So what was the announcement that caused a hectic world to briefly pause and listen? A new subatomic particle had been discovered, and its properties were consistent with those predicted for the long-sought Higgs boson. The Higgs boson, if it exists, is the experimental evidence needed to confirm the existence of the Higgs field, which is thought to give fundamental subatomic particles their mass.

Physicists were careful to not claim that they had conclusively discovered the Higgs boson. The Higgs boson was predicted in 1964 to have a litany of very specific properties. Until scientists are able to demonstrate that the newly-discovered particle matches all of the predictions, there remains the possibility that the new particle is something wholly unexpected. Of the properties that had been tested prior to the seminar, all of them pointed to this being the Higgs, which is why scientists said “consistent with the Higgs boson.” Using a metaphor involving the senses, what was found looked and smelled like the Higgs boson, but nobody had been able to taste, feel and touch it. So some uncertainty remained. This uncertainty still remains today, and it will be some time before scientists can definitively state that the observed particle was the Higgs boson.

But let’s imagine that the discovered particle, which is a boson of mass about 125 times that of the proton, is the Higgs boson. What then?

You’d think scientists would celebrate (and we did…more than a few champagne corks were popped), but once the confetti settled, there were some furrowed brows. Nobody understood why the mass of the Higgs boson was so low. Here’s the source of the conundrum.

A Higgs boson doesn’t always exist as a Higgs boson. Like other quantum particles, it can change forms. For instance, it can briefly convert into a pair of top quarks before coalescing back into a Higgs boson. These evanescent top quarks are called “virtual particles” and are just an example of the several kinds of particles into which a Higgs boson can temporarily fluctuate. So, if you want to predict the mass of the Higgs, you have to take all of these possible forms into account.

Higgs bosons can spontaneously convert into pairs of other subatomic particles. These pairs exist only for a very short time, but their existence will alter the mass of the Higgs boson.

To make things even more complicated, the effect of the fluctuations also comes in two pieces. These two terms are multiplied, not added, together. The first term involves the maximum energy for which the Higgs theory applies. This works out to be a huge number, about 10³⁸ GeV².

The second term is, roughly speaking, the sum of the effect of the bosons (W, Z & Higgs) minus the sum of the effect of the fermions (top quark). Let’s call this the fermion/boson sum.

So, let’s take a birds-eye view of the whole equation. The mass of the Higgs is equal to the theoretical mass plus a monstrously large number multiplied by the fermion/boson sum. Unless the fermion/boson sum is practically zero, the observed mass of the Higgs boson should be huge.

The only way to escape this conclusion is to somehow balance the fermion/boson sum to be exceedingly small. And to have the balance so perfect is utterly unnatural, as if we added up all the monthly paychecks of everyone in the United States and subtracted their monthly bills and those two huge numbers canceled out neatly.

That doesn’t happen in bookkeeping, and it shouldn’t happen in physics, either; unless, that is, there is some new and as-yet-undiscovered physical principle that enforces it. Thus, the small mass of the Higgs boson all but ensures that there is new physics to be discovered. Otherwise, we have to “tune” the masses of these particles to very precise values. Such precise balancing is utterly unnatural in physics theories, leading theoretical physicists to propose a series of ways in which this cancellation could occur naturally.

The most popular is a principle called supersymmetry. At the core of supersymmetry is the idea that, for every known fermion (quarks and leptons), there is a cousin boson (called squarks and sleptons) that we haven’t yet discovered. Similarly, for every known boson (e.g. photon, W, Z, gluon and Higgs boson), there is a cousin, also-undiscovered, fermion (called a photino, wino, zino, gluino and Higgsino). Because every fermion has a cousin boson (and vice versa), the fermion/boson sum is identically zero. Each particle cancels out exactly the effect of the cousin particle predicted by supersymmetry.

There are many technical issues that need to be addressed, not the least of which is that the predicted cousin particles have never been observed. But, so far, scientists can get around that problem. Thus supersymmetry remains an interesting idea.

If the particle found in July of 2012 is the Higgs boson, it definitely brings with it a very puzzling problem. As physicists begin to accept that the Higgs boson has likely been found, they are turning their attention to this most unnatural quandary. The main focus of the LHC is now becoming a search for a natural solution to this difficult question: Why is the Higgs so light?

The actual equation is the following: Mass(Higgs, observed)² = Mass(Higgs, theoretical)² + [k Λ]² × [Mass(Z boson)² + 2 × Mass(W boson)² +Mass(Higgs, theoretical)² – 4 × Mass(top quark)² ]. k is a technical constant and Λ is the maximum energy that the theory applies.

Go Deeper
Editor's picks for further reading

The Nature of Reality: Bittersweet Victory: Physics After the Higgs
A look at the implications of the Higgs on the future directions of physics research.

The Nature of Reality: Thanks, Mom! Finding the Quantum of Ubiquitous Resistance
In this blog post, physicist Frank Wilczek celebrates the July 4 Higgs announcement.

Quantum Diaries: Why The Higgs Boson Should Not Exist and Why This Is a Good Thing
Physicist Richard Ruiz asks why the Higgs boson is so light.

At such high temperatures, the protons and neutrons inside atomic nuclei literally melt, releasing the quarks and gluons inside them and creating a form of matter called a quark-gluon plasma. You can think of the quarks as the “matter” particles and gluons as the particles of force that hold the protons and neutrons together. A reasonable mental image of a proton or neutron would be like a few flecks of Styrofoam (quarks) inside a lottery ball machine. The wind in the lottery machine is analogous to the force field, while the air molecules represent the gluons.

Under ordinary conditions, quarks and gluons are forever locked inside protons and neutrons. They’re like the water held frozen into ice cubes in a glass. But just as ice can be melted into water when energy is added to the system (by pouring hot tea over them, for instance) allowing the molecules from one ice cube to mix with molecules from other cubes, so too it is possible to melt protons and neutrons and have the quarks and gluons scamper around willy-nilly.

To melt protons and neutrons, you need to heat them up to about a trillion degrees. The only way to generate this kind of temperature is to smash together atomic nuclei at high velocities in huge particle accelerators. That’s what physicists are now doing at the LHC and the RHIC, accelerators that take atoms (lead and gold, as well as some others), strip off all of the electrons, and then slam the bare atomic nuclei together. The most violent of these collisions can heat up the nuclear matter enough to free the quarks and gluons to wander as they will. Though experimental calibration issues add some uncertainty to the mix, the current temperature record seems to belong to the ALICE experiment at the LHC, which measured an astounding 5.5 trillion degrees centigrade.

Studying the phase transitions of quark-gluon plasma allows us to understand the behavior of matter in the early universe, just fractions of a second after the Big Bang, as well as conditions that might exist inside neutron stars. The fact that these two disparate phenomena are related demonstrates just how deeply the cosmic and quantum worlds are intertwined. Credit: Brookhaven National Laboratory

While the first lab-made quark-gluon plasma was created in 2000, physicists are only now beginning to understand how this form of matter behaves. At the LHC and RHIC, they are mapping out in more detail the temperatures and pressures at which ordinary matter transforms into quark-gluon plasma. They are also tracing the boundaries between quark-gluon plasmas and even more exotic forms of matter like the stuff of neutrons stars, which is thought to be so dense that, at the center of the stars, quarks get “smooshed” together into an exotic kind of solid.

In fact, physicists believe that there are many different phases of matter involving quarks. While I’ve focused on two states of matter, atomic nuclei and quark-gluon plasma, this just scratches the surface of the possible.

By studying quark-gluon plasma, physicists are able to explore a period in the history of the universe that has thus far eluded us—a period in which protons and neutrons, the basic ingredients of ordinary matter, were coalescing for the first time. Thanks to accelerators like the LHC and the RHIC, we are finally beginning to probe this pivotal chapter in the story of cosmos.

Go Deeper
Editor's picks for further reading

Brookhaven Lab: Quark-Gluon Plasma: a New State of Matter
In this video, physicist Peter Steinberg explains the nature of a quark-gluon plasma.

Physics Central: Quark-Gluon Plasma
Learn the basics of quark-gluon plasmas in this explainer from the American Physical Society.

Quark Matter 2012
Explore highlights from the August 2012 Quark Matter conference, held in Washington, DC.

There are many mysteries remaining in physics. While the Standard Model states that the Higgs boson gives subatomic particles their mass, it is quite silent on the specific masses held by each particle. Further, it doesn’t explain why so many different types of particles exist, nor does it explain why there are three forces and not two or 20.

Subatomic particles have a property called spin which can be usefully (and misleadingly) imagined as each particle being a tiny spinning ball, though the reality is that spin is an inherent property of these particles in the same way that electric charge is. Particles are divided into two classes based on their spin: Particles with half-number spin (1/2, 3/2, 5/2 and so on) are called fermions, and particles with whole-number spin are called bosons.

Supersymmetry proposes a new rule to govern the relationship between fermions and bosons. According to supersymmetry, the equations that describe the universe should work in exactly the same way if all fermion and boson terms are swapped. This implies that, for every particle known in the Standard Model, there should be an as-yet-undiscovered cousin particle. These cousin particles are identical to the known particles in every way except that they have different spin.

If supersymmetry is right, then the existing fermion quarks have cousin bosons called “squarks”; the lepton has a supersymmetric cousin called a slepton. For bosons, the naming convention is a little different: The bosons of the standard model (the gluon, photon and W and Z boson) have supersymmetric fermion cousins called the gluino, photino, wino and zino.

Though none of these particles has yet been observed, their very obscurity does offer us one important insight: If supersymmetry exists, it is not, in fact, symmetric. Recall that I said that the supersymmetric cousins of the familiar particles of the Standard Model were the same in every way except for their spin. This means that the selectron would have the same mass as the familiar electron and the up squark would have the same mass of the up quark. However, were this true, we would have discovered them already. Given that we haven’t, we can categorically say that supersymmetry in its ideal form has already been falsified.

However it could be that supersymmetry is mostly true, but “broken.” In the same way that an imperfect top might spin reasonably well, only to wobble a bit and end with a preferred side always touching the ground when it stops, perhaps the universe might have a supersymmetry that is mostly true. Just what mechanism breaks the symmetry between the Standard Model particles and the supersymmetric cousins is not known, although many ideas have been proposed.

So with this additional consideration, you are to be forgiven if you are suspicious of the whole idea. What is the reason for the interest in the idea of supersymmetry? Why have over ten thousand scientific papers (both experimental and theoretical) been written on the subject?

While there are several reasons to find the idea intriguing, one topical example is the way in which supersymmetry is thought to be linked to the Higgs boson. While we remain unsure if the boson we found in July is the Higgs boson, the new boson has a mass of about 125 GeV, or about 133 times heavier than a proton. This is an utterly unnatural value for the mass of the Higgs boson.

Why unnatural? The Higgs boson gains its own mass (in part) through its interaction with the other subatomic particles: the quarks and leptons and force carrying bosons. These particles should have a huge influence on the mass of the Higgs—on the scale of 10¹⁵ GeV. That’s over ten trillion times the observed mass of the new boson. So why isn’t the Higgs weighing in at that enormous mass?

First, we are helped because the contribution from the fermions and bosons are of opposite sign, so they can cancel each other out. But without invoking supersymmetry, it seems pretty suspicious that they would be so close in value. It’s uncanny, like a big bank simultaneously taking in a deposit of about a trillion dollars and making a loan of almost exactly the same amount down to a few bucks.

Supersymmetry can explain this quite easily, though. After all, for each particular fermion (say an electron), there is a corresponding boson (a selectron). Given the symmetry and the fact that fermions and bosons contribute with opposite signs, it is easier to see how these two corresponding particles could cancel each other out exactly. If supersymmetry were in fact perfectly symmetric, they would cancel each other perfectly and mass of the Higgs boson would be caused solely by its interaction with other Higgs bosons.

This example is but one in the myriad of phenomena which can be explained by supersymmetry. You should remember that we don’t know that supersymmetry is actually present in the universe; just because it works on paper doesn’t make it real. It makes it a cool idea. However scientists at the Large Hadron Collider are hot on supersymmetry’s trail. If supersymmetry is the answer to why the mass of the Higgs boson is small but not zero, we will find it at the LHC.

Go Deeper
Editor's picks for further reading

Nature's Blueprint: Supersymmetry and the Search for a Unified Theory of Matter and Force
Theoretical astrophysicist Dan Hooper's book on supersymmetry and the LHC's role in the search for evidence of supersymmetry.

Scientific American: Is Supersymmetry Dead?
Davide Castelvecchi asks what it will mean for physics if the LHC does not turn up evidence of supersymmetry.

Supersymmetry: Unveiling the Ultimate Laws of Nature
Physicist Gordon Kane's accessible 2001 book on supersymmetry.

Inside the tunnel of the Large Hadron Collider, particles speed through a 27-kilometer ring of superconducting magnets. Credit: David Parker/Photo Researchers, Inc.

Physicists had to overcome three challenges to discover the Higgs particle: producing it, detecting it, and proving that they really had produced and detected it.

To put these challenges in context, let me introduce another perspective on what the Higgs particle is: The Higgs particle is The Quantum of Ubiquitous Resistance. I’m referring here to a universe-filling medium that offers resistance to the motion of many elementary particles, thus producing what we commonly think of as their mass.

The Standard Model of physics—our best-yet model of the matter and forces that make our universe—requires, for consistency of its equations, that many of its ingredients are particles with zero mass. These particles should travel at the speed of light in empty space, but in reality, some of them—like quarks, leptons, and W and Z bosons—travel more slowly. What is slowing them down?

Our Standard Model comes equipped with a Standard Reconciliation: Space is never empty! Space is filled with a material that resists the motion of those particles. Over the past decades, physicists have deduced many of the properties of the Ubiquitous Resistance by observing its effects on the forms of matter we can see. They even gave it a name: the Higgs field. But none of the known particles had the right properties to build up the Ubiquitous Resistance. So theorists drew up the specifications for a particle that would do the job. They called it the Higgs particle.

But wishing doesn’t make it so. Only experiments can grant (or deny) theorists’ wishes. With that in mind, let us consider the three challenges facing experimental observation of the Higgs particle.

Producing it

Any physical material, hit hard enough, is bound to break. The smallest possible shard reveals the most basic unit of the material: its “quantum.” For the Ubiquitous Resistance, that quantum is the Higgs particle.

To break off a piece of the Ubiquitous Resistance, though, requires producing disturbances of unprecedented intensity, albeit confined to tiny volumes of space for tiny intervals of time. That is what the Large Hadron Collider (LHC) is all about. By accelerating beams of protons to extremely high energy, and bringing them into collision, the LHC creates “Little Bangs” systematically.

Detecting it

Once you’ve produced a Higgs particle, the next challenge is to detect it. This isn’t as easy as it sounds, as the Higgs rapidly decays into other particles. We can look for those secondary particles, but most of them are useless for detection because they are produced more abundantly by other processes. The Higgs’ tiny signal competes with a cacophony of noise. The most likely mode of Higgs decay, into a bottom quarks and its antiparticle, in particular, is diluted by garden-variety strong interaction processes which produce those particles in droves.

So detection requires cunning.

Some decay processes that we might be able to detect are sketched below. Each has its own advantages and limitations, and each adds information, so experimenters have pursued them all. (For more information on the characters you’ll encounter below—W bosons, Z bosons, and the rest of the particle zoo, this is a good starting point.)

#1: Photon pairs

After a Higgs particle is created, quantum fluctuations convert it into a particle-antiparticle pair, which recombines into two photons.

The observable signal, in this case, is the pair of photons emerging from the decay. From the energy and momentum of the two photons, one can reconstruct the mass of the Higgs particle. This is significant because there are many other ways to make photons in collisions at the LHC that don’t require the production and decay of Higgs particles. The Higgs signal would be swamped, if not for the redeeming feature that randomly produced photons will “add up” to indicate random masses for their hypothetical progenitors, and only by rare accident land on the Higgs particle mass, whatever it happens to be. The signature of the Higgs, then, is an excess of photon pairs in a very narrow mass range. The mass where there’s an excess is fingered as the Higgs particle mass. Since the energy and momentum of photons can be measured accurately, this method gives an excellent measurement of the Higgs particle mass.

The main limitation of this technique, besides the unavoidable background “noise,” is the fact that this decay process is quite rare compared to other possibilities.

#2: W boson+ (Higgs -> bottom-antibottom)

Here is one of those other possibilities: In this case, the Higgs particle is produced as a byproduct of the creation of a W boson. The W boson itself decays, but in ways that experimentalists are thoroughly familiar with, and can often identify with confidence. The presence of the W boson, itself a relatively rare occurrence, helps this class of event to stand out above the strong interaction background. Thus the most common Higgs decay, into bottom-antibottom pairs, becomes discernable when you demand an accompanying W.

There are two more possibilities:

#3: Higgs -> WW -> lepton + antilepton + neutrino + antineutrino

#4: H -> ZZ -> 2 leptons + 2 antileptons

In Processes 3 and 4, the observed particles are leptons (l), which is just another way of saying that they might be either electrons or muons, and their antiparticles; the ghostly neutrinos escape detection. The Higgs boson barely interacts with those light particles, but it can communicate with them indirectly, through fluctuations in the W and Z boson fields (a.k.a. “virtual particles”). Process 4 is special, in that it is the only case where the background is so small that individual events, as opposed to enhanced probabilities, can be ascribed with confidence to Higgs particles.

By measuring the rates of all of these processes, one can determine how powerfully the Higgs communicates with many different things: two gluons, two photons, two Z bosons, two W bosons, and bottom-antibottom pairs. Their different rates are logically independent, of course, but theory connects them.

Proving it

This is the final challenge. Finding the Higgs boson depends on assuming that the Standard Model is reliable, so we can work around the “background noise”. Here years of hard bread-and-butter work at earlier accelerators—especially the Large Electron-Positron Collider (LEP), which previously occupied the same CERN tunnel in which the LHC resides today, and the Tevatron at Fermilab, as well as at the LHC itself—pays off big. Over the years, many thousands of quantitative predictions of the Standard Model have been tested and verified. Its record is impeccable; it has earned our trust.

The next step is to search for data that the Standard Model can’t explain, like excesses of the decay products discussed earlier, and compare them against our predictions for yields from a hypothetical Higgs boson. Insofar as these quantitative predictions match the observations, which they do, one can speak of proof.

Future observations may reveal new effects, or small quantitative discrepancies in the effects already observed. (I’ll be surprised if they don’t!) But the original, simplest sketch of what The Quantum of Ubiquitous Resistance could possibly be resembles reality enough to pass muster, at least as its first draft.

Finally, I’d like to reprise the conclusion of my earlier piece, in which I considered what might happen if the hints of the Higgs did not pan out:

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.

Thanks, Mom!

The construction of the CMS detector at the LHC. CMS is one of the detectors involved in the hunt for the Higgs. Credit: Mark Thiessen/National Geographic Society/Corbis

As physicists gathered in Melbourne, Australia, for the International Conference on High Energy Physics, one of the most anticipated conferences of the year, the two large collaborations at CERN made an extraordinary announcement. In back-to-back seminars held at CERN and simulcast to the conference (and the world), the leaders of two different experiments, CMS and ATLAS, gave strong evidence that we found something that can’t be explained by well-understood physics—something which could (and it’s worth emphasizing the “could”) be the Higgs boson.

The Higgs boson is the missing piece in the current best model of the universe, the Standard Model. In the Standard Model, building blocks called quarks and leptons are held together by the four known forces: gravity, electromagnetism and the strong and weak nuclear forces. Using these basic ideas, physicists can explain most of the measurements we have made. But one thing we have not been able to explain is one of the most fundamental and vexing questions in physics: Why do those building blocks have mass?

In 1964, Peter Higgs took some ideas that were floating around at the time, added a few of his own, and proposed a solution to this conundrum, which included a new particle that we now call the Higgs boson. The search for the Higgs boson is an energetic activity, directly involving as many as six thousand physicists—myself included—and the most powerful particle collider on Earth, the Large Hadron Collider (LHC) at CERN.

One of the fantastic benefits of being a physicist doing research at CERN and Fermilab is that I have been privileged to see this discovery evolve with an insider’s perspective in more than one world-class experiment and in collaboration with some of the finest minds on the planet. Over the past few years, we have searched through the data at both laboratories. Our measurements so far have shown where the Higgs boson isn’t. The results released today may finally show where it is.

The first tantalizing suggestions of the Higgs came in December of 2011, when scientists working with CMS and ATLAS announced that their data contained hints that the Higgs boson might be starting to show its face, and that it could have a mass about 125 times heavier than a proton. However, neither experiment had enough data to claim a discovery—or even to be certain that they were seeing anything at all.

In March, the search picked up again. This time, though, the LHC’s energy level and beam intensity were dialed up. If the LHC had been making Higgs bosons before, it would be making even more of them now—about 25% more, depending on the boson’s mass. The CERN management made their plans for 2012 so that both CMS and ATLAS would have enough “beam time” to independently discover the Higgs boson—if, that is, our hypotheses about its mass and other properties were correct. However, given the intellect and work ethic of the scientists involved, nobody really thought it would take the whole year to see a signal that “looked like” a Higgs boson, although proving anything we found was the actual Higgs boson predicted by the Standard Model could well take the entire years’ worth of data.

By June of this year, both LHC experiments had already recorded as much data in 2012 as in all of 2011. The accelerator and its detectors were performing superbly. Now the race was on to be the first to finish the job and find—or rule out—the Higgs boson.

ATLAS and CMS won’t find the Higgs itself, though; it disappears too quickly, decaying into other subatomic particles. It’s those particles that we’re looking for in the ATLAS and CMS data. Depending on the true mass of the Higgs boson, it could decay in several different ways. Seeing an excess of these decay products is an indication that we might have discovered the Higgs.

And that’s what we found! In the shrapnel of the LHC’s powerful collisions, the CMS experiment detected more pairs of photons and Z bosons than we can explain without some new kind of physics appearing. CMS also looked for supporting evidence in predicted decays to bottom quarks, W bosons and tau leptons. The ATLAS experiment also found an excess of events decaying into two photons and two Z bosons, but the ATLAS did not announce the results of their investigations into other decay modes.

To be certain that we didn’t adjust our analysis techniques to produce a preconceived result, we did the searches “blind,” meaning that we designed the analysis before we looked at the relevant data. This was especially important given that we saw hints in December 2011. We didn’t want that information to bias our searches in any way. That way, if the 2012 data told the same story as that of 2011, it would tell us something about the universe and not ourselves.

When all of our results are combined, CMS claims to have found a new boson with a mass of 125 GeV (or about 133 times heavier than a proton) and a statistical significance of about five sigma (which means that this result could happen 1 time in 3.5 million by accident), while ATLAS’ measurement indicates the existence of a particle with about the same mass (126 GeV) and the same statistical significance. While both experiments’ results are significant individually, the fact that both experiments are announcing similar observations and the 2011 and 2012 measurements are compatible lends tremendous credence to today’s announcement.

It is very important to stress that neither experiment team has claimed to have observed the Higgs boson. They have observed something without a doubt, but the Standard Model Higgs boson is a very specific thing. To be sure we’re seeing the Higgs boson and not a lookalike, we need to see it in all of the predicted decay modes.

For instance, the Higgs theory makes specific predictions about the relative probabilities of the Higgs decaying into pairs of bottom quarks, tau leptons and a whole myriad of possibilities. If all of the predicted possibilities aren’t seen, or aren’t seen in the right ratio, it might be that what we’re observing isn’t the Higgs boson after all. Furthermore, the Higgs boson is predicted to have exactly zero quantum mechanical spin. Until those and other properties are confirmed, it is possible that the experiments might be picking up traces of something entirely different. So, although what has been observed is consistent with being a Higgs boson, these measurements cannot rule out some other possibilities. In fact, this announcement is not the end of the story but rather the very beginning.

Wednesday, July 4: CERN

Come back at 3 am ET on July 4, 2012 for a live webcast from CERN revealing the latest results in the search for the Higgs boson. A scientific seminar will begin at 3 am ET followed by a press conference at 5 am ET. Stay tuned!

Latest update in the search for the Higgs boson ©CERN

Press Conference: Update on the search for the Higgs boson at CERN on 4 July 2012 ©CERN

Monday, July 2: Fermilab

Tune in at 10 am ET on July 2, 2012 for a live webcast from Fermilab revealing the latest results from the Tevatron's CDF and DZero experiments in the search for the Higgs boson.

This week, the search for the Higgs boson—the elusive subatomic particle that is a critical piece of the Standard Model of physics—may reach its climax when, on Wednesday, two research teams announce the results of their work at the Large Hadron Collider (LHC) at CERN.

But before there was the LHC, there was the Tevatron, a particle accelerator at Fermilab. And before the LHC’s big announcement, there was a not-quite-so-big announcement from the Tevatron teams as they gathered with colleagues this morning to announce the results of the most detailed analysis so far of ten years'-worth of their Higgs search data.

The Tevatron at Fermilab. Image courtesy of Fermilab.

The Tevatron shut down last year, passing the baton to the newer, more powerful LHC. But the scientists working on two of the Tevatron’s detectors, CDF and DZero, haven’t given up searching for traces of the Higgs in their own data. Using ever-smarter computer algorithms, they aim to wring as much information as they can out of the data they’ve accumulated. As Wade Fisher, the Michigan State University scientist representing DZero at this morning’s conference, put it: “We’re still working, we’re not stopping….There’s still gas in the tank.”

What they’ve found so far is suggestive of the Higgs, but doesn’t rise to the level of discovery. Combining data from both CDF and DZero, they’ve eked out a signal that might be due to the Higgs, but there is also a one-in-550 chance that it is down to random fluctuations.

To claim a discovery, the physicists need to whittle that random-chance number down to one in three and a half million—“five sigma,” in stat-speak.

That’s what the physics world will be holding its breath for on Wednesday, when two LHC collaborations release their results.

Will they confirm the hints that the Tevatron has seen? Or will these inklings—and our hopes of completing the Standard Model of physics--evaporate into the mist of random fluctuations?

As Fermilab’s Eric James put it this morning: “We’re likely, after all this time, to find something out one way or the other.”

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

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

“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.

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:

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.

An invisible army of black holes? A cosmic graveyard of burned-out stars? A swarm of rogue planets that roam the depths of interstellar space? While examples of objects like these have been observed, we now know that they can’t account for the enormous mass of dark matter required to explain why galaxies rotate so fast. Following Sherlock Holmes’ dictum that once you have ruled out the impossible, whatever remains, however improbable, is the answer, scientists have been forced to conclude that dark matter is an entirely new form of matter, never before observed.

Here is what we think: Every galaxy is engulfed by a cloud of dark matter particles that extends far beyond that galaxy’s visible edge. Each dark matter particle is electrically neutral and has a mass tens or thousands of times that of the familiar proton. Finally, there is a lot of this dark matter. Our best estimate is that there is about five times as much dark matter as there is luminous matter, making our visible universe a thin frosting on a dark matter cake.

But physicists will need to observe dark matter first-hand before anyone should believe it is real. Our search for dark matter takes three distinct approaches: direct, indirect, and production—that is, actually making our own dark matter particles.

The search for dark matter rests on a three-legged stool, with direct, indirect and collider experiments all promising approaches to find it. Credit: Don Lincoln/Fermilab

The direct approach starts with a detector cooled to more than 459 degrees below zero Fahrenehit, so close to absolute zero that the atoms that make up the detector are nearly stationary. The detector is buried as much as a mile underground to protect it from ordinary cosmic rays, high-energy particles that are constantly bombarding the Earth. Though these detectors can’t actually “capture” a dark matter particle, should one happen to pass through and collide with the nucleus of an atom inside the detector, the detector will ring like a bell and the passage of the dark matter particle will be observed.

There are dozens of experiments underway using this approach, including one, called the DAMA (DArk MAtter) experiment, that has made a provocative finding. Scientists think that dark matter flows past the solar system like a wind, so DAMA uses the motion of the Earth around the Sun to winnow out a dark matter signal. For half a year, the Earth is moving into the dark matter wind, and for the other half, it is moving with the wind. Therefore, we expect to see an annual variation in the number of dark matter hits. This is exactly what DAMA has seen for many years now.

The problem is that other experiments which are nominally more sensitive don’t see this annual variation. This has led to considerable confusion and it will take additional work to understand if DAMA has seen the first hints of dark matter or merely an unexplained measurement artifact.

Indirect searches exploit the notion that dark matter might consist of both a matter and antimatter component. If so, occasionally a pair of matter and antimatter dark matter particles might meet and annihilate each other in a flash of gamma rays or matter/antimatter pairs that can be observed by satellites that are designed to detect gamma rays or antimatter in the cosmos. In fact, two such experiments, PAMELA and GLAST, have observed signals that could be the signature of dark matter, but could also have more prosaic explanations. Meanwhile, other experiments see no such signals.

Rather than waiting for dark matter to come to us, though, some physicists are hoping to make their own dark matter right here on Earth. Currently the only particle accelerator capable of making dark matter is the Large Hadron Collider at CERN. By exploiting Einstein’s famous equation E = mc², we hope to convert the kinetic energy of the beams directly into dark matter. Because dark matter is electrically neutral, it would escape our detectors undetected, but upon adding up the energy contained in all the particles that we can detect, we would notice that the energy books are unbalanced and that some energy is missing.

The scientists working on two of the LHC’s detectors, the ATLAS (A Toroidal Large Apparatus) experiment and my own CMS (Compact Muon Solenoid), are now searching their data tirelessly, looking for collisions with these characteristics. The situation is evolving rapidly as the LHC delivers a torrent of particles to the detectors.

It’s a race between the three different approaches to see which one will be the first to observe a reliable signature of dark matter. No one should be the slightest bit convinced until at least two of the approaches begin to tell a consistent story. One thing is certain; with five times as much dark matter as ordinary matter, the race is on for discovery and Nobel Prize glory.

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