Collisions Research Articles

Testing Bell inequalities and probing quantum entanglement at CEPC

We study quantum entanglement and test violation of Bell-type inequality at the Circular Electron Positron Collider (CEPC), which is one of the most attractive future collides. It is a promising particle collider designed to search new physics, make Standard Model (SM) precision measurements, and serving as a Higgs factory. Our study is based on a fast simulation of the Z boson pair production from Higgs boson decay at s=250 GeV. The detector effects are also included in the simulation. The spin density matrix of the joint ZZ system is parametrized using irreducible tensor operators and reconstructed from the spherical coordinates of the decay leptons. To test Bell inequalities, we construct observable quantities for the H→ZZ* process in CEPC by using the Collins-Gisin-Linden-Massar-Popescu (CGLMP) inequality, whose value is determined from the density matrix of the Z boson pairs. The sensitivity of the Bell inequality violation is observed with more than 1σ and the presence of the quantum entanglement is probed with more than 2σ confidence level. Published by the American Physical Society 2025

The impact of data from future lepton colliders on light hadrons fragmentation functions

In this work, we study the constraining power of future lepton colliders on fragmentation functions (FFs) to light charged hadrons from quarks and gluon in the framework of QCD collinear factorization. We perform analyses of FFs at NLO by including a wide range of pseudo-data from future lepton colliders, such as measurements on hadron multiplicities in the production of two jets and W boson pairs, at various center of mass energies, and from hadronic decays of the Higgs boson, including both to heavy quarks and to gluons. The high luminosity and high energies of future lepton colliders allow for quark flavor separations and ensure a precise determination of FFs based solely on data from electron-positron collisions. We find that either the CEPC, FCC-ee or ILC can significantly reduce the uncertainties of FFs in a wide kinematic range, compared to the NPC23 set obtained with a global analysis to current world data. We also discuss the impact of higher-order QCD corrections, and the potential constraints from measurements of three-jet production. Furthermore, we describe an update of the FMNLO program allowing for calculating hadron production cross sections at next-to-next-to-leading order in QCD, which is used in this study.

A unified machine learning approach for reconstructing hadronically decaying tau leptons

Tau leptons serve as an important tool for studying the production of Higgs and electroweak bosons, both within and beyond the Standard Model of particle physics. Accurate reconstruction and identification of hadronically decaying tau leptons is a crucial task for current and future high energy physics experiments. Given the advances in jet tagging, we demonstrate how tau lepton reconstruction can be decomposed into tau identification, kinematic reconstruction, and decay mode classification in a multi-task machine learning setup. Based on an electron-positron collision dataset with full detector simulation and reconstruction, we show that common jet tagging architectures can be effectively used for these sub-tasks. We achieve comparable momentum resolutions of 2–3% with all the tested models, while the precision of reconstructing individual decay modes is between 80–95%. We find ParticleTransformer to be the best-performing approach, significantly outperforming the heuristic baseline. This paper also serves as an introduction to a new publicly available Fuτure dataset for the development of tau reconstruction algorithms. This allows to further study the resilience of ML models to domain shifts and the efficient use of foundation models for such tasks.

Measurement of the nonlinear diffusion of the proton beam halo at the CERN LHC

In circular particle accelerators, storage rings, or colliders, mitigating beam losses is critical to ensuring optimal performance, particularly for rings that include superconducting magnets. A thorough understanding of beam halo dynamics is essential for this purpose. This paper presents recent results for the measurement of the nonlinear diffusion process of the beam halo at the CERN Large Hadron Collider (LHC). The novel approach used in this paper is based on the analytical framework of the Nekhoroshev theorem, which provides a functional form for the diffusion coefficient. By monitoring the beam loss signal during controlled movements of the collimator jaws, we determine the beam losses at equilibrium for various amplitudes and analyse the beam halo distribution. Postprocessing of these measurements provides the nonlinear diffusion coefficient, which is found to be in excellent agreement with the theoretical assumptions. Measurements from an experiment investigating the effectiveness of beam-beam compensation using beam-beam compensation wires also provide a direct assessment of the compensation’s effectiveness on beam-tail diffusion.

Core software for the super tau-charm facility

The Super Tau Charm Facility (STCF) is a new generation electron–positron collider, proposed to study various physics topics in the tau charm energy region. STCF will operate within a center-of-mass energy spanning 2 to 7[Formula: see text]GeV, with a peak luminosity of [Formula: see text] [Formula: see text]cm[Formula: see text]s[Formula: see text], which means STCF will produce two orders of magnitude more data compared to BESIII each year, posing great challenges for the data processing software. To support the intricate requirements from the offline data processing tasks, as well as to tackle this immense challenge caused by the huge data volume, the offline software for STCF (OSCAR) is designed and developed, based on the state-of-the-art technology and toolkits in the high energy physics community. This paper provides an overview of the design and implementation of OSCAR, highlighting its event data management system, parallel data processing capabilities leveraging SNiPER and TBB, and its geometry management framework powered by DD4hep. Presently, OSCAR is fully operational, effectively supporting the conceptual design of the STCF detector and exploring its vast physics potential.

Measurements of decay branching fractions of the Higgs boson to hadronic final states at the CEPC

Abstract The Circular Electron Positron Collider (CEPC) is a large-scale particle accelerator designed to collide electrons and positrons at high energies. One of the primary goals of the CEPC is to achieve high-precision measurements of the properties of the Higgs boson, facilitated by the large number of Higgs bosons that can be produced with significantly low contamination. The measurements of Higgs boson branching fractions into bb/cc/gg and tautau/WW*/ZZ*, where the W or Z bosons decay hadronically, are presented in the context of the CEPC experiment, assuming a scenario with 5600 fb-1 of collision data at a center-of-mass energy of 240 GeV. In this study the Higgs bosons are produced in association with a Z boson, with the Z boson decaying into a pair of muons (μ+μ-), which have high efficiency and high resolution. In order to separate all decay channels simultaneously with high accuracy, the Particle Flow Network (PFN), a graph-based machine learning model, is considered. The precise classification provided by the PFN is employed in measuring the branching fractions using the migration matrix method, which accurately corrects for detector effects in each decay channel. The statistical uncertainty of the measured branching ratio is estimated to be 0.55% in H → bb final state, and approximately 1.5%-16% in H → cc/gg/tautau/WW*/ZZ* final states. In addition, the main sources of systematic uncertainties to the measurement of the branching fractions are discussed. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.

Study of D_{s}^{+}→f_{0}(980)ρ^{+} and ϕπ^{+} Decays through D_{s}^{+}→π^{+}π^{+}π^{-}π^{0}.

We perform the first amplitude analysis of D_{s}^{+}→π^{+}π^{+}π^{-}π^{0} decays based on data samples of electron-positron collisions recorded with the BESIII detector at center-of-mass energies between 4.128 and 4.226GeV, corresponding to an integrated luminosity of 7.33 fb^{-1}. We report the observation of D_{s}^{+}→f_{0}(980)ρ(770)^{+} with a statistical significance greater than 10σ and determine the branching fractions B(D_{s}^{+}→π^{+}π^{+}π^{-}π^{0}|_{non-η})=(2.04±0.08_{stat}±0.05_{syst})% and B(D_{s}^{+}→ηπ^{+})=(1.56±0.09_{stat}±0.04_{syst})%. Moreover, we measure the relative branching fraction between ϕ→π^{+}π^{-}π^{0} and ϕ→K^{+}K^{-} to be [B(ϕ(1020)→π^{+}π^{-}π^{0})/B(ϕ(1020)→K^{+}K^{-})]=0.230±0.014_{stat}±0.010_{syst}., which deviates from the world average value by more than 4σ.

Development of a high gradient superconducting quadrupole experimental magnet in CEPC interaction region

Compact high-gradient quadrupole magnets are required on both sides of the interaction points (IP) in the Circular Electron Positron Collider (CEPC) interaction region (IR). The dual-aperture superconducting quadrupole magnet Q1a is positioned closest to the IP with a crossing angle of 33 mrad between the two apertures. The electromagnetic design and development of Q1a faces great challenges due to the difficulties associated with the small aperture, high gradient and the magnetic field interference. This paper presents the electromagnetic design scheme of CEPC IR quadrupole magnets and the development of a high gradient superconducting quadrupole experimental magnet. Firstly, by comparing the electromagnetic properties and technological characteristics of three commonly used quadrupole superconducting coils, the design scheme of the superconducting quadrupole magnet for the CEPC final focusing system is determined. Then, the electromagnetic design and optimization of a cos 2θ experimental magnet is presented, the manufacturing process of the experimental magnet is introduced. Finally, the cryogenic excitation test of the experimental magnet is described and the physical design scheme of the high-gradient superconducting magnet for final focusing system in CEPC IR is verified.

A methodology for timing performance optimizationof the pre-amplifier design in High Energy Physics

Particle colliders operating at an increasing luminosity for High Energy Physics (HEP) experiments often present greater challenges in the design of the front-end readout circuits, especially when exploring the possibility of employing Depleted Monolithic Active Pixel Sensors (DMAPS) for timing in HEP. This paper presents a methodology for the timing performance optimization in the context of the future collider experiments. First a simplified model is presented for theoretical performance analysis and the validity is tested. Then an optimization process is done to find the best parameters regarding the trade-offs from the application requirements.

95 GeV Higgs boson and spontaneous CP -violation at the finite temperature

The ATLAS and CMS collaborations reported a diphoton excess in the invariant mass distribution around the 95.4 GeV with a local significance of 3.1σ. Moreover, there is another 2.3σ local excess in the bb¯ final state at large electron-positron collider in the same mass region. A plausible solution is that the Higgs sector is extended to include an additional Higgs boson with a mass of 95.4 GeV. We study a complex singlet scalar extension of the two-Higgs-doublet model in which the 95.4 GeV Higgs is from the mixing of three CP-even Higgs fields. In addition, the extended Higgs potential can achieve spontaneous CP-violation at the finite temperature and restore CP symmetry at the present temperature of the Universe. We find that the model can simultaneously explain the baryon asymmetry of the Universe, the diphoton and bb¯ excesses around the 95.4 GeV while satisfying various relevant constraints including the experiments of collider and electric dipole moment. Published by the American Physical Society 2024

Generalised antenna functions for higher-order calculations

In this paper we discuss the definition, the construction and the implementation of generalised antenna functions for final-state radiation up to Next-to-Next-to-Leading Order (NNLO) in QCD. Generalised antenna functions encapsulate the singular behaviour of unresolved emissions when these occur within multiple hard radiators and not just two of them, as for traditional antenna functions. The construction of such objects is possible thanks to the recently proposed algorithm for building idealised antenna functions from a target set of infrared limits. Generalised antenna functions bring major simplifications in the assemblage of subtraction terms in the context of the antenna scheme at NNLO and beyond, as well as a substantial computational speedup of higher-order calculations. We discuss in detail the improvements on the formal and practical side for the computation of the NNLO correction to three-jet production at electron-positron colliders, providing a thorough numerical validation of the newly proposed scheme. For this calculation one can expect almost an order of magnitude speedup with respect to the original implementation.

W mass and muon g−2 in an inert 2HDM extended by a singlet complex scalar

The deviations of the recent measurements of the muon magnetic moment and the W-boson mass from their SM predictions hint to new physics beyond the SM. In this article, we address the observed discrepancies in the W-boson mass and muon anomalous magnetic moment in the Inert Two Higgs Doublet Model extended by a complex scalar field singlet under the SM gauge group. The model is constrained from the existing Large Electron Positron Collider (LEP) data and the measurements of partial decay widths to gauge bosons at LHC. It is shown that a large subset of the constrained parameter space of the model can accommodate both the experimentally measured as well as SM global fit value of W-boson mass while simultaneously explaining the observed muon g−2 anomaly. Published by the American Physical Society 2024

Unified interpretation of the muon g−2 anomaly, the 95 GeV diphoton, and bb¯ excesses in the general next-to-minimal supersymmetric standard model

We investigate three intriguing anomalies within the framework of the general next-to-minimal supersymmetric standard model. These anomalies include a significant deviation of the experimental results for the muon anomalous magnetic moment from its standard model prediction, with a confidence level of 5.1σ; a joint observation by the CMS and ATLAS Collaborations of a diphoton excess with a local significance of 3.1σ in the invariant mass distribution around 95.4 GeV; and a reported excess in the bb¯ production at the Large Electron-Positron Collider with a local significance of 2.3σ. Through analytical and numerical analyses, we provide unified interpretations across an extensive parameter space that remain consistent with current experimental restrictions from data on the Higgs boson at 125 GeV, B-physics measurements, and dark matter observables, as well as existing searches for supersymmetry and extra Higgs bosons. We attribute the muon anomaly to loops involving muon-smuon-neutralino and muon-sneutrino-chargino interactions, while attributing the diphoton and bb¯ excesses to the resonant production of a singlet-dominated scalar. These proposed solutions are poised for experimental tests at the high-luminosity LHC and future linear colliders. Published by the American Physical Society 2024

Online reinforcement learning control for maintaining an optimum beam collision on an electron-positron collider

For double-ring colliders, it is a vital task to make the two beams meeting each other properly at the interaction point. BEPCII is a double-ring collider that operates in the decay mode, as the beam currents decrease over time, the beam orbits need to be continuously adjusted to maintain the optimum collision conditions. Originally, this task was carried out manually, with operators adjusting three offset control knobs (x,y,y′) according to the luminosity. There is a crucial necessity to implement advanced automated control approaches. However, the feedback methods are ineffective due to the constraints imposed by machine characteristics. Nevertheless, the optimization methods are also limited by the slow response speed of BEPCII and have intrinsic limitations. Reinforcement learning (RL), which learns from past experiences, provides a new approach for handling such problems. In this paper, we implemented two automated control methods: the dither method and the deep Q-network (DQN) RL method. The dither method is a numerical optimization method used to provide more online data for DQN training. With the help of the historical data from the dither method, we successfully trained a DQN agent to control the offset knobs to optimize the luminosity. The DQN method exhibited significantly better performance than the dither method, achieving faster optimization speeds and yielding higher integral luminosity. Furthermore, the DQN method has been integrated into the daily operations of BEPCII, achieving long-term stable deployment and effectively substituting manual labor. Published by the American Physical Society 2024

95 GeV excesses in the Z3 -symmetric next-to-minimal supersymmetric standard model

Recent analyses by CMS and ATLAS suggest a deviation in the diphoton channel at approximately 95 GeV, alongside a previously observed excess in bb¯ signals at a similar mass by the Large Electron-Positron collider, potentially hinting at a new scalar particle. This study explores this possibility within the framework of the Z3-symmetric next-to-minimal supersymmetric standard model. A comprehensive parameter scan was conducted, integrating constraints from dark matter relic density, direct detection experiments, and the properties of the observed 125 GeV Higgs boson. The results demonstrate that the model can accommodate the observed excesses with a singlet-dominated CP-even scalar boson near 95 GeV. The model accurately predicts signal strengths of the diphoton and bb¯ channels at a level of 1σ. Furthermore, it accounts for the measured dark matter relic abundance through bino-dominated neutralinos coannihilation with winolike electroweakinos, all while remaining consistent with existing Large Hadron Collider (LHC) constraints. These findings pave the way for future validation at the high-luminosity LHC and linear colliders, which may offer crucial tests of the model’s predictions. Published by the American Physical Society 2024

Search potential of the high energy-large hadron collider for spin-1/2 excited quarks in di-jet final state

Composite models, which suggest a possible substructure of fundamental particles, can be directly proven by the discovery of the excited quark. Higher energy and higher-luminosity particle colliders are needed to discover the composite structure predicted in the proposed models. The High Energy Large Hadron Collider (HE-LHC) has the potential to be a possible discovery machine for composite models. In this collider, with a center-of-mass energy of 27 TeV and integrated luminosity between 750 and 15000 fb\textsuperscript{-1}, we calculated the exclusion, observation, and discovery limits for the mass of spin-1/2 excited quark in the \textit{di-jet} final state, as well as the attainable compositeness scale values. In addition to these calculations, we scanned free parameters from 0.06 to 1 to determine the HE-LHC potential to reveal spin-1/2 excited quark.

Mixed QCD-EW corrections to W-pair production at electron-positron colliders

The discrepancy between the CDF measurement and the Standard Model theoretical prediction for the W-boson mass underscores the importance of conducting high-precision studies on the W boson, which is one of the predominant objectives of proposed future e+e− colliders. We investigate in detail the production of W-boson pairs at e+e− colliders, and compute the next-to-next-to-leading order mixed QCD-EW corrections to both the integrated cross section and various kinematic distributions. By employing the method of differential equations, we analytically calculate the two-loop master integrals for the mixed QCD-EW virtual corrections to e+e− → W+W−. Utilizing the Magnus transformation, we derive a set of canonical master integrals for each integral family. This canonical basis satisfies a system of differential equations in which the dependence on the dimensional regulator is linearly factorized from the kinematics. We then express all these canonical master integrals as Taylor series in ϵ up to ϵ4, with coefficients articulated in terms of Goncharov polylogarithms up to weight four. Upon applying our analytic expressions of these master integrals to the phenomenological analysis of W-pair production, we observe that the O(ααs) corrections are significantly impactful in the α(0) scheme, particularly in certain phase-space regions. However, these mixed QCD-EW corrections can be heavily suppressed by adopting the Gμ scheme.

Scaling Violation in Power Corrections to Energy Correlators from the Light-Ray Operator Product Expansion.

In recent years, energy correlators have emerged as a powerful tool to explore the field theoretic structure of strong interactions at particle colliders. In this Letter we initiate a novel study of the nonperturbative power corrections to the projected N-point energy correlators in the limit where the angle between the detectors is small. Using the light-ray operator product expansion as a guiding principle, we derive the power corrections in terms of two nonperturbative quantities describing the fragmentation of quarks and gluons. In analogy with their perturbative leading-power counterpart, we show that power corrections obey a classical scaling behavior that is violated at the quantum level. This crucially results in a dependence on the hard scale Q of the problem that is calculable in perturbation theory. Our analytic predictions are successfully tested against MonteCarlo simulations for both lepton and hadron colliders, marking a significant step forward in the understanding of these observables.

Raoul Gatto and Bruno Touschek’s joint legacy in the rise of electron–positron physics

Raoul Gatto and Bruno Touschek’s collaboration in the establishment of electron–positron colliders as a fundamental discovery tool in particle physics will be illustrated. In particular, we will tell the little-known story of how Gatto and Touschek’s pioneering visions combined to provide the theoretical foundation for AdA, the first matter–antimatter collider, and how their friendship with Wolfgang Pauli and Gerhard Lüders was crucial to their understanding of the CPT theorem, the basis for AdA’s success. We will see how these two exceptional scientists shaped physics between Rome and Frascati, from the proposal to build AdA and soon after the larger machine ADONE in 1961, to the discovery of the J/Ψ particle in 1974. We will also highlight Gatto and Touschek’s contribution in mentoring an extraordinary cohort of students and collaborators whose work contributed to the renaissance of Italian theoretical physics after the Second World War and to the establishment of the Standard Model of particle physics.

First Stability Characterization for a CZT Detection System in an e+e- Collider Environment.

The SIDDHARTA-2 collaboration has developed a novel X-ray detection system based on cadmium-zinc-telluride (CZT, CdZnTe), marking the first application of this technology at the DAΦNE electron-positron collider at INFN-LNF. This work aims to demonstrate the stability of the detectors' performance in terms of linearity and resolution over short and long periods, thereby establishing their suitability for precise spectroscopic measurements within a collider environment. A reference calibration spectrum is presented in association with findings from assessments of linearity and resolution stability. Additionally, this study introduces a validated model of the response function of the detector. The relative deviations from the nominal values for the source transitions, obtained by fitting the entire spectrum with a background function and the previously introduced response function, are reported. Finally, a comparison of the calibration performance with and without beams circulating in the collider's rings is presented. These promising results pave the way for applying CZT detectors in kaonic atom studies and, more generally, in particle and nuclear physics spectroscopy.