Since the beginning of humanity, our eagerness to learn and to understand how the Universe came together has always been the main key of many scientific breakthroughs. However, like Albert Einstein once said, the more we learn, the less we know. There are still great unanswered questions such as what is dark matter? What is dark energy? What exactly is gravity? How did the Universe begin? All these mysteries may be solved by one creation: The Large Hadron Collider. Indeed, the world’s largest and most powerful particle collider is the best hope for scientists to understand what happened right after the big bang. In 2015, for the first time in 2 years, the particle accelerator has resumed the delivery of data to physicists around the world. It is now running at an unprecedented level to take the research further and get closer to the conditions that gave birth to the macrocosm. How is this possible? What can we expect to discover and what have we already found out?

What is the Large Hadron Collider (LHC)?

The LHC is the last addition to a complex of accelerators built by the European Council for Nuclear Research, abbreviated CERN. Concretely, it’s a 27 kilometres ring of superconducting magnets built deep underground, between Switzerland and France.

pic 1Its main purpose is quite simple: accelerate up to 99.9 percent of the speed of light[1] 2 beams of particles (protons or ions) going in opposite direction and smash them together. 

The collisions are made at four different locations, corresponding to the positions of four particle detectors –ATLAS, CMS, ALICE and LHCb (CERN, 2009). Doing so, physicists are able to investigate new energetic particles and many aspects of particle physics whose probability varies with collision energy, and which are often rare (CERN, 2009).

How does it work?

It goes without saying that making infinitely small particles moving in opposite direction collide at almost the speed of light demands an extreme amount of high precision, therefore advanced technology is needed. “The particles are so tiny that the task of making them collide is akin to firing two needles 10 kilometres apart with such precision that they meet halfway” (CERN, 2015).

The job is to speed up and increase the energy of a beam of particles by generating electric fields that accelerate the particles, and magnetic fields that steer and focus them (CERN, 2015). In order to make it possible, superconducting electromagnetic devices are used to manipulate the particles. A total of about 9600 magnets are to keep the particles in their nearby circular orbits, compress them together, and focus or steer the beam. The real challenge is to produce an electromagnetic field strong enough to properly conduct the beams. Indeed, the faster the particles go, the more energy they carry, therefore the more powerful the electromagnetic field has to be.

The very famous Einstein’s equation E = MC2 says it best. Hence, the electro-magnets need to be made superconducting[2] by pumping super-fluid helium into the magnet systems and cool them down, as no “warm” magnets can do the job. The temperature reached is 1.9 K (-271.3° C), which is lower than outer space! As you would have guessed it, only positively or negatively charged particles such as protons, ions and electrons -sensitive to electromagnetic fields- can be used in the accelerator. The type of particle used depends on the aim of the experiment.

The path of the particles

At CERN, a number of accelerators are joined together in sequence to reach successively higher energies, with the LHC being the last and most powerful one. A simple bottle of hydrogen gas marks the beginning of an extraordinary journey. First of all, an electric field is used to strip hydrogen atoms off their electrons to yield protons (CERN, The accelerator complex, 2015). The protons positively charged are now able to be accelerated by an electric field in the linear accelerator LINAC 2, the first in the chain. By the time the beam leaves LINAC 2, they will be travelling at 1/3 the speed of light and with the energy of 50 Mega eV[3].

From that point, straight acceleration is now impractical and the beam is injected into the next stage, the BOOSTER of 150 metres in circumference which takes the level of energy to 1.4 Giga eV (we are now at 91.6 percent of the velocity of light). Followed by the 628 metres in circumference PROTON SYNCHOTRON (PS), stage 3 by analogy, which pushes the beam to 25 GeV. It’s here that the point of transition is reached. The energy added to the protons by the pulsing electric field can no longer be translated into increase of velocity, as they’re already approaching the limiting speed of light (99.9 percent).

Instead, the added energy manifests itself as an increase in the mass of the protons. In short, remember the good old E = MC2. The packet of protons is then channeled into stage 4, the SUPER PROTON SYNCHROTON (SPS), a huge ring of 7 km. The energy is increased to 450 GeV and soon, the particles will be energized sufficiently to be launched into the orbit of the gigantic LHC.

acceleration chain

The protons are finally transferred to the two vacuum tubes of the LHC. Indeed, the beam pipes vacuum pressure will be 10-13 atm[4] (ultrahigh vacuum) because we want to avoid any type of collisions with gas molecules. The beam in one pipe circulates clockwise while the beam in the other circulates anticlockwise.

Last summer, for the first time, the LHC reached the impressive energy level of 6.5 Tera eV per beam, with a total energy at the collision point of 13 TeV[5]! It’s almost double the collision energy of the LHC’s first, three-year run. This allows a whole new range of experiments.

The Standard Model of physics particles

The objective of particle physics is to understand the basic structure and laws of nature all the way from the largest scale of the Universe to the smallest dimension. A great simplification was made when we realized that all the atoms are made of three different particles: the protons, the neutrons and the electrons. We thought that just by combining diverse number of protons or electrons we could get all the elements. It’s unfortunately naive to think of building a very simple Universe. It became much more complicated in the beginning of the 20th century when physicists discovered many new particles. They had to organize them somehow according to the properties they had such as the spin of the particles, the lifetime, the electrical charge and the mass.

They came up with the Standard Model, a collection of theories that embodies all of our current understanding of fundamental particles and forces (The Science Channel, 2010). It leads us to an insight of the subatomic particles. Three main groups are the base of all matter. First the Quarks, which are the fundamental ingredients to protons and neutrons. These particles composed of Quarks are also referred as Hadrons, from where the name “Large Hadron Collider” originated. Second, the Leptons such as electrons, and finally the Bosons which are the strong force carriers that bind together Quarks (Physics Academy, 2012).

standard model of physics

What are the main goals of the LHC?

The Standard Model has been tested by a great deal of experiments and it has proven particularly successful in anticipating the existence of previously undiscovered particles (CERN, 2009, p. 22). Actually, most of the subatomic particles are only found at high energies in particle accelerators. However, the Standard model leaves many unsolved questions, which the LHC will help to answer. Cosmological and astrophysical observations have shown that all of the visible matter accounts for only 4% of the Universe. Therefore, the search is open for particles or phenomena responsible for dark matter[6] and dark energy[7] (CERN, 2009). Also, our Model doesn’t explain the origin of mass, why some particles are heavy while some have no mass at all. That was until the 4 of July 2012 when experiments in the LHC revealed the Higgs boson, the missing piece in the Standard Model that explains the origin of fundamental particles’ mass (CERN, 2015). When a particle traverses the Higgs field, it interacts and gets mass. The more it interacts, the more mass it has. That was definitively the most important observation of CERN’s physicist.

Furthermore, the LHC collisions will provide a window onto the state of matter that would have existed in the early Universe. It will also help us to investigate the mystery of antimatter. Physicists believe that at the time of the Big Bang, matter and antimatter must have been produced in the same amounts. Nonetheless, from what we have observed so far, our Universe is made only of matter.

Finally, what could be more exciting and mind blowing than thinking of the existence of wormholes, allowing space and time transversal? Contrary to what is believed, the sky isn’t the limit.

References

[1] No particle can move with speeds faster than the speed of light, but there is no limit to the energy a particle can attain.

[2] Efficiently conducting electricity without resistance or loss of energy.

[3] In particle physics, the unit used for energy is the electronvolt. A single electron accelerated by a potential difference of 1 volt will have a discreet amount of energy 1 eV = 1.602 x 10-19 Joules.

[4] 1 atm being the normal atmospheric pressure, also equivalent to 101.325 kilopascal.

[5] In absolute terms, these energies, if compared to the energies we deal with every day, are not impressive. In fact, 1 TeV is about the energy of motion of a flying mosquito.

[6] Dark matter is thought to be some still undiscovered supersymmetric particles that have a gravitational effect that makes galaxies spin faster than expected.

[7] Dark energy is a form of energy that appears to be associated with the vacuum in space, and makes up approximately 70% of the Universe.

Sources

CERN. (2009). LHC the guide. Retrieved from CERN: http://cds.cern.ch/record/1164451/files/CERN-Brochure-2009-003-Fre.pdf

CERN. (2015). How an accelerator works. Récupéré sur home.cern: http://home.cern/about/how-accelerator-works

CERN. (2015). The accelerator complex. Récupéré sur home.cern: http://home.cern/about/accelerators

CERN. (2015). The basics of the Higgs boson. Récupéré sur home.cern: http://home.cern/about/updates/2013/05/basics-higgs-boson

CERN. (2015). The Large Hadron Collider. Retrieved from home.cern: http://home.cern/topics/large-hadron-collider

Physics Academy. (2012). The Standard Model of Particle Physics 001 : Overview. Récupéré sur youtube: https://www.youtube.com/watch?v=2xnsMGNicho

The Science Channel. (2010). CERN: The Standard Model of Particle Physics. Récupéré sur youtube: https://www.youtube.com/watch?v=V0KjXsGRvoA