CMS-PAS-HIG-18-006

CMS-PAS-HIG-18-006
Search for a pseudoscalar boson in the mass range from 4 to 15 GeV produced in decays of the 125 GeV Higgs boson in the final states with two muons and two nearby tracks at $\sqrt{s}=$ 13 TeV
CMS Collaboration
March 2019

Abstract: A search is presented for pairs of light pseudoscalar bosons, in the mass range between 4 and 15 GeV, produced in decay of the 125 GeV Higgs boson. The decay mode where one light boson decays into $\tau$ leptons, while the other one decays into a pair of $\tau$ leptons or muons, is considered. The search is based on proton-proton collision data collected by the CMS experiment at a centre-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 35.9 fb $^{-1}$ . No significant excess of events above the standard model background expectation obtained from control regions in data is observed. The 95% confidence level observed (expected) upper limits on the signal production cross section times branching fraction into the 4 $\tau$ final state, relative to the standard model Higgs boson production cross section are set between 0.022 (0.027) and 0.23 (0.19) in the probed mass range.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 800 (2019) 135087. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Illustration of the signal topology. The H(125) decays into two ${\mathrm {a}_1}$ bosons, where one 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 leg.
png pdf	Figure 2: Binning of the 2D ( $m_1$ , $m_2$ ) distribution. The last bin is an overflow bin and includes also masses up to 15 GeV, while values in parentheses are given in GeV.
png pdf	Figure 3: The observed invariant mass distribution, normalized to unity, of the first muon and the softest (left plot) or hardest (right plot) accompanying track for different isolation requirements imposed on the second muon: when the second muon has only one accompanying track ( $N_\mathrm {trk,2}=$ 1; circles); or when it has two or three accompanying tracks ( $N_\mathrm {trk,2}=$ 2, 3; squares).
png pdf	Figure 3-a: The observed invariant mass distribution, normalized to unity, of the first muon and the softest accompanying track for different isolation requirements imposed on the second muon: when the second muon has only one accompanying track ( $N_\mathrm {trk,2}=$ 1; circles); or when it has two or three accompanying tracks ( $N_\mathrm {trk,2}=$ 2, 3; squares).
png pdf	Figure 3-b: The observed invariant mass distribution, normalized to unity, of the first muon and the hardest accompanying track for different isolation requirements imposed on the second muon: when the second muon has only one accompanying track ( $N_\mathrm {trk,2}=$ 1; circles); or when it has two or three accompanying tracks ( $N_\mathrm {trk,2}=$ 2, 3; squares).
png pdf	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).
png pdf	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). Each event in the observed and expected signal distributions contributes two entries, corresponding to the two muon-track systems in each event passing the requirements. 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 statistical uncertainties.
png pdf	Figure 6: The ( $m_1$ , $m_2$ ) correlation factors $C(i,j)$ with their statistical uncertainties, derived from data in the CR {{Loose-Iso}}.
png pdf	Figure 7: Left (Right) : The ( $m_1$ , $m_2$ ) correlation factors $C(i,j)$ along with their MC statistical uncertainties, derived from simulated samples in the signal region (Loose-Iso CR).
png pdf	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.
png pdf	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.
png pdf	Figure 8: The distribution of the signal templates $f_\text {2D}(i,j)$ in one row for mass hypotheses $m_{{\mathrm {a}_1}}=$ 4 GeV (left plot) and 10 GeV (right plot). The ${\mathrm {H}} (125)\to {\mathrm {a}_1} {\mathrm {a}_1}\to 2\mu 2\tau$ (blue histogram) and ${\mathrm {H}} (125)\to {\mathrm {a}_1} {\mathrm {a}_1}\to 4\tau$ (red histogram) contributions are shown.
png pdf	Figure 8-a: The distribution of the signal templates $f_\text {2D}(i,j)$ in one row for mass hypotheses $m_{{\mathrm {a}_1}}=$ 10 GeV. The ${\mathrm {H}} (125)\to {\mathrm {a}_1} {\mathrm {a}_1}\to 2\mu 2\tau$ (blue histogram) and ${\mathrm {H}} (125)\to {\mathrm {a}_1} {\mathrm {a}_1}\to 4\tau$ (red histogram) contributions are shown.
png pdf	Figure 8-b: The distribution of the signal templates $f_\text {2D}(i,j)$ in one row for mass hypotheses $m_{{\mathrm {a}_1}}=$ 4 GeV. The ${\mathrm {H}} (125)\to {\mathrm {a}_1} {\mathrm {a}_1}\to 2\mu 2\tau$ (blue histogram) and ${\mathrm {H}} (125)\to {\mathrm {a}_1} {\mathrm {a}_1}\to 4\tau$ (red histogram) contributions are shown.
png pdf	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 blue histogram with the shaded error band. The distribution for the background is obtained after performing a maximum likelihood fit to the observed data under the background-only hypothesis. Signal expectations are shown as dotted histograms for the mass hypotheses of $m_{{\mathrm {a}_1}}=$ 4, 7, 10, and 15 GeV. The signal normalization is computed assuming that the H(125) 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.
png pdf	Figure 10: The observed and expected upper 95% confidence level limits on the signal cross section times the branching fraction $\sigma (pp \rightarrow {\mathrm {H}} (125)+X) \cdot {\cal {B}} ({\mathrm {H}} (125) \rightarrow {\mathrm {a}_1} {\mathrm {a}_1}) \cdot {\cal {B}}^{2} ({\mathrm {a}_1}\rightarrow \tau \tau)$ , 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 upper limits expected assuming no signal is present. The shaded area indicates the excluded value of the branching fraction of the H(125) decay into non-SM particles at 95% CL from Ref. [24].

Tables
png pdf	Table 1: The signal acceptance and the number of expected signal events after final selection. The former is presented for the final state with two same-sign muons, while the signal yields are shown for the benchmark value of branching fraction, ${\cal {B}}({\mathrm {H}} (125) \rightarrow {\mathrm {a}_1} {\mathrm {a}_1}) \cdot {\cal {B}}^{2} ({\mathrm {a}_1}\rightarrow \tau \tau)=$ 0.2. The quoted uncertainties for predictions from simulation include only statistical ones. The number of observed events selected in the signal region amounts to 2035.
png pdf	Table 2: The list of control regions used to model background shape. $N_\mathrm {trk}$ denotes the number of close-by tracks within a cone of $\Delta \rm {R}=$ 0.5 around the muon momentum direction. $N_\mathrm {trk,soft}$ is the number of soft tracks with 1.0 GeV $< {p_{\mathrm {T}}} <$ 2.5 GeV in the cone of $\Delta \rm {R}=$ 0.5 around the muon momentum direction.
png pdf	Table 3: Systematic uncertainties and their effect on the estimates of the QCD multijet background and signal. The effect of the uncertainties in $C(i,j)$ on the total background yield is absorbed by the overall background normalization, which is allowed to vary freely in the fit.

Summary

A search for a light pseudoscalar boson

${\mathrm{a}_1}$ , produced in decays of the H(125) boson is presented, using a data set corresponding to an integrated luminosity of 35.9 fb

$^{-1}$ of proton-proton collisions at a centre-of-mass energy of 13 TeV. The analysis targets the inclusive production of the H(125) boson and exploits the

$\mathrm{H}(125)\to {\mathrm{a}_1} {\mathrm{a}_1} \to 4\tau$ decay mode. The contribution of the

$\mathrm{H}(125)\to {\mathrm{a}_1} {\mathrm{a}_1} \to 2\mu 2\tau$ decay channel is also taken into account in the search. No evidence of signal is found in data. The observed 95% confidence level upper limit on the signal cross section times branching fraction, relative to the cross section of the H(125) boson production predicted in the SM ranges from 0.023 at

$m_{{\mathrm{a}_1}}=$ 9 GeV to 0.26 at

$m_{{\mathrm{a}_1}}=$ 4 GeV. The expected upper limit ranges from 0.028 at

$m_{{\mathrm{a}_1}}=$ 9 GeV to 0.19 at

$m_{{\mathrm{a}_1}}=$ 15 GeV.

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