arXiv subjects

Junya Nakamura

Publications and source records attributed to Junya Nakamura.

31 records · Page 2Linked to original sources

arXiv2020

The Power of Global Knowledge on Self-stabilizing Population Protocols

In the population protocol model, many problems cannot be solved in a self-stabilizing way. However, global knowledge, such as the number of nodes in a network, sometimes allows us to design a self-stabilizing protocol for such problems. In this paper, we investigate the effect of global knowledge on the possibility of self-stabilizing population protocols in arbitrary graphs. Specifically, we clarify the solvability of the leader election problem, the ranking problem, the degree recognition problem, and the neighbor recognition problem by self-stabilizing population protocols with knowledge of the number of nodes and/or the number of edges in a network.

cs.DC↗

arXiv2019

Role of the $Z$ polarization in the $H \rightarrow b\bar{b}$ measurement

It is widely known that the $ZH$ production channel provides a promising probe to the $H \rightarrow b\bar{b}$ decay at the LHC, when the Higgs and the $Z$ bosons are highly boosted along the transverse direction. We show how information on the $Z$ boson polarization -- that is disregarded in the current LHC analyses -- can relevantly improve the signal from background discrimination, helping us to observe and achieve a larger precision for the $H \rightarrow b\bar{b}$ measurement in the $Z(\ell\ell)H$ channel.

hep-ph↗

arXiv2018

Direct measurement of the Higgs self-coupling in $e^+e^- \to ZH$

A new method to measure the trilinear Higgs self-coupling $λ_3^{}$ in a single Higgs production process is proposed. Time-reversal-odd (T-odd) asymmetries in the process $e^+_{}e^-_{} \to ZH$, $Z \to f\bar{f}$ are computed from the absorptive part of the electroweak one-loop amplitude. Since the T-odd asymmetries measure the tree-level $t$-channel $ZH \to ZH$ scattering, they can be direct probes of $λ_3^{}$. The proposed method is quite challenging; a relatively large statistics and polarized $e^+_{}e^-_{}$ beams are demanded. However, this is probably the only approach to directly measure $λ_3^{}$ in $e^+_{}e^-_{}$ collisions, when a beam energy above the $ZHH$ production threshold is not available.

hep-ph↗

arXiv2018

Boosting the $H\to$ invisibles searches with $Z$ boson polarization

It is argued that, in $H \to $ invisibles searches with $Z(\ell\ell)H$ associated production at the LHC, the signal efficiency can be sensibly improved via a detailed study of the $Z$ boson polarization, discriminating between the signal and the dominant-irreducible $Z(\ell\ell)Z(νν)$ background. We first present a comprehensive polarization study, obtaining the complete set of angular coefficients $A_i$ in the Collins-Soper reference frame and identifying the dominant phenomenological effects. Then, we show the results for a realistic Monte Carlo study to $H\to$ invisibles, taking the polarization analysis into account. We obtain about $20\%$ improvement in the upper bound for the branching ratio of the Higgs boson to invisible particles, assuming $300\ \mathrm{fb}^{-1}$ of data at the 13 TeV LHC.

hep-ph↗

arXiv2017

Polarisations of the $Z$ and $W$ bosons in the processes $pp \to ZH$ and $pp \to W^{\pm}_{}H$

The $Z$ boson in the process $pp \to ZH$ and the $W^+_{}$ and $W^-_{}$ in the process $pp \to W^{\pm}_{}H$ can be in polarised states. The polarisation density matrix of the $Z$ ($W$) boson contains the complete information about a state of polarisation of the $Z$ ($W$) boson, and $HZZ$, $HZγ$ and $HWW$ interactions may be studied in detail from a careful analysis of these matrices. In this paper, a systematic approach to analyse these polarisation density matrices is presented. With the aim of making maximum use of the polarisation information, all of the elements of the polarisation density matrices are related with observables, which are measurable at the environment of $pp$ collisions. Consequences of non-standard $HZZ$, $HZγ$ and $HWW$ interactions for these observables are discussed.

hep-ph↗

arXiv2017

A set of new observables in the process $e^+_{}e^-_{}\to ZHH$

Consequences of non-standard Higgs couplings in the final-state distributions of the process $e^+_{}e^-_{} \to ZHH$ are studied. We derive an analytic expression for the differential cross section, which has in the most general case 9 non-zero functions. These functions are the coefficients of 9 angular terms, depend on the Higgs couplings, and can be experimentally measured as observables. Symmetry properties of these 9 functions are carefully discussed, and they are divided into 4 categories under CP and $\mathrm{CP\tilde{T}}$. The relations between our observables and the observables which exist in the literature are also clarified. We numerically study the dependence of our observables on the parameters in an effective Lagrangian for the Higgs couplings. It is shown that these new observables depend on most of the effective Lagrangian parameters in different ways from the total cross section. A benefit from longitudinally polarized $e^+_{}e^-_{}$ beams is also discussed.

hep-ph↗

arXiv2016

Jet azimuthal angle correlations in the production of a Higgs boson pair plus two jets at hadron colliders

Azimuthal angle correlations of two jets in the process $pp\to HHjj$ are studied. The loop induced $\mathcal{O}(α_s^4 α_{}^2)$ gluon fusion (GF) sub-process and the $\mathcal{O}(α_{}^4)$ weak boson fusion (WBF) sub-process are considered. The GF sub-process exhibits strong correlations in the azimuthal angles $ϕ_{1,2}^{}$ of the two jets measured from the production plane of the Higgs boson pair and the difference between these two angles $ϕ_1^{}-ϕ_2^{}$, and a very small correlation in the sum of them $ϕ_1^{}+ϕ_2^{}$. The impact of using a finite top mass $m_t^{}$ value on the correlations is found crucial. The transverse momentum of the Higgs boson can be used to enhance or suppress the correlations. The impact of a non-standard value for the triple Higgs self-coupling on the correlations is found much smaller than that on other observables, such as the invariant mass of the two Higgs bosons. The peak shifts of the azimuthal angle distributions reflect the magnitude of parity violation in the $gg\to HH$ amplitude and the dependence of the distributions on parity violating phases is analytically clarified. The WBF sub-process also produces correlated distributions and it is found that they are not induced by the quantum effect of the intermediate weak bosons but mainly by a kinematic effect. This kinematic effect is a characteristic feature of the WBF sub-process and is not observed in the GF sub-process. It is found that the correlations are different in the GF and in the WBF sub-processes. As part of the process dependent information, they will be helpful in the analyses of both the GF sub-process and the WBF sub-process at the LHC.

hep-ph↗

arXiv2015

A simple merging algorithm for jet angular correlation studies

A tree level merging algorithm which guarantees the leading order (LO) accuracy of angular correlations between jets is proposed and studied. The algorithm is designed so that n-jet events are generated exclusively according to the LO n-parton production cross section and each of the n-jet is close to each of the n-parton in terms of the jet measure. As a result, the LO accuracy of angular correlations between the n-jet is robust. Furthermore, as long as the n-jet events are exclusively subjects to a study, only the LO n-parton production cross section is needed and hence event generation is efficient. Correlations in the azimuthal angle difference between the two highest transverse momentum jets with large rapidity separations in the top quark pair production are evaluated as examples. The algorithm is validated by discussing numerical differences between its predictions and the predictions of a well-established merging algorithm.

hep-ph↗

arXiv2015

Study on the azimuthal angle correlation between two jets in the top quark pair production

An azimuthal angle correlation between the two hardest jets is studied in the ttbar production process at the 14 TeV LHC. The event samples are generated by merging the tree level matrix elements for the ttbar plus up to 2 or 3 partons with parton showers. The generated event samples show a strong correlation in the azimuthal angle difference between the two hardest jets, as predicted in the analysis based on the tree level matrix elements for the ttbar+2 partons. The effects of merging the matrix elements for the ttbar+3 partons on the correlation are studied in detail. It is found that they play important roles in improving the prediction of the correlation.

hep-ph↗

arXiv2014

TauDecay: a library to simulate polarized tau decays via FeynRules and MadGraph5

TauDecay is a library of helicity amplitudes to simulate polarized tau decays, constructed in the FeynRules and MadGraph5 framework. Together with the leptonic mode, the decay library includes the main hadronic modes, τ\to ν_τ+π, 2π, and 3π, which are introduced as effective vertices by using FeynRules. The model file allows us to simulate tau decays when the on-shell tau production is kinematically forbidden. We also demonstrate that all possible correlations among the decay products of pair-produced taus through a Z boson and a scalar/pseudoscalar Higgs boson are produced automatically. The program has been tested carefully by making use of the standard tau decay library Tauola.

hep-ph↗

arXiv2013

10 GeV neutralino dark matter and light stau in the MSSM

It has recently been pointed out that a component of the observed gamma ray emission in the low-latitudes of Fermi Bubble has a spectral shape that can be explained by a 10 GeV dark matter (DM) annihilating to tau leptons with a cross-section of 2 x 10^{-27} cm^3/s. Motivated by this possibility, we revisit the annihilation of a 10 GeV neutralino DM in the MSSM via stau exchange. The required stau masses and mixing, consistent with LEP direct search and electroweak precision constraints, are correlated with a possible enhancement of the Higgs decay rate to two photons. We also explore the implications of such a scenario for DM relic density and the muon anomalous magnetic moment, taking into account the recent ATLAS bounds on the chargino and the first two generation slepton masses, as well as the constraints on the Higgsino fraction of a 10 GeV neutralino.

hep-ph↗

arXiv2013

Diquark contributions to Top quark charge asymmetry at the Tevatron and LHC

We study contributions of a scalar diquark particle in a color representation of anti-triplet and sextet to the top quark pair production at the Tevatron and the Large hadron collider (LHC). The model can give Forward-Backward (FB) asymmetry at the Tevatron while can avoid the same sign top quark production at the LHC by assuming the top-number conserving diquark couplings. We study compatibility between the large positive FB asymmetry observed at the Tevatron and non-observation of the charge asymmetry at the LHC, by including contributions from the single and pair production of diquarks. We find that the whole parameter space of the models can soon be explored at the LHC by measuring the total t-tbar production cross section and the inclusive charge asymmetry with smaller uncertainties. In addition, we compare the statistical significance of the charge asymmetry measured at the LHC with that of the optimal observable of the subprocess FB asymmetry, and find that they are comparable even when we ignore the uncertainty in the parton distribution functions.

hep-ph↗

arXiv2013

Properties of 125 GeV Higgs boson in non-decoupling MSSM scenarios

Tantalizing hints of the Higgs boson of mass around 125 GeV have been reported at the LHC. We explore the MSSM parameter space in which the 125 GeV state is identified as the heavier of the CP even Higgs bosons, and study two scenarios where the two photon production rate can be significantly larger than the standard model (SM). In one scenario, $Γ(H\to γγ)$ is enhanced by a light stau contribution, while the $WW^{\ast}$ ($ZZ^{\ast}$) rate stays around the SM rate. In the other scenario, $Γ(H\to b\bar{b})$ is suppressed and not only the $γγ$ but also the $WW^{\ast}$ ($ZZ^{\ast}$) rates should be enhanced. The $τ\barτ$ rate can be significantly larger or smaller than the SM rate in both scenarios. Other common features of the scenarios include top quark decays into charged Higgs boson, single and pair production of all Higgs bosons in $e^+e^-$ collisions at $\sqrt{s}\lesssim 300$ GeV.

hep-ph↗