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Explaining the Higgs: on TV last night!

“We knew that we had indeed done something that was very different and very exciting, but we still didn’t expect it to have something to do with physical reality.” -Gerald Guralnik, co-developer of the Higgs mechanism

Might as well make this entire week “Higgs week” here on Starts With A Bang, given how important yesterday’s discovery/announcement was! It isn’t every day, after all, that you see a theoretical physicist on the 7PM news. (Video here.)

Ethan on KGW explaining the Higgs

Image credit:

(So proud of Portland, OR’s local TV station, KGW NewsChannel 8, for being willing to promote science to the whole city and have me on last night!)

Let’s just take a little time today — now that we’ve confidently announced the discovery of the Higgs — to recap what we now know.

Inside matter

Image credit: ETH Zurich, Institute for Particle Physics.

All the matter we know of is made out of quarks and leptons of various types, with the most common being the up-and-down quarks (that bind together to make protons and neutrons) and electrons (that bind with nuclei to make atoms). There are heavier fundamental quarks and leptons that will decay into the light ones with very short lifetimes, but they’re just as fundamental to the Universe as the ones that make us up. All quarks and leptons have an intrinsic angular momentum — known as spin — equal to Planck’s constant (ħ) divided by 2.

Fundamental Forces

Image credit: Fundamental Forces, via

There are also the gauge bosons: the fundamental particles responsible for the fundamental forces. There are the gluons, responsible for the strong force holding atomic nuclei and individual nucleons together, the weak bosons, responsible for radioactive decay and neutrino interactions, and the photon, responsible for electromagnetic interactions, radiation, and light. All of these gauge bosons have a quantum mechanical spin of Planck’s constant (ħ), double the value for quarks and leptons.

Curved spacetime

Image credit: curved spacetime, courtesy of

On the level of individual particles, we do not have a complete understanding of how gravity works. Our best theory for that is Einstein’s general relativity, which treats space-time as a fabric, and where matter and energy are responsible for curving this fabric. The matter and energy in the Universe determine the shape of the fabric, and then the particles in the Universe follow the path determined by that fabric.

Hawking Radiation

Curved spacetime is necessary for understanding quantum effects like Hawking radiation. Image credit:

The shape of the fabric is also important for quantum field theory; all of these particles we know of exist and interact in this curved spacetime, and the shape of this spacetime must be taken into account to get the correct predictions for the behavior of particles in the Universe. That’s our best understanding of gravitation.

And finally, as of yesterdaythere’s the Higgs.

Standard model particles

Image credit: The standard model by Fermilab, modifications by me.

The first and only fundamental particle with no spin. The particle that comes from the mechanism responsible for the masses of all the other fundamental particles, including the Higgs bosonitself. The final piece of the standard model puzzle required to explain the strong, weak, and electromagnetic forces and all of the particles therein.

CERN with detectors CMS and ATLAS

Image credit: © 2012 The School of Physics and Astronomy, The University of Edinburgh.

It took us decades of recreating temperatures and energies here at particle accelerators on Earth not found anyplace else to figure this out. The conditions we create in our most powerful accelerators are not found in the center of the Sun, nor in the central core of the Milky Way galaxy, nor around neutron stars and black holes, nor in the cosmic supernova explosions that give rise to all the heavy elements.

Supernova 1987a

Image credit: SN 1987a by NASA, ESA, P. Challis and R. Kirshner, by the Hubble Space Telescope.

These conditions have not existed* in the Universe, in fact, since the very early stages of the Big Bang, when the Universe was less than a microsecond old!

Quark-Gluon plasma

Image credit: CERN.

But yet, here we are, having successfully accelerated protons up to a record 299,792,450 m/s, just 8 m/s shy of the true speed of light, and collided them with protons moving the same speed in the opposite direction. Do this billions upon billions of times with a giant particle detector around the collision point, and on very rare occasions, you’ll be fortunate enough to create a Higgs boson, whose decay remnants we can detect.

Flying Spaghetti Monster, is that you?

Image credit: the CMS detector at CERN, 2009.

And now, at long last, the Higgs boson — the last undetected particle from the standard model — has been discovered. We’ve measured its mass and its spin, but not its width or its lifetime, yet, and there are still a bunch of unanswered questions. But for now, at least, I’m still celebrating the one question we did answer: the Higgs mechanism is correct, the Higgs boson (a fundamental, spinless scalar particle) does exist, it has a mass of 125-126 GeV, and the standard model is now complete!

We’ll take a look at the unanswered questions — and what’s next for physics and the LHC — very soon, but in the meantime, enjoy the continue Higgs-celebration, and if you’ve made it this far, enjoy my appearance on yesterday’s evening news!

Video | News | Weather | Sports

Wed Jul 04 19:37:02 PDT 2012

Hot Box: Explaining the Higgs boson

Theoretical astrophysicist Ehan Siegel explains the big Higgs boson announcement and what it means for the rest of us. view full article

* — Okay, so occasionally there are ultra-high-energy cosmic rays that are energetic enough to produce a particle such as this. But they’re extremely rare and their origin is not understood, so for all intents and purposes, these conditions don’t exist in the Universe today.

Views: 1436

Replies to This Discussion

San Diegos Giant Fireball

Das Feuerwerk am 4. Juli in San Diego ist wegen ‘nes technischen Fehlers komplett auf einmal hochgegangen. Epic Boom is epic. Die LA Times hat alles dazu, das Instagram-Foto oben kommt von jemandem namens Ben Baller.

The discovery of the Higgs Boson? Garrett Lisi explains

I voted, it's 83% percent yes; probably because the sheep haven't been fucked yet.

Still 83 percent - Goat's Rule!!!

does that look tasty?

Goats are the enemies of sheep, who are the blessed of the lord.  It has nothing to do  with eating.

From Starts with a Bang

“Even if there is only one possible unified theory, it is just a set of rules and equations. What is it that breathes fire into the equations and makes a universe for them to describe?” -Stephen Hawking

After a long search spanning more than my entire lifetime (so far), the Higgs boson has finally been discovered at both detectors — CMS and ATLAS — at the Large Hadron Collider at CERN.

LHC from the air, with detectors

Image credit: CERN / Particle Physics for Scottish Schools.

For a little more on this, check out the earlier posts here celebrating Higgs week:

Our standard model of elementary particles and interactions is now complete, with every single particle that’s a part of it having been discovered.

Standard Model Particles

Image credit: The standard model by Fermilab, modifications by me.

Combined with our knowledge of gravitation through general relativity, and the framework of quantum field theory describing all the standard model particles and how they interact with one another, it’s no stretch to say we’re sitting pretty, and that we’ve come an incredibly long way to arrive at our present understanding, especially considering practically none of this was known a century ago.

Fundamental Forces

Image credit: Fundamental Forces, via

But it isn’t like physics is over now, not by a long shot. There are still a great many mysteries to uncover, and a great many things we don’t fully understand about the Universe. In addition, there are also a great many hypotheses in physics that could lead us towards solving some of these questions. All of them are speculative, none of them are certain, and — unfortunately — a few of them have been (and are being) grossly oversold to the general public.

In no particular order, here are the questions we have on our plate at the moment.

Particle mass spectrum

Image credit: Gordon Kane, Scientific American, June 2003.

Why do the fundamental particles have the masses that they do? The standard model allows the Higgs to give rest mass to all of the particles (or leave them massless), but the reason they have these values is unknown.

What’s more, is that these values are not what we would have expected based on our current understanding of how the laws of physics work. Based on that, the masses of these particles should be on the order of 1019 GeV, some quadrillion times heavier than even the top quark, which is the heaviest known particle. This problem is known in physics as the hierarchy problem, and the standard model has no answer for it.

Neutrino mass hierarchy

Image credit: from a talk by Steve King, retrieved from Luca Merlo.

There’s also the (much newer) problem of neutrino masses: why are they so mind-bogglingly light, and yet of non-zero mass? Do they get their mass from the Higgs? If so, why is it so much smaller than all the other particles, and if not, why not?

Grand Unification Theory

Image credit: J. Barrow and J. Silk, 1993, Oxford Press.

We know that the electromagnetic force and the weak force are, at very high energies, different manifestations of the same fundamental force: the electroweak force. It’s only in our low-energy Universe that they appear so different from one another. Another way of saying this is that “the electroweak symmetry is broken,” and the separate electromagnetic force and weak force are how that broken symmetry is manifested.

Well, it’s possible that at very high energies, the strong force unifies with the electroweak force, giving us what’s known as a Grand Unified Theory. Extending even beyond that, it’s possible that at even higher energies, gravity unifies with the other three forces; this is the basic idea behindstring theory, the most promising framework for unifying all of the known forces and interactions into one theory.

Matter antimatter asymmetry

We live in a Universe where there's more matter than antimatter, but why?

Where does the origin of the matter-antimatter asymmetry come from? In other words, why do we have a Universe full of matter and only a tiny amount of antimatter, rather than equal amounts of both. We know how this is possible, of course, but we do not yet know how it actually happened. This problem of baryogenesis is yet another great unsolved problem of physics, and finding the Higgs and completing the Standard Model sheds no light on it.

CP Violation

Image credit: James Schombert at University of Oregon.

If you look at the fundamental interactions of matter, particles and antiparticles have a great deal in common. (Known as C-symmetry.) Particles spinning (or with angular momentum) in one direction have a lot in common with those spinning in the opposite direction. (Known as P-symmetry.) But in the weak interactions, not only do particles and antiparticles behave differently, and not only do particles spinning in one orientation behave differently that particles spinning in the opposite one, but particles spinning in one direction have slightly different physics than antiparticles spinning in the opposite direction! In other words, not only are C and P symmetries violated, they’re both violated together.

But for some unknown reason, even though there’s nothing in the standard model that prevents it, the CP-symmetry is not violated in the strong interactions. Known as the strong-CP problem, this is yet another physical reality that is unexplained by our current understanding of the Universe.

Baryon Acoustic Oscillation Visualization

Image credit: Amit Chourasia, Visservices, SDSC, Robert Harkness, Mike Norman and Pascal Paschos.

And finally, there’s dark matter and dark energy. They are both required for the Universe to look the way it does, but our current understanding of things does not explain where either one of them comes from.

Don’t let anyone tell you that “Physics is over now that we’ve found the Higgs.” On the contrary, it’s only the physics that we had every right to expect was correct that’s over. Now’s where the fun begins.

And I say “fun” fully recognizing that even our best ideas for what comes next have severe problems, and they may all be wrong.


Supersymmetric particles

Image credit: DESY at Hamburg.

Supersymmetry — SUSY for short — is the best candidate theory to solve the hierarchy problem. If it’s correct, it can also conceivably provide a dark matter candidate, give evidence for the potential unification of the strong force, and give circumstantial evidence for superstring theory(which requires SUSY).

Unfortunately, in order to actually solve the hierarchy problem, the masses of the superparticles need to be of the same magnitude as the masses of the normal, known, standard model particles. If the masses of the superparticles are beyond the reach of the LHC, then SUSY, even if it exists, no longer solves the hierarchy problem, which was the original motivation for SUSY in the first place! (In fact, even the best-case scenario for SUSY already has a little hierarchy problem.)

The longer the LHC goes without finding any of them, the more disfavored SUSY is going to become. Now that we’ve discovered the Higgs and we know its mass, it’s conceivable (although it would be a true nightmare scenario) that there are no new particles to be found until we get up to a monstrous 1010 GeV in energy! (It could’ve been even worse if the Higgs were a few GeV heavier!)

How stable is the Standard Model?

Image credit: Giuseppe Degrassi et al., 2012.

(And good luck getting a particle accelerator the size of Saturn’s orbit to find it!)

There could have been multiple Higgs particles (a Higgs multiplet), as predicted by practically all grand unified theories.

CERN CMS Higgs Signal

Image credit: CERN / CMS collaboration.

But the data very convincingly shows that there is just one, singlet, spin=0 Higgs, which is what the standard model alone predicts. I have written before about how the lack of proton decaydisfavors nearly all GUTs, but the lone standard model Higgs may be an even more damning observation.

So while I may not be impressed with string theory, SUSY, or grand unification, the cracks and unsolved problems are where the new, exciting (and probably surprising) discoveries are bound to happen. The first thing to check — when sufficient data comes in (and not before) — is whether the “particle-consistent-with-the-Higgs” that we’re producing at CERN does, in fact, behave like the Higgs is supposed to!

Figure 3 from P.P. Giardino

Image credit: P. P. Giardino et al., 2012.

Because if it doesn’t, you can add that to the list of unsolved problems in physics, too!

It may not be what you want to hear, that the leading attempts to extend the standard model have severe problems with them, but in physics, as always, data must be the ultimate decider of the veracity of our theories. There are some great possibilities for this Universe, and — just like you — I can’t wait until we find out more. Hope you’ll continue to join me as we keep looking!

The Hunt for Higgs (complete) - BBC Horizon 2012

CERN is the headquarters of the European Organisation for Nuclear Research. It's home to some of the thousands of scientists who have been doggedly hunting the elusive Higgs boson and the £6 billion experiment that they're using to do it. Especially built to find the one particle that's thought to give substance to everything in the universe. 

Horizon has been following the final stages of the hunt for this most important and elusive of particles.


Great article by Ethan, as usual. Great posting, doone. Great and useful comments to understand this important event. 

Jul. 12, 2012

It doesn’t get any simpler than this.


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