What happens in hydrocarbon stays in hydrocarbon (sometimes)

This shows what an event in the MINERvA detector looks like when a neutrino comes in from the left and interacts with a proton in the detector, creating a pion that goes backwards, in addition to a proton and a muon.

This shows what an event in the MINERvA detector looks like when a neutrino comes in from the left and interacts with a proton in the detector, creating a pion that goes backwards, in addition to a proton and a muon.

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When a neutrino enters the nucleus of an atom, it can interact with the protons and neutrons inside and impart enough energy to create completely new particles. Often a pion (a particle made of a quark and an antiquark) is produced. However, the nucleus is such a dense place that sometimes the pions never make it out of the atom!

Figuring out how many pions are produced and how many exit the nucleus is very important in the field of neutrino physics because it determines how well the energy of the incoming neutrino can be measured. Experiments such as LBNE will measure how neutrinos oscillate as a function of neutrino energy, but they will need to understand what those pions are doing in order to get the neutrino energies right.

 

Brandon Eberly (left) plays his trombone in front of a MINERvA detector prototype, taking advantage of the experimental cavern's acoustics. He will describe MINERvA's latest results in more detail in today's Wine and Cheese seminar at 4 p.m.

Brandon Eberly (left) plays his trombone in front of a MINERvA detector prototype, taking advantage of the experimental cavern’s acoustics. He will describe MINERvA’s latest results in more detail in today’s Wine and Cheese seminar at 4 p.m.

This plot shows the cross section (probability) of producing a pion of a given energy for the neutrinos coming from the NuMI beamline. A few different models of that probability are shown, where the models have been normalized to the data.

This plot shows the cross section (probability) of producing a pion of a given energy for the neutrinos coming from the NuMI beamline. A few different models of that probability are shown, where the models have been normalized to the data.

Particle physicists have been measuring pions and constructing models of how they interact for a long time, but the neutrino interactions that produce these pions and what happens to them as they exit the nucleus is not nearly as well modeled. The interactions felt by the pions on their way out of the nucleus are called final-state interactions, and they are difficult to calculate because there are so many moving parts — all the protons and neutrons in the nucleus. We do have a few models, but it is important to verify them with experimental data from neutrino experiments. When the MiniBooNE measurement of pion production was first released, it was clear that the most complete models of what happens inside the nucleus were not describing the data. MINERvA now has a sample of several thousand events where a pion, proton and muon are produced when a neutrino interacts with a neutron or proton in the detector’s plastic scintillator, which is made of hydrocarbons (see top figure).

By studying the energy distribution of the pions that make it out of the nucleus, MINERvA can determine how big an effect the nucleus has on those pions. The better we understand (and then model) that effect, the better the whole field will be able to measure neutrino energies.