New constraints and discovery potential for Higgs to Higgs cascade decays through vectorlike leptons

Journal of High Energy Physics, Oct 2016

One of the cleanest signatures of a heavy Higgs boson in models with vectorlike leptons is \( H\to {e}_{{}^4}^{\pm }{\ell}^{\mp}\to h{\ell}^{+}{\ell}^{-} \) which, in two Higgs doublet model type-II, can even be the dominant decay mode of heavy Higgses. Among the decay modes of the standard model like Higgs boson, h, we consider \( b\overline{b} \) and γγ as representative channels with sizable and negligible background, respectively. We obtained new model independent limits on production cross section for this process from recasting existing experimental searches and interpret them within the two Higgs doublet model. In addition, we show that these limits can be improved by about two orders of magnitude with appropriate selection cuts immediately with existing data sets. We also discuss expected sensitivities with integrated luminosity up to 3 ab−1 and present a brief overview of other channels.

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New constraints and discovery potential for Higgs to Higgs cascade decays through vectorlike leptons

Received: August New constraints and discovery potential for Higgs to Radovan Derm sek 0 1 2 4 Enrico Lunghi 0 1 4 Seodong Shin 0 1 3 4 leptons is H ! e 0 1 Open Access 0 1 c The Authors. 0 1 0 Seoul National University , Seoul 151-747 , Korea 1 Bloomington , IN 47405 , U.S.A 2 Department of Physics and Astronomy and Center for Theoretical Physics 3 Department of Physics & IPAP, Yonsei University 4 Physics Department, Indiana University One of the cleanest signatures of a heavy Higgs boson in models with vectorlike Beyond Standard Model; Higgs Physics - which, in two Higgs doublet model type-II, can even be the dominant decay mode of heavy Higgses. Among the decay modes of the standard model like Higgs boson, h, we consider bb and as representative channels with sizable and negligible background, respectively. We obtained new model independent limits on production cross section for this process from recasting existing experimental searches and interpret them within the two Higgs doublet model. In addition, we show that these limits can be improved by about two orders of magnitude with appropriate selection cuts immediately with existing data sets. We also discuss expected sensitivities with integrated luminosity up to 3 ab 1 and present a brief overview of other channels. 1 Introduction Analysis method New constraints from the 8 TeV LHC data 3.1 Recast of the bb + Recast of the Recast of the Sensitivity of bb + Expected experimental sensitivities Constraints and future prospects in two Higgs doublet model Heavy Higgs above the top threshold In models with vectorlike fermions, even a very small mixing with one of the Standard SM fermion. If there are more Higgs bosons, as in models with extended Higgs sector, the same mixing allows the heavy Higgses to decay into a vectorlike and a SM fermion. This leads to many new opportunities to search for new Higgs bosons and vectorlike matter simultaneously [1]. Limits from direct searches for vectorlike leptons are signi cantly weaker than for vectorlike quarks [1{6]. In addition, leptons in nal states typically result in clean signatures. Thus searching for combined signatures of vectorlike leptons and new Higgs bosons is especially advantageous. In this work, we focus on the process: where H is the heavy CP even Higgs and the e4 is a new charged lepton (note that, in a small region of the parameter space e4 is also a possible decay mode for the SM Higgs [7]). We obtain new constraints on this process from recasting existing experimental nd future experimental sensitivities by optimizing the selection cuts. This process appears for example in a two Higgs doublet model type-II with vectorlike leptons mixing with second SM family introduced in ref. [2, 8] and it was identi ed as Higgs decay mode ` ` (` = e; ) `+` ! 2`+2` (` = e; ) Table 1. The 13 TeV LHC production rates for H ! h + ) = 0:5. The value for h ! assumes that the not modi ed; in our model however it can be suppressed or enhanced, see ref. [8]. for various decay channels of the = 1 and BR(H ! h Yukawa coupling is one of the cleanest signatures of heavy Higgses in this class of models [1]. It was found can be the dominant decay mode of the heavy Higgs in a large range of parameters [1]. Moreover, as we will show, the high luminosity LHC is sensitive to this process even for branching ratio Depending on the decay mode of the SM-like Higgs boson, h, the process (1.1) leads to several interesting nal states with rates summarized in table 1 for a representative set of parameters: mH = 200 GeV, tan = 1, BR(H ! e4 mode of the SM-like Higgs boson h ! bb; W W ; ZZ ; ) = 0:5. Each decay provides its unique signal [1]. A prominent feature of all these channels is that the dimuon pair produced with the SM Higgs does not peak at the Z boson invariant mass as is the case for most backgrounds. Moreover, in most channels, it is possible to reconstruct the H and e4 masses. Although speci c searches for the process (1.1) do not exist, the particle content in nal states is the same as for pp ! Zh or pp ! A ! Zh and thus related Higgs searches constrain our process. We recast experimental searches for A ! hZ ! bb`+` , where model independent limits on production cross section of (1.1) in bb + nal states as functions of masses of H and e4. Then we suggest a simple modi cation of existing searches, the addition of the \o -Z" cut, which takes advantage of the two muons in the nal state not originating from a Z boson, and show how the limits could be improved immediately with current data and indicate experimental sensitivities with [12] performed at ATLAS. We set future data sets. After deriving model independent limits we interpret them within the two Higgs doublet model type-II. We rst use the scan of the parameter space of this model for mH < 2mt presented in ref. [1], where constraints from electroweak precision observables (oblique corrections, muon lifetime, Z-pole observables, W ), constraints on pair production of vectorlike leptons obtained from searches for anomalous production of multilepton events [3], H ! (W W; [1, 13] have been included. In addition, we extend 1Considering mixing with the rst SM family, or considering the CP odd Higgs instead of H would lead to very similar results. the scan for mH > 2mt and whole range of tan . We show how current experimental studies constrain the allowed parameter space and what we can achieve by means of optimized search strategies. This paper is organized as follows. In section 2 we brie y summarize our analysis method such as implementing the event simulations and setting the limits on our parameter space. The new constraints recasted from the existing searches are shown in section 3 and the expected experimental sensitivities in the future with our suggested cuts are discussed in section 4. We study the impact of the new constraints and future prospects of existing and suggested searches on the two Higgs doublet model type-II with vectorlike leptons in section 5. We further analyze the parameters of heavy Higgs above the tt threshold in section 6. Finally we give conclusions in section 7. Analysis method In this section we discuss the tools used for the event simulation and the statistical approach we adopt to set the limits. The new physics model is implemented in FeynRules [14], events are generated with MadGraph5 [15] and showered with Pythia6 [16]. The resulting StdHEP event les are converted into CERN root format using Delphes [17]. Jets are identi ed using the anti-kt algorithm of FastJet [18, 19] with angular separation R = 0:4. We present 95% C.L. upper limits calculated using a modi ed frequentist construction (CLs) [20, 21]. In recasting the searches presented in refs. [9, 11, 12], we follow the method described in refs. [3, 22] where the Poisson likelihood is assumed. In order to calculate the number of events, Ns95, that corresponds to the 95% C.L. upper limits, we consider a set of event numbers (fnig) corresponding to a Poisson distribution with expectation value b equal to the number of background events. For each ni the signal-plus-background hypothesis is tested using the CLs method. The expected upper limit Ns95 is the median of the fnig that pass the test.2 The 95% C.L. upper limits on the total pp ! H ! h + cross section normalized to the production cross section of a SM-like heavy Higgs (HSM) are given by BR(h ! bb) where ANPbb and ANP are the MC level acceptances (calculated using the selection cuts of the analyses that we recast) of the bb + detector level e ciencies and L is the integrated luminosity. Note that the negligible background to the channels, bb and mode implies Ns95( ) = 3 (with Poisson statistics, a null observation over a null background is compatible with up to three 2The median discovery signi cance is obtained by testing the background-only hypothesis with a data set with expectation value s + b where s is the number of expected signal events. signal events at 95% C.L. [3, 22]).3 For this reason the bb + can provide a stronger constraint as long as Ns95(bb) < Ns95( where the ratio of experimental e ciencies ( bb= ) is about one, the ratio of Monte Carlo and Ns95(bb) increases with the integrated luminosity as can be seen in table 2. ) is almost constant, New constraints from the 8 TeV LHC data In this section we extract upper bounds on the heavy Higgs cascade decays we consider from existing searches with 20.3 fb 1 of integrated luminosity at 8 TeV. The process H ! is constrained by searches for A ! hZ ! bb`+` , where A is a heavy (we focus on the former because they provide the explicit number of observed and expected events, allowing us to investigate the impact of the di erent cuts). The process H is constrained by the h ! ATLAS search [11] where the results with an inclusive lepton cut are presented (pp ! h`X ! `X) and also pp ! Z The results that we obtain and describe in details in the next three subsections are gures 1{3. The constraints on ( H = HSM ) ) are mostly constant as a function of the H and e4 masses and vary in the range [0:1; 0:3]. We steeply loose sensitivity for e4 close in mass to either the SM or the heavy Higgs (the transverse momentum of one the muons becomes too soft), or for a lighter heavy Higgs (the maximum value of the dilepton invariant mass is mH mh, see eq. (4.1) and the related discussion, and the requirement of an on-shell Z cuts all signal events for small mH ). Recast of the bb + From the results presented in ref. [9] we extract the observed upper limit Ns95(bb). We extract the detector level e ciency bb by comparing the expected number of the Higgsstrahlung (pp ! hZ) events given in ref. [9] to the ducial number of events that we calculate. In this way our bb includes the e ect of the pro le likelihood t of MC background events to the data in the control region. Using the Higgsstrahlung cross section presented in refs. [24, 25] and the acceptances we calculate, we nd bb ' 32%. The ducial region adopted in ref. [9] is de ned as follows. The two muons are required to have pseudorapidity j j < 2:5 and transverse momenta larger than 25 and 7 GeV. Their invariant mass is required to lie in the range 83 GeV < m`` < 99 GeV; note that this requirement cuts out a large part of our signal because we do not have an on-shell Z. A missing transverse energy cut Emiss < 60 GeV is imposed to reject the tt background. In 3With 3 ab 1 at 13 TeV the number of background events is non-zero and we take this into account in our limit setting. 4See also ref. [23] for resonances heavier than & 500 GeV. Figure 1. The 95% C.L. upper bounds on pp ! H ! e4 mH and me4 obtained by recasting the ATLAS search for A ! hZ ! bb`+` [9] from the 8 TeV for various choices of order to reduce the Z+jets background the transverse momentum of the dilepton system (pTZ ) is required to satisfy p 100 GeV where mVH is the invariant mass of the two leptons and two b-jets. The two b tagged-jets are required to have j j < 2:5 and pT > 45; 30 GeV to suppress Z+jets background. The invariant mass of the bb system is required to lie in the range 105 GeV < mbb < 145 GeV. Finally, in order to improve the mh = 125 GeV. Using the observed and expected background events given in table 1 of ref. [9] we obtain gure 1 we present the upper limits on pp ! H ! e4 various choices of mH and me4 . The limits become very weak for mH . 215 GeV because of the hard lepton selection cuts. Note that in the type-II two Higgs doublet model the ratio of Higgs production cross sections, that we show on the vertical axes, depends on tan . For tan values of tan < 7 this ratio is given by cot2 to a good approximation. At larger the bottom Yukawa coupling increases implying a non-negligible impact on the bb and gluon fusion production cross sections. We express our result in terms of ( H = HSM ) ) because the limits on this quantity are model independent. Recast of the The ducial region adopted in ref. [11] to study the nal state is de ned as follows. The diphoton event is selected when the invariant mass is in the range 105 GeV 160 GeV and p T > 0:35 (0.25) of m one isolated lepton with p this inclusive lepton selection cut (N` 95% C.L.. The limit on BR(H ! h + for the leading (next-to-leading) photon. At least T > 15 GeV is requested. The majority of our signal events pass 1) leading to large acceptance. ) is obtained from ANP ) < 0:80 fb, and is presented in gure 2 for various values of mH and From table 3 of ref. [11] the upper limit on the ducial cross section is about 0.80 fb at me4 . We can see that the strength of this constraint is similar to that of the bb + and me4 obtained by recasting the ATLAS search for H ! for various choices of mH 1 [11] from the 8 TeV full and me4 obtained by recasting the ATLAS search for pp ! for various choices of mH [12] from the 8 TeV full data. Recast of the In ref. [12] ATLAS presented a study of the T > 15 GeV, pT > 25 GeV, m nal state. The ducial cuts adopted > 40 GeV and > 0:4. Muons and photons are required to be isolated from nearby hadronic activity within a cone of size R = 0:4. In order to place a constraint on our signal we consider only three bins with m [100; 160] GeV (from the right panel of gure 4 of ref. [12]). The observed number of events is 8 over a background of 5 (mainly from pp ! Z( + limit on a new physics signal is 9.6 events. ). The implied 95% C.L. upper Using a detector level e ciency of 37.7% (as given in table 6 of ref. [12]), we obtain the bounds shown in gure 3. These bounds are slightly worse than those from the ysis in part because in that analysis the observed limit was slightly better than the expected one, while in the analysis there was a small excess for 100 GeV < m < 160 GeV. All the cuts described in section 3.1 (with the exception of the m`` one) Expected experimental sensitivities In this section we suggest new selection cuts to improve the sensitivity to our signal. First let us discuss the distribution of the invariant mass of the dilepton system m``. In our process the two oppositely charged muons are not produced from a Z decay. The analytic formula for m`` is m`` = m2H + me24 + (m2H where is the angle between the two muons in the heavy Higgs rest frame. The maximum value of m``, obtained for cos 1 and me4 = pmH mh, is mH mh. The detailed distribution of m`` depends on the masses mH and me4 . Examples are shown in gure 4 for at xed me4 = 260 GeV. As discussed in ref. [1] a large part of our signal lies in the region jm`` MZ j > 15 GeV allowing us to veto a major background process, Z + (heavy avored) jets with Z ! For this reason we propose to consider separately m`` > MZ + 15 GeV and 20 < m`` < 15 GeV cuts (we added a lower limit m`` > 20 GeV to suppress the background events ). We call these cuts \o -Z below" and \o -Z above" cuts. In each channel. All the cuts described in section 3.1 (with the exception of the m`` one) are imposed. panel of gures 4 and 5 we show such regions with blue vertical lines and arrows. We see that for small mH mh and/or mH me4 the \above" cut is depleted of events. channel we keep the rest of the cuts in ref. [9] other than 83 GeV < m`` < 99 GeV. Additionally we request that the invariant mass of all the nal states m should be within 10% of each mH hypothesis. Pro le likelihood ts can be used once actual data are available. channel we further impose a missing transverse energy cut Emiss < 60 GeV to suppress the background from the top-quark decays. Moreover we request two leptons with pT > 15 GeV. Sensitivity of bb + We begin by studying how the sensitivity of the existing 8 TeV 20 fb 1 bb + changes with the adoption of the new cuts we propose. This is controlled by the change in the expected number of background event that is obtained by computing the ratio of acceptances of new and original cuts: b = in eq. (2.1). where b0 is the number of background events given in ref. [9], AnBew and AoBriginal are the MC level acceptances for the new and original cuts, respectively. The ratio of acceptances are obtained from the sample including Z + b-jets, tt, and Higgsstrahlung processes.5 The 95% CLs median upper limits Ns95 obtained from the number of expected background and the ratio of acceptances are shown in table 2 for our reference parameters -Z above" cuts. Finally, the experimental sensitivities are obtained by inserting these limits 5We do not include the subdominant background channels like single top quark and diboson (V V ) mH [GeV] Ns95 (\o -Z below") Ns95 (\o -Z above") 8 TeV 13 TeV To estimate the sensitivity at 13 TeV we start with considering the cuts used in the recent ATLAS analysis [26] performed with 3.2 fb 1 of integrated luminosity at 13 TeV. Since we are interested in mH < 340 GeV for now, we consider the low p The basic cuts adopted in this search are the following. One of the two leptons must have pT > 25 GeV with j j < 2:5 and the invariant mass of the dilepton should be in the 70 GeV < m`` < 110 GeV window. Events with two b-tagged jets are selected when one of them satis es pT > 45 GeV on top of their basic b-jet selection criteria. The invariant mass of the two b-tagged jets must be in the range 110 GeV < mbb < 140 GeV. In order to suppress the tt background the missing transverse energy should be in the range GeV where HT is the scalar sum of the pT of the leptons and b-tagged jets. To improve the resolution of the bb`+` resonance signal the four momentum of Because the main goal of the search in ref. [26] is nding the resonant signal A ! hZ, with MZ = 91:2 GeV: this requirement strongly suppresses the acceptance of our signal implying the absence of Our proposed cuts involve adding the \o -Z above" and \o -Z below" cuts described above, removing the rescaling of the four momentum of the dimuon system and including the invariant mass cut jm mH j < 0:1 mH . The number of expected background events with integrated luminosities of 100 fb 1 and 3 ab 1 are calculated analogously to the 8 TeV case and the corresponding Ns95 are summarized in table 2. The cuts that we suggest are those considered in ref. [11] (and described in section 3.2) with the inclusion of the \o -Z below"/\o -Z above" cuts, a missing transverse energy cut Emiss < 60 GeV (to suppress the htt T nal state) and the requirement of a second isolated muon with pT > 15 GeV. Additionally one could add a veto on high pT b-jets (for an additional suppression of the htt background) and a cut on the invariant mass of the system. The latter, in particular, could turn useful if non-irreducible sources of background turns out to be larger than expected. The background to the channel has been studied in detail in ref. [12] (E > 15 GeV and pT > 25 GeV) and it is found to be dominated by pp ! Z( + vetos on b-jets, hence any Z + j; j ; jj with one or two jets misidenti ed as isolated photons. These backgrounds are also found to decrease steeply with the transverse energy of the photon. The E we suggest are much stronger (the hardest photon has E 56 GeV depending on the diphoton invariant mass) than those considered in ref. [12] and make this background completely negligible (also taking into account the further reduction due to the o -Z cuts). Two more sources of background (that are not suppressed by a stronger E (Presently we do not require nal state is a background). At 8 TeV the combined total cross section for these two processes is about 35 ab corresponding to 0.7 events with 20 fb 1 before applying any selection cut; therefore, we set this background to zero and and 240 events with 100 fb 1 and 3 ab 1, respectively; in this case a discussion of ducial acceptances and detector e ciencies is crucial to estimate the expected background. Using these selection cuts we nd that the ducial acceptances for the \o -Z below" and \o -Z above" cases are 1.4% and 1.2%, respectively. Assuming an overall detector e ciency of about 37.7% (as suggested in ref. [12]), we then nd that the expected number of background events at 13 TeV with 100 fb 1 and 3 ab 1 are 0 and 1, respectively: the corresponding Ns95 are 3 and 4 events. Constraints and future prospects in two Higgs doublet model In this section we study the impact of the limits derived in previous sections (and indicate future prospects of existing and suggested searches) on the two Higgs doublet model type-II with vectorlike pairs of new leptons introduced in ref. [2]. We assume that the new leptons mix only with one family of SM leptons and we consider the second family as an example. In gure 6 we present the parameter space scan of this model in the plane spanned by me4 and ( (pp ! H)= (pp ! HSM)) range [-0.5, 0.5], as described in ref. [1]. Each point satis es precision EW data constraints related to the muon and muon neutrino: muon lifetime, Z-pole observables, the W partial width and oblique observables. In addition, we impose constraints on pair production of vectorlike leptons obtained from searches for anomalous production of multilepton events [3] ) for four di erent heavy Higgs masses and constraints from searches for heavy Higgs bosons in H ref. [1, 13, 22] and for the SM Higgs h ! discussed in ref. [1]. The solid red, blue and green contours in gure 6 are the new constraints obtained from recasting the existing bb + constraints dominate at low mH because the bb + searches. Note that the search looses sensitivity due to a strong cut on the transverse momentum of the hardest muon. Dashed contours indicate expected sensitivities using our proposed o -Z cuts for three scenarios of LHC energies and integrated luminosities: (8 TeV, 20 fb 1), (13 TeV, 100 fb 1) and (13 TeV, 6We calculate the cross sections assuming that H is the heavy CP even Higgs. Note, however, that the limits are still model independent. discussed in model discussed in ref. [2] from bb + X (blue) and dashed lines are the expected constraints for the bb + (green) searches. The (blue) channels. We ((8 TeV,20 fb 1), (13 TeV,100 fb 1) and (13 TeV,3 ab 1)). result in similar bounds. A direct inspection of gure 6 shows that the analysis strategy we propose has the potential to improve the experimental sensitivity between one and two orders of magnitude depending on the heavy Higgs and vectorlike lepton masses. From the sensitivities shown in gure 6 we see that the impact of the o -Z cuts is much more pronounced for the bb + nal state rather for the one and the expected bounds converge at very high integrated luminosity. The reason is that the background to search is very small at all luminosities and, therefore, is not a ected much by the additional o -Z cuts; in the bb + channel the background is large and is sizably reduced by the cuts we propose. At very large luminosity the expected number of background events increases much more for bb + and the sensitivity of the two channels become comparable. At very high luminosities (beyond what is planned for the LHC) the di-photon channel would dominate. Overall the potential for exclusion (discovery) of new physics in these channels in the next few years seems very strong: sensitivity to branching ratios of order O(10 4 is within reach and, correspondently, a very large part of this model parameter space will ) for mH 2 [350; 800] GeV are shown with (purple, blue, green, and red colors) can be compared with (H ! anything but e4 We should note that, in ref. [27], ATLAS presented a search for bb + that makes use of multivariate techniques to massively reduce the irreducible background. While we were not able to use this analysis to place constraints on our model, we expect that a dedicated experimental study of the signal we propose using a similar approach has the potential to improve signi cantly the bounds we presented. The sensitivity could be additionally increased by looking for the e4 ! h ) resonance. Finally let us brie y discuss the decay H ! h + the other possible channels we mention in table 1. The h ! ZZ with the SM Higgs decaying into decay yields a 4` + nal state that has negligible SM background; nevertheless the small branching ratio makes this channel less sensitive than the one. On the other hand, the sizable h ! branching ratio (about 6.3%) makes the + nal state competitive with the bb + one; a detailed study of this nal state from pp ! A ! hZ has been performed by both ATLAS [28] and CMS [29]. The h ! expected to yield sensitivities even higher than the mode yields the 2`2 2 nal state and is channel (both have negligible background and the former has a larger branching ratio). Finally the h ! nal state with a rate that depends strongly on the model Yukawa couplings (see the discussion in refs. [7, 8]). Heavy Higgs above the top threshold In this section we discuss the constraints and prospects for mH & 2mt where the H ! tt contribution to the heavy Higgs decay width reduces its branching ratio into vectorlike ) for mH = 750 GeV are shown with the yellow scattered points which correspond to the red points in gure 7. Partial widths of H to various SM particles are shown with di erent colored lines. leptons. In this mass range, the heavy Higgs width into SM fermions is dominated by the tt channel at moderate tan < 7 and by the bb at larger tan . From the analysis presented in ref. [1] (see bottom-left panel of gure 3 of that paper) it is clear that the H ! e branching ratio can easily be dominant for all values of . 20 and mH < 2mt. This implies immediately that we expect BR(H ! e4 be sizable for Higgs masses above the tt threshold at large tan width is suppressed with respect to the H ! bb one). For tan (where the H ! tt partial < 7 the H ! tt partial width becomes dominant and we need a detailed numerical calculation in order to assess the size of the H ! e4 In order to check whether large BR(H ! e4 ) are allowed, we rescan the parameters for mH above the tt threshold up to 800 GeV and allow only parameter space points that satisfy all the constraints discussed in ref. [1]: electroweak precision data, anomalous multilepton production with missing ET , SM Higgs data for h ! , and heavy Higgs searches in the and W W channels. As in the previous case the charged sector Yukawa couplings are scanned in the range [-0.5, 0.5]. gure 7 we show the resulting heavy Higgs partial widths (calculated assuming H is the heavy CP even Higgs) as a function of tan . The widths mH 2 [350; 800] GeV are shown with the yellow scattered points. For comparisons of these widths with those for H ! SM particles we consider four di erent representative H masses these reference H masses the widths (H ! anything but e4 ) are shown with the solid lines; they are dominated by H ! tt for tan . 7 and by H ! bb for tan & 7 which can be directly read from We show estimates of the bound (black solid line) recasted from the 13 TeV A ! hZ resonance search [26] and future experimental sensitivities (black dashed lines) for integrated luminosities L = 300 fb 1 and 3 ab 1 at 13 TeV. see that the H ! e4 decay mode can be dominant for 4 . tan . 17. However, this region depends on the allowed range of Yukawa couplings. Increasing the range to [-1, 1], can dominate for 4 . tan Note that searches for heavy CP even neutral Higgs are extremely challenging because, as explained in ref. [30], the gg ! H ! tt resonant peak can be destroyed by interference with the SM background (especially for 400 GeV . mH . 900 GeV and tan in the aligned two Higgs doublet model type-II). For the CP odd Higgs this e ect leads to more dip-like signals in a large range of parameters but it is still hard to observe for mH < 600 GeV and tan . 5. If the heavy Higgs couples to vectorlike leptons, as in our BR(H ! e the models we consider, the H ! e4 channel o ers a new and very promising avenue ) plane. For simplicity we do not vary the vectorlike lepton mass and set it to ! e4e4 channel. Green, red, blue and magenta point correspond to tan < 3, 3 < tan < 20, and 20 < tan < 50 respectively. From gure 8 we see that ) can be larger than 0.25 for 3 < tan < 20. For larger tan of ( H = HSM ) heavy Higgs production cross section is enhanced compared to (pp ! HSM) so the values ) are as large as those for 3 < tan < 20. The solid black contour is the recasted constraint from the 13 TeV A ! hZ resonance search [26]. The expected sensitivities of future bb + studies are displayed as dashed lines. We conclude that recasted searches barely touch the allowed parameter space around ( H = HSM ) that we propose have the potential to constrain this quantity at 10 5 level. 0:05. However, future searches employing the o -Z cuts In this paper we discuss the Higgs cascade decay pp ! H ! e in models with extra vectorlike leptons and an extended Higgs sector. Among the various decay channels of the SM Higgs h we considered the bb and ones, which yield bb + nal states. These are two representative channels with sizable and negligible background, respectively. We were able to recast existing pp ! A ! hZ searches into constraints on the two modes we consider. We also presented the expected sensitivities of dedicated searches in the full 8 and 13 TeV data sets. A unique feature of cascade decay we consider is that the two leptons do not reconstruct a Z boson, while the h invariant masses peak at me4 and mH , respectively. Therefore, we suggest to employ two o -Z cuts that focus on the region above and below the Z resonance: 20 GeV < m`` < MZ 15 GeV and m`` > MZ + 15 GeV. In addition to these suggested cuts, the searches for two resonances corresponding to the H and e4 masses will lead further to higher sensitivities. We nd that this analysis strategy has the potential to improve the experimental sensitivity between one and two orders of magnitude depending on the heavy Higgs and vectorlike lepton masses. We discussed an explicit realization of a new physics model in which this cascade decay is allowed to proceed with sizable branching ratio. The model has been introduced in ref. [1] and involves a new family of vectorlike leptons and an extra Higgs doublet. We found that a vast majority of this model parameter space that survives various indirect and direct constraints can be easily tested by searches for heavy Higgs cascade decays. One major result of our analysis is that the bb + channel dominates the in most of the parameter space up to an integrated luminosity of 3 ab 1 at 13 TeV. We also brie y discussed other possible channels and found that the and 2`2 2 have the potential to o er constraints comparable to those obtained from the bb + Furthermore we discuss the reach of our search strategy for a heavy Higgs with mass above the tt threshold. We both H ! tt and H ! bb for 4 . tan branching ratio can dominate over . 17 (4 . tan . 32) when charged sector Yukawa couplings are allowed in [-0.5, 0.5] ([-1, 1]). However, even in the range of parameters where our process has only a small branching ratio, it can be the most promising search channel since the usual search strategies for H ! tt su er from interference e ect with the SM background. Rough estimates of future experimental sensitivities are extremely promising. EL thanks Narei Lorenzo Martinez for discussions on the channel. SS thanks Bogdan A. Dobrescu and Zhen Liu for useful discussions. SS also thanks Fermi National Accelerator Laboratory for hospitality and support during the completion of this work. The work of RD and EL was supported in part by the U.S. Department of Energy under grant number DE-SC0010120. RD is supported in part by the Ministry of Science, ICT and Planning (MSIP), South Korea, through the Brain Pool Program. SS is supported in part by BK21 plus program. Open Access. This article is distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited. doublet model with vectorlike leptons, JHEP 05 (2016) 148 [arXiv:1512.07837] [INSPIRE]. 92 (2015) 115018 [arXiv:1510.03456] [INSPIRE]. phenomenology, JHEP 05 (2014) 092 [arXiv:1312.5329] [INSPIRE]. extensions of the Standard Model and their phenomenological implications, JHEP 09 (2014) 130 [arXiv:1404.4398] [INSPIRE]. 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Radovan Dermíšek, Enrico Lunghi, Seodong Shin. New constraints and discovery potential for Higgs to Higgs cascade decays through vectorlike leptons, Journal of High Energy Physics, 2016, 81, DOI: 10.1007/JHEP10(2016)081