Monday, December 31, 2018

A New Particle Lurking?

In 1995 when I was a post-doctoral researcher at the Stanford Linear Accelerator Center (SLAC) in California, the top quark was discovered by two experiments at Fermilab near Chicago. This was an exciting event, for the top quark was the sixth and last quark predicted by the standard model of particle physics and had been searched for since 1977 when the bottom quark was discovered. (Quarks come in pairs so when the bottom quark was discovered, which was the fifth known quark at the time, a sixth quark was predicted to also exist. For a brief discussion of quarks and the standard model of physics see a previous post about the top quark.)

A physicist who was collaborating on one of the experiments that discovered the top quark at Fermilab came to give a lecture at SLAC to explain the details of this significant discovery to a packed auditorium. When the presentation ended I was walking out of the auditorium with a graduate student and I remarked, "Wow, that was great. They discovered the top quark." The student replied, "No it wasn't great. It looks exactly what we expected to find." In that moment, I realized the graduate student was correct and had taught me a lesson about scientific inquiry. In general, the most significant discoveries are not those that are predicted based on what we already know, but rather those that are completely unexpected. Though the discovery of the top quark was extremely important in the field of particle physics, it was indeed, exactly what we had expected to find.

An unexpected phenomena may be starting to show itself in two experiments being conducted near the south pole. One of the experiments, the Antarctic Impulsive Transient Antenna (ANITA) is an experiment carried by a balloon high into the atmosphere in order to detect high energy cosmic rays coming from outer space. ANITA has detected two strange events with extremely high energy that have come from space and passed through the earth, thus impacting the detector from the bottom rather than the top. These "upward" going events with such extremely high energies are not expected to occur given our current understanding of particle physics. In general, particles called neutrinos do mostly pass completely through the earth without any interaction, but if the neutrinos have enough energy then even they should interact with the matter in the earth and stop before completely passing through the earth and hitting the bottom of the ANITA detector.

When ANITA announced that they had observed these two strange events, collaborators from another experiment at the south pole looked at their data again. This other experiment is also designed to study cosmic rays but it is buried deep in the south pole ice rather than being lofted in a balloon. It seems that this other detector, appropriately named IceCube, has also seen three similar upward traveling strange events.

In a paper submitted for publication last September, scientists from Penn State University analyzed the results of the ANITA and Ice Cube experiment and calculated the probability these events could be produced by known physics is 1 in 3.5 million. So it seems very likely we are seeing evidence of some undiscovered particle. Theoretical physicists are proposing different options for what these particles might be including "sterile" neutrinos (a new kind of neutrino that interacts even less than those we have discovered), supersymmetric particles (SUSY), or some other form of the mysterious dark matter that we know exists but we don't know what it is composed of.

In my career as a particle physicist I have seen other apparent unexpected discoveries turn out to be only statistical anomalies or unexpected effects based on the detection technique. So it is possible that this apparent signal will not hold up to further scrutiny. Particle physicists are anxious to find something not explained by the standard model of particle physics. Many physicists expected that the Large Hadron Collider (LHC) at CERN would find something new almost immediately after it was turned on in 2010. Unfortunately nothing unexpected has been discovered there although the discovery of the Higgs Boson in 2012 was significant since it was the final prediction of the standard model.

Also in my career as a particle physicist I have been amazed that many secrets of the universe we have unlocked are initially outside the scope of human conjecture. Bizarre aspects of nature such as those that comprise quantum mechanics, special relativity, and general relativity were not anticipated until actually observed. The creativity and imagination of the Creator continually exceed what humans initially conceive. But as our understanding of these phenomena increases we conclude that these aspects of the universe are perfectly designed and incorporated into our universe so that their unusual and unexpected nature is actually required for our existence. It will be exciting to see how the hints of new physics seen by ANITA and IceCube unfold, whether they turn out to be revolutionary or nothing. However, I am confident that there are many aspects of the universe yet to be discovered that will reveal even more about the power and ingenuity of the Creator.

The opening picture is the ANITA experiment near the south pole

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