October 16, 2012

Two Cents: The Higgs Boson

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CERN researchers announced to the world on July 4 that with their Large Hadron Collider they had finally found the Holy Grail of physics—the elusive Higgs Boson, an elementary particle capable of adding mass to matter. But what does the finding mean for the field of physics?  What exactly is the so-called “God-Particle” and how does its discovery bring scientists closer to understanding the universe? This week in Science, we turn to Cornell physicists and particle scientists to explain the Higgs Boson discovery and why this half-century long search is a crowning achievement in physics.

What is the Higgs-Boson exactly, and why is it important?

The Scottish physicist Peter Higgs postulated that particles are fundamentally massless, as predicted by the Standard Model, but appear effectively massive due to interactions with an all- penetrating field that fills all of space. The interaction with this so-called Higgs field slows down the motion of the elementary particles mimicking the behavior of a massive particle.  A crude analogy for this effect is to think of a ping-pong ball that is being dragged in honey: due to the viscous force of the honey (the analog of the Higgs field) one might think that the ping-pong ball is actually abnormally heavy: it moves more slowly and behaves as if it were much more massive due to the interaction with the medium.  This is what we call the Higgs mechanism: the all-penetrating Higgs field generates masses for the elementary particles.

The main importance of the discovery is that it verifies the picture we have developed over the past fifty years for how particles acquire masses, and also suggests that the all-penetrating Higgs field originates from a single weakly interacting elementary scalar field much like in the original proposal of Higgs.

––– Prof. Csaba Csaki, elementary particle physics

Why is the Higgs Boson reffered to as the “God Particle”?

Physicists actually almost never call this particle the “God particle” – we call it the Higgs boson. It is mostly the journalists who called it the “God particle”. The term was originally coined by Nobel laureate Leon Lederman in a popular science book. The justification for the name is that this is the particle that (loosely speaking) controls the mass of all other particles.

–– Prof. Csaba Csaki, elementary particle physics

Why was the Higgs Boson so difficult to find?

Since it is the interaction of the all-penetrating Higgs field that is responsible for the elementary particle masses, it turns out that the interaction strength of the Higgs boson to other particles is fixed by particles’ masses  – the lighter the particle the weaker the interaction. Ordinary matter consists of very light particles, so the probability of actually producing a Higgs boson is extremely small. For example, at the LHC, only one in about every ten trillion events will actually contain a Higgs boson. In order to discover the Higgs, one first has to collide many tens of trillions of protons, and then be able to sift through all the data. But this is not all: the Higgs is a very short-lived particle and will decay almost instantaneously. Thus, instead of directly observing the Higgs one has to look for its decay products.  It turns out that the most common decay products of the Higgs are actually hard to distinguish from other events with no Higgs bosons in them, and the type of Higgs decays that one can use most reliably are very rare.

–– Prof. Csaba Csaki,  elementary particle physics

What is the Standard Model?

The Standard Model of particle physics is one of the most fundamental theories of physics we have. It answers two of the most basic questions about nature:  1) What is matter made from? and 2) What are the forces, or interactions, between the matter particles?

––– Prof. Anders Ryd, elementary particle physics

The ‘Standard model’ embodies our understanding of what are the most fundamental, smallest building blocks of matter and how they interact with one another. In the model we can classify particles as either matter particles or force particles. The matter particles are further subdivided into quarks (the things that neutrons and protons are made of) and leptons (things like electrons and neutrinos.) The force particles include the photon (it’s responsible for the electromagnetic force), the W and Z bosons (they are responsible for radioactive decay, among other things) and the gluon (carrier of the strong force, which binds the quarks together inside nuclei.) In addition to these particles there is a Higgs particle (also known as the Higgs Boson). It is in interactions with it that the particles become massive.

––– Prof. Peter Wittich, elementary particle physics

What other alternative theories are there besides the Standard Model?

Most of the alternative theories don’t replace the standard model but instead extend it. If you want to replace it your new theory has to also pass the many tests we have already subjected it to, since the standard model has passed all those tests. Some theories that extend the standard model are ones like Supersymmetry (which predicts that each known partner has a new super-partner that goes with it), extra dimensions (a theory which posits that there are additional space-like dimensions in addition to the three we know and love) and string theories (which try to connect gravity with the other forces that are in the standard model.) There are many more, but these are some of the more popular ones.

–– Prof. Peter Wittich, elementary particle physics

The Standard Model (SM) is a proven theory, just like any other theory that we call proven, like quantum mechanics or electromagnetism. The SM passed many tests and it describes Nature amazingly well. So, why are we talking about alternatives? In fact, we are talking about extensions, not alternatives. An extension is something more general than what we have now, but it is not a replacement. Let me give a trivial example:

“Cows in NY State have four legs” is for sure a good theory, but one may like to extend it by saying: “Cows have four legs.” This new theory is also correct and it extends the original correct theory. You may ask: why did one even have to make the first theory? The answer is that if you never left NY State, you would just make the first theory. But if you decided to leave and explore the world beyond NY State, you would find that the first theory can be extended. So this is where we are. We have a great theory and we have many reasons to think it is just part of a more fundamental one, and we are searching for these other theories. We are now working with energies we did not have before, so we can finally “explore outside NY State” and we hope to find a more fundamental theory.

–– Prof. Yuval Grossman, theoretical physics

How does the discovery of the Higgs Boson affect these other theories?

Well, your new theory better predict a Higgs-like particle –– otherwise, it just doesn’t work. There are some theories that have been ruled out by the discovery of the new particle. For instance, one called ‘technicolor’ tried to be just like the standard

model, but without a Higgs particle. We can say that it’s pretty likely that this theory has been ruled out. Otherwise, most of the new theories can accommodate this discovery because many people were assuming that the Higgs would be discovered.

–– Prof. Peter Wittich, elementary particle physics

What is next for Particle Physics?

An important goal for particle physics, and the LHC in particular, is to record more data to allow more detailed studies of the Higgs Boson’s properties. Among other things this involves measuring more of the decay modes of the Higgs. At the observed mass of about 125 GeV, or about 133 proton masses, we have access to many different decays of the Higgs. The discovery was made in the final state where the Higgs decayed to two photons or two Z bosons. The Standard Model Higgs is also expect to decays to a pair of W^+W^- bosons and pairs of quarks and lepton. More data is required to establish these decay modes of the Higgs. But confirmation of these decays at the expected rate would solidify the interpretation of the newly discovered boson as the Higgs.

–– Prof. Anders Ryd, elementary particle physics

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Original Author: Shauntle Barley

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