The Large Hadron Collider (LHC) - A Primer

When a future civilization looks back at ours, hopefully they would look at the LHC and wonder at our ingenuity, the way we look at the Pyramids and wonder. Every civilization has spent money and man power to build huge monuments to the Gods - from the Great Pyramids to the Mayan Temples all the way to the great modern cathedrals, temples and mosques of worship. The LHC is our contemporary society’s monument to answer the ultimate questions that our species has been struggling with all of human history – how did the universe start and what is the underlying nature of reality? And LHC is indeed a very appropriate monument of our modern times, since unlike our ancestors, we are trying to answer these profound questions with empirical evidence rather than faith based belief systems. Here is a short primer on LHC in celebration of it's resuming experiments as of today (April 5, 2015)

LHC stands for Large Hadron Collider. Large due to its size (approximately 27 km in circumference), Hadron because it accelerates protons or ions, which are hadrons, and Collider because these particles from two beams travelling in opposite directions, which collide at four points where the two rings of the machine intersect. Hadrons (from the Greek ‘adros’ meaning ‘bulky’) are particles composed of quarks. The protons and neutrons that atomic nuclei are made of belong to this family. On the other hand, leptons are particles that are not made of quarks. Electrons and muons are examples of leptons (from the Greek ‘leptos’ meaning ‘thin’).

How does LHC help with answering such fundamental questions like the origin of universe? A little primer on particle physics and big bang cosmology is needed to appreciate this so allow me to get in to some details here. Particle physics studies the tiniest objects of Nature. Apart from looking into the very small and fundamental particles, it also looks very far back into the past, just a few moments after the Big Bang. A particle accelerator studies these ultra tiny particles as well as the forces acting between them by accelerating them to very high speeds, and bombarding them with each other. Note that no particle can move with speeds faster than the speed of light in a vacuum; however, there is no limit to the energy a particle can attain. In high-energy accelerators, particles normally travel very close to the speed of light. Protons at full energy in the LHC will be travelling at 0.999999991 times the speed of light.  At this near light-speed, a proton in the LHC will make 11 245 circuits every second. And a beam of protons might circulate for 10 hours, travelling more that 10 billion kilometers, enough to get to the planet Neptune and back again! The protons that are accelerated at CERN is obtained from standard hydrogen. Although proton beams at the LHC are very intense, only 2 nanograms of hydrogen are accelerated each day. Therefore, it would take the LHC about 1 million years to accelerate just 1 gram of hydrogen!

It is not the absolute energy but the energy concentration or density that makes particle collisions so special. In absolute terms, these energies, if compared to the energies we deal with everyday, are not impressive. In fact, 1 TeV (1 trillion electron volt) is about the energy of motion of a flying mosquito. What makes the LHC so extraordinary is that it squeezes energy into a space about a million million times smaller than a mosquito, so the “energy concentration” is unprecedented. At near 13 TeV, the energy concentration achieved at LHC this time around is about 1000,000 times the energy concentration in the center of our sun, or equal to the energy concentration just 1 pico second (1 trillionth of a second) after Big Bang - allowing us to investigate questions that so far had been beyond the grasp of human experiments.

The Standard Model

The following pictures summarize the Standard Model’s basic points.

Our current understanding of the Universe is incomplete. The Standard Model of particles and forces summarizes our present knowledge of particle physics. According to the theory, which is supported by a great deal of experimental evidence, quarks are the building blocks of matter, and forces act through carrier particles exchanged between the particles of matter. 

The Standard Model has been tested by various experiments and it has proven particularly successful in anticipating the existence of previously undiscovered particles. However, it leaves many unsolved questions, which the LHC will help to answer. The Standard Model does not explain the origin of mass, nor why some particles are very heavy while others have no mass at all. The answer was postulated to be the so-called Higgs mechanism. According to the theory of the Higgs mechanism, the whole of space is filled with a ‘Higgs field’, and by interacting with this field, particles acquire their masses. Particles that interact intensely with the Higgs field are heavy, while those that have feeble interactions are light. The Higgs field has at least one new particle associated with it, the Higgs boson. It was at LHC that we were finally was able to detect this particle at last during the last round of experiments done at LHC in 2012, resulting in 2013 Nobel prize being awarded for this discovery .

What are we expecting from LHC this time around?

When the LHC started today (April 5, 2015), the energies of collisions would be even higher than what was achieved in 2012. The questions the scientists are trying to answer are to fill in the gaps in the Standard Model and includes ideas like Super Symmetry, Dark Matter, Anti Matter and Structure of Early Universe and even Alternate Dimensions as predicted by String Theory.

Super symmetry: A theory that hypothesizes the existence of more massive partners of the standard particles we know — could facilitate the unification of fundamental forces including gravity, as Standard Model does not explain gravity. If super symmetry is right, then the lightest super symmetric particles should be found at the LHC.

Dark matter: Cosmological and astrophysical observations have shown that all of the visible matter accounts for only 4% of the Universe. The search is open for particles or phenomena responsible for dark matter and dark energy making up 23% and 73 % of the whole universe! The first hint of the existence of dark matter came in 1933, when astronomical observations and calculations of gravitational effects revealed that there must be more ‘stuff’ present in the Universe than we could account for by sight. Researchers now believe that the gravitational effect of dark matter makes galaxies spin faster than expected, and that its gravitational field deviates the light of objects behind it. Measurements of these effects show the existence of dark matter, and can be used to estimate its density even though we cannot directly observe it. Dark energy is a form of energy that appears to be associated with the vacuum in space, and makes up approximately 73% of the Universe. Dark energy is homogeneously distributed throughout the Universe and in time. In other words, its effect is not diluted as the Universe expands. The even distribution means that dark energy does not have any local gravitational effects, but rather a global effect on the Universe as a whole. This leads to a repulsive force, which tends to accelerate the expansion of the Universe. The rate of expansion and its acceleration can be measured by experiments using the Hubble law. These measurements, together with other scientific data, have confirmed the existence of dark energy and have been used to estimate its quantity.

Anti matter: The LHC will also help us to investigate the mystery of antimatter. Matter and antimatter must have been produced in the same amounts at the time of the Big Bang, but from what we have observed so far, our Universe is made only of matter. Why? The LHC could help to provide an answer. It was once thought that antimatter was a perfect ‘reflection’ of matter — that if you replaced matter with antimatter and looked at the result as if in a mirror, you would not be able to tell the difference. We now know that the reflection is imperfect, and this could have led to the matter-antimatter imbalance in our Universe.

Very Early universe: In addition to the studies of proton–proton collisions, heavy-ion collisions at the LHC will provide a window onto the state of matter that would have existed in the early Universe, called ‘quark-gluon plasma’. When heavy ions collide at high energies they form for an instant a ‘fireball’ of hot, dense matter that can be studied by the experiments. According to the current theories, the Universe, born from the Big Bang, went through a stage during which matter existed as a sort of extremely hot, dense soup — called quark-gluon plasma (QGP) — composed of the elementary building blocks of matter. See the pictures below of the current model of Big Bang cosmology in it's first fraction of a second. 

As the Universe cooled, the quarks became trapped into composite particles such as protons and neutrons. This phenomenon is called the confinement of quarks. The LHC is able to reproduce the QGP by accelerating and colliding together two beams of heavy ions. In the collisions, the temperature will exceed 100 000 times that of the centre of the Sun. In these conditions, the quarks are freed again and the detectors can observe and study the primordial soup, thus probing the basic properties of the particles and how they aggregate to form ordinary matter. What is so incredible remarkable is that with the collision speeds anticipated at 13 TeV, we can create, albeit at an extremely small scale, the condition of the universe just 1 pico second (1 trillionth) after the Big Bang itself!

What is LHC made of?

Dipole magnets cooled to near absolute zero forms the heart of LHC. The magnet coils for the LHC are wound from a cable consisting of up to 36 twisted 15-mm strands, each strand being made up in turn of 6000-9000 individual filaments, each filament having a diameter as small as 7 micrometers (for comparison, a human hair is about 50 micrometers thick). The 27-km circumference of the LHC calls for some 7600 km of cable, corresponding to about 270 000 km of strand — enough to circle the Earth six times at the Equator. If all the component filaments were unravelled, they would stretch to the Sun and back five times with enough left over for a few trips to the Moon!

Cryogenics:

As noted this magnets at LHC are kept near absolute zero at the LHC, similar to the magnets of a MRI machine. The LHC is the largest cryogenic system in the world and one of the coldest places on Earth. To maintain its 27-km ring (4700 tonnes of material in each of the eight sectors) at super fluid helium temperature (1.9 K, –271.3°C), the LHC’s cryogenic system will have to supply an unprecedented total refrigeration capacity.

Detectors:

What are the detectors at the LHC? There are six experiments installed at the LHC: A Large Ion Collider Experiment (ALICE), ATLAS, the Compact Muon Solenoid (CMS), the Large Hadron Collider beauty (LHCb) experiment, the Large Hadron Collider forward (LHCf) experiment and the Total Elastic and diffractive cross section Measurement (TOTEM) experiment.

Data capture from LHC:

LHC is not only the largest man made machine ever, but also tests the computing capabilities by taking it to the limit. (Note here that it was at CERN the world wide web was invented). LHC creates more data to be analyzed per second than humans have ever been dealt with. The data flow from all four experiments will be about 700 MB/s, that is around 15 000 000 GB (=15 PetaByte) per year, corresponding to a stack of CDs about 20 km tall each year! (around twice the height of Everest!) This enormous amount of data will be accessed and analyzed by thousands of scientists around the world. 

Are the LHC collisions dangerous?

The LHC can achieve energies that no other particle accelerators have reached before. The energy of its particle collisions has previously only been found in Nature. And it is only by using such a powerful machine that physicists can probe deeper into the key mysteries of the Universe. Some people have expressed concerns about the safety of whatever may be created in high-energy particle collisions. However there are really no reasons for such concern. Accelerators only recreate the natural phenomena of cosmic rays under controlled laboratory conditions. Cosmic rays are particles produced in outer space in events such as supernovae or the formation of black holes, during which they can be accelerated to energies far exceeding those of the LHC. Cosmic rays travel throughout the Universe, and have been bombarding the Earth’s atmosphere continually since its formation 4.5 billion years ago. Despite the impressive power of the LHC in comparison with other accelerators, the energies produced in its collisions are greatly exceeded by those found in some cosmic rays. Since the much higher-energy collisions provided by nature for billions of years have not harmed the Earth, there is no reason to think that any phenomenon produced by the LHC will do so. The LHC’s energy, although powerful for an accelerator, is modest by nature’s standards.

Will LHC make Black holes?

Massive black holes are created in the Universe by the collapse of massive stars, which contain enormous amounts of gravitational energy that pulls in surrounding matter. The gravitational pull of a black hole is related to the amount of matter or energy it contains — the less there is, the weaker the pull. However these microscopic black holes produced inside the LHC will not generate a strong enough gravitational force to pull in surrounding matter. If the LHC can produce microscopic black holes, cosmic rays of much higher energies would already have produced many more. Since the Earth is still here, there is no reason to believe that collisions inside the LHC are harmful. Black holes lose matter through the emission of energy via a process discovered by Stephen Hawking. Any black hole that cannot attract matter, such as those that might be produced at the LHC, will shrink, evaporate and disappear. The smaller the black hole, the faster it vanishes. If microscopic black holes were to be found at the LHC, they would exist only for a fleeting moment. They would be so short-lived that the only way they could be detected would be by detecting the products of their decay

Can we investigate at energy levels higher than even the LHC?

Though LHC is indeed the most marvellous and powerful machine ever made by humans, and the energy concentration obtained at LHC can investigate early universe almost a trillionth of a second after the big bang, to achieve even higher collision speeds is limited by the size of the accelerator that can be made. To make an accelerator to recreate the situation a femtosecond (quadrillionth of a second) after the Big Bang will have to be bigger than the whole earth! And taking it to energy levels to ever smaller fractions of time after big bang would be truly beyond humans even theoretically (one calculation suggest an accelerator to test the inflationary period after big bang would need a particle accelerator almost a 1000 light years across!). However  we may be able to update our technology so even higher power accelerators than LHC can be made humanly possible, and one of the candidates for such an accelerator is based on new techniques like Laser Plasma accelerators. It is quite conceivable that in another generation we will have a particle accelerator that would help us see even farther back in time, even deeper in to the mind of God. But as of now, it is LHC and the brilliant scientists working there whom we are looking up to come up with answers to the deepest and hardest questions ever faced by humanity.