Measuring absolute branching ratios of charmed baryons in B decays

The B factories are expected to provide huge samples of single B decay events with little background by reconstructing one of the B mesons produced in decays. This represents a new experimental paradigm: such samples will allow to make measurements of a quality previously thought unrealistic. As example we discuss how absolute branching ratios for exclusive as well as inclusive charm baryon decays can be extracted. One starts out by observing decays like as a signature for etc. and then exploits various correlations of the flavour of the B meson with the baryon number of the (anti)proton and other observables like the charge of a lepton, baryon number of another baryon etc. An integrated luminosity of about 500 fb^-1 as could be available by 2005 should be sufficient for the task. Received: 21 December 2001 / Published online: 8 May 2002     

Inclusive semileptonic branching ratios of b hadrons produced in Z decays

A measurement of the inclusive semileptonic branching ratios of b hadrons produced in Z decay is presented, using four million hadronic events collected by the ALEPH detector from 1991 to 1995. Electrons and muons are selected opposite to b-tagged hemispheres. Two different methods are explored to distinguish the contributions from direct and cascade decays to the total lepton yield. One is based on the lepton transverse momentum spectrum, the other makes use of the correlation between the charge of the lepton and charge estimators built from tracks in the opposite hemisphere of the event. The latter method reduces the dependence on the modelling of semileptonic b decays. The results obtained by averaging the two techniques are Received: 1 August 2001 / Published online: 14 December 2001     

Measurement of the semileptonic b branching fractions and average b mixing parameter in Z decays

The semileptonic branching fractions for primary and cascade b decays , and were measured in hadronic Z decays collected by the DELPHI experiment at LEP. The sample was enriched in b decays using the lifetime information and various techniques were used to separate leptons from direct or cascade b decays. By fitting the momentum spectra of di-leptons in opposite jets, the average b mixing parameter was also extracted. The following results have been obtained: Received: 3 March 2000 / Revised version: 23 April 2001 / Published online: 28 May 2001     

A study of B meson oscillations using hadronic Z0 decays containing leptons

An event sample enriched in semileptonic decays of b hadrons is selected using an inclusive lepton selection from approximately 3.0 million hadronic Z⁰ decays collected with the OPAL detector. This sample is used to investigate B meson oscillations by reconstructing a proper decay time for the parent of each lepton, using a jet charge method to estimate the production flavour of this parent, and using the lepton charge to tag the decay flavour. We measure the mass difference between the two B⁰ _d mass eigenstates     

Measurement of the forward backward asymmetry of c andb quarks at the Z pole using reconstructed D mesons

A measurement of the forward–backward asymmetry of and on the Z resonance is performed using about 3.5 million hadronic Z decays collected by the DELPHI detector at LEP in the years 1992 to 1995. The heavy quark is tagged by the exclusive reconstruction of several D meson decay modes. The forward–backward asymmetries for c andb quarks at the Z resonance are determined to be: The combination of these results leads to an effective electroweak mixing angle of: Received: 9 February 1999 / Revised version: 26 April 1999 / Published online: 3 August 1999     

Tau decays with neutral kaons

The branching ratio of the lepton to a neutral kaon meson is measured from a sample of approximately 200,000 decays recorded by the OPAL detector at centre-of-mass energies near the resonance. The measurement is based on two samples which identify one-prong decays with and mesons. The combined branching rat ios are measured to be where the first error is statistica l and the second systematic. Received: 29 October 1999 / Published online: 24 February 2000     

Measurement of B d 0 - oscillations

B _d ⁰ meson oscillations are measured in hadronic Z⁰ decays using the charge of a lepton or the mean charge of an event hemisphere to sign the presence of a b or a b? quark when it is produced, and using the charge of a lepton emitted at large pt or of a D*^± to sign the presence of a B or a B? meson when it decays. With 3.2 million hadronic Z⁰ decays registered by DELPHI between 1991 and 1994, the mass difference Δm _d between the two physical B _d ⁰ states is measured in four channels: Taking into account the statistical overlap between these measurements and the common systematic uncertainties, the combined result is:      

Measurement of the {\rm\bf B_{\bf d}^{\bf 0}- {\bf \bar{B}}_{\bf d}^{\bf 0}} oscillation frequency

Time-dependent mixing is studied using about two million hadronic Z decays registered by L3 in 1994 and 1995. For this study three techniques are used. Tagging of the b-quark charge at decay time is performed by identifying leptons from semileptonic B decays. The flavour of the b quark at production time is determined from the charge of the lepton in the opposite hemisphere or by using a jet-charge technique. The proper time of the B-particle decay is obtained by reconstructing the production and decay vertices or by a measurement of the lepton impact parameter. The combined result for the frequency of meson oscillations is Received: 20 February 1998 / Revised version: 23 March 1998 / Published online: 12 August 1998     

Measurement of the Z resonance parameters at LEP

Abstract. The properties of the Z resonance are measured from the analysis of 4.5 million Z decays into fermion pairs collected with the Aleph detector at LEP. The data are consistent with lepton universality. The resonance parameters are measured to be , GeV, nb and, combining the three lepton flavours, . The corresponding number of light neutrino species is and the strong coupling constant is . The lepton pair forward-backward asymmetry is measured to be from which the effective weak mixing angle is derived: . The measurement of the leptonic width MeV leads to a determination of the effective parameter . The data support the Standard Model and favour a light Higgs. Received: 16 July 1999 / Published online: 17 March 2000     

On lepton pair production in proton-antiproton collisions at intermediate energies

The lepton pair production via the quark-antiquark annihilation subprocess in collisions of beam antiproton with the proton target at E _beam = 14 GeV is studied on the basis of the event sample simulated by PYTHIA6 generator. Different kinematical variables which may be useful for the design of the muon system and the electromagnetic calorimeter of the detector of PANDA experiment at FAIR, as well as for the study of proton structure functions in the available x-Q ² kinematical region, are considered. It is also argued that the measurement of the total transverse momentum of a lepton-antilepton system may provide important information about the intrinsic transverse momentum 〈k _T 〉 that appears due to the Fermi motion of quarks inside the nucleon. Another interesting possibility is the measurement of the production rate of two or three lepton pairs in one event that can give the information about the rate of multiple quarks interactions and the proton space structure. The problems due to the presence of fake leptons that appear from meson decays, as well as due to the contribution of background QCD processes and minimum bias events, are also discussed. The set of cuts which allows one to separate the events with the signal lepton pairs from different kind of background events is proposed. The article is published in the original.     

Charged-lepton-flavour violation in kaon decays in supersymmetric theories

We discuss rare kaon decays that violate charged-lepton flavour conservation in supersymmetric theories with and without R parity, in view of possible experiments using an intense proton source as envisaged for a neutrino factory. In the minimal supersymmetric standard model, such decays are generated by box diagrams involving charginos and neutralinos, but the limits from –e conversion and constrain the branching ratios to challengingly small values. However, this is no longer the case in R-violating theories, where such decays may occur at tree level at rates close to the present experimental limits. Within this framework, we obtain bounds on products of and operators from the experimental upper limits on and decays. We also note the possibility of like-sign lepton decays in the presence of non-zero – mixing. We conclude that rare kaon decays violating charged-lepton flavour conservation could be an interesting signature of R violation. Received: 8 September 2000 / Published online: 17 December 2001     

Measurements of inclusive semileptonic branching fractions of b hadrons in \boldmath Z^0z decays

A measurement of inclusive semileptonic branching fractions of b hadrons produced in Z decays is presented. An enriched Z sample is obtained with a lifetime flavour-tagging technique. The leptonic events are then selected from this sample, and classified according to their origin, which is determined by comparing the distribution of several kinematic variables using artificial neural network techniques. Using 3.6 million multihadronic events collected with the OPAL detector at energies near the Z resonance, the values are measured, where b denotes all weakly decaying b hadrons and represents either e or . The second error includes all experimental systematic uncertainties whereas the last error is due to uncertainties in modelling of the lepton momentum spectrum in semileptonic decays and b quark fragmentation. The average fraction of the beam energy carried by the weakly decaying b hadron, , is measured to be where the modelling error is dominated by the choice of b fragmentation model. The agreement between data and various semileptonic decay models and fragmentation functions is also investigated. Received: 11 June 1999 / Published online: 3 February 2000     

Inclusive semileptonicB decays in QCD including lepton mass effects

Starting from an operator product expansion in the heavy quark effective theory up to order 1/m _b ² we calculate the inclusive semileptonic decays of unpolarized bottom hadrons including lepton mass effects. We calculate the differential decay spectra d/(dE ), and the total decay rate forB meson decays to final states containing a lepton.Supported by the Graduiertenkolleg Teilchenphysik, Universität Mainz     

Discovering the Higgs through highly-displaced vertices

We suggest that the Higgs could be discovered at the Tevatron or the LHC (perhaps at the LHCb detector) through decays with one or more substantially displaced vertices from the decay of new neutral particles. This signal may occur with a small but measurable branching fraction in the recently-described “hidden valley” models, hep-ph/0604261; weakly-coupled models with multiple scalars, including those of hep-ph/0511250, can also provide such signals, potentially with a much larger branching fraction. This decay channel may extend the Higgs mass reach for the Tevatron. Unusual combinations of b jets, lepton pairs and/or missing energy may accompany this signal.     

A study of the Lorentz structure in tau decays

This paper describes a measurement of the Michel parameters, , , , , and the average helicity, , in lepton decays together with the first measurement of the tensor coupling in the weak charged current. The pairs were produced at the LEP collider at CERN from 1992 through 1995 in the DELPHI detector. Assuming lepton universality in the decays of the the measured values of the parameters were: , , , , . The strength of the tensor coupling was measured to be . The first error is statistical and the second error is systematic in all cases. The results are consistent with the structure of the weak charged current in decays of the lepton. Received: 14 February 2000 / Revised version: 7 March / Published online: 8 May 2000     

Determination of heavy quark non-perturbative parametersfrom spectral moments in semileptonic B decays

Moments of the hadronic invariant mass and of the lepton energy spectra in semileptonic B decays have been determined with the data recorded by the DELPHI detector at LEP. From measurements of the inclusive b-hadron semileptonic decays, and imposing constraints from other measurements on b- and c-quark masses, the first three moments of the lepton energy distribution and of the hadronic mass distribution, have been used to determine parameters which enter into the extraction of |V_cb| from the measurement of the inclusive b-hadron semileptonic decay width. The values obtained in the kinetic scheme are: and include corrections at order 1/m_b³. Using these results, and present measurements of the inclusive semileptonic decay partial width of b-hadrons at LEP, an accurate determination of |V_cb| is obtained: Received: 26 April 2005, Revised: 16 September 2005, Published online: 16 November 2005     

Measurement of A_{\rm FB}^b using inclusive b-hadron decays

Based on a sample of four million events collected by ALEPH from 1991 to 1995, a measurement of the forward-backward asymmetry in decays using inclusive final states is presented. High-performance tagging of events in a wide angular range is achieved using neural network techniques. An optimal hemisphere charge estimator is built by merging primary and secondary vertex information, leading kaon identification and jet charge in a neural network. The average charge asymmetry, the flavour tagging efficiencies and mean b-hemisphere charges are measured from data and used to extract the pole b asymmetry in the Standard Model corresponding to a value of the effective weak mixing angle of Received: 4 July 2001 / Published online: 12 November 2001     

Measurement of the $b\overline{b}$ production cross section in 920 GeV fixed-target proton-nucleus collisions

Using the HERA - B detector, the production cross section has been measured in 920 GeV proton collisions on carbon and titanium targets. The production was tagged via inclusive bottom quark decays into by exploiting the longitudinal separation of decay vertices from the primary proton-nucleus interaction. Both e ⁺ e ^- and channels have been reconstructed and the combined analysis yields the cross section . Received: 6 June 2002 / Revised version: 2 September 2002 / Published online: 8 November 2002     

Consequences of diffractive heavy flavor production

We calculate the lepton spectrum inpp collisions from the production and semileptonic decay ofΛ _c andΛ _b , and find limits on their production cross sections. Total cross sections in excess of 1 mb forΛ _c and 2 1/2μb forΛ _b violate the experimental fact that (μ/π)?10^?4. Abundant production ofΛ _c has led to speculations on the existence of a large intrinsic charm sea inside the proton. We discuss alternative ways of probing such a structure inside the proton: jets opposite highp _T direct photons and central production of ψ.     

Determination of the b -quark mass and nonperturbative parameters in semileptonic B -meson decays at BABAR⋆

Knowing the mass of the b -quark is essential to the study of the structure and decays of B -mesons as well as to future tests of the Higgs mechanism of mass generation. We present recent preliminary measurements of the b -quark mass and related nonperturbative parameters from moments of kinematic distributions in charmed and charmless semileptonic B -meson decays. Their determination from charmless semileptonic B -meson decays is the first measurement in this mode. The data were collected by the BABAR detector at the PEP-II asymmetric-energy e ⁺ e ^- -collider at the Stanford Linear Accelerator Center at a center-of-mass energy of 10.58GeV. Original article based on material presented at HADRON 2007.