123148086
Charm counting in b decays
The inclusive production of charmed particles in decays has been measured from the yield of D0, D+, Ds+ and Λc+ decays in a sample of events with high b purity collected with the ALEPH detector from 1992 to 1995. From these measurements, adding the charmonia production rate and an estimate of the charmed strange baryon contribution, the average number of charm quarks per b decay is determined to be nc = 1.230 ± 0.036 ± 0.038 ± 0.053, where the uncertainties are due to statistics, systematic effects and branching ratios, respectively.
122230572
A measurement of the ratio of longitudinal and transverse structure functions in neutrino interactions between 30 and 200 GeV
Abstract A new measurement of the ratio R = σ L / σ T of longitudinal and transverse structure functions in neutrino interactions on iron between 30 and 190 GeV neutrino energy is reported. The result is given as a function of the scale parameter x and the inelasticity ν of the interaction. The average value is R = 0.10 ± 0.07 around ν ≈ 50 GeV and is in accordance with a prediction from the QCD theory.
122188618
Charged particle pair production associated with a lepton pair in Z decays. indication of an excess in the tau channel
In a sample of 200 000 Z decays, events with two leptons and an additional pair of charged particles are studied. The 35 events found show a possible excess in the tau channel compared with the expectation from electroweak processes. The features of the events are consistent with radiation of virtual photons.
123101518
Studies of quantum chromodynamics with the ALEPH detector
Abstract Previously published and as yet unpublished QCD results obtained with the ALEPH detector at LEP1 are presented. The unprecedented statistics allows detailed studies of both perturbative and non-perturbative aspects of strong interactions to be carried out using hadronic Z and tau decays. The studies presented include precise determinations of the strong coupling constant, tests of its flavour independence, tests of the SU(3) gauge structure of QCD, study of coherence effects, and measurements of single-particle inclusive distributions and two-particle correlations for many identified baryons and mesons.
122041721
Production of excited beauty states inZ decays
AbstractA data sample of about 3.0 million hadronicZ decays collected by the ALEPH experiment at LEP in the years 1991 through 1994 is used to make an inclusive selection of B hadron events. In this event sample 4227±140±252B* mesons in the decayB*→Bγ and 1944±108±161B**u,d mesons decaying into a B meson and a charged pion are reconstructed. Here and in the followingB**u,d denotes the eightL=1(bū) and (bd) states and their charge conjugate.For the well establishedB* meson the following quantities are obtained:ΔM=MB*−MB=(45.30±0.35±0.87) MeV/c2 andNB*/(NB+NB*)=(77.1±2.6±7.0)%. The angular distribution of the photons in theB* rest frame is used to measure the relative contribution of longitudinalB* polarization states to beσL/(σL+σT)=(33±6±5)%.In theM(Bπ)−M(B) mass difference resonance structure is observed at (424±4±10) MeV/c2, for which the shape and position is consistent with the expectation forB**u,d states decaying intoB(*)π±. The relative production rate is determined to be
$$\begin{gathered} \frac{{BR(Z \to b \to B_{u,d}^{ * * } )}}{{BR(Z \to b \to B_{u,d} )}} \hfill \\ = [27.9 \pm 1.6(stat) \pm 5.9(syst) {}_{ - 5.6}^{ + 3.9} (model)]\% \hfill \\ \end{gathered} $$
 where the third error reflects the uncertainty due to different production and decay models for the broadB**u,d states.
122204997
First measurement of the quark-to-photon fragmentation function
Earlier measurements at LEP of isolated hard photons in hadronic Z decays, attributed to radiation from primary quark pairs, have been extended in the ALEPH experiment to include hard photon productioninside hadron jets. Events are selected where all particles combine democratically to form hadron jets, one of which contains a photon with a fractional energyz≥0.7. After statistical subtraction of non-prompt photons, the quark-to-photon fragmentation function,D(z), is extracted directly from the measured 2-jet rate. By taking into account the perturbative contributions toD(z) obtained from anO(ααs) QCD calculation, the unknown non-perturbative component ofD(z) is then determined at highz. Provided due account is taken of hadronization effects nearz=1, a good description of the other event topologies is then found.
53355048
Multipacting in 1 . 5-GHz superconducting niobium cavities of the CEBAF shape ∗
We present definitive thermometric evidence of two-point multipacting in the CEBAF cavity shape, starting at Epk = 30 MV/m (at 1.5 GHz). With the aid of high speed thermometry, we were able to record fleeting multipacting events near the cavity equator. Short term electron bombardment eliminates the multipacting barrier. Numerical trajectory simulations presented here also confirm the experimental data. Although multipacting itself could be processed away and did not limit the cavity performance, it is shown that the quenches due to the electron bombardment increase the cavity’s residual resistance by creating and trapping magnetic flux.
121421972
STARS: A Cascaded High-Gain Harmonic-Generation FEL
Femtosecond pulses from linac-based free-electron lasers are unique tools for future time-resolved experiments. In March 2004, BESSY published the Technical Design Report (TDR) for a free-electron-laser user facility that covers the VUV to soft X-ray range (BESSY FEL) [1]. This second-generation FEL facility is seeded and uses the High-Gain Harmonic-Generation (HGHG) [2] scheme to produce coherent radiation down to the 1 nm range.
33237644
STARS - a two-stage high-gain harmonic generation FEL demonstrator
BESSY is proposing a demonstration facility, called STARS, for a two-stage high-gain harmonic generation free electron laser (HGHG FEL). STARS is planned for lasing in the wavelength range 40 to 70 nm, requiring a beam energy of 325 MeV and peak current of 500 A. The facility consists of a normal conducting gun, three superconducting TESLA-type acceleration modules modified for CW operation, a single stage bunch compressor and finally a two-stage HGHG cascaded FEL. This paper describes the faciliy layout and the rationale behind the operation parameters.
121749411
Beam breakup instability suppression in multicell superconducting rf guns
Helmholtz-Zentrum, Berlin, Germany(Received 28 November 2009; revised manuscript received 28 November 2010; published 19 May 2011)In this paper, we provide an analytical description for transverse coupling impedances with dipole highorder modes (HOMs) and beam breakup (BBU) instability suppression of relativistic, high-current beamsundergoing strong acceleration, such as those typically produced by rf photoinjectors. The model adoptedis based on the accepted theory of coupling impedances extended to the case of beams characterized by afast transition due to strong acceleration, from the nonrelativistic to the relativistic regime in whicha bunch trajectory may not be rigidly directed parallel to the axis of the electric ﬁeld. The trajectoryoscillations in a dipole HOM ﬁeld in the transverse plane are effective in perturbing the bunch energy,which causes an increase of the coupling impedance up to the BBU instability. This BBU instabilityanalysis is based on equations obtained by Volkov [Phys. Rev. ST Accel. Beams 12, 011301 (2009)]inwhich external focusing due to both the applied transverse electric (TE) HOM and the acceleratingfundamental mode as well as dipole HOM damping by external loads are investigated. This analysis isvalid in the limit in which the weak nonlinearity of applied ﬁelds near the axis may be neglected. Thesolution suggests a means of enhancing the BBU threshold current. The possibility of obtaining BBUinstability suppression in a long, integrated photoinjector and linac structure, such as the superconductingrf source at Forschungs Zentrum Dresden (FZD), are numerically examined. New designs of photo-injectors that provide BBU instability suppression of all dipole HOMs to enhance the threshold currentsare suggested.
55994461
FIRST CHARACTERIZATION OF A FULLY SUPERCONDUCTING RF PHOTOINJECTOR CAVITY
As a first step towards a high brightness, high average current electron source for the BERLinPro ERL a fully superconducting photo-injector was developed by HZB in collaboration with JLab, DESY and the A. Soltan Institute. This cavity-injector ensemble is made up of a 1.6-cell superconducting cavity with a superconducting lead cathode deposited on the half-cell backwall. A superconducting solenoid is used for emittance compensation. This system, including a diagnostics beamline, has been installed in the HoBiCaT facility to serve as a testbed for beam dynamics studies and to test the combination SRF cavity and superconducting solenoid. This paper summarizes the characterization of the cavity in this configuration including Q measurements, dark current tests and field-stability analyses.