The decay chain \(b \to \bar{\mathrm{B}} \rightarrow \mathrm{D}^{**0} \ell^- \bar{\nu} X , \mathrm{D}^{**0}\rightarrow \mathrm{D}^{*+}\pi^-,\mathrm{D}^{*+} \to \mathrm{D}^0\pi^+, D^0 \to ...(\mathrm{K}\pi\; \mathrm{or} \;\mathrm{K}3\pi)\) is identified in a sample of 3.9 million hadronic Z decays collected with the OPAL detector at LEP. The branching ratio BR \((b \to \bar{B}) \times \mathrm{BR}(\bar{B} \to \mathrm{D}^0_1 \ell^-\bar{\nu} X) \times \mathrm{BR}(\mathrm{D}^0_1 \to \mathrm{D}^{*+}\pi^-)\) is measured to be \((2.64 \pm 0.79 (\mathrm{stat}) \pm 0.39 (\mathrm{syst})) \times 10^{-3}\) for the JP=1+ (D01) state. For decays into the JP=2+ (D2*0) state, an upper limit of 1.4 x 10-3 is placed on the branching ratio at the 95% confidence level.
The muon detectors of the experiments at the Large Hadron Collider (LHC) have to cope with unprecedentedly high neutron and gamma ray background rates. In the forward regions of the muon spectrometer ...of the ATLAS detector, for instance, counting rates of 1.7 kHz/square cm are reached at the LHC design luminosity. For high-luminosity upgrades of the LHC, up to 10 times higher background rates are expected which require replacement of the muon chambers in the critical detector regions. Tests at the CERN Gamma Irradiation Facility showed that drift-tube detectors with 15 mm diameter aluminum tubes operated with Ar:CO2 (93:7) gas at 3 bar and a maximum drift time of about 200 ns provide efficient and high-resolution muon tracking up to the highest expected rates. For 15 mm tube diameter, space charge effects deteriorating the spatial resolution at high rates are strongly suppressed. The sense wires have to be positioned in the chamber with an accuracy of better than 50 ?micons in order to achieve the desired spatial resolution of a chamber of 50 ?microns up to the highest rates. We report about the design, construction and test of prototype detectors which fulfill these requirements.
The ATLAS experiment at the Large Hadron Collider (LHC) at CERN is currently being assembled to be ready to take data in 2007. Its muon spectrometer is designed to achieve a momentum resolution of ...better than 10% at 1 TeV. The barrel part of the muon spectrometer consists of a toroidal air-core magnet which is instrumented with three layers of monitored drift tube (MDT) chambers as precision tracking detectors. The installation of the muon detectors has started in January 2005. The Max-Planck-Institut fur Physik and the Ludwig-Maximilians-University in Munich have built 88 MDT chambers for the outermost barrel region, each covering an area of 8 m 2 . To ensure their proper operation in the experiment, the MDT chambers have to pass a set of stringent tests both at the production site and after their delivery to CERN; at CERN, the MDT chambers are also integrated with resistive plate chambers (RPCs) of the trigger system to form muon stations. After their installation in the detector, the muon stations are further tested and commissioned with cosmic rays. We report on our experience with the chamber tests, the integration procedure and installation of the muon stations in the experiment. First results of their commissioning in the ATLAS detector will be presented, as well as results of the MDT chamber calibration and alignment studies performed with cosmic rays with and without magnetic field.
Pressurized drift-tube chambers are efficient detectors for high-precision tracking over large areas. The Monitored Drift-Tube (MDT) chambers of the muon spectrometer of the ATLAS detector at the ...Large Hadron Collider (LHC) reach a spatial resolution of 35 micons and almost 100% tracking efficiency with 6 layers of 30 mm diameter drift tubes operated with Ar:CO2 (93:7) gas mixture at 3 bar and a gas gain of 20000. The ATLAS MDT chambers are designed to cope with background counting rates due to neutrons and gamma-rays of up to about 300 kHz per tube which will be exceeded for LHC luminosities larger than the design value of 10-34 per square cm and second. Decreasing the drift-tube diameter to 15 mm while keeping the other parameters, including the gas gain, unchanged reduces the maximum drift time from about 700 ns to 200 ns and the drift-tube occupancy by a factor of 7. New drift-tube chambers for the endcap regions of the ATLAS muon spectrometer have been designed. A prototype chamber consisting of 12 times 8 layers of 15 mm diameter drift tubes of 1 m length has been constructed with a sense wire positioning accuracy of 20 microns. The 15 mm diameter drift-tubes have been tested with cosmic rays in the Gamma Irradiation Facility at CERN at counting rates of up to 1.85 MHz.
The effects of the hadronisation of partons on the distribution of event
shape observables are associated with corrections which are suppressed by
reciprocal powers of the energy scale of the ...process. The correction is
determined by one non-calculable parameter \alpha_0 for which an universal
value of 0.5 +/- 20% is found from the investigation of the distribution of
event shape observables and their mean values measured in e^+e^- annihilation.
Determinations of the strong coupling constant alpha_S at centre-of-mass
energies of 192 through 202 GeV at LEP are presented. The energy evolution of
alpha_S is in agreement with the prediction of ...QCD. The combined investigation
of
OPAL and JADE data in the energy range of 35 through 189 GeV yields
alpha_S(m_Z)=0.1187^{+0.0034}_{-0.0019}. The strenght of the strong coupling is
flavour independent if quark mass effects are taken into account.
The potential of the TESLA linear e+e- collider to determine the strong
coupling constant, alpha_S, at 500 GeV is investigated. Experimental
complications due to background from W- and Z-pairs, ...top-production, initial
state photon radiation and from beamstrahlung are considered. The hadronic
event selection procedures used by the experiments at LEP II are reviewed for
the applicability at TESLA. An estimate of the various error contributions to
the total uncertainty of an $\alpha_S$ determination is presented. It confirms
that hadronisation effects are diminished while the uncertainty from the choice
of the renormalisation scale will dominate. Fits of the ln(R)-matched second
order (O(alpha_S^2)) and resummed calculation (NLLA) to six observables are
used to estimate the error contributions. This yields the expectation of the
precision for alpha_S(500 GeV) of +-0.0025.
Data recorded by the JADE experiment at the PETRA collider were used to measure the event shape observables thrust, heavy jet mass, wide and total jet broadening and the differential 2-jet rate in ...the Durham scheme. For the latter three observables, no experimental results have previously been presented at these energies. The distributions were compared with resummed QCD calulations (+NLLA), and the strong coupling constant was determined at different energy scales . The results, are in agreement with previous combined results of PETRA albeit with smaller uncertainties. Together with corresponding data from LEP, the energy dependence of is significantly tested and is found to be in good agreement with the QCD expectation. Similarly, mean values of the observables were compared to analytic QCD predictions where hadronisation effects are absorbed in calculable power corrections.
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