Search

We study lepton flavour universality violation in SM boson decays in low-scale seesaw models of neutrino mass generation, also addressing other electroweak precision observables. We compute the electroweak next-to-leading order corrections, which turn out to be important - notably in the case of the invisible decay width of the $Z$ boson, for which the corrections can be as large as the current experimental uncertainty. As a well-motivated illustrative study case, we choose a realisation of the Inverse Seesaw mechanism, and discuss the complementary role of lepton flavour conserving, lepton flavour violating and precision observables, both in constraining and in probing such models of neutrino mass generation. Our findings suggest that invisible $Z$ decays are especially important, potentially at the origin of the most stringent constraints for certain regimes of the Inverse Seesaw (while complying with charge lepton flavour violation and other electroweak precision tests). We also discuss the probing power of the considered observables in view of the expected improvement in experimental precision at FCC-ee., 48 pages, 12 figures, 4 appendices

One of the major upcoming challenges in particle physics is achieving precise measurements of the Z, W, and H bosons, as well as the top quark. To meet these targets, the next e\textsuperscript{+}e\textsuperscript{-} collider complex, FCC-ee, will need to achieve unprecedented luminosities. The FCC-IS European Study is investigating the feasibility of these challenges, with a cornerstone of the study being the design and optimization of the high-energy booster (HEB). This paper provides an update on the status of the HEB of FCC-ee in light of recent developments in the injector and collider survey, as well as an overview of ongoing work on longitudinal stability and design robustness in relation to field, alignment, and diagnostics errors. Constraints and effects related to the design frequency of the accelerating cavities, as well as collective effects, are also highlighted. Lastly, the paper presents an investigation into an alternative arcs cell design., Comment: pre-print of ICFA Newsletter 2023

International audience; After the discovery of the Higgs boson at the LHC, particle physics community is exploring and proposing next accelerators, to address the remaining open questions on the underlying mechanisms and constituents of the present universe. One of the studied possibilities is FCC (Future Circular Collider), a 100 km long collider at CERN. The feasibility study of this future proposed accelerator implies the definition of tolerances on magnets imperfections and of the strategies of correction in order to guarantee the target performances of the High Energy Booster ring. The efficiency of the correction scheme, used to control the orbit, directly bounds the corrector needs and magnet tolerances. Analytic formulae give a first estimation of the average rms value of the required linear correctors' strengths and of the allowed magnets misalignments and field quality along the entire ring. The distribution of the correctors along the ring is simulated in order to verify the quality of the residual orbit after the proposed correction strategy and compared with the analytical predictions. First specifications of the orbit correctors strength and tolerances for the alignment of the main elements of the ring are presented. The limits of the studied correction scheme and method are also discussed.

The Future Circular electron-positron Collider (FCC-ee) is a proposal for a 91.17 km collider, which would operate in four modes with energies ranging from 45.6 GeV (Z-pole) to 182.5 GeV (ttbar-production). At high energies the beam lifetime could be as low as 6 minutes, requiring the beam to be continuously topped up to reach a high integrated luminosity. This top-up injection would use a separate booster ring in the same tunnel as the collider, which would accelerate the beams to the collider energy. The booster ring should achieve a lower equilibrium emittance than the collider, despite challenges such as a long damping time and no magnet-strength tapering to compensate for the impact of synchrotron radiation. For top-up injection into the collider, we consider two strategies: conventional bump injection, employing a closed orbit bump, and injection using a multipole kicker magnet. On-axis and off-axis sub-schemes will be studied for both. We compare these injection strategies on aspects including spatial constraints, machine protection, perturbation to the stored beam and hardware parameters., Proceedings of the 65th ICFA Advanced Beam Dynamics Workshop on High Luminosity Circular e+e- Colliders, eeFACT2022, Frascati, Italy

International audience; The physics potential of measuring $B^0_{(s)}\to\pi^0\pi^0$ and $B^0_{(s)}\to\eta\eta$ decays via four-photon final states at Tera-$Z$ phase of CEPC or FCC-ee is investigated in this paper. We propose an electromagnetic calorimeter (ECAL) energy resolution of $\frac{3\%}{\sqrt{E}} \oplus 0.3\%$ to efficiently reconstruct $\pi^0$ and $\eta$ from hadronic final states with high photon multiplicity. The resulting $B$-meson mass resolution is approximately 30 MeV, allowing 2 $\sigma$ separation between $B^0$ and $B_s^0$. With the assistance of the $b$-jet tagging, the relative sensitivities to $B^0\to\pi^0\pi^0$, $B^0_s\to\pi^0\pi^0$, $B^0\to\eta\eta$, and $B^0_s\to\eta\eta$ signal strengths at Tera-$Z$ are projected as 0.45%, 4.5%, 18%, and 0.95%, respectively. Their dependence on various detector performances is also discussed. In addition, $B^0\to\pi^0\pi^0$ and its two isospin-related modes are paid special attention due to their role in the determination of the CKM angle $\alpha$ ($\phi_2$). The anticipated precisions of their branching-ratio and $CP$-asymmetry measurements at Tera-$Z$ are evaluated. We show that the measurement of the time-integrated $B^0\to\pi^0\pi^0$ $CP$ asymmetry at Tera-$Z$ is complementary to $B$-factory ones. The precision on $\alpha$ combining $Z$- and $B$-factory results reaches $0.4^\circ$, lower than the systematic uncertainties attached to isospin breaking.

We underline the theoretical interest of the vector-vector penguin decay $B_s \to \phi \phi$, very clean from the experimental point of view. The CP-violation asymmetry $A_{CP}^{mix}$ comes from the interference of mixing and decay $\lambda_{\phi \phi} = {q \over p} {\bar{A} \over A}$. In the Standard Model (SM) and in the Naive Factorization limit, the CP phase from the mixing ${q \over p}$ exactly cancels the CP phase from the decay ratio ${\bar{A} \over A}$. Therefore, this mode is suitable to look for possible New Physics (NP) because $A_{CP}^{mix}$ would directly indicate the departure from the SM in mixing. We estimate the deviation from this cancellation by analyzing possible small effects in the SM, using in particular the QCD Factorization scheme. We compare the theoretical expectation for $A_{CP}^{mix}$ to the measurement of LHCb, and the implications for NP. We pay also special attention to the transverse amplitude $h = -$, the longitudinal and transverse polarization fractions, and the interesting helicity-dependent observables $\lambda_{\phi \phi}^{h=0}$ and $\lambda_{\phi \phi}^{h=-}$. On the other hand, we make an estimation of the expected sensitivity at the future FCCee experiment for the CP phase and modulus of $\lambda_{\phi \phi}$. We find $\delta(\mid \lambda_{\phi \phi}\mid) = 0.004$ and $\delta(\phi_{\phi \phi}) = 0.009$ rad and, comparing to the LHCb data, we point out the expectations at FCCee in the search of NP., Comment: 49 pages, 12 figures

International audience; The FCC-ee could measure the electron Yukawa coupling in a dedicated run at $\sim $125 GeV collision energy, provided that the center-of-mass (CM) energy spread can be reduced by means of monochromatization, e.g., through introducing nonzero horizontal dispersion of opposite sign at the interaction point (IP), for the two colliding beams. If the IP dispersion is nonzero, beamstrahlung blows up the horizontal emittance, and self-consistent IP parameters need to be determined. Two configurations are being studied. The first uses crab cavities to establish effective head-on collisions. The second configuration maintains the standard FCC-ee crossing angle, which, together with the IP dispersion, introduces a correlation between the local collision energy and the longitudinal location inside the detector, thereby allowing for an integrated scan of the Higgs resonance curve. We compare both approaches.

With its high luminosity, its clean experimental conditions, and a range of energies that cover the four heaviest particles known today, FCC-ee offers a wealth of physics possibilities, with high potential for discoveries. The FCC-ee is an essential and complementary step towards a 100 TeV hadron collider, and as such offers a uniquely powerful combined physics program. This vision is the backbone of the 2020 European Strategy for Particle Physics. One of the main challenges is now to design experimental systems that can, demonstrably, fully exploit these extraordinary opportunities., Comment: Approved for publication in EPJ+ special issue: A future Higgs and Electroweak factory (FCC): Challenges towards discovery, Focus on FCC-ee

New Physics models with either an elementary or composite origin are often associated with a similar imprint in a direct search at colliders, case in point being the production of a light pseudoscalar in association with a monochromatic photon from the decay of a Z boson at future $e+e-$ colliders. We exploit the correlation between the discovery of a signal in the Z decays and electroweak precision measurements as a tool to distinguish a composite model from an elementary scalar one. Our results offer an appealing and rich physics case for future colliders and demonstrate how a lepton collider at the Z mass can be a discovery machine for new physics in the Higgs sector., Comment: 7 Pages, 2 Figures

The European physical journal / Plus 137(1), 116 (2022). doi:10.1140/epjp/s13360-021-01848-4, Learning from the first twelve years of LHC running, this essay offers a brief journey through the FCC-ee physics programme from refined precision measurements to probes of new physics, highlighting some of the commentaries between the different runs of FCC-ee at various energies as well as the synergies between the two FCC-ee and FCC-hh collider stages., Published by Springer, Heidelberg

The worldwide High Energy Physics community widely agrees that the next collider should be a Higgs factory. Acknowledging this priority, in 2021 CERN has launched the international Future Circular Collider (FCC) Feasibility Study (FS). The FCC Integrated Project foresees, in a first stage, a high-luminosity high-energy electron-positron collider, serving as Higgs, top and electroweak factory, and, in a second stage, an energy frontier hadron collider, with a centre-of-mass energy of at least 100 TeV. In this paper, we address a few key elements of the FCC-ee accelerator design, its performance reach, and underlying technologies, as requested by the Snowmass process. The Conceptual Design Report for the FCC, published in 2019, serves as our primary reference. We also summarize a few recent changes and improvements., Contribution to Snowmass 2021

The great progress made recently in the sector of Flavor Physics has enabled to establish CP violation in the B-meson decays. The unitarity triangle derived from the unitarity relation $V_{ub}^* V_{ud} + V_{cb}^* V_{cd} + V_{tb}^* V_{td} = 0$ has been measured very precisely. To further asses our understanding of CP violation, it would be useful to carry out similar measurement of other triangles. In this note, we investigate the triangle derived from the relation $V_{ub}^* V_{us} + V_{cb}^* V_{cs} + V_{tb}^* V_{ts} = 0$. Two angles of this triangle ($\alpha_s$ and $\beta_s$) could be measured very accurately at FCCee using the decays $B_s(\overline{B_s})\rightarrow D^\pm_sK^\mp$ and $B_s(\overline{B_s})\rightarrow J/\psi \phi$ respectively, as discussed elsewhere by us. This note concentrates on the measurement of the third angle $\gamma_s$ using the modes $B^\pm \to \overline{D^0}(D^0)K^\pm$. We show that a direct measurement of the angle $\gamma_s$ is possible with some specific $B^\pm$ decays with an estimated resolution of the order of 1$^\circ$., Comment: 22 pages, 9 figures

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 $5\times 10^{12}$ Z bosons and a beam-energy calibration with relative uncertainty of $10^{-6}$. 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 - $m_{\rm Z}$, $\Gamma_{\rm Z}$, $N_\nu$, $\sin^2\theta_{\rm W}^{\rm eff}$, $\alpha_{\rm S}(m_{\rm Z}^2)$, and $\alpha_{\rm QED}(m^2_{\rm Z})$ - 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, $3\times 10^{-6}$ on $\sin^2\theta_{\rm W}^{\rm eff}$, a relative $3\times 10^{-5}$ on $\alpha_{\rm QED}$, and less than 0.0001 on $\alpha_{\rm S}$) 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., Comment: Submitted to EPJ+ special issue: A future Higgs and Electroweak factory (FCC): Challenges towards discovery, Focus on FCC-ee

The abundant production of beauty and charm hadrons in the $5 \times 10^{12}$ $Z^0$ decays expected at FCC-ee offers outstanding opportunities in flavour physics that in general exceed those available at Belle II, and are complementary to the heavy-flavour programme of the LHC. A wide range of measurements will be possible in heavy-flavour spectroscopy, rare decays of heavy-flavoured particles and $C\!P$-violation studies, which will benefit from the low-background experimental environment, the high Lorentz boost, and the availability of the full spectrum of hadron species. This essay first surveys the important questions in heavy-flavour physics, and assesses the likely theoretical and experimental landscape at the turn-on of FCC-ee. From this certain measurements are identified where the impact of FCC-ee will be particularly important. A full exploitation of the heavy-flavour potential of FCC-ee places specific constraints and challenges on detector design, which in some cases are in tension with those imposed by the other physics goals of the facility. These requirements and conflicts are discussed., Accepted for publication in the EPJ Plus Focus Point on "A future Higgs and electroweak factory (FCC): challenges towards discovery". This version includes minor modifications and corrections following review

We investigate the sensitivity with which two angles of the "flat" unitarity triangle, defined by $V_{ub}^* V_{us} + V_{cb}^* V_{cs} + V_{tb}^* V_{ts} =0$, can possibly be measured directly at FCCee. We show that the measured errors on the angle $\alpha_s = \arg(- V_{ub}^* V_{us} /V_{tb}^* V_{ts})$ and $\beta_s = \arg(-V_{tb}^* V_{ts} / V_{cb}^* V_{cs})$ should be better than $0.4^\circ$ and $0.035^\circ$, respectively. These measurements, combined with the measurement of the 3rd angle $\gamma_s = \arg(- V_{cb}^* V_{cs}/ V_{ub}^* V_{us})$, discussed in a different paper, will contribute to probe further the consistency of the CP sector of the Standard Model with unprecedented level of accuracy., Comment: 27 pages, 24 figures

The FCC-ee offers powerful opportunities to determine the Higgs boson parameters, exploiting over $10^6$ ${\rm e^+e^- \to ZH}$ events and almost $10^5$ ${\rm WW \to H}$ events at centre-of-mass energies around 240 and 365 GeV. This essay spotlights the important measurements of the ZH production cross section and of the Higgs boson mass. The measurement of the total ZH cross section is an essential input to the absolute determination of the HZZ coupling -- a "standard candle" that can be used by all other measurements, including those made at hadron colliders -- at the per-mil level. A combination of the measured cross sections at the two different centre-of-mass energies further provides the first evidence for the trilinear Higgs self-coupling, and possibly its first observation if the cross-section measurement can be made accurate enough. The determination of the Higgs boson mass with a precision significantly better than the Higgs boson width (4.1 MeV in the Standard Model) is a prerequisite to either constrain or measure the electron Yukawa coupling via direct ${\rm e^+e^- \to H}$ production at $\sqrt{s} = 125$ GeV. Approaching the statistical limit of 0.1% and $\mathcal{O}(1)$ MeV on the ZH cross section and the Higgs boson mass, respectively, sets highly demanding requirements on accelerator operation (ZH threshold scan, centre-of-mass energy measurement), detector design (lepton momentum resolution, hadronic final state reconstruction performance), theoretical calculations, and analysis techniques (efficiency and purity optimization with modern tools, constrained kinematic fits, control of systematic uncertainties). These challenges are examined in turn in this essay., To be published in EPJ+ special issue: A future Higgs and Electroweak factory (FCC): Challenges towards discovery, Focus on FCC-ee

We have studied three realistic benchmark geometries for a new far detector GAZELLE to search for long-lived particles at the \superkekb accelerator in Tsukuba, Japan. The new detector would be housed in the same building as Belle II and observe the same $e^+e^-$ collisions. To assess the discovery reach of GAZELLE, we have investigated three new physics models that predict long-lived particles: heavy neutral leptons produced in tau lepton decays, axion-like particles produced in $B$ meson decays, and new scalars produced in association with a dark photon, as motivated by inelastic dark matter. We do not find significant gains in the new physics discovery reach of GAZELLE compared to the Belle II projections for the same final states. The main reasons are the practical limitations on the angular acceptance and size of GAZELLE, effectively making it at most comparable to Belle II, even though backgrounds in the far detector could be reduced to low rates. A far detector for long-lived particles would be well motivated in the case of a discovery by Belle II, since decays inside GAZELLE would facilitate studies of the decay products. Depending on the placement of GAZELLE, searches for light long-lived particles produced in the forward direction or signals of a confining hidden force could also benefit from such a far detector. Our general findings could help guide the design of far detectors at future electron-positron colliders such as the ILC, FCC-ee or CEPC., Fixed a mistake in the detector size of the L-GAZELLE configuration (contribution to Snowmass)

In the last years there has been intense linear electron-accelerator development driven by different communities as the X-FEL community, the High Energy Physics (HEP) linear-collider community: ILC and CLIC as well as HEP circular-collider community: FCC-ee and CEPC. Furthermore, there are also many other applications from medical science to industry that will use such a linac as main accelerators. In all these studies, a high-efficient e-linac with energies from 10 to 1000 MeV is needed as driver or injector. Even if the linac technology to cope with the performances needed is very well known, an important R&D effort on more compact, simpler, cost-effective, efficient, robust and reliable is in progress. In this frame, this thesis will optimize the linac and its associated transfer lines in two cases: (1) The injector linac for FCC-ee (Future electron-positron Circular Collider), in particular the positron one. (2) The linac for applications known as PRAE project (Platform for Research and Application with Electrons). The Future Circular Collider (FCC) hosted by CERN, is an international collaboration to explore the feasibility of different particle collider scenarios with the aim of significantly increasing the energy and luminosity compared to existing colliders, in the search for new physics. In the case of the Future Circular electron-positron Collider (FCC-ee), 2.13*10^10 (3.5 nC) electron and positron particles per bunch are needed for the most demanding full filling of Z running mode in the conceptual design report (CDR). The baseline selection for FCC-ee injector linac is based on the SuperKEKB one, which gives us a positron yield of 0.2 e+/e- against 0.4 e+/e- design in recent experiment. The main objective of this thesis is to perform the start-to-end design and optimization for beam production, acceleration and transport from the feeding electron source to the positron damping ring as well as to increase the efficiency and the flexibility of the positron production (the positron yield needs to be larger than 0.7) for FCC-ee injector linac. A complementary study for positron target optimisation using conventional target or hybrid target is also shortly summarized. Considered the drawbacks of the current SuperKEKB injector scheme, three new different bypass scheme designs have been finished to transfer electron and positron particles separately for a better transmission and improved flexibility of the whole system, which finally could gives us a positron yield of around 1.2 e+/e- at the e+ damping ring in theoretical simulations. In conclusion, this work is a first step in the optimization of the FCC-ee injector system from the point of view of the efficiency of the transport and optics design. The various schemes proposed are based on established technologies and different paths for electrons and positrons are used in order to improve the efficiency of the transport from the point of view of losses and cost. In the second part of this thesis, the design for a radiobiology and nuclear physics application linac platform PRAE(Platform for Research and Application with Electrons) based on a high-quality pulsed electron beam of energy up to 70 MeV in phase 1 and 140 MeV in phase 2 has been realized. 2 nC electron bunches in the PRAE accelerator phase 1 are produced in a RF gun at 50 Hz frequency, post-accelerated by a S-band linac to 50-70 MeV and injected into the direct beam line plus a deviated line. The optics design of the beam lines has to be as flexible as possible to cope with different kinds of beam characteristics (beam size, energy, dispersion, current…) and operation modes depending on the application. The study of the different optics options and the implementation of these two beam lines as well as the beam-water interaction for pre-clinical studies for the case of the radiobiology experiments has also been done and presented in this thesis., Au cours des dernières années, il y a eu un développement intense d'accélérateurs d'électrons linéaires conduit par différentes communautés comme la communauté X-FEL, la communauté des collisionneurs linéaires de la physique des hautes énergies (HEP) : ILC et CLIC ainsi que la communauté des collisionneurs circulaires HEP: FCC- ee et CEPC. En outre, il existe également de nombreuses autres applications de la science médicale à l'industrie qui utiliseront un tel linac comme accélérateurs principaux. Dans toutes ces études, un e-linac à haut rendement avec des énergies de 10 à 1000 MeV est nécessaire comme conducteur ou injecteur. Même si la technologie linac pour faire face aux performances recherchées est très connue, un important effort de R&D sur des solutions plus compactes, plus simples, plus rentables, efficaces, robustes et fiables est en cours. Dans ce cadre, cette thèse optimisera le linac et ses lignes de transfert associées dans deux cas: (1) L'injecteur linac pour FCC-ee (Future Electron-Positon Circular Collider), en particulier le positron. (2) Le linac pour les applications dites projet PRAE (Platform for Research and Application with Electrons). Le Future Circular Collider (FCC) hébergé par le CERN, est une collaboration internationale visant à explorer la faisabilité de différents scénarios de collisionneurs de particules dans le but d'augmenter considérablement l'énergie et la luminosité par rapport aux collisionneurs existants, dans la recherche d'une nouvelle physique. L'objectif principal de cette thèse est de réaliser la conception et l'optimisation de bout en bout pour la production de faisceaux, l'accélération et le transport de la source d'électrons d'alimentation à l'anneau d'amortissement de positons ainsi que d'augmenter l'efficacité et la flexibilité de la production de positons (le rendement en positons doit être supérieur à 0,7) pour l'injecteur linac FCC-ee. Une étude complémentaire pour l'optimisation de la cible positron utilisant une cible conventionnelle ou une cible hybride est également brièvement résumée. Compte tenu des inconvénients du schéma d'injecteur SuperKEKB actuel, trois nouveaux modèles de schémas de dérivation différents ont été finis pour transférer les particules d'électrons et de positrons séparément pour une meilleure transmission et une flexibilité améliorée de l'ensemble du système, ce qui pourrait finalement nous donner un rendement en positons d'environ 1,2 e + / e- à l'anneau d'amortissement e + dans les simulations théoriques. En conclusion, ce travail est une première étape dans l'optimisation du système d'injecteur FCC-ee du point de vue de l'efficacité du transport et de la conception optique. Les différents schémas proposés sont basés sur des technologies établies et différents chemins pour les électrons et les positrons sont utilisés afin d'améliorer l'efficacité du transport du point de vue des pertes et du coût. Dans la deuxième partie de cette thèse, la conception d'une plateforme d'application de radiobiologie et de physique nucléaire PRAE (Platform for Research and Application with Electrons) linac basée sur un faisceau d'électrons pulsé de haute qualité d'énergie jusqu'à 70 MeV en phase 1 et 140 MeV en phase 2 a été réalisé. 2 paquets d'électrons nC dans l'accélérateur PRAE phase 1 sont produits dans un canon RF à une fréquence de 50 Hz, post-accélérés par un linac en bande S à 50-70 MeV et injectés dans la ligne directe du faisceau plus une ligne déviée. La conception optique des lignes de faisceau doit être aussi flexible que possible pour faire face à différents types de caractéristiques de faisceau et modes de fonctionnement en fonction de l'application. L'étude des différentes options optiques et la mise en œuvre de ces deux lignes de faisceau ainsi que l'interaction faisceau-eau pour les études précliniques pour le cas des expériences de radiobiologie ont également été réalisées et présentées dans cette thèse.

Discussion on the FCC-ee potential for searches for long-lived particles, including Heavy Neutral Leptons. Talk given at the session "Experiments and detectors: FCC-ee PE&D" Link

The capability to determine the FCC-ee centre-of-mass energies (ECM) at the ppm level using resonant depolarization of the beams is essential for the Z line shape measurements, the W mass and the possible observation of the Higgs boson s-channel production. A first analysis (Blondel A et al Polarization and centre-of-mass energy calibration at FCC-ee. arXiv:1909.12245) demonstrated the feasibility of this programme, conditional to careful preparation and a number of further developments. The existing simulation codes must be unified; the analysis and design of the instrumentation must be developed; and a detailed planning must be developed for the simultaneous and coordinated operation of the accelerator, of the continuous polarization and depolarization measurements, and of the beam monitoring devices, ensuring a precise extrapolation from beam energies to centre-of-mass energy and energy spread.

International audience; Injectors and in particular the positron production systems are a crucial element in linear and circular colliders. The proposed FCC-ee injector linac accelerates both electrons and positrons up to 6 GeV in order to be injected into the Pre-Booster Ring (PBR) with a high intensity of 3.5 nC per bunch for the Z running mode. The current design baseline of the FCC-ee injector linac is based on the SuperKEKB scheme, in which the electron and the positron beams share the same linacs with a fixed target configuration on-axis hole for electron beam passage. One of the main drawbacks of this scheme is the positron production efficiency at the exit of the injector linac. In order to achieve a better performance and a higher positron yield, in this paper we study different bypass injection options. Start-to-end design and optimization for beam production, acceleration and transport from the feeding electron source to the positron damping ring are also being presented.

Improving the accuracy of the impedance model of an accelerator is important for keeping beam instabilities and power loss under control. Here, by means of the PyHEAD- TAIL tracking code, we first review the longitudinal mi- crowave instability threshold for FCC-ee by taking into ac- count the longitudinal impedance model evaluated so far. Moreover, we present the results of beam dynamics simula- tions, including both the longitudinal and transverse wake- fields due to the resistive wall, in order to evaluate the influ- ence of the bunch length on the transverse mode coupling instability. The results of the transverse beam dynamics are also compared with the Vlasov solver DELPHI., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

The FCC-ee is a proposed future high-energy, high-intensity and high precision lepton collider. Here, we present the latest developments for the FCC-ee interaction regions, which shall ensure optimum conditions for the particle physics experiments. We discuss measures of background reduction and a revised interaction region layout including a low impedance compact beam chamber design. We also discuss the possible impact of the radiation generated in the interaction region including beamstrahlung., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

The European physical journal / Plus 136(10), 1066 (2021). doi:10.1140/epjp/s13360-021-02034-2, With centre-of-mass energies covering the Z pole, the WW threshold, the HZ production, and the top-pair threshold, the FCC-ee offers unprecedented possibilities to measure the properties of the four heaviest particles of the standard model (the Higgs, Z, and W bosons, and the top quark) and also those of the b and c quarks and of the $\tau $ lepton. At these moderate energies, the role of the calorimeters is to complement the tracking systems in an optimal (a.k.a. particle-flow) event reconstruction. In this context, precision measurements and searches for new particles can fully profit from the improved electromagnetic and hadronic object reconstruction offered by new technologies, finer transverse and longitudinal segmentation, timing capabilities, multi-signal readout, modern computing techniques and algorithms. The corresponding requirements arise in particular from the resolution on reconstructed hadronic masses, energies, and momenta, for example, of H, W, Z, needed to reach the FCC-ee promised precision. Extreme electromagnetic energy resolutions are also instrumental for $\pi ^0$ identification, $\tau $ exclusive decay reconstruction, and physics sensitivity to processes accessible via radiative return. We present state of the art, challenges and future developments on some of the currently most promising technologies: high-granularity silicon and scintillator readout, dual readout, noble-liquid and crystal calorimeters., Published by Springer, Heidelberg

In this report, we compared different matching device field profiles for the FCC-ee positron source. The matching device is used to capture positrons with magnetic field. A flux concentrator was designed with a conical inner chamber. A smaller aperture and a larger aperture were studied. An analytic field profile was also studied using an adiabatic formula. The peak field of the analytic profile as well as beam and target parameters was optimised to achieve a maximum positron yield. A safe energy deposition in the target was guaranteed by requiring a constraint on the deposited power and peak energy deposition density., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

In this essay we discuss the possibilities and associated challenges concerning beyond the Standard Model searches at FCC-ee, such as rare decays of heavy-flavoured particles and long-lived particles. The Standard Model contains several suppression mechanisms, which cause a given group of processes to happen rarely, resulting in rare decays. The interest in these decays lies in the fact that the physics beyond the Standard Model does not need to be affected by the same suppression mechanism and therefore can naturally manifest in these decays. Their interest is reinforced by the recent report of several measurements of $b$-flavoured rare decays, showing deviations with respect to the Standard Model predictions. We will show how the FCC-ee project has unique capabilities to address these scientific questions and will consider the related detector design challenges to meet. Another group of processes discussed are those that produce new particles with relatively long lifetimes, that travel substantial distances inside the detectors before decaying. Models containing long-lived particles can give answers to many open questions of the Standard Model, such as the nature of dark matter, or the neutrino masses, among others; while providing an interesting experimental complement to mainstream searches. Long-lived particles often display unique experimental signatures, such as displaced tracks and vertices, disappearing tracks, or anomalously charged jets. Due to this, they are affected by very low background levels but in exchange, they often require dedicated reconstruction algorithms and triggers., Submitted to the EPJ Plus "Focus Point on A Future Higgs & Electroweak Factory (FCC): Challenges towards Discovery - Part II: Physics Opportunities and Challenges"

This paper presents the prospects for a precise measurement of the branching fraction of the leptonic $$ {B}_c^{+} $$ B c + → τ+ντ decay at the Future Circular Collider (FCC-ee) running at the Z -pole. A detailed description of the simulation and analysis framework is provided. To select signal candidates, two Boosted Decision Tree algorithms are employed and optimised. The first stage suppresses inclusive $$ b\overline{b} $$ b b ¯ , $$ c\overline{c} $$ c c ¯ , and $$ q\overline{q} $$ q q ¯ backgrounds using event-based topological information. A second stage utilises the properties of the hadronic τ+→ π+π+π−$$ \overline{\nu} $$ ν ¯ τ decay to further suppress these backgrounds, and is also found to achieve high rejection for the B+→ τ+ντ background. The number of $$ {B}_c^{+} $$ B c + → τ+ντ candidates is estimated for various Tera-Z scenarios, and the potential precision of signal yield and branching fraction measurements evaluated. The phenomenological impact of such measurements on various New Physics scenarios is also explored.

Rare and Precision Frontier at the FCC-ee Talk given at the Rare Processes and Precision Frontier Townhall Meeting in the context of Snowmass 2021 https://indico.fnal.gov/event/45713/

We study the decay of the $Z$ vector boson into a photon and a massless (invisible) dark photon in high-energy collisions. The photon can be used as trigger for the event, while the dark photon is detected indirectly as missing momentum in the event final state. We investigate the possibility of searching for such a dark photon at the LHC, HL-LHC and future lepton colliders, and compare the respective sensitivities. As expected, the best result is found for the lepton colliders running at the $Z$ mass, FCC-ee and CEPC, with a final sensitivity to branching ratios of order $O(10^{-11})$. We also discuss how to use the photon angular distribution of the events in lepton collisions to discriminate between the dark photon and a pseudo-scalar state like the axion., 12 pages, 8 figures, G. Panizzo corresponding author

Circular electron positron colliders, such as the CEPC and FCC-ee, have been proposed to measure Higgs boson properties precisely, test the Standard Model, search for physics beyond the Standard Model, and so on. One of the important goals of these colliders is to measure the $W$ boson mass with great precision by taking data around the $W$-pair production threshold. In this paper, the data-taking scheme is investigated to maximize the achievable precisions of the $W$ boson mass and width with a threshold scan, when various systematic uncertainties are taken into account. The study shows that an optimal and realistic data-taking scheme is to collect data at three center-of-mass energies and that precisions of 1.0 MeV and 3.4 MeV can be achieved for the mass and width of the $W$ boson, respectively, with a total integrated luminosity of $\mathcal{L}=3.2$~\mbox{ab}$^{-1}$ and several assumptions of the systematic uncertainty sources., Comment: 12 pages, 10 figures and 4 tables

The mixing of neutral mesons is sensitive to some of the highest scales probed in laboratory experiments. In light of the planned LHCb Upgrade II, a possible upgrade of Belle II, and the broad interest in flavor physics in the tera-Z phase of the proposed FCC-ee program, we study constraints on new physics contributions to Bd and Bs mixings which can be obtained in these benchmark scenarios. We explore the limitations of this program, and identify the measurement of |Vcb| as one of the key ingredients in which progress beyond current expectations is necessary to maximize future sensitivity. We speculate on possible solutions to this bottleneck. Given the current tension with the standard model (SM) in semileptonic B decays, we explore how its resolution may impact the search for new physics in mixing. Even if new physics has the same Cabibbo-Kobayashi-Maskawa and loop suppressions of flavor changing processes as the SM, the sensitivity will reach 2 TeV, and it can be much higher if any SM suppressions are lifted. We illustrate the discovery potential of this program.

We review the capabilities of two projects that have been proposed as the next major European facility, for consideration in the upcoming update of the European Strategy for Particle Physics: CLIC and FCC. We focus on their physics potentials and emphasise the key differences between the linear or circular approaches. We stress the uniqueness of the FCC-ee programme for precision electroweak physics at the $Z$ peak and the $WW$ threshold, as well as its unequalled statistics for Higgs physics and high accuracy for observing possible new phenomena in Higgs and $Z$ decays, whereas CLIC and FCC-ee offer similar capabilities near the $t \overline t$ threshold. Whilst CLIC offers the possibility of energy upgrades to 1500 and 3000 GeV, FCC-ee paves the way for FCC-hh. The latter offers unique capabilities for making direct or indirect discoveries in a new energy range, and has the highest sensitivity to the self-couplings of the Higgs boson and any anomalous couplings. We consider the FCC programme to be the best option to maintain Europe's place at the high-energy frontier during the coming decades., Contribution to the update of the European Strategy for Particle Physics

The precise measure of the W boson mass is an essential input to a crucial test of the overall consistency of the Standard Model, whose failure might reveal the emergence of new physics. With millions of W bosons expected, future experiments will be W boson factories allowing the measurement of the W mass with unparalleled precision. The W mass measurement is discussed in the context of two experiments: the upgrade of CMS at the LHC, the HL-LHC, with a new endcap calorimeter, the HGCal, and a detector for the FCC-ee, a circular post-LHC project. In proton-proton collisions, the precise measurement of the W mass relies on a precise measurement of the hadronic recoil. Its accurate measurement mainly depends on its definition model and detector effects. The recoil definition is improved with machine learning techniques, using a multivariate quantile regression. The effects of the HGCal granularity, acceptance and resolution on the recoil reconstruction are evaluated. This study gives an estimate of the precision that might be reached on the W mass measurement at HL-LHC. Before evaluating the effect of the detector performance on the recoil, the full geometry and parameters had to be tuned and optimised. An accurate fast simulation tool, complement to the full CMS simulation, implemented to undertake such a study, is presented. In electron-positron collisions, the W mass can be determined from the W-pair decay products. The statistical uncertainties on the W mass and width are estimated using a kinematic fit technique in the hadronic and semi-leptonic channels at 162.6 GeV, 240 GeV and 365 GeV. Reaching a statistical precision below the MeV/c² level at all energies for both channels, the W mass measurement becomes limited by the systematic uncertainties. A treatment to reduce the systematic uncertainty coming from QCD effects, the largest source of systematic uncertainty at LEP, and its impact on the statistical uncertainty are also studied., La mesure précise de la masse du boson W est une composante essentielle pour tester la cohérence globale du Modèle Standard, dont une défaillance pourrait mettre à jour la nouvelle physique. Avec des millions de bosons W prévus, les futures expériences seront des usines à bosons permettant de mesurer sa masse avec une précision inégalée. La mesure de la masse du W est discutée dans le contexte de deux expériences : l'amélioration du détecteur CMS au LHC, le HL-LHC, avec un nouveau bouchon calorimétrique, le HGCal, et à un détecteur pour FCC-ee, un projet circulaire post-LHC. En collision proton-proton, la mesure précise de la masse du boson W dépend de la précision de la mesure du recul hadronique. Cette précision dépend principalement de la définition du recul et des effets induits par le détecteur. La définition du recul est améliorée par des méthodes de machine learning, en utilisant une régression quantile multivariée. Les effets de la granularité, de l'acceptance et de la résolution du HGCal sur le recul sont évalués. Cette étude donne une estimation de la précision sur la masse du boson W qui pourrait être atteinte au HL-LHC. Avant d'estimer les effets de la performance du détecteur sur le recul, la géométrie complète et les paramètres du détecteur ont été ajustés et optimisés. Un fidèle outil de simulation rapide, complément au logiciel de simulation complet de CMS, implémenté pour mener cette étude, est présenté. En collision électron-positron, la masse du boson W peut être déterminée à partir du produit de désintégration de la paire de W. Les incertitudes statistiques sur la masse et la largeur sont estimées en utilisant la méthode du fit cinématique, dans les canaux de désintégration hadronique et semi-leptonique à 162,6 GeV, 240 GeV et 365 GeV. Atteignant une incertitude statistique inférieure au niveau du MeV/c² à toutes les énergies et pour les deux canaux, la mesure de la masse du W devient limitée par les incertitudes systématiques. Un traitement pour réduire l'incertitude systématique engendrée par les effets QCD, la plus large source d'incertitude systématique à LEP, et son impact sur l'incertitude statistique sont également étudiées.

At the high luminosity electron-positron circular colliders like FCC-ee in CERN and CEPC in China it will possible to measure very precisely $e^+e^-\to Z\gamma$ process with subsequent Z decay into particles invisible in the detector, that is into three neutrina of the Standard Model and possibly into other weakly coupled neutral particles. Apart from the measurement of the total invisible width (which is not the main subject of this work) this process may be used as a source of $Z$ coupling to electron neutrino -- known very poorly. This is possible due to the presence of the $t$-channel $W$ exchange in the $e^+e^-\to \nu_e \bar\nu_e \gamma$ channel which deforms slightly spectrum of the photon. We are going to show experimental investigation of this effect, for $\sim 10$ inverse atobarn accumulated luminosity, which can provide measurement of the $Z-\nu_e$ coupling with statistical error of order 1\%. The estimation of the systematic experimental error will require more work, but most likely it will be of similar size., Comment: Ref. [10] updated

International audience; In early 2014 a design study started at CERN for a Future Circular Collider. A new tunnel with a circumference of about 100 km for the collider magnets is foreseen as well as new general-purpose particle detectors to probe electron-positron (e$^-$e$^+$), electron-hadron (eh), and hadron-hadron (hh) collisions, housed in large underground caverns. In the last four years baseline designs for the various detector magnets were developed. For the FCC-ee detector two magnet variants were defined: a 7.6-m bore and 7.9-m-long classical 2 T solenoid with 600 MJ stored energy, surrounding the calorimeters, and also a very challenging 4-m bore, 6-m-long, some 100-mm-thick ultrathin and radiation transparent 2 T solenoid with a stored energy of some 170 MJ, that surrounds only the inner tracker of the detectors. For the FCC-eh detector, the detector solenoid is combined with forward and backward dipole magnets required to guide the electron beam in and out of the collision point. This detector requires a 3.5 T solenoid, 2.6-m free bore and 9.2-m length with about 230 MJ of stored energy. Most demanding is the FCC-hh detector with a 14 GJ stored energy magnet system comprising three series connected solenoids, requiring 4 T in the main solenoid with 10-m free bore and a length of 20 m, in line with two 3.2 T forward solenoids with 5.1-m free bore and 4-m length. A quite challenging series of detector magnets is proposed, that needs to be further engineered in the coming years. The superconductor technology though is essentially the same in all the solenoids proposed: conductors comprising Rutherford type cables made of NbTi/Cu strands, stabilized by nickel doped pure aluminum and structurally reinforced with a high yield strength aluminum alloy. The cold masses are conduction cooled through helium cooling pipes welded to their outer support cylinder. The designs of the various baseline magnets as well as their engineering are presented.

International audience; In the Standard Model theory, the mass of the W boson is predicted with an uncertainty of 4 MeV/c$^{2}$ whereas the current experimental accuracy is of 12 MeV/c$^{2}$. The present uncertainty is currently limiting the sensitivity to possible effects of new physics through the global fit of the electroweak observables. With more than $2 \times 10^8$ W pairs produced at the W threshold energy and above, the FCC-ee collider will be a W boson factory allowing for W mass measurement with unparalleled precision. The W mass can be directly measured at and above the threshold from the kinematic reconstruction of the W-pair decay products. In addition, $e^+e^-$ collisions offer the possibility to derive the W mass from the WW cross-section measured at the pair-production threshold. The precise measurement of the W mass and width, with both methods, is presented in the context of a future experiment at FCC-ee. A statistical uncertainty on the W mass below 1 MeV/c$^{2}$ is expected and the experimental and theoretical systematic uncertainties must be reduced to match such a level of precision.

The FCC-ee is a high-luminosity, high-precision circular collider to be constructed in a new 100 km tunnel in the Geneva area. The physics case is well established and the FCC-ee operation is foreseen at 91 GeV (Z-pole), 160 GeV (W pair production threshold), 240 GeV (Higgs resonance) and 365 GeV (t-tbar threshold). Due to the large 6D production emittance and important thermal load in the production target, the positron injector, in particular the positron source, is one of the key elements of the FCC-ee, requiring special attention. To ensure high reliability of the positron source, conventional and hybrid targets are currently under study. The final choice of the positron target will be made based on the estimated performances. In this framework, we present a preliminary design of the FCC-ee positron source, with detailed simulation studies of positron production, capture and primary acceleration., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

International audience; The operation of the Large Hadron Collider (LHC) has been a great success with Run 1 culminating in the discovery of the Higgs boson by the ATLAS and CMS experiments in 2012. In Run 2, the LHC experiments have moved towards performing high precision measurements with uncertainties reaching below percent level for certain observables. Looking forward to the Future Circular Collider with electron beams (FCC-ee), which will operate in the experimentally much cleaner environment of electron-positron initial states, there will be an even more dramatic increase in experimental precision. In order to exploit the precision measurements, the theory community will need to provide high-precision predictions that match the experimental uncertainties. This requires the development of efficient ways to compute these corrections, breaking through the current computational bottlenecks.

The high luminosity electron-positron collider FCC-ee is part of the Future Circular Collider (FCC) study at CERN and it has been designed to cover the beam energy range from 45.6 GeV to 182.5 GeV to study the properties of the Higgs boson and other particles. Electron cloud build up simulations on the Z resonance revealed the necessity of minimising the Secondary Electron Yield (SEY) of the pipe walls by applying a Ti-Zr-V Non-Evaporable Getter (NEG) coating in the entire ring. Beam dynamics simulations at 45.6 GeV pointed out that minimising the thickness of this layer is mandatory to reduce the resistive wall (RW) impedance, thus increasing the single bunch instability thresholds and ensuring beam stability during operation. However, reducing the coating thickness can affect the performance of the material and therefore the SEY. For this reason, an extensive measurement campaign was performed at CERN to characterise NEG thin films with thicknesses below 250 nm in terms of activation performance and SEY measurements. This paper also presents the FCC-ee longitudinal impedance model which includes all the current machine components., Proceedings of the 62th ICFA ABDW on High Luminosity Circular e$^+$e$^-$ Colliders, eeFACT2018, Hong Kong, China

The first stage of the FCC (Future Circular Collider) is a high-luminosity electron-positron collider (FCC-ee) with centre-of-mass energy ranging from 88 to 365 GeV, to study with high precision the Z, W, Higgs and top particles, with samples of $5 \times 10^{12}$ Z bosons, $10^8$ W pairs, $10^6$ Higgs bosons and $10^6$ top quark pairs. A cornerstone of the physics program lays in the precise (ppm) measurements of the W and Z masses and widths, as well as forward-backward asymmetries. To this effect the centre-of-mass energy distribution should be determined with the high precision. This document describes the capacity offered by FCC-ee, starting with transverse polarization of the beams around the Z pole and the W pair threshold. A running scheme based on regular measurements of the beam energy by resonant depolarization of pilot bunches, during physics data taking, is proposed. The design for polarization wigglers, polarimeter and depolarizer is outlined. The $e^\pm$ beam energies will be monitored with a relative precision of $10^{-6}$. The centre-of-mass energy is derived subject to further corrections, related to the beam acceleration, synchrotron radiation and beamstrahlung; these effects are identified and evaluated. Dimuon events $e^+e^- \to ��^+ ��^-$, recorded in the detectors, provide with great precision the beam crossing angle, the centre-of-mass energy spread, and the $e^+$ and $e^-$ energy difference. Monitoring methods to minimize absolute error and relative uncertainties are discussed. The impact on the physics measurements is given. A programme of further simulations, design, monitoring and R&D is outlined., 107 pages, 15 tables, 58 figures