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Register a new userMagnifying the ATLAS Stealth Stop Splinter: Impact of Spin Correlations and Finite Widths
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In this paper, we recast a stealth stop search in the notoriously difficult region of the stop-neutralino Simplified Model parameter space for which $m(tilde{t}) - m(tilde{chi}) simeq m_t$. The properties of the final state are nearly identical for tops and stops, while the rate for stop pair production is $mathcal{O}(10%)$ of that for $tbar{t}$. Stop searches away from this stealth region have left behind a splinter of open parameter space when $m(tilde{t}) simeq m_t$. Removing this splinter requires surgical precision: the ATLAS constraint on stop pair production reinterpreted here treats the signal as a contaminant to the measurement of the top pair production cross section using data from $sqrt{s} = 7 text{ TeV}$ and $8 text{ TeV}$ in a correlated way to control for some systematic errors. ATLAS fixed $m(tilde{t}) simeq m_t$ and $m(tilde{chi})= 1 text{ GeV}$, implying that a careful recasting of these results into the full $m(tilde{t}) - m(tilde{chi})$ plane is warranted. We find that the parameter space with $m(tilde{chi})lesssim 55 text{ GeV}$ is excluded for $m(tilde{t}) simeq m_t$ --- although this search does cover new parameter space, it is unable to fully pull the splinter. Along the way, we review a variety of interesting physical issues in detail: (i) when the two-body width is a good approximation; (ii) what the impact on the total rate from taking the narrow width is a good approximation; (iii) how the production rate is affected when the wrong widths are used; (iv) what role the spin correlations play in the limits. In addition, we provide a guide to using MadGraph for implementing the full production including finite width and spin correlation effects, and we survey a variety of pitfalls one might encounter.
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The discovery of the stop - the Supersymmetric partner of the top quark - is a key goal of the physics program enabled by the Large Hadron Collider. Although much of the accessible parameter space has already been probed, all current searches assume the top mass is known. This is relevant for the stealth stop regime, which is characterized by decay kinematics that force the final state top quark off its mass shell; such decays would contaminate the top mass measurements. We investigate the resulting bias imparted to the template method based ATLAS approach. A careful recasting of these results shows that effect can be as large as 2.0 GeV, comparable to the current quoted uncertainty on the top mass. Thus, a robust exploration of the stealth stop splinter requires the simultaneous consideration of the impact on the top mass. Additionally, we explore the robustness of the template technique, and point out a simple strategy for improving the methodology implemented for the semi-leptonic channel.
The top squarks (stops) may be the most wanted particles after the Higgs boson discovery. The searches for the lightest stop have put strong constraints on its mass. However, there is still a search gap in the low mass region if the spectrum of the stop and the lightest neutralino is compressed. In that case, it may be easier to look for the second stop since naturalness requires both stops to be close to the weak scale. The current experimental searches for the second stop are based on the simplified model approach with the decay modes $tilde{t}_2 to tilde{t}_1 Z$ and $tilde{t}_2 to tilde{t}_1 h$. However, in a realistic supersymmetric spectrum there is always a sbottom lighter than the second stop, hence the decay patterns are usually more complicated than the simplified model assumptions. In particular, there are often large branching ratios of the decays $tilde{t}_2 to tilde{b}_1 W$ and $tilde{b}_1 to tilde{t}_1 W$ as long as they are open. The decay chains can be even more complex if there are intermediate states of additional charginos and neutralinos in the decays. By studying several MSSM benchmark models at the 14 TeV LHC, we point out the importance of the multi-$W$ final states in the second stop and the sbottom searches, such as the same-sign dilepton and multilepton signals, aside from the traditional search modes. The observed same-sign dilepton excesses at LHC Run 1 and Run 2 may be explained by some of our benchmark models. We also suggest that the vector boson tagging and a new kinematic variable may help to suppress the backgrounds and increase the signal significance for some search channels. Due to the complex decay patterns and lack of the dominant decay channels, the best reaches likely require a combination of various search channels at the LHC for the second stop and the lightest sbottom.
Recently ATLAS reported a $3.3sigma$ excess in the stop search with $ell+jets+E_T^{miss}$ channel. We try to interpret the signal by a light stop pair production in the MSSM. We find: (1) simple models where stop decays into a higgsino or a bino are not favored. (2) an extension of them can explain the data at $2sigma$ level without conflicting with the other search channels. A surviving possibility includes a light stop and a light higgsino, which is expected in a natural SUSY scenario.
We consider the $X(3872)$ resonance as a $J^{PC}=1^{++}$ $Dbar D^*$ hadronic molecule. According to heavy quark spin symmetry, there will exist a partner with quantum numbers $2^{++}$, $X_{2}$, which would be a $D^*bar D^*$ loosely bound state. The $X_{2}$ is expected to decay dominantly into $Dbar D$, $Dbar D^*$ and $bar D D^*$ in $d$-wave. In this work, we calculate the decay widths of the $X_{2}$ resonance into the above channels, as well as those of its bottom partner, $X_{b2}$, the mass of which comes from assuming heavy flavor symmetry for the contact terms. We find partial widths of the $X_{2}$ and $X_{b2}$ of the order of a few MeV. Finally, we also study the radiative $X_2to Dbar D^{*}gamma$ and $X_{b2} to bar B B^{*}gamma$ decays. These decay modes are more sensitive to the long-distance structure of the resonances and to the $Dbar D^{*}$ or $Bbar B^{*}$ final state interaction.
Stealth bosons are relatively light boosted particles with a cascade decay $S to A_1 A_2 to q bar q q bar q$, reconstructed as a single fat jet. In this work, we establish minimal extensions of the Standard Model that allow for such processes. Namely, we consider models containing a new (leptophobic) neutral gauge boson $Z$ and two scalar singlets, plus extra matter required to cancel the $text{U}(1)$ anomalies. Our analysis shows that, depending on the model and benchmark scenario, the expected statistical significance of stealth boson signals (yet uncovered by current searches at the Large Hadron Collider) is up to nine times larger than for the most sensitive of the standard leptophobic $Z$ signals such as dijets, $t bar t$ pairs or dibosons. These results provide strong motivation for model-independent searches that cover these complex signals.
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