In the wee hours of tomorrow morning, the latest crop of recipients for the Nobel Prize in Physics will be announced by the Royal Swedish Academy of Sciences. As always, there is considerable speculation about who will get the nod. Contrary to what many might assume, it is doubtful that Peter Higgs and his cohorts will be announced as recipients, at least not this year. More on why in a bit.
So, who will be snagging the prize? As outlined at ScienceWatch (as well as numerous other places), the front runners are:
- Stephen E. Harris and Lene V. Hau for their work with “slow light“.
- Leight T. Canham for his discovery of photoluminescence in porous silicon.
- Charles H. Bennett, Gilles Brassard, and William K. Wootters for their work on quantum teleportation. (Cue non-applicable Star Trek references in 3…2…1.)
For other predictions, see the following:
- Thomson Reuters Predicts 2012 Nobel Laureates | Reuters
- Nobel Prize 2012: The scientists, writers, and peace-mongers who are favored to win the famous awards. – Slate Magazine
- Nobel prize predictions released – USATODAY.com
- Nobel Prizes 2012: predicted winners | Dean Burnett | Science | guardian.co.uk
- Physics Buzz: Standing at the Wavefront of Scientific Discovery: A leadup to the Nobel Prize in Physics
So, why not Peter Higgs?
Given that the discovery of the Higgs boson has been hailed as one of the greatest scientific breakthroughs thus far this century, why wouldn’t the prize go to those who predicted it almost half a century ago?
Well, eventually Higgs and his cohorts will undoubtedly be recognized by the Nobel Committee. The problem right now is that the discovery of the predicted Higgs boson is still somewhat preliminary, and the Nobel Committee prefers to award the Physics prize on the basis of work that is on a firm footing. A new boson has been discovered at the Large Hadron Collider. Of that there is no doubt at this time. At least at a cursory level, it looks pretty much like the Higgs boson as predicted by the Standard Model, but more data collection and analysis are needed to confirm this. That confirmation needs to take place on two fronts: branching ratios, and spin. It is these final confirmations which will ultimately determine whether it is the Standard Model Higgs that has been discovered (which, frankly, would be a rather anti-climactic outcome), a variant of the Higgs from an extension of the Standard Model (wee, new physics), or something completely unexpected (double wee, REALLY new physics).
A long-standing maxim in particle physics is that if it is possible for something to happen, it will happen. It might happen rarely, but it will happen.
When a particle decays, there are generally many decay paths it can take. A sufficiently large population of such particles will take all available decay paths. The percentages of events that follow each decay path are collectively referred to as the branching ratios for that decay, and a given theoretical model can provide predictions of what those branching ratios will be.
This is no less the case with the Higgs boson. Of the decays observed thus far for the newly-discovered boson, there seems to be a slight excess in the rates for the di-photon decay mode compared to Standard Model predictions. There is also a lack of already-rare double-tau decays. If these deviations are just statistical anomalies, they should go away as more data is collected. Otherwise, we may be looking at new physics! The remainder of this year’s data collection run at the LHC should settle this issue once and for all.
The other aspect the requires clarification is spin. LHC researchers know from the decay products detected thus far that the new particle is not a fermion (a particle with half-integer spin, such as electrons, protons, and neutrons), but is instead a boson (a particle with integer spin). There is simply no way to add up the spins of the decay products to add up to a fermionic parent particle. The new particle must have integer spin. The question now is what that integer is. Intermediate vector bosons such as photons and W and Z bosons have a vector spin of 1. The hypothetical quantum intermediary for gravitation, the graviton, is expected to have a spin of 2 if it exists. (Why this is the case is, in of itself, an interesting story, but we’ll save that for another day.)
The Higgs boson, as predicted by the Standard Model, should be a spin 0 scalar boson. If the new boson has any spin other than 0, we know right off the bat that we are not dealing with a Standard Model Higgs. So, how do we make that determination?
One somewhat complicated way is by studying the angular distribution of the decay products. This approach is described here. A somewhat more straightforward approach is to take advantage of the data collected for confirming the branching ratios, because the decay products help constrain the possibilities. Here’s how.
First of all, there is something basic to the way these spins work that needs to be understood to follow the line of reasoning that I’m about to describe. Due to the fact that they have mass, the W and Z bosons can do something that photons can’t do. Whereas a photon can be measured as having a spin of +1 (spin “up”) or -1 (spin “down”) along a given axis (usually taken to be the direction in which the particle is travelling), W and Z bosons can also have a “sideways” spin, and can thus be measured as having a spin of +1, -1, or 0 along a given axis (with the spin vector of length 1 being perpendicular to the measured axis). This impacts the spin math below.
Supposing we have a spin 0 particle (like the SM Higgs boson). What combinations of decay products can it have to balance that spin?
|Photons||+1 + -1|
|Fermions||+½ + -½|
|W, Z bosons||0 + 0
+1 + -1
Note that a spin 1 particle cannot decay to two photons. The math doesn’t add up. Since the new particle has been seen to decay to two photons, we can rule it out as being a spin 1 particle.
For the spin 2 case, such a particle cannot decay to a pair of fermions. Again, the spins don’t add up properly. A spin 2 particle CAN decay to a pair of fermions plus a gluon (spin 1) though. If we see, for example, a decay to two tau leptons without anything else (which is predicted by the Standard Model for Higgs decays, but has not yet been detected), we know that we can’t be looking at a spin 2 particle.
In summary, to crib a graphic from Aidan Randle-Conde over at the Quantum Diaries blog, here is the summary of our quest to determine the spin of our new boson:
For more info on this, have a look at this lovely paper.
So, Who Gets It?
So, assuming that all of this does get hammered out (which could quite possibly happen by the end of this year), a Nobel Prize is quite likely in the offing for this breakthrough, at least at some point in the near future. But, as I’ve pointed out in a previous post, this raises the question of who gets it. Although the name of this new boson is inextricably tied to that of Peter Higgs, he did not work on this in isolation. Hammering out the theoretical underpinnings of the Higgs mechanism involved contributions from several individuals, and a single Nobel Prize can only be split among three (living) people at most. Those involved include:
- Phillip Anderson, who first crafted the broad strokes of the mechanism
- Peter Higgs, who first pointed out that a boson would be produced by the field in question
- Robert Brout (deceased)
- François Englert
- Gerald Guralnik
- C. R. Hagen
- Tom Kibble
One of these gentlemen, sadly, has already passed away. All are well advanced in years. With the passage of a little time, the difficulty of choosing which three to recognize may be rendered moot. But that hardly seems a fair way to settle the issue.
For more on the history of the Higgs mechanism, see Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism (history) – Scholarpedia.