A search has been performed, using the full 20.3 fb(-1) data sample of 8 TeV proton-proton collisions collected in 2012 with the ATLAS detector at the LHC, for photons originating from a displaced vertex due to the decay of a neutral long-lived particle into a photon and an invisible particle. The analysis investigates the diphoton plus missing transverse momentum final state, and is therefore most sensitive to pair production of long-lived particles. The analysis technique exploits the capabilities of the ATLAS electromagnetic calorimeter to make precise measurements of the flight direction, as well as the time of flight, of photons. No excess is observed over the Standard Model predictions for background. Exclusion limits are set within the context of gauge mediated supersymmetry breaking models, with the lightest neutralino being the next-to-lightest supersymmetric particle and decaying into a photon and gravitino with a lifetime in the range from 250 ps to about 100 ns.

pdf/search-for-nonpointing-and-delayed-photons-in-the-diphoton-2eb2q2avle.pdf

Search for nonpointing and delayed photons in the diphoton and missing transverse momentum final state in 8 TeV pp collisions at the LHC using the ATLAS detector

To calibrate the energy response of the ATLAS liquid argon calorimeter, an electronics calibration board has been designed; it delivers a signal whose shape is close to the calorimeter ionization current signal with amplitude up to 100 mA in 50 Ω with 16 bit dynamic range. The amplitude of this signal is designed to be uniform over all calorimeters channels, stable in time and with an integral linearity much better that the electronics readout. The various R&D phases and most of the difficulties met are discussed and illustrated by many measurements. The custom design circuits are described and the layout of the ATLAS calibration board presented. The procedure used to qualify the boards is explained and the performance obtained illustrated: a dynamic range up to 3 TeV in three energy scales with an integral linearity better than 0.1% in each of them, a response uniformity better than 0.2% and a stability better than 0.1%. The performance of the board is well within the ATLAS requirements. Finally, in situ measurements done on the ATLAS calorimeter are shown to validate these performances.

Electronics calibration board for the ATLAS liquid argon calorimeters

The response of the ATLAS electromagnetic calorimeter to muons has been studied in this paper. Results on signal over noise ratio, assessment of the detector response uniformity, and position resolution are presented. The possibility to study fine details of the structure of the detector through its response to muons is illustrated on a specific example. Finally, the performance obtained on muons in test-beam is used to estimate the detector uniformity and time alignment precision that will be reachable after the commissioning of the ATLAS detector with cosmic rays.

Study of the response of ATLAS electromagnetic liquid argon calorimeters to muons

/pdf/in-situ-commissioning-of-the-atlas-electromagnetic-3ue75cpfu9.pdf

In situ commissioning of the ATLAS electromagnetic calorimeter with cosmic muons

We investigate the collider signatures of heavy, long-lived, neutral particles that decay to charged particles plus missing energy. Specifically, we focus on the case of a neutralino NLSP decaying to Z and gravitino within the context of General Gauge Mediation. We show that a combination of searches using the inner detector and the muon spectrometer yields a wide range of potential early LHC discoveries for NLSP lifetimes ranging from 10^(-1)-10^5 mm. We further show that events from Z(l+l-) can be used for detailed kinematic reconstruction, leading to accurate determinations of the neutralino mass and lifetime. In particular, we examine the prospects for detailed event study at ATLAS using the ECAL (making use of its timing and pointing capabilities) together with the TRT, or using the muon spectrometer alone. Finally, we also demonstrate that there is a region in parameter space where the Tevatron could potentially discover new physics in the delayed Z(l+l-)+MET channel. While our discussion centers on gauge mediation, many of the results apply to any scenario with a long-lived neutral particle decaying to charged particles.

Long-Lived Neutralino NLSPs

The time reconstruction performance of the ATLAS electromagnetic calorimeter readout is studied. The contribution of the electronics to the time resolution is estimated to be about 20 ps, thus demonstrating the possibility of achieving a small constant term in the time resolution for particles. The resolution to electromagnetic showers produced by an electron beam is also measured. After correction for the effects due to the calorimeter geometry, a 100 ps constant term is found for a typical cell.

/pdf/time-resolution-of-the-atlas-barrel-liquid-argon-1bx7j6q8yd.pdf

Time resolution of the ATLAS barrel liquid argon electromagnetic calorimeter

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Time resolution of the ATLAS barrel liquid argon electromagnetic calorimeter