Heavy neutrinos: Leave no stone unturned

“Seesaw mechanism” theories explain the nearly invisible mass of the neutrinos as being balanced with a much heaver, yet undetected family of neutrinos. The CMS experiment continues to search for such heavy neutrinos. Photo: Mike Leary

“Seesaw mechanism” theories explain the nearly invisible mass of the neutrinos as being balanced with a much heaver, yet undetected family of neutrinos. The CMS experiment continues to search for such heavy neutrinos. Photo: Mike Leary

While the discovery of the Higgs boson at the LHC yielded considerable evidence that the Higgs mechanism is responsible for some particles having mass and others not, it does not help explain why massive particles have the specific masses they do. Over a decade prior to the discovery of the Higgs boson, experiments studying neutrinos produced by the sun and by particle accelerators made the astounding discovery that neutrinos have mass, albeit in incredibly tiny amounts. The question du jour about neutrino masses shifted immediately from “Do neutrinos have mass?” to “Why are neutrino masses what they are?”

Physicists naturally attack this question from as many angles as possible. A significant focus of the scientific efforts of Fermilab center on studying neutrinos produced by the Fermilab accelerator complex in order to probe this question. An experiment like CMS, designed to measure highly interactive particles, can’t directly detect neutrinos at all and might seem to be left on the sidelines in this quest. However, a popular family of theories suggests that there is an additional family of neutrino linked to the garden-variety neutrinos we know of. This linking mechanism between the known neutrinos and their exotic cousins is known as a “seesaw mechanism,” as it forces one type to become massive when the others become lightweight. Searching for unknown but massive particles is exactly what the CMS detector was designed to do.

These U.S. physicists contributed to this analysis.

These U.S. physicists contributed to this analysis.

The CMS experiment has searched for such heavy neutrinos, focusing on the case where the heavy neutrino is of the Majorana type, meaning that it is its own antiparticle. As Don Lincoln explains about one of the first such searches, the production and decay of a heavy Majorana neutrino results in the signature of two leptons (electrons or muons) of the same electric charge along with jets. A more recent search at CMS used the full 8-TeV data set and focused on events in which the same-charged leptons were muons.

To ensure that no stone remains unturned in the search for heavy Majorana neutrinos, the analysis of 8-TeV data has been updated to include events with like-charged electron pairs and like-charged pairings of an electron and a muon.

Unfortunately, as with the previous searches, no evidence of a heavy neutrino was seen. However, the inclusion of electron and electron-muon pair events allowed CMS physicists to place significantly more stringent limits on the possible masses of heavy Majorana neutrinos. With Run 2 of the LHC under way, you can expect searches for Majorana neutrinos to push into ever higher masses.