science: the LHC experiments

on 10 september 2008, an experiment was done with colliding protons traveling at near light-speed at Conseil Européen pour la Recherche Nucléaire (CERN)'s (Large Hadron Collider) LHC. CERN's LHC is the most powerful particle accelerator on earth, the second largest being the Fermilab.

Contents

• The Nature of Science
• Fundamental Forces & Interactions
• Elementary Particles
• The Standard Model
• Why Observe Particle Collisions?
• Relevance to The Big Bang
• The God Particle
• What are the Main Goals of the LHC?
• CERN Press Release: First beam in the LHC - accelerating science
• References & Links
• Videos

The Nature of Science

The purpose of Science is to explain natural phenomena in a logical manner. Unlike, religion or even philosophy, Science requires a Hypothesis which explains a natural phenomena which should be experimentally verified. The Hypothesis must be able to predict results of similar phenomena. Furthermore, results obtained from experiments should be repeatable and should correspond to predictions made by the Hypothesis. Only then would a Hypothesis be accepted as a Scientific Theory. In Science, the Theory is not merely a hunch or an idea.

Scientific Theories are not complete or absolute. They are not able to explain natural phenomena with absolute and complete accuracy. Theories have been reformed over time as technology allows better measurements and experimental observation of natural phenomena. The Atomic Model is one such example.

Science does not accept theories which cannot be backed by natural phenomena. That is, String Theory and all other "Theoretical Physical Models" are not accepted Scientific Theories. Experiments such as those done at Fermilab and CERN's LHC are attempts to explore if theoretical predictions made are actually natural phenomena or if they are merely mathematical concepts.

Fundamental Forces & Interactions

What is a Force? And What are Interactions? What has Force & Interactions got to do with the CERN's LHC Experiments?

In school we learnt that a force is something that can pull or push a mass. This definition would suffice explaining most natural phenomena we observe on a daily basis. In a subatomic level, the above definition of Force does not make sense. Instead Force is something which interact with particles. In Particle Physics, Force is often termed an Interaction. And there are four fundamental (classical) Interactions: Weak, Strong, Electromagnetic and Gravitation⁶. We are able to experience Electromagnetic and Gravitational Interaction because their Forces extend a much longer range than Weak and Strong Interactions. Weak and Strong Interactions are effective only at sub-atomic distances.

The Strong force binds quarks together to make protons and neutrons (and other particles). It also binds protons and neutrons in nuclei, where it overcomes the enormous electrical repulsion between protons. The Electromagnetic force holds electrons to nuclei in atoms, binds atoms into molecules, and is responsible for the properties of solids, liquids and gases. 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. 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.⁵

There is one more Interaction proposed by Peter Higgs⁶, which has been theoretically proven but not been experimentally verified. This is one of the hypothesises in Particle Physics being tested at CERN's LHC.

Elementary Particles

Hadrons are types of Quarks, such as Protons and Neutrons. There are two families of particles that make matter: Leptons and Quarks. For example, electrons are made of Lepton Particles and Protons are made of Quark Particles. Quarks respond to all four Fundamental Interactions, unlike Leptons. The hypothetical Higgs Bosons are a types of Quark. There are several types of Higgs Bosons predicted by Particle Physics. Finding these particles would allow us to explore the Higgs Interaction. This is one of the tests carried out at CERN's LHC.

Elementary Particles have a property (such as mass, electrical charge, etc.) called Spin. Bosons and Fermions may be either elementary, like the Photon, or composite, like Mesons. All observed Bosons have integer Spin, as opposed to Fermions, which have half-integer Spin. The Theory of Super-symmetry states that for every type of Boson there exists a corresponding type of Fermion, and vice-versa. This is another type of the tests carried out at CERN's LHC.

The Standard Model

The known Particles and the Forces that act between them as we understand today are called the Standard Model. It has well defined rules and theories that agree with experimental observations. However, some recent phenomena remain unexplained. Furthermore, some theoretical predictions have yet to be experimentally verified.

The Standard Model is a collection of theories that embodies all of our current understanding of fundamental particles and forces. 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. Forces also differ in their strength. The following pictures summarize the Standard Model’s basic points. Although the Standard Model is a very powerful theory, some of the phenomena recently observed — such as dark matter and the absence of antimatter in the Universe — remain unexplained and can not be accounted for in the model.⁵

Why Observe Particle Collisions?

"To us space-time and the laws of quantum mechanics are like the decor, the setting of a play. The elementary particles are the actors, and physics is what they do. A door that we see on the stage is not a door until we see an actor going through it. Else it might be fake, just painted on. Thus, elementary particles are the central objects. They are the actors that we look at, and they play a fascinating piece."⁶.

By observing particle collisions we are able to understand and test various hypothesis of physics. There might be a hypothesis suggesting that there is a door in the set but to test if the door is fake or true, we need to send an actor through the door.

Finding Higgs is one of the main goals of the LHC. Because that would give insight on most of the other tests carried out.

Relevance to The Big Bang

The Big Bang¹ is a Scientific Model of the Universe: how it began, how it's existing and how it will behave in the future. It consists of various Theories. Each Theory being able to explain various phenomena. Extrapolations from Einstein's General Relativity suggest that the universe is expanding². Furthermore, the expansion suggests that the universe was once contracted. Few moments after the Big Bang, matter is believed to have been in a phase called Quark-Gluon Plasma (QGP).

Protons were accelerated as much as 99.9999991% of light-speed⁵. The energy released during these collisions will generate temperatures as much as 15,000,000 K for a fraction of a second. At temperatures that high, Quarks are present. The LHC will be able to study these Quarks and verify the above hypothesis of the Big Bang model.

Contrary to media misinterpretation, the CERN LHC does not recreate the Big Bang or Super Massive Black Holes. It recreates conditions similar to moments after the Big Bang and it may create micro black holes, which decay almost instantly.

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.⁵

Although the energy concentration (or density) in the particle collisions at the LHC is very high, in absolute terms the energy involved is very low compared to the energies we deal with every day or with the energies involved in the collisions of cosmic rays. However, at the very small scales of the proton beam, this energy concentration reproduces the energy density that existed just a few moments after the Big Bang—that is why collisions at the LHC are sometimes referred to as mini big bangs. ⁵

The God Particle

By now you must be aware that the LHC experiments are not about recreating the Big Bang but a series of experiments aimed and answering some fundamental predictions made by Theoretical Physics. The God Particle as the media calls is the Higgs Boson, which we discussed above.

What are the Main Goals of the LHC?

Quoting Wikipedia⁷:

• Is Higgs Mechanism for generating elementary particles as predicted in the Standard Model a natural phenomena? If so, how many Higgs Bosons are there, and what are their masses?
• Are electromagnetism, the strong nuclear force and the weak nuclear force just different manifestations of a single unified force, as predicted by various Grand Unification Theories?
• Why is gravity so many orders of magnitude weaker than the other three fundamental forces?
• Is Supersymmetry realised in nature, implying that the known Standard Model particles have supersymmetric partners?
• Will the more precise measurements of the masses and decays of the quarks continue to be mutually consistent within the Standard Model?
• Why are there apparent violations of the symmetry between matter and antimatter?
• What is the nature of dark matter and dark energy?
• Are there Extra Dimensions in our Universe, as predicted by various models inspired by String Theory, and can we detect them?

Quoting CERN⁵:
Our current understanding of the Universe is incomplete. The Standard Model of particles and forces summarizes our present knowledge of particle physics. 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 may 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. If such a particle exists, experiments at the LHC will be able to detect it.

The Standard Model does not offer a unified description of all the fundamental forces, as it remains difficult to construct a theory of gravity similar to those for the other forces. Supersymmetry — a theory that hypothesises the existence of more massive partners of the standard particles we know — could facilitate the unification of fundamental forces. If supersymmetry is right, then the lightest supersymmetric particles should be found at the LHC.

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 (23%) and dark energy (73%). A very popular idea is that dark matter is made of neutral — but still undiscovered — supersymmetric particles.

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 70% of the Universe. Dark energy is homogenously 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.

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.

The strongest limits on the amount of antimatter in the Universe come from the analysis of the ‘diffuse cosmic gamma-rays’ and the inhomogeneities of the cosmic microwave background (CMB). Assuming that after the Big Bang, the Universe separated somehow into different domains where either matter or antimatter was dominant, it is evident that at the boundaries there should
be annihilations, producing cosmic (gamma) rays. Taking into account annihilation cross-sections, distance, and cosmic red-shifts, this leads to a prediction of the amount of diffuse gamma radiation that should arrive on Earth. The free parameter in the model is the size of the domains. Comparing with the observed gamma-ray flux, this leads to an exclusion of any domain size below 3.7 giga light years, which is not so far away from the entire Universe. Another limit comes from analyzing the inhomogeneities in the CMB — antimatter domains (at any size) would cause heating of domain boundaries and show up in the CMB as density fluctuations. The observed value of ~10-5 sets strong boundaries to the amount of antimatter in the 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. 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.

CERN Press Release: First beam in the LHC - accelerating science

A historic moment in the CERN Control Centre: the beam was successfully steered around the accelerator.
Souce: CERN Web Site

Geneva, 10 September 2008. The first beam in the Large Hadron Collider at CERN1 was successfully steered around the full 27 kilometres of the world's most powerful particle accelerator at 10h28 this morning. This historic event marks a key moment in the transition from over two decades of preparation to a new era of scientific discovery.

"It's a fantastic moment," said LHC project leader Lyn Evans, "we can now look forward to a new era of understanding about the origins and evolution of the universe."

Starting up a major new particle accelerator takes much more than flipping a switch. Thousands of individual elements have to work in harmony, timings have to be synchronized to under a billionth of a second, and beams finer than a human hair have to be brought into head-on collision. Today's success puts a tick next to the first of those steps, and over the next few weeks, as the LHC's operators gain experience and confidence with the new machine, the machine's acceleration systems will be brought into play, and the beams will be brought into collision to allow the research programme to begin.

Once colliding beams have been established, there will be a period of measurement and calibration for the LHC's four major experiments, and new results could start to appear in around a year. Experiments at the LHC will allow physicists to complete a journey that started with Newton's description of gravity. Gravity acts on mass, but so far science is unable to explain the mechanism that generates mass. Experiments at the LHC will provide the answer. LHC experiments will also try to probe the mysterious dark matter of the universe – visible matter seems to account for just 5% of what must exist, while about a quarter is believed to be dark matter. They will investigate the reason for nature's preference for matter over antimatter, and they will probe matter as it existed at the very beginning of time.

"The LHC is a discovery machine," said CERN Director General Robert Aymar, "its research programme has the potential to change our view of the Universe profoundly, continuing a tradition of human curiosity that's as old as mankind itself."

Tributes have been coming in from laboratories around the world that have contributed to today's success.

"The completion of the LHC marks the start of a revolution in particle physics," said Pier Oddone, Director of the US Fermilab. "We commend CERN and its member countries for creating the foundation for many nations to come together in this magnificent enterprise. We appreciate the support that DOE and NSF have provided throughout the LHC's construction. We in the US are proud to have contributed to the accelerator and detectors at the LHC, together with thousands of colleagues around the world with whom we share this quest."

"I congratulate you on the start-up of the Large Hadron Collider," said Atsuto Suzuki, Director of Japan's KEK laboratory, "This is a historical moment."

"It has been a fascinating and rewarding experience for us," said Vinod C. Sahni, Director of India's Raja Ramanna Centre for Advanced Technology, "I extend our best wishes to CERN for a productive run with the LHC machine in the years to come."

"As some might say: 'One short trip for a proton, but one giant leap for mankind!' TRIUMF, and indeed all of Canada, is delighted to bear witness to this amazing feat," said Nigel S. Lockyer, Director of Canada's TRIUMF laboratory. "Everyone has been involved but CERN is to be especially congratulated for bringing the world together to embark on such an incredible adventure."

In a visit to CERN shortly before the LHC's start-up United Nations Secretary General, Ban Ki-moon said: "I am very honored to visit CERN, an invaluable scientific institution and a shining example what international community can achieve through joint efforts and contribution. I convey my deepest admiration to all the scientists and wish them all the success for their research for peaceful development of scientific progress."

CERN, the European Organization for Nuclear Research, is the world's leading laboratory for particle physics. It has its headquarters in Geneva. At present, its Member States are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. India, Israel, Japan, the Russian Federation, the United States of America, Turkey, the European Commission and UNESCO have Observer status.

References & Links

1. Wikipedia: Big Bang
2. Wikipedia: Metric expansion of space
3. SciAm.com: Misconceptions about the Big Bang
4. SciAm.com: The First Few Microseconds
5. CERN FAQ: LHC - the guide
6. Martinus Veltman. (2003). Facts and Mysteries in Elementary Particle Physics. ISBN 981-238-148-1
7. Wikipedia: Large Hadron Collider
8. CERN Web Site
9. Print Quality Posters by ParticleAdventure.org
10. CosmicVariance.com: What Will the LHC Find by Sean Carroll

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