Another window on ‘new physics’ closes

This reconstructed image of two high-energy protons colliding at the LHC shows a B_s meson (blue) produced that then decays into two muons (pink), about 50 mm from the collision point.
This reconstructed image of two high-energy protons colliding at the LHC shows a B_s meson (blue) produced that then decays into two muons (pink), about 50 mm from the collision point. Image: LHCb/CERN

The Standard Model of particle physics is a theory that has been pieced together over the last 40 years after careful experiments. It accurately predicts the behaviour of various subatomic particles across a range of situations. Even so, it’s not complete: it can explain neither gravity nor anything about the so-called dark universe.

Physicists searching for a theory that can have to pierce through the Standard Model. This can be finding some inconsistent mathematics or detecting something that can’t be explained by it, like looking for particles ‘breaking down’, i.e. decaying, into smaller ones at a rate greater than allowed by the Model.

The Large Hadron Collider, CERN, on the France-Switzerland border, produces the particles, and particle detectors straddling the collider are programmed to look for aberrations in their decay, among other things. One detector in particular, called the LHCb, looks for signs of a particle called B_s (read “B sub s”) meson decaying into two smaller particles called muons.

On July 19, physicists from the LHCb experiment confirmed at an ongoing conference in Stockholm that the B_s meson decays to two muons at a rate consistent with the Model’s predictions (full paperhere). The implication is that one more window through which physicists could have a peek of the physics beyond the Model is now shut.

The B_s meson

This meson has been studied for around 25 years, and its decay-rate to two muons has been predicted to be about thrice every billion times, 3.56 ± 0.29 per billion to be exact. The physicists’ measurements from the LHCb showed that it was happening about 2.9 times per billion. A team working with another detector, the CMS, reported it happens thrice every billion decays. These are number pretty consistent with the Model’s. In fact, scientists think the chance of an error in the LHCb readings is 1 in 3.5 million, low enough to claim a discovery.

However, this doesn’t mean the search for ‘new physics’ is over. There are many other windows, such as the search for dark matter, observations of neutrino oscillations, studies of antimatter and exotic theories like Supersymmetry, to keep scientists going.

The ultimate goal is to find one theory that can explain all phenomena observed in the universe – from subatomic particles to supermassive black holes to dark matter – because they are all part of one nature.

In fact, physicists are fond of Sypersymmetry, a theory that posits that there is one as-yet undetected particle for every one that we have detected, because it promises to retain the naturalness. In contrast, the Standard Model has many perplexing, yet accurate, explanations that is keeping physicists from piecing together the known universe in a smooth way. However, in order to find any evidence for Supersymmetry, we’ll have to wait until at least 2015, when the CERN supercollider will reopen upgraded for higher energy experiments.

And as one window has closed after an arduous 25-year journey, the focus on all the other windows will intensify, too.

(This blog post first appeared at The Copernican on July 19, 2013.)