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

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−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 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

A high data rate readout system for particle detectors based on FPGA-to-server ethernet connections and the eXpress Data Path technology

With the Large Hadron Collider (LHC) at CERN being upgraded to the high luminosity LHC (HL-LHC), the four major experiment (ATLAS, ALICE, CMS and LHCb) use the opportunity to also upgrade their systems. In these so-called Phase II upgrades, the systems of each experiment are reworked or even replaced to enable the recording of a considerably increased event rate. In parallel, the resolution of the tracking subsystems will be increased to reach a better event separation. The ITk Pixel detector of the ATLAS experiment is used as example for this. The increased resolution and the increased number of front-end (FE) chips also leads to a drastically increased data rate, which is necessary to transfer all event information from the detector in a timely manner. The upgrade is also used for replacing the readout and processing chain outside of the detector volume. Thereby, a unification of the different subsystems as well as a shift towards commercial products and software can be observed. This is due to the immense costs and increasing problems in maintaining the systems over a long time period due to the lack of system experts. But there are some parts of the processing that still need to be done in hardware (e.g. within a field programmable gate array (FPGA)). Therefore, an interface between the hardware systems and the software running on a remote server needs to be implemented. While the PCIe interface was quite common for this, it limits the bandwidth as each processing card will only have a single PCIe interface. With the server hosting such cards is already busy with forwarding the data via a commercial network, it cannot provide any data processing capability. Thereby, it would be more efficient to sent directly from the FPGA as this would also allow for better scaling of bandwidth. However, the standard protocols for reliable data transmissions were designed with a software implementation in mind. That’s why their implementation in hardware would be rather complicated. Therefore, the question about the necessity of guaranteed data transmission arose. To answer this question, a network stack was implemented within this thesis to investigate the level of packet drop occurring in the transmission and how this could be reduced to an acceptable level. In this context, an emerging technique named eXpress Data Path (XDP) was evaluated. With its help, the transmission of 5.2PB (i.e. 2.92 × 10e12 packets) within 168h (i.e. a week) with not a single missing packet was demonstrated

The ITk interlock hardware portection system

For the upgrade of the Large Hadron Collider to the High-Luminosity Large Hadron Collider (HL-LHC) the ATLAS detector will install a new Inner Tracker (ITk), which consists completely of silicon detectors. Although di erent technologies were chosen for the inner and outer part, the major risk for all silicon detectors are heat-ups, which can cause irreparable damages. As, once the detector is installed, detector elements are not accessible for several years or even for the lifetime of the detector, such damages must be avoided by all means. The ITk interlock system is a hardwired safety system, it acts as last line of defense and is designed to protect the sensitive detector elements against upcoming risks. The core of the interlock system consists of distributed FPGAs, housing the interlock matrix decision tables. They collect signals from interlock protected devices and distribute signals onto interlock controlled units (e.g. power supplies). Additionally, signals from external systems can be integrated. To keep the number of detector elements, which are out of operation, at a minimum, the power supplies are controlled with a high granularity. The resulting large number of channels also explains why no commercial solution was selected. We report from the concept to the realization

Search for direct top squark pair production in final states with two leptons in s=13\sqrt{s} = 13 TeV pppp collisions with the ATLAS detector

International audienceThe results of a search for direct pair production of top squarks in events with two opposite-charge leptons (electrons or muons) are reported, using $36.1~\hbox {fb}^{-1}$ of integrated luminosity from proton–proton collisions at $\sqrt{s}=13$ TeV collected by the ATLAS detector at the Large Hadron Collider. To cover a range of mass differences between the top squark $\tilde{t}$ and lighter supersymmetric particles, four possible decay modes of the top squark are targeted with dedicated selections: the decay $\tilde{t} \rightarrow b \tilde{\chi }_{1}^{\pm }$ into a b-quark and the lightest chargino with $\tilde{\chi }_{1}^{\pm } \rightarrow W \tilde{\chi }_{1}^{0}$ , the decay $\tilde{t} \rightarrow t \tilde{\chi }_{1}^{0}$ into an on-shell top quark and the lightest neutralino, the three-body decay $\tilde{t} \rightarrow b W \tilde{\chi }_{1}^{0}$ and the four-body decay $\tilde{t} \rightarrow b \ell \nu \tilde{\chi }_{1}^{0}$ . No significant excess of events is observed above the Standard Model background for any selection, and limits on top squarks are set as a function of the $\tilde{t}$ and $\tilde{\chi }_{1}^{0}$ masses. The results exclude at 95% confidence level $\tilde{t}$ masses up to about 720 GeV, extending the exclusion region of supersymmetric parameter space covered by previous searches

Measurements of ttˉt\bar{t} differential cross-sections of highly boosted top quarks decaying to all-hadronic final states in pppp collisions at s=13 \sqrt{s}=13\, TeV using the ATLAS detector

Measurements are made of differential cross-sections of highly boosted pair-produced top quarks as a function of top-quark and $t\bar{t}$ system kinematic observables using proton--proton collisions at a center-of-mass energy of $\sqrt{s} = 13$ TeV. The data set corresponds to an integrated luminosity of $36.1$ fb$^{-1}$, recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Events with two large-radius jets in the final state, one with transverse momentum $p_{\rm T} > 500$ GeV and a second with $p_{\rm T}>350$ GeV, are used for the measurement. The top-quark candidates are separated from the multijet background using jet substructure information and association with a $b$-tagged jet. The measured spectra are corrected for detector effects to a particle-level fiducial phase space and a parton-level limited phase space, and are compared to several Monte Carlo simulations by means of calculated $\chi^2$ values. The cross-section for $t\bar{t}$ production in the fiducial phase-space region is $292 \pm 7 \ \rm{(stat)} \pm 76 \rm{(syst)}$ fb, to be compared to the theoretical prediction of $384 \pm 36$ fb

Measurements of ttˉt\bar{t} differential cross-sections of highly boosted top quarks decaying to all-hadronic final states in pppp collisions at s=13 \sqrt{s}=13\, TeV using the ATLAS detector

Measurements are made of differential cross-sections of highly boosted pair-produced top quarks as a function of top-quark and $t\bar{t}$ system kinematic observables using proton--proton collisions at a center-of-mass energy of $\sqrt{s} = 13$ TeV. The data set corresponds to an integrated luminosity of $36.1$ fb$^{-1}$, recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Events with two large-radius jets in the final state, one with transverse momentum $p_{\rm T} > 500$ GeV and a second with $p_{\rm T}>350$ GeV, are used for the measurement. The top-quark candidates are separated from the multijet background using jet substructure information and association with a $b$-tagged jet. The measured spectra are corrected for detector effects to a particle-level fiducial phase space and a parton-level limited phase space, and are compared to several Monte Carlo simulations by means of calculated $\chi^2$ values. The cross-section for $t\bar{t}$ production in the fiducial phase-space region is $292 \pm 7 \ \rm{(stat)} \pm 76 \rm{(syst)}$ fb, to be compared to the theoretical prediction of $384 \pm 36$ fb