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The super τ-charm facility (STCF) is an electron−positron collider proposed by the Chinese particle physics community. It is designed to operate in a center-of-mass energy range from 2 to 7 GeV with a peak luminosity of 0.5 × 10 35 cm −2·s −1 or higher. The STCF will produce a data sample about a factor of 100 larger than that of the present τ-charm factory - the BEPCII, providing a unique platform for exploring the asymmetry of matter-antimatter (charge-parity violation), in-depth studies of the internal structure of hadrons and the nature of non-perturbative strong interactions, as well as searching for exotic hadrons and physics beyond the Standard Model. The STCF project in China is under development with an extensive R&D program. This document presents the physics opportunities at the STCF, describes conceptual designs of the STCF detector system, and discusses future plans for detector R&D and physics case studies.

We present first results for a parameter study of the plasma linac of HALHF, a novel electron-positron collider concept combining plasma wakefield acceleration and mature RF acceleration to reach a centre of mass energy of 250 GeV. This study is a preliminary extension of the previous studies that indicated promising performance, by including transverse instability. Transverse instability was simulated using start-to-end simulations, where PIC simulations were combined with a simplified model to efficiently model transverse instability in the plasma-acceleration stages. No interstage optics were included. With a small initial transverse offset, the currently proposed parameter set will result in emittance growth and charge losses. If additional transverse misalignments are introduced between stages in the form of drive beam transverse jitter, the emittance growth is even more severe. Mitigation mechanisms to reduce the instability to an acceptable level are suggested.

The construction of an electron–positron collider ‘Higgs factory’ has been stalled for a decade, not because of feasibility but because of the cost of conventional radio-frequency (RF) acceleration. Plasma-wakefield acceleration promises to alleviate this problem via significant cost reduction based on its orders-of-magnitude higher accelerating gradients. However, plasma-based acceleration of positrons is much more difficult than for electrons. We propose a collider scheme that avoids positron acceleration in plasma, using a mixture of beam-driven plasma-wakefield acceleration to high energy for the electrons and conventional RF acceleration to low energy for the positrons. We emphasise the benefits of asymmetric energies, asymmetric bunch charges and asymmetric transverse emittances. The implications for luminosity and experimentation at such an asymmetric facility are explored and found to be comparable to conventional facilities; the cost is found to be much lower. Some of the areas in which R&D is necessary to make HALHF a reality are highlighted, including estimates for the improvement required in key technologies. These range from a factor of 10 to a factor of 1000.

Momentum maps connecting phase spaces of different multiplicities are widely used in precision calculations in QCD and collider phenomenology in general. The antenna mapping is a colour-ordered map which allows to absorb the recoil of an arbitrary number of emissions onto two hard radiators. Due to the rapid convergence in soft and collinear configurations, the antenna mapping is extensively used for fixed-order calculations within the antenna subtraction method, as well as in parton showers such as the VINCIA shower. However, colour-ordered momentum mappings cannot be directly applied in the presence of multiple unordered emissions of quarks and gluons, which typically appear beyond the leading-colour approximation. To address this technical challenge, we present, in the context of the antenna subtraction method, a simple decomposition of the phase space into sectors to isolate classes of infrared configurations which can be unambiguously assigned to a specific momentum ordering. In each sector, the singularities of any matrix element can be subtracted relying on a colour-ordered map. We illustrate the required phase-space sectors for up to three unordered emissions. The mapping algorithm described here has been recently employed for the first differential N 3 LO calculation of jet production at electron–positron colliders.

Despite mesons being one of the longest known type of particles, there are still many open questions. Besides well understood states that can be clearly attributed to meson nonets, there are many candidates which could have an exotic nature instead. Such exotic particles e.g. glueballs, hybrids and tetraquarks can be favorably studied in clean, gluon rich environments. The BE-SIII experiment, which is in operation at the BEPCII electron-positron collider in Beijing since 2009, has collected world leading high statistic data samples in the charmonium region. This allows to study rare reactions that are considered to be suppressed. This offers unique possibilities to study exotic QCD states in the charmonium sector, but also the light meson spectrum which can be ac cessed via charmonium decays. Especially radiative J/ψ decays offer a gluon rich environment in which glueballs and hybrid states can be expected. Since these states are often hard to identify and disentangle, partial wave analysis are needed to determine the different contributions.

Volume 1 of the FCC Feasibility Report presents an overview of the physics case, experimental programme, and detector concepts for the Future Circular Collider (FCC). This volume outlines how FCC would address some of the most profound open questions in particle physics, from precision studies of the Higgs and EW bosons and of the top quark, to the exploration of physics beyond the Standard Model. The report reviews the experimental opportunities offered by the staged implementation of FCC, beginning with an electron-positron collider (FCC-ee), operating at several centre-of-mass energies, followed by a hadron collider (FCC-hh). Benchmark examples are given of the expected physics performance, in terms of precision and sensitivity to new phenomena, of each collider stage. Detector requirements and conceptual designs for FCC-ee experiments are discussed, as are the specific demands that the physics programme imposes on the accelerator in the domains of the calibration of the collision energy, and the interface region between the accelerator and the detector. The report also highlights advances in detector, software and computing technologies, as well as the theoretical tools/reconstruction techniques that will enable the precision measurements and discovery potential of the FCC experimental programme. The content and structure of this report are guided by the scope and priorities defined in the mandate of the FCC Feasibility Study. It is therefore not intended to serve as an exhaustive review of the full physics potential of FCC. Several topics, already covered in earlier reports such as the FCC CDR, are not reiterated here or are addressed only briefly, in alignment with the study’s focus. This volume reflects the outcome of a global collaborative effort involving hundreds of scientists and institutions, aided by a dedicated community-building coordination, and provides a targeted assessment of the scientific opportunities and experimental foundations of the FCC programme.

The compatibility of W -boson mass measurements performed by the ATLAS, LHCb, CDF, and D0 experiments is studied using a coherent framework with theory uncertainty correlations. The measurements are combined using a number of recent sets of parton distribution functions (PDF), and are further combined with the average value of measurements from the Large Electron–Positron collider. The considered PDF sets generally have a low compatibility with a suite of global rapidity-sensitive Drell–Yan measurements. The most compatible set is CT18 due to its larger uncertainties. A combination of all m W measurements yields a value of m W = 80 , 394.6 ± 11.5  MeV with the CT18 set, but has a probability of compatibility of 0.5% and is therefore disfavoured. Combinations are performed removing each measurement individually, and a 91% probability of compatibility is obtained when the CDF measurement is removed. The corresponding value of the W boson mass is 80 , 369.2 ± 13.3  MeV, which differs by 3.6 σ from the CDF value determined using the same PDF set.

Particle physics has arrived at an important moment of its history. The discovery of the Higgs boson has completed the Standard Model, the core theory behind the known set of elementary particles and fundamental interactions. However, the Standard Model leaves important questions unanswered, such as the nature of dark matter, the origin of the matter–antimatter asymmetry in the Universe, and the existence and hierarchy of neutrino masses. To address these questions and the origin of the newly discovered Higgs boson, high-energy colliders are required. Future generations of such machines must be versatile, as broad and powerful as possible with a capacity of unprecedented precision, sensitivity and energy reach. Here, we argue that the Future Circular Colliders offer unique opportunities, and discuss their physics motivation, key measurements, accelerator strategy, research and development status, and technical challenges. The Future Circular Collider integrated programme foresees operation in two stages: initially an electron–positron collider serving as a Higgs and electroweak factory running at different centre-of-mass energies, followed by a proton–proton collider at a collision energy of 100 TeV. The interplay between measurements at the two collider stages underscores the synergy of their physics potentials. The Future Circular Colliders are proposed as a future step after the Large Hadron Collider has stopped running. The first stage foresees collision of electron–positron pairs before a machine upgrade to allow proton–proton operation.

A bstract The Compact Linear Collider (CLIC) is a proposed future high-luminosity linear electron-positron collider operating at three energy stages, with nominal centre-of-mass energies s = 380 GeV, 1 . 5 TeV, and 3 TeV. Its aim is to explore the energy frontier, providing sensitivity to physics beyond the Standard Model (BSM) and precision measurements of Standard Model processes with an emphasis on Higgs boson and top-quark physics. The opportunities for top-quark physics at CLIC are discussed in this paper. The initial stage of operation focuses on top-quark pair production measurements, as well as the search for rare flavour-changing neutral current (FCNC) top-quark decays. It also includes a top-quark pair production threshold scan around 350 GeV which provides a precise measurement of the top-quark mass in a well-defined theoretical framework. At the higher-energy stages, studies are made of top-quark pairs produced in association with other particles. A study of t ̄ tH production including the extraction of the top Yukawa coupling is presented as well as a study of vector boson fusion (VBF) production, which gives direct access to high-energy electroweak interactions. Operation above 1 TeV leads to more highly collimated jet environments where dedicated methods are used to analyse the jet constituents. These techniques enable studies of the top-quark pair production, and hence the sensitivity to BSM physics, to be extended to higher energies. This paper also includes phenomenological interpretations that may be performed using the results from the extensive top-quark physics programme at CLIC.