arXiv subjects

Anirban Kundu

Publications and source records attributed to Anirban Kundu.

At least 55 records · Page 3Linked to original sources

arXiv2016

Novel method to deal with off-shell particles in cascade decays

We propose a novel algorithm to compute the width of any generic $n$-body decay involving multiple off-shell particles having zero and non-zero spins. Starting from a toy example, we show the computations for three different processes that contain spin-0, $\frac{1}{2}$, and 1 off-shell particles. We have checked that our results match with the existing results at the analytical level. This proposal can be automatized and should be useful to compute the phase space for long cascade decays, without any Monte Carlo sampling.

hep-ph↗

arXiv2016

Diphoton excess at 750 GeV: Singlet scalars confront triviality

We suggest that the recently observed diphoton excess at 750 GeV comes from a quasi-degenerate bunch of gauge singlet scalars produced and decaying through one or more vector-like fermions. This explains the broad nature of the resonance, even though the decay is loop-mediated. At the same time, the model keeps the new Yukawa couplings in the perturbative region, which is necessary for the stability of the potential.

hep-ph↗

arXiv2015

On the scalar potential of two-Higgs doublet models

We perform a detailed analysis of the Two-Higgs Doublet Model (2HDM) potential. At the tree-level, the potential may accommodate more than one minima, one of them being the electroweak (EW) minimum where the universe lives. The parameter space allowed after the data from the Large Hadron Collider (LHC) came in almost excludes those cases where the EW vacuum is shallower than the second minimum. We extend the analysis by including terms in the 2HDM potential that break the $Z_2$ symmetry of the potential by dimension-4 operators and show that the conclusions remain unchanged. Furthermore, a one-loop analysis of the potential is performed for both cases, namely, where the $Z_2$ symmetry of the potential is broken by dimension-2 or dimension-4 operators. For quantitative analysis, we show our results for the Type-II 2HDM, qualitative results remaining the same for other 2HDMs. We find that the nature of the vacua from the tree-level analysis does not change; the EW vacuum still remains deeper.

hep-ph↗

arXiv2015

A Hybrid Approach to Word Sense Disambiguation Combining Supervised and Unsupervised Learning

In this paper, we are going to find meaning of words based on distinct situations. Word Sense Disambiguation is used to find meaning of words based on live contexts using supervised and unsupervised approaches. Unsupervised approaches use online dictionary for learning, and supervised approaches use manual learning sets. Hand tagged data are populated which might not be effective and sufficient for learning procedure. This limitation of information is main flaw of the supervised approach. Our proposed approach focuses to overcome the limitation using learning set which is enriched in dynamic way maintaining new data. Trivial filtering method is utilized to achieve appropriate training data. We introduce a mixed methodology having Modified Lesk approach and Bag-of-Words having enriched bags using learning methods. Our approach establishes the superiority over individual Modified Lesk and Bag-of-Words approaches based on experimentation.

cs.CL↗

arXiv2015

Indirect Probe of Electroweak-Interacting Particles at Future Lepton Colliders

Various types of electroweak-interacting particles, which have non-trivial charges under the $\mathrm{SU}(2)_L \times \mathrm{U}(1)_Y$ gauge symmetry, appear in various extensions of the Standard Model. These particles are good targets of future lepton colliders, such as the International Linear Collider (ILC), the Compact LInear Collider (CLIC) and the Future Circular Collider of electrons and positrons (FCC-ee). An advantage of the experiments is that, even if their beam energies are below the threshold of the production of the new particles, quantum effects of the particles can be detected through high precision measurements. We estimate the capability of future lepton colliders to probe electroweak-interacting particles through the quantum effects, with particular focus on the wino, the Higgsino and the so-called minimal dark matters, and found that a particle whose mass is greater than the beam energy by 100-1000 GeV is detectable by measuring di-fermion production cross sections with $O(0.1)$\% accuracy. In addition, with the use of the same analysis, we also discuss the sensitivity of the future colliders to model independent higher dimensional operators, and found that the cutoff scales corresponding to the operators can be probed up to a few ten TeV.

hep-ph↗

arXiv2015

Two more hidden scalars around 125 GeV and $h \to μτ$

We show that the $2.4σ$ signal of the leptonic flavor violating (LFV) Higgs boson decay $h\toμτ$, as observed by the CMS collaboration recently, can be explained by a certain class of two-Higgs doublet models that allow controllable flavor-changing neutral current with minimal number of free parameters. We postulate that (i) the alignment limit is maintained, which means the lightest neutral scalar ($h$) has identical couplings to that of the Standard Model Higgs boson and (ii) the signal comes from two other neutral scalars, the CP-even $H$ and the CP-odd $A$, almost degenerate with $h$ at 125 GeV. We also show that (i) it is entirely possible that these scalars are hidden, apart from this LFV signal; (ii) the signal strengths of $b\bar{b}$, $τ^+τ^-$ and $γγ$ around 125 GeV put severe constraints on the parameter space of such models; (iii) the constraint is further enhanced by the non-observation of processes like $μ\to eγ$, and we predict that the branching ratio of $μ\to eγ$ cannot be even an order below the present experimental limit, highlighting the role it plays in forcing $H$ and $A$ to be near-degenerate; (iv) an enhancement in the $τ^+τ^-$ production cross-section at around 125 GeV is expected in the gluon fusion channel, and should be observed during the next run of the LHC; (v) the branching ratio in the $eτ$ channel is enhanced and is expected to be at least about $2\%$. The constrained parameter space and minimum number of free parameters, along with such strong predictions, make this model easily testable and falsifiable.

hep-ph↗

arXiv2014

Two-Higgs doublet models confront the naturalness problem

The conjecture that some unknown symmetry is responsible for keeping the Higgs boson light at 125 GeV does not hold for the Standard Model, where the coefficient of the quadratic divergence of Higgs boson self-energy is far from zero. We show that such a cancellation can be achieved in two-Higgs doublet models, by virtue of which all the scalars remain at the electroweak scale and the naturalness problem is avoided. We explore the consequences of such cancellations in different two-Higgs doublet models with no flavour-changing neutral current, and show that the parameter space becomes tightly constrained; in particular, the ratio of two vacuum expectation values, $\tanβ$, no longer remains a free parameter but turns out to be a function of the quartic couplings.

hep-ph↗

arXiv2014

LHC signatures of neutrino mass generation through R-parity violation

We consider a model of neutrino mass based on R-parity violating (RPV) supersymmetry, with three $μ_i$, relevant for bilinear RPV terms, and three $λ'_{ijk}$, relevant for the trilinear terms. The present neutrino data, after a precise determination of the mixing angle $θ_{13}$, severely constrain such models. We make a thorough study of one such class of models that may have interesting signatures at the upgraded LHC. In this class of models, the relevant trilinear couplings are of the form $λ'_{i33}$, so if the lighter stop squark $\tilde{t}_1$ is the lightest supersymmetric particle (LSP), it will decay only through these couplings, giving rise to events with isolated hard leptons and jets. Even when $\tilde{t}_1$ is the next-to-LSP (NLSP), it can decay via the tiny $λ'$ couplings allowed by the neutrino data, although it may face stiff competition from some R-parity conserving decay modes. Using a simple Pythia based simulation, we find that in both the cases the signal consisting of a pair of oppositely charged leptons ($ee$, $μμ$ or $e μ$) plus jets may be observable at the upgraded LHC experiments for a reasonable range of the $\tilde{t}_1$ mass.

hep-ph↗

arXiv2014

Feasibility of light scalars in a class of two-Higgs-doublet models and their decay signatures

We demonstrate that light charged and extra neutral scalars in the (100-200) GeV mass range pass the potentially dangerous flavor constraints in a particular class of two-Higgs-doublet model which has appropriately suppressed flavor-changing neutral currents at tree level. We study their decay branching ratios into various fermionic final states and comment on the possibility of their detection in the collider experiments. We also remark on how their trademark decay signatures can be used to discriminate them from the light nonstandard scalars predicted in other two-Higgs-doublet models.

hep-ph↗

arXiv2014

Triplet-extended scalar sector and the naturalness problem

We consider the extension of the Standard Model by a complex scalar triplet field, which occurs naturally in several models of leptogenesis and see-saw mechanism for neutrino mass generation, in the context of ameliorating the fine-tuning problem of the fundamental scalars through the Veltman Condition, i.e. by demanding the sum of the quadratically divergent corrections to vanish (or be at a reasonable level) by virtue of some possible symmetry of the underlying theory. We show that it is possible to cancel all the scalar one-loop quadratic divergences and hence obtain a viable solution for the fine-tuning problem, while satisfying the electroweak precision observables, including the $ρ$ parameter, and successfully generating the neutrino masses. The stability of the scalar potential puts important constraints on the model.

hep-ph↗

arXiv2014

Z-pole observables in an effective theory

There are two Z-peak observables related to the pair production of bottom quarks that show a deviation of about 2.5σ each from Standard Model expectations. While the discrepancy in the forward-backward asymmetry is a long-standing one, the tension for the second observable, namely the ratio of the partial width for a Z decaying to a pair of bottom quarks to the total hadronic decay width of the Z, has recently gone up due to a full two-loop evaluation of the Standard Model contributions. We show how both these discrepancies may be explained in the framework of new physics that couples only to the third generation of quarks. In the paradigm of effective operators, the Wilson coefficients of some of the possible operators are already very tightly constrained by flavour physics data. However, there still remain certain operators, particularly those involving right-chiral quark fields, which can successfully explain the anomalies. We also show how the footprints of such operators may be observed at the upgraded LHC.

hep-ph↗

arXiv2014

Minimal supersymmetry confronts $R_b$, $A^b_{FB}$ and $m_h$

We study the impact of the measurements of three sets of observables on the parameter space of the constrained minimal supersymmetric Standard Model (cMSSM), its slightly general variant, the non-universal scalar model (NUSM), and some selected benchmark points of the 19-parameter phenomenological MSSM (pMSSM): ($i$) the direct measurement of the Higgs boson mass $m_h \approx 125$ GeV at the CERN Large Hadron Collider (LHC); ($ii$) $Z$ boson decay width in the $b \bar{b}$ channel normalized to its hadronic width ($R_b$), and the forward-backward asymmetry on the $Z$-peak in the same channel $(A_{\rm FB}^b)$; and ($iii$) several $B$-physics observables, along with $(g-2)$ of muon. In addition, there are constraints from non-observation of superparticles from direct searches at the LHC. In view of the recently re-estimated standard model (SM) value of $R_b$ with improved higher order corrections, the measured value of $R_b$ has a 1.2$σ$ discrepancy with its SM value, while the corresponding discrepancy in $A_{\rm FB}^b$ is 2.5$σ$. MSSM contributions from light superpartners improve the agreement of $R_b$ but worsen that of $A_{\rm FB}^b$. We project this ($R_b$-$A_{\rm FB}^b$) tension {\em vis-à-vis} the constraints arising from other observables in the parameter space of cMSSM and NUSM. We also consider a few well-motivated pMSSM benchmark points and show that pMSSM does not fare any better than the SM.

hep-ph↗

arXiv2013

Anomalous Higgs Couplings as a Window to New Physics

The initial data on the production and decay of the Higgs boson reported significant deviations from the Standard Model (SM) expectations, prompting much speculation about its couplings to the other particles. Although the latest data has veered towards conformity with the SM, there is yet room for a sizable deviation from the SM values of the coupling of the Higgs boson with $t\bar{t}$, and to a smaller extent, of that with $W^+W^-$ and $ZZ$. Keeping the fluid nature of the data in mind, this opens up an interesting avenue to explore regarding unitarity of gauge boson scattering and the stability of the electroweak vacuum in the presence of anomalous couplings. We show that, for some typical benchmark points, unitarity in gauge boson scattering breaks down between 1 and 10 TeV. We also show that if there are no new light degrees of freedom, the Higgs quartic coupling becomes negative at around the same point, making the electroweak vacuum unstable. Thus, some new ultraviolet completing new physics is demanded at that scale to cancel both these anomalous behaviours if such deviations from the SM couplings are indeed established.

hep-ph↗

arXiv2013

CPT violation and triple-product correlations in B decays

The T-odd triple product (TP) asymmetries in B decays to a pair of vector mesons are treated as a good probe of CP violation because of the CPT symmetry. If CPT is no longer a good symmetry, such correlations between T-odd and CP-odd observables do not exist, and one might get unexpected nonzero TP asymmetries as a signal for CPT violation. We give a general formalism of TP asymmetries in the presence of CPT violation, either in decay or in neutral meson mixing. We also discuss how the observables depending on the transversity amplitudes are modified, and compare our expressions with the LHCb results, showing that the study of TP asymmetries might turn out to be one of the best probes for CPT violation.

hep-ph↗

arXiv2013

Probing CPT Violation in B Systems

We discuss how a possible violation of the combined symmetry CPT in the B meson system can be investigated at the LHC. We show how a tagged and an untagged analysis of the decay modes of both B(d) and B(s) mesons can lead not only to a possible detection of a CPT-violating new physics but also to an understanding of its precise nature. The implication of CPT violation to a large mixing phase in the B(s) system is also discussed.

hep-ph↗

arXiv2013

Controlling the fine-tuning problem with singlet scalar dark matter

Assuming that no other conventional new physics is found immediately at the LHC, we investigate how just the consistent solution of the scalar mass hierarchy problem points towards the minimal necessary field content. We show that to ameliorate the fine-tuning problem, one needs to introduce more scalar degrees of freedom. The simplest solution is one or more real singlets (with the possibility of combining two of them in a complex singlet), which may act as viable cold dark matter candidates, because the constraints on the scalar potential disfavor any mixing between the new scalar(s) with the SM doublet. Furthermore, the fine-tuning problem of the new scalars necessitates the introduction of vector-like fermions. Thus, singlet scalar(s) and vector fermions are minimal enhancements over the Standard Model to alleviate the fine-tuning problem. We also show that the model predicts Landau poles for all the scalar couplings, whose positions depend only on the number of such singlets. Thus, introduction of some new physics at that scale becomes inevitable. We also discuss how the model confronts the LHC constraints and the latest XENON100 data.

hep-ph↗

arXiv2013

B decay anomalies in an effective theory

We investigate how far a new physics scenario affecting primarily the third generation fermions can ameliorate the tension between B-decay observables and Standard Model expectations. Adopting a model-independent approach, we find that among the three observables that show signs of such a tension, viz. the branching fractions for $B^+\toτν$, $B_d\to D(D^\ast)τν$, and the like-sign dimuon anomaly in neutral B decays, the first two can be explained adequately, while there is only a marginal improvement for the third. As a spin-off, it is shown that one can also accommodate a change in the branching fraction of the Higgs boson to a $τ$ lepton pair from the SM expectation, if such a change is established in future data.

hep-ph↗

arXiv2013

$B_s\to D_s K$ as a Probe of CPT Violation

We discuss some possible signals of CPT violation in the $B_s$ system that may be probed at the Large Hadron Collider (LHC). We show how one can construct combinations ofobservables coming from tagged and untagged decay rates of $B_s\to D_s^\pm K^\mp$ that can unambiguously differentiate between CPT violating and CPT conserving new physics (NP) models contributing in $B_s-\bar B_s$ mixing. We choose this particular mode as an illustrative example for two reasons: (i) In the Standard Model, there is only one decay amplitude, so it is easier to untangle any new physics; (ii) $B_s$ being a neutral meson, it is possible to unambiguously identify any sign of CPT violation that occurs only in mixing but not in decay. We define an observable which is useful to extract the CPT violating parameter in $B_s$ decay, and also discuss how far the results are applicable even if CPT violation is present in both mixing and decay.

hep-ph↗