arXiv subjects

Tanumoy Mandal

Publications and source records attributed to Tanumoy Mandal.

At least 55 records · Page 3Linked to original sources

arXiv2019

Hunting scalar leptoquarks with boosted tops and light leptons

The LHC search strategies for leptoquarks that couple dominantly to a top quark are different than for the ones that couple mostly to the light quarks. We consider charge $1/3$ ($ϕ_1$) and $5/3$ ($ϕ_5$) scalar leptoquarks that can decay to a top quark and a charged lepton ($t\ell$) giving rise to a resonance system of a boosted top and a high-$p_{\rm T}$ lepton. We introduce simple phenomenological models suitable for bottom-up studies and explicitly map them to all possible scalar leptoquark models within the Buchmüller-Rückl-Wyler classifications that can have the desired decays. We study pair and single productions of these leptoquarks. Contrary to the common perception, we find that the single production of top-philic leptoquarks $ϕ= \{ϕ_1,ϕ_5\}$ in association with a lepton and jets could be significant for order one $ϕt\ell$ coupling in certain scenarios. We propose a strategy of selecting events with at least one hadronic-top and two high-$p_{\rm T}$ same flavour opposite sign leptons. This captures events from both pair and single productions. Our strategy can significantly enhance the LHC discovery potential especially in the high-mass region where single productions become more prominent. Our estimation shows that a scalar leptoquark as heavy as $\sim1.7$ TeV can be discovered at the $14$ TeV LHC with 3 ab$^{-1}$ of integrated luminosity in the $t\ell\ell+X$ channel for $100\%$ branching ratio in the $ϕ\to t\ell $ decay mode. However, in some scenarios, the discovery reach can increase beyond $2$ TeV even though the branching ratio comes down to about $50\%$.

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arXiv2019

$R_{D^{(*)}}$ motivated $\mathcal{S}_1$ leptoquark scenarios: Impact of interference on the exclusion limits from LHC data

Motivated by the persistent anomalies in the semileptonic $B$-meson decays, we investigate the competency of LHC data to constrain the $R_{D^{(*)}}$-favoured parameter space in a charge $-1/3$ scalar leptoquark ($\mathcal S_1$) model. We consider some scenarios with one large free coupling to accommodate the $R_{D^{(*)}}$ anomalies. As a result, some of them dominantly yield nonresonant $ττ$ and $τν$ events at the LHC through the $t$-channel $\mathcal S_1$ exchange. So far, no experiment has searched for leptoquarks using these signatures and the relevant resonant leptoquark searches are yet to put any strong exclusion limit on the parameter space. We recast the latest $ττ$ and $τν$ resonance search data to obtain new exclusion limits. The nonresonant processes strongly interfere (destructively in our case) with the Standard Model background and play the determining role in setting the exclusion limits. To obtain precise limits, we include non-negligible effects coming from the subdominant (resonant) pair and inclusive single leptoquark productions systematically in our analysis. To deal with large destructive interference, we make use of the transverse mass distributions from the experiments in our statistical analysis. In addition, we also recast the relevant direct search results to obtain the most stringent collider bounds on these scenarios to date. These are independent bounds and are competitive to other known bounds. Finally, we indicate how one can adopt these bounds to a wide class of models with $\mathcal S_1$ that are proposed to accommodate the $R_{D^{(*)}}$ anomalies.

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arXiv2018

Heavy charged scalars from $c\bar{s}$ fusion: A generic search strategy applied to a 3HDM with $\mathrm{U}(1) \times \mathrm{U}(1)$ family symmetry

We describe a class of three Higgs doublet models (3HDMs) with a softly broken $\mathrm{U}(1) \times \mathrm{U}(1)$ family symmetry that enforces a Cabibbo-like quark mixing while forbidding tree-level flavour changing neutral currents. The hierarchy in the observed quark masses is partly explained by a softer hierarchy in the vacuum expectation values of the three Higgs doublets. As a consequence, the physical scalar spectrum contains a Standard Model (SM) like Higgs boson $h_{125}$ while exotic scalars couple the strongest to the second quark family, leading to rather unconventional discovery channels that could be probed at the Large Hadron Collider. In particular, we describe a search strategy for the lightest charged Higgs boson $H^{\pm}$, through the process $c\bar s\to H^{+}\to W^+\,h_{125}$, using a multivariate analysis that leads to an excellent discriminatory power against the SM background. Although the analysis is applied to the proposed class of 3HDMs, we employ a model-independent formulation such that it can be applied to any other model with the same discovery channel.

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arXiv2018

Exclusion limits on a scalar decaying to photons and distinguishing its production mechanisms

LHC run-II has a great potential to search for new resonances in the diphoton channel. Latest 13 TeV data already put stringent limits on the cross sections in the diphoton channel assuming the resonance is produced through the gluon-gluon fusion. Many beyond the Standard Model (SM) theories predict TeV-scale scalars, which copiously decay to diphotons. Apart from the gluon-gluon fusion production, these scalars can also be dominantly produced in other ways too at the LHC namely through the quark-quark fusion or the gauge boson fusions like the photon-photon, photon-$Z$, $WW$ or $ZZ$ fusions. In this paper, we use an effective field theory approach where a heavy scalar can be produced in various ways and recast the latest ATLAS diphoton resonance search to put model-independent limits on its mass and effective couplings to the SM particles. If a new scalar is discovered at the LHC, it would be very important to identify its production mechanism in order to probe the nature of the underlying theory. We show that combining various kinematic variables in a multivariate analysis can be very powerful to distinguish different production mechanisms one from the other.

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arXiv2017

Minimal anomalous $\mathrm{U}(1)$ theories and collider phenomenology

We study the collider phenomenology of a neutral gauge boson $Z'$ arising in minimal but anomalous $\mathrm{U}(1)$ extensions of the Standard Model (SM). To retain gauge invariance of physical observables, we consider cancellation of gauge anomalies through the Green-Schwarz mechanism. We categorize a wide class of $\mathrm{U}(1)$ extensions in terms of the new $\mathrm{U}(1)$ charges of the left-handed quarks and leptons and the Higgs doublet. We derive constraints on some benchmark models using electroweak precision constraints and the latest 13 TeV LHC dilepton and dijet resonance search data. We calculate the decay rates of the exotic and rare one-loop $Z'$ decays to $ZZ$ and $Z$-photon modes, which are the unique signatures of our framework. If observed, these decays could hint at anomaly cancellation through the Green-Schwarz mechanism. We also discuss the possible observation of such signatures at the LHC and at future ILC colliders.

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arXiv2017

LHC probes of TeV-scale scalars in $\mathrm{SO}(10)$ grand unification

We investigate the possibility of TeV-scale scalars as low energy remnants arising in the non-supersymmetric $\mathrm{SO}(10)$ grand unification framework where the field content is minimal. We consider a scenario where the $\mathrm{SO}(10)$ gauge symmetry is broken into the gauge symmetry of the Standard Model (SM) through multiple stages of symmetry breaking, and a colored and hypercharged scalar $χ$ picks a TeV-scale mass in the process. The last stage of the symmetry breaking occurs at the TeV scale where the left-right symmetry, i.e. $\mathrm{SU}(2)_L\otimes \mathrm{SU}(2)_R\otimes \mathrm{U}(1)_{B-L}\otimes \mathrm{SU}(3)_C$, is broken into that of the SM by a singlet scalar field $\mathcal{S}$ of mass $M_{\mathcal{S}}\sim 1$ TeV, which is a component of an $\mathrm{SU}(2)_R$-triplet scalar field, acquiring a TeV-scale vacuum expectation value. For the LHC phenomenology, we consider a scenario where $\mathcal{S}$ is produced via gluon-gluon fusion through loop interactions with $χ$ and also decays to a pair of SM gauge bosons through $χ$ in the loop. We find that the parameter space is heavily constrained from the latest LHC data. We use a multivariate analysis to estimate the LHC discovery reach of $\mathcal{S}$ into the diphoton channel.

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arXiv2017

Heavy photophilic scalar at the LHC from a varying electromagnetic coupling

We investigate the phenomenology of a heavy scalar $ϕ$ of the type involved in Bekenstein's framework for varying electromagnetic coupling theories, with the difference that the scalar in our model has a large mass. The model has only two free parameters, the mass $M_ϕ$ of the scalar and the scale $Λ$ of new physics. The scalar is dominantly produced through photon-photon fusion at the LHC and leads to a diphoton final state. It can also be produced by quark-antiquark fusion in association with a photon or a fermion pair. Its dominating decay is to diphotons, but it also has a large three-body branching to a fermion pair and a photon, which can provide an interesting search channel with a dilepton-photon resonance. We derive exclusion limits on the $M_ϕ-Λ$ plane from the latest 13 TeV LHC diphoton resonance search data. For a benchmark mass of $M_ϕ\sim 1$ TeV, we find a lower limit on $Λ$ of 18 TeV. We discuss the more complex possibility of varying couplings in the full electroweak theory and comment on the possibility that the new physics is related to extra dimensions or string theory.

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arXiv2017

Effect of strong magnetic field on competing order parameters in two-flavor dense quark matter

We study the effect of strong magnetic field on competing chiral and diquark order parameters in a regime of moderately dense quark matter. The inter-dependence of the chiral and diquark condensates through nonperturbative quark mass and strong coupling effects is analyzed in a two-flavor Nambu-Jona-Lasinio (NJL) model. In the weak magnetic field limit, our results agree qualitatively with earlier zero-field studies in the literature that find a critical coupling ratio $G_D/G_S\sim 1.1$ below which chiral or superconducting order parameters appear almost exclusively. Above the critical ratio, there exists a significant mixed broken phase region where both gaps are non-zero. However, a strong magnetic field $B\gtrsim 10^{18}$ G disrupts this mixed broken phase region and changes a smooth crossover found in the weak-field case to a first-order transition for both gaps at almost the same critical density. Our results suggest that in the two-flavor approximation to moderately dense quark matter, strong magnetic field enhances the possibility of a mixed phase at high density, with implications for the structure, energetics and vibrational spectrum of neutron stars.

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arXiv2016

Phenomenological signatures of additional scalar bosons at the LHC

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 $χ$ have been introduced in a two Higgs doublet model (2HDM), where $S$ is a portal to dark matter (DM) through its interaction with $χ$, a DM candidate and a possible source of missing transverse energy (\MET). Previous studies focused on a heavy scalar $H$ decay mode $H \to hχχ$, which was studied using an effective theory in order to explain a distortion in the Higgs ($h$) transverse momentum spectrum [16]. In this work, the effective decay is understood more deeply by including a mediator $S$, and the focus is changed to $H \to h S,~SS$ with $S \to χχ$. Phenomenological signatures of all the new scalars in the proposed 2HDM are discussed in the energy regime of the LHC, and their mass bounds have been set accordingly. Additionally, we have performed several analyses with final states including leptons and \MET, with $H \to 4 W$, $t\bar t H \to 6 W$ and $A \to ZH$ channels, in order to understand the impact these scalars have on current searches.

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arXiv2016

Effect of temperature and magnetic field on two-flavor superconducting quark matter

We investigate the effect of turning on temperature for the charge neutral phase of two-flavor color superconducting (2SC) dense quark matter in presence of constant external magnetic field. Within the Nambu-Jona-Lasinio model, by tuning the diquark coupling strength, we study the interdependent evolution of the quark Bardeen-Cooper-Schrieffer gap and dynamical mass as functions of temperature and magnetic field. We find that magnetic field $B \gtrsim 0.02$ GeV$^2$ ($10^{18}$ G) leads to anomalous temperature behavior of the gap in the gapless 2SC phase (moderately strong coupling), reminiscent of previous results in the literature found in the limit of weak coupling without magnetic field. The 2SC gap in the strong coupling regime is abruptly quenched at ultrahigh magnetic field due to the mismatched Fermi surfaces of up and down quarks imposed by charge neutrality and oscillation of the gap due to Landau level quantization. The dynamical quark mass also displays strong oscillation and magnetic catalysis at high magnetic field, although the latter effect is tempered by nonzero temperature. We discuss the implications for newly born compact stars with superconducting quark cores.

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arXiv2016

Constraining minimal anomaly free $\mathrm{U}(1)$ extensions of the Standard Model

We consider a class of minimal anomaly free $\mathrm{U}(1)$ extensions of the Standard Model with three generations of right-handed neutrinos and a complex scalar. Using electroweak precision constraints, new 13 TeV LHC data, and considering theoretical limitations such as perturbativity, we show that it is possible to constrain a wide class of models. By classifying these models with a single parameter, $κ$, we can put a model independent upper bound on the new $\mathrm{U}(1)$ gauge coupling $g_z$. We find that the new dilepton data puts strong bounds on the parameters, especially in the mass region $M_{Z'}\lesssim 3~ \mathrm{TeV}$.

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arXiv2016

Phenomenology of additional scalar bosons at the LHC

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 $χ$ and $S$ have been introduced in the THDM where $χ$ is treated as a candidate for dark matter. $χ$ does not receive any vacuum expectation value ($vev$) in the model whereas the Higgs-like scalar $S$ acquires $vev$. This allows small mixing between the $CP$-even scalars of the THDM, $h$, $H$ and $S$. In this study the mass spectrum of new scalars is taken to be $2 m_h < m_H < 2 m_t$, $m_χ< m_h/2$, $m_h \lesssim m_S \lesssim m_H - m_h$, $m_A > 2 m_t$ and $m_H^\pm < m_A$, where $m_h$ and $m_t$ is masses of the SM Higgs and top-quark respectively, $m_H, m_A$ and $m_{H^\pm}$ are the masses of the heavy $CP$-even scalar $H$, $CP$-odd scalar $A$, and charged Higgs $H^\pm$, respectively. A partial list of potential search channels at the LHC has been provided with possible phenomenological consequences. The expected phenomenology and constraints on parameters are also discussed in a model-independent approach .

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arXiv2016

The 750 GeV excess from photon-photon and quark-quark processes

The observed excess in the diphoton mass spectrum around 750 GeV at the 13 TeV LHC possibly indicates the presence of a photonphilic resonance. We show that the excess can be explained by a scalar of the type involved in Bekenstein's framework for varying electromagnetic coupling theories. The scalar, in our model, couples dominantly to photons and is mainly produced by the quark-quark fusion at the LHC. In addition, it can also be produced in photon-photon fusion. Our model has only two free parameters, the mass of the scalar and the scale of the new physics, which are fixed by the LHC excess to 750 GeV and 1.5 - 2 TeV, respectively. The scalar has a large three-body decay to a fermion pair and a photon, which provides an interesting search channel with a dilepton-photon resonance around 750 GeV.

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arXiv2016

Probing Compositeness with the CMS $eejj$ & $eej$ Data

Quark-lepton compositeness is a well-known beyond the Standard Model (SM) scenario with heavy exotic particles like leptoquarks (LQs) and leptogluons (LGs) etc. These particles can couple to leptons and jets simultaneously. In this letter, we use the recent CMS scalar LQ search data in the $eejj$ and $eej$ channels to probe this scenario. We recast the data in terms of a color octet partner of the SM electron (or a first generation spin-1/2 LG) that couples to an electron and a gluon via a dimension five operator suppressed by the quark-lepton compositeness scale ($Λ$). By combining different production processes of the color octet electron ($e_8$) at the LHC, we use the CMS 8TeV data to obtain a simultaneous bound on $Λ$ and the mass of the $e_8$ ($M_{e_8}$). We also study the reach of the 13 TeV LHC to discover the $e_8$ and interpret the required luminosity in terms of $M_{e_8}$ and $Λ$.

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arXiv2016

The impact of additional scalar bosons at the LHC

In this study we consider an effective model by introducing two hypothetical real scalars, $H$ and $χ$ - a dark matter candidate, where the masses of these scalars are $2 m_h < m_H < 2 m_t$ and $m_χ\approx m_h/2$ 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 $p p \to H \to hχχ$ could be observed in this model. An additional scalar, $S$, has been postulated to explain large $H \to hχχ$ branching ratios, assuming $m_h \lesssim m_S \lesssim m_H-m_h$ and $m_S > 2 m_χ$. Furthermore, a scenario of a two Higgs doublet model (2HDM) is introduced and a detailed proposal at the present energies of the Large Hadron Collider to study the extra CP-even ($h, H$), CP-odd ($A$) and charged ($H^\pm$) scalars has been pursued. With possible phenomenological implications, all possible spectra and decay modes for these scalars are discussed. Based on the mass spectrum of $H, A$ and $H^\pm$, the production of multi-leptons and $Z$+jets+missing-energy events are predicted. A specific, Type-II 2HDM model is discussed in detail.

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arXiv2016

Pair Production of Scalar Leptoquarks at the LHC to NLO Parton Shower Accuracy

We present the scalar leptoquark pair production process at the LHC computed at the next-to-leading order in QCD, matched to the PYTHIA parton shower using the MC@NLO formalism. We consider the leading order decay of a leptoquark to a lepton ($e,μ,τ$ or $ν_e,ν_μ,ν_τ$) and a jet and observe the effects of parton shower on the final states. For demonstration, we display the kinametical distributions of a selection of observables along with their scale uncertainties for the 13 TeV LHC. We also present pair production cross sections and $K$-factors using massless five-quark flavor scheme for different LHC center-of-mass energies. The complete stand-alone code is available online.

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arXiv2015

The compatibility of LHC Run 1 data with a heavy scalar of mass around 270\,GeV

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. Using a minimalistic model, we can predict the kinematics of these final states and compare the prediction against data directly. A statistical combination of these results shows that a best fit point is found for a heavy scalar having a mass of 272$^{+12}_{-9}$\,GeV. This result has been quantified as a three sigma effect, based on analyses which are not necessarily optimized for the search of a heavy scalar. The smoking guns for the discovery of this new heavy scalar and the prospects for Run 2 are discussed.

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arXiv2015

Neutrality of a magnetized two-flavor quark superconductor

We investigate the effect of electric and color charge neutrality on the two-flavor color superconducting (2SC) phase of cold and dense quark matter in presence of constant external magnetic fields and at moderate baryon densities. Within the framework of the Nambu-Jona-Lasinio (NJL) model, we study the inter-dependent evolution of the quark's BCS gap and constituent mass with increasing density and magnetic field. While confirming previous results derived for the highly magnetized 2SC phase with color neutrality alone, we obtain new results as a consequence of imposing charge neutrality. In the charge neutral gapless 2SC phase (g2SC), a large magnetic field drives the color superconducting phase transition to a crossover, while the chiral phase transition is first order. At larger diquark-to-scalar coupling ratio $G_D/G_S$, where the 2SC phase is preferred, we see hints of the Clogston-Chandrasekhar limit at a very large value of the magnetic field ($B\sim 10^{19}$G), but this limit is strongly affected by Shubnikov de Haas-van Alphen oscillations of the gap, indicating the transition to a domain-like state.

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