Finding the truth, whether that means solving a crime or describing the nature of fundamental particles, is just as much about eliminating the wrong answers as it is about finding the right ones. The same way that ruling out an alibi for a suspect is an important step toward finding the bad guy, disproving a theoretical prediction is necessary in order to find the correct theory that explains the whole story.
In the search for a better understanding of neutrinos, the Main Injector Neutrino Oscillation Search, MINOS, recently put forth results that help rule out a theorized fourth neutrino and strengthen the case against the hypothesis of neutrino decay. MINOS co-spokesperson Rob Plunkett says the results “really start to close the loop” on some major theories that neutrino experiments set out to investigate.
The MINOS experiment begins at Fermilab, in Batavia, Illinois, where a neutrino beam is generated, and its composition measured by the MINOS near detector. The beam then travels 735 kilometers to the Soudan mine in northern Minnesota, where the MINOS far detector catches it.
MINOS examines the neutrinos primarily through a process called charged current interaction. This method reveals the flavor of a neutrino--muon, tau, or electron--observed in a detector. But, from detection of charged current interactions, MINOS only has the capability to identify the muon and electron neutrinos in the beam. However, the experiment also detects what’s called neutral current interactions, which count all neutrinos, but do not reveal their flavors.
The first theory examined in this paper is the prediction of the existence of a fourth neutrino--known as the sterile neutrino. The standard model comprises only three neutrinos, which interact with ordinary matter via the weak nuclear force. But scientists know that the standard model isn't perfect. It predicts that neutrinos have no mass, yet experiments like MINOS tell us that they do. Hence there could also be a fourth type of neutrino that has eluded experimental detection so far, especially one like the sterile neutrino, which is immune to the effect of the weak force.
The sterile neutrino didn’t have a strong case going into the MINOS analysis. Results from the indirect observation of neutrinos at experiments at the European laboratory CERN pinned the number of light neutrinos at three, and so far only three neutrino flavors have been observed. Then, in 2001, an experiment at Los Alamos known as the Liquid Scintillator Neutrino Detector, LSND, published some puzzling results that seemed to indicate the presence of a sterile neutrino. Since then, no one has been able to reproduce the LSND findings. A few years ago, data from the MiniBooNE experiment (Booster Neutrino Experiment) at Fermilab seemed to refute most of the LSND results.
Still, the ghost of a sterile neutrino continues to wander among scientists, and MINOS is looking for either its fingerprint or evidence that it does not exist.
“[Our result] strongly disfavors the existence of a sterile neutrino,” Plunkett said about the analysis presented in the recent MINOS paper.
But the same way that it is difficult to put the rumors about ghosts to rest, ruling out the existence of a sterile neutrino is nearly impossible.
The MINOS paper strengthens the constraints on the sterile neutrino’s existence. Past analyses have shown that if muon neutrinos are oscillating into sterile neutrinos, only 68% of the disappearing neutrinos can do so. The new analysis shrinks that percentage to 50%, and more data will most likely reduce it further.
The second hypothesis up for investigation is the process of muon neutrino decay. Scientists know that the three neutrino flavors can oscillate among themselves. So, for example, a muon neutrino can turn into a tau neutrino. The transformations take place when neutrinos travel long distances, such as the 735 kilometers from Fermilab to Soudan. When scientists use neutral current detection techniques to analyze the beam, they find that the total number of neutrinos detected in Soudan agrees with expectations, given the measured number of neutrinos leaving the Fermilab site. But the composition of the beam changes dramatically. While the beam traversing the near detector at Fermilab consists almost entirely of muon neutrinos, the fraction of the beam that arrives at Soudan as muon neutrinos, as determined by their charged current interactions, is way down. The accepted interpretation is that the total number of neutrinos remains the same and that the missing muon neutrinos oscillated into tau (and possibly electron) neutrinos.
An alternative theory says that the missing muon neutrinos may have decayed. The MINOS scientists looked at two scenarios: first, the possibility that only decay and no oscillation takes place. Second, the situation when both decay and oscillation contribute to the observed effect.
In both analyses, the MINOS collaboration found its data to be inconsistent with neutrino decay. Plunkett said the results provide strong evidence against the existence of neutrino decay.
“This paper is really a terrific summary of a lot of stuff that’s going on [in neutrino physics] right now,” said Plunkett. “The paper shows the consistency of the whole picture. At the same time, it explores ways that the picture might be wrong or puts limitations on how wrong it might be.”
The paper has been submitted to Physical Review D, and is available on the physics website arxiv.org. The analysis was lead by Alexandre Sousa of Harvard University, Brian Rebel of Fermilab, and Anthony Mann of Tufts University.
