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キーワード:
ATLAS
要旨:
The properties of the Higgs boson couplings have been studied in decays into four leptons, $H\to ZZ^{*} \to 4 \ell$ ($\ell=e,\mu$), using proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider (LHC) in the years $2015$ to $2018$ at a centre-of-mass energy of $13$~TeV (LHC Run~2). Measurements have been performed using different data sets corresponding to integrated luminosities of $36.1$~fb$^{-1}$, $79.8$~fb$^{-1}$~and $139$~fb$^{-1}$.\\ The cross sections for the four main Higgs boson production modes have been measured inclusively in the $4\ell$ final states as well as in exclusive phase space regions probing the Higgs boson couplings. The inclusive Higgs boson production cross section times branching ratio into $4\ell$ decays in the rapidity range $|\eta|<2.5$ of $1.34 \pm 0.12$~pb measured with the full set of \Rtwo~data set is in very good agreement with the Standard Model (SM) prediction of $1.33 \pm 0.08$~pb. All measurements in exclusive phase space regions are compatible with the SM predictions as well. The sensitivity of the measurements was projected to the ultimate integrated luminosity of the high-luminosity LHC (HL-LHC) of $3000$~\invfb. With the HL-LHC data, the precision of the cross section measurements is expected to improve by a factor four for gluon fusion production and by a factor of six for weak gauge boson fusion production and for the associated production with a weak gauge boson. The results provided input for the long-term planning of particle physics infrastructure in the $2020$ European Strategy process.\\ The tensor structure of the Higgs boson couplings to massive vector bosons and to gluons has been studied in two effective field theory approaches to probe for small beyond-SM (BSM) CP-even and CP-odd admixtures to the CP-even SM coupling. In the first analysis performed with $36.1$~\invfb~of \Rtwo~data, the BSM couplings are defined within the so-called Higgs Characterisation framework. From the measurements of the rates of the four main Higgs boson production modes, constraints on the CP-odd BSM coupling to gluons ($\kAgg$) and on the CP-even and CP-odd BSM couplings to heavy vector bosons ($\kappa_{HVV}$ and $\kappa_{AVV}$) have been obtained: $-0.68<\sin\alpha\cdot \kappa_{Agg} < 0.68$, $-0.6<\cos\alpha\cdot \kappa_{HVV} < 4.2$ and $-4.4<\sin\alpha\cdot \kappa_{AVV} < 4.4$ at $95$\% confidence level (CL), where the mixing angle $\alpha$ describes the relative contributions of the CP-even or CP-odd terms. These are the first limits on BSM Higgs boson couplings in an EFT framework based on the measurement of the Higgs boson production rates providing substantially sensitivity.\\ The second analysis is based on the production cross section measurements in exclusive phase space regions using the full Run~2~data set ($139$~fb^${-1}$) interpreted in the so-called Standard Model Effective Field Theory (SMEFT). Constraints have been derived on the CP-even and CP-odd BSM coupling parameters to gluons ($c_{HG}$ and $c_{H\widetilde{G}}$), heavy vector bosons ($c_{HW},c_{HB},c_{HWB}$ and $c_{H\widetilde{W}},c_{H\widetilde{B}},c_{H\widetilde{W}B}$) and top quarks ($c_{uH}$ and $c_{\tilde{u}H}$). The most stringent limits at $95$\%~CL are $-0.0074< c_{HG} <0.0080$ on the CP-even BSM coupling to gluons, as well as $-2.4 < c_{H\widetilde{W}} < 2.4$, $-0.56 < c_{H\widetilde{B}} < 0.56$ and $-1.03 < c_{H\widetilde{W}B} <1.03$ on the CP-odd BSM coupling parameters to weak gauge bosons. The constraints on the CP-even BSM couplings to weak gauge bosons are weaker. These are the first constraints on SMEFT coupling parameters using the $H\to ZZ^{*} \to 4 \ell$ decay channel, which will be combined with the results in other Higgs boson decay channels. \\ Finally, the sensitivity of kinematic variables of the final state particles for CP-odd contributions to the Higgs boson coupling in vector boson fusion production has been evaluated in the Higgs Characterisation framework. Limits of $-2.56 < \kappa_{AVV}<2.64$ at $68$\% CL are expected for the full Run~2~data set. The measurement will provide the first limits on CP-violation in the $H\to ZZ^{*} \to 4 \ell$ decays.
