Suat Baris Tuncay

Abstract:

This paper presents the first differential fiducial measurements of γγ → ττ using 1.93 nb−1 of Pb+Pb data at sNN=5.02$$ \sqrt{s_{\mathrm{NN}}}=5.02 $$ TeV recorded by the ATLAS detector. Events in which one of the τ -leptons decays into a muon and two neutrinos τ→ντν¯μμ$$ \tau \to {\nu}_{\tau }{\overline{\nu}}_{\mu}\mu $$ are selected and are categorized into three regions by the presence of an electron or either one or three charged-particle track(s) from the second τ-lepton decay. The measurement is performed in events where both Pb ions remain intact and no neutrons are emitted. Differential cross-sections are measured for seven variables in three fiducial regions at particle level. The measurements are compared to theory predictions with different photon flux models and spin correlation effects. For the fiducial region with one muon and one electron in the final state, comparisons to next-to-leading-order electroweak predictions are also made. The transverse momentum (pT) of the decay muon, the pT of the visible decay particles of the other τ-lepton, the total pT, invariant mass, and pseudorapidity of the visible particles from the di-τ system, and the rapidity and acoplanarity of the visible decay particles from either τ-lepton are measured. A maximum-likelihood fit to the muon transverse-momentum distributions in the three regions before unfolding is performed to extract the τ-lepton anomalous magnetic moment aτ and electric dipole moment dτ, the latter for the first time in heavy ion collisions. The observed 95% confidence level intervals are −0.057 < aτ< 0.035 and |dτ| < 2.7 × 10−16ecm.

Abstract:

This paper presents a measurement of the production cross-section of a W boson associated with one or two jets, where at least one jet originates from a b-quark. The data are collected with the ATLAS detector at the LHC in proton-proton collisions at s=13$$ \sqrt{s}=13 $$ TeV, corresponding to an integrated luminosity of 140 fb-1. Differential cross-sections are presented as a function of the transverse momentum of the b-jet for both the electron and muon decay modes of the W boson. The results corrected for all detector effects are presented in a fiducial region defined with basic lepton and jet kinematics, and compared with theoretical predictions. The combined fiducial cross-section of W + b-jets for the electron and muon channels is measured to be 16.6 ± 1.9 pb, which is consistent with the next-to-leading-order QCD prediction of 16.8 ± 2.3 pb. The relative precision of this measurement is improved by approximately a factor of two compared with the previous ATLAS measurement at s=7$$ \sqrt{s}=7 $$ TeV.

Abstract:

The first observation of a new meson state decaying into $B_c^{+}$ and a photon is presented using a dataset of $pp$ collisions at a center-of-mass energy $\sqrt{s}=13\mathrm{TeV}$ collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of $140{\mathrm{fb}}^{-1}$ . The $B_c^{+}$ mesons are reconstructed from a three-muon final state from $B_c^{+}\to J/\psi ({\mu }^{+}{\mu }^{-}){\mu }^{+}{\nu }_{\mu }X$ decays, along with photons reconstructed via conversions to electron-positron pairs in the detector material. The new state is observed with a significance that exceeds 8 standard deviations. The mass difference between the new meson state and the ground-state $B_c^{+}$ meson is measured to be $64.5\pm 1.4(\mathrm{stat}){}_{-1.4}^{+1.0}(\mathrm{syst})\mathrm{MeV}$ . This corresponds to a mass for the observed state of $6339.0\pm 1.4(\mathrm{stat}){}_{-1.4}^{+1.0}(\mathrm{syst})\pm 0.3(m_{B_c^{+}})\mathrm{MeV}$ , where the last uncertainty is due to the precision of the best knowledge of the $B_c^{+}$ meson mass. The low mass difference value matches the theory expectations for the lowest vector state of the $\overline{b}c$ system, the $B_c^{*+}$ meson.

Abstract:

A search for leptoquark signals is performed in final states with a hadronically decaying τ-lepton and missing transverse momentum, using data from proton–proton collisions at a centre-of-mass energy of s = 13 TeV recorded by the ATLAS detector at the Large Hadron Collider during Run 2 (2015–18), corresponding to an integrated luminosity of 140 fb − 1 . The analysis is designed to probe both resonant production and non-resonant t-channel exchange of the leptoquark, covering a wide range of coupling scenarios. No excess above the Standard Model background prediction is observed. Limits are set on the couplings in the benchmark U 1 vector-leptoquark model at 95% confidence level for masses between 1.5 TeV and 3.0 TeV.

Abstract:

This Letter presents a combination of searches for Higgs boson pair ( $HH$ ) production performed by the ATLAS and CMS Collaborations using proton-proton collision datasets recorded at $\sqrt{s}=13\mathrm{TeV}$ during the Large Hadron Collider Run 2, corresponding to integrated luminosities ranging between 126 and $140{\mathrm{fb}}^{-1}$ . The upper limit at the 95% confidence level on the total $HH$ signal strength, defined as the ratio of the measured cross section to the standard model (SM) prediction, corresponds to 2.5, with an expected value of 1.7 (2.8) assuming the absence (presence) of the SM $HH$ signal. The strength of the $HH$ signal is measured to be ${0.8}_{-0.7}^{+0.9}$ relative to the SM prediction. The observed significance is found to be 1.1 standard deviations whereas 1.3 are expected for the SM $HH$ signal. Constraints are set on the Higgs boson trilinear self-coupling and on the couplings of two Higgs bosons to two vector bosons, both normalized to the SM predictions and denoted as ${\kappa }_{\lambda }$ and ${\kappa }_{2V}$ , respectively. The observed individual constraints at the 95% confidence level are $-0.71<{\kappa }_{\lambda }<6.1$ and $0.73<{\kappa }_{2V}<1.3$ , while the expected constraints assuming the presence of the SM $HH$ signal are $-1.3<{\kappa }_{\lambda }<6.7$ and $0.66<{\kappa }_{2V}<1.4$ .