Huge progress made in attempt to upgrade standard model of particles

Finding the next big theory of everything will require updating our current best model of all existing particles and forces. Researchers just put new bounds on 132 possible ways to do that update, opening a door to new experiments at the frontier of known physics. In the 1900s, physicist Hendrik Lorentz argued that experiments involving electric charges must look the same in all frames of reference. This means such experiments taking place in a room ought to give the same result whether the room is stationary, is moving at constant speed or has been rotated through some angle, such as during Earth’s rotation about its axis. Albert Einstein later built this idea into his theory of special relativity. This eventually cemented the place of “Lorentz symmetry” in the standard model of particle physics, which is currently our best explanation for the behaviour of all known particles and forces other than gravity. Advertisement But we now know of phenomena that the standard model fails to fully explain, such as dark matter and dark energy. This has sent researchers looking for ways to upgrade or amend it. One possible path towards a new model starts with looking for novel quantum fields that violate Lorentz’s argument. Researchers tabulated all such theoretical candidates in the late 1990s, in what is known as the standard model extension (SME), identifying 132 that would most likely appear in near-term experiments. Now, for the first time, Jay Tasson at Carleton College in Minnesota and his colleagues have calculated just how precisely researchers ought to look for every single one of those 132 candidates. Tasson says that searching for Lorentz violations is akin to identifying some special direction in the universe such that moving in this direction or rotating through it changes the outcome of an experiment. This would be similar to noticing that a pencil that usually falls to the ground when placed against the vertical surface of your palm in fact remains in contact with your hand when you move in a particular direction, because of the influence of some new force. If such a direction is discovered, it would strongly hint that subatomic particles are interacting with a previously unknown quantum field – an entity that extends through all space and interacts with nearby objects, similar to the electromagnetic fields that fill our homes. Mathematically, the search becomes like a jigsaw puzzle – researchers have to fit a hypothetical missing field, or fields, into the standard model without disturbing the parts of the model that have been proven to work well. “The SME is a sort of test framework for looking for Lorentz-violating effects in a comprehensive way across all of physics,” says Tasson. “You get the standard model of particle physics plus a bunch of extra stuff that you can go look for in experiments.” Instead of considering all subatomic particles, Tasson and his team focused on effects relevant to protons, neutrons and electrons. These are particles that comprise all matter and most commonly feature in high-precision experiments. Accordingly, their analysis referenced experiments with rotating helium and potassium atoms where properties of their protons and neutrons had been carefully measured, as well as others where electrons were studied within a pendulum that would theoretically twist, or rotate on its thread, because of some novel force. They also accounted for the fact that these experiments were conducted on Earth, which is always rotating. As the planet rotates around its axis, it has different velocities at different times of day and at different latitudes, so a Lorentz violation could look like a property of a particle changing with those two factors. Tasson says that mathematically modelling all 132 scenarios didn’t necessarily require unconventional mathematical techniques, but did involve a lot of patience. “The problem is big and the bookkeeping is big and challenging,” he says. “This work has been under way for something like seven years, and at the beginning of that time, we didn’t know exactly how it was going to turn out.” These calculations represent a spectacular advance, says Alan Kostelecky at Indiana University Bloomington, the original architect of the SME. Over the past 30 years or so, experiments by many research groups found ways to study 89 of the 132 effects and effectively ruled them out. “But this left 43 primary ways that the behaviour of matter could, in principle, still deviate from the laws of special relativity,” he says. The work of Tasson and his colleagues, all of whom were undergraduate students, puts limits on the size of those 43 effects. Again, they were found to be very small. Neil Russell at Northern Michigan University maintains the reference table of Lorentz violations and says its past editions had notable gaps – those 43 unstudied violations – which have now been filled. “The authors have unearthed a treasure trove of new information. I was startled to see how extensive this analysis turned out to be,” he says. “I find the implications impressive.” Because the new analysis has shown that many of the effects would be remarkably small, Russell says it may motivate new experiments with more sensitive instruments. Tasson says part of the project has always been to inspire new experiments, and any experimental sign of a Lorentz violation would be truly extraordinary. It could ultimately teach us something about building the next theory of everything, such as some iterations of string theory, he says. Physical Review Letters DOI: 10.1103/ckyj-bbfl