Inclusive and differential cross-section measurements of t\bartZ production in pp collisions at √s=13 TeV with the ATLAS detector, including EFT and spin-correlation interpretations

Measurements of both the inclusive and differential production cross sections of a top-quark-top-antiquark pair in association with a Z boson (tt¯Z) are presented. Final states with two, three or four isolated leptons (electrons or muons) are targeted. The measurements use the data recorded by the ATLAS detector in pp collisions at s√=13 TeV at the Large Hadron Collider during the years 2015-2018, corresponding to an integrated luminosity of 140 fb−1. The inclusive cross section is measured to be σtt¯Z=0.86±0.04 (stat.)±0.04 (syst.) pb and found to be in agreement with the most advanced Standard Model predictions. The differential measurements are presented as a function of a number of observables that probe the kinematics of the tt¯Z system. Both the absolute and normalised differential cross-section measurements are performed at particle level and parton level for specific fiducial volumes, and are compared with NLO+NNLL theoretical predictions. The results are interpreted in the framework of Standard Model effective field theory and used to set limits on a large number of dimension-6 operators involving the top quark. The first measurement of spin correlations in tt¯Z events is presented: the results are in agreement with the Standard Model expectations, and the null hypothesis of no spin correlations is disfavoured with a significance of 1.8 standard deviations

Observation of quantum entanglement in top-quark pairs using the ATLAS detector

We report the highest-energy observation of entanglement, in top−antitop quark events produced at the Large Hadron Collider, using a proton−proton collision data set with a center-of-mass energy of s√=13 TeV and an integrated luminosity of 140 fb−1 recorded with the ATLAS experiment. Spin entanglement is detected from the measurement of a single observable D, inferred from the angle between the charged leptons in their parent top- and antitop-quark rest frames. The observable is measured in a narrow interval around the top−antitop quark production threshold, where the entanglement detection is expected to be significant. It is reported in a fiducial phase space defined with stable particles to minimize the uncertainties that stem from limitations of the Monte Carlo event generators and the parton shower model in modelling top-quark pair production. The entanglement marker is measured to be D=−0.547±0.002 (stat.)±0.021 (syst.) for 340<mtt¯<380 GeV. The observed result is more than five standard deviations from a scenario without entanglement and hence constitutes both the first observation of entanglement in a pair of quarks and the highest-energy observation of entanglement to date

Mechanism of neurogenesis and neuro-regeneration in the adult teleost brain

Fish retain a remarkable potential of neuro-regeneration throughout life, whereas injury to neuronal system in the mammalian central nervous system (CNS) results in degeneration and loss of function. Hypothetically, the brain of teleost will regenerate after injury, however, the molecular mechanism underlying this phenomena as well as the reason for the lack of such regenerative capability in the mammalian CNS is still unknown. To address this issue, three main study objectives were placed, (i) to understand the mechanism involved in neuro-regeneration in teleosts brain (Chapter 2); (ii) to identify novel molecular factors involved in neuro-regeneration in the zebrafish brain (Chapter 3); and (iii) to investigate the potential of identified novel molecular factors in associating adult neurogenesis in the zebrafish brain (Chapter 4). In chapter 2, the study aimed to characterize the neuro-regenerative process by observing changes in apoptotic and proliferative activities in the brain of teleost, zebrafish (Danio rerio) and tilapia (Oreochromis niloticus). Morphological observations showed complete neuro-regeneration of the habenula region by 40 days and 60 days post-damage for zebrafish and tilapia respectively. Recovery of neuronal projections and axonal re-integration further proved complete neuro-regeneration in adult teleost. In chapter 3, the study aimed to identify the molecular factor(s) that play role in the early stage of regeneration, before first batch of newborn cells were significantly detected at the injury site, protein profiling through two-dimensional difference gel electrophoresis was done in the following chapter 3. Ten proteins were identified, which included cytoskeleton protein, transcription factors and binding proteins. Among these proteins identified, sprouty-related EVH1 domain-containing protein 2 (Spred-2) was chosen for further study as there is no study between Spred-2 and neurogenesis has been reported. Finally in chapter 4, the expression of spred-2 mRNA in the adult zebrafish brain was analyzed by in situ hybridization and their histological changes were examined during the brain injury. The zebrafish spred-2 mRNA containing cells were observed in most of the cell proliferative zones, which suggests the potential role of Spred-2 in the regulation of neurogenesis. On the other hand, histological study showed a decrease of spred-2 expression with a concurrent increase in apoptotic cells at the lesion site on the first two days after the injury. Damaged and dead neural cells release growth factors and neurotrophins which can down-regulate Spred-2 expression to unlock the activation of the ERK pathway. Proliferative and phosphorylated-ERK-immunoreactive cells significantly increased on day-4 post-lesion and co-expression of spred-2 mRNA in phosphorylated-ERK-immunoreactive cells were observed in neurogenic zones near the lesion site. These observations suggest that a decrease in spred-2 after injury activates the ERK pathway to stimulate cell proliferation in the adult zebrafish brain. Collectively, these studies showed the teleost CNS possesses excellent neuro-regenerative capability. The apoptotic neural cells could be the earliest source to induce cell proliferation. Differential protein profiling revealed that Spred-2 could be a novel molecule which can regulate neurogenesis in adult teleost CNS. High conservation of Spred-2 protein sequence in vertebrates suggests the potential role for Spred-2 in neurogenesis-regulation in vertebrates including mammals. Taken together, these results provide better cellular and molecular processes underlying adult neuro-regeneration of injured CNS

Search for events with a pair of displaced vertices from long-lived neutral particles decaying into hadronic jets in the ATLAS muon spectrometer in collisions at

A search for events with two displaced vertices from long-lived particle (LLP) pairs using data collected by the ATLAS detector at the LHC is presented. This analysis uses 139 fb(-1) of proton-proton collision data at root s=13 TeV recorded in 2015-2018. The search employs techniques for reconstructing vertices of LLPs decaying to jets in the muon spectrometer displaced between 3 and 14 m with respect to the primary interaction vertex. The observed numbers of events are consistent with the expected background and limits for several benchmark signals are determined. For the Higgs boson with a mass of 125 GeV, the paper reports the first exclusion limits for branching fractions into neutral long-lived particles below 0.1%, while branching fractions above 10% are excluded at 95% confidence level for LLP proper lifetimes ranging from 4 cm to 72.4 m. In addition, the paper present the first results for the decay of LLPs into (tt) over bar in the ATLAS muon spectrometer

Constraints on Higgs boson production with large transverse momentum using decays in the ATLAS detector

This paper reports constraints on Higgs boson production with transverse momentum above 1 TeV. The analyzed data from proton-proton collisions at a center-of-mass energy of 13 TeV were recorded with the ATLAS detector at the Large Hadron Collider from 2015 to 2018 and correspond to an integrated luminosity of 136 fb(-1.) Higgs bosons decaying into b (b) over bar are reconstructed as single large-radius jets recoiling against a hadronic system and are identified by the experimental signature of two b-hadron decays. The experimental techniques are validated in the same kinematic regime using the Z -> b (b) over bar process. The 95% confidence-level upper limit on the cross section for Higgs boson production with transverse momentum above 450 GeV is 115 fb, and above 1 TeV it is 9.6 fb. The Standard Model cross section predictions for a Higgs boson with a mass of 125 GeV in the same kinematic regions are 18.4 fb and 0.13 fb, respectively

Search for neutral long-lived particles in pp collisions at s \sqrt{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(-1) of proton-proton collision data collected by the ATLAS detector at the LHC in 2015-2018 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

Search for heavy particles in the -tagged dijet mass distribution with additional -tagged jets in proton-proton collisions at with the ATLAS experiment

A search optimized for new heavy particles decaying to two b-quarks and produced in association with additional b-quarks is reported. The sensitivity is improved by b-tagging at least one lower-pT jet in addition to the two highest-pT jets. The data used in this search correspond to an integrated luminosity of 103 fb-1 collected with a dedicated trijet trigger during the 2017 and 2018 pffisffi = 13 TeV proton-proton collision runs with the ATLAS detector at the LHC. The search looks for resonant peaks in the b-tagged dijet invariant mass spectrum over a smoothly falling background. The background is estimated with an innovative data-driven method based on orthonormal functions. The observed b-tagged dijet invariant mass spectrum is compatible with the background-only hypothesis. Upper limits at 95% confidence level on a heavy vector-boson production cross section times branching ratio to a pair of b-quarks are derived

Search for Higgs boson decays into a pair of pseudoscalar particles in the final state with the ATLAS detector in collisions at

This paper presents a search for decays of the Higgs boson with a mass of 125 GeV into a pair of new pseudoscalar particles, H -> aa, where one a-boson decays into a b-quark pair and the other into a muon pair. The search uses 139 fb(-1) of proton-proton collision data at a center-of-mass energy root s = 13 TeV recorded between 2015 and 2018 by the ATLAS experiment at the LHC. A narrow dimuon resonance is searched for in the invariant mass spectrum between 16 GeV and 62 GeV. The largest excess of events above the Standard Model backgrounds is observed at a dimuon invariant mass of 52 GeV and corresponds to a local (global) significance of 3.3 sigma (1.7 sigma). Upper limits at 95% confidence level are placed on the branching ratio of the Higgs boson to the bb mu mu final state, beta(H -> aa -> bb mu mu), and are in the range 0.2-4.0 x 10(-4), depending on the signal mass hypothesis

Measurement of the inclusive isolated-photon cross section in pp collisions at √s = 13 TeV using 36 fb⁻¹ of ATLAS data

The differential cross section for isolated-photon production in pp collisions is measured at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC using an integrated luminosity of 36.1 fb−1. The differential cross section is presented as a function of the photon transverse energy in different regions of photon pseudorapidity. The differential cross section as a function of the absolute value of the photon pseudorapidity is also presented in different regions of photon transverse energy. Next-to-leading-order QCD calculations from Jetphox and Sherpa as well as next-to-next-to-leading-order QCD calculations from Nnlojet are compared with the measurement, using several parameterisations of the proton parton distribution functions. The predictions provide a good description of the data within the experimental and theoretical uncertainties. [Figure not available: see fulltext.

Measurements of jet observables sensitive to -quark fragmentation in events at the LHC with the ATLAS detector

Several observables sensitive to the fragmentation of b quarks into b hadrons are measured using 36 fb(-1) of root s = 13 TeV proton-proton collision data collected with the ATLAS detector at the LHC. Jets containing b hadrons are obtained from a sample of dileptonic (tt) over bar events, and the associated set of charged-particle tracks is separated into those from the primary pp interaction vertex and those from the displaced b-decay secondary vertex. This division is used to construct observables that characterize the longitudinal and transverse momentum distributions of the b hadron within the jet. The measurements have been corrected for detector effects and provide a test of heavy-quark-fragmentation modeling at the LHC in a system where the top-quark decay products are color connected to the proton beam remnants. The unfolded distributions are compared with the predictions of several modern Monte Carlo parton-shower generators and generator tunes, and a wide range of agreement with the data is observed, with p values varying from 5 x 10(-4) to 0.98. These measurements complement similar measurements from e(+)e(-) collider experiments in which the b quarks originate from a color singlet Z/gamma*