Closing in on the tip of the CMSSM stau coannihilation strip

Near the tip of the $\tilde{\tau }$ coannihilation strip in the CMSSM with a neutralino lightest supersymmetric particle $\chi$, the astrophysical cold dark matter density constraint forces the $\tilde{\tau }-\chi$ mass difference to be small. If this mass difference is smaller than $m_{\tau }$, the $\tilde{\tau }$ may decay either in the outer part of an LHC detector—the “disappearing track” signature—or be sufficiently long-lived to leave the detector before decaying—the long-lived massive charged-particle signature. We combine searches for these signatures with conventional $E_T^{\text{miss}}$ searches during LHC Run 1, identifying the small remaining parts of the CMSSM $\tilde{\tau }$ coannihilation strip region that have not yet been excluded and discussing how they may be explored during Run 2 of the LHC.

Figure 1

Overviews of the regions of the CMSSM parameter space with small mass difference $\mathrm{\Delta }m\equiv m_{\tilde{\tau }}-m_{\chi }$ for $\tan \beta =10$ (upper panels) and $\tan \beta =40$ (lower panels), and for $A_0=0$ (left panels) and $2.5m_0$ (right panels). The bands with $\mathrm{\Delta }m<m_{\tau }$ are shaded beige, and the coannihilation strips where $0.125>{\mathrm{\Omega }}_{\chi }{h}^2>0.115$ as calculated using SoftSUSY 3.3.7 [23] coupled to MicrOMEGAs 3.5.5 [24] are shaded pink. The lower limit on $m_{1/2}$ from the ATLAS $E_T^{\text{miss}}$ search during Run 1 at the LHC [16] is represented in each panel by a maroon line, and contours of $m_h$ calculated using FeynHiggs 2.10.0 [22] are shown as green (dashed or dotted) lines. Parameter regions excluded by searches for the direct and total production of metastable charged particles [15] are shaded darker and lighter blue, respectively, and regions excluded by searches for particles leaving disappearing tracks are shaded grey (see Sec. 3 for details).

Figure 2

Left panel: the $\tilde{\tau }$ lifetime as a function of the ${\tilde{\tau }}_L-{\tilde{\tau }}_R$ mixing angle, ${\theta }_{\tilde{\tau }}$, for $\mathrm{\Delta }m=1.2,1.3,\cdots ,1.7\mathrm{GeV}$. Right panel: the $\tilde{\tau }$ decay branching ratios as functions of ${\theta }_{\tilde{\tau }}$ for $\mathrm{\Delta }m=1.5\mathrm{GeV}$. The blue, orange, brown, yellow, and red lines are for the final states with $a_1(1260)$, $\rho (770)$, $\pi$, $\mu$, and $e$, respectively, indicated by the labels adjacent to the corresponding curves. In both panels, a pure binolike $\chi$ with a mass of 300 GeV is assumed.

Figure 3

Some $\tilde{\tau }$ lifetime contours as functions of $m_{1/2}$ and $\mathrm{\Delta }m$, for the four choices of the CMSSM parameters used in Fig. 1, namely, $\tan \beta =10,40$ and $A_0=0,2.5m_0$. The widths of the contours span ranges of the $\tilde{\tau }$ lifetime for the different choices of $\tan \beta$ and $A_0$.

Figure 4

Scatter plot of $E_T^{\text{miss}}$ and $m_{\text{eff}}$ in the four-jet channel for CMSSM scenarios with metastable staus ($\mathrm{\Delta }m=0.5\mathrm{GeV}$, red points) and with rapid $\tilde{\tau }\to \tau +\chi$ decays ($\mathrm{\Delta }m=1.9\mathrm{GeV}$, blue points). The left plot is for $\tan \beta =10$, $A_0=0$ and the right plot is for $\tan \beta =40$, $A_0=2.5m_0$, both with $m_{1/2}=800\mathrm{GeV}$. The solid diagonal lines correspond to the ATLAS cut $E_T^{\text{miss}}>0.25m_{\text{eff}}$ [16].

Figure 5

The dependence of the fraction of ‘stable’ staus, i.e., those exiting CMS without decaying (left), and the fraction of staus decaying within the ATLAS detector (right) on the $\tilde{\tau }-\chi$ mass difference. The solid lines correspond to direct stau-pair production only whereas the dashed are for both direct and indirect stau production (i.e., all SUSY processes). The value of $m_{1/2}$ is fixed to 800 GeV.

Figure 6

Projected limits from the 14 TeV LHC Run 2 with $300/\mathrm{fb}$ integrated luminosity. The sensitivity of the $\text{jets}+E_T^{\text{miss}}$ search is sufficient to explore the rest of the coannihilation strip, and the search for metastable charged tracks from direct stau-pair production is also strong enough to explore independently the portion of the coannihilation strip where $\mathrm{\Delta }m\lesssim 1.6\mathrm{GeV}$.