121366770
Measurement of the strong coupling constant using τ decays
The strong coupling constant is determined from the leptonic branching ratios, the lifetime, and the invariant mass distribution of the hadronic final state of the τ lepton, using data accumulated at LEP with the ALEPH detector. The strong coupling constant measurement, αs(mτ2) = 0.330±0.046, evolved to the Z mass yields αs(MZ2) = 0.188±0.005. The error includes experimental and theoretical uncertainties, the latter evaluated in the framework of the Shifman, Vainshtein and Zakharov (SVZ) approach. The method allows the non-perturbative contribution to the hadronic decay rate to be determined to be 0.3±0.5%.
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.
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.
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.
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.
36916632
Basic Concepts of Measurements Made on Superconducting RF Cavities
Presented here is an introduction to some of the concepts used in the tests performed on superconducting RF cavities in the SRF group at Cornell. It is intended for the 'novice' to this field and by no means attempts a rigorous derivation. Assumed is some previous exposure to the physics of waveguides and cavities. Explained is the concept of the Quality (Q) of a cavity, the coupling parameters and how these quantities are determined by experiment. Furthermore a short overview of the results obtained for a typical cell is presented.
121235219
Start to end simulations for the BESSY FEL project
Abstract BESSY plans to construct a FEL multi user facility for the Soft-X-Ray spectral range from 20 eV to 1 keV . Central part of the project is a superconducting linac, based on the TESLA technology but operating in CW mode. A photo injector and a bunch compression scheme is introduced, allowing to reach the required electron beam parameters at the undulators beginning: up to 4 kA peak current ( 1 nC bunch charge), 1.5π mm mrad normalized transverse emittance and an energy spread below 1%. Extensive Start to End (S2E) simulations have been performed, using the programs ASTRA for the injector part, ELEGANT for the linac and bunch compression studies and GENESIS for the FEL process. Results of these simulations are presented and discussed.
55935509
Status of the HoBiCaT Superconducting Cavity Test Facility at BESSY
BESSY recently constructed the HOBICAT cryogenic test facility for superconducting TESLA cavity units, including all ancillary devices (helium tank, input coupler, tuner, magnetic shielding). It is designed to house two such units in a configuration similar to that envisaged for the superconducting CW linac of the BESSY FEL. These units are being fabricated, prepared and assembled by industry. HOBICATwill be used to address many of the issues that must be considered prior to finalizing the design of the proposed linac. Rapid turn-around-tests permit the investigation of items such as RF regulation, microphonic detuning and cryogenic parameters/achievable pressure stability. These test will also serve as the first step towards qualifying the industrialproductionofassembled cavity units. The commissioning of HOBICAT began in Spring 2004 and the current status is presented here.
15393287
Explosive field emitter processing in superconducting rf cavities
Superconducting niobium radiofrequency cavities for e + e accelerators presently are limited to accelerating gradients of 25 MV/m — far less than their theoretical capability of 50 MV/m. Field emission presents the main impediment to higher gradients. In many cases, in situ high-pulsed-power processing is used to explode emission sites (rf processing), thereby improving the cavity performance. Thermometric and microscopic results summarized here are used to develop a qualitative theory of the events leading up to rf processing. It is demonstrated that the ionization of gases from the emitter is crucial to the initiation of the explosion, similar to past observations made with dc emitters. This fact is underscored by the examination of emitters processed with intentionally administered helium gas in the cavity (helium processing). Numerical simulations of rf processing, including the ionization of gases by the field emission current, illustrate the conditions required for emitter explosion, and they confirm the importance of a plasma during such events. Most importantly, the calculations demonstrate that the positive space charge of the ions enhances the field emission current, and that the production of ions is a self amplifying process. A runaway situation can ensue, which explains the submicrosecond rf processing times observed experimentally. The rapid release of monolayer surface adsorbates at the emission site, in particular, can play a critical role in initiating voltage breakdown.