The Large Hadron Collider (LHC) used in the CERN project is a particle accelerator located on the border between

Correct — D, (d) France and Switzerland.

THE LARGE HADRON COLLIDER SITS IN A RING TUNNEL ABOUT 27 KILOMETRES ROUND, roughly 100 metres underground, on the plain between Lake Geneva and the Jura mountains. The ring is large enough that it crosses the international boundary, so part of it lies under France and part under Switzerland. That is the whole answer, and the geography is worth fixing properly because the question can be asked about either the laboratory or the machine.

CERN, the European Organization for Nuclear Research, has its main site at Meyrin, just outside Geneva in SWITZERLAND, and its acronym comes from the French name of the provisional council that set it up, the Conseil Européen pour la Recherche Nucléaire. It was founded in 1954 by twelve European states and is one of the earliest and most successful pieces of post-war European scientific co-operation. Its accelerator complex, however, is not confined to Swiss territory: the LHC tunnel runs out under the French communes of the Pays de Gex and back again.

WHY THE MACHINE IS CALLED WHAT IT IS, since each word of the name carries information. LARGE — the 27-kilometre circumference, inherited from the tunnel dug for the earlier Large Electron-Positron collider, which the LHC replaced. HADRON — a hadron is a composite particle made of quarks held together by the strong nuclear force. Protons and neutrons are hadrons. The LHC collides beams of protons, and for part of each year beams of lead nuclei, rather than the electrons of the machine it replaced. COLLIDER — two beams travel in opposite directions and are brought into head-on collision, which puts far more energy into the interaction than firing one beam at a stationary target.

HOW IT WORKS, in outline. Superconducting dipole magnets, cooled with liquid helium to under 2 kelvin — colder than the background temperature of outer space — bend the two counter-rotating proton beams round the ring, while radio-frequency cavities accelerate them to within a whisker of the speed of light. The beams are crossed at four points, and at each of those points sits a detector: ATLAS, CMS, ALICE and LHCb.

WHAT IT IS FOR. The LHC recreates, in a very small volume, energy densities of the kind that existed a fraction of a second after the Big Bang, so that physicists can look for particles too massive to be produced in any other way. Its best-known result came on 4 July 2012, when the ATLAS and CMS collaborations announced the discovery of a new particle consistent with the HIGGS BOSON, the quantum of the field that gives elementary particles their mass. Peter Higgs and François Englert received the Nobel Prize in Physics in 2013. The word 'boson' in that name honours the Indian physicist SATYENDRA NATH BOSE, whose work with Einstein described the statistics that this whole class of particles obeys.

INDIA'S CONNECTION is examinable in its own right. India held observer status at CERN from 2002 and became an ASSOCIATE MEMBER in January 2017, and Indian institutions and the Department of Atomic Energy have contributed hardware, magnets, software and physicists to the CMS and ALICE experiments. One more piece of CERN trivia is worth carrying: the World Wide Web was invented there in 1989 by Tim Berners-Lee, as a way of sharing documents between physicists.

The stem prints 'LHC' and 'CERN' in capitals and ends on the words 'on the border between' with no punctuation, as the booklet sets it.

Why the others are wrong

• (a)Spain and France — Spain and France do share a real and well-known border — the Pyrenees, running from the Bay of Biscay to the Mediterranean — so unlike one of the other options this is at least geographically possible. It is simply not where the collider is. There is no comparable accelerator on that frontier: Spain's contribution to European big science runs through its membership of CERN and of the European Southern Observatory, not through hosting a collider, and the large machines on French soil away from Geneva are the neutron source and the synchrotron at Grenoble, which lie well inside France. The option works as a distractor because a candidate who remembers only that the LHC is 'somewhere on a French border' has a one-in-several chance of landing here. The fix is to attach the machine to Geneva, which is the single fact that settles it: CERN sits at the edge of Geneva, and the ring runs out from there into France.
• (b)France and Germany — Another genuine border, running largely along the Rhine, and another plausible-sounding home for a European laboratory — but not this one. Germany's own large accelerator facilities are elsewhere: DESY and the European XFEL are at Hamburg, in the north, and the GSI heavy-ion centre with its FAIR project is at Darmstadt. Neither is on the French frontier and neither is the LHC. Germany is a founder member of CERN and its largest financial contributor, which may be what pulls a candidate towards this option, but contributing to a laboratory is not the same as hosting it. Keep the two facts apart: MEMBERSHIP of CERN is spread across more than twenty European states, while the SITE of CERN is a single place on the Franco-Swiss border outside Geneva.
• (c)Spain and Switzerland — This option can be eliminated without knowing anything at all about particle physics, because Spain and Switzerland DO NOT SHARE A BORDER. France lies between them across their whole width, so there is no Spanish-Swiss frontier for anything to straddle. Switzerland's neighbours are France, Germany, Austria, Italy and Liechtenstein, and Spain's land neighbours in Europe are France, Andorra, Portugal and Gibraltar. An option that asserts a border which does not exist is a free elimination, and spotting it is a useful general habit on questions of this shape: before weighing which frontier a facility sits on, check that each named pair of countries actually meets. Here that single check removes one of the four options and leaves a three-way choice for a candidate who knows no more than European geography.

CERN — the EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH — is the world's largest particle physics laboratory, founded in 1954 and headquartered at Meyrin on the outskirts of Geneva, Switzerland. Its acronym preserves the French name of the provisional body that created it. It operates a chain of accelerators in which each machine feeds the next, ending at the Large Hadron Collider.

THE LARGE HADRON COLLIDER is a circular proton-proton collider in a tunnel of about 27 kilometres circumference, roughly 100 metres below the surface, crossing the FRANCO-SWISS BORDER. It occupies the tunnel excavated for the Large Electron-Positron collider, which ran until 2000. First beams circulated in September 2008 and the first collisions followed in 2009 after a magnet fault was repaired.

HOW A COLLIDER WORKS. Two beams of protons are accelerated in opposite directions around the same ring. Superconducting dipole magnets, cooled by superfluid helium to about 1.9 kelvin, bend the beams into a circle; radio-frequency cavities supply the acceleration. The beams are brought into head-on collision at four crossing points. A head-on collision between two moving beams delivers far more usable energy than firing a beam at a fixed target, because the centre-of-mass energy is the sum of the two beam energies rather than a small fraction of one. Collision debris is recorded by detectors built around each crossing point.

THE FOUR MAIN EXPERIMENTS. ATLAS and CMS are general-purpose detectors and were the two that independently observed the Higgs boson. ALICE studies the quark-gluon plasma produced in lead-ion collisions. LHCb studies the small asymmetry between matter and antimatter through the behaviour of particles containing the bottom quark.

WHAT A HADRON IS. Matter's fundamental constituents divide into QUARKS and LEPTONS. Quarks are never found alone; they bind through the strong force into composite particles called HADRONS. Hadrons made of three quarks are baryons, of which the proton and the neutron are the familiar examples; hadrons made of a quark and an antiquark are mesons. The electron is a LEPTON, not a hadron, which is precisely the difference between the LHC and the LEP machine that preceded it in the same tunnel.

THE HIGGS BOSON. The Standard Model of particle physics explains how elementary particles acquire mass through their interaction with a field that permeates space, proposed independently in 1964 by several groups including Peter Higgs and the pair of Robert Brout and François Englert. The quantum of that field is the Higgs boson. Its discovery was announced on 4 July 2012 by ATLAS and CMS, and the Nobel Prize in Physics followed in 2013 for Higgs and Englert. The generic term BOSON honours Satyendra Nath Bose, whose 1924 paper, forwarded and extended by Einstein, gave the statistics obeyed by all particles of integer spin.

INDIA AND CERN. India was granted observer status in 2002 and became an ASSOCIATE MEMBER in January 2017, which allows Indian industry to bid for CERN contracts and Indian nationals to apply for staff positions. Indian participation is concentrated in the CMS and ALICE experiments and in accelerator technology, with the Department of Atomic Energy and the Department of Science and Technology as the nodal agencies.

ONE MORE CERN FACT THAT IS ASKED OFTEN. Tim Berners-Lee invented the World Wide Web at CERN in 1989 to let physicists share documents across a network, and CERN placed the underlying software in the public domain in 1993.

Key facts

• The Large Hadron Collider lies in a tunnel about 27 kilometres in circumference, around 100 metres underground, crossing the border between France and Switzerland.
• CERN, the European Organization for Nuclear Research, was founded in 1954 and is headquartered at Meyrin near Geneva, Switzerland.
• A hadron is a composite particle made of quarks bound by the strong force; protons and neutrons are hadrons, while the electron is a lepton.
• The LHC occupies the tunnel built for the earlier Large Electron-Positron collider and collides protons, and lead nuclei for part of each year.
• Its superconducting magnets are cooled with helium to about 1.9 kelvin, colder than the background temperature of outer space.
• The four main experiments are ATLAS, CMS, ALICE and LHCb.
• The discovery of the Higgs boson was announced on 4 July 2012 by the ATLAS and CMS collaborations; Peter Higgs and François Englert received the 2013 Nobel Prize in Physics.
• The term 'boson' honours the Indian physicist Satyendra Nath Bose.
• India held observer status at CERN from 2002 and became an associate member in January 2017.
• The World Wide Web was invented at CERN by Tim Berners-Lee in 1989.
• Spain and Switzerland share no border; France lies between them.

Common traps

• Assuming that because CERN's headquarters are in Switzerland, the whole machine is Swiss. The ring crosses into France.
• Selecting a pair of countries that do not share a border at all, as option (c) invites.
• Confusing CERN with the European Space Agency or with the European Southern Observatory, which are different organisations with different sites.
• Thinking a hadron is any subatomic particle. It is specifically a particle built from quarks, which excludes the electron.
• Attributing the Higgs boson's 'boson' to a European physicist. The word honours Satyendra Nath Bose.
• Mixing up India's observer status of 2002 with its associate membership of January 2017.

• practice — not a real PYQ

  The 'Higgs boson', whose discovery was announced by the ATLAS and CMS collaborations in 2012, is associated with which one of the following ?

  • (a)The force that binds quarks inside a proton
  • (b)The mechanism by which elementary particles acquire mass
  • (c)The decay of a free neutron into a proton
  • (d)The bending of light by a massive body

  Answer(b) The mechanism by which elementary particles acquire mass — the Higgs boson is the quantum of the field whose interaction with elementary particles gives them mass. The binding of quarks is the work of the strong force and its gluons, neutron decay is a weak-force process, and the bending of light by mass is a prediction of general relativity.

• practice — not a real PYQ

  India's status at CERN, the European Organization for Nuclear Research, is that of

  • (a)Full member since 1954
  • (b)Associate member since January 2017
  • (c)Observer with no further status
  • (d)Non-participating state

  Answer(b) Associate member since January 2017 — India had been an observer at CERN from 2002 and was admitted as an associate member in January 2017, which allows Indian industry to bid for CERN contracts and Indian nationals to apply for staff posts.