What Is LHC? Here Is Everything You Want To Know About!

 Last century, we have seen many technological advancements which has allowed human to build incredible machines and to do incredible things. These technologies had made it possible for us to go to the Moon, or to produce nuclear energy on Earth and many more insane engineering marvels. Of those insane machines, one of the most remarkable machine ever built by us is the LHC, the Large Hadron Collider. The Large Hadron Collider is a gigantic particle accelerator built under the CERN, the largest laboratory of particle physics in the world. The LHC is 27 kilometres long ring, with such a huge circumference, it is the largest particle collider in the world. 

What is a collider actually?

 To dig deeper into the fundamental structure of the nature, scientists have to create a situation which are similar to that of the Big Bang. To achieve that using our current understanding, one of the ways out could be to accelerate elementary particles to extremely high speeds (almost at the speed of light), and then make them clashing together and this is exactly what a collider does. Then after the collision scientists analysed the outcome of these collisions which led them to the discovery of new particles and to improve our understanding of the Universe. In LHC, there are two beams of high energy protons traveling in opposite direction which are made to collide at very high speeds. These particles are accelerated up to a speed of 99.999999% of the speed of light.

 How is it done? 

The main method used to accelerate particles is to use “radio frequency cavities”.  Radio Frequency cavities are tubes which are placed all along the collider, containing an electric field which changes direction periodically. 16 such radio frequency cavities are there in LHC. When  a charged particle is released in the electric field of LHC it is accelerated and it direction depends on the sign of the charge. Positively charged particles move along with the field where as the negatively charged particles move opposite to it. Neutral particles don't interact with electric fields, so they are not used. Every time protons enter a new RF cavity, they gain some  energy due to the acceleration caused by electric field. After they have traveled thousands of Eventually, after they have completed thousands of kilometres under acceleration due to electric field in these cavities, they reach a speed 99.999999% of light.

Well, do you think that is enough enough to make a collider? You may wonder how are we keeping them in the circle even though they are being accelerated. Normally particles tend to go straight. If there were no external aid, these protons would quickly crash against the walls of the ring. So, what keeps them in the circular trajectory? Some of you may have guessed it correct, magnetic fields! Magnetic fields cause no change in speed of the particles but the change in velocity due to its change in direction. Here, strong magnets are used to deflect the protons and keep them along the circular ring of LHC. But to create fields strong enough to deflect these energetic protons, the scientists need to use special and different types of magnets. To bring out the best performance these magnets are operated in super conducting state in which the loss of electrical energy is almost zero. To achieve this super conducting state the magnets have to be kept at very low temperatures, something few Kelvin. To sustain such a temperature they constantly refrigerate it using liquid helium. 

 The magnetic fields produced by these strong magnets have an intensity of 8 Tesla which is 20,000 times stronger than the Earth's magnetic field intensity.  Similar magnets are also used to “squeeze” the bunches of protons and keep them collimated. Once protons are accelerated at the desired speed, then it’s time to make them colliding! The collisions between the proton beams occur at four designated points along the ring. When the two beams collide, the protons “break apart” and hundreds of new particles are produced. In order to detect and identify them, and to discover new things about nature, scientists built huge detectors at these points of collision. The main four detectors are ATLAS, CMS, ALICE and LHCb. ATLAS and CMS are the biggest ones, and they have multiple purposes. One of the biggest achievement of these two experiments is the discovery of the Higgs boson in 2012, the particle that gives mass to all the others.

 The ALICE experiment aims to study a particular form of matter, called “quark-gluon plasma”, while the LHCb experiment was built to investigate why the amount of matter in the Universe is much larger than the amount of antimatter. However, don’t think that the task of these experiments is that easy! In fact, the amount of data they need to record(and then analyse) is impressive. Every second, in fact, there are about 1 billion collisions per second at LHC. And in each of these collisions, hundreds of particles are produced. It’s therefore impossible to record all the events, otherwise the CERN computing system would run out of space pretty soon. So, these detectors need to identify only the interesting events within this chaos, and to record those only. Indeed, more than 99.99% of the events produced in a collision at LHC are immediately discarded, and only the rest of them is stored for the analysis. Even with this system, more than one million gigabyte of data is recorded on average every day at the LHC!

But how do scientists use the data obtained from the LHC?

During each collision, hundreds of particles are produced. However, many of them are “not interesting” for scientists. In fact, their target is to identify new particles in this mess. Here it’s where the detectors play a very important role, because their sensitivity allows to identify only the potentially interesting particles produced at the collision point. Each of the four detectors consists of several sub-detectors, each of them having a different purpose. For example, one of the sub-systems of CMS is the “tracker”: its purpose is to track the trajectory of the particles produced inthe collision. This tracker consists of several silicon layers,and when a charged particle hits one of these layers, it leaves a “trace”. By checking all the hits left by the particle on the layers of the tracker, it is possible to reconstruct its whole trajectory from the collision point. Another sub-detector of CMS is the “calorimeter”, its purpose is to measure the energy released by the particles produced in the collision. It is made of special crystals of a particular material, lead tungstate. When a particle passes through it, this material emits a “cascade” of electrons and light. And by measuring the amount of light produced,scientists can infer the initial energy of the particle. Thanks to these special sub-detectors, scientists are able to collect a lot of important information about the particles produced in the collisions. Then, they finally analyse the data. 

 The purpose of the analysis can vary. For example, one of the goals can be the discovery of a new particle. This is done by identifying the correct “signature”of the hunted particle. In fact, each type of particle has different properties (different electric charge, different mass), so it leaves a different signature in the sub-systems of the detector. For example, an electron releases a lot of light in the calorimeter, while a neutron does not leave any. When scientists were searching for the Higgs boson, they knew exactly what was the “signature” of this particle (because the theory predicted exactly how this particle was going to behave). However, often it is not that easy to distinguish it from the tracks of other known particles. For example, the Higgs boson decays very quickly(it means that it “breaks apart”, producing new particles). Since this process is fast and occurs very close to the collision point, when a Higgs boson is produced, it decays even before reaching the detector. So, how is it possible to detect it? The trick is, when the Higgs boson decays,it produces particles that are well-known and easily recognisable. For instance, the Higgs boson sometimes decays into two photons. These photons have a very clear signature,because they don’t leave any sign in the tracker, but they release large amounts of light when they hit the crystals of the calorimeter. This is what scientists do if they want to search for a Higgs boson that decayed into two photons, they search for large signals produced in the calorimeter, without any hit in the tracker.

 However, as you can imagine, life is not that easy! In fact, dozens of other photons are produced in a single proton-proton collision. And we also have to take into account other particles that are produced in the collision and that can also decay into two photons,because these particles can “mimic” the signature of the Higgs boson. So, how to distinguish them from the actual signal of a real Higgs boson? This is done by requiring additional constraints on the signature released by the two photons. For instance, the energy and the direction of the photons produced by the decay of a Higgs boson are generally different from those of the photons produced directly at the point of collision, so it is possible to use these characteristics to discriminate between photons that are produced by the decay of a Higgs boson and those that are not. Of course, this is just a simplification, in reality, physicists have to use very complicate algorithms in order to run their analysis to identify something interesting. Thanks to all these techniques, scientists are able to do important discoveries about the Universe, even in this chaos of particles produced during the collisions. The discovery of the Higgs boson is just one such example.

 The LHC has also been able to provide many additional measurements, used to test the validity of the Standard Model, currently the most comprehensive theory of physics that describes the particles that build up matter and the forces that act between them. But this is not the end of the story, because LHC aims to answer some of the fundamental questions about our Universe: what is the nature of the dark matter? Why is there more matter than antimatter inthe Universe? Are there new forces of nature that we don’tknow yet? To answer all these questions, scientists need to collect much more data, that’s why LHC is set to continue to operate for many years, also with some important machine upgrades that will allow to increase the energy of the collisions and hopefully to observe new rare processes that we haven’t seen so far. The journey of LHC through knowledge has just begun, and we hope to see many new exciting discoveries in the future!