| Brief Description of the LHC. Energising the quest for 'big theory' By Paul Rincon BBC News science reporter, Geneva |
"We are at a point where experiments must guide us, we cannot make progress without them," explains Jim Virdee, a particle physicist at Imperial College London.
Enlarge Image "We must wait for the data to speak."
Over a coffee in the lobby of building 40 at Cern, the sprawling experimental facility situated on the Swiss-French border, Professor Virdee says physics has reached a critical juncture.
In the 1970s, the theory known as the Standard Model was considered a triumph of theoretical physics, incorporating all that was then known about the interactions of sub-atomic particles.
Today it is regarded as incomplete, a mere stepping stone to something else.
The Standard Model cannot explain the best known of the so-called four fundamental forces: gravity; and it describes only ordinary matter, which makes up but a small part of the total Universe.
The Large Hadron Collider (LHC) at Cern (The European Centre for Nuclear Research) is costing some 3bn Swiss francs (1.9bn euros; £1.3bn), which is paid for by contributions from Cern's member countries (including the UK) with support from international partners such as the US, Japan, China and India.
It should reinvigorate physics' biggest endeavour: a grand theory to describe all physical phenomena in nature.
About 100m below us, in a tunnel that runs in a ring for 27km (17 miles), the LHC is being assembled
from its constituent parts like a vast, impossibly complex Meccano set.
When it is switched on for a pilot run in summer 2007, this huge physics experiment will collide two beams of particles head-on at super-fast speeds, recreating the conditions in the Universe moments after the Big Bang.
The beam collisions should create showers of new particles, revealing new physics beyond the Standard Model. In order for that to happen, the LHC needs to reach much higher energies than previous colliders.
Sealed vacuum
The particle beams, composed of either protons or lead ions, will be created in Cern's existing chain of particle accelerators and then injected into the LHC. Here they will receive an additional electrical impulse to boost them up to their final energy; the equivalent of seven trillion volts.
Some 1,232 "dipole magnets" will carry these high energy beams through their interior and bend them around the LHC.
Each one undergoes a rigorous quality test before it can be lowered into the tunnel. At the Cern site known as SM18, Dr Mike Lamont, from Cern's beam operations group, shows us round the hangar-like facility where the magnets are put through their paces, 12 at a time.
The tests are run at 1.9 Kelvin (-271C), the eventual operating temperature of the LHC. This is just a shade above "absolute zero" and colder than the vacuum of outer space. The magnets are cooled to this ultra-low temperature by bathing them in liquid helium.
When helium is cooled to 2.17 Kelvin, it exhibits remarkable properties. In this "superfluid" state, it flows with almost zero viscosity and an unusually high thermal conductivity. This makes it ideal for cooling and stabilising a large superconducting system like the one at the LHC.
In 2007, Mike Lamont will be one of the machine co-ordinators "driving" the LHC: "It's a huge challenge," he says. "The magnets are one thing, but then you've got 27km of instrumentation and controls. Everything's got to be synchronised incredibly well."
At four points around the LHC ring, the two beams cross each other, causing some of the particles to collide head-on. Near each of the crossing points will sit a detector, an experiment the size of a mansion, to capture and measure new particles produced in the collisions.
The LHC's four detectors are named LHCb, Alice, Atlas and the Compact Muon Solenoid (CMS). Each is worked on by a dedicated team of physicists.
While LHCb and Alice are designed to investigate specific physical phenomena, Atlas and CMS are designated "general purpose" detectors.
They will both aim to identify the elusive Higgs boson (known as the "God particle" because of its importance to the Standard Model), look for so-called supersymmetric particles and seek out the existence of extra dimensions.
As such, the scientists working on Atlas will to some extent be competing with those on CMS. Both teams aim to be first to find the Higgs, leaving the other to "verify" their discovery
(a scientific euphemism, one suspects, for "eat humble pie").
Mass giver
The Higgs boson explains why all other particles have mass. According to the theory, particles acquire their mass through interactions with an all-pervading field, called the Higgs field, which is carried by the Higgs boson.
At Point One in Switzerland, we don hard hats, squeeze into a lift with the construction workers and descend into the enormous cavern that will house Atlas, the bigger of the two general purpose detectors.
The modules that make up this giant experiment have been built separately in laboratories around the world and transported to Cern for assembly.
"The same happens with big aircraft such as the Airbus A380," observes Dr Alan Barr, a physicist at University College London and Atlas team member. "They make all the bits separately, bring them together and it all fits.
"The thing is, we only ever build one," comments Cern scientist and Atlas team member Dr Pippa Wells. "We don't make one Atlas, then another, and find out we know what we're doing by the fifth."
"We've got one shot, it's got to work first time," says Alan Barr.
Testing time
Over the border at Cessy in France, the CMS team will soon begin testing individual "slices" of its experiment to see whether each can detect cosmic rays from space.
Much of the assembly of the CMS is taking place above ground. Large elements will then be lowered by crane into the underground cavern built to house it. This process will take about six months.
A wrangle over the supply of crystals lining the detector's electromagnetic calorimeter (which measures the energies of particles produced in collisions) has now been resolved. But part of this component won't be ready for installation in CMS by the LHC's pilot run in 2007. 
However, says team member Jim Virdee, the calorimeter will be complete and installed in the CMS by April 2008, ready for the start of the LHC's main science run. The delay won't put the CMS at a disadvantage in the race to find the Higgs, he adds, since over a year's worth of data will be needed to announce a discovery.
Cern's chief theorist Professor John Ellis even thinks finding the Higgs could shed light on another great mystery in physics: dark energy.
Dark discovery
In 1998, two teams studying supernovae showed that this dark energy is accelerating the expansion of the Universe. Subsequent work revealed that dark energy may make up about 70% of the Universe, but the best theories could not explain it.
According to John Ellis, however, the Higgs field is the perfect candidate for the source of dark energy.
"The Higgs mechanism fills all of space with a field. Unlike the gravitational field, which is strong around the Sun and the centre of the galaxy, the Higgs field would have essentially the same value everywhere,"
he explains.
"It would give you dark energy, in the sense that dark energy is energy density in empty space a long way away from any matter."
There's just a small problem with the idea, says John Ellis: it gives 120 orders of magnitude too much dark energy.
"If we find the Higgs, it would corroborate this whole theory that there's this Higgs field sitting throughout the Universe providing dark energy. Then we can get on to the next question which is why it has the value it does."
Such a breakthrough would energise debate on a "unified theory" to describe all natural phenomena. Discoveries such as supersymmetry may also bridge gaps between experimental evidence and string theory, one attempt at building a grand scheme.
The LHC might even reveal something completely unexpected about the workings of our Universe. And that, say physicists, might be even more satisfying.
No comments:
Post a Comment