Diphoton decays of stoponium at the Large Hadron Collider

Abstract

If the lighter top squark has no kinematically allowed two-body decays that conserve flavor, then it will form hadronic bound states. This is required in models that are motivated by the supersymmetric little hierarchy problem and obtain the correct thermal relic abundance of dark matter by top-squark-mediated neutralino annihilations, or by top-squark-neutralino co-annihilations. It is also found in models that can accommodate electroweak-scale baryogenesis within minimal supersymmetry. I study the prospects for detecting scalar stoponium from its diphoton decay mode at the Large Hadron Collider, updating and correcting previous work. Under favorable circumstances, this signal will be observable over background, enabling a uniquely precise measurement of the superpartner masses through a narrow peak in the diphoton invariant mass spectrum, limited by statistics and electromagnetic calorimeter resolutions.

Figures (9)

• Figure 1: The binding energies for the 1S, 2S, and 1P stoponium states as a function of the stoponium mass, as computed from the potential model of ref. Hagiwara:1990sq with $\Lambda^{(4)}_{\rm \overline{MS}} = 300$ MeV.
• Figure 2: The differential cross-section $d\sigma/dM_{\gamma\gamma}$ for the diphoton backgrounds in $pp$ collisions at $\sqrt{s} = 14$ TeV, due to the parton-level processes $q\bar{q} \rightarrow \gamma\gamma$ and $gg \rightarrow \gamma\gamma$, at leading order. Here $M_{\gamma\gamma}$ is the diphoton invariant mass. The cuts imposed on the angle with respect to the beam axis are $|\cos\theta_*| < 0.7$ in the diphoton center-of-momentum frame and $|\cos\theta| < 0.95$ in the lab frame. The kink in the $gg$ background at $M_{\gamma\gamma} = 2 m_t$ is due to the threshold in the top-quark box loop.
• Figure 3: The $pp \rightarrow \eta_{\tilde{t}} \rightarrow \gamma\gamma$ cross-section and the irreducible background in a bin with $|M_{\gamma\gamma} - m_{\eta_{\tilde{t}}}| < 0.02 m_{\eta_{\tilde{t}}}$, for $pp$ collisions at $\sqrt{s} = 14$ TeV, as a function of the stoponium mass $m_{\eta_{\tilde{t}}}$. Both are computed at leading order, with the same angular cuts as in Figure \ref{['fig:back']}. The signal assumes an idealized limit in which BR($\eta_{\tilde{t}} \rightarrow gg$) $+$ BR($\eta_{\tilde{t}} \rightarrow \gamma\gamma$) is 100%.
• Figure 4: Total integrated luminosity yielding an expected $S/\sqrt{B} = 2,3,4,5$ for $M_{\gamma\gamma}$ in a bin with $|M_{\gamma\gamma} - m_{\eta_{\tilde{t}}}| < 0.02 m_{\eta_{\tilde{t}}}$, as a function of the stoponium mass $m_{\eta_{\tilde{t}}}$, for $pp$ collisions at $\sqrt{s} = 14$ TeV, in the idealized limit that BR($\eta_{\tilde{t}} \rightarrow gg$) $+$ BR($\eta_{\tilde{t}} \rightarrow \gamma\gamma$) is 100%. The integrated luminosity needed to achieve a given $S/\sqrt{B}$ can be obtained by scaling by $1/[{\rm BR}(\eta_{\tilde{t}} \rightarrow gg)]^2$.
• Figure 5: The branching ratios of scalar stoponium into the most important final states, for some representative models of the type described in the text. In all cases, $\tan\beta = 10$, $\mu>0$, and $M_1$ varies, with $m_0$ adjusted to give $\Omega_{\rm DM} h^2 = 0.11$. The upper left, upper right, lower left, and lower right panels have respectively $(C_{24}, -A_0/M_1) = (0.19, 1)$, $(0.21, 1)$, $(0.21, 1.5)$, and $(0.24, 1.5)$. The thicker part of the $gg$ line indicates the range of stoponium mass for which stop-mediated annihilations $\tilde{N}_1 \tilde{N}_1 \rightarrow t \overline t$ contribute more than 50% to $1/\Omega_{\rm DM} h^2$.