Today we will be exploring an exciting field in current physics research: neutrinos! The goal of this activity is to get you acquainted with these mysterious little particles and some recent developments regarding them. We will then ultimately analyze some real neutrino data from the Super-Kamiokande neutrino detector located in Japan.
Please take some time to try to understand the important concepts discussed on this page before moving on. Enjoy!
Well, a neutrino (denoted by the Greek letter ν ) is a "ghostly" elementary particle with some important properties...
There are also three different neutrino types, or "FLAVORS" which each interact differently with matter.
Currently, neutrino mass has not been measured directly. Neutrino physicists are trying to find out if neutrino flavors can change from and into another ("oscillate"). Neutrino oscillation is possible only if they have mass.
So... where do neutrinos come from?
Well, they're everywhere! Neutrinos are the products of nuclear interactions, ranging from radiation to cosmic rays to fusion. They fly right through you, rarely ever interacting. In fact, more than 50 trillion electron neutrinos from the sun are passing through the human body every second! It's a good thing we can't feel them.
We can study neutrinos from various sources:
Neutrinos from the Atmosphere
High energy neutrinos can be created through cosmic ray collisions with atoms in the upper atmosphere. Because it is the neutrinos we want to detect (and not cosmic rays), the best place to do this is deep underground, where thick layers of rock shield from other cosmic rays.
Okay, that was sort of a trick question. We can't exactly detect neutrinos directly, but we can detect their interactions with stuff around them. Neutrinos very rarely interact with ordinary matter, but once in a while, they do. When this happens, a CHARGED PARTICLE such as an electron or muon is created - that's what we can detect. A neutrino interaction is called an "EVENT", and during an event, neutrinos produce charged particles according to their respective flavors:
We detect these charged particles through their emission of "Cherenkov Light". By utilizing this phenomenon and also the technology of photomultiplier tubes to detect the light emitted from this radiation, a number of detectors have been built with the goal of detecting and learning more about these ghostly neutrinos.
Cherenkov light is produced when a charged particle moves faster than the speed of light in that medium (although of course slower than the speed of light in vacuum). This creates shock waves of light that emanate out in a cone shape, similar to how an object traveling faster than the speed of sound can produce a sonic boom.
PHOTOMULTIPLIER TUBES (PMTs) turn single photons into measurable electrical signals. The voltage of the electric pulse depends on the number of photons detected, which depends on the energy of the particle.
One of these detectors is Super-Kamiokande (or Super-K for short). Super-K is is a huge cylindrical water Cherenkov neutrino detector (it uses water as the medium in which to detect neutrinos) located in Mozumi, Japan. It was designed to search for the theoretical proton decay and also study solar, atmospheric, and supernova neutrinos.Here are some stats:
Tasting flavors in Water
We can use Super-K to distinguish between different flavors and directions of neutrinos because each has a distinct pattern of Cherenkov light.
The difference in time between the top of the cone reaching the detector wall and the bottom can be used to calculate the direction that the particle came from; the bigger the difference, the greater the angle from the horizontal of the particle's path.
The type/flavor of particle can also be inferred from the shape and sharpness of the edge of the cone:
Something unexpected happens when we compare the number of μ-like and e-like events. We find that there aren't enough muon neutrino interactions! We can call this "TOO FEW NU MUS" or, if you wish to be scientific, "The ATMOSPHERIC NEUTRINO ANOMALY".
Does this mean muon neutrinos are "disappearing" on their journey between the atmosphere and the detector?
Neutrinos from All Directions
Since neutrinos can sail right through the earth, we see neutrinos produced in the atmosphere coming from ALL DIRECTIONS. We would expect to find the same number of up-going and down-going neutrinos of each flavor, right?
However, this is not the case! Instead, we find a discrepancy between up-going muon neutrinos and down-going muon neutrinos. Why is that?
Let's think about this:
What we will do
We will look at up- and down-going neutrinos by viewing the data from Super-K. From the Cherenkov light patterns, we must then decide:
|This project supported by the National Science Foundation|