Thursday, July 5, 2012

The Higgs Is Different: It's Spinless!

Yesterday's much anticipated announcement of the discovery, at CERN, of a particle that is "consistent with the Higgs boson" has generated an unprecedented amount of comment, both technical and popular. Even the most casual observer of the scene for the past year could have anticipated what was coming, and indeed even I, in tweeting a summary of the Phenomenology 2012 Symposium at the University of Pittsburgh last May noted that the CMS and ATLAS experiments at CERN, as well as the CDF Experiment at Fermilab, had found a resonance at 125 GeV consistent with the Higgs.

While many post-July 4  2012 commentators have focused on the role of Higgs in particle physics, as the short-lived 'heavy' particle (field) that gives other lighter particles from quarks to electrons their masses, and others have noted such things as the difficulty of deciding who, among the six physicists who published papers predicting and/or analyzing the phenomenon of the Higgs should get the Nobel Prize(s) [since not more than 3 can get it any given year], and others yet have focused on the analogies in condensed matter physics (with superconductivity and superfluidity), I found precious little commentary on what is truly different about the Higgs - that it is a boson, and a scalar boson at that - a spinless (i.e. spin zero) particle.

Jonathan Bagger's piece in the ILC Newsline, however, lays this out in crystal clear prose:

Every other fundamental particle discovered to date – the quarks, leptons and gauge bosons of the standard model – has spin, an intrinsically quantum mechanical property that determines its fate. The Higgs, however, does not. It is an entirely new form of matter. The spin of the quarks and leptons is ultimately responsible for the structure of matter, including the properties of nuclei and the electronic structures that govern all of chemistry. The spin of the gauge bosons gives rise to the forces of nature, ranging from electricity and magnetism to nuclear reactions and gravity. The Higgs, though, is different; it has no spin. Its spinless state allows it to condense and fill the vacuum, much like steam condenses to form the sea. It is this Higgs condensate that is responsible for mass: particles travelling through the condensate experience a drag that slows their motion and gives them mass. The more the drag, the greater the mass.
But even more importantly, he lays out the possible cosmological consequences of the other Higgs-like particles predicted in supersymmetry and Grand Unified Theories for Dark Matter, and Dark Energy:
Higgs-like particles are ubiquitous in theories of physics that extend beyond the standard model.  They are predicted by supersymmetry and by theories of grand unification.  Their condensates contribute to the dark energy that is accelerating the expansion of the universe, and they determine the geometry of the extra dimensions in string theory.  Higgs-like particles might even be responsible for cosmological inflation, the change in time of dark energy, the missing dark matter, or even the puzzling properties of neutrinos.
The reference to Higgs-like particles must be emphasized: he does not mean the Higgs particle currently under intense discussion (the "Standard Model Higgs", which refers only to the particle-field responsible for the generation of masses for the two W and one Z bosons through electroweak symmetry breaking). He refers here to similar particles, yet to be discovered, that are responsible for mass generation in theories of supersymmetry and Grand Unified Symmetry breaking. Also, that 'Higgs-like particles might even be responsible for cosmological inflation' must be clarified: shortly after the original suggestion by Alan Guth of MIT of cosmological inflation, the scalar field required for inflation, subsequently called the 'inflaton' was initially identified with the electroweak Higgs field, but after more theoretical developments ,the inflaton, is no longer thought to result solely from the electroweak Higgs. [A related field, known as the curvaton, has also been proposed that, while not driving inflation itself, creates instead curvature perturbations in spacetime, and thus might have a role in, for example, the formation of structure at cosmological scales. In some theoretical models, the curvaton acts independently of the inflaton, for example, after it has decayed, while in others, inflaton and curvaton fields are considered to act jointly in creating the primordial perturbations.]. It still remains to explain, however, the origin of the field that drives inflation itself.  Here the proposal that Higgs-like particles from supersymmetric or Grand Unified symmetry breaking could be responsible for cosmological inflation, and analogously, could also have a role in explaining Dark Energy, an acceleration similar to inflation, but smaller in magnitude, in the rate of expansion of the universe, can still be considered.

With the near-complete convergence today in cosmological and particle physics related questions, any discussion of the significance of the recent confirmation of the (electroweak) Higgs is incomplete without also mentioning the possible significance of the (generalized) Higgs mechanisms and Higgs particle-fields in cosmology, and  for this we have Professor Bagger's insightful essay to thank. Prof. Bagger also argues that the proposed Electron-Positron International Linear Collider (ILC) could become a Higgs factory, producing enough of them for a full description of all its properties, and related issues such as whether the Higgs is part of a family of particles, or even whether, while being consistent with all the known properties of the electroweak Higgs, the current discovery at CERN might be a Higgs-imposter! (one of the minor disappointments of the discovery is that nothing 'new' was discovered, the particle seems to be a 'mere' confirmation of something predicted nearly a half century ago.)

A few personal notes - (i) Prof. Bagger is a faculty member at Johns Hopkins, my PhD alma mater, though I have known of him from much earlier [during 1985-86, he was a postdoc in the SLAC Theory Group, while I was a graduate student with the MaRC-II Experiment, and he was already well-known then for his work on supersymmetry]. I vividly remember also the seminar he presented when he visited Johns Hopkins before formally joining the Department - back in 1989!
(ii) Peter Ware Higgs was born the year of my father's birth; (exactly one month later!); and the Higgs mechanism was first proposed in 1962, the year of my own birth - though by Anderson; Higgs himself did it two short years later. [For a long time, the idea was known as the Anderson-Higgs mechanism. The six physicists who published papers nearly at the same time on the subject are Englert, Brout, Higgs, Guralnik, Hagen and Kibble. Nambu, Jona-Lasinio and Goldstone did fundamental groundlaying work].
(iii) I first encountered the Higgs mechanism back in '83, Peter Higgs is 83 now. And on a final personal note, I was named 'Satyen' for Satyen Bose by my mother, herself a scientist. Satyen Bose had been named National Professor in India three years before I was born, and his fame in India at the time as the discoverer of the Bose quantum statistics applicable to all integer spin particles was very high (the generic words 'boson' and 'fermion', however, appear to have first been used by Paul Dirac in 1946). Incidentally, Bose's paper on his calculation of the statistical distribution of energies in the photon gas, translated into German and transmitted to Zeitschrift fur Physik by Albert Einstein himself, was received by ZfP on 2 July 1924, almost exactly 88 years ago this day!
Update: The Higgs event detected might be consistent both with minimal (MMSM) and next-to-minimal supersymmetry (NMSSM).

Update March 2014 on Prof. Jonathan Bagger: He has just been named the next Director of TRIUMF, March 2014. The appointment, among other things, signals the very strong interest, both at TRIUMF and at other Canadian particle physics groups, in the International Linear Collider Collaboration.