Synopsis: How to find a “leptophobic” Z′ boson at the LHC

A striking new six-lepton signal is proposed to find a so-far unseen heavy spin-1 boson that does not couple directly to leptons.

The first collisions have just been observed at the Large Hadron Collider (LHC), which is now the world’s highest energy accelerator. Two of the central goals of the LHC are to find the Higgs boson and to look for physics beyond the standard model. Among its primary targets for the latter is finding a new heavy neutral spin-1 particle, called a $Z\prime$ boson, since such a particle arises in any extension of the standard model to include an additional local phase-rotation, or $\text{U}\left(1\right)$, symmetry.

Searches for a $Z\prime$ boson at hadron colliders usually look for decays of the $Z\prime$ into two leptons (e.g., an electron and a positron) because such $Z\prime$ events stand out from the flood of background events. But what if the $Z\prime$ boson, unlike the standard model $Z$ boson, is “leptophobic,” meaning it doesn’t couple very strongly to leptons? This would make the $Z\prime$ very difficult to detect. In a paper published in Physical Review Letters, Vernon Barger at the University of Wisconsin, Paul Langacker at Princeton, and Hye-Sung Lee at the University of California, Riverside, all in the US, propose an interesting new model-independent way to study such leptophobic $Z\prime$ bosons at the LHC. They show that the $Z\prime$ can decay, via a Higgs boson, into three $Z$ bosons, each of which can then decay into two leptons. So a $Z\prime$ boson that does not decay into two leptons could paradoxically be found by this indirect decay into six leptons.

There will undoubtedly be surprises at the LHC, and this striking channel could be the discovery mode for new physics, if it takes the form of a leptophobic $Z\prime$ boson. – Robert Garisto

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