Intergranular flux pinning in underdoped and overdoped R(1-x)CaxBa2Cu3Oz (R=Y,Gd; x=0,0.2) samples

The temperature dependences of AC magnetic susceptibility at different magnetic field amplitudes and frequencies are investigated for underdoped and overdoped R(1-x)Ca(x)Ba(2)Cu(3)O(7-delta)(R=Y; Gd and x=0; 0.2) polycrystalline samples. The activation energy, Ea, for thermally assisted flux flow (TAFF) in intergranular region is determined. It was established that a correlation exists between the intergranular critical current and flux pinning activation energy. In underdoped samples the intergranular current shows S-I-S behaviour and the activation energy is small, while in overdoped samples the intergranular current is changed to S-N-S type and the activation energy increases. 2D pancake vortices are characteristic of underdoped samples, while 3D vortex system exists in overdoped samples. In fact we demonstrate that Ca substitution not only increases carrier concentration, but improves intergranular activation energy for TAFF.Comment: 20 pages including 7 figures and one Table. accepted in Physica

Effects of Substituting Calcium for Yttrium on the Superconducting Properties of YBa2Cu3Oz Bulk Samples

We report systematic studies of AC magnetic susceptibility and transport properties of Y(1-x)Ca(x)Ba2Cu3Oz bulk samples with 0\leq x\leq 0.4. Single phase materials, reduction of carrier concentration and decrease of Tc to 85 K were obtained at doping levels up to 20%. For Y0.7Ca0.3Ba2Cu3Oz sample the improvement of grains boundary transport and screening capabilities has been observed as a result of the optimal ratio between carrier concentration and impurity phase BaCuO2 presence. The appearence of bulk pinning and nonlinear effects starting at the highest temperature were detected also.Comment: 12 pages, 8 figures, accepted in Physica

Direct observation of the dead-cone effect in quantum chromodynamics

At particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD) [1]. The vacuum is not transparent to the partons and induces gluon radiation and quark pair production in a process that can be described as a parton shower [2]. Studying the pattern of the parton shower is one of the key experimental tools in understanding the properties of QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass m and energy E, within a cone of angular size m/E around the emitter [3]. A direct observation of the dead-cone effect in QCD has not been possible until now, due to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible bound hadronic states. Here we show the first direct observation of the QCD dead-cone by using new iterative declustering techniques [4, 5] to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD, which is derived more generally from its origin as a gauge quantum field theory. Furthermore, the measurement of a dead-cone angle constitutes the first direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics.The direct measurement of the QCD dead cone in charm quark fragmentation is reported, using iterative declustering of jets tagged with a fully reconstructed charmed hadron.In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD). These partons subsequently emit further partons in a process that can be described as a parton shower which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass $m_{\rm{Q}}$ and energy $E$, within a cone of angular size $m_{\rm{Q}}$/$E$ around the emitter. Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics

Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC

The article is the pre-print version of the final publishing paper that is available from the link below.Results are presented from searches for the standard model Higgs boson in proton–proton collisions At √s = 7 and 8 TeV in the Compact Muon Solenoid experiment at the LHC, using data samples corresponding to integrated luminosities of up to 5.1 fb−1 at 7TeV and 5.3 fb−1 at 8 TeV. The search is performed in five decay modes: γγ, ZZ, W+W−, τ+τ−, and bb. An excess of events is observed above the expected background, with a local significance of 5.0 standard deviations, at a mass near 125 GeV, signalling the production of a new particle. The expected significance for a standard model Higgs boson of that mass is 5.8 standard deviations. The excess is most significant in the two decay modes with the best mass resolution, γγ and ZZ; a fit to these signals gives a mass of 125.3±0.4(stat.)±0.5(syst.) GeV. The decay to two photons indicates that the new particle is a boson with spin different from one

A New Boson with a Mass of 125 GeV Observed with the CMS Experiment at the Large Hadron Collider

The Higgs boson was postulated nearly five decades ago within the framework of the standard model of particle physics and has been the subject of numerous searches at accelerators around the world. Its discovery would verify the existence of a complex scalar field thought to give mass to three of the carriers of the electroweak force-the W+, W-, and Z(0) bosons-as well as to the fundamental quarks and leptons. The CMS Collaboration has observed, with a statistical significance of five standard deviations, a new particle produced in proton-proton collisions at the Large Hadron Collider at CERN. The evidence is strongest in the diphoton and four-lepton (electrons and/or muons) final states, which provide the best mass resolution in the CMS detector. The probability of the observed signal being due to a random fluctuation of the background is about 1 in 3 x 10(6). The new particle is a boson with spin not equal to 1 and has a mass of about 1.25 giga-electron volts. Although its measured properties are, within the uncertainties of the present data, consistent with those expected of the Higgs boson, more data are needed to elucidate the precise nature of the new particle