arXiv subjects

Syuhei Iguro

Publications and source records attributed to Syuhei Iguro.

At least 55 records · Page 3Linked to original sources

arXiv2022

Scrutinizing the 95-100 GeV di-tau excess in the top associated process

Recently, the CMS collaboration has reported a di-tau excess with a local significance of 2.6-3.1$σ$ where the invariant mass is $m_{ττ}=$95-100 GeV. This excess can be interpreted as a light scalar boson that couples to the third generation fermions, particularly top and $τ$. Based on the simplest model that can account for the CMS di-tau excess, we evaluate experimental sensitivities to the additional light resonance, using the results reported by the ATLAS collaboration. We see that a search for the top-quark associated production of the SM Higgs boson that decays into $τ\barτ$ sets a strong model-independent limit. We also find that the CP-even scalar interpretation of the light resonance is excluded by the ATLAS results, while the CP-odd interpretation is not.

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arXiv2022

Testing Leptoquark/EFT in $\bar B \to D^{(*)} l\barν$ at the LHC

We investigate the current LHC bounds on New Physics (NP) that contributes to $\bar B \to D^{(*)} l\barν$ for $l = (e,μ,τ)$ by considering both leptoquark (LQ) models and an effective-field-theory (EFT) Hamiltonian. Experimental analyses from $l+\text{missing}$ searches with high $p_T$ are applied to evaluate the NP constraints with respect to the Wilson coefficients. A novel point of this work is to show difference between LQs and EFT for the applicable LHC bound. In particular, we find that the EFT description is not valid to search for LQs with the mass less than $\lesssim 10\,\text{TeV}$ at the LHC and leads to overestimated bounds. We also discuss future prospects of high luminosity LHC searches including the charge asymmetry of background and signal events. Finally, a combined summary for the flavor and LHC bounds is given, and then we see that in several NP scenarios the LHC constraints are comparable with the flavor ones.

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arXiv2022

Importance of vector leptoquark-scalar box diagrams in Pati-Salam unification with vector-like families

We study lepton flavor violating meson decays induced by box diagrams involving a vector leptoquark (LQ) and scalar fields in Pati-Salam (PS) unification with vector-like families. The vector LQ corresponds to the massive gauge boson associated with the PS gauge symmetry breaking and the scalar fields are the physical degrees of freedom of the PS breaking field. The LQ generally causes the rapid flavor violating decays, such as $K_L \to μe$, at the tree-level unless its mass scale is higher than PeV scale. The vector-like families are introduced to suppress the tree-level contributions mediated by the LQ and explain the realistic fermion mass matrices. In this paper, we point out that there are inevitable one-loop contributions to those meson decays from the box diagrams mediated by both one LQ and one scalar field, even if the tree-level contributions are suppressed. We consider a concrete model for demonstration, and find that the vector-like fermion masses have an upper bound for a given LQ mass when the one-loop induced meson decays are consistent with the experimental limits. The vector-like fermion mass should be lighter than 3 TeV for 20 TeV LQ, if a combination of the couplings does not suppress $K_L \to μe$ decay. Our findings would illustrate importance of the box diagrams involving both LQ and physical modes of symmetry breaking scalars in generic models.

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arXiv2022

Interplay between the $R_{D^{(*)}}$ anomaly and the LHC

The current discrepancy in $R_{D^{(*)}}=BR(\bar{B} \to D^{(*)}τ\barν)/BR(\bar{B}\to D^{(*)} \ell \barν)$ where $\ell=e,\,μ$ may imply a know particle with the mass around the order of TeV. Therefore it is well motivated to test those scenarios at the large hadron collider. Thanks to the size of the discrepancy and the analysis based on the effective field theory, the variety of the possible new physics candidates is limited. We focus on the charged Higgs and leptoquark possibility and discuss the collider physics.

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arXiv2022

Towards ruling out the charged Higgs interpretation of the $R_{D^{(*)}}$ anomaly

Motivated by the notorious anomaly in the lepton flavor universality ratios $R_{D^{(*)}}$, we study the sensitivity of the Large Hadron Collider (LHC) to a low-mass charged Higgs boson $H^-$ lighter than $400\,$GeV in a generic two Higgs doublet model. A combination of current constraints from the $B_c\toτν$ decay, $B_{s}$ meson mixing data, tau sleptons and di-jet searches at the LHC allows to explain the $R_{D^{(*)}}$ anomaly at the $1\,σ$ level by a low-mass charged Higgs. In this context, we estimate the reach of an LHC search for resonant $H^-$ production, where the final state contains an energetic $τ$ lepton decaying hadronically, a neutrino with large transverse momentum, and an additional $b$-jet ($pp \to b+τ_h + ν$). Requiring the additional $b$-tagged jet in the $τν$ resonance search profits from the suppression of the Standard Model background, and therefore it allows us to judge the low-mass $H^-$ interpretation of the $R_{D^{(*)}}$ anomaly. To demonstrate this, we perform a fast collider simulation for the $τν$ resonance search with an additional $b$-tagged jet, and find that most of the interesting parameter region of the whole mass range can already be probed with the current integrated luminosity of $139\,$fb$^{-1}$.

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arXiv2022

Revival of $H^-$ interpretation of $R_{D^{(*)}}$ anomaly and closing low mass window

Thanks to the recent careful revisit of the theoretical prediction of the $B_c$ meson lifetime, the conservative upper bound on the branching ratio (BR) of $τν$ mode is found to be $\simeq 63\%$ due to the large charm quark mass uncertainty. Although it is well known that a charged Higgs ($H^-$) interpretation of the $R_{D^{(*)}}$ anomaly is excluded by the previously proposed bounds, BR$(B_c\toτν)\le 30\%$ and $\le10\%$, $H^-$ can still explain the anomaly within $1σ$ if we adopt the 63$\%$ one. The scalar contribution is also favored by the polarization data $F_L^{D^*}$ measured at the Belle. Since the implied NP scale is within the reach of the Large Hadron Collider (LHC), collider searches are powerful tools to test the scenario. For instance, the $τν$ resonance search has already put the more stringent bound for $m_{H^-}\ge 400\,$GeV. In this work we revisit the further lighter mass range, $180\,$GeV$\le m_{H^-}\le 400\,$GeV which has not been covered yet. We will see that a combination of the conventional stau search and low mass flavor inclusive and bottom flavored di-jet resonance searches can place a new limit on the interpretation. We summarize the current status of the low mass region and discuss the future sensitivity in the high luminosity (HL)-LHC based on the existent collider constraints.

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arXiv2022

Monopoles From an Atmospheric Fixed Target Experiment

Magnetic monopoles have a long history of theoretical predictions and experimental searches, carrying direct implications for fundamental concepts such as electric charge quantization. We analyze in detail for the first time magnetic monopole production from collisions of cosmic rays bombarding the atmosphere. This source of monopoles is independent of cosmology, has been active throughout Earth's history, and supplies an irreducible monopole flux for all terrestrial experiments. Using results for robust atmospheric fixed target experiment flux of monopoles, we systematically establish direct comparisons of previous ambient monopole searches with monopole searches at particle colliders and set leading limits on magnetic monopole production in the $\sim 5-100$ TeV mass-range.

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arXiv2022

Non-resonant new physics search at the LHC for the $b \to c τν$ anomalies

Motivated by the $b \to c τ\overlineν$ anomalies, we study non-resonant searches for new physics at the large hadron collider (LHC) by considering final states with an energetic and hadronically decaying $τ$ lepton, a $b$-jet and large missing transverse momentum ($pp \to τ_h \overline{b} + E_{\rm T}^{\rm miss}$). Such searches can be useful to probe new physics contributions to $b \to c τ\overlineν$. They are analyzed not only within the dimension-six effective field theory (EFT) but also in explicit leptoquark (LQ) models with the LQ non-decoupled. The former is realized by taking a limit of large LQ mass in the latter. It is clarified that the LHC sensitivity is sensitive to the LQ mass for $O(1)$ TeV even in the search of $pp \to τ_h \overline{b} + E_{\rm T}^{\rm miss}$. Although the LQ models provide a weaker sensitivity than the EFT limit, it is found that the non-resonant search of $pp \to τ_h \overline{b} + E_{\rm T}^{\rm miss}$ can improve the sensitivity by $\approx 40\%$ versus a conventional mono-$τ$ search ($pp \to τ_h + E_{\rm T}^{\rm miss}$) in the whole LQ mass region. Consequently, it is expected that most of the parameter regions suggested by the $b \to c τ\overlineν$ anomalies can be probed at the HL-LHC. Also, it is shown that $\text{R}_2$ LQ scenario is accessible entirely once the LHC Run 2 data are analyzed. In addition, we discuss a charge selection of $τ_h$ to further suppress the standard-model background, and investigate the angular correlations among $b,\, τ$ and the missing transverse momentum to discriminate the LQ scenarios.

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arXiv2022

Revisiting rescattering contributions to $\overline{B}_{(s)}\to D^{(*)}_{(s)} M$ decays

Motivated by the reported discrepancies between experimental data and Standard Model predictions for the branching ratios of the color-allowed decays $\overline{B}^0 \to D^{(*)+}K^-$ and $\overline{B}^0_s \to D^{(*)+}_s π^-$, we study final-state rescattering effects on $\overline{B}_{(s)}\to D^{(*)}_{(s)} P$ and $\overline{B}_{(s)}\to D_{(s)} V$, where $P$ ($V$) is a light pseudoscalar (vector) meson. We consider quasi-elastic rescatterings in the framework of $SU(3)$ and $U(3)$ flavor symmetries, and find that the effects cannot explain the measured branching ratios of the color-allowed and color-suppressed decays simultaneously. We also perform global fits to the $\overline{B}_{(s)}\to D^{(*)}_{(s)} P$ and $\overline{B}_{(s)}\to D_{(s)} V$ data, allowing for new physics contributions to the Wilson coefficient $a_1$ associated with the color-allowed tree amplitudes. It is shown that the fits prefer a downward shift of $\mathcal{O}(10\%)$ in $a_1$ even in the presence of the quasi-elastic rescattering contributions.

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arXiv2022

Bayesian fit analysis to full distribution data of $\bar B \to D^{(*)} \ell\barν$: $|V_{cb}|$ determination and New Physics constraints

We investigate the semi-leptonic decays of $\bar B \to D^{(*)} \ell\barν$ in terms of the Heavy-Quark-Effective-Theory (HQET) parameterization for the form factors, which is described with the heavy quark expansion up to $\mathcal O(1/m_c^2)$ beyond the simple approximation considered in the original CLN parameterization. An analysis with this setup was first given in the literature, and then we extend it to the comprehensive analyses including (i) simultaneous fit of $|V_{cb}|$ and the HQET parameters to available experimental full distribution data and theory constraints, and (ii) New Physics (NP) contributions of the $V_2$ and $T$ types to the decay distributions and rates. For this purpose, we perform Bayesian fit analyses by using Stan program, a state-of-the-art public platform for statistical computation. Then, we show that our $|V_{cb}|$ fit results for the SM scenarios are close to the PDG combined average from the exclusive mode, and indicate significance of the angular distribution data. In turn, for the $\text{SM} + \text{NP}$ scenarios, our fit analyses find that non-zero NP contribution is favored at the best fit point for both $\text{SM} + V_2$ and $\text{SM} + T$ depending on the HQET parameterization model. A key feature is then realized in the $\bar B \to D^{(*)} τ\barν$ observables. Our fit result of the HQET parameters in the $\text{SM} (+T)$ produces a consistent value for $R_D$ while smaller for $R_{D^*}$, compared with the previous SM prediction in the HFLAV report. On the other hand, $\text{SM}+V_2$ points to smaller and larger values for $R_D$ and $R_{D^*}$ than the SM predictions. In particular, the $R_{D^*}$ deviation from the experimental measurement becomes smaller, which could be interesting for future improvement on measurements at the Belle II experiment.

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arXiv2021

Higgs flavor phenomenology in a supersymmetric left-right model with parity

In this paper, we focus on the supersymmetric model with left-right (LR) symmetry, that is especially proposed in our previous work [1]. In this model, there are four Higgs doublets in order to realize the Standard Model (SM) fermion masses and the Cabibbo-Kobayashi-Maskawa matrix. The heavy Higgs doublets unavoidably have flavor changing couplings to the SM fermions and induce flavor-changing neutral currents at tree level. We study broader parameter space than the previous work with including the renormalization group corrections to the Yukawa couplings between the LR breaking scale, $\mathcal{O}(10^{13})$ GeV, and the supersymmetry breaking scales, $\mathcal{O}(100)$ TeV. The CP violating observable in $K$-$\overline{K}$ mixing, $ε_K$, strongly constrains the model, so that heavy Higgs mass should be heavier than $\mathcal{O}(100)$ TeV. We study the lepton flavor violating (LFV) processes setting heavy Higgs masses to be 170 TeV. The branching ratios of $μ\to 3 e$ and the $μ$-$e$ conversion can be larger than $10^{-16}$ that could be covered by the future experiments. We also study the degree of fine-tuning in the parameter region that predicts testable LFV processes.

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arXiv2021

TeV-scale vector leptoquark from Pati-Salam unification with vectorlike families

In this paper, we show an explicit way to realize a TeV-scale vector leptoquark from the Pati-Salam (PS) unification with extra vectorlike families. The leptoquark mass is constrained to be heavier than PeV-scale by the measurement of a flavor violating kaon decay, $K_L \to μe$, in conventional models. This strong constraint can be avoided by introducing vectorlike families consistently with the quark and lepton masses and CKM and PMNS matrices. The other flavor violating processes are also suppressed. In this model, the vector leptoquark can be sufficiently light to explain the recent $b\to sμμ$ anomaly, while the $b\to cτν$ anomaly is difficult to be explained due to the strong constraints from the $Z^\prime$ boson and vectorlike quark searches at the LHC. When the $b\to sμμ$ anomaly is explained, we show that $\mathcal{O}({0.2})$ % tuning is required in the fermion matrix, the future experiments in $μ\to eγ$ and $μ$-$e$ conversions will cover the most available parameter space, and sizable neutral meson mixings, induced by the extra Higgs doublets, are unavoidable.

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arXiv2020

Implications for new physics from a novel puzzle in $\bar{B}_{(s)}^0 \to D^{(\ast)+}_{(s)} \lbrace π^-, K^- \rbrace$ decays

Recently, the standard model predictions for the $B$-meson hadronic decays, $\bar{B}^0 \to D^{(\ast)+}K^-$ and $\bar{B}^0_s \to D^{(\ast)+}_s π^-$, have been updated based on the QCD factorization approach. This improvement sheds light on a novel puzzle in the $B$-meson hadronic decays: there are mild but universal tensions between data and the predicted branching ratios. Assuming the higher-order QCD corrections are not huge enough to solve the tension, we examine several new physics interpretations of this puzzle. We find that the tension can be partially explained by a left-handed $W^\prime$ model, which can be compatible with other flavor observables and collider bounds.

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arXiv2020

Probing $eμ$ flavor-violating ALP at Belle II

Recently, it was pointed out that the electron and muon g-2 discrepancies can be explained simultaneously by a flavor-violating axion-like particle (ALP). We show that the parameter regions favored by the muon g-2 are already excluded by the muonium-antimuonium oscillation bound. In contrast, those for the electron g-2 can be consistent with this bound when the ALP is heavier than 1.5 GeV. We propose to search for a signature of the same-sign and same-flavor lepton pairs and the forward-backward muon asymmetry to test the model at the Belle II experiment.

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arXiv2020

Probing $μτ$ flavor-violating solutions for the muon $g-2$ anomaly at Belle II

The discrepancy between the measured value and the Standard Model prediction of the muon anomalous magnetic moment is one of the most important issues in the particle physics. It is known that introducing a mediator boson X with the $μτ$ lepton flavor violating (LFV) couplings is one good solution to explain the discrepancy, due to the $τ$ mass enhancement in the one-loop correction. In this paper, we study the signal of this model, i.e. the same-sign leptons, in the Belle II experiment, assuming the flavor-diagonal couplings are suppressed. We show that the Belle II experiment is highly sensitive to the scenario in the mediator mass range of ${\cal O}(1-10)$~GeV, using the $e^+e^- \to μ^\pmτ^\mp X \toμ^\pm μ^\pm τ^\mp τ^\mp$ process induced by the $X$.

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arXiv2020

Detecting a μτ-philic Z' boson via photon initiated processes at the LHC

Discrepancy between the measured value and the Standard Model prediction of the muon anomalous magnetic moment is a possible hint for new physics. A $Z^\prime$ particle with $μτ$ flavor violating couplings can give a large contribution to the muon anomalous magnetic moment due to the $τ$ mass enhancement at the one-loop level, and is known to explain the above discrepancy. In this paper, we study the potential of the Large Hadron Collider (LHC) for detecting such a $Z^\prime$ boson via the $p p \toμ^-μ^-τ^+τ^+ $ process. Earlier studies in the literature only considered the production channel with quark initial states ($p p \to q \bar q \toμ^-μ^-τ^+τ^+ $). Here, we show that the photon initiated process, $p p \to γγ\to μ^-μ^-τ^+τ^+ $, is in fact the dominant production mode, for a heavy $Z^\prime$ boson of mass greater than a few hundred GeV. The potential of the high luminosity (HL) LHC is also considered.

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arXiv2019

Testing the 2HDM explanation of the muon g-2 anomaly at the LHC

The discrepancy between the measured value and the Standard Model prediction for the muon anomalous magnetic moment is one of the important issues in the particle physics. In this paper, we consider a two Higgs doublet model (2HDM) where the extra Higgs doublet couples to muon and tau in lepton flavor violating (LFV) way and the one-loop correction involving the scalars largely contributes to the muon anomalous magnetic moment. The couplings should be sizable to explain the discrepancy, so that the extra Higgs bosons would dominantly decay into $μτ$ LFV modes, which makes the model testable at the LHC through multi-lepton signatures even though they are produced via the electroweak interaction. We discuss the current status and the future prospect for the extra Higgs searches at the LHC, and demonstrate the reconstruction of the mass spectrum using the multi-lepton events.

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arXiv2019

Test of the $R(D^{(*)})$ anomaly at the LHC

There are discrepancies between the experimental results and the Standard Model predictions, in the lepton flavor universality of the semileptonic $B$ decays: $B \to D^{(*)} \ell ν$. As the new physics interpretations, new charged vector and charged scalar fields, that dominantly couple to the second and third generations, have been widely discussed. In this paper, we study the signals of the new particles at the LHC, and test the interpretations via the direct search for the new resonances. In particular, we see that the $τν$ resonance search at the LHC has already covered most of the parameter regions favored by the Belle and BaBar experiments. We find that the bound is already stronger than the one from the $B_c$ decay depending on the mass of charged scalar.

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