It seems that at every turn, the Standard Model of physics reinforces its death choke around the Universe as we know it, but are physicists beginning to see data that bucks this trend? According to one experiment in the Large Hadron Collider (LHC), it appears that there’s a slight deviation from the “norm,” hinting that the Standard Model ain’t all that.
Before we go into the details of this tentative discovery, let’s quickly review why some physicists are excited while others… are, well, not so much.
A Flawed — Yet Reliable — Recipe
The Standard Model is the recipe book of the Universe that’s matured over decades. It explains how subatomic particles should act and predicts interactions within particle colliders such as the LHC with incredible accuracy. But it’s not a perfect description of the Universe, it has some major shortfalls.
For example, the Standard Model does not explain dark matter and dark energy. Also, it has a gaping hole where gravity should be — for an all-encompassing theory of the Universe, the Standard Model is like a cake recipe that mysteriously forgets to add flour.
These shortcomings to one side, however, the Standard Model has had some huge victories in recent months. For one — and this is a biggie — the Higgs boson, a particle that mediates mass in all matter, has (to a high degree of certainty) been discovered at an exact energy where the Standard Model predicts it should be. Also, more recently, the extremely rare decay of the BS meson — a particle that decays into two muons at the ridiculously low rate of three decays out of every billion — was measured by the LHC’s crazy-high resolution detectors at the exact same rate as predicted by the Standard Model. In both these cases, if there were some weirdness in the results, physicists would be getting pretty excited about the possibility of “new physics.”
New (or exotic) physics is basically experimental results that scientists cannot explain with current (Standard Model) ideas, thereby the requirement of coming up with new ideas. One key theory beyond the Standard Model is that of supersymmetry (a.k.a. SUSY) that predicts the existence of more massive superpartner particles for all normal particles. But to be able to detect these exotic particles, you need powerful accelerators like the LHC to generate the necessary energies to dig deeper into increasingly energetic regimes. It’s a bit like a high energy archaeological dig; the more energy you generate, the more exotic and primordial the particle interactions become.
So, in an effort to detect any hints of any new physics, the LHC has been collecting data on countless trillions of particle collisions to see if the resulting decays do anything out of the (Standard Model) ordinary. Physicists from Spain and France have been pouring over this data and they have just reported the possible discovery of a deviation from the Standard Model.
New Physics Fingerprint?
While analyzing data from the LHCb detector, Sébastien Descotes-Genon of the University of Paris teamed up with Joaquim Matias and Javier Virto of the Autonomous University of Barcelona to report on some weirdness in the results of B-meson decays. B-mesons, which are hadrons composed of a quark and anti-quark, are generated inside the LHCb experiments and rapidly decay into a kaon (K*) particle and two muons (muons are the larger cousins of electrons), i.e. B → K*μ+μ–
In their results, the team noted a deviation in the angular distribution of the B-meson’s decay products. What’s more, this deviation isn’t random, there’s a pattern, a pattern not predicted by the Standard Model.
Though exciting, we’re not exactly at the champagne cork-popping stage quite yet. The team has found a 4.5σ significance with their statistical results, just shy of the 5σ required for a bona fide discovery. Still, the statistical certainty in these results are certainly suggestive of something odd going on.
So what could explain this coherent pattern not predicted by the Standard Model? In a preprint of their published results uploaded to the arXiv service, the team suggest that it could be proof of the existence of a Z’ boson. The hypothetical Z’ is a more massive supersymmetric cousin of the Standard Model Z boson — a particle that mediates the weak force. So could this by the fingerprint of supersymmetry in LHC results?
Speaking with PhysicsWorld.com, B-meson specialist and LHCb collaboration physicists Tim Gershon of the University of Warwick urged caution.
“I for one would not bet my house on this being a first sign of new physics, but it is certainly very interesting,” he said. So far, according to Gershon, data through 2011 have been analyzed — a huge quantity of 2012 data have yet to be analyzed. “They will more than triple the size of the sample,” he added.
In the long game of tracking down new physics in the LHC, we’ll unlikely have an answer to the mysterious anomalies in some B-meson decays for some time to come. But still, there’s experimental hope for SUSY yet…
Image: The LHCb detector in the LHC ring of supercooled magnets. Credit: CERN/Fermilab