The associative memory (AM) system of fast tracker (FTK) processor has been designed for the tracking trigger upgrade to the ATLAS detector at the Conseil Europeen Pour La Recherche Nucleaire large ...hadron collider. The system performs pattern matching (PM) using the detector hits of particles in the ATLAS silicon tracker. The AM system is the main processing element of FTK and is mainly based on the use of application-specified integrated circuits (ASICs) (AM chips) designed to execute PM with a high degree of parallelism. It finds track candidates at low resolution which become seeds for a full resolution track fitting. The AM system implementation is based on a collection of large 9U Versa Module Europa (VME) boards, named "serial link processors" (AMBSLPs). On these boards, a huge traffic of data is implemented on a network of 900 2-Gb/s serial links. The complete AM-based processor consumes much less power (~50 kW) than its CPU equivalent and its size is much smaller. The AMBSLP has a power consumption of ~250 W and there will be 16 of them in a crate. This results in unusually large power consumption for a VME crate and the need for complex custom infrastructure in order to have sufficient cooling. This paper reports on the design and testing of the infrastructures needed to run and cool the system which will include 16 AMBSLPs in the same crate, the integration of the AMBSLP inside a first FTK slice, the performance of the produced prototypes (both hardware and firmware), as well as their tests in the global FTK integration. This is an important milestone to be satisfied before the FTK production.
The use of tracking information at the trigger level in the LHC Run II period is crucial for the trigger and data acquisition system and will be even more so as contemporary collisions that occur at ...every bunch crossing will increase in Run III. The Fast TracKer is part of the ATLAS trigger upgrade project; it is a hardware processor that will provide every Level-1 accepted event (100 kHz) and within 100mus, full tracking information for tracks with momentum as low as 1 GeV . Providing fast, extensive access to tracking information, with resolution comparable to the offline reconstruction, FTK will help in precise detection of the primary and secondary vertices to ensure robust selections and improve the trigger performance.
A direct search for the standard model Higgs boson decaying to a pair of charm quarks is presented. Associated production of the Higgs and Z bosons, in the decay mode ZH→ + -cc is studied. A data set ...with an integrated luminosity of 36.1 fb-1 of pp collisions at s=13TeV recorded by the ATLAS experiment at the LHC is used. The H→cc signature is identified using charm-tagging algorithms. The observed (expected) upper limit on σ(pp→ZH)×B(H→cc) is 2.7 (3.9-1.1+2.1) pb at the 95% confidence level for a Higgs boson mass of 125 GeV, while the standard model value is 26 fb.
A search for the standard model Higgs boson produced in association with a top-quark pair, ttH, is presented. The analysis uses 36.1 fb-1 of pp collision data at s=13 TeV collected with the ATLAS ...detector at the Large Hadron Collider in 2015 and 2016. The search targets the H→bb decay mode. The selected events contain either one or two electrons or muons from the top-quark decays, and are then categorized according to the number of jets and how likely these are to contain b-hadrons. Multivariate techniques are used to discriminate between signal and background events, the latter being dominated by tt+jets production. For a Higgs boson mass of 125 GeV, the ratio of the measured ttH signal cross-section to the standard model expectation is found to be μ=0.84-0.61+0.64. A value of μ greater than 2.0 is excluded at 95% confidence level (C.L.) while the expected upper limit is μ<1.2 in the absence of a ttH signal.
A search for the narrow structure, X(5568), reported by the D0 Collaboration in the decay sequence X→Bs0π±, Bs0→J/ψφ, is presented. The analysis is based on a data sample recorded with the ATLAS ...detector at the LHC corresponding to 4.9 fb-1 of pp collisions at 7 TeV and 19.5 fb-1 at 8 TeV. No significant signal was found. Upper limits on the number of signal events, with properties corresponding to those reported by D0, and on the X production rate relative to Bs0 mesons, ρX, were determined at 95% confidence level. The results are N(X)<382 and ρX<0.015 for Bs0 mesons with transverse momenta above 10 GeV, and N(X)<356 and ρX<0.016 for transverse momenta above 15 GeV. Limits are also set for potential Bs0π± resonances in the mass range 5550 to 5700 MeV.
The production of a top quark in association with a Z boson is investigated. The proton–proton collision data collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy ...of s=13TeV are used, corresponding to an integrated luminosity of 36.1fb-1. Events containing three identified leptons (electrons and/or muons) and two jets, one of which is identified as a b-quark jet are selected. The major backgrounds are diboson, tt¯ and Z+jets production. A neural network is used to improve the background rejection and extract the signal. The resulting significance is 4.2σ in the data and the expected significance is 5.4σ. The measured cross-section for tZq production is 600±170(stat.)±140(syst.)fb.
A search for long-lived, massive particles predicted by many theories beyond the Standard Model is presented. The search targets final states with large missing transverse momentum and at least one ...high-mass displaced vertex with five or more tracks, and uses 32.8 fb-1 of s=13 TeV pp collision data collected by the ATLAS detector at the LHC. The observed yield is consistent with the expected background. The results are used to extract 95% C.L. exclusion limits on the production of long-lived gluinos with masses up to 2.37 TeV and lifetimes of O(10-2)-O(10) ns in a simplified model inspired by split supersymmetry.
During the current LHC shutdown period the ATLAS experiment will upgrade the Trigger and Data Acquisition system to include a hardware tracker coprocessor: the Fast TracKer (FTK). The FTK receives ...data from the 80 million of channels of the ATLAS silicon detector, identifying charged tracks and reconstructing their parameters at a rate of up to 100 KHz and within 100 microseconds. To achieve this performance, the FTK system identifies candidate tracks utilizing the computing power of a custom ASIC chip with associative memory (AM) designed to perform “pattern matching” at very high speed; track parameters are then calculated using modern FPGAs. A detailed simulation of this massive system has been developed with the goal of supporting the hardware design and studying its impact in the ATLAS online event selection at high LHC luminosities. We present the issues related to emulating FTK on a general-purpose CPU platform, using ATLAS computing Grid resources, and the solutions developed in order to mitigate these problems and allow the studies required to support the system design, construction and installation.
Additional jet activity in dijet events is measured using Formula: see text collisions at ATLAS at a centre-of-mass energy of Formula: see text, for jets reconstructed using the Formula: see text ...algorithm with radius parameter Formula: see text. This is done using variables such as the fraction of dijet events without an additional jet in the rapidity interval bounded by the dijet subsystem and correlations between the azimuthal angles of the dijet s. They are presented, both with and without a veto on additional jet activity in the rapidity interval, as a function of the scalar average of the transverse momenta of the dijet s and of the rapidity interval size. The double differential dijet cross section is also measured as a function of the interval size and the azimuthal angle between the dijet s. These variables probe differences in the approach to resummation of large logarithms when performing QCD calculations. The data are compared to powheg, interfaced to the pythia 8 and herwig parton shower generators, as well as to hej with and without interfacing it to the ariadne parton shower generator. None of the theoretical predictions agree with the data across the full phase-space considered; however, powheg+pythia 8 and hej+ariadne are found to provide the best agreement with the data. These measurements use the full data sample collected with the ATLAS detector in Formula: see textFormula: see text collisions at the LHC and correspond to integrated luminosities of Formula: see text and Formula: see text for data collected during 2010 and 2011, respectively.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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