115138605
Measurements of the charged particle multiplicity distribution in restricted rapidity intervals
Charged particle multiplicity distributions have been measured with the ALEPH detector in restricted rapidity intervals |Y| ≤0.5, 1.0, 1.5, 2.0 along the thrust axis and also without restriction on rapidity. The distribution for the full range can be parametrized by a log-normal distribution. For smaller windows one finds a more complicated structure, which is understood to arise from perturbative effects. The negative-binomial distribution fails to describe the data both with and without the restriction on rapidity. The JETSET model is found to describe all aspects of the data while the width predicted by HERWIG is in significant disagreement.
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.
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.
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.
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.
11679008
MIRCO-STRUCTURES OF RF SURFACES IN THE ELECTRON-BEAM-WELD REGIONS OF NIOBIUM
Micro-structures of RF surfaces in the electron-beam-weld (EBW) regions of niobium samples were studied. These surfaces were polished with different techniques,namely buffered chemical polish (1:1:2 BCP) and electrolytic polish (EP). It was found that there are three distinctively different regions near the EBW, characterized by different grain sizes, shapes, and orientations. Strikingly, we found grain boundaries within the EBW are nicely lined up so that they form an angle of about 60 degree with respect to the weld seam. Upon being polished with BCP, these grain boundaries evolve into micro-steps with sharp edges, due to preferential etching of different crystallographic planes. There is evidence that the step height increases up to possibly more than 30 microns with repeated chemical polish. In contrast to the result with BCP, dimensions of surface irregularities were reduced with EP and sharp edges were rounded off.These observations have important implications in explanations of the high-field-Q-drop of niobium superconducting cavities.
18885837
Beam test of a superconducting cavity for the CESR luminosity upgrade
The prototype superconducting cavity system for CESR-Phase III was tested in CESR in August 1994. The performance of the system was very gratifying. The cavity operated gradients of 4.5-6 MV/m and accelerated beam currents up to 220 mA. This current is a factor of 3 above the world record 67 mA for SRF[1]. The high circulating beam current did not increase the heat load or present any danger to the cavity. No instability attributable to the SRF cavity was encountered. A maximum of 155 kW of rf power was transferred to a 120 mA beam. The window was subjected to 125 kW reflected power and processed easily. In the travelling wave mode, vacuum bursts and are trips prevented us from going above 165 kW. The maximum HOM power extracted was 2 kW. Beam stability studies were conducted for a variety of bunch configurations. In other tests a 120 mA beam was bumped horizontally and vertically by /spl plusmn/10 mm. While supporting a 100 mA beam, the cavity was axially deformed with the tuner by 0.4 mm to sweep the HOM frequencies across dangerous revolution harmonics. In all such tests, no resonant excitation of HOMs or beam instabilities were observed, which confirms that the potentially dangerous modes were damped strongly enough to be rendered harmless.
123667389
Trapped magnetic flux in superconducting niobium samples
Trapped magnetic flux is known to be one cause of residual losses in bulk niobium superconducting radio frequency cavities. In the Meissner state an ambient magnetic field should be expelled from the material. Disturbances such as lattice defects or impurities have the ability to inhibit the expulsion of an external field during the superconducting transition so that the field is trapped. We have investigated the effect the treatment history of bulk niobium has on the trapped flux and which treatment leads to minimal flux trapping. For that purpose, we measured the fraction of trapped magnetic flux in niobium samples representing cavities with different typical treatment histories. The differences between single crystal and polycrystalline material as well as the influence of spatial temperature gradients and different cooling rates were investigated. In addition, the progression of the release of a trapped field during warm-up was studied. We found that heat treatment reduces trapped flux considerably and that single crystal samples trap less flux than polycrystalline niobium. As a consequence, the single crystal sample with $1200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ baking trapped the smallest amount of field which is about 42%. Moreover, the release of the trapped field during warm-up was observed to progress over a broad temperature range for the baked single crystal samples.
24618282
CW Operation of the TTF-III Input Coupler
Several newly proposed superconducting linacs are designed to use TESLA technology operating CW rather than pulsed for which the system was developed. It must therefore be demonstrated that CW operation is feasible. Here we describe a series of CW tests with the TTF-III power coupler to determine the average power limit. These tests were performed on a coupler test stand both at room temperature at the FZ Rossendorf and under cryogenic operating conditions in BESSY’s HOBICAT[1] facility. An extrapolation of the measurements suggests that the coupler can handle 5 kW CW standing wave. It was also demonstrated that even higher power levels are feasible if the cooling of the inner conductor is modified.
198946223
CHARACTERISATION OF MICROPHONICS IN HOBICAT
The HoBiCaT test-facility at BESSY, which is designed for cryogenic testing of superconducting TESLA units has been equipped with a 9-cell TESLA-type cavity. Mechanical vibrations in the cryostat result in microphonic detuning of the cavity resonance. These microphonics have been characterized, and their sources analyzed, including the impact of operating conditions such as LHe pressure, cavity field and heater power. Furthermore, the mechanical transfer functions needed for the eventual compensation of the microphonics have been recorded. INTRODUCTION For CW operation of a superconducting, TESLA-type cavity microphonics represent the largest disturbance of the resonant frequency. In particular at a small bandwidth Δf = ω/QL, according to the equation P ∝ 1 + (δf) 2 (Δf)2 (1) the microphonics δf lead to a significant increase of the generator power P, which is required to maintain a constant cavity field. Therefore it is highly desirable to minimize the microphonics. Measurements characterizing the microphonics and their impact on the cavity operation are presented below. The perspectives on possible countermeasures are described elsewhere [1]. MICROPHONICS MEASUREMENTS Microphonics have been recorded in two different ways. In a first method, the RF-feedback signal, see Figure 1, is recorded over time. This is preferably done in open loop configuration, because obtaining a calibration factor is more straightforward here, than in closed loop configuration. A second method utilizes the sensor-actuator design of the piezo tuner to record the induced piezoelectric voltage over time. In this design, two high-voltage piezos are attached to the tuner, thus piezoelectric forces can be exercised on the cavity and measured simultaneously. The latter is only an indirect method, as the mechanical influence from cavity and tank on the frequency response does not necessarily lead to a linear relationship between cavityand piezo position. The results presented here were solely obtained from the RF-feedback signal. ∗Work funded by the European Commission in the Sixth Framework Program, contract No. 011935 EUROFEL. Input coupler piezo