Heavy Flavor Quarks Research Articles

A FB for bb and cc

At LEP the forward-backward asymmetries in the distribution of the polar angle of charm (A c FB ) and bottom (A b FB ) quarks produced in the Z boson decay have been measured in about 4 million events collected by each experiment. From the measurement of heavy flavour quark asymmetry a value of sin 2 θ l W,e f f is determined with relative accuracy of 1.6 × 10 −3 .

Multiplicity of identified particles in e+e− at the Z 0

The production rates as a function of Ξ p for the identified hadrons π, K and p have been measured by DELPHI in inclusive hadronic Z 0 decays, as well as separately in decays into light and heavy flavors. The RICH detector (Ring Imaging Čerenkov detector) of DELPHI was used to identify charged hadrons. The vertex detector was used to tag high-impurity samples of light and heavy quark flavour events. The results, based on a data sample of 1 400 000 Z 0 decays collected by DELPHI during 1994, have been compared with the predictions from the JETSET parton shower model and the HERWIG cluster fragmentation model. In general, JETSET describes the measured spectra better. In particular, HERWIG seems to fail in the case of the proton spectra in all flavor samples; overestimates the spectra for all charged particles, pions and kaons in Z 0 → b b events and underestimates the corresponding spectra in Z 0 → u u , d d , s s events. The DELPHI results on pions, kaons and protons in the inclusive flavor sample agree well with the results from the ALEPH and SLD collaborations, whereas the proton spectrum in the low Ξ p region is in disagreement with the results from the OPAL collaboration. The particle multiplicities in Z 0 → q q , Z 0 → b b , and Z 0 → u u , d d , s s events have been measured; improved results were obtained for kaons and protons in the inclusive and b samples and new results were obtained on the multiplicity of pions, kaons and protons in Z 0 → u u , d d , s s events and on the multiplicity of pions in Z 0 → b b events.

Leading log radiative corrections to polarized heavy flavor leptoproduction

O(\alpha) QED radiative corrections to polarized neutral current leptoproduction of heavy quark flavours are calculated in the leading log approximation for HERMES, COMPASS, and HERA kinematics. Besides the inclusive case, we derive general $z_h$-differential formulae for the polarized boson gluon fusion process and use them to calculate radiative corrections to semi-inclusive polarized heavy flavour production in the case of HERA kinematics.

Charge-coupled devices as particle tracking detectors

Following a few years after the invention of the charge-coupled device (CCD) in 1970, the discovery of charmed particles established the need for very high precision (few μm) detectors for tracking high energy charged particles. This review describes the work which has evolved over the past 20 years from these disconnected events, both as regards the application of increasingly refined CCDs to particle tracking (in particular as vertex detectors for identifying heavy flavor quarks and tau leptons), and also the advances in CCD detector design stimulated by these requirements. The lessons learned in this work should provide guidance for the construction of large arrays of CCDs or active pixel devices in the future in a number of areas of science and technology.

Note on the mass square of the eta′ meson

We propose to use as a mathematical tool to study the mass square of the etaprime meson an infinitely heavy supplementary quark flavor Q. This is understood in the framework of QCD with one light quark flavor (or n_fl light flavors) and the gauge group SU3 colour (or SUN colour). The full system thus consists of n_fl + 1 flavors, including the infinitely heavy one. The purpose of this note is to show how the heavy flavor can be made to represent the anomalies of the subtheory describing the remnant physical degrees of freedom. In the large N_c limit the mass square of etaprime tends to a finite limit. The essential deviations from (semi) perturbative derivations are related to simple properties of the 'heavy' flavor.

Fragmentation functions for baryons in a quark-diquark model

A perturbative QCD calculation of heavy flavor quark fragmentation into heavy flavor baryons is developed along the lines of corresponding heavy meson models. The non-perturbative formation of the baryon is accomplished by implementing the quark-diquark model of the baryons. Diquark color form factors are used to enable the integration over the virtual heavy quark momentum. The resulting spin independent functions for charmed and bottom quarks to fragment into charmed and bottom baryons with spin 1/2 and 3/2 are compared with recent data. Predictions are made for the spin dependent fragmentation functions as well, particularly for the functions $\hat{g}_1$ and $\hat{h}_1$ in the case of spin 1/2 baryons.

Testing a Model with Additional Vector Fermions at the LEP2 Collider

Our aim is to test a model recently presented, motivated by the reported Rb and Rc "crisis", which contains extra vector fermions. We suggest an alternative indirect test of the possible existence of new heavy quark flavors at the LEP2 collider, which turns out to give the clearest signal. We calculate [Formula: see text] cross-sections within this framework, including one-loop corrections, and find measurable differences compared to the Standard Model predictions.

CKM-favored semileptonic decays of heavy hadrons at zero recoil.

We study the properties of Cabibbo-Kobayashi-Maskawa (CKM) favored semileptonic decays of mesons and baryons containing a heavy quark at the point of no recoil. We first use a diagrammatic analysis to rederive the result observed by earlier authors that at this kinematic point the $B$ meson decays via $b\to c$ transitions can only produce a $D$ or $D^*$ meson. The result is generalized to include photon emissions which violate heavy quark flavor symmetry. We show that photons emitted by the heavy quarks and the charged lepton are the only light particles that can decorate the decays $\bar{B}\to D(D^*) + \ell\nu$ at zero recoil, and the similar processes of heavy baryons. Implications for the determinations of the CKM parameter $V_{cb}$ are discussed. Also studied in this paper is the connection between our diagrammatic analysis of suppression of particle emission and the formal observation based on weak currents at zero recoil being generators of heavy quark symmetry. We show that the two approaches can be unified by considering the Isgur-Wise function in the presence of an external source.

Effective hadron theory

The low-energy flavour dynamics is approximated by the NJL model from which effective hadron theories are derived. In the light meson sector emphasis is put on a recent one-meson loop calculation of the π-π phase shifts and the electromagnetic form factor of the pion, where isospin symmetry breaking is included, giving rise to the ϱ-ω-mixing. I also discuss the relation between the two different common phenomenological forms of vector meson dominace. The description of baryons either as chiral solitons of the meson fields or as bound quark-diquark states or as hybrids of both is briefly reviewed. Recent results for the hyperon radiative decays obtained within the Skyrme model are presented. Finally the NJL model is extended to heavy quark flavours and converted into an effective meson theory, which describes the interplay between chiral and heavy quark symmetry.

Calculation of Z plus four jet production at the Tevatron

We present the first calculation of Z + 4 jet production with heavy quark flavor identification at the Tevatron p p ̄ collider. The Z + 4 jet channel is especially interesting as a normalizer for the W + 4 jet background to top quark signals, as a background to a possible t → cZ flavor-changing neutral-current (FCNC) decay signal, and as a background to missing- p T signals from gluino pairs. We also calculate the contributions to W + 4 jet production from all the different heavy-flavor final states. The MADGRAPH program is used to generate all leading order subprocess helicity amplitudes. We present Monte Carlo results with separation and acceptance criteria suitable for the Tevatron experimental analyses. The dependence of the cross sections on experimental cuts and the theoretical ambiguities due to the scale dependence are discussed. The predicted (W + 4 jet) (Z + 4 jet) ratio is insensitive to most of these choices.

Signatures of scalar top with R-parity breaking coupling at HERA

In the framework of the minimal supersymmetric standard model with an R-parity breaking coupling of the scalar top quark (stop) we investigate production processes and decay properties of the stop at HERA energies. The model is characterized by a light stop possibly lighter than the other squarks. We show that the stop could be singly produced not only in the neutral current processes but also in associated processes whose final states contain some heavy flavor quarks, bottom and top quarks. These signatures would be useful to discriminate the stop from leptoquarks.

Anathematizing the Guralnik-Manohar bound for [formula omitted

There is a recent claim by Guralnik and Manohar [Phys. Lett. B 302 (1993) 103] to have established a rigorous lower bound on Λ , the asymptotic difference between the mass of a heavy flavour hadron and that of the heavy flavour quark. We point out the flaw in their reasoning and discuss the underlying physical problem. An explicit counterexample to the GM bound is given; therefore, one cannot count on a refined proof to re-establish this bound.

Tests of the Electroweak Theory at LEP

LEP offers a rich choice of tests of the electroweak theory such as the measurement of hadronic and leptonic cross sections, leptonic forward-backward asymmetries, τ polarization asymmetries, partial widths and forward-backward asymmetries of heavy quark flavours, of the inclusive qq charge asymmetry and of final state radiation in hadronic events. We discuss experimental aspects of these measurements and their theoretical parametrization and summarize the results available so far. We present several analyses which reveal specific aspects of the results, such as their constraints on Standard Model parameters and on new particles, the sensitivity to deviations from the Standard Model multiplet structure and an analysis in a framework which provides a model independent search for new physics.

QCD predictions for lepton spectra in inclusive heavy flavor decays.

We derive the lepton spectrum in semileptonic beauty decays from a nonperturbative treatment of QCD; it is based on an expansion in $1/m_Q$ with $m_Q$ being the heavy flavour quark mass. The leading corrections arising on the $1/m_Q$ level are completely expressed in terms of the difference in the mass of the heavy hadron and the quark. Nontrivial effects appear in $1/m_Q^2$ terms affecting mainly the endpoint region; they are different for meson and baryon decays as well as for beauty and charm decays.

Recent heavy flavor physics results from fixed target experiments

Recent results from fixed target experiments in the field of heavy quark flavors, as published or otherwise disseminated in the last year, are reviewed. Emphasis is placed on distilling the main conclusions from these results.

Superflavor symmetry for heavy particles

We extend the heavy quark spin and flavor symmetry discussed by Voloshin and Shifman and by Isgur and Wise to systems in which the heavy colored particles have different spins. We discuss in detail the superflavor symmetry between a heavy quark and a heavy color triplet scalar.

QCD corrections to vector boson self-energies in the standard model

The O (α s) corrections to the vacuum polarization tensor of the gauge bosons are presented for both light and heavy quark flavours in the standard model of electroweak interactions on the basis of a running coupling constant α s. The influence of ϒ, T b and θ resonance as well as the proper threshold behaviour of the imaginary part of the vacuum polarization function are incorporated in the analysis. The real part is calculated numerically through the properly subtracted dispersion relation. A set of FORTRAN routines that implements the scheme is described.

A possible mechanism of heavy quark flavour production

A nonperturbative mechanism of heavy quark flavour production is proposed. It is shown that it explains some features of charm production in hadron-hadron processes.

A vertex detector for SLD

The SLAC Linear Collider is currently being commissioned. A second-generation detector for SLC, known as SLD, is now under construction. In the centre of this 4000 ton detector there will be a vertex detector (VXD) consisting of 4 barrels of 2-dimensional CCDs, approximately 250 CCDs in total. This detector will be used as a tracking microscope, able to pinpoint the outgoing tracks with a precision of about 5 μm, and thus to distinguish between particles produced at the primary vertex and those which result from the decay of heavy-flavour quarks (charm, bottom and possibly others) or from the decay of heavy leptons. This paper describes the present state of the VXD project, with particular emphasis on the signal processing procedures which will reduce the 60 million measurements of pixel contents for each event to a manageable level (some tens of kilobytes).

Penetrating flavors

Hadrons made of heavy flavor quarks are expected to have small cross sections and perhaps reduced energy loss in collisions, leading to their being more “penetrating” than ordinary hadrons. Some effects of this in cosmic ray phenomena are considered. If the heavy particle lives long enough it can become the dominant hadron component in the atmosphere. With particles of short lifetime there is a threshold effect in energy where the penetrating hadron travels far enough to become important. Some of the anomalous effects in 10 2 – 10 3 TeV phenomena could be caused via this mechanism by a particle with lifetime ∼ 10 −11 s. An ∼ 150 cm thick iron absorber would provide a substantial enhancement in the charm/proton ratio in the 10–100 TeV range. If free quarks are produced and have the same cross section as bound quarks, they act as highly penetrating hadrons.