What Happens at the Large Hadron Collider : the Big-Bang machine?

The Large Hadron Collider (LHC) is one of the hottest place of the galaxy since the temperature of the system produced after collision is more than 100000 times the temperature of core of Sun. It is also one of the coolest place because LHC magnets operate at 1.9 K (-271.25 C).

We all have listened about discovery of some God particle or Higgs boson at the Large Hadron Collider. You must be curious that what really Large Hadron Collider is or how it works, what it do. In this blog, I will try to explain in a simple way. Before going in details about the LHC, it will be good to discuss why we need such a huge machine.

Big-Bang (origin of universe):

According to Big-Bang theory which is the most accepted theory of universe evolution, our universe formed around 13.7 billion years ago by an explosion known as Big-Bang. Please see image below to understand the evolution of universe.


After Big-Bang, the temperature of universe cooled down and atoms, star, galaxies formed as shown in above figure.

Fundamental particles and fundamental interactions:

We see or feel presence of matter around us like Earth, plants, animals, air etc. Let us pick any object and start observing it. In first see, we find its a single object. Now let us break it, what we see? Now the object is not single anymore so we can conclude original was basically made up of many small pieces. Now a question arise, if we keep breaking it, after how many pieces it can’t be broken more or in simple language, what are the smallest or fundamental building blocks of matter? Well we have already answer of this question, thanks to our scientists and scientific experiments.


The matter is formed by small building blocks which are known as Atom. But an atom is not fundamental particle since it is made up of nucleus and electrons (see the above figure). Again a nucleus is composed of protons and neutrons and these are made up of up and down quarks. So fundamental particles or building blocks of matter are electrons and u,d quarks. There are other fundamental particles too. All building blocks of visible universe are divided into two group which are fermions (spin 1/2 particles):

  • quarks : there are six type of quarks – up, down, charm, strange, top and bottom.
  • leptons : there are six types of leptons too – electron, muon, tau, electron neutrino, muon neutrino and tau neutrino.

These quarks and leptons have their anti-particles which are known as anti-quark and anti-lepton. An anti-particle has just opposite properties of it’s particle like charge etc but same mass.

Well, there is another question come – how these particles were discovered? Obviously by various experiments and observations but can we really discover a particle if it doesn’t interact with our experimental system or more generally, with other particles? Every particle interact with other by some rules of nature or fundamental interactions. There are four types of fundamental interactions by which, particle interact with each other. Particles interact with each other by exchanging some force carrier which are bosons. The four interactions and their force careers are :

  1. Strong force (felt by quarks, carrier: gluons) – this force is responsible to bind quarks inside nucleons (proton and neutrons) and other particles. It is also responsible to bind nucleons inside the nucleus. Particles that interact via the strong force are also called ‘hadrons’.
  2. Electromagnetic force (felt by quarks and charged leptons, carrier: photons) – It holds electrons to nuclei in atoms, binds atoms into molecules, and is responsible for the properties of solids, liquids and gases.
  3. Weak force (felt by quarks and leptons, carrier : intermediate vector bosons- W,Z) -The weak force underlies natural radioactivity, for example in the Earth beneath our feet. It is also essential for the nuclear reactions in the centres of stars like the Sun, where hydrogen is converted into helium.
  4. Gravitation (felt by: all particles, Carrier: Graviton (Not yet discovered)) –
    Gravity makes apples fall to the ground. It is an attractive force. On an astronomical scale it binds matter in planets and stars, and holds stars together in galaxies.

So why LHC?

So far we know that:

  • Our universe formed by Big-Bang around 13.7 billion years ago (predicted by Big-Bang theory). It’s still expanding.
  • The current visible universe is made up by leptons and quarks which are fermion and they interact by exchanging force carrier bosons
  • Mass–energy equivalence : states that anything having mass has an equivalent amount of energy and vice versa, with these fundamental quantities directly relating to one another by Albert Einstein‘s famous formula: E = mc2 where c (3×108 m/s) is the speed of light which is also upper limit of the attainable speed of a particle.

Our current understanding of the Universe is incomplete. There are lot of questions still need to be answered like: why there is matter-antimatter asymmetry, the visible matter creates only 5% percent of universe, rest of universe is made of dark matter, dark energy so what is dark matter, dark energy? and many more. To find the answers of these, the Large Hadron Collider was constructed. Apart from unanswered questions of physics, the studies of proton–proton collisions, heavy-ion collisions at the LHC will provide a window onto the state of matter that existed in the early Universe, called ‘quark-gluon plasma’. When heavy ions collide at high energies they form for an instant a ‘fireball’ of such hot, dense matter that can be studied by the experiments.

The LHC:

The Large Hadron Collider (LHC) is the most powerful and biggest particle accelerator ever built with the aim of allowing physicists to test the predictions of different theories of particle physics and high-energy physics. This is also most complex and biggest single machine made by human. This is a masterpiece example of engineering and technology. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and engineers from over 100 countries, as well as hundreds of universities and laboratories.


The LHC lies in a tunnel 27 km (17 mi) in circumference, as deep as 175 m (574 ft) beneath the Franco-Swiss border near Geneva, Switzerland. Its synchrotron is designed to collide opposing particle beams of either protons at up to 14 TeV (2.24 micro-joules) per nucleon, or lead nuclei at an energy of 574 TeV (184.0J) per nucleus (5.5 TeV per nucleon-pair).

The collider tunnel contains two adjacent parallel beam-lines (or 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, made of copper-clad niobium-titanium, at their operating temperature of 1.9 K (271.25 C). 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. 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 accelerator complex at CERN is a succession of machines with increasingly higher energies. Each machine injects the beam into the next one, which takes over to bring the beam to an even higher energy , and so on.

LHC Collisions:

Hydrogen atom is taken from a standard bottle of hydrogen. By stripping orbiting electrons off hydrogen atoms, we get proton. Protons are injected into PS Booster (PSB) at an energy of 50 MeV from Linac2. The booster accelerates them to 1.4 GeV. The beam is then fed to Proton Synchrotron (PS) where it is accelerated to 25 GeV. Protons are then sent to the Super Proton Synchrotron (SPS) where they are accelerated to 450 GeV. They are finally transfered to the LHC (both in a clockwise and anticlockwise direction) where they are accelerated for 20 minutes to their nominal.

In LHC, beams of p-p, p-Pb and Pb-Pb are allowed to collide.


A proton contains three quarks, two up and one down which move around very close to each other. The quarks are kept together by number of gluons interacting with quarks and also with other gluns. Gluons give rise to gluon field which keeps the constituents of proton together. In Proton-Proton Collisions, two protons approach each other with relativistic speed. When two colliding protons get very close and overlap, an energetic gluon is exchanged between two of the quark of two protons. The scatter quark and two other quarks in each of the protons get separated more and more. Now a strange thing happen, the quarks cannot get separated from each-other very much because of strong gluon field. Instead of stretching the gluon fields between the quarks further, the gluon field breaks by creating a new pair of quark and anti-quark. The quarks continue to move apart from each other, once again stretching the gluon field between each other. The gluon field breaks again by forming quark and anti-quark pair. This process is repeated over and over again until all quarks are moving close together in pairs or triplets forming bound state of quark and anti-quark.

At the end of the process, initiated by exchange of a gluon between two quarks of colliding protons, four or more jets of particles create. Each jet contains dozen of particles like pions, kaons etc. When these particles pass through detectors, they interact with the detector medium and produced tracks as shown in figure below.


The experimentalist analyze and search for new physics using this data after reconstruction of tracks.

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