123042372
Measurement of the tau lepton lifetime with the three-dimensional impact parameter method
A new method is presented for the measurement of the mean $\tau$ lepton lifetime using events in which $\tau$'s are pair-produced and both $\tau$'s decay to hadrons and $\nu_\tau$. Based on the correlation between the two $\tau$'s produced at a symmetric $e^+ e^-$ collider, the 3DIP method relies on the three-dimensional information from a double-sided vertex detector and on kinematic constraints for the precise measurement of the $\tau$ decay angles. Using the data collected from 1992 to 1994 with the ALEPH detector at LEP, a $\tau$ lifetime of $288.0 \pm 3.1 \pm 1.3 $\fs is obtained from the sample in which both $\tau$'s decay to one charged track, and $292.8 \pm 5.6 \pm 3.0 $\fs from the sample in which one $\tau$ decays to one prong and the other to three prongs. The results show small statistical correlations with those derived from other methods. When combined with the previously published ALEPH measurements, the resulting $\tau$ lifetime is $291.2 \pm 2.0 \pm 1.2 $\fs .
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Abstract Previously published and as yet unpublished QCD results obtained with the ALEPH detector at LEP1 are presented.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.
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.
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.
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.
120575134
EUROFEL: Europe Tackles Key Issues of Free Electron Lasers
During the last few years Europe has made enormous progress towards free electron laser (FEL)-based research infrastructures. The FLASH facility [1] at DESY in Hamburg, Germany, has been in user operation since the summer of 2005; FERMI@Elettra [2] in Trieste, Italy, is under construction; the European XFEL [3] in Hamburg will start construction in summer 2008; and several other FEL projects are being prepared in France, Germany, Italy, Poland, Sweden, Switzerland and the UK. In addition, a number of dedicated test facilities have become available, including the photo-injector test facility (PITZ) [4] and the cryomodule test bench (CMTB) [5] at DESY, the HoBiCaT facility for testing superconducting cavities [6] at BESSY, Berlin, Germany, the SPARC facility [7] at INFN-LNF in Frascati, Italy, and the ERL Prototype (ERLP) [8] at Daresbury Laboratory, UK. They have been strongly supported by the national stakeholders in order to prepare the design of new FEL sources.
58910854
RF CONTROL OF THE SUPERCONDUCTING LINAC FOR THE BESSY FEL
For the BESSY-FEL superconducting linac, precise RF control of the cavities’ voltage is imperative to maintain a bunch-to-bunch time jitter of less than 50 fs for synchronization in the HGHG section. Noise, in particular microphonic detuning, strongly impact the achievable level of control. Presented here are simulations of the cavityfeedback system, taking into account beam loading and noise sources such as measurement noise, microphonics and injection jitter. These simulations are used to estimate the resultant time and energy jitter of the bunches as they enter the HGHG section of the BESSY-FEL. It will be shown, that under the given assumptions these requirements can be fulfilled.
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
122656487
Microscopic investigation of high gradient superconducting cavities after reduction of field emission
Abstract In the previous companion paper we showed that high power RF processing (HPP) is an effective technique to reduce field emission in superconducting cavities, so higher accelerating gradients can be reached. In this work we show improved understanding of the mechanisms at work when field emitters process. Thermometry measurements of the outer wall of single-cell cavities reveal the field emission from localized sites and also the reduction in field emission by processing. Subsequent scanning electron microscope (SEM) examination of the RF surface at the emission/processed sites reveals 5–10 μm sized molten craters, micron sized molten particles of foreign elements, and sub-mm sized spots shaped like starbursts. These features indicate that processing occurs through a violent melting/vaporization phenomenon. A “model” for RF processing is presented based upon the experimental evidence, both from this study and from others.
40909893
Enhanced susceptibility of Nb cavity equator welds to the hydrogen related Q-virus ∗
Thermometric studies of chemically treated niobium cavities have demonstrated that preferential niobium-hydride precipitation takes place along the electron-beam-welded equator (“Q-virus”). High residual losses are recorded as a result of the hydride in this region. Such precipitation can take place even when the cavity is rapidly cooled from 300 K to 4.2 K, a procedure that is known to be effective at preventing the Q-virus in bulk niobium.