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 km 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 15 m length which are used to bend the beams, and 392 quadrupole magnets, each 5–7 m 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 10 km 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 will be made to collide at four locations around the accelerator ring, corresponding to the positions of the
particle detectors.
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 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.
LHC Computing Grid
The Large Hadron Collider will produce roughly 15 petabytes (15 million gigabytes) of data annually – enough to fill more than 1.7 million dual-layer DVDs a year!
Thousands of scientists around the world want to access and analyse this data, so CERN is collaborating with institutions in 33 different countries to operate a distributed computing and data storage infrastructure: the LHC Computing Grid (
LCG).
Data from the LHC experiments is distributed around the globe, with a primary backup recorded on tape at CERN. After initial processing, this data is distributed to eleven large computer centres – in Canada, France, Germany, Italy, the Netherlands, the Nordic countries, Spain, Taipei, the UK, and two sites in the USA – with sufficient storage capacity for a large fraction of the data, and with round-the-clock support for the computing grid.
These so-called “Tier-1” centres make the data available to over 120 “Tier-2” centres for specific analysis tasks. Individual scientists can then access the LHC data from their home country, using local computer clusters or even individual PCs.
The LCG collaborates closely with the other CERN grid projects:
The LHC Computing Grid has been the driving force behind the European multi-science grid Enabling Grids for E-SciencE (
EGEE), which continues to grow in size and diversity of usage. EGEE currently involves more than 240 institutions in 45 countries, supporting science in more than 20 disciplines, including bioinformatics, medical imaging, education, climate change, energy, agriculture and more.
CERN openlab: The LCG project also works with industry, in particular through the CERN openlab, where leading IT companies are testing and validating cutting-edge grid technologies using the LCG environment.
The safety of the LHC
The Large Hadron Collider (LHC) can achieve an energy that no other particle accelerators have reached before, but Nature routinely produces higher energies in cosmic-ray collisions. Concerns about the safety of whatever may be created in such high-energy particle collisions have been addressed for many years. In the light of new experimental data and theoretical understanding, the LHC Safety Assessment Group (LSAG) has updated a review of the analysis made in 2003 by the LHC Safety Study Group, a group of independent scientists.
LSAG reaffirms and extends the conclusions of the 2003 report that LHC collisions present no danger and that there are no reasons for concern. Whatever the LHC will do, Nature has already done many times over during the lifetime of the Earth and other astronomical bodies. The LSAG report has been reviewed and endorsed by CERN’s Scientific Policy Committee, a group of external scientists that advises CERN’s governing body, its Council.
The following summarizes the main arguments given in the
LSAG report. Anyone interested in more details is encouraged to consult it directly, and the technical scientific papers to which it refers.
Cosmic rays
The LHC, like other particle accelerators, recreates the natural phenomena of cosmic rays under controlled laboratory conditions, enabling them to be studied in more detail. Cosmic rays are particles produced in outer space, some of which are accelerated to energies far exceeding those of the LHC. The energy and the rate at which they reach the Earth’s atmosphere have been measured in experiments for some 70 years. Over the past billions of years, Nature has already generated on Earth as many collisions as about a million LHC experiments – and the planet still exists. Astronomers observe an enormous number of larger astronomical bodies throughout the Universe, all of which are also struck by cosmic rays. The Universe as a whole conducts more than 10 million million LHC-like experiments per second. The possibility of any dangerous consequences contradicts what astronomers see - stars and galaxies still exist.
Microscopic black holes
Nature forms black holes when certain stars, much larger than our Sun, collapse on themselves at the end of their lives. They concentrate a very large amount of matter in a very small space. Speculations about microscopic black holes at the LHC refer to particles produced in the collisions of pairs of protons, each of which has an energy comparable to that of a mosquito in flight. Astronomical black holes are much heavier than anything that could be produced at the LHC.
According to the well-established properties of gravity, described by Einstein’s relativity, it is impossible for microscopic black holes to be produced at the LHC. There are, however, some speculative theories that predict the production of such particles at the LHC. All these theories predict that these particles would disintegrate immediately. Black holes, therefore, would have no time to start accreting matter and to cause macroscopic effects.
Although stable microscopic black holes are not expected in theory, study of the consequences of their production by cosmic rays shows that they would be harmless. Collisions at the LHC differ from cosmic-ray collisions with astronomical bodies like the Earth in that new particles produced in LHC collisions tend to move more slowly than those produced by cosmic rays. Stable black holes could be either electrically charged or neutral. If they had electric charge, they would interact with ordinary matter and be stopped while traversing the Earth, whether produced by cosmic rays or the LHC. The fact that the Earth is still here rules out the possibility that cosmic rays or the LHC could produce dangerous charged microscopic black holes. If stable microscopic black holes had no electric charge, their interactions with the Earth would be very weak. Those produced by cosmic rays would pass harmlessly through the Earth into space, whereas those produced by the LHC could remain on Earth. However, there are much larger and denser astronomical bodies than the Earth in the Universe. Black holes produced in cosmic-ray collisions with bodies such as neutron stars and white dwarf stars would be brought to rest. The continued existence of such dense bodies, as well as the Earth, rules out the possibility of the LHC producing any dangerous black holes.
Strangelets
Strangelet is the term given to a hypothetical microscopic lump of ‘strange matter’ containing almost equal numbers of particles called up, down and strange quarks. According to most theoretical work, strangelets should change to ordinary matter within a thousand-millionth of a second. But could strangelets coalesce with ordinary matter and change it to strange matter? This question was first raised before the start up of the Relativistic Heavy Ion Collider, RHIC, in 2000 in the United States. A study at the time showed that there was no cause for concern, and RHIC has now run for eight years, searching for strangelets without detecting any. At times, the LHC will run with beams of heavy nuclei, just as RHIC does. The LHC’s beams will have more energy than RHIC, but this makes it even less likely that strangelets could form. It is difficult for strange matter to stick together in the high temperatures produced by such colliders, rather as ice does not form in hot water. In addition, quarks will be more dilute at the LHC than at RHIC, making it more difficult to assemble strange matter. Strangelet production at the LHC is therefore less likely than at RHIC, and experience there has already validated the arguments that strangelets cannot be produced.
Vacuum bubbles
There have been speculations that the Universe is not in its most stable configuration, and that perturbations caused by the LHC could tip it into a more stable state, called a vacuum bubble, in which we could not exist. If the LHC could do this, then so could cosmic-ray collisions. Since such vacuum bubbles have not been produced anywhere in the visible Universe, they will not be made by the LHC.
Magnetic monopoles
Magnetic monopoles are hypothetical particles with a single magnetic charge, either a north pole or a south pole. Some speculative theories suggest that, if they do exist, magnetic monopoles could cause protons to decay. These theories also say that such monopoles would be too heavy to be produced at the LHC. Nevertheless, if the magnetic monopoles were light enough to appear at the LHC, cosmic rays striking the Earth’s atmosphere would already be making them, and the Earth would very effectively stop and trap them. The continued existence of the Earth and other astronomical bodies therefore rules out dangerous proton-eating magnetic monopoles light enough to be produced at the LHC.
Reports and reviews
Facts and figures
The largest machine in the world...
The precise circumference of the LHC accelerator is 26 659 m, with a total of 9300 magnets inside. Not only is the LHC the world’s largest particle accelerator, just one-eighth of its cryogenic distribution system would qualify as the world’s largest fridge. All the magnets will be pre‑cooled to -193.2°C (80 K) using 10 080 tonnes of liquid nitrogen, before they are filled with nearly 60 tonnes of liquid helium to bring them down to -271.3°C (1.9 K).
The fastest racetrack on the planet...
At full power, trillions of protons will race around the LHC accelerator ring 11 245 times a second, travelling at 99.99% the speed of light. Two beams of protons will each travel at a maximum energy of 7 TeV (tera-electronvolt), corresponding to head-to-head collisions of 14 TeV. Altogether some 600 million collisions will take place every second.
The emptiest space in the Solar System...
To avoid colliding with gas molecules inside the accelerator, the beams of particles travel in an ultra-high vacuum – a cavity as empty as interplanetary space. The internal pressure of the LHC is 10-13 atm, ten times less than the pressure on the Moon!
The hottest spots in the galaxy, but even colder than outer space...
The LHC is a machine of extreme hot and cold. When two beams of protons collide, they will generate temperatures more than 100 000 times hotter than the heart of the Sun, concentrated within a minuscule space. By contrast, the 'cryogenic distribution system', which circulates superfluid helium around the accelerator ring, keeps the LHC at a super cool temperature of -271.3°C (1.9 K) – even colder than outer space!
The biggest and most sophisticated detectors ever built...
To sample and record the results of up to 600 million proton collisions per second, physicists and engineers have built gargantuan devices that measure particles with micron precision. The LHC's detectors have sophisticated electronic trigger systems that precisely measure the passage time of a particle to accuracies in the region of a few billionths of a second. The trigger system also registers the location of the particles to millionths of a metre. This incredibly quick and precise response is essential for ensuring that the particle recorded in successive layers of a detector is one and the same.
The most powerful supercomputer system in the world...
The data recorded by each of the big experiments at the LHC will fill around 100 000 dual layer DVDs every year. To allow the thousands of scientists scattered around the globe to collaborate on the analysis over the next 15 years (the estimated lifetime of the LHC), tens of thousands of computers located around the world are being harnessed in a
distributed computing network called the Grid.
LHC Milestones
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 LHC 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…
From CERN
