Matter = antimatter?

Ordinary thinking is that matter and antimatter are diametrically different, like good and evil. CMS searched for a Majorana neutrino, a particle that is both a matter particle and an antimatter particle. It is equivalent in this analogy to something being both good and evil. The Dirac neutrino starts out as a matter particle and stays a matter particle throughout. Since for a Majorana neutrino the particle and antiparticle are the same, it is possible for an emitted matter neutrino to subsequently act like an antimatter neutrino. This behavior would result in striking decay patterns that would show up in CMS data.

Fermilab has an enormous experimental program that studies neutrinos by shooting beams of them at detectors both near and far. It is also possible to study neutrino physics using LHC data. Given that the neutrino sector is still pretty mysterious (for instance, establishing that neutrinos have mass occurred less than a decade and a half ago), there are many questions still to be answered.

The familiar electron, muon and tau neutrinos all have very low mass. This observation underscores the fact that we do not have a coherent picture of how some particles have huge masses while others have very tiny ones. One method to explain neutrinos’ small mass is to postulate that there is another form of neutrino that is extremely massive. According to this idea, the masses of the familiar neutrinos and the new hypothetical heavy neutrino are tied together in such a way that if one gets very small, the other must get very large. For this reason, the idea is called the seesaw mechanism.

The subject of one ongoing debate in the neutrino community is whether neutrinos and antineutrinos are distinctly different particles or the same particle. As explained in an earlier article, if matter and antimatter particles are different, they are called Dirac particles; if they are the same, they are called Majorana particles.

The typical understanding of matter and antimatter is that they retain their “matter” or “antimatter” identity. Once something is “antimatter-like,” it can never again be “matter-like.”

For instance, let’s consider the decay of a W boson into a positively charged lepton and a theoretical heavy Dirac neutrino (ND) – that is, (W+l+ + ND(matter)). The W+ boson is neither matter nor antimatter, so the sum of its decay products must similarly be matter-antimatter-neutral. The positively charged lepton is antimatter (all positive leptons are antimatter), so the heavy neutrino would have to be a matter neutrino to cancel out the antimatter property of the charged lepton. Then, if this heavy matter neutrino subsequently decayed into a W boson and charged lepton (ND(matter) → W+ + l ), the charged lepton would have to be a matter particle, reflecting the matter property of the neutrino.

In contrast, if the heavy neutrino is a Majorana neutrino (NM), it is its own antiparticle. In this case, when the W+ boson decays into a positive lepton and heavy neutrino (W+l+ + NM(matter)), this initial decay is the same as that for the Dirac neutrino – the heavy neutrino must be a matter particle to counterbalance the antimatter property of the positively charged lepton. However, in the subsequent process where it’s the heavy neutrino’s turn to decay, since the Majorana neutrino is its own antiparticle, it could then decay as if it were instead an antimatter neutrino (NM(antimatter) → W + l+), resulting in a positively charged (antimatter) lepton. The net result is that an initial W boson decays into a state where there is extra matter or extra antimatter.

CMS searched for events with two positively or two negatively charged leptons, along with jets that further identified the kinds of collisions that would indicate a Majorana neutrino was formed. No evidence for heavy Majorana neutrinos was observed.

—Don Lincoln

These physicists contributed to this analysis.
Many CMS physics analyses are deeply dependent on a thorough understanding of statistics in order to extract a meaningful result. These U.S. scientists are members of the CMS committee, which works to ensure that all measurements have correctly employed statistical methods.