Discovery Science with the Large Hadron Collider

Professor Emmanuel Tsesmelis

Directorate Office, CERN

Department of Physics, University of Oxford


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Normal matter is made up of up and down quarks (found in protons and neutrons), electrons and electron neutrinos. The force particles include the graviton (which may be responsible for gravity), photons (responsible for electromagnetic forces), Gluons (elementary particles that act as the exchange particles or gauge bosons for the strong force between quarks i.e. responsible for the strong nuclear force), W and Z bosons (responsible for the weak nuclear force).

The bottom left slide shows the standard model in a simple mathematical form. This gave predictions for the masses of the W, Z and Higgs bosons.

The formula expresses the so-called Lagrangian of the Standard Model of Particle Physics (Glashow-Salam-Weinberg Model). In general, a Lagrangian defines a theory by summarising the dynamics of the system. The Lagrangian is given by T – V, the difference between the kinetic energy and the potential energy. The Lagrangian given in the slide consists of the following 4 terms (one on each line):

1)      W, Z and gamma kinetic energies and self-interactions.

2)      Lepton and quark kinetic energies and their interactions with W, Z and gamma.

3)      W, Z, gamma and Higgs boson masses and couplings.

4)      Lepton and quark masses and couplings to Higgs boson.

The study of the Standard Model is based on these mathematical foundations!

The full derivation of the Lagrangian takes several pages of mathematical rigour but outlines can be found in many university undergraduate-level textbooks of modern elementary particle physics.

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The Large Hadron Collider is looking at the origin of the universe and the difference between matter and anti-matter.

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The electro-weak phase transition (which will be investigated by ATLAS and CMS) was when the quark-gluon plasma universe cooled and the current fundamental forces we know took their present forms through further symmetry breaking – notably the breaking of electroweak symmetry – and the full range of complex and composite particles we see around us today became possible, leading to a matter dominated universe, the first neutral atoms (almost all of them hydrogen), and the cosmic microwave background radiation we can detect today.

Quark matter or QCD matter refers to any of a number of theorized phases of matter whose degrees of freedom include quarks and gluons. These theoretical phases would occur at extremely high temperatures and densities, billions of times higher than can be produced in equilibrium in laboratories. Under such extreme conditions, the familiar structure of matter, where the basic constituents are nuclei (consisting of nucleons which are bound states of quarks) and electrons, is disrupted. In quark matter it is more appropriate to treat the quarks themselves as the basic degrees of freedom. In the standard model of particle physics, the strong force is described by the theory of quantum chromodynamics (QCD). At ordinary temperatures or densities this force just confines the quarks into composite particles (hadrons) of size around 10E−15 m = 1 femtometer = 1 fm (corresponding to the QCD energy scale ΛQCD ≈ 200 MeV) and its effects are not noticeable at longer distances. However, when the temperature reaches the QCD energy scale (T of order 10E12 kelvins) or the density rises to the point where the average inter-quark separation is less than 1 fm (quark chemical potential μ around 400 MeV), the hadrons are melted into their constituent quarks, and the strong interaction becomes the dominant feature of the physics. Such phases are called quark matter or QCD matter. The strength of the colour force makes the properties of quark matter unlike gas or plasma, instead leading to a state of matter more reminiscent of a liquid. At high densities, quark matter is a Fermi liquid, but is predicted to exhibit colour superconductivity at high densities and temperatures below 10E12 K. ALICE, ATLAS and CMS will investigate this.

Cosmic rays are very high-energy particles, mainly originating outside the Solar System. They may produce showers of secondary particles that penetrate and impact the Earth’s atmosphere and sometimes even reach the surface. Comprised primarily of high-energy protons and atomic nuclei, their origin has been a mystery. Data from the Fermi space telescope (2013) have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernovae of massive stars. However, this is not thought to be their only source. Active galactic nuclei probably also produce cosmic rays. The term ray is a historical accident, as cosmic rays were at first, and wrongly, thought to be mostly electromagnetic radiation. In modern common usage high-energy particles with intrinsic mass are known as “cosmic” rays, and photons, which are quanta of electromagnetic radiation (and so have no intrinsic mass) are known by their common names, such as “gamma rays” or “X-rays”, depending on their frequencies.

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The Higgs boson or Higgs particle is an elementary particle initially theorised in 1964, and tentatively confirmed to exist on 14 March 2013. The discovery has been called “monumental” because it appears to confirm the existence of the Higgs field, which is pivotal to the Standard Model and other theories within particle physics. In this discipline, it explains why some fundamental particles have mass when the symmetries controlling their interactions should require them to be massless, and—linked to this—why the weak force has a much shorter range than the electromagnetic force. Its existence and knowledge of its exact properties are expected to impact scientific knowledge across a range of fields, and should eventually allow physicists to determine whether the Standard Model or a competing theory is more likely to be correct, guide other theories and discoveries in particle physics, and—as with other fundamental discoveries of the past—potentially over time lead to developments in “new” physics, and new technologies.

Self-interaction gives the Higgs boson its mass and the Higgs field permeates space. It is the field that gives sub-atomic particles their mass. Remove the field and you are left with the Higgs particle. Bits of the Higgs field can be “stolen” and are no longer available. 3 out of 4 parts of the field get gobbled up by the W and Z bosons.

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In astronomy and cosmology, dark matter is a type of matter hypothesised to account for a large part of the total mass in the universe. Dark matter cannot be seen directly with telescopes; evidently it neither emits nor absorbs light or other electromagnetic radiation at any significant level. Instead, its existence and properties are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. According to the Planck mission team, and based on the standard model of cosmology, the total mass–energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Thus, dark matter is estimated to constitute 84.5% of the total matter in the universe and 26.8% of the total content of the universe. Dark matter came to the attention of astrophysicists due to discrepancies between the mass of large astronomical objects determined from their gravitational effects, and the mass calculated from the “luminous matter” they contain: stars, gas and dust. It was first postulated by Jan Oort in 1932 to account for the orbital velocities of stars in the Milky Way, and by Fritz Zwicky in 1933 to account for evidence of “missing mass” in the orbital velocities of galaxies in clusters.

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In particle physics, supersymmetry (often abbreviated SUSY) is a proposed symmetry of nature relating two basic classes of elementary particles: bosons, which have an integer-valued spin, and fermions, which have a half-integer spin. Each particle from one group is associated with a particle from the other, called its superpartner, whose spin differs by a half-integer. In a theory with unbroken supersymmetry each pair of superpartners shares the same mass and internal quantum numbers besides spin, but since no superpartners have been observed yet, supersymmetry must be a spontaneously broken symmetry.

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Following his research on general relativity, Einstein entered into a series of attempts to generalize his geometric theory of gravitation to include electromagnetism as another aspect of a single entity. In 1950, he described his “unified field theory” in a Scientific American article entitled “On the Generalized Theory of Gravitation”. Although he continued to be lauded for his work, Einstein became increasingly isolated in his research, and his efforts were ultimately unsuccessful. In his pursuit of a unification of the fundamental forces, Einstein ignored some mainstream developments in physics, most notably the strong and weak nuclear forces, which were not well understood until many years after his death. Mainstream physics, in turn, largely ignored Einstein’s approaches to unification. Einstein’s dream of unifying other laws of physics with gravity motivates modern quests for a theory of everything and in particular string theory, where geometrical fields emerge in a unified quantum-mechanical setting.

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Why have more dimensions?

It’s not so hard to construct higher dimensional worlds using the Einstein equations. But the question is then: WHY BOTHER? It’s because physicists dream of a unified theory: a single mathematical framework in which all fundamental forces and units of matter can be described together in a manner that is internally consistent and consistent with current and future observation. And it turns out that having extra dimensions of space makes it possible to build candidates for such a theory.

Extra dimensions in string theory

Superstring theory is a possible unified theory of all fundamental forces, but superstring theory requires a 10 dimensional spacetime, or else bad quantum states called ghosts with unphysical negative probabilities become part of the spectrum. Now this creates a problem in d=10 string theory: how to get the d=4 world as we know it out of the theory. So far there are two main proposals: 1. Roll up the extra dimensions into some very tiny but nonetheless interesting space of their own. This is called Kaluza Klein compactification. 2. Make the extra dimensions really big, but constrain all the matter and gravity to propagate in a three dimensional subspace called the three brane. (For an analogy, your computer screen could be said to be a two brane of three dimensional space). These types of theories are called braneworlds.

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Two bunches of protons are used. One moves clockwise and the other moves anti-clockwise.

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An LHC cryodipole is a superconducting dipole magnet (“dipole cold mass”) housed inside a cryostat. These magnets are necessary to keep the particles moving around the circular path. They must be kept cold to be superconducting.

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In particle physics, the Higgs mechanism is a kind of mass generation mechanism, a process that gives mass to elementary particles. According to this theory, particles gain mass by interacting with the Higgs field that permeates all space. Self-interaction actually gives the Higgs boson its own mass.

The Standard Model states that a field (the Higgs field) exists throughout space which breaks certain symmetry laws of the electroweak interaction. The field’s existence triggers the Higgs mechanism, and therefore the gauge bosons corresponding to these symmetries—those responsible for the weak force—are massive, and consequently have a very short range.

Some years after the original theory, scientists realised that the same field would also explain, in a different way, why other fundamental constituents of matter (including electrons and quarks) have mass.

The existence of the Higgs field can be proven by searching for a matching particle associated with it, which should also exist—the “Higgs boson”. Detecting Higgs bosons would automatically prove the Higgs field exists, and that the Standard Model is essentially correct—the crucial question. As of 2013, scientists are virtually certain that they have proved the Higgs boson exists, and further testing over the coming years should eventually tell us more about it and show which version of the theory best matches the experimental results.

The Higgs Field is an invisible energy field that exists throughout the universe. The field is accompanied by a fundamental particle called the Higgs Boson, which it uses to continuously interact with other particles. As particles pass through the field they are endowed with the property of mass, much as an object passing through treacle (or molasses) will become slower.

Although apparent, mass is not generated by the Higgs field, as creation of matter or energy would conflict with the laws of conservation; mass is, however, transferred to particles from the field, which contains the relative mass in the form of energy. Once the field has endowed a formerly massless particle the particle slows down because it has become heavier. Bits of the Higgs field therefore get “stolen” and are no longer available.

If the Higgs field did not exist particles would not have the mass required to attract one another, and would simply float around freely at light-speed.

The process of endowing a particle with mass is known as the Higgs Effect.

The Higgs particle is actually what is left if you take away the field. Three out of four parts of the Higgs field is believed to be gobbled up by the W and Z bosons.

The W and Z bosons are the elementary particles that mediate the weak interaction. The W bosons are named after the weak force (they have a positive and negative electric charge of 1 elementary charge respectively and are each other’s antiparticles). The physicist Steven Weinberg named the additional particle the “Z particle”, later giving the explanation that it was the last additional particle needed by the model – the W bosons had already been named – and that it has zero electric charge.

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Experiments with the Large Hadron Collider (LHC) may be able to detect WIMPs (weakly interacting Massive Particles) produced in collisions of the LHC proton beams. Because a WIMP has negligible interactions with matter, it may be detected indirectly as (large amounts of) missing energy and momentum which escape the LHC detectors, provided all the other (non-negligible) collision products are detected. These experiments could show that WIMPs can be created, but it would still require a direct detection experiment to show that they exist in sufficient numbers in the galaxy to account for dark matter.

The Large Hadron Collider (LHC) has a design energy of 14 TeV for proton-proton collisions and 1150 TeV for Pb-Pb collisions. It was argued in 2001 that in these circumstances black hole production could be an important and observable effect at the LHC or future higher-energy colliders. Such quantum black holes should decay emitting sprays of particles that could be seen by detectors at these facilities. A paper by Choptuik and Pretorius, published on March 17, 2010 in Physical Review Letters, presented a computer-generated proof that micro black holes must form from two colliding particles with sufficient energy, which might be allowable at the energies of the LHC if additional dimensions are present other than the customary four (three space, one time).

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The International Linear Collider (ILC) is a proposed linear particle accelerator. It is planned to have a collision energy of 500 GeV initially, with the possibility for a later upgrade to 1000 GeV (1 TeV). The host country for the accelerator has not yet been chosen and proposed locations are Japan, Europe (CERN) and the USA (Fermilab). Japan is considered the most likely candidate, as the Japanese government is willing to contribute half of the costs, according to a representative for the European Commission on Future Accelerators. Construction could begin in 2015 or 2016 and will not be completed before 2026.

It is widely expected that effects of physics beyond that described in the current Standard Model will be detected by experiments at the proposed ILC. In addition, particles and interactions described by the Standard Model are expected to be discovered and measured. At the ILC physicists hope to be able to: Measure the mass, spin, and interaction strengths of the Higgs boson; If existing, measure the number, size, and shape of any TeV-scale extra dimensions; Investigate the lightest supersymmetric particles, possible candidates for dark matter. To achieve these goals, new generation particle detectors are necessary.

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