No abstract available

[en] Since the bb production is predicted to form the dominant contribution to the physics events taken at the ATLAS-Experiment at LHC precisely tuned b trigger and tagging algorithms are essential for online data taking as well as for the offline analysis. This study comprises two different methods to discriminate b-quark events from underlying background, tagging the large invariant mass or the large decay length of the B-meson, respectively. The first rough selection strategy focusing on the semi-leptonic B-decay with muons in the final state is mainly based on the transverse muon momentum relativ to the associated jets axis p_T^rel. The other possibility relies on the inclusive B → J/ψX decay and calculates impact parameters of muons from the J/ψ decay as well as its secondary vertex. First results based on PYTHIA-B Monte Carlo studies for selection strategies at the higher trigger level and on reconstructed data will be shown. (orig.)

[en] The LHC experiments will perform sensitive tests of physics phenomena beyond the Standard Model (BSM). Investigation of decays of beauty hadrons represents an alternative approach in addition to direct BSM searches. The ATLAS efforts concentrate on those B decays that can be selected already at the first and second trigger levels. The most favorable trigger signature will be for B hadrons decaying to J/ψ→ μμ. Using this trigger ATLAS will be able to accommodate unprecedently high statistics in so called Golden LHC channel B_s → J/ψφ allowing a measurement of the CP violation effect, where BSM models predicted values are significantly higher than SM. In the rare decays sector, these are purely di-muon decays, and families of semi-muonic exclusive channels. Already with 1 fb^-1 the ATLAS sensitivity in the di-muon channels will be comparable to today worlds statistics. The strategy is to carry on the di-muon channel program up to nominal LHC luminosity. In particular the B_sμμX signal with 4.3 σ significance can be measured combining low luminosity samples with those of one year of LHC operation at a luminosity of 10³⁴ cm^-2s^-1. This precision allows excluding or confirming the SM unambiguously

[en] The ATLAS trigger system consists of the hardware-based Level 1 (LVL1), and the software-based high-level trigger (HLT), containing Level 2 (LVL2) and event filter (EF). In order to extract information of interest from high-cross-section background in p-p collisions, this system needs to be highly selective. The trigger decision is taken in a consecutive step decision system. Implementation of monitoring software for HLT is necessary for ensuring its correct functionality. The HLT runs on a complex farm system, therefore, gathering information on farm parameters, calculating averages, standard deviations, implementing warning mechanisms and presenting information from each node is of great importance. All of the monitoring information needs to be presented in a user friendly way, which would enable fast and easy error handling for shifter and expert. In addition, it needs to be prepared for different physics scenarios, and expected major changes in the first year of ATLAS data taking. A flexible and easily readable system for presenting both, HLT and farm information, is needed to give a relevant overview of operational status of this complex system. In the scope of getting ready for first ATLAS measurements planned for 2008, the HLT monitoring software, software for gathering information from farms and software for presenting HLT overall information to the shift are to be tested in DESY-Zeuthen and during technical runs in CERN at the beginning of 2007. (orig.)

[en] In many high-energy physics experiments, online selection is vital to collect the most interesting collisions out of the large data volume. The ATLAS experiment at the Large Hadron Collider (LHC) utilizes a trigger system that consists of a hardware Level-1 (L1) and a software based high-level trigger (HLT), reducing the event rate from the LHC bunch-crossing rate of 40 MHz to an average recording rate of around 1000 Hz. In Run 2 of the LHC, started in spring 2015, the LHC is operating at a centre-of-mass energy of 13 TeV providing an instantaneous luminosity up to 1.4 ⋅ 10³⁴ cm⁻²s⁻¹ so far. The ATLAS trigger system has to cope with these challenges, while maintaining or improving upon the efficiency to select relevant physics processes. In this paper, the ATLAS trigger system for the LHC Run 2 is reviewed. Secondly, the impressive performance improvements in the HLT trigger algorithms used to identify leptons, hadrons and global event quantities like missing transverse energy is shown. Electron, muon and photon triggers covering transverse energies from a few GeV to several TeV are essential for signal selection in a wide variety of ATLAS physics analyses to study Standard Model (SM) processes and to search for new phenomena. Finally, further developments planned for the rest of the LHC Run 2 are discussed. These include two new hardware subsystems for topological selections at L1 and full-scan tracking at the input to the HLT.

[en] The total cross section for the photoproduction process with a leading proton in the final state has been measured at γp centre-of-mass energies W of 91, 181 and 231 GeV. The measured cross sections apply to the kinematic range with the transverse momentum of the scattered proton restricted to p_T≤0.2 GeV and 0.68≤z≤0.88, where z=E_p'/E_p is the scattered proton energy normalised to the beam energy. The cross section dσ_γp→Xp'(W,z)/dz is observed to be independent of W and z within the measurement errors and amounts to (8.05±0.06 (stat)±0.89 (syst)) μb on average. The data are well described by a Triple Regge model in which the process is mediated by a mixture of exchanges with an effective Regge trajectory of intercept α_i(0)=0.33±0.04 (stat)±0.04 (syst). The total cross section for the interaction of the photon with this mixture (γα_i→X) can be described by an effective trajectory of intercept α_k(0)=0.99±0.01 (stat)±0.05 (syst). Predictions based on previous triple Regge analyses of pp→pX data assuming vertex factorisation are broadly consistent with the γp data. The measured cross sections are compared with deep inelastic scattering leading proton data in the same region of z and p_T for photon virtuality Q²>2.5 GeV². The ratio of the cross section for leading proton production to the total cross section is found to rise with Q²

[en] A study of the angular distributions of leptons from decays of J/ψ's produced in p-C and p-W collisions at √(s)=41.6 GeV has been performed in the J/ψ Feynman-x region -0.34< x_F<0.14 and for J/ψ transverse momenta up to 5.4 GeV/c. The data were collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The results, based on a clean selection of 2.3 x 10⁵ J/ψ's reconstructed in both the e⁺e^- and μ⁺μ^- decay channels, indicate that J/ψ's are produced polarized. The magnitude of the effect is maximal at low p_T. For p_T>1 GeV/c a significant dependence on the reference frame is found: the polar anisotropy is more pronounced in the Collins-Soper frame and almost vanishes in the helicity frame, where, instead, a significant azimuthal anisotropy arises. (orig.)

[en] Measurements of the kinematic distributions of J/ψ mesons produced in p-C, p-Ti and p-W collisions at √(s)=41.6 GeV in the Feynman-x region -0.34< x_F<0.14 and for transverse momentum up to p_T=5.4 GeV/c are presented. The x_F and p_T dependencies of the nuclear suppression parameter, α, are also given. The results are based on 2.4 x 10⁵J/ψ mesons reconstructed in both the e⁺e^- and μ⁺μ^- decay channels. The data have been collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The measurement explores the negative region of x_F for the first time. The average value of α in the measured x_F region is 0.981±0.015. The data suggest that the strong nuclear suppression of J/ ψ production previously observed at high x_F turns into an enhancement at negative x_F. (orig.)

[en] Inclusive doubly differential cross sections d²σ_pA/dx_Fdp_T² as a function of Feynman-x(x_F) and transverse momentum (p_T) for the production of K_S⁰, Λ and anti Λ in proton-nucleus interactions at 920 GeV are presented. The measurements were performed by HERA-B in the negative x_F range (-0.12< x_F<0.0) and for transverse momenta up to p_T=1.6 GeV/c. Results for three target materials: carbon, titanium and tungsten are given. The ratios of production cross sections are presented and discussed. The Cronin effect is clearly observed for all three V⁰ species. The atomic number dependence is parameterized as σ_pA=σ_pN.A^α where σ_pN is the proton-nucleon cross section. The measured values of α are all near one. The results are compared with EPOS 1.67 and PYTHIA 6.3. EPOS reproduces the data to within ∼20% except at very low transverse momentum. (orig.)