Top Quark The top quark, also known as the t quark (symbol: t) or truth quark, is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-1⁄2, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. It has an electric charge of +2⁄3 e, and is the most massive of all observed elementary particles. It has a mass of 173.34 ± 0.27 (stat) ± 0.71 (syst) GeV/c2, which is about the same mass as an atom of tungsten. The antiparticle of the top quark is the top antiquark (symbol: t, sometimes called antitop quark or simply antitop), which differs from it only in that some of its properties have equal magnitude but opposite sign. The top quark interacts primarily by the strong interaction but can only decay through the weak force. It decays almost exclusively to a W boson and a bottom quark, but it can decay also into a strange quark, and on the rarest of occasions, into a down quark. The Standard Model predicts its mean lifetime to be roughly 5×10−25 s. This is about 20 times shorter than the timescale for strong interactions, and therefore it does not form hadrons, giving physicists a unique opportunity to study a bare quark (all other quarks hadronize, meaning they combine with other quarks to form hadrons, and can only be observed as such). Because it is so massive, the properties of the top quark allow predictions to be made of the mass of the Higgs boson under certain extensions of the Standard Model (see Mass and coupling to the Higgs boson below). As such, it is extensively studied as a means to discriminate between competing theories. Its existence (and that of the bottom quark) was postulated in 1973 by Makoto Kobayashi and Toshihide Maskawa to explain the observed CP violations in kaon decay, and was discovered in 1995 by the CDF and DØ experiments at Fermilab. Kobayashi and Maskawa won the 2008 Nobel Prize in Physics for the prediction of the top and bottom quark, which together form the third generation of quarks. •At the final Tevatron energy of 1.96 TeV, top–antitop pairs were produced with a cross section of about 7 picobarns (pb).[21] The Standard Model prediction (at next-to-leading order with mt = 175 GeV/c2) is 6.7–7.5 pb. •The W bosons from top quark decays carry polarization from the parent particle, hence pose themselves as a unique probe to top polarization. •In the Standard Model, the top quark is predicted to have a spin quantum number of 1⁄2 and electric charge +2⁄3. A first measurement of the top quark charge has been published, resulting in approximately 90% confidence limit that the top quark charge is indeed +2⁄3. Because top quarks are very massive, large amounts of energy are needed to create one. The only way to achieve such high energies is through high energy collisions. These occur naturally in the Earths upper atmosphere as cosmic rays collide with particles in the air, or can be created in a particle accelerator. As of 2011, the only operational accelerator that generates a beam of sufficient energy to produce top quarks is the Large Hadron Collider at CERN, with a center-of-mass energy of 7 TeV. There are multiple processes that can lead to the production of a top quark. The most common is production of a top–antitop pair via strong interactions. In a collision a highly energetic gluon is created which subsequently decays into a top and antitop. This process was responsible for the majority of the top events at Tevatron and was the process observed when the top was first discovered in 1995.[23] It is also possible to produce pairs of top–antitop through the decay of an intermediate photon or Z-boson. However, these processes are predicted to be much rarer and have a virtually identical experimental signature in a hadron collider like Tevatron. A distinctly different process is the production of single tops via weak interaction. This can happen in two ways (called channels): either an intermediate W-boson decays into a top and antibottom quark (s-channel) or a bottom quark (probably created in a pair through the decay of a gluon) transforms to top quark by exchanging a W-boson with an up or down quark (t-channel). The first evidence for these processes was published by the DØ collaboration in December 2006, and in March 2009 the CDF and DØ collaborations released twin papers with the definitive observation of these processes. The main significance of measuring these production processes is that their frequency is directly proportional to the |Vtb|2 component of the CKM matrix. en.wikipedia.org/wiki/Top_quark fnal.gov/pub/science/historical-results/ fnal.gov/pub/presspass/press_releases/2014/Top-Quark-Puzzle-20140224.html Scientists complete the top quark puzzle phys.org/news/2014-02-scientists-quark-puzzle.html CMS presents a new precise measurement of the top-quark mass cms.web.cern.ch/news/cms-presents-new-precise-measurement-top-quark-mass First combination of Tevatron and LHC measurements of the top-quark mass arxiv.org/abs/1403.4427 The bottom quark or b quark (from its symbol, b), also known as the beauty quark, is a third-generation quark with a charge of −1⁄3 e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quarks large bare mass (around 4.2 GeV/c2, a bit more than four times the mass of a proton), combined with low values of the CKM matrix elements Vub and Vcb, gives it a distinctive signature that makes it relatively easy to identify experimentally (using a technique called B-tagging). Because three generations of quark are required for CP violation (see CKM matrix), mesons containing the bottom quark are the easiest particles to use to investigate the phenomenon; such experiments are being performed at the BaBar, Belle and LHCb experiments. The bottom quark is also notable because it is a product in almost all top quark decays, and is a frequent decay product for the Higgs boson. The bottom quark was theorized in 1973 by physicists Makoto Kobayashi and Toshihide Maskawa to explain CP violation. The name bottom was introduced in 1975 by Haim Harari. The bottom quark was discovered in 1977 by the Fermilab E288 experiment team led by Leon M. Lederman, when collisions produced bottomonium.[2][6][7] Kobayashi and Maskawa won the 2008 Nobel Prize in Physics for their explanation of CP-violation. On its discovery, there were efforts to name the bottom quark beauty, but bottom became the predominant usage. The bottom quark can decay into either an up or charm quark via the weak interaction. Both these decays are suppressed by the CKM matrix, making lifetimes of most bottom particles (~10−12 s) somewhat higher than those of charmed particles (~10−13 s), but lower than those of strange particles (from ~10−10 to ~10−8 s). en.wikipedia.org/wiki/Bottom_quark First-time observation of photon polarization emitted in the weak decay of a bottom quark phys.org/news/2014-03-first-time-photon-polarization-emitted-weak.html Quark asymmetries hint at physics beyond the Standard Model phys.org/news/2013-08-quark-asymmetries-hint-physics-standard.html A bound on the natural width of the Higgs boson phys.org/news/2014-04-bound-natural-width-higgs-boson.html The discovery of CP violation in B-meson decays https://www6.slac.stanford.edu/news/2012-11-19-babar-trv.aspx Light Axigluon Contributions to b-bbar and c-cbar Asymmetry and Constraints on Flavor Changing Axigluon Currents arxiv.org/abs/1301.3990 Measurement of the isospin asymmetry in B→K(∗)μ+μ− decays arxiv.org/abs/1205.3422 To B or not to Bbar: b-Jet Identification quantumdiaries.org/2011/05/12/to-b-or-not-to-bbar-b-jet-identification/ B–Bbar oscillation Neutral B meson oscillations (or B–B oscillations) is one of the manifestations of the neutral particle oscillation, a fundamental prediction of the Standard Model of particle physics. It is the phenomenon of B mesons changing (or oscillating) between their matter and antimatter forms before their decay. The B s meson can exist as either a bound state of a strange antiquark and a bottom quark, or a strange quark and bottom antiquark. The oscillations in the neutral B sector are analogous to the phenomena that produces long and short-lived neutral kaons. en.wikipedia.org/wiki/B%E2%80%93Bbar_oscillation Measurement of the forward-central bb¯ production asymmetry https://cds.cern.ch/record/1517497?ln=en BaBar Experiment Confirms Time Asymmetry https://www6.slac.stanford.edu/news/2012-11-19-babar-trv.aspx LHCb lhcb-public.web.cern.ch/lhcb-public/ CMS experiment observes new Xi_b beauty particle phys.org/news/2012-04-xib-beauty-particle.html#nRlv Puzzling asymmetries in B decays hint at deviations from the Standard Model phys.org/news/2012-05-puzzling-asymmetries-hint-deviations-standard.html Two new excited states of the Lambda-b beauty particle observed by LHCb phys.org/news/2012-05-states-lambda-b-beauty-particle-lhcb.html#nRlv LHCb experiment observes new matter-antimatter difference phys.org/news/2013-04-lhcb-matter-antimatter-difference.html Particle physicists measure the spin contribution of the protons antiquark phys.org/news/2014-08-particle-physicists-contribution-proton-antiquark.html New measurement of electron–quark scattering phys.org/news/2014-02-electronquark.html Proton weak charge determined for first time: Initial Q-weak experiement results phys.org/news/2013-09-proton-weak-q-weak-results.html Experiments reveal a neutron halo around neutron-rich magnesium nuclei phys.org/news/2014-08-reveal-neutron-halo-neutron-rich-magnesium.html Zeroing in on the protons magnetic moment phys.org/news/2014-05-zeroing-proton-magnetic-moment.html The first supercomputer simulations of spin–orbit forces between neutrons and protons in an atomic nucleus phys.org/news/2014-07-supercomputer-simulations-spinorbit-neutrons-protons.html Exotic atoms hold clues to unsolved physics puzzle at the dawn of the universe phys.org/news/2013-05-exotic-atoms-clues-unsolved-physics.html Measurements of the t t-bar charge asymmetry using the dilepton decay channel in pp collisions at sqrt(s) = 7 TeV arxiv.org/abs/1402.3803 Picture from hadron.physics.fsu.edu/~crede/IMAGES/atoms_to_quarks.gif
Posted on: Wed, 17 Sep 2014 18:54:16 +0000
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