Welcome!

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!

What is this "Neutrino" You Speak Of?

The first question you may have is... what exactly is a NEUTRINO?

Well, a neutrino (denoted by the Greek letter ν ) is a "ghostly" elementary particle with some important properties...

• - Zero electric charge
• - Previously thought to have zero mass
• - Travels near or at the speed of light
• - Hardly interacts with matter at all, only via the WEAK FORCE (hence the "ghostly")

There are also three different neutrino types, or "FLAVORS" which each interact differently with matter.

• - ELECTRON neutrinos: ν_e
• - MUON neutrinos: ν_μ
• - TAU neutrinos: ν_τ

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:

• - The Sun
• - The Atmosphere
• - Supernovae
• - Particle Accelerators

The neutrinos we will be focusing on the most in this activity will be atmospheric neutrinos. You can read about the other types in the "Links" section.

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.

How Do We Detect Neutrinos?

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:

• - ν_e produces electrons
• - ν_μ produces muons
• - ν_τ produces tau particles

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 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 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.

Super-Kamiokande

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:

• - 1 kilometer underground in the Mozumi Mine mine (shielding from cosmic rays)
• - 50,000 tons (!!) of very pure water (for comparison, the average swimming pool holds about 80 tons)
• - 42 meters high and 40 m in diameter
• - Contains about 13,000 PMTs (11,000 looking inward, 2000 looking outward)
• - Despite the enormous size of the detector and trillions and trillions of neutrinos passing through, only about 8 atmospheric neutrino interactions occur per day

Flavors

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:

• - MUON NEUTRINOS ⇒ Highly relativistic muons travel almost straight through the detector and produce clean rings with sharp edges
  •     ν_μ + n → μ + p
• - ELECTRON NEUTRINOS ⇒ The multiple scattering of electrons is large, so electromagnetic showers produce fuzzy rings
  •     ν_e + n → e + p
• (we usually only observe "μ-like" and e-like" events)
  
  
• - TAU NEUTRINOS are invisible, because much more energy is needed to produce a tau lepton than needed for an electron or muon and also because taus decay immediately (3 x 10^-13 sec, after travelling 0.1 mm) after production.

Counting Flavors
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?

The Atmospheric Neutrino Anomaly

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:

• - The neutrinos that come from above travel ~10 km
• - The neutrinos that come from below travel ~10,000 km
• ⇒ The only difference is the distance in which they travel. If the flavor changes (oscillates) depending on distance traveled, then the answer to this "anomaly" could be that NEUTRINOS FROM ABOVE AND BELOW HAVE DIFFERENT PROBABILITIES OF CHANGING FLAVOR. Thus, these "missing" neutrinos have oscillated.

The Experiment

What we will do

• - We're going to look at muon and electron neutrinos that have traveled different distances... in real Super-K event displays
• - LOOK UP to see neutrinos from a short distance away (10 km). How many muon neutrinos have reached us (from the atmosphere above)? How many electron neutrinos?
• - LOOK DOWN to see neutrinos from a long distance away (10000 km). How many muon neutrinos have reached us (through the earth, from the atmosphere below)? How many electron neutrinos?
• - If neutrinos are massless and don't oscillate, we expect about the same number of muon electrons coming up as coming down.
• - BUT if neutrinos oscillate (and therefore have mass) there will be an asymmetry, different for different flavors.

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:

• 1. Is it a "clean" event? Clean events have single Cherenkov rings. (i.e. only one charged particle making Cherenkov light).
• 2. Is it mu-like (sharp) or e-like (fuzzy)?
• 3. Is it up-going or down-going?

At the end, we'll count up and compare the number of up and down single ring muon and electron neutrinos.