Search

This is the third out of five chapters of the final report [1] of theWorkshop on Physics at HL-LHC, and perspectives on HE-LHC [2]. It is devoted tothe study of the potential, in the search for Beyond the Standard Model (BSM)physics, of the High Luminosity (HL) phase of the LHC, defined as$3~\mathrm{ab}^{-1}$ of data taken at a centre-of-mass energy of$14~\mathrm{TeV}$, and of a possible future upgrade, the High Energy (HE) LHC,defined as $15~\mathrm{ab}^{-1}$ of data at a centre-of-mass energy of$27~\mathrm{TeV}$. We consider a large variety of new physics models, both in asimplified model fashion and in a more model-dependent one. A long list ofcontributions from the theory and experimental (ATLAS, CMS, LHCb) communitieshave been collected and merged together to give a complete, wide, andconsistent view of future prospects for BSM physics at the consideredcolliders. On top of the usual standard candles, such as supersymmetricsimplified models and resonances, considered for the evaluation of futurecollider potentials, this report contains results on dark matter and darksectors, long lived particles, leptoquarks, sterile neutrinos, axion-likeparticles, heavy scalars, vector-like quarks, and more. Particular attention isplaced, especially in the study of the HL-LHC prospects, to the detectorupgrades, the assessment of the future systematic uncertainties, and newexperimental techniques. The general conclusion is that the HL-LHC, on top ofallowing to extend the present LHC mass and coupling reach by $20-50\%$ on mostnew physics scenarios, will also be able to constrain, and potentiallydiscover, new physics that is presently unconstrained. Moreover, compared tothe HL-LHC, the reach in most observables will generally more than double atthe HE-LHC, which may represent a good candidate future facility for a finaltest of TeV-scale new physics.

We study the influence of theoretical systematic uncertainties due to the quark density on LHC experimental searches for Z ′ -bosons. Using an approach originally proposed in the context of the ABMP16 PDF set for the high- x behaviour of the quark density, we present results on observables commonly used to study Z ′ signals in dilepton channels. [ABSTRACT FROM AUTHOR]

Charged lepton pairs are produced copiously in high-energy hadron collisions via electroweak gauge boson exchange, and are one of the most precisely measured final states in proton-proton collisions at the Large Hadron Collider (LHC). We propose that measurements of lepton angular distributions can be used to improve the accuracy of theoretical predictions for Higgs boson production cross sections at the LHC. To this end, we exploit the sensitivity of the lepton angular coefficient associated with the longitudinal Z -boson polarization to the parton density function (PDF) for gluons resolved from the incoming protons, in order to constrain the Higgs boson cross section from gluon fusion processes. By a detailed numerical analysis using the open-source platform xFitter, we find that high-statistics determinations of the longitudinally polarized angular coefficient at the LHC Run III and high-luminosity HL-LHC improve the PDF systematic uncertainties of the Higgs boson cross section predictions by 50% over a broad range of Higgs boson rapidities. [ABSTRACT FROM AUTHOR]

Precision determinations of Standard Model (SM) Electro-Weak (EW) parameters at the Large Hadron Collider (LHC) are dominated by uncertainties due to Parton Distribution Functions (PDFs). Reweighting and profiling techniques are routinely employed to treat this. We explore approaches based on combining measurements of charged current and neutral current Drell-Yan (DY) asymmetries to improve PDF uncertainties. We present the results of a numerical analysis performed with the open-source platform xFitter. PDF uncertainties are examined for lepton-charge and forward-backward asymmetries in regions of transverse and invariant masses near the vector-boson peak, based on LHC Run III and HL-LHC luminosity scenarios. We discuss the complementarity of the asymmetries in reducing PDF uncertainties in observables relevant to both SM and Beyond the SM (BSM) physics. [ABSTRACT FROM AUTHOR]

Di-lepton searches for Beyond the Standard Model (BSM) Z ′ bosons that rely on the analysis of the Breit-Wigner (BW) line shape are appropriate in the case of narrow resonances, but likely not sufficient in scenarios featuring Z ′ states with large widths. Conversely, alternative experimental strategies applicable to wide Z ′ resonances are much more dependent than the default bump search analyses on the modelling of QCD higher-order corrections to the production processes, for both signal and background. For heavy Z ′ boson searches in the di-lepton channel at the CERN Large Hadron Collider (LHC), the transverse momentum q T of the di-lepton system peaks at q T ≲ 10 − 2 M l l , where M l l is the di-lepton invariant mass. We exploit this to treat the QCD corrections by using the logarithmic resummation methods in M l l / q T to all orders in the strong coupling constant α s. We carry out studies of Z ′ states with large width at the LHC by employing the program reSolve, which performs QCD transverse momentum resummation up to Next-to-Next-to-Leading Logarithmic (NNLL) accuracy. We consider two benchmark BSM scenarios, based on the Sequential Standard Model (SSM) and dubbed 'SSM wide' and 'SSM enhanced'. We present results for the shape and size of Z ′ boson signals at the differential level, mapped in both cross section (σ) and Forward-Backward Asymmetry (A FB), and perform numerical investigations of the experimental sensitivity at the LHC Run 3 and High-Luminosity LHC (HL-LHC). [ABSTRACT FROM AUTHOR]