Sterile neutrinos could explain dark matter – if we can find them

There are good causes to suppose that neutrinos have a shy cousin that might clarify darkish matter, however searches have thus far come up empty, writes Chanda Prescod-Weinstein


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10 November 2021

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You might need seen latest headlines a few “sterile neutrino” and been confused. Common readers know that I’m a fan of neutrinos, partly as a result of they’re non-trinary: they arrive in three flavours and randomly change between them for causes we now have but to totally clarify. The sorts of neutrinos I’ve mentioned on this column earlier than are these we now have detected: electron neutrinos, muon neutrinos and tau neutrinos.

These three flavours are basic particles in the usual mannequin of particle physics, our greatest image of matter and its interactions. They’re members of the lepton household together with electrons, muons and taus. Like their leptonic kin, the usual mannequin neutrinos are in a category of particles known as fermions, outlined by having an inner quantum sense of rotation – spin – that may be a a number of of a half. Importantly, these neutrinos work together with different particles via gravity and the weak nuclear power.

Sterile neutrinos are, as we are saying within the US, an entire new ball recreation. In contrast to customary mannequin neutrinos, we don’t know if they’re actual. And in contrast to the neutrinos we all know, they appear to solely work together via gravity. Sounds boring, you would possibly suppose. Why hassle? Initially, all people is aware of that I like a very good darkish matter candidate. I’m particularly fond of 1 that I can argue ought to exist anyway, no matter our lacking, invisible matter drawback. Sterile neutrinos share two of my favorite qualities for a hypothetical particle: they’re well-motivated they usually occur to be attention-grabbing darkish matter candidates.

We expect sterile neutrinos ought to exist due to a property of normal mannequin neutrinos: handedness. Particularly, the neutrinos we all know are lefties (and antimatter antineutrinos are all righties). Although I’m referring to this as handedness, this property – formally referred to as chirality – isn’t fairly like on a regular basis life as a result of it isn’t classical. Like particle spin, it’s a quantum function.

Each recognized particle can are available each left and right-handed varieties – aside from neutrinos. They arrive solely as left-handed particles. Naturally, over time, physicists have puzzled whether or not there are right-handed neutrinos (and left-handed antineutrinos).

The sterile neutrino is that hypothetical right-handed neutrino. It’s named “sterile” as a result of it solely interacts via gravity. Whereas this property makes sterile neutrinos totally different from different neutrinos, they do have a mass and aren’t electrically charged, similar to customary mannequin neutrinos. This implies they may very well be darkish matter and, not like customary mannequin neutrinos, they probably have adequate mass to clarify the obvious gravitational impression of darkish matter’s presence.

“Detecting unusual neutrinos is troublesome sufficient. That work is much more sophisticated with sterile neutrinos”

These of us who’re theorists get the thrilling work of determining how the concept sterile neutrinos may very well be darkish matter would work mathematically. Experimentalists get the enjoyment – and unbelievable problem – of going out and searching for bodily proof.

One in every of these searches lately prompted some headlines by discovering a null end result: no sterile neutrinos. Detecting unusual neutrinos is troublesome sufficient. That work is much more sophisticated with sterile neutrinos, which may solely be “seen” via their interactions with quantum fluctuations of their customary mannequin counterparts. To seek out sterile neutrinos, you must search for a selected kind of behaviour in on a regular basis neutrinos.

The experiment that lately introduced outcomes, MicroBooNE, is situated at Fermilab, not removed from Chicago. It consists of a big container of argon hooked up to a beamline the place neutrinos are produced by colliding protons collectively. It’s simpler to comply with the trajectory of neutrino occasions in argon, as a consequence of its excessive density and sensitivity to the charged particles which can be produced within the collisions.

MicroBooNE’s major process is to raised perceive how neutrinos work together with argon and to attempt to replicate the hints seen in earlier experiments that sterile neutrinos are actual. Two experiments, MiniBooNE and LSND, noticed an extra of muon neutrinos oscillating into electron neutrinos over distances that didn’t bodily make sense. This oddity may very well be defined if the muon neutrinos have been first turning into sterile neutrinos, earlier than becoming electron neutrinos.

Sadly for some, the MicroBooNE group introduced lately that it hadn’t, thus far, seen the identical electron neutrino extra. That is in keeping with knowledge from different experiments, leaving us with fairly the thriller. Why are totally different experiments getting totally different outcomes? We don’t know.

However even when nothing turns up as soon as we now have explored each place this hypothetical particle may very well be hiding, that may nonetheless be helpful. If sterile neutrinos prove to solely be a figment of the particle theorist’s creativeness, we’ll know it’s time to transfer on.

Chanda’s week

What I’m studying

I acquired an advance copy of the brand new comedian guide LUNAR ROOM #1, and it’s glorious.

What I’m watching

I’m very excited that Succession is again!

What I’m engaged on

We’re into letters of advice and grant proposal season right here, so I’m doing plenty of each.

  • This column seems month-to-month. Up subsequent week: Graham Lawton

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