World's most ambitious experiment about to start

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Purpose of the Experiment

The existence of the Higgs boson would be a significant step in the search for a Grand Unified Theory, which seeks to unify three of the four known fundamental forces: electromagnetism, the strong nuclear force and the weak nuclear force, leaving out only gravity. The Higgs boson may also help to explain why gravitation is so weak compared with the other three forces.

Higgs boson may be produced at the LHC. Here, two gluons decay into a top/anti-top pair which then combine to make a neutral Higgs (H0).

When in operation, about seven thousand scientists from eighty countries will have access to the LHC. Physicists hope to use the collider to answer the following questions:

* Is the popular Higgs mechanism for generating elementary particle masses in the Standard Model realised in nature? If so, how many Higgs bosons are there, and what are their masses?

* Will the more precise measurements of the masses of the quarks continue to be mutually consistent within the Standard Model?

* Do particles have supersymmetric ("SUSY") partners?

* Why are there apparent violations of the symmetry between matter and antimatter?

* Are there extra dimensions, as predicted by various models inspired by string theory, and can we "see" them?

* What is the nature of dark matter and dark energy?

* Why is gravity so many orders of magnitude weaker than the other three fundamental forces?

Renowned British astrophysicist Stephen Hawking has bet against the mega-experiment finding the elusive Higgs particle. "I think it will be much more exciting if we don't find the Higgs. That will show something is wrong, and we need to think again. I have a bet of $100 that we won't find the Higgs," Hawking speculated, but the experiment could discover superpartners, particles that would be supersymmetric partners to particles already known. "Their existence would be a key confirmation of string theory, and they could make up the mysterious dark matter that holds galaxies together. Whatever the LHC finds, or fails to find, the results will tell us a lot about the structure of the universe," he said.

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Test timeline

September 2008

The LHC physics program is mainly based on proton–proton collisions. The first beam was circulated through the collider on the morning of 10 September 2008. CERN successfully fired the protons around the tunnel in stages, 3 km at a time. The particles were fired in a clockwise direction into the accelerator and successfully steered around it at 10:28 am local time. The LHC successfully completed its first major test: after a series of trial runs, two white dots flashed on a computer screen showing the protons traveled the full length of the collider. It took less than one hour to guide the stream of particles around its inaugural circuit. CERN next successfully sent a beam of protons in a counterclockwise direction.

October 2008

The first high-energy collisions are planned to take place after the LHC is officially unveiled on 21 October 2008.

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Cost of the Experiment

The total cost of the project is anticipated to be € 3.2–6.4 billion. The construction of LHC was approved in 1995 with a budget of 2.6 billion Swiss francs (€ 1.6 billion), with another 210 million francs (€ 140 million) towards the cost of the experiments. However, cost over-runs, estimated in a major review in 2001 at around 480 million francs (€ 300 million) for the accelerator, and 50 million francs (€ 30 million) for the experiments, along with a reduction in CERN's budget, pushed the completion date from 2005 to April 2007. The superconducting magnets were responsible for 180 million francs (€ 120 million) of the cost increase. There were also engineering difficulties encountered while building the underground cavern for the Compact Muon Solenoid, in part due to faulty parts loaned to CERN by fellow laboratories Argonne National Laboratory and Fermilab.

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Operational safety

The size of the LHC constitutes an exceptional engineering challenge with unique operational issues on account of the huge energy stored in the magnets and the beams. While operating, the total energy stored in the magnets is 10 GJ (equivalent to 2.4 tons of TNT) and the total energy carried by the two beams reaches 724 MJ.

Loss of only one ten-millionth part (10−7) of the beam is sufficient to quench a superconducting magnet, while the beam dump must absorb an energy equivalent to that of a typical air-dropped bomb. These immense energies are even more impressive considering how little matter is carrying it: under nominal operating conditions (2,808 bunches per beam, 1.15×10 11 protons per bunch), the beam pipes contain 1.0×10-9 gram of hydrogen, which, in standard conditions for temperature and pressure, would fill the volume of one grain of fine sand.

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Superconductivity

Superconductivity was discovered in 1911, a practical superconducting electromagnet had to await the discovery of superconductors that could stand high magnetic fields. The first successful superconducting magnet was built by George Yntema in 1954 using Niobium wire and achieved a field of 0.71 T at 4.2 K. Widespread interest was sparked by Kunzler's 1961 discovery of the advantages of niobium-tin as a high Hc, high current winding material. In 2007 a magnet with windings of YBCO achieved a world record field of 26.8 Tesla.

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Superconducting magnet

A superconducting magnet is an electromagnet that is built using superconducting coils. They must be cooled to cryogenic temperatures during operation. Their advantages are that they can produce stronger fields than ordinary iron-core electromagnets, and can be cheaper to operate, since no power is lost to ohmic resistance in the windings.

Liquid helium is used as a coolant for superconducting windings with critical temperatures around its boiling point of 4.2 K. The magnet and coolant are contained in a thermally insulated container (dewar) called a cryostat. To keep the helium from boiling away, the cryostat is usually constructed with an outer jacket containing (significantly cheaper) liquid nitrogen at 77 K.

The superconducting portions of most such magnets are composed of niobium-titanium. This material has critical temperature of 10 Kelvins and remains in this state until about 15 Teslas. More expensive magnets can be made of niobium-tin (Nb3Sn). These have a Tc of 18 K. When operating at 4.2 K they are able to withstand a much higher magnetic field intensity, up to 25 to 30 Teslas. Unfortunately, it is far more difficult to make the required filaments from this material. This is why sometimes a combination of Nb3Sn for the high field sections and Nb3Ti for the lower field sections is used. High temperature superconductors (BSCCO or YBCO) may be used for high-field inserts when magnetic fields are required which are higher than Nb3Sn can manage. BSCCO, YBCO or magnesium diboride may also be used for current leads, conducting high currents from room temperature into the cold magnet without an accompanying large heat leak.

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Use of superconducting magnet

Superconducting magnets have a number of advantages over resistive electromagnets. They can achieve an order of magnitude stronger field than ordinary ferromagnetic-core electromagnets, which are limited to fields of around 2 T. The field is generally more stable, resulting in less noisy measurements. They can be smaller, allowing more freedom in the configuration of the rest of the device (such as the cryostat), and for large magnets consume much less power - in fact, power consumption is negligible in the steady field state. Higher fields, however can be achieved with special cooled resistive and hybrid magnets, as the superconducting coils will enter the normal (non-superconducting) state at high fields.

Superconducting magnets are widely used in MRI machines, and also in bubble chamber magnets and particle accelerator magnets, and soon in tokamak fusion reactors.

One of the most challenging use of SC magnets is in the LHC particle accelerator. The niobium-titanium (Nb-Ti) magnets will operate at 1.9 K to allow them to run safely at 8.3 T. Each magnet will store 7 MJ. In total the magnets will store 10.4 GJ. Once or twice a day, as the protons are accelerated from 450 GeV to 7 TeV, the field of the superconducting bending magnets will be increased from 0.54 T to 8.3 T.

The central solenoid and toroidal field superconducting magnets designed for the ITER fusion reactor use niobium-tin (Nb3Sn) as a superconductor. The Central Solenoid coil will carry 46 kA and produce a field of 13.5 Tesla. The 18 Toroidal Field coils at max field of 11.8 T will store 41 GJ. They have been tested at a record 80 kA. Other lower field ITER magnets (PF and CC) will use niobium-titanium. Most of the ITER magnets will have their field varied many times per hour.

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