What the Higgs happened?


The scientific community was recently stunned by the announcement that the Higgs boson – said to be the agent that gives particles mass – has finally been discovered. Penangite Khoo Teng Jian, a PhD student and member of the Atlas (A Toroidal LHC Apparatus experiment) searching for supersymmetric particles at the European Organization for Nuclear Research (CERN), takes us through what just happened.

What did we discover?

The jury is still out! The measurements we have made have not yet achieved sufficient precision to say conclusively whether or not this is the Higgs. At the moment it's like waiting for your school bus in the middle of the monsoon season. Through the pouring rain, you see headlights, and eventually you see enough to be sure that it is a bus coming, but it has to get closer before you can see the number. We can't see the number yet, although the shape and size hint that it's the right bus.

Particle physics is quantum mechanical, and at our present level of understanding, this means there is an element of randomness involved, hence the need for a lot of data and sophisticated statistical analyses. In order to be certain, it is necessary to test the detailed predictions of the theory far more carefully and collect a bigger data sample. This will take time.

But some of the results do not match so neatly with the theoretical expectations. In particular, although the total number of “Higgs” events observed is broadly in agreement with theory, the breakdown into the different channels shows some tension with the models. So far these discrepancies are not conclusive enough to rule out the Standard Model (SM) Higgs. If, however, the inconsistencies remain as we sharpen our analyses, this could mean that we have some sort of “special” Higgs, and this would be very interesting indeed!

How was this discovery possible?

The announcement made last Wednesday ( July 4) was by the Atlas and CMS (Compact Muon Solenoid) experiments, which are the two biggest at the Large Hadron Collider (LHC). About 3,000 authors (including me!) are currently active on Atlas, and about 2,100 on CMS. Many of these are students and academic staff at universities and laboratories from about 40 countries. CERN itself employs 2,400 staff, including experimenters, engineers, computer scientists and other technical staff, not to forget the theorists.

But the discovery is truly a culmination of scientific and technological progress. If a single Nobel Prize comes out of the discovery, it's worth noting how many were awarded for research contributing to development of the accelerator and experiments. Many of these came out of basic (as opposed to applied) research, underlining the worth of science for its own sake. The list includes superconductors, transistors, liquid helium and beam cooling.

The cost of building the LHC was around US$6.5bil, or RM20bil at today's exchange rate. That's not quite five times the cost of the Petronas Twin Towers—but Petronas makes about three times in annual net profits! Estimates put Malaysian economic losses to corruption at RM10bil-RM25bil. If this money were recovered, we could fund our own LHC in one to two years!

It is worth remembering, however, that this money does not enter a vacuum. For example,Japan, which supplies a substantial amount of the CERN budget, receives economic benefits as Japanese companies are contracted to build many of the silicon devices for tracking charged particles. It would not surprise me if many of the chips powering the computer farms on which we run our analyses come from the semiconductor factories in Penang.

Some more local interest: To my knowledge, six Malaysians have worked on the LHC or its experiments since it started up, and at least five of them are Penangites!

How does this affect our understanding of how the universe works? If it were never discovered, where would that have left us?

If this newly discovered particle really is the Higgs, it will confirm one of the most established theoretical mechanisms in particle physics, which is the production of particle masses. In a sense, it does not change our understanding of the world by adding something new. However, empirical tests are critical to the acceptance of a theory, so theorists can now focus on extending this theory, rather than on devising alternatives.

At its heart, the Higgs boson is the manifestation of a “field” that causes particles to have masses. The mathematical structure describing the interactions of matter and force particles in its simplest form does not allow all of the particles to be given the masses that we observe them to have. If you now add the Higgs field, and let it interact with the massless particles, they gain mass proportionally to their interaction strength.

A massive particle moving in the Higgs field can be thought of like a feather duster being waved in air—its feathers catch the air (interacting strongly) and its motion is impeded. A light particle is like a smooth stick—you can whirl it around much more easily.

Where would we have been had we not found this particle? In a very interesting place! We’d have to change track and look for something completely different on the experimental side. The theorists would also have a ton of work to do, because the Higgs mechanism is the foundation for many of the models that they have come up with.

The world was similarly set abuzz a few months ago when scientists "discovered" that neutrinos could travel faster than light. This has since been discredited. How can we be sure this is the real deal?

The reason the superluminal neutrinos made the news was that it was wholly unexpected and completely at odds with our understanding of physics. But big claims require big proof. The neutrino result was the work of a single experiment, and the people responsible were themselves not totally convinced of its veracity. One of the reasons they publicised the result is that they thought they had run out of mistakes to check, and invited others to provide a better explanation.

By contrast, there are heaps of corroborative information for this discovery. The CMS/Atlas announcement came only two days after the two Tevatron experiments, CDF and D0, reported similar results, but at a lower level of significance. Particle physicists measure the significance of a result in units of “sigma”, which describe the probability that there is no actual signal, and the result is basically a completely random coincidence.

The Tevatron experiments had 3 sigma, which means that they had a 0.2% probability of a false positive. The LHC experiments each have approximately 5 sigma, which translates to 1 chance in 3 million. When you put together the consistent results from four independent experiments, it’s pretty convincing.

It’s conceivable that this will turn out to be something besides the Higgs, or a Higgs that has different properties to the predicted ones. But we’re pretty sure this isn’t a discovery that will just “go away” with more data!

If this is indeed the Higgs boson, does that mean that the SM is finally complete?

In a sense, yes. The Higgs boson has long been considered the last piece in the SM. The SM was put together in the 1970s, encapsulating everything we thought we knew then about particle interactions. However, there are some cracks in the SM. First and foremost, it is largely empirical. There are a lot of free parameters whose values are determined solely by experimental measurements. The ultimate description of Nature should have no free parameters, but tell us why every number is what it is. At most, the SM is merely an iTheory—“it just works”, but don’t ask why or tinker with it...

Aside from needing to be "fine-tuned", the SM is known to be incomplete. It doesn’t explain dark matter, and is irreconcilable with general relativity and gravitation.

When the head of the Cavendish Laboratory and the UK Science Minister were in Westminster listening to the announcement, the minister, amidst the jubilation, asked Prof Stirling, “But it seems to me the book is not closed. Surely this is just the beginning?” Prof Stirling replied, echoing the opinions of every particle physicist, “YES!”

What’s next on the list to discover?

Theorists have come up with a plethora of “Beyond-the-Standard-Model” theories that aim to solve the problems of the SM, such as supersymmetry, one of the theories that can produce dark matter.

Some supersymmetric theories include a dark matter particle, and furthermore patch some, if not all, of the problematic aspects of the SM. What’s most interesting for experimentalists like myself is that supersymmetry postulates a superpartner for every particle (and in fact five different Higgs bosons!), which would mean lots of interesting searches and measurements done if ever we find it. It’s like SM, keh leow (“with added ingredients”).

Dark matter would be high on the list, but it also happens to be pretty hard to find, because it doesn’t interact with much!

Do you agree with calling the Higgs the “God particle”?

Short answer: No!

To be fair, the Higgs mechanism is of profound importance in our theoretical understanding of physics, and is accepted as a critical component of most modern theories. But “God particle” irritates the religious enough to be problematic, and the media seizes on the name to build up disproportionate hype.

How many bad Higgs jokes have you heard since the discovery was announced?

Hmm, the one I hadn’t heard was how the Higgs boson was asked to leave the Catholic Church. His reply: “But without me, how can you have mass?”

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