Enhancing Long-Lived Particles Searches at the LHC with Precision Timing Information

We explore the physics potential of using precision timing information at the LHC in searches for long-lived particles (LLPs). In comparison with the light standard model particles, the decay products of massive LLPs arrive at detectors with time delays around the nanosecond scale. We propose new strategies to take advantage of this time delay feature by using initial state radiation to time stamp the collision event and require at least one LLP to decay within the detector. This search strategy is effective for a broad range of models. In addition to outlining this general approach, we demonstrate its effectiveness with the projected reach for two benchmark scenarios: a Higgs boson decaying into a pair of LLPs, and pair production of long-lived neutralinos in the gauge mediated supersymmetry breaking models. Our strategy increases the sensitivity to the lifetime of the LLP by two orders of magnitude or more and particularly exhibits a better behavior with a linear dependence on the lifetime in the large lifetime region compared to traditional LLP searches. The timing information significantly reduces the standard model background and provides a powerful new dimension for LLP searches.

Figure 1

An event topology with an LLP $X$ decaying into two light SM particles $a$ and $b$. A timing layer, at a transverse distance $L_{T_2}$ away from the beam axis (horizontal gray dotted line), is placed at the end of the detector volume (shaded region). The trajectory of a reference SM background particle is also shown (blue dashed line). The gray polygon indicates the primary vertex.

Figure 2

The differential $\mathrm{\Delta }t$ distribution for typical signals and backgrounds at 13 TeV LHC. The plot is normalized to the fraction of events per bin with varying bin sizes, in linear ($\mathrm{\Delta }t<1\mathrm{ns}$) and logarithmic scale ($>1\mathrm{ns}$), respectively. Two representative signal models are shown with different masses. The LLP proper lifetime is set to 10 m, and the distribution only counts events decayed within $[L_{T_1},L_{T_2}]$ of [0.2, 1.17] m in the transverse direction, following the geometry of the CMS MTD in the barrel region. For the background distribution shown in gray curves, we assume bunch spacing of 25 ns. The solid and dashed gray curves represent backgrounds from the same hard collision vertex and hence with a precision timing uncertainty of ${\delta }_t^{\mathrm{PT}}=30\mathrm{ps}$ and from the pileup with a spread of ${\delta }_t=190\mathrm{ps}$, respectively.

Figure 3

The 95% C.L. limit on $\mathrm{BR}(h\to XX)$ for signal process $pp\to jh$ with subsequent decay $h\to XX$ and $X\to jj$. Different colors indicate different masses of the particle $X$. The thick solid and dotted (thick long dashed) lines indicate MS (MTD) searches with different timing cuts. The numbers in parentheses are the assumed timing resolutions. Other 13 TeV LHC projections [29, 51] are plotted in thin lines.

Figure 4

The projected 95% C.L. limit on the Higgsino mass–lifetime plane for signal process of Higgsino pair production in association with jets, with subsequent decay of the lightest Higgsino ${\tilde{\chi }}^0\to h\tilde{G}$ and $h\to bb$ in GMSB scenario. We decouple other electroweakinos and have Higgsino-like chargino ${\tilde{\chi }}^{\pm }$ and neutralino ${\tilde{\chi }}_2^0$ nearly degenerate with ${\tilde{\chi }}_1^0$.