Bar Bb Research Articles

Search for single production of a vector-like quark via a heavy gluon in the 4b final state with the ATLAS detector in pp collisions at s=8 TeV

A search is performed for the process $pp \to G^* \to B_H\bar b/\bar B_H b \to H b \bar b \to b\bar bb\bar b$, predicted in composite Higgs scenarios, where $G^*$ is a heavy colour octet vector resonance and $B_H$ a vector-like quark of charge $-1/3$. The data were obtained from $pp$ collisions at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.5 fb$^{-1}$, recorded by the ATLAS detector at the LHC. The largest background, multijet production, is estimated using a data-driven method. No significant excess of events with respect to Standard Model predictions is observed, and upper limits on the production cross section times branching ratio are set. Comparisons to the predictions from a specific benchmark model are made, resulting in lower mass limits in the two-dimensional mass plane of $m_{G^*}$ vs. $m_{B_H}$.

Determination of the mass spectrum and the decay constant mesons consisting of c and b quarks

On the basis of an investigation into the asymptotic behavior of the correlation functions of the corresponding field currents with the necessary quantum numbers, an analytic method for determining the mass spectrum and decay constants of mesons consisting of c and b quarks with relativistic corrections is proposed. The dependence of the constituent mass of quarks on the current mass and on the orbital and radial quantum numbers is analytically derived. The mass and the wave function (WF) mesons are determined from the Schrodinger equation with a mass of constituent components particles. We calculate the splitting of the mass spectrum for the singlet and triplet states mesons, as well as to determine the width of the lepton and radiation decays due E1 transition for \(\left( {\bar cc} \right)\), \(\left( {\bar bb} \right)\), \(\left( {\bar bc} \right)\) systems. Our results for the mass spectrum of mesons consisting of c and b quarks are in satisfactory agreement with the available experimental data.

$b{\bar b}b{\bar b}$ production in proton-proton and proton-nucleus collisions at the CERN LHC

Given the large parton luminosities, sizable rate of events, with several pairs of b-quarks produced contemporarily by multiple parton interactions, are foreseen in hadronic and nuclear collisions at very high energies. We compare the different contributions to \( b\bar bb\bar b \) production, due to single and double parton scatterings, in pp and in p A collisions at the CERN LHC within the acceptance of the ALICE and of the LHCb detectors.

Effects of new neutral currents at linear electron-positron colliders

Effects that are induced by contact four-fermion interactions in the processes e+e− → μ+μ−, \(\bar bb\), and \(\bar cc\) at \(\sqrt s = 0.5\)TeV linear electron-positron colliders are investigated for the case of longitudinally polarized initial beams. This analysis employs new integrated observables constructed from the polarized cross sections for the scattering of final fermions into the forward (σF) and the backward (σB) hemisphere in such a way that they single out the helicity cross sections for the processes in question. This property of the observables makes it possible to perform, in the most general form, a model-independent analysis of contact four-fermion interactions and to set constraints on their parameters. It is also shown that the sensitivity of new polarization observables to contact interactions is noticeably higher than the corresponding sensitivity of canonical observables like σ, AFB, ALR, and ALR,FB.

Can the trilinear Higgs self-coupling be measured at future linear colliders?

We assess the possibility of measuring the trilinear self-coupling of the Standard Model Higgs boson at future electron-positron linear colliders with centre-of-mass energies between 500 and 1500 GeV. We consider rather light Higgs scalars, with mass below the \(W^+W^-\) threshold, produced in the double Higgs-strahlung channel \(e^+e^-\to HHZ\) and decaying via the dominant mode \(H\rightarrow b\bar b\). Assuming ideal vertex tagging of the heavy-quark jets and mass reconstruction of the Z boson produced in association with the Higgs pair, we compare the yield of the above process to those of all irreducible electroweak and strong backgrounds proceeding through a \(b\bar bb\bar b Z\) intermediate state. Total cross sections and differential spectra of phenomenological relevance to the selection and analysis of the signal are given and discussed.

A one-dimensional lattice model for a quantum mechanical free particle

Two types of particles, A and B with their corresponding antiparticles, are defined in a onedimensional cyclic lattice with an odd number of sites. In each step of time evolution, each particle acts as a source for the polarization field of the other type of particle with nonlocal action but with an effect decreasing with the distance: \(A \to \cdots \bar BB\bar BB\bar B \cdots ;B \to \cdots A\bar AA\bar AA \cdots \). It is shown that the combined distribution of these particles obeys the time evolution of a free particle as given by quantum mechanics.

Higher orders in the color-octet model ofJ/ψproduction

We study the hadro- and photo-production of $\bar cc$ and $\bar bb$ mesons at low transverse momentum to high orders in the relative velocity of the pair, v, in non-relativistic QCD. We evaluate cross sections to order v^7 for $\eta_c$, sufficient for studies of photo-production in the almost-elastic region. For all other charmonium states we find the cross section to order v^9, sufficient for studies of the ratio of $\chi_{c1}$ and $\chi_{c2}$ production rates. We find recurrence formulae for generating terms at even higher orders, should they be needed.

Top and bottom quark condensates

It is possible that electroweak symmetry might be broken dynamically by a condensate of top quarks. We extend a previous study of top quark dynamics to include the bottom quark. The simultaneous gap equations in dressed ladder approximation for∑ t (p),∑ b (p) are studied numerically and analytically in various regions of parameter space, and the solutions are used to determine $$\langle \bar tt\rangle ,\langle \bar bb\rangle $$ and the decay constantsF ±,F 0, as well as the top and bottom masses. We discuss the question of fine-tuning, and show how our approach can be extended to other quark and lepton masses.

Calculation of $$n^3 S_1 (Q\bar Q) \to \gamma + $$ (pseudo)scalar

The decays of3S1 quarkonia into a photon and a scalar or pseudoscalar Higgs particle are examined, taking into account the bound-state dynamics in the framework of a nonrelativistic potential model. We find that for realistic quark potentials the naive calculation [1] overestimates the scalar rate. Numerical results are obtained for the\(\bar bb\) and\(\bar tt\) quark systems.

Photonic decays involving the $$2^{ + + } \left( {\bar bb} \right)$$ : Partial widths and jets from tensor meson dominance

Tensor meson dominance combined with vector meson dominance, QCD-potentials and the experimental leptonic widths of Γ and Γ′ predicts $$\Gamma _{\Upsilon '\left( {10.01} \right) \to \gamma 2^{ + + } \left( {\bar bb} \right)} = 2.8keV$$ and $$\Gamma _{2^{ + + } \left( {\bar bb} \right) \to \gamma \Upsilon \left( {9.46} \right)} = 134keV.$$ The angular distributions of the γ and the jetsj resulting from the decays $$e^ + e^ - \to \Upsilon '\left( {10.01} \right) \to \gamma 2^{ + + } \left( {\bar bb} \right) \to \gamma gg \to \gamma jj$$ and $$e^ + e^ - \to \Upsilon '\left( {10.01} \right) \to \gamma 2^{ + + } \left( {\bar bb} \right) \to \gamma \bar qq \to \gamma jj$$ with massless vector gluonsg, (coupled gauge invariantly) and quarksq are uniquely determined in TMD. The result for the first process agrees with that of perturbative QCD. No perturbative QCD-prediction for the latter is known.