arXiv subjects

Kaustubh Agashe

Publications and source records attributed to Kaustubh Agashe.

At least 19 recordsLinked to original sources

arXiv2026

Probing New Degrees of Freedom with the Universal Tail of Primordial Black Hole Mass Functions

The primordial black hole (PBH) mass function today develops a low-mass evaporation tail whose shape is universal with respect to the initial PBH mass distribution. This universality is fixed by the continuity equation and the Hawking mass-loss rate, but the tail is distorted if additional particle degrees of freedom participate in Hawking evaporation. Since this tail controls high-energy PBH photon emission, such distortions leave characteristic features in the gamma-ray spectrum. We show that these features provide a robust probe of new degrees of freedom, even for subdominant PBHs, within reach of future experiments and independent of visible-sector couplings or relic abundance.

hep-ph↗

arXiv2025

Searching for hadronic scale baryonic and dark forces at $(g-2)_μ$'s lattice-vs-dispersion front

The anomalous magnetic moment of the muon ($\,a_μ\,$) provides a stringent test of the quantum nature of the Standard Model (SM) and its extensions. To probe beyond the SM physics, one needs to be able to subtract the SM contributions, which consists of a non-perturbative part, namely, the hadronic vacuum polarization (HVP) of the photon. The state of the art is to predominantly use two different methods to extract this HVP: lattice computation, and dispersion relation-based, data-driven method. Thus one can construct different forms of the ``$a_μ$ test" which compares the precise measurement of $a_μ$ to its theory prediction. Additionally, this opens the possibility for another subtle test, where these two ``theory" predictions themselves are compared against each other, which is denoted as the ``HVP-test". This test is particularly sensitive to hadronic scale new physics. Therefore, in this work, we consider a SM extension consisting of a generic, light $\sim(100~{\rm MeV}-1~{\rm GeV})$ vector boson and study its impact on both tests. We develop a comprehensive formalism for this purpose. We find that in the case of data-driven HVP being used in the $a_μ$ test, the new physics contributions effectively cancels for a flavor-universal vector boson. As an illustration of these general results, we consider two benchmark models: i)~the dark photon ($\,A'\,$) and ii)~a gauge boson coupled to baryon-number ($\,B\,$). Using a combination of these tests, we are able to constrain the parameter space of $B$ and $A'$, complementarily to the existing limits. As a spin-off, our preliminary analysis of the spectrum of invariant mass of $3π$ in events with ISR at the $B-$ factories (BaBar, Belle) manifests the value of such a study in searching for $B\to 3π$ decay, thus motivating a dedicated search by experimental collaborations.

hep-ph↗

arXiv2025

Detecting Axion-Like Particles with Primordial Black Holes

Future gamma-ray experiments, such as the e-ASTROGAM and AMEGO telescopes, can detect the Hawking radiation of photons from primordial black holes (PBHs) if they make up a fraction or all of dark matter. PBHs can analogously also Hawking radiate new particles, which is especially interesting if these particles are mostly secluded from the Standard Model (SM) sector, since they might therefore be less accessible otherwise. A well-motivated example of this type is axion-like particles (ALPs) with a tiny coupling to photons. We assume that the ALPs produced by PBHs decay into photons well before reaching the earth, so these will augment the photons directly radiated by the PBHs. Remarkably, we find that the peaks in the energy distributions of ALPs produced from PBHs are different than the corresponding ones for Hawking radiated photons due to the spin-dependent greybody factor. Therefore, we demonstrate that this process will in fact distinctively modify the PBHs' gamma-ray spectrum relative to the SM prediction. We use monochromatic asteroid-mass PBHs as an example to show that e-ASTROGAM can observe the PBH-produced ALP gamma-ray signal (for masses up to ~60 MeV) and further distinguish it from Hawking radiation without ALPs. By measuring the gamma-ray signals, e-ASTROGAM can thereby probe yet unexplored parameters in the ALP mass and photon coupling.

hep-ph↗

arXiv2025

Partial Compositeness: from Anarchy to Symmetry

Within the Composite Higgs paradigm, Partial Compositeness has emerged as an elegant mechanism for generating large flavor hierarchies such as are observed in the quark and lepton masses and mixings. This mechanism exploits the strong renormalization group effects of the compositeness dynamics when these are {\it not} flavor-symmetric. Despite its remarkable properties, at this point it is stringently constrained by the body of flavor- and CP-violation tests, so that the compositeness scale must be at least $O(100)$ TeV, beyond the direct reach of proposed colliders. On the other hand, Composite Higgs theories with flavor-symmetric strong dynamics, but with realistic flavor-violating hierarchies introduced in an ad hoc manner, can extend the GIM mechanism of the standard model and thereby be far less constrained, at the edge of LHC reach and well within reach of future colliders. We show how the best features of both these types of dynamics can be combined if flavor-symmetries of the strong composite dynamics are emergent in the IR near the compositeness scale but absent in the far UV. In this case, flavor hierarchies can be generated by the renormalization group flow in the UV, followed by an IR stage in which the dynamics flows towards accidental flavor and CP symmetries. We point out how the collider and low-energy phenomenology is significantly impacted by the IR stage. Our analysis includes a discussion of the distinctive features of the small neutrino masses and their large mixings.

hep-ph↗

arXiv2025

Conformal Leptogenesis in Composite Higgs Models

We study the generation of the baryon asymmetry in Composite Higgs models with partial compositeness of the Standard Model (SM) fermions and heavy right-handed neutrinos, developing for the first time a complete picture of leptogenesis in that setup. The asymmetry is induced by the out of equilibrium decays of the heavy right-handed neutrinos into a plasma of the nearly conformal field theory (CFT), i.e. the deconfined phase of the Composite Higgs dynamics. This exotic mechanism, which we call Conformal Leptogenesis, admits a reliable description in terms of a set of ``Boltzmann equations'' whose coefficients can be expressed in terms of correlation functions of the CFT. The asymmetry thus generated is subsequently affected by the supercooling resulting from the confining phase transition of the strong Higgs sector as well as by the washout induced by the resonances formed after the transition. Nevertheless, a qualitative description of the latter effects suggests that conformal leptogenesis can successfully reproduce the observed baryon asymmetry in a wide region of parameter space. A distinctive signature of our scenarios is a sizable compositeness for all the generations of SM neutrinos, which is currently consistent with all constraints but may be within reach of future colliders.

hep-ph↗

arXiv2024

Light in the Shadows: Primordial Black Holes Making Dark Matter Shine

We consider the possibility of indirect detection of dark sector processes by investigating a novel form of interaction between ambient dark matter (DM) and primordial black holes (PBHs). The basic scenario we envisage is that the ambient DM is ``dormant'', \ie, it has interactions with the SM, but its potential for an associated SM signal is not realized for various reasons. We argue that the presence of PBHs with active Hawking radiation (independent of any DM considerations) can act as a catalyst in this regard by overcoming the aforementioned bottlenecks. The central point is that PBHs radiate all types of particles, whether in the standard model (SM) or beyond (BSM), which have a mass at or below their Hawking temperature. The emission of such radiation is ``democratic" (up to the particle spin), since it is based on a coupling of sorts of gravitational origin. In particular, such shining of (possibly dark sector) particles onto ambient DM can then activate the latter into giving potentially observable SM signals. We illustrate this general mechanism with two specific models. First, we consider asymmetric DM, which is characterized by an absence of ambient anti-DM, and consequently the absence of DM indirect detection signals. In this case, PBHs can ``resurrect'' such a signal by radiating anti-DM, which then annihilates with ambient DM in order to give SM particles such as photons. In our second example, we consider the PBH emission of dark gauge bosons which can excite ambient DM into a heavier state (which is, again, not ambient otherwise), this heavier state later decays back into DM and photons. Finally, we demonstrate that we can obtain observable signals of these BSM models from asteroid-mass PBHs (Hawking radiating currently with $\sim \mathcal{O}(\mathrm{MeV})$ temperatures) at gamma-ray experiments such as AMEGO-X.

hep-ph↗

arXiv2024

"Unification" of BSM Searches and SM Measurements: the case of lepton$+MET$ and $m_W$

We develop the idea that the unprecedented precision in Standard Model (SM) measurements, with further improvement at the HL-LHC, enables new searches for physics Beyond the Standard Model (BSM).As an illustration, we demonstrate that the measured kinematic distributions of the lepton$+MET$ final state not only determine the mass of the $W$ boson, but are also sensitive to light new physics. Such a search for new physics thus requires a simultaneous fit to the BSM and SM parameters, "unifying" searches and measurements at the LHC and Tevatron. In this paper, we complete the program initiated in our earlier work arXiv:2310.13687. In particular, we analyze ($i$) novel decay modes of the $W$ boson with a neutrinophilic invisible scalar or with a heavy neutrino; ($ii$) modified production of $W$ bosons, namely, associated with a hadrophilic invisible $Z^\prime$ gauge boson; and ($iii$) scenarios without an on-shell $W$ boson, such as slepton-sneutrino production in the Minimal Supersymmetric Standard Model (MSSM). Here, we complement our previous MSSM analysis in arXiv:2310.13687 by considering a different kinematic region. Our results highlight that new physics can still be directly discovered at the LHC, including light new physics,via SM precision measurements. Furthermore, we illustrate that such BSM signals are subtle, yet potentially large enough to affect the precision measurements of SM parameters themselves, such as the $W$ boson mass.

hep-ph↗

arXiv2023

A new purpose for the $W$-boson mass measurement: searching for New Physics in lepton+$MET$

We show that the $m_W$ measurement is a direct probe of New Physics (NP) contributing to lepton and missing transverse momentum ($\ell+MET$), independently from indirect tests via the electroweak fit. Such NP modifies the kinematic distributions used to extract $m_W$, necessitating a simultaneous fit to $m_W$ and NP. This effect can in principle bias the $m_W$ measurement, but only to a limited extent for our considered models. Given that, we demonstrate that the agreement at high-precision with SM-predicted shapes results in bounds competitive to, if not exceeding, existing ones for two examples: anomalous $W$ decay involving a $L_μ - L_τ$ gauge boson and $\tildeν_{l} \tilde{l}$ production in the MSSM.

hep-ph↗

arXiv2023

Energy-peak based method to measure top quark mass via B-hadron decay lengths

We develop a method for the determination of the top quark mass using the distribution of the decay length of the $B$-hadrons originating from its decay. This technique is based on our earlier observation regarding the location of the peak of the $b$ quark energy distribution. Such "energy-peak" methods enjoy a greater degree of model-independence with respect to the kinematics of top quark production compared to earlier proposals. The CMS experiment has implemented the energy-peak method using associated $b$-jet energy as an approximation for $b$ quark energy. The new method uses $B$-hadron decay lengths, which are related to $b$ quark energies by convolution. The advantage of the new decay length method is that it can be applied in a way that evades jet-energy scale (JES) uncertainties. Indeed, CMS has measured the top quark mass using $B$-hadron decay lengths, but they did not incorporate the energy-peak method. Therefore, mismodeling of top quark transverse momentum remains a large uncertainty in their result. We demonstrate that, using energy-peak methods, this systematic uncertainty can become negligible. We show that with the current LHC data set, a sub-GeV statistical uncertainty on the top quark mass can be attained with this method. To achieve a comparable systematic uncertainty as is true for many methods based on exclusive or semi-inclusive observables using hadrons, we find that the quark-hadron transition needs to be described significantly better than is the case with current fragmentation functions and hadronization models.

hep-ph↗

arXiv2023

The International Linear Collider: Report to Snowmass 2021

The International Linear Collider (ILC) is on the table now as a new global energy-frontier accelerator laboratory taking data in the 2030s. The ILC addresses key questions for our current understanding of particle physics. It is based on a proven accelerator technology. Its experiments will challenge the Standard Model of particle physics and will provide a new window to look beyond it. This document brings the story of the ILC up to date, emphasizing its strong physics motivation, its readiness for construction, and the opportunity it presents to the US and the global particle physics community.

physics.acc-ph↗

arXiv2023

Correlating Gravitational Wave and Gamma-ray Signals from Primordial Black Holes

Asteroid-mass primordial black holes (PBH) can explain the observed dark matter abundance while being consistent with the current indirect detection constraints. These PBH can produce gamma-ray signals from Hawking radiation that are within the sensitivity of future measurements by the AMEGO and e-ASTROGAM experiments. PBH which give rise to such observable gamma-ray signals have a cosmic origin from large primordial curvature fluctuations. There must then be a companion, stochastic gravitational wave (GW) background produced by the same curvature fluctuations. We demonstrate that the resulting GW signals will be well within the sensitivity of future detectors such as LISA, DECIGO, BBO, and the Einstein Telescope. The multi-messenger signal from the observed gamma-rays and GW will allow a precise measurement of the primordial curvature perturbation that produces the PBH. Indeed, we argue that the resulting correlation between the two types of observations can provide a smoking-gun signal of PBH.

astro-ph.CO↗

arXiv2022

Report of the Topical Group on Top quark physics and heavy flavor production for Snowmass 2021

This report summarizes the work of the Energy Frontier Topical Group on EW Physics: Heavy flavor and top quark physics (EF03) of the 2021 Community Summer Study (Snowmass). It aims to highlight the physics potential of top-quark studies and heavy-flavor production processes (bottom and charm) at the HL-LHC and possible future hadron and lepton colliders and running scenarios.

hep-ph↗

arXiv2022

Report of the Topical Group on Physics Beyond the Standard Model at Energy Frontier for Snowmass 2021

This is the Snowmass2021 Energy Frontier (EF) Beyond the Standard Model (BSM) report. It combines the EF topical group reports of EF08 (Model-specific explorations), EF09 (More general explorations), and EF10 (Dark Matter at Colliders). The report includes a general introduction to BSM motivations and the comparative prospects for proposed future experiments for a broad range of potential BSM models and signatures, including compositeness, SUSY, leptoquarks, more general new bosons and fermions, long-lived particles, dark matter, charged-lepton flavor violation, and anomaly detection.

hep-ph↗

arXiv2022

Sleptonic SUSY: From UV Framework to IR Phenomenology

We study an attractive scenario, "Sleptonic SUSY", which reconciles the $125$ GeV Higgs scalar and the non-observation of superpartners thus far with potentially pivotal roles for slepton phenomenology: providing viable ongoing targets for LHC discovery, incorporating a co-annihilation partner for detectable thermal relic dark matter, and capable of mediating the potential muon $g-2$ anomaly. This is accomplished by a modestly hierarchical spectrum, with sub-TeV sleptons and electroweakinos and with multi-TeV masses for the other new states. We study new elements in the UV MSSM realization of Sleptonic SUSY based on higher-dimensional sequestering and the synergy between the resulting gaugino-mediation, hypercharge $D$-term mediation and Higgs-mediation of SUSY-breaking, so as to more fully capture the range of possibilities. This framework stands out by harmoniously solving the flavor, CP and $μ- Bμ$ problems of the supersymmetric paradigm. We discuss its extension to orbifold GUTs, including gauge-coupling and $b$-tau unification. We also develop a non-minimal model with extra Higgs fields, in which the electroweak vacuum is more readily cosmologically stable against decay to a charge-breaking vacuum, allowing a broader range of sleptonic spectra than in the MSSM alone. We survey the rich set of signals possible at the LHC and future colliders, covering both $R$-parity conservation and violation, as well as for dark matter detection. While the multi-TeV squarks imply a Little Hierarchy Problem, intriguingly, small changes in parameter space to improve naturalness result in dramatic phase transitions to either electroweak-preservation or charge-breaking. In a Multiverse setting, the modest unnaturalness may then be explained by the "principle of living dangerously".

hep-ph↗

arXiv2022

Snowmass2021 - White Paper, Implications of Energy Peak for Collider Phenomenology: Top Quark Mass Determination and Beyond

We first review the decade-old, broad collider physics research program dubbed energy-peaks. We consider the energy distribution of a massless particle in the lab frame arising from the two-body decay of a heavy particle produced unpolarized, whose boost distribution is arbitrary. Remarkably, the location of the peak of this child particle's energy distribution is identical to its single-valued energy in the rest frame of the parent, which is a function of the parent's mass and that of the other decay product. We summarize generalizations to other types of decay and a variety of applications to BSM. The energy-peak idea can also furnish a measurement of the top quark via the energy of the bottom quark from its decay, which, based on the "parent-boost-invariance," is less sensitive to details of the production mechanism of the top quark (cf.~most other methods assume purely SM production of the top quarks, hence are subject to uncertainties therein, including a possible BSM contribution). The original proposal along this line was to simply use the $b$-jet energy as a very good approximation to the bottom quark energy. This method has been successfully implemented by the CMS collaboration. However, the $b$-jet energy-peak method is afflicted by the jet-energy scale (JES) uncertainty. Fortunately, this drawback can be circumvented by using the decay length of a $B$-hadron contained in the $b$-jet as a proxy for the bottom quark energy. An interesting proposal is to then appropriately dovetail the above two ideas resulting in a "best of both worlds" determination of the top quark mass, i.e., based on a measurement of the $B$-hadron decay length, but improved by the energy-peak concept: this would be free of JES uncertainty and largely independent of the top quark production model. We summarize here the results of such an analysis which is to appear in a forthcoming paper.

hep-ph↗

arXiv2022

Snowmass2021 White Paper: Collider Physics Opportunities of Extended Warped Extra-Dimensional Models

While the warped extra-dimensional models provide an attractive solution to both the gauge and the flavor hierarchy problems, the mass scale of new particles predicted by the minimal models would be beyond the reach of the LHC. Models of extended warped extra dimensions have been proposed to evade these issues and their collider implications have been investigated for the last decade. This white paper summarizes the recent developments in the context of collider phenomenology. The strategies and lessons are broad, and provide a template to extend the experimental program, to cover a wider class of signals in other new physics scenarios as well.

hep-ph↗

arXiv2021

Phase Transitions from the Fifth Dimension

We study the cosmological transition of 5D warped compactifications, from the high-temperature black-brane phase to the low-temperature Randall-Sundrum I phase. The transition proceeds via percolation of bubbles of IR-brane nucleating from the black-brane horizon. The violent bubble dynamics can be a powerful source of observable stochastic gravitational waves. While bubble nucleation is non-perturbative in 5D gravity, it is amenable to semiclassical treatment in terms of a "bounce" configuration interpolating between the two phases. We demonstrate how such a bounce configuration can be smooth enough to maintain 5D effective field theory control, and how a simple ansatz for it places a rigorous lower-bound on the transition rate in the thin-wall regime, and gives plausible estimates more generally. When applied to the Hierarchy Problem, the minimal Goldberger-Wise stabilization of the warped throat leads to a slow transition with significant supercooling. We demonstrate that a simple generalization of the Goldberger-Wise potential modifies the IR-brane dynamics so that the transition completes more promptly. Supercooling determines the dilution of any (dark) matter abundances generated before the transition, potentially at odds with data, while the prompter transition resolves such tensions. We discuss the impact of the different possibilities on the strength of the gravitational wave signals. Via AdS/CFT duality the warped transition gives a theoretically tractable holographic description of the 4D Composite Higgs (de)confinement transition. Our generalization of the Goldberger-Wise mechanism is dual to, and concretely models, our earlier proposal in which the composite dynamics is governed by separate UV and IR RG fixed points. The smooth 5D bounce configuration we introduce complements the 4D dilaton/radion dominance derivation presented in our earlier work.

hep-th↗

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.

hep-ph↗