arXiv subjects

Debjyoti Bardhan

Publications and source records attributed to Debjyoti Bardhan.

14 recordsLinked to original sources

arXiv2023

Searching for dark jets with displaced vertices using weakly supervised machine learning

If "dark quarks" from a confining hidden sector are produced at the LHC, they will shower and hadronize to dark sector hadrons, which may decay back to Standard Model particles within the detector, possibly resulting in a collimated spray of particles resembling a QCD jet. In this work we address scenarios in which dark hadrons decay with a measurable small displacement, such that the relevant background is dominated by heavy-flavor jets. Since dark sector parameters are largely unconstrained, and the precise properties of a dark QCD-like theory are difficult to compute or simulate reliably in any case, model-independent, data-based searches for such scenarios are desirable. We explore a search strategy employing weakly supervised machine learning to search for anomalous jets with displaced vertices. The method is tested on several toy signals, demonstrating the feasibility of such a search. Our approach has potential to outperform simple cut-based methods in some cases and has the advantage of being more model-independent.

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arXiv2023

$R_{K^{(\ast)}}$ from RPV-SUSY sneutrinos

We analyze the lepton flavor universality violation (LFV) in process $b\to s \ell^+\ell^-$ in an R-parity violating supersymmetric (RPV-SUSY) scenario. The most recent update on $\rk$ from the LHCb collaboration suggests approximately $3.1σ$ deviation from the Standard Model predictions, strengthening the case of LFV New Physics. In this work, we show that $R_{K^{(*)}}$ anomaly can be addressed via only sneutrinos within the framework of R-parity violating interactions assuming phenomenologically viable values of the couplings. While our proposed solution is by no means the only solution to these anomalies, it is a phenomenologically plausible one and, notably, quite a minimal one in the context of RPV-SUSY.

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arXiv2023

Bounds on boosted dark matter from direct detection: The role of energy-dependent cross sections

The recoil threshold of Direct Detection experiments limits the mass range of Dark Matter (DM) particles that can be detected, with most DD experiments being blind to sub-MeV DM particles. However, these light DM particles can be boosted to very high energies via collisions with energetic Cosmic Ray electrons. This allows Dark Matter particles to induce detectable recoil in the target of Direct Detection experiments. We derive constraints on scattering cross section of DM and electron, using XENONnT and Super-Kamiokande data. Vector and scalar mediators are considered, in the heavy and light regimes. We discuss the importance of including energy dependent cross sections (due to specific Lorentz structure of the vertex) in our analysis, and show that the bounds can be significantly different than the results obtained assuming constant energy-independent cross-section, often assumed in the literature for simplicity. Our bounds are also compared with other astrophysical and cosmological constraints.

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arXiv2021

Towards constraining triple gluon operators through tops

Effective field theory techniques provide us important tools to probe for physics beyond the Standard Model in a relatively model-independent way. In this work, we revisit the CP-even dimension-6 purely gluonic operator to investigate the possible constraints on it by studying its effect on top-pair production at the LHC, in particular the high $p_T$ and $m_{t\bar{t}}$ tails of the distribution. Cut-based analysis reveals that the scale of New Physics when this operator alone contributes to the production process is greater than 3.4 TeV at 95% C.L., which is a much stronger bound compared to the bound of 850 GeV obtained from Run-I data using the same channel. This is reinforced by an analysis using Machine Learning techniques. Our study complements similar studies that have focussed on other collider channels to study this operator.

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arXiv2019

B-meson charged current anomalies: the post-Moriond status

In this note, we discuss the impact of the recent Belle result on the various theoretical explanations of the $R_D$ and $R_{D^*}$ anomalies. The pure tensor explanation, which was strongly disfavoured by the measurements of $F_L^{D^*}$ and high-$p_T$ $p \, p \to τ\, ν$ searches before Moriond, is now completely allowed because of reduction of the experimental world-average. Moreover, the pure right-chiral vector solution (involving right-chiral neutrinos) has now moved into the $2σ$ allowed range of the LHC $p \, p \to τ\, ν$ searches. We also critically re-examine the bound on $\mathcal{B}(B_c^- \to τ^- \barν_τ)$ from LEP data and show that the bound is considerably weaker than the number $10\%$ often used in the recent literature.

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arXiv2018

Anatomy of $b \to c \, τ\, ν$ anomalies

In recent times, one of the strongest hints of Physics Beyond the Standard Model (BSM) has been the anomaly in the ratios $R_D$ and $R_{D^\ast}$ measured in the charged current decays of $B$-mesons. In this work, we perform a comprehensive analysis of these decay modes, first in a model independent way and subsequently, in the context of composite Higgs models. We discuss in depth as to how linearly realised $\rm SU(2)_L \times U(1)_Y$ symmetry imposes severe constraint on the various scenarios because of correlations with other $ΔF =1$ processes and $Z\,τ\,τ$ and $Z\,ν\,ν$ couplings. In the composite Higgs paradigm with partial compositeness, we show that, irrespective of the flavour structure of the composite sector, constraints from $ΔF =2$ processes bring the compositeness scale down to $\sim 650$ GeV which is in tension with electroweak precision observables. In the presence of composite leptoquarks, the situation improves only marginally (a factor of $\sqrt{2}$ in the compositeness scale), thus making the new states soon discoverable by direct searches at the LHC. We also comment on the possible explanation of the $R_{K,K^*}$ anomalies within the composite Higgs framework.

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arXiv2018

A closer look at the $R_D$ and $R_{D^*}$ anomalies

The measurement of $R_D$ ($R_{D^*}$), the ratio of the branching fraction of $\overline{B} \to D τ\barν_τ(\overline{B} \to D^* τ\barν_τ)$ to that of $\overline{B} \to D l \barν_l (\overline{B} \to D^* l \barν_l)$, shows $1.9 σ$ $(3.3 σ)$ deviation from its Standard Model (SM) prediction. The combined deviation is at the level of $4 σ$ according to the Heavy Flavour Averaging Group (HFAG). In this paper, we perform an effective field theory analysis (at the dimension-6 level) of these potential New Physics (NP) signals assuming $\rm SU(3)_{C} \times SU(2)_{L} \times U(1)_{Y}$ gauge invariance. We first show that, in general, $R_D$ and $R_{D^*}$ are theoretically independent observables and hence, their theoretical predictions are not correlated. We identify the operators that can explain the experimental measurements of $R_D$ and $R_{D^*}$ individually and also together. Motivated by the recent measurement of the $τ$ polarisation in $\overline{B} \to D^* τ\barν_τ$ decay, $P_τ(D^*)$ by the Belle collaboration, we study the impact of a more precise measurement of $P_τ(D^*)$ (and a measurement of $P_τ(D)$) on the various possible NP explanations. Furthermore, we show that the measurement of $R_{D^*}$ in bins of $q^2$, the square of the invariant mass of the lepton-neutrino system, along with the information on $τ$ polarisation and the forward-backward asymmetry of the $τ$ lepton, can completely distinguish the various operator structures. We also provide the full expressions of the double differential decay widths for the individual $τ$ helicities in the presence of all the 10 dimension-6 operators that can contribute to these decays.

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arXiv2017

Role of Tensor operators in $R_K$ and $R_{K^*}$

The recent LHCb measurement of $R_{K^*}$ in two $q^2$ bins, when combined with the earlier measurement of $R_K$, strongly suggests lepton flavour non-universal new physics in semi-leptonic $B$ meson decays. Motivated by these intriguing hints of new physics, several authors have considered vector, axial vector, scalar and pseudo scalar operators as possible explanations of these measurements. However, tensor operators have widely been neglected in this context. In this paper, we consider the effect of tensor operators in $R_K$ and $R_{K^*}$. We find that, unlike other local operators, tensor operators can comfortably produce both of $R_{K^*} ^{\rm low}$ and $R_{K^*} ^{\rm central}$ close to their experimental central values. However, a simultaneous explanation of $R_K$ is not possible with only Tensor operators, and other vector or axial vector operators are needed. In fact, we find that combination of vector and tensor operators can provide simultaneous explanations of all the anomalies comfortably at the $1 σ$ level, a scenario which is hard to achieve with only vector or axial vector operators. We also comment on the compatibility of the various new physics solutions with the measurements of the inclusive decay $B_d \to X_s \ell^+ \ell^-$.

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arXiv2017

Search for bottom squarks in the baryon-number violating MSSM

We consider a scenario of Minimal Supersymmetric Standard Model (MSSM) with $R$-parity violation, where the lightest supersymmetric particle (LSP) is the lighter sbottom $(\tilde b_1)$. We study the production of a sbottom pair at the LHC and their subsequent decays through the baryon number violating $(U D D)$ operators leading to a top pair with two light quarks. Looking for both semi-leptonic and fully hadronic (no leptons) final states, we perform cut-based as well as multivariate analyses (MVA) to estimate the signal significance at the 13 TeV run of the LHC. We find that a cut-based analysis can probe sbottom mass upto $\sim 650 $ GeV which may be extended upto $\sim 750 $ GeV using MVA with $300~{\rm fb}^{-1}$ integrated luminosity. The fully hadronic final state, however, is not as promising.

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arXiv2017

A Closer Look at $R_D$ and $R_{D^\ast}$

The measurement of $R_D$ ($R_{D^*}$), the ratio of the branching fraction of $\overline{B} \to D τ\barν_τ(\overline{B} \to D^* τ\barν_τ)$ to that of $\overline{B} \to D l \barν_l (\overline{B} \to D^* l \barν_l)$, shows $1.9 σ$ $(3.3 σ)$ deviation from its Standard Model (SM) prediction. The combined deviation is at the level of $4 σ$ according to the Heavy Flavour Averaging Group (HFAG). We perform an effective field theory analysis (at the dimension 6 level) of these potential New Physics (NP) signals assuming $ SU(3)_{C} \times SU(2)_{L} \times U(1)_{Y}$ gauge invariance. We first show that, in general, $R_D$ and $R_{D^*}$ are theoretically independent observables and hence, their theoretical predictions are not correlated. We identify the operators that can explain the experimental measurements of $R_D$ and $R_{D^*}$ individually and also together. Motivated by the recent measurement of the $τ$ polarisation in $\overline{B} \to D^* τ\barν_τ$ decay, $P_τ^{D^*}$ by the {\sc Belle} collaboration, we study the impact of a more precise measurement of $P_τ^{D^*}$ (and a measurement of $P_τ^D$) on the various possible NP explanations. Furthermore, we show that the measurement of $R_{D^*}$ in bins of $q^2$, the square of the invariant mass of the lepton neutrino system, along with the information on $τ$ polarisation, can completely distinguish the various operator structures.

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arXiv2017

FCNC Decays of the Top Quark

If new physics (e.g. SUSY) does not show up as direct evidence at the LHC, it could still be observable in FCNC processes involving the $t$-quark. We take a close look at the process $t\to c + h/Z$ and show that its branching ratio in the Standard Model is subject to three mechanisms of suppression. To obtain an observable signal, one needs to evade all these mechanisms in a theory beyond the Standard Model. We show that a theory like the cMSSM cannot provide a big enough enhancement. However, in a framework like $R$-parity-violating SUSY, observable signals are a distinct possibility.

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arXiv2016

A Detailed Analysis of Flavour-changing Decays of Top Quarks as a Probe of New Physics at the LHC

If the LHC should fail to observe direct signals for new physics, it may become necessary to look for new physics effects in rare events such as flavour-changing decays of the top quark, which, in the Standard Model, are predicted to be too small to be observed. We set up the theoretical framework in which experimentally accessible results can be expected in models of new physics, and go on to discuss two models of supersymmetry -- one with conserved $R$-parity, and one without $R$-parity -- to illustrate how the flavour-changing signals are predicted in these models. In the latter case, there is a distinct possibility of detecting the rare decay $t \to c + Z^0$ at the LHC. We also present a detailed set of very general formulae which can be used to make similar calculations in diverse models of new physics.

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arXiv2016

The 750 GeV diphoton resonance as an sgoldstino: a reappraisal

Among the various explanations of the possible 750 GeV diphoton resonance, the possibility of it being an sgoldstino is an attractive one, as it is related to the spontaneous breaking of global supersymmetry. We discuss this possibility in this paper and point out the various theoretical issues associated with it. In particular, we indicate the difficulties of this explanation in realistic models of gauge mediated supersymmetry breaking.

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arXiv2016

Radion Candidate for the LHC Diphoton Resonance

The recent observation of a modest excess in diphoton final states at the LHC, by both the ATLAS and CMS Collaborations, has sparked off the expected race among theorists to find the right explanation for this proto-resonance, assuming that the signal will survive and not prove to be yet another statistical fluctuation. We carry out a general analysis of this `signal' in the case of a scalar which couples only to pairs of gluons (for production) and photons (for diphoton decay modes), and establish that an explanation of the observed resonance, taken together with the null results of new physics searches in all the other channels, requires a scalar with rather exotic behaviour. We then demonstrate that a fairly simple-minded extension of the minimal Randall-Sundrum model can yield a radion candidate which might reproduce this exotic behaviour.

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