CMS-PAS-EXO-20-003

CMS-PAS-EXO-20-003
Search for Higgs boson decays into long-lived particles in associated Z boson production
CMS Collaboration
May 2021

Abstract: We present a search for long-lived particles (LLPs) produced in association with a Z boson. The search is performed with data from 13 TeV proton-proton collisions recorded by the CMS experiment during 2016-2018, corresponding to an integrated luminosity of 117 fb$^{-1}$. The LLPs are assumed to decay into a pair of standard model fermions inside the tracker volume, which results in displaced jets. A trigger and selections based on Z boson decays to electron or muon pairs provide sensitivity to light (15 GeV or less) LLPs, which have up to now been difficult to access. Decays of LLPs are selected by requiring the presence of displaced jets which are identified using information from the CMS tracking system. The results are interpreted in the context of exotic decays of the Higgs boson to LLPs ($\mathrm{H} \to \mathrm{S}\mathrm{S}$). The search is sensitive to branching fractions $\mathcal{B}( \mathrm{H} \to \mathrm{S}\mathrm{S})$ of 4-5% (less than 20%) for LLP masses of 40 (15) GeV and mean proper decay lengths of 10-100 mm (10-50 mm).
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 03 (2022) 160. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: A simplified model for the Higgs boson decaying to a pair of long-lived scalars (S). The Higgs boson is produced in association with a Z boson, where the Z boson decays to pair of leptons. The long-lived scalars decay to a pair of fermions (f).
png pdf	Figure 2: The background estimate and the observed data in the $ {\mathrm {N}_{\mathrm {dis}}^{\mathrm {j}}} \geq$ 2 bins, for each of the seven validation samples along with the signal sample. Two signal model distributions for scalar masses of 15 and 55 GeV are also shown, where the Higgs boson branching ratio to long-lived scalars ($ {\mathcal {B}} (\mathrm{H} \rightarrow {\mathrm {S}} {\mathrm {S}})$) is set to 20%.
png pdf	Figure 3: 95% CL exclusion limits on the Higgs boson branching ratio to long-lived scalars ($ {\mathcal {B}} (\mathrm{H} \rightarrow {\mathrm {S}} {\mathrm {S}})$). Limits are presented for scalar decays to d quarks (left) and b quarks (right) as a function of the mean proper decay length of the scalar. Different scalar masses are shown in different colors for each scalar decay mode.
png pdf	Figure 3-a: 95% CL exclusion limits on the Higgs boson branching ratio to long-lived scalars ($ {\mathcal {B}} (\mathrm{H} \rightarrow {\mathrm {S}} {\mathrm {S}})$). Limits are presented for scalar decays to d quarks as a function of the mean proper decay length of the scalar. Different scalar masses are shown in different colors for each scalar decay mode.
png pdf	Figure 3-b: 95% CL exclusion limits on the Higgs boson branching ratio to long-lived scalars ($ {\mathcal {B}} (\mathrm{H} \rightarrow {\mathrm {S}} {\mathrm {S}})$). Limits are presented for scalar decays to b quarks as a function of the mean proper decay length of the scalar. Different scalar masses are shown in different colors for each scalar decay mode.

Tables
png pdf	Table 1: Summary of the systematic uncertainties in the background and signal yield predictions. Dashes indicate that the systematic effect is not applicable for either the signal or the background, or is negligible.

Summary

We have performed a search for long-lived particles that are produced via Higgs boson decays to a pair of scalars in association with a Z boson, resulting in displaced jets. The Higgs boson decay branching ratio is constrained to be below 5-10% for proper decays lengths of 10-100 mm and masses between 40 and 55 GeV. This result yields stringent exclusion limits on exotic Higgs decays to long-lived scalars. In particular, this analysis provides the most stringent CMS limits for the branching fractions $\mathcal{B}( \mathrm{H} \to \mathrm{S}\mathrm{S})$ for low mass scalar particles of around 15 GeV with mean proper decay lengths of 2-30 mm, where the scalars decay to a pair of b quarks.

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