Nivedita Ghosh

We come up with a novel search strategy for long-lived doubly charged scalars at future proposed lepton colliders. The doubly charged scalar studied in this work belongs to an S U ( 2 ) L complex scalar triplet that accounts for tiny neutrino masses via the type II seesaw mechanism. For scalar masses ≲ 200 GeV and appropriate values of the triplet vacuum expectation value, this state can be long-lived and decay predominantly into like-sign muon pairs (e.g. μ + μ + or μ − μ − ), producing distinctive displaced-vertex signals. We investigate the pair production of these scalars at the International Linear Collider (ILC) and a prospective muon collider, considering their planned center-of-mass energies. Incorporating theoretical and experimental constraints, we study the resulting signature of four leptons accompanied by missing transverse energy. Displaced vertices offer direct evidence of the scalar’s long lifetime, while we further show that the invariant mass distribution of same-sign dilepton pairs serves as a powerful complementary probe for discovering doubly charged Higgs bosons at both the ILC and muon collider.

We study the lepton flavor conserving and violating couplings of a doubly charged scalar at a 3 TeV muon collider. Using a model-independent Lagrangian, we analyze the e e , μ μ , and τ τ final states mediated by the doubly charged scalar to probe individual couplings to μ e , μ μ , and μ τ . We find that for a doubly charged scalar of mass greater than 1 TeV and O ( 1 ) couplings, we achieve high signal significance in these channels. We delineate the collider’s sensitivity in the mass-coupling plane, highlighting the extensive reach of the muon collider in probing these couplings far beyond the current experimental limits. We also propose an angular distribution variable to discriminate between the exchange of a doubly charged scalar from that of a neutral scalar, which gives identical signals.

In the search for beyond the Standard Model (SM) physics, long-lived particles (LLPs) have emerged as potential candidates and are being explored in various ongoing experiments. Future lepton colliders, such as the FCC-ee, shall provide an excellent opportunity to probe LLPs, owing to their clean environment and improved particle identification. This study investigates the potential of the proposed Innovative Detector for an Electron-Positron Accelerator (IDEA) detector at FCC-ee in the detection of LLPs produced from B -meson and Higgs boson decays. We explore benchmark scenarios for different final states resulting from LLP decays, including a detailed analysis of the SM long-lived hadronic background. Additionally, we propose dedicated LLP detectors with different configurations, dimensions, and locations with respect to the IDEA detector. DELIGHT B, originally proposed as a dedicated LLP detector for the FCC-hh, stands out as the detector with the maximum efficiency for detecting LLPs produced at FCC-ee. We find that cylindrical detector configurations, if feasible to construct around the IDEA detector, would also enhance sensitivity for LLPs mostly decaying outside it.

As the particle physics community has explored most of the conventional avenues for new physics, the more elusive areas are becoming increasingly appealing. One such potential region, where new physics might be hiding, involves light and weakly interacting long-lived particles (LLPs). To probe deeper into this region, where the possibility of highly displaced scenarios weakens the role of general-purpose collider detectors, dedicated LLP detectors are our best option. However, their potential can only be fully realized if we optimize their position and dimensions to suit our physics goals. This is possible at the upcoming Future Circular Collider (FCC) facility, where the feasibility and design studies are still ongoing and can accommodate new proposals focused specifically on LLP searches. We propose optimized dedicated detectors in both the transverse and forward directions, significantly enhancing the sensitivity to previously unchartered regions of the new physics parameter space. Our proposed DELIGHT detector can additionally serve as a shared transverse detector during both the FCC-ee and FCC-hh runs. The concept of a shared transverse detector is novel and sustainable, utilizing the proximity of the interaction points of the lepton and hadron colliders at the FCC. This minimizes costs and boosts the LLP physics case at FCC. Furthermore, we emphasize the impact of our proposed forward detector, FOREHUNT, along the FCC-hh beam pipe, in probing LLPs from meson decays.

We revisit the generalized two-Higgs-doublet model (2HDM) with a lepton specific (Type-X) Yukawa sector in context of the muon collider. The model offers a large region of parameter space where it can provide a solution to the observed muon ( g − 2 ) excess. Specifically the low mass C P odd scalar of this model plays an important role to fit this anomaly. Interestingly, the coupling of the pseudoscalar with the muons is proportional to tan β (which equals to the ratio of vacuum expectation values of the Higgs doublets) which dominantly controls the one loop and two loop contributions of the muon magnetic moment, therefore it can be probed directly at a muon collider with greater sensitivity in comparison to the Large Hadron Collider. Primarily focusing on a parameter space where muon g − 2 anomaly is satisfied, we explore the available theoretical and experimental constraints arising from vacuum stability, unitarity, lepton flavor violation, electroweak precision constraint, B-meson decay, and collider experiments on the model. In the parameter space where all these constraints are satisfied, we propose to study the production of the light pseudoscalar A in association with a photon at a 3 TeV muon collider. Subsequent decay of A into a τ pair will produce a striking l + l − γ plus missing energy signature.