A new particle to break the Standard Model?

The Wire
July 2, 2015

Scientists at the Large Hadron Collider particle-smasher have unearthed data from an experiment conducted in 2012 that shows signs of a new particle. If confirmed, its discovery could herald a new period of particle physics research.

On June 2, members of the ATLAS detector collaboration uploaded a paper to the arXiv pre-print server discussing the possible sighting of a new particle, which hasn’t been named yet. If the data is to be believed, it weighs as much as about 2,000 protons, making it 12-times heavier than the heaviest known fundamental particle, the top quark. It was spotted in the first place when scientists found an anomalous number of ‘events’ recorded by ATLAS at a particular energy scale, more than predicted by the Standard Model set of theories.

Actually, the Standard Model is more like a collection of principles and rules that dictate the behaviour of fundamental particles. Since the 1960s, it has dominated particle physics research but of late has revealed some weaknesses by not being able to explain the causes behind some of its own predictions. For example, two physicists – Peter Higgs and Francois Englert – used the Standard Model to predict the existence of a Higgs boson in 1964. The particle was found at the LHC in 2012. However, the model has no explanation for why the particle is much lighter than it was thought to be.

If its existence is confirmed, the new probable-particle sighted by ATLAS could force the Standard Model to pave way for a more advanced, and comprehensive, theory of physics and ultimately of nature. However, proving that it exists could take at least a year.

The scientists found the probable-particle in data that was recorded by a detector trained to look for the decays of W and Z bosons. These are two fundamental particles that mediate the weak nuclear force that’s responsible for radioactivity. A particle’s mass is equivalent to its energy, which every particle wants to lose if it has too much of it. So heavier particle often break down into smaller clumps of energy, which manifest as smaller particles. Similarly, at the 2 TeV energy scale, scientists spotted a more-than-predicted clumping of energy that’s often the sign of a new particle, in the W/Z channel.

The chance of the telltale spike in the data belonging to a fluke or impostor event, on the other hand, was 0.00135 (with 0 being ‘no chance’ and 1, certainty) – enough to claim evidence but insufficient to claim a discovery. For the latter, the chances will have to be reduced to at least 0.000000287. In the future, this is what scientists intent on zeroing in on the particle will be gunning for.

The LHC shut in early 2013 for upgrades, waking up in May 2015 to smash protons together at almost twice the energy and detect them with twice the sensitivity as before. The ATLAS data about the new particle was gathered in 2012, when the LHC was still smashing protons at a collision energy of 8 TeV (more than 8,000 proton-masses). In its new avatar, it will be smashing them at 13 TeV and with increased intensity as well. As a result, rarer events like this probable-particle’s formation could happen more often, making it easier for scientists to spot and validate them.

If unfortunately the probable-particle is found to have been something else, particle physicists will be disappointed. Since the LHC kicked off in 2009, physicists have been eager to find some data that will “break” the Standard Model, expose cracks in its foundations, that could be taken advantage of to build a theory that can explain the Higgs boson’s mass or why gravity among the four fundamental forces is so much more weaker than the other three.

The ATLAS team acknowledges a paper from members of the CMS collaboration, also at the LHC, from last year that found similar but weaker signs of the same particle.