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Probing the Quantum Universe ) July06-final.qxd 9/20/2006 9:29 PM Page 185 ARTICLE DE FOND ( PROBING THE QUANTUM UNIVERSE ) PROBING THE QUANTUM UNIVERSE by Alain Bellerive This article summarizes the activities encompassed by the the composition of our Universe, and the understanding of Particle Physics Division (PPD) at the 2006 Canadian elementary particle properties. The on-going Canadian par- Association of Physicists (CAP) annual congress that took ticipation in CLEO-c, ZEUS, D0 and CDF projects are paving place at Brock University. The summary is targeted toward the road for understanding the physics awaiting our commu- the general readership of Physics in Canada as it is meant to nity at the LHC. The knowledge gained by the future ILC be a review for non-experts in the experiments and the upcoming LHC form of a comprehensive overview of experiments is complementary since the physics research at the subatomic both facilities are needed to develop a scale. In a way the ultimate goal of A comprehensive summary more complete theory of fundamental particle physics is to establish the link of the research topics in particles and interactions. The study between our Quantum Universe and of particle properties and the probable Cosmology, and the research innova- subatomic physics and a discovery of the constituents of dark tions proposed by the PPD are critical look at the future challenges matter at accelerator complexes is for advancing this mission. The con- tightly coupled to direct investigation nection between research in particle of the Particle Physics of weakly interacting particles with physics and cosmology has never Division (PPD). underground detectors to be located in been closer, with the mysterious dark a deep and ultra-clean environment at matter and dark energy which is now the newly funded SNOLAB. Neutrino known to account for about 95% of beam experiments will also provide the mass of the Universe. The synergy between the upcom- new insights about particle mixing in the lepton sector; which ing colliders operating at the energy frontier and the under- are complementary to the ongoing investigation in the quark ground observatories monitoring weakly interacting messen- sector by the BaBar experimental programs at the high-lumi- gers are giving us the means to investigate in detail the gene- nosity B-Factory at the Stanford Linear Accelerator Center sis of massive elementary particles, to study new symmetries, (SLAC). The TWIST experiment at Canada’s National and to discover what dark matter and dark energy actually Laboratory for Nuclear and Particle Physics (TRIUMF) are, thus creating a breakthrough in our understanding of strengthens the elucidation of the weak interaction by study- nature. The effort to understand our natural world is a colos- ing the decay of polarized muons; while the top-cited sal enterprise and it calls for Canada to promote the develop- Sudbury Neutrino Observatory (SNO) is probing how the ment of precise instrumentation at universities across the Sun is producing neutrinos. country and to engage research in the best laboratories around the globe. Precise measuring devices are found at the Astronomical observations are complementary to the quest of heart of all forefront scientific investigations. Their develop- probing matter at the subatomic scale. As most of the galaxies ment, driven by cutting-edge requirements of basic research, seem to be filled with dark matter and dark energy, the study has numerous applications beyond pure physics research and of fluctuations in the cosmic microwave background together leads to social and economic benefits. This effort is not an with data from supernova and galaxy cluster surveys might easy task and hence represents the very best of what we, confirm the original concept of Einstein's cosmological con- physicists, can learn about being human, while providing the stant, in which empty space is filled with a mysterious ener- otherwise unimaginable insights that illuminate our brief gy that increases as the universe expands. Furthermore anni- existence here on Earth. hilation of dark matter particles might be detected in high- energy gamma ray telescopes such as the PPD project VERI- INTRODUCTION TAS. In an attempt to reveal the ultimate link between the cosmos and the subatomic world, particle physicists will hunt The community of particle physicists in Canada [1] has formed dark matter particles at accelerators and underground exper- a consensus opinion that the next major facilities that are iments in order to investigate their quantum properties and required to advance the understanding of matter, space, and consequently understand the profound nature of the uni- time are the near term proton-proton Large Hadron Collider verse. (LHC) at the European Laboratory for Particle Physics (CERN) near Geneva, the neutrino beam at the KEK High Energy Accelerator Research Organization in Japan, the Deep Underground Laboratory in Sudbury (SNOLAB), and the Alain Bellerive <[email protected]> is the Chair of longer term International Linear Collider (ILC). Those proj- the Particle Physics Division of CAP, and a Canada Research ects are planning to answer the key open questions at the Chair in Experimental Particle Physics, Carleton University, forefront of research in particle physics: the origin of mass, 1125 Colonel By Drive, Ottawa, Ontario, K1S 5B6 LA PHYSIQUE AU CANADA juillet / août 2006 185 July06-final.qxd 9/20/2006 9:29 PM Page 186 FEATURE ARTICLE ( PROBING THE QUANTUM UNIVERSE ) For non-experts, the previous two paragraphs are only jargon about the basic physical laws that govern the interaction and acronyms. Let’s pause and see what it all means and let’s between elementary particles. Those questions, at the same try to put all of this into perspective. Everybody knows the time eminently familiar and profoundly revolutionary, irony of quantum mechanics. Particle microscopes required define the path for particle physics in the upcoming decades. to probe the subatomic scale are huge experimental appara- tus that physicists like to call gothic cathedrals of modern ZEUS AND ITS DESCRIPTION OF PARTICLE BAGS time for the hunt of the “God” particle. To add to this grandiose scheme, particle physicists are sometime a little bit The very high energy electrons produced by the Hadron arrogant with their multi-million international large scale Electron Ring Accelerator (HERA) at the German Electron projects. Is this all vanity or is it following a logical path for Synchroton (DESY) offers the possibility to probe deep inside the ultimate truth? the proton and to study the structure of its constituents, the quarks and gluons. Following Heisenberg’s uncertainty rela- THE STANDARD MODEL tion, an investigation at the smallest scale is only possible with high momentum electron impacts on the proton. The Throughout human history, scientific theories and experi- ZEUS detector can probe 10-16 cm for a transverse momen- ments of increasing power and sophistication have tum of q2 = 40,000 GeV2, which is a factor 1,000 smaller than addressed the basic questions about our Universe. The the proton radius. With this resolving power, numerous resulting knowledge has led to revolutionary insights into physical phenomena can be studied, such as possible sub- the nature of the world around us. In the last 40 years, physi- structures of quarks and electrons, neutral and charged cur- cists have achieved a profound understanding of subatomic rent processes mediated by the weak interaction, tests of physics with a simple and comprehensive framework that strong forces via quantum chromodynamics (QCD), and explains the hundreds of composite particles in nature searches for new particles. Ultimately, HERA provides a together with the complex reactions taking place between method of describing the properties of a system of confined them. In the modern study of the fundamental constituents quarks and gluons (i.e. partons) with the famous bag model of matter and their interactions, the Standard Model succeeds and the associated Parton Density Functions (PDFs). This is to explain all the phenomena of particle physics in terms of very important for the next phase of physics exploration at three distinct types of elementary particles. The first two are the LHC. The ZEUS Collaboration reported at CAP06 [2] pre- the leptons and the quarks, both spin ½ fermions. The other liminary cross-sections that can be fitted simultaneously for is the gauge bosons, carriers of four distinct types of funda- the mass of the charged mediator of the weak interaction (i.e. mental force: gravity, electromagnetism as well as the weak the W boson) as well as PDF parameters. The fit leads an and strong interactions. impressive value of MW = 79.10 ± 0.77 (stat) ± 0.99 (syst) GeV/c2 and unprecedented precision over a wide range of Researchers have subjected the Standard Model to countless transverse momentum for the PDF parameters. ZEUS is experimental tests; and, again and again, its predictions have scheduled to acquire data until 2007. held true. The series of experimental and theoretical break- throughs that combined to produce the Standard Model can TWIST & SHOOT truly be celebrated as one of the great scientific triumphs of the past century. Consequently, one of the most pressing TWIST is an experiment at TRIUMF in Vancouver. It is questions in particle physics at this time has to do with the designed to measure the decay distributions of polarized way the electromagnetic and weak forces, which are unified muons to high precision. The muon is a particle that appears at very high energies, appear as distinct forces at low ener- identical to an electron in all regards except that it is heavier. gies. The breaking of the electroweak symmetry is explained Physicists of the TWIST Collaboration are investigating dif- in the Standard Model by the Higgs mechanism, which pre- ferential energy and angular distribution with a precision of dicts the existence of a fundamental so-called God particle, one part in 10,000, allowing a determination of the parame- known as the Higgs boson.
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