Wednesday, October 9, 2013

'God Particle' Theorists Get Nobel Prize in Physics - But Pieces of Higgs Puzzle May Still Be Missing

The announcement yesterday of the award of the Nobel Prize in Physics created quite a media storm. Half a century after he formulated the theory that would change the world, Peter Higgs,  84, of Edinborough University, shared the 8 million Swedish kronor (£775,000) prize with Fran├žois Englert at the Free University of Brussels.

The pair received the prize for showing how fundamental particles get their masses. Before the theory, the answer to this basic question was unknown After all, they had postulated the existence of the "God Particle" - otherwise known as the Higgs boson. It can be thought as the "glue" which holds particles of mass together.

In the parlance of The Royal Swedish Academy, the prize was awarded  for "the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the Atlas and CMS experiments at Cern's Large Hadron Collider."

What is the background here? It basically began when the two theorists produced a series of papers in 1964 that described how an invisible field that lurks in the vacuum of space interacts with elementary particles and gives some of them mass.  Now, this was still before Arno Penzias and  Robert Wilson's detection of the 2.7 K isotropic radiation subsequently traced to the "Big Bang". And hence, before the cohering process that resulted in what's been called the Standard Model.

Elaborating a bit: this so-called 'Standard Model' is generally defined as the symmetry:

SU(3) x SU(2) X U(1)

where each of the above denotes a specific matrix, or more exactly a group. See, e.g.

In the case of SU(2) we describe it as the "special unitary group" which has the form:

S =


where a*, b* are complex conjugates and we have (aa* + b*b) = 1. Thus the elements of SU(2) are the unitary 2 x 2 matrices with DET (determinant) = 1. These groups thus define the behavior of a specific class of subatomic particles. Spontaneous symmetry breaking would therefore resolve this combination into constituent parts, e.g.: SU(3) associated with the 'color force' of quarks:

 SU(2) x U(1)

associated with the electro-weak force.

One possible symmetry breaking (quark -boson format) is:

SU(3) x SU(2) X U(1) -> SU(3) + SU(2) x U(1)

which would occur at a particular ambient temperature (T_qb) for the universe at some epoch (E_qb) in the past. In the foregoing, the synthesis of SU(2) and U(1) into the locally gauge invariant electro-weak theory requires a mechanism which confers mass to three vector bosons while leaving the photon massless. This 'mass-giving' mechanism is called the Higgs Field or Higgs mechanism, and it demands the existence of one or more massive, spin-0 bosons otherwise called Higgs bosons.

Last year, theory evidently transmuted into reality when the discovery (thanks to the large hadron collider)  was announced at CERN. As I noted (in a July 4 blog post), Dr. Rolf Heuer, director general of CERN, while referring to the new discovery as "a historic milestone"  nevertheless cautioned that it was too soon to know for sure if the new particle (coming in at 125 billion electron volts) is actually the long sought particle. (Also, was it a unique Higgs, or just one of several?)

The problem? The culmination of analyses of over 800 trillion proton-proton collisions over the 2 years leading up to the announcement generated a quandary. When buttonholed,  the physicists admitted they  actually knew little. The CERN results were mostly based on measurements of two or three of the dozen different ways, or “channels,” by which a Higgs boson could be produced and then decay. Worse, there were hints that some of the channels were overproducing the Higgs while others might have been underproducing. In either case, false positives or false negatives, one had to look askance at the initial results.

The upshot? There may not have been a real Higgs discovered but a spurious 'mirage' imitating some of its properties but more a confection of the data than based in reality. Also, assuming a genuine signal or find, it may not have been unique but only ONE of two or three different Higgs bosons. Much like the case of the neutrino, which was once believed to be one entity only, but we now know is THREE: the electron neutrino, the tau neutrino and the muon neutrino, see e.g.

By March 15 of this year the issue appeared settled, to the extent that at least that ONE Higgs had indeed been  found,  when CERN's brain trust stopped dithering and announced that the particle described in July 2012 was, in fact, a Higgs Boson. Spokesman Joe Incandela said in a statement issued March 15 :

The preliminary results with the full 2012 data set are magnificent and to me it is clear that we are dealing with a Higgs boson though we still have a long way to go to know what kind of Higgs boson it is,

 To make this final determination, the dataset was analyzed to see if the quantum properties of the boson discovered in July matched the properties that are currently predicted by physics. After tests in two different detectors, it was confirmed that the particle possessed those properties

Ben Allanach, a theoretical physicist at Cambridge University, said:

"This is the recognition of a triumph for fundamental physics that will stay in the history books for millennia to come. I am thrilled about the prize, and Englert and Higgs both deserve it well. I cannot over-stress the importance of the discovery. The mass mechanism that the Higgs boson is a signal for has had a huge impact on particle physics over the last 50 years. I think many of us felt that it had to be correct, although we were willing to let data dissuade us."

Maybe. But it would still be nice to finally also settle the issue of whether the putative discovery is unique, or if other Higgs bosons may be lurking in the vacuum. To that end further experiments using an upgraded Large Hadron Collider may be called for. Unfortunately,  the LHC is  currently closed down for repairs. But the team is working on making it faster and more powerful. By then, the final questions may be settled at last and we can then know the extent to which the Standard Model needs to be revised.

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