CMS-PAS-FTR-18-002

CMS-PAS-FTR-18-002
Search sensitivity for dark photons decaying to displaced muons with CMS at the high-luminosity LHC
CMS Collaboration
October 2018

Abstract: This note presents sensitivity studies for a search for pairs of displaced muons originating from long-lived dark photons using the Phase-2 CMS detector at the high-luminosity LHC with an integrated luminosity of 3000 fb$^{-1}$. Projected sensitivities are obtained for broad ranges of dark photon masses (1-30 GeV) and lifetimes ($c\tau = $ 0.01 -10 m) in the context of Dark Supersymmetry models.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram of the decay of SM Higgs boson to a final state containing two or more muons in Dark SUSY models [11]. (a) Decay chain leading to a final state containing exactly two muons. (b) Decay chain leading to a final state containing exactly four muons.
png	Figure 1-a: Feynman diagram of the decay of SM Higgs boson to a final state containing two or more muons in Dark SUSY models [11]. (a) Decay chain leading to a final state containing exactly two muons. (b) Decay chain leading to a final state containing exactly four muons.
png	Figure 1-b: Feynman diagram of the decay of SM Higgs boson to a final state containing two or more muons in Dark SUSY models [11]. (a) Decay chain leading to a final state containing exactly two muons. (b) Decay chain leading to a final state containing exactly four muons.
png pdf	Figure 2: Branching ratio of dark photons to muons. The branching ratio calculations include the impact of hadronic resonances, such as $\rho $, $\omega $, $\phi $ and $\rho ^{\prime}$, as well as other leptonic decay modes of the dark photon. Narrow hadronic resonances (e.g. $\Upsilon, J/\psi $ and $\psi $(2S)), which are shown as gray areas, do not enter the branching ratio calculations.
png pdf	Figure 3: Event display of a $\mathrm{t\bar{t}}$ event with high pileup. All reconstructed muons fulfilling $p_{\mathrm{T}} > $ 1 GeV are shown. Muons from pileup are predominantly in the forward region of the detector. The tracks going horizontally through detector with no origin at the center of the detector are muons from beam halo. Both types of muons, from pileup and beam halo, are very low-$p_{\mathrm{T}}$ objects and are rejected by the muon $p_{\mathrm{T}}$ criterion applied in the analysis.
png pdf	Figure 4: Distribution of the significance of the transverse impact parameter, ${^\circ}limiter 69640972 {d_{0}} {^\circ}limiter 86418188 / {\sigma ({d_{0}}})$, for signal and background samples. Displaced standalone muons passing the kinematic selection ($p_{\mathrm{T}}$ and $\eta $) of the object selection are shown.
png pdf	Figure 5: Sketch illustrating the three dimensional distances of the closest approach of the displaced muon track to the primary interaction vertex, $R_{\text {Muon-1}}$ and $R_{\text {Muon-2}}$, for the two selected muons. PV denotes the primary interaction vertex. $(x,y)$ illustrates the transverse plane and the $z$-axis is along the beam line.
png pdf	Figure 6: Distribution of the distance of the closest approach of the displaced muon track to the primary interaction vertex, $R_{\rm Muon-1}$, for signal and background samples after the final event selection. The distance of the highest $p_{\mathrm{T}}$ muon is shown.
png pdf	Figure 7: 95% CL upper limits on production cross section $\sigma / \sigma _{\text {theory}}$ for various dark photon mass hypotheses and a fixed decay length of $c\tau = $ 1000 mm (a) and a fixed mass of $M_{\gamma _D} = $ 20 GeV as a function of the dark photon decay length (b). Green and yellow shaded bands show the one and two sigma range of variation of the expected 95% CL limits, respectively. Phase-2 results with 3000 fb$^{-1}$ (red) are compared to results obtained with 300 fb$^{-1}$ (violet) of integrated luminosity, which corresponds to the end of Phase-1 data taking. Another median of an excluded limit is shown here which represents the scenario with the reduced standalone reconstruction efficiency with 3000 fb$^{-1}$ (black) of integrated luminosity. Additionally, Phase-2 results with 3000 fb$^{-1}$ are determined without any systematic uncertainties (blue). The theoretical Dark SUSY cross section for 14 TeV is shown as a solid line. The gray lines indicate the regions of narrow hadronic resonances where the analysis does not claim any sensitivity (see Fig. 2).
png pdf	Figure 7-a: 95% CL upper limits on production cross section $\sigma / \sigma _{\text {theory}}$ for various dark photon mass hypotheses and a fixed decay length of $c\tau = $ 1000 mm (a) and a fixed mass of $M_{\gamma _D} = $ 20 GeV as a function of the dark photon decay length (b). Green and yellow shaded bands show the one and two sigma range of variation of the expected 95% CL limits, respectively. Phase-2 results with 3000 fb$^{-1}$ (red) are compared to results obtained with 300 fb$^{-1}$ (violet) of integrated luminosity, which corresponds to the end of Phase-1 data taking. Another median of an excluded limit is shown here which represents the scenario with the reduced standalone reconstruction efficiency with 3000 fb$^{-1}$ (black) of integrated luminosity. Additionally, Phase-2 results with 3000 fb$^{-1}$ are determined without any systematic uncertainties (blue). The theoretical Dark SUSY cross section for 14 TeV is shown as a solid line. The gray lines indicate the regions of narrow hadronic resonances where the analysis does not claim any sensitivity (see Fig. 2).
png pdf	Figure 7-b: 95% CL upper limits on production cross section $\sigma / \sigma _{\text {theory}}$ for various dark photon mass hypotheses and a fixed decay length of $c\tau = $ 1000 mm (a) and a fixed mass of $M_{\gamma _D} = $ 20 GeV as a function of the dark photon decay length (b). Green and yellow shaded bands show the one and two sigma range of variation of the expected 95% CL limits, respectively. Phase-2 results with 3000 fb$^{-1}$ (red) are compared to results obtained with 300 fb$^{-1}$ (violet) of integrated luminosity, which corresponds to the end of Phase-1 data taking. Another median of an excluded limit is shown here which represents the scenario with the reduced standalone reconstruction efficiency with 3000 fb$^{-1}$ (black) of integrated luminosity. Additionally, Phase-2 results with 3000 fb$^{-1}$ are determined without any systematic uncertainties (blue). The theoretical Dark SUSY cross section for 14 TeV is shown as a solid line. The gray lines indicate the regions of narrow hadronic resonances where the analysis does not claim any sensitivity (see Fig. 2).
png pdf	Figure 8: Parameter scan in the $\epsilon -m_{\gamma _D}$ plane. (a) Collection of existing limits taken from Ref. [8]. (b) Results from this analysis for Phase-2 with 3000 fb$^{-1}$. The ranges with exclusion and discovery sensitivity are shown in light and dark red color, respectively. The gray lines indicate the regions of narrow hadronic resonances where the analysis does not claim any sensitivity (see Fig. 2).
png pdf	Figure 8-a: Parameter scan in the $\epsilon -m_{\gamma _D}$ plane. (a) Collection of existing limits taken from Ref. [8]. (b) Results from this analysis for Phase-2 with 3000 fb$^{-1}$. The ranges with exclusion and discovery sensitivity are shown in light and dark red color, respectively. The gray lines indicate the regions of narrow hadronic resonances where the analysis does not claim any sensitivity (see Fig. 2).
png pdf	Figure 8-b: Parameter scan in the $\epsilon -m_{\gamma _D}$ plane. (a) Collection of existing limits taken from Ref. [8]. (b) Results from this analysis for Phase-2 with 3000 fb$^{-1}$. The ranges with exclusion and discovery sensitivity are shown in light and dark red color, respectively. The gray lines indicate the regions of narrow hadronic resonances where the analysis does not claim any sensitivity (see Fig. 2).
png pdf jpg	Figure 9: Feynman diagram of the process leading to smuon pair production at a hadron collider. The decay of the smuons leads to the final state including two muons.
png pdf	Figure 10: (a) Discovery significance and p-value for a fixed smuon mass of $M_{\tilde{\mu}} = $ 200 GeV. (b) Discovery sensitivity in the parameter space of mass and decay length.
png pdf	Figure 10-a: (a) Discovery significance and p-value for a fixed smuon mass of $M_{\tilde{\mu}} = $ 200 GeV. (b) Discovery sensitivity in the parameter space of mass and decay length.
png pdf	Figure 10-b: (a) Discovery significance and p-value for a fixed smuon mass of $M_{\tilde{\mu}} = $ 200 GeV. (b) Discovery sensitivity in the parameter space of mass and decay length.

Tables
png pdf	Table 1: Signal and background event yields with statistical uncertainties in different search regions after the final event selection. The systematic uncertainties can be found in Sec. 8.

Summary

Present searches for displaced muons show no significant deviation with respect to the standard model expectation. However, there is quite a large phase-space which has not been explored yet and is unreachable with the current LHC conditions due to low signal cross sections and limited statistics. The high-luminosity LHC will provide a unique opportunity to search for new physics with a striking signature of highly displaced muons. This study presents the search sensitivity for pairs of displaced muons with an integrated luminosity of 3000 fb$^{-1}$. The transverse impact parameter significance and the three-dimensional distance between the extrapolated displaced muon track and the primary vertex are used as the discriminating variables in the search for two largely displaced muons that are reconstructed with a standalone algorithm using only muon chamber hits.

The search in this note is performed within a model belonging to a class of Dark SUSY models where dark photons decay into a pair of displaced muons. The study shows that searches at the high-luminosity LHC will be able to probe phase-space which has not been explored yet.

Additional Figures
png pdf	Additional Figure 1: Exclusion and discovery sensitivity of the search for dark photons for the Phase-2 CMS detector and an integrated luminosity of 3000 fb$^{-1}$. The search is assuming $\mathcal{B}(\mathrm{H} \to 2\gamma _D + \mathrm {X}) = $ 20%. The sensitive regions are shown in light and dark red color for exclusion and discovery, respectively. The gray lines indicate the regions of narrow hadronic resonances where the analysis does not claim any sensitivity. New points in the dark photon mass and lifetime parameter space are included compared to previous results. This extends the investigated mass range up to $M_{\gamma _{D}} = $ 45 GeV.
png pdf	Additional Figure 2: Exclusion sensitivity of the search for dark photons for the Phase-2 CMS detector and an integrated luminosity of 3000 fb$^{-1}$. The different shades of red indicate the sensitive areas for different values of $\mathcal{B}(\mathrm{H} \to 2\gamma _D + \mathrm {X})$. The gray lines indicate the regions of narrow hadronic resonances where the analysis does not claim any sensitivity.

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