A search for heavy neutral Higgs bosons and
Article funded by SCOAP3
\SI{15}{\GeV}$} and to be in the fiducial volume of the inner detector, $|\eta|<2.47$. The transition region between the barrel and endcap calorimeters ($1.37<|\eta|<1.52$) is excluded. Muon candidates are reconstructed in the range $|\eta|<2.5$ by matching tracks found in the muon spectrometer to tracks found in the inner detector~\cite{PERF-2015-10}. The tracks of the muon candidates are re-fitted using the complete track information from both detector systems. They are required to have a transverse momentum $\pt>\SI{7}{\GeV}$ and to pass a ``loose'' muon identification requirement. The selected lepton (electron or muon) in the \lephad channel must then have $\pt>\SI{30}{\GeV}$ and pass a ``medium'' identification requirement. This lepton is considered isolated if it meets $\pt$- and $\eta$-dependent isolation criteria utilising calorimetric and tracking information. The criteria correspond to an efficiency of 90\% (99\%) for a transverse momentum of $\pt=25~(60)\,\GeV$. The efficiency increases with lepton \pt as the requirements are relaxed to account for the decreased background from misidentified jets. Jets are reconstructed from topological clusters of energy depositions~\cite{Aad-ml-2016upy} in the calorimeter using the anti-$k_t$ algorithm~\cite{AntiKT, IsolationPileUp2}, with a radius parameter value $R =0.4$. The average energy contribution from pile-up is subtracted according to the jet area and the jets are calibrated as described in ref.~\cite{PERF-2016-04}. They are required to have $\pt > \SI{20}{\GeV}$ and $|\eta| < 2.5$. To reduce the effect of pile-up, a jet vertex tagger algorithm is used for jets with $\pt < \SI{60}{\GeV}$ and $|\eta| < 2.4$. It employs a multivariate technique based on jet energy, vertexing and tracking variables to determine the likelihood that the jet originates from or is heavily contaminated by pile-up~\cite{PERF-2014-03}. In order to identify jets containing $b$-hadrons ($b$-jets), a multivariate algorithm is used, which is based on the presence of tracks with a large impact parameter with respect to the primary vertex, the presence of displaced secondary vertices and the reconstructed flight paths of $b$- and $c$-hadrons associated with the jet~\cite{Aad-ml-2015ydr, ATL-PHYS-PUB-2016-012}. The algorithm has an average efficiency of 70\% for $b$-jets and rejections of approximately 13, 56 and 380 for $c$-jets, hadronic tau decays and jets initiated by light quarks or gluons, respectively, as determined in simulated \ttbar events. Hadronic tau decays are composed of a neutrino and a set of visible decay products (\tauhadvis), typically one or three charged pions and up to two neutral pions. The reconstruction of the visible decay products is seeded by jets~\cite{PERF-2013-06}. The \tauhadvis candidates must have $\pt > 25~(45)\,\GeV$ in the \lephad (\hadhad) channel, $|\eta|<2.5$ excluding $1.37<|\eta|<1.52$, one or three associated tracks and an electric charge of $\pm 1$. The leading-\pt \tauhadvis candidate in the \lephad channel and the two leading-\pt \tauhadvis candidates in the \hadhad channel are then selected and all remaining candidates are considered as jets. A Boosted Decision Tree (BDT) identification procedure, based on calorimetric shower shapes and tracking information is used to reject backgrounds from jets~\cite{ATL-PHYS-PUB-2015-045-fixed,ATLAS-CONF-2017-029}. Two $\tauhadvis$ identification criteria are used: ``loose'' and ``medium'', specified in section~\ref{sec:sel}. The criteria correspond to efficiencies of about 60\% (50\%) and 55\% (40\%) in \DYtt events and rejections of about 30 (30) and 50 (100) in \multijet events, for one-track (three-track) $\tauhadvis$ candidates, respectively. An additional dedicated likelihood-based veto is used to reduce the number of electrons misidentified as $\tauhadvis$ in the \lephad channel, providing 95\% efficiency and a background rejection between 20 and 200, depending on the pseudorapidity of the $\tauhadvis$ candidate. Geometrically overlapping objects are removed in the following order: \begin{enumerate*}[(a)] \item jets within $\Delta R = 0.2$ of selected \tauhadvis candidates are excluded, \item jets within $\Delta R = 0.4$ of an electron or muon are excluded, \item any \tauhadvis candidate within $\Delta R = 0.2$ of an electron or muon is excluded, \item electrons within $\Delta R$ = 0.2 of a muon are excluded. \end{enumerate*} The missing transverse momentum, \metvec, is calculated as the negative vectorial sum of the $\mathbf{p}_\mathrm{T}$ of all fully reconstructed and calibrated physics objects~\cite{ATL-PHYS-PUB-2015-027,ATL-PHYS-PUB-2015-023}. This procedure includes a ``soft term'', which is calculated using the inner-detector tracks that originate from the hard-scattering vertex but are not associated with reconstructed objects. ]]>
\SI{65}{\GeV}$ and no electrons or muons. The leading-\pt \tauhadvis candidate must be geometrically matched to the trigger signature and must exceed the trigger \pt threshold by \SI{5}{\GeV}. The leading and sub-leading \tauhadvis candidates must satisfy the ``medium'' and ``loose'' identification criteria, respectively. They must also have opposite electric charge and be back to back in the transverse plane: \mbox{$|\Delta\phi(\ptvec^{\tau_1},\ptvec^{\tau_2})| >\SI{2.7}{rad}$,} as tau leptons from the decay of heavy neutral resonances are typically produced back to back in the transverse plane. The signal acceptance times efficiency for this selection (calculated with respect to all possible \ditau final states) varies between 1\% and 7\% for signals with masses of \SI{0.35}{\TeV} or higher. The maximum occurs for signals with masses of around \SI{0.9}{\TeV}, degradations occur at lower masses due to the \tauhadvis \pt thresholds and at higher masses due to the \tauhadvis reconstruction and identification efficiencies. A summary of the selection is given in table~\ref{tab:regions:hadhad} of section~\ref{sec:bkg}. ]]>
\SI{2.4}{rad}$.} To reduce background from \Wjets production, the transverse mass of the isolated lepton and the missing transverse momentum, \begin{equation*} \mt(\ptvec^{\ell},\metvec) \equiv \sqrt{2 \pt^{\ell}\met \left [ 1 - \cos\Delta\phi(\ptvec^{\ell}, \metvec) \right ] }\,, \end{equation*} must be less than $\SI{40}{\GeV}$. To reduce background from \Zee production in the \ehad channel, events where the isolated lepton and identified \tauhadvis candidate have an invariant mass between 80 and \SI{110}{\GeV} are rejected. The signal acceptance times efficiency for this selection also varies between 1\% and 7\%, but the maximum occurs at lower masses due to the lower \pt thresholds on the tau decay products. A summary of the selection is given in table~\ref{tab:regions:lephad} of section~\ref{sec:bkg}. \end{sloppypar} ]]>
110\, (100)\,\GeV$} in the \ehad (\muhad) channel. The tighter selection in the \ehad channel is used to help suppress the larger \multijet contamination. The region has $\sim$90\% \ttbar purity. ]]>
2.7$ \\ \hline CR-1 & Pass SR except: $\tau_2$ (fail loose) \\ DJ-FR & jet trigger, $\tau_1$+$\tau_2$ (no identification), $q(\tau_1)\times q(\tau_2)<0$, $|\Delta\phi(\ptvec^{\tau_1},\ptvec^{\tau_2})|>2.7$, $\pt^{\tau_2}/\pt^{\tau_1} > 0.3$ \\ W-FR & $\mu$ (trigger, isolated), $\tau_1$ (no identification), $|\Delta\phi(\ptvec^{\mu},\ptvec^{\tau_1})|>2.4$, $\mT(\ptvec^{\mu},\metvec)>\SI{40}{\GeV}$ \\ & \bveto category only \\ T-FR & Pass W-FR except: \btag category only \\ \hline \end{tabular}} ]]>
\SI{85}{\GeV}$ and also exceeds the trigger threshold by $10\%$, and the sub-leading candidate has $\pt>\SI{65}{\GeV}$. The \tauhadvis candidates must have opposite charge sign, be back to back in the transverse plane, $|\Delta\phi(\ptvec^{\tau_1},\ptvec^{\tau_2})| > 2.7$ rad and the \pt of the sub-leading \tauhadvis must be at least 30\% of the leading \tauhadvis \pt. The fake-factors are measured using the sub-leading \tauhadvis candidate to avoid trigger bias and to be consistent with their application in \failcr. They are parameterised as functions of the sub-leading \tauhadvis \pt and the sub-leading \tauhadvis track multiplicity. The purity of \multijet events that pass the \tauhadvis identification is 98--99\% (93--98\%) for the \bveto (\btag) categories. The non-\multijet contamination is subtracted using simulation. The fake-factors are shown in figure~\ref{fig:hadhad_fakefactors}. \begin{figure} \centering \includegraphics[width=0.49\columnwidth]{\figpath/c520_hadhad_ff_split.pdf} ]]>
\SI{55}{\GeV}$ that fired the trigger, no electrons and at least one \tauhadvis candidate with $\pt>\SI{50}{\GeV}$. The events must also satisfy $|\Delta\phi(\ptvec^{\mu},\ptvec^{\tauhadvis})|>\SI{2.4}{rad}$ and $\mT(\ptvec^{\mu},\metvec)>\SI{40}{\GeV}$. The events are then categorised into \btag and \bveto categories, defining T-FR and W-FR, respectively. Backgrounds from non-\ttbar (non-\Wjets) processes are subtracted from T-FR (W-FR) using simulation. The fake-rates are measured using the leading-\pt \tauhadvis candidate and are parameterised as functions of the \tauhadvis \pt and track multiplicity. ]]>
2.4$, \\ & $\mT(\ptvec^{\ell},\metvec)<\SI{40}{\GeV}$, veto $80 < m(\pvec^{\ell},\pvec^{\tau_1}) < \SI{110}{\GeV}$ (\ehad channel only) \\ \hline CR-1 & Pass SR except: $\tau_1$ (very-loose, fail medium) \\ CR-2 & Pass SR except: $\tau_1$ (very-loose, fail medium), $\ell$ (fail isolation) \\ MJ-FR & Pass SR except: $\tau_1$ (very-loose), $\ell$ (fail isolation) \\ W-FR & Pass SR except: 70\,(60)$ < \mT(\ptvec^{\ell},\metvec) < \SI{150}{\GeV}$ in \ehad (\muhad) channel \\ CR-T & Pass SR except: $\mT(\ptvec^{\ell},\metvec)> 110\,(100)\,\GeV$ in the \ehad (\muhad) channel,\\ &\btag category only \\ L-FR & $\ell$ (trigger, selected), jet (selected), no loose $\tauhadvis$, $\mT(\ptvec^{\ell},\metvec) < \SI{30}{\GeV}$ \\ \hline \end{tabular}} ]]>
\SI{150}{\GeV}$ is assigned. The \tauhadvis trigger efficiency uncertainty is 3--14\%. The uncertainty in the \tauhadvis energy scale is 2--3\%. The probability for electrons to be misidentified as \tauhadvis is measured with a precision of 3--14\%~\cite{ATLAS-CONF-2017-029}. The electron, muon, jet and \metvec systematic uncertainties described above are found to have a very small impact. Theoretical cross-section uncertainties are taken into account for all backgrounds estimated using simulation. For \DYjets production, uncertainties are taken from ref.~\cite{EXOT-2015-07} and include variations of the PDF sets, scale, \alphas, beam energy, electroweak corrections and photon-induced corrections. A single 90\% CL eigenvector variation uncertainty is used, based on the CT14nnlo PDF set. The variations amount to a $\sim$$5\%$ uncertainty in the total number of \DYjets events within the acceptance. For \diboson production, an uncertainty of $10\%$ is used~\cite{Campbell-ml-2011bn,Gleisberg-ml-2008ta}. For \ttbar~\cite{Czakon-ml-2011xx} and \singletop~\cite{Aliev-ml-2010zk, Kant-ml-2014oha} production, the assigned $6\%$ uncertainty is based on PDF, scale and top-quark mass variations. Additional uncertainties related to initial- and final-state radiation modelling, tune and (for \ttbar only) the choice of \texttt{hdamp} parameter value in \POWHEGBOXV{ v2}, which controls the amount of radiation produced by the parton shower, are also taken into account~\cite{ATL-PHYS-PUB-2016-004}. The uncertainty due to the hadronisation model is evaluated by comparing \ttbar events generated with \POWHEGBOXV{ v2} interfaced to either \HERWIGpp~\cite{Herwigpp} or \PYTHIAV{6}. To estimate the uncertainty in generating the hard scatter, the \POWHEG{} and MG5\_aMC@NLO event generators are compared, both interfaced to the \HERWIGpp parton shower model. The uncertainties in the \Wjets cross section have a negligible impact in the \hadhad channel and the \Wjets simulation is not used in the \lephad channel. For MSSM Higgs boson samples, various sources of uncertainty which impact the signal acceptance are considered. The impact from varying the factorisation and renormalisation scales up and down by a factor of two, either coherently or oppositely, is taken into account. Uncertainties due to the modelling of initial- and final-state radiation, as well as multiple parton interaction are also taken into account. These uncertainties are estimated from variations of the \PYTHIAV{8} A14 tune~\cite{ATL-PHYS-PUB-2014-021} for the $b$-associated production and the AZNLO \PYTHIAV{8} tune~\cite{AZNLO-ml-2014} for the gluon-gluon fusion production. The envelope of the variations resulting from the use of the alternative PDFs in the PDF4LHC15$\_$nlo$\_$nf4$\_$30\,(PDF4LHC15$\_$nlo$\_$100)~\cite{Ball-ml-2014uwa} set is used to estimate the PDF uncertainty for the $b$-associated (gluon-gluon fusion) production. The total uncertainty for the MSSM Higgs boson samples is typically 1--4\%, which is dominated by variations of the radiation and multiple parton interactions, with minor impact from scale variations. The \Zprime signal acceptance uncertainties are expected to be negligible. For both the MSSM Higgs boson and \Zprime samples, uncertainties in the integrated cross section are not included in the fitting procedure used to extract experimental cross-section limits. The uncertainty for \Zprime is included when overlaying model cross sections, in which case it is calculated using the same procedure as for the \DYjets background. ]]>
\limmhmodpobslow$ (\limmhmodpexplow) for $\MA = \SI{0.25}{\TeV}$ and $\tanb> \limmhmodpobshigh$ (\limmhmodpexphigh) for $\MA = \SI{1.5}{\TeV}$. Constraints in the \hmssm scenario are stronger due to the presence of low-mass neutralinos in the \mhmod scenario that reduce the $H/A\to\tau\tau$ branching fraction and which are absent in the \hmssm scenario. In the \hmssm scenario, the most stringent observed (expected) constraints on $\tanb$ for the combined search exclude $\tanb > \limmssmobslow$ (\limmssmexplow) for $\MA = \SI{0.25}{\TeV}$ and $\tanb> \limmssmobshigh$ (\limmssmexphigh) for $\MA = \SI{1.5}{\TeV}$ at 95\% CL. The expected exclusion limit and bands around $\MA = \SI{350}{\GeV}$ reflect the behaviour of the $A\to\tau\tau$ branching fraction close to the $A\to\ttbar$ kinematic threshold for low $\tanb$, allowing for some exclusion in this region. However, when \MA is above the $A\to\ttbar$ production threshold, this additional decay mode reduces the sensitivity of the $A\to\tau\tau$ search for low $\tanb$. \begin{figure} \centering \subfloat[\mhmod scenario.]{\includegraphics[width=0.450\textwidth]{\figpath/c202_limit_mhmodp.pdf}}\hfill \subfloat[\hmssm scenario.]{\includegraphics[width=0.450\textwidth]{\figpath/c200_limit_hMSSM.pdf}} ]]>
\limmssmobslow$ for $\MA = \SI{0.25}{\TeV}$ and $\tanb> \limmssmobshigh$ for $\MA = \SI{1.5}{\TeV}$ at 95\% CL. In the context of the Sequential Standard Model, \ZprimeSSM bosons with masses less than \SI{\limssm}{\TeV} are excluded at 95\% CL, while \mbox{$m_{\ZprimeSFM}< \limsfmlow$--\SI{\limsfmhigh}{\TeV}} is excluded in the range $0.03 < \sinsquarephi < 0.5$ in the non-universal $G(221)$ model. ]]>