STANDARD MODEL OF PARTICLE PHYSICS

BY:UPVITA PANDEY

OUTLINE• INTRODUCTION• HISTORICAL BACKGROUND• WHAT IS STANDARD MODEL OF PHYSICS ?• PARTICLE CONTENT• FUNDAMENTAL FORCES• TESTS AND PREDICTIONS• LIMITATIONS OF SM• CHALLENGES• THEORETICAL PROBLEMS• BEYOND SM

INTRODUCTION• In particle physics, an elementary particle  is a particle

whose substructure is unknown, thus it is unknown whether it is composed of other particles.

• Known elementary particles include the fundamental fermions(quarks, leptons, antiquarks and antileptons), which generally are "matter particles" and “antimatterparticles",

• As well as the fundamental bosons (gauge bosons and Higgs bosons), which generally are "force particles" that mediate interactions among fermions.

Ancient times People think that earth, air, fire, and water are the fundamental elements.

1802 Dalton’s Atomic theory began forming.

1897 J. J. Thompson discovered the electron.

1911 Rutherford discovered positive nucleus.

1930 Pauli invented the neutrino particle.

1932 James Chadwick discovered the neutron.

1937 The muon was discovered by J. C. Street and E. C. Stevenson.

1956 First discovery of the neutrino by an experiment: the electron neutrino.

1962 Discovery of an other type of neutrino: the muon neutrino.

1969 Friedman, Kendall, and Taylor found the first evidence of quarks.

1974 The charmed quark was observed.

1976 The tau lepton was discovered at SPEAR.

1977 Experimenters found proof of the bottom quark.

1983 Carlo Rubbia and Simon Van der Meer discovered the W and Z bosons.

1991 LEP experiments show that there are only three light neutrinos.

1995 The top quark was found at Fermilab.

1998 Neutrino oscillations may have been seen in LSND and Super-Kamiokande.

2000 The tau neutrino was observed at Fermilab.

2003 A Five-Quark State has been discovered. 2013 Discovery of Higgs Boson.

HISTORICAL BACKGROUND• The first step towards the Standard Model was Sheldon Glashow’s discovery

in 1961 of a way to combine the electromagnetic and weak interactions.• In 1967 Steven Weinberg and Abdus Salam incorporated the Higgs

mechanism into Glashow’s electroweak theory, giving it a modern form.• The Higgs mechanism is believed to give rise to the masses of all the 

elementary particles in the Standard Model. This includes the masses of the W and Z bosons, and the masses of the fermions, i.e. the quarks and leptons.

• After the neutral weak currents caused by Z boson exchange were discovered at CERN in 1973, the electroweak theory became widely accepted and Glashow, Salam, and Weinberg shared the 1979 Nobel Prize in Physics for discovering it.

• The W and Z bosons were discovered experimentally in 1981, and their masses were found to be as the Standard Model predicted.

WHAT IS STANDARD MODEL ?

The Standard Model explains how the basic

building blocks of matter interact, governed by four

fundamental forces and classifies all the subatomic particles known. Because

of its success in explaining a wide variety of

experimental results, the Standard Model is

sometimes regarded as a "theory of almost

everything".

PARTICLE CONTENT The Standard Model includes members of several classes of elementary

particles (fermions, gauge bosons, and the Higgs boson), which in turn can be distinguished by other characteristics, such as color charge.

FERMIONS GAUGE BOSONS HIGGS BOSON

FERMIONS

Fermions are divided into two groups of six, Those that must bind together are called Quarks and those that can exist independently are called Leptons.

QUARKS Six quarks (up, down, charm,

strange, top, bottom). they carry color charge. they carry electric charge and

weak isospin.

LEPTONS Six leptons (electron, electron

neutrino, muon, muon neutrino, tau, tau neutrino)

do not carry colour charge three neutrinos do not carry

electric charge

Fermions obey the pauli exclusion principle. They are characterized by Fermi-Dirac statistics. They have half integer spin.

GAUGE BOSONS

•Gauge Bosons are of four types and are classified on the basis of force they interact with- Photon- Electromagnetic ForceGluon- Strong ForceW and Z boson- Weak Force

•They have integral spins and the spin of photon, gluon, W and Z boson is 1.

HIGGS BOSON The Higgs particle is a massive scalar elementary particle theorized by Robert

Brout, Francois Englert, Peter Higgs, Gerald Guralnik, C. R. Hagen, and Tom Kibble in 1964 and is a key building block in the Standard Model

It has no intrinsic spin, and for that reason is classified as a boson.

Because the Higgs boson is a very massive particle and also decays almost immediately when created, only a very high-energy particle accelerator can observe and record it. 

On 14 March 2013 the Higgs Boson was tentatively confirmed to exist.

On December 10, 2013, two of them, Peter Higgs and François Englert, were awarded the Nobel Prize in Physics for their work and prediction.

WHAT ARE FUNDAMENTAL

FORCES ?

The Standard Model classified all four

fundamental forces in nature. In the Standard

Model, a force is described as an exchange

of bosons between the objects affected, such as

a photon for the electromagnetic force and

a  gluon for the strong interaction. Those particles

are called force carriers.

TESTS AND PREDICTIONS• The Standard Model (SM) predicted the existence of the W and Z bosons, gluon,

and the top and charm quarks before these particles were observed.

QUANTITY MEASURED (GeV) SM PREDICTION (GeV)Mass of W boson 80.387 ± 0.019 80.390 ± 0.018

Mass of Z boson 91.1876 ± 0.0021 91.1874 ± 0.0021

• The SM also makes several predictions about the decay of Z bosons, which have been experimentally confirmed by the Large Electron- Positron Collider at CERN.

LIMITATIONS• The model does not incorporate the full theory of gravitation, as described

by general relativity or account for the accelerating expansion of the universe.

• The model does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology.

• It also does not incorporate neutrino oscillations (and their non-zero masses).

CHALLENGES• GRAVITY - The standard model does not explain gravity. The approach of

simply adding a "graviton" to the Standard Model does not recreate what is observed experimentally without other modifications. Moreover, instead, the Standard Model is widely considered to be incompatible with the most successful theory of gravity to date, general relativity.

• DARK MATTER AND DARK ENERGY - Cosmological observations tell us the standard model explains about 5% of the energy present in the universe. About 26% should be dark matter, which would behave just like other matter, but which only interacts weakly (if at all) with the Standard Model fields. Yet, the Standard Model does not supply any fundamental particles that are good dark matter candidates. The rest (69%) should be dark energy, a constant energy density for the vacuum. Attempts to explain dark energy in terms of vacuum energy of the standard model lead to a mismatch of 120 orders of magnitude.

•MATTER-ANTIMATTER ASYMMETRY - The universe is made out of mostly matter. However, the standard model predicts that matter and antimatter should have been created in (almost) equal amounts if the initial conditions of the universe did not involve disproportionate matter relative to antimatter. Yet, no mechanism sufficient to explain this asymmetry exists in the Standard Model.

•MUONIC HYDROGEN - Standard Model makes precise theoretical predictions regarding the atomic radius size of ordinary hydrogen (a proton-electron system) and that of muonic hydrogen (a proton-muon system in which a muon is a "heavy" variant of an electron). However, the measured atomic radius of muonic hydrogen differs significantly from that of the radius predicted by the Standard Model.

THEORETICAL PROBLEMSSome features of the standard model are added in an ad hoc way. These are not

problems per se, but they imply a lack of understanding.

• Number of parameters — Standard model depends on 19 numerical parameters. Their values are known from experiment, but the origin of the values is unknown. Some theorists have tried to find relations between different parameters, for example, between the masses of particles in different generations.

• Quantum triviality - Suggests that it may not be possible to create a consistent quantum field theory involving elementary scalar Higgs particles.

• Strong CP problem - Theoretically it can be argued that the standard model should contain a term that breaks CP symmetry, relating matter to antimatter, in the strong interaction sector. Experimentally, however, no such violation has been found, implying that the coefficient of this term is very close to zero. This fine tuning is also considered unnatural.

BEYOND STANDARD MODEL

While the Standard Model goes a long way towards explaining the "whys" of physical interactions, there are still many mysteries yet to be solved.

Due to these shortcomings of the standard model, a need for theories beyond the standard model arose. These theories attempt to resolve the

shortcomings of standard model.

GRAND UNIFICATION• The standard model has three gauge symmetries; the colour SU(3), the weak

isospin SU(2), and the hypercharge U(1) symmetry, corresponding to the three fundamental forces.

• Due to renormalization the coupling constants of each of these symmetries vary with the energy at which they are measured. Around 10^16 GeV these couplings become approximately equal.

• This has led to speculation that above this energy the three gauge symmetries of the standard model are unified in one single gauge symmetry with a simple gauge group, and just one coupling constant. Below this energy the symmetry is spontaneously broken to the standard model symmetries.

• Theories that unify the standard model symmetries in this way are called Grand Unified Theories (or GUTs), and the energy scale at which the unified symmetry is broken is called the GUT scale.

SUPERSYMMETRY Supersymmetry extends the Standard Model by adding another class of

symmetries to the Lagrangian. These symmetries exchange fermionic particles with bosonic ones. Such a symmetry predicts the existence of supersymmetric particles, abbreviated as sparticles, which include the sleptons, squarks, neutralinos, and charginos. Each particle in the Standard Model would have a superpartner whose spin differs by 1/2 from the ordinary particle. Due to the breaking of supersymmetry, the sparticles are much heavier than their ordinary counterparts; they are so heavy that existing particle colliders would not be powerful enough to produce them. However, some physicists believe that sparticles will be detected by the Large Hadron Collider at CERN.

STRING THEORY• String theory is a theoretical framework in which the point-like

particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and interact with each other.

• On distance scales larger than the string scale, a string looks just like an ordinary particle, with its mass, charge, and other properties determined by the vibrational state of the string.

• In string theory, one of the vibrational states of the string corresponds to the graviton, a quantum mechanical particle that carries gravitational force. Thus string theory is a theory of quantum gravity .

TECHNICOLOR Technicolor theories try to modify the Standard Model in a minimal way

by introducing a new QCD-like interaction. This means one adds a new theory of so-called Techniquarks, interacting via so called Technigluons. The main idea is that the Higgs-Boson is not an elementary particle but a bound state of these objects.

PREON THEORY According to preon theory there are one or more orders of particles more

fundamental than those (or most of those) found in the Standard Model. The most fundamental of these are normally called preons, which is derived from "pre-quarks". In essence, preon theory tries to do for the Standard Model what the Standard Model did for the particle zoo that came before it. Most models assume that almost everything in the Standard Model can be explained in terms of three to half a dozen more fundamental particles and the rules that govern their interactions. Interest in preons has waned since the simplest models were experimentally ruled out in the 1980s.

ACCELERON THEORY

• Accelerons are the hypothetical subatomic particles that integrally link the newfound mass of the neutrino and to the dark energy conjectured to be accelerating the expansion of the universe.

• In theory, neutrinos are influenced by a new force resulting from their interactions with accelerons. Dark energy results as the universe tries to pull neutrinos apart.