Higgs and top physics reconstruction challenges and opportunities at FCC-ee

The Higgs bosons and the top quark decay into rich and diverse final states, containing both light and heavy quarks, gluons, photons as well as W and Z bosons. This article reviews the challenges involved in reconstructing Higgs and top events at the FCC-ee and identifies the areas where novel developments are needed. The precise identification and reconstruction of these final states at the FCC-ee rely on the capability of the detector to provide excellent flavour tagging, jet energy and angular resolution, and global kinematic event reconstruction. Excellent flavour tagging performance requires low-material vertex and tracking detectors, and advanced machine learning techniques as successfully employed in LHC experiments. In addition, the Z pole run will provide abundant samples of heavy flavour partons that can be used for calibration of the tagging algorithms. For the reconstruction of jets, leptons, and missing energy, particle-flow algorithms are crucial to explore the full potential of the highly granular tracking and calorimeter systems, and give access to excellent energy–momentum resolution and precise identification of heavy bosons in their hadronic decays. This enables, among many other key elements, the reconstruction of Higgsstrahlung processes with leptonically and hadronically decaying Z bosons, and an almost background-free identification of top quark pair events. Exploiting the full available kinematic constraints together with exclusive jet clustering algorithms will allow for the optimisation of global event reconstruction with kinematic fitting techniques.

Polarized Z bosons from the decay of a Higgs boson produced in association with two jets at the LHC

Investigating the polarization of weak bosons provides an important probe of the scalar and gauge sector of the Standard Model. This can be done in the Higgs decay to four leptons, whose Standard-Model leading-order amplitude enables to generate polarized observables from unpolarized ones via a fully-differential reweighting method. We study the $$\text {Z} $$ Z -boson polarization from the decay of a Higgs boson produced in association with two jets, both in the gluon-fusion and in the vector-boson fusion channel. We also address the possibility of extending the results of this work to higher orders in perturbation theory.

Studies on dimension-eight genuine Quartic Boson Coupling operators

Vector Boson Scattering (VBS) processes provide a great source of information on the structure of the Quartic Gauge Boson Couplings (QGCs). The Standard Model allows self interactions of the charged vector gauge bosons, although vertices with neutral-only bosons are forbidden. In this paper we use Monte Carlo samples containing VBS events with two Z-bosons in association with two jets, and we present preliminary studies for the setting of constraints on anomalous quartic couplings. In these studies we investigate typical kinematic variables and we classify them according to their sensitivity to aQGC effects. Finally, we evaluate the cross-section enhancement by each one of the dimension-eight QGC operators in the ZZjj channel.

NLO EW and QCD corrections to polarized ZZ production in the four-charged-lepton channel at the LHC

Measuring the polarization of electroweak bosons at the LHC allows for important tests of the electroweak-symmetry-breaking mechanism that is realized in nature. Therefore, precise Standard Model predictions are needed for the production of polarized bosons in the presence of realistic kinematic selections. We formulate a method for the calculation of polarized cross-sections at NLO that relies on the pole approximation and the separation of polarized matrix elements at the amplitude level. In this framework, we compute NLO-accurate cross-sections for the production of two polarized Z bosons at the LHC, including for the first time NLO EW corrections and combining them with NLO QCD corrections and contributions from the gluon-induced process.

Testing freeze-in with axial and vector Z′ bosons

The freeze-in production of Feebly Interacting Massive Particle (FIMP) dark matter in the early universe is an appealing alternative to the well-known — and constrained — Weakly Interacting Massive Particle (WIMP) paradigm. Although challenging, the phenomenology of FIMP dark matter has been receiving growing attention and is possible in a few scenarios. In this work, we contribute to this endeavor by considering a Z′ portal to fermionic dark matter, with the Z′ having both vector and axial couplings and a mass ranging from MeV up to PeV. We evaluate the bounds on both freeze-in and freeze-out from direct detection, atomic parity violation, leptonic anomalous magnetic moments, neutrino-electron scattering, collider, and beam dump experiments. We show that FIMPs can already be tested by most of these experiments in a complementary way, whereas WIMPs are especially viable in the Z′ low mass regime, in addition to the Z′ resonance region. We also discuss the role of the axial couplings of Z′ in our results. We therefore hope to motivate specific realizations of this model in the context of FIMPs, as well as searches for these elusive dark matter candidates.

The Z lineshape challenge: ppm and keV measurements

The FCC-ee offers powerful opportunities for direct or indirect evidence for physics beyond the standard model, via a combination of high-precision measurements and searches for forbidden and rare processes and feebly coupled particles. A key element of FCC-ee physics program is the measurement of the Z lineshape from a total of $$5\times 10^{12}$$ 5 × 10 12 Z bosons and a beam-energy calibration with relative uncertainty of $$10^{-6}$$ 10 - 6 . With this exceptionally large event sample, five orders of magnitude larger than that accumulated during the whole LEP1 operation at the Z pole, the defining parameters—$$m_\mathrm{Z}$$ m Z , $$\Gamma _\mathrm{Z}$$ Γ Z , $$N_\nu $$ N ν , $$\sin ^2\theta _\mathrm{W}^\mathrm{eff}$$ sin 2 θ W eff , $$\alpha _\mathrm{S}(m_\mathrm{Z}^2)$$ α S ( m Z 2 ) , and $$\alpha _\mathrm{QED}(m^2_\mathrm{Z})$$ α QED ( m Z 2 ) —can be extracted with a leap in accuracy of up to two orders of magnitude with respect to the current state of the art. The ultimate goal that experimental and theory systematic errors match the statistical accuracy (4 keV on the Z mass and width, $$3\times 10^{-6}$$ 3 × 10 - 6 on $$\sin ^2\theta _\mathrm{W}^\mathrm{eff}$$ sin 2 θ W eff , a relative $$3\times 10^{-5}$$ 3 × 10 - 5 on $$\alpha _\mathrm{QED}$$ α QED , and less than 0.0001 on $$\alpha _\mathrm{S}$$ α S ) leads to highly demanding requirements on collider operation, beam instrumentation, detector design, computing facilities, theoretical calculations, and Monte Carlo event generators. Such precise measurements also call for innovative analysis methods, which require a joint effort and understanding between theorists, experimenters, and accelerator teams.

Anomalous Z′ bosons for anomalous B decays

Motivated by the intriguing discrepancies in b → sℓℓ transitions, the fermion mass problem, and a desire to preserve the accidental symmetries of the Standard Model (SM), we extend the SM by an anomalous U(1)X gauge symmetry where X = Y3 + a(Lμ− Lτ)/6. The heavy Z′ boson associated with spontaneously breaking U(1)X at the TeV scale mediates the b → sℓℓ anomalies via $$ {\mathcal{O}}_9^{\mu}\sim \frac{1}{\Lambda^2}\left(\overline{s}{\gamma}_{\rho }{P}_Lb\right)\left(\overline{\mu}{\gamma}^{\rho}\mu \right) $$ O 9 μ ~ 1 Λ 2 s ¯ γ ρ P L b μ ¯ γ ρ μ . We show that this model, which features mixed gauge anomalies involving U(1)X and hypercharge, can be made anomaly-free for any a ∈ ℤ by integrating in a pair of charged fermions whose masses naturally reside somewhere between 1 and 30 TeV. The gauge symmetry permits only the third family Yukawas at the renormalisable level, and so the light quark masses and mixings are controlled by accidental U(2)3 flavour symmetries which we assume are minimally broken alongside U(1)X. The lepton sector is not governed by U(2) symmetries, but rather one expects a nearly diagonal charged lepton Yukawa with me,μ « mτ. The model does not explain the hierarchy me « mμ, but it does possess high quality lepton flavour symmetries that are robust to the heavy physics responsible for generating me,μ. We establish the viability of these models by checking agreement with the most important experimental constraints. We comment on how the model could also explain neutrino masses and the muon g − 2.

Searching for Z′ bosons at the P2 experiment

The P2 experiment aims at high-precision measurements of the parity-violating asymmetry in elastic electron-proton and electron-12C scatterings with longitudinally polarized electrons. We discuss here the sensitivity of P2 to new physics mediated by an additional neutral gauge boson Z′ of a new U(1)′ gauge symmetry. If the charge assignment of the U(1)′ is chiral, i.e., left- and right-handed fermions have different charges under U(1)′, additional parity-violation is induced directly. On the other hand, if the U(1)′ has a non-chiral charge assignment, additional parity-violation can be induced via mass or kinetic Z-Z′ mixing. By comparing the P2 sensitivity to existing constraints, we show that in both cases P2 has discovery potential over a wide range of Z′ mass. In particular, for chiral models, the P2 experiment can probe gauge couplings at the order of 10−5 when the Z′ boson is light, and heavy Z′ bosons up to 79 (90) TeV in the proton (12C) mode. For non-chiral models with mass mixing, the P2 experiment is sensitive to mass mixing angles smaller than roughly 10−4, depending on model details and gauge coupling magnitude.

Global analysis of leptophilic Z′ bosons

New neutral heavy gauge bosons (Z′) are predicted within many extensions of the Standard Model. While in case they couple to quarks the LHC bounds are very stringent, leptophilic Z′ bosons (even with sizable couplings) can be much lighter and therefore lead to interesting quantum effects in precision observables (like (g − 2)μ) and generate flavour violating decays of charged leptons. In particular, $$ \mathrm{\ell}\to \mathrm{\ell}^{\prime }v\overline{v} $$ ℓ → ℓ ′ v v ¯ decays, anomalous magnetic moments of charged leptons, ℓ → ℓ′γ and ℓ → 3ℓ′ decays place stringent limits on leptophilic Z′ bosons. Furthermore, in case of mixing Z′ with the SM Z, Z pole observables are affected. In light of these many observables we perform a global fit to leptophilic Z′ models with the main goal of finding the bounds for the Z′ couplings to leptons. To this end we consider a number of scenarios for these couplings. While in generic scenarios correlations are weak, this changes once additional constraints on the couplings are imposed. In particular, if one considers an Lμ− Lτ symmetry broken only by left-handed rotations, or considers the case of τ − μ couplings only. In the latter setup, on can explain the (g − 2)μ anomaly and the hint for lepton flavour universality violation in $$ \tau \to \mu v\overline{v}/\tau \to ev\overline{v} $$ τ → μv v ¯ / τ → ev v ¯ without violating bounds from electroweak precision observables.