Massless dark photons are predicted in hidden-sector models with an unbroken dark U(1) gauge symmetry. A particular class of these models, aiming to solve both the Yukawa-hierarchy and the dark-matter problems of the standard model, manifests natural Higgs nondecoupling properties for the dark photon. As a consequence, we show that the Higgs-boson production at colliders followed by the Higgs decay into a photon and a dark photon provides a very promising dark-photon production mechanism. This decay gives rise to an unconventional Higgs signature characterized by a resonating gamma-plus-missing-momentum system with a monochromatic photon. We discuss the sensitivity of the LHC to the corresponding signal for a Higgs boson produced in both gluon-fusion and vector-boson-fusion channels. © Copyright owned by the author(s).Peer reviewe

Biswas, S.

Gabrielli, E.

Heikinheimo, M.

Mele, B.

English

Helsingin yliopiston digitaalinen arkisto

PoS(EPS-HEP2017)315Searching for massless Dark Photons at the LHC viaHiggs productionS. BiswasKIAS, Seoul, Republic of KoreaE-mail: sanjoy@kias.re.krE. GabrielliUniversità di Trieste, and INFN, Sezione di Trieste, Trieste, Italy, and NICPB, Tallinn, EstoniaE-mail: emidio.gabrielli@cern.chM. HeikinheimoHelsinki Institute of Physics, University of Helsinki, Helsinki, FinlandE-mail: matti.heikinheimo@helsinki.fiB. Mele∗INFN, Sezione di Roma, Rome, ItalyE-mail: barbara.mele@roma1.infn.itMassless dark photons are predicted in hidden-sector models with an unbroken dark U(1) gaugesymmetry. A particular class of these models, aiming to solve both the Yukawa-hierarchy and thedark-matter problems of the standard model, manifests natural Higgs nondecoupling propertiesfor the dark photon. As a consequence, we show that the Higgs-boson production at colliders fol-lowed by the Higgs decay into a photon and a dark photon provides a very promising dark-photonproduction mechanism. This decay gives rise to an unconventional Higgs signature characterizedby a resonating gamma-plus-missing-momentum system with a monochromatic photon. We dis-cuss the sensitivity of the LHC to the corresponding signal for a Higgs boson produced in bothgluon-fusion and vector-boson-fusion channels.EPS-HEP 2017, European Physical Society conference on High Energy Physics5-12 July 2017Venice, Italy∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/PoS(EPS-HEP2017)315Searching for massless Dark Photons B. Mele1. IntroductionMost theoretical models introduced to solve the hierarchy problem of the Higgs scalar fieldin the standard model (SM) of particle interactions would require the observation of some hintsof inconsistency and/or new physical states in collisions at the TeV energy scale. Despite that, inthe first few years of running at 7, 8 and 13 TeV, the CERN Large Hadron Collider (LHC) hasexplored collisions at the TeV scale in many many different channels, and found up to now a globalpicture of consistency with the SM. While the predictions of the most popular beyond-the-standard-model (BSM) models are being thoroughly scrutinized, and are presently sort of late in showing up,one should try to have a coverage as extensive as possible of new non-conventional experimentalsignatures in order to make the search for either new particles or inconsistencies (or both) as modelindependent as possible.The class of models containing hidden (or dark) sectors (where hidden/dark means made up offields uncharged under the SM gauge group) can indeed provide new signatures not yet covered bypresent searches. When including a dark U(1) gauge group, the dark sector predicts the existenceof a dark photon. The way the dark photon can show up in experiments depends on its mass (incase the extra U(1) is broken), coupling and hidden-sector structure. Most attention in colliderphysics has been given up to now to the case of massive dark photons (see e.g. [1]), where the U(1)gauge field naturally develops a tree-level interaction with ordinary matter mediated by kinetic-mixing. In case the dark photon is massless the latter interaction can be rotated away [2, 3].The dark-photon interaction with ordinary matter occurs then via higher-order operators, and issuppressed by typically large new-physics energy scales. Correspondingly, the search for masslessdark photons must proceed via different phenomenological strategies with respect to the massivecase.We consider a particular class of hidden sectors, described in [4], that aims to explain thehierarchy in the Yukawa couplings of fermions in the SM, and, at the same time, can providerealistic dark-matter candidates. In this class of models, a nonperturbative mechanism involvinga dark unbroken U(1) gauge symmetry generates an exponential hierarchy in the dark-fermionmass spectrum, that is in turn transmitted to the ordinary-fermion mass spectrum via a messengersector. Dark fermions are only charged under the dark U(1) [with an integer-charge sequence],and might be natural dark-matter candidates. While the predicted massless dark photon γ̄ hasin general suppressed interactions with the ordinary matter, for typical values of the charges andmixing parameters of the model the Higgs boson can enter in a nondecoupling regime. Then, theone-loop interaction with the dark photon (mediated by the messengers) is not suppressed by a largeenergy scale but by just the Higgs-boson field vacuum expectation value v [5, 6]. This regime givesrise to new peculiar signatures in the Higgs decays that could be observed in Higgs production athigh-energy collisions. In the following, we will consider the corresponding phenomenology at theLHC.2. Higgs decay into massless dark photonsThe new Higgs boson decay channels involving a massless dark photon are H→ γγ̄,Zγ̄, γ̄ γ̄ [3].In a nondecoupling regime, they can have sizeble rates [5]. The signature of a massless dark photon1PoS(EPS-HEP2017)315Searching for massless Dark Photons B. Meleγ/Zγ̄HSiSiSiγ/Zγ̄HCV γ̄Figure 1: Effective Hγγ̄ (HZγ̄) interaction described as the low energy approximation of a one-loop diagrammediated by heavy messengers Si [6].in collider experiments will be the same as the neutrino one, thanks to the suppressed interactionwith ordinary matter, and to its massless nature. The Higgs decay into two dark photons will thenaffect the Higgs invisible decay width. Here we consider the single dark photon Higgs signature,as detailed in Figure 1, where the effective Hγγ̄ (HZγ̄) interaction is described as the low energyapproximation of a one-loop diagram mediated by heavy messengers Si. We focus in particular onthe H → γγ̄ channel giving rise to a resonating gamma-plus-missing-momentum signature, char-acterized by one monochromatic photon with energy mH/2, and same amount of missing energy.Note that the photon energy is more than twice the one in the resonance corresponding to the rareSM decay H → γ Z → γνν̄ . While a quantitative prediction for the H → γγ̄ decay width will bein general dependent on quite a few model parameters, in a simplified-model analysis [5] one canobtain typical allowed ranges which correspond to branching ratios (BRs) up to about 5%. This isshown in Figure 2 [5], where BR(H→ γγ̄) is plotted versus the dark U(1) fine structure constant ᾱ(details on the model assumed and on the definition of relevant input parameters can be foundin [5]). Such large BR(H → γγ̄) might indeed lead to an observable signal in Higgs production atthe LHC, as discussed in the next section.10.80.60.40.200.050.0450.040.0350.030.0250.020.0150.010.005rγγ = ΓNPγγ /ΓSMγγ0.70.60.50.40.30.2BRinv = 0.1rγγ = 1rγγ = 0.5rγγ = 0.3rγγ = 0.1ᾱBR(H→γγ̄)Figure 2: BR(H → γγ̄) versus the dark U(1) fine structure constant ᾱ . The black solid lines show theallowed BR ranges as a function of the relative new physics contribution to the H → γγ width, and of theH→ γ̄ γ̄ contribution to the Higgs invisible BR [5].3. Dark-photon production via Higgs production at the LHCIn [5] the potential of the Run-1 dataset at the LHC has been studied at parton level by consid-ering the gluon fusion process gg→ H → γγ̄ at the c.m. proton energy of 8 TeV with 20 fb−1 of2PoS(EPS-HEP2017)315Searching for massless Dark Photons B. Meleintegrated luminosity. It is particularly informative to look at a comparison of the distribution of themissing transverse mass MT =√2pγT EmissT (1− cos∆φ) of the resonating γγ̄ system in the Higgssignal (where ∆φ is the azimuthal distance between the photon and dark-photon three momenta)and of the main irreducible and reducible backgrounds, as shown in Figure 3 [5]. In that figure, the60 70 80 90 100 110 120 13010501005001000MT @GeVDdΣdMT@fbGeVDFigure 3: Distribution of the γγ̄ transverse invariant mass assuming BR(H → γγ̄) ' 0.5% (red) comparedto the main irreducible and reducible backgrounds, as from [5].γ +EmissT transverse invariant mass distribution (in fb/GeV) of the signal for BR(H → γγ̄) = 5%(shown in red) is compared with: i) the γ j channel (grey) [where large apparent EmissT is created bya combination of real EmissT from neutrinos in heavy quark decays and mismeasured jet energy]; ii)the γ Z → γνν̄ channel (blue); iii) jZ → jνν̄ (green), where the jet is misidentified as a photon;and iiii)W → eν (yellow), where the electron (positron) is misidentified as a photon. The resonantstructure of the signal emerges clearly from the background. The definition of the relevant basicobjects, corresponding cut-flows, and detection efficiencies applied in Figure 3 are described in [5].With a parton-level event-generator analysis, one finds that a 5σ discovery can be reached with theexisting 8 TeV LHC data-set down to BR(H→ γγ̄)' 0.5% [5].An improved analysis (including parton shower effects, and a more realistic background sim-ulation) essentially confirms this estimate for the Run-1 potential [7]. When increasing the LHCcollision energy up to√s = 14 TeV, beyond the gluon fusion channel also the vector-boson-fusionHiggs production channel deserves to be considered. A thorough analysis of the two channelswith a comparison with the corresponding backgrounds at 14 TeV has been carried out in [7].In Table 1, one can find the LHC sensitivity to BR(H → γγ̄) for different integrated luminosi-ties in the two channels. One can see that the gluon-fusion process at 14 TeV considerablyextends the Run-1 potential, and turns out to be about one order of magnitude more sensitivethan the vector-boson-fusion channel. In particular, for an integrated luminosity of 100 (300)fb−1 at 14 TeV, the signal significance S/√S+B for the gluon-fusion channel approaches the5σ level at BRγγ̄ =BR(H → γγ̄) ' 1.6 (0.9)× 10−3. At the High-Luminosity LHC, with an in-tegrated luminosity of 3 ab−1, the 5σ reach is extended down to BRγγ̄ ' 2.9× 10−4. On theother hand, in absence of any signal observation, a 95% C.L. exclusion bound could be set atBRγγ̄ > (6.4,3.7,1.2)×10−4, with (100, 300, 3000) fb−1 of integrated luminosity, respectively.Of course detailed detector-level studies will be needed to draw realistic conclusions on theLHC sensitivity to the new striking signature of a single monochromatic photon resonating withmissing momentum that is associated to the Higgs decay into massless dark photons.3PoS(EPS-HEP2017)315Searching for massless Dark Photons B. MeleBRγγ̄ (%) L= 100 fb−1 L=300 fb−1 L=3 ab−1Significance 2σ 5σ 2σ 5σ 2σ 5σBRγγ̄ (VBF ) 0.76 1.9 0.43 1.1 0.14 0.34BRγγ̄ (ggF) 0.064 0.16 0.037 0.092 0.012 0.029Table 1: Reach in BR(H → γγ̄) (in percentage) for a 95% C.L. exclusion and a 5σ discovery at the 14TeV LHC, for a Higgs produced either in the VBF or in the gluon-fusion channels, for different integratedluminosities L.Finally, it should be stressed that, apart from LHC, other collider setups, like e+e− collidersor fixed-target experiments, can also be invaluable to look for footprints of a massless dark photonin high-energy collisions [6],[8]-[10].Acknowledgments : The work of M.H. has been supported by the Academy of Finland projectnumber 267842.References[1] G. Aad et al. [ATLAS Collaboration], A search for prompt lepton-jets in pp collisions at√s = 8 TeVwith the ATLAS detector, JHEP 1602 (2016) 062 [arXiv:1511.05542 [hep-ex]].[2] B. Holdom, Two U(1)’s and Epsilon Charge Shifts, Phys. Lett. 166B (1986) 196.[3] B. A. Dobrescu, Massless gauge bosons other than the photon, Phys. Rev. Lett. 94 (2005) 151802[hep-ph/0411004].[4] E. Gabrielli and M. Raidal, Exponentially spread dynamical Yukawa couplings from nonperturbativechiral symmetry breaking in the dark sector, Phys. Rev. D 89 (2014) no.1, 015008 [arXiv:1310.1090[hep-ph]].[5] E. Gabrielli, M. Heikinheimo, B. Mele and M. Raidal, Dark photons and resonant monophotonsignatures in Higgs boson decays at the LHC, Phys. Rev. D 90 (2014) no.5, 055032 [arXiv:1405.5196[hep-ph]].[6] S. Biswas, E. Gabrielli, M. Heikinheimo and B. Mele, Dark-photon searches via ZH production ate+e− colliders, Phys. Rev. D 96 (2017) no.5, 055012 [arXiv:1703.00402 [hep-ph]].[7] S. Biswas, E. Gabrielli, M. Heikinheimo and B. Mele, “Dark-Photon searches via Higgs-bosonproduction at the LHC, Phys. Rev. D 93 (2016) no.9, 093011 [arXiv:1603.01377 [hep-ph]].[8] S. Biswas, E. Gabrielli, M. Heikinheimo and B. Mele, Higgs-boson production in association with adark photon in e+e− collisions, JHEP 1506 (2015) 102 [arXiv:1503.05836 [hep-ph]].[9] E. Gabrielli, B. Mele, M. Raidal and E. Venturini, FCNC decays of standard model fermions into adark photon, Phys. Rev. D 94 (2016) no.11, 115013 [arXiv:1607.05928 [hep-ph]].[10] M. Fabbrichesi, E. Gabrielli and B. Mele, Hunting down massless dark photons in kaon physics, Phys.Rev. Lett. 119 (2017) no.3, 031801 [arXiv:1705.03470 [hep-ph]].4

Searching for massless dark photons at the lhc via higgs production

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Searching for massless dark photons at the lhc via higgs production

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