This is one discovery that will likely excite and disappoint physicists in equal measure. Large Hadron Collider (LHC) scientists have confirmed the detection of an ultra-rare subatomic decay for the first time, a decay that is predicted by the Standard Model. Unfortunately for supersymmetry proponents, that’s one hefty blow against their theory.
But before we can understand the bad news, it’s best to start with the good news.
The Good: Standard Model Glory
On its ongoing mission to explore the most primordial of matter of the Universe, the LHC slams particles (usually protons, sometimes higher-mass hadrons like lead nuclei) together at close to the speed of light. By doing this, for the briefest of moments, the energy conditions that existed shortly after the Big Bang are created. From this energetic soup, particles that were last seen buzzing around the ancient universe some 13.75 billion years ago condense from the blast of energy, like raindrops condensing inside a raincloud.
By their nature, these newly-condensed particles buzzing inside the LHC’s monstrous detectors are unstable, so they quickly decay into other particles. These decays are extremely important to physics as they provide a very privileged view into how particle interactions worked during the earliest moments of the universe and bolster decades of scientific theory.
The Standard Model of physics is the theoretical framework by how all matter should act. And in this case, the Standard Model predicts that a very, very rare decay should occur for a specific particle in a very specific way. The LHC — with its vast energies, ultra-high resolution detectors and epic computing power — is the first machine available to mankind that can probe and detect these extremely rare and specific decays.
So, after analyzing two years worth of data from the LHC, physicists from two LHC experiments, LHCb and CMS, have announced the discovery of the decay of the Bs meson into two muons. (A meson is a hadron and composed of a quark and anti-quark. Muons are the larger cousins of electrons.)
To see this Bs decay, however, you need to be patient — the particle only decays into two muons three times out of every billion decays. For a particle collider that produces hundreds of millions of collisions every second, that’s countless trillions of particle interactions that need to be analyzed to weed out the desired Bs decays to any statistical significance.
“Finding particle decays this rare makes hunting for a needle in a haystack seem easy,” the LHC physicists said in this morning’s news release (July 24).
In short, the detection of Bs meson decaying into two muons at the exact rate predicted is a huge triumph for the Standard Model of physics. Add that to the recent confirmed discovery of the Higgs boson that appears to exist at exactly the energy predicted by the Standard Model.
This model may have its restrictions, but it has once again proven that it’s a very good “recipe book” for how the universe works on a subatomic level.
The Bad: Supersymmetry Woes
But it’s not all good news. In fact, it rather depends on your definition of “good.”
Physicists have long been concerned by the Standard Model’s inability to account for gravity, dark matter and dark energy. So, as the Standard Model is pushed to its limits by particle accelerators like the LHC, physicists have been carefully watching for any slight oddities in particle collision data. In the hope that supersymmetry theory (or “SUSY”) may help explain dark matter, for example, they’ve been expecting small signatures of supersymmetry revealing itself in experimental results. SUSY should skew the Bs decay rate slightly, but, as this most recent discovery has once again proven, the Standard Model isn’t budging and there’s no sign of any experimental evidence for supersymmetry — the Bs meson decay rate is spot-on.
“Measurements of this very rare decay significantly squeeze the places new physics (i.e. SUSY) can hide,” said Val Gibson, leader of the Cambridge particle physics group and member of the LHCb experiment. “It is the dedication of our students and post-docs that make such measurements possible. The UK LHCb team are now looking forward to the LHC returning at even higher energy and to an upgrade to the experiment so that we can investigate why new physics is so shy.”
While this isn’t the end for supersymmetry, it is a blow for our understanding of what lies beyond the Standard Model.
The Ugly: A Dark Dilemma
Understanding the nature of dark matter and dark energy are two of the biggest challenges for all of physics, from the quantum world to cosmology. Supersymmetry — which predicts that for every particle of “normal” matter there’s a “superpartner” particle — may help explain why 95 percent of the mass-energy of the universe is invisible. (Dark matter has been detected indirectly by its gravitational impact on normal matter and dark energy is the invisible force that has been indirectly observed as the force that accelerates the expansion of the universe.)
But if we cannot detect any evidence of supersymmetry, what the heck is dark matter and dark energy? Let alone gravity; that ‘everyday’ force remains as mysterious as ever.
Unfortunately, until the LHC turns up supersymmetry evidence, or spits out some totally unpredictable exotic dark matter particle, dark matter and dark energy will remain a mystery. And as for supersymmetry theory, well, it will soldier on while physicists push particle accelerators to ever higher energies in the hope of seeing over the Standard Model horizon and into a new landscape of exotic physics that may, or may not be, supersymmetrical in nature.
Image: Simulation of particle collisions and Bs decay inside the LHCb detector. Credit: CERN/LHCb Collaboration