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| CMS-HIG-18-006 ; CERN-EP-2019-105 | ||
| Search for light pseudoscalar boson pairs produced from decays of the 125 GeV Higgs boson in final states with two muons and two nearby tracks in pp collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 16 July 2019 | ||
| Phys. Lett. B 800 (2019) 135087 | ||
| Abstract: A search is presented for pairs of light pseudoscalar bosons, in the mass range from 4 to 15 GeV, produced from decays of the 125 GeV Higgs boson. The decay modes considered are final states that arise when one of the pseudoscalars decays to a pair of tau leptons, and the other one either into a pair of tau leptons or muons. The search is based on proton-proton collisions collected by the CMS experiment in 2016 at a center-of-mass energy of 13 TeV that correspond to an integrated luminosity of 35.9 fb$^{-1}$. The 2$\mu$ 2$\tau$ and 4$\tau$ channels are used in combination to constrain the product of the Higgs boson production cross section and the branching fraction into 4$\tau$ final state, ${\sigma \mathcal{B}}$, exploiting the linear dependence of the fermionic coupling strength of pseudoscalar bosons on the fermion mass. No significant excess is observed beyond the expectation from the standard model. The observed and expected upper limits at 95% confidence level on ${\sigma \mathcal{B}}$, relative to the standard model Higgs boson production cross section, are set respectively between 0.022 and 0.23 and between 0.027 and 0.19 in the mass range probed by the analysis. | ||
| Links: e-print arXiv:1907.07235 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Illustration of the signal topology, in which the H decays into two $\mathrm{a}_{1}$ bosons, where one $\mathrm{a}_{1}$ boson decays into a pair of tau leptons, while the other one decays into a pair of muons or a pair of tau leptons. The analyzed final state consists of one muon and an oppositely charged track in each $\mathrm{a}_{1}$ decay. |
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Figure 2:
Binning of the 2D ($m_1,m_2$) distribution. |
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Figure 3:
The observed invariant mass distribution, normalized to unity, of the first muon and the softest (left) or hardest (right) accompanying "signal" track for different isolation requirements imposed on the second muon: when the second muon has only one accompanying "isolation" track ($N_\text {iso,2}=1$; circles); or when it has two or three accompanying "isolation" tracks ($N_\text {iso,2}=2,3$; squares). |
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Figure 3-a:
The observed invariant mass distribution, normalized to unity, of the first muon and the softest accompanying "signal" track for different isolation requirements imposed on the second muon: when the second muon has only one accompanying "isolation" track ($N_\text {iso,2}=1$; circles); or when it has two or three accompanying "isolation" tracks ($N_\text {iso,2}=2,3$; squares). |
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Figure 3-b:
The observed invariant mass distribution, normalized to unity, of the first muon and the hardest accompanying "signal" track for different isolation requirements imposed on the second muon: when the second muon has only one accompanying "isolation" track ($N_\text {iso,2}=1$; circles); or when it has two or three accompanying "isolation" tracks ($N_\text {iso,2}=2,3$; squares). |
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Figure 4:
The observed invariant mass distribution, normalized to unity, of the muon-track invariant mass in control regions $N_{23}$ (circles) and $N_{45}$ (squares). |
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Figure 5:
Normalized invariant mass distribution of the muon-track system for events passing the signal selection. Observed numbers of events are represented by data points with error bars. The QCD multijet background model is derived from the control region $N_{23}$. Also shown are the normalized distributions from signal simulations for four mass hypotheses, $m_{\mathrm{a}_{1}}=$ 4, 7, 10, and 15 GeV (dashed histograms), whereas for higher masses the analysis has no sensitivity. Each event in the observed and expected signal distributions contributes two entries, corresponding to the two muon-track systems in each event passing the selection. The signal distributions include 2$\mu $2$\tau $ and 4$\tau $ contributions. The lower panel shows the ratio of the observed to expected number of background events in each bin of the distribution. The grey shaded area represents the background model uncertainty. |
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Figure 6:
The ($m_1,m_2$) correlation factors $C(i,j)$ with their statistical uncertainties, derived from data in the CR {{Loose-Iso}}. |
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Figure 7:
The ($m_1,m_2$) correlation factors $C(i,j)$ along with their MC statistical uncertainties, derived from simulated samples in the (left: signal region, right: Loose-Iso CR). |
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Figure 7-a:
The ($m_1,m_2$) correlation factors $C(i,j)$ along with their MC statistical uncertainties, derived from simulated samples in the signal region. |
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Figure 7-b:
The ($m_1,m_2$) correlation factors $C(i,j)$ along with their MC statistical uncertainties, derived from simulated samples in the Loose-Iso CR. |
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Figure 8:
The distribution of the signal templates $f_\text {2D}(i,j)$ in one row for mass hypothesis $m_{\mathrm{a}_{1}} = $ 4 GeV (left) and 10 GeV (right). The $\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1} \to 2\mu 2\tau $ (blue histogram) and $\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1} \to 4\tau $ (red histogram) contributions are shown. The notation of the bins follows that of Fig. 2. |
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Figure 8-a:
The distribution of the signal templates $f_\text {2D}(i,j)$ in one row for mass hypothesis $m_{\mathrm{a}_{1}} = $ 4 GeV. The $\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1} \to 2\mu 2\tau $ (blue histogram) and $\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1} \to 4\tau $ (red histogram) contributions are shown. The notation of the bins follows that of Fig. 2. |
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Figure 8-b:
The distribution of the signal templates $f_\text {2D}(i,j)$ in one row for mass hypothesis $m_{\mathrm{a}_{1}} = $ 10 GeV. The $\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1} \to 2\mu 2\tau $ (blue histogram) and $\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1} \to 4\tau $ (red histogram) contributions are shown. The notation of the bins follows that of Fig. 2. |
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Figure 9:
The ($m_1,m_2$) in one row distribution used to extract the signal. Observed numbers of events are represented by data points with error bars. The background with its uncertainty is shown as the blue histogram with the shaded error band. The shape and the normalization of the background distribution are obtained by applying a fit to the observed data under the background-only hypothesis. Signal expectations for the 4$ \tau $ and 2$\mu $2$\tau $ final states are shown as dotted histograms for the mass hypotheses $m_{\mathrm{a}_{1}}=$ 4, 7, 10 and 15 GeV. The relative normalisation of the 4$ \tau $ and 2$\mu $2$\tau $ final states are given by Eq. (1) as explained in Section 6. The signal normalization is computed assuming that the H boson is produced in pp collisions with a rate predicted by the SM, and decays into $ \mathrm{a}_{1} \mathrm{a}_{1} \to 4\tau $ final state with the branching fraction of 20%. The lower plot shows the ratio of the observed data events to the expected background yield in each bin of the ($m_1,m_2$) distribution. |
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Figure 10:
The observed and expected upper limits at 95% confidence levels on the product of signal cross section and the branching fraction $\sigma ({\mathrm{p}} {\mathrm{p}} \to \mathrm{H} +X) {\mathcal {B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) {\mathcal {B}}^{2} (\mathrm{a}_{1} \to \tau \tau)$, relative to the inclusive Higgs boson production cross section $\sigma _\text {SM}$ predicted in the SM. The green and yellow bands indicate the regions that contain 68% and 95% of the distribution of limits expected under the background-only hypothesis. The shaded area in blue indicates the excluded region of $ > $34% for the branching fraction of the H decay into non-SM particles at 95% CL from Ref. [26]. |
| Tables | |
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Table 1:
The signal acceptance and the number of expected signal events after selection in the SR. The number of expected signal events is computed for a benchmark value of branching fraction, ${\mathcal {B}}(\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) {\mathcal {B}}^{2} (\mathrm{a}_{1} \to \tau \tau)=0.2$ and assuming that the H production cross section is the one predicted in the SM. The quoted uncertainties for predictions from simulation include only statistical ones. |
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Table 2:
Control regions used to construct and validate the background model. The symbols $N_\text {sig}$, $N_\text {iso}$ and $N_\text {soft}$ denote the number of "signal", "isolation" and "soft" tracks, respectively, within a cone of $\Delta \mathrm {R}=0.5$ around the muon momentum direction. The last row defines the SR. |
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Table 3:
Systematic uncertainties and their effect on the estimates of the QCD multijet background and signal. |
| Summary |
| A search is presented for light pseudoscalar $ \mathrm{a}_1 $ bosons, produced from decays of the 125 GeV Higgs boson (H) in a data set corresponding to an integrated luminosity of 35.9 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV. The analysis is based on the H inclusive production and targets the $\mathrm{H}\to \mathrm{a}_{1} \mathrm{a}_{1} \to 4\tau/2\mu 2\tau$ decay channels. Both channels are used in combination to constrain the product of the inclusive signal production cross section and the branching fraction into the 4$\tau$ final state, exploiting the linear dependence of the fermionic coupling strength of $ \mathrm{a}_1 $ on the fermion mass. With no evidence for a signal, the observed 95% confidence level upper limit on the product of the inclusive signal cross section and the branching fraction, relative to the SM H production cross section, ranges from 0.022 at $m_{\mathrm{a}_{1} }=$ 9 GeV to 0.23 at $m_{\mathrm{a}_{1} }=$ 4 GeV and reaches 0.16 at $m_{\mathrm{a}_{1} }=15$ GeV. The expected upper limit ranges from 0.027 at $m_{\mathrm{a}_{1} }=$ 9 GeV to 0.16 at $m_{\mathrm{a}_{1} }=$ 4 GeV and reaches 0.19 at $m_{\mathrm{a}_{1} }=15$ GeV. |
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Compact Muon Solenoid LHC, CERN |
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