Large Hadron Collider (LHC), is the world’s most powerful particle accelerator. The LHC was constructed by the European Organization for Nuclear Research (CERN) in the same 27-km (17-mile) tunnel that housed its Large Electron-Positron Collider (LEP). The tunnel is circular and is located 50–175 metres (165–575 feet) below ground, on the border between France and Switzerland. The LHC ran its first test operation on Sept. 10, 2008. An electrical problem in a cooling system on September 18 resulted in a temperature increase of about 100 °C (180 °F) in the magnets, which are meant to operate at temperatures near absolute zero (−273.15 °C, or −459.67 °F). Early estimates that the LHC would be quickly fixed soon turned out to be overly optimistic; it restarted on Nov. 20, 2009. Shortly thereafter, on Nov. 30, 2009, it supplanted the Fermi National Accelerator Laboratory’s Tevatron as the most powerful particle accelerator, when it boosted protons to energies of 1.18 teraelectron volts (TeV; 1 × 10¹² electron volts). In March 2010 scientists at CERN announced that a problem with the design of superconducting wire in the LHC required that the collider could only run at half-energy (7 TeV) until the end of 2011. The LHC is scheduled to be shut down in 2012 to fix the problem and is expected to run at its full energy of 14 TeV in 2013.

The heart of the LHC is a ring that runs through the circumference of the LEP tunnel; the ring is only a few centimetres in diameter, evacuated to a higher degree than deep space and cooled to within two degrees of absolute zero. In this ring, two counter-rotating beams of heavy ions or protons are accelerated to speeds within one millionth of a percent of the speed of light. (Protons belong to a category of heavy subatomic particles known as hadrons, which accounts for the name of this particle accelerator.) At four points on the ring, the beams can intersect and a small proportion of particles crash into each other. At maximum power, collisions between protons will take place at a combined energy of up to 14 TeV, about seven times greater than has been achieved previously. At each collision point are huge magnets weighing tens of thousands of tons and banks of detectors to collect the particles produced by the collisions.

The project took a quarter of a century to realize; planning began in 1984, and the final go-ahead was granted in 1994. Thousands of scientists and engineers from dozens of countries were involved in designing, planning, and building the LHC, and the cost for materials and manpower was nearly $5 billion; this does not include the cost of running experiments and computers.

One goal of the LHC project is to understand the fundamental structure of matter by recreating the extreme conditions that occurred in the first few moments of the universe according to the big bang model. For decades physicists have used the so-called standard model for fundamental particles, which has worked well but has weaknesses. First, and most important, it does not explain why some particles have mass. In the 1960s British physicist Peter Higgs postulated a particle that had interacted with other particles at the beginning of time to provide them with their mass. The Higgs particle has never been observed—it should be produced only by collisions in an energy range not available for experiments before the LHC. Second, the standard model requires some arbitrary assumptions, which some physicists have suggested may be resolved by postulating a further class of supersymmetric particles—these might be produced by the extreme energies of the LHC. Finally, examination of asymmetries between particles and their antiparticles may provide a clue to another mystery: the imbalance between matter and antimatter in the universe.

As with all groundbreaking experiments, the most exciting results may well be unexpected ones. As British physicist Stephen Hawking said, “It is more exciting if we don’t find the Higgs. That will show that something is wrong and we need to think again.”

By David G.C. Jones

Large Hadron Collider finds new variant of particle – 24.12.2011

The Large Hadron Collider (LHC), famously engaged in the quest for the Higgs boson, has turned up a heavier variant of a sub-atomic particle first discovered a quarter-century ago, scientists reported on 23.12.2011.

Newcomer

The newcomer is called Chi-b(3P), which was uncovered in the debris from colliding protons, according to research published in the open-access online journal arxiv. Like the elusive Higgs and the photon, it is a boson, meaning it is a particle that carries force. But while the Higgs is not believed to be made of smaller particles, the Chi-b(3) comprises two relatively heavy particles, the beauty quark and its anti-quark. They are bonded by the so-called “strong” force which also causes the atomic nucleus to stick together. The Chi-b(3P) is a heavier version of a particle that was first observed around 25 years ago. “The Chi-b(3P) is a particle that was predicted by many theorists, but was not observed at previous experiments,” said James Walder, a British physicist quoted by the University of Birmingham in a press release. Described by some as the world’s largest machine, the LHC is located in a 27-km ring-shaped tunnel near Geneva that straddles the Franco-Swiss border up to 580 feet below ground. Streams of protons are fired in opposite, but parallel, directions in the tunnel.

Powerful magnets

The beams are then bent by powerful magnets so that some of the protons collide in four giant labs, which are lined with detectors to record the sub-atomic debris that results. On December 13, physicists at the European Organisation for Nuclear Research (CERN) said they had narrowed the search for the Higgs — the so-called “God particle” that may confer mass. The theory behind the Higgs is that mass does not derive from particles themselves. Instead, it comes from a boson that interacts strongly with some particles but less, if at all, with others. Finding the Chi-b(3P) is a further test of the powers of the LHC, which became the world’s biggest particle collider when it was completed in 2008. “Our new measurements are a great way to test theoretical calculations of the forces that act on fundamental particles, and will move us a step closer to understanding how the Universe is held together,” said Miram Watson, a British research fellow working on the CHi-b(3) investigation.

Holy grail of particle physics – The Hindu Editorial dt 24.12.2011

Physicists like to describe nature in the simplest and most elegant theoretical framework. A significant development towards such a description of the sub-atomic world during the 1970s was the unification of disparate forces of nature (excluding gravity) in a single theoretical framework. But this unification came at a price. The elegant mathematical symmetry that made it possible required all elementary particles to be massless, which is not the real world we know. So the underlying universal symmetry had to be ‘broken’ to some degree for particles to have a range of masses and forces to have different strengths, and yet described by a single theory. This was achieved through the introduction of a hypothetical particle called Higgs — after Peter Higgs who proposed it — and an associated force. One can imagine Higgs as an all-pervasive ether-like force-field, which endows particles with mass (or inertia) because of the drag that the field exerts as particles move through space. This model of the universe, called the Standard Model (SM), seems to be along the right lines; this is so particularly after the discovery during the 1980s of particles W and Z — the carriers of the weak nuclear force just as the massless photon carries the electromagnetic force — with masses exactly as predicted by SM (about 100 times the mass of the proton). Since then the model has held up superbly in experimental tests. But Higgs itself has remained elusive and is the only missing piece in this otherwise enormously successful theory. The model itself does not predict a mass for the Higgs. So, for the last three decades physicists have been combing the entire energy ranges available to particle accelerators for signatures of Higgs in the decays of particles produced in these high-energy particle collisions. With the advent in 2009 of the highest energy accelerator, the Large Hadron Collider (LHC) at CERN, Geneva, which opened up a new energy domain, hopes of discovering Higgs have been high.

A discovery in particle physics is a long, painstaking process. It requires sifting through data related to trillions of collisions and picking out a statistically significant signal that stands out as an excess over the background of events from other processes that mimic a decaying Higgs. Two entirely independent experiments at CERN, ATLAS and CMS, have seen an excess of events that are attributable to Higgs. By summer, these experiments had excluded vast regions of mass where Higgs could exist, leaving just a narrow window. The latest results, announced on December 13, have squeezed the window further to around 125 times the mass of a proton. Since two independent experiments have arrived at the same conclusions, these are tantalising signals — but not good enough to be called a discovery. At present there is just about one per cent chance of the excess being due to fluctuations in the background. The golden rule for discovery in particle physics is that such a chance should be less than one in a million. A definitive statement on the existence or non-existence of Higgs requires more LHC data running through 2012. If Higgs does not show up even then, there will be an upheaval in the current understanding of the sub-atomic world, with the crucial question on the origin of mass remaining unanswered. But that, as we have seen before in the history of physics, is only likely to throw up even more revolutionary ideas.

Dream Dare Win

www.jeywin.com

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