The Large Hadron Collider
Our understanding of the Universe is about to change...
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.
Why the LHC
A few unanswered questions...
The LHC was built to help scientists to answer key unresolved
questions in particle physics. The unprecedented energy it achieves may
even reveal some unexpected results that no one has ever thought of!
For the past few decades, physicists have been able to describe
with increasing detail the fundamental particles that make up the
Universe and the interactions between them. This understanding is
encapsulated in the
Standard Model
of particle physics, but it contains gaps and cannot tell us the whole
story. To fill in the missing knowledge requires experimental data, and
the next big step to achieving this is with LHC.
Newton's unfinished business...
What is mass?
What is the origin of mass? Why do tiny particles weigh the
amount they do? Why do some particles have no mass at all? At present,
there are no established answers to these questions. The most likely
explanation may be found in the
Higgs boson,
a key undiscovered particle that is essential for the Standard Model to
work. First hypothesised in 1964, it has yet to be observed.
The
ATLAS and
CMS experiments will be actively searching for signs of this elusive particle.
An invisible problem...
What is 96% of the universe made of?
Everything we see in the Universe, from an ant to a galaxy, is
made up of ordinary particles. These are collectively referred to as
matter, forming 4% of the Universe.
Dark matter and dark energy
are believed to make up the remaining proportion, but they are
incredibly difficult to detect and study, other than through the
gravitational forces they exert. Investigating the nature of dark matter
and dark energy is one of the biggest challenges today in the fields of
particle physics and cosmology.
The
ATLAS and
CMS experiments will look for supersymmetric particles to test a likely hypothesis for the make-up of dark matter.
Nature's favouritism...
Why is there no more antimatter?
We live in a world of matter – everything in the Universe, including ourselves, is made of matter.
Antimatter
is like a twin version of matter, but with opposite electric charge. At
the birth of the Universe, equal amounts of matter and antimatter
should have been produced in the Big Bang. But when matter and
antimatter particles meet, they annihilate each other, transforming into
energy. Somehow, a tiny fraction of matter must have survived to form
the Universe we live in today, with hardly any antimatter left. Why does
Nature appear to have this bias for matter over antimatter?
The
LHCb
experiment will be looking for differences between matter and
antimatter to help answer this question. Previous experiments have
already observed a tiny behavioural difference, but what has been seen
so far is not nearly enough to account for the apparent
matter–antimatter imbalance in the Universe.
Secrets of the Big Bang
What was matter like within the first second of the Universe’s life?
Matter, from which everything in the Universe is made, is
believed to have originated from a dense and hot cocktail of fundamental
particles. Today, the ordinary matter of the Universe is made of
atoms, which contain a nucleus composed of protons and neutrons, which
in turn are made of quarks bound together by other particles called
gluons. The bond is very strong, but in the very early Universe
conditions would have been too hot and energetic for the gluons to hold
the quarks together. Instead, it seems likely that during the first
microseconds after the Big Bang the Universe would have contained a
very hot and dense mixture of quarks and gluons called quark–gluon
plasma.
The
ALICE
experiment will use the LHC to recreate conditions similar to those
just after the Big Bang, in particular to analyse the properties of the
quark-gluon plasma.
Hidden worlds…
Do extra dimensions of space really exist?
Einstein showed that the three dimensions of space are related to time. Subsequent theories propose that further
hidden dimensions
of space may exist; for example, string theory implies that there are
additional spatial dimensions yet to be observed. These may become
detectable at very high energies, so data from all the detectors will be
carefully analysed to look for signs of extra dimensions.
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 the
particle detectors.
Heavy-ion physics at 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
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.
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