1,801,308 research outputs found
Probing the composition of sub-millisecond rotating compact stars by r-modes instability
No full textWe investigate the implications of the r-modes instability on the composition of a compact star rotating at a sub-millisecond period. In particular, the only viable astrophysical scenario for such an object, wich might present inside the Low Mass X-ray Binary associated with the x-ray transient XTE J1739-285, is that it has a strangeness content. Since previous analysis indicate that hyperonic stars or stars containing a kaon condensate are unlikely because of the mass-shedding constraint, the only remaining possibility is that such an object is either a strange quark star or a hybrid quark-hadron star
Search for top quark decays t → qH with H → γγ using the ATLAS detector
Get PDFA search is performed for flavour-changing neutral currents in the decay of a top quark to an up-type (c, u) quark and a Higgs boson, where the Higgs boson decays to two photons. The proton-proton collision data set used corresponds to 4.7 fb-1 at √ = 7TeV and 20.3fb-1 at √ = 8TeV collected by the ATLAS experiment at the LHC. Top quark pair events are searched for in which one top quark decays to qH and the other decays to bW. Both the hadronic and the leptonic decay modes of the W boson are used. No significant signal is observed and an upper limit is set on the t → qH branching ratio of 0.79 at the 95% confidence level. The corresponding limit on the tqH coupling combination λtcH 2 + λtuH 2 is 0.17
Heavy quarkonia in quark gluon plasma as open quantum systems
Get PDFDutta N. Heavy quarkonia in quark gluon plasma as open quantum systems. Bielefeld: Universitätsbibliothek; 2013
Quarkonium states in an anisotropic quark-gluon plasma
No full textIn this work we study the properties of quarkonium states in a quark-gluon plasma which, due to expansion and non-zero viscosity, exhibits a local anisotropy in momentum space. We determine the hard-loop resummed gluon propagator in an anisotropic QCD plasma in general linear gauges and define a potential between heavy quarks from the Fourier transform of its static limit. This potential which arises due to one-gluon exchange describes the force between a quark and anti-quark at short distances. It is closer to the vacuum potential as compared to the isotropic Debye screened potential which indicates the reduced screening in an anisotropic QCD plasma. In addition, angular dependence appears in the potential; we find that there is stronger attraction on distance scales on the order of the inverse Debye mass for quark pairs aligned along the direction of anisotropy than for transverse alignment. The potential at long distances, however, is non-perturbative and modeled as a QCD string which is screened at the same scale as the Coulomb field. At asymptotic separation the potential energy is non-zero and inversely proportional to the temperature. With a phenomenological potential model which incorporates the different behaviors at short and long distances, we solve the three-dimensional Schrödinger equation. Our numerical results show that quarkonium binding is stronger at non-vanishing viscosity and expansion rate, and that the anisotropy leads to polarization of the P-wave states. Furthermore, we determine viscosity corrections to the imaginary part of the heavyquark potential in the weak-coupling hard-loop approximation. The imaginary part is found to be smaller (in magnitude) than at vanishing viscosity. This implies a smaller decay width of quarkonium bound states in an anisotropic plasma.In dieser Arbeit untersuchen wir die Eigenschaften gebundener Zustände zweier schwerer Quarks (“Quarkonium”) in einem Quark-Gluon Plasma, das aufgrund seiner kollektiven Expansion und nichtverschwindender Viskosität lokal eine Anisotropie im Impulsraum aufweist. Wir bestimmen den resummierten Gluonpropagator in der sogenannten “harte thermische Schleifen” (engl.: “hard thermal loop”) Hochtemperaturnäherung in einem solchen anisotropen Plasma. Wir berechnen den Propagator in linearen Eichungen und definieren aus seinem statischen Limes durch Fouriertransformation das Potential zwischen (unendlich) schweren Quarks. Dieses durch Eingluonaustausch bestimmte Potential beschreibt die Wechselwirkung des Quark-Antiquark Paares bei kleinen Abständen. Wir finden, dass die Anisotropie die Debye-Abschirmung der Wechselwirkung reduziert und sich das Potential somit wieder dem Vakuumpotential annähert. Darüber hinaus hängt die Stärke der attraktiven Wechselwirkung nicht nur vom Abstand der Quarks ab sondern ist auch winkelabhängig: ein Quark-Antiquark Paar das entlang der Plasmaanisotropie ausgerichtet ist zieht stärker an als ein dazu transversal ausgerichtetes Quarkpaar. Bei grossen Abständen dominieren nichtperturbative Beiträge. Wir modellieren hier das Potential als QCD String der auf der gleichen Längenskala wie das perturbative Coulombpotential abgeschirmt wird. Das Potential verschwindet nicht bei asymptotischen Abständen sondern ist vielmehr umgekehrt proportional zur Temperatur. Nach Konstruktion des Potentials bestimmen wir numerische Lösungen der Schrödingergleichung in drei Raumdimensionen. Diese Lösungen bestätigen, dass Quarkoniumzustände in einem expandierenden, viskosen Plasma stärker gebunden sind als in einem idealen Plasma im Gleichgewicht. Die Anisotropie führt zudem zu einer Polarisation der P-Wellen. Im Anschluss bestimmen wir auch Viskositätskorrekturen zum Imaginärteil des Potentials. Auch hier finden wir, dass die Nichtgleichgewichtseffekte die thermischen Effekte reduzieren, d.h., dass der Imaginärteil des Potentials absolut kleiner ist als im perfekten Gleichgewicht. Dies impliziert eine kleinere Breite der gebundenen Zustände im anisotropen Plasma
Off-Shell Effects for Single Top-Quark Production Processes at Hadron Colliders
Get PDFWe present a calculation of s- and t-channel single top-quark production and decay at next-to-leading order in QCD for the Tevatron and for the LHC. The calculation includes the off-shell effects of the intermediate top-quark and the non-factorisable corrections arising from interferences between the production and decay subprocesses, extending the results beyond the narrow-width approximation. A general method for including such effects is outlined. The method comprises a simultaneous expansion in the coupling constants, as in a standard perturbative approach, along with an expansion in the virtuality of the intermediate heavy-particle. This expansion makes it possible to identify the contributions relevant to the calculation, up to a desired accuracy, before computation of the amplitudes, allowing significant simplifications to be made to the final calculation. Results obtained using this method are presented, along with results obtained using both the standard and improved narrow-width approximations. This enables us to investigate the impact of both off-shell and spin-correlation effects on the cross section and various kinematic distributions. In general, both effects are found to be small except close to kinematic boundaries or for specific distributions, such as the top-quark invariant-mass, where their effects can become sizeable
Measurement of the top quark charge in pp collisions at s√=7 TeV with the ATLAS detector
Get PDFA measurement of the top quark electric charge is carried out in the ATLAS experiment at the Large Hadron Collider using 2.05 fb−1 of data at a centre-of-mass energy of 7 TeV. In units of the elementary electric charge, the top quark charge is determined to be 0.64 ± 0.02 (stat.) ± 0.08 (syst.) from the charges of the top quark decay products in single lepton tt¯ candidate events. This excludes models that propose a heavy quark of electric charge −4/3, instead of the Standard Model top quark, with a significance of more than 8σ
Screening of the QCD heavy quark potential at finite temperature
Get PDFKarsch F, Wyld HW. Screening of the QCD heavy quark potential at finite temperature. Physics Letters, B. 1988;213(4):505-510
High-energy dileptons from an anisotropic quark-gluon plasma
No full textWe calculate leading-order dilepton yields from a quark-gluon plasma which has a time-dependent anisotropy in momentum space. Such anisotropies can arise during the earliest stages of quark-gluon plasma evolution due to the rapid longitudinal expansion of the created matter. A phenomenological model for the proper time dependence of the parton hard momentum scale, p_hard, and the plasma anisotropy parameter, xi, is proposed. The model describes the transition of the plasma from a 0+1 dimensional collisionally-broadened expansion at early times to a 0+1 dimensional ideal hydrodynamic expansion at late times. We find that high-energy dilepton production is enhanced by pre-equilibrium emission up to 50% at LHC energies, if one assumes an isotropization/thermalization time of 2 fm/c. Given sufficiently precise experimental data this enhancement could be used to determine the plasma isotropization time experimentally
Flavor physics in the quark sector
Get PDFIn the past decade, one of the major challenges of particle physics has been to gain an in-depth understanding of the role of quark flavor. In this time frame, measurements and the theoretical interpretation of their results have advanced tremendously. A much broader understanding of flavor particles has been achieved; apart from their masses and quantum numbers, there now exist detailed measurements of the characteristics of their interactions allowing stringent tests of Standard Model predictions. Among the most interesting phenomena of flavor physics is the violation of the CP symmetry that has been subtle and difficult to explore. In the past, observations of CP violation were confined to neutral K mesons, but since the early 1990s, a large number of CP-violating processes have been studied in detail in neutral B mesons. In parallel, measurements of the couplings of the heavy quarks and the dynamics for their decays in large samples of K, D, and B mesons have been greatly improved in accuracy and the results are being used as probes in the search for deviations from the Standard Model.
In the near future, there will be a transition from the current to a new generation of experiments; thus a review of the status of quark flavor physics is timely. This report is the result of the work of physicists attending the 5th CKM workshop, hosted by the University of Rome "La Sapienza", September 9-13, 2008. It summarizes the results of the current generation of experiments that are about to be completed and it confronts these results with the theoretical understanding of the field which has greatly improved in the past decade
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