arXiv subjects

Maxim Pospelov

Publications and source records attributed to Maxim Pospelov.

At least 91 records · Page 5Linked to original sources

arXiv2019

Foraging for dark matter in large volume liquid scintillator neutrino detectors with multiscatter events

We show that dark matter with a per-nucleon scattering cross section $\gtrsim 10^{-28}~{\rm cm^2}$ could be discovered by liquid scintillator neutrino detectors like BOREXINO, SNO+, and JUNO. Due to the large dark matter fluxes admitted, these detectors could find dark matter with masses up to $10^{21}$ GeV, surpassing the mass sensitivity of current direct detection experiments (such as XENON1T and PICO) by over two orders of magnitude. We derive the spin-independent and spin-dependent cross section sensitivity of these detectors using existing selection triggers, and propose an improved trigger program that enhances this sensitivity by two orders of magnitude. We interpret these sensitivities in terms of three dark matter scenarios: (1) effective contact operators for scattering, (2) QCD-charged dark matter, and (3) a recently proposed model of Planck-mass baryon-charged dark matter. We calculate the flux attenuation of dark matter at these detectors due to the earth overburden, taking into account the earth's density profile and elemental composition, and nuclear spins.

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arXiv2019

Novel direct detection constraints on light dark matter

All attempts to directly detect particle dark matter (DM) scattering on nuclei suffer from the partial or total loss of sensitivity for DM masses in the GeV range or below. We derive novel constraints from the inevitable existence of a subdominant, but highly energetic, component of DM generated through collisions with cosmic rays. Subsequent scattering inside conventional DM detectors, as well as neutrino detectors sensitive to nuclear recoils, limits the DM-nucleon scattering cross section to be below $10^{-31}$ cm$^2$ for both spin-independent and spin-dependent scattering of light DM.

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arXiv2019

Long-Lived Particles at the Energy Frontier: The MATHUSLA Physics Case

We examine the theoretical motivations for long-lived particle (LLP) signals at the LHC in a comprehensive survey of Standard Model (SM) extensions. LLPs are a common prediction of a wide range of theories that address unsolved fundamental mysteries such as naturalness, dark matter, baryogenesis and neutrino masses, and represent a natural and generic possibility for physics beyond the SM (BSM). In most cases the LLP lifetime can be treated as a free parameter from the $μ$m scale up to the Big Bang Nucleosynthesis limit of $\sim 10^7$m. Neutral LLPs with lifetimes above $\sim$ 100m are particularly difficult to probe, as the sensitivity of the LHC main detectors is limited by challenging backgrounds, triggers, and small acceptances. MATHUSLA is a proposal for a minimally instrumented, large-volume surface detector near ATLAS or CMS. It would search for neutral LLPs produced in HL-LHC collisions by reconstructing displaced vertices (DVs) in a low-background environment, extending the sensitivity of the main detectors by orders of magnitude in the long-lifetime regime. In this white paper we study the LLP physics opportunities afforded by a MATHUSLA-like detector at the HL-LHC. We develop a model-independent approach to describe the sensitivity of MATHUSLA to BSM LLP signals, and compare it to DV and missing energy searches at ATLAS or CMS. We then explore the BSM motivations for LLPs in considerable detail, presenting a large number of new sensitivity studies. While our discussion is especially oriented towards the long-lifetime regime at MATHUSLA, this survey underlines the importance of a varied LLP search program at the LHC in general. By synthesizing these results into a general discussion of the top-down and bottom-up motivations for LLP searches, it is our aim to demonstrate the exceptional strength and breadth of the physics case for the construction of the MATHUSLA detector.

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arXiv2018

Cosmological beam dump: constraints on dark scalars mixed with the Higgs boson

Precision cosmology provides a sensitive probe of extremely weakly coupled states due to thermal freeze-in production, with subsequent decays impacting physics during well-tested cosmological epochs. We explore the cosmological implications of the freeze-in production of a new scalar $S$ via the super-renormalizable Higgs portal. If the mass of $S$ is at or below the electroweak scale, peak freeze-in production occurs during the electroweak epoch. We improve the calculation of the freeze-in abundance by including all relevant QCD and electroweak production channels. The resulting abundance and subsequent decay of $S$ is constrained by a combination of X-ray data, cosmic microwave background anisotropies and spectral distortions, $N_{\rm eff}$, and the consistency of BBN with observations. These probes constrain technically natural couplings for such scalars from $m_S \sim$ keV all the way to $m_S \sim 100$ GeV. The ensuing constraints are similar in spirit to typical beam bump limits, but extend to much smaller couplings, down to mixing angles as small as $θ_{Sh} \sim 10^{-16}$, and to masses all the way to the electroweak scale.

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arXiv2018

Directly Detecting MeV-scale Dark Matter via Solar Reflection

If dark matter (DM) particles are lighter than a few MeV/$c^2$ and can scatter off electrons, their interaction within the solar interior results in a considerable hardening of the spectrum of galactic dark matter received on Earth. For a large range of the mass vs. cross section parameter space, $\{m_e, σ_e\}$, the "reflected" component of the DM flux is far more energetic than the endpoint of the ambient galactic DM energy distribution, making it detectable with existing DM detectors sensitive to an energy deposition of $10-10^3$ eV. After numerically simulating the small reflected component of the DM flux, we calculate its subsequent signal due to scattering on detector electrons, deriving new constraints on $σ_e$ in the MeV and sub-MeV range using existing data from the XENON10/100, LUX, PandaX-II, and XENON1T experiments, as well as making projections for future low threshold direct detection experiments.

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arXiv2018

Muon Beam Experiments to Probe the Dark Sector

A persistence of several anomalies in muon physics, such as the muon anomalous magnetic moment and the muonic hydrogen Lamb shift, hints at new light particles beyond the Standard Model. We address a subset of these models that have a new light scalar state with sizable couplings to muons and suppressed couplings to electrons. A novel way to search for such particles would be through muon beam-dump experiments by (1) missing momentum searches; (2) searches for decays with displaced vertices. The muon beams available at CERN and Fermilab present attractive opportunities for exploring the new scalar with a mass below the di-muon threshold, and potentially covering a range of relevant candidate models. For the models considered in this paper, both types of signals, muon missing momentum and anomalous energy deposition at a distance, can probe a substantial fraction of the unexplored parameter space of the new light scalar, including a region that can explain the muon anomalous magnetic moment discrepancy.

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arXiv2018

New Physics in the Rayleigh-Jeans Tail of the CMB

We show that despite stringent constraints on the shape of the main part of the CMB spectrum, there is considerable room for its modification within its Rayleigh-Jeans (RJ) end, $ω\ll T_{\rm CMB}$. We construct explicit New Physics models that give an order one (or larger) increase of photon count in the RJ tail, which can be tested by existing and upcoming experiments aiming to detect the cosmological 21 cm emission/absorption signal. This class of models stipulates the decay of unstable particles to dark photons, $A'$, that have a small mass, $m_{A'} \sim 10^{-14} - 10^{-9}$ eV, non-vanishing mixing angle $ε$ with electromagnetism, and energies much smaller than $T_{\rm CMB}$. The non-thermal number density of dark photons can be many orders of magnitude above the number density of CMB photons, and even a small probability of $A'\to A$ oscillations, for values of as small as $ε\sim 10^{-9}$, can significantly increase the number of RJ photons. In particular, we show that resonant oscillations of dark photons into regular photons in the interval of redshifts $20 < z < 1700$ can be invoked as an explanation of the recent tentative observation of a stronger-than-expected absorption signal of 21 cm photons. We present a model that realizes this possibility, where milli-eV mass dark matter decays to dark photons, with a lifetime longer than the age of the Universe.

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arXiv2018

Light Scalars and Dark Photons in Borexino and LSND Experiments

Bringing an external radioactive source close to a large underground detector can significantly advance sensitivity not only to sterile neutrinos but also to "dark" gauge bosons and scalars. Here we address in detail the sensitivity reach of the Borexino-SOX configuration, which will see a powerful (a few PBq) $^{144}$Ce$-^{144}$Pr source installed next to the Borexino detector, to light scalar particles coupled to the SM fermions. The mass reach of this configuration is limited by the energy release in the radioactive $γ$-cascade, which in this particular case is 2.2 MeV. Within that reach one year of operations will achieve an unprecedented sensitivity to coupling constants of such scalars, reaching down to $g\sim 10^{-7}$ levels and probing significant parts of parameter space not excluded by either beam dump constraints or astrophysical bounds. Should the current proton charge radius discrepancy be caused by the exchange of a MeV-mass scalar, then the simplest models will be decisively probed in this setup. We also update the beam dump constraints on light scalars and vectors, and in particular rule out dark photons with masses below 1 MeV, and kinetic mixing couplings $ε\gtrsim 10^{-5}$.

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arXiv2018

Signatures of Dark Radiation in Neutrino and Dark Matter Detectors

We consider the generic possibility that the Universe's energy budget includes some form of relativistic or semi-relativistic dark radiation (DR) with non-gravitational interactions with Standard Model (SM) particles. Such dark radiation may consist of SM singlets or a non-thermal, energetic component of neutrinos. If such DR is created at a relatively recent epoch, it can carry sufficient energy to leave a detectable imprint in experiments designed to search for very weakly interacting particles: dark matter and underground neutrino experiments. We analyze this possibility in some generality, assuming that the interactive dark radiation is sourced by late decays of an unstable particle, potentially a component of dark matter, and considering a variety of possible interactions between the dark radiation and SM particles. Concentrating on the sub-GeV energy region, we derive constraints on different forms of DR using the results of the most sensitive neutrino and dark matter direct detection experiments. In particular, for interacting dark radiation carrying a typical momentum of $\sim30$~MeV$/c$, both types of experiments provide competitive constraints. This study also demonstrates that non-standard sources of neutrino emission (e.g. via dark matter decay) are capable of creating a "neutrino floor" for dark matter direct detection that is closer to current bounds than is expected from standard neutrino sources.

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arXiv2018

Dipole portal to heavy neutral leptons

We consider generic neutrino dipole portals between left-handed neutrinos, photons, and right-handed heavy neutral leptons (HNL) with Dirac masses. The dominance of this portal significantly alters the conventional phenomenology of HNLs. We derive a comprehensive set of constraints on the dipole portal to HNLs by utilizing data from LEP, LHC, MiniBooNE, LSND as well as observations of Supernova 1987A and consistency of the standard Big Bang Nucleosynthesis. We calculate projected sensitivities from the proposed high-intensity SHiP beam dump experiment, and the ongoing experiments at the Short-Baseline Neutrino facility at Fermilab. Dipole mediated Primakoff neutrino upscattering and Dalitz-like meson decays are found to be the main production mechanisms in most of the parametric regime under consideration. Proposed explanations of LSND and MiniBooNE anomalies based on HNLs with dipole-induced decays are found to be severely constrained, or to be tested in the future experiments.

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arXiv2017

Light Resonances and the Low-$q^2$ Bin of $R_{K^*}$

LHCb has reported hints of lepton-flavor universality violation in the rare decays $B \to K^{(*)} \ell^+\ell^-$, both in high- and low-$q^2$ bins. Although the high-$q^2$ hint may be explained by new short-ranged interactions, the low-$q^2$ one cannot. We thus explore the possibility that the latter is explained by a new light resonance. We find that LHCb's central value of $R_{K^*}$ in the low-$q^2$ bin is achievable in a restricted parameter space of new-physics scenarios in which the new, light resonance decays preferentially to electrons and has a mass within approximately $10$ MeV of the di-muon threshold. Interestingly, such an explanation can have a kinematic origin and does not require a source of lepton-flavor universality violation. A model-independent prediction is a narrow peak in the differential $B \to K^* e^+e^-$ rate close to the di-muon threshold. If such a peak is observed, other observables, such as the differential $B \to K e^+e^-$ rate and $R_K$, may be employed to distinguish between models. However, if a low-mass resonance is not observed and the low-$q^2$ anomaly increases in significance, then the case for an experimental origin of the lepton-flavor universality violating anomalies would be strengthened. To further explore this, we also point out that, in analogy to $J/ψ$ decays, $e^+e^-$ and $μ^+μ^-$ decays of $ϕ$ mesons can be used as a cross check of lepton-flavor universality by LHCb with $5$ fb$^{-1}$ of integrated luminosity.

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arXiv2017

Search for domain wall dark matter with atomic clocks on board global positioning system satellites

Cosmological observations indicate that 85% of all matter in the Universe is dark matter (DM), yet its microscopic composition remains a mystery. One hypothesis is that DM arises from ultralight quantum fields that form macroscopic objects such as topological defects. Here we use GPS as a ~ 50,000 km aperture DM detector to search for such defects in the form of domain walls. GPS navigation relies on precision timing signals furnished by atomic clocks hosted on board GPS satellites. As the Earth moves through the galactic DM halo, interactions with topological defects could cause atomic clock glitches that propagate through the GPS satellite constellation at galactic velocities ~ 300 km/s. Mining 16 years of archival GPS data, we find no evidence for DM in the form of domain walls at our current sensitivity level. This allows us to improve the limits on certain quadratic scalar couplings of domain wall DM to standard model particles by several orders of magnitude.

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arXiv2017

BBN for the LHC: constraints on lifetimes of the Higgs portal scalars

LHC experiments can provide a remarkable sensitivity to exotic metastable massive particles, decaying with significant displacement from the interaction point. The best sensitivity is achieved to models where the production and decay occur due to different coupling constants, and the lifetime of exotic particles determines the probability of decay within a detector. The lifetimes of such particles can be independently limited from standard cosmology, in particular the Big Bang Nucleosynthesis. In this paper, we analyze the constraints on the simplest scalar model coupled through the Higgs portal, where the production occurs via $h\to SS$, and the decay is induced by the small mixing angle of the Higgs field $h$ and scalar $S$. We find that throughout the most part of the parameter space, $2 m_μ< m_S < m_h/2$, the lifetimes of exotic particle has to be less than 0.1 seconds, while below $2m_μ$ it could grow to about a second. The strong constraints on lifetimes are induced by the nucleonic and mesonic decays of scalars that tend to raise the $n/p$ ratio. Strong constraints on lifetimes of the minimal singlet extensions of the Higgs potential is a welcome news for the MATHUSLA proposal that seeks to detect displaced decays of exotic particles produced in the LHC collisions. We also point out how more complicated exotic sectors could evade the BBN lifetime constraints.

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arXiv2017

Light dark matter in neutrino beams: production modelling and scattering signatures at MiniBooNE, T2K and SHiP

We analyze the prospects for detection of light sub-GeV dark matter produced in experiments designed to study the properties of neutrinos, such as MiniBooNE, T2K, SHiP, DUNE etc. We present an improved production model, when dark matter couples to hadronic states via a dark photon or baryonic vector mediator, incorporating bremsstrahlung of the dark vector. In addition to elastic scattering, we also study signatures of light dark matter undergoing deep inelastic or quasi-elastic NC$π^0$-like scattering in the detector producing neutral pions, which for certain experiments may provide the best sensitivity. An extensive appendix provides documentation for a publicly available simulation tool {\tt BdNMC} that can be applied to determine the hidden sector dark matter production and scattering rate at a range of proton fixed target experiments.

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arXiv2016

Dressing L_mu - L_tau in Color

We consider a new massive vector-boson Z' that couples to leptons through the L_mu - L_tau current, and to quarks through an arbitrary set of couplings. We show that such a model can be obtained from a renormalizable field theory involving new heavy fermions in an anomaly-free representation. The model is a candidate explanation for the discrepancy observed recently by the LHCb collaboration in angular distributions of the final state particles in the rare decay B \to K* mu^+ mu^-. Interestingly, the new vector-boson contribution to the decay tau \to mu nu_tau \bar nu_mu can also remove a small tension in the measurement of the corresponding branching ratio. Constraints from light flavor meson-mixing restrict the coupling to the up- and down-quarks to be very small and thus direct production of the vector-boson at hadron colliders is strongly suppressed. The most promising ways to test the model is through the measurement of the Z decay to four leptons and through its effect on neutrino trident production of muon pairs. This latter process is a powerful but little-known constraint, which surprisingly rules out explanations of (g-2)_mu based on Z' gauge bosons coupled to muon number, with mass of at least a few GeV.

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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].

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arXiv2016

Shedding Light on Neutrino Masses with Dark Forces

Heavy right-handed neutrinos (RHNs) provide the simplest explanation for the origin of light neutrino masses and mixings. If the RHN masses are at or below the weak scale, direct experimental discovery of these states is possible at accelerator experiments such as the LHC or new dedicated beam dump experiments; in such experiments, the RHN decays after traversing a macroscopic distance from the collision point. The experimental sensitivity to RHNs is significantly enhanced if there is a new "dark" gauge force connecting them to the Standard Model (SM), and detection of RHNs can be the primary discovery mode for the new dark force itself. We take the well-motivated example of a B-L gauge symmetry and analyze the sensitivity to displaced decays of the RHNs produced via the new gauge interaction in two experiments: the LHC and the proposed SHiP beam dump experiment. In the most favorable case in which the mediator can be produced on-shell and decays to RHNs, the sensitivity reach is controlled by the square of the B-L gauge coupling. We demonstrate that these experiments could access neutrino parameters responsible for the observed SM neutrino masses and mixings in the most straightforward implementation of the see-saw mechanism.

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arXiv2016

The Leptonic Higgs Portal

An extended Higgs sector may allow for new scalar particles well below the weak scale. In this work, we present a detailed study of a light scalar $S$ with enhanced coupling to leptons, which could be responsible for the existing discrepancy between experimental and theoretical determinations of the muon anomalous magnetic moment. We present an ultraviolet completion of this model in terms of the lepton-specific two-Higgs doublet model and an additional scalar singlet. We then analyze a plethora of experimental constraints on the universal low energy model, and this UV completion, along with the sensitivity reach at future experiments. The most relevant constraints originate from muon and kaon decays, electron beam dump experiments, electroweak precision observables, rare $B_d$ and $B_s$ decays and Higgs branching fractions. The properties of the leptonic Higgs portal imply an enhanced couplings to heavy leptons, and we identify the most promising search mode for the high-luminosity electron-positron colliders as $e^+{+}e^-\toτ^+{+}τ^-{+}S \to τ^+{+}τ^-{+}\ell{+}\bar \ell$, where $\ell =e,μ$. Future analyses of existing data from BaBar and Belle, and from the upcoming Belle II experiment, will enable tests of this model as a putative solution to the muon $g-2$ problem for $m_S < 3.5$ GeV.

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