Possible new particle hints that universe may not be left-handed

Mirroring the universe (Image: Claudia Marcelloni/CERN)

PHYSICS may be shifting to the right. Tantalising signals at CERN’s Large Hadron Collider near Geneva, Switzerland, hint at a new particle that could end 50 years of thinking that nature discriminates between left and right-handed particles.

Like your hands, some fundamental particles are different from their mirror images, and so have an intrinsic handedness or “chirality”. But some particles only seem to come in one of the two handedness options, leading to what’s called “left-right symmetry breaking”.

In particular, W bosons, which carry the weak nuclear force, are supposed to come only in left-handed varieties. The debris from smashing protons at the LHC has revealed evidence of unexpected right-handed bosons.

After finding the Higgs boson in 2012, the collider shut down for upgrades, allowing collisions to resume at higher energies earlier this year. At two of the LHC’s experiments, the latest results appear to contain four novel signals. Together, they could hint at a W-boson-like particle, the W’, with a mass of about 2 teraelectronvolts. If confirmed, it would be the first boson discovered since the Higgs.

The find could reveal how to extend the successful but frustratingly incomplete standard model of particle physics, in ways that could explain the nature of dark matter and why there is so little antimatter in the universe.

The strongest signal is an excess of particles seen by the ATLAS experiment (arxiv.org/abs/1506.00962), at a statistical significance of 3.4 sigma. This falls short of the 5 sigma regarded as proof of existence (see “Particle-spotting at the LHC“), but physicists are intrigued because three other unexpected signals at the independent CMS experiment could point to the same thing.

“The big question is whether there might be some connection between these,” says Bogdan Dobrescu at Fermilab in Chicago. In a paper posted online last month, Dobrescu and Zhen Liu, also at Fermilab, showed how the signals could fit naturally into modified versions of left-right symmetric models (arxiv.org/abs/1507.01923). They restore left-right symmetry by introducing a suite of exotic particles, of which this possible W’ particle is one.

Another way to fit the right-handed W’ into a bigger theory was proposed last week by Bhupal Dev at the University of Manchester, UK, and Rabindra Mohapatra at the University of Maryland. They invoke just a few novel particles, then restore left-right symmetry by giving just one of them special properties (arxiv.org/abs/1508.02277).

Some theorists have proposed that these exotic particles instead hint that the Higgs boson is not fundamental particle. Instead, it could be a composite, and some of its constituents would account for the observed signals.

“In my opinion, the most plausible explanation is in the context of composite Higgs models,” says Adam Falkowski at CERN. “If this scenario is true, that would mean there are new symmetries and new forces just around the corner.”

“If the Higgs is really a composite particle, that would mean new forces just around the corner”

The next step is for the existence of the right-handed W’ boson to be confirmed or ruled out. Dobrescu says that should be possible by October this year. But testing the broader theories could take a couple of years.

Other LHC anomalies have disappeared once more data became available. That could happen again, but Raymond Volkas at the University of Melbourne, Australia, says this one is more interesting.

“The fact that the data hint at a very sensible and well-motivated standard model extension that has been studied for decades perhaps is reason to take this one a bit more seriously,” he says.


Particle-spotting at the LHC

How does the LHC see new particles?

It smashes protons together at practically the speed of light, fleetingly creating exotic particles. Analysing the collision debris can help identify them.
What is 3.4 sigma?

A sigma represents one standard deviation, a statistical measure of whether an observation is important or the result of random noise. Physicists have agreed that a particle is confirmed if measurements hint at its presence with a “5 sigma” significance – indicating that the chance the signal is simply noise is one in several million.
What is the W boson?

The fundamental particles are split into two categories: fermions and bosons. Fermions are things like quarks and electrons, which make up ordinary matter. Bosons, like the famous Higgs, carry the fundamental forces. The W boson is one particle that carries the weak nuclear force, which is involved in radioactive decay.

This article appeared in print under the headline “New boson caught right-handed?”