Particle Physics

28
Jun

The Higgs Boson Explained

Editor’s note: An earlier version of this article originally appeared here on December 15, 2011. We are featuring it again, updated for context, in anticipation of the July 4, 2012 announcement on the latest results from the ATLAS and CMS instruments.

What is all the buzz about the Higgs boson, aka the “God particle”?

“Higgs” is Peter Higgs, a professor at Edinburgh, who made some interesting suggestions along the lines I’ll discuss below in 1964. The name “Higgs particle,” though standard, is not entirely fair, for several reasons: the basic idea has a significant pre-history; what’s original with Higgs has co-claimants; and the modern, mature version of the theory involves many ideas that were not anticipated in 1964. I’ll leave those issues for historians of science and the Swedish Academy to sort out.

Atlas detector

The construction of the ATLAS detector at the LHC. ATLAS is one of the detectors involved in the hunt for the Higgs. Credit: Martial Trezzini/epa/Corbis

God on the other hand deserves full credit, or blame.

Herewith a brief introduction, in question and answer format, for the buzz-curious.

What’s the basic idea?

Suppose that a species of fish evolved to the point that some of them became physicists, and began to ponder how things move. At first the fish-physicists would, by observation and measurement, derive very complicated laws. But eventually a fish-genius would imagine a different, ideal world ruled by much simpler laws of motion–the laws we humans call Newton’s laws. The great new idea would be that motion looks complicated, in the everyday fish-world, because there’s an all-pervasive medium–water!–that complicates how things move.

Modern physics proposes something very similar for our world. We can use much nicer equations if we’re ready to assume that the “space” of our everyday perception is actually a medium whose influence complicates how matter is observed to move.

Are there precedents for such an outrageous dodge?

Yes. In fact it’s a time-honored, successful strategy.

For example: In its basic equations, Newtonian mechanics postulates complete symmetry among the three dimensions of space. Yet in everyday experience there’s a big difference between motion in vertical, as opposed to horizontal, directions. The difference is ascribed to a medium: a pervasive gravitational field.

A much more modern example occurs in quantum chromodynamics (QCD), our fundamental theory of the strong force between quarks and gluons. There we discover that the universe is filled with a medium, the sigma (σ) field, that forms a sort of cosmic molasses for protons and neutrons. The σ field slows protons and neutrons down. Allowing a bit of poetic license, we can say that the σ field gives protons and neutrons mass. Many consequences of the σ field have been calculated and successfully observed, so that to modern physicists it is now every bit as real as Earth’s gravity field. But the σ field exists everywhere and everywhen; it is not tied to Earth.

What’s the new idea, then?

In the theory of the weak force, we need to do a similar trick for less familiar particles, the W and Z bosons. We could have beautiful equations for those particles if their masses were zero; but their masses are observed not to be zero. So we postulate the existence of a new all-pervasive field, the so-called Higgs condensate, which slows them down. This proposal, which here I’ve described only loosely and in words, comes embodied in specific equations and leads to many testable predictions. This proposal has been resoundingly successful.

What is the Higgs particle, conceptually?

Trouble is, no known form of matter has the right properties to make the Higgs condensate. In order to build that medium, we need to add to our inventory of world-ingredients. The simplest, “minimal” implementation introduces exactly one new elementary particle: the Higgs particle.

What is the Higgs particle, specifically?

There’s a quotation I love from Heinrich Hertz, about Maxwell’s equations, that’s relevant here.

To the question: “What is Maxwell’s theory?” I know of no shorter or more definite answer than the following: “Maxwell’s theory is Maxwell’s system of equations.”

Similarly, Higgs particles are the entities that obey the equations of Higgs particle theory. Those equations prescribe everything about how Higgs particles move, interact with other particles, and decay—with just one, albeit glaring, exception: The equations do not determine the mass of the Higgs particle. The theory can accommodate a wide range of values for that mass.

What is a Higgs particle, operationally?

A Higgs particle is a highly unstable particle, visible only through its decay products. It has zero electric charge, and—unlike all other known elementary particles—no intrinsic rotation, or “spin.” These null properties reflect the fact that many Higgs particles, uniformly distributed through space, build up the Higgs condensate, which we sense as emptiness or pure vacuum. (Although individual Higgs particles are highly unstable, a uniform distribution of them is stabilized through their mutual interactions. Visible Higgs particles are disturbances above that uniform background.)

As mentioned before, theory does not predict what mass a Higgs particle should have. Masses anywhere from 10 Giga-electron Volts (GeV) to 800 GeV might be accommodated, though problems start to emerge near either extreme. (Physicists commonly use GeV as the unit of mass for elementary particles. One GeV is close to, but slightly more than, the mass of one proton.)

Because Higgs particles are unstable, to study them one must produce them. That requires concentrating lots of energy into a very small space to create enormous energy density. The required concentration of energy is achieved at particle colliders. At the LHC, two counter-rotating beams of high energy protons are made to pass through one another, or cross, at a few points. At each crossing some fraction of the protons, which are moving in opposite directions at very close to the speed of light, collide. The collisions produce fireballs that explode into tens or hundreds of stable or near-stable particles including electrons and positrons, pi mesons, photons, protons and antiprotons, and several other possibilities.

Known physical processes account for the vast majority of this debris. Production and decay of Higgs particles, if they exist, will produce some additional debris. To get evidence for the existence of Higgs particles, therefore, one must identify some distinctive patterns in the observed debris that could result from Higgs particle decays but which are difficult to produce with conventional processes.

Putting it another way: If you’re looking for needles in a haystack, you’d better have a really good grip on what hay can look like—and it helps to look for needles that are hard to mistake!

Several patterns play an important role in the analysis, but I’ll discuss just one—a crucial one—to give a flavor of what’s involved. One process of Higgs particle production and decay is depicted in this sketch:
Morley-231x300

The sequence of events in the sketch above unfolds reading upwards. Gluons inside the fast-moving protons convert, by quantum fluctuations, into a “virtual” top quark and its antiparticle. The virtual top quark and antiquark swiftly recombine into a Higgs particle. Then the Higgs particle decays by a similar mechanism: quantum fluctuations convert it into a particle-antiparticle pair, which recombine into two photons. At the end of the day, it is those two photons that are observed. (I’m particularly fond of this exotically beautiful quantum process, which I discovered theoretically in 1977.) The point is that more conventional processes, i.e. processes that don’t involve Higgs particles, but which produce two energetic photons are fairly rare. Thus the calculated contribution from Higgs particles, should they exist, can be discerned above the background.

What did we know about the Higgs before July 4, 2012?

Prior to the July 4 announcement, we already knew that a very large range of potential mass-values had been ruled out. Only a small window in the range between 115 and 127 GeV remains viable.

On the other hand, an excess of events, above expectations from known processes, had been observed in the two-photon channel mentioned above and (less clearly) in several others. The excesses are compatible with, and could be explained by, the existence of Higgs particles with mass close to 125 GeV.

The observed excess might also be compatible with a statistical fluctuation in the background processes—e.g., an improbable run of normal processes leading to photon pairs, comparable to rolling four consecutive sixes at dice.

What will it mean if we find the Higgs?

First of all, it will be a dazzling triumph for theoretical physics. Physicists will have used intricate equations and difficult calculations to predict not only the mere existence of the Higgs particle, but also (given its mass) its rate of production in the complex, extreme conditions of ultra high energy proton-proton collisions. Those equations will also have accurately rendered the relative rates at which the Higgs particle decays in different ways. Yet the most challenging task of all may be computing the much larger, competing background “noise” from known processes, in order to successfully contrast the Higgs’ “signal.” Virtually every aspect of our current understanding of fundamental physics comes into play, and gets a stringent workout, in crafting these predictions.

The animating spirit of research in fundamental physics, captured in the maxim “Today’s sensation is tomorrow’s calibration,” will not rest in that triumph, however. A Higgs particle at mass 125 GeV would portend a new level of fundamental understanding and discovery. Let me explain why.

Within our current theories of the fundamental interactions, embodied in the so-called Standard Model, the Higgs particle mass might, as previously mentioned, have any value within a wide range. Yet there are good reasons to suspect that despite its many virtues, the Standard Model is incomplete. Notably, its equations postulate four different forces (strong, weak, electromagnetic and gravitational) and six different materials they act on. It would be prettier to have a more coherent, unified theory. And in fact there are beautiful, concrete proposals for unified field theories, within which we have just one force and just one kind of material. But to make the unified theory work quantitatively, in detail, we need to expand the equations of the Standard Model so that they integrate a concept called supersymmetry.

Supersymmetry has many aspects and ramifications, but two are most relevant here. First, supersymmetry (for experts: more specifically, focus point supersymmetry) predicts that the Higgs particle mass should lie in the range 120-130 GeV. Finding Higgs particles with mass in that range would give strong circumstantial evidence both for supersymmetry and for the unification that supersymmetry enables.

Second, supersymmetry predicts the existence of many additional new fundamental particles, besides the Higgs particle, that should be accessible to the LHC. So if supersymmetry is right, the LHC will have many more years of brilliant discovery in front of it.

And if not?

I’ll be heartbroken. Mother Nature will have shown that Her taste is very different from mine. I don’t doubt that it’s superior, but I’ll have to struggle to understand it.

Tell us what you think on Twitter, Facebook, or email.

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Frank Wilczek

    Frank Wilczek has received many prizes for his work in physics, including the Nobel Prize of 2004 for work he did as a graduate student at Princeton University, when he was only 21 years old. He is known, among other things, for the discovery of asymptotic freedom, the development of quantum chromodynamics, the invention of axions, and the exploration of new kinds of quantum statistics. Frank is currently the Herman Feshbach professor of physics at MIT. His latest book is The Lightness of Being.

    • Anonymous

      As a lay person with great interest in theoretical physics, I feel like asking – in both a light-hearded, sci-fi-ish way as well as in a more serious way – whether confirmation of the existence of the Higgs boson moves us any closer to developing a unified field theory that explains the relationship between electro-magnetism, the strong and weak nuclear forces and gravitation such that one force can be coverted into another, e.g. use of EM energy to create anti-gravity, etc. Obviously there would be lots of practical applications.

      Also, the notion that space is actually a somewhat uniform field of mass engendering “stuff” is quite intriguing. When I first learned of the Higg field the first thing I thought of is Einstein’s theory that any object approaching the speed of light becomes infinitely massive. The Higgs-field seems to explain this – the faster you move, the more Higgs particles you interact with and the more massive you become. In addition, we clearly see here that the Higgs is involved with conversion of one type of particle into others.

      My passion for theoretical physics has always been fueled by a desire to know more about what we are here and whether by learning more about the fundamental realities of physical existence we can somehow perceive a little bit more of the mind of God, the Creator – just catch the slightest more of a glimpse of understanding ourselves and this “why.” Thanks to all the physicists who are making this possible.

    • Edward M Billinghurst

      Isaac Newton opined that a body has mass. He also observed that it can be accelerated by applied force for a duration of time. I. e., Mass is a property of a body, acceleration is an effect due to applied force. Mass then was taken to be invariant; acceleration was variable.
      Today’s moderns now define Mass as an effect, and acceleration a cause.
      I have no problem with this, as long as everyone has the same understanding of such arcane logic, and the elders and the youngsters can agree on Physics matters as regards Mass and Acceleration.

    • thylawyer

      I also have always been fascinated by physics, and if my high school had had math teachers who could teach beyond algebra, I night have gone there.

      I too await with bated breath the announcement that the Higgs has been observed.

      Of course, none of this helps with either dark energy or dark matter, nor with having to have an infinite number of universes. It’s just another version of the turtle, and yes, it’s turtles all the way down!

    • Constantin Ioannides

      We know what a proton is. What are the next particles which derive or result from braking a proton ?

    • 1bruce2u

      Having no physics background, I have always been curious about the “how and why” from as far back as memory recollects, after reading an Einstein book in high school, it took my curiosity to higher ground. I’m glad to have just learned of the Higgs theory, the diagram shown looks similar to the one Ice seen used to explain the relationship of all people to each other due to a period of time that changed our gene pools from multiples, to this time in history. Which asks the question, will the human race survive, or expand by diversification. Perhaps thru this learning stage we enter an answer.

    • eap

      Yes, Thats right

    • Marcial Losada

      Brilliant, clear and compelling explanation.

      What is focus-point supersymmetry?

    • Alexs

      does this mean that the ether is back?

    • Dr Komor

      I never, ever comment on articles. I just don’t have time or inclination. This article, however, which deals with a critical subject at the interface of lay understanding of deep science just makes matters more confusing. The article attempts to be cute and glitzy with just the right daub of hype to attract human house plants who watch “reality” television. In addition the author tries to draw in so many different nuances that they end up saying nothing helpful and confusing the matter more. Now if the article was appropriately titled something like “Everything You Ever Wanted to Know About Anything That Has Ever Been Related to the Higgs Boson But Nothing About the Higgs boson Itself” then it would be much more readable. Our expectations would be fulfilled. As a lay person who is neither a House Plant or a Physicist I am the audience this article should have been written for – those possibly capable of understanding if you just clear away a little of the quantum debris for us. Unfortunately the author did not and I still don’t know my Higgs form a Hole In the Ground.

    • BilB

      I was a bit lost at the point where matter had no weight, until I realized that parcel of coelescence (parcel) of energy that resides in space that is rotating or vibrating in the Higgs field (inertial field) will acquire weight.

      Well that is the way that I am choosing to understand all of this for the time being. And if that is the way that it is then that is indeed a beautiful thing.

      Again by way of trying to visualise this I am imagining that space is filled with energy and anti (dark) energy (this was once called the ether). The 2 coexist in a manner similar to oil in water, the oil representing energy. For some reason energy cannot exist in large masses so disperses into small packages which have stable levels (quanta). Energy can move around in the form of photons. The difference between energy and anti energy is that energy collects and anti energy disperses (opposite signs). Energy moving through anti energy creates drag which accumulates at high speed into the form of a particle, a bit like the compression zone at the nose tip of an aeroplane or the bunching up of a carpet as something heavy is dragged over it.

      Well that is as far as I have got. If anyone has a correct or better visualisation I am very keen to hear it.

    • ProfessorNow

      Quantum Ocean
      Matter and Energy with Information
      Appear at Once to Bind Observation.

      Our understanding of ‘Reality’ is incomplete without a clarification of information and individuality. Matter we can measure in four dimensions (length, width, height and depth – magnification); energy we can measure in four (gravitational, temperature, magnetic and electro-static); information for each particle or location has its own assignments of direction, speed, torque/spin and density if you like.
      As for the observation: it has to be done by some kind of living thing, human, animial, plants, etc.

    • Novice

      so 2 protons, each 1 GeV, collide to form a particle of 125 GeV which then decays to 2 protons. wow. that’s a lot of E=mc2 to a novice. i must look up the mass of gluons and quarks.

    • Anonymous

      Good article. It explains everything except what a Higg’s Boson is.

    • Rick Carter

      So,just turn this Higgs Boson field to “OFF”, and you can supposedly go as fast as you want, even as fast as you want to go there (supposedly no inertia, either!). Gee, that would appear to agree with much of our observed ET phenomena, so what can I say, but, “GO FER IT, EVERYONE” !!!!!!!!!!