Explanation of MINOS Event Display Web Page

The latest MINOS Far Detector events can be viewed here. This web page tries to better explain what you are looking at.

The picture below is a diagram of a typical event seen in the MINOS Far Detector. The title at the top (circled in yellow)lists the time (in GMT, 6 hours ahead of CST), run and "snarl" (event) numbers (how we keep track of things internally), and a first guess at what caused the event. Almost all of the tracks shown in this display are produced by cosmic rays, which are produced by high energy particles from space hitting the upper atmosphere. They are around us on both the surface and for about two miles underground all the time. Only about one in a million events observed in the MINOS far detector comes from the neutrino beam from Fermilab. In fact, this is why the experiment is so far underground. On the surface (without all that rock to slow down the cosmic rays) seeing the very rare neutrino would be 100,000 times harder.

The big window to the right of the image above (circled in green) is an "end-on" view, what you see when looking at the octagonal face of MINOS. The little circle right in the middle is the hole where the magnetic coil goes, this hole is not instrumented.

The rectangle at upper left (“U-Z”) is the view from the upper left of the detector, the one below that (“V-Z”) is the view from the upper right of the detector. Note the gap in the middle - that is the physical gap between the two halves of the detector, where we do not have any instrumentation.

What are the colored dots? Each one is a place where a charged particle passed through a scintillator strip, causing some light which the electronics noticed. The color represents the amount of light – blue through red, with red being the brightest. Since most of the objects you see in this display are muons, they travel through many layers of the detector, leaving a long track of dots. If the muon is going slowly enough, you will see it curve due to the force of the magnetic field. The more slowly the muon travels, the more it curves. The fastest, most energetic muons hardly curve at all, for the same reasons that the faster you drive your car the harder it is to corner sharply. This particular event entered right at the top center of the detector, was moving fast so didn't curve much, and exited out the bottom left.

In the face on view, the "hat" above the octagon represents the veto shield. These scintillator modules are placed on the top and sides of the detector to help tag particles that come in from outside, as these cosmic ray muons are doing. If this is the case, you will see some dots on the veto shield where the track is pointing, and know that it came in from outside. This is true for the example event shown here.

The two graphs in the lower left are time versus horizontal position (helps us to see if it's going up or down), and how much energy was left in the detector versus horizontal position. In this case, the lowest (first) times are to the right, the higher times to the left. Looking above at the side views, we know then that the first dots are on the right, and the last dots on the left. So, this muon is indeed going right to left (in the side view), which is downwards. In the bottom graph, we see that nearly all the track has about the same, medium-low level of light for each of the dots. Except for one spot in the middle that was quite bright. Looking at the face on view, you can see where all that energy is, just to the upper left of the coil hole. This tells us two things. First, that it's an ordinary muon, which loses a pretty regular amount of energy as it crosses the detector. Second, where it did give us a blip, it was a “brehmstrallung”, something that happens occasionally to muons which gives us a hint to their total energy.

Now let's look at our first neutrino event, linked to here. In this picture, you see a muon coming into the detector from the left (in the side view). That's the direction Fermilab is. Also notice that it's coming slightly up – from Soudan's perspective, Fermilab is a bit down (it's a spherical Earth). Look at the timing – the first times are on the left, the later ones on the right. Yep, it really is going left to right. Notice the muon track curve under the influence of the magnetic field – it must not be traveling too fast. Indeed, it eventually runs out of steam and stops about ¾ of the way through the detector, after having curved all the way over from the left side to close to the center. Part of the puzzle not shown in this display – it happened at the same time as we know the beam fired.

This neutrino is somewhat atypical of our “ideal” neutrino event. It is from a neutrino which hit some rock in front of the detector, making a muon which flew in. What we'd like to see a lot of are neutrinos which hit steel inside the experiment. That way we get a picture of the whole thing, and don't have to wonder how much of it was off in the rock somewhere. If this happened, we would see nothing coming in – just something start in the middle and move off from there.

Here is a 3D rendering of just such a contained beam muon-neutrino interaction seen in the far detector. Note that it starts inside the detector where the neutrino interacted with an iron nucleus to produce a charge muon lepton. The resulting muon travels through many layers of steel, producing light as it goes (which are reconstructed to be the dots seen in the image). As the muon loses energy the detector's magnetic field causes it to curve more strongly, and the muon eventually runs out of steam and stops. Both the distance traveled and the degree of curvature produce a muon momentum estimate of 3.5 GeV/c. A different view of this event, showing how alternating U and V views are used to localize the path of the particle in MINOS is below