A search is performed for WH production with a light Higgs boson decaying to hidden-sector particles resulting in clusters of collimated electrons, known as electron-jets. The search is performed with 2.04 fb −1 of data collected in 2011 with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at √ s = 7 TeV. One event satisfying the signal selection criteria is observed, which is consistent with the expected background rate. Limits on the product of the WH production

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... ss section and the branching ratio of a Higgs boson decaying to prompt electron-jets are calculated as a function of a Higgs boson mass in the range from 100 to 140 GeV. Recently, the production of a boson with a mass of about 125 GeV has been observed by the ATLAS [1] and CMS [2] collaborations. The observation is compatible with the production and decay of the Standard Model (SM) Higgs boson [3] [4] [5] at this mass. Strengthening or rejecting the SM Higgs boson hypothesis is currently of utmost importance and thus a search for non-SM Higgs boson decays is of high interest. In this paper, a search for a Higgs boson decaying to a new hidden sector of particles is presented. The masses and couplings of the hiddensector particles are chosen such that the Higgs boson decay cascade results in jets consisting exclusively of electrons ('electron-jets') and weakly interacting neutral particles [6, 7] . This is the first search performed for this particular channel. Moreover, in addition to a recently discovered state consistent with the SM Higgs boson, there may be other scalar fields that couple to the W boson and decay to electron-jets. These scalars arise in the Higgs boson sector in many extensions of the SM, and electron-jets could be the primary discovery channel for these new states. The search is performed in the Higgs boson mass range between 100 and 140 GeV. The analysis examines the associated Higgs boson production mechanism, pp → WH , assuming SM couplings between the Higgs boson and the W boson. Many models of physics beyond the SM contain a light hidden sector, which is composed of as yet unobserved fields that are singlets under the SM group SU (3) × SU (2) × U (1) and that can be probed at the Large Hadron Collider (LHC) [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] . Models of this hidden sector vary from simple modifications of the SM [12, 13] to models motivated by string theory [14] to so-called unparticle models [15] . In the present analysis two models, discussed in [7], are considered. These differ in the way the Higgs boson decays, either via a three-step cascade (figure 1 left) or a two-step cascade (figure 1 right) to hidden-sector particles. In both models the masses of particles in a hiddensector cascade are taken to be substantially lower than the Higgs boson mass, thus the Higgs boson decay has a two-jet topology. The models feature a dark photon γ d that kinetically mixes with the SM photon [16, 17] , a neutral weakly interacting stable scalar n d and two hidden New Journal of Physics 15 (2013) 043009 (http://www.njp.org/) G Azuelos 93,184 , Y Azuma 155 , M A Baak 30 , G Baccaglioni 89a , C Bacci 134a,134b , A