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An innovative Cylindrical Gas Electron Multiplier Inner Tracker for the BESIII Spectrometer

Periodic Reporting for period 2 - BESIIICGEM (An innovative Cylindrical Gas Electron Multiplier Inner Tracker for the BESIII Spectrometer)

Reporting period: 2017-01-01 to 2018-12-31

Inner Trackers (IT) are key detectors in Particle Physics experiments; excellent spatial resolution, radiation transparency and hardness, and operability under high occupancies are main requirements. While planar Gas Electron Multiplier (GEM) detectors are common in modern spectrometers, only one Cylindrical-GEMs (CGEM) detector has been produced up to now by the KLOE-2 Collaboration and has been only recently commissioned.

The BESIII collaboration has built and is operating the BESIII Spectrometer, hosted on the intersection region of the Beijing Electron-Positron Collider II (BEPCII).

The BESIIICGEM Consortium (our logo in the picture) is composed of researchers, many junior and some more senior, from four Institutions: the Institute of High Energy Physics of Beijing (IHEP, People’s Republic of China), the National Institute of Nuclear Physics (INFN, Italy), the University of Mainz and the Helmholtz-Institut Mainz (HIM, Germany) and the University of Uppsala (Sweden). We are a subset of the BESIII Collaboration, and we are designing, building, and we are going to install within the end of 2018 and commission within the end of 2019, the CGEM-IT, that should become new IT of the BESIII Spectrometer.

While the above described research activities clearly aim to produce a detector whose main use is to reconstruct the trajectories of reaction products to perform Particle Physics studies, the project requires a complex multidisciplinary approach and makes use and can correspondingly improve the state of the art knowledge related to:
- engineering one of the most advanced gas detector of its kind;
- advanced design of readout and front end electronics and in particular of an ASIC;
- distributed and virtualized computing techniques, beyond commercial standards.

Exploring new uses of the GEM detectors, a technology fast and reliable, may open new path to cutting-edge applications in medical therapies or in homeland security applications.

Last but not least, a project of this kind can significantly enhance an existing collaboration, or give birth to new ones, among the involved European and Chinese Institution, offering precious career opportunities to young researchers in all the involved Countries.
While designing and constructing the CGEM-IT we faced the unprecedented challenge of reaching high level tracking performance in a strong magnetic field (1 Tesla) with a cylindrical GEM detector. That was supposed to be achieved with an analog readout that has been proved to be very effective in the case of no magnetic field. But no measurement of GEM spatial resolution with analog readout in large magnetic field was published in literature before our work, and performing our extensive tests (Figure 1 shows part of team) we discovered that the effect of a large magnetic field (larger than 0.5 Tesla) is disruptive for the shape of the electron avalanche inside the GEM detectors, worsening their resolution. We hence adopted a clusterization method originally developed by the ATLAS collaboration for the Micromegas detector: the micro Time Projection Chamber (micro-TPC readout).

The unexpected adoption of the micro-TPC readout had a strong impact to most of the Working Packages of the project, leading to delays ranging from few months to almost an year depending on the considered tasks, and of about one year on the installation schedule, while retaining the possibility to install a fully operative CGEM-IT by the end of 2018, i.e. still within the temporal scope of the project.

The CGEM-IT design has been completed. Of the three layers composing the CGEM-IT, the middle one has been built in 2016 (Figure 2), and has been characterized by mean of extensive tests conducted with cosmic rays, sources and beam tests (Figure 3 and 4); it will be now rebuilt exploiting the information collected in the tests in order to maximize its performances. The three layers will be completed within the end of 2017 to form the CGEM-IT and integrated with its readout electronics.

Dedicated boards, chips and an optimized high voltage power supply system, allow to readout the CGEM-IT signal to the data acquisition system of the whole BESIII experiment, with which a perfect integration is needed. Most of the boards and a dedicated ASIC have been already designed, and their prototypes are now being tested before the final production.

Dedicated softwares for the retrieval of the data from the CGEM-IT electronics and for their analysis and reconstruction are in a quite advanced stage of development.

Such kind of detectors introduces new challenges in the computing techniques needed to analyze the data they produce. We developed tools that allow for a fast (few hours) automatic deployment of micro and middle-size cloud infrastructures that can be centrally controlled by a VMDIRAC instance, hence creating a network of interconnected cloud infrastructures at an intercontinental scale.

Our outreach activities included two Summer Doctoral Schools on Cloud Computing and outreach activities specifically targeted to high school students and/or to the general public.

We aim to install the CGEM-IT within the end of 2018, in order to proceed within 2019 to its commissioning, that will be performed right after and out of the temporal scope of this Project.
The same significant co-founding lines that have allowed within our Institutions for the significant progresses performed up to now, will support the commissioning of the CGEM-IT in 2019.

This project has already allowed for a terrific enhancement in the cooperation of the involved Institutions, and for a very effective knowledge transfer within the network of researchers that compose the Consortium.
The CGEM-IT represents a significant evolution w.r.t. the state of the art of GEM (and CGEM) detectors, since:
- the new kind of supports (Rohacell) for GEM foils reduces the total radiation length of the detector improving its tracking performance;
- an innovative design of the CGEM anode allows for smaller capacitances and bigger signals;
- the new micro-TPC readout is applied for the first time on a GEM detector;
- specific clusterization and digitization algorithms are being developed now;
- the tools for the automatic deployment of cloud infrastructures allow for a quick and easy way to those sites characterized by limited computing power and/or managing manpower, or by limited cloud computing skills, to be easily part of distributed computing scenarios on an intercontinental scale.

The CGEM-IT may become the reference for a new generation of detectors able to operate in large magnetic fields and yet with very good resolutions, and to stand high radiation. A kind of detector interesting for the next generation of colliders as the future Circular Electron Positron Collider (CEPC) or the Future Circular Collider (FCC).

Other applications are possible for CGEM detectors, since they reconstruct very precisely the trajectories of particles and their points of origin. As mere examples: high resolution quasi-real time three-dimensional dose imaging in hadron therapy; GEM-based Muon Tomography for homeland security.
Moreover most likely in the future the impact of the automatic deployment tools for micro cloud infrastructures we have developed will extend beyond the HEP community, such tools being the optimal solution addressing the typical computing needs of Small Business Enterprises.