Lhc(large Hadron Collider)

The Large Hadron Collider Introduction: 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 100 m underground. It is a particle accelerator used by physicists to study the smallest known particles – the fundamental building blocks of all things. It will revolutionize 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 – will travel in opposite directions inside the circular accelerator, gaining energy with every lap. Physicists will 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 will analyze 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, but what's for sure is that a brave new world of physics will emerge from the new accelerator, as knowledge in particle physics goes on to describe the

workings of the Universe. 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 higher 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 hypothesized 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 LHC experiment will be looking for differences between matter and antimatter to help answer this question. Previous experiments have already observed a tiny behavioral 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.

Design: The LHC is the world's largest and highest-energy particle accelerator. 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. The 3.8 m wide concrete-lined tunnel, constructed between 1983 and 1988, was formerly used to house the Large Electron-Positron Collider. It crosses the border between Switzerland and France at four points, with most of it in France. Surface buildings hold ancillary equipment such as compressors, ventilation equipment, control electronics and refrigeration plants. The collider tunnel contains two adjacent parallel beam pipes that intersect at four points, each containing a proton beam, which 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 maximize

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. Approximately 96 tonnes of liquid helium is needed to keep the magnets at their operating temperature of 1.9 K, making the LHC the largest cryogenic facility in the world at liquid helium temperature.

Superconducting quadrupole electromagnets are used to direct the beams to four intersection points, where interactions between protons will take place. Once or twice a day, as the protons are accelerated from 450 GeV to 7 TeV, the field of the superconducting dipole magnets will be increased from 0.54 to 8.3 tesla (T). The protons will each have an energy of 7 TeV, giving a total collision energy of 14 TeV (2.2 μJ). At this energy the

protons have a Lorentz factor of about 7,500 and move at about 99.9999991% of the speed of light. It will take less than 90 microsecond (μs) for a proton to travel once around the main ring – a speed of about 11,000 revolutions per second. Rather than continuous beams, the protons will be bunched together, into 2,808 bunches, so that interactions between the two beams will take place at discrete intervals never shorter than 25 nanoseconds (ns) apart. However it will be operated with fewer bunches when it is first commissioned, giving it a bunch crossing interval of 75 ns. Prior to being injected into the main accelerator, the particles are prepared by a series of systems that successively increase their energy. The first system is the linear particle accelerator LINAC 2 generating 50 MeV protons, which feeds the Proton Synchrotron Booster (PSB). There the protons are accelerated to 1.4 GeV and injected into the Proton Synchrotron (PS), where they are accelerated to 26 GeV. Finally the Super Proton Synchrotron (SPS) is used to further increase their energy to 450 GeV before they are at last injected (over a period of 20 minutes) into the main ring. Here the proton bunches are accumulated, accelerated (over a period of 20 minutes) to their peak 7 TeV energy, and finally stored for 10 to 24 hours while collisions occur at the four intersection points. The LHC will also be used to collide lead (Pb) heavy ions with a collision energy of 1,150 TeV. The Pb ions will be first accelerated by the linear accelerator LINAC 3, and the Low-Energy Injector Ring (LEIR) will be used as an ion storage and cooler unit. The ions then will be further

accelerated by the PS and SPS before being injected into LHC ring, where they will reach an energy of 2.76 TeV per nucleon.

Detectors Six detectors have been 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.[16] A Large Ion Collider Experiment (ALICE) and LHCb have more specific roles and the last two TOTEM and LHCf are very much smaller and are for very specialized research. The BBC's summary of the main detectors is: * ATLAS – one of two so-called general purpose detectors. Atlas will be used to look for signs of new physics, including the origins of mass and extra dimensions. * CMS – the other general purpose detector will, like ATLAS, hunt for the Higgs boson and look for clues to the nature of dark matter. * ALICE – will study a "liquid" form of matter called quark-gluon plasma that existed shortly after the Big Bang.

* LHCb – equal amounts of matter and anti-matter were created in the Big Bang. LHCb will try to investigate what happened to the "missing" anti-matter.

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! The CERN Control CentreAll 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.

1.ALICE A Large Ion Collider Experiment For the ALICE experiment, the LHC will collide lead ions to recreate the conditions just after the Big Bang under laboratory conditions. The data obtained will allow physicists to study a state of matter known as quark-gluon plasma, which is believed to have existed soon after the Big Bang. All ordinary matter in today’s Universe is made up of atoms. Each atom contains a nucleus composed of protons

and neutrons, surrounded by a cloud of electrons. Protons and neutrons are in turn made of quarks which are bound together by other particles called gluons. This incredibly strong bond means that isolated quarks have never been found. Collisions in the LHC will generate temperatures more than 100 000 times hotter than the heart of the Sun. Physicists hope that under these conditions, the protons and neutrons will 'melt', freeing the quarks from their bonds with the gluons. This should create a state of matter called quark-gluon plasma, which probably existed just after the Big Bang when the Universe was still extremely hot. The ALICE collaboration plans to study the quark-gluon plasma as it expands and cools, observing how it progressively gives rise to the particles that constitute the matter of our Universe today. A collaboration of more than 1000 scientists from 94 institutes in 28 countries works on the ALICE experiment (March 2006).

ALICE setup

ALICE detector * Size: 26 m long, 16 m high, 16 m wide * Weight: 10 000 tonnes * Design: central barrel plus single arm forward muon spectrometer * Location: St Genis-Pouilly, France. See ALICE in Google Earth.

2.ATLAS: A Toroidal LHC ApparatuS ATLAS is one of two general-purpose detectors at the LHC. It will investigate a wide range of physics, including the search for the Higgs boson, extra dimensions, and particles that could make up dark matter.

With the same goals in physics as CMS, ATLAS will record similar sets of measurements on the particles created in the collisions – their paths, energies, and their identities. However, the two experiments have adopted radically different technical solutions and designs for their detectors' magnet systems. The main feature of the ATLAS detector is its enormous doughnut-shaped magnet system. This consists of eight 25-m long superconducting magnet coils, arranged to form a cylinder around the beam pipe through the centre of the detector. During operation, the magnetic field is contained within the central cylindrical space defined by the coils. More than 1700 scientists from 159 institutes in 37 countries work on the ATLAS experiment (March 2006). ATLAS setup

ATLAS detector

* Size: 46 m long, 25 m high and 25 m wide. The ATLAS detector is the largest volume particle detector ever constructed. * Weight: 7000 tonnes * Design: barrel plus end caps * Location: Meyrin, Switzerland.

3.CMS: Compact Muon Solenoid The CMS experiment uses a general-purpose detector to investigate a wide range of physics, including the search for the Higgs boson, extra dimensions, and particles that could make up dark matter. Although it has the same scientific goals as the ATLAS experiment, it uses different technical solutions and design of its detector magnet system to achieve these. The CMS detector is built around a huge solenoid magnet. This takes the form of a cylindrical coil of superconducting cable that generates a magnetic field of 4 teslas, about 100 000 times that of the Earth. The magnetic field is confined by a steel 'yoke' that forms the bulk of the detector's weight of 12 500 tonnes. An unusual feature of the CMS detector is that instead of being built in-situ underground, like the other giant detectors of the LHC experiments, it was constructed on the surface, before being lowered underground in 15 sections and reassembled.

More than 2000 scientists collaborate in CMS, coming from 155 institutes in 37 countries (October 2006). CMS setup

CMS detector * Size: 21 m long, 15 m wide and 15 m high. * Weight: 12 500 tonnes * Design: barrel plus end caps * Location: Cessy, France. See CMS in Google Earth.

4.LHCb:

Large Hadron Collider beauty The LHCb experiment will help us to understand why we live in a Universe that appears to be composed almost entirely of matter, but no antimatter. It specialises in investigating the slight differences between matter and antimatter by studying a type of particle called the 'beauty quark', or 'b quark'. Instead of surrounding the entire collision point with an enclosed detector, the LHCb experiment uses a series of sub-detectors to detect mainly forward particles. The first sub-detector is mounted close to the collision point, while the next ones stand one behind the other, over a length of 20 m. An abundance of different types of quark will be created by the LHC before they decay quickly into other forms. To catch the b-quarks, LHCb has developed sophisticated movable tracking detectors close to the path of the beams circling in the LHC. The LHCb collaboration has 650 scientists from 48 institutes in 13 countries (April 2006).

LHCb setup

LHCb detector * Size: 21m long, 10m high and 13m wide * Weight: 5600 tonnes * Design: forward spectrometer with planar detectors * Location: Ferney-Voltaire, France.

5.TOTEM: TOTal Elastic and diffractive cross section Measurement The TOTEM experiment studies forward particles to focus on physics that is not accessible to the generalpurpose experiments. Among a range of studies, it will measure, in effect, the size of the proton and also monitor accurately the LHC's luminosity.

To do this TOTEM must be able to detect particles produced very close to the LHC beams. It will include detectors housed in specially designed vacuum chambers called 'Roman pots', which are connected to the beam pipes in the LHC. Eight Roman pots will be placed in pairs at four locations near the collision point of the CMS experiment. Although the two experiments are scientifically independent, TOTEM will complement the results obtained by the CMS detector and by the other LHC experiments overall. The TOTEM experiment involves 50 scientists from 10 institutes in 8 countries (2006). TOTEM detector * Size: 440 m long, 5 m high and 5 m wide * Weight: 20 tonnes * Design: Roman pot and GEM detectors and cathode strip chambers * Location: Cessy, France (near CM)

6.LHCf: Large Hadron Collider forward The LHCf experiment uses forward particles created inside the LHC as a source to simulate cosmic rays in laboratory conditions. Cosmic rays are naturally occurring charged particles from outer space that constantly bombard the Earth's atmosphere. They collide with nuclei in the upper atmosphere, leading to a cascade of particles that reaches ground level. Studying how collisions inside the LHC cause similar cascades of particles will help scientists to interpret and calibrate large-scale cosmic-ray experiments that can cover thousands of kilometres. The LHCf experiment involves 22 scientists from 10 institutes in 4 countries (September 2006). LHCf detector * Size: two detectors, each measures 30 cm long, 80 cm high, 10 cm wide * Weight: 40 kg each * Design: * Location: Meyrin, Switzerland (near ATLAS)

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 cuttingedge 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 theory predicts that microscopic black holes decay rapidly, even hypothetical stable black holes can be shown to be harmless by studying the consequences of their production by cosmic rays. Whilst 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, one can still demonstrate their safety. The specific reasons for this depend whether the black holes are electrically charged, or neutral. Many stable black holes would be expected to be electrically charged, since they are created by charged particles. In this case they would interact with ordinary matter and be stopped while traversing the Earth or Sun, whether produced by cosmic

rays or the LHC. The fact that the Earth and Sun are 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 thousandmillionth 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 protoneating magnetic monopoles light enough to be produced at the LHC.

Other aspects of LHC safety: Concern has recently been expressed that a 'runaway fusion reaction' might be created in the LHC carbon beam dump. The safety of the LHC beam dump had previously been reviewed by the relevant regulatory authorities of the CERN host states, France and Switzerland. The specific concerns expressed more recently have been addressed in a technical memorandum by Assmann et al. As they point out, fusion reactions can be maintained only in material compressed by some external pressure, such as that provided by gravity inside a star, a fission explosion in a thermonuclear device, a magnetic field in a Tokamak, or by continuing isotropic laser or particle beams in the case of inertial fusion. In the case of the LHC beam dump, it is struck once by the beam coming from a single direction. There is no countervailing pressure, so the dump material is not compressed, and no fusion is possible. Concern has been expressed that a 'runaway fusion reaction' might be created in a nitrogen tank inside the LHC tunnel. There are no such nitrogen tanks. Moreover, the arguments in the previous paragraph prove that no fusion would be possible even if there were.

Finally, concern has also been expressed that the LHC beam might somehow trigger a 'Bose-Nova' in the liquid helium used to cool the LHC magnets. A study by Fairbairn and McElrath has clearly shown there is no possibility of the LHC beam triggering a fusion reaction in helium. We recall that 'Bose-Novae' are known to be related to chemical reactions that release an infinitesimal amount of energy by nuclear standards. We also recall that helium is one of the most stable elements known, and that liquid helium has been used in many previous particle accelerators without mishap. The facts that helium is chemically inert and has no nuclear spin imply that no 'Bose-Nova' can be triggered in the superfluid helium used in the LHC.

Comments on the papers by Giddings and Mangano, and by LSAG The papers by Giddings and Mangano and LSAG demonstrating the safety of the LHC have been studied, reviewed and endorsed by leading experts from the CERN Member States, Japan, Russia and the United States, working in astrophysics, cosmology, general relativity, mathematics, particle physics and risk analysis, including several Nobel Laureates in Physics. They all agree that the LHC is safe. The paper by Giddings and Mangano has been peerreviewed by anonymous experts in astrophysics and particle physics and published in the professional scientific

journal Physical Review D. The American Physical Society chose to highlight this as one of the most significant papers it has published recently, commissioning a commentary by Prof. Peskin from the Stanford Linear Accelerator Laboratory in which he endorses its conclusions. The Executive Committee of the Division of Particles and Fields of the American Physical Society has issued a statement endorsing the LSAG report. The LSAG report has been published by the UK Institute of Physics in its publication Journal of Physics G. The conclusions of the LSAG report were endorsed in a press release that announced this publication. The conclusions of LSAG have also been endorsed by the Particle and Nuclear Physics Section (KET) of the German Physical Society. A translation into German of the complete LSAG report may be found on the KET website, as well as here. (A translation into French of the complete LSAG report is also available.) Thus, the conclusion that LHC collisions are completely safe has been endorsed by the three respected professional societies of physicists that have reviewed it, which rank among the most highly respected professional societies in the world. World-renowned experts in astrophysics, cosmology, general relativity, mathematics, particle physics and risk analysis, including several Nobel Laureates in Physics,

have also expressed clear individual opinions that LHC collisions are not dangerous:

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 oneeighth 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…

References: • Wikipedia • www.scotland.gov.uk (Scotland govt. official website) • (http://lhc.web.cern.ch/lhc/)LHC homepage • CERN(European Organization for Nuclear Research) • The ALICE website (aliceinfo.cern.ch/Public/) • The ATLAS website(http://atlas.ch/) • (http://cmsinfo.cern.ch/outreach/)The CMS public website • http://lhcb-public.web.cern.ch/lhcb-public/ • http://totem.web.cern.ch/Totem/