The search for evidence of beyond Standard Model Higgs bosons is an integral
part of the Higgs boson studies at the LHC. This article reviews recent beyond
Standard Model Higgs boson searches using Run I LHC proton-proton collision
data recorded by the ATLAS detector. In particular, searches for Higgs boson
cascades, double Higgs boson production, scalar particles decaying to
$\gamma\gamma$ pairs, flavor changing neutral currents involving Higgs bosons,
and Higgs bosons decaying to invisible particles are discussed. No significant
deviations from the background expectations are found and corresponding
constraints on physics beyond the Standard Model are obtained.Comment: 5 pages, LHCP Conference Proceedings, Columbia University, New York,
  U.S.A. June 2-7, 201

Rompotis, Nikolaos

English

arXiv

The search for evidence of beyond Standard Model Higgs bosons is an integral part of the Higgs boson studies at the LHC. This article reviews recent beyond Standard Model Higgs boson searches using Run I LHC proton-proton collision data recorded by the ATLAS detector. In particular, searches for Higgs boson cascades, double Higgs boson production, scalar particles decaying to $\gamma\gamma$ pairs, flavor changing neutral currents involving Higgs bosons and Higgs bosons decaying to invisible particles are discussed. No significant deviations from the background expectations are found and corresponding constraints on physics beyond the Standard Model are obtained.The search for evidence of beyond Standard Model Higgs bosons is an integral part of the Higgs boson studies at the LHC. This article reviews recent beyond Standard Model Higgs boson searches using Run I LHC proton-proton collision data recorded by the ATLAS detector. In particular, searches for Higgs boson cascades, double Higgs boson production, scalar particles decaying to $\gamma\gamma$ pairs, flavor changing neutral currents involving Higgs bosons, and Higgs bosons decaying to invisible particles are discussed. No significant deviations from the background expectations are found and corresponding constraints on physics beyond the Standard Model are obtained

CERN Document Server

Beyond Standard Model Higgs boson physics with the ATLAS experiment at the LHC

The search for evidence of beyond Standard Model Higgs bosons is an integral
part of the Higgs boson studies at the LHC. This article reviews recent beyond
Standard Model Higgs boson searches using Run I LHC proton-proton collision
data recorded by the ATLAS detector. In particular, searches for Higgs boson
cascades, double Higgs boson production, scalar particles decaying to
$\gamma\gamma$ pairs, flavor changing neutral currents involving Higgs bosons,
and Higgs bosons decaying to invisible particles are discussed. No significant
deviations from the background expectations are found and corresponding
constraints on physics beyond the Standard Model are obtained

University of Liverpool Repository

arXiv:1408.3521v1  [hep-ex]  15 Aug 2014Proceedings of the Second Annual LHCPATL-PHYS-PROC-2014-097August 12, 2014Beyond Standard Model Higgs boson physics with the ATLAS experimentat the LHCNikolaos RompotisOn behalf of the ATLAS Experiment,Department of Physics,University of Washington, Box 351560, Seattle WA 98195-1560, USAABSTRACTThe search for evidence of beyond Standard Model Higgs bosons is an integralpart of the Higgs boson studies at the LHC. This article reviews recent beyondStandard Model Higgs boson searches using Run I LHC proton-proton collisiondata recorded by the ATLAS detector. In particular, searches for Higgs bosoncascades, double Higgs boson production, scalar particles decaying to γγ pairs,flavor changing neutral currents involving Higgs bosons, and Higgs bosonsdecaying to invisible particles are discussed. No significant deviations from thebackground expectations are found and corresponding constraints on physicsbeyond the Standard Model are obtained.PRESENTED ATThe Second Annual Conferenceon Large Hadron Collider PhysicsColumbia University, New York, U.S.AJune 2-7, 20141 IntroductionThe recent observation of a new particle with mass 125–126 GeV at the LHC [1,2] opens a new era for particlephysics. The detailed study of the production and decay modes of the new particle provides invaluable inputto answer the question of whether this is indeed the long-sought Standard Model (SM) Higgs boson [3–8].The first measurements indicate that the new particle is indeed compatible with the SM Higgs boson, seee.g., [9]. Nevertheless, many more measurements and data will be needed to extract reliable conclusions.This task is further complicated by the fact that many beyond SM physics scenarios include a SM-like Higgsboson, which is part of an extended scalar sector. In that case, searches for beyond SM Higgs bosons arevery interesting, since they provide direct information on a possibly extended scalar sector, and hence theyare complementary to the precise measurements of the properties of the new particle.This article reviews some of the recent searches for beyond SM Higgs bosons or exotic properties of therecently discovered Higgs boson using proton-proton collision data at 7 and 8 TeV center-of-mass energiesas recorded by the ATLAS detector [10].2 Higgs Bosons in beyond SM physics scenariosThe scalar sector of the SM consists of a complex Higgs doublet, which after the electroweak symmetrybreaking leaves a single scalar boson in the theory. Possible extensions of the scalar sector are restricteddue to the fact that, in general, they include unacceptably large corrections to the well-measured quantityρ ≡ m2W/(m2Zcos2 θ), where θ is the weak mixing angle [11]. The addition of doublets or singlets is such thatleading order corrections to ρ vanish and, hence they can be compatible with precision electroweak tests [12].The introduction of one additional Higgs doublet defines a class of models, which are collectively knownas 2 Higgs doublet models (2HDM) [13]. These models introduce five Higgs bosons, three of which are neutraland two of which are charged. The Minimal Supersymmetric Standard Model (MSSM) [14–17], which is avery popular realization of supersymmetry, is a particular case of a 2HDM. The MSSM, as well as other2HDMs, are compatible with existing measurements, including the recently discovered Higgs boson [18, 19].Apart from the introduction of new particles, extensions of the SM scalar sector may affect the propertiesof the SM-like Higgs boson discovered at the LHC. Enhancement of rare decays, completely new decays andproduction mechanisms, and different couplings with respect to the SM expectations are possible and mayindicate connections to other puzzles, such as dark matter (e.g., see Ref. [20] and references therein).In the following, only a few recent ATLAS searches are described. As a convention, the symbol h willrefer to the newly discovered Higgs boson with mass ∼ 125 GeV.3 Some recent searches for beyond SM Higgs bosons with ATLASThe existence of an extended scalar sector with more than one Higgs bosons opens the possibility of cascadedecays in which heavier Higgs bosons decay into lighter ones. In this context, a Higgs boson cascade hasbeen looked for in Ref. [21] using 20.3 fb−1 of 8 TeV proton-proton collision data. A heavy Higgs boson isproduced via gluon-gluon fusion and initiates a cascade decay that includes a charged scalar, H±, a lightneutral Higgs boson, h, and W bosons: H → W−H+ → W−W+h, see Fig. 1(a). The light neutral Higgsboson decays to bb and it is assumed to have mass 125 GeV. The first of theW bosons decays leptonically andprovides a way to trigger the events whereas the second one decays hadronically maximizing the branchingratio. The final state shares the same topology with tt events with similar W boson decays, which is themain background process for the search. A multivariate discriminant is employed to exploit the kinematicdifferences between signal and tt events. Upper limits on the production cross section of heavy neutral Higgsboson, H , times the branching ratio BR(H → W∓H± → W∓W±h→ W∓W±bb) are derived as a functionof its mass and the charged scalar mass, as shown in Fig. 1(b).The pair production of Higgs bosons is very interesting and can be enhanced due to the resonant decay ofa heavy CP-even Higgs boson (H → hh), e.g., in a 2HDM, or even in a non-resonant way, e.g., in compositeHiggs boson models [22]. The analysis described in Ref. [23] uses 20.3 fb−1 of 8 TeV proton-proton collision1❜ø❜❲❲❣❣❍✵❍➧❤✵(a) Mass [GeV]0H400 500 600 700 800 900 1000 Mass [GeV]±H30040050060070080090095% C.L. Upper Limits [pb]-110110-1Ldt = 20.3 fb∫ATLAS = 8 TeVs±W±Wbb→±W±W0h→±H±W→0H(b)Figure 1: The topology of a Higgs cascade search, described in Ref. [21], is shown in (a). The observed upperlimit for this process is shown in (b).data to look for hh production in the h → bb and h → γγ channel. Both resonant and non-resonantanomalous production of hh pairs is searched for. Upper limits on the production of a narrow-width heavyscalar boson decaying to hh as a function of its mass are shown in Fig. 2. The cross section for non-resonanthh production is constrained to be less than 2.2 pb at 95% confidence level. As a reminder, the SM hhproduction cross section is ∼ 10 fb, i.e., about two orders of magnitude smaller than the sensitivity of thissearch. [GeV]Xm300 350 400 450 500hh) [pb]→ BR(X× Xσ11.522.533.544.5ATLAS∫  = 8 TeVs at -1Ldt = 20 fbObserved 95% CL Limitσ1±Expected Limit σ2±Expected Limit Type I 2HDM:)=-0.05α-β=1, cos(β     tanFigure 2: Upper limits from the search for a scalar particle decaying to hh in the bbγγ channel from Ref [23].A search for a scalar particle that decays to a γγ pair and is in the mass range 65–600 GeV is describedin Ref. [24] and uses 20.3 fb−1 of 8 TeV proton-proton collision data. This analysis considers the SM-likeHiggs boson at ∼ 125 GeV as a background to the search. Analytical functions are employed to describethe shape of the γγ invariant mass similar to those used in the SM Higgs boson search in the γγ channel [1].The γγ mass range of this search is constrained by trigger and background estimation requirements fromthe low mass side and by the data statistics in the high mass side. Upper limits on the fiducial cross sectiontimes the branching ratio H → γγ are quoted, see Fig. 3.Flavor changing neutral currents (FCNC) are in general enhanced in extended Higgs sectors. In severaloccasions, models can be built that evade the tight constraints on FCNC by flavor physics, like the type-III2HDM [19]. In those cases, the thc coupling is predicted to be sizeable and close to the LHC sensitivity [25].A search for top quark decaying to hq, where q denotes some light quark, is reported in Ref. [26] and uses4.7 fb−1 of 7 TeV and 20.3 fb−1 of 8 TeV proton-proton collision data. This analysis looks for tt productionin which one top decays to hq and the other semileptonically or in a fully hydronic way. An example ofthe γγ invariant mass after all selection requirements for the case where one of the tops decays in a fully2 [GeV]Xm100 200 300 400 500 600 BR [fb]⋅ fidσ95% CL limit on -110110210310ATLAS-1Ldt = 20.3 fb∫ = 8 TeV, sObservedExpectedσ 1 ±σ 2 ±60 80 100 120 140 16050100Figure 3: Upper limits at 95% confidence level of a search for a scalar particle decaying to a γγ pair fromRef. [24].hydronic way is shown in Fig. 4(a). The observed (expected) upper limit at 95% confidence level for theflavor changing branching ratio is BR(t→ hq) < 0.79(0.51)%, see Fig. 4(b). [GeV]γγm100 110 120 130 140 150 160Events/4 GeV0246810121416 ATLASs = 8 TeV√, -1L dt = 20.3 fb∫s = 7 TeV√,  -1L dt =  4.7 fb∫Hadronic SelectionData 2011+2012Sig.+SM Higgs+continuum bkg. fit = 125.5 GeV)H(mSM Higgs+continuum bkg.Continuum bkg.(a)qH → tB0 0.002 0.004 0.006 0.008 0.01 0.012 0.014sCL-210-1101ATLASs = 8 TeV√, -1L dt = 20.3 fb∫s = 7 TeV√, -1L dt =  4.7 fb∫ObservedExpectedσ 1 ±σ 2 ±(b)Figure 4: The distribution of the γγ mass for tt→ hqWb candidates in which the W boson decays hadron-ically is shown in (a). Observed and expected CLs for the t → hq search is shown in (b). Both figures arefrom Ref. [26].A search for invisible decays of the Higgs boson discovered at the LHC can provide access to dark matterthrough the Higgs portal [27]. A search reported in Ref. [28] uses 4.5 fb−1 of 7 TeV and 20.3 fb−1 of8 TeV proton-proton collision data to look for the associated production of a Higgs boson with a Z bosonas shown pictorially in Fig. 5(a). The Z boson decays to an electron or a muon pair and the Higgs bosonis assumed to decay to invisible particles. The final event sample is dominated by Z+jet events and themissing transverse momentum distribution is examined for discrepancies with respect to the SM prediction,see Fig. 5(b) Upper limits for the production of a Higgs boson in association with a Z boson, assumingthat the Higgs boson decays always to invisible particles, are shown in Fig. 5(c). This result is combinedwith indirect constraints on invisible Higgs boson decays from Higgs boson coupling measurements [29]. Theobserved (expected) combined constraint for the 125 GeV Higgs boson on the invisible branching ratio isBR(h→ invisible) < 37(39)% at 95% confidence level.3(a) (b)(c)Figure 5: The topology of the search for invisible Higgs boson decays described in Ref. [28] is shown in (a).The missing transverse momentum after the full selection and the upper limits on this process are shown in(b) and (c), respectively.4 ConclusionsThe search for beyond SM Higgs bosons is highly motivated and has just started having sensitivity torealistic scenarios. The discovery of a Higgs boson with mass around 125 GeV has opened new possibilitiesfor searches, especially those that include the new particle in the final state. More data are needed in orderto explore the possibility of an extended scalar sector and, hence the community is looking forward to thenew LHC run.References[1] ATLAS Collaboration, Phys. Lett. B 716, 1 (2012) [arXiv:1207.7214].[2] CMS Collaboration, Phys. Lett. B 716, 30 (2012) [arXiv:1207.7235].[3] F. Englert and R. Brout, Phys. Rev. Lett. 13, 321 (1964).[4] P. W. Higgs, Phys. Lett. 12, 132 (1964).[5] P. W. Higgs, Phys. Rev. Lett. 13, 508 (1964).4[6] G. S. Guralnik, C. R. Hagen and T. W. B. Kibble, Phys. Rev. Lett. 13, 585 (1964).[7] P. W. Higgs, Phys. Rev. 145, 1156 (1966).[8] T. W. B. Kibble, Phys. Rev. 155, 1554 (1967).[9] ATLAS Collaboration, Phys. Lett. B 726, 88 (2013) [arXiv:1307.1427].[10] ATLAS Collaboration, JINST 3, S08003 (2008).[11] J. F. Gunion, H. E. Haber, G. L. Kane and S. Dawson, Front. Phys. 80, 1 (2000).[12] M. Baak, M. Goebel, J. Haller, A. Hoecker, D. Kennedy, R. Kogler, K. Moenig and M. Schott et al.,Eur. Phys. J. C 72, 2205 (2012) [arXiv:1209.2716].[13] T. D. Lee, Phys. Rev. D 8, 1226 (1973).[14] P. Fayet, Phys. Lett. B 64, 159 (1976).[15] P. Fayet, Phys. Lett. B 69, 489 (1977).[16] G. R. Farrar and P. Fayet, Phys. Lett. B 76, 575 (1978).[17] P. Fayet, Phys. Lett. B 84, 416 (1979).[18] P. Bechtle, S. Heinemeyer, O. Stal, T. Stefaniak, G. Weiglein and L. Zeune, Eur. Phys. J. C 73, 2354(2013) [arXiv:1211.1955].[19] G. C. Branco, P. M. Ferreira, L. Lavoura, M. N. Rebelo, M. Sher and J. P. Silva, Phys. Rept. 516, 1(2012) [arXiv:1106.0034].[20] D. Curtin, R. Essig, S. Gori, P. Jaiswal, A. Katz, T. 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Beyond Standard Model Higgs boson physics with the ATLAS experiment at the LHC

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