Large Hadron Collider

Dr S M Paranjape

ON September 10, 2008 the whole world was agog with the excitement over the switching on of the largest man-made �atom-smasher� that has been built to date. That is when the �Large Hadron Collider� (LHC) was first �started� and produced the fastest circulating proton beam. Its commencement which is scheduled for October will enable physicists and scientists all over the globe to carry out experiments that will provide clues to some of the deepest questions which have arisen in physics.

Is the excitement justified, or is it just media hype? Although there is an element of the latter (considering that the �news� vanished from the front pages and �breaking news� strips within two days!) there are some things every person will want to know: Where is this mammoth �machine� and who built it? What role has India played in its construction and operation? And above all what are the questions that scientists are expecting answers to, from the enormous volumes of data that is expected to flow from the LHC once it is fully functional? These are some of the issues that we will take up in this article and also provide some basic information about the accelerator.

But, first, it should be made absolutely clear that some persons acting either out of sheer ignorance or worse, malice, painted doomsday scenarios associated with the switching on date. �The world will come to an end�, �A black hole is going to be created that will swallow the earth,� are some of the rumours circulated through the internet and media, which were unfortunately taken seriously by the gullible. Absolutely nothing of the sort has happened, nor was it expected to happen � definitely not on September 10. In India, just as people stay indoors on days of solar eclipse, many locked themselves in at Bhubaneshwar, and other places.

ENERGY ACHIEVED BY THIS MACHINE

Let us return to the main theme. The word �Hadron� in Large Hadron Collider refers to the species of elementary particles akin to those that reside in the nuclei of atoms, namely, protons and neutrons. Of these the protons are positively charged while neutrons are electrically neutral. A nucleus of Helium called as alpha particle has two of each packed into a tiny volume less than a billionth of a cubic metre. The electrical repulsion between the protons would have caused this nucleus and all heavier nuclei to burst. But this does not happen because of the �strong interaction force� which exists between protons and neutrons. These and others that interact strongly like them are called �Hadrons�.

Atoms have these positively charged nuclei as their core and are electrically neutral because of negatively charged electrons circling them. The simplest of these is the Hydrogen atom, with just one proton in the core and one electron surrounding it. Their binding is sufficiently strong so that ordinary matter is completely neutral electrically � otherwise we would be feeling electrical shocks all the time, as it happens sometimes on dry days. The separation of an electron from the proton inside the Hydrogen atom requires energy to be supplied from outside. This is often called chemical energy associated with chemical reactions, and is measured in �electron-volts�, abbreviated as �eV�. It takes about 14eV of energy to separate the electron from the nucleus of the Hydrogen atom. Chemical dissociation and combinations typically involve energy exchange of 5 to 15 electron volts. The Large Hadron Collider will accelerate protons in a huge ring to energy of 7 trillion electron volts (TeV)! And there will be two such giant rings which will intersect each other. This will cause the protons to collide head-on to produce energy of 14 TeV that is a trillion times the dissociation energy of hydrogen.

WHY BUILD SUCH HIGH ENERGY MACHINES?

Several developments took place on the theoretical as well as experimental front during the 1960s and early 1970s which spurred on the construction of high energy accelerators designed to cause collisions of such tremendous force as to expose the inner structure of hadrons. The Linear Accelerator at Stanford, USA (called SLAC) led to the discovery of �scaling� and the formulation of the quark-parton model. On the other hand construction of the Bevatron at Fermilab and study of electron-positron annihilations immensely enriched our understanding of the microworld. The theories formulated during this period gave rise to a compelling description of the unified nature of three of the main forces which govern matter at very small distances and very high speeds � the relativistic microparticles. These are: Electromagnetic, Weak and Strong Interactions.

Only the first of these, namely �electromagnetic interactions� lies in the familiar domain of everyday experience. Indeed, modern human society is inconceivable without electricity and its constant companion, magnetism. Light from the sun, ultraviolet radiation, X-rays, TV & radio signals are all examples of electromagnetic radiation. The recent debate over the India-US nuclear deal and viability of nuclear energy has again brought into focus �Nuclear Reactors�, nuclear fuel and similar concepts. These depend very much on the �Strong� nuclear force. Moreover, the destruction unleashed by this force in Hiroshima and Nagasaki, which is the only instance when nuclear weapons were ever used (by the United States at the end of World War II), will never be forgotten by mankind. The third member of this trilogy of forces � �Weak Interactions� � is a more subtle force which was discovered by Madame Curie at the end of the nineteenth century during the study of radioactivity. But even these processes have been pressed into service due to the progress in medical science, especially radiation therapy for treatment of cancer.

UNIFICATION OF WEAK AND ELECTROMAGNETIC FORCES & PHILOSOPHICAL ISSUES

At first sight there seems to be nothing in common in these three interactions. In fact, until the 1970s even professional physicists had no clue that they would have common ground. Physicists Steven Weinberg, Abdus Salaam (who hailed from Pakistan), and Sheldon Glashow, shared the Nobel Prize for their path breaking research papers which showed how the �Weak� and �Electromagnetic� interactions were two sides of the same coin. Their work became the foundation for the unification of all the three � Weak, Electromagnetic and Strong!

�Electromagnetism� itself is not a stand-alone entity, but a concrete unification of seemingly very diverse forces of electricity and magnetism (which had been achieved by James Clerk Maxwell in the 1870s), Weinberg, Salaam and Glashow demonstrated after a gap of 100 years the unification of the Weak and Electromagnetic forces. However, these theoretical models predicted the existence of new types of elementary particles called Intermediate Vector Bosons (IVB�s) whose masses were expected to be much larger � up to 1000 times larger � than what had been observed in the entire array of particles with strong, weak and electromagnetic interactions. This too became a key factor in the necessity to build very high energy machines.

The theory of unified weak and electromagnetic interactions received a boost when the predicted IVB�s were found in experiments conducted at the Fermilab Bevatron. This generation of accelerators with capability of several hundred Billion electron volts (BeV�s) also gave corroboration of the quark structure of strongly interacting particles.

At the turn of the 19th century positivists and idealists had pounced upon the electromagnetic nature of matter to pronounce the death of philosophical materialism. Comrade Lenin totally debunked this lie in his famous treatise �Materialism and Empirio-Criticism�. He went on to add that materialism is not based on a particular form of matter � be it electromagnetic or something else. The only property of matter that is relevant to it philosophically is that it exists independent of consciousness! He also went on to elaborate the infinity of matter, in terms of its forms and its extent. The discovery of still further forms of matter and their unity fully corroborates Lenin�s insightful observations. We will come back to the manner in which progression of science corroborates scientific materialist dialectics later in this article.

The paradigm of unification requires for its intricate theoretical structure a method devised in the 1960s by British physicist Peter Higgs and named after him as the �Higgs mechanism�. This makes it possible for other (observed) particles to acquire their mass through their interactions. The web of elementary particles thereby becomes interdependent and self-sustaining. However, the working out of this mechanism necessitates the existence of particles of still higher mass � including the Higgs particle � which would have been inaccessible even to the highest energy machines which were being contemplated at that time, namely the Tevatron, or the Trillion electron Volt accelerator.

THE LARGE HADRON COLLIDER � WHAT IS IT?

This was one of the principal motivating factors behind the conception of the Large Hadron Collider which would be an enormous ramp up of the existing Interacting Storage Ring facility at CERN. Two beams of particles are made to collide with each other in chambers where the rings intersect. This leads to doubling of the available energy, but also leads to a considerable increase in the complexity of design and execution. But the enormous success of the ISRs at CERN was clearly behind the proposal to build the next level of �Colliding beams� that was mooted in the last decade of the 20th century. This design has the advantage of achieving double the energy of conventional (Femilab type) accelerating machines in which a beam hits a stationary target. We now reproduce some material from Wikipedia to shed light on the LHC.

�The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator complex, intended to collide opposing beams of protons charged with approximately 7 TeV of energy. Its main purpose is to explore the validity and limitations of the Standard Model (refinement of Weinberg-Salaam-Glashow model to include also the strong interactions), the current theoretical picture for particle physics. It is theorized that the collider will produce the Higgs boson, the observation of which could confirm the predictions and missing links in the Standard Model, and could explain how other elementary particles acquire properties such as mass�

�The LHC was built by the European Organisation for Nuclear Research (CERN), and lies underneath the Franco-Swiss border near Geneva, Switzerland. It is funded by and built in collaboration with over eight thousand physicists from over eighty-five countries as well as hundreds of universities and laboratories. The LHC is already operational and is presently in the process of being prepared for collisions. The first beams were circulated through the collider on September 10, 2008, and the first high-energy collisions

are planned to take place after the LHC is officially unveiled on October 21, 2008�

The accelerator chain of the Large Hadron Collider (LHC) is depicted below (followed by some technical details about its construction)

HOW DOEST IT WORK?

The collider is contained in a circular tunnel with a circumference of 27 kilometres (17 mi) at a depth ranging from 50 to 175 metres underground. It crosses the border between Switzerland and France at four points, but most of it is in France. Surface buildings hold ancillary equipment such as compressors, ventilation equipment, control electronics and refrigeration plants. The collider tunnel contains two adjacent beam pipes, each containing a proton beam � remember a proton is one type of hadron. The two beams travel in opposite directions around the ring. Some 1,232 dipole magnets keep the beams on their circular path, while an additional 392 quadrupole magnets are used to keep the beams focused, in order to maximise the chances of interaction between the particles in the four intersection points, where the two beams will cross. In total, over 1,600 superconducting magnets are installed, with most weighing over 27 tonnes. All this will be cooled with liquid helium and liquid nitrogen, making it the coldest and loneliest place on earth. The Hindustan Times reported on September 11, 2008 that Indian laboratories and scientists accounted for 1848 tracking magnets, 2 liquid nitrogen tanks and ancillary equipment. In addition, 200 Indian scientists have clocked in 41 man years for the historic event! A significant part of the software support also has been provided by Indian scientists.

WHAT DO WE EXPECT TO FIND?

When operational, the LHC will have protons circulating at 99.999999 per cent of light speed that is about 11,000 revolutions per second! The LHC will also be used to collide lead (which has over two hundred hadrons!) (Pb) heavy ions with collision energy of 1,150 TeV. The Pb ions will be first accelerated by the linear accelerator Linac 3; the ions will then be further accelerated by the PS and SPS before being injected into LHC ring, where they will reach energy of 2.76 TeV per nucleon. This is why the LHC will be the highest energy accelerator for many years to come.

Six detectors are being constructed at the LHC, located underground in large caverns excavated at the LHC's intersection points. Two of them, the ATLAS experiment and the Compact Muon Solenoid (CMS), are large, general purpose particle detectors. "A Large Ion Collider Experiment" (ALICE) is designed to study the properties of quark-gluon plasma from the debris of heavy-ion collisions.

This last prospect is pregnant with many possibilities, and is also the most controversial, philosophically. The energy concentration which will be achieved when beams of lead ions collide in tiny volumes is expected to simulate conditions of the hypothesised �primordial� quark-gluon soup. This will be discussed in the sequel to the present article.

When activated, it is theorised that the collider will produce the elusive Higgs boson. The verification of 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.