They do not interact very often; except on rare occasions, they do not leave any signs of their passage. So, when my adviser tells me to “go find a neutrino,” what can I do?
If I want any chance of finding neutrinos at all, I need a lot of neutrinos, a very large target, and a sensitive detector. Even with this setup, I still won’t see any neutrinos; I will only see the footprints, the particles produced after neutrinos interact.
Luckily, unlike the neutral neutrino, some outgoing particles produced by neutrino interactions are electrically charged, and so they leave tracks in detectors, as in the bubble chamber photo shown on page 6. Each charged particle makes a unique track, and just as you can tell the difference between bird, rabbit, and deer tracks in the snow, we can tell the difference between the tracks of the electron and those of its bigger counterparts, the muon and the tau, in our detector. This means we can identify which type of neutrino we see by identifying its outgoing particles: an electron neutrino makes an electron; a muon neutrino makes a muon; and a tau neutrino makes a tau. In some cases, we can also reconstruct the energy and momentum of the interacting neutrino by measuring the energy and momentum of its byproducts’ tracks.
Today’s most popular type of neutrino detector looks for charged byproducts via Cherenkov radiation, light that is emitted when a charged particle travels faster than the speed of light in a particular medium. (Note that no particle goes faster than the speed of light in a vacuum despite recent excitement over the OPERA neutrino experiment.) This light emission is analogous to a sonic boom but instead produces a cone of light: a “photonic boom.” The light is then captured by a large number of photon detectors called “photomultiplier tubes” (PMTs) lining the detector. The Cherenkov rings in this case are the “tracks” of the neutrino byproducts, and the rings’ sizes and shapes tell us about the identities and energies of the charged particles produced in the interaction.
This type of detector has been used very successfully in a number of important neutrino experiments, but it does have limitations. For example, Fermilab’s MiniBooNE experiment, which looks for electron neutrinos appearing in a beam of muon neutrinos (through a phenomenon known as “neutrino oscillations”), detected an unexpected excess of events that looked like electron neutrinos at low energies. These events can result either from real electron neutrino interactions or from background events, events that produce a signal in the detector mimicking a neutrino byproduct. The primary source of uncertainty in MiniBooNE’s measurement is background events, which have a higher occurrence at these lower energies. These background events largely consist of photons, which quickly convert into electron/antielectron (aka positron) pairs. These pairs are so close together that their Cherenkov rings overlap, making them indistinguishable from the lone electrons or positrons produced when electron neutrinos or antineutrinos interact.
To address this issue, we are developing a new type of detector technology called a “Liquid Argon Time Projection Chamber” (LArTPC) in a new experiment called MicroBooNE. Rather than inferring the nature of charged particles from their Cherenkov rings, TPCs can actually map out their full tracks in three dimensions, very much like a bubble chamber. Furthermore, TPCs can distinguish between one track and two overlaid tracks, thus between an electron and an electron/positron pair.
Like the Cherenkov detector, this technology works by exploiting the fact that when neutrinos interact with a material, charged particles are released. When these charged particles travel through the liquid argon, they ionize the argon atoms, ripping off “ionization electrons.” These ionization electrons, therefore, trace the “track” of the charged particle. The electrons are then drifted to the end of the detector with an applied electric field and measured, allowing for a beautiful 3D reconstruction of the particles that traveled through the liquid argon and their energies.
We use liquid argon in MicroBooNE because it is a noble element. This is important because ionization electrons won’t recombine with the noble argon as they drift to their detection point. Liquid argon is also dense, making a good target material for neutrinos. We will eventually need 34,000 tons of the stuff for the next generation detector, so it is lucky that liquid argon is relatively cheap as a byproduct of making liquid nitrogen. We do have to keep it very cold, however, because liquid argon has a boiling point of 87 Kelvin (-303º Fahrenheit), just above liquid nitrogen.
The Cherenkov detector used light to detect and measure charged neutrino byproducts. In the LArTPC detector, we primarily use the ionization electrons to measure these charged byproducts, but the ionization process does lead to light production through a different mechanism — scintillation. This scintillation light is used to measure the timing of neutrino events and allows for more accurate energy measurements and background rejection. The wavelength of light produced is 128 nm, which is so far into the UV spectrum that it cannot go through the glass of PMTs (or even air). Because of this, we must shift the wavelength of the light using a fluorescent material, tetraphenyl butadiene (TPB), which we apply to acrylic plates placed in front of the PMTs. This material produces blue light at 425 nm, which the PMTs are then able to detect. We have also invented a light detector made out of TPB-coated light guides. This system will address many of the issues that we expect to arise when trying to scale up the current system for the next generation of LArTPCs. The light guides will have a larger light collection area and will guide the light to PMTs located outside of the main detector region, leaving more volume for particle interactions.
LArTPC technology is still in the R&D phase, but it has many advantages that can contribute greatly to the field. It provides bubble-chamber quality images of events that will allow us to address mysteries like the MiniBooNE excess. My work will ultimately help us to determine the feasibility of scaling up these detectors to many kilotons. It is exciting to be involved in inventing such a cool (87 K cool!) new technology. So, when my adviser tells me to “go find a neutrino,” I can do a lot!