by Don Lincoln
June 13, 2015

from Space Website
 

The Large Hadron Collider is the world's most powerful particle accelerator.

In June 2015, the LHC was restarted at nearly twice the energy

at which it operated during its first run, which ended in 2013.
Credit: CERN

 

Somewhere under the French-Swiss border, two protons have a date with destiny.

Trapped inside the Large Hadron Collider (LHC), the world's largest and most powerful particle accelerator, they follow a circular path in opposite directions with velocities very near the speed of light.

As they approach each other, their fate is clear: A collision is inevitable. One could imagine that an impact between two protons might look like a collision between subatomic billiard balls.

But the rules of the micro-realm are quite different from what familiar intuition developed in the corner pub would suggest.
 

One of the first recorded particle collisions at an energy of 13 trillion electron volts,

(tera-electronvolts or TeV), recorded by the CMS experiment on the LHC.
Credit: CERN
 

Colliding with success

After a hiatus of more than two years, the LHC is up and running again.

After a broad program of refurbishments, retrofits and upgrades, the accelerator is essentially an entirely new facility. Operating at nearly double the energy and triple the number of collisions per second, the LHC will create collisions within the centers of four huge experiments, each ready to make the discovery of the century.

Since Einstein's 1905 papers on relativity, physicists have known of the equivalence between energy and mass.

As described by Einstein's famous equation (E = mc²), energy can be converted into matter and vice versa. And that's one of the big things that happens inside a particle accelerator.

The huge kinetic (i.e., moving) energy of the two incoming beam particles is converted into the mass of particles that didn't exist before the collision.

It is in this manner that two protons, each having a low mass (about 1 billion electron volts for the techno-crowd), can collide and make the Higgs boson, which is a particle with a mass about 125 times heavier than that of a proton.

The motion energy of the protons is literally transformed into a very heavy particle.
 

A collision recorded by the CMS experiment in 2011

 that was a candidate for Higgs data.
Credit: CERN
 

When the LHC began operations in 2010, it had a clear mission.

Two large experiments, each comprised of around 3,000 scientists, were focused predominantly on finding the Higgs boson. Predicted in 1964, the Higgs boson is connected to the Higgs field, which is thought to give the mass to fundamental (i.e., pointlike) subatomic particles.

Finding the Higgs boson meant that the idea of the Higgs field was validated.

Prior to its discovery, the Higgs boson was the last missing component of the wildly successful Standard Model of particle physics. When combined with Einstein's theory of general relativity, the Standard Model can describe the behavior all of the matter ever observed - from the matter in you and me, to majestic galaxies careening through the cosmos.

While the discovery of the Higgs boson in 2012 was indeed an enormous success for the scientific community, the triumph came with a disappointment.

Explaining this is simple:

Essentially, the Higgs boson was like a final piece that completed the Standard Model  puzzle.

 

However, as any puzzle enthusiast will tell you, it is the tabs and blanks of pieces that allow one to build a puzzle. The hanging tab gives you a hint as to what the next piece will be.

But a completed puzzle is silent on what to do next.

The mysteries that remain

It's not like we don't have mysteries in the world of physics.

From our observation of galaxies, we know that they rotate faster than can be explained by the known laws of gravity and the matter we can detect. To explain that mystery, we invented an unobserved form of matter called dark matter.

The fundamental nature of dark matter is certainly a big mystery.

Another mystery stems from that famous Einstein equation, E = mc². It actually says that when energy is converted into matter, an equal amount of antimatter will be made.

During the Big Bang, the universe was full of energy, and this energy transformed into equal amounts of matter and antimatter. Yet when scientists look at the universe, they see only matter. So where did the antimatter go?

While physicists have had a few hints from previous experiments, we don't really know the answer. This is another mystery.

There are other mysteries, too, like wondering if there are smaller building blocks of the universe than those with which we are now familiar. Following the history of investigations into that question, we have learned of molecules and then atoms. Research in the early 1900s revealed protons, neutrons and electrons, and the 1960s brought to light the quarks and leptons that are currently considered the smallest particles of nature.

However, it is natural to ask if there might be even smaller building blocks. While scientists don't know the answer, there must be some sort of deeper and more fundamental physics that can explain the patterns seen in the quarks and leptons.

The answer to that question is yet another mystery.
 

Engineers work on one of the CMS detectors

during the scheduled two-year shutdown of the LHC.
Credit: CERN
 

The curious Higgs boson mass

Physicists don't know the answer to any of those fundamental questions, and, to be honest, it is possible that the LHC won't teach us about any of those secrets of nature.

But there is one question for which LHC data is a surer bet.

It stems from mysteries that arise in calculations of the Higgs boson's mass. When scientists try to calculate this value directly from the theory, the result is much higher than the LHC data suggest.

Because of the laws of quantum mechanics, the Higgs boson can fluctuate into other types of particles (e.g., the top quark, the W and Z bosons, and even pairs of Higgs bosons).

This behavior leads to predictions of the mass of the Higgs boson that are closer to the Planck mass which is a hundred quadrillion times heavier than the mass that scientists have measured.

(The Planck mass is the highest mass our current theories could possibly apply and marks a frontier beyond which we are certain that we will have to rethink everything.)

Obviously, this is a problem, and physicists have spent several decades imagining possible explanations, even before the Higgs boson's discovery. (After all, it was clear even early on that this problem would exist if the Higgs boson had a mass that could be discovered.)

Supersymmetry

The most popular theoretical explanation is a principle called supersymmetry.

This idea essentially postulates that the force-carrying bosons (particles with a subatomic spin that is integer multiple of ħ (which is the natural unit for spin in the quantum world).

For example, photons of spin 1 × ħ and the matter-carrying fermions (particles with half integer subatomic spin, e.g. electrons of spin 1/2 x ħ) should appear in the theory in a symmetric way. That means if you swap all the fermion and boson symbols, the equation will remain unchanged.

Essentially this puts forces and matter on equal footing, making them conceptually interchangeable.
 

Credit: Symmetry magazine
 

And in theories with supersymmetry, a new set of particles emerge, cousins of the familiar particles of the Standard Model.

Supersymmetry says that the familiar quarks and leptons must come with new, related particles physicists now call squarks and sleptons. Similarly, supersymmetric analogs of the photon and gluon, called photinos and gluinos, must exist.

Mind you, no direct evidence for the existence of these supersymmetric particles has ever been found. However, if they do exist, scientists can use these particles' quantum mechanical properties to cancel the contribution of the familiar particles in calculations of the mass of the Higgs boson.

With supersymmetry accounting for the other particles, the calculations result in a predicted mass of the Higgs boson that is small, in accordance with measurements.
 

Scientists celebrate the first recorded collisions

at 13TeV in CERN's LHC Control Center on June 3, 2015.
Credit: CERN
 

Some scientists' enthusiasm for supersymmetry has been dampened by the fact that supersymmetric particles haven't been observed.

Thus, researchers are exploring other possibilities, for example, the ideas that there might exist additional dimensions of space or that the Higgs boson might contain smaller particles within it.

These ideas and others are alternative approaches for taming the unruly predictions of the mass of the Higgs boson.

To quote the famous philosopher Yogi Berra,

it's hard to make predictions, especially about the future.

Thus it is difficult to know exactly what discoveries will be made at the LHC.

However, it seems probable that the mystery of the mass of the Higgs boson is the most promising thread at which scientists can tug. Hopefully, the right tug will let us unravel the existing Standard Model and allow us to knit an even better theory.

Only time will tell if we will be successful.