Search for long-lived particles using delayed photons in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$

A search for long-lived particles decaying to photons and weakly interacting particles, using proton-proton collision data at $\sqrt{s}=13\mathrm{TeV}$ collected by the CMS experiment in 2016–2017 is presented. The data set corresponds to an integrated luminosity of $77.4{\mathrm{fb}}^{-1}$. 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.

Figure 1

Example Feynman diagrams for SUSY processes that result in diphoton (left) and single photon (middle and right) final states via squark (upper) and gluino (lower) pair-production at the LHC.

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. The horizontal bars on the data represent the bin widths, which are treated as uncertainties in the fit.

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: $\mathrm{\Lambda }=200\mathrm{TeV}$, $c\tau =2\mathrm{m}$). The $p_{\mathrm{T}}^{\text{miss}}$ distribution for data is separated into events with $t_{\gamma }\ge 1\mathrm{ns}$ (blue, darker) and $t_{\gamma }<1\mathrm{ns}$ (red, lighter), scaled to match the total number of events with $t_{\gamma }\ge 1\mathrm{ns}$. The $t_{\gamma }$ distribution for data is separated into events with $p_{\mathrm{T}}^{\mathrm{miss}}\ge 100\mathrm{GeV}$ (blue, darker) and $p_{\mathrm{T}}^{\mathrm{miss}}<100\mathrm{GeV}$ (red, lighter), scaled to match the total number of events with $p_{\mathrm{T}}^{\mathrm{miss}}\ge 100\mathrm{GeV}$. The signal (black, dotted) is shown in the left plot only for events with $t_{\gamma }\ge 1\mathrm{ns}$, and in the right plot only for events with $p_{\mathrm{T}}^{\mathrm{miss}}\ge 100\mathrm{GeV}$. 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.

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: $\mathrm{\Lambda }=200\mathrm{TeV}$, $c\tau =2\mathrm{m}$). The $p_{\mathrm{T}}^{\text{miss}}$ distribution for data is separated into events with $t_{\gamma }\ge 1\mathrm{ns}$ (blue, darker) and $t_{\gamma }<1\mathrm{ns}$ (red, lighter), scaled to match the total number of events with $t_{\gamma }\ge 1\mathrm{ns}$. The $t_{\gamma }$ distribution for data is separated into events with $p_{\mathrm{T}}^{\mathrm{miss}}\ge 100\mathrm{GeV}$ (blue, darker) and $p_{\mathrm{T}}^{\mathrm{miss}}<100\mathrm{GeV}$ (red, lighter), scaled to match the total number of events with $p_{\mathrm{T}}^{\mathrm{miss}}\ge 100\mathrm{GeV}$. The signal (black, dotted) is shown in the left plots only for events with $t_{\gamma }\ge 1\mathrm{ns}$, and in the right plots only for events with $p_{\mathrm{T}}^{\mathrm{miss}}\ge 100\mathrm{GeV}$. 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.

Figure 5

The 95% C.L. exclusion contours for the GMSB neutralino production cross section, shown as functions of the neutralino mass, or equivalently the SUSY breaking scale, $\mathrm{\Lambda }$, in the GMSB SPS8 model, and the neutralino proper decay length, ${c\tau }_{{\tilde{\chi }}_1^0}$.