SCT and TRT Performance from Cosmic Ray Runs

The ATLAS SCT and TRT detectors have been integrated into one barrel and two end-cap parts. Cosmic ray runs of the combined detectors are used to study the individual and combined detector integration and performance. This has taken place both in the surface building (SR1) and after installation in the ATLAS cavern. This article focuses on the most resent results, which includes timing in and synchronisation procedures, noise studies as well as initial efficiency, tracking and alignment results

The detector control system for the ATLAS semiconductor tracker assembly phase

The ATLAS Semiconductor Tracker (SCT) consists of 4088 silicon microstrip modules, with a total of 6.3 million readout channels. These are arranged into 4 concentric barrel layers and 2 endcaps of 9 disks each. The coherent and safe operation of the SCT during commissioning and subsequent operation is an essential task of the Detector Control System (DCS). The main building blocks of the SCT DCS, the cooling system, the power supplies and the environmental system, are described. First results from DCS testing are presented

Overcoming limitations to ALP parameter inference using Neural Ratio Estimation

In the hunt for new physics phenomena, such as Axion-like particles (ALPs), it is crucial to compare experimental data to theoretical models. This involves inferring the most likely values of a model’s parameters — such as particle masses and cross sections. However, traditional likelihood-based inference techniques are oftentimes not practically feasible without making significant simplifying assumptions, which decrease the reliability of the inference. This is especially the case for ALP-searches with gamma-ray telescopes such as the upcoming Cherenkov Telescope Array. Recently however, new likelihood-free inference (LFI) techniques based on machine learning have emerged to help overcome these limitations. In particular, “Neural Ratio Estimation” (NRE) stands out with its reported accuracy and efficiency. In this contribution, we have applied NRE to simulated CTA-data of the active galactic nucleus NGC1275 in the Perseus Cluster, in order to probe the viability of this technique for ALP-searches with cosmic gamma-rays. Our example-inferences provide encouraging evidence that NRE will be applicable to deriving sensitive and accurate limits. We also identify some challenges in the practical execution of such an analysis, as well as concrete next steps towards deriving formal and reliable limits on the ALP mass and coupling to photons.publishedVersio

Coherent laser cooling with trains of ultrashort laser pulses

We propose to extend coherent laser cooling from narrow-band to broad-band transitions by using trains of ultrashort broadband pulses. We study analytically two possible methods to reduce the momentum spread of a distribution by several units of photon momentum in a single spontaneous emission lifetime. We report on numerical simulations of one-dimensional laser cooling of a two-level system with realistic parameters. The technique introduced here is of high interest for efficient laser cooling of fast species with short lifetime such as positronium

Measurement of the production cross section of prompt J/ψ mesons in association with a W ± boson in pp collisions at √s = 7 TeV with the ATLAS detector

The process pp → W ± J/ψ provides a powerful probe of the production mechanism of charmonium in hadronic collisions, and is also sensitive to multiple parton interactions in the colliding protons. Using the 2011 ATLAS dataset of 4.5 fb−1 of s√ = 7 TeV pp collisions at the LHC, the first observation is made of the production of W ± + prompt J/ψ events in hadronic collisions, using W ± → μν μ and J/ψ → μ + μ −. A yield of 27.4+7.5−6.5 W ± + prompt J/ψ events is observed, with a statistical significance of 5.1σ. The production rate as a ratio to the inclusive W ± boson production rate is measured, and the double parton scattering contribution to the cross section is estimated.publishedVersio

Search for heavy lepton resonances decaying to a ZZ boson and a lepton in pppp collisions at s=8\sqrt{s}=8 TeV with the ATLAS detector

A search for heavy leptons decaying to a $Z$ boson and an electron or a muon is presented. The search is based on $pp$ collision data taken at $\sqrt{s}=8$ TeV by the ATLAS experiment at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, Three high-transverse-momentum electrons or muons are selected, with two of them required to be consistent with originating from a $Z$ boson decay. No significant excess above Standard Model background predictions is observed, and 95% confidence level limits on the production cross section of high-mass trilepton resonances are derived. The results are interpreted in the context of vector-like lepton and type-III seesaw models. For the vector-like lepton model, most heavy lepton mass values in the range 114-176 GeV are excluded. For the type-III seesaw model, most mass values in the range 100-468 GeV are excluded

Search for dark matter in events with heavy quarks and missing transverse momentum in pppp collisions with the ATLAS detector

This article reports on a search for dark matter pair production in association with bottom or top quarks in $20.3 fb^{-1}$ of $pp$ collisions collected at $\sqrt{s} = 8$ TeV by the ATLAS detector at the LHC. Events with large missing transverse momentum are selected when produced in association with high-momentum jets of which one or more are identified as jets containing $b$-quarks. Final states with top quarks are selected by requiring a high jet multiplicity and in some cases a single lepton. The data are found to be consistent with the Standard Model expectations and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between dark matter and Standard Model particles. Limits on the dark-matter--nucleon cross-section for spin-independent and spin-dependent interactions are also provided. These limits are particularly strong for low-mass dark matter. Using a simplified model, constraints are set on the mass of dark matter and of a coloured mediator suitable to explain a possible signal of annihilating dark matter

Measurement of the muon reconstruction performance of the ATLAS detector using 2011 and 2012 LHC proton–proton collision data

This paper presents the performance of the ATLAS muon reconstruction during the LHC run with pp collisions at √s = 7–8 TeV in 2011–2012, focusing mainly on data collected in 2012. Measurements of the reconstruction efficiency and of the momentum scale and resolution, based on large reference samples of J/ψ → μμ, Z → μμ and ϒ → μμ decays, are presented and compared to Monte Carlo simulations. Corrections to the simulation, to be used in physics analysis, are provided. Over most of the covered phase space (muon |η| < 2.7 and 5 ≲pT ≲pT 100 GeV) the efficiency is above 99% and is measured with per-mille precision. The momentum resolution ranges from 1.7% at central rapidity and for transverse momentum pT ≃ 10 GeV, to 4% at large rapidity and pT ≃ 100 GeV. The momentum scale is known with an uncertainty of 0.05% to 0.2% depending on rapidity. A method for the recovery of final state radiation from the muons is also presented

Measurement of the b-hadron production cross section using decays to D*+ μ − X final states in pp collisions at √s = 7 TeV with the ATLAS detector

The b-hadron production cross section is measured with the ATLAS detector in pp collisions at √s = 7 TeV, using 3.3 pb−1 of integrated luminosity, collected during the 2010 LHC run. The b-hadrons are selected by partially reconstructing D*+μ−X final states. Differential cross sections are measured as functions of the transverse momentum and pseudorapidity. The measured production cross section for a b-hadron with pT > 9 GeV and |η| < 2.5 is 32.7±0.8(stat.)+4.5−6.8(syst.) μb, higher than the next-to-leadingorder QCD predictions but consistent within the experimental and theoretical uncertainties