The Large Hadron Collider.... LHC

The Large Hadron Collider --- LHC

Our understanding of the Universe is about to change...

 Sulekha Rani R,PGT Chemistry,KV NTPC Kayamkulam 

 

 

The Large Hadron Collider (LHC) is a gigantic scientific instrument near Geneva, where it spans the border between Switzerland and France about 100m underground. It is a particle accelerator used by physicists to study the smallest known particles – the fundamental building blocks of all things. It will revolutionise our understanding, from the minuscule world deep within atoms to the vastness of the Universe.

Two beams of subatomic particles called "hadrons" – either protons or lead ions – travel in opposite directions inside the circular accelerator, gaining energy with every lap. Physicists use the LHC to recreate the conditions just after the Big Bang, by colliding the two beams head-on at very high energy. Teams of physicists from around the world then analyse the particles created in the collisions using special detectors in a number of experiments dedicated to the LHC.

There are many theories as to what will result from these collisions. For decades, the Standard Model of particle physics has served physicists well as a means of understanding the fundamental laws of Nature, but it does not tell the whole story. Only experimental data using the high energies reached by the LHC can push knowledge forward, challenging those who seek confirmation of established knowledge, and those who dare to dream beyond the paradigm

 

 

How the LHC works

The LHC, the world’s largest and most powerful particle accelerator, is the latest addition to CERN’s accelerator complex. It mainly consists of a 27-kilometre ring of superconducting magnets with a number of accelerating structures to boost the energy of the particles along the way.

Inside the accelerator, two beams of particles travel at close to the speed of light with very high energies before colliding with one another. The beams travel in opposite directions in separate beam pipes – two tubes kept at ultrahigh vacuum. They are guided around the accelerator ring by a strong magnetic field, achieved using superconducting electromagnets. These are built from coils of special electric cable that operates in a superconducting state, efficiently conducting electricity without resistance or loss of energy. This requires chilling the magnets to about ‑271°C – a temperature colder than outer space. For this reason, much of the accelerator is connected to a distribution system of liquid helium, which cools the magnets, as well as to other supply services.

Thousands of magnets of different varieties and sizes are used to direct the beams around the accelerator. These include 1232 dipole magnets of 15m length which are used to bend the beams, and 392 quadrupole magnets, each 5–7m long, to focus the beams. Just prior to collision, another type of magnet is used to "squeeze" the particles closer together to increase the chances of collisions. The particles are so tiny that the task of making them collide is akin to firing needles from two positions 10km apart with such precision that they meet halfway!

All the controls for the accelerator, its services and technical infrastructure are housed under one roof at the CERN Control Centre. From here, the beams inside the LHC are made to collide at four locations around the accelerator ring, corresponding to the positions of theparticle detectors.

 Model of an LHC superconducting dipole magnet

 

Heavy-ion... at the LHC

 Preparation for injection in the LINAC 3 of the LEAD source material used to create heavy ions for the LHC.

 

In the LHC heavy-ion programme, beams of heavy nuclei ("ions") collide at energies up to 30 times higher than in previous laboratory experiments. In these heavy-ion collisions, matter is heated to more than 100,000 times the temperature at the centre of the Sun, reaching conditions that existed in the first microseconds after the Big Bang. The aim of the heavy-ion programme at the LHC is to produce this matter at the highest temperatures and densities ever studied in the laboratory, and to investigate its properties in detail. This is expected to lead to basic new insights into the nature of the strong interaction between fundamental particles.

The strong interaction is the fundamental force that binds Nature's elementary particles, called quarks, into bigger objects such as protons and neutrons, which are themselves the building blocks of the atomic elements. Much is known today about the mechanism with which the elementary force-carriers of the strong interaction, the gluons, bind quarks together into protons and neutrons. However, two aspects of the strong interaction remain particularly intriguing.

First, no quark has ever been observed in isolation: quarks and gluons seem to be confined permanently inside composite particles, such as protons and neutrons. Second, protons and neutrons contain three quarks, but the mass of these three quarks accounts for only one percent of the total mass of a proton or neutron. So while the Higgs mechanism could give rise to the masses of the individual quarks, it cannot account for most of the mass of ordinary matter.

The current theory of strong interactions, called quantum chromodynamics, predicts that at very high temperatures, quarks and gluons are deconfined and can exist freely in a new state of matter known as the quark-gluon plasma. Theory also predicts that at the same temperature, the mechanism that is responsible for giving composite particles most of their mass ceases to act.

In the LHC heavy-ion programme, three experiments – ALICE, ATLAS and CMS – aim to produce and study this extreme, high-temperature phase of matter and provide novel access to the question of how most of the mass of visible matter in the Universe was generated in the first microseconds after the Big Bang.

The LHC experiments

 A worker inside the LHC tunnel

 

 

The six experiments at the LHC are all run by international collaborations, bringing together scientists from institutes all over the world. Each experiment is distinct, characterised by its unique particle detector.

The two large experiments, ATLAS and CMS, are based on general-purpose detectors to analyse the myriad of particles produced by the collisions in the accelerator. They are designed to investigate the largest range of physics possible. Having two independently designed detectors is vital for cross-confirmation of any new discoveries made.

Two medium-size experiments, ALICE and LHCb, have specialised detectors for analysing the LHC collisions in relation to specific phenomena.

Two further experiments, TOTEM and LHCf, are much smaller in size. They are designed to focus on "forward particles" (protons or heavy ions). These are particles that just brush past each other as the beams collide, rather than meeting head-on.

The ATLAS, CMS, ALICE and LHCb detectors are installed in four huge underground caverns located around the ring of the LHC. The detectors used by the TOTEM experiment are positioned near the CMS detector, whereas those used by LHCf are near the ATLAS detector.