8 August 2008

Cosmic roulette

Cosmos Magazine
As physicists around the world are staking the lot on the Large Hadron Collider, we review the odds of success.
Cosmic gamble

Credit: iStockphoto

ASPIRING PUNTERS ARE currently placing bets at close to the speed of light around the biggest, costliest and most capricious roulette wheel ever conceived. There’s a pile of chips banking on the Higgs boson cropping up, and an almost equally tall stack on the supersymmetry square. Other gamblers are hedging their bets by dividing their chips between the two, figuring both will be winners but unsure where the skittering ball will drop first.

Some hopeful betters are wagering on an entire row labelled with the exotic figures of the Standard Model, while the more adventuresome are drawn to a single long-odds square, formidably labelled ‘Dark Matter’. But the really astronomical odds are found on the squares marked quantum gravity, extra dimensions, microscopic black holes, or the longest shot of all: unparticles.

As you’ve no doubt guessed, it’s a very special game of roulette and the punters are rather unusual folk. The ‘wheel’ is the Large Hadron Collider (LHC), the most powerful particle accelerator ever built. It consists of a roughly circular tunnel 27 kilometres in circumference buried beneath the Swiss and French countryside near Geneva at the headquarters of CERN (European Organisation for Nuclear Research), the 20-nation body that oversaw the building of the collider and will be responsible for its operation.

The gamblers are theorists and experimentalists in particle physics, some who straddle both those worlds, as well as a coterie of cosmologists. And the croupier is about to declare “Les jeux sont fait”.

WHEN THE LHC IS POWERED UP sometime in September 2008 (see, Large Hadron Collider to start within weeks, Cosmos Online), dense beams of protons will be accelerated in opposite directions around the tunnel at nearly the speed of light and nudged into head-on collisions inside four complex detectors. Over the next few years, the punters will pore over vast reams of records of the fleeting subatomic debris from those collisions, using a purpose-built, globe-spanning computing grid.

Their expectation is that they’ll resolve some of the biggest puzzles in physics, shed light on related (and even bigger) mysteries about the fundamental laws of nature, and be completely surprised by revelations unsuspected by even the wildest theories that currently exist.

“My hope is that we end up being confused at a much higher level than we are now,” says Michigan State University physicist Joey Huston in mock seriousness, quoting the caption from a cartoon that adorns the office walls of many scientists. “But I also think we’ll have found answers to some of the profound questions in physics.”

All this is on the gaming table because the LHC is about to transport physicists across a previously impassable energy frontier into the terascale, the promised land of New Physics. The terascale is the realm of physics opened up when two elementary particles smash into one another with a combined energy of at least a trillion electronvolts, or one teraelectronvolt (TeV). An electronvolt is the energy an electron acquires when moving between the terminals of a one-volt battery.

Existing accelerators have allowed scientists to investigate fundamental properties in the realm of billions of electronvolts, abbreviated as GeV for gigaelectronvolts. The LHC boosts the ante to the terascale, boasting 10 times the energy and 100 times the collision rate of the world’s current accelerator heavyweight, the Tevatron at Fermi National Accelerator Laboratory (Fermilab) near Chicago, USA.

Since energy and mass are related by Einstein’s famous E = mc2 equation, the more energy that goes into the collisions, the heavier the particles that can be spewed out. Physicists refer to the range of available particle energies as the ‘scale’.

“There’s something special about the scale that’s being probed. There are good reasons to expect this particular scale to be a threshold where things should happen,” says Nima Arkani-Hamed, a wunderkind theoretical physicist from Harvard University now spending a year at the Institute for Advanced Study in Princeton, New Jersey.

Speaking at the American Association for the Advancement of Science (AAAS) meeting in Boston in February 2008, string theory guru Edward Witten divided the LHC’s list in two: there are all the old riddles, which have troubled scientists for decades; and the new riddles, such as dark energy, which have only recently emerged.

And these riddles have not troubled only scientists, he noted, because particle physics is really nothing more than the age-old quest to understand the fundamental laws of nature, transferred to the subatomic realm.

“Particle physics today is on the brink of a very big jump into the unknown,” says Witten, this year’s co-winner of the US$500,000 Crafoord Prize from the Royal Swedish Academy of Sciences for basic research in astronomy and mathematics.

Here are five of the biggest puzzles that the Large Hadron Collider stands a good chance of untangling:

1) Does the Standard Model still hold up at the terascale?

At the LHC, revelations are expected to begin squarely in the realm of the known: the Standard Model, which is the detailed explanation of the relationships between particles that make up matter – such as quarks and leptons – and particles that mediate the forces between matter – such as the photon for electromagnetism (see “The whole shebang”, Cosmos 16, p62).

The Standard Model may now be taught in undergraduate courses but its predictions have never been tested in the terascale. Those tests involve probing such esoterica as the mass effects of charm and bottom quarks, ‘parton shower models’ and ‘renormalisation group invariance’.

Because of its higher energy, the LHC will also produce familiar particles in far greater numbers than any other accelerator, allowing investigators to check interactions with increased precision.

Harvard’s Arkani-Hamed says that it is far from a safe bet that the Standard Model will continue to hold up. “It’s not clear how to read the tea leaves. Nature is what it is.”

2) Why do the known elementary particles, such as quarks and electrons, have such different masses?

One pivotal building block of the Standard Model has yet to be observed. It’s called the Higgs boson, after British theoretician Peter Higgs, who theorised the eponymous particle as an elegant after-the-fact fudge to explain why the mass of elementary particles spans such a humongous range, with the top quark being 370,000 times heavier than the electron.

The nature of mass is truly an old riddle. Newton and Einstein both plunked down m for mass in their famous equations, but neither explained where the property came from. The puzzle is actually even deeper than that, entering into a nether region called spontaneous symmetry breaking, a physicist’s catchphrase for an especially perplexing conundrum.

At the fundamental level, the same mathematical equations describe both the electromagnetic force, such as light waves, and the weak force inside the nucleus, which is involved in radioactivity. But they look nothing alike.

The belief is that in the early universe some unknown agent came along and spontaneously broke a natural symmetry that then existed between the two forces, causing them to look different. That symmetry breaking might have involved the Higgs boson, or a Higgs wannabe.

Current thinking is that the as-yet-undetected Higgs permeates space like an invisible quantum fluid, exuding a sticky force that slows the motion of other particles and which they register as mass. So the top quark becomes like Britney Spears pushing through a crush of paparazzi, while the electron enjoys the easy passage guaranteed a tax collector anywhere.

Since these various particles are the building blocks for the constituent parts of atoms and molecules, the Higgs would eventually explain the mass of everything. Yet there are serious shortcomings with the Higgs as postulated (such as it should weigh more than is suggested by indirect evidence) and physicists have long been straining for answers in the terascale.

Probably no sooner than a year after the first trial proton beams its course around the LHC racetrack, more than 3,500 scientists and engineers on two large teams will launch the hunt for the Higgs in a serious fashion. The current wisdom is that it should show up at an energy somewhere between 115 GeV and 182 GeV, levels that are child’s play for the multibillion-dollar collider.

“The LHC goes where no accelerator has ever gone before. Any way the Higgs chooses to behave we have it covered, all the way from below 200GeV to several thousand GeV if necessary,” says John Ellis, who is CERN’s resident guru of the New Physics.

Yet the Higgs hunt is still much more complicated than non-physicists can possibly fathom, involving aspects such as gluon-gluon production and four-lepton decay modes. There’s also the question of sheer numbers. Because the Higgs boson gives off weak signals against a noisy background, lots of collisions are needed to generate enough new particles to reach the statistical power that would support a claimed discovery.

For example, particle collisions in the LHC detectors are projected to occur at a rate of 600 million a second, but scientists estimate that a single Higgs boson will be produced only every 2.5 seconds – under optimal conditions.

But since the LHC was specifically designed to find the Higgs, or a particle that performs the Higgs role in creating mass, failure is almost inconceivable to most investigators. “If we don’t find the Higgs or a stand-in, then something is drastically wrong with our current laws of physics. Or else everything is wrong, or else quantum mechanics is wrong, or else so much is wrong that we don’t even have the language to describe it,” says Arkani-Hamed.

Yet the often-predicted Higgs discovery could also be a big let-down, notes William Trischuk, who directs Canada’s Institute of Particle Physics from an office at the University of Toronto. “Deep down in our hearts if we found a Standard Model Higgs boson that tied up all the loose ends, we might ask ‘is that all there is?'”

3) Are there still more basic building blocks of nature, heavier cousins of the known particles making up an extended family of superparticles?

Probably not. Quite a few physicists are talking about a ‘Higgs package deal’ at the LHC. In these scenarios, the Higgs could also be somehow linked to supersymmetry, the theory that all the 20 fundamental particles are paired with a ‘sparticle’. These sparticles are replicas, except in two respects: they are heavier because of spin and exist as a kind of quantum mirror image. What does that mean? All particles are classed as either fermions or bosons.

Supersymmetry assumes that a particle belonging to one category has a superpartner in the other, making nature more balanced. The theoretical superpartner of an electron (a fermion), for instance, is known as a selectron (which is a boson).

A large part of supersymmetry’s attraction to many physicists lies in its elegance – a grand dance of particles throughout the universe, of which we currently see only half. Yet there are more practical reasons why CERN’s Ellis has focussed his attention on determining what readings from the collider’s detectors would amount to a telltale ‘signature’ of sparticles such as the selectron, photino or wino (pronounced we-no).

“Why the hell should one believe in such an exotic theory?” Ellis asks, and then reels off four areas where supersymmetry would help advance the New Physics: the unification of fundamental interactions; a neutral candidate for cold dark matter; explaining why the Higgs isn’t heavier than it is; and resolving what’s known as the hierarchy problem – a bland label that hides deep befuddlement about why gravity is 40 orders of magnitude weaker than the three other elementary forces.

“This is infinitely more exciting than just finding some extra dimension, which I consider boring,” says Ellis. “It would complicate life, but it’s really only one new principle, a new kind of symmetry between particles – some with spin and some without. It’s a very powerful way of solving some of the fundamental problems in physics.”

Identifying evidence for supersymmetry particles depends more on measuring what’s missing from the LHC’s detectors than on measuring what’s there. The investigators total the momentum of particles that smash and the momentum of the resulting debris. (This isn’t something you can watch, since the cascade of decaying particles flashes past in 10 to 27 seconds.) If these two momentum totals don’t balance, that suggests some energy has been carried away by a new type of particle that’s invisible to the normal detectors.

Says Ellis: “In the New Physics the most important clue is the energy that you cannot see.”

4) What exactly is dark matter, the mysterious substance that, observations suggest, is five times more common in the universe than all the visible stuff?

Just as the Higgs and supersymmetry could turn out to be conjoined twins in the New Physics, so too could supersymmetry be integral to solving The Mystery of the Missing Mass. In an embarrassing development, over the past decade scientists have come to realise that they’ve been overlooking about 96 per cent of the universe.

The visible “ordinary” matter constitutes a mere four per cent, according to current thinking. Observations and theory say the rest is either dark energy (about 74 per cent) or cold dark matter (some 22 per cent). Since cold dark matter emits neither heat nor light, no one has yet been able to directly detect it.

However, if ordinary matter were the only thing holding clusters of galaxies together, then astronomers say they would have long ago flown apart. There isn’t anywhere near enough matter to produce the necessary gravitational pull. Ergo, the pressing need for dark matter to close the gravitational deficit.

One widely accepted hypothesis is that dark matter particles could have been produced in copious quantities in the Big Bang, and that enough of them survived until today to constitute the invisible cosmological soup.

Since the LHC has the capacity to reach energies last experienced when the universe was being born, the hope is that the colliding proton beams will recreate dark matter particles inside the detectors. At the same time, several underground detectors around the world are tuning ultrasensitive receivers to try to capture the passage of a dark matter particle.

“We have at least a chance of unravelling the mystery of dark matter,” says string theory guru Witten, who does his thinking at the Institute for Advanced Study in Princeton, New Jersey.

5) Is a ‘theory of everything’, with gravity and the three other fundamental forces uniting at much higher energies than the terascale, possible?

Among the longest odds on the roulette table are for what’s sometimes called the Theory of Everything, or quantum gravity. Einstein pursued this particular Holy Grail for the last two decades of his life, attempting to write equations that would unite the very small realm of quantum physics inside the atom with the very large realm of gravity, which spans the universe.

This would require a coming together, or unification, of the four forces: electromagnetism, the strong nuclear force, the weak nuclear force (all encompassed in the Standard Model) and gravity, which appears to be many times weaker than the others.

Most theories predict that unification of the forces would take place at very high energies (1019 GeV), so could be sensed only indirectly at the collider.

But a few physicists speculate that nature has more tricks up her sleeve, such as making total gravity stronger by having it operate in extra dimensions. If the threshold to those extra dimensions lies in the low end of the terascale, then experiments at the LHC might produce some indirect evidence for the ultimate unification of the four forces. “This is a big maybe question,” says Ellis.

THE LONG SHOTS: Some results would be given extra tough scrutiny. In addition to the five most popular bets already outlined, the LHC roulette game also features a plethora of long-odds wagers, such as finding mini-black holes, antimatter, clues to the nature of cosmic inflation or the longest shot of all, unparticles, proposed by Harvard physicist Howard Georgi.

Unparticles are a completely new kind of matter which have no definite mass and yet theoretically can have every possible mass simultaneously, a trait allowing them to masquerade as fractions of particles. Picking up on this notion, other theorists have proposed that unparticles could even exert an “ungravity” force on normal particles, providing an alternative explanation for dark matter.

If this sounds too bizarre to be credited, even by the sometimes flaky standards of high-energy physics, consider that Georgi pioneered the theory of supersymmetry in 1981, along with Savas Dimopoulos at California’s Stanford University. As well, other theorists have so far produced more than 100 papers exploring various aspects of unparticles.

Says Georgi: “Unparticles would be a much more striking discovery than supersymmetry or extra dimensions. Unparticle stuff would astonish us immediately.”

Unparticle theory is still very much in its infancy. There’s no consensus, for instance, on whether they’re all of a kind or come in different varieties, as do protons and electrons. Yet current thinking says unparticles are immune from even E = mc2, because their chameleon-like nature eliminates any fixed relationship between their mass, energy and speed of travel.

The largest bet at the roulette table, however, has nothing to do with which competing theories about the New Physics will eventually prove correct. It centres on whether governments see a payoff from terascale particle accelerators that’s worth the billions invested. And the signs are not encouraging: late last year, both the U.S. and Britain eliminated from their national budgets their share of research and development funding for the International Linear Collider, the follow-on to the LHC.

BEFORE ANY OF THESE GRAND PUZZLES can be tackled comes the task of breaking in the Large Hadron Collider. That’s the first order of business once the slender packets of protons start zooming around the accelerator’s octagonal tunnel; they will be coaxed along by 1,700 magnets chilled with liquid helium to two degrees above absolute zero so they are superconducting.

The shakedown will likely still have another full year to run on 21 October 2008, the LHC’s official inauguration date. In attendance will be some of the politicians who approved the funding for the collider, a figure that lies somewhere between US$4 billion, counting only cash outlay, to $8 billion, including contributions in kind and personnel. For that kind of money, the politicians have been told they’re getting two beams of protons each carrying 7 TeV that will produce a 14 TeV smash-up.

That’s not the complete story. The sought-after new particles are produced not by the collisions of the protons themselves, but by collisions between the quarks and the gluons from inside the protons. These carry only a fraction of the energy of their parent protons, so a quark–quark smash won’t release much more than 2 TeV, energy which is then divvied up among all the collision debris.

During this commissioning phase, engineers and scientists from 80 countries have to become adept in fine-tuning those proton beams, slowly nudging the apparatus up to ‘full luminosity’. They also need to be experts in the challenging vagaries of the four underground detectors.

Two of these, ATLAS and CMS, are complex general purpose apparati for which initial target will be the Higgs. The third, LHCb, is tackling the mysterious absence of antimatter in the universe by looking at bottom-flavoured quarks and antiquarks. The fourth, ALICE, probes collisions of lead ions which will produce a substance called a quark-gluon plasma that’s assumed to be similar to the Big Bang’s primordial fireball.

Taking extra care to understand precisely what these detectors are detecting (and not detecting) is crucial because discoveries at the LHC won’t feature a filmic ‘Eureka’ moment with a white-coated researcher pointing excitedly to a telltale particle track on a computer screen. In the past, impatience and rivalry have led to premature claims of discovery at CERN, an embarrassment chronicled in the 1986 book Nobel Dreams by Gary Taubes.

To guard against any such slips, research groups usually have a member who plays the role of advocatus diaboli (Devil’s advocate), somewhat like the Vatican-appointed sceptics who picked holes in the evidence of miracles attributed to candidates for sainthood. As the overall detector quality co-ordinator on ATLAS, Rob McPherson acknowledges the importance of such scepticism.

“I’ve done rare-particle searches for my whole career,” says McPherson, who was based at CERN for 10 years and is now a research scientist with Canada’s Institute of Particle Physics, a virtual body. “I’ve never done an analysis in my life where I haven’t discovered something. But it didn’t always work out to be quite true.”

Also providing further insurance against mistakes are the large collaborations at the LHC. About 1,700 investigators are associated with ATLAS, for example, so any results will be double-checked “from Istanbul to Kentucky,” in the words of one physicist.

Funding cutbacks have also rocked other big international science projects such as the Gemini telescopes in Hawaii and Chile, and ITER, the experimental nuclear fusion reactor being built in the south of France (see “Nuclear 2.0″, Cosmos 19, p50). One person troubled by these trends is physicist Lawrence Krauss, who ran a session on international collaborations at the AAAS meeting in Boston.

“LHC may be our last hope to find answers,” says Krauss, also a best-selling popular science author from Case Western Reserve University in Cleveland, Ohio. “It may be the last accelerator ever built.” If so, the Large Hadron Collider will have inspired a new generation of particle physicists and then abandoned them, with no way of crossing over the next energy threshold.

Typical of these is 33-year-old Brigitte Vachon, who worked on the DZero experiment at Fermilab before being lured home to Canada with an appointment as a Canada Research Chair in particle physics at McGill University in Montréal. Interviewed during one of her periodic forays to work on the ATLAS detector at the LHC, Vachon radiates enthusiasm across the transoceanic phone link.

“It’s a fascinating project from every aspect, even if you don’t care about the physics – the engineering challenge, the computing challenge and the social challenge of working with thousands of people from scores of countries.

“We’ll be turning on a great big toy and seeing how it works.”

Peter Calamai is the national science reporter for Canada’s Toronto Star and is a graduate in physics from McMaster University in Hamilton, Canada.

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