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| CMS-EXO-19-005 ; CERN-EP-2019-185 | ||
| Search for long-lived particles using delayed photons in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 13 September 2019 | ||
| Phys. Rev. D 100 (2019) 112003 | ||
| Abstract: A search for long-lived particles decaying to photons and weakly interacting particles, using proton-proton collision data at $\sqrt{s} = $ 13 TeV collected by the CMS experiment in 2016-2017 is presented. The data set corresponds to an integrated luminosity of 77.4 fb$^{-1}$. Results are interpreted in the context of supersymmetry with gauge-mediated supersymmetry breaking, where the neutralino is long-lived and decays to a photon and a gravitino. Limits are presented as a function of the neutralino proper decay length and mass. For neutralino proper decay lengths of 0.1, 1, 10, and 100 m, masses up to 320, 525, 360, and 215 GeV are excluded at 95% confidence level, respectively. We extend the previous best limits in the neutralino proper decay length by up to one order of magnitude, and in the neutralino mass by up to 100 GeV. | ||
| Links: e-print arXiv:1909.06166 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Example Feynman diagrams for SUSY processes that result in a diphoton (left) and single photon (middle and right) final state via squark (upper) and gluino (lower) pair-production at the LHC. |
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Figure 1-a:
Example Feynman diagrams for SUSY processes that result in a diphoton (left) and single photon (middle and right) final state via squark (upper) and gluino (lower) pair-production at the LHC. |
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Figure 1-b:
Example Feynman diagrams for SUSY processes that result in a diphoton (left) and single photon (middle and right) final state via squark (upper) and gluino (lower) pair-production at the LHC. |
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Figure 1-c:
Example Feynman diagrams for SUSY processes that result in a diphoton (left) and single photon (middle and right) final state via squark (upper) and gluino (lower) pair-production at the LHC. |
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Figure 1-d:
Example Feynman diagrams for SUSY processes that result in a diphoton (left) and single photon (middle and right) final state via squark (upper) and gluino (lower) pair-production at the LHC. |
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Figure 1-e:
Example Feynman diagrams for SUSY processes that result in a diphoton (left) and single photon (middle and right) final state via squark (upper) and gluino (lower) pair-production at the LHC. |
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Figure 1-f:
Example Feynman diagrams for SUSY processes that result in a diphoton (left) and single photon (middle and right) final state via squark (upper) and gluino (lower) pair-production at the LHC. |
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Figure 2:
The time resolution between two neighboring ECAL crystals as a function of the effective amplitudes of the signals in the two crystals for the 2016 and 2017 data sets. The lines shown reflect the fits described in the text. |
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Figure 3:
The ${{p_{\mathrm {T}}} ^\text {miss}}$ (left) and ${t_{\gamma}}$ (right) distributions for the 2016 event selection, shown for data and a representative signal benchmark (GMSB: $\Lambda =$ 200 TeV, ${c\tau}=$ 2 m). The ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution for data is separated into events with $ {t_{\gamma}}\geq $ 1 ns (blue, darker) and $ {t_{\gamma}} < $ 1 ns (red, lighter), scaled to match the total number of events with $ {t_{\gamma}}\geq $ 1 ns. Signal (black, dotted) is shown only for events with $ {t_{\gamma}}\geq $ 1 ns. The ${t_{\gamma}}$ distribution for data is separated into events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV (blue, darker) and $ {{p_{\mathrm {T}}} ^\text {miss}} < $ 100 GeV (red, lighter), scaled to match the total number of events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. Signal (black, dotted) is shown only for events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. The entries in each bin are normalized by the bin width. The horizontal bars on data indicate the bin boundaries. The last bin in each plot includes overflow events. |
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Figure 3-a:
The ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution for the 2016 event selection, shown for data and a representative signal benchmark (GMSB: $\Lambda =$ 200 TeV, ${c\tau}=$ 2 m). The ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution for data is separated into events with $ {t_{\gamma}}\geq $ 1 ns (blue, darker) and $ {t_{\gamma}} < $ 1 ns (red, lighter), scaled to match the total number of events with $ {t_{\gamma}}\geq $ 1 ns. Signal (black, dotted) is shown only for events with $ {t_{\gamma}}\geq $ 1 ns. The ${t_{\gamma}}$ distribution for data is separated into events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV (blue, darker) and $ {{p_{\mathrm {T}}} ^\text {miss}} < $ 100 GeV (red, lighter), scaled to match the total number of events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. Signal (black, dotted) is shown only for events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. The entries in each bin are normalized by the bin width. The horizontal bars on data indicate the bin boundaries. The last bin includes overflow events. |
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Figure 3-b:
The ${t_{\gamma}}$ distribution for the 2016 event selection, shown for data and a representative signal benchmark (GMSB: $\Lambda =$ 200 TeV, ${c\tau}=$ 2 m). The ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution for data is separated into events with $ {t_{\gamma}}\geq $ 1 ns (blue, darker) and $ {t_{\gamma}} < $ 1 ns (red, lighter), scaled to match the total number of events with $ {t_{\gamma}}\geq $ 1 ns. Signal (black, dotted) is shown only for events with $ {t_{\gamma}}\geq $ 1 ns. The ${t_{\gamma}}$ distribution for data is separated into events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV (blue, darker) and $ {{p_{\mathrm {T}}} ^\text {miss}} < $ 100 GeV (red, lighter), scaled to match the total number of events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. Signal (black, dotted) is shown only for events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. The entries in each bin are normalized by the bin width. The horizontal bars on data indicate the bin boundaries. The last bin includes overflow events. |
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Figure 4:
The ${{p_{\mathrm {T}}} ^\text {miss}}$ (left) and ${t_{\gamma}}$ (right) distributions for the 2017$\gamma$ (upper row) and 2017$\gamma \gamma$ (lower row) event selections shown for data and a representative signal benchmark (GMSB: $\Lambda =$ 200 TeV, ${c\tau}=$ 2 m). The ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution for data is separated into events with $ {t_{\gamma}}\geq $ 1 ns (blue, darker) and $ {t_{\gamma}} < $ 1 ns (red, lighter), scaled to match the total number of events with $ {t_{\gamma}}\geq $ 1 ns. Signal (black, dotted) is shown only for events with $ {t_{\gamma}}\geq $ 1 ns. The ${t_{\gamma}}$ distribution for data is separated into events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV (blue, darker) and $ {{p_{\mathrm {T}}} ^\text {miss}} < $ 100 GeV (red, lighter), scaled to match the total number of events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. Signal (black, dotted) is shown only for events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. The entries in each bin are normalized by the bin width. The horizontal bars on data indicate the bin boundaries. The last bin in each plot includes overflow events. |
png pdf |
Figure 4-a:
The ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution for the 2017$\gamma$ event selection shown for data and a representative signal benchmark (GMSB: $\Lambda =$ 200 TeV, ${c\tau}=$ 2 m). The ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution for data is separated into events with $ {t_{\gamma}}\geq $ 1 ns (blue, darker) and $ {t_{\gamma}} < $ 1 ns (red, lighter), scaled to match the total number of events with $ {t_{\gamma}}\geq $ 1 ns. Signal (black, dotted) is shown only for events with $ {t_{\gamma}}\geq $ 1 ns. The ${t_{\gamma}}$ distribution for data is separated into events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV (blue, darker) and $ {{p_{\mathrm {T}}} ^\text {miss}} < $ 100 GeV (red, lighter), scaled to match the total number of events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. Signal (black, dotted) is shown only for events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. The entries in each bin are normalized by the bin width. The horizontal bars on data indicate the bin boundaries. The last bin includes overflow events. |
png pdf |
Figure 4-b:
The ${t_{\gamma}}$ distribution for the 2017$\gamma$ event selection shown for data and a representative signal benchmark (GMSB: $\Lambda =$ 200 TeV, ${c\tau}=$ 2 m). The ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution for data is separated into events with $ {t_{\gamma}}\geq $ 1 ns (blue, darker) and $ {t_{\gamma}} < $ 1 ns (red, lighter), scaled to match the total number of events with $ {t_{\gamma}}\geq $ 1 ns. Signal (black, dotted) is shown only for events with $ {t_{\gamma}}\geq $ 1 ns. The ${t_{\gamma}}$ distribution for data is separated into events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV (blue, darker) and $ {{p_{\mathrm {T}}} ^\text {miss}} < $ 100 GeV (red, lighter), scaled to match the total number of events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. Signal (black, dotted) is shown only for events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. The entries in each bin are normalized by the bin width. The horizontal bars on data indicate the bin boundaries. The last bin includes overflow events. |
png pdf |
Figure 4-c:
The ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution for the 2017$\gamma \gamma$ event selection shown for data and a representative signal benchmark (GMSB: $\Lambda =$ 200 TeV, ${c\tau}=$ 2 m). The ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution for data is separated into events with $ {t_{\gamma}}\geq $ 1 ns (blue, darker) and $ {t_{\gamma}} < $ 1 ns (red, lighter), scaled to match the total number of events with $ {t_{\gamma}}\geq $ 1 ns. Signal (black, dotted) is shown only for events with $ {t_{\gamma}}\geq $ 1 ns. The ${t_{\gamma}}$ distribution for data is separated into events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV (blue, darker) and $ {{p_{\mathrm {T}}} ^\text {miss}} < $ 100 GeV (red, lighter), scaled to match the total number of events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. Signal (black, dotted) is shown only for events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. The entries in each bin are normalized by the bin width. The horizontal bars on data indicate the bin boundaries. The last bin includes overflow events. |
png pdf |
Figure 4-d:
The ${t_{\gamma}}$ distribution for the 2017$\gamma \gamma$ event selection shown for data and a representative signal benchmark (GMSB: $\Lambda =$ 200 TeV, ${c\tau}=$ 2 m). The ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution for data is separated into events with $ {t_{\gamma}}\geq $ 1 ns (blue, darker) and $ {t_{\gamma}} < $ 1 ns (red, lighter), scaled to match the total number of events with $ {t_{\gamma}}\geq $ 1 ns. Signal (black, dotted) is shown only for events with $ {t_{\gamma}}\geq $ 1 ns. The ${t_{\gamma}}$ distribution for data is separated into events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV (blue, darker) and $ {{p_{\mathrm {T}}} ^\text {miss}} < $ 100 GeV (red, lighter), scaled to match the total number of events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. Signal (black, dotted) is shown only for events with $ {{p_{\mathrm {T}}} ^\text {miss}} \geq $ 100 GeV. The entries in each bin are normalized by the bin width. The horizontal bars on data indicate the bin boundaries. The last bin includes overflow events. |
png pdf |
Figure 5:
The 95% CL exclusion contours for the GMSB neutralino production cross section, shown as functions of the neutralino mass, or equivalently the SUSY breaking scale, $\Lambda $, in the GMSB SPS8 model, and the neutralino proper decay length, $ {c\tau}_{\tilde{\chi}^0_1}$. |
| Tables | |
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Table 1:
The fitted ECAL timing resolution parameters for the 2016 and 2017 data sets. |
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Table 2:
The optimized bin boundaries for ${t_{\gamma}}$ (first number, in units of ns) and ${{p_{\mathrm {T}}} ^\text {miss}}$ (second number, in units of GeV), for different GMSB SPS8 signal model benchmark points considered in the search and for each data set category. |
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Table 3:
Summary of systematic uncertainties in the analysis. Also included are notes on whether each source affects signal yields (Sig) or background (Bkg) estimates, to which bins each uncertainty applies, and how the correlations of the uncertainties between the different data sets are treated. We assign different values for the uncertainty in the closure of the background prediction for short and long lifetime signal models. The column labeled 2017 includes both the 2017$\gamma$ and 2017$\gamma \gamma$ categories. |
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Table 4:
Observed number of events (${N_{\mathrm {obs}}^{\text {data}}}$) and predicted background yields from the background-only fit (${N_{\mathrm {bkg}}^{\text {post-fit}}}$) in bins A, B, C, and D in data for the 2016 category and for the different ${t_{\gamma}}$ and ${{p_{\mathrm {T}}} ^\text {miss}}$ bin boundaries summarized in Table 2. In addition, the predicted post-fit yields from the background-only fit not including bin C (${N_{\text {bkg(no C)}}^{\text {post-fit}}}$) are provided as a test of the closure. Uncertainties in the ${N_{\mathrm {bkg}}^{\text {post-fit}}}$ and ${N_{\text {bkg(no C)}}^{\text {post-fit}}}$ values are the sums in quadrature of the statistical and systematic components, with the former being dominant. |
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Table 5:
Observed number of events (${N_{\mathrm {obs}}^{\text {data}}}$) and predicted background yields from the background-only fit (${N_{\mathrm {bkg}}^{\text {post-fit}}}$) in bins A, B, C, and D in data for the 2017$\gamma$ (upper table) and 2017$\gamma \gamma$ (lower table) categories and for the different ${t_{\gamma}}$ and ${{p_{\mathrm {T}}} ^\text {miss}}$ bin boundaries summarized in Table 2. Additional details are described in the caption of Table 4. |
| Summary |
| A search for long-lived particles that decay to a photon and a weakly interacting particle has been presented. The search is based on proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS experiment in 2016-2017. The photon from this particle's decay would enter the electromagnetic calorimeter at non-normal impact angles and with delayed times, and this striking combination of features is exploited to suppress backgrounds. The search is performed using a combination of the 2016 and 2017 data sets, corresponding to a total integrated luminosity of 77.4 fb$^{-1}$. Both single-photon and diphoton event samples are used for the search, with each sample providing a complementary sensitivity at larger and smaller long-lived particle proper decay lengths, respectively. The results are interpreted in the context of supersymmetry with gauge-mediated supersymmetry breaking, using the SPS8 benchmark model. For neutralino proper decay lengths of 0.1, 1, 10, and 100{ m}, masses up to about 320, 525, 360, and 215 GeV are excluded at 95% confidence level, respectively. The previous best limits are extended by one order of magnitude in the neutralino proper decay length and by 100 GeV in the mass reach. |
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