COURTESY: CERN

One of the first images from the Compact Muon Solenoid, one of the six experiments of the LHC, shows the debris of particles picked up in the detector’s calorimeters and muon chambers after the first beam on September 10 was steered into the collimator (tungsten block).

SEPTEMBER 10 is a red-letter day for physicists around the world. Though a couple of years overdue in coming, the stage was set on that date for the beginning of the largest and costliest ever international scientific experiment, which was 14 years in the making, involving nearly 9,000 physicists from 60 countries and about $8 billion at the European Centre for Nuclear Research (CERN) in Geneva.

The first particle beams were injected into what will be the most powerful particle accelerator and steered around the full 26.659 km circumference of this ringed underground machine in both clockwise and anticlockwise directions. Called the Large Hadron Collider (LHC), the accelerator lies at an average depth of 100 metre in a 3.7 m diameter tunnel straddling the Swiss-French border near the Alps. To bring down the cost considerably, the LHC is reusing the tunnel that housed the previous high-energy accelerator, the Large Electron-Positron (LEP) collider. The LEP was shut down in 2000.

The instrument will cause counter-circulating beams of very high energy hadrons travelling close to the velocity of light to collide at four points of intersection of the two beams where large detectors are set up. (Hadrons are a class of sub-atomic particles to which the familiar neutrons and protons (collectively called nucleons) belong.) A collider has a big advantage over accelerators where beams strike a stationary target. In a collider, the collision energy is the sum of the energies of the two beams whereas in accelerators a beam of same energy will result in collisions of much less energy. In the first phase, the LHC will accelerate protons to peak energy of 7 tera or trillion (10{+1}{+2}) electron volt (TeV), which means that collisions will have a total energy of 14 TeV. In a couple of years, beams of positively charged lead ions (atoms stripped of negatively charged electrons) will also be made to collide at a total of 1150 TeV. These scales of energies have never before been attained in the laboratory.

In the debris of these collisions there will be a multitude of particles – known, expected and the unexpected – which will be tracked and analysed to study the behaviour of the fundamental building blocks of matter at energies that mimic in the laboratory the conditions that prevailed in the early universe. The success of the start-up procedure (with no particle collisions) was critical to ensure that all the systems worked properly, the beams did not diverge and damage the walls of the circular beam pipe and the powerful superconducting magnets that bend the beam but are well collimated and focussed. The energy of the injected beam at the start-up was only 450 GeV, about a 16th of the designed peak value of 7 TeV and less than a tenth of 5 TeV, which it is slated to achieve by the year end.

The LHC was approved by the CERN Council in December 1994. The original plan was to build the machine in two stages, first a total energy of 10 TeV and to be upgraded later to 14 TeV. But substantial contributions (amounting to a total of half a billion Swiss Francs) that came during 1995-96 from non-member states – India, Israel, Japan, the Russian federation, the United States and Turkey – led the Council to decide in December 1996 to construct a 14 TeV machine in one go. The non-member states’ agreements included financial contributions or in-kind contributions, or both. India’s contribution, both in the construction of the LHC and two of the associated experiments, is entirely in-kind, which includes hardware, software and skilled manpower, and is valued at 60.4 million Swiss Francs (about $50 m at 2001 rates).

Particle physics is today perhaps on the threshold of a revolution in understanding what the universe is made of and how it all works. Though physicists have done remarkably well in describing in increasing detail the fundamental particles and the forces of interaction with what is called the Standard Model, the basic correctness of which has been verified by high-precision experiments, it is evidently incomplete. There are many questions that the model is unable to answer. By studying particle interactions at terascale energies that are not reached in existing accelerators, the physics community expects to find answers to these as well as clues to new physics that perhaps lies beyond the model. The highest energy attained so far is in the Tevatron collider in which counter-circulating beams of protons and antiprotons (the antimatter counterpart of proton) at 0.98 TeV each collide.

What is the Standard Model and where does it falter? In the universe that the Standard Model describes, there are six quarks, which are the constituents of hadrons, and six leptons, which include electron, which interact through three forces: the strong nuclear force, which binds the nuclei, the weak nuclear force, which causes radioactivity and stars to shine and the electromagnetic force, which causes chemical reactions. The model has been tested by various experiments and it has come out with flying colours, particularly in predicting undiscovered particles. However, it does not explain the origin of mass and why some particles are heavy while others very light or have no mass at all. It endows mass to the particles through a theoretical mechanism called the Higgs mechanism. According to the theory, the whole space is filled with the “Higgs field” and particles acquire mass by interacting with this field. The catch is that the Higgs field has a particle associated with it, the Higgs boson.

The Higgs boson is one of the crucial missing links in the Standard Model and has eluded all experimental searches so far. The currently accepted estimate of the mass limits of the Higgs particle (117 GeV to 251 GeV) implies that it was just beyond the reach of Tevatron, though physicists at Fermilab are still hoping to see at least some evidence for it. But the LHC has the right energy scale to detect it if it exists and should find it even if it weighed 1 TeV. The model does not unify all the forces of nature; it does not include or explain gravity. Super-symmetry – a theory that hypothesises the existence of more massive partners to the particles of the Standard Model – could explain gravity.

The LHC should be able to find or pick up signals of these super-symmetric particles. Cosmological and astrophysical observations show that all visible matter in the universe accounts for only 4 per cent of the universe. The rest is some mysterious dark matter (23 per cent) and dark energy (73 per cent). A popular idea is that dark matter is made of neutral, yet undiscovered, super-symmetric particles.

The LHC will also help us to investigate the mystery of the universe being made only of matter and a complete absence of antimatter. If matter and antimatter were produced in equal amounts at the Big Bang, as the laws of physics tell you, how did the two separate?

In addition to proton-proton collisions, heavy-ion collisions will provide a window to the state of matter that would have existed in the early universe, called the quark-gluon plasma (QGP). When heavy ions (such as lead ions in the LHC) collide, an instant “fireball” of hot, dense matter will be formed, which can be studied at the LHC because temperatures in these fireballs will exceed 100,000 times that of the centre of the sun.

To study all the above expected phenomena, the LHC is equipped with six experiments, four large and two small, which were added subsequently: A Large Ion Collider (ALICE), ATLAS, the Compact Muon Solenoid (CMS), the Large Hadron Collider beauty (LHCb) experiment, the Large Hadron Collider forward (LHCf) experiment and the Total Elastic and Diffractive Cross-section Measurement (TOTEM) experiment.

The first four are installed in huge underground caverns at the four collision points of the LHC beams. TOTEM will be installed close to CMS and LHCf will be installed near ATLAS. Both ATLAS and CMS will look for the Higgs. ALICE is aimed at the study of QGP and LHCb will investigate the matter-antimatter question.

The LHC thus marks the ushering in of a new era of discovery. As Nobel Laureate Sheldon Glashow said in an interview (Frontline, February 1, 2008), “There are three possibilities at the LHC. No new physics whatsoever; or a Higgs and nothing new but a Higgs; or lots of new physics. I would bet on the third.” The switching-on was, therefore, a landmark event for the particle physics community around the world, which greeted the news with cheers. And for the physicists directly involved with the LHC, which included groups from several Indian institutions, it was a moment to celebrate.

It is, however, unfortunate that the significance of the event was lost in the misplaced coverage in the Indian media much of which was focussed on the baseless fear that experiments at the LHC would destroy the earth. Equally unfortunate is the total lack of dissemination of information to the public and the media by the various institutes participating in the LHC activities, notwithstanding their frequent grandiose pronouncements about public outreach.

In a message soon after start-up to LHC physicists, Robert Aymar, the director-general of CERN, said: “The star of the show was the LHC itself, and all those who have worked tirelessly over the years to ensure such a remarkably smooth transition from construction to operation. They deserve the highest praise for their professionalism and dedication…The most important news of this day is that the successful circulation of a beam of particles in each direction of LHC brings proof that all of the components have been up to the task. All systems from magnets to cryogenics, from power supplies to vacuum, from instrumentation to control, met or exceeded expectations, and the operators very rapidly demonstrated their skill in mastering this complex machine. I would like to thank all those involved.” A complex machine indeed it is. The LHC uses some of the most powerful dipoles and radiofrequency (RF) resonator cavities in existence.

The design energy of the machine is constrained only by the radius of the machine and the size of the tunnel; the magnetic field of the dipoles that keep the particles in their orbits; and other focussing quadrupole magnets, cavities that give RF kicks to the beam and accelerate it as well as keep the energy constant by compensating for energy losses and other elements of the machine. In all there are about 9,600 magnets and all of them are superconducting. Besides the large dipoles (of which there are 1,232) and quadrupoles (which number 392), there are sextupoles, octupoles and decapoles that function as trajectory corrector magnets.

The dipoles, in fact, represent the most important technological challenge in the LHC design. Being superconducting magnets, they are able to produce the high magnetic field of 8.3 Tesla over their length. The dipoles use niobium-titanium superconducting cables operating at an extremely low temperature of 1.9 K (-271.3 {+0} C), which is even lower than outer space temperature of 2.7 K. The first use of superconducting magnets was in CERN’s Intersecting Storage Rings (ISR) accelerator, which operated until 1984, but at a slightly higher temperature of 4.5 K.

It was, however, becoming clear that higher energies could be attained only with superconducting magnets at lower temperatures. The French Tokamak Tore II Supra was the first to use superconductors at 1.9 K, which was then proposed for the LHC. At 4.5 K, on the other hand, the dipoles will be able to generate a magnetic field of only 6.3 T. The coolant used in the LHC is superfluid helium II and it took six years from the concept stage to validation of these magnets.

The above requirement makes the LHC’s cryogenic system particularly special. It is the largest cryogenic system in the world and one of the coldest places on the earth. In fact, one-eighth its size will make it the world’s biggest fridge. Each of the eight sectors of the 27-km ring contains 4,700 tonnes of material. To maintain these at superfluid temperature, the LHC’s cryogenic system uses two sets of refrigerators, one to liquefy helium, which has already been cooled down to 80 K (with 10,000 tonnes of liquid nitrogen temperature) to 4.5 K (which consumes 150 kW of power) and the other to cool the liquid helium further down to 1.9 K (which consumes 20 kW) to make it superfluid. In all, the LHC will use 120 tonnes of helium, 90 tonnes of which will be used in the magnets and the rest in the pipes and refrigerators.

The protons in the LHC actually circulate in the ultra-high vacuum conditions (10{+-}{+1}{+3} bar pressure) in bunches, a direct consequence of acceleration with RF cavities, and each beam has 2,808 bunches, with each bunch containing 100 billion (10{+1}{+1}) protons. More the number of bunches, the higher the intensity or luminosity of the beam, which has a direct bearing on the number of collisions that can occur. (The LEP, for instance, had as few as four bunches). In the LHC, the bunch spacing is about 7 m; that is, a bunch will pass every 25 nanoseconds.

At peak energy, a proton beam can last for about 10 hours during which time each proton, travelling at near-light speed, will make about 11,425 circuits per second. The focussing magnets are so designed that away from collision points, bunches are a few centimetres long and a millimetre wide, but as they approach a collision point, they are squeezed to about 16 micrometre to increase the probability of proton-proton collisions. In all about 600 million collisions are likely to occur every second.

From the 600 million collisions in the LHC, after discarding uninteresting events, only about 100 per second are expected to be of interest. But even these would result in unprecedented rate of data flow. The four main experiments of the LHC represent about 150 million sensors transmitting data at about 40 million times per second.

The data flow from all the four experiments will be about 700 MB/s, which is about 15 Peta Bytes (PB) or 15 million GB a year, enough to fill 1.7 m dual layer DVDs that will stack up to 20 km high pile of disks every year. This enormous amount of data, over the next 15 years, will be accessed and analysed by thousands of physicists around the world via the World LHC Computing Grid (WLCG) network that has been established, which is now up and running.

After initial processing, CERN will distribute this data to 11 major computing centres, the so-called Tier-1 centres, in Canada, France, Germany, Italy, the Netherlands, the Nordic countries, Spain, Taipei, the United Kingdom, and two sites in the U.S. These centres in turn will make data available to 120 Tier-2 centres in over 30 countries, including India.

With this global concerted effort, glimpses of new physics certainly are what the particle physicist community expects. And with this expectation, the CERN Council has decided on a luminosity upgrade to the LHC, the Super LHC, by 2016.