In particle physics, the gold standard for experimental significance is a threshold of 5-sigma. Yet BaBar physicists achieved a significance of 14-sigma: a remarkable accomplishment. The reason you’ve likely never heard about it? It was overshadowed by slightly bigger particle physics news occurring in the same year: the discovery of the Higgs boson. But this result maybe Nobel-worthy, too. The laws of nature are not the same forwards and backwards in time. After seven years, it’s time the world felt the impact of this discovery.”
Imagine you took a ball and threw it off of a tower, watching its trajectory as it flew through the air and eventually hit the ground. If you were to take that same ball and throw it with the right speed and angle from the ground, it would fly through the air and wind up exactly at the point it was launched from in the first example. It would, in fact, follow the exact same trajectory as if you video recorded the first throw and ran the video backwards in time. This is called time-reversal invariance, and it’s valid for Newton’s laws of motion. But it isn’t valid for all laws of physics!
“Will it be successful? Regardless of what we find, that answer is unequivocally yes. In experimental physics, success does not equate to finding something, as some might erroneously believe. Instead, success means knowing something, post-experiment, that you did not know before you did the experiment. To push beyond the presently known frontiers, we’d ideally want both a lepton and a proton collider, at the highest energies and collision rates we can achieve.
There is no doubt that new technologies and spinoffs will come from whichever collider or colliders come next, but that’s not why we do it. We are after the deepest secrets of nature, the ones that will remain elusive even after the Large Hadron Collider finishes. We have the technical capabilities, the personnel, and the expertise to build it right at our fingertips. All we need is the political and financial will, as a civilization, to seek the ultimate truths about nature.”
With the discovery of the Higgs boson and nothing else at the LHC, many physicists are legitimately entertaining what’s been called the “nightmare scenario,” where no new particles exist beyond the Standard Model that can be discovered by terrestrial colliders. But it isn’t a foregone conclusion that there aren’t such particles, and there are two generic types of plan for how we might find any new particles that do exist beyond the LHC’s reach. If the experimental particle physics community comes together to develop a single, coherent proposal for their future, we could probe the frontiers of nature as never before.
This Is Why It’s Meaningless That Dark Matter Experiments Haven’t Found Anything
“To date, the direct detection efforts having to do with dark matter have come up empty. There are no interaction signals we’ve observed that require dark matter to explain them, or that aren’t consistent with Standard Model-only particles in our Universe. Direct detection efforts can disfavor or constrain specific dark matter particles or scenarios, but does not affect the enormous suite of indirect, astrophysical evidence that leaves dark matter as the only viable explanation.
Many people are working tirelessly on alternatives, but unless they’re misrepresenting the facts about dark matter (and some do exactly that), they have an enormous suite of evidence they’re required to explain. When it comes to looking for the great cosmic unknowns, we might get lucky, and that’s why we try. But absence of evidence is not evidence of absence. When it comes to dark matter, don’t let yourself be fooled.”
If dark matter is so successful, then why haven’t we directly detected the particles that make it up yet? Doesn’t the failure of all these experiments attempting to directly detect dark matter point to a failure of the dark matter hypothesis.
Ask Ethan: What Is The Fine Structure Constant And Why Does It Matter?
“When we do our best to measure the Universe — to greater precisions, at higher energies, under various conditions, at lower temperatures, etc. — we often find details that are intricate, rich, and puzzling. It’s not the devil that’s in those details, though, but rather that’s where the deepest secrets of reality lie.
The particles in our Universe aren’t just points that attract, repel, and bind together with one another; they interact through every subtle means that the laws of nature permit. As we reach greater precisions in our measurements, we start uncovering these subtle effects, including intricacies to the structure of matter that are easy to miss at low precisions. Fine structure is a vital part of that, but learning where even our best predictions of fine structure break down might be where the next great revolution in particle physics comes from. Doing the right experiment is the only way we’ll ever know.“
This week, I was asked to explain the fine structure constant as simply as possible. It’s actually a story more than a century in the making, as the previously-observed fine structure of matter let us know that Niels Bohr’s model of the atom was insufficient from the outset! Today, our understanding of how the spin of matter, the relativistic effects that come from moving close to the speed of light, and the inherently fluctuating nature of the quantum fields permeating the Universe come together enables us to probe the structure and nature of matter more deeply than ever before.
Dark Matter Search Discovers A Spectacular Bonus: The Longest-Lived Unstable Element Ever
“Whenever you build an experiment that can take you beyond your previous sensitivity limits, you open yourself up to the possibility of discovery. In robustly detecting this extraordinarily rare decay with a longer lifetime than any other we’ve ever seen, the XENON collaboration has demonstrated how capable their apparatus is. Although it was designed to search for dark matter, it’s also sensitive to a number of other possibilities which might herald rare or even entirely new physics.
While the direct detection of the longest-lived unstable decay is an incredible feat, its implications go far beyond a simple discovery. It’s a demonstration of XENON’s sensitivity, and its ability to tease out even a tiny signal against a well-understood, low-magnitude background. It gives us every reason to be hopeful that, if nature is kind, XENON may reveal some of its even more profound secrets.”
Naturally-occurring xenon is made up of nine isotopes. While only one of them was known to be unstable, transmuting into barium through double beta decay, some of the isotopes should theoretically be unstable to decays through an even rarer pathway: double electron capture. For the first time, double electron capture has now been observed in the element xenon, thanks to the incredible work and the unprecedented detector sensitivities provided by the experiment XENON1T. Xenon-124 has now broken the record, and is officially the longest-lived unstable isotope ever to have its decay measured.
Is There Really A Fourth Neutrino Out There In The Universe?
“Regardless of what the ultimate explanation is, it’s quite clear that the normal Standard Model, with three neutrinos that oscillate between electron/muon/tau types, cannot account for everything we’ve observed up to this point. The LSND results, once dismissed as a baffling experimental result that must surely be wrong, have been confirmed in a big way. With reactor deficiencies, MiniBooNe’s results, and three new experiments on the horizon to gather more data about these mysteriously misbehaving particles, we may be poised for a new revolution in physics.
The high-energy frontier is only one way we have of learning about the Universe on a fundamental level. Sometimes, we just have to know what the right question to ask truly is. By looking at the lowest-energy particles at different distances from where they’re generated, we just might take the next great leap in our knowledge of physics. Welcome to the era of the neutrino, which is taking us, at last, beyond the Standard Model.”
No matter how good, compelling, elegant, or successful our theories about the Universe are, they must always be confronted with experiments. If there’s a new, conflicting experimental result, it must be verified and validated independently to make sure it’s correct. Well, that was what the LSND experiment was for neutrinos and the Standard Model: an outlier that couldn’t be explained consistently with the other observations. After 16 years, the MiniBooNe experiment has released their final results: validating LSND and presenting a combined 6.0-sigma significance. Neutrinos don’t behave as they should.
How Many Fundamental Constants Does It Take To Explain The Universe?
“Our Universe is an intricate, amazing place, and yet our greatest hopes of a unified theory — a theory of everything — seek to decrease the number of fundamental constants we need. In reality, though, the more we learn about the Universe, the more parameters we’re learning it takes to fully describe it. It’s important to recognize where we are and what it takes, today, to describe the entirety of what’s known.
But we still don’t know everything, and so it’s also important to keep searching for a more complete paradigm. If we’re successful, it will give us absolutely everything the Universe has in it, including solutions to our current mysteries. The hope of many, but not a requirement, is that the Universe will wind up being simpler than we currently know. Right now, unfortunately, anything simpler than what’s been put forth here is too simple to work. Our Universe may not be elegant, after all.”
Think about everything that exists in our Universe. We have the four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. We have all the particles and antiparticles of the Standard Model; we have the bosons; we have the ways that particle behavior changes dependent on energy. We have hundreds of known composite particles and the ways that they interact, couple and decay. For everything that’s known, there are at least 26 fundamental constants required to explain the Universe on top of the laws of physics themselves, and still, they don’t give us everything.
Why Physics Needs, And Deserves, A Post-LHC Collider
“If what we observe and measure is identical to what the Standard Model predicts, then we haven’t found anything new. So far, that’s what the LHC has revealed: particles that behave in perfect accord with the Standard Model.
But there might be new particles out there. There might be new physics, new forces, new interactions, new couplings, or any slew of exotic scenarios. Some of them are scenarios we haven’t even yet envisioned, but the dream of particle physics is that new data will lead the way. As we peel back the veil of our cosmic ignorance; as we probe the energy frontiers; as we produce more and more events, we start obtaining data like we’ve never had before.”
There are some big differences between theorists and experimentalists. Theorists look at the big picture, come up with their preferred hypotheses and ideas, and work to create a consistent, predictive framework that provide possible signatures of what might extend our knowledge of the Universe. But experimentalists have, as their main goal, to gather more data and probe what is currently unknown. Both work hard to extend our knowledge of the Universe, but experimental results are useful and interesting in their own right, regardless of what truths they do or do not reveal. To some, the LHC’s results, discovering a Standard Model Higgs, and nothing else new, have led to a nightmare scenario.