arXiv subjects

Diptimoy Ghosh

Publications and source records attributed to Diptimoy Ghosh.

At least 55 records · Page 3Linked to original sources

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

Is the Higgs Mechanism of Fermion Mass Generation a Fact? A Yukawa-less First-Two-Generation Model

It is now established that the major source of electroweak symmetry breaking (EWSB) is due to the observed Higgs particle. However, whether the Higgs mechanism is responsible for the generation of all the fermion masses, in particular, the fermions of the first two generations, is an open question. In this letter we present a construction where the light fermion masses are generated through a secondary, subdominant and sequestered source of EWSB. This fits well with the approximate U(2) global symmetry of the observed structure of the flavor sector. We first realize the above idea using a calculable two Higgs doublet model. We then show that the first two generation masses could come from technicolor dynamics, while the third generation fermions, as well as the electroweak gauge bosons get their masses dominantly from the Higgs mechanism. We also discuss how the small CKM mixing between the first two generations and the third generation, and soft mixing between the sequestered EWSB components arise in this setup. A typical prediction of this scenario is a significant reduction of the couplings of the observed Higgs boson to the first two generation of fermions.

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arXiv2016

The CP-Violating pMSSM

We investigate the sensitivity of the next generation of flavor-based low-energy experiments to probe the supersymmetric parameter space in the context of the phenomenological MSSM (pMSSM), and examine the complementarity with direct searches for Supersymmetry at the 13 TeV LHC in a quantitative manner. To this end, we enlarge the previously studied pMSSM parameter space to include all physical non-zero CP-violating phases, namely those associated with the gaugino mass parameters, Higgsino mass parameter, and the tri-linear couplings of the top quark, bottom quark and tau lepton. We find that future electric dipole moment and flavor measurements can have a strong impact on the viability of these models even if the sparticle spectrum is out of reach of the 13 TeV LHC. In particular, the lack of positive signals in future low-energy probes would exclude values of the phases between ${\cal O}(10^{-2})$ and ${\cal O}(10^{-1})$. We also find regions of parameter space where large phases remain allowed due to cancellations. Most interestingly, in some rare processes, such as BR($B_s \to μ^+ μ^-$ ), we find that contributions arising from CP-violating phases can bring the potentially large SUSY contributions into better agreement with experiment and Standard Model predictions.

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arXiv2015

CP Violation Tests of Alignment Models at LHCII

We analyse the low-energy phenomenology of alignment models both model-independently and within supersymmetric (SUSY) scenarios focusing on their CP violation tests at LHCII. Assuming that New Physics (NP) contributes to K-Kbar and D-Dbar mixings only through non-renormalizable operators involving SU(2)_L quark-doublets, we derive model-independent correlations among CP violating observables of the two systems. Due to universality of CP violation in Delta F=1 processes the bound on CP violation in Kaon mixing generically leads to an upper bound on the size of CP violation in D mixing. Interestingly, this bound is similar in magnitude to the current sensitivity reached by the LHCb experiment which is starting now to probe the natural predictions of alignment models. Within SUSY, we perform an exact analytical computation of the full set of contributions for the D-Dbar mixing amplitude. We point out that chargino effects are comparable and often dominant with respect to gluino contributions making their inclusion in phenomenological analyses essential. As a byproduct, we clarify the limit of applicability of the commonly used mass insertion approximation in scenarios with quasi-degenerate and split squarks.

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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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arXiv2015

Same sign di-lepton candles of the composite gluons

Composite Higgs models, where the Higgs boson is identified with the pseudo-Nambu-Goldstone-Boson (pNGB) of a strong sector, typically have light composite fermions (top partners) to account for a light Higgs. This type of models generically also predicts the existence of heavy vector fields (composite gluons) which appear as an octet of QCD. These composite gluons generically become very broad resonances once phase-space allows them to decay into two composite fermions. This makes their traditional experimental searches, which are designed to look for narrow resonances, quite ineffective. In this paper we, as an alternative, propose to utilize the impact of composite gluons on the production of top partners to constrain their parameter space. We place constraints on the parameters of the composite resonances using the 8 TeV LHC data and also assess the reach of the 14 TeV LHC. We find that the high luminosity LHC will be able to probe composite gluon masses up to $\sim 6$ TeV, even in the broad resonance regime.

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arXiv2015

The Dimension Six Triple Gluon Operator in Higgs+Jet Observables

Recently a lot of progress has been made towards a full classification of new physics effects in Higgs observables by means of effective dimension six operators. Specifically, Higgs production in association with a high transverse momentum jet has been suggested as a way to discriminate between operators that modify the Higgs-top coupling and operators that induce an effective Higgs-gluon coupling---a distinction that is hard to achieve with signal strength measurements alone. With this article we would like to draw attention to another source of new physics in Higgs+jet observables: the triple gluon operator $O_{3g}$ (consisting of three factors of the gluon field strength tensor). We compute the distortions of kinematic distributions in Higgs+jet production at a 14 TeV LHC due to $O_{3g}$ and compare them with the distortions due to dimension six operators involving the Higgs doublet. We find that the transverse momentum, the jet rapidity and the difference between the Higgs and jet rapidity are well suited to distinguish between the contributions from $O_{3g}$ and those from other operators, and that the size of the distortions are similar if the Wilson coefficients are of the same order as the expected bounds from other observables. We conclude that a full analysis of new physics in Higgs+jet observables must take the contributions from $O_{3g}$ into account.

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arXiv2015

Global Bayesian Analysis of the Higgs-boson Couplings

We present preliminary results of a bayesian fit to the Wilson coefficients of the Standard Model gauge invariant dimension-6 operators involving one or more Higgs fields, using data on electroweak precision observables and Higgs boson signal strengths.

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arXiv2014

Hint of Lepton Flavour Non-Universality in $B$ Meson Decays

The LHCb collaboration has recently presented their result on R_K = BR(B+ -> K+ mu+ mu-)/ BR(B+ -> K+ e+ e-) for the dilepton invariant mass bin m_{ll}^2 = 1-6 GeV^2 (l = mu, e). The measurement shows an intriguing 2.6 sigma deviation from the Standard Model (SM) prediction. In view of this, we study model independent New Physics (NP) explanations of R_K consistent with other measurements involving b -> s l l transition, relaxing the assumption of lepton universality. We perform a Bayesian statistical fit to the NP Wilson Coefficients and compare the Bayes Factors of the different hypotheses in order to quantify their goodness-of-fit. We show that the data slightly favours NP in the muon sector over NP in the electron sector.

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arXiv2014

Nonuniversality of indirect CP asymmetries in $D \to ππ, KK$ decays

We point out that, if the direct CP asymmetries in the $D \to π^+ π^-$ and $D \to K^+ K^-$ decays are unequal, the indirect CP asymmetries as measured in these modes are necessarily unequal. This nonuniversality of indirect CP asymmetries can be significant with the right amount of new physics contributions, a scenario that may be fine-tuned, but is still viable. A model-independent fit to the current data allows different indirect CP asymmetries in the above two decays. This could even be contributing to the apparent tension between the difference CP asymmetries $ΔA_{\rm CP}$ measured through the pion-tagged and muon-tagged data samples at the LHCb. This also implies that the measurements of $A_Γ$ and $y_{\rm CP}$ in the $π^+ π^-$ and $K^+ K^-$ decay modes can be different, and averaging over these two modes should be avoided. In any case, the complete analysis of CP violation measurements in the $D$ meson sector needs to take into account the possibility of different indirect CP asymmetries in the $π^+π^-$ and $K^+ K^-$ channels.

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arXiv2014

Multi-leptons and Top-jets in the Hunt for Gluinos in R-parity Violating Supersymmetry

The presence of R-parity ($\mathcal{R}_p$) violation offers intersting decay channels for the gluinos. In this work we present a new search strategy for the gluinos in the presence of semileptonic $\mathcal{R}_p$ violating couplings $λ^{'}_{133}$ and $λ^{'}_{233}$. We consider two scenarios (i) $λ^{'}$ induced 3-body decay of gluinos to a top quark ($t$), a bottom quark ($b$) and a light lepton ($\ell$) (ii) cascade decay of gluinos to top quarks and neutralinos ($\widetildeχ_1^0$) followed by the decay of $\widetildeχ_1^0$ to $t$, $b$ and $\ell$ through $λ^{'}$ couplings. We present two different search procedures which are common to both the scenarios. While the first one involves the traditional approach with multi-leptons and $b$-tagged jets, the second one employs the more recent technique to reconstruct highly energetic hadronically decaying top quarks. We perform a detailed simulation of the signal as well as all the relevant Standard Model backgrounds to show that the second procedure offers slightly better sensitivity for gluino discovery. In both the procedures, a $\geq$ 5$σ$ discovery is possible for the gluino mass in the range 1.5 -1.7 TeV at 14 TeV LHC with 50 fb$^{-1}$ integrated luminosity.

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arXiv2014

Explaining the $B \to K^\ast μ^+ μ^-$ data with scalar interactions

Recent LHCb results on the decay $B \to K^\ast μ^+ μ^-$ show significant deviations from the SM estimates in some of the angular correlations. In this paper we study the possibility of explaining these deviations using new scalar interactions. We show that new dimension-6 four-quark operators of scalar and pseudo-scalar type can successfully account for the discrepancies even after being consistent with other experimental measurements. We also briefly discuss possible extensions of the Standard Model where these operators can be generated.

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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

Boosted di-boson from a mixed heavy stop

The lighter mass eigenstate ($\widetilde{t}_1$) of the two top squarks, the scalar superpartners of the top quark, is extremely difficult to discover if it is almost degenerate with the lightest neutralino ($\widetildeχ_1^0$), the lightest and stable supersymmetric particle in the R-parity conserving supersymmetry. The current experimental bound on $\widetilde{t}_1$ mass in this scenario stands only around 200 GeV. For such a light $\widetilde{t}_1$, the heavier top squark ($\widetilde{t}_2$) can also be around the TeV scale. Moreover, the high value of the higgs ($h$) mass prefers the left and right handed top squarks to be highly mixed allowing the possibility of a considerable branching ratio for $\widetilde{t}_2 \to \widetilde{t}_1 h$ and $\widetilde{t}_2 \to \widetilde{t}_1 Z$. In this paper, we explore the above possibility together with the pair production of $\widetilde{t}_2$ $\widetilde{t}_2^*$ giving rise to the spectacular di-boson + missing transverse energy final state. For an approximately 1 TeV $\widetilde{t}_2$ and a few hundred GeV $\widetilde{t}_1$ the final state particles can be moderately boosted which encourages us to propose a novel search strategy employing the jet substructure technique to tag the boosted $h$ and $Z$. The reconstruction of the $h$ and $Z$ momenta also allows us to construct the stransverse mass $M_{T2}$ providing an additional efficient handle to fight the backgrounds. We show that a 4--5$σ$ signal can be observed at the 14 TeV LHC for $\sim$ 1 TeV $\widetilde{t}_2$ with 100 fb$^{-1}$ integrated luminosity.

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arXiv2013

The CP-violating pMSSM at the Intensity Frontier

In this Snowmass whitepaper, we describe the impact of ongoing and proposed intensity frontier experiments on the parameter space of the Minimally Supersymmetric Standard Model (MSSM). We extend a set of phenomenological MSSM (pMSSM) models to include non-zero CP-violating phases and study the sensitivity of various flavor observables in these scenarios Future electric dipole moment and rare meson decay experiments can have a strong impact on the viability of these models that is relatively independent of the detailed superpartner spectrum. In particular, we find that these experiments have the potential to probe models that are expected to escape searches at the high-luminosity LHC.

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arXiv2013

Stop and sbottom search using dileptonic $M_{T2}$ variable and boosted top technique at the LHC

The ATLAS and CMS experiments at the CERN LHC have collected about 20 $fb^{-1}$ of data each at the end of their 8 TeV run, and ruled out a huge swath of parameter space in the context of Minimally Supersymmetric Standard Model(MSSM). Limits on masses of the gluino ($\tilde{g}$) and the squarks of the first two generations($\tilde{q}$) have been pushed to the TeV range. Light third generation squarks namely stop and sbottom of sub-TeV masses, on the other hand, are still allowed by their direct search limits. Interestingly, the discovery of a Standard Model(SM) higgs boson like particle with a mass of $\sim$ 125 GeV favours a light third generation which is also motivated by naturalness arguments. Decays of stop and sbottom quarks can in general produce a number of distinct final states which necessitate different search strategies in the collider experiments. In this paper we, on the other hand, propose a general search strategy to look for third generation squarks in the final state which contains a top quark in the sample along with two additional hard leptons and substantial missing transverse momentum. We illustrate that a search strategy using the dileptonic $M_{T2}$, the effective mass $m_{eff}$ and jet substructure to reconstruct the hadronic top quark can be very effective to reduce the SM backgrounds. With the proposed search strategy, we estimate that the third generation squarks with masses up to about 800 GeV can be probed at the 14 TeV LHC with a 100 $fb^{-1}$ data set. We also interpret our results in two simplified scenarios where we consider the stop (sbottom) pair production followed by their subsequent decay to a top quark and the second lightest neutralino (lightest chargino). In this case also we find that stop (sbottom) mass up to 1 TeV (0.9 TeV) can be discovered at the 14 TeV LHC with 100 $fb^{-1}$ integrated luminosity.

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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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