The ATLAS Level-1 topological processor: Experience and upgrade plans

During Run 2 (2015–2018) the Large Hadron Collider has provided, at the World’s highest energy frontier, proton–proton collisions to the ATLAS experiment with high instantaneous luminosity (up to [Formula: see text]), placing stringent operational and physics requirements on the ATLAS trigger system in order to reduce the 40 MHz collision rate to a manageable event storage rate of 1 kHz, while not rejecting interesting collisions. The Level-1 trigger is the first rate-reducing step in the ATLAS trigger system with an output rate of up to 100 kHz and decision latency of less than 2.5 [Formula: see text]s. In Run 2, an important role was played by the Level-1 Topological Processor (L1Topo). This innovative system consists of two blades designed in AdvancedTCA form factor, mounting four individual state-of-the-art processors, and providing high input bandwidth and low latency data processing. Up to 128 topological trigger algorithms can be implemented to select interesting events by applying kinematic and angular requirements on electromagnetic clusters, hadronic jets, muons and total energy reconstructed in the ATLAS apparatus. This resulted in a significantly improved background rejection and enhanced acceptance of physics signal events, despite the increasing luminosity. The L1Topo system has become more and more important for physics analyses making use of low energy objects, commonly present in the Heavy Flavor or Higgs physics events, for example. An overview of the L1Topo architecture, simulation and performance results during Run 2 is presented alongside with upgrade plans for the L1Topo system to be installed for the future Run 3 data taking period.

Search for excited electrons singly produced in proton–proton collisions at $$\sqrt{s} ~=~13~\text {Te}\text {V}$$ with the ATLAS experiment at the LHC

A search for excited electrons produced in pp collisions at $$\sqrt{s}$$s = 13 $$\text {Te}\text {V}$$Te via a contact interaction $$q{\bar{q}}\rightarrow ee^*$$qq¯→ee∗ is presented. The search uses 36.1 fb$$^{-1}$$-1 of data collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider. Decays of the excited electron into an electron and a pair of quarks ($$eq{\bar{q}}$$eqq¯) are targeted in final states with two electrons and two hadronic jets, and decays via a gauge interaction into a neutrino and a $$W$$W boson ($$\nu W$$νW) are probed in final states with an electron, missing transverse momentum, and a large-radius jet consistent with a hadronically decaying $$W$$W boson. No significant excess is observed over the expected backgrounds. Upper limits are calculated for the $$pp \rightarrow ee^*\rightarrow eeq{\bar{q}} $$pp→ee∗→eeqq¯ and $$pp \rightarrow ee^*\rightarrow e\nu W $$pp→ee∗→eνW production cross sections as a function of the excited electron mass $$m_{e^*}$$me∗ at 95% confidence level. The limits are translated into lower bounds on the compositeness scale parameter $$\Lambda $$Λ of the model as a function of $$m_{e^*}$$me∗. For $$m_{e^*} <0.5$$me∗<0.5 $$\text {Te}\text {V}$$Te, the lower bound for $$\Lambda $$Λ is 11 $$\text {Te}\text {V}$$Te. In the special case of $$m_{e^*} =\Lambda $$me∗=Λ, the values of $$m_{e^*} <4.8$$me∗<4.8 $$\text {Te}\text {V}$$Te are excluded. The presented limits on $$\Lambda $$Λ are more stringent than those obtained in previous searches.

Production of prompt photons associated with jets at LHC in kT-factorization

We use the kT–factorization approach to calculate total and differential cross sections of associated production of prompt photons and hadronic jets at the LHC energies. Our consideration relies on the pegasus Monte-Carlo generator with implemented ℴ(αα2s) off-shell gluon–gluon fusion subprocess g*g* → γqq− and several subleading quark-initiated contributions from ℴ(ααs) and ℴ(αα2s) subprocesses, taken into account in the collinear limit. Using Monte-Carlo generators CASCADE and PYTHIA, we investigate parton showering effects and compare our predictions with the data, taken by CMS and ATLAS collaborations at the LHC. We demostrate reasonabledescription of the data and the importance of parton shower effects in the kT–factorization.

Simulations of Gamma-Ray Emission from Magnetized Microquasar Jets

In this work, we simulate γ-rays created in the hadronic jets of the compact object in binary stellar systems known as microquasars. We utilize as the main computational tool the 3D relativistic magnetohydrodynamical code PLUTO combined with in-house derived codes. Our simulated experiments refer to the SS433 X-ray binary, a stellar system in which hadronic jets have been observed. We examine two new model configurations that employ hadron-based emission mechanisms. The simulations aim to explore the dependence of the γ-ray emissions on the dynamical as well as the radiative properties of the jet (hydrodynamic parameters of the mass-flow density, gas-pressure, temperature of the ejected matter, high energy proton population inside the jet plasma, etc.). The results of the two new scenarios of initial conditions for the microquasar stellar system studied are compared to those of previously considered scenarios.

Explore the Lifetime Frontier with MATHUSLA

Many extensions of the Standard Model (SM) include particles that are neutral, weakly coupled, and long-lived that can decay to final states containing several hadronic jets. Long-lived particles (LLPs) can be detected as displaced decays from the interaction point, or missing energy if they escape. ATLAS and CMS have performed searches at the LHC and significant exclusion limits have been set in recent years. However, the current searches performed at colliders have limitations. An LLP does not interact with the detector and it is only visible once it decays. Unfortunately, no existing or proposed search strategy will be able to observe the decay of non-hadronic electrically neutral LLPs with masses above GeV and lifetimes near the limit set by Big Bang Nucleosynthesis (cπ ~ 107 - 108 m). Therefore, ultra-long-lived particles (ULLPs) produced at the LHC will escape the main detector with extremely high probability. MATHUSLA (MAssive Timing Hodoscope for Ultra Stable neutraL pArticles) is a surface detector, which can be implemented with existing technology and in time for the high luminosity LHC upgrade to find such ultra-long-lived particles, whether produced in exotic Higgs decays or more general production modes. The MATHUSLA detector will consist of resistive plate chambers (RPC) and scintillators with a total sensitive area of 200 x 200 m2. It will be installed on the surface, close to the ATLAS or CMS detectors. A small-scale test detector (~ 6m2) is going to be installed on the surface above ATLAS in November 2017. It will consist of three layers of RPCs used for timing/tracking and two layers of scintillators for timing measurements. It will be placed above the ATLAS interaction point to estimate cosmic backgrounds and proton-proton backgrounds coming from ATLAS during nominal LHC operations.