Extensions of electroweak Higgs sector received a great attention by both experimentalist and phenomenologist after the discovery of the Higgs boson announced by both ATLAS and CMS Collaborations. This work presents two models, namely the additional singlet model and the two-Higgs-doublet model, discussing the model parameters that are crucial in experimental analyses. Higgs boson

searches at high mass, made by the ATLAS Collaboration using pp collisions data recorded during Run I of LHC, are also shown

Zurzolo, G.

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DOI 10.1393/ncc/i2015-15008-3Colloquia: IFAE 2014IL NUOVO CIMENTO 38 C (2015) 8Beyond the Standard Model Higgs searches at high massG. Zurzolo(∗)INFN, Sezione di Napoli and Universita` di Napoli “Federico II” - Napoli, Italyreceived 7 January 2015Summary. — Extensions of electroweak Higgs sector received a great attentionby both experimentalist and phenomenologist after the discovery of the Higgs bo-son announced by both ATLAS and CMS Collaborations. This work presents twomodels, namely the additional singlet model and the two-Higgs-doublet model, dis-cussing the model parameters that are crucial in experimental analyses. Higgs bosonsearches at high mass, made by the ATLAS Collaboration using pp collisions datarecorded during Run I of LHC, are also shown.PACS 12.60.Fr – Extensions of electroweak Higgs sector.1. – IntroductionThe discovery of the Higgs boson at the Large Hadron Collider at CERN, announcedby both ATLAS and CMS Collaborations [1,2], is actually a milestone of theoretical andexperimental particle physics. The discovered particle has been analyzed to evaluate itsproperties showing that the Higgs boson is a JP = 0+ particle with a mass of about125GeV [3]. Nevertheless, theory does not exclude the existence of additional singletsor duplets of Higgs scalar fields and these hypotheses are considered in many extensionsof the Standard Model. With a view to the forthcoming LHC upgrade, exploration ofextended theories Beyond the Standard Model (BSM) will be crucial for the next physicsanalyses.2. – Theoretical modelsIn the Standard Model (SM) the scalar sector is defined in the simplest possible wayby assuming the existence of just one SU(2) doublet. The crucial theoretical aspect aboutscalar sector is the ρ parameter that was experimentally demonstrated to be very close toone. Since both SU(2) singlets with Y = 0 and SU(2) doublets with Y = ±1 give ρ = 1,the simplest way to extend Higgs sector is to add a real singlet or a complex doublet ofscalar fields.(∗) E-mail: zurzolog@na.infn.itCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 G. ZURZOLO2.1. Additional electroweak singlet . – In addition to the doublet Higgs field of the SMa real electroweak (EW) singlet of SU(2) is assumed. Spontaneous symmetry breakingleads to mixing between the two states, resulting in two CP -even Higgs bosons assumedto be non-degenerate. The two Higgs bosons, h (the lighter) and H (the heavier), coupleto fermions and vector bosons as the SM Higgs boson but with strengths reduced by ascale factor, denoted as k (k′) for h (H), and constrained by unitarity to be k2+(k′)2 = 1.In this model, the lighter Higgs boson h is assumed to have identical production anddecay modes to the SM Higgs boson, but with decay rates σh and total decay width Γhreduced by a factor k2. Since the heavier Higgs boson H can decay in new modes, e.g.H → hh, if they are kinematically accessible, its decay rates σH and total decay widthsΓH are defined, with respect to those of the SM Higgs boson with equal mass and interms of branching ratio of all new decay modes BRnew, as(1) σH = (k′)2 × σH,SM and ΓH = (k′)21−BRnew × ΓH,SM.2.2. Two-Higgs-Doublet Model (2HDM). – In the two-Higgs-Doublet model (2HDM)the Higgs sector is extended by an additional doublet of SU(2) and it predicts the exis-tence of five physical bosons: two neutral CP -even bosons (h, H), one neutral CP -oddboson (A) and two charged bosons (H±). Furthermore, this model can be described by sixparameters: four Higgs boson masses (mh,mH ,mA and mH±), the ratio between the vac-uum expectation values of the two fields tanβ = v1/v2, with v21 + v22 = v2 ≈ (246GeV)2,and the mixing angle of the two neutral CP -even states (α). The couplings of the twoneutral CP -even Higgs bosons to vector bosons relative to their SM values are constrainedfrom gauge invariance to be(2) g2HDMhV V /gSMhV V = sin(β − α) and g2HDMHV V /gSMHV V = cos(β − α).Depending on assumptions made about Higgs doublets coupling to fermions and vec-tor bosons, it is possible to define four different types of 2HDM as described in [4].3. – Experimental analysesThe ATLAS experiment at the LHC has determined the coupling of the Higgs bosonto other particles, as well as its mass, and it has exploited a wide mass range in manyproduction and decay channels by using both 4.8 fb−1 at√s = 7TeV and 20.3 fb−1 at√s = 8TeV of pp collision data.In the case of the additional EW singlet model, using the signal strength of the lightHiggs boson, μh = k2, as measured combining all studied channels [5], it is possible toextract an upper limit on the coupling of the heavy Higgs boson, (k′)2. The maximumsignal strength for contamination of a heavy Higgs boson into the light Higgs boson signalobserved (expected) at 95% CL is k′ < 0.12 (0.29). The approach for 2HDMs is quitedifferent since the rescaling of production and decay rates is a function of the light Higgsboson couplings to vector bosons, fermions and leptons, which in turn are functions of αand β; thus, analyses usually scan the [cos(β − α), tan(β)] plane. Results [5] show thatdata are consistent with the SM limit at cos(β − α) = 0 within ∼ 1–2 σ in each of thefour models considered.BEYOND THE STANDARD MODEL HIGGS SEARCHES AT HIGH MASS 3The H → ZZ → 4l decay channel [6] is sensitive across a wide range of mass valuesmH and it is characterized by a high signal to background ratio S/B and a very sharpinvariant mass resolution; nevertheless, this channel is affected by statistical limitationsdue to small branching ratio. The Higgs mass range has been scanned up to 1TeVand, through this, it is possible to extract constraints on heavier Higgs boson crosssections and, consequently, its couplings. Indeed, results show that 95% CL limit on(σH × BR)/(σH × BR)SM at Higgs boson mass mH = 500GeV is ∼ 0.3 (∼ 1.5) forgluon-gluon fusion (vector boson fusion plus associated production) channel.Also the H → ZZ → llqq decay channel [7] is sensitive to a wide range of mass valuesmH and, furthermore, it is favored with respect to the 4l channel by the higher branchingratios; nevertheless, this channel is affected by a very huge irreducible background mostlyconstituted by Z boson produced in association with one or more jets and top quarksproductions. In this case, the 95% CL limit on (σH × BR)/(σH × BR)SM at Higgsboson mass mH = 500GeV is ∼ 2.5 for gluon-gluon fusion and vector boson fusionproduction channels extracted by using only 4.7 fb−1 of integrated luminosity collectedat√s = 7TeV.The H → WW → lνlν decay channel [8] is characterized by the missing transverseenergy, EmissT , due to the neutrinos produced in the W boson leptonic decays and thecrucial variable used to study the Higgs boson production is the W bosons transversemass mT . Thus, the major background to this channel is constituted by top quarks anddiboson productions. The Higgs boson mass range mH has been scanned up to 1TeVand results show that 95% CL limit on (σH × BR)/(σH × BR)SM at Higgs boson massmH = 500GeV is ∼ 0.7 for gluon-gluon fusion production channel.4. – ConclusionsBeyond the Standard Model extensions of the Higgs sector has been discussed, namelythe additional electroweak singlet model and the two-Higgs-Doublet-Model (2HDM),presenting the most important parameters for searches of heavier Higgs bosons. Theexperimental analyses of the ATLAS Collaboration, done by using pp collisions datarecorded at the LHC at a center-of-mass energy of√s = 7 and 8TeV, have been alsoshown confirming the feasibility of BSM Higgs searches at high mass values that will becrucial in the study of the Higgs sector during the planned Run II of the LHC.REFERENCES[1] ATLAS Collaboration, Phys. Lett. B, 716 (2012) 1.[2] CMS Collaboration, Phys. Lett. B, 716 (2012) 30.[3] ATLAS Collaboration, Phys. Lett. B, 726 (2013) 120.[4] Branco G., Ferreira P., Lavoura L., Rebelo M., Sher M. et al., Phys. Rep., 516(2012) 1.[5] ATLAS Collaboration, ATLAS-CONF-2014-010, http://cds.cern.ch/record/1670531.[6] ATLAS Collaboration, ATLAS-CONF-2013-013, http://cds.cern.ch/record/1523699.[7] ATLAS Collaboration, ATLAS-CONF-2012-017, http://cds.cern.ch/record/1429666.[8] ATLAS Collaboration, ATLAS-CONF-2013-067, http://cds.cern.ch/record/1562879.

Beyond the Standard Model Higgs searches at high mass

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Beyond the Standard Model Higgs searches at high mass

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