Search for high mass photon pairs in e+ e- ---> f anti-f gamma gamma (f = e, mu, tau, neutrino, q) at LEP

title: 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abstract: Previously published and as yet unpublished QCD results obtained with the ALEPH detector at LEP1 are presented.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.

title: Charged particle pair production associated with a lepton pair in Z decays. indication of an excess in the tau channel
abstract: 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.

title: Production of excited beauty states inZ decays
abstract: 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.

title: First measurement of the quark-to-photon fragmentation function
abstract: 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.

title: 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.

title: Enhanced susceptibility of Nb cavity equator welds to the hydrogen related Q-virus ∗
abstract: 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.

title: MICROSCOPIC INVESTIGATION OF FIELD EMITTERS LOCATED BY THERMOMETRY IN 1.5 GHz SUPERCONDUCTING NIOBIUM CAVITIES
abstract: A new high resolution, high speed thermometry system has been built to permit the study of eld emitters in 1.5 GHz superconducting niobium cavities. Following a cavity test, the cavity is dissected for examination of the emitters in an electron microscope. Presented below is a survey of the topographical characterstics and composition of various defects found so far. A correlation between the current characteristics of emitters and their ability to process away was observed. It has been found that plasma production in the vicinity of an emitter is of importance in determining how much heat an emitter deposits on the rf surface as the electric eld in the cavity is increased. In particular, this plasma plays a pivotal role in the extinction of emitters.

title: Basic Concepts of Measurements Made on Superconducting RF Cavities
abstract: 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.

title: Trapped magnetic flux in superconducting niobium samples
abstract: 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.

title: MICROPHONICS MEASUREMENTS IN A CW-DRIVEN TESLA-TYPE CAVITY ∗
abstract: Superconducting cavities with a high quality factor exhibit a very low bandwidth in their resonant frequency, which makes their operation very sensitive to mechanical oscillations. In CW mode of operation, as it is intended for the BESSY-FEL Linac, microphonics are therefore the dominant error source for field stability. In order to compensate the detuning, it is necessary to properly characterize amplitude and frequency with respect to all involved mechanical and electrical components. Such measurements have been performed at the HoBiCaT test facility at BESSY and will be described in detail.