Wednesday, August 15, 2012

Higgs Boson

NOTE: This article has been rewritten from the original August version. My husband, a curious scientist in his own right, told me quite correctly I dropped the ball on this one. The original article is just too damned confusing and impossible to follow. So, please bear with me as I try to make the pesky Higgs boson accessible to us (and hopefully make certain I understand it myself!). So here goes:

The Higgs boson, discovered on July 4, 2012, is, according to some researchers, the science breakthrough of the century. It has even captured the imagination of laypeople, like me. A lot of us, however, feel like we don't quite get it - what exactly is a Higgs boson and what's so great about it? What makes it  the "God Particle" described by the media? It's not easy to get in on the excitement when even the subject - theoretical physics - feels intimidating.

This discovery is kind of a neat story about how science is evolving. People want to be involved in the process, and they want to understand the buzz around the latest breakthroughs in physics, and that has made this discovery one of the fastest transitions of scientific knowledge from laboratory to general public ever. It topped the list of trending twitter topics for weeks, beginning with a flurry of rumours showing up on various physics blogs. You could even find trendy articles on how to talk about the Higgs boson at barbeques and sound smart this summer.

There is a bit of a catch when we try to discuss the Higgs boson, however. We may understand it is a particle in physics, a particle that gives other particles mass, and that kind of makes some sense, but when we ask a few simple questions such as why or how it does this and why it is so important, we start to find ourselves waist-deep in complex theory, an unsettling feeling to say the least. And it's usually here where curious minds run aground and give up.

I'm going to try to understand the Higgs boson right along with you. I'll try to put this discovery in some context with other important discoveries in physics. Things are (still) about to get complicated, but I hope to get us to the end feeling comfortable with a working understanding of the Higgs boson. Let's start with the basics and then build on that:

A Beginner's Sketch of the Higgs Boson:

As I mentioned, you may have heard that this boson is responsible for giving other particles mass. Right here is an interesting breakthrough in itself when you think about it - it means that particles don't inherently have mass - they have to acquire it somehow. How do they do that? According the the theory behind the Higgs boson, particles with mass somehow get mired down in a "sticky" Higgs field, a kind of force field like gravity or electromagnetism, that permeates the universe. Even completely empty space is filled with various force fields like these.

Particles, like electrons and protons inside atoms of matter for example, acquire mass as they interact with this Higgs field, and because they do, they have to slow down. They can't travel at light speed like massless photons of light can. Photons are particles that don't have mass. They don't interact with the Higgs field. They're invisible to it. And because they don't interact, they can zip right along at the speed limit of the universe. The reason why the speed of light is the speed limit of the universe is whole other story; check out this introduction to special relativity, if you'd like to explore that a bit further. This whole mass acquisition business works through a mysterious Higgs mechanism. Now, if you are like me, a bunch of new questions pop into your head when you read this. How does it really work? Why do some particles have mass and not others? Where does this Higgs particle originate? Where is it now and why was it so hard to find? The Higgs mechanism is one of the most difficult concepts to understand in physics but it is also one of the most rewarding. If you can get a handle on the Higgs mechanism, you will gain some deep insight into the how our universe began, and how it came to be the way it is today.

The Higgs boson lives in a weird and fascinating world of particle physics.  Let's get acquainted with this world with this concept-in-a-nutshell introduction by Ian Sample, science correspondent at

Why Were Physicists Looking for The Higgs Boson?

We're going to need to build a little background to understand why an enormously expensive technologically challenging piece of hardware called the Large Hadron Collider was built, in large, just to find this particle.

The foundation of modern particle physics is what is called the Standard Model. This theoretical model, is the result of theories and discoveries made by literally thousands of physicists over the past century, and it is very useful. It provides us with a great deal of insight into the fundamental nature of matter and forces in our universe. Our current understanding is that we have 12 basic building blocks of matter and they are governed by four fundamental forces. The matter particles are the purple and green blocks and the force particles are shown in red:

Source: MissMJ (Wikipedia)

Don't worry about knowing all the names, like muon and so forth. This image is just to give you the idea that all of physics is governed by particles of force and matter.

The Standard Model above contains three generations of matter. The matter particles on the left make up all stable matter. These are the particles of everyday atoms and neutrinos. They are called first generation particles. Heavier unstable particles belong to second and third generations, which quickly decay into second and then first generation particles. These heavier unstable particles, muon neutrinos and bottom quarks for example, are briefly found in extreme high energy environments like particle accelerators or in cosmic rays.

The Standard Model explains how subatomic particles behave and how they are related to one other. However, as successful as it is, there has always been a deep underlying mystery as to why some particles, such as electrons and quarks (quarks make up the protons and neutrons inside the nucleus) inside atoms have mass and others, such as photons of light, do not. You could say, well they just do, but curious physicists are not satisfied with that answer. As I mentioned earlier, photons and some other particles without mass zip around at light speed, Einstein's speed limit of the universe, while others, those with mass, seem to slog through space as if they were slopping through molasses in rubber boots. Why?

We know that gravity acts on mass. Photons and other massless particles do not interact with gravity at all. And here, I should clarify something you may already know about photons. You may know that black holes, objects of infinite mass and gravity, "suck" light into them. What's really happening here is that the photons are not interacting with gravity at all BUT they are moving through space-time. Gravity bends space-time, and where a black hole lies, space-time is bent into a funnel with an endlessly long bottom point to it. The photons simply follow that bent space-time and they get trapped and can't get back out. This geometric concept of gravity is Albert Einstein's general theory of relativity.

What quality gives rise to mass in the first place?

In 1964, physicist Peter Higgs predicted a mechanism by which particles could attain mass. Some particles do not interact with this mechanism - photons for example - and others do - electrons and quarks for example. Particles gain mass by interacting with a field that permeates all of space, called the Higgs field.

Where Higgs theory starts to get interesting is when we talk about force particles. Some of them are massless like the photon, but others are not. Take a look at the particle diagram once again. You will see 4 different force particles there on the right - the photon, the gluon and two kinds of boson. Do you notice the GeV/c2 in some of the red blocks? That's a fancy measure of mass that physicists use. The bosons have mass but the photon and gluon don't. How can that be? Weren't all the fundamental forces created right at the beginning to the universe when the laws of physics took shape? What kind of events took place to give only some of the force particles mass? If a force field took shape back then to give mass to particles, why didn't all of them get painted with the same brush? That question has perplexed physicists for decades and it's what made the whole Higgs adventure so interesting, like reading a great detective mystery.

Let's dive a little deeper into the force particle story:

Now, according to the Standard Model, each fundamental force has a particle associated with it. This means that there are particles of matter and there are particles of force. A force particle mediates or carries out a force. It is also the smallest whole (or quantum) unit of the force possible. For example, massless particles called gluons carry out the strong force. This is one of the four fundamental forces in the universe. The strong force holds quarks together inside protons and neutrons. Gluons also hold the protons and neutrons themselves together inside the nucleus. Gluons, like photons, are massless and because of that they travel at light speed. Physicists don't deal directly with gluons much. Their range of influence is so short that it is confined to the insides of atoms. But gluons are very powerful, as implied by the name "strong" force. It isn't easy to tear apart the nuclei of atoms, as illustrated by the enormous energy released by a fission bomb (and that's just prying protons and neutrons loose, not ripping them apart into quarks). The strong force is about 100 X stronger than the electromagnetic force, another of the 4 fundamental forces.

Photons mediate the electromagnetic force, which means they are the particles that carry out all the forces between electrically charged particles, such as electrons and protons inside atoms. Photons are the force particles behind all of chemistry, electricity and magnetism. Almost all the phenomena we experience day to day is due to the electromagnetic force.

Physicists call all the force-mediating particles, including the photon and the gluon, gauge bosons. In addition to photons and gluons there are other gauge bosons in the universe. For example, particles called W and Z bosons really mystify researchers because they have mass. Unlike the photon and the gluon, these particles interact with the Higgs field and, as a result, they cannot travel at light speed. Their masses are about 80 and 90 GeV* respectively, so they are influenced by gravity, while photons and gluons are "invisible" to gravity. W and Z bosons, together, carry out the weak fundamental force. This is the force responsible for radioactive decay and the fusion of hydrogen into helium inside stars.

*A GeV is a unit of energy, a giga (billion) electron volts to be exact. To give you an idea of how much energy that is, when two protons collide with each other at the Large Hadron Collider (LHC) at CERN (shown below), they can release an energy of 14 TeV (14 thousand billion electron volts). That's about 12 thousand times more energy than a Z boson has. A lot of that energy is kinetic energy that's released in the collider. But it's also in the mass of various newly created particles that shoot off in all directions. In particle physics, it is easier to describe the masses of particles in terms of energy instead of something like grams, because particles are so small. The mass-energy equivalence Einstein described as E = mc2 plays out over and over in collision-type experiments. Inside colliders, particles are annihilated, releasing energy, and they are created, consuming energy, as a matter of course.

Credit: Julian Herzog (Wikipedia)

You might have noticed I said 4 fundamental forces. I left out the fourth force, gravity, on purpose. Gravity is a real mystery to physicists - they haven't yet found the particle that carries it out IF there is a particle. Some physicists think gravity might not be force per se at all. It might just be a property of space-time itself. The stretching of space-time simply looks like a force to us. Others think that gravity, rather than being an out and out particle, might hint at a mysterious deeper reality of physics, perhaps some kind of fundamental string-based universe, where gravity and all the other forces and particles are strings, each vibrating at a particular frequency.

A motley crew of massive and massless force particles in the universe present physicists with a real puzzle. How did just some of them get mass? Let's take a closer look at the Higgs field and how it might offer us some clues to solving this puzzle.

What is the Higgs Field?

Thanks to Higgs' and others work, the Standard Model in physics predicts a field. This force field has energy, even in its ground (or lowest possible energy) state. Even in the vacuum of empty space, this field implies a small but definable lowest possible energy exists there. This energy is what physicists often refer to as the Higgs mechanism.

The Higgs Mechanism Tells Us About the Very Mysterious Beginning of Our Universe - And It's All About Symmetry

If we measure the current expansion of the universe, we can extrapolate backwards to its beginning. The universe exploded from a perfect single point. The laws of physics imply that all the forces acting on this point should be equal all around. This is just a way of saying that the baby universe should have been a perfectly even, homogenous, expanding sphere. It should, in fact, have created equal amounts of matter and antimatter. And if it did, then both matter and antimatter would have annihilated each other right off the bat and our universe would have nothing in it today, no stars, no gas, nothing. The fact that matter somehow won out gave physicists a clue to how the universe unfolded. If we look at the laws of thermodynamics, the universe must have cooled as it expanded, thanks to the conservation of energy. And when it cooled, physicists think it went through some phase changes. These phase changes were not that much different from the phase change of water freezing into ice, and we can think of them in the same way.

Here we get to a common theme underlying the story of how forces and particles came to be in our universe. New forces and particles come into existence when basic symmetries break. These symmetries are often referred to as gauge symmetries and there is a whole complicated gauge theory behind them. This is why we call the fundamental force particles gauge bosons. They can all be described using the mathematics of gauge theory. In order to understand the Higgs (another gauge) boson, we don't need to get all muddled up in complex gauge theory. We can think of symmetry-breaking as a phase change, and I'll show you soon how this concept works for us when we try to explain why some particles have mass and others don't.

For now, lets make sure we get the idea of symmetry-breaking: a glass of water, after cooling sufficiently (ie. after it loses enough energy) begins to freeze. At this point the symmetry, the uniformity of the water if you will, begins to break into two different sections: water and ice. Uniformity breaks into two non-uniform parts. (If you are curious about how the universe got more matter than antimatter, take a look at my article on antimatter. I also talk about matter/antimatter symmetry-breaking when I explore Our Universe in the same-titled series of articles elsewhere in this blog.) Similarly, the universe "broke" into different forces as it began to expand and cool just after the Big Bang.

Higgs: Looking for a Needle in a Haystack?

From a starting point of infinite energy, the universe began to cool after the Big Bang. As it did so, several fundamental symmetries broke. For example, physicists have documented that two fundamental forces, the electromagnetic force and the weak force ultimately came from a once unified force called the electroweak force. Very soon after the Big Bang, the energy of the universe "cooled" to about 100 GeV. Today the energy of the universe, the cosmic background radiation in other words, is around 0.235meV.  That's a difference on the order of about 1012. At around 100 GeV, the electroweak force broke into the two forces we know today - electromagnetism and the weak force. That was a breaking of symmetry analogous to a phase change. And just like a water phase change, physicists can reverse the process by adding energy back into a system. Tremendous energies can be achieved inside particle accelerators, and it is here where physicists have observed evidence for the unified electroweak force.

It is here too where we are about to find out how our motley crew of massive and massless fundamental force bosons came from one single (homogenous if you want to call it that) progenitor force particle.

When you create a collision with enough energy (100 GeV) the W bosons (there are 2 of these, each with an opposite charge), the Z boson and, remarkably, the photon all revert into one indistinguishable particle, which mediates a single force - the electroweak force. This is right where the needle in the haystack can be found. It is here where somehow a particle breaks into massive and massless particles. But how? This is where the Higgs mechanism comes in.

High-energy experiments from the 1980's up to today at the Large Hadron Collider have produced millions of particles behaving the way we would expect if the electromagnetic and weak forces were unified. It seems when you first think about it that the mystery of the boson's masses has been all sewn up. These massive and massless bosons come from electroweak symmetry breaking into the weak force and the electromagnetic force. But with this success came an unexpected problem. The electroweak theory can explain what happens at energies where the force first unifies (100 GeV), but if higher energies are inserted into the mathematical gauge theory describing these particles, the results become nonsense. Some new force field, some new specific energy was needed to "fix" the theory at very high energies. And this is, once again, where the Higgs mechanism comes in. Remember that baseline energy of the Higgs field I mentioned earlier. It comes into play now.

In physics, fundamental forces are described as fields, and each is associated with a particle that mediates the field. The particle, a gauge boson, is a quantum unit of the force. At high enough energies you can "shake" these particles out. What I mean is, you can see evidence for them inside colliders, where there is tremendous energy. Physics at "everyday" energies does not encounter them. You won't stumble into a Higgs boson while walking down the street. I'll go back to my gluon example to explain what I mean. As I mentioned earlier, this particle mediates the strong fundamental force. It acts like a glue that holds quarks together inside protons and neutrons and neutrons and protons together inside atomic nuclei. It is the field that we indirectly experience as atomic nuclei holding themselves together inside atoms. The gluons are there in every-day atoms but only in "force form." Only at very high energies can we "experience" the gluon particle itself. Gluons were first found in 1978 in the collider called DORIS. We could argue that gluons are hidden inside atoms so that's why we can't "see" these particles. But that's not the whole story: We can't see W, Z or Higgs bosons either, not because they are hidden or confined somewhere but because they aren't there. At ordinary energies, we can experience and test only the forces they convey. To "see" the particles themselves in action, we need to create the high-energy environment in which they "shake out." So, as we go up in energy, force particles shake out, and the forces they mediate come together.

If you are wondering where the electrons and quarks that make up atoms fit into all this, I can give you a short version of the story, and here you will see that in contrast to energy particles, matter particles "shake out" as you go down in energy, or as the universe cooled.  As the electroweak force and strong force broke from a once single unified fundamental force, a kind of proto-matter made its first appearance in the universe. It was a disordered quark-gluon soup that, as the universe cooled further, became seeded with electrons and neutrinos. The LHC is now focused on studying quark-gluon plasma now that the Higgs discovery was made. The universe had to further cool before quarks could slow down enough to bind to each other and create protons and neutrons. As the universe continued to cool, these first simple nuclei attracted electrons to create the first simple atoms - hydrogen and helium. Check out my articles, Our Universe Parts 6, 7 and 8, for a more detailed description of how ordinary matter came to be.

Why The Higgs Mechanism is so Important

Getting back to our Higgs story, when the electroweak force broke into the electromagnetic force and the weak force, four subatomic particles "shook" out as a result of the "phase change." These particles all mediate forces. They are the W- and W+ bosons, the Z boson and the photon. The W bosons are charged and have a different mass than the neutral Z boson, while the photon has no charge or mass. How do you get from a single homogenous state, one force with one associated electroweak boson, to this? And why do some of the products have mass while one does not? The Standard Model, as good as it is, could not account for this until the Higgs mechanism was introduced.

Researchers think that the Higgs mechanism is what spontaneously broke the electroweak gauge symmetry when the universe was very young, somewhere between 10-35 seconds and 10-12 seconds old. When this happened, the Higgs boson itself made its appearance in the universe. The Higgs boson is a sign, if you will, of the Higgs mechanism happening. Physicists do not yet know how the mechanism was triggered but its triggering is associated with the sudden appearance of a pervasive new field, the Higgs field, which is mediated by a particle, the Higgs boson. Remember that a boson is a quantum unit of a field, in the same way a gluon, for example, is a quantum unit of the strong force holding quarks together in neutrons. Before this, no particles had mass. Remember too I mentioned that quarks and electrons made their first appearances just after the Higgs mechanism was triggered, and force bosons with mass (the W and Z bosons) appear simultaneously with the mechanism. The Higgs boson is itself is a massive boson, and most researchers think it attained its mass the same way other particles did. After this symmetry broke, leptons and quarks as well as the W and Z bosons and the Higgs boson itself (and possibly neutrinos too if they are proven to have mass) all attained mass, while some bosons did not, such as the photon and gluon.

A Mathematical Formula Describes the Higgs Mechanism

According to Standard Model theory, which itself is an elegant mathematical formulation, the Higgs field consists of four component fields. Two of these fields are neutral and two are charged. Two of the charged fields and one of the neutral fields are mediated by what are called Goldstone bosons which in theoretical models act like W+, W- and Z bosons, all of which have mass. The last neutral field component is predicted to be mediated by a neutral boson of mass - the Higgs boson. Remember that the photon does not interact with the Higgs field. It is not described by these four component fields. To give you an idea of what the math looks like, here is an excerpt from Wikipedia which mathematically describes the symmetry breaking of the electroweak force which gives rise to the W and Z boson masses, fermion (quark and lepton) masses as well as an energy value for the Higgs field, formally called the vacuum expectation value:

You certainly don't need to understand all this math but I hope it gives you a feel for how physicists can  use it to describe how one (electroweak) boson can turn into four unique new bosons, some making their appearance with a new quality called mass for the first time in the universe. And the quality that gives them mass is the same thing that triggered their appearance in the first place.

The Standard Model  has been in development since the 1950's and many brilliant minds have gone into its making, both on the theoretical front and on the experimental front. Physicists use this important theoretical framework to refine their search for new particles. Quarks and neutrinos were theorized well before they were discovered in collider experiments. The Higgs boson is a key building block in the Standard Model. This boson was first theorized in 1964 in a series of papers on symmetry breaking. This gauge theory symmetry-breaking extends the Standard Model by incorporating the models of the grand unified theory. This theory is based on symmetry-breaking processes. These grand unified theory models seek to unify the fundamental forces into one grand force thought to exist when the universe first exploded into being - before gravity, the strong force and the electroweak force broke into existence, when they were all unifed into one "mother" force. This mother force implies some kind of mother boson but it is almost impossible to imagine what kind of mother particle could mediate such a force. What does seem clear is that the energy required to create such a particle would be far beyond our current technology, if it even exists. It seems to me if any particle should merit the name "God particle," it should be this one.

Meanwhile, the Higgs discovery puts some very essential pieces of the Standard Model and our understanding of cosomolgy and particle physics in general into place for us. It not only gives us powerful verification of our current understanding of how particles and forces work, it gives us our first glimpse of how mass first arose in our universe. To me the best part of the Higgs story is the symmetry-breaking part - the Higgs boson allows us to understand how our current (low energy) unsymmetrical universe, filled with some massive particles and other massless ones, arose from a once perfectly symmetrical point of origin, the Big Bang.

How To Catch a Higgs Boson

How did physicists finally catch this elusive high-energy massive particle? The Higgs boson itself, as mentioned, lives only in a very high-energy environment. The math in Higgs theory predicts the Higgs boson should have a very high mass. There is more than one mathematical solution to the theory, so what you get is a range of possible masses to look for in the collider. The LHC can attain energies sufficient to create particles with masses around what the Higgs boson is expected to have, by smashing two protons together at tremendous speed. That part of the hunt for the Higgs wasn't all that terribly difficult. The problem is the Higgs boson is likely to be created only rarely during these collisions and it doesn't last long - it immediately decays into other lighter particles. The trick is to accurately predict what its decay particles should be and look for them instead. It's not an easy process.

Physicists at the LHC were looking for the Higgs boson within a mass range of 120 to 140 GeV. The LHC has more than enough energy to create particles within this range but it gets difficult to find the Higgs boson as you approach the lower end of the range. If it's over twice the mass of the W boson, for example, it should decay into two W bosons, and they can be spotted. If it's not quite massive enough to do this, it could decay into two Z bosons, which are a bit less massive. This would be easy to spot too because the two Z bosons would quickly decay into 4 leptons that are easy to detect. But, if it is lighter than this it could decay into two heavy bottom quarks. This would result in a bunch of hadrons being detected (hadrons are combinations of quarks bound to each other; protons and neutrons are examples) and hadrons are so common as decay products in collisions that it would be very difficult to find a signature of a Higgs boson in all that hadron debris, even given that you would expect many more Higgs bosons to be created with the same energy than if it were more massive, say with a mass equivalent of two W bosons for example.

The big questions here then is: if you find, for example, a spray of two Z or W bosons as well as perhaps many other lighter and massless particles as your result, how do you know for sure a Higgs boson left its particular signature in it?

The Elusive Higgs Signature

The Higgs boson, according to Standard Model theory is expected to create a particular pattern based on something called its coupling energy. This is the energy with which it couples to the Higgs field. All force-mediating particles couple to their force fields. In this case, actual results are complicated by two things. First, systematic errors - is the detector working correctly, is the device calculating the energies correctly, etc. Second, every subatomic event plays out in the strange environment of quantum mechanics, so there is a random component to identical repeated collisions. These reasons are why the Higgs boson is described as being discovered with a 5-sigma accuracy. This accuracy scale which physicists use is equivalent to a 1 in 3.5 million chance of being wrong. Given the challenges involved those are astoundingly good odds! Here is one possible Higgs signature according to CERN:

In this simulated data, two protons have just collided, creating two jets of hadrons and two electrons. The lines reperesent possible paths of particles produced and the energy of these particles is shown in light blue.

All this is why the Higgs boson has been so hard to find and why physicists have been carefully running many tests to see that it recreates the expected result, and to attain almost irrefutable odds that they actually found it.

The great announcement you heard in the news was when physicists discovered evidence of a particle right in the range where the Higgs mass should be. That is the Big Breakthrough in a nutshell. Like other particles, the Higgs boson is a quantum particle, and that means it not only has mass but it has other qualities too like spin and charge.  For example, the Higgs boson should have zero spin, zero charge and no colour charge. Physicists still have to verify that this particle matches all these qualities before they say with 100% certainty it is the Higgs boson.

The Standard Model has so far been extremely accurate in predicting particles that have later been discovered experimentally (the W and Z bosons, gluon, top and charm quarks). The predicted Higgs boson rounds out the theory - it is the last missing puzzle piece to add.

What is the Higgs Mechanism Precisely? More Questions

We've learned in this article that the Higgs field switched on and when it did so, it "cracked" the electroweak field into photons (mediating particles of electromagnetism) and into W and Z bosons (mediating the weak field), imparting mass on the W and Z bosons (as well as on quarks and electrons and the Higgs boson itself) but not photons (or gluons). We know when the Higgs mechanism switched on, but we are left wondering how it imparts a new quality (mass) on some particles, making them suddenly "visible" to gravity. We're left to wonder how this new quality of mass "hooks into" or interacts with gravity. Gravity and the Higgs field have one thing in common - an interaction with mass.

Can we connect gravity with the Higgs mechanism?

The Higgs Mechanism Has a Mysterious Connection to Gravity

I've hinted at what gravity is earlier in this article. Now we need to really understand it in detail. Isaac Newton thought that masses attract each other, and that attraction is experienced as gravity. Einstein, in his theory of general relativity, describes gravity in terms of geometry, as the curvature of space-time. Space-time is a four-dimensional framework of the universe, consisting of three spatial dimensions plus one dimension of time. Mass curves spacetime. What Newton describes as mutual attraction of masses to each other can be described by general relativity as two masses falling into each other's gravity wells.

Now, if we look back at our description of particle mass, we will remember that mass and energy are equivalent. So energy too must curve spacetime and therefore must also be a source of gravity. This means too that when we consider the mass of a molecule, for example, we are measuring not only the combined mass of its quarks and electrons but also its binding energy, chemical bond energy, its temperature, its momentum (a quality closely associated with energy), and its pressure and even tension. All these qualities tell spacetime how much to curve. They are all components or facets of a general description called the stress-energy tensor. So, we can describe our molecule mathematically in terms of a stress-energy tensor and we can calculate just how much it should bend spacetime (it won't be much!). This works in reverse too. The geometry of spacetime also tells our molecule how much it can move. For example, the molecule will be accelerated in an external gravitational field, just like a ball falling to the ground after you throw it up in the air.

Hopefully this gives us a good understanding of how gravity and mass work on each other. Space, time and mass are all intimately intertwined through general relativity. But notice that we have not yet incorporated the Higgs mechanism. There is a good reason, but I warn you it won't be satisfying. The Higgs mechanism is a quantum phenomenon, as we currently understand it. Gravity is understood only as a relativistic phenomenon. Physics doesn't yet have a way of getting these two theoretical frameworks to talk to each other. Their mathematics will not work together without giving us nonsense answers. I'd like to add a little clarification here: We can describe the Higgs field as a sticky surface, where the motion of particles with mass is impeded, but massless particles are perfectly slippery to it and zip right through at light speed. This impediment of motion is not the same as the effect of gravity on mass - you need to apply energy to accelerate our molecule because it has inertia. That energy has nothing to do with the Higgs field. Instead it is the energy described by general relativity. Sometimes in the literature I've noticed these two concepts get mixed up.

There is no way yet to bridge the gap between the Higgs mechanism and gravity. We will need a quantum description of gravity in order to relate the two phenomena to each other. They have the common denominator of mass and maybe through that, there might be a way to explore the quantum nature of gravity.

The Higgs Boson Wasn't All They Were Looking For: Big Questions Remain

If you think the mystery of the universe has been wrecked by the Higgs discovery, don't fret one bit. Besides the Higgs boson, there are other high mass particles physicists are hunting for, and they are hoping one or more of them might explain pesky dark matter. Dark matter has mass. In fact, that is how physicists know it's there. Though it neither emits nor absorbs any electromagnetic radiation, meaning it is undetectable, its gravitational effects on visible ordinary matter, radiation and on the motion of galaxies betray its existence. Based on these effects, about 84% of all the matter in the universe is calculated to be dark matter. Is there a particle with mass that has not yet been discovered, one that interacts only through gravity and perhaps through the weak force, but not with electromagnetism? This rules out almost all ordinary (baryonic) matter, that is, matter made of atoms. It doesn't necessarily rule out non-emitting atoms in things such as non-luminous gas and condensed objects like black holes, neutron stars, and brown dwarfs. These very faint-to-invisible objects have mass but physicists don't believe there is nearly enough of them to account for so much matter in the universe. This leaves us with nonbaryonic matter, such as neutrinos (if they have mass), hypothetical particles with mass called axions (which are not part of the Standard Model but instead are a theoretical solution to an unrelated problem in theoretical physics) and something called supersymmetric particles. That means none of the Standard Model matter particles is a candidate, with the possible exception of neutrinos, and right now neutrino mass is standing on pretty iffy ground. Therefore, many researchers have turned to an extension of the Standard Model called supersymmetry theory. This idea comes from a theoretical extension of the Standard Model that embraces at least some string theory.

There is an elegant mathematical framework that describes this symmetry; in fact it provides possible solutions to several current theoretical problems in particle physics, including those in string theory. It suggests that there is a symmetry underlying all force-carrying particles and particles of matter. In other words, all bosons (all of which have whole integer spins and are force-carrying particles) have supersymmetric partner fermions (all of which have half or multiples of half integer spins and tend to be particles of matter) and vice versa.

For example, the photon would have a corresponding mirror particle called the photino, with a spin 1/2, and a mass between 10 and 100 times that of a photon. There is as yet no direct evidence for supersymmetry but the photino is one of a handful of possible candidate particles for dark matter. It would be the lightest supersymmetric particle produced, and therefore it itself cannot decay, so it would be expected to exist in our current low energy universe. Supersymmetry, if it were verified, would fatten up our current standard model by doubling the number of particles in the universe. The promise of these particles as candidates for dark matter is their mass. Each of these supersymmetric particles would have an expected mass much higher than its partner, most falling between 100 and 1000 times more than that of the proton. The LHC can muster enough energy to produce some of these particles, particularly the photino, and it is looking for them now. The fact that photinos haven't been found yet in fact makes supersymmetry theorists a bit nervous.

Supersymmetric particles fall into five classes with funky names such as squarks, gluinos, charginos, neutralinos and sleptons. Their expected interactions and decays are described in detail by the minimal supersymmetric standard model, giving physicists characteristic signatures they can look for in the collider. Here's an example:

(image captured from Wikipedia: Minimal Supersymmetric Standard Model)

The photino, not shown in the above chart, has an expected mass of between 500 MeV to 1.6 GeV. Physicists are currently attempting to work out the photino's relic abundance in the universe to see if it could be a suitable dark matter candidate.

The idea of supersymmetry suggests that the universe might have gone through a very brief state in which matter and energy were unified before splitting into supersymmetric particles and their partners, and at that time, energy may have first existed as a single "mother" force before it too split into the fundamental forces today. Almost all of these supersymmetric particles would have decayed soon after they formed, thanks to their high mass-energy values. They would be strictly high-energy entities. The Higgs boson fits right in the middle of this early and unimaginably energetic slew of particles, suggesting that it or some relative particle of it (perhaps the Higgsino?) was around along with its mysterious mass-imparting mechanism to impart mass on these supersymmetric particles.

In terms of explaining the Higgs mechanism, the dark matter/supersymmetry trail also leaves us unsatisfied. Somehow the Higgs boson, or perhaps a Higgs superpartner, interacts with these supersymmetric particles, if they exist, and contributes all the mysterious mass of dark matter. However, it does not get us any closer to understanding exactly how mass is inferred upon particles. How does this Higgs field interact with particles exactly? And how do the various specific masses of the particles arise? In other words, why do some (heavy) particles interact with the Higgs field more strongly than other (light) particles? Saying the field acts like a sticky (3-dimensional) surface gives us a way to visualize what might be going on but it doesn't tell us how that happens.

I wonder too if the Higgs mechanism was some kind of inevitable physical process (perhaps analogous to magnetic field lines setting up in cooling rock) or was it an historical accident, without which our universe would have no mass today. It would have remained a frothy mess of particles instead, all of them zipping around at light speed. There would be no clumping of matter, so no planets, stars or galaxies. This doesn't preclude gravity's existence however. Our current understanding is that gravity is either the first force expected to have broken from the universe's initial perfect state of symmetry, or it is an artifact of the fabric of spacetime itself. But without mass, gravity would have nothing to "hook" itself to. Every particle would be invisible to its effects.

Finally, it doesn't seem intuitively obvious that a force with a single boson should break into two forces, one governed by three bosons, two of which are the same mass but opposite charges, along with a third neutral slightly less massive particle, and another force governed by a massless particle. There's no obvious pattern to these particles. One splitting into two with opposite charges would somehow be much more satisfying. This randomness isn't confined to these particles either. If you look once again at the particle diagram earlier and compare energies between generations of Standard Model particles you get the sense someone blindly picked them out of a bag. And yet, the fact that matter comes in different particles at different energies might be a clue to how mass works regarding the Higgs boson.

Conclusion: Is the Higgs Boson the "God Particle"?

Theoretical physicist Peter Higgs, from Edinburgh University, now 83, is enjoying the rare pleasure of being alive to see the experimental confirmation of a mathematical theory he began working on 48 years ago.

(credit: Gert-Martin Greuel (Wikipedia))

Some of the theoretical significance of this discovery, however, is yet to come. It completes the Standard Model, but already it is presenting physicists with many new questions about the nature of mass, matter and gravity in our ever-enigmatic universe. The Higgs boson is a significant piece snapped into the great physics puzzle, but "God particle"? The name, coined by a publicist and disliked by many physicists, doesn't seem appropriate.  The particle itself seems less revolutionary than the process to which it speaks, that is, symmetry-breaking. Symmetry-breaking does nothing short of telling a good chunk of the story of how the universe evolved from the Big Bang to present day, and the Higgs boson discovery does much to confirm that story.

Playing Around With Higgs

With the Higgs discovery comes some intriguing possibilities. For example, if we could figure out how the Higgs field "hooks" onto some particles but not others, maybe we could manipulate that process and, let's say, unhook a whole spacecraft. We could slip through space invisible to gravity, at light speed! Or, if the Higgs boson imparts mass to particles by bouncing off of them, perhaps some kind of Higgs boson imager might be possible, analogous to an electron microscope in which electrons are bounced off objects providing us with high resolution images. Here, Higgs bosons could be beamed into space (at very high energy before they have a chance to decay; in fact it would probably require enormous energy to accelerate these massive beasts to near light speed), illuminating any object with mass, regardless of its electromagnetic luminosity, so dark matter could be visualized and mapped. The interior of our and other planets and stars could be mapped too, telling us what really is inside a neutron star, for example.

None of these fantasies is anywhere near realizing. In the meantime I hope not only that the Higgs boson now makes some sense to us but that we can also see how it's a discovery in progress. It hints at a much larger story about the amazing evolution of matter and energy that took place when the universe was just a tiny fraction of a second old.


  1. I have a question about this paragraph:

    "Photons mediate the electromagnetic force, which means they are the particles that carry out all the forces between electrically charged particles, such as electrons and protons inside atoms. Photons are the force particles behind all of chemistry, electricity and magnetism."

    So does that mean ferromagnetism too? I thought that was due to electron spin, but then, I'm not sure of that.

    1. You are exactly correct SceinceMan! My mistake - The electromagnetic force between 2 electrons is carried out by the exchange of virtual photons BUT photons are NOT exchanged during ferromagnetism. The mechanisms responsible for ferromagnetism is something called the exchange interaction. This is a quantum mechanical effect where the wave functions (in this case the spins) of two nearby electrons align with each other, and its actually much stronger than the dipole-dipole magnetic interaction you see when you bring magnets together, even though the exchange mechanism is not a true force because no force carriers (photons) are involved. it's more like an interesting side effect of certain special (iron) atoms when they are chemically bound in certain ways.