Feature - A neutrino's journey: From accelerator to analysis

The first T2K event seen in Super-Kamiokande. Each dot is a photo multiplier tube which has detected light. The two circles of hits indicate that a neutrino has probably produced a particle called a π 0, perfectly in time with the arrival of a pulse of neutrinos from J-PARC. Another faint circle surrounds the viewpoint of this image, showing a third particle was created by the neutrino. Image courtesy of T2K.

Neutrinos are the introverts of the particle physics world. They travel through the universe largely unnoticed, except for the very rare interaction. Every day, neutrinos pass through you, and you don’t even notice. Don’t panic – they can’t hurt you, because they don’t interact with your body’s matter.

Neutrinos are neutral – free of charge. That means that electricity and magnetism can’t draw them out and force them to interact. Likewise, they have so little mass that for a long time, scientists believed that they might be massless.

Since then, we’ve learned a lot about these elusive particles. We know that there are three types of neutrinos – the electron neutrino is the smallest, the tau neutrino the largest, with the muon neutrino caught in the middle.

We also know that when no one’s looking, neutrinos go ‘fuzzy.’ Just as Schrodinger’s cat is both dead and alive while it is still sealed in its box, an unobserved neutrino is all three types of neutrinos at the same time. As an example, imagine that a scientist has just measured a subset of a neutrino beam, and found that it is 99 percent muon neutrinos, with one percent electron neutrinos. In between measurements, the neutrino beam reverts to a fuzzy state, where it is all three types of neutrinos at once. According to one theory, when the scientist measures the beam again in order to bring it back into focus, the scientist may find that the ratio of neutrino types has changed to include more electron neutrinos.

The likelihood that a scientist will see a particular type of neutrino changes periodically over time, oscillating like the rise and fall of a merry-go-round. Three different constant angles determine the rate at which those probabilities oscillate. Scientists have already seen muon and tau neutrino oscillation, and measured two of the three angles. The third angle, theta13, is much tricker to measure, however, because it is very small. And that’s where the Tokai-to-Kamioka (T2K) experiment in Japan comes into the picture.

A schematic of a neutrino's journey from the neutrino beamline at J-PARC, through the near detectors (yellow dot) which are used to determine the properties of the neutrino beam, and then 295 km underneath Japan to Super-Kamiokande. Image courtesy of T2K.

Measuring the unknown

At the Japan Proton Accelerator Research Complex in Tokai, protons are accelerated to extraordinarily high speeds before striking a fixed target. The collision with the target produces positively charged pi mesons, or pions for short.

The pions don’t last for long before they decay, but while they do, magnets direct them into a beam pointing westward, towards the Super-Kamiokande detector in Kamioka, 295 km away.

When the pions do decay, 99.9877 percent of them decay into a muon and a muon neutrino. These are recorded by the Near Detector in Tokai for later comparison with the neutrinos observed at Super-Kamiokande. By measuring the change in the percentage of electron neutrinos, scientists will be able to calculate the value of theta13, confirming that the electron neutrino percentage oscillates.

Because neutrinos rarely interact with other particles, they don’t need a tunnel to travel through the ground to Kamioka. But by the same token, they cannot be shaped into a straight beam. The neutrinos spread out, much like the light from a flashlight. By the time they get to Kamioka, the beam is spread thin, so that only a fraction actually reach the Super-Kamiokande detector. And, since neutrinos rarely interact with other particles, only a fraction of those will be detected at Super-Kamiokande.

A cutaway drawing of the Super-Kamiokande Detector. The detector is a 40 meter diameter by 40 meter high cylinder filled with ultrapure water and surrounded by more than 10,000 50 centimeter phototubes, each sensitive enough to see a single photon. Image courtesy of Super-Kamiokande.

Scintillating at Super-Kamiokande

Super-Kamiokande is, in essence, a giant cylindrical tank filled with 50,000 tons of pure water located 1,000 meters underground. The inside walls of the tank are covered with photomultiplier tubes, which detect any sparks of light that occur inside the tank.

The lepton is ejected, traveling at extremely high speeds. Although it does not travel as quickly as light does in a vacuum, it does travel faster than light does in water, creating Cerenkov radiation – the visual equivalent of a sonic boom. The photomultiplier tubes detect the scintillating light of the Cerenkov radiation, and in so doing, they indirectly detect the neutrino.

In the control room at Super-Kamiokande, physicists monitor the experiment around the clock, watching for potential events.

“It is set up in such a way that if a candidate event arrives, it notifies that there is a candidate event,” explained Chang Kee Jung, US spokesperson for T2K. “So in some ways it is in real time that it is detected, but it is analyzed a couple of days later.”

Super-Kamiokande events are analyzed on site in Kamioka using local computational resources. But to answer the question of how rapidly neutrinos change from muon neutrinos into electron neutrinos, they need a basis for comparison.

Drawing comparisons

Back in Tokai, the Near Detector recorded a great deal of information on the outgoing muon neutrinos. That data is transmitted to a computer cluster at KEK in Tsukuba, Japan.

“A lot of processing will be done at KEK,” said Ian Taylor, a researcher with the US T2K collaboration. “The file output from that will be distributed to the Tier 1 and Tier 2 sites.”

Taylor and Jung caution not to confuse their tier system with the more familiar LHC Computing Grid (LCG) system. Within T2K, Tier 1 sites are distinguished by the fact that they have complete copies of the raw data, the reconstructions, and even the calibrations data. Tier 2 sites will not have the calibration data, and Tier 3 sites will have to download data as needed.

The North American Tier 1 site is located at TRIUMF, a subatomic physics laboratory in Vancouver, Canada. In Europe, the Tier 1 site is located at the Rutherford Appleton Laboratory in Didcot, UK.

“The different countries have taken different approaches,” Taylor said. “It’s a lot of work to set up our software on the grid, and for the amount of monte carlos we need to do, it's just not worthwhile” for the US collaboration.

The Europeans have chosen a different approach, however. The multinational T2K collaboration is already using a global EGEE virtual organization, t2k.org, to authenticate and distribute data.

“Collaborators all over the globe are accessing this data using LCG grid tools and processing small amounts locally,” said Ben Still, a researcher with the UK T2k collaboration. In other words, at the moment, the grid is being used to distribute data, but not to process it.

In the UK, there are plans for that to change. “I am developing scripts for running over data and producing monte carlo simulation data,” Still said. “We hope to be up and running on the Grid in time for the next physics data taking beam runs.”

—Miriam Boon, iSGTW