arXiv subjects

Juri Smirnov

Publications and source records attributed to Juri Smirnov.

At least 37 records · Page 2Linked to original sources

arXiv2022

White Dwarfs in Dwarf Spheroidal Galaxies: A New Class of Compact-Dark-Matter Detectors

Recent surveys have discovered a population of faint supernovae, known as Ca-rich gap transients, inferred to originate from explosive ignitions of white dwarfs. In addition to their unique spectra and luminosities, these supernovae have an unusual spatial distribution and are predominantly found at large distances from their presumed host galaxies. We show that the locations of Ca-rich gap transients are well matched to the distribution of dwarf spheroidal galaxies surrounding large galaxies, in accordance with a scenario where dark matter interactions induce thermonuclear explosions among low-mass white dwarfs that may be otherwise difficult to ignite with standard stellar or binary evolution mechanisms. A plausible candidate to explain the observed event rate are primordial black holes with masses above $10^{21}$ grams.

astro-ph.CO↗

arXiv2022

Early-Universe Model Building

Theoretical investigations into the evolution of the early universe are an essential part of particle physics that allow us to identify viable extensions to the Standard Model as well as motivated parameter space that can be probed by various experiments and observations. In this white paper, we review particle physics models of the early universe. First, we outline various models that explain two essential ingredients of the early universe (dark matter and baryon asymmetry) and those that seek to address current observational anomalies. We then discuss dynamics of the early universe in models of neutrino masses, axions, and several solutions to the electroweak hierarchy problem. Finally, we review solutions to naturalness problems of the Standard Model that employ cosmological dynamics.

hep-ph↗

arXiv2022

Glueballs in a Thermal Squeezeout Model

It has been shown that a first order confinement phase transition can drastically change the relic dark matter abundance in confining dark sectors with only heavy dark quarks. We study the phenomenology of one such model with a Z' portal to Standard Model. We find that dark glueballs are long-lived in this setup and dilute the dark matter abundance after their decay to Standard Model. With this effect, the correct relic abundance is obtained with dark matter masses up to $\mathcal{O}(10^6)$~TeV. We find that while a part of the parameter space is already ruled out by direct detection and collider searches, there is still a broad space of viable scenarios that can be probed by future experiments.

hep-ph↗

arXiv2022

Thermal Squeezeout of Dark Matter

We carry out a detailed study of the confinement phase transition in a dark sector with a $SU(N)$ gauge group and a single generation of dark heavy quark. We focus on heavy enough quarks such that their abundance freezes out before the phase transition and the phase transition is of first-order. We find that during this phase transition the quarks are trapped inside contracting pockets of the deconfined phase and are compressed enough to interact at a significant rate, giving rise to a second stage of annihilation that can dramatically change the resulting dark matter abundance. As a result, the dark matter can be heavier than the often-quoted unitarity bound of $\sim100~$TeV. Our findings are almost completely independent of the details of the portal between the dark sector and the Standard Model. We comment briefly on possible signals of such a sector. Our main findings are summarized in a companion letter, while here we provide further details on different parts of the calculation.

hep-ph↗

arXiv2021

WIMPs Without Weakness: Generalized Mass Window with Entropy Injection

We study general freeze-out scenarios where an arbitrary number of initial and final dark matter particles participate in the number-changing freeze-out interaction. We consider a simple sector with two particle species undergoing such a thermal freeze-out; one of the relics is stable and gives rise to the dark matter today, while the other one decays to the Standard Model, injecting significant entropy into the thermal bath that dilutes the dark matter abundance. We show that this setup can lead to a stable relic population that reproduces the observed dark matter abundance without requiring weak scale masses or couplings. The final dark matter abundance is estimated analytically. We carry out this calculation for arbitrary temperature dependence in the freeze-out process and identify the viable dark matter mass and cross section ranges that explain the observed dark matter abundance. This setup can be used to open parameter space for both heavy (above the unitarity bound) or light (sub-GeV) dark matter candidates. We point out that the best strategy for probing most parts of our parameter space is to look for signatures of an early matter-dominant epoch.

hep-ph↗

arXiv2021

Accidentally Asymmetric Dark Matter

We study the effect of a first-order phase transition in a confining $SU(N)$ dark sector with heavy dark quarks. The baryons of this sector are the dark matter candidate. During the confinement phase transition the heavy quarks are trapped inside isolated, contracting pockets of the deconfined phase, giving rise to a second stage of annihilation that dramatically suppresses the dark quark abundance. The surviving abundance is determined by the local accidental asymmetry in each pocket. The correct dark matter abundance is obtained for $\mathcal{O}(1-100)$ PeV dark quarks, above the usual unitarity bound.

hep-ph↗

arXiv2020

Vainshtein Screening in Bimetric Cosmology

We demonstrate that the instabilities in linear cosmological perturbations in bimetric theory are the manifestation of the non-linear Vainshtein mechanism on an FRW background. The spin-2 mass serves as the cosmological Vainshtein scale in this case. This allows us to quantitatively address early universe cosmology. In particular, in a global analysis, we study data from the cosmic microwave background radiation and local measurements of the Hubble flow. We show that bimetric cosmology resolves the discrepancy in the local and early-time measurements of the Hubble scale via an effective phantom dark energy component.

gr-qc↗

arXiv2020

Co-SIMP Miracle

We present a new mechanism for thermally produced dark matter, based on a semi-annihilation-like process, $χ+ χ+\text{SM} \rightarrow χ+ \text{SM}$, with intriguing consequences for the properties of dark matter. First, its mass is low, $\lesssim 1$ GeV (but $\gtrsim 5$ keV to avoid structure-formation constraints). Second, it is strongly interacting, leading to kinetic equilibrium between the dark and visible sectors, avoiding the structure-formation problems of $χ+ χ+ χ\rightarrow χ+ χ$ models. Third, in the $3 \rightarrow 2$ process, one dark matter particle is consumed, giving the standard-model particle a monoenergetic recoil. We show that this new scenario is presently allowed, which is surprising (perhaps a "minor miracle"). However, it can be systematically tested by novel analyses in present and near-term experiments. In particular, the Co-SIMP model for thermal-relic dark matter can explain the XENON1T excess.

hep-ph↗

arXiv2020

The Thermal History of Composite Dark Matter

We study the thermodynamic history of composite Dark Matter models. We start with classifying the models by means of the symmetries partially protecting the composite Dark Matter decays and constrain their lifetimes. For each model, we determine the impact of number-changing and number-conserving operators on its thermal history. We also develop the analytic formalism to calculate the asymptotic abundance of stable relics. We show how the relative strength between number-changing and number-conserving interactions together with the dark plasma lifetime affect the thermal fate of the various composite models. Additionally, we discover that the final dark relic density of composite particles can be diluted due to an entropy increase stemming from dark plasma decay. Finally, we confront the models with experimental bounds. We find that indirect detection experiments are most promising in testing this large class of models.

hep-ph↗

arXiv2020

Dark Matter meets Quantum Gravity

We search for an extension of the Standard Model that contains a viable dark matter candidate and that can be embedded into a fundamental, asymptotically safe, quantum field theory with quantum gravity. Demanding asymptotic safety leads to boundary conditions for the non-gravitational couplings at the Planck scale. For a given dark matter model these translate into constraints on the mass of the dark matter candidate. We derive constraints on the dark matter mass and couplings in two minimal dark matter models: i) scalar dark matter coupled via the Higgs-portal in the $B$-$L$ model; ii) fermionic dark matter in a $U(1)_X$ extension of the Standard Model, coupled via the new gauge boson. For scalar dark matter we find 56 GeV $ < M_\text{DM} < 63$ GeV, and for fermionic dark matter $M_\text{DM} \leq 50$ TeV. Within our framework, we identify three benchmark scenarios with distinct phenomenological consequences.

hep-ph↗

arXiv2019

TeV-Scale Thermal WIMPs: Unitarity and its Consequences

We re-examine unitarity bounds on the annihilation cross section of thermal-WIMP dark matter. For high-mass pointlike dark matter, it is generic to form WIMP bound states, which, together with Sommerfeld enhancement, affects the relic abundance. We show that these effects lower the unitarity bound from 139 TeV to below 100 TeV for non-self-conjugate dark matter and from 195 TeV (the oft-quoted value of 340 TeV assumes $Ω_{DM} h^2 = 1$) to 140 TeV for the self-conjugate case. For composite dark matter, for which the unitarity limit on the radius was thought to be mass-independent, we show that the largest allowed mass is 1 PeV. In addition, we find important new effects for annihilation in the late universe. For example, while the production of high-energy light fermions in WIMP annihilation is suppressed by helicity, we show that bound-state formation changes this. Coupled with rapidly improving experimental sensitivity to TeV-range gamma rays, cosmic rays, and neutrinos, our results give new hope to attack the thermal-WIMP mass range from the high-mass end.

hep-ph↗

arXiv2019

Safe Clockwork

In this letter we demonstrate that safe quantum field theories can accommodate the clockwork mechanism upgrading it to a fundamental theory a lá Wilson. Additionally the clockwork mechanism naturally sources Yukawa hierarchies for safe completions of the Standard Model (SM). As proof of concept we investigate a safe Pati-Salam (PS) clockwork structure.

hep-ph↗

arXiv2019

Colored Dark Matter

We explore the possibility that Dark Matter is the lightest hadron made of two stable color octet Dirac fermions ${\cal Q}$. The cosmological DM abundance is reproduced for $M_{\cal Q}\approx 12.5$ TeV, compatibly with direct searches (the Rayleigh cross section, suppressed by $1/M_{\cal Q}^6$, is close to present bounds), indirect searches (enhanced by ${\cal Q}{\cal Q}+\bar{\cal Q}\bar{\cal Q}\to {\cal Q}\bar{\cal Q}+{\cal Q}\bar{\cal Q}$ recombination), and with collider searches (where ${\cal Q}$ manifests as tracks, pair produced via QCD). Hybrid hadrons, made of $\cal Q$ and of SM quarks and gluons, have large QCD cross sections, and do not reach underground detectors. Their cosmological abundance is $10^5$ times smaller than DM, such that their unusual signals seem compatible with bounds. Those in the Earth and stars sank to their centers; the Earth crust and meteorites later accumulate a secondary abundance, although their present abundance depends on nuclear and geological properties that we cannot compute from first principles.

hep-ph↗

arXiv2018

Long Range Effects in Gravity Theories with Vainshtein Screening

In this paper we study long range modifications of gravity in the consistent framework of bigravity, which introduces a second massive spin-2 field and allows to continuously interpolate between the regime of General Relativity (mediated by a massless spin-2 field) and massive gravity (mediated by a massive spin-2 field). In particular we derive for the first time the equations for light deflection in this framework and study the effect on the lensing potential of galaxy clusters. By comparison of kinematic and lensing mass reconstructions, stringent bounds can be set on the parameter space of the new spin-2 fields. Furthermore, we investigate galactic rotation curves and the effect on the observable dark matter abundance within this framework.

astro-ph.CO↗

arXiv2018

Cosmological Abundance of Colored Relics

The relic cosmological abundance of stable or long-lived neutral colored particles gets reduced by about a few orders of magnitude by annihilations that occur after QCD confinement. We compute the abundance and the cosmological bounds on relic gluinos. The same post-confinement effect strongly enhances co-annihilations with a lighter Dark Matter particle, provided that their mass difference is below a few GeV. Charged colored particles (such as stops) can instead form baryons, which can be (quasi)stable in some models.

hep-ph↗

arXiv2018

Cosmological Implications of Dark Matter Bound States

We present generic formulae for computing how Sommerfeld corrections together with bound-state formation affects the thermal abundance of Dark Matter with non-abelian gauge interactions. We consider DM as a fermion 3plet (wino) or 5plet under SU(2)$_L$. In the latter case bound states raise to 14 TeV the DM mass required to reproduce the cosmological DM abundance and give indirect detection signals such as (for this mass) a dominant $γ$-line around 85 GeV. Furthermore, we consider DM co-annihilating with a colored particle, such as a squark or a gluino, finding that bound state effects are especially relevant in the latter case.

hep-ph↗

arXiv2018

Asymptotically Safe Standard Model Extensions?

We consider theories with a large number $N_F$ of charged fermions and compute the renormalisation group equations for the gauge, Yukawa and quartic couplings resummed at leading order in $1/N_F$. We construct extensions of the Standard Model where SU(2) and/or SU(3) are asymptotically safe. When the same procedure is applied to the Abelian U(1) factor, we find that the Higgs quartic can not be made asymptotically safe and stay perturbative at the same time.

hep-ph↗

arXiv2018

Decoherence of Gravitational Wave Oscillations in Bigravity

Following up on our recent study, we consider the regime of graviton masses and gravitational wave propagation distances at which decoherence of the wave packets plays a major role for phenomenology. This regime is of particular interest, as it can lead to very striking phenomena of echo events in the gravitational waves coming from coalescence events. The power of the experimental search in this case lies in the fact that it becomes sensitive to a large range of graviton masses, while not relying on a specific production mechanism. We are thus able to place new relevant limits on the parameter space of the graviton mixing angle.

gr-qc↗