Same-sign charginos and Majorana neutralinos at the CERN LHC

We demonstrate the possibility of studying weakly interacting new particles in weak-boson fusion, using the example of supersymmetric same-sign charginos. This signal could establish the existence of Majorana neutralinos and give access to their electroweak couplings. It can be observed over (supersymmetric) QCD backgrounds provided the charginos are light and not too close to the squark mass. We finally show how same-sign fermion production can be distinguished from same-sign scalars or vectors arising in other models of new physics.

Figure 1

Feynman diagram for the pure WBF SUSY process $q{q}^{\prime }\to q{q}^{\prime }{\tilde{\chi }}_1^{+}{\tilde{\chi }}_1^{+}$ as described in the text. The complete set of diagrams sums over all Majorana neutralinos in the $t$-channel.

Figure 2

Representative Feynman diagrams for the electroweak non-WBF SUSY process $q{q}^{\prime }\to q{q}^{\prime }{\tilde{\chi }}_1^{+}{\tilde{\chi }}_1^{+}$. The complete set sums over all Majorana neutralinos in the $t$-channel.

Figure 3

Representative Feynman diagrams for the QCD SUSY process $q{q}^{\prime }\to q{q}^{\prime }{\tilde{\chi }}_1^{+}{\tilde{\chi }}_1^{+}$.

Figure 4

Distributions for the WBF same-sign chargino signal at SPS9, including the electroweak and QCD backgrounds from squark production. The jets are ordered according to their transverse momentum. Only WBF cuts Eqs. (1, 2) are used. The invariant-mass distribution can be used for long-lived charginos only. All distributions are normalized to unity; see Table  for total rates.

Figure 5

Kinematic distributions for the tagging jets in WBF production of charginos, scalars, and vectors. None of the distributions rely on the charged particles being long-lived.

Figure 6

Distributions comparing charged scalar pair production with a single neutral scalar of 500 GeV, to demonstrate identical spin structure.

Figure 7

Comparison between standard model $W$ pairs, $W^{\prime }$ pairs, both with the full set of Feynman diagrams and the $W^{\prime }$ case (wrongly) omitting all heavy-quark diagrams. Bad high-energy behavior can clearly be seen in the transverse momentum distribution of the left panel. The lack of gauge cancellations also leads to incorrect angular distributions (right).