Overall view of the LHC. (Image courtesy of CERN.)
On March 30, 2010, physicists at the Large Hadron Collider (LHC), near Geneva, Switzerland, coaxed two beams of protons—those tiny, positively-charged particles found in the nuclei of atoms—into slamming into each other. Moving close to the speed of light, these protons hit head-on with a combined force of seven trillion electron volts of energy. To you and me, this much energy is like two swarms of kamikaze mosquitoes aiming for each other, but on the scale of subatomic particles it was a new record for high energy collisions. It was also "a great day to be a particle physicist," according to Rolf Heuer, Director General of CERN, the European particle physics laboratory that built and runs the LHC.
Rolf Heuer's great day marked the official start of the LHC research program after more than two decades of planning, prototyping, construction, and testing. There have also been some very bad days, such as September 19, 2008, a mere month before the LHC's official inauguration, when a faulty electrical connection melted and a ton of liquid helium vented into the tunnel housing the accelerator.
Great days, bad days, go-home-and-kick-your-dog days, Nobel Prize-winning days—it's a safe bet that the LHC will deliver all these days and more, but how the moods of particle physicists translate into scientific discoveries is another question. And not just particle physicists, but high energy physicists, condensed matter physicists, theoretical physicists, cosmologists, astrophysicists—physicists from almost every field are waiting to see what the LHC will find.
Much attention has been given to two possibilities: "God particles" and mini black holes. Physicists can't ask for more than new insights into these potent enigmas, can they?
Yes, they can. They can, they do, and they want the LHC to deliver.
The Numbers Game (Part 1)
What the LHC can deliver first and foremost is more energy than any other particle accelerator in history, able to speed up particles to unheard of velocities before smashing together. When it's running at full capacity some time in 2013, the 17-mile (27 km) ring buried under the Swiss-French border will accelerate two beams of protons to 99% the speed of light before slamming them into each other with a total energy of 14 trillion electron volts.
Image of a 7 TeV proton-proton collision in CMS (Compact Muon Solenoid) producing more than 100 charged particles. (Image of courtesy CERN.)
High energies are crucial because quantum mechanics, our theoretical model of the infinitesimal world, says almost all events are possible when dealing with fundamental particles, but some events are more probable than others. The higher energies delivered by the LHC improve the chances that nice, big, never-before-seen particles will pop into existence from the energy left over from proton collisions. These new particles are what physicists want to see. Such particles are needed to fill out the mathematical description of all known particles—a list including electrons, quarks, and neutrinos—called the Standard Model. The picture the Standard Model paints is highly precise as far as it goes, but some areas are incorrect, some incomplete, and a vital element just does not fit: gravity. Theorists are hard at work proposing ways to smooth out the wrinkles, but without data, all patchwork is suspect.
"The theoretical underpinnings are not strong enough to support a lack of experimental confirmation," says Leonard Susskind, director of the Stanford Institute for Theoretical Physics, and he should know. Generally credited as one of the fathers of string theory, Susskind recently emerged triumphant from the Black Hole War, an epic (among physicists) disagreement with Stephen Hawking regarding whether quantum effects hold sway over black holes. (They do.)
As Susskind explains it, "There's a hell of a lot that we do know that we don't understand. We can't use quantum mechanics to study the universe. Big pieces are missing."
Searching for Symmetry
The universe gives every indication of wanting to be a tidy place. There are four forces, but three of them—the strong force, the weak force, and electromagnetism—have been shown to be manifestations of the same force.
The fourth, gravity, is a holdout. Why?
The forces act on particles using intermediaries called bosons, in effect using the bosons to "tell" particles how the particles ought to behave. The boson for electromagnetism, the photon, is massless.
The rest are not without mass, even though the Standard Model says they should be. Why not?
These scuff-marks on the neatly-wrapped package of the Standard Model can be attributed to something called symmetry breaking, and exploring symmetry breaking, according to some physicists, is what the LHC is really all about.
The Higgs boson, the aforementioned "God particle," is the result of breaking the symmetries of the Standard Model, and finding it could settle the mass question. Dubbed the "God particle" by Leon Lederer, a Nobel Prize winner who was apparently a bit more media-savvy than your average physicist, the Higgs is the only particle of the Standard Model which has not been shown experimentally to exist.
CMS: Simulated Higgs to two jets and two electrons. (Image courtesy of CERN.)
The Higgs boson "tells" other particles about the presence of the Higgs field, the real source of the mass. Howard Haber, of the Santa Cruz Institute for Particle Physics (SCIPP) at the University of California, Santa Cruz and one of the authors of The Higgs Hunter's Guide, likens the effect of the Higgs field on a particle to that of a vat of molasses on a speeding bullet. "Of course, a bullet in a vat of molasses would continue to decelerate and that doesn't happen to a particle in a Higgs field, so it's not a perfect analogy."
Analogies involving the bizarre world of subatomic particles rarely are. Perhaps Lederer knew what he was doing when he gave the Higgs its divine moniker. Regardless, the importance of the particle to the Standard Model is not up for debate. "If the Higgs isn't discovered," says Susskind, "we're really screwed up."
But the Standard Model displays only one type of symmetry breaking. The big hope is that the LHC will show the cracks in a type of symmetry not yet proven to exist: supersymmetry, or SUSY.
SUSY posits that each fundamental particle of the Standard Model has an almost-twin—a supersymmetric partner particle, or "sparticle." (No wonder physicists have trouble finding the right words.) If SUSY exists, it must be broken because each particle/sparticle pair should have the same mass. If each known particle had an almost-twin with the same mass, those twins would already be tucked into their enclosures at the particle zoo of the Standard Model.
Yet many physicists do believe that evidence for SUSY will be forthcoming from the LHC. "Of all the ideas that are on the table, I think that supersymmetry in a general sense has the best chance of being right," says Michael Dine, also from SCIPP.
Kim Griest, an astrophysicist at the University of California, San Diego, puts it a little more strongly. "They need to find supersymmetry. The future of fundamental particles and particle physics is on the line."
What makes SUSY so compelling?
One reason is that "supersymmetry is the only way to join the Standard Model and gravity," says Griest.
Another reason, a little less dramatic but still of great importance to cosmologists and astrophysicists, was first proposed by by Joel Primack of SCIPP in 1983. Primack suggested that the LSP, the lightest supersymmetric particle, could explain the 80% of matter in the universe that's hidden from our sight and only apparent through its gravitational effects on the stuff we can see. Apparently this dark matter, as it's called, cocoons our galaxy, and the galaxies next door in our Local Group, and all the galaxies in all the superclusters around us. If Primack is right, SUSY and dark matter are a two-for-one deal. This possibility has cosmologists and astrophysicists waiting right alongside the particle physicists.
SUSY could explain why neutrinos have mass (the Standard Model says they shouldn't but they do).
SUSY could tell us why the universe ended up full of matter instead of antimatter—or nothing at all.
SUSY could reveal why the strength of the weak force doesn't match up with what the Standard Model says it should be.
There are a lot of reasons to hope the LHC uncovers supersymmetry.
Of course, to answer all these questions the LHC has to find the right supersymmetry.
If We Knew SUSY
First problem: there are a lot of different versions of SUSY. Some have candidates for dark matter, some do not. Some have extra dimensions, and some do not. Even the Minimal Supersymmetric Standard Model (MSSM), the smallest possible rewrite of the Standard Model to contain a viable version of supersymmetry, adds about 120 new parameters, the values of which can vary.
Second problem: SUSY may exist, but require an even more powerful accelerator to find.
Susskind, even though he considers the mathematics behind SUSY "strange" and "tedious," hopes that isn't the case. If SUSY exists but the LHC can't find it, Susskind says, "It (SUSY) may not be doing what we hope it's doing." In other words, the answers to some questions might require "fine-tuning," a term even more distasteful to physicists than "God particle." Fine- tuning the value of a cosmic parameter means admitting that our universe is the way it is "just because" instead of as a logical consequence of a deeper theory. In other words, "Nature could have just done it," says Susskind, an idea that is anathema to the people who choose to live their lives looking for the reasons why.
According to both Dine and Tom Banks, another SCIPP physicist, what the LHC will really do is help cross possible SUSY models off the list. Neither is willing to wager on what might remain.
"You know what they say," says Banks. "Ask 10 physicists, get 11 opinions."
"We should back up," says Dine. "There's no guarantee that SUSY is the correct underlying theory for anything."
But the LHC is a six-billion-dollar bet that it is.
The Numbers Game (Part II)
View of the ATLAS experiment in April, 2006. (Image courtesy of CERN.)
Physicists are going to have to wait a bit longer to see whether they will rake in the chips or leave the table with empty pockets. The first order of business for the LHC is to spend 2010 and 2011 rediscovering all the known particles of the Standard Model. This will demonstrate not only that the machine is working, but that we're capable of correctly interpreting the tremendous amounts of data the LHC generates. 2012 will be spent on upgrades. The LHC will start running at full capacity some time in 2013.
At the recent West Coast LHC Theory Group meeting at the Santa Cruz Institute for Particle Physics, hosted by the University of California at Santa Cruz, the morning sessions covered the LHC in rediscovery mode. According to the presentation given by Ian Hinchcliffe, a member of the Theoretical Physics Group of the Lawrence Berkeley National Laboratory, the accelerator and—possibly even more important—the detectors are working well. "I was amazed that the detectors worked out of the box."
However, there is no plan to speed up the commissioning process. No one wants another day as bad as September 19, 2008.
Dine thinks the time waiting should be spent writing more papers with more testable predictions. Banks has another idea.
"You can bet on the mass of the Higgs boson with the London bookies," he says.
Would they bet on their own theories? In true physicist fashion, they debate the most logical way to wager.
"The thing to do," says Banks, "is bet against it (his own theory). Then if it's right you're excited, but if it's wrong you've made money."
Letting It Ride
What if the entire physics community loses the bet? The Higgs is too heavy, or SUSY doesn't exist at energies the LHC can reach?
Everyone interviewed zeroed in on the main problem—no money to try again.
Yet Daniele Alves, a Brazilian graduate student in the Theoretical Group at SLAC, is sanguine about the future of her chosen profession. "There are other experiments working at the forefront of high energy particle physics. It wouldn't be the end of the world." The LHC is designed to address certain questions and the people working on those questions might be in trouble, she allows, but "I must say that most people aren't pessimistic, I think, about what the LHC might discover."
Banks is looking forward to the unexpected. "That's usually the best time—when an experiment produces something where you say, 'What is that?'"
Susskind admits that there will be "tremendous disappointment" if the LHC does not deliver, but "I do not think that will be the end of physics." In fact, "It's probably wrong to be discouraged," because the questions will still be there, even if the technology to answer them experimentally is not. The end of physics will come with the end of questions, and the end of questions, Susskind believes, will only come with the end of people.