Beyond Einstein with neutrinos

Post by Teppei Katori, Janet Conrad and Carlos Argüelles.
Beyond Einstein with neutrinos
The IceCube Laboratory at the South Pole with the aurora australis.
The IceCube Laboratory at the South Pole with
the aurora australis.
Photo courtesy: Martin Wolf
(IceCube, National Science Foundation)

There is a website well-known to physicists that asks, “Are you a Crackpot?”  A leading question in the test is:  “Does your paper start with: Einstein is wrong?”  It’s a good cautionary tale to those of us who search for Lorentz violation.  The ground is littered with false claims that Einstein was wrong.

In fact, by the requirements of science, Einstein was clearly right.  His theory of space-time has withstood many, many tests, to very high precision.  It is a great description of our universe and still accessible today.  At this point, the question is not, “Was Einstein wrong?”  The real question now is, “Is Einstein’s theory sufficient?”

There is a famous example of a beautiful theory that was not wrong, but was not sufficient, and that is Maxwell’s equations.  These equations are a perfect description of how light behaves.  Since the 1800s, they have not been proven wrong.  What was proven wrong, by the influential Michelson and Morley experiment, was the worldview in which these equations were being interpreted:  light does not travel through an ether — its speed is the same from all directions.  Just because Maxwell’s equations are right, it does not mean there is an ether.

We love the Michelson and Morley experiment for many reasons.  First and foremost, of course, is the world-changing view of the meaning of Maxwell’s equations that this experiment demanded.  In fact, that changing worldview led directly to Einstein’s space-time theory.  But also, the interferometry of this experiment is a great analogy to the approach we use in our paper.  In addition, Michelson and Morley demonstrated the power of limits — although it found nothing, this is one of the most consequential experiments ever.  Limits are as important as signals.

What the Michelson and Morley experiment showed was the so-called Lorentz symmetry. Lorentz symmetry guarantees that physical observations are consistent for all inertial (non-accelerating) observers. This is a fundamental principle of Einstein’s theory of special relativity, which is a foundation for both the quantum field theory, the Standard Model of particle physics, and for general relativity, the description of gravity for the universe. Although both branches of physics are working well, physicists have not yet been successful in unifying these two theories. This leads physicists to believe there may be an ultra-high-energy scale, called the Planck scale, where the highest-energy theories, such as string theory, unify all matter, forces and space-time. And these theories suggest Lorentz symmetry violation, or Lorentz violation, may be hidden in our world at extremely small scales.

Our group has worked together for many years on Beyond Standard Model searches, including for Lorentz violation.  The first to pick up this topic was Teppei Katori, who chose it for his research during graduate school.  As a PhD student at Indiana University (IU), Bloomington, USA, he was exposed to these ideas through faculty at the IU Center for Space-Time Symmetries.  He continued to pursue this work as a postdoc in Janet Conrad’s group at MIT and hooked her on the question.  This has been his passion in physics ever since.

During the summer of 2011, Teppei met Carlos Argüelles.  Carlos came to Fermilab, USA, as a visiting theory student from PUCP (Pontificia Universidad Católica del Perú), Peru.  It took no time for them to recognize their common interest in Lorentz violation, and even in their first meeting, they discussed how to test it.  They decided the best source was IceCube neutrino data.  This is a natural conclusion, because the effects of Lorentz violation are expected to grow larger for neutrinos propagating longer distances and at higher energies, and IceCube was famous for searching high-energy astrophysical neutrinos.  So it was easy for Teppei and Carlos to see that this experiment might be able to ‘go beyond Einstein.’

Despite the initial excitement, though, real world intervened.  Carlos went back to Peru, and Teppei got a job in the UK.  They wandered off in opposite directions.  But sometimes the fate associated with chance-meetings-that-bring-great-ideas strikes twice.  Carlos started graduate school in 2012 at the University of Wisconsin–Madison (UW–Madison), the US center for the IceCube Neutrino Observatory.  Then Janet joined IceCube in 2013, and Teppei in 2014.  Without a plan or discussion, IceCube acted like a magnet to bring us together again!

We began the Lorentz violation search described in this paper in 2016, with Carlos now a postdoc in Janet’s group and Teppei a junior faculty member.  At first, we naturally focused on the highest-energy neutrinos in IceCube — the astrophysical neutrinos. This turned out to be very difficult, because we don’t yet understand the astrophysical neutrino flux very well. Janet suggested using the atmospheric neutrino data that extends to 20 TeV energies instead. This is a beautiful data set because it is well understood and is IceCube’s largest data set.

Carlos Argüelles (left) and Teppei Katori (right)
Carlos Argüelles (left) and Teppei Katori (right). Photo courtesy: Anatoli Fedynitch

At first we feared that the lower energies compared to those of astrophysical neutrinos would dilute the sensitivity. But graduate student Ali Kheirandish (UW–Madison) suggested that we look at our sensitivity to so-called high-dimension operators that we had not considered before. We discovered that our sensitivity on dimension-six operator is beyond any existing test of Lorentz violation on elementary particles, from table-top experiments to cosmology!  MIT graduate student Gabriel Collin brought Bayesian statistics to this analysis, which is common in astrophysics but still new for particle physics.  Graduate student Shivesh Mandalia (Queen Mary University) did most of the hard-core coding.  We did our work within the IceCube Beyond Standard Model working group, and our collaborators were excited and supportive, even if they may have seen it as an (ice) fishing expedition! In the end, the result of the search stunned all of the IceCube Collaboration, including us, with its reach — up to the Planck scale!

Along this two-year journey, our little group of Lorentz violation hunters has grown to six.  Are we a half-dozen crackpots?   We don’t think so.  Are we finished with Lorentz violation searches?  No, we have more ideas to pursue, even with IceCube!   Someday, we would love for the powerful tool of neutrino interferometry to reveal Lorentz violation.  If we do find it, we will raise our glasses to Michelson and Morley, who showed us that, sometimes, great theories that are necessary…are not sufficient!

Teppei Katori, Janet Conrad and Carlos Argüelles

The original paper in Nature Physics can be read here.

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