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The Hunting of the Higgs: what is it and why does it matter?

On the eve of the rumoured announcement from Geneva, we are proud to have commissioned this special article explaining why it is potentially so important, from Lawrence Krauss.

There has been a lot of news lately about the possible discovery at the Large Hadron Collider of the long awaited Higgs Boson, but at the same time there has been a lot of confusion about what it is, why we have had to work hard to find it, and why we should care. Here’s why.

First, the short answer:

If the Higgs is discovered, it will represent perhaps one of the greatest triumphs of the human intellect in recent memory, vindicating 50 years of the building of one of the greatest theoretical edifices in all of science, and requiring the building of the most complicated machine that has ever been built. That’s the good news. But if the Higgs is all that is found at the LHC, it will mean that the other crucial empirical guidance that physicists now need to try and understand truly fundamental questions about our existence—from understanding whether all four forces in nature are unified in some grand theory, to determining what may have caused the Big Bang—will still be absent. Answering these questions may be beyond our technical and financial capabilities in this generation.

Now for the long answer:

Getting something from nothing is one of the great developments in physics in the last century, from understanding how to create a Universe from nothing, to our current understanding of how one might endow another form of nothing—namely empty space—with energy. But perhaps there is no better example relevant to our direct experience of how to get something from nothing than the phenomenon called “spontaneous symmetry breaking” that the Higgs Boson represents.

If our ideas about the Higgs turn out to be true, then everything we see is a kind of window dressing based on an underlying fabric of reality in which we shouldn’t exist. The particles that make us up are massive and bind together to form protons, neutrons, nuclei, and ultimately atoms. But without the Higgs, these particles would actually be massless, like photons, which are required to move restlessly at the speed of light and cannot be confined, except perhaps in a black hole.

We have all experienced the fact that how heavy something feels depends on where it is located. In water, for example, with buoyant forces present, objects that are heavy on the land appear lighter. Similarly, if you try and push something through a very thick fluid it may appear heavier (giving you more resistance to the force of your pushing) than it would appear in pushing it through the air.

The Standard Model of particle physics implies that there is an otherwise invisible background “Higgs field” that permeates all of space. This field interacts with other particles with varying degrees of strength. As particles move through space, they interact with the background Higgs field, and those that interact more strongly will experience more resistance to their motion, and will act heavier. Some particles, like the photon, do not interact with the field at all, and remain massless. In this way, the mass of everything we see is determined by the existence of this field, and if it didn’t exist, essentially all particles would be massless. In this case, mass is an ‘accident’ of our circumstances because we exist in a universe in which such a background field happens to have arisen.

But why a Higgs “particle”? Well, it turns out that relativity tells us that no signal can travel faster than light. Incorporating this in quantum mechanics tells us that forces which we think of as being due to fields like the electric field are actually transmitted between objects by the exchange of particles, and that these particles travel on average at the speed of light or slower. Why particles transmit forces is like thinking of playing catch. If I throw a ball to you and you catch it, then you will be pushed backwards by the force of my ball, and I will by pushed backward by the act of throwing the ball.. Thus we act like we repel each other in this case.

So, if there is a Higgs field, it turns out that there has to be a new particle associated with this field, and this is the Higgs particle.

This seems like a remarkable and fanciful framework, rather like concocting angels on the head of a pin. What would drive scientists to imagine such a scenario? One of the greatest theoretical successes of the last half of the 20th century has been the unification of two of the known forces in nature: electromagnetism and the weak interaction (responsible for the reactions that power the Sun). In this theory electromagnetic forces arise by the exchange of massless photons, and is long range, and the short-range weak force results due to the exchange of massive particles, called W and Z particles, discovered experimentally in the 1980’s after they were predicted to exist in the 1960’s.

In order for this theoretical unification to make mathematical sense, all three different kinds of particles would have to be massless in the underlying theory, and therefore the forces they mediated would be almost identical. However, only if the W and Z particle obtain a mass by interacting with a background field—the Higgs field—will the underlying unified theory be mathematically consistent, while at the same time implying that the two forces will appear different at the scales we measure them today.

As I have described Quantum Mechanics and Relativity tell us that with every field in nature we can associate an elementary particle. If the Higgs field exists, there should be a new “Higgs particle” that reflects its existence. Since the masses of the W and Z particles are almost 100 times the mass of the proton the theory suggests that the mass of a Higgs particle should also be near this value. However, the exact mass is not predicted even though the mass of the W and Z particles were predicted in advance to high accuracy. This is because their masses turn out to be the products of two unknowns multiplied together — the unknown Higgs mass and also the unknown strength of the coupling between these particles and the Higgs field.

For over 25 years after the discovery of the W and Z, experimental physicists have been trying to achieve the energy necessary, and the intensity of beams necessary to produce a real massive Higgs particle, if it exists. The Tevatron at Fermilab was able to reach up to about 120 times the mass of the proton (about 1 Giga electron-Volt or GeV) in its search.

The Large Hadron Collider was designed to probe for Higgs masses that are heavier than this, although it turns out that the lower mass range is most difficult to explore, due to the production of lots of other particles in collisions that make the interpretation of the results most difficult. Thus, in its earliest runs, the LHC was able to rule out the entire mass range for a Standard Model Higgs down to about 135 GeV or so. Things were beginning to look bleak for the Higgs, and many theorists were scurrying at their desks to figure out how to modify the standard model if no Higgs was discovered.

If the Higgs particle is announced at CERN, with a mass of 125 GeV, as present rumors suggest, it will be the crowning jewel of our theoretical understanding, not only of the electro-weak unified theory, but of our understanding of our own origins, and the origin of almost all mass we measure in the Universe.

All is not that rosy, however. The Standard Model gives no explanation of why the masses of the Higgs, the W, and Z have the scales that they do. Indeed, other arguments suggest that one needs new physics to ensure that this scale of masses is not driven up to much higher energies due to quantum mechanical effects that can be calculated. One of the most exciting ways in which this behavior might be kept in check involves a new possible symmetry in nature, called Supersymmetry. If supersymmetry is manifested in the real world, the number of elementary particles would double, and it turns out that because of this one would need not one Higgs particle but two particles to do the job of giving masses to the other particles in nature. Thus, many elementary particle physicists expected to find not one Higgs particle at CERN, but two.

Since supersymmetry is an essential ingredient that is built into the more speculative string theory models that attempt to unify gravity and quantum mechanics, there was even more reason for some theorists to hope that either two Higgs particles, or new particles, the super-partners of the particles making up ordinary matter, might be discovered at the LHC.

If a single Higgs and nothing else is discovered at the LHC it will therefore be a mixed blessing—indeed perhaps the worst empirical possibility we theorists can imagine. We will have discovered the origin of mass, as advertised, but there will be no new experimental guidance on how to take the next step, or where to search for empirical answers to the outstanding puzzles in particle physics, from the origin of the electroweak scale, or ultimately to a possible unification of all four known forces in the cosmos.

Lawrence M. Krauss is Foundation Professor and Director of the Origins Project at Arizona State University. His newest book, A Universe from Nothing, will appear on Jan 10, 2012.



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