[PDF] Search For Lepton Flavour Violating Decays Of The Higgs Boson To E Tau And E Mu In Proton Proton Collisions At Sqrts eBook
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A direct search for lepton flavour violating decays of the Higgs boson (H) in the H to e tau and H to e mu channels is described. The data sample used in the search was collected in proton-proton collisions at sqrt(s)=8 TeV with the CMS detector at the LHC and corresponds to an integrated luminosity of 19.7 inverse femtobarns. No evidence is found for lepton flavour violating decays in either final state. Upper limits on the branching fractions, B(H to e tau)
Abstract: This thesis presents a direct search for lepton-flavour violating (LFV) decays of the Higgs boson using data from proton-proton collisions at a center-of-mass energy of 13 TeV delivered by the Large Hadron Collider (LHC) and recorded by the ATLAS detector at CERN. The dataset corresponds to an integrated luminosity of 138.4 1/fb collected during Run 2 of the LHC which lasted from 2015 to 2018. The search in this thesis is focused on the decays H -> e tau and H -> mu tau. Only leptonic decays of the tau-lepton leading to different-flavour final states are considered in order to suppress background contributions from Drell-Yan production. Two signal regions (VBF SR and nonVBF SR) are defined, targeting different production processes of the Higgs boson thereby allowing for exploiting the characteristic topology of the vector-boson-fusion (VBF) production. Individual neural networks are trained in each of the two signal regions aiming at classifying each collision event to be signal or background. This is achieved by assigning a probability value to each event quantifying how signal-like it is. The resulting probability distribution is used as final discriminant in the statistical analysis. For background estimation, the so-called "Symmetry Method" is used which exploits the fact that Standard-Model processes are symmetric with respect to an exchange of electrons with muons and vice versa at the high energies prevalent at the LHC. The two LFV decays H -> e tau and H -> mu tau, on the contrary, break this symmetry as long as their branching ratios (BR) are of different size since systematic differences of the kinematic properties of the electron and muon are implied within each decay. The differing branching ratios are motivated by a bound on the product of the two LFV couplings derived from mu -> e gamma. The usage of the "Symmetry Method" allows for an estimation of the background mainly based on data and hence reduces the dependence on simulated events. However, it also implies that the analysis is only sensitive to the difference in branching ratios of H -> e tau and H -> mu tau and does not allow for measuring their absolute values. The difference in branching ratios, Delta = BR(H -> mu tau) - BR(H -> e tau), is measured to be (0.25+/-0.10)% which indicates that the data favours a larger branching ratio for the decay H -> mu tau than for the decay H -> e tau with a significance of 2.5 sigma. An upper limit at 95% confidence level of 0.42% is set on the difference Delta, the expected limit is (0.19+0.08/-0.05)%. Combinations with partner analyses which use templates from simulations for background estimation are performed, assuming the respective other LFV signal to be zero. Observed (expected) upper limits at 95% confidence level on the branching ratios of the decays H -> e tau and H -> mu tau are derived to be 0.23% ((0.12+0.05/-0.03)%) and 0.17% ((0.09+0.04/-0.03)%), respectively
This thesis presents the first direct search for lepton flavor violating decays of the recently discovered Higgs boson. The search is performed in the $H \rightarrow \mu \tau_{e}$ and $H \rightarrow \mu \tau_{h}$ decay channels, where $\tau_{e}$ and $\tau_{h}$ denote the electronic and hadronic tau decay channels respectively. The search is performed using the proton-proton collisions data gathered at the Compact Muon Solenoid detector at the Large Hadron Collider. The search uses 19.7 fb$^{-1}$ of $\sqrt{s}$ = 8 TeV data taken in 2012 as well as 2.3 fb$^{-1}$ of $\sqrt{s}$ = 13 TeV data taken in 2015. The limits on the branching fraction $B(H\rightarrow \mu\tau)$ are measured, and the existing limits from indirect searches are reduced by an order of magnitude. This thesis also presents a measurement of the differential cross section of a W boson produced in associated with jets. This measurement was performed with 2.5 fb$^{-1}$ of $\sqrt{s}$ = 13 TeV proton-proton collision data produced by the Large Hadron Collider in 2015 and recorded at the Compact Muon Solenoid detector. Cross sections are computed as a function of jet multiplicity, and the results are compared to theoretical predictions from leading order and next-to-leading order Monte Carlo generators.
This thesis presents the first direct search for lepton flavor violating decays of the recently discovered Higgs boson. The search is performed in the $H \rightarrow \mu \tau_{e}$ and $H \rightarrow \mu \tau_{h}$ decay channels, where $\tau_{e}$ and $\tau_{h}$ denote the electronic and hadronic tau decay channels respectively. The search is performed using the proton-proton collisions data gathered at the Compact Muon Solenoid detector at the Large Hadron Collider. The search uses 19.7 fb$^{-1}$ of $\sqrt{s}$ = 8 TeV data taken in 2012 as well as 2.3 fb$^{-1}$ of $\sqrt{s}$ = 13 TeV data taken in 2015. The limits on the branching fraction $B(H\rightarrow \mu\tau)$ are measured, and the existing limits from indirect searches are reduced by an order of magnitude. This thesis also presents a measurement of the differential cross section of a W boson produced in associated with jets. This measurement was performed with 2.5 fb$^{-1}$ of $\sqrt{s}$ = 13 TeV proton-proton collision data produced by the Large Hadron Collider in 2015 and recorded at the Compact Muon Solenoid detector. Cross sections are computed as a function of jet multiplicity, and the results are compared to theoretical predictions from leading order and next-to-leading order Monte Carlo generators.
The first direct search for lepton-flavour-violating decays of the recently discovered Higgs boson (H) is described. The search is performed in the H 2![mu][tau]e and H2!mu][tau]h channels, where [tau]e and [tau]h are tau leptons reconstructed in the electronic and hadronic decay channels, respectively. The data sample used in this search was collected in pp collisions at a centre-of-mass energy of [arrow]" = 8 TeV with the CMS experiment at the CERN LHC and corresponds to an integrated luminosity of 19.7 fb−1. The sensitivity of the search is an order of magnitude better than the existing indirect limits. A slight excess of signal events with a significance of 2.4 standard deviations is observed. The p-value of this excess at MH=125 GeV is 0.010. The best fit branching fraction is [Beta](H 2![mu][tau])=(0.84+0.39-0.37)%. A constraint on the branching fraction, [Beta](H 2![mu][tau])
A direct search for lepton-flavour-violating H → [mu][tau] decays of the recently discovered Higgs boson with the ATLAS detector at the LHC is presented. The analysis is performed in the H → [mu][tau] had channel, where [tau] had is a hadronically decaying [tau] -lepton. The search is based on the data sample of proton-proton collisions collected by the ATLAS experiment corresponding to an integrated luminosity of 20.3 fb-1 at a centre-of-mass energy of √s = 8 TeV. No statistically significant excess of data over the predicted background is observed. As a result, the observed (expected) 95% confidence-level upper limit on the branching fraction, Br( H → [mu][tau] ), is 1.85% (1.24%).
This dissertation presents a search for lepton-flavor-violating (LFV) decays of the 125 GeV Higgs boson in final states of \mu\tau_{\mathrm{had}} and e\tau_{\mathrm{had}} in the full dataset collected by the ATLAS detector in 2012. The search is based on data samples of proton--proton collisions at \sqrt{s} = 8 TeV, corresponding to an integrated luminosity of 20.3 fb^{-1}. The topology of LFV events is exploited through an optimized selection of objects and events, with signal and control regions defined by event kinematics. A binned likelihood fit searching for the presence of LFV Higgs decays is performed using the reconstructed Higgs boson mass as the discriminating variable. No such decays are observed. The expected (observed) upper limits at 95% confidence on the branching ratios for the muon and electron final states are found to be Br(H \rightarrow \mu\tau) = 1.24% (1.85%) and Br(H \rightarrow e\tau) = 2.07% (1.81%).
Abstract: This Letter presents direct searches for lepton flavour violation in Higgs boson decays, and , performed with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy TeV, corresponding to an integrated luminosity of . No significant excess is observed above the expected background from Standard Model processes. The observed (median expected) 95% confidence-level upper limits on the lepton-flavour-violating branching ratios are 0.47% () and 0.28% () for and , respectively
We present a search for high-mass resonances decaying into two leptons of different flavor: e[mu], e[tau], and [mu][tau]. These resonances are predicted by several models beyond the standard model, such as the R-parity-violating MSSM. The search is based on 1 fb−1 of data collected at the Collider Detector at Fermilab (CDF II) in proton anti-proton collisions. Our observations are consistent with the standard model expectations. The results are interpreted to set 95% C.L. upper limits on [sigma] x BR of {tilde {nu}}{sub [tau]} → e[mu], e[tau], [mu]{tau}.