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CMS-HIG-19-012 ; CERN-EP-2020-120

Search for decays of the 125 GeV Higgs boson into a Z boson and a $\rho$ or $\phi$ meson

CMS Collaboration

9 July 2020

JHEP 11 (2020) 039

Abstract: Decays of the 125 GeV Higgs boson into a Z boson and a $\rho^{0}(770)$ or $\phi(1020)$ meson are searched for using proton-proton collision data collected by the CMS experiment at the LHC at $\sqrt{s} = $ 13 TeV. The analysed data set corresponds to an integrated luminosity of 137 fb$^{-1}$. Events are selected in which the Z boson decays into a pair of electrons or a pair of muons, and the $\rho$ and $\phi$ mesons decay into pairs of pions and kaons, respectively. No significant excess above the background model is observed. As different polarization states are possible for the decay products of the Z boson and $\rho$ or $\phi$ mesons, affecting the signal acceptance, scenarios in which the decays are longitudinally or transversely polarized are considered. Upper limits at the 95% confidence level on the Higgs boson branching fractions into Z$\rho$ and Z$\phi$ are determined to be 1.04-1.31% and 0.31-0.40%, respectively, where the ranges reflect the considered polarization scenarios; these values are 740-940 and 730-950 times larger than the respective standard model expectations. These results constitute the first experimental limits on the two decay channels.

Links: e-print arXiv:2007.05122 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures
png pdf	Figure 1: Feynman diagrams that contribute to the decay of a Higgs boson into a heavy vector boson and a vector meson. The grey oval shape represents the meson. The two indirect processes (left and middle), where the meson originates from an off-shell Z boson or $\gamma ^{*}$, contribute the most to the total branching fraction in the SM.
png pdf	Figure 1-a: Feynman diagram that contributes to the decay of a Higgs boson into a heavy vector boson and a vector meson. The grey oval shape represents the meson. This indirect process, where the meson originates from an off-shell Z boson, is one of two that contribute the most to the total branching fraction in the SM.
png pdf	Figure 1-b: Feynman diagram that contributes to the decay of a Higgs boson into a heavy vector boson and a vector meson. The grey oval shape represents the meson. This indirect process, where the meson originates from a $\gamma ^{*}$, is one of two that contribute the most to the total branching fraction in the SM.
png pdf	Figure 1-c: Feynman diagram that contributes to the decay of a Higgs boson into a heavy vector boson and a vector meson. The grey oval shape represents the meson.
png pdf	Figure 2: The angular distance $\Delta R$ between the two tracks from the meson decay in $\mathrm{H} \to \mathrm{Z} \rho $ events (dashed red) and in $\mathrm{H} \to \mathrm{Z} \phi $ events (dotted blue). The separation is calculated between reconstructed tracks that are matched to the generator-level pions (kaons) to ensure that the tracks originate from the $\rho$ ($\phi$) decay. Both contributions are normalized to the same area.
png pdf	Figure 3: The transverse momentum distribution for the track that has the larger ${p_{\mathrm {T}}}$ out of the two tracks selected as the $\rho$ or $\phi$ candidate. The distribution is shown for events that pass the meson candidate selection described in the text, but not the requirement that one of the tracks must have $ {p_{\mathrm {T}}} > $ 10 GeV. This distribution is shown for the $\mathrm{H} \to \mathrm{Z} \rho $ decay (dashed red), the $\mathrm{H} \to \mathrm{Z} \phi $ decay (dotted blue) and the background from Drell-Yan events (solid black). All contributions are normalized to the same area.
png pdf	Figure 4: The ditrack isolation sum in the $\ell \ell \pi \pi $ (left) and $\ell \ell \mathrm{K} \mathrm{K} $ (right) channels, combining the $\mu \mu $ and ee channels for all the data-taking years. The distribution in data, as well as in the simulated $\mathrm{H} \to \mathrm{Z} \rho $ and $\mathrm{H} \to \mathrm{Z} \phi $ signals is shown. A branching fraction of 10 (5)% for the $\mathrm{H} \to \mathrm{Z} \rho $ ($\mathrm{H} \to \mathrm{Z} \phi $) signal is assumed. The isolation sum is shown after applying all selection criteria apart from the ditrack isolation requirement. The ditrack invariant mass requirement is also applied. Only events in which the dilepton plus ditrack invariant mass is in the range 120-130 GeV are considered. The dashed line indicates the boundary of the region used in the analysis, for which the isolation sum is required to be smaller than 0.5 GeV.
png pdf	Figure 4-a: The ditrack isolation sum in the $\ell \ell \pi \pi $ channel, combining the $\mu \mu $ and ee channels for all the data-taking years. The distribution in data, as well as in the simulated $\mathrm{H} \to \mathrm{Z} \rho $ signal is shown. A branching fraction of 10% for the $\mathrm{H} \to \mathrm{Z} \rho $ signal is assumed. The isolation sum is shown after applying all selection criteria apart from the ditrack isolation requirement. The ditrack invariant mass requirement is also applied. Only events in which the dilepton plus ditrack invariant mass is in the range 120-130 GeV are considered. The dashed line indicates the boundary of the region used in the analysis, for which the isolation sum is required to be smaller than 0.5 GeV.
png pdf	Figure 4-b: The ditrack isolation sum in the $\ell \ell \mathrm{K} \mathrm{K} $ channel, combining the $\mu \mu $ and ee channels for all the data-taking years. The distribution in data, as well as in the simulated $\mathrm{H} \to \mathrm{Z} \phi $ signal is shown. A branching fraction of 5% for the $\mathrm{H} \to \mathrm{Z} \phi $ signal is assumed. The isolation sum is shown after applying all selection criteria apart from the ditrack isolation requirement. The ditrack invariant mass requirement is also applied. Only events in which the dilepton plus ditrack invariant mass is in the range 120-130 GeV are considered. The dashed line indicates the boundary of the region used in the analysis, for which the isolation sum is required to be smaller than 0.5 GeV.
png pdf	Figure 5: Distribution of the ditrack invariant mass in simulated $\mathrm{H} \to \mathrm{Z} \rho $ events passing the $\ell \ell \pi \pi $ selection criteria (left) and in simulated $\mathrm{H} \to \mathrm{Z} \phi $ events passing the $\ell \ell \mathrm{K} \mathrm{K} $ selection criteria (right). These masses are calculated assuming the charged particle mass equals the pion mass in the $\ell \ell \pi \pi $ selection and assuming the charged particle mass equals the kaon mass in the $\ell \ell \mathrm{K} \mathrm{K} $ selection. The events pass all selection criteria described in the text, apart from the requirements on the ditrack invariant mass window. The dashed lines indicate the region selected in the analysis.
png pdf	Figure 5-a: Distribution of the ditrack invariant mass in simulated $\mathrm{H} \to \mathrm{Z} \rho $ events passing the $\ell \ell \pi \pi $ selection criteria.These masses are calculated assuming the charged particle mass equals the pion mass. The events pass all selection criteria described in the text, apart from the requirements on the ditrack invariant mass window. The dashed lines indicate the region selected in the analysis.
png pdf	Figure 5-b: Distribution of the ditrack invariant mass in simulated $\mathrm{H} \to \mathrm{Z} \rho $ events passing the $\ell \ell \pi \pi $ selection criteria. These masses are calculated assuming the charged particle mass equals the kaon mass. The events pass all selection criteria described in the text, apart from the requirements on the ditrack invariant mass window. The dashed lines indicate the region selected in the analysis.
png pdf	Figure 6: Distributions of $m_{\ell \ell \pi \pi}$ (left) and $m_{\ell \ell \mathrm{K} \mathrm{K}}$ (right). For visualization the $\mu \mu $ and ee channels, as well as all three data-taking periods, are combined. Also shown are the $\mathrm{H} \to \mathrm{Z} \rho $ and $\mathrm{H} \to \mathrm{Z} \phi $ signals, in the isotropic-decay scenario and assuming branching fractions of 3.0 and 0.7%, respectively. The ratio between the data and the background model is shown in the lower panels.
png pdf	Figure 6-a: Distribution of $m_{\ell \ell \pi \pi}$. For visualization the $\mu \mu $ and ee channels, as well as all three data-taking periods, are combined. Also shown is the $\mathrm{H} \to \mathrm{Z} \rho $ signal, in the isotropic-decay scenario and assuming branching fractions of 3.0% The ratio between the data and the background model is shown in the lower panel.
png pdf	Figure 6-b: Distribution of $m_{\ell \ell \mathrm{K} \mathrm{K}}$. For visualization the $\mu \mu $ and ee channels, as well as all three data-taking periods, are combined. Also shown is the $\mathrm{H} \to \mathrm{Z} \phi $ signal, in the isotropic-decay scenario and assuming branching fractions of 0.7% The ratio between the data and the background model is shown in the lower panel.

Tables
png pdf	Table 1: The effect on the signal yields of reweighting to the extreme polarization scenarios, described in more detail in the text, relative to the scenario with isotropic decays. The change in the fraction of signal events that pass the selection criteria affects the final results of the analysis.
png pdf	Table 2: Effect of systematic uncertainties on the simulated signal. The ranges reflect differences between channels and data-taking periods.
png pdf	Table 3: Observed and expected 95% CL upper limits on $\mathcal {B}(\mathrm{H} \to \mathrm{Z} \rho)$, for different polarizations.
png pdf	Table 4: Observed and expected 95% CL upper limits on $\mathcal {B}(\mathrm{H} \to \mathrm{Z} \phi)$, for different polarizations.

Summary

A search has been presented for the rare decay of the Higgs boson (H) into a Z boson and a $\rho$ or a $\phi$ meson in the dilepton-$\pi^{+}\pi^{-}$ final states of the $\mathrm{H}zrho$ decay, and in the dilepton-$\mathrm{K^{+}}\mathrm{K^{-}}$ final states of the $\mathrm{H} \to \mathrm{Z}\phi$ decay. The search used a sample of proton-proton collisions, collected by the CMS experiment at a centre-of-mass energy of 13 TeV from 2016 to 2018 and corresponding to an integrated luminosity of 137 fb$^{-1}$. Upper limits on the branching fractions $\mathcal{B}(\mathrm{H} \to \mathrm{Z}\rho)$ and $\mathcal{B}(\mathrm{H} \to \mathrm{Z}\phi)$ have been set at the 95% confidence level for various polarization scenarios. The upper limits on $\mathcal{B}(\mathrm{H} \to \mathrm{Z}\rho)$ are in the range 1.04-1.31%, or 740-940 times the standard model expectation, while the upper limits on $\mathcal{B}(\mathrm{H} \to \mathrm{Z}\phi)$ range from 0.31 to 0.40%, or 730-950 times the standard model expectation. These results constitute the first experimental limits on the two decay channels.

Additional Figures
png pdf	Additional Figure 1: Distribution of $m_{\ell \ell {\pi} {\pi}}$, combining the dielectron and dimuon channels for all three data taking years. The $ {\mathrm {H}} \to {\mathrm {Z}} {\rho} $ signal, in the isotropic-decay scenario assuming a branching fraction of 3%, is also shown. This figure only shows the mass range containing most of the signal, which is a subset of the mass range used in the fit.
png pdf	Additional Figure 2: Distribution of $m_{\ell \ell {\mathrm {K}} {\mathrm {K}}}$, combining the dielectron and dimuon channels for all three data taking years. The $ {\mathrm {H}} \to {\mathrm {Z}} $ signal, in the isotropic-decay scenario assuming a branching fraction of 0.7%, is also shown. This figure only shows the mass range containing most of the signal, which is a subset of the mass range used in the fit.
png pdf	Additional Figure 3: Observed and expected 95% CL upper limits on the branching ratio $\mathcal {B}({\mathrm {H}} \to {\mathrm {Z}} {\rho})$, for the different polarization scenarios considered. The Z boson and the $ {\rho} $ meson are either both transversely polarized (transverse) or both longitudinally polarized (longitudinal). The limits for each polarization scenario are obtained using the same data; differences in the limits arise only from changes in the expected signal yield.
png pdf	Additional Figure 4: Observed and expected 95% CL upper limits on the branching ratio $\mathcal {B}({\mathrm {H}} \to {\mathrm {Z}})$, for the different polarization scenarios considered. The Z boson and the $\phi$ meson are either both transversely polarized (transverse) or both longitudinally polarized (longitudinal). The limits for each polarization scenario are obtained using the same data; differences in the limits arise only from changes in the expected signal yield.
png pdf	Additional Figure 5: View of a candidate event with two muons (red lines) compatible with having originated from a Z boson, and two charged kaon tracks compatible with having originated from a $\phi$ meson. In this view all charged particle tracks in the event are shown.
png pdf	Additional Figure 6: View of a candidate event with two muons (red lines) compatible with having originated from a Z boson, and two charged kaon tracks (yellow lines) compatible with having originated from a $\phi$ meson. In this view only the two charged kaon tracks are shown.

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