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Register a new userQED radiative corrections and their impact on H -> tau tau searches at the LHC
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In this paper we show that the excess of the tau tau events with respect to the Standard Model background predictions, observed by the ATLAS and CMS collaborations and interpreted as the evidence of the Higgs-boson decay into a pair of tau-leptons, may be accounted for by properly taking into account QED radiative corrections in the modelling of the Z/gamma* -> tau tau background.
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In phenomenological preparation for new measurements one searches for the carriers of quality signatures. Often, the first approach quantities may be difficult to measure or to provide sufficiently precise predictions for comparisons. Complexity of necessary details grow with precision. To achieve the goal one can not break the theory principles, and take into account effects which could be ignored earlier. Mixed approach where dominant effects are taken into account with intuitive even simplistic approach was developed. Non dominant corrections were controlled with the help of Monte Carlo simulations. Concept of Optimal Variables was successfully applied for many measurements. New techniques, like Machine Learning, offer solutions to exploit multidimensional signatures. Complementarity of these new and old approaches is studied for the example of Higgs Boson CP-parity measurements in H to tau^+tau^-, tau^pm to nu (3pi)^pm cascade decays.
We study the effective field theory sensitivity of an LHC analysis for the $tau u$ final state with an associated b-jet. To illustrate the improvement due to the b-tagging, we first recast the recent CMS analysis in the $tau u$ channel, using an integrated luminosity of 35.9 fb$^{-1}$ at $sqrt{s}=13$ TeV, and provide limits on all the dimension-six effective operators which contribute to the process. The expected limits from the b-tagged analysis are then derived and compared. We find an improvement of approximately $sim 30%$ in the bounds for operators with a b quark. We also discuss in detail possible angular observables to be used as a discriminator between dimension-six operators with different Lorentz structure. Finally, we study the impact of these limits on some simplified scenarios aimed at addressing the observed deviations from the Standard Model in lepton flavor universality ratios of semileptonic B-meson decays. In particular, we compare the collider limits on those scenarios set by our analysis either with or without the b-tagging, assuming an integrated luminosity of 300 fb$^{-1}$, with relevant low-energy flavor measurements.
We consider the possibility of studying new physics that singles out the tau-lepton at the LHC. We concentrate on the tau-lepton charge asymmetry in tau+tau- pair production as a tool to probe this physics beyond the Standard Model. We consider two generic scenarios for the new physics. We first study a non-universal Z boson as an example of a new resonance that can single out tau-leptons. We then consider vector lepto-quarks coupling of the first generation quarks with the third generation leptons as an example of non-resonant new physics. We find that in both cases the charge asymmetry can be sufficiently sensitive to the new physics to provide useful constraints at the LHC.
We consider the radiative corrections to the impact factors of electron and photon. According to a generalized eikonal representation the ebar e scattering amplitude at high energies and fixed momentum transfers is proportional to the electron form factor. But we show that this representation is violated due to the presence of non-planar diagrams. One loop correction to the photon impact factor for small virtualities of the exchanged photon is obtained using the known results for the cross section of the ebar e production at photon-nuclei interactions.
The magnetic moment ($a_gamma$) and weak magnetic moment ($a_W$) of charged leptons and quarks are sensitive to quantum effects of new physics heavy resonances. In effective field theory $a_gamma$ and $a_W$ are induced by two independent operators, therefore, one has to measure both the $a_gamma$ and $a_W$ to shed lights on new physics. The $a_W$s of the SM fermions are measured at the LEP. In this work, we analyze the contributions from magnetic and weak magnetic moment operators in the processes of $ppto H gamma$ and $ggto H to tau^+ tau^- gamma$ at the High-Luminosity Large Hadron Collider. We demonstrate that the two processes could cover most of the parameter space that cannot be probed at the LEP.
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