Search results for "Quark"
showing 10 items of 2905 documents
Semileptonic B ->pi decays from an Omnes improved nonrelativistic constituent quark model
2005
The semileptonic $B\to \pi l^+ \nu_l$ decay is studied starting from a simple quark model which includes the influence of the $B^*$ pole. To extend the predictions of a nonrelativistic constituent quark model from its region of applicability near $q^2_{\rm max}=(m_B-m_\pi)^2$ to all $q^2$ values accessible in the physical decay, we use a novel multiply-subtracted Omn\`es dispersion relation, which considerably diminishes the form factor dependence on the elastic $\pi B \to \pi B$ scattering amplitudes at high energies. By comparison to the experimental branching fraction we extract $|V_{ub}| = 0.0034 \pm 0.0003 ({\rm exp}) \pm 0.0007 ({\rm theory})$. To further test our framework, we also s…
Measurement of the chi(b) (3 P) mass and of the relative rate of chi(b1) (1 P) and chi(b2) (1 P) production
2014
The production of $\chi_b$ mesons in proton-proton collisions is studied using a data sample collected by the LHCb detector, at centre-of-mass energies of $\sqrt{s}=7$ and $8$ TeV and corresponding to an integrated luminosity of 3.0 fb$^{-1}$. The $\chi_b$ mesons are identified through their decays to $\Upsilon(1S)\gamma$ and $\Upsilon(2S)\gamma$ using photons that converted to $e^+e^-$ pairs in the detector. The $\chi_b(3P)$ meson mass, and the relative prompt production rate of $\chi_{b1}(1P)$ and $\chi_{b2}(1P)$ mesons as a function of the $\Upsilon(1S)$ transverse momentum in the $\chi_b$ rapidity range 2.0< $y$<4.5, are measured. Assuming a mass splitting between the $\chi_{b1}(3P)$ an…
Identified charged particles in quark and gluon jets
2000
A sample of 2.2 million hadronic Z decays, selected from the data recorded by the Delphi detector at LEP during 1994-1995 was used for an improved measurement of inclusive distributions of pi+, K+ and p and their antiparticles in gluon and quark jets. The production spectra of the individual identified particles were found to be softer in gluon jets compared to quark jets, with a higher multiplicity in gluon jets as observed for inclusive charged particles. A significant proton enhancement in gluon jets is observed indicating that baryon production proceeds directly from colour objects. The maxima, xi^*, of the xi-distributions for kaons in gluon and quark jets are observed to be different.
Semileptonic decays of double heavy baryons
2001
We study the semileptonic decays of the lowest lying double heavy baryons using the relativistic three-quark model. We do not employ a heavy quark mass expansion but keep the masses of the heavy quarks and baryons finite. We calculate all relevant form factors and decay rates.
DETERMINATION OF ALPHA(S) FOR B-QUARKS AT THE Z(0) RESONANCE
1993
The strong coupling constant for b quarks has been determined, and its flavour independence, as predicted by QCD, investigated. The analysis involved events with lepton candidates selected from approximately 356 000 hadronic decays of the Z0, collected by the DELPHI detector at LEP in 1990 and 199 1. A method based on a direct comparison of the three-jet fraction in a b enriched sample, selected by requiring leptons with large momenta and transverse momenta, to that of the entire hadronic sample, illustrated the significant effect of the b quark mass on the multi-jet cross section, and verified the flavour independence of the strong coupling constant to an accuracy of +/- 6%. A second proce…
Diffractive dijet production and Wigner distributions from the color glass condensate
2019
Experimental processes that are sensitive to parton Wigner distributions provide a powerful tool to advance our understanding of proton structure. In this work, we compute gluon Wigner and Husimi distributions of protons within the Color Glass Condensate framework, which includes a spatially dependent McLerran-Venugopalan initial configuration and the explicit numerical solution of the JIMWLK equations. We determine the leading anisotropy of the Wigner and Husimi distributions as a function of the angle between impact parameter and transverse momentum. We study experimental signatures of these angular correlations at a proposed Electron Ion Collider by computing coherent diffractive dijet p…
Neutral pion production at threshold and low-energy theorems
1990
Abstract Recent experiments of neutral pion photoproduction show a strong discrepancy between experimental data and the predictions of low-energy theorems (LET). The basic ingredients of these theorems are the conservation of the electromagnetic current and the hypothesis of a partially conserved axial current (PCAC). Both, anomalies in the t-channel, and rescattering in a simple R-matrix formalism, lead to small corrections of the order of 10 – 20 %. However, the investigation of the effects of chiral symmetry breaking and isospin symmetry breaking at the quark level in a current algebraic approach leads to substantial contributions of the right order of magnitude.
Spectrum of SU(2) lattice gauge theory with two adjoint Dirac flavours
2008
An SU(2) gauge theory with two fermions transforming under the adjoint representation of the gauge group may appear conformal or almost conformal in the infrared. We use lattice simulations to study the spectrum of this theory and present results on the masses of several gauge singlet states as a function of the physical quark mass determined through the axial Ward identity and find indications of a change from chiral symmetry breaking to a phase consistent with conformal behaviour at beta_L ~ 2. However, the measurement of the spectrum is not alone sufficient to decisively confirm the existence of conformal fixed point in this theory as we show by comparing to similar measurements with fun…
Unified QCD determination of all vector meson coupling constants
1992
Coupling constants of all vector mesons are determined in QCD by means of a new QCD sum-rule method. The results are found to be sensitive to the binding energy (i.e. meson and quark mass differences). For standard values of quark masses we obtainf ϱ=202 MeV,f K*=216 MeV,f D*=212±22 MeV,f B*=138±42 MeV, $$f_{D_s^* } = 238 \pm 21\,MeV$$ and $$f_{B_s^* } = 163 \pm 45\,MeV$$ . Based on this approach the spin-flavour symmetry for the heavy quark systems are also discussed.
Search for anomalous Wtb couplings in single top quark production in pp¯ collisions at s=1.96 TeV
2012
We present new direct constraints on a general $Wtb$ interaction using data corresponding to an integrated luminosity of 5.4 fb$^{-1}$ collected by the D0 detector at the Tevatron $p\bar{p}$ collider. The standard model provides a purely left-handed vector coupling at the $Wtb$ vertex, while the most general, lowest dimension Lagrangian allows right-handed vector and left- or right-handed tensor couplings as well. We obtain precise limits on these anomalous couplings by comparing the data to the expectations from different assumptions on the $Wtb$ coupling.