101 research outputs found

    Angular distribution and rotations of frame in vector meson decays into lepton pairs

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    We discuss how the angular distribution of lepton pairs from decays of vector mesons depends on the choice of reference frame, and provide a geometrical description of the transformations of the coefficients of the angular distribution. Invariant expressions involving all coefficients are discussed, together with bounds and consistency relations.Comment: 7 pages, 3 figure

    Measurement of direct CP violation with the experiment NA48 at CERN

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    Direct CP violation is studied in two pion decays of the neutral kaon. Using data collected in the first beam period in 1997, the result for the parameter Re(epsilon'/epsilon) is (18.5+/-4.5(stat)+/-5.8(syst))x10^(-4).Comment: 7 pages, 7 figures, talk presented at EPS-HEP 9

    Space charge in ionization detectors and the NA48 calorimeter

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    The effects of space charge due to slowly drifting ions can be relevant for detectors operated at high intensity, or for relatively low values of the bias voltage. Accurate measurements have been obtained with the liquid krypton calorimeter of the NA48 experiment, from data collected in 1997. The build-up of space charge takes place during the first part of the beam extraction burst, and causes a dependence of the response on the transverse coordinate of the axis of electromagnetic showers, and a small reduction of average amplitude. The effects are well reproduced by a computation, where the only free parameter is the value of the ion mobility. The model can be applied a wide range of operating conditions, and generalized to detectors with different geometry and active medium.The effects of space charge due to slowly drifting ions can be relevant for detectors operated at high intensity, or for relatively low values of the bias voltage. Accurate measurements have been obtained with the liquid krypton calorimeter of the NA48 experiment, from data collected in 1997. The build-up of space charge takes place during the first part of the beam extraction burst, and causes a dependence of the response on the transverse coordinate of the axis of electromagnetic showers, and a small reduction of average amplitude. The effects are well reproduced by a computation, where the only free parameter is the value of the ion mobility. The model can be applied a wide range of operating conditions, and generalized to detectors with different geometry and active medium

    Space Charge Effects in Noble Liquid Calorimeters and Time Projection Chambers

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    The subject of space charge in ionization detectors is reviewed, showing how the observations and the formalism used to describe the effects have evolved, starting with applications to calorimeters and reaching recent, large-size time projection chambers. General scaling laws, and different ways to present and model the effects are presented. The relation between space-charge effects and the boundary conditions imposed on the side faces of the detector are discussed, together with a design solution that mitigates part of the effects. The implications of the relative size of drift length and transverse detector size are illustrated. Calibration methods are briefly discussed.Comment: Review paper submitted to Instruments, special issue: Liquid Argon Detectors: Instrumentation and Application

    Space Charge in Ionization Detectors

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    The effect on the operation of a vacuum diode of a spatially varying distribution of free electrons, commonly called space charge, was first considered by Child [1] and Langmuir [2]. In that example electrons are released from a hot cathode at low velocity, and then drift freely under the influence of the local electric field as modified by the drifting electrons. 1 In this problem, deduce the steady-state electric field distribution E = E(x) ňÜx in an ionization detector (whose nominal field, E0 ňÜx, is uniform over the length D of the detector) as affected by the positive-ion space-charge density ŌĀ(x). Suppose that ionization is created uniformly throughout the volume of the detector (by cosmic rays or by secondary particles from a particle accelerator) at a volume rate of K ion pairs/volume/sec. The ionization electrons are quickly collected at the anode and the associated electron space-charge density can be ignored. That is, you may assume the positive-ion space-charge density is not so great as to prevent the ionization electrons from reaching the anode. The positive ions drift slowly to the cathode with a velocity given by v = őľE, (1) where the positive-ion mobility őľ is independent of the electric field strength. 2

    Space charge in liquid argon time-projection chambers: a review of analytical and numerical models, and mitigation methods

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    The subject of space charge in ionisation detectors is reviewed, with particular attention to the case of liquid argon time projection chambers. Analytical and numerical description of the effects on the reconstructed coordinates along the drift and the transverse directions are presented. The cases of limited electron lifetime, of dual-phase detectors with ion feedback, and of detectors with small and comparable ratio between drift length and width are considered. Two design solutions that mitigates the effects are discussed.Comment: 26 pages, 15 figure

    Measurement of the decay rate and the parameter őĪK* of the decay KL ‚Üí őľőľő≥

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    The rate of the decay of the neutral K meson into KL0 ‚Üí őľőľő≥ has been measured with the NA48 detector at the CERN SPS. A total of 59 events has been observed with an estimated background of 14 events. The overall Kaon flux was determined to be 1.2 √ó 109. This observation corresponds to a branching ratio of (3.4 ¬Ī 0.6stat ¬Ī 0.4sys √ó 10-7. Using this branching ratio the parameter őĪK* describing the relative strength of the two contributing amplitudes to this decay, through intermediate pseudoscalar or vectorlike mesons, was measured to be őĪK* = -0.04+-0.21+0.24

    Search for dark matter produced in association with bottom or top quarks in ‚ąös = 13 TeV pp collisions with the ATLAS detector

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    A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb‚ąí1 of proton‚Äďproton collision data recorded by the ATLAS experiment at ‚ąös = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

    Measurement of the correlation between the polar angles of leptons from top quark decays in the helicity basis at ‚ąös = 7 TeV using the ATLAS detector