arXiv subjects

M. Cai

Publications and source records attributed to M. Cai.

5 recordsLinked to original sources

arXiv2025

Irradiation Study Using QA Test Pieces of ATLAS18 ITk Strip Sensors with 80MeV Protons

The ATLAS experiment is planning a complete replacement of its inner detector(ID) with a new all-silicon inner tracker (ITk) for the ATLAS Inner Tracker Phase-2 upgrade. The ATLAS18 silicon strip sensors are designed to operate up to the integrated luminosity of 4000 fb$^{-1}$, which corresponds to the maximum fluence of $1.6 \times 10^{15} \, \text n_{\text{eq}} / \text{cm}^2$ (including safety factor). To enhance the quality assurance (QA) program to monitor the key properties of the sensors, the strip sensor community is considering to include China Spallation Neutron Source (CSNS) as a proton irradiation site and Institute of High Energy Physics (IHEP) as a QA test site. A total of 18 ATLAS18 ITk QA test pieces were irradiated with $6.0 \times 10^{14}$, $1.6 \times 10^{15}$, and $2.6 \times 10^{15} \, \text n_{\text{eq}} / \text{cm}^2$ protons at CSNS, and measured at IHEP, including IV (leakage current-voltage), CV (capacitance-voltage) and CCE (charge collection efficiency) measurements. The upgraded irradiation setup at CSNS and measurement setup at IHEP are shown in this paper. Irradiated samples were exchanged between IHEP, Ljubljana and Birmingham to cross-check CCE measurements.

physics.ins-det↗

arXiv2023

Subthreshold production of $J/\psi$ mesons from the deuteron with SoLID

The electro- and photo-production of $J/\psi$ meson near the threshold from the proton is relevant to the search of hidden charm pentaquark candidates reported by the LHCb collaboration, and the study of the QCD trace anomaly's contribution to the proton mass. It is also expected to be sensitive to the QCD van der Waals interaction, that is mediated by multi-gluon exchanges and expected to dominate the interaction between two hadrons with no common valence quarks. Subthreshold production of $J/\psi$ from a nuclear target is expected to enhance such attractive interaction, and also allows for a direct probe of short range correlations inside a nucleus. With the high luminosity capability of the 12-GeV CEBAF facility at Jefferson Lab, high-precision data on $J/\psi$ meson production from the proton is becoming available, providing also a reference for subthreshold $J/\psi$ production from the deuteron. Data from the deuteron will establish the baseline for subthreshold $J/\psi$ production from other nuclear targets. In this paper, we present our findings from a feasibility study of subthreshold $J/\psi$ production from the deuteron using the proposed Solenoidal Large Intensity Device (SoLID), and discuss the potential physics impact of such data.

nucl-ex↗

arXiv2021

First demonstration of in-beam performance of bent Monolithic Active Pixel Sensors

A novel approach for designing the next generation of vertex detectors foresees to employ wafer-scale sensors that can be bent to truly cylindrical geometries after thinning them to thicknesses of 20-40$\mu$m. To solidify this concept, the feasibility of operating bent MAPS was demonstrated using 1.5$\times$3cm ALPIDE chips. Already with their thickness of 50$\mu$m, they can be successfully bent to radii of about 2cm without any signs of mechanical or electrical damage. During a subsequent characterisation using a 5.4GeV electron beam, it was further confirmed that they preserve their full electrical functionality as well as particle detection performance. In this article, the bending procedure and the setup used for characterisation are detailed. Furthermore, the analysis of the beam test, including the measurement of the detection efficiency as a function of beam position and local inclination angle, is discussed. The results show that the sensors maintain their excellent performance after bending to radii of 2cm, with detection efficiencies above 99.9% at typical operating conditions, paving the way towards a new class of detectors with unprecedented low material budget and ideal geometrical properties.

physics.ins-det↗

arXiv2013

Efficient Variable-Coefficient Finite-Volume Stokes Solvers

We investigate several robust preconditioners for solving the saddle-point linear systems that arise from spatial discretization of unsteady and steady variable-coefficient Stokes equations on a uniform staggered grid. Building on the success of using the classical projection method as a preconditioner for the coupled velocity-pressure system [B. E. Griffith, J. Comp. Phys., 228 (2009), pp. 75657595], as well as established techniques for steady and unsteady Stokes flow in the finite-element literature, we construct preconditioners that employ independent generalized Helmholtz and Poisson solvers for the velocity and pressure subproblems. We demonstrate that only a single cycle of a standard geometric multigrid algorithm serves as an effective inexact solver for each of these subproblems. Contrary to traditional wisdom, we find that the Stokes problem can be solved nearly as efficiently as the independent pressure and velocity subproblems, making the overall cost of solving the Stokes system comparable to the cost of classical projection or fractional step methods for incompressible flow, even for steady flow and in the presence of large density and viscosity contrasts. Two of the five preconditioners considered here are found to be robust to GMRES restarts and to increasing problem size, making them suitable for large-scale problems. Our work opens many possibilities for constructing novel unsplit temporal integrators for finite-volume spatial discretizations of the equations of low Mach and incompressible flow dynamics.

math.NA↗

arXiv2006

Gravitational collapse of magnetized clouds II. The role of Ohmic dissipation

We formulate the problem of magnetic field dissipation during the accretion phase of low-mass star formation, and we carry out the first step of an iterative solution procedure by assuming that the gas is in free-fall along radial field lines. This so-called ``kinematic approximation'' ignores the back reaction of the Lorentz force on the accretion flow. In quasi steady-state, and assuming the resistivity coefficient to be spatially uniform, the problem is analytically soluble in terms of Legendre's polynomials and confluent hypergeometric functions. The dissipation of the magnetic field occurs inside a region of radius inversely proportional to the mass of the central star (the ``Ohm radius''), where the magnetic field becomes asymptotically straight and uniform. In our solution, the magnetic flux problem of star formation is avoided because the magnetic flux dragged in the accreting protostar is always zero. Our results imply that the effective resistivity of the infalling gas must be higher by several orders of magnitude than the microscopic electric resistivity, to avoid conflict with measurements of paleomagnetism in meteorites and with the observed luminosity of regions of low-mass star formation.

astro-ph↗