Searches for a heavy scalar boson H decaying to a pair of 125 GeV Higgs bosons hh or for a heavy pseudoscalar boson A decaying to Zh, in the final states with h→ττ

A search for a heavy scalar boson H decaying into a pair of lighter standard-model-like 125 GeV Higgs bosons h and a search for a heavy pseudoscalar boson A decaying into a Z and an h boson are presented. The searches are performed on a dataset corresponding to an integrated luminosity of 19.7 inverse femtobarns of pp collision data at a centre-of-mass energy of 8 TeV, collected by CMS in 2012. A final state consisting of two tau leptons and two b jets is used to search for the H to hh decay. A final state consisting of two tau leptons from the h boson decay, and two additional leptons from the Z boson decay, is used to search for the decay A to Zh. The results are interpreted in the context of both the minimal supersymmetric extension of the standard model and two-Higgs-doublet models. No excess is found above the standard model expectation and upper limits are set on the heavy boson production cross sections in the mass ranges 260


Introduction
The discovery of additional Higgs bosons at the LHC would provide direct evidence of physics beyond the standard model (SM). There are several types of models that require two Higgs doublets [1][2][3]. For example the minimal supersymmetric extension of the SM (MSSM) requires the introduction of an additional Higgs doublet, where one Higgs doublet couples to up-type quarks and the other to down-type quarks [4][5][6][7][8][9][10][11]. This leads to the prediction of five Higgs particles: one light and one heavy CP-even Higgs boson, h and H, one CP-odd Higgs boson A, and two charged Higgs bosons H ± [2,12]. The masses and couplings of these bosons are interrelated and, at tree level, can be described by two parameters, which are often chosen to be the mass of the pseudoscalar boson m A and the ratio of the vacuum expectation values of the neutral components of the two Higgs doublets tan β. However, radiative corrections [13][14][15][16][17] introduce dependencies on other parameters namely the mass of the top quark m t , the scale of the soft supersymmetry breaking masses M SUSY , the higgsino mass parameter μ, the wino mass parameter M 2 , the third-generation trilinear couplings, A t , A b , and A τ , the mass of the gluino mg, and the third-generation slepton mass parameter M˜ 3 . E-mail address: cms-publication-committee-chair@cern.ch.
Direct searches for the neutral MSSM Higgs bosons have been performed by the CMS and ATLAS Collaborations [18][19][20] using the benchmark scenarios proposed in Ref. [21]. In these scenarios the parameters involved in the radiative corrections for the Higgs boson masses and couplings have been fixed, and only the two parameters m A and tan β remain free. The value of M SUSY was fixed at around 1 TeV, which produces a lightest CP-even Higgs boson with a mass m h lower than the observed Higgs boson mass of 125.09 ± 0.21 (stat) ± 0.11 (syst) GeV [22], for values of tan β 6.
If, however, M SUSY is much larger than 1 TeV, as suggested by the non-observations of SUSY partner particles at the LHC so far, low values of tan β can produce an h boson with m h 125 GeV [23,24]. The interpretation of the Higgs boson measurements in the framework of the recently developed MSSM benchmark scenarios [24][25][26][27] suggests that the mass of the CP-odd Higgs boson, m A , can be smaller than 2m t . In the mass region below 2m t and at low values of tan β, the decay mode of the heavy scalar H → hh and that of the pseudoscalar A → Zh can have sizeable branching fractions.
This encourages a programme of searches in the so-called "low tan β" channels [23,28]: • for 220 GeV < m A < 2m t : A → Zh; • for 260 GeV < m A < 2m t : H → hh; • for m A > 2m t : A/H → tt. The decay modes H → hh and A → Zh, studied in this paper, are also present in other types of two-Higgs-doublet models (2HDM) [2,3]. There are different types of 2HDM with those most similar to the MSSM (i.e. where up-type fermions couple to one doublet and down-type fermions to the other) being "Type II" 2HDM. The discovery of a Higgs boson at the LHC [29-31] with a mass around 125 GeV pushes the 2HDM parameter space towards either the alignment or decoupling limits [24]. In these limits the properties of h are SM-like.
In the alignment limit of 2HDM when cos(β − α) 1 (where α is the mixing angle between the two neutral scalar fields), the Hhh and AZh couplings vanish at Born level [32]. However, in the MSSM, the Hhh and AZh couplings do not vanish, even in the alignment limit, because of the large radiative corrections that arise in the model. In the decoupling limit of 2HDM the scalar Higgs boson H has a very large mass and the decay H → tt dominates [32]. This paper reports the results of searches for the decays H → hh → bbτ τ and A → Zh → τ τ (where denotes μμ or ee). The choice of τ pair final state was driven by its quite clean signature and by the most recent results, which gave stronger evidence of the 125 Higgs boson coupling to the fermions [33]. This analysis exploits similar techniques as used for the search for the SM Higgs boson at 125 GeV [34] and several different τ τ signatures [39][40][41] in di-photon, multilepton and bb final states.
This analysis has the power to bring important results in the low tan β region for the m A range, which has been previously discussed and where these processes have an enhanced sensitivity [23]. This region has not yet been excluded by the direct or indirect searches for a heavy scalar or pseudoscalar Higgs boson, that have been mentioned above, therefore the described decay modes look to be quite promising. For simplicity of the paper, we are neither indicating the charge of the leptons nor the particle-antiparticle nature of quarks.

The CMS detector, simulation and data samples
A detailed description of the CMS detector can be found in Ref. [42]. The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter providing a field of 3.8 T. Within the field volume are a silicon pixel and strip tracker, a crystal electromagnetic calorimeter (ECAL), and a brass/scintillator hadron calorimeter. Muons are measured in gas-ionisation detectors embedded in the steel return yoke of the magnet. The CMS coordinate system has the origin centred at the nominal collision point and is oriented such that the x-axis points to the centre of the LHC ring, the y-axis points vertically upward and the z-axis is in the direction of the beam. The azimuthal angle φ is measured from the x-axis in the xy plane and the radial coordinate in this plane is denoted by r. The polar angle θ is defined in the rz plane and the pseudorapidity is η = − ln[tan(θ/2)] [42]. The momentum component transverse to the beam direction, denoted by p T , is computed from the x-and y-components.
The first level (L1) of the CMS trigger system, composed of custom hardware processors, uses information from the calorimeters and muon detectors to select the most interesting events in a fixed time interval of less than 4 μs. The high-level Trigger processor farm decreases the L1 accept rate from around 100 kHz to less than 1 kHz before data storage.
The data used for this search were recorded with the CMS detector in proton-proton collisions at the CERN LHC and correspond to an integrated luminosity of 19.7 fb −1 at a centre-of-mass energy of √ s = 8 TeV. The H → hh signals are modelled with the pythia 6.4.26 [43] event generator while the A → Zh signals were modelled with MadGraph 5.1 [44]. When modelling background processes, the MadGraph 5.1 generator is used for Z+jets, W+jets, tt, and diboson production, and powheg 1.0 [45][46][47][48] for single top quark production. The powheg and MadGraph generators are interfaced with pythia for parton showering and fragmentation using the Z2* tune [49]. All generators are interfaced with tauola [50] for the simulation of the τ decays. All generated events are processed through a detailed simulation of the CMS detector based on Geant4 [51] and are reconstructed with the same algorithms as the data. Parton distribution functions (PDFs) CT10 [52] or CTEQ6L1 [53] for the proton are used, depending on the generator in question, together with MSTW2008 [54] according to PDF4LHC prescriptions [55].

Event reconstruction
During the 2012 LHC run there were an average of 21 protonproton interactions per bunch crossing. The collision vertex that maximises the sum of the squares of momenta components perpendicular to the beamline (transverse momenta) of all tracks associated with it, p 2 T , is taken to be the vertex of the primary hard interaction. The other vertices are categorised as pileup vertices.
A particle-flow algorithm [56,57] is used to reconstruct individual particles, i.e. muons, electrons, photons, charged hadrons and neutral hadrons, using information from all CMS subdetectors.
Composite objects such as jets, hadronically decaying τ leptons, and missing transverse energy are then constructed using the lists of individual particles.
Muons are reconstructed by performing a simultaneous global track fit to hits in the silicon tracker and the muon system [58]. Electrons are reconstructed from clusters of ECAL energy deposits matched to hits in the silicon tracker [59]. Muons and electrons assumed to originate from W or Z boson decays are required to be spatially isolated from other particles [59,60]. The presence of charged and neutral particles from pileup vertices is taken into account in the isolation requirement of both muons and electrons. Muon and electron identification and isolation efficiencies are measured via the tag-and-probe technique [61] using inclusive samples of Z → events from data and simulation. Correction factors are applied to account for differences between data and simulation.
Jets are reconstructed from all particles using the anti-k T jet clustering algorithm implemented in fastjet [62,63] with a distance parameter of 0.5. The contribution to the jet energy from particles originating from pileup vertices is removed following a procedure based on the effective jet area described in Ref. Jets from the hadronisation of b-quarks (b jets) are identified with the combined secondary vertex (CSV) b tagging algorithm [67], which exploits the information on the decay vertices of long-lived mesons and the transverse impact parameter measurements of charged particles. This information is combined in a likelihood discriminant. The medium value of the CSV discriminator, corresponding to a b jet misidentification probability of 1%, has been used in this analysis.
Hadronically decaying τ leptons are reconstructed using the hadron-plus-strips algorithm [68], which considers candidates with one charged pion and up to two neutral pions, or three charged pions. The neutral pions are reconstructed as "strips" of electromagnetic particles taking into account possible broadening of calorimeter energy depositions in the φ direction from photon conversions. The τ h candidates that are also compatible with muons or electrons are rejected. Jets originating from the hadronisation of quarks and gluons are suppressed by requiring the τ h candidate to be isolated. The contribution of charged and neutral particles from pileup interactions is removed when computing the isolation.

Event selection
The events are selected with a combination of electron, muon and τ trigger objects [34,59,60,69]. The identification criteria of these objects were progressively tightened and their transverse momentum thresholds raised as the LHC instantaneous luminosity increased over the data taking period. A tag-and-probe method was used to measure the efficiencies of these triggers in data and simulation, and correction factors are applied to the simulation.
Electrons, muons, and τ h are selected using the criteria defined in the CMS search for the SM Higgs boson at 125 GeV [34]. Specific requirements for the selection of the H → hh → bbτ τ and the A → Zh → τ τ channels are described below.

Event selection of H → hh → bbτ τ
In the H → hh → bbτ τ channel, the three most sensitive final states are analysed, distinguished by the decay mode of the two τ leptons originating from the h boson (μτ h , eτ h and τ h τ h ).
In the μτ h and eτ h final states, events are selected with a muon with p T > 20 GeV and |η| < 2.1 or an electron of p T > 24 GeV and |η| < 2.1, and an oppositely charged τ h of p T > 20 GeV and |η| < 2.3. To reduce the Z → μμ, ee contamination, events with two muons or electrons of p T > 15 GeV, of opposite charges, and passing loose isolation criteria are rejected.
In the μτ h and eτ h final states, the transverse mass of the muon or electron and p miss where p T is the lepton transverse momentum and φ is the difference in the azimuthal angle between the lepton momentum and p miss T , is required to be less than 30 GeV to reject events coming from W+jets and tt backgrounds. The m T distribution for the μτ h final state is shown in Fig. 1.
In the τ h τ h final state, events with two oppositely charged hadronically decaying τ leptons with p T > 45 GeV and |η| < 2.1 are selected.
In addition to the τ τ selection, each selected event must contain at least two jets with p T > 20 GeV and |η| < 2.4. These p T and η requirements are necessary to select jets that have a well defined value of the CSV discriminator (Section 3), which is important for categorising signal-like events with two b jet candidates coming from the 125 GeV Higgs boson decaying to bb. Simulation studies show that the majority of signal events will have at least one jet passing the medium working point of the CSV discriminator. The jets are ordered by CSV discriminator value, such that the leading and subleading jets are defined as those with the two highest CSV values. Then the events are separated into categories, defined as: • 2jet-0tag when neither the leading nor subleading jet passes the medium CSV working point. Only a small amount of signal is collected in this category, which is background-dominated.
• 2jet-1tag when only the leading but not the subleading jet passes the medium CSV working point.
• 2jet-2tag when both the leading and subleading jets pass the medium CSV working point.
The signal extraction is performed using the distribution of the reconstructed mass of the H boson candidate.

Event selection of A → Zh → τ τ
In the A → Zh → τ τ channel eight final states are analysed.
These are categorised according to the decay mode of the Z boson and the decay mode of the τ leptons originating from the h boson.
The Z boson is reconstructed from two same-flavour, isolated, and oppositely charged electrons or muons. In the Z → μμ (ee) final state the muons (electrons) are required to have |η| < 2.4 (2.5) with p T > 20 GeV for the leading lepton and p T > 10 GeV for the subleading lepton. The invariant mass of the two leptons is required to be between 60 GeV and 120 GeV. When more than one pair of leptons satisfy these criteria, the pair with an invariant mass closest to the Z boson mass is selected.
After the Z candidate has been chosen, the h → τ τ decay is selected by combining the decay products of the two τ leptons in the four final states μτ h , eτ h , τ h τ h , eμ. The combination of the large contribution from the irreducible ZZ background and of the small branching fractions of leptonic tau decays makes the μμ and ee final states less sensitive to the signal, and therefore they are not used in the analysis. Depending on the final state, a muon with p T > 10 GeV and |η| < 2.4, or an electron of p T > 10 GeV and |η| < 2.5, or a τ h of p T > 21 GeV and |η| < 2.3 are combined to In order to reduce the tt background, events containing a jet with p T > 20 GeV, |η| < 2.4 and passing the medium working point of the CSV b tagging discriminator are removed. The four final objects are further required to be separated from each other by R = ( η) 2 + ( ϕ) 2 larger than 0.5 (where ϕ is in radians), and to come from the same primary vertex. In this channel the signal extraction is performed using the distribution of the reconstructed mass of the A boson candidate.

Background estimation for H → hh → bbτ τ
The backgrounds to the H → hh → bbτ τ final state consist predominantly of tt events, followed by Z → τ τ +jets events, W+jets events, and QCD multijet events, with other small contributions from Z → , diboson, and single top quark production. The estimation of the shapes of the reconstructed H mass and of the yields of the major backgrounds is obtained from data wherever possible.
The Z → τ τ process constitutes an irreducible background due to its final state involving two τ leptons, which only differ from the h → τ τ signal by having an invariant mass closer to the mass of the Z boson instead of the Higgs boson. Requiring two jets in the event greatly reduces this background and the b tagging requirements reduce it even further. Nevertheless, it still remains an important source of background events, in particular in the 2jet-1tag and 2jet-0tag categories. This background is estimated using a sample of Z → μμ events from data, obtained by requiring two oppositely charged isolated muons, where the reconstructed    muons are replaced by the reconstructed particles from simulated τ decays. A correction for a contamination from tt events is applied to the Z → μμ selection. This technique substantially reduces the systematic uncertainties due to the jet energy scale and the missing transverse energy, as these quantities are modelled with data.
For the tt background, both shape and normalisation are taken from Monte Carlo simulation (MC), and the results are checked against data in a control region where the presence of tt events is enhanced by requiring eμ in the final state instead of a ditau, and at least one b tagged jet.
Another significant source of background is from QCD multijet events, which can mimic the signal in various ways, e.g. where one or more jets are misidentified as τ h . In the μτ h and eτ h channels, the shape of the QCD background is estimated using an observed sample of same-sign (SS) τ τ events. The yield is obtained by scaling the observed number of SS events by the ratio of the opposite-sign (OS) to SS event yields obtained in a QCD-enriched region with relaxed lepton isolation. In the τ h τ h channel, the shape is obtained from OS events with relaxed τ isolation. The yield is obtained by scaling these events by the ratio of SS events with tighter and relaxed τ isolation.
In the μτ h and eτ h channels, W+jets events in which there is a jet misidentified as a τ h are another sizeable source of background. The W+jets shape is modelled using MC simulation and the yield is estimated using a control region of events with large m T close to the W mass. In the τ h τ h channel this background has been found to be less relevant and its shape and yield are taken from MC simulation.
The contribution of Drell-Yan production of muon and electron pairs is estimated from simulation after rescaling the simulated yield to that measured from observed Z → μμ events. In the eτ h channel, the Z → ee simulation is further corrected using the e → τ h misidentification rate measured in data using a tag-andprobe technique [61] on Z → ee events.
Finally the contributions of other minor backgrounds such as diboson and single top quark events are estimated from simulation. Possible contributions from SM Higgs boson production are estimated and found to have a negligible effect on the final result.

Background estimation for
The backgrounds to the A → Zh channel can be divided into a reducible component and an irreducible component which contribute in equal parts. The predominant source of irreducible background is from ZZ production that yields exactly the same final states as the expected signal. Other "rare" sources of irreducible background are SM Higgs boson associated production with a Z boson, ttZ production where the Z boson decays into a muon or an electron pair and both top quarks decay leptonically (to e, μ, or τ h ), and triboson events (WWZ, WZZ, ZZZ). The contributions of all the irreducible backgrounds after the final selection are estimated from simulation.
The reducible backgrounds have at least one lepton in the final state that is due to a misidentified jet that passes the lepton identification. In τ h τ h final states, the reducible background is essentially composed of Z+jets events with at least two jets, whereas in μτ h and eτ h final states, the main contribution to the reducible background comes from WZ+jets with three light leptons. The contribution from these processes to the final selected events is estimated using control samples in data.
The probabilities for a jet that passes relaxed lepton selection criteria to pass the final identification and isolation criteria of electrons, muons, and τ leptons are measured in a signal-free region as a function of the transverse momentum of the object closest to the candidate, f (p fake T ). In this region, events are required to pass all the final state selections, except that the reconstructed τ candidates are required to have the same sign and to pass relaxed identification and isolation criteria. This effectively eliminates any possible signal, while maintaining roughly the same proportion of reducible background events.
In order to use the misidentification probabilities f (p fake T ), sidebands are defined for each channel, where, unlike the relaxed criterion, the final identification or isolation criterion is not satisfied for one or more of the final state lepton candidates. The number of reducible background events due to a lepton being misidentified in the final selection is estimated by applying the weight f (p fake T )/(1 − f (p fake T )) to the observed events with lepton candidates in the sideband that satisfy the relaxed but not the final identification or isolation criterion. Finally, the reducible background shape of the reconstructed A mass is obtained from a SS signal-free region where the τ candidates have the same charge and relaxed isolation criteria. Possible contributions from SM Higgs boson production are estimated and found to have a negligible effect on the final result.

Systematic uncertainties
The shape of the reconstructed mass of the A and H boson candidates, used for signal extraction, and the normalisation are sensitive to various systematic uncertainties.
The main contributions to the normalisation uncertainty that affect the signal and the simulated backgrounds include the uncertainty in the total integrated luminosity, which amounts to 2.6% [70], and the identification and trigger efficiencies of muons (2%) and electrons (2%). The τ h identification efficiency has a 6% uncertainty (8% in the τ h τ h channel), which is measured in Z/γ * → τ τ → μτ h events using a tag-and-probe technique. There is a 3% uncertainty in the efficiency on the hadronic part of the μτ h and eτ h triggers, and a 4.5% uncertainty on each of the two τ h candidates required by the τ h τ h trigger. The b tagging efficiency has an uncertainty of 2-7%, and the mistag rate for light-flavour partons is accurate to 10-20% depending on η and p T [67]. The background normalisation uncertainties from the estimation methods discussed in Section 5 are also considered. In the H → hh → bbτ τ channel this uncertainties amount to 2-40% depending on the event category and on the final state. The uncertainties of reducible backgrounds to the A → Zh channel are estimated by evaluating an individual uncertainty for each lepton misidentification rate and applying it to the background calculation. This amounts to 15-50% depending on the final τ τ state considered. The main uncertainty in the estimation of the ZZ background arises from the theoretical uncertainty in the ZZ production cross section.
Uncertainties that contribute to variations in the shape of the mass spectrum include the jet energy scale, which varies with jet p T and jet η [71], and the τ lepton (3%) energy scale [34].
Theoretical uncertainties on the cross section for signal derive from PDF and QCD scale uncertainties and depend on the choice of signal hypothesis. For model independent results no choice of cross section is made and hence no theoretical uncertainties are considered. For the MSSM interpretation the uncertainties depend on m A and tan β and amount to 2-3% for PDF uncertainties and 5-9% for scale uncertainties, evaluated as described in [27] and using the PDF4LHC recommendations [55]. No theoretical uncertainties are considered in the 2HDM interpretation.

Results and interpretation
The ditau (m τ τ ) mass is reconstructed using a dedicated algorithm called SVFit [72], which combines the visible four-vectors of For the H → hh → bbτ τ process, the chosen distribution for signal extraction is the four-body mass. The decay products of the two h bosons need to fulfill stringent kinematic constraints, due to the small natural width of the h. These constraints can be used in a kinematic fit in order to improve the event reconstruction and to better separate signal events from background. The collinear approximation for the decay products of the τ leptons is assumed in the fit, since the τ leptons are highly boosted as they originate from an object that is heavy when compared to their own mass. Furthermore, it is assumed that the reconstruction of the directions of all final state objects is accurate and the uncertainties can be neglected compared to the uncertainties on the energy recon-  from reducible and irreducible backgrounds, while the eμ final states are dominated by the irreducible ZZ production. The background labelled as "rare" collects together the smaller contributions from the triboson processes as discussed in the previous section.
In neither search do the invariant mass spectra show any evidence of a signal. Model independent upper limits at 95% confidence level (CL) on the cross section times branching fraction are set using a binned maximum likelihood fit for the signal plus background and background-only hypotheses. The limits are determined using the CL s method [73,74] and the procedure is described in Refs. [75,76].
Systematic uncertainties are taken into account as nuisance parameters in the fit procedure: normalisation uncertainties affect the signal and background yields. Uncertainties on the τ energy scale and jet energy scale are propagated as shape uncertainties.
The model independent expected and observed cross section times branching fraction limits for the H → hh → bbτ τ process are shown in Fig. 8 and for the A → Zh → LLτ τ process in Figs. 9 and 10 where L = e, μ or τ in order to reflect the small Z → τ τ contribution to the signal acceptance.
We interpret the observed limits on the cross section times branching fraction in the MSSM and 2HDM frameworks, discussed in Section 1.
In the MSSM we interpret them in the "low tan β" scenario [27,78] in which the value of M SUSY is increased until the mass of the lightest Higgs boson is consistent with 125 GeV over a range of low tan β and m A values. The exclusion region in the m A -tan β plane for the combination of the H → hh → bbτ τ and A → Zh → τ τ analyses, in such a scenario, is shown in Fig. 11. The limit falls off rapidly as m A approaches 350 GeV because decays of the A to two top quarks are becoming kinematically allowed.
The interpretation of the observed limits in a Type II 2HDM is performed in the "physics basis". The inputs to this interpretation are the physical Higgs boson masses (m h , m H , m A , m H ± ), the ratio of the vacuum expectation energies (tan β), the CP-even Higgs mixing angle (α) and m 2 The cross sections and branching fractions in the 2HDM were calculated as described by the LHC Higgs Cross Section Working Group [77,78]. The exclusion regions, calculated using the combination of the H → hh → bbτ τ and A → Zh → τ τ analyses, in the cos(β − α) vs. tan β plane for such a Type II 2HDM scenario with a heavy Higgs boson mass of 300 GeV are shown in Fig. 12. This can be compared to Fig. 5 in Ref. [41].

Summary
A search for a heavy scalar Higgs boson (H) decaying into a pair of SM-like Higgs bosons (hh) and a search for a heavy neutral pseudoscalar Higgs boson (A) decaying into a Z boson and a SMlike Higgs boson (h), have been performed using events recorded by the CMS experiment at the LHC. The data set corresponds to an integrated luminosity of 19.7 fb −1 , recorded at 8 TeV centre-ofmass energy in 2012. No evidence for a signal has been found and exclusion limits on the production cross section times branching fraction for the processes H → hh → bbτ τ and A → Zh → LLτ τ are presented. The results are also interpreted in the context of the MSSM and 2HDM models.

Acknowledgements
We congratulate our colleagues in the CERN accelerator departments for the excellent performance of the LHC and thank the technical and administrative staffs at CERN and at other CMS institutes for their contributions to the success of the CMS effort. In addition, we gratefully acknowledge the computing centres and personnel of the Worldwide LHC Computing Grid for delivering so effectively the computing infrastructure essential to our analyses.