Large Hadron Collider: Touring the physics frontier
With the Large Hadron Collider experiments under way, physicists are getting new data to anchor their theories.
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GENEVA--There are two kinds of physicists in the world, broadly speaking: those with the equation-covered blackboards, and those with the scales, thermometers, and pressure gauges.
The theoretical physicists have had the upper hand for years, but something new has begun tilting the balance toward the experimentalists: the Large Hadron Collider.
This mammoth, $8 billion particle accelerator is housed in a ring 27km in circumference bored about 100 meters beneath a somewhat pastoral valley west of Geneva and operated by a multinational nuclear physics organization called CERN, which was founded in 1954.
The LHC is now speeding protons nearly to the velocity of light and smashing them into each other. Thousands of researchers involved with the LHC's experiments hope to distill the secrets of the universe from the results: everything from Higgs bosons and quark-gluon plasma to supersymmetry and dark matter.
Despite serious setbacks in the LHC's early operation, and even though it's still only running at half the energy level planned, there is a swell of optimism as the accelerator operators iron out its wrinkles and the data starts pouring in.
"The LHC is positioned so that we're almost guaranteed to get something new out of it," said Tom LeCompte, physics coordinator for one of the major LHC experiments, ATLAS, and scientists have begun preparing papers based on some early results.
And with experimental data now arriving, it's time to start giving the theoreticians new food for thought.
"The data side has not spoken for a long time," said Guy Wilkinson, physics coordinator for another experiment, LHCb. "There are lots of ideas floating around but nothing to tether them to."
Looking back in time
The LHC is a time machine. It's designed to look back at the earliest moments of the universe.
During the Big Bang, the amount of energy in a given volume was colossal, but the neighborhood has been going downhill as the universe expanded over time. The LHC attempts to reclaim a tiny bit of that lost glory.
To do so, it moves clumps of protons very fast in two beams that travel both directions around the ring. Today, each proton can reach an energy level of 3.5 tera-electron-volts, so, when two collide from opposite directions at carefully controlled points along the ring, they have a total energy of 7TeV.
When they collide, they make a mess.
Other particles cascade out from the impact, triggering detectors to pour data into computer storage systems. Further data processing lets the researchers reconstruct what happened--particles generated by the collision and more particles produced as short-lived progenitors decay.
With each new generation of particle accelerators, energy levels get closer to the Big Bang's conditions. Initial accelerators were called atom smashers, since they broke atoms into subatomic particles such as protons and neutrons. Now, with more energy on hand, subatomic particles are being smashed into even smaller bits--quarks, for example, three of which make up each proton and neutron, but also a lot more.
Energy levels diminished enough for atomic nuclei to form something like 3 minutes after the Big Bang. Individual protons and neutrons formed earlier--about a thousandth of a second afterward. Earlier accelerators, such as the 1 TeV Tevatron at Fermilab in Illinois, have been able to peer into this regime, and indeed that's where the last of the six varieties of quark, the top quark, was first observed in 1995.
The LHC should be able to peer yet earlier into the history of the universe by operating at yet higher energy--all the way back to the time of the quark-gluon plasma, when the universe was only a trillionth of a second old.
Wait, gluons? OK, this is where the particles start sounding more unfamiliar. Under quantum physics' "standard model," there is a sizable family of elementary particles. They include the six quarks, the lepton group that includes electrons and three varieties of neutrinos, and another group called the bosons.
Bosons include photons--light--and gluons, which effectively bind quarks together into protons and neutrons. Other varieties of bosons, the W and Z, were discovered in 1983 at an LHC predecessor at CERN.
The Higgs boson
But it's the Higgs boson, hypothesized but as yet undetected, that is one of the main reasons the LHC exists. The standard model is fine as far as it goes, but it doesn't explain everything. It's like Newtonian physics: it works well in one regime, where velocities are small, but for objects traveling closer toward the speed of light, Einstein's equations come into play.
The Higgs boson--or more likely, at least five of them--could be the first glimpse into what's beyond the standard model. Many physicists believe in "supersymmetry," in which the conventional elementary particles in the standard model have companions, including the Higgs.
"If there's only one Higgs boson, it's on our side of the symmetry. In supersymmetry, you can't make a consistent theory with just one. You need at least five," LeCompte said
This is where the LHC's two general-purpose instruments come into play, ATLAS and CMS. They're designed to detect the wide variety of possible signatures that indicate the various Higgs bosons were produced.
"A light Higgs might decay into two gamma rays. A heavy one might decay into two W bosons and a Z," LeCompte said.
What would you do all day if you were a Higgs boson? Imbue other particles with mass, perhaps.
The stock simile goes something like this: a pervasive Higgs Field acts as a drag on some particles, making them harder to accelerate or decelerate. It's like a celebrity at a party that accumulates groupies: it's hard to get moving because of the groupies, and once they all get moving, they're hard to stop. Some particles--those with little mass--have feeble interactions with the Higgs field, like ordinary people at a party.
The LHC is geared to excite this Higgs field enough for it to produce Higgs bosons, illuminating the physics mystery that is mass.
Supersymmetric particles that LHC produces generally aren't expected to last long--indeed, most of them will decay within the detectors. But eventually, the decaying particles will leave something stable behind. But if we can't detect the dark matter that pervades our very bodies, how can the LHC researchers see it?
Perversely, by its absence. Because of conservation of momentum, there's essentially a recoil the detector observes that balances the unseen activity.
"The other side is unbalanced. That tells us, wow, there's a lot of energy that escapes," said Albert de Roeck, one of the CMS managers. "From studying this part that one can see, one can learn a lot about that [other unseen] part."
Antimatter imbalance
The Higgs gets the headlines, but it's not all that's going on at the LHC. Antimatter, which looks like regular matter but carries an opposite electrical charge, is another.
Matter and antimatter are famously opposite: upon contact with each other, matter and antimatter destroy each other and leave only a highly energetic frequency of light called gamma rays. Since 1964, physicists have known that antimatter and matter aren't exact mirror images, as was earlier thought.
Specifically, the LHCb experiment will investigate the decay of one type of short-lived quark, variously called the bottom or beauty quark. These b quarks vanished long ago from the regular universe, but the LHC produces them in abundance.
LHCb precisely measures the difference in decay times of the b and anti-b quarks. This subtle asymmetry is partly responsible for the fact that the universe today is made of matter, not merely the gamma rays that would be the sole survivor of a universe with matter and antimatter in balance.
"Something in the early history of the universe caused matter and antimatter to behave in slightly different manner," Wilkinson said, and physicists call it CP violation. What's been so far observed is not sufficient to explain the imbalance, he said.
LHCb is sensitive enough to see effects not predicted by the standard model. Specifically, physicists hope to find evidence of dark matter, invisible material that pervades the universe. Dark matter doesn't generally interact with the ordinary matter we're made of, except through gravitational effects such as the rate galaxies rotate, but its influence could be detected at LHCb.
"These very superheavy articles may influence the decay of these light particles in a ghostly manner," Wilkinson said.
Quarks unconfined: ALICE
Another major CERN experiment, ALICE, is designed to illuminate the quark-gluon plasma era of the universe. Today, through a concept called confinement, quarks are found only confined within particles such as neutrons and protons.
With enough energy, though--specifically, a temperature about 100,000 times hotter than the sun's center--quarks become unconfined.
Most LHC experiments rely on proton collisions, but ALICE (A Large Ion Collider Experiment) requires something much heavier: lead atoms. The collision of two lead atoms traveling nearly at the speed of light should produce the quark-gluon plasma.
But not for long: as the plasma expands, it cools back into ordinary matter. Scientists have only about 0.00000000000000000000001 of a second to make their direct observations.
ALICE also is designed to shed light on another quark mystery: why is it that the combined mass of the three quarks needed to make a proton or neutron is about 1 percent of an actual proton or neutron?
ALICE, CMS, ATLAS, and LHCb are the major experiments at the LHC. Two other smaller ones will operate, though.
First is TOTEM, which measures the effective size and structure of protons. Second is LHCf, which will study accelerator-produced particle cascades similar to those from rare ultra-high energy cosmic rays that strike the Earth.
What's perhaps most important about the LHC's experiments, though, is that they've grown beyond the design and construction idea phases.
"We are in a stage we're where producing physics," de Roeck said.
See also:
• What makes the LHC tick?
• A scientific subculture thrives at LHC