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| CMS-EXO-20-015 ; CERN-EP-2021-125 | ||
| Search for long-lived particles decaying in the CMS endcap muon detectors in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 10 July 2021 | ||
| Phy. Rev. Lett. 127 (2021) 261804 | ||
| Abstract: A search for long-lived particles (LLPs) produced in decays of standard model (SM) Higgs bosons is presented. The data sample consists of 137 fb$^{-1}$ of proton-proton collisions at $\sqrt{s} = $ 13 TeV, recorded at the LHC in 2016-2018. A novel technique is employed to reconstruct decays of LLPs in the endcap muon detectors. The search is sensitive to a broad range of LLP decay modes and to masses as low as a few GeV. No excess of events above the SM background is observed. The most stringent limits to date on the branching fraction of the Higgs boson to LLPs subsequently decaying to quarks and $\tau^{+}\tau^{-}$ are found for proper decay lengths greater than 6, 20, and 40 m, for LLP masses of 7, 15, and 40 GeV, respectively. | ||
| Links: e-print arXiv:2107.04838 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ; | ||
| Figures | Summary | Additional Figures & Material | References | CMS Publications |
|---|
| Instructions for reinterpretation can be found here |
| Figures | |
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Figure 1:
The signal efficiency of the combined cluster reconstruction, veto, and identification selections as a function of the simulated $r$ and $z$ decay positions of S decaying to $\mathrm{b} {}\mathrm{\bar{b}} $, for a mass of 15 GeV and a uniformly distributed mixture of events with $c\tau $ between 1-10 m. The barrel and endcap muon stations are drawn as black boxes and labeled by their station names, showing the geometry of the muon detectors. Regions occupied by the steel return yoke are shaded in gray. |
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Figure 2:
Distributions of $N_\text {hits}$ (left) and $\Delta \phi _{\mathrm {c}}$ (right) in the search region. The background predicted by the fit is shown in blue with the shaded region showing the fitted uncertainty. The expected signal with $ {\mathcal {B}({\mathrm{H} \to \mathrm{S} \mathrm{S}})} = $ 1%, $\mathrm{S} \to {\mathrm{d} \mathrm{\bar{d}}} $, and $c\tau = $ 1 m is shown for $m_\mathrm{S} $ of 7, 15, 40, and 55 GeV in various colors and dotted lines. The $N_\text {hits}$ distribution includes only events in bins C and D, while the $\Delta \phi _{\mathrm {c}}$ includes only events in bins A and D. The last bin in the $N_\text {hits}$ distributions includes overflow events. |
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Figure 2-a:
Distribution of $N_\text {hits}$ in the search region. The background predicted by the fit is shown in blue with the shaded region showing the fitted uncertainty. The expected signal with $ {\mathcal {B}({\mathrm{H} \to \mathrm{S} \mathrm{S}})} = $ 1%, $\mathrm{S} \to {\mathrm{d} \mathrm{\bar{d}}} $, and $c\tau = $ 1 m is shown for $m_\mathrm{S} $ of 7, 15, 40, and 55 GeV in various colors and dotted lines. The $N_\text {hits}$ distribution includes only events in bins C and D, while the $\Delta \phi _{\mathrm {c}}$ includes only events in bins A and D. The last bin in the $N_\text {hits}$ distributions includes overflow events. |
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Figure 2-b:
Distribution of $\Delta \phi _{\mathrm {c}}$ in the search region. The background predicted by the fit is shown in blue with the shaded region showing the fitted uncertainty. The expected signal with $ {\mathcal {B}({\mathrm{H} \to \mathrm{S} \mathrm{S}})} = $ 1%, $\mathrm{S} \to {\mathrm{d} \mathrm{\bar{d}}} $, and $c\tau = $ 1 m is shown for $m_\mathrm{S} $ of 7, 15, 40, and 55 GeV in various colors and dotted lines. The $N_\text {hits}$ distribution includes only events in bins C and D, while the $\Delta \phi _{\mathrm {c}}$ includes only events in bins A and D. The last bin in the $N_\text {hits}$ distributions includes overflow events. |
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Figure 3:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ {\mathcal {B}({\mathrm{H} \to \mathrm{S} \mathrm{S}})} $ as functions of $c\tau $ for the $\mathrm{S} \to {\mathrm{d} \mathrm{\bar{d}}} $ (left) and $\mathrm{S} \to \tau^{+} \tau^{-} $ (right) decay modes. The exclusion limits are shown for four different mass hypotheses: 7, 15, 40, and 55 GeV. |
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Figure 3-a:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ {\mathcal {B}({\mathrm{H} \to \mathrm{S} \mathrm{S}})} $ as a function of the $c\tau $ for the $\mathrm{S} \to {\mathrm{d} \mathrm{\bar{d}}} $ decay mode. The exclusion limits are shown for four different mass hypotheses: 7, 15, 40, and 55 GeV. |
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Figure 3-b:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ {\mathcal {B}({\mathrm{H} \to \mathrm{S} \mathrm{S}})} $ as a function of the $c\tau $ for the $\mathrm{S} \to \tau^{+} \tau^{-} $ decay mode. The exclusion limits are shown for four different mass hypotheses: 7, 15, 40, and 55 GeV. |
| Summary |
| In summary, proton-proton collision data at $\sqrt{s} = $ 13 TeV recorded by the CMS experiment in 2016--2018, corresponding to an integrated luminosity of 137 fb$^{-1}$, have been used to conduct the first search for beyond the standard model (SM) long-lived particles (LLPs) using the CMS endcap muon detectors as a calorimeter. Based on a unique detector signature, the search is largely model-independent, with sensitivity to a broad range of LLP decay modes and to LLP masses as low as a few GeV. With the excellent shielding provided by the inner CMS detector, the background is suppressed to a low level and a search for a single LLP decay is possible. No significant deviation from the SM background is observed, and the most stringent limits on the branching fraction of Higgs boson to LLP decaying to $\mathrm{d\bar{d}}$, $\mathrm{b\bar{b}}$, and $\tau^{+}\tau^{-}$ are set for proper decay lengths $c\tau > $ 6, 20, and 40 m, and LLP masses of 7, 15, and 40GeV, respectively. For $c\tau > $ 100 m, this search outperforms the previous best limits [30,31] by a factor of 6 (2) for an LLP mass of 7 ($\ge$15) GeV. |
| Additional Figures | |
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Additional Figure 1:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ {\mathcal {B}({{\mathrm {h}^0} \rightarrow {\mathrm {S}} {\mathrm {S}}})} $ as a function of $c\tau $ for the $ {\mathrm {S}} \rightarrow {{\mathrm {b}} {\overline {\mathrm {b}}}} $ decay mode. The exclusion limits are shown for three different mass hypotheses: 15 (black), 40 (red), and 55 (blue) GeV. |
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Additional Figure 2:
The $N_\mathrm {hits}$ (left) and $\Delta \phi ({\vec{p}_{\mathrm {T}}^{\,\text {miss}}},\mathrm {cluster})$ (right) distributions. The signal (assuming $ {\mathcal {B}({{\mathrm {h}^0} \rightarrow {\mathrm {S}} {\mathrm {S}}})} = $ 1%, $ {\mathrm {S}} \rightarrow {{\mathrm {d}} {\overline {\mathrm {d}}}} $, and $c\tau =$ 1 m) and data (black) distributions have $\Delta \phi < $ 0.75 ($N_\mathrm {hits} > $ 130) selections applied in the left (right) plot. The data distribution shown in magenta includes the inverted cut $\Delta \phi > $ 0.75 ($N_\mathrm {hits} < $ 130) applied in the left (right) plot. The last bin in the $N_\mathrm {hits}$ distributions includes all events in the overflow. |
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Additional Figure 2-a:
The $N_\mathrm {hits}$ distribution. The signal (assuming $ {\mathcal {B}({{\mathrm {h}^0} \rightarrow {\mathrm {S}} {\mathrm {S}}})} = $ 1%, $ {\mathrm {S}} \rightarrow {{\mathrm {d}} {\overline {\mathrm {d}}}} $, and $c\tau =$ 1 m) and data (black) distributions have the $\Delta \phi < $ 0.75 selection applied. The data distribution shown in magenta applies the inverted cut $\Delta \phi > $ 0.75. The last bin includes all events in the overflow. |
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Additional Figure 2-b:
The $\Delta \phi ({\vec{p}_{\mathrm {T}}^{\,\text {miss}}},\mathrm {cluster})$ distribution. The signal (assuming $ {\mathcal {B}({{\mathrm {h}^0} \rightarrow {\mathrm {S}} {\mathrm {S}}})} = $ 1%, $ {\mathrm {S}} \rightarrow {{\mathrm {d}} {\overline {\mathrm {d}}}} $, and $c\tau =$ 1 m) and data (black) distributions have the $N_\mathrm {hits} > $ 130 selection applied. The data distribution shown in magenta applies the inverted cut $N_\mathrm {hits} < $ 130. |
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Additional Figure 3:
The signal (assuming $ {\mathcal {B}({{\mathrm {h}^0} \rightarrow {\mathrm {S}} {\mathrm {S}}})} = $ 1%, $ {\mathrm {S}} \rightarrow {{\mathrm {d}} {\overline {\mathrm {d}}}} $, and $c\tau =$ 1 m), in-time data ($N_\mathrm {hits} < $ 130), and out-of-time data (inclusive in $N_\mathrm {hits}$) distributions of $N_\mathrm {hits}$ (left) and $\Delta \phi ({\vec{p}_{\mathrm {T}}^{\,\text {miss}}},\mathrm {cluster})$ (right). The signal distributions include events from regions A, B, C, and D (as defined in the analysis summary), the in-time data distributions include events from regions B and C, and the OOT data are out-of-time clusters from previous bunch crossings, with the yield normalized to that of the in-time distribution. The last bin in the $N_\mathrm {hits}$ distributions includes all events in the overflow. |
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Additional Figure 3-a:
The signal (assuming $ {\mathcal {B}({{\mathrm {h}^0} \rightarrow {\mathrm {S}} {\mathrm {S}}})} = $ 1%, $ {\mathrm {S}} \rightarrow {{\mathrm {d}} {\overline {\mathrm {d}}}} $, and $c\tau =$ 1 m), in-time data ($N_\mathrm {hits} < $ 130), and out-of-time data (inclusive in $N_\mathrm {hits}$) distributions of $N_\mathrm {hits}$. The signal distributions include events from regions A, B, C, and D (as defined in the analysis summary), the in-time data distributions include events from regions B and C, and the OOT data are out-of-time clusters from previous bunch crossings, with the yield normalized to that of the in-time distribution. The last bin includes all events in the overflow. |
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Additional Figure 3-b:
The signal (assuming $ {\mathcal {B}({{\mathrm {h}^0} \rightarrow {\mathrm {S}} {\mathrm {S}}})} = $ 1%, $ {\mathrm {S}} \rightarrow {{\mathrm {d}} {\overline {\mathrm {d}}}} $, and $c\tau =$ 1 m), in-time data ($N_\mathrm {hits} < $ 130), and out-of-time data (inclusive in $N_\mathrm {hits}$) distributions of $\Delta \phi ({\vec{p}_{\mathrm {T}}^{\,\text {miss}}},\mathrm {cluster})$. The signal distributions include events from regions A, B, C, and D (as defined in the analysis summary), the in-time data distributions include events from regions B and C, and the OOT data are out-of-time clusters from previous bunch crossings, with the yield normalized to that of the in-time distribution. |
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Additional Figure 4:
The event display of a simulated signal event for $m_ {\mathrm {S}} = $ 40 GeV, $c\tau =$ 1 m, and $ {\mathrm {S}} \rightarrow {{\mathrm {b}} {\overline {\mathrm {b}}}} $ in $rz$-plane (left) and $r\phi $-plane (right). The LLP decayed at the position $x, y, z =$ $-$364, 92, 796 cm and produced a CSC hit cluster with 711 hits (orange dots). The green lines represent the tracks. The yellow lines represent jets. The red arrow represents the MET direction. The red and blue cones represent the ECAL and HCAL energy deposits, respectively. |
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Additional Figure 4-a:
The event display of a simulated signal event for $m_ {\mathrm {S}} = $ 40 GeV, $c\tau =$ 1 m, and $ {\mathrm {S}} \rightarrow {{\mathrm {b}} {\overline {\mathrm {b}}}} $ in $rz$-plane. The LLP decayed at the position $x, y, z =$ $-$364, 92, 796 cm and produced a CSC hit cluster with 711 hits (orange dots). The green lines represent the tracks. The yellow lines represent jets. The red arrow represents the MET direction. The red and blue cones represent the ECAL and HCAL energy deposits, respectively. |
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Additional Figure 4-b:
The event display of a simulated signal event for $m_ {\mathrm {S}} = $ 40 GeV, $c\tau =$ 1 m, and $ {\mathrm {S}} \rightarrow {{\mathrm {b}} {\overline {\mathrm {b}}}} $ in $r\phi $-plane. The LLP decayed at the position $x, y, z =$ $-$364, 92, 796 cm and produced a CSC hit cluster with 711 hits (orange dots). The green lines represent the tracks. The yellow lines represent jets. The red arrow represents the MET direction. The red and blue cones represent the ECAL and HCAL energy deposits, respectively. |
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Additional Figure 5:
Distributions of cluster time in data and in simulated $ {\mathrm {S}} \rightarrow {{\mathrm {b}} {\overline {\mathrm {b}}}} $ signal events. The signal distribution contains equal fractions of events with mass of 15, 40, and 55 GeV, each simulated with a uniform mixture of $c\tau $ between 0.1 and 100 m. Both distributions are normalized to unit area and are required to pass the veto requirements as defined in the analysis summary. |
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Additional Figure 6:
The clustering efficiency requiring $ > 50$ hits (left) and the overall signal efficiency of the clustering requiring $ > $50 hits, veto, and identification selections (right), as defined in the analysis summary, as a function of the simulated $z$ decay positions of ${\mathrm {S}}$ decaying to $ {{\mathrm {b}} {\overline {\mathrm {b}}}} $, for a mass of 15 GeV and a uniform mixture of $c\tau $ between 1 and 100 m. The endcap muon stations are labeled by their station names. Regions occupied by steel shielding are shaded in gray. |
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Additional Figure 6-a:
The clustering efficiency requiring $ > 50$ hits, as defined in the analysis summary, as a function of the simulated $z$ decay positions of ${\mathrm {S}}$ decaying to $ {{\mathrm {b}} {\overline {\mathrm {b}}}} $, for a mass of 15 GeV and a uniform mixture of $c\tau $ between 1 and 100 m. The endcap muon stations are labeled by their station names. Regions occupied by steel shielding are shaded in gray. |
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Additional Figure 6-b:
The overall signal efficiency of the clustering requiring $ > $50 hits, veto, and identification selections, as defined in the analysis summary, as a function of the simulated $z$ decay positions of ${\mathrm {S}}$ decaying to $ {{\mathrm {b}} {\overline {\mathrm {b}}}} $, for a mass of 15 GeV and a uniform mixture of $c\tau $ between 1 and 100 m. The endcap muon stations are labeled by their station names. Regions occupied by steel shielding are shaded in gray. |
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Additional Figure 7:
The cluster efficiency in bins of hadronic and EM energy in region A (left) and B (right), estimated with LLPs decaying to ${{\tau}^{+} {\tau}^{-}}$. The sample contains equal fractions of events with LLP mass of 7, 15, 40, and 55 GeV and LLP lifetime of 0.1, 1, 10, and 100m. The first hadronic energy bins correspond to LLPs that decayed leptonically with 0 hadronic energy. Region A is defined as 391 cm $ < r < $ 695.5 cm and 400 cm $ < |z| < $ 671 cm. Region B is defined as 671 cm $ < |z| < $ 1100 cm, $r < $ 695.5 cm and $|\eta | < $ 2. The cluster efficiency includes all cluster-level selections described in the paper, except for the jet veto, time cut, and $\Delta \phi ({\vec{p}_{\mathrm {T}}^{\,\text {miss}}},\mathrm {cluster})$ cut. The full simulation signal yield prediction for samples with various LLP mass between 7 - 55 GeV, lifetime between 0.1 - 100 m, and decay mode to $ {{\mathrm {d}} {\overline {\mathrm {d}}}} $ and $ {{\tau}^{+} {\tau}^{-}} $ can be reproduced using this parameterization to within 35% and 20% for region A and B, respectively. The statistical uncertainty for each bin is documented in Figure 7 of the HEPData record of this paper. |
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Additional Figure 7-a:
The cluster efficiency in bins of hadronic and EM energy in region A, estimated with LLPs decaying to ${{\tau}^{+} {\tau}^{-}}$. The sample contains equal fractions of events with LLP mass of 7, 15, 40, and 55 GeV and LLP lifetime of 0.1, 1, 10, and 100m. The first hadronic energy bins correspond to LLPs that decayed leptonically with 0 hadronic energy. Region A is defined as 391 cm $ < r < $ 695.5 cm and 400 cm $ < |z| < $ 671 cm. The cluster efficiency includes all cluster-level selections described in the paper, except for the jet veto, time cut, and $\Delta \phi ({\vec{p}_{\mathrm {T}}^{\,\text {miss}}},\mathrm {cluster})$ cut. The full simulation signal yield prediction for samples with various LLP mass between 7 - 55 GeV, lifetime between 0.1 - 100 m, and decay mode to $ {{\mathrm {d}} {\overline {\mathrm {d}}}} $ and $ {{\tau}^{+} {\tau}^{-}} $ can be reproduced using this parameterization to within 35% for region A. The statistical uncertainty for each bin is documented in Figure 7 of the HEPData record of this paper. |
png pdf |
Additional Figure 7-b:
The cluster efficiency in bins of hadronic and EM energy in region B, estimated with LLPs decaying to ${{\tau}^{+} {\tau}^{-}}$. The sample contains equal fractions of events with LLP mass of 7, 15, 40, and 55 GeV and LLP lifetime of 0.1, 1, 10, and 100m. The first hadronic energy bins correspond to LLPs that decayed leptonically with 0 hadronic energy. Region B is defined as 671 cm $ < |z| < $ 1100 cm, $r < $ 695.5 cm and $|\eta | < $ 2. The cluster efficiency includes all cluster-level selections described in the paper, except for the jet veto, time cut, and $\Delta \phi ({\vec{p}_{\mathrm {T}}^{\,\text {miss}}},\mathrm {cluster})$ cut. The full simulation signal yield prediction for samples with various LLP mass between 7 - 55 GeV, lifetime between 0.1 - 100 m, and decay mode to $ {{\mathrm {d}} {\overline {\mathrm {d}}}} $ and $ {{\tau}^{+} {\tau}^{-}} $ can be reproduced using this parameterization to within 20% for region B. The statistical uncertainty for each bin is documented in Figure 7 of the HEPData record of this paper. |
png pdf |
Additional Figure 8:
The efficiency of $N_{station} > 1$ requirement in bins of hadronic energy in region B, estimated with LLPs decaying to ${{\tau}^{+} {\tau}^{-}}$. The sample contains equal fractions of events with LLP mass of 7, 15, 40, and 55 GeV and LLP lifetime of 0.1, 1, 10, and 100m. This efficiency is independent of the EM energy. The first hadronic energy bin corresponds to LLPs that decayed leptonically with 0 hadronic energy. Region B is defined as 671 cm $ < |z| < $ 1100 cm, $r < $ 695.5 cm and $|\eta | < $ 2. The efficiency is calculated with respect to clusters that pass all the cluster-level cuts described in the paper, except for the jet veto, time cut, and $\Delta \phi ({\vec{p}_{\mathrm {T}}^{\,\text {miss}}},\mathrm {cluster})$ cut. The full simulation signal yield prediction for samples with various LLP mass between 7 - 55 GeV, lifetime between 0.1 - 100 m, and decay mode to $ {{\mathrm {d}} {\overline {\mathrm {d}}}} $ and $ {{\tau}^{+} {\tau}^{-}} $ can be reproduced using this parameterization to within 10%. |
png pdf |
Additional Figure 9:
The geometric signal acceptance as a function of $c\tau $. The acceptance is shown for four different LLP mass hypotheses: 7 (green), 15 (black), 40 (red), and 55 (blue) GeV. The acceptance is defined by requiring at least 1 LLP to decay in the region defined as 400 cm $ < |z| < $ 1100 cm, $r < $ 695.5 cm, and $|\eta | < $ 2.4. |
| Instructions for Reinterpretation |
|
We provide signal efficiency parameterization for the cluster-level selections that allows for reproduction of the full-simulation signal yield for various LLP masses (7-55 GeV), lifetimes (0.1 - 100 m) and decay modes (dd̅ and τ+τ-). In order to recast this analysis, only the generator level LLP hadronic energy, EM energy, and decay position are needed. The following selection efficiencies are needed to account for all cluster-level selections mentioned in the paper:
cut_based_id.py attached. The function uses the LLP decay position to predict the average station of the cluster (AvgStation function in the code) and the Nstation > 1 efficiency parameterization (Additional Figure 8). As described in the paper, the cluster ID requirement applies different η cuts depending on the Nstation and average station number. We need a parametrization of the efficiency of the Nstation > 1 requirement and a transfer function that takes gen-level LLP decay position to RECO-level cluster average station (only for clusters with Nstation = 1 ). Since the entire region A is in an η region (|η| < 1.3) that passes all the η selections used in the cut-based ID (tightest cut is |η| < 1.6), the cut-based ID efficiency in this region is 100%. The focus of the parameterization will be on region B only. The efficiency of Nstation > 1 in region B is provided in bins of LLP hadronic energy (Additional Figure 8) and the efficiency is independent of the LLP EM energy. The average station transfer function is provided as the AvgStation function in the code attached. The full simulation signal yield prediction for samples with various LLP mass between 7 - 55 GeV, lifetime between 0.1 - 100 m, and decay mode to dd̅ and τ+τ- can be reproduced using this parameterization to within 10%.
|
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