[Paper Review] A Cylindrical GEM Inner Tracker for the BESIII experiment at IHEP

R. Farinelli, M. Alexeev+44 more|arXiv (Cornell University)|Jul 2, 2018

Particle Detector Development and Performance2 references3 citations

TL;DR

This paper presents a cylindrical GEM (CGEM) inner tracker for the BESIII experiment at IHEP, designed to provide high-resolution tracking in a magnetic field. It employs two reconstruction algorithms—charge centroid (CC) and time-based mu-TPC—optimized for orthogonal and inclined tracks or magnetic field conditions, achieving stable spatial resolution below 130 μm across all incident angles and up to 1 T magnetic field.

ABSTRACT

The Beijing Electron Spectrometer III (BESIII) is a multipurpose detector that collects data provided by the collision in the Beijing Electron Positron Collider II (BEPCII), hosted at the Institute of High Energy Physics of Beijing. Since the beginning of its operation, BESIII has collected the world largest sample of J/ψ and ψ(2s). Due to the increase of the luminosity up to its nominal value of 10^33 cm-2 s-1 and aging effect, the MDC decreases its efficiency in the first layers up to 35% with respect to the value in 2014. Since BESIII has to take data up to 2022 with the chance to continue up to 2027, the Italian collaboration proposed to replace the inner part of the MDC with three independent layers of Cylindrical triple-GEM (CGEM). The CGEM-IT project will deploy several new features and innovation with respect the other current GEM based detector: the μTPC and analog readout, with time and charge measurements will allow to reach the 130 μm spatial resolution in 1 T magnetic field requested by the BESIII collaboration. In this proceeding, an update of the status of the project will be presented, with a particular focus on the results with planar and cylindrical prototypes with test beams data. These results are beyond the state of the art for GEM technology in magnetic field.

Motivation & Objective

• To develop a cylindrical GEM tracker compatible with the BESIII experiment's high-rate, high-magnetic-field environment.
• To address the challenge of maintaining high spatial resolution under varying incident angles and magnetic fields.
• To validate the performance of a large-area triple-GEM detector in cylindrical geometry using test beam data.
• To demonstrate the compatibility of planar GEM reconstruction algorithms with cylindrical GEM geometry.
• To characterize the transition between optimal performance of charge centroid and mu-TPC algorithms under different conditions.

Proposed method

• Design and construction of a large-area (20 cm radius, 80 cm length) triple-GEM detector in cylindrical geometry at INFN workshops.
• Use of gas sealing and electrical stability techniques to ensure operational reliability in high-rate environments.
• Employment of two independent reconstruction algorithms: charge centroid (CC) for orthogonal tracks and mu-TPC for inclined or magnetic field-affected tracks.
• Application of charge centroid method using weighted average of strip charges to determine position, optimized for cluster size >2.
• Implementation of mu-TPC algorithm using drift time and simulated drift velocity (Garfield) to assign 2D positions to fired strips.
• Performance evaluation via test beam measurements at CERN, comparing CGEM and planar GEM response across gain settings and cluster sizes.

Experimental results

Research questions

• RQ1 How does the spatial resolution of a cylindrical GEM tracker vary with incident angle in the absence of a magnetic field?
• RQ2 How does the presence of a 1 T magnetic field affect the performance of the charge centroid and mu-TPC reconstruction algorithms?
• RQ3 To what extent does the charge distribution shape (Gaussian vs. box-like) degrade the resolution at large incident angles?
• RQ4 Can the mu-TPC algorithm compensate for resolution loss in the charge centroid method under magnetic field or oblique incidence?
• RQ5 Is the performance of the cylindrical GEM consistent with that of planar GEMs in terms of charge linearity and cluster size response?

Key findings

• The charge centroid (CC) method achieves a spatial resolution of 110 μm under orthogonal incidence and no magnetic field.
• The CC resolution degrades above 100 μm at large incident angles due to distortion of the charge distribution from Gaussian to box-like shape.
• The mu-TPC algorithm improves resolution at large angles and in magnetic fields, becoming the dominant method above a Lorentz angle of 26°.
• The combination of CC and mu-TPC algorithms maintains stable spatial resolution below 130 μm across all incident angles and up to 1 T magnetic field.
• The mean collected charge in the CGEM shows linearity with cluster size and is compatible with planar GEMs, confirming consistent electron avalanche behavior.
• The CGEM demonstrates full efficiency plateau at 97% for both views, with signal charge and fired strip count increasing with gain.