CMSEXO20015 ; CERNEP2021125  
Search for longlived particles decaying in the CMS endcap muon detectors in protonproton collisions at $\sqrt{s} = $ 13 TeV  
CMS Collaboration  
10 July 2021  
Phy. Rev. Lett. 127 (2021) 261804  
Abstract: A search for longlived particles (LLPs) produced in decays of standard model (SM) Higgs bosons is presented. The data sample consists of 137 fb$^{1}$ of protonproton collisions at $\sqrt{s} = $ 13 TeV, recorded at the LHC in 20162018. 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: eprint arXiv:2107.04838 [hepex] (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 110 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 2a:
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 2b:
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 3a:
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 3b:
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, protonproton collision data at $\sqrt{s} = $ 13 TeV recorded by the CMS experiment in 20162018, corresponding to an integrated luminosity of 137 fb$^{1}$, have been used to conduct the first search for beyond the standard model (SM) longlived particles (LLPs) using the CMS endcap muon detectors as a calorimeter. Based on a unique detector signature, the search is largely modelindependent, 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 2a:
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 2b:
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), intime data ($N_\mathrm {hits} < $ 130), and outoftime 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 intime data distributions include events from regions B and C, and the OOT data are outoftime clusters from previous bunch crossings, with the yield normalized to that of the intime distribution. The last bin in the $N_\mathrm {hits}$ distributions includes all events in the overflow. 
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Additional Figure 3a:
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), intime data ($N_\mathrm {hits} < $ 130), and outoftime 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 intime data distributions include events from regions B and C, and the OOT data are outoftime clusters from previous bunch crossings, with the yield normalized to that of the intime distribution. The last bin includes all events in the overflow. 
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Additional Figure 3b:
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), intime data ($N_\mathrm {hits} < $ 130), and outoftime 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 intime data distributions include events from regions B and C, and the OOT data are outoftime clusters from previous bunch crossings, with the yield normalized to that of the intime 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 $x, y, z =$ $$364, 92, 796 cm and produced a CSC hit cluster with 711 hits (orange dots). The green lines represent the tracks. The yellow lines represent jets. The red arrow represents the MET direction. The red and blue cones represent the ECAL and HCAL energy deposits, respectively. 
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Additional Figure 4a:
The event display of a simulated signal event for $m_ {\mathrm {S}} = $ 40 GeV, $c\tau =$ 1 m, and $ {\mathrm {S}} \rightarrow {{\mathrm {b}} {\overline {\mathrm {b}}}} $ in $rz$plane. The LLP decayed at the position $x, y, z =$ $$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 4b:
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 6a:
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 6b:
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 clusterlevel 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 7a:
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 clusterlevel 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. 
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Additional Figure 7b:
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 clusterlevel 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. 
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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 clusterlevel 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 clusterlevel selections that allows for reproduction of the fullsimulation signal yield for various LLP masses (755 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 clusterlevel selections mentioned in the paper:
The LLP EM and hadronic energy is assigned by matching stable (status 1) GenParticles that are produced 0.1 m from the LLP decay vertex. Neutral pions, electrons, and photons are assigned as EM energy. All other particles, except for neutrinos and muons, are assigned as hadronic energy. Neutrinos and muons are ignored because they do not produce showers in the muon system. The LLP decay region is categorized into 2 regions. These 2 regions have qualitatively different behavior. Within each region, they have quantitatively similar behavior, so we will provide the efficiency parameterization for each region separately. 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 η < 2. The fraction of LLPs that decay in each region are dependent on the LLP mass and cτ. However, the signal efficiency in B is much larger than A, so for the models considered, more than 90% of clusters passing all selections are from LLPs that decay in region B. The cluster efficiency is parameterized in bins of LLP hadronic energy and EM energy in each LLP decay region (Additional Figure 7). The efficiency includes clusterlevel selections mentioned in the paper, including the segment and rechit vetos, muon veto, time spread cut, and N_{hits} ≥ 130, except for the jet veto, time cut, and Δφ(p_{T}^{miss}, cluster) cut. These latter requirements are model dependent, but they can be calculated accurately for each model using generator level information. When recasting the analysis, these additional selections need to be implemented, to be consistent with applying all selections described in the paper. 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 35% and 20% for region A and B, respectively. The parameterization of the cutbased ID is provided in python file 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 N_{station} > 1 efficiency parameterization (Additional Figure 8). As described in the paper, the cluster ID requirement applies different η cuts depending on the N_{station} and average station number. We need a parametrization of the efficiency of the N_{station} > 1 requirement and a transfer function that takes genlevel LLP decay position to RECOlevel cluster average station (only for clusters with N_{station} = 1 ). Since the entire region A is in an η region (η < 1.3) that passes all the η selections used in the cutbased ID (tightest cut is η < 1.6), the cutbased ID efficiency in this region is 100%. The focus of the parameterization will be on region B only. The efficiency of N_{station} > 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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