Tag Archive | "standard model"

Quantum Queries: Where Does The Higgs Boson Fit In?

Higgs Hoopla

Last 4th of July, scientists at the European Organization for Nuclear Research (also known as CERN  — don’t ask me why) made the announcement that they have detected a particle that could possibly be the long sought after Higgs boson.

As a non-hipster science fan, I find it heartwarming that a scientific discovery made in the French-Swiss underground scene is finally making it into the mainstream. However, I noticed that many people are at a loss when it comes to comprehending the excitement surrounding this Higgs thingy. After all, where in the big picture of science does this so-called “God particle” fit in?

[Photo credit: betabeat.com]

The Higgs boson is one of the few missing pieces of the Standard Model of particle physics. If the particle detected this week was indeed a Higgs boson, that’s +100 points for the Standard Model. The Standard Model is currently our best theory when it comes to explaining the behavior of our universe’s basic ingredients. Over past decades, it has been very successful at predicting how every known particle behaves and interacts.

If the universe is a stage, the Standard Model gives us the best insider story about the cast of characters and the role each character plays. Before we can describe what part the Higgs boson plays, we must first introduce the other members of the cast.


Enter the Leptons

The first members of the cast are the light leptons. There are six kinds of free-living leptons. The first three have charges, and they are called electrons, muons and tau particles. The next three don’t have charges, and they are called neutrinos. There are three kinds of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos.

Electrons are part of the atoms that make up most of the material things we handle everyday. In fact, electrons are the first subatomic particles to be discovered. You can read this article on a computer screen only because humans have mastered the art of making electrons the way they want it to move.

An electron.

Muons are similar to electrons, only they are heavier and short-lived. Tau particles are even heavier and more short-lived! In particle physics jargon, we say that the electron is stable while the muon and tau particle are unstable. (Most people are, unfortunately, like muons in more ways than one.) It is because of their short lives that we do not meet muons and tau particles in our daily affairs.

A muon.

Neutrinos are very light and elusive particles. They are also neutrally charged, which means that they do not get repelled or attracted by other charges. In fact, they very seldom interact with other particles. This is why it took scientists a while before they finally detected them. In this regard, neutrinos are basically ninja particles!

Neutrino = ninja particle.

Their elusiveness aside, neutrinos are actually everywhere! Right this instant, there are billions upon billions of neutrinos whizzing through your body like bullets flying though mist. You are not feeling it precisely because neutrinos mostly ignore other particles and are ignored by other particles. In fact, they can pass through the Earth like the Earth is not there.

Neutrinos recently made the news when some scientists thought they found neutrinos traveling faster than the speed of light. It was later discovered that neutrinos don’t break the universe’s speed limit after all.


Six Quarks for Muster Mark

The next members of our cast of characters are the quarks. There are also six of them: the up, down, charm, strange, top, and bottom quarks (aaawww yeah).

The six quarks are grouped according to “generation”. The up and down quarks belong to the first generation, the charm and strange to the second, and the top and bottom to the third. Quarks in each generation are heavier than those in the previous generation.

What distinguishes the quarks from the leptons is the fact that we do not find free-living quarks. Quarks are always tightly glued to other quarks to form hadrons. When a hadron is composed of a quark and its anti-quark glued together, we call it a meson. Meanwhile, when a hadron is composed of a triad of quarks, we call it a baryon.

You have quadrillions of hadrons in you, and so is the computer screen you are staring at right now. Why? Because the nucleus of atoms are made of protons and neutrons, and protons and neutrons are hadrons. To be more specific, they are baryons; protons and neutrons are made of three quarks glued together very tightly. The proton is made of two up quarks and one down quark while the neutron is made of one up quark and two down quarks.

A proton composed of three quarks, two up quarks and one down quark.


The Large Hadron Collider (LHC) of CERN is so-called because it was designed to smash together hadrons at very high speed. And also because it’s very large, as far as lab equipment go – it is found in a more or less circular tunnel 27 kilometers in circumference!


May the Force Carriers be with You

There are four fundamental forces: gravity, electromagnetic, weak, and strong. According to the Standard Model, the three forces aside from gravity are mediated by particles called force carriers.

Photons are the force carriers of the electromagnetic force. Photons are massless particles that travel at the speed of light, which is not surprising given that photons are the particles of light; light is but a stream of photons. Photons are also responsible for making like charges repel and unlike charges to attract. This means that without photons, atoms won’t exist either, because photons are what keep the electron around the nucleus! Without photons, the universe will be a very dark place indeed.

A photon.

The force carriers of the strong force are called gluons, so-called because they form the “glue” that tightly binds quarks to form hadrons. Like photons, gluons are also massless. Without gluons, protons and neutrons won’t exist.

A gluon.

The weak force, on the other hand, is mediated by heavy force carriers called the W and Z bosons. These particles are around 80-90 times heavier than protons. The obesity of these force carriers is the reason why the weak force, unlike the electromagnetic force, has a very short range. The weak force can only act across distances smaller than an atom. But exotic as it may sound, the weak force is in fact very important to life on Earth. The weak force is responsible for some forms of radioactivity without which our Sun wouldn’t shine and the Earth’s interior wouldn’t be a dynamic fluid.

A W boson.

Of the three forces of the Standard Model, the weak is the weakest and the strong is the strongest (like duh). Compared to the electromagnetic force, the weak force is a trillion times weaker while the strong force is a hundred times stronger.


The Punch Line

The Standard Model makes many now well-confirmed predictions about the behavior of the particles that make up our world, but there’s a catch: it seems to say that all the particles of the model (the six leptons, six quarks and the force carriers) have to be massless. Except for photons and gluons, which are indeed massless, this is clearly not the case. This is a problem of the theory. And it’s a major one, too.

This is where the Higgs boson comes to the Standard Model’s rescue. Higgs bosons provide a mechanism that imbues some particles with mass. This happens because Higgs bosons, which are everywhere in the universe, “couple” with some particles and thus supply them mass. The stronger the coupling of the Higgs bosons with a certain particle, the more massive that particle becomes. (Unfortunately, for people who want to lose weight really quickly, changing how you couple with Higgs bosons is not an option.)

In a universe without Higgs bosons, the Standard Model predicts that all particles will be massless and they will all zip across space at the speed of light. Since we find ourselves living in a universe where only photons and gluons can travel at the speed of light, then either Higgs bosons exist or the Standard Model is wrong after all. The discovery of the Higgs boson is therefore a major triumph of the Standard Model.

Higgs boson.


In Search of a New Standard

To date, the Standard Model is one of two best theories about the universe. However, it still has a lot of problems. For one, it does not say anything about gravity. For another, it goes haywire when combined with the other theory we have of the universe, General Relativity.

Gravity is the weakest of the four fundamental forces; it is literally weaker than weak. In fact, it is weaker than the weak force by a factor of 10^25 or a thousand million quadrillions! That is why in the world of tiny particles, gravity is negligible. Another problem with gravity is that the Standard Model says nothing about it. But it is the force that keeps you anchored to the Earth, the force that keeps the planets tethered to the Sun, and the force that herds stars into galaxies and galaxies into clusters. Gravity, weak as it may be, is a force to be reckoned with.

Our best theory for gravity is Einstein’s General Relativity, which explains that gravity is the curvature of space and time. General Relativity has passed all experimental and observational tests with flying colors. It powerfully explains the behavior of the universe as a whole from its earliest stages up to the present. But it is not friends with the Standard Model, something that bothers physicists to no end. This is especially bothersome given that the origin of our universe, the moments approaching the Big Bang, is subject to both the laws of General Relativity and the Standard Model.

Another problem with the Standard Model is that it accounts for only 4% of the universe! As for the other 96%, it has nothing to say. In fact, the other 96% is so mysterious to us that we decided to simply call them “dark matter” and “dark energy,” which just goes to show that we know next to nothing about them, except that they exist. (IMHO, calling the other 96% “love” would have been apt.)

The universe pie.


The Search Goes On

Let us summarize what we have talked about. The Standard Model is our best theory about the composition of our universe. It tells us that the universe is composed of six leptons that can fly around freely (like electrons and neutrinos), six quarks that are always glued to other quarks (protons and neutrons are just quarks glued together), and force carriers that mediate the interactions between the other particles. But the Standard Model can only account for the mass of some of the particles if a particle known as the Higgs boson exists. If the particle detected last week was a Higgs boson, it would be a major triumph for the Standard Model.

However, it is apparent that the Standard Model cannot be the last say. It has its own problems, chief among these is that it cannot explain gravity, it is not compatible with our best theory explaining gravity, and it can account for only 4% of the universe. And so the search for the solution to the problem of existence has not ended. In fact, the discovery of the Higgs boson opens the door for more furious research; in other words, the search has only begun.

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Why is the Higgs Boson a Massive Deal?

After around 50 years since the Higgs boson’s existence was proposed in the 1960’s, scientists at CERN have now confirmed at a 4.9 sigma significance level (more than 99.9999% confidence) that a particle that looks like the Higgs boson exists. Evidence from the ATLAS and the CMS experiments at CERN show a particle with the predicted Higgs boson mass of around 125 GeV resulting from the head-on collision of two protons.

Remember from Albert Einstein’s famous E = mc^2 equation that mass is energy divided by the square of the speed of light. Particle masses are measured using the voltage (or energy) of the electron as the standard. At over 125 x 10^9 electrons worth of energy, the Higgs boson is the heaviest particle that has ever been produced at the Large Hadron Collider (LHC). Indeed, finding this particle was one of the main motivations of building a particle accelerator with the size and power of the LHC in the first place.

A boson is a particle whose behavior can be described by a system called Bose-Einstein statistics. The Higgs boson is a special kind of boson called a “gauge” boson. This class of bosons brings about the fundamental forces of nature, with the photon mediating electromagnetism, gluons mediating the strong force (which holds quarks, the building blocks of protons and neutrons, together), and the Z and W bosons mediating the weak force (which is involved in radioactive decay and hydrogen fusion in stars). The as-yet-undetected graviton is not predicted by the Standard Model, but is a gauge boson that mediates the force of gravity in some quantum mechanical descriptions of gravity.

The Higgs boson has been called in the media as the “God particle,” as it will solve a key problem in the current Standard Model of particle physics. The Standard Model describes the nature of matter through dozens of subatomic particles. The initial problem with the model, however, is that it shows that particles initially have zero mass and cannot explain why some particles have mass. If particles don’t have mass, they will whiz through the universe at the speed of light. This is clearly not the universe we live in.

To solve this matter, Peter Higgs, among other independent teams of scientists, proposed what came to be known as the “Higgs mechanism.” Higgs suggested that throughout the universe exists a field, now called the Higgs field. Fields can be described using particles, like how the electromagnetic field can be represented by light particles called photons. The particle of the Higgs field is the legendary Higgs boson.

Peter Higgs (middle) shedding tears, which have masses from a particle that bears his name, at CERN when the ATLAS and CMS results were publicly announced


The Higgs mechanism acts similarly to how photons, or light particles, travel at 299,792,458 m/s in a vacuum, but not when there are particles in the way. Light appears to slow down (and bends) in a medium, such as glass, because it actually travels a longer-than-apparent path.


The black lines represent the true path of the photon as it goes through a medium. At each portion of its journey, it travels at the speed of light. The red line represents its apparent shorter path.


Photons hit glass particles and get deflected from their path, resulting in a longer true distance traveled despite appearing to have traveled a shorter distance. Since speed is distance over time, it appears to us as if the photon has taken a longer travel time, when it actually just traveled a longer distance at the same speed it always does. Higgs bosons act like glass particles on a photon, obstructing the path of Higgs-interacting particles, such as electrons, in the universe. The “God particle” isn’t magical or supernatural as its name suggests it to be, but without the Higgs boson, there would be no people to wonder why particles have mass.

“Why are things so heavy in the future? Is there a problem with the Earth’s gravitational pull?”


Recall from Newton’s first law that inertia is the tendency to remain at rest, if initially at rest, and tendency to remain in motion, if initially in motion. Since mass is simply the measure of inertia and particles naturally have no mass and fly around at the speed of light, all Higgs-interacting particles will be slowed down by the Higgs field and look to us as if they had gained a resistance to change in motion, which we measure as mass. The photon does not interact with the Higgs field, so it retains its speed and lack of mass.

The Higgs mechanism explains how many particles, such as electrons, gain inertia (or mass). Quarks, which make up neutrons and protons, also gain some of their mass through the Higgs mechanism (though most of their mass comes from interacting with gluons). To be clear, the Higgs boson is not the reason for all mass in the universe.

The Higgs boson is hoped to be the final piece of the Standard Model of physics. Further work will be done to establish that the discovered massive 125–126 GeV particle does indeed have the properties that have been predicted for the Higgs boson. However, even just finding a 99.9999% credible Higgs boson candidate makes scientists more confident that we have in the Standard Model a good picture of the universe at its smallest and weirdest scales.


Peter Higgs Image and Tear Metaphor Credit: Reddit

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