arXiv subjects

Rohini Godbole

Publications and source records attributed to Rohini Godbole.

14 recordsLinked to original sources

arXiv2024

Current status of the light neutralino thermal dark matter in the phenomenological MSSM

In a previous publication, we studied the parameter space of the phenomenological Minimal Supersymmetric Standard Model (pMSSM) with a light neutralino thermal dark matter ($M_{\tilde{\chi}_1^0} \leq M_h/2$) and observed that the recent results from the dark matter and collider experiments put strong constraints on this scenario. In this work, we present in detail the arguments behind the robustness of this result against scanning over the large number of parameters in pMSSM. The Run-3 of LHC will be crucial in probing the surviving regions of the parameter space. We further investigate the impact of light staus on our parameter space and also provide benchmarks which can be interesting for Run-3 of LHC. We analyse these benchmarks at the LHC using the machine learning framework of \texttt{XGBOOST}. Finally, we also discuss the effect of non-standard cosmology on the parameter space.

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arXiv2021

Unveiling Hidden Physics at the LHC

The field of particle physics is at the crossroads. The discovery of a Higgs-like boson completed the Standard Model (SM), but the lacking observation of convincing resonances Beyond the SM (BSM) offers no guidance for the future of particle physics. On the other hand, the motivation for New Physics has not diminished and is, in fact, reinforced by several striking anomalous results in many experiments. Here we summarise the status of the most significant anomalies, including the most recent results for the flavour anomalies, the multi-lepton anomalies at the LHC, the Higgs-like excess at around 96 GeV, and anomalies in neutrino physics, astrophysics, cosmology, and cosmic rays. While the LHC promises up to 4/ab of integrated luminosity and far-reaching physics programmes to unveil BSM physics, we consider the possibility that the latter could be tested with present data, but that systemic shortcomings of the experiments and their search strategies may preclude their discovery for several reasons, including: final states consisting in soft particles only, associated production processes, QCD-like final states, close-by SM resonances, and SUSY scenarios where no missing energy is produced. New search strategies could help to unveil the hidden BSM signatures, devised by making use of the CERN open data as a new testing ground. We discuss the CERN open data with its policies, challenges, and potential usefulness for the community. We showcase the example of the CMS collaboration, which is the only collaboration regularly releasing some of its data. We find it important to stress that individuals using public data for their own research does not imply competition with experimental efforts, but rather provides unique opportunities to give guidance for further BSM searches by the collaborations. Wide access to open data is paramount to fully exploit the LHCs potential.

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arXiv2021

Current bounds and future prospects of light neutralino dark matter in NMSSM

Unlike its minimal counterpart, the Next to Minimal supersymmetric Standard Model (NMSSM) allows the possibility that the lightest neutralino could have a mass as small as $\sim 1 {\rm GeV}$ while still providing a significant component of relic dark matter (DM). Such a neutralino can provide an invisible decay mode to the Higgs as well. Further, the observed SM-like Higgs boson ($H_{125}$) could also have an invisible branching fraction as high as $\sim 19\%$. Led by these facts, we first delineate the region of parameter space of the NMSSM with a light neutralino ($M_{{\tildeχ}_{1}^{0}} < 62.5 {\rm GeV}$) that yields a thermal neutralino relic density smaller than the measured relic density of cold dark matter, and is also compatible with constraints from collider searches, searches for dark matter, and from flavor physics. We then examine the prospects for probing the NMSSM with a light neutralino via direct DM detection searches, via invisible Higgs boson width experiments at future $e^+e^-$ colliders, via searches for a light singlet Higgs boson in $2b2μ$, $2b2τ$ and $2\mu2τ$ channels and via pair production of winos or doublet higgsinos at the high luminosity LHC and its proposed energy upgrade. For this last-mentioned electroweakino search, we perform a detailed analysis to map out the projected reach in the $3l+{\rm E{\!\!\!/}_T}$ channel, assuming that chargino decays to $W {\tildeχ}_{1}^{0}$ and the neutralino(s) decay to $Z$ or $H_{125}$ + ${\tildeχ}_{1}^{0}$. We find that the HL-LHC can discover SUSY in just part of the parameter space in each of these channels, which together can probe almost the entire parameter space. The HE-LHC probes essentially the entire region with higgsinos (winos) lighter than 1 TeV (2 TeV) independently of how the neutralinos decay, and leads to significantly larger signal rates.

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arXiv2020

Jets with electrons from boosted top quarks

We propose a method to identify jets consisting of all the visible remnants of boosted top particles when these decay semileptonically to electrons. Within these jets, the electron shower overlaps with the shower initiated by the $b$ quark, which makes the identification of the electron hard. Even if an electron inside a jet is identified, it is difficult to pinpoint whether the electron rich jet is indeed due to top quark decay or not, since the invisible neutrino carries away a nontrivial part of the energy-momentum of the original top quark. Broadly speaking, the method proposed here has three key components. It uses the distribution of energy in various parts of the detector to identify whether the observed jet is consistent with a jet containing an energetic electron. It uses the substructure of the jet to determine the momentum associated with the electron. Finally, it constructs new variables that carry tell-tale features of top quark decay kinematics using an extra ansatz that, there exists a massless invisible four-momentum \emph{roughly} collimated to the electron, which reconstructs a $W$ and a top when it is combined with the electron and the full jet respectively. We demonstrate the efficacy of this proposal using simulated data and show that our method not only reduces the backgrounds from light flavor jets, $b$ jets from QCD, and hadronic top jets, it can also tell apart jets rich in electrons but not due to top quark decays.

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arXiv2019

Boosted Top quark polarization

In top quark production, the polarization of top quarks, decided by the chiral structure of couplings, is likely to be modified in the presence of any new physics contribution to the production. Hence the same is a good discriminator for those new physics models wherein the couplings have a chiral structure different than that in the Standard Model (SM). In this note we construct probes of the polarization of a top quark decaying hadronically, using easily accessible kinematic variables such as the energy fraction or angular correlations of the decay products. Tagging the boosted top quark using the usual jet sub structure technique we study robustness of these observables for a benchmark process, $W^{\prime} \to tb$. We demonstrate that the energy fraction of b-jet in the laboratory frame and a new angular variable, constructed by us in the top rest frame, are both very powerful tools to discriminate between the left and right polarized top quarks. Based on the polarization sensitive angular variables, we construct asymmetries which reflect the polarization. We study the efficiency of these variables for two new physics processes where which give rise to boosted top quarks: (i) decay of the top squark in the context of supersymmetry searches, and (ii) decays of the Kaluza-Klein(KK) graviton and KK gluon, in Randall Sundrum(RS) model. Remarkably, it is found that the asymmetry can vary over a wide range about +20\% to -20\%. The dependence of asymmetry on top quark couplings of the new particles present in these models beyond the SM (BSM) is also investigated in detail.

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arXiv2019

Determining the space-time structure of bottom-quark couplings to spin-zero particles

We present a general argument that highlights the difficulty of determining the space-time structure of the renormalizable bottom quark Yukawa interactions of the Standard Model Higgs boson, or for that matter of any hypothetical spin-zero particle, at high energy colliders. The essence of the argument is that, it is always possible, by chiral rotations, to transform between scalar and pseudoscalar Yukawa interactions without affecting the interactions of bottom quarks with SM gauge bosons. Since these rotations affect only the $b$-quark mass terms in the Standard Model Lagrangian, any differences in observables for scalar versus pseudoscalar couplings vanish when $m_b \rightarrow 0$, and are strongly suppressed in high energy processes involving the heavy spin-zero particle where the $b$-quarks are typically relativistic. We show, however, that the energy dependence of, for instance, $e^+e^- \rightarrow b\bar{b} X$ (here $X$ denotes the spin-zero particle) close to the reaction threshold may serve to provide a distinction between the scalar versus pseudoscalar coupling at electron-positron colliders that are being proposed, provided that the $Xb\bar{b}$ coupling is sizeable. We also note that while various kinematic distributions for $t \bar{t} h$ are indeed sensitive to the space-time structure of the top Yukawa coupling, for a spin-0 particle $X$ of an arbitrary mass, the said sensitivity is lost if $m_{X} >> m_t$.

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arXiv2018

Proceedings of the 2014 Asia-Europe-Pacific School of High-Energy Physics

The Asia-Europe-Pacific School of High-Energy Physics is intended to give young physicists an introduction to the theoretical aspects of recent advances in elementary particle physics. These proceedings contain lecture notes on quantum field theory and the electroweak Standard Model, the theory of quantum chromodynamics, flavour physics and CP violation, neutrino physics, heavy-ion physics, cosmology and a brief introduction to the principles of instrumentation and detectors for particle physics.

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arXiv2017

The invisible decay of Higgs boson in the context of a thermal and non-thermal relic in MSSM

We study the decay of 125 GeV Higgs boson to light LSP neutralino in the phenomenological minimal supersymmetric standard model in the context of collider searches and astrophysical experiments. We consider the parameter space for light neutralinos that can be probed via the invisible Higgs decays and higgsino searches at the ILC. We consider the cases where the light neutralino is compatible with the observed relic density or where the thermal relic is over-abundant, pointing to non-standard cosmology. In the former case, when the neutralino properties give rise to under-abundant relic density, the correct amount of relic abundance is assumed to be guaranteed by either additional DM particles or by non-thermal cosmology. We contrast these different cases. We assess what astrophysical measurements can be made, in addition to the measurements made at the ILC, which can provide a clue to the nature of the light neutralino. We find that a number of experiments, including Xenon-nT, PICO-250, LZ in conjunction with measurements made at the ILC on invisible Higgs width can pin down the nature of this neutralino, along with its cosmological implications. Additionally, we also point out potential LHC signatures that could be complementary in this region of parameter space.

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arXiv2016

Future Collider Signatures of the Possible 750 GeV State

If the recent indications of a possible state $Φ$ with mass $\sim 750$ GeV decaying into two photons reported by ATLAS and CMS in LHC collisions at 13 TeV were to become confirmed, the prospects for future collider physics at the LHC and beyond would be affected radically, as we explore in this paper. Even minimal scenarios for the $Φ$ resonance and its $γγ$ decays require additional particles with masses $\gtrsim \frac12 m_Φ$. We consider here two benchmark scenarios that exemplify the range of possibilities: one in which $Φ$ is a singlet scalar or pseudoscalar boson whose production and $γγ$ decays are due to loops of coloured and charged fermions, and another benchmark scenario in which $Φ$ is a superposition of (nearly) degenerate CP-even and CP-odd Higgs bosons in a (possibly supersymmetric) two-Higgs doublet model also with additional fermions to account for the $γγ$ decay rate. We explore the implications of these benchmark scenarios for the production of $Φ$ and its new partners at colliders in future runs of the LHC and beyond, at higher-energy $pp$ colliders and at $e^+ e^-$ and $γγ$ colliders, with emphasis on the bosonic partners expected in the doublet scenario and the fermionic partners expected in both scenarios.

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arXiv2015

Light stop in the MSSM after LHC Run 1

The discovery of a Higgs boson with a mass of 126 GeV at the LHC when combined with the non-observation of new physics both in direct and indirect searches imposes strong constraints on supersymmetric models and in particular on the top squark sector. The experiments for direct detection of dark matter have provided with yet more constraints on the neutralino LSP mass and its interactions. After imposing limits from the Higgs, flavour and dark matter sectors, we examine the feasibility for a light stop in the context of the pMSSM, in light of current results for stop and other SUSY searches at the LHC. We only require that the neutralino dark matter explains a fraction of the cosmologically measured dark matter abundance. We find that a stop with mass below $\sim$ 500 GeV is still allowed. We further study various probes of the light stop scenario that could be performed at the LHC Run - II either through direct searches for the light and heavy stop, or SUSY searches not currently available in simplified model results. Moreover we study the characteristics of heavy Higgs for the points in the parameter space allowed by all the available constraints and illustrate the region with large cross sections to fermionic or electroweakino channels. Finally we show that nearly all scenarios with a small stop$-$LSP mass difference will be tested by Xenon1T provided the NLSP is a chargino, thus probing a region hard to access at the LHC.

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arXiv2013

Probing the flavor violating scalar top quark signal at the LHC

The Large Hadron Collider(LHC) has completed its run at 8 TeV with the experiments ATLAS and CMS having collected about 25 $\rm fb^{-1}$ of data each. Discovery of a light Higgs boson, coupled with lack of evidence for supersymmetry at the LHC so far, has motivated studies of supersymmetry in the context of naturalness with the principal focus being the third generation squarks. In this work, we analyze the prospects of the flavor violating decay mode $\rm \tilde{t}_1\to cχ_{1}^{0}$ at 8 and 13 TeV center of mass energy at the LHC. This channel is also relevant in the dark matter context for the stop-coannihilation scenario, where the relic density depends on the mass difference between the lighter stop quark ($\tilde{t}_1$) and the lightest neutralino($χ_{1}^{0}$) states. This channel is extremely challenging to probe, specially for situations when the mass difference between the lighter stop quark and the lightest neutralino is small. Using certain kinematical properties of signal events we find that the level of backgrounds can be reduced substantially. We find that the prospect for this channel is limited due to the low production cross section for top squarks and limited luminosity at 8 TeV, but at the 13 TeV LHC with 100 $fb^{-1}$ luminosity, it is possible to probe top squarks with masses up to $\sim$ 450 GeV. We also discuss how the sensitivity could be significantly improved by tagging charm jets.

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arXiv2013

Looking for an Invisible Higgs Signal at the LHC

While the recent discovery of a Higgs-like boson at the LHC is an extremely important and encouraging step towards the discovery of the {\it complete} standard model(SM), the current information on this state does not rule out possibility of beyond standard model (BSM) physics. In fact the current data can still accommodate reasonable values of the branching fractions of the Higgs into a channel with `invisible' decay products, such a channel being also well motivated theoretically. In this study we revisit the possibility of detecting the Higgs in this invisible channel for both choices of the LHC energies, 8 and 14 TeV, for two production channels; vector boson fusion(VBF) and associated production($ZH$). In the latter case we consider decays of the $Z$ boson into a pair of leptons as well as a $b \bar b$ pair. For the VBF channel the sensitivity is found to be more than $5 σ$ at both the energies up to an invisible branching ratio ${\cal B}r_{invis} \sim 0.80$, with luminosities $\sim 20/30 {\rm fb}^{-1}$. The sensitivity is further extended to values of ${\cal B}r_{invis} \sim 0.25$ for $300 {\rm fb}^{-1}$ at 14 TeV. However the reach is found to be more modest for the $ZH$ mode with leptonic final state; with about $3.5 σ$ for the planned luminosity at 8 TeV, reaching $8 σ$ only for 14 TeV for $50 {\rm fb}^{-1}$. In spite of the much larger branching ratio of the $Z$ into a $b \bar b$ channel compared to the dilepton case, the former channel, can provide useful reach upto ${\cal B}r_{invis} \gsim 0.75$, only for the higher luminosity ($300{\rm fb}^{-1}$) option using jet-substructure and jet clustering methods for $b$-jet identification.

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arXiv2013

Boosting Higgs CP properties via VH Production at the Large Hadron Collider

We consider ZH and WH production at the Large Hadron Collider, where the Higgs decays to a bb pair. We use jet substructure techniques to reconstruct the Higgs boson and construct angular observables involving leptonic decay products of the vector bosons. These efficiently discriminate between the tensor structure of the HVV vertex expected in the Standard Model and that arising from possible new physics, as quantified by higher dimensional operators. This can then be used to examine the CP nature of the Higgs as well as CP mixing effects in the HZZ and HWW vertices separately.

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arXiv2011

Clarifications on the impact of theoretical uncertainties on the Tevatron Higgs exclusion limits

In this note, we respond to the comments and criticisms made by the representatives of the CDF and D0 collaborations on our recent papers in which we point out that the theoretical uncertainties in the Higgs production cross section have been largely un- derestimated and, if properly taken into account, will significantly loosen the Tevatron Higgs exclusion bounds. We show that our approach to the theoretical uncertainties is reasonable and fully justified. In particular, we show that our procedure is not very different from that adopted by the LHC experiments and if the latter is used in the Tevatron case, one obtains much larger uncertainties that those assumed by the CDF and D0 collaborations. Furthermore, we provide additional details on our statistical analysis of the CDF and D0 exclusion limit and show that it is conceptually correct.

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