CMS-EXO-20-003

CMS-EXO-20-003 ; CERN-EP-2021-198
Search for long-lived particles produced in association with a Z boson in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
25 October 2021
JHEP 03 (2022) 160
Abstract: A search for long-lived particles (LLPs) produced in association with a Z boson is presented. The study is performed using data from proton-proton collisions with a center-of-mass energy of 13 TeV recorded by the CMS experiment during 2016-2018, corresponding to an integrated luminosity of 117 fb$^{-1}$. The LLPs are assumed to decay to a pair of standard model quarks that are identified as displaced jets within the CMS tracker system. Triggers and selections based on Z boson decays to electron or muon pairs improve the sensitivity to light LLPs (down to 15 GeV{}). This search provides sensitivity to beyond the standard model scenarios which predict LLPs produced in association with a Z boson. In particular, the results are interpreted in the context of exotic decays of the Higgs boson to a pair of scalar LLPs (H $\to$ SS). The Higgs boson decay branching fraction is constrained to values less than 6% for proper decay lengths of 10-100 mm and for LLP masses between 40 and 55 GeV. In the case of low-mass ($\approx$15 GeV) scalar particles that subsequently decay to a pair of b quarks, the search is sensitive to branching fractions ${\mathcal{B}} (\mathrm{H \to SS}) < $ 20% for proper decay lengths of 10-50 mm. The use of associated production with a Z boson increases the sensitivity to low-mass LLPs of this analysis with respect to gluon fusion searches. In the case of 15 GeV scalar LLPs, the improvement corresponds to a factor of 2 at a proper decay length of 30 mm.
Links: e-print arXiv:2110.13218 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram of a simplified model for the Higgs boson decaying to a pair of long-lived scalar particles (S). The Higgs boson is produced in association with a Z boson, where the Z boson decays to a pair of leptons. The long-lived scalars decay to a pair of quark jets (q).
png pdf	Figure 2: Distributions of the three tagging variables for data and for four signal samples, where the decay lengths of the signal range from 1 to 1000 mm. The left figure displays the distributions for the ${\hat{\mathrm {IP}}^\mathrm {2D}_\text {sig}}$ tagging variable, while the center and right figures display the distributions for the ${\hat{\Theta}^\mathrm {2D}}$ and ${\alpha _\text {max}}$ tagging variables, respectively. Overlaid on the figures is a line with an arrow pointing to the region of where values of the variable are used to aid in distinguishing possible displaced jets from background jets.
png pdf	Figure 2-a: The figure displays the distributions for the ${\hat{\mathrm {IP}}^\mathrm {2D}_\text {sig}}$ tagging variable for data and for four signal samples, where the decay lengths of the signal range from 1 to 1000 mm. Overlaid on the figure is a line with an arrow pointing to the region of where values of the variable are used to aid in distinguishing possible displaced jets from background jets.
png pdf	Figure 2-b: The figure displays the distributions for the ${\hat{\Theta}^\mathrm {2D}}$ tagging variable for data and for four signal samples, where the decay lengths of the signal range from 1 to 1000 mm. Overlaid on the figure is a line with an arrow pointing to the region of where values of the variable are used to aid in distinguishing possible displaced jets from background jets.
png pdf	Figure 2-c: The figure displays the distributions for the ${\alpha _\text {max}}$ tagging variable for data and for four signal samples, where the decay lengths of the signal range from 1 to 1000 mm. Overlaid on the figure is a line with an arrow pointing to the region of where values of the variable are used to aid in distinguishing possible displaced jets from background jets.
png pdf	Figure 3: The background estimate and the observed data in the $ {N_{\mathrm {j}}^{\text {dis}}} \geq $ 2 bin, for each of the seven validation samples (VS$_{1}$ through VS$_{7}$), along with the signal sample (Sig S). Signal model distributions for scalar masses of 15 and 55 GeV with a proper mean decay length of 20 mm are also shown. The Higgs boson branching fraction to long-lived scalars (${\mathcal{B}} (\mathrm{H \to SS})$) is set to 20%.
png pdf	Figure 4: Exclusion limits at 95% CL on the Higgs boson branching fraction to long-lived scalars ${\mathcal{B}} (\mathrm{H \to SS})$. 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. The limits for the different scalar masses are shown in different colors for each scalar decay mode.
png pdf	Figure 4-a: Exclusion limits at 95% CL on the Higgs boson branching fraction to long-lived scalars ${\mathcal{B}} (\mathrm{H \to SS})$. Limits are presented for scalar decays to d quarks as a function of the mean proper decay length of the scalar. The limits for the different scalar masses are shown in different colors for each scalar decay mode.
png pdf	Figure 4-b: Exclusion limits at 95% CL on the Higgs boson branching fraction to long-lived scalars ${\mathcal{B}} (\mathrm{H \to SS})$. Limits are presented for scalar decays to b quarks as a function of the mean proper decay length of the scalar. The limits for the different scalar masses are shown in different colors for each scalar decay mode.

Tables
png pdf	Table 1: The systematic uncertainties in the background and signal yield expectations. Dashes indicate that the systematic effect is not applicable or is negligible.
png pdf	Table 2: Summary of the track-based displaced-jet tagging requirements to define the seven validation samples, VS$_{1}$ through VS$_{7}$, and the signal sample Sig S.
png pdf	Table 3: Observed data and estimated background yields in the search sample for the three ${N_{\mathrm {j}}^{\text {dis}}}$ bins. The estimated background yields are obtained using the fit procedure described in Sec. 5. Signal yields for two model points are also shown, where the Higgs boson decay branching fraction ${\mathcal{B}} (\mathrm{H \to SS})$ is assumed to be 10%.

Summary

A search for long-lived particles (LLPs) produced in association with a Z boson decaying to a pair of electrons or muons has been performed. The decays of LLPs in the tracker volume result in a displaced-jet signature, which is used to distinguish signal from SM background. No excess over the expected SM event rate is observed. The results of this search provide sensitivity to beyond the SM scenarios that predict LLPs produced in association with a Z boson. In particular, stringent exclusion limits on exotic Higgs boson decays to long-lived scalars are obtained. The Higgs boson decay branching fraction ${\mathcal{B}} (\mathrm{H \to SS})$ is constrained to values less than 6% for proper decay lengths of 10-100 mm and for long-lived particle masses between 40 and 55 GeV. In the case of low-mass ($\approx$15 GeV) scalar particles that subsequently decay to a pair of b quarks, the search is sensitive to branching fractions ${\mathcal{B}} (\mathrm{H \to SS}) < $ 20% for mean proper decay lengths of 10-50 mm. This corresponds to an improvement in sensitivity with respect to gluon fusion searches by a factor of 2 at a proper decay length of 30 mm.

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