arXiv subjects

Bibhushan Shakya

Publications and source records attributed to Bibhushan Shakya.

At least 55 records · Page 3Linked to original sources

arXiv2017

Searching for Confining Hidden Valleys at the LHC(b)

We explore strategies for probing Hidden Valley scenarios exhibiting confinement. Such scenarios lead to a multiplicity of light hidden hadrons from showering processes. Their decays are typically soft and displaced, making them challenging to probe with traditional LHC searches. We show the low trigger thresholds and excellent track and vertex reconstruction at LHCb provide an ideal environment to search for such signals -- in both muonic and hadronic channels. We also explore the potential of ATLAS/CMS and discuss modifications to present searches that might make these experiments competitive with the LHCb reach. Our proposed searches can probe $Z^{\prime}$ models with dominant decays to dark sectors as well as exotic Higgs boson decays in Twin Higgs models.

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arXiv2017

On NMSSM Higgs Search Strategies at the LHC and the Mono-Higgs Signature in Particular

We study the collider phenomenology of the extended Higgs sector of the Next-to-Minimal Supersymmetric Standard Model (NMSSM). The region of NMSSM parameter space favored by a 125\,GeV SM-like Higgs and naturalness generically features a light Higgs and neutralino spectrum as well as a large $\mathcal{O}(1)$ coupling between the Higgs doublets and the NMSSM singlet fields. In such regimes, the heavier Higgs bosons can decay dominantly into lighter Higgs bosons and neutralinos. We study the prospects of observing such decays at the 13\,TeV LHC, focusing on mono-Higgs signatures as probes of such regions of parameter space. We present results in a model-independent framework as well as in the NMSSM. In the NMSSM, we find that the mono-Higgs channel can probe TeV scale Higgs bosons and has sensitivity even in the low tan$β$, large $m_A$ regime that is difficult to probe in the MSSM. Unlike for many conventional Higgs searches, the reach of the mono-Higgs channel will improve significantly with the increased luminosity expected to be collected at the LHC in the ongoing and upcoming runs.

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arXiv2017

Cosmological Imprints of Frozen-In Light Sterile Neutrinos

We investigate observable cosmological aspects of sterile neutrino dark matter produced via the freeze-in mechanism. The study is performed in a framework that admits many cosmologically interesting variations: high temperature production via annihilation processes from higher dimensional operators or low temperature production from decays of a scalar, with the decaying scalar in or out of equilibrium with the thermal bath, in supersymmetric or non-supersymmetric setups, thus allowing us to both extract generic properties and highlight features unique to particular variations. We find that while such sterile neutrinos are generally compatible with all cosmological constraints, interesting scenarios can arise where dark matter is cold, warm, or hot, has nontrivial momentum distributions, or provides contributions to the effective number of relativistic degrees of freedom Neff during Big Bang nucleosynthesis large enough to be probed by future measurements.

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arXiv2017

Spacetime Dynamics of a Higgs Vacuum Instability During Inflation

A remarkable prediction of the Standard Model is that, in the absence of corrections lifting the energy density, the Higgs potential becomes negative at large field values. If the Higgs field samples this part of the potential during inflation, the negative energy density may locally destabilize the spacetime. We use numerical simulations of the Einstein equations to study the evolution of inflation-induced Higgs fluctuations as they grow towards the true (negative-energy) minimum. These simulations show that forming a single patch of true vacuum in our past light cone during inflation is incompatible with the existence of our Universe; the boundary of the true vacuum region grows outward in a causally disconnected manner from the crunching interior, which forms a black hole. We also find that these black hole horizons may be arbitrarily elongated---even forming black strings---in violation of the hoop conjecture. By extending the numerical solution of the Fokker-Planck equation to the exponentially suppressed tails of the field distribution at large field values, we derive a rigorous correlation between a future measurement of the tensor-to-scalar ratio and the scale at which the Higgs potential must receive stabilizing corrections in order for the Universe to have survived inflation until today.

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arXiv2016

Sterile Neutrino Dark Matter with Supersymmetry

Sterile neutrino dark matter, a popular alternative to the WIMP paradigm, has generally been studied in non-supersymmetric setups. If the underlying theory is supersymmetric, we find that several interesting and novel dark matter features can arise. In particular, in scenarios of freeze-in production of sterile neutrino dark matter, its superpartner, the sterile sneutrino, can play a crucial role in early Universe cosmology as the dominant source of cold, warm, or hot dark matter, or of a subdominant relativistic population of sterile neutrinos that can contribute to the effective number of relativistic degrees of freedom Neff during Big Bang nucleosynthesis.

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arXiv2016

MSSM A-funnel and the Galactic Center Excess: Prospects for the LHC and Direct Detection Experiments

The pseudoscalar resonance or "A-funnel" in the Minimal Supersymmetric Standard Model~(MSSM) is a widely studied framework for explaining dark matter that can yield interesting indirect detection and collider signals. The well-known Galactic Center excess (GCE) at GeV energies in the gamma ray spectrum, consistent with annihilation of a $\lesssim 40$ GeV dark matter particle, has more recently been shown to be compatible with significantly heavier masses following reanalysis of the background. In this paper, we explore the LHC and direct detection implications of interpreting the GCE in this extended mass window within the MSSM A-funnel framework. We find that compatibility with relic density, signal strength, collider constraints, and Higgs data can be simultaneously achieved with appropriate parameter choices. The compatible regions give very sharp predictions of 200-600 GeV CP-odd/even Higgs bosons at low tan$β$ at the LHC and spin-independent cross sections $\approx 10^{-11}$ pb at direct detection experiments. Regardless of consistency with the GCE, this study serves as a useful template of the strong correlations between indirect, direct, and LHC signatures of the MSSM A-funnel region.

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arXiv2015

Sterile Neutrino Dark Matter from Freeze-In

A sterile neutrino is a well-motivated and widely studied dark matter candidate. The most straightforward realization of sterile neutrino dark matter, through the Dodelson-Widrow mechanism, is now ruled out by a combination of X-ray and Lyman-αmeasurements. An alternative production mechanism that is becoming increasingly popular in the literature is the freeze-in mechanism, involving frameworks where a feeble coupling to a particle - usually a scalar beyond the Standard Model - in the thermal bath results in a gradual accumulation of the sterile neutrino dark matter abundance. This article reviews the various motivations for realizing such frameworks in the literature, their common characteristic features, and phenomenological signatures.

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arXiv2015

Hints of a PeV Scale Supersymmetric Neutrino Sector: Neutrino Masses, Dark Matter, keV and PeV Anomalies

Neutrino masses and light (keV-GeV) sterile neutrinos can arise naturally via a modified, low energy seesaw mechanism if the right-handed neutrinos are charged under a new symmetry broken by a PeV scale vacuum expectation value, presumably tied to supersymmetry breaking. The additional field content also allows for freeze-in production of sterile neutrino dark matter. This framework can accommodate the recently observed 3.5 keV X-ray line, while a straightforward extension of the framework, using the new symmetry and the PeV energy scale, can explain the PeV energy neutrino events at IceCube. Together, these can therefore be taken as hints of the existence of a PeV scale supersymmetric neutrino sector.

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arXiv2015

Neutrino Masses and Sterile Neutrino Dark Matter from the PeV Scale

We show that active neutrino masses and a keV-GeV mass sterile neutrino dark matter candidate can result from a modified, low energy seesaw mechanism if right-handed neutrinos are charged under a new symmetry broken by a scalar field vacuum expectation value at the PeV scale. The dark matter relic abundance can be obtained through active-sterile oscillation, freeze-in through the decay of the heavy scalar, or freeze-in via non-renormalizable interactions at high temperatures. The low energy effective theory maps onto the widely studied nuMSM framework.

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arXiv2015

PeV Neutrinos and a 3.5 keV X-Ray Line from a PeV Scale Supersymmetric Neutrino Sector

Recent measurements of PeV energy neutrinos at IceCube and a 3.5 keV X-ray line in the spectra of several galaxies are both tantalizing signatures of new physics. This paper shows that one or both of these observations can be explained within an extended supersymmetric neutrino sector. Obtaining light active neutrino masses as well as phenomenologically interesting (keV-GeV) sterile neutrino masses without any unnaturally small parameters hints at a new symmetry in the neutrino sector that is broken at the PeV scale, presumably tied to supersymmetry breaking. The same symmetry and structure can sufficiently stabilize an additional PeV particle, produce its abundance through the freeze-in mechanism, and lead to decays that can give the energetic neutrinos observed by IceCube. The lightest sterile neutrino, if at 7 keV, is a non-resonantly produced fraction of dark matter, and can account for the 3.5 keV X-ray line. The two signals could therefore be the first probes of an extended supersymmetric neutrino sector.

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arXiv2015

Probable or Improbable Universe? Correlating Electroweak Vacuum Instability with the Scale of Inflation

Measurements of the Higgs boson and top quark masses indicate that the Standard Model Higgs potential becomes unstable around $Λ_I \sim 10^{11}$ GeV. This instability is cosmologically relevant since quantum fluctuations during inflation can easily destabilize the electroweak vacuum if the Hubble parameter during inflation is larger than $Λ_I$ (as preferred by the recent BICEP2 measurement). We perform a careful study of the evolution of the Higgs field during inflation, obtaining different results from those currently in the literature. We consider both tunneling via a Coleman-de Luccia or Hawking-Moss instanton, valid when the scale of inflation is below the instability scale, as well as a statistical treatment via the Fokker-Planck equation appropriate in the opposite regime. We show that a better understanding of the post-inflation evolution of the unstable AdS vacuum regions is crucial for determining the eventual fate of the universe. If these AdS regions devour all of space, a universe like ours is indeed extremely unlikely without new physics to stabilize the Higgs potential; however, if these regions crunch, our universe survives, but inflation must last a few e-folds longer to compensate for the lost AdS regions. Lastly, we examine the effects of generic Planck-suppressed corrections to the Higgs potential, which can be sufficient to stabilize the electroweak vacuum during inflation.

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arXiv2014

Light Higgsino Decays as a Probe of the NMSSM

In the Next-to Minimal Supersymmetric Standard Model (NMSSM), a sizable coupling $λ$ between the singlet and Higgs fields can naturally accommodate the observed Higgs boson mass of 125 GeV. This large coupling also results in a large separation between the Higgsino and singlino mass scales in the neutralino sector to evade stringent constraints from direct detection experiments. Most of the natural parameter space in this setup therefore contains light Higgsinos that can decay into the singlino and the 125 GeV Higgs. If the Higgsinos are to be light enough to be produced at the LHC, the mass gap $m_{\tildeχ^0_2} -(m_{\tildeχ^0_1} +m_h)$ is generally fairly low, resulting in small missing energy. We study the collider phenomenology of this process and demonstrate that the 4$b+j+{E\!\!\!\!/_{\rm T}}$ signal arising from $pp\rightarrow\tildeχ_i^0\tildeχ_j^0jj$ process can be a viable channel to search for this low mass gap region at the 14 TeV LHC. In addition, we find that a potential signal from the LHC search, in tandem with direct detection experiments, can distinguish the NMSSM from the analogous process in the MSSM, where the bino plays the role of the singlino, for positive values of the $μ$ parameter even when the additional singlet-dominated (pseudo)scalars are absent or indistinguishable from the Higgs in the decay.

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arXiv2014

Dark Matter at the Pseudoscalar Higgs Resonance in the pMSSM and SUSY GUTs

We study dark matter at the MSSM pseudoscalar Higgs resonance (A-funnel), which is one of the few remaining MSSM thermal dark matter candidates in the $100-1000$ GeV range safe from direct detection constraints. To illustrate the various factors at play, this study is performed in two contrasting set-ups: a bottom-up phenomenological MSSM (pMSSM) approach that allows significant freedom, and the top-down, highly constrained Yukawa unified $SO(10)$ GUT model. In the pMSSM, for $μ> 0$, the entire parameter space lies above the coherent neutrino background and mostly within reach of XENON1T and LZ, while blind spots exist at $m_A\,> \, 800\,$GeV for $μ< 0$; the strongest constraints come from $A/H \rightarrow ττ$ searches at the LHC. For Yukawa unified models, the confluence of $B_s \rightarrow μ^+ μ^-$ constraints, fits to the bottom quark and Higgs masses, and gluino mass bounds from the LHC result in a prediction: realizing the pseudoscalar resonance $requires$ gaugino mass non-universality, with a mass ratio that is determined to within a small range.

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arXiv2013

The Status of Neutralino Dark Matter

The lightest neutralino in supersymmetry is the most studied dark matter candidate. This writeup reviews the status of neutralino dark matter in minimal and nonminimal supersymmetric models in light of recent null results at the XENON100 experiment and the observation of a 130 GeV gamma ray signal from the Galactic Center by the Fermi LAT.

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arXiv2013

Higgs Couplings and Naturalness in $λ$-SUSY

We study Higgs boson couplings in the large-$λ$ version of the Next-to-Minimal Supersymmetric Standard Model, known as $λ$-SUSY. We find that the predicted deviations from the Standard Model (SM) in these couplings are inversely correlated with the amount of fine-tuning needed to accommodate a 126 GeV Higgs. In the most natural regions of parameter space, the 126 GeV Higgs has large admixtures of both the SM-singlet and the non-SM Higgs doublet scalars, and such regions are already ruled out by the LHC. Future improvements in the Higgs coupling measurements will either discover deviations from the SM, or put further stress on naturalness in $λ$-SUSY.

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arXiv2013

XENON100 Implications for Naturalness in the MSSM, NMSSM and lambda-SUSY

In a recent paper arXiv:1107.5048, we discussed the correlation between the elastic neutralino-nucleon scattering cross section, constrained by dark matter direct detection experiments, and fine-tuning at tree-level in the electroweak symmetry breaking sector of the Minimal Supersymmetric Standard Model (MSSM). Here, we show that the correlation persists in the Next-to-Minimal Supersymmetric Standard Model (NMSSM), and its variant, lambda-SUSY. Both models are strongly motivated by the recent discovery of a 125 GeV Higgs-like particle. We also discuss the implications of the recently published bound on the direct detection cross section from 225 live days of XENON100 experiment. In both the MSSM and the NMSSM, most of the parameter space with fine-tuning less than 10% is inconsistent with the XENON100 bound. In lambda-SUSY, on the other hand, large regions of completely natural electroweak symmetry breaking are still allowed, primarily due to a parametric suppression of fine-tuning with large λ. The upcoming XENON1T experiment will be able to probe most of the parameter space with less than 1% fine-tuning in all three models.

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arXiv2012

Neutralino Oscillations at the LHC

We study a supersymmetric phenomenon that can give spectacular signals at the LHC: oscillations of neutralinos. Such oscillations can be naturally realised in R-symmetric models, where additional fields are introduced as Dirac mass partners of gauginos and Higgsinos. Majorana masses for gauginos, necessarily generated from anomaly mediation, can create tiny mass splittings between degenerate mass eigenstates, causing states produced at the LHC to oscillate between neutralinos and their Dirac partner fields. Scenarios where such states decay with displaced vertices can lead to striking signatures at the LHC, where the oscillation can be visible directly in the distribution of displaced vertex lengths. We elaborate on the theory and LHC phenomenology of this feature within a specific scenario of a Higgsino decaying with a displaced vertex into a gravitino and a Z boson.

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arXiv2012

A 130 GeV Gamma Ray Signal from Supersymmetry

The viability of neutralino dark matter as an explanation of the 130 GeV gamma ray signal from the Galactic Center recently observed by the Fermi Large Area Telescope is examined. It is found that the signal can be compatible with a sharp feature from internal bremsstrahlung from a mostly bino dark matter particle of mass around 145 GeV, augmented by a contribution from annihilation into gamma+Z via a small wino admixture. This scenario circumvents the problematic overproduction of lower energy continuum photons that plague line interpretations of this signal. Sleptons approximately degenerate in mass with the neutralino are required to enhance the internal bremsstrahlung feature.

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