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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.

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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 mixtureof 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.

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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 analyzed 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 inopposite 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

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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

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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.

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

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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 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.

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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 detector

  Size: 46 m long, 25 m high and 25 m wide. The ATLAS detector is the largestvolume particle detector ever constructed.

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  Weight: 7000 tonnes  Design: barrel plus end caps  Location: Meyrin, Switzerland.

CMS

Compact Muon SolenoidThe 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).

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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.

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 noantimatter. 

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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 detector

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

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 TOTEM

TOTal Elastic and diffractive cross section Measurement

The TOTEM experiment studies forward particles to focus on physics that is not

accessible to the general-purpose 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 CMS)

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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, soCERN 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

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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 Europeanmulti-science grid Enabling Grids for E-SciencE (EGEE), which continues togrow 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 particularthrough the CERN openlab, where leading IT companies are testing andvalidating 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 thelight 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

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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. 

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 245times 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. Theinternal 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...

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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 thanouter 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 adistributed computing network called the Grid.

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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…