Michihisa Takeuchi

Heavy fermion bound states for diphoton excess at 750 GeV — collider and cosmological constraints

A bstractA colored heavy particle with sufficiently small width may form non-relativistic bound states when they are produced at the large hadron collider (LHC), and they can annihilate into a diphoton final state. The invariant mass of the diphoton would be around twice of the colored particle mass. In this paper, we study if such bound state can be responsible for the 750 GeV diphoton excess reported by ATLAS and CMS. We found that the best-fit signal cross section is obtained for the SU(2)L singlet colored fermion X with YX = 4/3. Having such an exotic hypercharge, the particle is expected to decay through some higher dimensional operators, consistent with the small width assumption. The decay of X may involve a stable particle χ, if both X and χ are odd under some conserved Z2 symmetry. In that case, the particle X suffers from the constraints of jets + missing ET searches by ATLAS and CMS at 8 TeV and 13 TeV. We found that such a scenario still survives if the mass difference between X and χ is above ∼ 30 GeV for mX ∼ 375 GeV. Even assuming pair annihilation of χ is small, the relic density of χ is small enough if the mass difference between X and χ is smaller than ∼ 40 GeV.

Flavor-Changing Neutral-Current Decays in Top-Specific Variant Axion Model

A bstractThe invisible variant axion model is very attractive as it is free from the domain wall problem. This model requires two Higgs doublets at the electroweak scale where one Higgs doublet carries a nonzero Peccei-Quinn (PQ) charge and the other is neutral under the PQ U(1) symmetry. We consider the most interesting and less constrained scenario of the variant axion model, where only the right-handed top quark is charged under the PQ symmetry and couples with the PQ-charged Higgs doublet. As a result, the top quark can decay to the observed standard-model-like Higgs boson h and the charm or up quark, t → h c/u, which is testable soon at the LHC Run-II. Moreover, we propose a method to probe the chiral nature of the Higgs flavor-changing interaction using the angular distribution of t → ch decays if a sufficient number of such events are observed. We also show that our model has the capacity to explain the h → τ μ decay reported by the CMS collaboration, if the right-handed tau lepton also carries a PQ charge and couples to the PQ-charged Higgs boson.

LHC τ -rich tests of lepton-specific 2HDM for (g − 2) μ

A bstractThe lepton-sepcific (or type X) 2HDM (L2HDM) is an attractive new physics candidate explaining the muon g − 2 anomaly requiring a light CP-odd boson A and large tan β. This scenario leads to τ -rich signatures, such as 3τ , 4τ and 4τ + W/Z, which can be readily accessible at the LHC. We first study the whole L2HDM parameter space to identify allowed regions of extra Higgs boson masses as well as two couplings λhAA and ξhl which determine the 125 GeV Higgs boson decays h → τ+τ− and h → AA/AA∗(τ+τ−), respectively. This motivates us to set up two regions of interest: (A) mA ≪ mH ∼ mH± , and (B) mA ∼ mH± ∼ O

Surviving scenario of stop decays for ATLAS ℓ+jets+E̸Tmiss search

mathcal{O}

t \bar t H

(100)GeV ≪ mH , for which derive the current constraints by adopting the chargino-neutralino search at the LHC8, and then analyze the LHC14 prospects by implementing τ -tagging algorithm. A correlated study of the upcoming precision determination of the 125 GeV Higgs boson decay properties as well as the observation of multi-tau events at the next runs of LHC will be able to shed light on the L2HDM option for the muon g − 2.

Channel

We study the feasibility of probing a region of natural supersymmetry where the stop and Higgsino masses are compressed. Although this region is most effectively searched for in the monojet channel, this signature is present in many other nonsupersymmetric frameworks. Therefore, another channel that carries orthogonal information is required to confirm the existence of the light stop and Higgsinos. We show that a supersymmetric version of the

Higgs mediated CLFV processes

toverline{t}H

\mu N (eN) \rightarrow \tau X

process,