Category: particle physics

How Certain Are We That Protons Don’t Decay?

“There is no arguing, however, that in all our endeavors to measure the stability of the proton, we’ve never observed even one event of a proton spontaneously decaying into lighter particles and violating the conservation of baryon number. If the proton is truly stable and will never decay, it means that a whole lot of proposed extensions to the Standard Model — Grand Unification Theories, supersymmetry, supergravity and string theory among them — cannot describe our Universe.

Regardless of whether the proton is truly stable forever and ever or “only” stable for a septillion times the current age of the Universe, the only way we’ll figure it out is by performing the critical experiments and watching how the Universe behaves. We have a matter-filled Universe almost completely devoid of antimatter, and nobody knows why. If the proton turns out to be truly stable, many of our best ideas for what could cause it will be ruled out.

The secrets of nature may remain a mystery for a little while longer, but as long as we keep looking, there’s always the hope of a new, revolutionary discovery.”

Do protons decay? If they do, we’d have a hint of where our matter-antimatter asymmetry comes from. We’d have an idea that grand unification might be correct, either with or without supersymmetry, extra dimensions, or string theory. And we’d learn that nothing, not even the humble atom, will be stable forever; much like galaxies and black holes, they’ll also eventually decay.

But if the proton is stable, as we’ve observed so far, it’s back to the drawing board on all of it. Here’s how certain we are, at the start of 2020, that the proton really is stable.

The ‘Strong CP Problem’ Is The Most Underrated Puzzle In All Of Physics

“At almost every frontier in theoretical physics, scientists are struggling to explain what we observe. We don’t know what composes dark matter; we don’t know what’s responsible for dark energy; we don’t know how matter won out over antimatter in the early stages of the Universe. But the strong CP problem is different: it’s a puzzle not because of something we observe, but because of the observed absence of something that’s so thoroughly expected.

Why, in the strong interactions, do particles that decay match exactly the decays of antiparticles in a mirror-image configuration? Why does the neutron not have an electric dipole moment? Many alternative solutions to a new symmetry, such as one of the quarks being massless, are now ruled out. Does nature just exist this way, in defiance of our expectations?

Through the right developments in theoretical and experimental physics, and with a little help from nature, we just might find out.”

The list of unsolved problems in physics is long and glorious, reflecting our curiosity about every yet-to-be-understood facet of our Universe. Most of what remains to be understood is because we observe something that we can’t explain: we see a phenomenon that our best current theories won’t predict without adding something new. But the strong CP problem is different: our best current theories predict that there should be CP violation in the strong interactions, but we don’t see any at all!

This is a puzzle that never gets the prominence it’s due based on its merits. Don’t be intimidated; find out what all the fuss is about today!

This One Award Was The Biggest Injustice In Nobel Prize History

“Every October, the Nobel foundation awards prizes celebrating the greatest advances in numerous scientific fields. With a maximum of three winners per prize, many of history’s most deserving candidates have gone unrewarded. However, the greatest injustices occurred when the scientists behind the most worthy contributions were snubbed.”

Imagine this scenario: you work hard all your life investigating some aspect of reality with as much scientific rigor as anyone ever has. You make a great breakthrough working on a very hard problem, and you push your scientific field forward in a novel, important, and unprecedented way. And then, when the time comes to evaluate the quality and impact of your work, it’s chosen as being Nobel-worthy. 

Only, when they announce the winners of the Nobel Prize, your name isn’t called at all. Instead, other scientists are awarded the prize, while both your name and your decisive work are omitted from every aspect of the award. Sounds like a pretty big injustice, yes? 

Well, it’s happened to many people over the years, including Chien-Shiung Wu in perhaps the greatest injustice of them all. Come get the full story on the eve of the 2019 Nobel Prize in physics being awarded!

Ask Ethan: Why Are There Only Three Generations Of Particles?

“It is eminently possible that there are more particles out there than the Standard Model, as we know it, presently predicts. In fact, given all the components of the Universe that aren’t accounted for in the Standard Model, from dark matter to dark energy to inflation to the origin of the matter-antimatter asymmetry, it’s practically unreasonable to conclude that there aren’t additional particles.

But if the additional particles fit into the structure of the Standard Model as an additional generation, there are tremendous constraints. They could not have been created in great abundance during the early Universe. None of them can be less massive than 45.6 GeV/c^2. And they could not imprint an observable signature on the cosmic microwave background or in the abundance of the light elements.

Experimental results are the way we learn about the Universe, but the way those results fit into our most successful theoretical frameworks is how we conclude what else does and doesn’t exist in our Universe. Unless a future accelerator result surprises us tremendously, three generations is all we get: no more, no less, and nobody knows why.”

There are three generations of (fermionic) particles in the Universe. In addition to the lightest quarks (up and down), the electron and positron, and the electron neutrino and anti-neutrino, there are two extra, heavy “copies” of this structure. The charm-and-strange quarks plus the top-and-bottom quarks fill the remaining generations of quarks, while the muon and muon neutrino and anti-neutrino plus the tau and tau neutrino and anti-neutrino comprise the next generation of leptons.

Theoretically, there’s nothing demanding three and only three generations, but experiments have shown that there are no more to within absurd constraints. Here’s the full story of how we know there are only three generations.

This Is Why Neutrinos Are The Standard Model’s Greatest Puzzle

“But this is where the big puzzle comes in: if neutrinos and antineutrinos have mass, then it should be possible to turn a left-handed neutrino into a right-handed particle simply by either slowing the neutrino down or speeding yourself up. If you curl your fingers around your left thumb and point your thumb towards you, your fingers curl clockwise around your thumb. If you point your left thumb away from you, though, your fingers appear to curl counterclockwise instead.

In other words, we can change the perceived spin of a neutrino or antineutrino simply by changing our motion relative to it. Since all neutrinos are left-handed and all antineutrinos are right-handed, does this mean that you can transform a left-handed neutrino into a right-handed antineutrino simply by changing your perspective? Or does this mean that left-handed anti-neutrinos and right-handed neutrinos exist, but are beyond our current detection capabilities?”

Every fermion in the Standard Model has a number of properties inherent to it. Mass, charge, baryon number, lepton number, lepton family number, etc. All the fermions that exist in the Standard Model have non-zero masses, even the neutrino, and all of them can have their intrinsic angular momentum go in any direction… except the neutrino. Unlike all the other fermions, we’ve only ever seen left-handed neutrinos and right-handed antineutrinos. But if we’re clever enough, we can design an experiment that will reverse the perceived spin of these neutrinos.

What will we see then? Believe it or not, the answer could unlock the mystery of why our Universe is filled with matter and not anti-matter. Let’s do what we can to solve this puzzle; the entire Universe may be at stake!

Forget About Electrons And Protons; The Unstable Muon Could Be The Future Of Particle Physics

“Humanity can always choose to build a bigger ring or invest in producing stronger-field magnets; those are easy ways to go to higher energies in particle physics. But there’s no cure for synchrotron radiation with electrons and positrons; you’d have to use heavier particles instead. There’s no cure for energy being distributed among multiple constituent particles inside a proton; you’d have to use fundamental particles instead.

The muon is the one particle that could solve both of these issues. The only drawback is that they’re unstable, and difficult to keep alive for a long time. However, they’re easy to make: smash a proton beam into a piece of acrylic and you’ll produce pions, which will decay into both muons and anti-muons. Accelerate those muons to high energy and collimate them into beams, and you can put them in a circular collider.”

There are lots of possibilities being discussed for how we could build a next-generation particle collider, capable of pushing past the frontiers where the LHC will be fundamentally limited. We could go to a larger proton collider, we could go back to doing high-precision collisions of electrons and positrons to create large numbers of the known, existing particles, or we could push the frontiers in an entirely new way: by colliding muons with anti-muons.

“But they only live for 2.2 microseconds,” you correctly object. Good thing we understand physics. If we can get the technology there, it’s the best option imaginable.

This Is Why Two Higgs Bosons Don’t Have The Same Mass As One Another

“In this quantum Universe, every particle will have properties that are inherently uncertain, as many of the measurable properties are changed by the act of measurement itself, even if you measure a property other than the one you wish to know. While we might talk about photon or electron uncertainties most commonly, some particles are also unstable, which means their lifetime is not pre-determined from the moment of their creation. For those classes of particles, their inherent energy, and therefore their mass, is inherently variable, too.

While we might be able to state the mass of the average unstable particle of a particular variety, like the Higgs boson or the top quark, each individual particle of that type will have its own, unique value. Quantum uncertainty can now be convincingly extended all the way to the rest energy of an unstable, fundamental particle. In a quantum Universe, even a property as basic as mass itself can never be set in stone.”

Create an electron, and there will be a certain set of properties that you’ll know for certain, irrespective of any quantum uncertainty. You’ll know its mass, its electric charge, its intrinsic angular momentum, and many other properties as well. But that’s because the electron itself is a fundamentally stable particle: it’s lifetime is infinite, with no uncertainty. This isn’t true for many of the particles of the Standard Model, though, with the heaviest particles like the Higgs boson, the W and Z bosons, and the top quark having the shortest lifetime. Well, there’s also an energy-time uncertainty relation, and that means that the shorter your lifetime is, the bigger your inherent uncertainty in your energy is. Now, combine that with the knowledge that E = mc^2, and what do you get? 

An inherently uncertain mass. Yes, it’s true: every top quark you create has a unique mass that’s different from every other top quark. Come find out the science behind this remarkable property of nature!

Ask Ethan: Can Free Quarks Exist Outside Of A Bound-State Particle?

“In our low-energy, modern-day Universe, we only find quarks and antiquarks in bound, hadronic states: baryons, anti-baryons and mesons. But that’s only because the quarks that conventionally exist are long-lived, at low densities, and at low enough energies and temperatures. If we change any one of those three, the existence of free quarks is not only possible, but mandatory.

If the conditions for forming a bound state aren’t met, then confinement is impossible. The four ways we know how to get there are to create a top quark, to look to the early stages of the hot Big Bang, to collide heavy ions together at relativistic speeds, or to look inside the densest objects (like neutron stars or the hypothetical strange quark stars) to find the quark-gluon plasma inside. It’s not an easy feat to accomplish, but if you want to create matter in the most extreme states we know of, you have to go to extreme ends to get there.”

Have you ever wondered, if protons and neutrons are made of quarks, whether it’s possible to have a quark (or antiquark) exist outside of a bound-state system? There are lots of ways that we’ve tried to separate quarks out from their bound states that fail. Split a proton apart and it will split, but into other bound states. Take a meson and pull the quark and antiquark apart, and a new antiquark/quark pair will snap into existence to give you two new mesons instead. Even if you create a quark/antiquark pair in a collider that move in opposite directions, they hadronize and only produce the baryons and mesons we can detect: bound states.

But that’s not the end of the tricks up our (and the Universe’s) sleeve. We can create free quarks after all. If you’re curious, you can now find out how.

No, The Laws Of Physics Are Not The Same Forwards And Backwards In Time

“It took the creation of over 400 million ϒ(4s) particles to detect time-reversal violation directly, and this was accomplished by the BaBar collaboration back in 2012. The test for the reversal of initial and final entangled states is, to date, the only direct test ever performed to see if T-symmetry is conserved or violated in a direct fashion. Just as anticipated, the weak interactions violate this T-symmetry, proving that the laws of physics are not identical whether time runs forwards or backwards.

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! 

We once thought they were, and there’s only ever been one experiment to measure its violation directly: BaBar, in 2012. Here’s what we know and how we know it!

Does Particle Physics Have A Future On Earth?

“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.

Does particle physics have a future on Earth? It should, and here’s what I would recommend they choose if they have the political and financial will to do so.