Fresh evidence suggests particle discovered in 2012 is the Higgs
boson
Findings confirm that a
particle decays to fermions, as predicted by the Standard Model.
Deepak Kumar, MIT News Office
June 22, 2014
June 22, 2014
Fresh evidence that a new particle discovered
in 2012 is the Higgs boson has been unveiled today by an international
collaboration led by researchers at MIT.
The findings, published in the journal Nature
Physics, confirm that the bosons decay to fermions — a group of particles
that includes all leptons and quarks — as predicted by the Standard Model of
particle physics.
“This is an enormous breakthrough,” says
Markus Klute, an assistant professor of physics at MIT and leader of the
international effort. “Now we know that particles like electrons get their mass
by coupling to the Higgs field, which is really exciting.”
In July 2012 researchers from the ATLAS and
Compact Muon Solenoid (CMS) experiments at CERN, the European Organization for
Nuclear Research, revealed that they had observed a new particle in the mass
region of 125 to 126 gigaelectronvolts (GeV).
Preliminary studies revealed that the new
particle’s properties were consistent with those predicted for the Higgs boson
by the Standard Model, but much more work was needed to confirm this. In
particular, researchers wanted to clarify whether there was a single Higgs or
many different Higgs particles, as predicted by various extensions of the
Standard Model, Klute says.
“What we are trying to do is establish whether
this particle is really consistent with the Higgs boson, the particle we
predict in our Standard Model, and not one of many Higgs bosons, or an imposter
that looks like it but has a different origin,” he says.
Previous analysis of the data produced by
experiments at CERN’s Large Hadron Collider, near Geneva in Switzerland, has
shown that like the Higgs boson of the Standard Model, the new particles have
no spin, and rapidly decay by splitting into pairs of photons, W bosons, or Z
bosons. But it remained uncertain whether they could also decay to fermion
pairs, Klute says.
Consistent with Standard Model
Now the team from the CMS Collaboration, which
also includes researchers from Imperial College London, Ecole Polytechnique in
Paris, the University of Wisconsin at Madison, and CERN, has demonstrated that
the bosons also decay to fermions in a way that is consistent with the Standard
Model Higgs.
“We have now established the main
characteristics of this new particle, in its coupling to fermions and to
bosons, and its spin-parity structure; all of these things are consistent with
the Standard Model,” Klute says.
To determine whether the particles could decay
to fermions, the researchers fired protons at each other in a 6-meter-diameter
solenoid and used specialized detectors to determine which particles were
produced in the resulting collisions.
The researchers were hunting for particles
called tau leptons, which have a mass of around 1.7 GeV, making them around
3,500 times heavier than their little sibling, the electron. They chose tau
leptons because the strength of a particle’s coupling with the Higgs field is
dependent on its mass. This means that decay to a heavier particle, such as a
tau lepton, will be much easier to spot than decay to a lighter one, such as an
electron.
They were able to confirm the presence of
decay to tau leptons with a confidence level of 3.8 standard deviations — a one
in 10,000 chance that the signal they saw would have appeared if there were no
Higgs particles.
Further research
To confirm the finding, the team plans to
increase this to a confidence level of five standard deviations — or a one in
nearly 2 million chance — once the LHC restarts particle collisions in 2015.
The LHC is currently in shutdown mode while researchers work on the facility,
including upgrading the available energy for collisions from 8
teraelectronvolts (TeV) to 13 TeV. This will allow the team to accumulate much
more data and produce more accurate results, Klute says.
“Within the current level of precision there
is still room for other models with particles that look like the Standard Model
Higgs, so we need to accumulate more data to figure out if there is a
deviation,” Klute says. “Although if we do find a deviation from the Standard
Model, it is likely to be a very closely related one,” he adds.
Once the LHC begins operating again in 2015,
collision experiments will go on for another three years, Klute says. “At the
end of that time we might have some indications for physics beyond the Standard
Model. This period will be extremely interesting.”
Tejinder Virdee, a professor of physics at
Imperial College London who was not involved in this measurement, says that
physicist Stephen Weinberg’s famous paper of 1967, which lay the foundations
for the Standard Model, first pointed out that the Higgs mechanism not only
generates masses for the weak force carriers, such as W and Z bosons, but also
for fermions, the fundamental matter particles.
As a result, it is vital to establish whether
the recently discovered Higgs boson couples to fermions, and so behaves as
described in the Standard Model, he says: “This CMS paper provides clear
evidence for the coupling to tau leptons and, more generally, fermions.”