New evidence strengthens Higgs boson finding
By Anthony Wood
June 29, 2014
Fresh evidence has come to light supporting the theory that the particle detected at CERN's Large Hadron Collider (LHC) in 2012 is indeed the elusive Higgs boson. The work is the result of an international collaboration led by researchers from MIT, and confirms that the potential Higgs boson does exhibit the decay characteristics that would be expected under the Standard Model.
The Standard Model predicts that the Higgs boson is the last elementary particle waiting to be discovered. In this view, all particles gain mass through their interaction with the uniform Higgs field, which exists throughout the universe. (Why uniform? Otherwise mass would vary depending on which direction it was traveling through space.) This is the simplest, but not the only, approach to explain why particles, kings, and cabbages have mass. If you can find the Higgs boson, the Higgs field also exists. But if the new particle discovered at CERN is not the Higgs boson, this could be the first solid indication that the Standard Model is wrong.
The initial discoveryOriginally detected back in 2012 by both the A Toroidal LHC Apparatus (ATLAS) and the Compact Muon Solenoid (CMS) experiments installed in the LHC, the potential discovery of the fabled particle swept through the media with all the ferocity of a typhoon. This furore is understandable when one considers the implications of such a discovery. The Higgs boson represents the final elementary particle to be discovered under the Standard Model of particle physics, which accounts for three of the four forces that govern the behavior of our universe. These are the strong and weak nuclear forces, electromagnetism, with gravity being the only force which does not neatly fit inside the standard model.
However, even discounting the inability to integrate gravity within its structure, without the Higgs boson the Standard Model would fail to explain one of the vital facets of how our universe behaves – how particles have mass. The discovery of the key particle would account for how all particles, from toasters to people, have mass.
The theory is that subatomic particles gain their mass via interaction with the Higgs field, a theoretical field of energy that is present throughout the universe (this video provides a useful explanation of the Higgs field). By proving the existence of the Higgs particle, we would also prove the existence of the Higgs field. So in July 2012, when scientists working at the LHC announced that they had detected a never before seen Higgs-like boson with a mass between 125 and 126 GeV, people naturally got a little excited.
Confirming the breakthroughThe newly discovered particle displayed many characteristics consistent with the Standard Model. Alongside being the correct mass for the theorized particle, it also had no spin, and decayed into pairs of photons, W bosons or Z bosons.
However, whilst the indicators were in favor of the new discovery being the Higgs boson there were other possibilities as to the true identity of the particle. Therefore, even in March 2013, when scientists at CERN were confident that they had accrued enough data to tentatively name the new particle the Higgs boson, further research was required in order to determine whether the boson discovered fitted into the description of the Standard Model Higgs.
The evidence recently published in the journal Nature Physics aims to strengthen this case.
Specifically, the new research demonstrated that the bosons also decay to fermion pairs (in this case tau-lepton pairs). Fermions are important because they are fundamental matter particles, unlike photons, or W and Z bosons that were observed in the earlier experiment, which are force carriers or gauge particles.
"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," says Markus Klute, Assistant Professor of physics at MIT and leader of the international research project.
The researchers say that further work is needed to confirm the findings when the LHC fires up again in 2015.
"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."