Abstract
This paper presents a search for Higgs bosons decaying to four leptons, either electrons or muons, via one or two light exotic gauge bosons $Z_d$, $H\to Z Z_d \to 4\ell$ or $H\to Z_d Z_d \to 4\ell$. The search was performed using $pp$ collision data corresponding to an integrated luminosity of about 20 fb$^{-1}$ at the center-of-mass energy of $\sqrt{s}=8 $TeV recorded with the ATLAS detector at the Large Hadron Collider. The observed data are well described by the Standard Model prediction. Upper bounds on the branching ratio of $H\to Z Z_d \to 4\ell$ and on the kinetic mixing parameter between the $Z_d$ and the Standard Model hypercharge gauge boson are set in the range $(1$--$9)\times10^{-5}$ and $(4$--$17)\times10^{-2}$ respectively, at 95% confidence level assuming the Standard Model branching ratio of $H\to Z Z^* \to 4\ell$, for $Z_d$ masses between 15 and 55 GeV. Upper bounds on the effective mass mixing parameter between the $Z$ and the $Z_d$ are also set using the branching ratio limits in the $H \to ZZ_d\to 4\ell$ search, and are in the range $(1.5$--$8.7)\times 10^{-4}$ for $15<m_{Z_d} < 35$ GeV. Upper bounds on the branching ratio of $H\rightarrow Z_dZ_d \to 4\ell$ and on Higgs portal coupling parameter, controlling the strength of the coupling of the Higgs boson to dark vector bosons, are set in the range $(2$--$3)\times10^{-5}$ and $(1$--$10)\times 10^{-4}$ respectively, at 95% confidence level assuming the Standard Model Higgs boson production cross sections, for $Z_d$ masses between 15 and 60 GeV.
Highlights
Hidden sector or dark sector states appear in many extensions to the Standard Model (SM) [1,2,3,4,5,6,7,8,9,10], to provide a candidate for the dark matter in the Universe [11] or to explain astrophysical observations of positron excesses [12,13,14]
In the “m12 fit method,” the m12 distribution of tt is fitted with a second-order Chebychev polynomial, and the Z þ jets component is fitted with a
For all relevant backgrounds (H → ZZÃ → 4l, ZZÃ, tt and Z þ jets) the m34 distribution is obtained from simulation
Summary
Hidden sector or dark sector states appear in many extensions to the Standard Model (SM) [1,2,3,4,5,6,7,8,9,10], to provide a candidate for the dark matter in the Universe [11] or to explain astrophysical observations of positron excesses [12,13,14]. The discovery of the Higgs boson [21,22,23] during Run 1 of the Large Hadron Collider (LHC) [24,25] opens a new and rich experimental program that includes the search for exotic decays H → ZZd → 4l and H → ZdZd → 4l This scenario is not entirely excluded by electroweak constraints [5,6,7,8,9,10,18,20]. In the absence of a significant signal, upper bounds are set on the relative branching ratios BRðH → ZZd → 4lÞ=BRðH → 4lÞ and BRðH → ZdZd → 4lÞ=BRðH → ZZÃ → 4lÞ as functions of the mass of the dark vector boson mZd. The branching ratio limits are used to set upper bounds on the kinetic mixing, mass mixing, and Higgs boson mixing parameters [5,6].
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