Higgsapalooza

In the beginning there was light.
Sort of.
When energies were high enough, particles were effectively massless and the universe was a nice seething mess of particle/anti-particle creation and annihilation.
As the universe cools, a symmtery, the Electroweak symmetry breaks, a field condenses out, and interesting stuff starts happening.
Hence we get chemistry, and the autocatalytic evolving goo that reaches out and ties to puzzle out where it all came from.

In the Standard Model of Particle Physics, the rest mass of the spectrum of normal matter particles is dynamically generated. The mass we observe is not an intrinsic property of the particles, rather much of the mass of the elementary particles comes from the interaction of the particles with a putative elementary scalar field – the Higgs.
Specifically, the elementary vector bosons in the theory acquire mass by interacting with a massive Higgs scalar.
One of the vector bosons remains massless, as a gauge field remains fully symmetric. In the standard model this is electromagnetism, and the photon remains exactly massless.
The masses of the W and Z bosons are in good accord with calculations assuming a Higgs mechanism, which has long been good indirect evidence for the existence of a Higgs mechanism, or something effectively equivalent.

It gets complicated; the mass of the Higgs itself is affected by the interaction, it has an effective mass that has long been known to have to be somewhere between 114 GeV and 1,400 GeV.
A low mass Higgs permits the Standard Model to be consistent all the way to the Planck scale.
The vacuum expectation value of the Higgs is 246 GeV, and the effective mass, which is generally different, has over the years been constrained to be at the low end of the permitted range.
Interestingly, that is also where extended Higgs models from Supersymmetric theories tend to favour as the value of the effective mass of the Higgs.

Compound particles, eg the proton and neutron, also have mass terms from their internal binding energy, so the effective mass of matter is due to a mixture of mass of the vector bosons generated by the Higgs fields, mass of the fermions from flavour state mixing, but much of what we think of as normal particle mass is just binding energy.

So, now we found it, last of the canonical Standard Models zoo of elementary particles. At nominal 5 σ.
Singlet standard model scalar Higgs.
Physics is Unitary.
No funky trinity or other fun new stuff.
We’re done.
We can go home now.
NOT.

So, both experiments see a signal consistent with a massive scalar boson; mass is ~125-126 GeV, and they look in five decay channels, looking at particle/anti-particle decay modes.
Excess signal is seen for photon-photon, b/b-bar, WW, and ZZ but not tau/tau-bar decay modes.
Signals are looked at based on models for Standard Model decay channels; there is some relative excess over model predictions in the photon decay channel, but not formally significant, and the tau channel is not showing at the amplitude expected.
Could be small number statistics, could be signature of new physics.
Future LHC runs will provide some additional constraints and improve significance of all the detections.

So, what does this tell us: well, it is a vindication of four decades of careful physics, not to mention the power of refereeing… it is also a little bit worrying.

We know the Standard Model is incomplete, there has to be something more to it – and if all the remaining action is up at the Planck scale, we are stuck.
Something like supersymmetry, or some extended Grand Unification Theory, or any new physics showing up as something more complicated than a minimal single scalar Higgs.
Nothing so far, though theorists will no doubt start speculating about the hints of anomalous decay ratios in the data so far in the hope that they’ll be confirmed.

There will also be an impetous to move on with the International Linear Collider, and get a lot of data at collision energies that probe the Higgs mass well, with room to upgrade natch.
I wouldn’t be surprised to see a proposal for a ~ 10 km machine that can be a “Higgs factory”.
There will also be intense scrutiny of astroparticle data in the appropriate energy range.

So, what about Dark Matter?
The Higgs ain’t Dark Matter.

Does it couple to Dark Matter.
Well, depends.
Since we don’t know wtf Dark Matter is, it might, or it might not.
A least massive supersymmetric partner as a dark matter particle might have interesting Higgs couplings. Maybe.
Especially if the mass is in the low mass range hinted at by recent cold dark matter experimental searches.
That would be very interesting indeed.

There is one other personal bugaboo of mine: the Higgs is quintessentially a scalar field – there is no charge, colour, flavour or any other internal quantum number stuck on it.
It is an ambient field.

So, apparently, maybe, is Dark Energy: it is looking like a very annoying ambient scalar field.
But not the same.
Or are they… somehow?
If they are, how come we don’t know that.
If they are not, then what is the difference between the two?

I’ve had this nagging sense for about twenty five years now that we’re missing something basic in all of this.

‘course that is why we need more data.
Always more bloody data.

Cosmic Variance live blog

Ye Olde Introduction to Higgs from few years ago

Resonaances blogging it

Matt Strassler

Starts With a Bang take on it

Comments

  1. #1 Doug Little
    July 5, 2012

    Hi Steinn,

    I was interested in where the LHC will double down on its efforts now that the Higgs (or somthing Higgs like) has been discovered. I know they are going to extend the run before the 2 year upgrade to full design energy to try and pin down some of the properties of the particle. After the upgrade what potentially can be done with all that extra energy? Is there anything specific that they are looking for that the LHC can’t reach in its current configuration?

  2. #2 Mike
    United States
    July 6, 2012

    Maybe I completely misunderstand the whole Higgs field, but the implication seems to be that particles *don’t* actually have mass, as such, but what we call mass is simply their interaction with the Higgs field.

  3. #3 Steinn Sigurðsson
    July 6, 2012

    @Doug – the next 15 month run is primarily to cumulate more signal to nail the result down, and to get statistical significance on the branching ratios for the different decay modes and whether they do differ from Standard Model predictions.
    The design energy for the LHC was higher ’cause they didn’t know what they’d find, and they might as well ramp up. Secretly they have to be hoping now to see signatures of supersymmetry at somewhat higher energies…

    @Mike – the Higgs field primarily gives mass to the vector bosons, specifically the weak force mediating W and Z bosons. The fermions, the electron, muon, tau and the quarks sort of get their mass indirectly from the Higgs mechanism but it gets complicated.
    But, most of the mass of normal matter, specifically most of the mass of the proton and neutron, is really just the binding energy of the gluons, and not directly Higgs related.

  4. #4 Doug Little
    July 6, 2012

    Secretly they have to be hoping now to see signatures of supersymmetry at somewhat higher energies

    What would those signatures look like? Or don’t they know what they are looking for until they see something that doesn’t fit with expectation.

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