The discovery of a new particle at CERN is the just the beginning, not the end, of the scientific work at the Large Hadron Collider (LHC). Two LHC experiments announced observation of a new particle that is consistent with the long-sought Higgs Boson, the famous last missing ingredient of the Standard Model of particle physics. This is the particle that the LHC and the experiments were built to discover. But are physicists therefore done with their quest to explain matter and its interactions on the smallest scales and highest energies? Put simply, this is only the beginning.
In the Standard Model, the Higgs Boson is the quantum manifestation of the space-filling Higgs field that gives fundamental particles their masses. As particles move through space, the strength of their interaction with the ever-present Higgs field gives them inertia: the stronger the interaction, the larger the mass. The electron has a small coupling to the Higgs field, while the top quark has a much larger coupling.
The Higgs field is also crucially important to our understanding of two of the forces in the Standard Model. The model's equations explain both electromagnetic and weak interactions in exactly the same way, unifying both of those forces; each is but one aspect of a deeper theory, with a common mathematical description. But the photon, the quantum of the electromagnetic field, is massless, while the W and Z bosons that mediate the weak interaction are much heavier. The symmetry of the equations is broken by -- you guessed it -- the Higgs field, which gives us a massless photon with no Higgs coupling and heavy W and Z bosons with large couplings. This comes about from the presence of the Higgs field everywhere in space. The Higgs Boson is a particle represented by ripples in the Higgs field that can be excited by the energetic collisions at the LHC.
On July 4, the ATLAS and CMS experiments at CERN's LHC announced they see the production of a new particle consistent with being the Higgs Boson, but a full dossier of properties for the new particle will be needed before identification as the Standard Model Higgs Boson can be made. More elaborate variants are possible, and even more interesting for particle physics. For example, studying our new particle may provide the first evidence for extra dimensions or a whole new zoo of supersymmetric particles mirroring the known particles.
What do we know about the new Higgs-like particle?
It is clearly observed decaying in two different ways, into two photons and into two Z bosons. Evidence is also there for decays to W boson pairs. All this is as we expect for the Higgs. The new particle is quite heavy at 125 billion electron volts, about 130 times the proton mass, but it is within the expected range based on what is known about the weak interaction. At this mass, it should be possible to see Higgs Boson decays to bottom quark and tau lepton pairs, but the evidence is not yet strong enough in these channels, which are experimentally difficult.
The relative strengths of the decays will tell us if this is a Standard Model Higgs Boson or something more complicated. In fact, the strength for two photon decays is larger than expected in the Standard Model, but within uncertainties due to the small number of Higgs decays observed. This may be a hint of a non-standard Higgs-like particle.
We know it must be a boson with spin 0 or spin 2 because it decays to two spin 1 photons. However, it remains to be seen if it has spin 0 and positive parity, as the Higgs Boson should.
To answer these questions about the new particle, more data are needed. Fortunately, the LHC is already producing that new data. This year experiments are expecting at least a factor of three more, which will be a great start. Then, the LHC will shutdown for more than a year to make improvements. In 2014, the energy of the beams should be almost doubled, and that will increase the rate of Higgs Boson production. Particle physicists will have more and more events containing the new particle to study, and that should help them determine the precise nature of this Higgs-like boson. Today's discovery has opened the detailed study of the Higgs sector of particle physics after nearly half a century of hypotheses.
What is the significance for humanity?
We are looking more deeply than ever before into the structure of physics at the smallest scales. We are investigating parts of our theories that bear on our own existence. If the Higgs coupling to electrons (read electron mass) were much different, atoms would not exist: too large or too small and electrons would orbit too close or too far from the nucleus for chemistry to work as we know it. The Universe would be a very different place. It is very likely, even more restrictive, to have intelligent beings in the universe that can reason all of this out and wonder at how it all came to be. This is where basic research like particle physics bears its first fruit -- in wonder and inspiration.
There are spinoffs too: training of students, development of technologies to undertake experiments and applications of discoveries themselves. In 1897 J.J. Thompson discovered the electron. Could he imagine the world we live in today, with electronics everywhere, cell phones, computers and the World Wide Web, which was developed at CERN to facilitate collaborative science? Basic research pays dividends on long timescales.
In particle physics, like all of science, an answer to an interesting question like, "Is there a Higgs boson?" leads to more interesting questions and the start of a whole new area of inquiry. Here's to the discovery of a new particle. May it bring us new questions and a deeper, broader understanding of our universe in the years to come.
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