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Experiment

What DAEδALUS Will Do: The Basic Structure

Since the value of δcp affects the way the three flavors of neutrinos mix, we can study the mixing between neutrinos to find δcp. For instance, the proportion of muon antineutrinos that turn into electron antineutrinos depends on the value of δcp, which indicates how much CP is violated. In reality, measuring this difference in probabilities is difficult, so obtaining an exact value for δcp will be a challenge.

To measure δcp, DAEδALUS will look at how muon antineutrinos turn into electron antineutrinos. The proportion of muon antineutrinos that turn into electron antineutrinos varies with distance (regardless of the value of δcp), so DAEδALUS will make measurements at three distances. Instead of using three detectors, DAEδALUS will use a single detector for three different cyclotron accelerators:

The three accelerators (labeled as "Acc" in the diagram) are located 1.5, 8, and 20 km away from the detector and they fire beams of muon antineutrinos towards the detector at staggered intervals. Data collected by the detector then determine how many muon antineutrinos have turned into electron antineutrinos for each accelerator.

In order to find δcp, measurements at multiple energies or distances must be made. DAEδALUS chooses to measure δcp by varying distance, which is why multiple accelerators at different distances will be used. Only two accelerators would be needed, except a third one is useful for calibration. The closest accelerator is used to calibrate the antineutrino flux of the other two accelerators.

Creating Muon Antineutrinos

Step 1: Shoot protons at a carbon target to create pions. Negative pions are more or less captured quickly. Positive pions remain.

Step 2: Positive pions decay into muon neutrinos and antimuons.

Step 3: Antimuons decay into positrons, electron neutrinos, and muon antineutrinos.

The beauty of this process is that it creates muon antineutrinos without creating electron antineutrinos! This way all electron neutrinos detected are from neutrino oscillations.

Finding Electron Antineutrinos

DAEδALUS's job is essentially counting electron antineutrinos, which appear from muon antineutrinos as a result of oscillations. The number of electron antineutrinos appearing from the three accelerators will indicate potential values for δcp, which affects how many muon antineutrinos turn into electron neutrinos at a given distance.

However, neutrinos are difficult to detect (as mentioned previously). How will DAEδALUS find electron antineutrinos?

In general, neutrinos do not interact much with ordinary matter, but they do sometimes participate in specific particle interactions, one of which is inverse beta decay. Inverse beta decay involves the following: an electron antineutrino combines with a proton to create a positron and a neutron. The detector then recognizes the positron and neutron as the signature of an electron antineutrino. This double detection--of the positron and the neutron--is essential. It will prevent DAEδALUS from mistaking background events as electron antineutrino events.

DAEδALUS will utilize inverse beta decay to determine the presence of electron antineutrinos. Electron antineutrino collisions with protons are relatively rare, so to increase collision likelihood, DAEδALUS will use an ultra-large water Cerenkov detector that contains 300 kilotons of water doped with gadolinium. The water will contain many protons for the electron antineutrinos to collide with, giving even rare events a chance of occurring.

Positrons from inverse beta decay are easy to detect in water. These positrons travel extremely fast; they're even faster than the speed of light in water. While no particle can exceed the speed of light in vacuum, it is possible to exceed the speed of light in water. Water slows down light to three quarters its vacuum speed--slow enough for the positron to surpass in speed. As a result, the positron creates a light analog of a sonic boom, called Cerenkov radiation, and this radiation is picked up as a signal by devices surrounding the detector.

Background events can give off Cerenkov radiation too. Most significantly, electron neutrinos (not antineutrinos) created in the accelerator can react with oxygen in water to create electrons, though at a much slower rate than the rate at which positrons are created. Still, given the volume of neutrinos coming through the beam, this would create significant error in the analysis if left uncorrected.

Instead, a clever workaround is used. Since in inverse beta decay, the electron antineutrino and proton interact to create a positron and a neutron, the detector can be set up to recognize an electron antineutrino only if it senses a simultaneous arrival of a positron and neutron. Neutrons can be detected through neutron capture; this is why gadolinium, element 64 of the periodic table, will be added to the water. Gadolinium can absorb neutrons, and releases a significant and noticeable amount of energy when it does so--a good signal for when a neutron is created from inverse beta decay.

Therefore, if both a positron and neutron are detected at the same time, then an electron antineutrino has probably passed through and has undergone inverse beta decay. Electron antineutrinos are not supposed to be created in the DAEδALUS accelerators; thus, the detected electron antineutrinos must be from neutrino oscillations of the original muon antineutrinos. Measuring the proportion of muon antineutrinos that have turned into electron antineutrinos will then make it experimentally possible to narrow down values of δcp.

Tentative Timeline

It will be several years before DAEδALUS can work on finding δcp. First, developmental work must be done on equipment needed for the experiment, such as on cyclotrons and gadolinium-doped water detectors. By 2020, Phase I of the experiment is planned to begin and several years will be spent on locating a CP violation signal. After that, in Phase II, DAEδALUS will be upgraded and used to measure the CP violation signal more exactly.

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