The τ lepton anomalous magnetic moment aτ=gτ−22 was measured, so far, with a precision of only several percents, despite its high sensitivity to physics beyond the Standard Model such as compositeness or supersymmetry. A new study is presented to improve the sensitivity of the aτ measurement with photon-photon interactions from ultraperipheral lead-lead collisions at the LHC. The theoretical approach used in this work is based on an effective Lagrangian and on a photon flux implemented in the adraph5 Monte Carlo simulation. Using a multivariate analysis to discriminate the signal from the background processes, a sensitivity to the anomalous magnetic moment aτ=0+0.015−0.019 is obtained at 95% confidence level with a dataset corresponding to an integrated luminosity of 2 nb−1 of lead-lead collisions and assuming a conservative 10% systematic uncorrelated uncertainty for signal and background. The present results using multivariate analysis are compared to similar results obtained using sequential cuts, as done in previous measurements, showing an improvement of about 35% in the sensitivity to aτ. Published by the American Physical Society 2024

We explore the application of concepts developed in high-energy physics (HEP) in a field of high social impact, i.e., advanced medical data analysis. More specifically, we focus on shortening the reconstruction times of a multi-parametric quantitative magnetic resonance imaging (MRI) technique: magnetic resonance fingerprinting (MRF). This technique has the potential to replace multiple qualitative MRI acquisitions with a single reproducible measurement for increased sensitivity and efficiency of the examination. In MRF, a fast acquisition is followed by a pattern-matching (PM) task, where signal responses are matched to entries from a dictionary of simulated, physically feasible responses, yielding multiple tissue parameters simultaneously. Each voxel signal response in the volume is compared through scalar products with all dictionary entries to choose the best measurement reproduction. MRF is limited by the PM processing time, which scales exponentially with the dictionary dimensionality, i.e., with the number of tissue parameters to be reconstructed. In the context of HEP, we developed a powerful, compact, embedded system, optimized for extremely fast PM. This system executes real-time particle trajectory (track) reconstruction for online event selection in the HEP experiments, exploiting maximum parallelism and pipelining. Track reconstruction is executed in two steps. The associative memory (AM) ASIC first implements a PM algorithm by recognizing track candidates at low resolution. The second step, which is implemented into field programmable gate arrays (FPGAs), refines the AM output finding the track parameters at full resolution. We propose to use this system to achieve a faster reconstruction time in MRF. This article proposes an adaptation of the HEP system for medical imaging and shows some preliminary results.

This paper presents a search for direct top squark pair production in events with missing transverse momentum plus either a pair of jets consistent with Standard Model Higgs boson decay into b-quarks or a same-flavour opposite-sign dilepton pair with an invariant mass consistent with a Z boson. The analysis is performed using the proton–proton collision data at \n$$\\sqrt{s}=13$$\n\n TeV collected with the ATLAS detector during the LHC Run-2, corresponding to an integrated luminosity of 139 fb\n$$^{-1}$$\n\n. No excess is observed in the data above the Standard Model predictions. The results are interpreted in simplified models featuring direct production of pairs of either the lighter top squark (\n$$\\tilde{t}_1$$\n\n) or the heavier top squark (\n$$\\tilde{t}_2$$\n\n), excluding at 95% confidence level \n$$\\tilde{t}_1$$\n\n and \n$$\\tilde{t}_2$$\n\n masses up to about 1220 and 875 GeV, respectively.

The Associative Memory (AM) system of the Fast Tracker (FTK) processor has been designed to perform pattern matching using the hit information of the ATLAS experiment silicon tracker. The system is one of the two main processing elements of FTK and is mainly based on the use of Application Specific Integrated Circuits, the AM chips, specifically designed to execute pattern matching with a high degree of parallelism. It finds track candidates at low resolution that are seeds for a full resolution track fitting. The AM system implementation is based on a collection of "AM boards", the "Serial Link Processors" (AMBSLP). The AMBSLP is based on a network of high speed serial links to sustain very high data traffic. It has a high power consumption (~250 W) because of its high performance requirements and, therefore, the AM system needs custom power and cooling.This proceedings reports on the performance studies of the system made with the first production of 64 AMBSLPs integrated in FTK and results from the first ATLAS data during 2018.

A measurement of production cross sections of the Higgs boson in proton-proton collisions is presentedin the H → ττ decay channel. The analysis is performed using 36.1 fb−1 of data recorded by the ATLASexperiment at the Large Hadron Collider at a center-of-mass energy of ffiffis p ¼ 13 TeV. All combinations ofleptonic (τ → lvv¯ with l ¼ e; μ) and hadronic (τ → hadrons v) τ decays are considered. The H → ττsignal over the expected background from other Standard Model processes is established with an observed(expected) significance of 4.4 (4.1) standard deviations. Combined with results obtained using data taken at7 and 8 TeV center-of-mass energies, the observed (expected) significance amounts to 6.4 (5.4) standarddeviations and constitutes an observation of H → ττ decays. Using the data taken at ffiffis p ¼ 13 TeV, thetotal cross section in the H → ττ decay channel is measured to be 3.77þ0.60 −0.59 ðstatÞ þ0.87 −0.74 ðsystÞ pb, for aHiggs boson of mass 125 GeV assuming the relative contributions of its production modes aspredicted by the Standard Model. Total cross sections in the H → ττ decay channel are determinedseparately for vector-boson-fusion production and gluon-gluon-fusion production to be σVBFH→ττ ¼ 0.280.09 ðstatÞ þ0.11 −0.09 ðsystÞ pb and σggFH→ττ ¼ 3.1 1.0 ðstatÞ þ1.6 −1.3 ðsystÞ pb, respectively. Similarly, results of a fitare reported in the framework of simplified template cross sections. All measurements are in agreementwith Standard Model expectations.

The centrality dependence of the mean chargedparticle multiplicity as a function of pseudorapidity is measured in approximately 1 μb−¹ of proton–lead collisions at a nucleon–nucleon centre-of-mass energy of √sNN=5.02 TeV using the ATLAS detector at the Large Hadron Collider. Charged particles with absolute pseudorapidity less than 2.7 are reconstructed using the ATLAS pixel detector. The p + Pb collision centrality is characterised by the total transverse energy measured in the Pb-going direction of the forward calorimeter. The charged-particle pseudorapidity distributions are found to vary strongly with centrality, with an increasing asymmetry between the proton-going and Pb-going directions as the collisions become more central. Three different estimations of the number of nucleons participating in the p+Pb collision have been carried out using the Glauber model as well as two Glauber–Gribov inspired extensions to theGlauber model. Charged-particle multiplicities per participant pair are found to vary differently for these three models, highlighting the importance of including colour fluctuations in nucleon–nucleon collisions in the modelling of the initial state of p + Pb collisions.

The rich set of data collected from HERA, Tevatron and LHC collisions offer an extraordinary possibility to test different aspects of perturbative Quantum Chromodynamics. A selection of recent results illustrates the variety and precision of these tests. The comparison of the experimental data with the most recent theoretical predictions demonstrates the advances made in the theoretical field.

The extended use of tracking information at the trigger level in the LHC is crucial for the trigger and data acquisition (TDAQ) system to fulfill its task.Precise and fast tracking is important to identify specific decay products of the Higgs boson or new phenomena, as well as to distinguish the contributions coming from the many collisions that occur at every bunch crossing.However, track reconstruction is among the most demanding tasks performed by the TDAQ computing farm; in fact, complete reconstruction at full Level-1 trigger accept rate (100 kHz) is not possible.In order to overcome this limitation, the ATLAS experiment is planning the installation of a dedicated processor, the Fast Tracker (FTK), which is aimed at achieving this goal.The FTK is a pipeline of high performance electronics, based on custom and commercial devices, which is expected to reconstruct, with high resolution, the trajectories of charged-particle tracks with a transverse momentum above 1 GeV, using the ATLAS inner tracker information.Pattern recognition and the track parameter extraction are expected to be performed in roughly 100 µs, allowing all the high level trigger selections to use the tracks provided by FTK in order to build high quality and robust triggering.