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Abstract:

A search for events with one displaced vertex from long-lived particles using data collected by the ATLAS detector at the Large Hadron Collider is presented, using $140{\mathrm{fb}}^{-1}$ of proton-proton collision data at $\sqrt{s}=13\mathrm{TeV}$ recorded in 2015–2018. The search employs techniques for reconstructing vertices of long-lived particles decaying into hadronic jets in the muon spectrometer displaced between 3 m and 14 m from the primary interaction vertex. The observed number of events is consistent with the expected background and limits for several benchmark signals are determined. A scalar-portal model and a Higgs-boson-portal baryogenesis model are considered. A dedicated analysis channel is employed to target Z-boson associated long-lived particle production, including an axionlike particle and a dark photon model. For the Higgs boson model, branching fractions above 1% are excluded at 95% confidence level for long-lived particle proper decay lengths ranging from 5 cm to 40 m. For the photophobic axionlike particle model considered, this search produces the strongest limits to date for proper decay lengths greater than $\mathcal{O}(10)\mathrm{cm}$ .

Abstract:

Measurements of jet substructure in Pb+Pb collisions provide key insights into the mechanism of jet quenching in the hot and dense QCD medium created in these collisions. This Letter presents a measurement of the suppression of large-radius jets with a radius parameter of R = 1.0 and its dependence on the jet substructure. The measurement uses 1.72 nb − 1 of Pb+Pb data and 255 pb − 1 of p p data, both at s NN = 5.02 TeV, recorded with the ATLAS detector at the Large Hadron Collider. Large-radius jets are reconstructed by reclustering R = 0.2 calorimetric jets and are measured for transverse momentum above 200 GeV. Jet substructure is evaluated using charged-particle tracks, and the overall level of jet suppression is quantified using the jet nuclear modification factor ( R AA ). The jet R AA is measured as a function of jet p T , the charged k t splitting scale ( d 12 ), and the angular separation ( Δ R 12 ) of two leading sub-jets. The jet R AA gradually decreases with increasing d 12 , implying significantly stronger suppression of large-radius jets with larger k t splitting scale. The jet R AA gradually decreases for Δ R 12 in the range 0.01 − 0.2 and then remains consistent with a constant for Δ R 12 ≳ 0.2 . The observed significant dependence of jet suppression on the jet substructure will provide new insights into its role in the quenching process.

Abstract:

The stellar velocity dispersion ( σ ) of massive elliptical galaxies is a key ingredient in breaking the mass-sheet degeneracy and obtaining precise and accurate cosmography from gravitational time delays. The relative uncertainty on the Hubble constant H 0 is double the relative error on σ . Therefore, time-delay cosmography imposes much more demanding requirements on the precision and accuracy of σ than galaxy studies. While precision can be achieved with an adequate signal-to-noise ratio (S/N), the accuracy critically depends on key factors such as the elemental abundance and temperature of stellar templates, flux calibration, and wavelength ranges. We carried out a detailed study of the problem using multiple sets of galaxy spectra of massive elliptical galaxies with S/N ∼ 30–160 Å −1 , along with state-of-the-art empirical and semi-empirical stellar libraries and stellar population synthesis templates. We show that the choice of stellar library is generally the dominant source of residual systematic errors. We propose a general recipe for mitigating and accounting for residual uncertainties. We show that a sub-percent level of accuracy can be achieved on individual spectra with our data quality, which we subsequently validated with simulated mock datasets. The covariance between velocity dispersions measured for a sample of spectra can also be reduced to sub-percent levels. We recommend this recipe for all applications that require high precision and accurate stellar kinematics. Thus, we have made all the software publicly available to facilitate its implementation. This recipe will also be used in future TDCOSMO collaboration papers.

Abstract:

The relationship between the already formed stellar mass in a galaxy and the gas reservoir of neutral atomic hydrogen, is a key element in our understanding of how gas is turned into stars in galaxy haloes. In this paper, we measure the relation based on a stellar-mass selected sample at and the MeerKAT International GHz Tiered Extragalactic Exploration-H i Data Release 1 spectral data. Using a powerful Bayesian stacking technique, for the first time we are also able to measure the underlying bivariate distribution of H i mass and stellar mass of galaxies with M, finding that an asymmetric underlying H i distribution is strongly preferred by our complete samples. We define the concepts of the average of the logarithmic H i mass, , and the logarithmic average of the H i mass, , and find that the difference between and can be as large as 0.5 dex for the preferred asymmetric H i distribution. We observe shallow slopes in the underlying scaling relations, suggesting the presence of an upper H i mass limit beyond which a galaxy can no longer retain further H i gas. From our bivariate distribution we also infer the H i mass function at this redshift and find tentative evidence for a decrease of 2–10 times in the comoving space density of the most H i massive galaxies up to .