A Heavy Gluino as the Lightest Supersymmetric Particle

TL;DR

The paper investigates whether a heavy gluino can be the lightest supersymmetric particle (LSP) by examining both cosmological relic-density considerations and collider signatures. It combines nonperturbative QCD effects (e.g., Sommerfeld enhancements) with a Boltzmann-based relic-density calculation to show that $ m \Omega h^2$ can vary over many orders of magnitude, leaving cosmological constraints ambiguous without a clearer understanding of low-energy gluino interactions. On the collider side, the authors model heavy gluino hadrons (R-hadrons) propagating through detectors, emphasizing how the observable jet energy and missing momentum depend on the charged-fragmentation probability $P$ and on hadronic-interaction lengths, and they reinterpret LEP and Tevatron data in light of these signatures. They derive bounds from LEP/LEP2 on light gluinos, and from Tevatron RunI data on moderate-to-heavy gluinos, with RunII extending the reach; in the NLSP scenario decaying to a gluon plus gravitino, they obtain strong exclusions up to about 280 GeV. The work also outlines how the jet-plus-missing-momentum strategy can be adapted to other stable strongly interacting particles, and discusses complementary heavy-ionizing-track searches; collectively these results map out the viable parameter space for a heavy gluino LSP and guide future experimental searches.

Abstract

We consider the possibility that the lightest supersymmetric particle is a heavy gluino. After discussing models in which this is the case, we demonstrate that the gluino-LSP could evade cosmological and other constraints by virtue of having a very small relic density. We then consider how neutral and charged hadrons containing a gluino will behave in a detector, demonstrating that there is generally substantial apparent missing momentum associated with a produced gluino-LSP. We next investigate limits on a (heavy) gluino-LSP deriving from LEP, LEP2 and RunI Tevatron experimental searches for excess events in the jets plus missing momentum channel and for stable heavily-ionizing charged particles. The range of gluino mass that can be excluded depends upon the path length of the gluino in the detector, the amount of energy it deposits in each hadronic collision, and the probability for the gluino to fragment to a pseudo-stable charged hadron after a given hadronic collision. We explore how the range of excluded gluino mass depends upon these ingredients, concluding that for non-extreme cases the range $3\gev\lsim\mgl\lsim 130-150\gev$ can be excluded at 95% CL based on currently available OPAL and CDF analyses. We find that RunII at the Tevatron can extend the excluded region (or discover the gluino) up to $\mgl\sim 160-180\gev$. For completeness, we also analyze the case where the gluino is the NLSP (as possible in gauge-mediated supersymmetry breaking) decaying via gluino -> g + gravitino. We find that the Tevatron RunI data excludes $\mgl\leq 240$ GeV. Finally, we discuss application of the procedures developed for the heavy gluino-LSP to searches for other stable strongly interacting particles, such as a stable heavy quark.

Figures (32)

• Figure 1: $T_F/m_{\widetilde{g}}$ as a function of $m_{\widetilde{g}}$ for the 11 cases described in the text. The solid lines correspond to results for cases (1), (2) and (3), respectively, in order of decreasing $T_F$. Results for cases (4) (I,a,i) (5) (II,a,i) (6) (I,b,i) (7) (II,b,i) are the lower dashed, dotted, dot-dot-dash and dash-dot lines, respectively. Results for cases (8) (I,a,ii) (9) (II,a,ii) (10) (I,b,ii) and (11) (II,b,ii) are the upper dashed, dotted, dot-dot-dash and dash-dot lines, respectively. This figure assumes $L=1~{\rm GeV}$, see text.
• Figure 2: $\Omega h^2$ as a function of $m_{\widetilde{g}}$ for the 11 cases described in the text. Line notation as in Fig. \ref{['relicxf']}, with solid lines for cases (1), (2) and (3) in order of decreasing $\Omega h^2$.
• Figure 3: Average energy loss, $\langle \Delta E \rangle$, in a collision as a function of $\beta$ for the three cases described in the text. Results are shown for $m_{R^0}=5$, $25$ and $140~{\rm GeV}$. At high $\beta$, curves are ordered according to increasing $m_{R^0}$.
• Figure 4: We plot the energy of an incident 100 GeV pion after a certain number of hadronic collisions for the case (1) and (2) cross section models.
• Figure 5: We plot the number of $\lambda_I=17$ cm ( i.e. in iron) path lengths required for 95% containment of the energy of a pion. Experimental results from the PDG, Fig. 24.2 of Ref. pdg, are compared to predictions based on Eq. (\ref{['veve']}) for the case (1) and (2) cross section models.