Supersymmetry search using $ Z^ 0$ bosons produced in neutralino decays at the ATLAS detector (original) (raw)
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Our universe is formed of radiant energy and matter. Matter assumes different forms called substances. The science of chemistry attempts to describe the properties of substances and the reactions that convert them into other substances. The Development of the Atomic Theory of Matter Near the beginning of the 19th century, the law of simple multuple proportions was enunciated by the English physicist and chemist, Dalton, stating that the weights of the constituents (elements) of substances are in the ratio of small integers. Dalton hypothesized that each certain element consists of small, identicle particles, which he called atoms, and explained the law of constant proportions by theorizing that atoms of particular elements combine in definite proportions to form specific compounds. It was discovered later that some compounds could be decomposed by electric current and that a certain quantity of electric current was required to decompose a certain amount of a particular substance (compound). This led researchers to theorize that electricity consisted of units of charge that were particles (electrons), and that atoms contained such particles along with other oppositely charged particles (protons). Late in the 19th century, J.J. Thompson, an English researcher, developed an experiment utilizing cathode rays to determine the charge to mass ratio of the electron. R.A. Millikan, an American, devised an experiment showing that all electrical charges are integral multiples of a definite, elementary unit, which he quantified: electron charge:-1.6 X 10-19 C Still, scientists did not have a clear idea of what the atom looked like. The English researcher, Ernest Rutherford, provided clearer focus when he bombarded a thin sheet of gold foil with alpha rays (beams of helium nuclei). If atoms were uniformly dense, as he expected, all of the rays would have passed directly through. That did not occur. He recorded a few large deflections, very few when compared to the total number of alpha particles emitted. He made sense of his results by postulating that most of the mass of the atom is concentrated in a very small particle, which he termed the atomic nucleus. The nucleus contains the positively charged protons (and also neutrons in most atoms) with electrons orbitting about it. Compared to the total volume of the atom, which is determined by the orbital space of the electrons, the volume of the nucleus is minute. For the most part, matter is filled with empty space! Quantum Theory: The arrival of quantum theory marks a departure from classical Newtonian physics. Before the twentieth century, phenomena such as electrical charge or light were conceived of as continuously divisible quantities. With quantum theory, we have the principle that certain physical quantities can only assume discrete quantities. For example, charge cannot have magnitude less than 1.6 X 10-19 C. Amounts of electrical charge, then, are always integral multiples of this quantity. The initial introduction of quantum theory is credited to Max Planck. At the turn of the cenury, physics was unable to explain certain characteristics of the curves of energy against wavelength for light emitted when a black-body is heated up (A black-body is an
Insights from the classical atom
Physics Today, 2012
R ichard Feynman once wrote that the concept of the atomic structure of the material world was the most fertile idea we inherited from antiquity. But although the so-called atomic hypothesis traces its beginnings to the fifth century BC (see box 1), it was only a century ago, in 1911, that the atom secured its place as the cornerstone of the modern physical sciences. That year marked two important advances in our understanding of the microscopic world. First, from the observation that α particles were deflected as they passed through a thin gold foil, Ernest Rutherford arrived at the planetary model of the atom, in which electrons orbit a massive nucleus. Second, he evaluated the angle-differential cross section for the deflection of the α particles. 1 Rutherford's formula was purely classical-he treated atomic particles as having trajectories influenced by Coulomb forces-yet it proved remarkably accurate. An identical expression can be derived quantum mechanically by calculating the scattering amplitude due to Coulomb interaction in the limit of weak scattering, where the first Born approximation holds (see box 2). The success of Rutherford's calculations, however, was soon overshadowed by the advent of quantum mechanics (QM), which triumphed in describing atomic and subatomic processes. Deemed generally inadequate, the classical approach was neglected until 1953, when Gregory Wannier, then at Bell Labs, used classical mechanics to derive a law describing near-threshold atom ionization due to electron impact. 2 In the decades that followed, various efforts from an assortment of groups demonstrated the enduring capacity of classical mechanics to elucidate the structure and interactions of atoms.
researchgate.net, 2021
In two exciting decades, Becquerel, the Curies, Thomson, Rutherford, and Bohr, became the undisputed protagonists of a new scientific revolution, one we might call “the atom is not the smallest unit of matter”. Scientists now had the huge task of trying to look inside the atom. The nuclear age had begun.
100th Anniversary of Bohr’s Model of the Atom
Angewandte Chemie International Edition, 2013
Dedicated to Professor W. Kutzelnigg on the occasion of his 80th birthday atomic spectroscopy • Bohr, Niels • model of the atom • periodic system • quantum chemistry One hundred years ago this autumn, the young 27-year-old Danish physicist Niels Bohr published his atomic models. [1] In 1911-1912 he had visited the centers of experimental and theoretical atomic physics: Cambridge (where Thomson, the Nobel Laureate of 1906, had discovered the electron in 1897) and Manchester (where Lord Rutherford, Chemistry Nobel Laureate of 1908 for studies in radioactivity, had discovered the atomic nucleus in 1911). The harvest of Bohrs postdoctoral stay comprised in particular three papers with the comprehensive title "On the Constitution of Atoms and Molecules" published in the Philosophical Magazine in the fall of 1913. [1a] The papers became famous as "Bohrs Trilogy". His "investigation of the structure of atoms" earned him the Nobel Prize in 1922 (Figure 1). [2] We shall begin the present account by reviewing the atomistic concepts in chemistry and physics up to the beginning of the 20th century (Section 1), a time when many atomistic phenomena were known in detail but remained physically unexplainable. Some scientists had concluded that classical physics needed revision and hypothetical suggestions inconsistent with classical physics appeared. Against this background we shall then describe Bohrs fundamental steps and achievements during the period 1912-1913 as regards the physical structure and spectra of atoms, the periodic ordering of the elements, and chemical bonding (Section 2). Finally, we shall identify Bohrs lasting results for chemistry while also noting those conjectures that were short-lived (Section 3). In this context we shall highlight the importance of subsequent physical discoveries for chemistry, notably the electron spin, the Pauli exclusion principle, and the Heisenberg uncertainty principle. We shall conclude with some scientific and philosophical observations and also call attention to certain definitive and chemically relevant insights of Bohr that still have not yet found their way into many chemistry textbooks. Recent short accounts of Bohrs atomic models are found in Refs. [3-5], a recommendable book in the present context is Ref. [6], the respective commented writings in "Niels Bohr Collected Works" are found in Refs. [7, 8]. We will focus on Bohrs chemically relevant electronic theory, without nuclear chemistry.