Study of the rare decays of B 0 s and B 0 mesons into muon pairs using data collected during 2015 and 2016 with the ATLAS detector The ATLAS Collaboration A study of the decays B 0 s → µ + µ − and B 0 → µ + µ − has been performed using 26.3 fb −1 of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016. Since the detector resolution in µ + µ − invariant mass is comparable to the B 0 s -B 0 mass difference, a single fit determines the signal yields for both decay modes. This results in a measurement of the branching fraction B(B 0 s → µ + µ − ) = 3.2 +1.1 −1.0 × 10 −9 and an upper limit B(B 0 → µ + µ − ) < 4.3 × 10 −10 at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding B(B 0 s → µ + µ − ) = 2.8 +0.8 −0.7 ×10 −9 and B(B 0 → µ + µ − ) < 2.1×10 −10 at 95% confidence level. The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the B(B 0 → µ + µ − )-B(B 0 s → µ + µ − ) plane.

A measurement of the total pp cross section at the LHC at √ s = 7 TeV is presented. In a special run with high-β beam optics, an integrated luminosity of 80 µb −1 was accumulated in order to measure the differential elastic cross section as a function of the Mandelstam momentum transfer variable t. The measurement is performed with the ALFA sub-detector of ATLAS. Using a fit to the differential elastic cross section in the |t| range from 0.01 GeV 2 to 0.1 GeV 2 to extrapolate to |t| → 0, the total cross section, σ tot (pp → X), is measured via the optical theorem to be:where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last is related to uncertainties in the extrapolation to |t| → 0. In addition, the slope of the elastic cross section at small |t| is determined to be B = 19.73 ± 0.14 (stat.) ± 0.26 (syst.) GeV −2 .

The ATLAS detector at the Large Hadron Collider is used to search for the lepton flavor violating process Z → eμ in pp collisions using 20.3 fb −1 of data collected at ffiffi ffi s p ¼ 8 TeV. An enhancement in the eμ invariant mass spectrum is searched for at the Z-boson mass. The number of Z bosons produced in the data sample is estimated using events of similar topology, Z → ee and μμ, significantly reducing the systematic uncertainty in the measurement. There is no evidence of an enhancement at the Z-boson mass, resulting in an upper limit on the branching fraction, BðZ → eμÞ < 7.5 × 10 −7 at the 95% confidence level. DOI: 10.1103/PhysRevD.90.072010 PACS numbers: 12.60.-i I. INTRODUCTIONLepton flavor conservation in the charged lepton sector is a fundamental assumption of the Standard Model (SM) but there is no associated symmetry. Thus, searches for lepton flavor violation (LFV) processes are good candidates for probing new physics. The observation of neutrino oscillations is a clear indication of LFV in the neutral lepton sector; however, such an oscillation mechanism cannot induce observable LFV in the charged lepton sector. All searches in the charged lepton sector have produced null results so far [1]. Lepton flavor violation in the charged lepton sector may have a different origin than LFV induced by neutrino oscillations and the search for this effect provides constraints on theories beyond the SM (see for example Refs. [2][3][4]).In this paper, a search for the lepton flavor violating decay Z → eμ is presented. There are stringent experimental limits on other charged lepton flavor violating processes, which can be used to derive an upper limit on the branching fraction for Z → eμ with some theoretical assumptions. For example, the upper limit on μ → 3e yields BðZ → eμÞ < 10 −12 [5] and on μ → eγ yields BðZ → eμÞ < 10 −10 [6]. The experiments at the Large Electron-Positron Collider (LEP) searched directly for the decay Z → eμ [7-10]. The most stringent upper limit is BðZ → eμÞ < 1.7 × 10 −6 at the 95% confidence level (C.L.) using a data sample of 5.0 × 10 6 Z bosons produced in e þ e − collisions at ffiffi ffi s p ¼ 88-94GeV [7]. The Large Hadron Collider (LHC) has already produced many more Z bosons in pp collisions, but with substantially more background. In this paper, the 20.3 AE 0.6 fb −1 [11] of data collected at ffiffi ffi s p ¼ 8 TeV by the ATLAS experiment corresponds to 7.8 × 10 8 Z bosons produced. Despite the larger background at the LHC, a more restrictive direct limit on the Z → eμ decay is reported in this paper. II. ATLAS DETECTORThe ATLAS detector [12] consists of an inner detector (ID) surrounded by a solenoid that produces a 2 T magnetic field, electromagnetic and hadronic calorimeters, and a muon spectrometer (MS) immersed in a magnetic field produced by a system of toroids. The ID measures the trajectories of charged particles over the full azimuthal angle and in a pseudorapidity [13] range of jηj < 2.5 using silicon pixel, silicon microstrip, and straw-tube transitionradiation ...

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