https://www.newscientist.com/article/mg ... ft-handed/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.
Paper:
http://arxiv.org/pdf/1506.00962v2.pdfAbstract
A search is performed for narrow resonances decaying into WW, WZ, or ZZ boson pairs
using 20.3 fb−1 of proton–proton collision data at a centre-of-mass energy of √s = 8 TeV
recorded with the ATLAS detector at the Large Hadron Collider. Diboson resonances with
masses in the range from 1.3 to 3.0 TeV are sought after using the invariant mass distribution
of dijets where both jets are tagged as a boson jet, compatible with a highly boosted W or Z
boson decaying to quarks, using jet mass and substructure properties. The largest deviation
from a smoothly falling background in the observed dijet invariant mass distribution occurs
around 2 TeV in the WZ channel, with a global significance of 2.5 standard deviations.
Exclusion limits at the 95% confidence level are set on the production cross section times
branching ratio for the WZ final state of a new heavy gauge boson, W0, and for the WW
and ZZ final states of Kaluza–Klein excitations of the graviton in a bulk Randall–Sundrum
model, as a function of the resonance mass. W0 bosons with couplings predicted by the
extended gauge model in the mass range from 1.3 to 1.5 TeV are excluded at 95% confidence
level.