In SUSY we trust: What the LHC is really looking for

AS DAMP squibs go, it was quite a spectacular one. Amid great pomp and ceremony - not to mention dark offstage rumblings that the end of the world was nigh - the Large Hadron Collider (LHC), the world's mightiest particle smasher, fired up in September last year. Nine days later a short circuit and a catastrophic leak of liquid helium ignominiously shut the machine down.

Now for take two. Any day now, if all goes to plan, proton beams will start racing all the way round the ring deep beneath CERN, the LHC's home on the outskirts of Geneva, Switzerland.

Nobel laureate Steven Weinberg is worried. It's not that he thinks the LHC will create a black hole that will engulf the planet, or even that the restart will end in a technical debacle like last year's. No: he's actually worried that the LHC will find what some call the "God particle", the popular and embarrassingly grandiose moniker for the hitherto undetected Higgs boson.

"I'm terrified," he says. "Discovering just the Higgs would really be a crisis."

Why so? Evidence for the Higgs would be the capstone of an edifice that particle physicists have been building for half a century - the phenomenally successful theory known simply as the standard model. It describes all known particles, as well as three of the four forces that act on them: electromagnetism and the weak and strong nuclear forces.

It is also manifestly incomplete. We know from what the theory doesn't explain that it must be just part of something much bigger. So if the LHC finds the Higgs and nothing but the Higgs, the standard model will be sewn up. But then particle physics will be at a dead end, with no clues where to turn next.

Hence Weinberg's fears. However, if the theorists are right, before it ever finds the Higgs, the LHC will see the first outline of something far bigger: the grand, overarching theory known as supersymmetry. SUSY, as it is endearingly called, is a daring theory that doubles the number of particles needed to explain the world. And it could be just what particle physicists need to set them on the path to fresh enlightenment.

So what's so wrong with the standard model? First off, there are some obvious sins of omission. It has nothing whatsoever to say about the fourth fundamental force of nature, gravity, and it is also silent on the nature of dark matter. Dark matter is no trivial matter: if our interpretation of certain astronomical observations is correct, the stuff outweighs conventional matter in the cosmos by more than 4 to 1.

Ironically enough, though, the real trouble begins with the Higgs. The Higgs came about to solve a truly massive problem: the fact that the basic building blocks of ordinary matter (things such as electrons and quarks, collectively known as fermions) and the particles that carry forces (collectively called bosons) all have a property we call mass. Theories could see no rhyme or reason in particles' masses and could not predict them; they had to be measured in experiments and added into the theory by hand.

These "free parameters" were embarrassing loose threads in the theories that were being woven together to form what eventually became the standard model. In 1964,Peter Higgs of the University of Edinburgh, UK, and François Englert and Robert Brout of the Free University of Brussels (ULB) in Belgium independently hit upon a way to tie them up.

That mechanism was an unseen quantum field that suffuses the entire cosmos. Later dubbed the Higgs field, it imparts mass to all particles. The mass an elementary particle such as an electron or quark acquires depends on the strength of its interactions with the Higgs field, whose "quanta" are Higgs bosons.

Fields like this are key to the standard model as they describe how the electromagnetic and the weak and strong nuclear forces act on particles through the exchange of various bosons - the W and Z particles, gluons and photons. But the Higgs theory, though elegant, comes with a nasty sting in its tail: what is the mass of the Higgs itself? It should consist of a core mass plus contributions from its interactions with all the other elementary particles. When you tot up those contributions, the Higgs mass balloons out of control.

The experimental clues we already have suggest that the Higgs's mass should lie somewhere between 114 and 180 gigaelectronvolts - between 120 and 190 times the mass of a proton or neutron, and easily the sort of energy the LHC can reach. Theory, however, comes up with values 17 or 18 orders of magnitude greater - a catastrophic discrepancy dubbed "the hierarchy problem". The only way to get rid of it in the standard model is to fine-tune certain parameters with an accuracy of 1 part in 1034, something that physicists find unnatural and abhorrent.
Three into one

The hierarchy problem is not the only defect in the standard model. There is also the problem of how to reunite all the forces. In today's universe, the three forces dealt with by the standard model have very different strengths and ranges. At a subatomic level, the strong force is the strongest, the weak the weakest and the electromagnetic force somewhere in between.

Towards the end of the 1960s, though, Weinberg, then at Harvard University, showed with Abdus Salam and Sheldon Glashow that this hadn't always been the case. At the kind of high energies prevalent in the early universe, the weak and electromagnetic forces have one and the same strength; in fact they unify into one force. The expectation was that if you extrapolated back far enough towards the big bang, the strong force would also succumb, and be unified with the electromagnetic and weak force in one single super-force (see graph).

In 1974 Weinberg and his colleagues Helen Quinn and Howard Georgi showed that the standard model could indeed make that happen - but only approximately. Hailed initially as a great success, this not-so-exact reunification soon began to bug physicists working on "grand unified theories" of nature's interactions.

It was around this time that supersymmetry made its appearance, debuting in the work of Soviet physicists Yuri Golfand and Evgeny Likhtman that never quite made it to the west. It was left to Julius Wess of Karlsruhe University in Germany and Bruno Zumino of the University of California, Berkeley, to bring its radical prescriptions to wider attention a few years later.

Wess and Zumino were trying to apply physicists' favourite simplifying principle, symmetry, to the zoo of subatomic particles. Their aim was to show that the division of the particle domain into fermions and bosons is the result of a lost symmetry that existed in the early universe.

According to supersymmetry, each fermion is paired with a more massive supersymmetric boson, and each boson with a fermionic super-sibling. For example, the electron has the selectron (a boson) as its supersymmetric partner, while the photon is partnered with the photino (a fermion). In essence, the particles we know now are merely the runts of a litter double the size (see diagram).

The key to the theory is that in the high-energy soup of the early universe, particles and their super-partners were indistinguishable. Each pair co-existed as single massless entities. As the universe expanded and cooled, though, this supersymmetry broke down. Partners and super-partners went their separate ways, becoming individual particles with a distinctive mass all their own.

Supersymmetry was a bold idea, but one with seemingly little to commend it other than its appeal to the symmetry fetishists. Until, that is, you apply it to the hierarchy problem. It turned out that supersymmetry could tame all the pesky contributions from the Higgs's interactions with elementary particles, the ones that cause its mass to run out of control. They are simply cancelled out by contributions from their supersymmetric partners. "Supersymmetry makes the cancellation very natural," says Nathan Seiberg of the Institute of Advanced Studies, Princeton.

That wasn't all. In 1981 Georgi, together with Savas Dimopoulos of Stanford University, redid the force reunification calculations that he had done with Weinberg and Quinn, but with supersymmetry added to the mix. They found that the curves representing the strengths of all three forces could be made to come together with stunning accuracy in the early universe. "If you have two curves, it's not surprising that they intersect somewhere," says Weinberg. "But if you have three curves that intersect at the same point, then that's not trivial."

This second strike for supersymmetry was enough to convert many physicists into true believers. But it was when they began studying some of the questions raised by the new theory that things became really interesting.

One pressing question concerned the present-day whereabouts of supersymmetric particles. Electrons, photons and the like are all around us, but of selectrons and photinos there is no sign, either in nature or in any high-energy accelerator experiments so far. If such particles exist, they must be extremely massive indeed, requiring huge amounts of energy to fabricate.

Such huge particles would long since have decayed into a residue of the lightest, stable supersymmetric particles, dubbed neutralinos. Still massive, the neutralino has no electric charge and interacts with normal matter extremely timorously by means of the weak nuclear force. No surprise then that it is has eluded detection so far.

When physicists calculated exactly how much of the neutralino residue there should be, they were taken aback. It was a huge amount - far more than all the normal matter in the universe.

Beginning to sound familiar? Yes, indeed: it seemed that neutralinos fulfilled all the requirements for the dark matter that astronomical observations persuade us must dominate the cosmos. A third strike for supersymmetry.

Each of the three questions that supersymmetry purports to solve - the hierarchy problem, the reunification problem and the dark-matter problem - might have its own unique answer. But physicists are always inclined to favour an all-purpose theory if they can find one. "It's really reassuring that there is one idea that solves these three logically independent things," says Seiberg.
Supersymmetry solves problems with the standard model, helps to unify nature's forces and explains the origin of dark matter

Supersymmetry's scope does not end there. As Seiberg and his Princeton colleague Edward Witten have shown, the theory can also explain why quarks are never seen on their own, but are always corralled together by the strong force into larger particles such as protons and neutrons. In the standard model, there is no mathematical indication why that should be; with supersymmetry, it drops out of the equations naturally. Similarly, mathematics derived from supersymmetry can tell you how many ways can you fold a four-dimensional surface, an otherwise intractable problem in topology.

All this seems to point to some fundamental truth locked up within the theory. "When something has applications beyond those that you designed it for, then you say, 'well this looks deep'," says Seiberg. "The beauty of supersymmetry is really overwhelming."

Sadly, neither mathematical beauty nor promise are enough on their own. You also need experimental evidence. "It is embarrassing," says Michael Dine of the University of California, Santa Cruz. "It is a lot of paper expended on something that is holding on by these threads."

Circumstantial evidence for supersymmetry might be found in various experiments designed to find and characterise dark matter in cosmic rays passing through Earth. These include the Cryogenic Dark Matter Search experiment inside the Soudan Mine in northern Minnesota and the Xenon experiment beneath the Gran Sasso mountain in central Italy. Space probes like NASA's Fermi satellite are also scouring the Milky Way for the telltale signs expected to be produced when two neutralinos meet and annihilate.

The best proof would come, however, if we could produce neutralinos directly through collisions in an accelerator. The trouble is that we are not entirely sure how muscular that accelerator would need to be. The mass of the super-partners depends on precisely when supersymmetry broke apart as the universe cooled and the standard particles and their super-partners parted company. Various versions of the theory have not come up with a consistent timing. Some variants even suggest that certain super-partners are light enough to have already turned up in accelerators such as the Large Electron-Positron collider - the LHC's predecessor at CERN - or the Tevatron collider in Batavia, Illinois. Yet neither accelerator found anything.

The reason physicists are so excited about the LHC, though, is that the kind of supersymmetry that best solves the hierarchy problem will become visible at the higher energies the LHC will explore. Similarly, if neutralinos have the right mass to make up dark matter, they should be produced in great numbers at the LHC.

Since the accident during the accelerator's commissioning last year, CERN has adopted a softly-softly approach to the LHC's restart. For the first year it will smash together two beams of protons with a total energy of 7 teraelectronvolts (TeV), half its design energy. Even that is quite a step up from the 1.96 TeV that the Tevatron, the previous record holder, could manage. "If the heaviest supersymmetric particles weigh less than a teraelectronvolt, then they could be produced quite copiously in the early stages of LHC's running," says CERN theorist John Ellis.

If that is so, events after the accelerator is fired up again could take a paradoxical turn. The protons that the LHC smashes together are composite particles made up of quarks and gluons, and produce extremely messy debris. It could take rather a long time to dig the Higgs out of the rubble, says Ellis.

Any supersymmetric particles, on the other hand, will decay in as little as 10-16seconds into a slew of secondary particles, culminating in a cascade of neutralinos. Because neutralinos barely interact with other particles, they will evade the LHC's detectors. Paradoxically, this may make them relatively easy to find as the energy and momentum they carry will appear to be missing. "This, in principle, is something quite distinctive," says Ellis.

So if evidence for supersymmetry does exist in the form most theorists expect, it could be discovered well before the Higgs particle, whose problems SUSY purports to solve. Any sighting of something that looks like a neutralino would be very big news indeed. At the very least it would be the best sighting yet of a dark-matter particle. Even better, it would tell us that nature is fundamentally supersymmetric.

There is a palpable sense of excitement about what the LHC might find in the coming years. "I'll be delighted if it is supersymmetry," says Seiberg. "But I'll also be delighted if it is something else. We need more clues from nature. The LHC will give us these clues."

11 November 2009 by Anil Ananthaswamy

Did a Time-Traveling Bird Sabotage the Collider?

Sometime on Nov. 3, the supercooled magnets in sector 81 of the Large Hadron Collider (LHC), outside Geneva, began to dangerously overheat. Scientists rushed to diagnose the problem, since the particle accelerator has to maintain a temperature colder than deep space in order to work. The culprit? "A bit of baguette," says Mike Lamont of the control center of CERN, the European Organization for Nuclear Research, which built and maintains the LHC. Apparently, a passing bird may have dropped the chunk of bread on an electrical substation above the accelerator, causing a power cut. The baguette was removed, power to the cryogenic system was restored and within a few days the magnets returned to their supercool temperatures.

While most scientists would write off the event as a freak accident, two esteemed physicists have formulated a theory that suggests an alternative explanation: perhaps a time-traveling bird was sent from the future to sabotage the experiment. Bech Nielsen of the Niels Bohr Institute in Copenhagen and Masao Ninomiya of the Yukawa Institute for Theoretical Physics in Kyoto, Japan, have published several papers over the past year arguing that the CERN experiment may be the latest in a series of physics research projects whose purposes are so unacceptable to the universe that they are doomed to fail, subverted by the future. 

The LHC, a 17-mile underground ring designed to smash atoms together at high energies, was created in part to find proof of a hypothetical subatomic particle called the Higgs boson. According to current theory, the Higgs is responsible for imparting mass to all things in the universe. But ever since the British physicist Peter Higgs first postulated the existence of the particle in 1964, attempts to capture the particle have failed, and often for unexpected, seemingly inexplicable reasons.

In 1993, the multibillion-dollar United States Superconducting Supercollider, which was designed to search for the Higgs, was abruptly canceled by Congress. In 2000, scientists at a previous CERN accelerator, LEP, said they were on the verge of discovering the particle when, again, funding dried up. And now there's the LHC. Originally scheduled to start operating in 2006, it has been hit with a series of delays and setbacks, including a sudden explosion between two magnets nine days after the accelerator was first turned on, the arrest of one of its contributing physicists on suspicion of terrorist activity and, most recently, the aerial bread bombardment from a bird. (A CERN spokesman said power cuts such as the one caused by the errant baguette are common for a device that requires as much electricity as the nearby city of Geneva, and that physicists are confident they will begin circulating atoms by the end of the year). 

In a series of audacious papers, Nielsen and Ninomiya have suggested that setbacks to the LHC occur because of "reverse chronological causation," which is to say, sabotage from the future. The papers suggest that the Higgs boson may be "abhorrent to nature" and the LHC's creation of the Higgs sometime in the future sends ripples backward through time to scupper its own creation. Each time scientists are on the verge of capturing the Higgs, the theory holds, the future intercedes. The theory as to why the universe rejects the creation of Higgs bosons is based on complex mathematics, but, Nielsen tells TIME, "you could explain it [simply] by saying that God, in inverted commas, or nature, hates the Higgs and tries to avoid them."

Many physicists say that Nielsen and Ninomiya's theory, while intellectually interesting, cannot be accurate because the event that the LHC is trying to recreate already happens in nature. Particle collisions of an energy equivalent to those planned in the LHC occur when high-energy cosmic rays collide with the earth's atmosphere. What's more, some scientists believe that the Tevatron accelerator at Fermi National Accelerator Laboratory (or Fermilab) near Chicago has already created Higgs bosons without incident; the Fermilab scientists are now refining data from their collisions to prove the Higgs' existence. 

Nielsen counters that nature might allow a small number of Higgs to be produced by the Tevatron, but would prevent the production of the large number of particles the LHC is anticipated to produce. He also acknowledges that Higgs particles are probably produced in cosmic collisions, but says it's impossible to know whether nature has stopped a great deal of these collisions from happening. "It's possible that God avoids Higgs [particles] only when there are very many of them, but if there are a few, maybe He let's them go," he says.

Nielsen and Ninomiya's theory represents one side of an intellectual divide between particle physicists today. Contemporary physicists tend to fall into one of two camps: the theorists, who posit ideas about the origins and workings of the universe; and experimentalists, who design telescopes and particle accelerators to test these theories, or provide new data from which novel theories can emerge. Most experimentalists believe that the theorists, due to a lack of new data in recent years, have reached a roadblock — the Standard Model, which is the closest thing the theorists have to an evidence-backed "theory of everything," provides only an incomplete explanation of the universe. Until theorists get further data and evidence to move forward, the experimentalists believe, they end up simply making wild guesses — like those concerning time-traveling saboteurs — about how the universe works. "Nielsen and Ninomiya's theories are clearly crazy theories," says Dmitri Denisov, a physicist and Higgs-hunter at the DZero experiment at Fermilab. "In recent years theorists have been starving for experimental input and as a result, theories of second type are propagating widely. The majority of them have nothing to do with world we live in." 

Nielsen concedes, "We have very little data, so theorists are going their own ways and making a lot of theories that may not be very plausible. We need guidance from experimentalists to make the theories more healthy."

"But," he adds, "in terms of our theory, we are submitting to a form of experiment. We are saying the LHC won't be allowed to produce a large number of Higgs. If it does, it would be very damaging to our theory."

Particle physics has a long history of zany theories that turned out to be true. Niels Bohr, the doyen of modern physicists, often told a story about a horseshoe he kept over his country home in Tisvilde, Denmark. When asked whether he really thought it would bring good luck, he replied, "Of course not, but I'm told it works even if you don't believe in it." In other words: if preposterous theories are mathematically sound and can be confirmed by observation, they are true, even if seemingly impossible to believe. To scientists in the early 20th century, for example, quantum mechanics may have seemed outrageous. "The concept that you could have a wave-particle duality — that an object could take on either wave-like properties or point-like properties, depending on how you observe it — takes a huge leap of imagination," says Roberto Roser, a scientist at Fermilab. "Sometimes outlandish papers turn out to be the laws of physics."

So what would Peter Higgs himself make of the intellectual controversy surrounding his eponymous particle? Speaking on behalf of his friend, Professor Richard Kenway, who holds Higgs' former position at the University of Edinburgh, says that the 78-year-old emeritus professor remains quietly confident that the LHC will discover the Higgs boson when it is eventually running at full strength. For his part, Kenway says the LHC's delays are to be expected given the size and intricacy of the $9 billion experiment. And he says if he ever needs further proof that the Higgs boson is not abhorrent to nature, he need only spend time with his friend and mentor. "If nature truly did not want us to discover the Higgs, a cosmic ray would have zapped the embryo that became Peter, preventing its development into a physicist," he says.

By Eben Harrell Wednesday, Nov. 11, 2009

How To See a Black Hole

A black hole is like a scary monster from children’s literature. It’s vividly imagined but never actually seen in real life.

A simulated image of the disk of gas surrounding the supermassive black hole at the centre of the Milky Way as it might appear with new methods designed to reveal the black hole's dark edge. The light-bending effects of the black hole's strong gravitational field as well as the disk's rapid rotation would produce a crescent shaped image wrapped around the event horizon.

Avery Broderick

This is true even for the largest black holes we know — the ones that reside at the centers of galaxies. The nearest of these lies some 30,000 light-years away, in the core of the Milky Way. If you placed it in our solar system it would probably span the orbit of Mercury. Yet, because of its great distance, it’s a mere speck against the sky, about 36 million times smaller than the full Moon. How could anyone see any detail when looking at something with an apparent size that small?

Amazingly, there is a way. And now it’s promising not only to reveal the giant black hole in our own galaxy, but also a much larger and more active one in the galaxy known as M87 in Virgo.

The nifty trick that puts this ambitious goal within reach is called very long baseline interferometry. VLBI involves two or more radio dishes that are spaced as far apart as possible — for example, in Arizona and Hawaii. The dishes observe the same radio sources in the sky, and when their signals are combined they form an image that’s as sharp as what you would get from a single receiver as big as the separation between the dishes. The idea is to show the way radio emission is spatial distributed across a small region of sky. It’s just what you need to “see” a black hole.

But what does that mean? Aren’t black holes supposed to be, well, black?

Yes and no. If completely isolated in space, a black hole would indeed be well camouflaged. But in the densely populated center of the Milky Way, there is plenty of hot gas swirling around the giant black hole there.

The energized ions in the gas give off radio waves. Seen up close, there should also be a dark sphere at the center of that swirl of gas, where matter funnels in and never comes out. The dark sphere is the infamous event horizon. It’s the point of no return, from which not even light can escape. In this case, “seeing” the black hole means seeing the event horizon silhouetted against the glowing gas.

Shep Doeleman of the Massachusetts Instistute of Technology has been leading the charge to image the Milky Way’s central black hole, and he and his collaborators have made astounding progress. They’ve looked deep into the heart of the radio source known as Sagittarius A*, where the black hole is believed to be lurking. What they’ve got is not exactly an image but a sense that there is some structure on the scale of a supermassive black hole deep in the heart of the radio source.

The complication in mapping out an image of this object is that Sgr A* is changing on a regular basis, presumably as clumps of gaseous matter go a-whirling around the black hole.

This is not going to be a problem when it comes to looking at even bigger black holes, like the giant monster at the center of the galaxy M87 in Virgo. It devours vast quantities of gas and spits out a spectacular jet that extends far into extragalactic space.

Recently, Karl Gebhardt of the University of Texas at Austin and Jens Thomas of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, set about measuring the size of M87 by running existing data through a new model that mimics the galaxy star by star. Unlike earlier efforts, their model also takes into account the unseen halo of dark matter that surround the visible portion of the galaxy. This turned out to have an unexpected affect on the way the model calculates the mass in the stars that illuminate the galaxy’s core. In the final analysis, it allocates far more mass — a whopping 6.4 billion suns — to the galaxy’s monstrous central black hole.

The surprising corollary to this is that M87’s black hole, if viewed from Earth, should be the same apparent size as the black hole in Sgr A* — just as the Sun and the Moon appear roughly the same size, though the Sun is larger and much farther way. That puts M87’s black hole within reach of Doeleman’s radio telescopes. It may even be easier to image than Sgr A* because it’s larger size means it doesn’t change nearly as rapidly.

What’s particularly exciting to theorists like Avery Broderick, of the Canadian Institute for Theoretical Astrophysics in Toronto, is that M87’s black hole is also violently active, with a vast disk of gas around it and a big jet of shooting out in one direction. A radio image of this black hole might not only reveal the event horizon but show us the region where the jet is launched.

“It’s kind of exciting as an alternate object because it is so different from the supermassive black hole in our backyard,” says Avery. “Between them, they span the range of what we expect from these objects.”

Stay tuned. Modern astronomy is increasingly become the science in which the unseen becomes seeable. Before long we’ll be adding black holes to that list.