Leptoquarks and contact interactions at HERA

Search for new physics in e±q contact interactions at HERA

Neutral and charged current cross sections at high [formula omitted]2 at HERA

Physics from the polarized electron or positron beam colliding with the unpolarized proton beam at HERA-II as well as from the unpolarized ep collisions at HERA-I is discussed. Results shown include (i) inclusive neutral and charged current cross sections and structure functions measurements at HERA-I, (ii) the impact of these measurements on parton density functions, on electroweak parameters and searches for new physics and (iii) the first results at HERA-II and its future prospects.

High precision electroweak tests, such as deviations from the Standard Model expectations of the Lepton Flavor Universality (LFU) breaking in K → ℓνℓ (with l = e or μ), represent a powerful tool to test the Standard Model and, hence, to constrain or obtain indirect hints of New Physics beyond it. We explore such a possibility within Supersymmetric theories. Interestingly enough, a process that in itself does not need lepton flavor violation to occur, i.e. the violation of μ−e non-universality in K → ℓνℓ, proves to be quite effective in constraining not only relevant regions of SUSY models where lepton flavor is conserved, but even those where specific lepton flavor violating contributions arise. Indeed, a comparison with analogous bounds coming from τ lepton flavor violating decays shows the relevance of the measurement of RKe/μ = Γ(K → eν)/Γ(K → μν) to probe Lepton Flavor Violation in SUSY. We outline the role and the interplay of the direct New Physics searches at the LHC with the indirect searches performed by LFU tests.

The conservation of lepton flavor is a prediction of the Standard Model and is still an excellent approximate symmetry despite our observation of neutrino oscillations. Lepton flavor violation by one or two units has been discussed for decades, with several dedicated experiments exploring the vast model landscape but no discoveries so far. Here, we explore operators and processes that violate at least one lepton flavor by three units and identify testable signatures. In the Standard Model effective field theory, such operators already arise at mass dimension 7 and can be tested through their contributions to Michel parameters in leptonic decays. True neutrinoless charged-lepton flavor violation arises at mass dimension 10 and can realistically only be seen in the tau decay channels τ→eeeμ¯μ¯ or τ→μμμe¯e¯, for example in Belle II. Testable rates for these tau decays require light new particles and subsequently predict an avalanche of remarkably clean but so-far unconstrained collider signatures.

A study of events with isolated leptons and missing transverse momentum in ep collisions is presented. Within the Standard Model (SM) such topologies are expected mainly from production of real W bosons with subsequent leptonic decay. This thesis continues the analysis of such events done in the HERA-1 period where an excess over the SM prediction was observed for events with high hadronic transverse momentum PTX > 25 GeV. New data of the HERA-2 period are added. The analysed data sample recorded in e+p collisions corresponds to an integrated luminosity of 220 pb-1 which is a factor of two more with respect to the HERA-1 analysis. The e-p data correspond to 186 pb-1 which is a factor of 13 more with respect to HERA-1. All three lepton generations (electrons muons and tau leptons) are analysed. In the electron and muon channels a total of 53 events are observed in 406 pb-1. This compares well to the SM expectation of 53.7 +- 6.5 events, dominated by W production. However a difference in the event rate is observed for different electron beam charges. In e+p data the excess of events with PTX > 25 GeV is sustained, while the e-p data agree with the SM. In the tau channel 18 events are observed in all HERA data, with 20+-3 expected from the SM. The events are dominated by irreducible background from charged currents. The contribution from W production amounts to about 22%. One event with PTX > 25 GeV is observed, where 1.4 +- 0.3 are expected from the SM.

A simultaneous Electroweak and QCD fit of electroweak parameters and parton distribution functions to HERA data on deep inelastic scattering is presented. The input data are neutral current and charged current inclusive cross sections measured by the H1 and ZEUS collaborations at the ep collider HERA. The polarisation of the electron beam was taken into account for the ZEUS and H1 data recorded between 2004 and 2007. Results are presented on the vector and axial-vector couplings of the Z boson to u- and d-type quarks. The values are in agreement with Standard Model predictions. The results on a_u and v_u represent the most precise measurements from a single process.

High-[formula omitted] NC and CC cross sections at HERA and proton structure

We investigate the sensitivity of electron-proton (ep) colliders for charged lepton flavor violation (cLFV) in an effective theory approach, considering a general effective Lagrangian for the conversion of an electron into a muon or a tau via the effective coupling to a neutral gauge boson or a neutral scalar field. For the photon, the Z boson and the Higgs particle of the Standard Model, we present the sensitivities of the LHeC for the coefficients of the effective operators, calculated from an analysis at the reconstructed level. As an example model where such flavor changing neutral current (FCNC) operators are generated at loop level, we consider the extension of the Standard Model by sterile neutrinos. We show that the LHeC could already probe the LFV conversion of an electron into a muon beyond the current experimental bounds, and could reach more than an order of magnitude higher sensitivity than the present limits for LFV conversion of an electron into a tau. We discuss that the high sensitivities are possible because the converted charged lepton is dominantly emitted in the backward direction, enabling an efficient separation of the signal from the background.

Neutral current cross sections in the deep inelastic region

On the determination of the Weinberg angle from neutrino reactions

The problem of generations (families) of fermions remains one of the central problems of elementary particle physics. The standard gauge model SU(3)⊗SU(2)⊗U(1), as well as its ‘vertical’ extensions, based on gauge groups of unification SU(5), SU(10), and so on, are built into one fermion generation. These models do not contain any deep physical causes of the mass hierarchy between generations of fermions and the observed smallness of mixing angles.KeywordsDark MatterYukawa CouplingNeutrino OscillationHide SectorMass HierarchyThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

\n The $$t$$-channel exchange of a first generation leptoquark could contribute to the cross section for $$q\\bar{q} \\rightarrow e^+e^-$$. The leptoquark is off-shell, so this process can be sensitive to leptoquarks beyond the mass reach of pair production searches at the LHC (currently $$m_\\mathrm{LQ}> 830$$ GeV). We attempt to analytically translate ATLAS bounds on $$ (\\bar{q} \\gamma ^\\mu q) (\\bar{e} \\gamma _\\mu e) $$ contact interactions to the various scalar leptoquarks, we but encounter two difficulties: the leptoquark momentum is not negligible, and the leptoquarks do not induce the contact interaction studied by ATLAS, so the interference with the standard model is different. If bounds were quoted on the functional dependence of the cross section on $$\\hat{s}$$, rather than on particular contact interaction models, these difficulties could be circumvented. We use the results of such a “form factor” fit to CMS plots to obtain bounds on the various leptoquarks’ quark–lepton coupling of order $$\\lambda ^2 \\lesssim (m_\\mathrm{LQ}/3$$ TeV)$$^2$$.\n

Deep inelastic scattering cross section measurements previously published by the H1 and ZEUS collaborations are combined. The procedure takes into account the systematic error correlations in a coherent approach, leading to a significantly reduced overall cross section uncertainty by cross calibrating the various data sets. The analysis is based on data with momentum transfers Q2 > 0.045 GeV2 collected by the H1 and ZEUS collaborations between the years 1995 and 2000. This combined HERA-I data set, of neutral and charged current inclusive cross sections for e+p and e-p scattering, is used as the sole input for a next-to-leading order (NLO) QCD parton distribution function (PDF) fit. The consistent treatment of systematic uncertainties in the joint data set ensures that experimental uncertainties on the PDFs can be calculated without need for an increased χ2 tolerance. This results in PDFs with greatly reduced experimental uncertainties compared to the separate analyses of the ZEUS and H1 experiments. Model uncertainties, including those arising from parametrization dependence, are also carefully considered.

We report the result of a search for $\tau^\pm\rightarrow\ell^\pm\gamma$ ($\ell=\mu,e$) using the full data sample at Belle. Since the observation of neutrino oscillations has unambiguously shown that the lepton flavor is no longer conserved, we can expect lepton flavor violation (LFV) in the charged lepton sector. Though the standard model (SM) does not predict charged LFV decays at an observable rate, $\tau^\pm\rightarrow\ell^\pm\gamma$ is predicted by many new physics scenarios and is thus one of the most promising LFV modes. Consequently, we have obtained the most stringent limit on the branching fraction of $\tau^\pm\rightarrow\mu^\pm\gamma$. In addition, we report the result of a search for tau electric dipole moment (EDM) evaluating $u$ vertex coupling using the full data sample at Belle. At present, the observed CP violation is insufficient to explain the prevalent matter-antimatter asymmetry in our universe. On the other hand, the EDM of leptons is predicted to be negligibly small in the SM and is expected as a source of CPV in the lepton sector induced by some new physics. We have obtained one order more sensitive result both for the real and imaginary parts of the $\tau$ EDM.