Theoretical physics is full of mysteries and unknowns. In the case of some particles, we can predict their existence even if we can’t find them.
Such is the status of the Higgs boson. And yet detecting this particle would revolutionise physics as we know it.
But let’s start with mass
When you place your suitcase on the scales at the airport check-in counter, you are hoping it weighs less than the limit so you won’t have to pay any excess baggage fees.
The force of Earth’s gravity acting on the suitcase’s mass determines its weight. A suitcase that weighs 20kg on Earth would weigh 3kg on the moon, while its mass remains the same. What determines the suitcase’s mass? And even more fundamentally, what is mass?
This is one of the most important questions in particle physics today. The leading explanation for the origin of mass is the Higgs mechanism developed in 1964, which involves the Higgs field and the Higgs boson.
My favourite description of the Higgs mechanism comes from David J. Miller, the winner of a competition among physicists to find the best way of explaining the physics to the UK Science Minister in 1993 to acquire funding. The analogy goes a something like this …
Imagine that a room full of physicists chattering quietly is like space filled with the Higgs field. A well-known scientist walks in, creating a disturbance as he moves across the room and attracting a group of admirers with every step.
This increases his resistance to movement. In other words, he acquires mass, just like a particle moving through the Higgs field. Now imagine if, instead of a well-known scientist entering, somebody started a rumour.
As the rumour spreads throughout the room, it creates the same kind of grouping, but this time it’s the scientists grouping together.
In this analogy, these groups are the Higgs bosons. If we find these groups, we can prove the Higgs field exists and thus explain the origin of mass.
To try finding the Higgs boson, scientists collide other particles together at very high energies and search the debris for traces of this elusive particle.
Scientists at the previous LEP particle accelerator at CERN near Geneva felt they came close before the machine shut down in 2000.
Scientists using the Tevatron accelerator at Fermilab near Chicago are hoping to publish a discovery before the machine shuts down later this year.
The most promising prospect for finally discovering the Higgs boson is the most powerful particle collider ever built: The Large Hadron Collider (LHC) at CERN, a 27km accelerator located 100m under the French-Swiss border that took 25 years to plan and $6 billion to build.
There haven’t been any hints of the particle yet, but it’s early days for the accelerator, which is expected to run for at least 10 years.
What’s at stake?
Both the discovery and exclusion of the Higgs boson at the LHC have vast and important consequences. It is the final missing piece of the Standard Model, the theory physicists use to describe the electromagnetic, strong and weak forces.
All the other particles in the Standard Model have been proven to exist through experiment.
An example of such a prediction and subsequent discovery are the W and Z bosons, which mediate the weak force.
These particles were predicted in 1968 and discovered in 1983, an achievement so significant that Carlo Rubbia and Simon van der Meer were awarded a Nobel Prize in 1984. The same reward may be waiting if we find the Higgs boson.
Finding the Higgs boson will provide insight into why particles have certain masses, and will help to develop subsequent physics.
What’s the hold-up?
The technical problem is that we do not know the mass of the Higgs boson itself, which makes it more difficult to identify.
Physicists have to look for it by systematically searching a range of mass within which it is predicted to exist.
The yet unexplored range is accessible using the LHC, which will determine the existence or otherwise of the Higgs boson.
If it turns out that we cannot find it, this will leave the field wide open for physicists to develop a completely new theory to explain the origin of particle mass.
It is a very exciting time to be in particle physics.