Unusually, we stand on the threshold of a complete rethinking of our answer to these questions, which will force us to discard the Standard Model — the theory that embodies the year-old consensus as to what these basic constituents and forces are. Four centuries of study reveal that Nature comes to us in an enormous hierarchy of scales, ranging from elementary particles at the smallest sizes up to the observable universe as a whole on the largest distances.
Similarly, an understanding of atoms does not depend on a detailed knowledge of nuclei, and so on.
Yet although the properties of Nature at any one scale are largely independent of most of the details of more microscopic physics, some of these details do turn out to be important for our explanation of the properties of larger systems. The properties of car engines do constrain the average speeds that are possible for traffic flows. Similarly, some chemical and thermal properties of matter depend on the size of atoms; atomic sizes depend on the properties — mass and couplings — of the constituent electrons and nuclei, and so on.
For this reason the properties of matter on the smallest scales play a fundamental role in science; they underpin many of the explanations of why larger things behave the way they do.
Elementary Particle Physics - Physics & Astronomy | McMaster University
The context for present-day research starts with a summary of what has been the paradigm up until now, together with the reasons why this is believed now to be inadequate. The twentieth century saw enormous progress in identifying the fundamental constituents of matter. In the early s these were thought to consist of several dozen types of atoms, together with some oddities like the then-recently-discovered electron and products of radioactive decay. The discovery of quantum mechanics and the nucleus then allowed the many properties of atoms to be inferred from those of nuclei and electrons.
The recognition that nuclei are themselves built from smaller things eventually led to a much more economical list of fundamental objects: protons, neutrons and electrons.
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New particles — muons, pions and a horde of other new particles — continued to be discovered, initially through studies of the cosmic rays that continuously bombard our atmosphere from space. This temporarily led to a much more complicated picture, whose underlying simplicity did not emerge until the s when many of the particles known by that point were themselves found to be made up of still smaller constituents.
The resulting list of particles is strangely redundant. Progress has also been made identifying the interactions through which these particles interact.
Four interactions are now recognized as fundamental, of which only two — gravity and electromagnetism — had been identified in the 19th century. The other two — the strong and weak forces — emerged later as the interactions responsible for binding quarks into protons and neutrons and these into nuclei , and for some of their radioactive decays. In the modern description each of these forces is associated with a field, whose quanta — gravitons, photons, gluons and W and Z bosons — can also be produced in reactions much like any other elementary particles.
The s saw a great synthesis of these constituents and interactions into a very successful theory — the Standard Model — which survives slightly battered in our own day as the best theoretical benchmark we have. The Standard Model describes in detail how the fundamental quarks and leptons interact through three of the four forces: the weak, strong and electromagnetic interactions.
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In particular it accounts for and explains several exact and approximate conservation laws that appear to work very well in practice. Although the Standard Model provides an impressive synthesis of three of the interactions, one force — gravity — remains beyond the pale. The discoveries leading to the Standard Model provide a precise snapshot of the constituents of matter and their interactions down to distance scales that are just beginning to be surpassed now.
Although this represents a major triumph, it is only the first step towards understanding the properties of systems involving two or more particles. Experience shows us that systems with many particles often aggregate into complicated states that exhibit a broad diversity and richness of properties. Typically it is particles that interact through the strongest forces that organize themselves into bound states on the smallest scales. Quarks and gluons who couple to the strong force typically bind into nuclear matter that can take on any of a myriad of forms — nuclei, protons or neutrons, a quark-gluon plasma — depending on the pressures and temperatures involved.
Larger systems built from these — such as atoms and molecules — are usually bound through the next-strongest interaction: electromagnetism.
The weakest long-range force — gravity — is the main player in the largest of objects, like planets, stars and galaxies. The study of how these different kinds of matter behave is of much interest in its own right, and is an important part of our confidence in the success of the Standard Model in describing experiments apart from a few recent examples, see below.
The study of matter in extreme environments — like exotic nuclei, inside neutron stars and stellar explosions, or the very early universe — is of ongoing interest for a variety of reasons. These include understanding qualitatively new forms of matter that can arise at extreme energies and densities; the past and current evolution of the universe as a whole; the role of extreme environments in the origins of the elements we find today in Nature; and so on. Despite the great success the Standard Model has enjoyed when tested over the decades since its discovery, recent years have revealed signs of incipient failure.
It is remarkable that Hadron Physics is well organized in Brazil, with a strong and active community that holds regular meetings of both national and international scope. With the changes in the frontiers of particle physics research described above, it is important that more communication and more merging develops between the areas. The amount of particle physics research presented in the hadronic meetings could enrich substantially the programs of ENFPC, and effort must be made in this direction. On the other hand, the interface between particle and nuclear physics, in large part provided by hadronic physics, must be incentivated in Brazil.
Modern Nuclear Physics must be QCD based, which requires rather profound changes in the research that is traditionally made in our laboratories in Brazil. The interface between particle and nuclear physics, provided by hadronic physics must be developed to impulsionate these changes. I find that the organizing committee was wise and lucky in choosing these topics for the plenary and parallel sessions in phenomenology.
Gay Ducati described the work of hers and her students on the difficult dynamics of QCD in the confinement regime. Including unitarity and screening corrections in the evolution equations, they obtain new equations, which are then solved with approximations, to obtain observable quantities of the proton structure. The screening corrections will be particularly important in the analysis of the heavy ion and hadronic collisions that will be observed in future accelerators. Rosner presented a beautiful and competent review of the studies of CP violation that date since the first discovery of neutral K 0 mesons decaying in wrong CP states.
The violation of CP symmetry, which is related to the behaviour of systems under time reversal, is now under intense scrutiny by experimentalists and theorists, who try to understand all aspects of the systems and dynamics involved. The studies will clarify the parameters of the standard model and the dynamics of the electroweak interaction. In spite of the frightening title how many people knew about the GDH sum rule, with all its simplicity and exactness? Interesting to hear how the low energy photon experiments can test the proton structure, complementing the studies with deep inelastic scattering made in the high energy machines.
Neutrino physics has been a very hot subject in the recent years, with several problems and intriguing questions, and a dramatic effect in our knowledge of the fundamental particles, due to the discovery of the non-zero neutrino masses and the neutrino oscillations. The discoveries are important also for cosmology as for the study of the physics processes inside the sun. The number and nature may some of them be of Majorana kind? The talk by Guzzo was a very elucidating and updated review of the situation.
ORAL07 : Beyond the standard model: exotic bosons of charge 2 decaying into 2 leptons in model. ORAL08 : Neutrino mass oscillations formulae more carefully evaluated with wavepackets formed in the creation process. P : possibility of t neutrino detection in the showers to be observed in Auger experiment.
P : spacial gluon distribution inside the proton and its influence in the observed gluon structure function. P : charmonium production as a mixture of the perturbative and the smaller nonperturbative contributions. P : soliton skyrmion configurations studied with three possible forms of stabilizing terms added in the chiral nonlinear sigma model.
P : Nambu-Jona Lasinio model to study spin 1 vector and axial mesons, their masses, decay constants and symmetry breaking parameters. P : complex real plus quaternionic potentials in nonrelativistic quantum mechanics describe decay processes, with time reversal non-invariance and thus possible connection to CP violation. P : potential models to describe heavy quarkonia bound states, with evaluation of mass spectrum and transition rates.
P : study of the representation of elastic differential cross sections by profile functions of gaussian form. P : Nambu-Jona-Lasinio model with parametrized constituent masses to study mesonic observables. P : Flux of nuclei in primary cosmic rays as origin of observed hadronic and electromagnetic intensities. P : Perturbative QCD calculation exchange of gluon ladders of pp and r p p cross sections. P : Analysis of solar neutrinos observed in Super-Kamiokand experiment, with determination of masses and oscillation parameters.
P : Beyond the standard model: supergravity model leads to scalar and pseudoscalar particles coupled to photons that affect cosmological observables. P : Nambu-Jona-Lasinio model with analytical extension of the 4-point function that produces quark confinement. P : Experimental : separation between electromagnetic and muonic components in observed showers in the Auger experiment. P : Effective chiral lagrangian for NN interaction with mass dependence in central force component.
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