arXiv subjects

Oleg Lebedev

Publications and source records attributed to Oleg Lebedev.

At least 55 records · Page 3Linked to original sources

arXiv2016

Multicomponent Dark Matter from Gauge Symmetry

The composition of Dark Matter (DM) remains an important open question. The current data do not distinguish between single- and multi-component DM, while in theory constructions it is often assumed that DM is composed of a single field. In this work, we study a hidden sector which naturally entails multicomponent DM consisting of spin-1 and spin-0 states. This UV complete set-up is based on SU(3) hidden gauge symmetry with the minimal scalar field content to break it spontaneously. The presence of multiple DM components is a result of a residual Z_2 x Z'_2 symmetry which is part of an unbroken global U(1) x Z'_2 inherent in the Yang-Mills systems. We find that the model exhibits various parametric regimes with drastically different DM detection prospects. In particular, we find that the direct detection cross section is much suppressed in large regions of parameter space as long as the Standard Model Higgs mixes predominantly with a single scalar from the hidden sector. The resulting scattering rate is often beyond the level of sensitivity of XENON1T, while still being consistent with the thermal WIMP paradigm.

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arXiv2016

Higgs--Flavon Mixing and $h \rightarrow μτ$

ATLAS and CMS have reported an excess in the flavor violating decay of the Higgs boson, $h \rightarrow μτ$. We show that this result can be accommodated through a mixing of the Higgs with a flavon, the field responsible for generating the Yukawa matrices in the lepton sector. We employ a version of the Froggatt-Nielsen mechanism at the electroweak scale, with only the leptons and the flavon transforming non--trivially under the corresponding symmetry group. Non--observation of charged lepton flavor violation (LFV) in other processes imposes important constraints on the model, which we find to be satisfied in substantial regions of parameter space.

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arXiv2015

Higgs-inflaton coupling from reheating and the metastable Universe

Current Higgs boson and top quark data favor metastability of our vacuum which raises questions as to why the Universe has chosen an energetically disfavored state and remained there during inflation. In this Letter, we point out that these problems can be solved by a Higgs-inflaton coupling which appears in realistic models of inflation. Since an inflaton must couple to the Standard Model particles either directly or indirectly, such a coupling is generated radiatively, even if absent at tree level. As a result, the dynamics of the Higgs field can change dramatically.

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arXiv2015

Non-Abelian gauge fields as dark matter

SU(N) Lie algebras possess discrete symmetries which can lead naturally to stable vector dark matter (DM). In this work, we consider the possibility that the dark SU(N) sector couples to the visible sector through the Higgs portal. We find that minimal CP-conserving hidden `Higgs sectors' entail stable massive gauge fields which fall into the WIMP category of dark matter candidates. For SU(N), N>2, DM consists of three components, two of which are degenerate in mass. In all of the cases, there are substantial regions of parameter space where the direct and indirect detection as well as relic abundance constraints are satisfied.

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arXiv2015

Higgsophilic gauge bosons and monojets at the LHC

We consider a generic framework where the Standard Model (SM) coexists with a hidden sector endowed with some additional gauge symmetry. When this symmetry is broken by a scalar field charged under the hidden gauge group, the corresponding scalar boson generally mixes with the SM Higgs boson. In addition, massive hidden gauge bosons emerge and via the mixing, the observed Higgs-like mass eigenstate is the only known particle that couples to these hidden gauge bosons directly. We study the LHC monojet signatures of this scenario and the corresponding constraints on the gauge coupling of the hidden gauge group as well as the mixing of the Higgs scalars.

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arXiv2015

A second Higgs from the Higgs portal

In the Higgs portal framework, the Higgs field generally mixes with the Standard Model singlet leading to the existence of two states, one of which is identified with the 125 GeV scalar observed at the LHC. In this work, we analyse direct and indirect constraints on the second mass eigenstate and the corresponding mixing angle. The existence of the additional scalar can be beneficial as it can stabilise the otherwise-metastable electroweak vacuum. We find parameter regions where all of the bounds, including the stability constraints, are satisfied. We also study prospects for observing the decay of the heavier state into a pair of the 125 GeV Higgs-like scalars.

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arXiv2015

Stabilizing the Higgs potential with a Z$'$

Current data point toward metastability of the electroweak vacuum within the Standard Model. We study the possibility of stabilizing the Higgs potential in U(1) extensions thereof. A generic Z$'$ boson improves stability of the scalar potential in two ways: it increases the Higgs self--coupling, due to a positive contribution to the beta--function of the latter, and it decreases the top quark Yukawa coupling, which again has a stabilizing effect. We determine the range of U(1) charges which leads to a stable electroweak vacuum. In certain classes of models, such stabilization is possible even if the Z$'$ does not couple to the Higgs and is due entirely to the reduction of the top Yukawa coupling. We also study the effect of the kinetic mixing between the extra U(1) and hypercharge gauge fields.

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arXiv2014

Axial Dark Matter: the case for an invisible Z'

We consider the possibility that fermionic dark matter (DM) interacts with the Standard Model fermions through an axial Z' boson. As long as Z' decays predominantly into dark matter, the relevant LHC bounds are rather loose. Direct dark matter detection does not significantly constrain this scenario either, since dark matter scattering on nuclei is spin--dependent. As a result, for a range of the Z' mass and couplings, the DM annihilation cross section is large enough to be consistent with thermal history of the Universe. In this framework, the thermal WIMP paradigm, which currently finds itself under pressure, is perfectly viable.

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arXiv2013

More on the Hypercharge Portal into the Dark Sector

If the hidden sector contains more than one U(1) groups, additional dim-4 couplings (beyond the kinetic mixing) between the massive U(1) fields and the hypercharge generally appear. These are of the form similar to the Chern-Simons interactions. We study the phenomenology of such couplings including constraints from laboratory experiments and implications for dark matter. The hidden vector fields can play the role of dark matter whose characteristic signature would be monochromatic gamma ray emission from the galactic center. We show that this possibility is consistent with the LHC and other laboratory constraints, as well as astrophysical bounds.

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arXiv2013

Metastable Electroweak Vacuum: Implications for Inflation

Within the Standard Model, the current Higgs and top quark data favor metastability of the electroweak vacuum, although the uncertainties are still significant. The true vacuum is many orders of magnitude deeper than ours and the barrier separating the two is tiny compared to the depth of the well. This raises a cosmological question: how did the Higgs field get trapped in the shallow minimum and why did it stay there during inflation? The Higgs initial conditions before inflation must be fine--tuned to about one part in 10^8 in order for the Higgs field to end up in the right vacuum. In this note, we show that these problems can be resolved if there is a small positive coupling between the Higgs and the inflaton.

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arXiv2012

First LHC Constraints on Neutralinos

The ATLAS and CMS collaborations have recently reported tantalizing hints of the existence of a 125 GeV Higgs--like particle, whose couplings appear to match well the Standard Model (SM) expectations. In this work, we study implications of this observation for the neutralino sector of supersymmetric models, assuming that the Higgs signal gets confirmed. In general, the Higgs decay into neutralinos can be one of its dominant decay channels. Since a large invisible Higgs decay branching ratio would be in conflict with the data, this possibility is now constrained. In particular, we find that most of the region mu < 170 GeV, M_1 < 70 GeV at tan beta ~ 10 and mu < 120 GeV, M_1 < 70 GeV at tan beta ~ 40 is disfavored.

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arXiv2012

On Stability of the Electroweak Vacuum and the Higgs Portal

In the Standard Model (SM), the Higgs mass around 125 GeV implies that the electroweak vacuum is metastable since the quartic Higgs coupling turns negative at high energies. I point out that a tiny mixing of the Higgs with a heavy singlet can make the electroweak vacuum completely stable. This is due to a tree level correction to the Higgs mass-coupling relation, which survives in the zero-mixing/heavy-singlet limit. Such a situation is experimentally indistinguishable from the SM, unless the Higgs self-coupling can be measured. As a result, Higgs inflation and its variants can still be viable.

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arXiv2012

Implications of LHC searches for Higgs--portal dark matter

The search for the a Standard Model Higgs boson at the LHC is reaching a critical stage as the possible mass range for the particle has become extremely narrow and some signal at a mass of about 125 GeV is starting to emerge. We study the implications of these LHC Higgs searches for Higgs portal models of dark matter in a rather model independent way. Their impact on the cosmological relic density and on the direct detection rates are studied in the context of generic scalar, vector and fermionic thermal dark matter particles. Assuming a sufficiently small invisible Higgs decay branching ratio, we find that current data, in particular from the XENON experiment, essentially exclude fermionic dark matter as well as light, i.e. with masses below 50 GeV, scalar and vector dark matter particles. Possible observation of these particles at the planned upgrade of the XENON experiment as well in collider searches is discussed.

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arXiv2012

Vector Higgs-portal dark matter and the invisible Higgs

The Higgs sector of the Standard Model offers a unique probe of the hidden sector. In this work, we explore the possibility of renormalizable Higgs couplings to the hidden sector vector fields which can constitute dark matter (DM). Abelian gauge sectors with minimal field content, necessary to render the gauge fields massive, have a natural Z_2 parity. This symmetry ensures stability of the vector fields making them viable dark matter candidates, while evading the usual electroweak constraints. We illustrate this idea with the Stueckelberg and Higgs mechanisms. Vector DM is consistent with the WMAP, XENON100, and LHC constraints, while it can affect significantly the invisible Higgs decay. Due to the enhanced branching ratio for the Higgs decay into the longitudinal components of the vector field, the vector Higgs portal provides an efficient way to hide the Higgs at the LHC. This could be the reason why the latest combined ATLAS/CMS data did not bring evidence for the existence of the Higgs boson.

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arXiv2012

Higgs-induced lepton flavor violation

Due to the smallness of the lepton Yukawa couplings, higher-dimensional operators can give a significant contribution to the lepton masses. In this case, the lepton mass matrix and the matrix of lepton-Higgs couplings are misaligned leading to lepton flavor violation (LFV) mediated by the Standard Model Higgs boson. We derive model-independent bounds on the Higgs flavor violating couplings and quantify LFV in decays of leptons and electric dipole moments for a class of lepton-Higgs operators contributing to lepton masses. We find significant Higgs-mediated LFV effects at both 1-loop and 2-loop levels.

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arXiv2011

Higgs Portal Inflation

The Higgs sector of the Standard Model offers a unique opportunity to probe the hidden sector. The Higgs squared operator is the only dimension two operator which is Lorentz and gauge invariant. It can therefore couple both to scalar curvature and the hidden sector at the dim--4 level. We consider the possibility that a combination of the Higgs and a singlet from the hidden sector plays the role of inflaton, due to their large couplings to gravity. This implies that the quartic couplings satisfy certain constraints which leads to distinct low energy phenomenology, including Higgs signals at the LHC. We also address the unitarity issues and show that our analysis survives the unitarization procedure.

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arXiv2011

CP violation with Higgs-dependent Yukawa couplings

The framework of Higgs-dependent Yukawa couplings allows one to eliminate small couplings from the Standard Model, which can be tested at the LHC. In this work, I study the conditions for CP violation to occur in such models. I identify a class of weak basis invariants controlling CP violation. The invariant measure of CP violation is found to be more than 10 orders of magnitude greater than that in the Standard Model, which can be sufficient for successful electroweak baryogenesis.

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arXiv2010

Constraints on a very light CP-odd Higgs of the NMSSM and other axion-like particles

In the NMSSM, a light CP-odd Higgs arises due to spontaneous breaking of approximate symmetries such as Peccei-Quinn or R-symmetry and is motivated by string theory. The case when it is heavier than two muons is well studied and constrained. We analyze various meson decay, g-2, beam dump and reactor bounds on the CP-odd Higgs with mass below the muon threshold, in particular, addressing the question how light a CP-odd Higgs can be. We find that it has to be heavier than 210 MeV or have couplings to fermions 4 orders of magnitude below those of the Standard Model Higgs. Our analysis applies more generally to couplings of a light pseudoscalar to matter.

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