Search for the lepton flavor violating decayZ→eμinppcollisions ats

Abstract

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 ...

Search for Lepton Flavor Violation in Z and Higgs decays with the CMS Experiment

“…Given these numbers, an observed (expected) upper limit on the branching fraction of B(Z → eµ) < 7.3 · 10 −7 (6.7 +2.8 −2.0 · 10 −7 ) at 95% confidence level (CL) is obtained. This result is similar to that obtained by the ATLAS Collaboration [10].…”

Impact of the top quark cross-section data on parton distribution functions

“…The recent measurements of tt pair production cross section by ATLAS [21][22][23]31] and CMS [24][25][26][27][28][29]32] collaborations at LHC and DØ [30] collaboration at Tevatron, are considered in the present study. The ATLAS experiment [21] at LHC has measured the tt production cross section in events containing an opposite-charge electron-µ (eµ) pair.…”

“…The recent measurements of the total tt pair production cross section are reported by ATLAS [21][22][23] and CMS [24-29] collaborations at LHC, and DØ [30] collaboration at Tevatron. The measurements of the differential tt cross section is provided by ATLAS [31] and CMS [32] collaborations at LHC. Both experimental measurements are used in this analysis to constrain the PDF's.…”

“…The central values and theoretical uncertainties for CT14, MMHT2014 and NNPDF3.0 are not all close to each other, except in the certain regions of x. The goal of this analysis is finding the impact of the new measurements of the production cross section of the top quark pair (tt) [21][22][23][24][25][26][27][28][29][30][31][32] on the modern CT14 [4], NNPDF3.0 [6] and MMHT2014 [5] PDF sets using the Hessian profiling technique [37], without need of having a complete baseline global PDF fit procedure.…”

Searches for new phenomena in multilepton final states in ATLAS and CMS

“…Many new physics models, for e.g., Dirac or Majorana neutrinos, R-parity violating SUSY can increase the branching fraction of lepton flavour violating Z decays which can be detected at LHC [10,11]. Both ATLAS [12] and CMS [13] searched for oppositely charged eµ pairs consistent with the Z mass. Dominant background to this analysis is Z → ττ.…”