• Physics 16, 20
Collider experiments have set new direct limits on the existence of hypothetical heavy neutrinos, serving to to constrain how strange neutrinos get their mass.
The invention over 20 years in the past that neutrinos can oscillate from one kind to a different got here as a shock to particle physicists, as these oscillations require that neutrinos have mass—opposite to the usual mannequin of particle physics. Whereas a number of neutrino experiments proceed to constrain oscillation charges and mass values (see Viewpoint: Lengthy-Baseline Neutrino Experiments March On), one basic query stays unanswered: How do neutrinos get their mass? Many theoretical fashions exist, however to this point none of them have been experimentally confirmed. Now the CMS Collaboration at CERN in Switzerland has offered outcomes on a seek for a hypothetical heavy neutrino that could possibly be tied to neutrino mass era . No signatures of this particle had been discovered, putting new constraints on a well-liked mannequin for the origin of neutrino mass, referred to as the seesaw mechanism. These outcomes pave a brand new approach to probe the neutrino mass origin at particle colliders sooner or later.
There are clues that neutrino lots may be particular. Neutrinos don’t have any electrical cost—making them distinct from different “matter” particles, comparable to electrons and quarks. Neutrinos are additionally distinctive in being noticed with only one kind of handedness (a property that emerges from the particle mass and spin). Different matter particles might be both left-handed or right-handed and procure their mass by way of their interplay with the so-called Higgs discipline (see Focus: Nobel Prize—Why Particles Have Mass). The truth that neutrinos are noticed to be solely left-handed might recommend that the Higgs mechanism doesn’t apply to them.
There may be one other particular characteristic of neutrinos: they’re much lighter than all the opposite elementary particles. Experiments constrain neutrinos to be no less than one million occasions lighter than electrons, the subsequent lightest particles. This disparity means that one thing is “pushing” the neutrino mass towards small values. The seesaw mechanism contains such a push [2–7]. Identical to on a playground, two gamers are concerned on this metaphorical seesaw: an noticed neutrino on one finish and a hypothetical neutrino on the opposite (Fig. 1). The quantum states of those two particles are combined—which means that one particle can doubtlessly oscillate into the opposite. This mixing results in an inverse relation between the lots of the 2 gamers: the heavier the hypothetical particle, the lighter the noticed neutrino.
The heavy hypothetical neutrinos are “sterile” in that they don’t take part in any of the recognized basic interactions. One other essential characteristic of the seesaw mechanism is that neutrinos should be their very own antiparticles—thus, proof that neutrinos annihilate with themselves would supply assist for this mechanism.
Researchers have for a few years been trying to find extra neutrinos. One probe entails observing neutrino oscillations and in search of indicators of “lacking” neutrinos. This situation might happen, for instance, if a few of the electron neutrinos from a nuclear reactor remodel into sterile neutrinos that may’t be detected. Some experiments have seen hints of sterile neutrinos (see Viewpoint: Neutrino Thriller Endures), however these probably noticed sterile neutrinos are a lot lighter than the expected sterile neutrinos from the seesaw mechanism. At larger lots, international research of neutrino information have uncovered no indicators of lacking neutrinos, permitting physicists to derive constraints on further neutrinos that stretch as much as extraordinarily excessive lots of 1015 GeV/c2.
A extra direct seek for extra neutrinos depends on producing them in high-energy experiments. The CMS Collaboration has hunted for indicators of additional neutrinos in collision information from the Giant Hadron Collider (LHC) at CERN. The crew’s goal sign was a violation of lepton quantity, which is a type of “cost” associated to electrons, muons, tauons, and neutrinos (Fig. 2). To this point, physicists have solely seen processes that preserve lepton quantity (see Viewpoint: The Hunt for No Neutrinos) , however devoted searches at excessive vitality have been uncommon . On this new research, the CMS Collaboration regarded for collisions between protons (lepton variety of 0) that produced both a pair of muons (lepton variety of +2) or a pair of antimuons (lepton variety of −2). Observing such dimuon occasions would indicate muon neutrinos annihilating with themselves by way of their coupling to sterile neutrinos.
The CMS crew didn’t discover proof of lepton quantity violation within the muon information, which allowed the researchers to derive new bounds on the blending of sterile neutrinos with muon neutrinos. These bounds enhance over current limits for sterile neutrino lots above 650 GeV/c2, and so they characterize the strongest bounds from colliders for sterile neutrino lots as much as 25 TeV/c2. (It ought to be famous that oscillation experiments present stronger bounds on sterile neutrinos, however these bounds don’t embody lepton quantity violation, and thus the relation to the seesaw mechanism will not be easy .)
Placing these outcomes into the context of the theoretical neutrino mass mechanism panorama, the CMS constraints on the blending parameters are roughly 10 orders of magnitude weaker than the expected values of those parameters within the easiest model of the seesaw mechanism. It’s subsequently unlikely that collider experiments can probe this straightforward model even sooner or later with extra statistics and upgraded particle accelerators. Nonetheless, these outcomes present vital constraints on standard variants of the seesaw mechanism, which typically characteristic bigger mixings. Actually, these variants will proceed to be a goal of alternative for future collider experiments, in addition to for research that mix searches for lepton quantity violation and searches for sterile neutrinos.
Whereas neutrino oscillation experiments are nearing the willpower of all of the oscillation parameters, understanding the underlying mechanism of neutrino mass era requires searches past oscillation experiments. These neutrino mass research are progressing on a number of fronts, however to this point they’ve offered solely constraints—fairly than a discovery of the origin of neutrino lots.
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