandaX (δ< 160 keV), iodine at PICO (160 < δ< 300 keV), and tungsten at CRESST (when δ> 300 keV). Amusingly, once δ> 200 keV, weak scale (and larger) DM - nucleon scattering cross sections are allowed. The relative competitiveness of these experiments is governed by the upper bound on the recoil energies employed by each experiment, as well as strong sensitivity to the mass of the heaviest element in the detector. Several implications, including sizable recoil energy-dependent annual modulation, and improvements for future experiments are discussed. We show that the xenon experiments can improve on the PICO results, if they were to analyze their existing data over a larger range of recoil energies, i.e., 20-500 keV. We also speculate about several reported events at CRESST between 45-100 keV, that could be interpreted as inelastic DM scattering. Future data from PICO, CRESST and xenon experiments can test this with anaylses of high energy recoil data.

hep-ph↗