B. Mukhopadhyaya - Harish-Chandra Research Institute

B. Mukhopadhyaya
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B. Mukhopadhyaya
Harish-Chandra Research Institute

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High Energy Physics - Phenomenology (49)
High Energy Physics - Experiment (22)
High Energy Physics - Theory (3)
General Relativity and Quantum Cosmology (1)
High Energy Astrophysical Phenomena (1)

Publications Authored By B. Mukhopadhyaya

Presence of the light gravitino as dark matter candidate in a supersymmetric (SUSY) model opens up interesting collider signatures consisting of one or more hard photons together with multiple jets and missing transverse energy from the cascade decay. We investigate such signals at the 13 TeV LHC in presence of compressed SUSY spectra, consistent with the Higgs mass as well as collider and dark matter constraints. We analyse and compare the discovery potential in different benchmark scenarios consisting of both compressed and uncompressed SUSY spectra, considering different levels of compression and intermediate decay modes. Read More

It is possible to ameliorate the Higgs vacuum stability problem by switching over to two Higgs doublet models (2HDM), ensuring a stable electroweak vacuum up to the Planck scale, even though the top quark mass may be on the high side. However, the simultaneous requirements of perturbative unitarity, and also compatibility with collider and flavour data, constrain the parameter space severely. We investigate the collider signals answering to the regions allowed by such constraints. Read More

In the light of the 125 GeV Higgs ($h$) discovery at the Large Hadron Collider (LHC), one of the primary goals of the LHC and possible future colliders is to understand its interactions more precisely. Here we have studied the $h$-$b$-$\bar b$-$\gamma$ effective interaction terms arising out of gauge invariant dimension six operators in a model independent setting, as a potential source of new physics. Their role in some detectable final states have been compared with those coming from anomalous $h$-$b$-$\bar b$ interactions. Read More

We study the prospects of measuring the CP property of the Higgs ($h$) coupling to tau leptons using the vector boson fusion (VBF) production mode at the high-luminosity LHC. Utilizing the previously proposed angle between the planes spanned by the momentum vectors of the $(\pi^+\pi^0)$ and $(\pi^- \pi^0)$ pairs originating in $\tau^\pm$ decays as the CP-odd observable, we perform a detailed Monte Carlo analysis, taking into account the relevant standard model backgrounds, as well as detector resolution effects. We find that excluding a pure CP-odd coupling hypothesis requires $\mathcal{O}(400 {~\rm fb}^{-1})$ luminosity at the 14 TeV LHC, and values of the CP-mixing angle larger than about $25^\circ$ can be excluded at $95\%$ confidence level using $3 {~\rm ab}^{-1}$ data. Read More

We show that the study of scalar resonances at various vector boson scattering processes at the Large Hadron Collider can serve as a useful tool to distinguish between different extensions of the scalar sector of the Standard Model. The recent measurement of the Higgs boson properties leaves enough room for the extended scalar sectors to be relevant for such studies. The shape of the resonances, being model dependent, can shed light on the viable parameter space of a number of theoretical models. Read More

The confirmation of the Higgs boson in Run I data at the Large Hadron Collider (LHC) and the excesses in recent Run II data suggest scenarios beyond the Standard Model (SM). We pursue a study in a minimal model which is an extension of a scalar doublet in the SM known as two-Higgs doublet model (THDM). Following earlier suggestions two real scalars $\chi$ and $S$ have been introduced in the THDM where $\chi$ is treated as a candidate for dark matter. Read More

We study the feasibility and advantages of investigating the $c\to s$ decay channel of the $B_c$ meson through the cascade decay $B_c\to B_s\pi$, $B_s\to D_s\ell\nu$. We take into account possible contaminations coming from the vector excitations (like $B_s^*$, $D_s^*$ or $\rho$ in the intermediate stages of the cascade) too, as well as the opposite cascade $B_c\to B_s\ell\nu$, $B_s\to D_s\pi$. We show how the $p_T$ distribution of the pion and the lepton can possibly help to differentiate between various form factor models, for which we either need more integrated luminosity at the LHC, or a high-luminosity $e^+e^-$ B factory producing $B_c$ mesons too. Read More

We investigate the search prospects for new scalars beyond the Standard Model (SM) at the Large Hadron Collider (LHC). In these studies two real scalars $S$ and $\chi$ have been introduced in a two Higgs doublet model (2HDM), where $S$ is a portal to dark matter (DM) through its interaction with $\chi$, a DM candidate and a possible source of missing transverse energy (\MET). Previous studies focused on a heavy scalar $H$ decay mode $H \to h\chi\chi$, which was studied using an effective theory in order to explain a distortion in the Higgs ($h$) transverse momentum spectrum [16]. Read More

The Kalb-Ramond field, identifiable with bulk torsion in a five-dimensional Randall Sundrum (RS) scenario, has Chern-Simons interactions with gauge bosons, from the requirement of gauge anomaly cancellation. Its lowest Kaluza Klein (KK) mode on the visible 3-brane can be identified with a spin-0 CP-odd field, namely, the axion. By virtue of the warped geometry and Chern-Simons couplings, this axion has unsuppressed interactions with gauge bosons in contrast to ultra-suppressed interactions with fermions. Read More

Current data (LHC direct searches, Higgs mass, dark matter-related bounds) severely affect the constrained minimal SUSY standard model (CMSSM) with neutralinos as dark matter candidates. But the evidence for neutrino masses coming from oscillations requires extending the SM with at least right-handed neutrinos with a Dirac mass term. In turn, this implies extending the CMSSM with right-handed sneutrino superpartners, a scenario we dub $\tilde\nu$CMSSM. Read More

We consider the observable effects of CP-violating anomalous $ZZh$ interaction arising from gauge invariant dimension-6 operators at the Large Hadron Collider (LHC), with the purpose of distinguishing them from not only the standard model effects but also those of CP-even anomalous interactions of similar nature. The postulation of a gauge invariant origin makes various couplings of this kind interrelated. The updated constraints from the LHC as well as limits from neutron and electron dipole moments are used in selecting the benchmark interaction strengths. Read More

We make an attempt to identify regions in a Type II Two-Higgs Doublet Model, which correspond to a metastable electroweak vacuum with lifetime larger than the age of the universe. We analyse scenarios which retain perturbative unitarity up to Grand unification and Planck scales. Each point in the parameter space is restricted using Data from the Large Hadron Collider (LHC) as well as flavor and precision electroweak constraints. Read More

In this study we consider an effective model by introducing two hypothetical real scalars, $H$ and $\chi$ - a dark matter candidate, where the masses of these scalars are $2 m_h < m_H < 2 m_t$ and $m_\chi \approx m_h/2$ with $m_h$ and $m_t$ being the Standard Model Higgs boson and top quark masses respectively. A distortion in the transverse momentum distributions of $h$ in the intermediate region of the spectrum through the processes $p p \to H \to h\chi\chi$ could be observed in this model. An additional scalar, $S$, has been postulated to explain large $H \to h\chi\chi$ branching ratios, assuming $m_h \lesssim m_S \lesssim m_H-m_h$ and $m_S > 2 m_\chi$. Read More

We consider a scenario where, along with the usual Higgs doublet, two scalar triplets are present. The extension of the triplet sector is required for the Type~II mechanism for the generation of neutrino masses, if this mechanism has to generate a neutrino mass matrix with two-zero texture. One CP-violating phase has been retained in the scalar potential of the model, and all parameters have been chosen consistently with the observed neutrino mass and mixing patterns. Read More

A compressed spectrum was initially proposed as an explanation for the elusiveness of low-energy supersymmetry (SUSY). Some characteristic signals at the Large Hadron Collider (LHC), such as mono-jet + \slashed{E}_T, had been propounded as its trademark signals. However, later investigations suggested that lower limits on the supersymmetric particle masses would be quite stringent in spite of compression. Read More

The first run of the LHC was successful in that it saw the discovery of the elusive Higgs boson, a particle that is consistent with the SM hypothesis. There are a number of excesses in Run 1 ATLAS and CMS results which can be interpreted as being due to the existence of another heavier scalar particle. This particle has decay modes which we have studied using LHC Run 1 data. Read More

We consider the most general set of $SU(2) \times U(1)$ invariant CP-violating operators of dimension six, which contribute to $VVh$ interactions ($V = W, Z, \gamma$). Our aim is to constrain any CP-violating new physics above the electroweak scale via the effective couplings that arise when such physics is integrated out. For this purpose, we use, in turn, electroweak precision data, global fits of Higgs data at the Large Hadron Collider and the electric dipole moments of the neutron and the electron. Read More

We suggest a way of improving the probes on dimension-6 CP-conserving $HVV$ interactions ($V$ = $W$, $Z$, $\gamma$), from the LHC data on the Higgs boson to be available in the 14 TeV run with an integrated luminosity of $3000$ fb$^{-1}$. We find that the ratios of total rates in different channels can be quite useful in this respect. This includes ratios of event rates in (a) different final states for the Higgs produced by the same production mechanism, and (b) the same final state from two different production modes. Read More

The type III seesaw mechanism for neutrino mass generation usually makes use of at least two $Y = 0$, $SU(2)_L$ lepton triplets. We augment such a model with a third triplet and a sterile neutrino, both of which are odd under a conserved $\Z_2$ symmetry. With all new physics confined to the $\Z_2$-odd sector, whose low energy manifestation is in some higher-dimensional operators, a fermionic dark matter candidate is found to emerge. Read More

We consider a two-Higgs doublet scenario containing three $SU(2)_L$ singlet heavy neutrinos with Majorana masses. The second scalar doublet as well as the neutrinos are odd under a $Z_2$ symmetry. This scenario not only generates Majorana masses for the light neutrinos radiatively but also makes the lighter of the neutral $Z_2$-odd scalars an eligible dark matter candidate, in addition to triggering leptogenesis at the scale of the heavy neutrino masses. Read More

Assuming that $SU(2)_L$ doublet sneutrinos are the lightest among the MSSM sparticles, we show that it is possible to obtain same-sign tri-lepton (SS3L) events in the R-parity conserving supersymmetric scenarios. We consider the cascade decay of the lightest stop or the gluino above it to the lightest sneutrino via an intermediate on-shell gaugino--like neutralino and/or chargino and show that such processes can lead to SS3L events. We discuss the prospect of observing these SS3L events at Large Hadron Collider (LHC) and comment on discovering or constraining the proposed spectrum from future LHC data. Read More

In view of the recent interest in the pMSSM with light third generation squarks, we consider a hitherto neglected scenario where the lighter bottom squark ($\widetilde{b}_1$) is the next lightest supersymmetric particle (NLSP) which co-annihilates with the lightest supersymmetric particle (LSP), the dark matter (DM) candidate. Since the co-annihilation cross section receives contributions from both electroweak and strong vertices, it is relatively large. As a result relatively large NLSP-LSP mass difference (25 - 35 GeV) is consistent with the PLANCK data. Read More

We attempt an answer to the question as to why the evolution of four-dimensional universe is governed by spacetime curvature but not torsion. An answer is found if there is an additional compact spacelike dimension with a warped geometry, with torsion caused by a Kalb-Ramond (KR) antisymmetric tensor field in the bulk. Starting from a Randall-Sundrum type of warped extra dimension, and including the inevitable back reaction ensuing from the radius stabilization mechanism, we show that there is always an extra exponential suppression of the KR field on the four-dimensional projection that constitutes our visible universe. Read More

A radion in a scenario with a warped extra dimension can be lighter than the Higgs boson, even if the Kaluza-Klein excitation modes of the graviton turn out to be in the multi-TeV region. The discovery of such a light radion would be gateway to new physics. We show how the two-photon mode of decay can enable us to probe a radion in the mass range 60 - 110 GeV. Read More

Higgs boson properties could be studied with a high accuracy at a muon collider via the s-channel resonant production. We consider the situation where the center-of-mass energy of the muon collider is off the resonance above the Higgs mass. We discuss the discovery potential for a generic heavy Higgs boson ($H$) and compare different production mechanisms, including the "radiative return" ($\gamma H$), $Z$-boson associated production ($ZH$) and heavy Higgs pair production ($HA$). Read More

We consider the conditions for the validity of a two-Higgs doublet model at high energy scales, together with all other low- and high-energy constraints. The constraints on the parameter space at low energy, including the measured value of the Higgs mass and the signal strengths in channels are juxtaposed with the conditions of vacuum stability, perturbativity and unitarity at various scales. We find that a scenario with an exact $\mathbb{Z}_2$ symmetry in the potential cannot be valid beyond about 10 TeV without the intervention of additional physics. Read More

Probing signatures of anomalous interactions of the Higgs boson with pairs of weak vector bosons is an important goal of an $e^+e^-$ collider commissioned as a Higgs factory. We perform a detailed analysis of such potential of a collider operating at $250 - 300$ GeV. Mostly using higher dimensional operators in a gauge-invariant framework, we show that substantial information on anomalous couplings can be extracted from the total rates of $s$-and $t$-channel Higgs production. Read More

An SU(2) vectorlike singlet quark with a charge either +2/3 (t') or -1/3 (b') is predicted in many extensions of the Standard Model. The mixing of these quarks with the top or bottom lead to Flavor Changing Yukawa Interactions and Neutral Current. The decay modes of the heavier mass eigenstates are therefore different from the Standard Model type chiral quarks. Read More

The inclusion of right-chiral sneutrino superfields is a rather straightforward addition to a supersymmetric scenario. A neutral scalar with a substantial right sneutrino component is often a favoured dark matter candidate in such cases. In this context, we focus on the tentative signal in the form of a monochromatic photon, which may arise from dark matter annihilation and has drawn some attention in recent times. Read More

We study the signals for a "fermiophobic" charged Higgs boson present in an extension of the standard model with an additional Higgs doublet and right handed neutrinos, responsible for generating Dirac-type neutrino masses. We study the pair production of the charged Higgs at the Large Hadron Collider (LHC), which can be relatively light and still allowed by experimental data. The charged Higgs decays dominantly into a $W$ boson and a very light neutral scalar present in the model, which decays invisibly and passes undetected. Read More

The inclusion of higher-dimensional gauge invariant operators induces new Lorentz structures in Higgs couplings with electroweak gauge boson pairs. This in principle affects the kinematics of Higgs production and decay, thereby modifying the efficiencies of the experimental cuts compared to what simulations based on the standard model interactions yield. Taking some sample cases, we perform a rigorous analysis of how the efficiencies differ for various strengths of the additional operator vis-a-vis the standard model interactions, scanning over the values of both of them. Read More

We explore the constraints on the parameter space of a Randall-Sundrum warped geometry scenario, where a radion field arises out of the attempt to stabilise the radius of the extra compact spacelike dimension, using the most recent data from higgs searches at the Large Hadron Collider (LHC) and the Tevatron. We calculate contributions from both the scalar mass eigenstates arising from radion-higgs kinetic mixing in all important search channels. The most important channel to be affected is the decay via WW*, where no invariant mass peak can discern the two distinct physical states. Read More

In the usual 5-dimensional Randall-Sundrum scenario with warped geometry of the extra compact dimension, the Goldberger-Wise mechanism for stabilisation of the radius of compactification can lead to a scalar field called the radion. The radion can have implications in TeV-scale physics, which can be especially noticeable if its vacuum expectation value (vev) is not far above a TeV. However a large mass of the first graviton excitation, which seems to be suggested by recent search limit, tends to make the radion vev, far too large in the minimal model. Read More

Within the framework of a constrained Minimal Supersymmetric Standard Model (cMSSM) augmented by an MSSM singlet-pair sector to account for the non-zero neutrino masses by inverse seesaw mechanism, the lightest supersymmetric particle (LSP) can be a mixed sneutrino with mass as small as 50 GeV, satisfying all existing constraints, thus qualifying as a light dark matter candidate. We study the possibility of the lightest neutral Higgs boson in this model decaying invisibly into a pair of sneutrino LSPs, thereby giving rise to novel missing energy signatures at the LHC. We perform a two-parameter global analysis of the LHC Higgs data available till date to determine the optimal invisible Higgs branching fraction in this scenario, and obtain a $2\sigma(1\sigma)$ upper limit of 0. Read More

The type II seesaw mechanism for neutrino mass generation usually makes use of one complex scalar triplet. The collider signature of the doubly-charged scalar, the most striking feature of this scenario,consists mostly in decays into same-sign dileptons or same-sign $W$ boson pairs. However, certain scenarios of neutrino mass generation, such as those imposing texture zeros by a symmetry mechanism, require at least two triplets in order to be consistent with type-II the seesaw mechanism. Read More

We study the possibility of a light Dark Matter (DM) within a constrained Minimal Supersymmetric Standard Model (cMSSM) framework augmented by a SM singlet-pair sector to account for the non-zero neutrino masses by inverse seesaw mechanism. Working within a 'hybrid' scenario with the MSSM sector fixed at high scale and the singlet neutrino sector at low scale, we find that, contrary to the case of the usual cMSSM where the neutralino DM cannot be very light, we can have a light sneutrino DM with mass below 100 GeV satisfying all the current experimental constraints from cosmology, collider as well as low-energy experiments. We also note that the supersymmetric inverse seesaw mechanism with sneutrino as the lightest supersymmetric partner can have enhanced same-sign dilepton final states with large missing transverse energy (mET) coming from the gluino- and squark-pair as well as the squark-gluino associated productions and their cascade decay through charginos. Read More

We perform a multi-parameter global analysis of all data available till date from the ATLAS, CMS and Tevatron experiments, on the signals of a Higgs boson, to investigate how much scope exists for departure from the standard model prediction. We adopt a very general and model-independent scenario, where separate deviations from standard model values are possible for couplings of the observed scalar with up-and down-type fermions, W-and Z-boson pairs, as well as gluon and photon pair effective interactions. An arbitrary phase in the coupling with the top-pair, and the provision for an invisible decay width for the scalar are also introduced. Read More

It was shown in a previous study that a lightest neutralino with mass below 30 GeV was severely constrained in the minimal supersymmetric standard model (MSSM), unless it annihilates via a light stau and thus yields the observed dark matter abundance. In such a scenario, while the stau is the next-to-lightest supersymmetric particle (NLSP), the charginos and the other neutralinos as well as sleptons of the first two families are also likely to be not too far above the mass bounds laid down by the Large Electron Positron (LEP) collider. As the branching ratios of decays of the charginos and the next-to-lightest neutralino into staus are rather large, one expects significant rates of tau-rich final states in such a case. Read More

We predict a clean signal at the Large Hadron Collider ($\sqrt s)$=14 TeV for a scenario where there is a top-like, charge +2/3 vectorlike isosinglet fermion. Such a quark, via mixing with the standard model top, can undergo decays via both flavour-changing Z-boson coupling and flavour-changing Yukawa interactions. We concentrate on the latter channel, and study the situation where, following its pair-production, the heavy quark pair gives rise to two tops and two Higgs boson. Read More

We examine the parameter space of the purely phenomenological minimal super- symmetric standard model (MSSM), without assuming any supersymmetry breaking scheme. We find that a large region of the parameter space can indeed yield the lightest neutral Higgs mass around 125 GeV, as suggested by the recent ATLAS data, and also lead to event rates around, or slightly higher than, the standard model expectation in the two-photon and four-lepton channels. Using a lightest neutralino that is considerably lighter than the Higgs, we find that the 'invisible' decay of the Higgs into a pair of neutralinos upto about 10% can be consistent with the current data from the Large Hadron Collider (LHC). Read More

Affiliations: 1Harish-Chandra Research Institute, 2Harish-Chandra Research Institute

We present a re-interpretation of the recent ATLAS limits on supersymmetry in channels with jets (with and without b-tags) and missing energy, in the context of light third family squarks, while the first two squark families are inaccessible at the 7 TeV run of the Large Hadron Collider (LHC). In contrast to interpretations in terms of the high-scale based constrained minimal supersymmetric standard model (CMSSM), we primarily use the low-scale parametrisation of the phenomenological MSSM (pMSSM), and translate the limits in terms of physical masses of the third family squarks. Side by side, we also investigate the limits in terms of high-scale scalar non-universality, both with and without low-mass sleptons. Read More

We present a detailed investigation to establish that lepton-number (L) violating supersymmetry (SUSY) can be effectively probed at the LHC in the practically background-free same-sign trilepton (SS3l) and same-sign four-lepton (SS4l) channels. With this in view, we extend our earlier analysis of SS3l and SS4l signals by considering situations based on minimal supergravity as well as a phenomenological SUSY model. We find that the R-parity violating scenario predicts large event rates, for both the 7 and 14 TeV runs. Read More

A sufficiently long lived warm dark matter could be a source of X-rays observed by satellite based X-ray telescopes. We consider axinos and gravitinos with masses between 1 keV and 100 keV in supersymmetric models with small R-parity violation. We show that axino dark matter receives significant constraints from X-ray observations of Chandra and SPI, especially for the lower end of the allowed range of the axino decay constant $f_{a}$, while the gravitino dark matter remains unconstrained. Read More

The role of a bulk graviton in predicting the signature of extra dimensions through collider-based experiments is explored in the context of a multiply warped spacetime. In particular it is shown that in a doubly warped braneworld model, the presence of the sixth dimension, results in enhanced concentration of graviton Kaluza Klein (KK) modes compared to that obtained in the usual 5-dimensional Randall-Sundrum model. Also, the couplings of these massive graviton KK modes with the matter fields on the visible brane turn out to be appreciably larger than that in the corresponding 5- dimensional model. Read More

We consider the collider phenomenology of a singlet Majoron model with softly broken lepton number. Lepton number is spontaneously broken when the real part of a new singlet scalar develops vacuum expectation value. With the additional soft terms violating lepton numbers, the imaginary part of this singlet scalar becomes a massive pseudo-Majoron which can account for the dark matter. Read More

We discuss large non-universality in the Higgs sector at high scale in supersymmetric theories, in the context of the Large Hadron Collider (LHC). In particular, we note that if ${m_{H_u}}^2-{m_{H_d}}^2$ is large and negative ($\simeq 10^6 {\rm ~GeV^2}$) at high scale, the lighter slepton mass eigenstates at the electroweak scale are mostly left chiral, in contrast to a minimal supergravity (mSUGRA) scenario. We use this feature to distinguish between non-universal Higgs masses (NUHM) and mSUGRA by two methods. Read More

We investigate the possibility of probing an anomalous CP-violating coupling in the HWW vertex at the LHC. We consider the production of the Higgs in association of a W and then decay via the $H \rightarrow WW$ channel taking into account the limits on the Higgs production cross section from the Tevatron. We select the same-sign dilepton final state arising from leptonic decays of two of the three Ws and apply cuts required to suppress the standard model background. Read More

We consider a scenario where supersymmetry (SUSY) is broken at a relatively low scale by modular fields of extra compact spacelike dimensions. The effect of both soft and hard SUSY breaking terms on the mass of the lightest neutral Higgs boson are investigated. An important conclusion is that the lightest neutral Higgs can be considerably more massive than what is expected in the MSSM, if the overseeing theory breaks SUSY at a scale not too far above a TeV. Read More

The problem of discriminating possible scenarios of TeV scale new physics with large missing energy signature at the Large Hadron Collider (LHC) has received some attention in the recent past. We consider the complementary, and yet unexplored, case of theories predicting much softer missing energy spectra. As there is enough scope for such models to fake each other by having similar final states at the LHC, we have outlined a systematic method based on a combination of different kinematic features which can be used to distinguish among different possibilities. Read More

We point out that same-sign multilepton events, not given due attention yet for new physics search, can be extremely useful at the Large Hadron Collider. After showing the easy reducibility of the standard model backgrounds, we demonstrate the viability of same-sign trilepton signals for R-parity breaking supersymmetry, at both 7 and 14 TeV. We find that same-sign four-leptons, too, can have appreciable rates. Read More