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Register a new userThe photon PDF from high-mass Drell Yan data at the LHC
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Achieving the highest precision for theoretical predictions at the LHC requires the calculation of hard-scattering cross-sections that include perturbative QCD corrections up to (N)NNLO and electroweak (EW) corrections up to NLO. Parton distribution functions (PDFs) need to be provided with matching accuracy, which in the case of QED effects involves introducing the photon parton distribution of the proton, $xgamma(x,Q^2)$. In this work a determination of the photon PDF from fits to recent ATLAS measurements of high-mass Drell-Yan dilepton production at $sqrt{s}=8$ TeV is presented. This analysis is based on the xFitter framework, and has required improvements both in the APFEL program, to account for NLO QED effects, and in the aMCfast interface to account for the photon-initiated contributions in the EW calculations within MadGraph5_aMC@NLO. The results are compared with other recent QED fits and determinations of the photon PDF, consistent results are found.
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We analyze the Drell-Yan lepton pair production at forward rapidity at the Large Hadron Collider. Using the dipole framework for the computation of the cross section we find a significant suppression in comparison to the collinear factorization formula due to saturation effects in the dipole cross section. We develop a twist expansion in powers of Q_s^2/M^2 where Q_s is the saturation scale and M the invariant mass of the produced lepton pair. For the nominal LHC energy the leading twist description is sufficient down to masses of 6 GeV. Below that value the higher twist terms give a significant contribution.
Spectral features in LHC dileptonic events may signal radiative corrections coming from new degrees of freedom, notably dark matter and mediators. Using simplified models, we show how these features can reveal the fundamental properties of the dark sector, such as self-conjugation, spin and mass of dark matter, and the quantum numbers of the mediator. Distributions of both the invariant mass $m_{ell ell}$ and the Collins-Soper scattering angle $costheta_{CS}$ are studied to pinpoint these properties. We derive constraints on the models from LHC measurements of $m_{ell ell}$ and $costheta_{CS}$, which are competitive with direct detection and jets + Missing Energy searches. We find that in certain scenarios the $costheta_{CS}$ spectrum provides the strongest bounds, underlying the importance of scattering angle measurements for non-resonant new physics.
We investigate the impact of high-statistics Drell-Yan (DY) measurements at the LHC on the study of non-perturbative QCD effects from parton distribution functions (PDF). We present the results of a PDF profiling analysis based on the neutral-current DY forward-backward asymmetry, using the open source fit platform xFitter.
We study the impact of the inclusion of Neutral Current (NC) DY data from LHC mapped in the Forward-Backward Asymmetry ($A_{rm FB}$) observable on PDF uncertainties, using the open source platform texttt{xFitter}. We find that $A_{rm FB}$ enables new PDF sensitivity at current and future luminosity stages of LHC.
We consider a four site Higgsless model based on the $SU(2)_Ltimes SU(2)_1times SU(2)_2times U(1)_Y$ gauge symmetry, which predicts two neutral and four charged extra gauge bosons, $Z_{1,2}$ and $W^pm_{1,2}$. We compute the properties of the new particles, and derive indirect and direct limits on their masses and couplings from LEP and Tevatron data. In contrast to other Higgsless models, characterized by fermiophobic extra gauge bosons, here sizeable fermion-boson couplings are allowed by the electroweak precision data. The prospects of detecting the new predicted particles in the favoured Drell-Yan channel at the LHC are thus investigated. The outcome is that all six extra gauge bosons could be discovered in the early stage of the LHC low-luminosity run.
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