Understanding the Universe
Friday, March 31, 2023

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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.


Journey to a New Frontier:

The LHC accelerator was originally conceived in the 1980s and approved for construction by the CERN Council in late 1994. Turning this ambitious scientific plan into reality proved to be an immensely complex task.

Civil engineering work to excavate underground caverns to house the huge detectors for the experiments started in 1998. Five years later, the last cubic metre of ground was finally dug for the whole project.

Numerous state-of-the-art technologies were pushed even further to meet the accelerator's exacting specifications and unprecedented demands. Anticipating the colossal amount of data the LHC's experiments would produce (nearly 1% of the world's information production rate), a new approach to data storage, management, sharing and analysis was created in the LHC Computing Grid project.  For more than a decade, building the enormous particle accelerator on the Swiss-French border had been a dream for many who have worked hard to bring it to completion. Finally we can retell the story of this adventure in a journey, from a dream to a reality.

How an Accelerator Works:

Accelerators were invented to provide energetic particles to investigate the structure of the atomic nucleus. Since then, they have been used to investigate many aspects of particle physics. Their job is to speed up and increase the energy of a beam of particles by generating electric fields that accelerate the particles, and magnetic fields that steer and focus them.


The linear accelerator at SLAC, StanfordAn accelerator comes either in the form of a ring (circular accelerator), where a beam of particles travels repeatedly round a loop, or in a straight line (linear accelerator), where the beam travels from one end to the other. A number of accelerators may be joined together in sequence to reach successively higher energies, as at the accelerator complex at CERN.

The Main Components of an Accelerator Include:

Radiofrequency (RF) cavities and electric fields – these provide acceleration to a beam of particles. RF cavities are located intermittently along the beam pipe. Each time a beam passes the electric field in an RF cavity, some of the energy from the radio wave is transferred to the particles.

Vacuum chamber – this is a metal pipe (also known as the beam pipe) inside which a beam of particles travels. It is kept at an ultrahigh vacuum to minimise the amount of gas present to avoid collisions between gas molecules and the particles in the beam.

Magnets – various types of magnets are used to serve different functions. For example, dipole magnets are usually used to bend the path of a beam of particles that would otherwise travel in a straight line. The more energy a particle has, the greater the magnetic field needed to bend its path. Quadrupole magnets are used to focus a beam, gathering all the particles closer together (similar to the way that lenses are used to focus a beam of light).  Collisions at accelerators can occur either against a fixed target, or between two beams of particles. Particle detectors are placed around the collision point to record and reveal the particles that emerge from the collision.

An Accelerator at Home:

A cathode ray tube (CRT) television set has the basic features of CERN's accelerators. A filament inside the glass vacuum tube of the television set acts as a source of particles. When the filament is heated, electrons are set free by the increase in energy. The electrons are accelerated and guided through the vacuum of the CRT by an electromagnetic field, generated by a coil of wires. The television screen acts as a particle detector. As the high-energy electrons hit the back of the screen, they are detected and made visible in the colour pixels that make up the image.


The LHC Experiments:

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.