arXiv subjects

Kaustubh Agashe

Publications and source records attributed to Kaustubh Agashe.

At least 37 records · Page 2Linked to original sources

arXiv2020

LHC Signals for KK Graviton from an Extended Warped Extra Dimension

We analyze signals at the Large Hadron Collider (LHC) from production and decay of Kaluza-Klein (KK) gravitons in the context of "extended" warped extra-dimensional models, where the standard model (SM) Higgs and fermion fields are restricted to be in-between the usual ultraviolet/Planck brane and a $\sim O(10)$ TeV (new, "intermediate") brane, whereas the SM gauge fields (and gravity) propagate further down to the $\sim O( \hbox{TeV} )$ infrared brane. Such a framework suppresses flavor violation stemming from KK particle effects, while keeping the KK gauge bosons and gravitons accessible to the LHC. We find that the signals from KK graviton are significantly different than in the standard warped model. This is because the usually dominant decay modes of KK gravitons into top quark, Higgs and longitudinal $W/Z$ particles are suppressed by the above spatial separation between these two sets of particles, thus other decay channels are allowed to shine themselves. In particular, we focus on two novel decay channels of the KK graviton. The first one is the decay into a pair of radions, each of which decays (dominantly) into a pair of SM gluons, resulting in a resonant 4-jet final state consisting of two pairs of dijet resonance. On the other hand, if the radion is heavier and/or KK gluon is lighter, then the KK graviton mostly decays into a KK gluon and a SM gluon. The resulting KK gluon has a significant decay branching fraction into radion and SM gluon, thereby generating (again) a 4-jet signature, but with a different underlying event topology, i.e., featuring now three different resonances. We demonstrate that the High-Luminosity LHC (HL-LHC) has sensitivity to KK graviton of (up to) $\sim 4$ TeV in both channels, whereas it is unlikely to have sensitivity in the standard dijet resonance search channel from KK graviton decay into two gluons.

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arXiv2020

"Non-Local" Effects from Boosted Dark Matter in Indirect Detection

Indirect dark matter (DM) detection typically involves the observation of standard model (SM) particles emerging from DM annihilation/decay inside regions of high dark matter concentration. We consider an annihilation scenario in which this reaction has to be initiated by one of the DMs involved being boosted while the other is an ambient non-relativistic particle. This "trigger" DM must be created, for example, in a previous annihilation or decay of a heavier component of DM. Remarkably, boosted DM annihilating into gamma-rays at a specific point in a galaxy could actually have traveled from its source at another point in the same galaxy or even from another galaxy. Such a "non-local" behavior leads to a non-trivial dependence of the resulting photon signal on the galactic halo parameters, such as DM density and core size, encoded in the so-called "astrophysical" $J$-factor. These non-local $J$-factors are strikingly different than the usual scenario. A distinctive aspect of this model is that the signal from dwarf galaxies relative to the Milky Way tends to be suppressed from the typical value to various degrees depending on their characteristics. This feature can thus potentially alleviate the mild tension between the DM annihilation explanation of the observed excess of $\sim$ GeV photons from the Milky Way's galactic center vs. the apparent non-observation of the corresponding signal from dwarf galaxies.

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arXiv2020

Cosmological Phase Transition of Spontaneous Confinement

The dynamics of a cosmological (de)confinement phase transition is studied in nearly conformally invariant field theories, where confinement is predominantly spontaneously generated and associated with a light "dilaton" field. We show how the leading contribution to the transition rate can be computed within the dilaton effective theory. In the context of Composite Higgs theories, we demonstrate that a simple scenario involving two renormalization-group fixed points can make the transition proceed much more rapidly than in the minimal scenario, thereby avoiding excessive dilution of matter abundances generated before the transition. The implications for gravitational wave phenomenology are discussed. In general, we find that more (less) rapid phase transitions are associated with weaker (stronger) gravitational wave signals. The various possible features of the strongly coupled composite Higgs phase transition discussed here can be concretely modeled at weak coupling within the AdS/CFT dual Randall-Sundrum extra-dimensional description, which offers important insights into the nature of the transition and its theoretical control. These aspects will be presented in a companion paper.

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arXiv2018

Natural Seesaw and Leptogenesis from Hybrid of High-Scale Type I and TeV-Scale Inverse

We develop an extension of the basic inverse seesaw model which addresses simultaneously two of its drawbacks, namely, the lack of explanation of the tiny Majorana mass term $μ$ for the TeV-scale singlet fermions and the difficulty in achieving successful leptogenesis. Firstly, we investigate systematically leptogenesis within the inverse (and the related linear) seesaw models and show that a successful scenario requires either small Yukawa couplings, implying loss of experimental signals, and/or quasi-degeneracy among singlets mass of different generations, suggesting extra structure must be invoked. Then we move to the analysis of our new framework, which we refer to as hybrid seesaw. This combines the TeV degrees of freedom of the inverse seesaw with those of a high-scale ($M_N\gg$ TeV) seesaw module in such a way as to retain the main features of both pictures: naturally small neutrino masses, successful leptogenesis, and accessible experimental signatures. We show how the required structure can arise from a more fundamental theory with a gauge symmetry or from warped extra dimensions/composite Higgs. We provide a detailed derivation of all the analytical formulae necessary to analyze leptogenesis in this new framework, and discuss the entire gamut of possibilities our scenario encompasses: including scenarios with singlet masses in the enlarged range $M_N \sim 10^6 - 10^{16}$ GeV. The idea of hybrid seesaw was proposed by us in arXiv:1804.06847; here, we substantially elaborate upon and extend earlier results.

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arXiv2018

Detecting a Boosted Diboson Resonance

New light scalar particles in the mass range of hundreds of GeV, decaying into a pair of $W/Z$ bosons can appear in several extensions of the SM. The focus of collider studies for such a scalar is often on its direct production, where the scalar is typically only mildly boosted. The observed $W/Z$ are therefore well-separated, allowing analyses for the scalar resonance in a standard fashion as a low-mass diboson resonance. In this work we instead focus on the scenario where the direct production of the scalar is suppressed, and it is rather produced via the decay of a significantly heavier (a few TeV mass) new particle, in conjunction with SM particles. Such a process results in the scalar being highly boosted, rendering the $W/Z$'s from its decay merged. The final state in such a decay is a "fat" jet, which can be either four-pronged (for fully hadronic $W/Z$ decays), or may be like a $W/Z$ jet, but with leptons buried inside (if one of the $W/Z$ decays leptonically). In addition, this fat jet has a jet mass that can be quite different from that of the $W/Z$/Higgs/top quark-induced jet, and may be missed by existing searches. In this work, we develop dedicated algorithms for tagging such multi-layered "boosted dibosons" at the LHC. As a concrete application, we discuss an extension of the standard warped extra-dimensional framework where such a light scalar can arise. We demonstrate that the use of these algorithms gives sensitivity in mass ranges that are otherwise poorly constrained.

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arXiv2018

Hybrid seesaw leptogenesis and TeV singlets

The appealing feature of inverse seesaw models is that the Standard Model (SM) neutrino mass emerges from the exchange of TeV scale singlets with sizable Yukawa couplings, which can be tested at colliders. However, the tiny Majorana mass splitting between TeV singlets, introduced to accommodate small neutrino masses, is left unexplained. Moreover, we argue that these models suffer from a structural limitation that prevents a successful leptogenesis if one insists on having unsuppressed Yukawa couplings and TeV scale singlets. In this work we propose a hybrid seesaw model, where we replace the mass splitting with a coupling to a high scale seesaw module including a TeV scalar. We show that this structure achieves the goal of filling both the above gaps with couplings of order unity. The necessary structure automatically arises embedding the seesaw mechanism in composite Higgs models, but may also be enforced by new gauge symmetries in a weakly-coupled theory. Our hybrid seesaw models have distinguishing features compared to the standard high scale type-I seesaw and inverse seesaw. Firstly, they have much richer phenomenology. Indeed, they generally predict new TeV scale physics (including scalars) potentially accessible at present and future colliders, whereas weakly-coupled versions may also have astrophysical and cosmological signatures due to the presence of a light Nambu-Goldstone boson coupled to neutrinos. Secondly, our scenario features an interesting interplay between high scale and TeV scale physics in leptogenesis and enlarges the range of allowed high scale singlet masses beyond the usual $\sim 10^9-10^{ 15 }$ GeV, without large hierarchies in the Yukawa couplings nor small mass splitting among the singlets.

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arXiv2017

Dedicated Strategies for Triboson Signals from Cascade Decays of Vector Resonances

New colorless electroweak (EW) charged spin-1 particles with mass of a few TeV arise in numerous extensions of the Standard Model (SM). Decays of such a vector into a pair of SM particles, either fermions or EW bosons, are well studied. Many of these models have an additional scalar, which can lead to (and even dominate in certain parameter regions) a novel decay channel for the heavy vector particles instead - into a SM EW boson and the scalar, which subsequently decays into a SM EW boson pair. In this work, we focus on the scalar being relatively heavy, roughly factor of two lighter than the vector particles, rendering its decay products well separated. Such a cascade decay results in a final state with three isolated bosons. We argue that for this "triboson" signal the existing diboson searches are not quite optimal due to combinatorial ambiguity for three identical bosons, and in addition, due to a relatively small signal cross-section determined by the heaviness of the decaying vector particle. In order to isolate the signal, we demonstrate that tagging all three bosons, followed by use of the full triboson invariant mass distribution as well as that of appropriate subsets of dibosons, is well motivated. We develop these general strategies in detail within the context of a specific class of models that are based on extensions of the standard warped extra-dimensional scenario. We also point out that a similar analysis would apply to models with an enlarged EW gauge sector in four dimensions, even if they involve a different Lorentz structure for the relevant couplings.

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arXiv2017

LHC Signals for Singlet Neutrinos from a Natural Warped Seesaw (II)

A natural seesaw mechanism for obtaining the observed size of SM neutrino masses can arise in a warped extra dimensional/composite Higgs framework. In a previous paper, we initiated the study of signals at the LHC for the associated $\sim$ TeV mass SM singlet neutrinos, within a canonical model of $SU(2)_L \times SU(2)_R \times U(1)_{ B - L }$ (LR) symmetry in the composite sector, as motivated by consistency with the EW precision tests. Here, we investigate LHC signals in a different region of parameter space for the same model, where production of singlet neutrinos can occur from particles beyond those in usual LR models. Specifically, we assume that composite $(B - L)$ gauge boson is lighter than all the others in the EW sector. We show that the composite $(B - L)$ gauge boson can acquire a significant coupling to light quarks simply via mixing with elementary hypercharge gauge boson. Thus, the singlet neutrino can be pair-produced via decays of $(B - L)$ gauge boson, without a charged current counterpart. Furthermore, there is no decay for $(B - L)$ gauge boson directly into dibosons, unlike for the usual case of $W_R^{ \pm }$ and $Z^{ \prime }$. Independently of the above extension of the EW sector, we analyze production of singlet neutrinos in decays of composite partners of $SU(2)_L$ doublet leptons, which are absent in the usual LR models. In turn, these doublet leptons can be produced in composite $W_L$ decays. We show that $4 - 5 σ$ signal can be achieved for both cases described above for the following spectrum with 3000 fb$^{-1}$ luminosity: $2 - 2.5$ TeV composite gauge bosons, $1$ TeV composite doublet lepton (for the second case) and $500 - 750$ GeV singlet neutrino.

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arXiv2016

LHC signals for Singlet Neutrinos from a Natural Warped Seesaw (I)

Recently, it was shown in arXiv:1512.06742 that a straightforward implementation of the type I seesaw mechanism in a warped extra dimensional framework is in reality a {\em natural} realization of "inverse" seesaw, i.e., the Standard Model (SM) neutrino mass is dominantly generated by exchange of pseudo-Dirac {\em TeV}-mass SM singlet neutrinos. By the AdS/CFT correspondence, this scenario is {\em dual} to these singlet particles being composites of some new strong dynamics, along with the SM Higgs boson, with the rest of the SM particles being mostly elementary. We study signals from production of these heavy neutrinos at the Large Hadron Collider (LHC). We focus on the scenario where the strong sector has a global $SU(2)_{\rm L} \times SU(2)_{\rm R} \times U(1)_{\rm X}$ symmetry; such a left-right (LR) structure being motivated by consistency with the electroweak (EW) precision tests. The singlet neutrinos are charged under $SU(2)_{\rm R} \times U(1)_{\rm X}$ symmetry, thus can be produced from $W^{ \pm }_R$ exchange, as in four-dimensional (4D) LR symmetric models. However, the direct coupling of light quarks to $W^{ \pm }_R$ is negligible, due to $W^{ \pm }_R$ also being composite; nonetheless, a sizable coupling can be induced by mixings among the various types of $W^{ \pm }$ bosons. Furthermore, $W^{ \pm }_R$ decays dominantly into the singlet and {\em composite} partner of charged lepton. This heavy charged lepton, in turn, decays into SM lepton, {\em plus} $Z$/Higgs, thus the latter can be used for extra identification of the signal. For a benchmark scenario with $W^{ \pm }_R$ of mass 2 TeV and singlet neutrino of mass 750 GeV, we find that, in both the di-lepton + di-jet + Higgs and tri-lepton + Higgs channels, significant evidence can be seen at the LHC14 for an integrated luminosity of 300/fb and that even discovery is possible with slightly more luminosity.

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arXiv2016

Flavor Universal Resonances and Warped Gravity

Warped higher-dimensional compactifications with "bulk" standard model, or their AdS/CFT dual as the purely 4D scenario of Higgs compositeness and partial compositeness, offer an elegant approach to resolving the electroweak hierarchy problem as well as the origins of flavor structure. However, low-energy electroweak/flavor/CP constraints and the absence of non-standard physics at LHC Run 1 suggest that a "little hierarchy problem" remains, and that the new physics underlying naturalness may lie out of LHC reach. Assuming this to be the case, we show that there is a simple and natural extension of the minimal warped model in the Randall-Sundrum framework, in which matter, gauge and gravitational fields propagate modestly different degrees into the IR of the warped dimension, resulting in rich and striking consequences for the LHC (and beyond). The LHC-accessible part of the new physics is AdS/CFT dual to the mechanism of "vectorlike confinement", with TeV-scale Kaluza-Klein excitations of the gauge and gravitational fields dual to spin-0,1,2 composites. Unlike the minimal warped model, these low-lying excitations have predominantly flavor-blind and flavor/CP-safe interactions with the standard model. Remarkably, this scenario also predicts small deviations from flavor-blindness originating from virtual effects of Higgs/top compositeness at $\sim O(10)$ TeV, with subdominant resonance decays into Higgs/top-rich final states, giving the LHC an early "preview" of the nature of the resolution of the hierarchy problem. Discoveries of this type at LHC Run 2 would thereby anticipate (and set a target for) even more explicit explorations of Higgs compositeness at a 100 TeV collider, or for next-generation flavor tests.

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arXiv2016

Mass Measurement Using Energy Spectra in Three-body Decays

In previous works we have demonstrated how the energy distribution of massless decay products in two body decays can be used to measure the mass of decaying particles. In this work we show how such results can be generalized to the case of multi-body decays. The key ideas that allow us to deal with multi-body final states are an extension of our previous results to the case of massive decay products and the factorization of the multi-body phase space. The mass measurement strategy that we propose is distinct from alternative methods because it does not require an accurate reconstruction of the entire event, as it does not involve, for instance, the missing transverse momentum, but rather requires measuring only the visible decay products of the decay of interest. To demonstrate the general strategy, we study a supersymmetric model wherein pair-produced gluinos each decay to a stable neutralino and a bottom quark-antiquark pair via an off-shell bottom squark. The combinatorial background stemming from the indistinguishable visible final states on both decay sides can be treated by an "event mixing" technique, the performance of which is discussed in detail. Taking into account dominant backgrounds, we are able to show that the mass of the gluino and, in favorable cases, that of the neutralino can be determined by this mass measurement strategy.

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arXiv2016

Top quark mass determination from the energy peaks of b-jets and B-hadrons at NLO QCD

We analyze the energy spectra of $single$ b-jets and B-hadrons resulting from the production and decay of top quarks within the SM at the LHC at the NLO QCD. For both hadrons and jets, we calculate the correlation of the peak of the spectrum with the top quark mass, considering the "energy-peak" as an observable to determine the top quark mass. Such a method is motivated by our previous work where we argued that this approach can have reduced sensitivity to the details of the production mechanism of the top quark, whether it is higher-order QCD effects or new physics contributions. As part of the NLO improvement over the original proposal, we assess the residual sensitivity of the extracted top quark mass to perturbative effects both in top quark production and decay. For a 1% jet energy scale uncertainty (and assuming negligible statistical error), the top quark mass can then be extracted using the energy-peak of b-jets with an error +- (1.2 (exp) + 0.6(th)) GeV. We note that recently the CMS collaboration reported a top quark mass measurement based on the original proposal (with b-jets) so that our result contributes to a precise evaluation of the associated theory uncertainty. In view of the dominant jet energy scale uncertainty in the measurement using b-jets, we also investigate the extraction of the top quark mass from the energy-peak of the corresponding B-hadrons which, in principle, can be measured without this uncertainty. The calculation of the B-hadron energy spectrum is carried out using fragmentation functions at NLO. The dependence on the fragmentation scale turns out to be the largest theoretical uncertainty in this extraction of top quark mass. Future improvement of the treatment of bottom quark hadronization can reduce this uncertainty, rendering methods based on the B-hadron energy-peak competitive for the top quark mass measurement.

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arXiv2015

Warped Seesaw is Physically Inverted

Warped extra dimensions can address both the Planck-weak and flavor hierarchies of the Standard Model (SM). In this paper we discuss the SM neutrino mass generation in a scenario in which a SM singlet bulk fermion - coupled to the Higgs and the lepton doublet near the IR brane - is given a Majorana mass of order the Planck scale on the UV brane. Despite the resemblance to a type I seesaw mechanism, a careful investigation based on the mass basis for the singlet 4D modes reveals a very different picture. Namely, the SM neutrino masses are generated dominantly by the exchange of the TeV-scale mass eigenstates of the singlet, that are pseudo-Dirac and have a sizable Higgs- induced mixing with the SM doublet neutrino: remarkably, in warped 5D models the anticipated type I seesaw morphs into a natural realization of the so-called "inverse" seesaw. This understanding uncovers an intriguing and direct link between neutrino mass generation (and possibly leptogenesis) and TeV-scale physics. We also perform estimates using the dual CFT picture of our framework, which back-up our 5D calculation.

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arXiv2015

(In)Direct Detection of Boosted Dark Matter

We present a new multi-component dark matter model with a novel experimental signature that mimics neutral current interactions at neutrino detectors. In our model, the dark matter is composed of two particles, a heavier dominant component that annihilates to produce a boosted lighter component that we refer to as boosted dark matter. The lighter component is relativistic and scatters off electrons in neutrino experiments to produce Cherenkov light. This model combines the indirect detection of the dominant component with the direct detection of the boosted dark matter. Directionality can be used to distinguish the dark matter signal from the atmospheric neutrino background. We discuss the viable region of parameter space in current and future experiments.

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arXiv2015

Energy spectra of massive two-body decay products and mass measurement

We have recently established a new method for measuring the mass of unstable particles produced at hadron colliders based on the analysis of the energy distribution of a massless product from their two-body decays. The central ingredient of our proposal is the remarkable result that, for an unpolarized decaying particle, the location of the peak in the energy distribution of the observed decay product is identical to the (fixed) value of the energy that this particle would have in the rest-frame of the decaying particle, which, in turn, is a simple function of the involved masses. In addition, we utilized the property that this energy distribution is symmetric around the location of peak when energy is plotted on a logarithmic scale. The general strategy was demonstrated in several specific cases, including both beyond the SM particles, as well as for the top quark. In the present work, we generalize this method to the case of a massive decay product from a two-body decay; this procedure is far from trivial because (in general) both the above- mentioned properties are no longer valid. Nonetheless, we propose a suitably modified parametrization of the energy distribution that was used successfully for the massless case, which can deal with the massive case as well. We establish the accuracy of this parametrization using concrete examples of energy spectra of Z bosons from the decay of a heavier stop into a Z boson and a lighter stop. We then study a realistic application for the same process, but now including dominant backgrounds and using foreseeable statistics at LHC14, in order to determine the performance of this method for an actual mass measurement. The upshot of our present and previous work is that, in spite of energy being a Lorentz-variant quantity, its distribution emerges as a powerful tool for mass measurement at hadron colliders.

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arXiv2015

A simple, yet subtle "invariance" of two-body decay kinematics

We study the two-body decay of a mother particle into a massless daughter. We further assume that the mother particle is unpolarized and has a generic boost distribution in the laboratory frame. In this case, we show analytically that the laboratory frame energy distribution of the massless decay product has a peak, whose location is identical to the (fixed) energy of that particle in the rest frame of the corresponding mother particle. Given its simplicity and "invariance" under variations of the boost distribution of the mother particle, our finding should be useful for the determination of masses of mother particles. In particular, we anticipate that such a procedure will then not require a full reconstruction of this two-body decay chain (or for that matter, information about the rest of the event). With this eventual goal in mind, we make a proposal for extracting the peak position by fitting the data to a well-motivated analytic function describing the shape of such energy distribution. This fitting function is then tested on the theoretical prediction for top quark pair production and its decay and it is found to be quite successful in this regard. As a proof of principle of the usefulness of our observation, we apply it for measuring the mass of the top quark at the LHC, using simulated data and including experimental effects.

hep-ph↗

arXiv2015

Photon Cascade Decay of the Warped Graviton at LHC14 and a 100 TeV Hadron Collider

In warped 5D models of hierarchy and flavor, the first Kaluza-Klein (KK) state of the graviton $G_1$ is heavy enough to decay into a photon and its first KK mode $γ_1$ on-shell: $G_1 \to γ_1 γ$. The volume-suppression of the rate for this process [relative to 2-body decay into heavy Standard Model (SM) final states ($W/Z/t/H$)] may be partially compensated by the simplicity of the photon final state. We consider $γ_1 \to W^+W^-$, with a typical O(1) branching fraction, and focus on the semi-leptonic final state $W(\to jj) W(\to \ell, ν)$ with $\ell=e,μ$. The SM background originates from $2\to 3$ parton processes and is relatively suppressed compared to those for 2-body decays of $G_1$. Moreover, to further reduce the background, we can impose an invariant mass window cut for $γ_1$ (in addition to that for $G_1$) in this new channel. We emphasize that this "photon cascade" decay probes a different combination of (bulk and brane) interactions of the KK states than the decays into two heavy SM states. Thus, in combination with other channels, the cascade decay could be used to extract the individual underlying geometric parameters. The $3σ$ reach for $G_1$ in our channel is up to 1.5 TeV at the high luminosity (14 TeV) LHC, and can be extended to about 4 TeV, at $5σ$, at a future 100 TeV hadron collider. Along the way, we point out the novel feature that the invariant mass distribution of KK graviton decay products becomes skewed from the Breit-Wigner form, due to the KK graviton coupling growing with energy.

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arXiv2014

(In)direct Detection of Boosted Dark Matter

We initiate the study of novel thermal dark matter (DM) scenarios where present-day annihilation of DM in the galactic center produces boosted stable particles in the dark sector. These stable particles are typically a subdominant DM component, but because they are produced with a large Lorentz boost in this process, they can be detected in large volume terrestrial experiments via neutral-current-like interactions with electrons or nuclei. This novel DM signal thus combines the production mechanism associated with indirect detection experiments (i.e. galactic DM annihilation) with the detection mechanism associated with direct detection experiments (i.e. DM scattering off terrestrial targets). Such processes are generically present in multi-component DM scenarios or those with non-minimal DM stabilization symmetries. As a proof of concept, we present a model of two-component thermal relic DM, where the dominant heavy DM species has no tree-level interactions with the standard model and thus largely evades direct and indirect DM bounds. Instead, its thermal relic abundance is set by annihilation into a subdominant lighter DM species, and the latter can be detected in the boosted channel via the same annihilation process occurring today. Especially for dark sector masses in the 10 MeV-10 GeV range, the most promising signals are electron scattering events pointing toward the galactic center. These can be detected in experiments designed for neutrino physics or proton decay, in particular Super-K and its upgrade Hyper-K, as well as the PINGU/MICA extensions of IceCube. This boosted DM phenomenon highlights the distinctive signatures possible from non-minimal dark sectors.

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