122693943
Kaon and pion emission in asymmetric C+Au and Au+C collisions at 1.0A GeV and 1.8A GeV
The emission of K{sup +} and {pi}{sup {+-}} mesons has been studied in the asymmetric collision system Au+C and in the inverse reaction C+Au at 1.0A GeV (and at 1.8A GeV for C+Au only) in order to extract the effective source rapidities based on their distribution in the p{sub t}-y plane. The extracted source rapidity of K{sup +} mesons is about y{sub s}/y{sub beam}=0.25 at both incident energies [y(Au)=0,y(C)=y{sub beam}]. This corresponds to a reaction volume consisting of the C nucleus and a tube cut out of the Au nucleus. In clear contrast, the source rapidities of {pi}{sup {+-}} mesons vary from y{sub s}/y{sub beam}=0.5 in peripheral collisions (corresponding to NN interactions) to about y{sub s}/y{sub beam}<0.33 in central reactions. The multiplicities of K{sup +} and {pi} are compared to symmetric collision systems and, together with detailed transport model calculations, are used to study the sensitivity to the nuclear equation of state.
25947222
Evidence for a Soft Nuclear Equation-of-State from Kaon Production in Heavy-Ion Collisions.
The production of pions and kaons has been measured in 197Au+197Au collisions at beam energies from 0.6 to 1.5A GeV with the kaon spectrometer at SIS/GSI. The K+ meson multiplicity per nucleon is enhanced in Au+Au collisions by factors up to 6 relative to C+C reactions, whereas the corresponding pion ratio is reduced. The ratio of the K+ meson excitation functions for Au+Au and C+C collisions increases with decreasing beam energy. This behavior is expected for a soft nuclear equation-of-state.
17217990
First measurement of antikaon phase-space distributions in nucleus–nucleus collisions at subthreshold beam energies
Abstract Differential production cross sections of K − and K + mesons have been measured as function of the polar emission angle in Ni + Ni collisions at a beam energy of 1.93  A GeV. In near-central collisions, the spectral shapes and the widths of the rapidity distributions of K − and K + mesons are in agreement with the assumption of isotropic emission. In non-central collisions, the K − and K + rapidity distributions are broader than expected for a single thermal source. In this case, the polar angle distributions are strongly forward–backward peaked and the nonisotropic contribution to the total yield is about one third both for K + and K − mesons. The K − /K + ratio is found to be about 0.03 independent of the centrality of the reaction. This value is significantly larger than predicted by microscopic transport calculations if in-medium modifications of K mesons are neglected.
54021017
Implications for compact stars of a soft nuclear equation of state from heavy-ion data
We study the implications on compact star properties of a soft nuclear equation of state determined from kaon production at subthreshold energies in heavy-ion collisions. On one hand, we apply these results to study radii and moments of inertia of light neutron stars. Heavy-ion data provides constraints on nuclear matter at densities relevant for those stars and, in particular, to the density dependence of the symmetry energy of nuclear matter. On the other hand, we derive a limit for the highest allowed neutron star mass of three solar masses. For that purpose, we use the information on the nucleon potential obtained from the analysis of the heavy-ion data combined with causality on the nuclear equation of state.
121748724
Kaon and antikaon production in dense nuclear matter
The production and propagation of kaons and antikaons in relativistic heavy-ion collisions have been systematically investigated with the Kaon spectrometer at SIS/GSI. Experimental results on the K+ and K− yield and on the azimuthal emission pattern of K+ mesons are presented. Within the framework of transport models the data can be explained assuming in-medium kaon–nucleon potentials. The comparison of K+ production excitation functions obtained for Au+Au and C+C collisions with results of transport model calculations favours a soft nuclear equation-of-state.
119373633
Review of the results of the KaoS Collaboration
The production of K+ and of K− mesons in heavy-ion collisions at beam energies of 1 to 2 A GeV has systematically been investigated with the kaon spectrometer KaoS. The ratio of the K+ production excitation function for Au+Au and for C+C reactions increases with decreasing beam energy, which is expected for a soft nuclear equation-of-state. A comprehensive study of the K+ and of the K− emission as a function of the size of the collision system, of the collision centrality, of the kaon energy, and of the polar emission angle has been performed. The K−/K+ ratio is found to be nearly constant as a function of the collision centrality and can be explained by the dominance of strangeness exchange. On the other hand, the spectral slopes and the polar emission patterns are different for K− and for K+. Furthermore the azimuthal distribution of the particle emission has been investigated. K+ mesons and pions are emitted preferentially perpendicular to the reaction plane as well in Au+Au as in Ni+Ni collisions. In contrast for K− mesons in Ni+Ni reactions an in-plane flow was observed for the first time at these incident energies.
119300543
Complete electroweak corrections to Higgs production in a Standard Model with four generations at the LHC
Abstract Complete electroweak two-loop corrections to the process gg → H are presented and discussed in a Standard Model with a fourth generation of heavy fermions. The latter is studied at the LHC to put exclusion limits on a fourth generation of heavy fermions. Therefore also a precise knowledge of the electroweak (EW) next-to-leading-order (NLO) corrections is important. The corrections due to the fourth generation are positive and large for a light Higgs boson, positive but relatively small around the t ¯ – t threshold and start to become negative for a Higgs boson mass around M H = 450 GeV . Increasing further the value of the Higgs boson mass, the EW NLO effects tend to become huge and negative, O ( − 100 % ) , around the heavy-fermion threshold, assumed at 1.2 TeV , so that gg-fusion becomes non-perturbative. Above that threshold they start to grow again and become positive around M H = 1.75 TeV . The behavior at even larger values of M H shows a positive enhancement, O ( + 100 % ) at M H = 3 TeV .
120793699
Quark masses from low-energy moments of heavy-quark current correlators
We present an update of the determination of charm- and bottom-quark masses from the lowenergy expansion of the vacuum polarization function in combination with experimental data for R(s). This analysis leads to the most precise determination of th e masses of the charm and bottom quarks: mc(3 GeV) = 986(13) MeV, mb(mb) = 4163(16) MeV.
119202777
Two-Loop Virtual Corrections to Drell-Yan Production at order $\alpha_s \alpha^3$
The Drell-Yan mechanism for the production of lepton pairs is one of the most basic processes for physics studies at hadron colliders. It is therefore important to have accurate theoretical predictions. In this work we compute the two-loop virtual mixed QCD x QED corrections to Drell-Yan production. We evaluate the Feynman diagrams by decomposing the amplitudes into a set of known master integrals and their coefficients, which allows us to derive an analytical result. We also perform a detailed study of the ultraviolet and infrared structure of the two-loop amplitude and the corresponding poles in epsilon.
427206
Four-loop moments of the heavy quark vacuum polarization function in perturbative QCD
New results at four-loop order in perturbative QCD for the first two Taylor coefficients of the heavy quark vacuum polarization function are presented. They can be used to perform a precise determination of the charm- and bottom-quark mass. Implications for the value of the quark masses are briefly discussed.
13361740
Heavy Quark Masses from Sum Rules in Four-Loop Approximation
Abstract New data for the total cross section σ ( e + e − → hadrons ) in the charm and bottom threshold region are combined with an improved theoretical analysis, which includes recent four-loop calculations, to determine the short distance MS ¯ charm and bottom quark masses. A detailed discussion of the theoretical and experimental uncertainties is presented. The final result for the MS ¯ -masses, m c ( 3 GeV ) = 0.986 ( 13 ) GeV and m b ( 10 GeV ) = 3.609 ( 25 ) GeV , can be translated into m c ( m c ) = 1.286 ( 13 ) GeV and m b ( m b ) = 4.164 ( 25 ) GeV . This analysis is consistent with but significantly more precise than a similar previous study.