Collisions Research Articles

Investigation of the concurrent effects of ALP-photon and ALP-electron couplings in Collider and Beam Dump Searches

Axion-like particles (ALPs) have been studied in numerous experiments to search for their interactions, but most studies have focused on deriving bounds for the single coupling. However, in ultraviolet (UV) models, these couplings can appear simultaneously, and their interplay could have important implications for collider and beam dump searches. In this study, we investigate the concurrent effects of the ALP-photon and ALP-electron couplings in a simplified model and examine how their simultaneous presence modifies existing bounds. We find that modifications to production cross-sections, decaying branching ratios, and the lifetime of the ALP are the major effects. Our results show that low-energy electron-positron colliders such as Belle-II and BaBar are primarily affected by the first two factors, while beam dump experiments such as E137 and NA64 are affected by the cross sections and lifetime. We also consider two UV models — the KSVZ-like model and a lepton-specific version of the DFSZ model — which have only one of the two couplings at tree-level. However, the other coupling can be generated at loops, and our analysis reveals that the simultaneous presence of the two couplings can significantly modify existing bounds on these models for 10−3< ma< 10 GeV, especially for beam dump experiments. Overall, our study highlights the importance of considering the concurrent effects of the ALP-photon and ALP-electron couplings in future collider and beam dump analyses.

Measurement of the effective weak mixing angle at the CEPC* *Supported by the "USTC Research Funds of the Double First-Class Initiative", the International Partnership Program of Chinese Academy of Sciences (113111KYSB20190030) and the Innovative Scientific Program of Institute of High Energy Physics

We present a study of the measurement of the effective weak mixing angle parameter () at the Circular Electron Positron Collider (CEPC). As a fundamental physics parameter, plays a key role not only in the global test of the standard model electroweak sector, but also in constraining the potential beyond standard model new physics at the high energy frontier. CEPC proposes a two year running period around the Z boson mass pole at high instataneous luminosity, providing a large data sample with candidates generated in total. It allows a high precision measurement of both in the lepton and quark final states, where the uncertainty can be one order of magnitude lower than any previous measurement at the LEP, SLC, Tevatron, and LHC. It will improve the overall precision of the experimental determination to be comparable to the preicision of the theoretical calculation with two-loop radiative corrections, and it will also provide direct comparisons between different final states. In this paper, we also study the measurement of in the high mass region. Taking data for one month, the precision of measured at 130 GeV from b quark final state is 0.00010, which will be an important experimental observation on the energy-running effect of .

Comprehensive dynamic numerical simulation of cooling process for a 1.3GHz 9-cell superconducting cavity based on different structures

As an indispensable component of X-ray free electron lasers and future ring electron-positron colliders, a 1.3 GHz 9-cell superconducting cavity provides higher acceleration voltage and higher RF power per unit length, and saves equipment space. During testing and operation, superconducting cavities often need to be cooled gradually from the ambient temperature (300 K) to the superconducting temperature (4.5 K or less). To avoid plastic deformation of materials and seal leaking caused by high thermal stress, the cooling rates in each stage and the temperature differences on the cavity must be properly controlled. Currently, a less effective manual control strategy is typically used, whose main limitations include a low level of automation, a high reliance on the experience of the operating personnel, and a fairly lengthy time due to low efficiency. In this paper, a 1.3 GHz 9-cell superconducting cavity and its helium vessel were taken as the research subjects, six different mechanical structures of the helium vessel were designed, and six different three-dimensional thermal-flow coupling models were established. For each structural model, the temperature distribution and cooling strategy were analyzed using fluid numerical simulation software. The influence of inlet parameters on the cooling process was analyzed, and the appropriate mechanical structure model was recommended. The results showed that: During the cooling process, the closer the superconducting cavity is to the inlet, the lower the temperature is. The cooling rate is faster in the early stage in which the temperature difference can be reduced by 50 % within the first 30 min. After experimental test verification, the total cooling time is about 650 min (10.8 h or so), and the deviation between the simulation results and the experimental test results is less than 7 %, which shows the simulation results can well predict the actual cooling process for 1.3 GHz 9-cell superconducting cavity. The research work in this paper can lay a good foundation for the further R&D status of the superconducting cavity.

Collimation, compression and acceleration of isotropic hot positrons by an intense vortex laser

Laser-driven positron sources, characterized by short pulse width, small focal spot and high energy, are promising for potential applications, e.g. electron–positron collider and positron annihilation spectroscopy. However, the broad divergence angle and wide pulse width during the laser-driven positron transport are extremely unfavorable for achieving high spatiotemporal resolution. In this paper, we propose a novel method to manipulate the positrons by using a left-hand circularly-polarized Laguerre–Gaussian (LG) laser pulse. Using the LG laser with a intensity of and a duration of a few cycles, three-dimensional particle-in-cell simulations reveal that isotropic hot positrons can be effectively captured, collimated, compressed, and accelerated due to the unique field structure of the LG laser. A high-quality positron bunch is obtained with a peak divergence angle of 1∘, an average pulse duration of 0.5 fs, a maximum energy of 450 MeV, and a density of 70 times that of the initial electron source. A damping vibration model is also formulated to explain qualitatively the quality improvement of the positrons.

Magnetic pinching of relativistic particle beams: a new approach to strong-field QED physics

Quantum electrodynamics (QED) is a foundation of modern physics, yet access to the strong-field QED regime in the laboratory remains a formidable challenge. Currently, high-power lasers at the multi-petawatt level and above are generally believed to be an important approach to test QED physics. Here, we present a different approach by use of an electron beam self-pinched to near-solid-density. The beam self-pinching is realized while it transports through a properly designed hollow cone target, where strong azimuthal magnetic fields are generated by the beam-induced plasma return currents at the inner surface of the cone target. In this way, the beam diameter can be reduced by more than an order of magnitude down to submicron and its density is increased by hundreds of times. The produced ultradense electron beams can unlock a new regime of QED-dominated beam–plasma interactions, for example, more than 60% of the beam energy can be converted into GeV gamma-rays with unprecedented brilliance when such a beam passes through a thin solid foil. Moreover, with proper parameter design, this beam-focusing scheme can also be applied to positron beams and thus may find applications in broad areas, such as particle colliders and strong-field physics.

A hybrid, asymmetric, linear Higgs factory based on plasma-wakefield and radio-frequency acceleration

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.

Conceptual Design of CEPC Detector Magnet

The Circular Electron Positron Collider (CEPC) will measure the Higgs boson properties in great detail. The detector needs a 3 Tesla superconducting solenoid followed by a muon detector embedded in a flux return yoke. This paper introduces the conceptual design of the superconducting solenoid includes two schemes based on LTS and HTS technology. The fundamental parameters and structures of magnets are discussed, and the advancements in electromagnetic design, quench calculation, cryogenic system design, and conductor development are outlined. The conductors used in the two schemes, the aluminum stabilized NbTi/Cu Rutherford cable and the Aluminum stabilized Stacked REBCO Tape Cable (ASTC), are also under development. The viability of the first-ever HTS aluminum stabilized cable proposal is confirmed, and the technology of co-extrusion aluminum coating has been preliminary mastered.

Radiation-dominated injection of positrons generated by the nonlinear Breit–Wheeler process into a plasma channel

Plasma acceleration is considered a prospective technology for building a compact multi-TeV electron–positron collider in the future. The challenge of this endeavor is greater for positrons than for the electrons because usually the self-generated fields from laser–plasma interaction are not well-suited for positron focusing and on-axis guiding. In addition, an external positron source is required, while electrons are naturally available in the plasma. Here, we study electron–positron pair generation by an orthogonal collision of a multi-PW laser pulse and a GeV electron beam by the nonlinear Breit–Wheeler process. We studied conditions favorable for positron deflection in the direction of the laser pulse propagation, which favors injection into the plasma for further acceleration. We demonstrate using the OSIRIS particle-in-cell framework that the radiation reaction triggered by ultra-high laser intensity plays a crucial role in the positron injection. It provides a suppression of the initial transverse momentum gained by the positrons from the Breit-Wheeler process. For the parameters used in this work, the intensity of at least 2.2×1023 W/cm2 is needed in order to inject more than 1% of positrons created. Above this threshold, the percentage of injected positrons rapidly increases with intensity. Moreover, subsequent direct laser acceleration of positrons in a plasma channel, using the same laser pulse that created them, can ensure a boost of the final positron energy by a factor of two. The positron focusing and guiding on the axis is provided by significant electron beam loading that changes the internal structure of the channel fields.

Exploring beam size measurement based on the Talbot effect at BEPCII.

Vertical beam size measurements were carried out at Beijing Electron-Positron Collider II (BEPCII) using a phase grating and an absorption grating based on the Talbot effect. The transverse coherence of synchrotron radiation is closely related to beam size. Due to the partial coherence of the synchrotron radiation source, the coherence length can be calculated by measuring the visibility decay of interferograms recorded at different distances behind the gratings. A vertical beam size of 68.19 ± 2 µm was obtained based on the relationship between the coherence length and beam size at the 3W1 beamline of BEPCII. A comparison of the vertical emittance derived from the grating Talbot method and the synchrotron radiation visible light interferometer method was presented. The vertical emittances from the two methods are 1.41 nm rad and 1.40 nm rad, respectively. The 0.1% difference indicates that the grating Talbot method for beam size measurement is reliable. This technique has great potential for small beam size measurement of fourth-generation synchrotron radiation light sources, considering its small diffraction limitation and simple experimental setups.

A spectral metric for collider geometry

By quantifying the distance between two collider events, one can triangulate a metric space and reframe collider data analysis as computational geometry. One popular geometric approach is to first represent events as an energy flow on an idealized celestial sphere and then define the metric in terms of optimal transport in two dimensions. In this paper, we advocate for representing events in terms of a spectral function that encodes pairwise particle angles and products of particle energies, which enables a metric distance defined in terms of one-dimensional optimal transport. This approach has the advantage of automatically incorporating obvious isometries of the data, like rotations about the colliding beam axis. It also facilitates first-principles calculations, since there are simple closed-form expressions for optimal transport in one dimension. Up to isometries and event sets of measure zero, the spectral representation is unique, so the metric on the space of spectral functions is a metric on the space of events. At lowest order in perturbation theory in electron-positron collisions, our metric is simply the summed squared invariant masses of the two event hemispheres. Going to higher orders, we present predictions for the distribution of metric distances between jets in fixed-order and resummed perturbation theory as well as in parton-shower generators. Finally, we speculate on whether the spectral approach could furnish a useful metric on the space of quantum field theories.

Probing lepton flavor violation at Circular Electron-Positron Colliders

Lepton flavor violation is one of the cleanest probes of physics beyond the standard model. In this work, we explore the sensitivity of the process e+e− → τμ to new physics above the TeV scale at the proposed circular electron-positron colliders FCC-ee and CEPC. We compute the e+e− → τμ cross-section in the Standard Model Effective Field Theory and assess the relevant backgrounds. We compare our sensitivity projections to existing and expected constraints from tau decays and Z decays and find that the future electron-positron colliders provide competitive probes of new physics. We highlight the complementarity of searches for resonant e+e−→ Z → τμ production on the Z pole and searches for non-resonant e+e−→ τμ at higher center-of-mass energies.

Characterization of a CMOS Pixel Sensor prototype for the CEPC vertex detector

The proposed Circular Electron Positron Collider poses significant challenges for the vertex detector in terms of material budget, spatial resolution, readout speed, and power consumption. To address these challenges, a Monolithic Active Pixel Sensor prototype, named TaichuPix, has been developed based on a column drain readout architecture. The performance of the second version of this prototype, TaichuPix2, has been evaluated using a radioactive source, which exhibits a time walk of less than 100ns corresponding to the threshold of 380 e− and amplitude variation of analog output from 300mV to 500mV. An infrared laser setup is utilized to estimate the single-point spatial resolution of this sensor for minimum-ionizing particles. The setup includes a high-precision three-dimensional translation stage and an optical system with a 1064nm laser diode. With this setup, the spatial resolution is estimated to be about 4µm. The performance for real particles is to be confirmed in a test beam.

Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic B-Meson Decays at Belle II.

We present the first measurement of the ratio of branching fractions of inclusive semileptonic B-meson decays, R(X_{e/μ})=B(B→Xeν)/B(B→Xμν), a precision test of electron-muon universality, using data corresponding to 189 fb^{-1} from electron-positron collisions collected with the Belle II detector. In events where the partner B meson is fully reconstructed, we use fits to the lepton momentum spectra above 1.3 GeV/c to obtain R(X_{e/μ})=1.007±0.009(stat)±0.019(syst), which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.

Belle II status and prospects for studies of neutral currents

The Belle II experiment at the SuperKEKB energy-asymmetric electron-positron collider is a substantial upgrade of the B factory facility at the Japanese KEK laboratory. Belle II collected a sample of 362 fb-1 at the Υ(4S) resonance between 2019 and 2022, with a maximum peak luminosity of 4.7 × 1034 cm-2 s -1. Belle II is currently facing a long shutdown period, required for several upgrades of the detector and the collider. Data taking will resume at the end of 2023. We report the recent measurements which involve neutral current transitions in B meson decays. In particular, we present the current status and future prospects for the branching fractions measurements of the radiative decays B → K*γ and the fully inclusive B → Xsγ, the search for B + → K + νν̅ decays, the measurement of the branching fractions of B → J/ψ(→ℓℓ)K and B → K * ℓℓ. Finally, we show the perspectives of the search for B → K * ττ and the searches of lepton flavor violating channels B → K (*) ℓℓ', with ℓ = e, μ, τ.

Updates on the Mechanical Design of FalconD, a Nb$_{3}$Sn Cos$\theta$ Short Model Dipole for FCC-hh

As part of the research project called FCC-hh led by CERN, which aims to study the possibility of building a new 100 TeV center of mass energy particle collider along an 80-100 km circumference tunnel, there are many collaborations underway to test different designs for the main bending dipole of the collider. One of these collaborations involves INFN, the Italian Institute of Nuclear Physics, which has been working on the cos <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\theta$</tex-math></inline-formula> design until 2019 as part of the European EuroCirCol project. This study consisted of a conceptual design of a 16 T, double-aperture, four-layer dipole made of Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn. Considering the challenge that such a cutting-edge magnet represents, this conceptual design resulted in a 5-year agreement between INFN and CERN in order to study and build a 1.5 m long, single aperture, cos <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\theta$</tex-math></inline-formula> , Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn dipole with a target magnetic field of 12 T. This project is called the Future Accelerator post-LHC Cos <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\theta$</tex-math></inline-formula> Optimized Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn Dipole (or FalconD for short). In this paper we present the mechanical analysis of this project after some major cross-sectional upgrades that have impacted the design by easing the assembly phase. One of the most challenging aspects of this design is its mechanical structure that must handle the intense magnetic forces generated by such a high magnetic field. The mechanical structure is based on the “bladder & key” (B&K) concept and it has never been adopted before in a cos <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\theta$</tex-math></inline-formula> dipole, so it needs to be validated. This contribution presents the 2D FEM of the FalconD, showing the mechanical behavior of the magnet during each construction step.

20 T Hybrid Nb3Sn-HTS Block-Coil Accelerator Dipole With Stress-Management

In the framework of studies for high energy particle colliders, design concepts for high field dipoles are being explored. In particular, relatively compact 20 T magnets can be achieved in a hybrid configuration, combining a High Temperature Superconductor (HTS) and a Low Temperature Superconductor (LTS). Preliminary concepts have been previously proposed using Bi2212 for the HTS and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn for the LTS. One of the main difficulties of 20 T magnets is the management of the very high stresses developing during operation. The design concepts rely on a rectangular block-coil layout, which offers the advantage of aligning the conductors with the main magnetic field, therefore submitting the conductors to a perpendicular electromagnetic force for a better control of the stresses. In addition, the layout allows a specific stress management, with adequate horizontal and vertical plates to intercept the stresses. The paper presents the improvements provided to the initial concept. In terms of magnetic design, the field quality has been improved. In terms of mechanical design, the stress management has been optimized to provide a compact coil with a reduced peak stress on the HTS. Concepts for flared-end coils with joints in the coil-ends are finally presented.

Measured and projected beam backgrounds in the Belle II experiment at the SuperKEKB collider

The Belle II experiment at the SuperKEKB electron–positron collider aims to collect an unprecedented data set of 50ab−1 to study CP-violation in the B-meson system and to search for Physics beyond the Standard Model. SuperKEKB is already the world’s highest-luminosity collider. In order to collect the planned data set within approximately one decade, the target is to reach a peak luminosity of 6 × 1035cm−2s−1 by further increasing the beam currents and reducing the beam size at the interaction point by squeezing the betatron function down to βy∗=0.3mm. To ensure detector longevity and maintain good reconstruction performance, beam backgrounds must remain well controlled. We report on current background rates in Belle II and compare these against simulation. We find that a number of recent refinements have significantly improved the background simulation accuracy. Finally, we estimate the safety margins going forward. We predict that backgrounds should remain high but acceptable until a luminosity of at least 2.8 × 1035cm−2s−1 is reached for βy∗=0.6mm. At this point, the most vulnerable Belle II detectors, the Time-of-Propagation (TOP) particle identification system and the Central Drift Chamber (CDC), have predicted background hit rates from single-beam and luminosity backgrounds that add up to approximately half of the maximum acceptable rates.

Demonstration of thermal limit mean transverse energy from cesium antimonide photocathodes

The mean transverse energy (MTE) of electrons emitted from cathodes is a critical parameter that determines the brightness of electron beams for applications, such as x-ray free electron lasers, particle colliders, and ultrafast electron scattering experiments. Achieving a MTE close to the thermal limit is a key step toward realizing the full potential of electron sources in these applications. Cesium antimonide (Cs3Sb) is a technologically important material with a long history of use in photon detection and electron sources. The smallest MTE of electrons photoemitted from Cs3Sb has always been appreciably greater than the thermal limit and was attributed to surface non-uniformities. In this work, we present comprehensive measurements of the photoemission electron energy spectra (PEES), quantum efficiency, and MTE from Cs3Sb in a wide photoexcitation energy range from 1.5 to 2.3 eV. Our PEES measurements demonstrate a notably low photoemission threshold of around 1.5 eV, which is in contrast with the previously perceived threshold of 1.8–2.0 eV. Moreover, we show that the MTE at this threshold of 1.5 eV nearly converges to the thermal limit at 300 K. At 1.8 eV, the MTE measured is 40 meV, which is comparable to the previously reported value. We conclude that this MTE value at 1.8 eV photon energy is not due to surface roughness effects as previously believed, but is a direct consequence of the excess energy.

First Direct Observation of Collider Neutrinos with FASER at the LHC.

We report the first direct observation of neutrino interactions at a particle collider experiment. Neutrino candidate events are identified in a 13.6TeV center-of-mass energy pp collision dataset of 35.4 fb^{-1} using the active electronic components of the FASER detector at the Large Hadron Collider. The candidates are required to have a track propagating through the entire length of the FASER detector and be consistent with a muon neutrino charged-current interaction. We infer 153_{-13}^{+12} neutrino interactions with a significance of 16 standard deviations above the background-only hypothesis. These events are consistent with the characteristics expected from neutrino interactions in terms of secondary particle production and spatial distribution, and they imply the observation of both neutrinos and anti-neutrinos with an incident neutrino energy of significantly above 200GeV.

MetaHEP: Meta learning for fast shower simulation of high energy physics experiments

For High Energy Physics (HEP) experiments, such as the Large Hadron Collider (LHC) experiments, the calorimeter is a key detector to measure the energy of particles. Incident particles interact with the material of the calorimeter, creating cascades of secondary particles, so-called showers. A detailed description of the showering process relies on simulation methods that precisely describe all particle interactions with matter. Constrained by the need for precision, the simulation of calorimeters is inherently slow and constitutes a bottleneck for HEP analysis. Furthermore, with the upcoming high luminosity upgrade of the LHC and a much-increased data production rate, the amount of required simulated events will increase. Several research directions have recently investigated the use of Machine Learning based models to accelerate particular calorimeter response simulation. These models typically require a large amount of data and time for training, and the result is a simulation tuned specifically to this configuration. Meta-learning has emerged recently as a fast learning algorithm using small training datasets. In this paper, we use a meta-learning approach that “learns to learn” to generate showers from multiple calorimeter geometries, using a first-order gradient-based algorithm. We present MetaHEP, the first application of the meta-learning approach to accelerate shower simulation using very high granular data and using one of the calorimeters proposed for the Future Circular Collider (FCC), a next-generation of high-performance particle colliders.