arXiv subjects

Nirmal Raj

Publications and source records attributed to Nirmal Raj.

54 records · Page 3Linked to original sources

arXiv2020

Warming Nuclear Pasta with Dark Matter: Kinetic and Annihilation Heating of Neutron Star Crusts

Neutron stars serve as excellent next-generation thermal detectors of dark matter, heated by the scattering and annihilation of dark matter accelerated to relativistic speeds in their deep gravitational wells. However, the dynamics of neutron star cores are uncertain, making it difficult at present to unequivocally compute dark matter scattering in this region. On the other hand, the physics of an outer layer of the neutron star, the crust, is more robustly understood. We show that dark matter scattering solely with the low-density crust still kinetically heats neutron stars to infrared temperatures detectable by forthcoming telescopes. We find that for both spin-independent and spin-dependent scattering on nucleons, the crust-only cross section sensitivity is $10^{-43} - 10^{-41}$~cm$^2$ for dark matter masses of 100 MeV $-$ 1 PeV, with the best sensitivity arising from dark matter scattering with a crust constituent called nuclear pasta (including gnocchi, spaghetti, and lasagna phases). For dark matter masses from 10 eV to 1 MeV, the sensitivity is $10^{-39} - 10^{-34}$~cm$^2$, arising from exciting collective phonon modes in a neutron superfluid in the inner crust. Furthermore, for any $s$-wave or $p$-wave annihilating dark matter, we show that dark matter will efficiently annihilate by thermalizing just with the neutron star crust, regardless of whether the dark matter ever scatters with the neutron star core. This implies efficient annihilation in neutron stars for any electroweakly interacting dark matter with inelastic mass splittings of up to 200 MeV, including Higgsinos. We conclude that neutron star crusts play a key role in dark matter scattering and annihilation in neutron stars.

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arXiv2020

Pre-Supernova Neutrinos in Large Dark Matter Direct Detection Experiments

The next Galactic core-collapse supernova (SN) is a highly anticipated observational target for neutrino telescopes. However, even prior to collapse, massive dying stars shine copiously in "pre-supernova" (pre-SN) neutrinos, which can potentially act as efficient SN warning alarms and provide novel information about the very last stages of stellar evolution. We explore the sensitivity to pre-SN neutrinos of large scale direct dark matter detection experiments, which, unlike dedicated neutrino telescopes, take full advantage of coherent neutrino-nucleus scattering. We find that argon-based detectors with target masses of $\mathcal{O}(100)$ tonnes (i.e. comparable in size to the proposed ARGO experiment) operating at sub-keV thresholds can detect $\mathcal{O}(10-100)$ pre-SN neutrinos coming from a source at a characteristic distance of $\sim$200 pc, such as Betelgeuse ($α$ Orionis). Large-scale xenon-based experiments with similarly low thresholds could also be sensitive to pre-SN neutrinos. For a Betelgeuse-type source, large scale dark matter experiments could provide a SN warning siren $\sim$10 hours prior to the explosion. We also comment on the complementarity of large scale direct dark matter detection experiments and neutrino telescopes in the understanding of core-collapse SN.

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arXiv2019

Dark matter astrometry at underground detectors with multiscatter events

We show that current and imminent underground detectors are capable of precision astrometry of dark matter. First we show that galactic dark matter velocity distributions can be obtained from reconstructed tracks of dark matter scattering on multiple nuclei during transit; using the liquid scintillator neutrino detector SNO+ as an example, we find that the dark matter velocity vector can be reconstructed event-by-event with such a small uncertainty, that the precision of dark matter astrometry will be limited mainly by statistics. We then determine the number of dark matter events required to determine the dispersion speed, escape speed, and velocity anisotropies of the local dark matter halo, and also find that with as few as $\mathcal{O}(10)$ events, dark matter signals may be discriminated from potential backgrounds arising as power-law distributions. Finally, we discuss the prospects of dark matter astrometry at other liquid scintillator detectors, dark matter experiments, and the recently proposed MATHUSLA detector.

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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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arXiv2018

Monochromatic dark neutrinos and boosted dark matter in noble liquid direct detection

If dark matter self-annihilates into neutrinos or a second component of ("boosted") dark matter that is nucleophilic, the annihilation products may be detected with high rates via coherent nuclear scattering. A future multi-ten-tonne liquid xenon detector such as DARWIN, and a multi-hundred-tonne liquid argon detector, ARGO, would be sensitive to the flux of these particles in complementary ranges of $10-1000$ MeV dark matter masses. We derive these sensitivities after accounting for atmospheric and diffuse supernova neutrino backgrounds, and realistic nuclear recoil acceptances. We find that their constraints on the dark neutrino flux may surpass neutrino detectors such as Super-Kamiokande, and that they would extensively probe parametric regions that explain the missing satellites problem in neutrino portal models. The XENON1T and Borexino experiments currently restrict the effective baryonic coupling of thermal boosted dark matter to $\lesssim 10-100 \ \times$ the weak interaction, but DARWIN and ARGO would probe down to couplings 10 times smaller. Detection of boosted dark matter with baryonic couplings $\sim 10^{-3}-10^{-2} \ \times$ the weak coupling could indicate that the dark matter density profile in the centers of galactic halos become cored, rather than cuspy, through annihilations. This work demonstrates that, alongside liquid xenon, liquid argon direct detection technology would emerge a major player in dark matter searches within and beyond the WIMP paradigm.

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arXiv2018

Hunting leptoquarks in monolepton searches

We show that stringent limits on leptoquarks that couple to first-generation quarks and left-handed electrons or muons can be derived from the spectral shape of the charged-current Drell-Yan process ($p p \to \ell^\pm ν$) at Run 2 of the LHC. We identify and examine all six leptoquark species that can generate such a monolepton signal, including both scalar and vector leptoquarks, and find cases where the leptoquark exchange interferes constructively, destructively or not at all with the Standard Model signal. When combined with the corresponding leptoquark-mediated neutral-current ($p p \to \ell^+ \ell^-$) process, we find the most stringent limits obtained to date, outperforming bounds from pair production and atomic parity violation. We show that, with 3000 fb$^{-1}$ of data, combined measurements of the transverse mass in $p p \to \ell^\pm ν$ events and invariant mass in $p p \to \ell^+ \ell^-$ events can probe masses between 8 TeV and 18 TeV, depending on the species of leptoquark, for electroweak-sized couplings. In light of such robust sensitivities, we strongly encourage the LHC experiments to interpret Drell-Yan (dilepton and monolepton) events in terms of leptoquarks, alongside usual scenarios like $Z'$ bosons and contact interactions.

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arXiv2018

Saturated Overburden Scattering and the Multiscatter Frontier: Discovering Dark Matter at the Planck Mass and Beyond

We show that underground experiments like LUX/LZ, PandaX-II, XENON, and PICO could discover dark matter up to the Planck mass and beyond, with new searches for dark matter that scatters multiple times in these detectors. This opens up significant discovery potential via re-analysis of existing and future data. We also identify a new effect which substantially enhances experimental sensitivity to large dark matter scattering cross-sections: while passing through atmospheric or solid overburden, there is a maximum number of scatters that dark matter undergoes, determined by the total number of scattering sites it passes, such as nuclei and electrons. This extends the reach of some published limits and future analyses to arbitrarily large dark matter scattering cross-sections.

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arXiv2017

Thermal dark matter via the flavon portal

Dark matter (DM) is added to the Froggatt-Nielsen (FN) mechanism, and conditions for its successful freezeout identified. Requesting the FN scale $Λ_{\text{FN}}$ to be the cutoff of the theory renders freezeout scenarios surprisingly few. Fermionic DM is typically charged under $U(1)_{\text{FN}}$, with the dominant annihilation channel a CP-even flavon + CP-odd flavon. A minimal case is when the DM-flavon coupling strength is $\mathcal{O}(1)$, with several implications: (1) the DM mass is $\mathcal{O}$(100 GeV - 1 TeV), thanks to the WIMP coincidence, (2) requiring perturbativity of couplings puts a lower $and$ upper limit on the flavor scale, 2 TeV $\lesssim Λ_{\text{FN}} \lesssim 14~$TeV, on account of its relation to DM mass and couplings, (3) DM is a "secluded WIMP" effectively hidden from collider and direct detection searches. Limits on the masses of dark matter and mediators from kaon mixing measurements constitute the best constraints, surpassing Xenon1T, Fermi-LAT, and the LHC. Future direct detection searches, and collider searches for missing energy plus a single jet/bottom/top, are promising avenues for discovery.

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arXiv2017

Characterizing dark matter at the LHC in Drell-Yan events

Spectral features in LHC dileptonic events may signal radiative corrections coming from new degrees of freedom, notably dark matter and mediators. Using simplified models, we show how these features can reveal the fundamental properties of the dark sector, such as self-conjugation, spin and mass of dark matter, and the quantum numbers of the mediator. Distributions of both the invariant mass $m_{\ell \ell}$ and the Collins-Soper scattering angle $cosθ_{CS}$ are studied to pinpoint these properties. We derive constraints on the models from LHC measurements of $m_{\ell \ell}$ and $cosθ_{CS}$, which are competitive with direct detection and jets + Missing Energy searches. We find that in certain scenarios the $cosθ_{CS}$ spectrum provides the strongest bounds, underlying the importance of scattering angle measurements for non-resonant new physics.

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arXiv2017

Neutron stars at the dark matter direct detection frontier

Neutron stars capture dark matter efficiently. The kinetic energy transferred during capture heats old neutron stars in the local galactic halo to temperatures detectable by upcoming infrared telescopes. We derive the sensitivity of this probe in the framework of effective operators. For dark matter heavier than a GeV, we find that neutron star heating can set limits on the effective operator cutoff that are orders of magnitude stronger than possible from terrestrial direct detection experiments in the case of spin-dependent and velocity-suppressed scattering.

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arXiv2017

Dark Kinetic Heating of Neutron Stars and An Infrared Window On WIMPs, SIMPs, and Pure Higgsinos

We identify a largely model-independent signature of dark matter interactions with nucleons and electrons. Dark matter in the local galactic halo, gravitationally accelerated to over half the speed of light, scatters against and deposits kinetic energy into neutron stars, heating them to infrared blackbody temperatures. The resulting radiation could potentially be detected by the James Webb Space Telescope, the Thirty Meter Telescope, or the European Extremely Large Telescope. This mechanism also produces optical emission from neutron stars in the galactic bulge, and X-ray emission near the galactic center, because dark matter is denser in these regions. For GeV - PeV mass dark matter, dark kinetic heating would initially unmask any spin-independent or spin-dependent dark matter-nucleon cross-sections exceeding $2 \times 10^{-45}$ cm$^2$, with improved sensitivity after more telescope exposure. For lighter-than-GeV dark matter, cross-section sensitivity scales inversely with dark matter mass because of Pauli blocking; for heavier-than-PeV dark matter, it scales linearly with mass as a result of needing multiple scatters for capture. Future observations of dark sector-warmed neutron stars could determine whether dark matter annihilates in or only kinetically heats neutron stars. Because inelastic inter-state transitions of up to a few GeV would occur in relativistic scattering against nucleons, elusive inelastic dark matter like pure Higgsinos can also be discovered.

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arXiv2016

Extending the Reach of Compressed Gluinos at the LHC

Conventional supersymmetry searches rely on large missing momentum and, on that account, are unsuitable for discovering superpartners nearly degenerate with the LSP. Such "compressed regions" are best probed by dedicated strategies that exploit their unique kinematic features. We consider a case study of a compressed gluino-bino simplified spectrum, motivated by its ability to set the dark matter relic abundance via co-annihilation. A kinematic variable suited to this spectrum is introduced, by which, for a gluino-bino mass splitting of 100 GeV, the discovery reach is extendable to m(gluino) = 850 GeV (1370 GeV) at LHC center-of-mass energy 8 TeV (13 TeV) with luminosity 20 inverse fb (3000 inverse fb). The non-trivial role played by soft triggers is also discussed.

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arXiv2016

Anticipating Non-Resonant New Physics in Dilepton Angular Spectra at the LHC

At the LHC, dileptonic events may turn up new physics interacting with quarks and leptons. The poster child for this scenario is a resonant $Z'$, much anticipated in $\ell^+ \ell^-$ invariant mass spectra. However, angular spectra of dileptons may play an equal or stronger role in discovering a non-resonant species. This paper avails of their LHC measurements to corner the couplings and masses of leptoquarks (LQs), that can mediate $q \bar{q} \rightarrow \ell^+ \ell^-$ in the $t$-channel and dramatically alter Standard Model angular spectra. Also derived are constraints from alterations to $m_{\ell \ell}$ distributions. These dilepton probes, exploiting the high rates and small uncertainties of the Drell-Yan process, rival or outdo dedicated LHC searches for LQs in single and pair production modes. The couplings of LQs with electronic interactions are best bound today by low-energy measurements of atomic parity violation, but can be probed better by $\ell^+ \ell^-$ measurements in the high luminosity runs of the LHC, with the angular spectra leading the way. This work also advocates the experimental presentation of boost-invariant angular asymmetries that vanish in the SM.

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arXiv2016

Forbidden Dark Matter at the Weak Scale via the Top Portal

At the tail of its velocity distribution, cold dark matter (DM) can annihilate at finite temperature to states heavier than itself. We explore the possibility that DM freezeout is dictated by these "forbidden annihilations" at the electroweak scale. Demanding that annihilation products be Standard Model particles, we find that for the forbidden mechanism to primarily set the DM relic abundance, DM must couple predominantly, if not solely, to top quarks. This can be arranged by invoking a non-trivial flavor structure such as Minimal Flavor Violation. We avail two avenues to achieve the correct thermal cross-section, requiring a mediator exchanged in the $s$- or $t$-channel. These simplified models submit easily to direct detection and collider searches, and necessarily hide from indirect detection signals. Viable supersymmetric spectra involving the forbidden mechanism may be found if combined with co-annihilation.

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arXiv2015

A 125 GeV Fat Higgs at large $\tan β$

We study the viability of regions of large $\tan β$ within the framework of Fat Higgs/$λ$-SUSY Models. We compute the one-loop effective potential to find the corrections to the Higgs boson mass due to the heavy non-standard Higgs bosons. As the tree level contribution to the Higgs boson mass is suppressed at large $\tan β$, these one-loop corrections are crucial to raising the Higgs boson mass to the measured LHC value. By raising the Higgsino and singlino mass parameters, typical electroweak precision constraints can also be avoided. We illustrate these new regions of Fat Higgs/$λ$-SUSY parameter space by finding regions of large $\tan β$ that are consistent with all experimental constraints including direct dark matter detection experiments, relic density limits and the invisible decay width of the $Z$ boson. We find that there exist regions around $λ= 1.25, \tan β= 50$ and a uniform psuedo-scalar $4~{\rm TeV} \lsim M_A \lsim 8$~TeV which are consistent will all present phenomenological constraints. In this region the dark matter relic abundance and direct detection limits are satisfied by a lightest neutralino that is mostly bino or singlino. As an interesting aside we also find a region of low $\tan β$ and small singlino mass parameter where a well-tempered neutralino avoids all cosmological and direct detection constraints.

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arXiv2014

Dark Matter Signals in Dilepton Production at Hadron Colliders

We show that new physics can show up in dileptonic events through its radiative contributions to the dilepton invariant mass, leading to unique "monocline" features in $m_{\ell\ell}$, as well as the angular distribution of the leptons. We focus in particular on the case of dark matter with scalar messengers coupling it to the quarks and leptons. Consistent thermal models require the dark matter to have masses of 100's GeV and have $\gsim 1$ couplings to the Standard Model (SM), implying that radiative corrections to the SM Drell-Yan rate can be sizeable. We consider the case of Majorana, Dirac, and pseudo-Dirac dark matter and show that there are regions of parameter space where the non-existence of a monocline, which starts at roughly twice the dark matter mass, $m_{\ell\ell} \sim 2 m_χ$, places the strongest constraint on the model. We make predictions for the sensitivities at the high luminosity 14 TeV LHC as well as a future 100 TeV proton-proton collider. We find that our dilepton signal is most sensitive when the mediator and the dark matter are nearly degenerate and conventional MET-based searches are least sensitive.

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arXiv2013

Mixed Gauginos Sending Mixed Messages to the LHC

Supersymmetric models with a Dirac gluino have been shown to be considerably less constrained from LHC searches than models with a Majorana gluino. In this paper, we generalize this discussion to models with a "Mixed Gluino", that acquires both Dirac and Majorana masses, as well as models in which electroweak gauginos contribute to squark production. Our primary interest is the degree of suppression of the cross section for first generation squarks compared with a gluino with a pure Majorana mass. We find that not all Majorana masses are alike -- a Majorana mass for the gluino can suppress the squark production cross section, whereas a Majorana mass for the adjoint fermion partner leads to an increased cross section, compared with a pure Dirac gluino. In the presence of electroweak gauginos, squark production can increase by at most a factor of a few above the pure Dirac gluino case when the electroweak gauginos have purely Majorana masses near to the squark masses. This unusual set of gaugino masses is interesting since the usual Higgs quartic coupling would not be suppressed. When the electroweak gaugino masses are much lighter than the squarks, there is a negligible change to the squark production cross section. We explain both of these cases in detail by considering the various subprocesses for squark production at the LHC. Continued searches for squark production with cross sections much smaller than expected in the MSSM are absolutely warranted, and we suggest new simplified models to fully characterize the collider constraints.

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arXiv2012

Revisiting Theories with Enhanced Higgs Couplings to Weak Gauge Bosons

Based on recent LHC Higgs analyses and in anticipation of future results we revisit theories where Higgs bosons can couple to weak gauge bosons with enhanced strength relative to the Standard Model value. Specifically, we look at the Georgi-Machacek model and its generalizations where higher "spin" representations of SU(2)_L break electroweak symmetry while maintaining custodial SU(2). In these theories, there is not only a Higgs-like boson but partner Higgs scalars transforming under representations of custodial SU(2), leading to a rich phenomenology. These theories serve as a consistent theoretical and experimental framework to explain enhanced couplings to gauge bosons, including fermiophobic Higgses. We focus on the phenomenology of a neutral scalar partner to the Higgs, which is determined once the Higgs couplings are specified. Depending on the parameter space, this partner could have i) enhanced fermion and gauge boson couplings and should be searched for at high mass (> 600 GeV), ii) have suppressed couplings and could be searched for at lower masses, where the Standard Model Higgs has already been ruled out, and iii) have fermiophilic couplings, where it can be searched for in heavy Higgs and top resonance searches. In the first two regions, the partner also has substantial decay rates into a pair of Higgs bosons. We touch briefly on the more model-dependent effects of the nontrivial SU(2)_C multiplets, which have exotic signals, such as a doubly-charged Higgs. We also discuss how the loop induced effects of these scalars tend to reduce the Higgs decay rate to photons, adding an additional uncertainty when extracting the couplings for the Higgs boson.

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