Searches for additional Higgs bosons decaying to tau leptons at the LHC

The searches for additional Higgs bosons decaying to tau leptons in scenarios beyond the standard model will be summarised, from the pp collision data collected by the ATLAS and CMS experiments at LHC Run-2. Copyright C. Caputo et al. This work is licensed under the Creative Commons Attribution 4.0 International License. Published by the SciPost Foundation. Received 21-12-2018 Accepted 17-01-2019 Published 20-02-2019 Check for updates doi:10.21468/SciPostPhysProc.1.022


Introduction
The discovery of a new particle in July 2012 by the ATLAS [1] and CMS [2] collaborations at the Large Hadron Collider (LHC) [3] [4,5], compatible with the standard model (SM) Higgs boson, is a fundamental step forward in our understanding of the electroweak spontaneous symmetry breaking.However, many open questions, including the problem of the large hierarchy between the Planck and electroweak scale, still need to be addressed.
In order to cope with this, many different extension of the SM have been proposed, like supersymmetry (SUSY) [6,7].
Extending the SM entails, in most of the cases, the extension of the Higgs sector.One of the simple extensions is described by the 2 Higgs Doublet Model (2HDM), where two scalar Higgs doublets are introduced.The spontaneous symmetry breaking give rise to five scalar bosons: a neutral CP-odd A, two neutral CP-even h and H, two charged bosons H ± .In the decoupling limit, the lightest scalar of 2HDM can have properties compatible with the discovered Higgs boson; in this scenario all other scalars have larger masses.
Considering how the two doublets can interact with other particles of the SM, different phenomenology scenarios can appear.One of this scenarios is the Type-II 2HDM, which supposes that the first doublet couples only with up-quarks, while the second doublet only with down-quarks and charged fermions.
The Minimal Supersymmetric Standard Model (MSSM) [8,9], which incorporate the supersymmetry, is a Type-II 2HDM.At tree level, all the phenomenology can be described by two parameters, conventionally chosen to be the mass of the pseudoscalar Higgs m A and the ratio between the two vacuum expectation values (VEVs) tanβ = ν 1 /ν 2 .
For A and H the dominant production process is still the gluon fusion, for small and medium values of tanβ, followed by the b b-associated production, that increase at high tanβ due to the second doublet couplings to down-type fermions.The H ± production mechanism is strictly connected to the mass of the charged boson.For masses below the top-quark mass (m H + < m t ) the decay mode in a τ lepton plus is neutrino dominate in a Type-II 2HDM scenario; for mass above the top-quark mass (m H + > m t ), decay mode τ ν increase with tan β.In this report, results of direct searches of MSSM Higgs bosons with tau leptons in the final state, from the ATLAS and CMS collaborations using the 2016 dataset, are presented.
A complex SU (2) L singlet field S can be added to 2HDM, with a small mixing with the doublets; such a model is called 2HDM+S.This leads to two additional singlet states, a CP-odd scalar a and a CP-even s, which inherit a mixture of the Higgs doublets fermion interactions.In such a model, also known as NMSSM, the branching fraction of the Higgs boson to a pair of a or s bosons can be sizeable, and a wide variety of exotic Higgs decays are allowed [10], especially h → aa.In this report, results of direct searches of h → aa with tau leptons in the final state, from the CMS collaborations using the 2016 dataset, are presented.

Search for a neutral MSSM Higgs boson decaying into τ τ
The coupling of the H and the A to down-type fermions, at leading-order (LO), is enhanced by tan β with respect to the expectation for an SM Higgs boson of the same mass, while the coupling to vector bosons and up-type fermions is suppressed.The enhanced coupling to down-type fermions makes searches for additional heavy neutral Higgs bosons that exploit final states containing τ τ particularly interesting.It also has consequences for the production: firstly, the production in association with b quarks dominates over the production via gluon fusion for large values of tanβ.Secondly, in gluon fusion production the kinematic properties of the Higgs boson change as a function of tanβ due to the increasing contribution of b quarks in the fermion loop.Diagrams for h, H, and A production at LO are shown in Figure 1.
The ATLAS and CMS collaborations performed the direct search in the most sensitive final states of the taus [11] [12].Both focus their attention on eτ h , µτ h and τ h τ h , where τ h The dominant background contribution comes from misidentification of jets as τ h , which is estimated using a data-driven technique called Fake-Factor Method.This method is extensively explained in [11,12].Other important background contributions come from Z/γ * → τ τ production in the b-veto category, t t production in the b-tag category, and to a lesser extent W (→ lν)+jets, single top-quark, diboson and Z(→ ll)+jets production.These contributions are estimated using simulation, in some cases re-normalized using control regions in data.Corrections are applied to the simulation to account for mismodelling of the trigger, reconstruction, identification and isolation efficiency, the electron to τ h misidentification rate and the momentum scales and resolutions.
The total transverse mass of the system is used as final discriminant to search for an excess due to signal, where τ 1 and τ 2 respectively, refer to the p T leading and sub-leading taus, while E miss T is the missing energy measured in the event considered.The m tot T binned distribution is fitted simultaneously in all the categories used in the analysis.No evidence for a signal is found.Both collaborations set upper limits at 95% confidence level (CL) on the crosssection times branching fraction for two dominant production modes, gluon fusion and b b-associated production.The limits are computed in the narrow width approximation.
Figure 2 shows the upper limits obtained by the ATLAS and CMS collaborations as a function of Higgs boson mass.
Results are re-interpreted in two different benchmark scenario models; the m mod+ h and the hMSSM scenarios [13,14].Figure 3 shows limits set on m A − tan β plane.and the (right) hMSSM scenarios.In the top left plot, the red shade area indicates the region that does not give a light h higgs boson consistent with a mass of 125 GeV within the theoretical uncertainties ±3 GeV.In the bottom left plot, the red dashed lines represent the different parameters value that give a particular m h value.In the bottom right plot, the purple area indicates the region already excluded by constrains on h(125) couplings.[11] [12] 3 Search for charged Higgs bosons with the H ± → τ ± ν The H ± production mechanism is strictly connected to the mass of the charged boson.
If the H ± mass is below the top-quark mass (m H + < m t ), the production mode goes through the decay of a top-quark, t → bH + , in a t t production.In this mass range, the decay mode in a τ lepton plus a neutrino dominate in a Type-II 2HDM scenario.If the H ± mass above the top-quark mass (m H + > m t ), the dominant production mode is gg → tbH + .In this mass range, the dominant decay is H + → tb, considering the alignment limit (cos β − α 0) [15]; however the branching fraction for H + → τ ν can reach up to 10 − 15% at high tan β.The mass region where the H ± and the top-quark masses are similar (m H + m t ) involves interference effects among the t t and H ± nonresonant top-quark productions.Recently theoretical prediction become available for this region [16], which now allows to compare directly the H ± model with data in proximity of the top-quark mass.In Figure 4  Backgrounds classification and estimation depends on the type of object that gives rise to the identified τ h .If τ h arise from a true hadronically decaying tau or electron/muon misidentification, simulation is used to estimate such backgrounds like Z+jets, W +jets or dibosons; however, in the case of t t events, the normalization is obtained from a fit to the data.If τ h arise from a misidentified gluon-jet or quark-jet, the Fake Factor Method is used to estimate such background [17].Figure 5 shows the BDTs output for the τ h +jets final state after estimating the different background contributions.
BDTs binned distribution are fitted simultaneously in all the three signal regions.The data are found to be consistent with the background-only hypothesis.Exclusion limits are set at 95% CL on σ(pp → tbH + ) × B(H + → τ ν) for the full mass range, as well as on B(t → bH + ) × B(H + → τ ν) for low mass range.Figure 6 shows the expected and observed exclusion limits as a function of the H ± mass hypothesis.

result
Observed Expected Figure 7: 95% CL exclusion limits on tan β as a function of the charged Higgs boson mass in the context of the hMSSM scenario, for the regions in which theoretical predictions are available (0.5 ≤ tan β ≤ 60).[17] 4 Search for new light bosons in decays of the h(125) The combination of data collected at center-of-mass energies of 7 and 8 TeV by ATLAS and CMS constrains branching fractions of the Higgs boson to particles beyond the SM to less than 34% at 95% CL [19].Decay chains h(125) → aa are allowed in 2HDM+S scenarios.
Among all the possible 2HDM+S sceaniros, only four type forbid flavour-changing neutral current at tree level.In Type-I, all SM particles couple to the first doublet.
In Type-II, up-type quarks couple to the first doublet, whereas leptons and down-type quarks couple to the second doublet.NMSSM is a particular case of 2HDM+S of Type-II.
In Type-III, quarks couple to the first doublet, and leptons to the second one.To increase the sensitivity of the analysis, events in each final state are separated into four categories with different signal-to-background ratios.The categories are defined on the basis of m vis τ τ b , the invariant mass of the visible decay products of the τ leptons and the b-tagged jet with the highest p T .This variable exploits the difference in the kinematics of the final objects in signal events and background events.Usually, m vis τ τ b has low values for the former and high for latter.
The dominant backgrounds, having these objects in the final state, are t t and Z → τ τ production.Another large background consists of events with jets misidentified as τ h , such as W +jets events, the background from SM events composed uniquely of jets produced through the strong interaction, referred to as QCD multijet events, or semileptonic tt events.The misidentified background is estimated through the Fake Rate Method described in [20].

h → aa → µµτ τ
The analysis focus on four different final states that cover the different possible τ lepton decay modes: µµ + eµ, µµ + eτ h , µµ + µτ h , and µµ + τ h τ h .The µµ + ee and µµ + µµ final states are not considered because of their smaller branching fractions and the large background contribution from ZZ production.
The background composed of events where at least one jet is misidentified as one of the final state leptons is estimated from data.Such events include mostly Z+jets and WZ+jets events, but there are also minor contributions from ZZ → 2l2q events, t t production, or from the background from SM QCD multijet events.
The analysis scans the reconstructed dimuon mass spectrum for a characteristic resonance structure.The event selection and signal extraction used in this analysis are optimized for the h → aa → µµτ τ decay channel, where h has a mass of 125 GeV.Events from the h → aa → τ τ τ τ process can also enter the signal region when at least two of the τ leptons decay leptonically to muons and neutrinos.These events are treated as a part of the signal even if they do not exhibit a narrow dimuon mass peak.

Results
For the h → aa → bbτ τ decay channel, a global binned maximum-likelihood fit based on the m vis τ τ distributions, in the different channels and categories, is performed for the search for an excess of signal events over the expected background.Unbinned maximumlikelihood fit to the dimuon invariant mass distribution is used in the h → aa → µµτ τ decay channel.
Figure 8 shows 95 % CL upper limits obtained from combining the different final states considered in each analysis.35.9 fb Figure 8: Upper limits at 95% CL on (left) (σ(h)/σ SM ) × B(h → aa → bbτ τ ) and on (right) (σ(h)/σ SM ) × B(h → aa → µµτ τ ) where the h → aa → 4τ process is considered as a part of the signal, and is scaled with respect to the h → aa → µµτ τ signal.[20,21] This translates to limits on (σ(h)/σ SM ) × B(h → aa) in the different 2HDM+S scenarios.As explained at the beginning of Section 4.2, the different scenarios are related to how the leptons, up-quark, and down-quark interact with the two doublets introduced.
The two analyses have different sensitivity in the m a − tan β plane due to the involvement of down-quarks and leptons in the bbτ τ and only leptons in µµτ τ .In the Type-I scenario, and Type-II scenario, with tan β > 1, assuming the SM production cross section and mechanisms for the Higgs boson, limits on (σ(h)/σ SM ) × B(h → aa) are reduced to 20% for bbτ τ and down to 33% for µµτ τ .For Type-III and Type-IV scenarios, the limits are depicted in Figure 9.The contours corresponding to a 95% CL exclusion of (σ(h)/σ SM ) × B(h → aa) = 1.00 and 0.34 are drawn with dashed lines.The number 34% corresponds to the limit on the branching fraction of the Higgs boson to beyond-the-SM particles at the 95% CL obtained with data collected at center-of-mass energies of 7 and 8 TeV by the ATLAS and CMS experiments [19].[20,21]

Figure 1 :
Figure 1: Feynman diagrams of the production modes of an neutral MSSM Higgs boson.(top) Gluon gluon fusion; (bottom-left) b b-associated production four-flavour scheme; (bottom-right) b b-associated production five-flavour scheme

Figure 2 :
Figure 2: (top) CMS expected and observed limits on σ(φ) × BR(φ → τ τ ) for (left) the gluon fusion and (right) the b b-associated production, resulting from the combination of all the four channels considered.(bottom) ATLAS expected and observed limits on σ(φ) × BR(φ → τ τ ) for (left) the gluon fusion and (right) the b b-associated production, resulting from the combination of all the three channels considered.[11] [12]

Figure 3 :
Figure 3: Model dependent exclusion limits in the m A − tan β plane for the (left) m mod+ h

Figure 4 :
Figure 4: Examples of leading-order Feynman diagrams contributing to the production of charged Higgs bosons in pp collisions: (left) non-resonant top-quark production, (center) single-resonant top-quark production that dominates at large H + masses, (right) double-resonant top-quark production that dominates at low H + masses.The interference between these three main diagrams becomes most relevant in the intermediate-mass region.The ATLAS and CMS collaboration searched for a charged Higgs boson in pp collision using a dataset corresponding to an integrated luminosity of ∼ 36f b −1 , at a center-of-mass energy of 13 TeV[17] [18].The results presented will refer to the ATLAS search, the only one with the full 2016 dataset public at the time of the conference.Two different channels are considered: τ h +jets and τ h +lepton, where both aim to different decays of the top-quark produced with the H ± .Furthermore, a multivariate discriminant is used to increase the search sensitivity, exploiting the kinematic variables that differentiate between signal and backgrounds.The output score of a Boosted Decision Trees (BDTs) is used as final discriminant.In order to take advantage of the different H ± decay products' kinematic regime, simulated signal sample are divide in five H ± mass bins: 90-120 GeV, 130-160 GeV, 160-180 GeV, 200-400 GeV and 500-2000 GeV.The BDTs are trained using a set of variables related to the particular final state.

Figure 7
shows 95% CL exclusion limits on tan β as a function of the charged Higgs boson mass in the context of the hMSSM scenario.

Figure 5 :Figure 6 :
Figure5: BDTs score distributions in the signal region of the τ h +jets channel, in the five mass ranges used for the BDTs trainings, after a fit to the data with the background-only hypothesis.The lower panel of each plot shows the ratio of data to the SM background prediction.The uncertainty bands include all statistical and systematic uncertainties.The normalisation of the signal (shown for illustration) corresponds to the integral of the background.[17] Finally, in Type-IV, leptons and up-type quarks couple to the first doublet, while down-type quarks couple to the second doublet.The analysis here presented are based on pp collisions collected in 2016 by the CMS experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9f b −1 .Decay chains considered are aa → b bτ τ [20] and aa → µµτ τ [21].Masses of the pseudoscalar boson between 15.0 and 62.5 GeV are probed.4.1 h → aa → b bτ τ Three different τ τ final states are considered: eµ, eτ h , and µτ h .They are additionally required to contain at least one b-tagged jet.
Several searches for BSM Higgs bosons, with tau leptons in the final state, have been carried out in the ATLAS and CMS experiments using 2015+2016 data at √ s = 13 TeV.No evidence of additional Higgs bosons has been observed.Upper limits are provided on the cross-section times branching fraction for different searches.The results are, furthermore, interpreted in the context of an extended Higgs sector, such as MSSM and NMSSM.The full Run-2 data, in which the integrated luminosity has reached ∼ 140 f b 1 will give an incredible boost to the sensitivity for searches of new physics in the Higgs sector.

Figure 9 :
Figure9: Observed 95% CL limits on (σ(h)/σ SM ) × B(h → aa) in 2HDM+S of type III (left), and type IV (right).The contours corresponding to a 95% CL exclusion of (σ(h)/σ SM ) × B(h → aa) = 1.00 and 0.34 are drawn with dashed lines.The number 34% corresponds to the limit on the branching fraction of the Higgs boson to beyond-the-SM particles at the 95% CL obtained with data collected at center-of-mass energies of 7 and 8 TeV by the ATLAS and CMS experiments[19].[20,21]