Over two years have passed since the much-heralded discovery of the Higgs boson, and research into the newly found particle is far from over. Researchers are still trying to ascertain whether the Higgs boson is exactly what had been predicted by the Standard Model theory, or whether it has slightly different properties - perhaps even making it one of a family of undiscovered particles, leading researchers to a new chapter in modern physics.
Pinpointing the particle's exact properties might have far-reaching implications for the Standard Model, the theory serving as the basis of modern particle physics.
Physicists at the the Large Hadron Collider - the 27-km large particle accelerator based at CERN (the Conseil Européen pour la Recherche Nucléaire) are working on drawing a more precise profile of the particle by analyzing data collected by the accelerator's two main detectors.
What the physicists are doing is like trying to distinguish between identical twins who can be told apart only by their pronouncement of a single word, says Rolf Heuer, the research center's director general, in a visit of Israeli journalists to the particle accelerator, initiated by the Israel Academy of Sciences and Humanities and CERN. Scientists are trying to get the Higgs to talk - and give away its true identity - by examining data collected from the proton collision experiments in the last few years, he says.
At the moment, it seems that the Higgs is looking like a Standard Model Higgs, but we'll get more accurate results only a few years from now, says Eliezer Rabinovici, a theoretical physicist at the Hebrew University's Racah Institute of Physics, who serves as Israel's representative at CERN and Chair of the Israeli Academy of Science's National Committee for High Energy Physics.
Siblings to the Higgs?
The discovery of the Higgs boson, popularly known as the 'God Particle', was announced in July 4, 2012. It was first predicted almost half a century earlier, in 1964, within the framework of the Standard Model, a theory of the fundamental forces and particles that make up matter.
But the particle found at CERN may not be exactly the Standard Model Higgs particle, in two different ways. Both of these possibilities hold the promise of going beyond the Standard Model.
The discovered particle wouldn't be a Standard Higgs if it does not fulfill its role as the major contributor of mass to two particles, known as W and Z, which function as carriers for one of the four fundamental interactions between particles, the weak interaction. If the discovered Higgs does not live up to this task, perhaps it has sibling particles which help contribute the mass to the two carrier particles.
Indeed, there are theoretical models which predict the existence of extra particles that are similar in certain aspects to the discovered Higgs.
Another possibility, which doesn't negate the first, is that the discovered Higgs may be able to interact with yet undiscovered particles, offering physicists a way beyond the Standard Model.
Something else out there
Identifying a Standard Model Higgs would be a success story in its own right, since it is the last fundamental particle missing according to the Standard Model. Whatever the identity of the new particle may be, it is clearly part of the long-sought-after mechanism that contributes to other fundamental particles' mass.
Yet theorists are hoping that the Higgs isn't the single particle predicted by the Standard Model, since they are eager to expand or even replace this model with a more comprehensive theory.
If the boson found is a "standard Higgs, it would amount to another corroboration of the Standard Model, which, for all its successful predictions throughout the twentieth century, still presents significant theoretical problems that cannot be set aside.
To begin with, the particle's mass is much lighter than what many theoretical predictions would suggest. In addition, the model as is incorporates only three of the four fundamental forces, leaving out gravitation. Nor does it offer any predictions concerning the missing particle of dark matter, which according to current estimates comprises a quarter of the total mass and energy in the universe.
The question is whether there is more to physics than the Standard Model, that can be observed in the properties of the particles we already know about," says Rabinovici. So far, all attempts to see something new have failed. But now the Higgs is the new kid on the block, so we're studying its properties with the hope that they will deviate from those predicted by the Standard Model. If these properties actually do deviate from the predictions, we'll know that there is something else out there and we'll try to identify it. This is what we are all hoping for."
A new chapter in physics
How, then, will researchers conclude whether they discovered a Standard Model Higgs or a slightly different boson which could lead to new theoretical boundaries beyond the Standard Model?
Researchers at CERN are working on characterizing the new particle according to at least two of its properties: a property known as spin, and a property more familiar to laymen: mass.
Physicists had already identified the particle's mass over two years ago, before declaring its discovery in July 2012. The particle's spin, on the other hand, is still a matter of ongoing research, but the preliminary evidence points to zero spin – which corresponds to the Standard Model's prediction for the Higgs boson.
In order to fully expose the new particle's identity, researchers are examining the products of its decay.
Generally speaking, unstable particles – and the Higgs is certainly that – cannot be directly discovered because they are too short-lived, so physicists search for the different particles into which they break down: their products of decay. Since each kind of particle has its unique set of decay products, these can serve as the fingerprints pointing to their origin.
The Standard Model makes predictions concerning the Higgs boson's decay products – which include photons, muons and quarks, among other particles – and indeed all these were discovered, and pointed at the existence of the Higgs.
So far, the preliminary evidence on this front points at a Standard Model Higgs boson, but the current evidence isn't conclusive. The Large Hadron Collider cannot yet offer a sufficiently high resolution of the decay products. Physicists are currently debating whether a future upgrade of the particle accelerator will provide the necessary resolution for concluding whether the products of decay point to a Standard Higgs - or whether billions of dollars will need to be invested in building a new, even more powerful accelerator than the Large Hadron Collider.
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