Were you told that there were only three types of neutrino? Perhaps, even by me?
You may have been lied to! Good evening, and welcome to Particle / Counter Particle,
Science YouYube's third most popular particle physics debate show on Tuesdays. This past
century has been an exciting time for physics. Neutrinos were predicted, discovered, and found
to oscillate or change among the three flavors. But some experiments have shown that neutrinos
are not oscillating as predicted. One possible culprit: an undiscovered fourth kind of neutrino.
Tonight, we're examining the ongoing mystery surrounding these so-called sterile neutrinos.
Do they exist? Do they not? We'll let you be the judge. I'm Kirsty Duffy, you're watching BNN.
Tonight, I'm joined by Mark, Chair of the Fourth Neutrino Society and Global Head of
Sterile Neutrinos Unanimous, and Elena, a fervent pro-standard modelist and Executive
Director of the Three Neutrino Coalition. You'll each have 30 seconds for an opening
statement and then we'll move into discourse. And please remember: no technical jargon
and if you interrupt I will mute you. Mark, you're up first. What do you mean by a sterile neutrino?
Mark: Well thanks for having me, Kirsty. Sterile here refers to this neutrino's
interaction properties. Physicists have seen some anomalous oscillation results
and we need a different type of neutrino to begin to explain these intriguing behaviors.
This light sterile neutrino wouldn't interact via the weak force like the other neutrinos do,
it would interact only through gravity. In other words, it wouldn't interact with any particle in
the standard model at all. No charged lepton the pair with, no protons, no atoms, nothing.
Kirsty: Thanks, Mark. Elena, your opening thoughts.
Elena: Thanks, Kirsty, it's wonderful to be here. Look, tradition is on my side here. Everything
we've learned so far tells us that nature likes to group fundamental particles into threes.
We have three positive quarks and three negative quarks that pair together,
and there are three charged leptons and three neutrinos that also pair to each other.
It's the Standard Model of Particle Physics. It's a theory that has accurately predicted
countless observations in the field. There's no room in the Standard Model for a fourth neutrino.
>Interesting. Mark, care to share your response? >Oh come on now, we already know the Standard
Model isn't complete. It doesn't even account for neutrino mass or oscillations, which I think we
all agree definitely exist. What's more, the idea of a fourth neutrino isn't new,
it was proposed over 50 years ago by Bruno Pontecorvo, the same guy who thought oscillations
were the solution to missing solar nutrinos in the Homestake experiment. Not only did he
consider oscillations between the two active neutrinos, he also theorized what this might
look like for an additional sterile particle. >Theorists think up new particles all the time,
that's not revolutionary. >Fair, to Elena's point,
the existence of a light sterile neutrino wasn't taken very seriously by the physics community up
until very recently when experimental observations thrust a possibility back into the foreground.
>So which experiments are you referring to? >Well, 20 years ago in New Mexico to start.
The Liquid Scintillator Neutrino Detector, or LSND, saw extra electron neutrinos in their
oscillation measurements. Way more than being described by a simple three neutrino framework.
A fourth neutrino provided an excellent explanation for this discrepancy.
>That could have been a source of misunderstood background.
>Maybe but that's why MiniBooNE, a neutrino experiment at Fermilab was built,
to figure out whether the LSND electron neutrino excess was real or not. And MiniBooNE
ended up observing something anomalous as well, a completely independent result.
>Sure, but that data wasn't convincing either, and there's plenty of other
explanations. Neither experiment has really proven that we've seen light sterile neutrinos.
>Right, and that's why MicroBooNE, another neutrino experiment at
Fermilab was built, to figure out how- >How many neutrino detectors do you need?
>Well, it's not like there's enough evidence to rule it out, yet.
>That's a good point, Mark. Elena, I am taking you off mute now.
Does any experimental evidence contradict the possibility of a fourth neutrino?
>Yeah, several experiments have verified that the total number of neutrinos is three by studying a
heavy neutral particle called z-boson. The z-boson will sometimes decay and produce neutrinos and
anti-neutrinos in- >Oh please,
not the invisible particle width again? >Jargon warning, Mark. Continue, Elena.
>As I was saying, there is a relationship between the z decay rate and the total
number of neutrinos, and measuring these rates revealed that the number of neutrinos is three.
>You do know that relation only accounts for active flavors, right? Doesn't disprove the
possibility of a sterile state at all. >Jargon.
>Sure, but then how do you explain away the constraint from the cosmic neutrino background?
>Have we even confirmed that exists, yet?
>Well the neutrino-to-photon density ratio parameterizes into three flavors.
>More like 3.045, which doesn't even begin to make sense.
>Jargon! >These are well-founded theories
with reliable experimental evidences. >You can't just choose to ignore
anomalous results whenever you want. >I'm not doing that! MiniBooNE can't even tell
the difference between electrons and photons. >Ah photons, no one cares about photons.
It's definitely an electron. >No, I mean you can't ignore
half of the possibilities. >No no no no no no, MiniBooNE
even works fine. It's been 30 years looking at this, plus we need sterile neutrinos, how else-
>No, the modeling of the scintillation >Ridiculous
>Hey, let's settle down, please. We can all spin out of control
when dealing with particle physics, but let's keep this debate friendly?
Now, I'm going to take you both off mute. Elena, let's go back to you.
>Okay, look. Beyond the convincing evidence that we have against this so-called sterile neutrinos,
there's another big issue here which is that they are invisible. Like, completely undetectable.
We can at least observe some of the effect of the active flavor neutrinos in our detectors,
but sterile neutrinos don't interact at all in any way that we can actually see,
or directly observe in our detectors. >That's a great point, sterile neutrinos wouldn't
produce visible charge and light like their active flavored counterparts, but they're still part of
the neutrino family. So they would still take place in neutrino oscillations. We've got a great
idea of how to expect oscillations to look in our detectors. What we'd be searching for then is a
discrepancy between what we think we know and what we actually measure in our collected data. That's
what happened with LSND and MiniBooNE. There were a lot more electron neutrinos than were
expected with the three neutrino flavor. >But as Elena noted, these experiments
have limitations that prevented us from confirming the existence of sterile neutrinos.
>Yeah, and on top of that, if there was oscillation going on we would see
a neutrino disappearing as well as electron neutrino appearing. And we don't see the neutrino
disappearing. The disappearance results are consistent with no sterile neutrinos.
>So how do you expect future efforts will overcome these challenges?
>State-of-the-art technology has been developed over the past years to address
exactly this mystery. We have a better understanding of the different types
of backgrounds and how they might cover up potential sterile signals.
We have developed new detector technologies, employing two detector systems and using different
materials better suited to probe this particle's existence. And finally, we have sophisticated
software algorithms that ultimately will help us filter and characterize all the different events
we see, and help us make a discovery. >Thank you. Elena, any final thoughts?
>We know we are seeing something interesting that happens with neutrinos and we need to explain
that. But is this sterile neutrino? I'm still not entirely convinced.
The oscillation results don't match up and there's still too many questions about
how neutrinos interact in our detectors, and this could really offer an alternative explanation.
Making room for a fourth neutrino overrides the entire symmetry of the Standard Model,
and that can't be right. Physics should be symmetric.
>Unless? >Unless?
>Unless there's a whole family of new particles and new symmetries that we've yet to discover.
>Well, I'll eagerly await those results.
>Thank you both for tonight's spirited debate, I'm certain this debate will be highly contentious for
years to come. I'm Kirsty Duffy and thanks for watching Particle Counter Particle. As always,
remember folks, science follows the data, not your opinions. Join us next week for one of the hottest
debates in particle physics: is it top and bottom or is it truth and beauty? Tune in to find out.