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Scientists solve neutrino mysteries by watching them interact with detectors — specifically, with the atomic nuclei in the detector material. Most of the time, a neutrino does not even shake hands with a nucleus. But when it does, the lightweight, neutral particle can transform into a charged particle and knock things out of the nucleus as it escapes — leaving a crime scene behind. It is the job of scientists at Fermilab’s MINERvA experiment to reconstruct the crime scene and figure out what has happened during the interaction.
The impact
Neutrinos are lightweight particles that rarely interact with matter. Their reluctance to interact makes them difficult to study, but they’re also the very particles that could answer longstanding questions about the creation of the cosmos, so they’re worth the pursuit. And it’s a tough one, since the neutrino can’t be studied directly. Rather, scientists must study the traces it leaves behind. The more information they can gather about the meaning of those traces, the better their neutrino measurements — not just at MINERvA, but at other neutrino experiments as well.
Neutrinos entering the MINERvA detector interact with the detector’s atoms, generating new particles before fleeing the scene. The MINERvA experiment recently used a new investigative technique to better trace those fleeing neutrinos that kicked everything off. Photo: Reidar Hahn
Summary
Neutrinos are lightweight, neutral particles, and they usually sail through matter without bumping into it. But once in a while, it does shake hands with a nucleus, and sometimes the handshake takes a destructive turn: A charged lepton (an electron or muon, sometimes called a “heavy electron”) is produced, while the constituents of the nucleus are knocked out. The traces of the charged lepton and the knock-out are collected by a particle detector.
MINERvA scientists study the resultant particles’ traces to reconstruct the interaction between the neutrinos and the nuclei. So far, this has not been an easy task: Nuclear effects have obscured much of the evidence of the intruding neutrinos, leaving researchers with complex and seemingly irrelevant information. Not all neutrinos misbehave but, unfortunately, the neutrinos we care about – those with energy comparable to the mass of the constituents of the nuclei and could possibly tell us about the creation of the cosmos – all have this modus operandi.
The transverse boosting angle δαT represents the direction of the net transverse motion of the charged lepton and the knock-out.
To reconstruct the resulting crime scene, scientists need a complete understanding of how the nuclear effects work.
Both the charged lepton and the knock-out retain partial fingerprints from the original neutrino, and those partial fingerprints lie ambiguously on top of the nuclear effect background.
Researchers have found that the fingerprints can be lifted via a novel neutrino CSI technique known as “final-state correlations.” Just as the sun’s corona is visible only during a solar eclipse, the fine details of the nuclear effects become clear only when other effects are removed.
To get a sense of the “final-state correlations” technique, let’s take a step back and look at the events leading to the crime scene: A neutrino bumps into a nucleus. The interaction produces other particles. Those new particles — charged lepton and knock-out — fly off in opposite directions, leaving traces of themselves in the detector.
In the absence of nuclear effects, the charged lepton and the knock-out would fly off in separate, roughly back-to-back paths, away from the incoming neutrino path. Picture a neutrino entering through, say, the south entrance of some tiny, subatomic building. It bumps into a nucleus. The resulting charged lepton flees through an east exit, and the knock-out particle flees through some west exit.
With no nuclear effects, the charged lepton heads east with as much determination as the knock-out particle heads west. That is, the charge lepton’s east-pointing momentum matches the knock-out particle’s west-pointing momentum.
But in reality, there are nuclear effects, and that means that the charged lepton’s eastward motion does not match the knock-out particle’s westward motion. These subtle momentum differences are clues; they reflect everything that happens inside the nucleus, like a shadow of the crime scene cast by the flashlight carried by the neutrino. Thus, neutrinos cast no shadows – only nuclear effects do.
The final-state correlations technique matches the nuclear effects with the postinteraction particles’ departures from the paths of equal east-west momenta.
In a recent MINERvA neutrino investigation, researchers used the new technique. They laid out a detailed reconstruction of the nuclear effects. The underlying phenomena – such as the initial state of the nucleus, additional knock-out mechanism, and final-state interactions between the knock-out and the rest of the nucleus – are now separated. New insights on the workings of nuclear effects have been reported in Phys. Rev. Lett. 121, 022504. Those interested are much encouraged to review MINERvA’s findings.
Xianguo Lu is a physicist at the University of Oxford.
Xianguo Lu from University of Oxford explains why neutrinos leave no shadows at the March 2, 2018 Fermilab Wine & Cheese Seminar. Photo: Kevin McFarland
All baseball fans know that probability is a huge component of their favorite sport. Just as, when you roll a die, you know that a certain outcome has a one in six chance of showing up, in baseball, each batter has a certain probability of hitting the ball based on their skills. Analogously, physicists are aware of the probabilistic nature of the interactions between particles and want to measure these probabilities to understand how nature works. A particle in a beam moving toward a fixed target can be imagined as a baseball, thrown by a pitcher, heading to a batter. The particle will not “hit” (interact) with a certainty of 100 percent. Depending on both the particle and the target, that probability changes, and it may be very low.
What do physicists do in that case? They simply throw a lot of identical particles at a specific target in order to collect a reasonable number of interactions to investigate. Studying how particles interact with different targets in a statistical way can unveil nature’s secrets.
The ArgoNeuT detector at Fermilab used liquid argon to detect mysterious particles called neutrinos. Photo: ArgoNeuT collaboration
The impact
The particle known as the neutrino interacts very rarely with matter. It comes in three types, and while traveling, there is a probability they morph from one of their types into another. This process is known as neutrino oscillation, and it’s one of the most active research topics related to these curious particles today.
Neutrino-nucleus interaction probabilities are a fundamental prerequisite for every neutrino oscillation experiment. In high-energy particle physics, such probability is expressed in terms of an area, called cross-section. In order to correctly interpret the outcome of neutrino oscillation experiments, researchers need precise neutrino cross-section measurements in the desired energy range.
Neutrinos that interact with a nucleus produce other particles that scientists study to learn more about the neutrino responsible for the interaction.
ArgoNeuT was a neutrino detector filled with 170 liters of liquid argon. It was designed to study neutrinos produced in a beam, but more specifically to exploit and fully understand what scientists now call liquid-argon technology, because it makes use of the liquid argon as the neutrino’s target.
Using data collected over six months by this detector at Fermilab, ArgoNeuT researchers measured the probability for a neutrino to interact with a nucleus of argon to produce a particular result: one muon, exactly one charged pion and any number of nucleons (protons and neutrons).
During the analysis of the ArgoNeuT data, scientists made fundamental improvements in the software that reconstructs the particles in the detector. These tools use the data to reconstruct – create a picture – and identify the particles produced in the interaction. The same reconstruction tools will be used by current and future neutrino experiments that use liquid argon as the detection material, such as the MicroBooNE and SBND experiments at Fermilab.
Moreover, these measurements provide new information about the neutrino single-pion production and can be used to improve the modeling of neutrino interactions with the argon nucleus.
A negative muon and positive pion candidate event in ArgoNeuT. The figure shows the 2-D projections in the two wire planes. The color of the track respects the charge read by the wire planes, wire by wire.
Summary
The ArgoNeuT experiment was the first ever to make cross-section measurements of neutrino and antineutrino (the neutrino’s antimatter counterpart) interactions resulting in a muon, a charged pion and any number of nucleons in the final state using argon as the target.
Charged particles moving in the detector leave behind marks of their passage that can be read and recorded. Because of the structure of the detector, this information can be interpreted as a quantity of electric charge, proportional to the particle’s energy, divided into small dots along the particle’s path. In order to consider a particle “reconstructed,” all the dots must be grouped in a cluster, more or less like solving a connect-the-dots puzzle (without the help given by numbered labels arranged in ascending order!).
Scientists can identify the types of particles that move through the detector based on their tracks.
Researchers on ArgoNeuT managed to solve a series of issues. One was to account for the pesky presence of muons that happened to have no affiliation with any neutrino interaction in the detector. Such muons would arise from neutrino interactions with the environment surrounding the detector. They also took on the challenge of optimizing the reconstruction software for this analysis. The improved software was able to clusterize all the dots in the neutrino events in a more consistent and realistic way.
Besides a chain of cuts able to remove the events that clearly didn’t respected the desired event structure, ArgoNeuT researchers implemented a boosted decision tree. This is a technique for creating a model that separates events according to several carefully chosen parameters given as inputs from the user. The boosted decision tree was trained using simulated signal and background samples, further improving the separation between signal and background data.
After correcting for selection efficiency, scientists carried out the measurements and compared them with four of the most commonly used neutrino event simulators. The comparison showed a mismatch between data and most of the neutrino simulation predictions, showing how much physicists still have to understand about neutrino-argon and neutrino-nucleus interactions. The results obtained in these measurements can help improve the simulators taking into account more recent data from neutrino-argon interactions. Furthermore, because of the software’s great performance, ArgoNeuT will aid larger neutrino experiments in their quest to understand the nature of the subtle neutrino.
Giacomo Scanavini is a Yale University physicist. Tingjun Yang is a Fermilab physicist.