Sunday, 18 November 2012

Your move, theorists

B-mesons' decay products

Physicists rather enjoy the friendly rivalry between theorists and experimentalists, and this week has been a fascinating week for both. Results presented at the Hadron Collider Physics symposium in Kyoto this week have proved a triumph for experimentalists working with the most powerful tool at their disposal - the LHC. But theorists have been left scratching their heads, as one of the most prominent theories of "new physics" took a major hit. Since the search for physics beyond the Standard Model is a leading priority in particle physics, this is highly significant for everyone.

Unsurprisingly, the hot topic of discussion at the Hadron Collider Physics symposium was the LHC's incendiary boson results from earlier this year.

Both the teams from the CMS detector and the ATLAS detector presented new analyses of the observations which led them to announce the discovery of a Higgs-like boson in July.  What's striking about the new data is that is backs up initial suspicions that the boson discovered at the LHC appears to be behaving precisely as the Standard Model predicted it would. As Tommaso Dorigo noted in Quantum Diaries, the new measurements, "confirm the standard model interpretation of the new found object."  Philip Gibbs at viXra provides some further technical analysis of the results here.

The LHCb Experiment. Photo courtesy of CERN.

But perhaps more sensational was the new results presented by their colleagues over at the LHCb Experiment. Johannes Albrecht reported that the LHCb team have observed one of the rarest particle decay events in physics, a Bs meson decaying into 2 muons. These events are so rare that the Standard Model predicts they should only occur about once in 300 million collisions.

That LHCb has observed one of these events at all is a stunning achievement for the experimentalists working on the detector. But the fact that their results suggest the the Bs meson decay is every bit as rare as the Standard Model predicted it should be, is a serious blow for one of the leading theories of new physics - supersymmetry.

The theory of supersymmetry, often referred to by its shortened nickname, SUSY, states that every fundamental matter particle should have a more massive, or 'super' force carrier particle, and every force carrier should have a 'super' matter particle.  These particles are often referred to as 'sparticles' (supersymmetric particles). Supersymmetry. has been championed by theorists such as Savas Dimopoulos and Gordon Kane, who memorably described the theory as a "wonderful, beautiful and unique" solution for the problems in our understanding of the subatomic world.

However, the LHCb results have cast serious doubt on the viability of supersymmetry as a theory. Supersymmetry predicts that if superparticles exist the Bs meson decay to a pair of muons should occur far more often than one in 300 million.  The work the LHCb team were doing has long been considered the most important experimental test for supersymmetry.  Whilst the LHCb results, which prove the rarity of the decay, don't rule supersymmetry out all together, the parameters for superparticles have narrowed dramatically, making the theory a much less likely explanation for the mysteries in our subatomic world, than many had hoped.

Why is this significant? Well, all physicists know that the Standard Model, despite its elegance, does not function as a complete explanation for the forces which govern our universe. It provides little explanation for gravity, and it noticeably fails to explain either dark energy or dark matter. Given that it is believed that dark matter may constitute up to 84% of all matter in the universe, and dark energy up to 73% of all the known energy in the universe, a theory which explains neither is clearly inadequate.

The science community at large had been hoping that the experiments running at the LHC would start to uncover evidence for "new physics" beyond the Standard Model, which would begin to explain these puzzling features of the universe.  But so far, not only have the main results not done so, they've simply provided ever-strengthening evidence for the veracity of the Standard Model

As Marc-Olivier Bettler from LHCb noted this week, "if new physics is present then it is hiding very well behind the Standard Model".

A typical candidate event for the Higgs boson measured in the CMS electromagnetic calorimeter. Image courtesy of CERN

The fact that CMS and ATLAS this week seemed to be describing a Higgs boson which looks awfully like the one predicted by the Standard Model, is compounding theoretical concern. Expressing this eloquently this week, Guido Altarelli from CERN stated that a Standard Model Higgs was, "a toy model to make the theory match the data, a crutch to allow the Standard Model to walk a bit further until something better comes along."

As Matthew Chalmers noted in an article which starkly set out the challenges the experimental results are raising, a Higgs boson at 125 GeV (the measurements both CMS and ATLAS have provided further evidence for this week) not only has a mass "vastly less than it should be, it is also about as small as it can possibly be without dragging the universe into another catastrophic transition. If it were just a few GeV lighter, the strength of the Higgs interactions would change in such a way that the lowest energy state of the vacuum would dip below zero. The universe could then at some surprise moment "tunnel" into this bizarre state, again instantly changing the entire configuration of the particles and forces and obliterating structures such as atoms."
He dramatically intoned, "as things stand, the universe is seemingly teetering on the cusp of eternal stability and total ruin."

All of this is to say that whilst results presented by ATLAS, CMS, and the LHCb are bringing relief in some quarters, as they certainly prove how exceptional the LHC is as a tool of discovery, they are causing some deep unease amongst theorists.

These are exciting times.

As particle physicist Ben Still observed earlier this year, "until theorists can come up with ways we can test their theories, they are just dealing with works of fiction."

So after some cracking moves this week in which the experimentalists have put pay to some of the most treasured literary works of physics theory, the ball is now back in the court of the theorists. They need to dream up new theories which help make sense of these results, and suggest new routes forward.


1 comment:

  1. I've read that the bilinears of a spin 1/2 field have a middle factor, written gamma^0. Although soldered to the time axis of the reference frame, it's treated like a constant scalar.

    Suppose, instead of a constant, it's a fiduciary vector. Assuming for simplicity that its fiber is a pure boost (relative to Fermi-Walker transport) and that the external spacetime is a first-order approximation of the earth's clock frame, would there be a theoretically observable precession that could be tested for?