arXiv subjects

Kuver Sinha

Publications and source records attributed to Kuver Sinha.

At least 73 records · Page 4Linked to original sources

arXiv2020

New Directions for Axion Searches via Scattering at Reactor Neutrino Experiments

Searches for pseudoscalar axion-like-particles (ALPs) typically rely on their decay in beam dumps or their conversion into photons in haloscopes and helioscopes. We point out a new experimental direction for ALP probes through their production via the Primakoff process or Compton-like scattering off of electrons or nuclei. We consider ALPs produced by the intense gamma ray flux available from megawatt-scale nuclear reactors at neutrino experiments through Primakoff-like or Compton-like channels. Low-threshold detectors in close proximity to the core will have visibility to ALP decays and inverse Primakoff and Compton scattering, providing sensitivity to the ALP-photon and ALP-electron couplings. We find that the sensitivity to these couplings at the ongoing MINER neutrino experiment exceeds existing limits set by laboratory experiments and, for the ALP-electron coupling, we forecast the world's best laboratory-based constraints over a large portion of the sub-MeV ALP mass range.

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arXiv2020

PASSAT: Particle Accelerator helioScopes for Slim Axion-like-particle deTection

We propose a novel method to search for axion-like particles (ALPs) at particle accelerator experiments (exps). ALPs produced at the target via the Primakoff effect subsequently enter a region with a magnetic field, where they are converted to photons that are then detected. Dubbed Particle Accelerator helioScopes for Slim Axion-like-particle deTection (PASSAT), our proposal uses the principle of the axion helioscope but replaces ALPs produced in Sun with those produced in a target material. Since we rely on ALP-photon conversions, our proposal probes light (slim) ALPs that are otherwise inaccessible to lab-based exps which rely on ALP decay, and complements astrophysical probes that are more model-dependent. We first reinterpret existing data from the NOMAD exp in light of PASSAT, and constrain the parameter space for ALPs lighter than ~100 eV and ALP-photon coupling larger than ~$10^{-4}$ GeV$^{-1}$. As benchmarks of feasible low-cost exps improving over the NOMAD limits, we study the possibility of reusing the magnets of the CAST and the proposed BabyIAXO exps and placing them at the proposed BDF facility at CERN, along with some new detectors. We find that these realizations of PASSAT allow for a direct probe of the parameter space for ALPs lighter than ~100 eV and ALP-photon coupling larger than ~4 x $10^{-6}$ GeV$^{-1}$, which are regions that have not been probed yet by exps with lab-produced ALPs. In contrast to other proposals aiming at detecting 1 or 2-photon only events in hadronic beam dump environments, that rely heavily on Monte Carlo simulations, the background (BG) in our proposal can be directly measured in-situ, its suppression optimized, and the irreducible BG statistically subtracted. Sensitivity studies with other beams will be the subject of future work. The measurements suggested here represent an additional physics case for the BDF beyond those already proposed.

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arXiv2019

Is the magnitude of the Peccei-Quinn scale set by the landscape?

Rather general considerations of the string theory landscape imply a mild statistical draw towards large soft SUSY breaking terms tempered by the requirement of proper electroweak symmetry breaking where SUSY contributions to the weak scale are not too far from m(weak)~ 100 GeV. Such a picture leads to the prediction that m_h~ 125 GeV while most sparticles are beyond current LHC reach. Here we explore the possibility that the magnitude of the Peccei-Quinn (PQ) scale f_a is also set by string landscape considerations within the framework of a compelling SUSY axion model. First, we examine the case where the PQ symmetry arises as an accidental approximate global symmetry from a more fundamental gravity-safe Z(24)^R symmetry and where the SUSY mu parameter arises from a Kim-Nilles operator. The pull towards large soft terms then also pulls the PQ scale as large as possible. Unless this is tempered by rather severe (unknown) cosmological or anthropic bounds on the density of dark matter, then we would expect a far greater abundance of dark matter than is observed. This conclusion cannot be negated by adopting a tiny axion misalignment angle theta_i because WIMPs are also overproduced at large f_a. Hence, we conclude that setting the PQ scale via anthropics is highly unlikely. Instead, requiring soft SUSY breaking terms of order the gravity-mediation scale m_{3/2}~ 10-100 TeV places the mixed axion-neutralino dark matter abundance into the intermediate scale sweet zone where f_a~ 10^{11}-10^{12} GeV. We compare our analysis to the more general case of a generic SUSY DFSZ axion model with uniform selection on theta_i but leading to the measured dark matter abundance: this approach leads to a preference for f_a~ 10^{12} GeV.

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arXiv2019

Astro2020 Science White Paper: Using X-Ray Polarimetry to Probe the Physics of Black Holes and Neutron Stars

This white paper highlights compact object and fundamental physics science opportunities afforded by high-throughput broadband (0.1-60 keV) X-ray polarization observations. X-ray polarimetry gives new observables with geometric information about stellar remnants which are many orders of magnitude too small for direct imaging. The X-ray polarimetric data also reveal details about the emission mechanisms and the structure of the magnetic fields in and around the most extreme objects in the Universe. Whereas the Imaging X-ray Polarimetry Explorer (IXPE) to be launched in 2021 will obtain first results for bright objects, a follow-up mission could be one order of magnitude more sensitive and would be able to use a broader bandpass to perform physics type experiments for representative samples of sources.

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arXiv2019

Probing Boson Stars with Extreme Mass Ratio Inspirals

We propose to use gravitational waves from extreme mass ratio inspirals (EMRI), composed of a boson star and a supermassive black hole in the center of galaxies, as a new method to search for boson stars. Gravitational waves from EMRI have the advantage of being long-lasting within the frequency band of future space-based interferometer gravitational wave detectors and can accumulate large signal-to-noise ratio (SNR) for very sub-solar mass boson stars. Compared to gravitational waves from boson star binaries, which fall within the LIGO band, we find that much larger ranges of the mass and compactness of boson stars, as well as the underlying particle physics parameter space, can be probed by EMRI. We take tidal disruption of the boson stars into account and distinguish those which dissolve before the inner-most-stable-circular-orbit (ISCO) and those which dissolve after it. Due to the large number of cycles recorded, EMRIs can lead to a very precise mass determination of the boson star and distinguish it from standard astrophysical compact objects in the event of a signal. Tidal effects in inspiralling binary systems, as well as possible correlated electromagnetic signals, can also serve as potential discriminants.

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arXiv2019

Collider and Gravitational Wave Complementarity in Exploring the Singlet Extension of the Standard Model

We present a dedicated complementarity study of gravitational wave and collider measurements of the simplest extension of the Higgs sector: the singlet scalar augmented Standard Model. We study the following issues: (i) the electroweak phase transition patterns admitted by the model, and the proportion of parameter space for each pattern; (ii) the regions of parameter space that give detectable gravitational waves at future space-based detectors; and (iii) the current and future collider measurements of di-Higgs production, as well as searches for a heavy weak diboson resonance, and how these searches interplay with regions of parameter space that exhibit strong gravitational wave signals. We carefully investigate the behavior of the normalized energy released during the phase transition as a function of the model parameters, address subtle issues pertaining to the bubble wall velocity, and provide a description of different fluid velocity profiles. On the collider side, we identify the subset of points that are most promising in terms of di-Higgs and weak diboson production studies while also giving detectable signals at LISA, setting the stage for future benchmark points that can be used by both communities.

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arXiv2019

X-Ray Polarization Signals from Magnetars with Axion-Like-Particles

Axion-like-particles (ALPs) produced in the core of a magnetar can convert to photons in the magnetosphere, giving rise to novel features in the X-ray spectrum. Since ALPs only mix with the parallel mode of the photon, the polarization of the soft and hard X-ray spectra is predicted to have an O-mode component, in addition to the mainly X-mode component given by most astrophysical models. The relative strength of the O-mode component depends on the intensity of ALPs produced in the core and the probability of conversion. We quantify our results by considering X-ray emission produced both by astrophysical processes and by ALP-photon conversion, in an uncorrelated fashion, and in different relative proportions, which we parametrize by the angle $χ_0$. We then define a normalized astrophysics-subtracted Stokes parameter $R$ which only acquires non-zero values in the presence of ALP-photon conversion. We find, remarkably, that the parameter $R$ factorizes into a product of the ALP-to-photon conversion probability and $\cos(2χ_0)$ and display $R$, as well as the usual Stokes parameter $Q$, as a function of the photon energy and relative fractions of ALP and photon intensities. For benchmark points currently allowed by the CAST experiment, the O-mode prediction can be tested in future X-ray polarimeters and used either to constrain ALPs or find evidence for them.

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arXiv2019

Constraining Axion-Like-Particles with Hard X-ray Emission from Magnetars

Axion-like particles (ALPs) produced in the core of a magnetar will convert to photons in the magnetosphere, leading to possible signatures in the hard X-ray band. We perform a detailed calculation of the ALP-to-photon conversion probability in the magnetosphere, recasting the coupled differential equations that describe ALP-photon propagation into a form that is efficient for large scale numerical scans. We show the dependence of the conversion probability on the ALP energy, mass, ALP-photon coupling, magnetar radius, surface magnetic field, and the angle between the magnetic field and direction of propagation. Along the way, we develop an analytic formalism to perform similar calculations in more general $n$-state oscillation systems. Assuming ALP emission rates from the core that are just subdominant to neutrino emission, we calculate the resulting constraints on the ALP mass versus ALP-photon coupling space, taking SGR 1806-20 as an example. In particular, we take benchmark values for the magnetar radius and core temperature, and constrain the ALP parameter space by the requirement that the luminosity from ALP-to-photon conversion should not exceed the total observed luminosity from the magnetar. The resulting constraints are competitive with constraints from helioscope experiments in the relevant part of ALP parameter space.

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arXiv2019

Anapole Dark Matter via Vector Boson Fusion Processes at the LHC

Dark matter that is electrically neutral but couples to the electromagnetic current through higher-dimensional operators constitutes an interesting class of models. We investigate this class of models at the Large Hadron Collider, focusing on the anapole moment operator in an effective field theory (EFT) framework, and utilizing the vector boson fusion (VBF) topology. Assuming proton-proton collisions at $\sqrt{s} = 13$ TeV, we present the VBF anapole dark matter (ADM) cross sections and kinematic distributions as functions of the free parameters of the EFT, the cutoff scale $Λ$ and the ADM mass $m_χ$. We find that the distinctive VBF topology of two forward jets and large dijet pseudorapidity gap is effective at reducing SM backgrounds, leading to a $5σ$ discovery reach for all kinematically allowed ADM masses with $Λ\le 1.65$ (1.15) TeV, assuming an integrated luminosity of 3000 (100) fb$^{-1}$.

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arXiv2019

LHC SUSY and WIMP dark matter searches confront the string theory landscape

The string theory landscape of vacua solutions provides physicists with some understanding as to the magnitude of the cosmological constant. Similar reasoning can be applied to the magnitude of the soft SUSY breaking terms in supersymmetric models of particle physics: there appears to be a statistical draw towards large soft terms which is tempered by the anthropic requirement of the weak scale lying not too far from ~100 GeV. For a mild statistical draw of m_{soft}^n with n=1 (as expected from SUSY breaking due to a single F term) then the light Higgs mass is preferred at ~125 GeV while sparticles are all pulled beyond LHC bounds. We confront a variety of LHC and WIMP dark matter search limits with the statistical expectations from a fertile patch of string theory landscape. The end result is that LHC and WIMP dark matter detectors see exactly that which is expected from the string theory landscape: a Standard Model-like Higgs boson of mass 125 GeV but as yet no sign of sparticles or WIMP dark matter. SUSY from the n=1 landscape is most likely to emerge at LHC in the soft opposite-sign dilepton plus jet plus MET channel. Multi-ton noble liquid WIMP detectors should be able to completely explore the n=1 landscape parameter space.

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arXiv2018

Resonant Di-Higgs Production at Gravitational Wave Benchmarks: A Collider Study using Machine Learning

We perform a complementarity study of gravitational waves and colliders in the context of electroweak phase transitions choosing as our template the xSM model, which consists of the Standard Model augmented by a real scalar. We carefully analyze the gravitational wave signal at benchmark points compatible with a first order phase transition, taking into account subtle issues pertaining to the bubble wall velocity and the hydrodynamics of the plasma. In particular, we comment on the tension between requiring bubble wall velocities small enough to produce a net baryon number through the sphaleron process, and large enough to obtain appreciable gravitational wave production. For the most promising benchmark models, we study resonant di-Higgs production at the high-luminosity LHC using machine learning tools: a Gaussian process algorithm to jointly search for optimum cut thresholds and tuning hyperparameters, and a boosted decision trees algorithm to discriminate signal and background. The multivariate analysis on the collider side is able either to discover or provide strong statistical evidence of the benchmark points, opening the possibility for complementary searches for electroweak phase transitions in collider and gravitational wave experiments.

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arXiv2018

Higgs and superparticle mass predictions from the string theory landscape

Predictions for the scale of SUSY breaking from the string landscape go back at least a decade to the work of Denef and Douglas on the statistics of flux vacua. The assumption that an assortment of SUSY breaking F and D terms are present in the hidden sector, and their values are uniformly distributed in the landscape of D=4, N=1 effective supergravity models, leads to the expectation that the landscape pulls towards large values of soft terms favored by a power law behavior P(m(soft))~ m(soft)^n. On the other hand, similar to Weinberg's prediction of the cosmological constant, one can assume an anthropic selection of weak scales not too far from the measured value characterized by m(W,Z,h)~ 100 GeV. Working within a fertile patch of gravity-mediated low energy effective theories where the superpotential mu term is << m(3/2), as occurs in models such as radiative breaking of Peccei-Quinn symmetry, this biases statistical distributions on the landscape by a cutoff on the parameter Delta(EW), which measures fine-tuning in the m(Z)-mu mass relation. The combined effect of statistical and anthropic pulls turns out to favor low energy phenomenology that is more or less agnostic to UV physics. While a uniform selection n=0 of soft terms produces too low a value for m(h), taking n=1 or 2 produce most probabilistically m(h)~125 GeV for negative trilinear terms. For n>=1, there is a pull towards split generations with m(squarks,sleptons)(1,2)~10-30 TeV whilst m(t1)~ 1-2 TeV. The most probable gluino mass comes in at ~ 3-4 TeV--apparently beyond the reach of HL-LHC (although the required quasi-degenerate higgsinos should still be within reach). We comment on consequences for SUSY collider and dark matter searches.

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arXiv2018

Theoretical Uncertainties in the Calculation of Supersymmetric Dark Matter Observables

We estimate the current theoretical uncertainty in supersymmetric dark matter predictions by comparing several state-of-the-art calculations within the minimal supersymmetric standard model (MSSM). We consider standard neutralino dark matter scenarios -- coannihilation, well-tempering, pseudoscalar resonance -- and benchmark models both in the pMSSM framework and in frameworks with Grand Unified Theory (GUT)-scale unification of supersymmetric mass parameters. The pipelines we consider are constructed from the publicly available software packages SOFTSUSY, SPheno, FeynHiggs, SusyHD, micrOMEGAs, and DarkSUSY. We find that the theoretical uncertainty in the relic density as calculated by different pipelines, in general, far exceeds the statistical errors reported by the Planck collaboration. In GUT models, in particular, the relative discrepancies in the results reported by different pipelines can be as much as a few orders of magnitude. We find that these discrepancies are especially pronounced for for cases where the dark matter physics relies critically on calculations related to electroweak symmetry breaking, which we investigate in detail, and for coannihilation models, where there is heightened sensitivity to the sparticle spectrum. The dark matter annihilation cross section today and the scattering cross section with nuclei also suffer appreciable theoretical uncertainties, which, as experiments reach the relevant sensitivities, could lead to uncertainty in conclusions regarding the viability or exclusion of particular models.

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arXiv2017

Collider Detection of Dark Matter Electromagnetic Anapole Moments

Dark matter that interacts with the Standard Model by exchanging photons through higher multipole interactions occurs in a wide range of both strongly as well as weakly coupled hidden sector models. We study the collider detection prospects of these candidates, with a focus on Majorana dark matter that couples through the anapole moment. The study is conducted at the effective field theory level with the mono-$Z$ signature incorporating varying levels of systematic uncertainties at the high-luminosity LHC. The projected collider reach on the anapole moment is then compared to the reach coming from direct detection experiments like LZ. Finally, the analysis is applied to a weakly coupled completion with leptophilic dark matter.

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arXiv2017

Can We Discover Double Higgs Production at the LHC?

We explore double Higgs production via gluon fusion in the $b\bar{b} γγ$ channel at the high-luminosity LHC using machine learning tools. We first propose a Bayesian optimization approach to select cuts on kinematic variables, obtaining a $30-50$ \% increase in the significance compared to current results in the literature. We show that this improvement persists once systematic uncertainties are taken into account. We next use boosted decision trees (BDT) to further discriminate signal and background events. Our analysis shows that a joint optimization of kinematic cuts and BDT hyperparameters results in an appreciable improvement in the significance. Finally, we perform a multivariate analysis of the output scores of the BDT. We find that assuming a very low level of systematics, the techniques proposed here will be able to confirm the production of a pair of Standard Model Higgs bosons at 5$σ$ level with 3 ab$^{-1}$ of data. Assuming a more realistic projection of the level of systematics, around 10\%, the optimization of cuts to train BDTs combined with a multivariate analysis delivers a respectable significance of 4.6$σ$. Even assuming large systematics of 20\%, our analysis predicts a 3.6$σ$ significance, which represents at least strong evidence in favor of double Higgs production. We carefully incorporate background contributions coming from light flavor jets or $c$-jets being misidentified as $b$-jets and jets being misidentified as photons in our analysis.

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arXiv2017

Black Holes, Dark Matter Spikes, and Constraints on Simplified Models with $t$-Channel Mediators

The density spike of dark matter (DM) in the subparsec region near the supermassive black hole at the Galactic Center can provide potentially observable gamma-ray signals coming from DM annihilations. Taking Fermi-$LAT$ data for the gamma-ray flux from the point source 3FGL J1745.6-2859c (Sgr A*), we calculate the resulting constraints on generic models of DM, allowing for the possibility of a non-negligible velocity-dependent component of the annihilation cross section. We consider a variety of selections for the astrophysical parameters that describe the spike profile and find that the gamma-ray flux is strongly dependent on these selections, particularly the modelling of spike depletion effects due to gravitational interactions with baryons, which affect the spike radius and steepness profile. We calculate constraints on the DM parameter space for both attenuated and idealized spikes, considering different choices for the steepness profiles in each case. We find that for the most conservative selection of parameters, corresponding to a depleted spike with an NFW cusp profile, the gamma-ray flux for a 100 GeV thermal relic is lower than current observational constraints by several orders of magnitude. For more optimistic choices of parameters corresponding to spikes that have not been attenuated, bounds on DM masses can be obtained for a variety of choices of steepness profiles. We then specialize to a class of simplified models of fermionic DM that annihilate dominantly through the $t-$channel exchange of two scalar mediators with arbitrary mixing angle $α$, and calculate the indirect detection constraints coming from the DM spike. Along the way, we discuss constraints on the astrophysical parameters describing the DM spike, taking the Galactic Center excess as a signal of DM annihilation.

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arXiv2016

Simplified Dark Matter Models with Charged Mediators: Prospects for Direct Detection

We consider direct detection prospects for a class of simplified models of fermionic dark matter (DM) coupled to left and right-handed Standard Model fermions via two charged scalar mediators with arbitrary mixing angle $α$. DM interactions with the nucleus are mediated by higher electromagnetic moments, which, for Majorana DM, is the anapole moment. After giving a full analytic calculation of the anapole moment, including its $α$ dependence, and matching with limits in the literature, we compute the DM-nucleon scattering cross-section and show the LUX and future LZ constraints on the parameter space of these models. We then compare these results with constraints coming from $Fermi$-LAT continuum and line searches. Results in the supersymmetric limit of these simplified models are provided in all cases. We find that future direct detection experiments will be able to probe most of the parameter space of these models for $\mathcal{O}(100-200)$ GeV DM and lightest mediator mass $\lesssim \mathcal{O}(5\%)$ larger than the DM mass. The direct detection prospects dwindle for larger DM mass and larger mass gap between the DM and the lightest mediator mass, although appreciable regions are still probed for $\mathcal{O}(200)$ GeV DM and lightest mediator mass $\lesssim \mathcal{O}(20\%)$ larger than the DM mass. The direct detection bounds are also attenuated near certain "blind spots" in the parameter space, where the anapole moment is severely suppressed due to cancellation of different terms. We carefully study these blind spots and the associated $Fermi$-LAT signals in these regions.

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arXiv2016

Diphotons at the $Z$-pole in Models of the 750 GeV Resonance Decaying to Axion-Like Particles

Models in which the 750 GeV resonance ($S$) decays to two light axion-like particles (ALPs $a$), which in turn decay to collimated photons mimicking the observed signal, are motivated by Hidden Valley scenarios and could also provide a mechanism by which a $S \to γγ$ signal persists while $S \to Z γ,\; ZZ$ and $WW$ remain subdued in the near future. We point out that these Hidden Valley like models invoking $S \to aa \to 4 γ$ must also contend with $Z \to a (\to γγ) γ$ constraints coming from CDF and ATLAS. Within an effective field theory framework, we work out the constraints on the couplings of $S$ to $a$ and gauge bosons coming from photonic $Z$ decays and ensuring that the ALPs decay inside the electromagnetic calorimeter, in two regimes - where $a$ decays primarily to photons, and where $a$ also has hadronic branchings. The analysis is done for both when $S$ has a large as well as a narrow width, and for different relative contributions to the signal coming from $S \to γγ$ and $a \to γγ$. Results for the particular case where $S$ and $a$ belong to the same complex field are also presented. A $γγ$ resonance at the $Z$-pole coming from $Z \to a γ$ is expected in this class of models. Taking benchmark ALP masses below around 0.4 GeV and, assuming reasonable values for the fake jet rate and the identification efficiency of the photon-jet, we find the prospects for the discovery of diphotons at the $Z$-pole.

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