Connection between soft and hard probes of small collision systems at RHIC and LHC

The expected connections between signatures in the soft and hard sectors of small collision systems, and the status of experimental attempts to identify them, are discussed. These proceedings summarize the talk as given at the International Symposium on Multi-Particle Dynamics in September 2019 in Santa Fe, NM (ISMD19). As such, the choice of content and focus are selective and not intended to be comprehensive.Comment: 6 pages, 3 figures, proceedings of the ISMD1

Inclusive jet production in ultrarelativistic proton-nucleus collisions

High-pT processes in proton- and deuteron-nucleus collisions at TeV energies are the best presently available way to study the partonic structure of the nucleus in a high-density regime. Jet production over a wide range of phase space can significantly constrain the current knowledge of nuclear parton distribution functions (nPDFs), which are substantially less well understood than the corresponding PDFs in protons and which have only recently begun to be treated in a spatially-dependent way. An accurate knowledge of nPDFs is crucial for a definitive control of perturbative processes in a cold nuclear environment, since high-pT probes are used to quantitatively investigate the hot QCD matter created in ultrarelativistic nucleus-nucleus collisions. Furthermore, jets from low Bjorken-x partons can probe the transition from the dilute to saturated nuclear regimes

Contribution to differential π0\pi^0 and γdir\gamma_\mathrm{dir} modification in small systems from color fluctuation effects

A major complication in the search for jet quenching in proton- or deuteron-nucleus collision systems is the presence of physical effects which influence the experimental determination of collision centrality in the presence of a hard process. For example, in the proton color fluctuation picture, protons with a large Bjorken-$x$ ($x \gtrsim 0.1$) parton interact more weakly with the nucleons in the nucleus, leading to a smaller (larger) than expected yield in large (small) activity events. A recent measurement by PHENIX compared the yield of neutral pion and direct photon production in $d$+Au collisions, under the argument that the photon yields correct for such biases, and the difference between the two species is thus attributable to final-state effects (i.e., jet quenching). The main finding suggests a significant degree of jet quenching for hard processes in small systems. In this paper, I argue that the particular photon and pion events selected by PHENIX arise from proton configurations with significantly different Bjorken-$x$ distributions, and thus are subject to different magnitudes of modification in the color fluctuation model. Using the results of a previous global analysis of RHIC and LHC data, I show that potentially all of the pion-to-photon difference in PHENIX data can be described by a proton color fluctuation picture at a quantitative level before any additional physics from final-state effects is required. This finding reconciles the interpretation of the PHENIX measurement with others at RHIC and LHC, which have found no observable evidence for jet quenching in small systems.Comment: 6 pages, 4 figures, final version as publishe

Two-particle Bose-Einstein correlations in pp collisions at root s=13 TeV measured with the ATLAS detector at the LHC

This paper presents studies of Bose&ndash;Einstein correlations (BEC) in proton&ndash;proton collisions at a centre-of-mass energy of 13&nbsp;TeV, using data from the ATLAS detector at the CERN Large Hadron Collider. Data were collected in a special low-luminosity configuration with a minimum-bias trigger and a high-multiplicity track trigger, accumulating integrated luminosities of 151&nbsp;\upmu b^{-1} and 8.4&nbsp;nb^{-1}, respectively. The BEC are measured for pairs of like-sign charged particles, each with |\eta | &lt; 2.5, for two kinematic ranges: the first with particle p_{\mathrm {T}} &gt; 100&nbsp;MeV and the second with particle p_{\mathrm {T}} &gt; 500 MeV. The BEC parameters, characterizing the source radius and particle correlation strength, are investigated as functions of charged-particle multiplicity (up to 300) and average transverse momentum of the pair (up to 1.5&nbsp;GeV). The double-differential dependence on charged-particle multiplicity and average transverse momentum of the pair is also studied. The BEC radius is found to be independent of the charged-particle multiplicity for high charged-particle multiplicity (above 100), confirming a previous observation at lower energy. This saturation occurs independent of the transverse momentum of the pair. &nbsp;</p

Search for neutral long-lived particles in pp collisions at root s = 13 TeV that decay into displaced hadronic jets in the ATLAS calorimeter

A search for decays of pair-produced neutral long-lived particles (LLPs) is presented using 139 fb&minus;1 of proton-proton collision data collected by the ATLAS detector at the LHC in 2015&ndash;2018 at a centre-of-mass energy of 13 TeV. Dedicated techniques were developed for the reconstruction of displaced jets produced by LLPs decaying hadronically in the ATLAS hadronic calorimeter. Two search regions are defined for different LLP kinematic regimes. The observed numbers of events are consistent with the expected background, and limits for several benchmark signals are determined. For a SM Higgs boson with a mass of 125 GeV, branching ratios above 10% are excluded at 95% confidence level for values of c times LLP mean proper lifetime in the range between 20 mm and 10 m depending on the model. Upper limits are also set on the cross-section times branching ratio for scalars with a mass of 60 GeV and for masses between 200 GeV and 1 TeV. &nbsp;</p

Modelling and computational improvements to the simulation of single vector-boson plus jet processes for the ATLAS experiment

This paper presents updated Monte Carlo configurations used to model the production of single electroweak vector bosons (W, Z/&gamma;&lowast;) in association with jets in proton-proton collisions for the ATLAS experiment at the Large Hadron Collider. Improvements pertaining to the electroweak input scheme, parton-shower splitting kernels and scale-setting scheme are shown for multi-jet merged configurations accurate to next-to-leading order in the strong and electroweak couplings. The computational resources required for these set-ups are assessed, and approximations are introduced resulting in a factor three reduction of the per-event CPU time without affecting the physics modelling performance. Continuous statistical enhancement techniques are introduced by ATLAS in order to populate low cross-section regions of phase space and are shown to match or exceed the generated effective luminosity. This, together with the lower per-event CPU time, results in a 50% reduction in the required computing resources compared to a legacy set-up previously used by the ATLAS collaboration. The set-ups described in this paper will be used for future ATLAS analyses and lay the foundation for the next generation of Monte Carlo predictions for single vector-boson plus jets production. &nbsp;</p

Observation of electroweak production of two jets in association with an isolated photon and missing transverse momentum, and search for a Higgs boson decaying into invisible particles at 13 TeV with the ATLAS detector

his paper presents a measurement of the elec-troweak production of two jets in association with a Z &gamma; pair,with the Z boson decaying into two neutrinos. It also presentsa search for invisible or partially invisible decays of a Higgsboson with a mass of 125 GeV produced through vector-boson fusion with a photon in the final state. These results usedata from LHC proton&ndash;proton collisions at &radic;s = 13 TeV col-lected with the ATLAS detector and corresponding to an inte-grated luminosity of 139 fb&minus;1 . The event signature, sharedby all benchmark processes considered for the measurementsand searches, is characterized by a significant amount ofunbalanced transverse momentum and a photon in the finalstate, in addition to a pair of forward jets. Electroweak Z &gamma;production in association with two jets is observed in thisfinal state with a significance of 5.2 (5.1 expected) standarddeviations. The measured fiducial cross-section for this pro-cess is 1.31 &plusmn; 0.29 fb. An observed (expected) upper limitof 0.37 (0.34+0.15&minus;0.10 ) at 95% confidence level is set on thebranching ratio of a 125 GeV Higgs boson to invisible parti-cles, assuming the Standard Model production cross-section.The signature is also interpreted in the context of decays ofa Higgs boson into a photon and a dark photon. An observed(expected) 95% CL upper limit on the branching ratio forthis decay is set at 0.018 (0.017+0.007&minus;0.005 ), assuming the Stan-dard Model production cross-section for a 125 GeV Higgsboson.</p

Performance of the ATLAS Level-1 topological trigger in Run 2

During LHC Run&nbsp;2 (2015&ndash;2018) the ATLAS Level-1 topological trigger allowed efficient data-taking by the ATLAS experiment at luminosities up to 2.110&nbsp;cms, which exceeds the design value by a factor of two. The system was installed in 2016 and operated in 2017 and 2018. It uses Field Programmable Gate Array processors to select interesting events by placing kinematic and angular requirements on electromagnetic clusters, jets, -leptons, muons and the missing transverse energy. It allowed to significantly improve the background event rejection and signal event acceptance, in particular for Higgs and B-physics processes.</p