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Development of Fast Timing tools for Event Reconstruction at the high luminosity frontier

Periodic Reporting for period 1 - FasTER (Development of Fast Timing tools for Event Reconstruction at the high luminosity frontier)

Reporting period: 2016-05-01 to 2018-04-30

The scientific program of high energy physics experiments, which includes precision measurements of the recently-discovered Higgs boson, consistency tests of the standard model of particle interactions, and searches for new heavy or exotic particles, will benefit greatly from the enormous dataset provided by high-intensity future proton-proton colliders. However, the projected beam intensities will result in several hundreds of concurrent proton-proton interactions per bunch crossing, spread over a luminous region of order 5 cm along the beam axis and of about 200 ps in time. At this high collision multiplicities, particle reconstruction and correct assignment to primary interaction vertices present a formidable challenge to the detectors that must be overcome in order to harvest the benefits of the increased beam intensity. Time tagging of minimum ionizing particles (MIPs) and of neutral particles in the calorimeters with a resolution a factor ten smaller than the time spread of the interaction vertices provides further discrimination of interaction vertices in the same bunch crossing beyond conventional spatial tracking algorithms.
The goal of this project was the definition of a viable way to exploit precision timing in event reconstruction at the High-Luminosity Large Hadron Collider, planned to begin operation in 2026, and provide the Compact Muon Solenoid (CMS) experiment with ultimate timing capabilities. On the hardware side, the project studied the ability of microchannel plates, commonly used as amplification stage in light detection systems, to time-tag MIPs and high energy electromagnetic showers. Satisfactory performance was achieved with small scale prototypes, with a complete understanding of the effects contributing to the detector response. The integration of this detection technique in a large scale experiment requires further work. This project also contributed, through the development of specific algorithms, to motivate the need for a dedicated timing layer showing the improvements in reconstructed observables afforded by four-dimensional (space and time) tracking. These improvements will enable future experiments to exploit the data delivered by future high-intensity colliders fully.
The work performed along the duration of the project focused on two main objectives: the evaluation of the performance gain with ultimate timing on selected benchmarks particularly relevant for the HL-LHC physics program and the optimisation of MCPs for precision timing in high rate and high radiation environments.

1. Evaluation of the performance gain with ultimate timing on selected physics benchmarks
The evaluation of the performance benefits exploited simulated data samples representing the CMS detector response and the beam spot and pileup conditions expected at the HL-LHC. The detector is assumed to provide also time information with 30 ps resolution for minimum ionising tracks and for electromagnetic showers. The inclusion of this information in the reconstruction of the events yields a factor five reduction in the effective multiplicity of collisions and makes the event purity at the HL-LHC comparable to that achieved at the LHC pileup level. In particular, a dedicated timing layer - at the interface between the tracker and the calorimeters and specialized to provide timing for each individual track crossing it - would optimally complement the timing abilities of the upgraded CMS calorimeters, thereby enabling a significant gain in performance with improved track and vertex reconstruction abilities, lepton isolation efficiencies, rejection of fake jets formed by the clustering of pileup tracks and improved resolution of the missing transverse energy. All these observables are key to precision measurements and searches for signatures of processes beyond the Standard Model.

2. Development and optimisation of microchannel plates for precision timing
The MCPs offer an attractive solution due to their renowned intrinsic time resolution and good radiation tolerance. Conventional MCPs are used in a PMT-MCP assembly with an optical window and a photocathode. In this project, we focused on an alternative option, namely the usage of MCPs in ionisation mode (iMCP), which exploits the secondary emission of electrons directly from the MCP surface (see Fig. 2). This configuration would dispense the photocathode and glass window with the advantage of a simpler, more robust and potentially more radiation tolerant structure. Different prototypes made of various materials and with different geometrical properties were exposed to high energy electron beams at the Beam Test Facility of the INFN Laboratori Nazionali di Frascati (Fig. 3) and at the CERN SPS in 2016 with the purpose to measure their efficiency and time resolution. Efficiencies up to 90% and time resolutions of a few tens of ps have been achieved for some of the tested configurations in the detection of single minimum ionising particles.
The studies performed in this project helped clarify that precision timing can make a significant impact on the event reconstruction at future colliders.
Because of the limited timing resolution and relatively modest amount of pileup at current LHC experiments, algorithms exploiting precision timing for pileup mitigation in event reconstruction were still at an early stage at the beginning of the project. Software tools have been developed and several studies on physics objects and benchmarks channels performed. These studies indicate that precision timing can have a major impact on the physics program of the HL-LHC, whose priorities will be the precision study of the Higgs boson properties and direct searches for new phenomena at the TeV scale. By exploiting precision timing, Higgs measurements will benefit from the improved determination of the hard interaction vertex and from the improved acceptance of isolated objects, which translate in a sizable gain in equivalent integrated luminosity. Searches for new phenomena will benefit both from the increase in acceptance and from ameliorations in the reconstruction of spatially extended or global event variables, like jets and missing transverse energy, which will considerably extend the sensitivity of these searches.
On the instrumental side, we have proven the viability of microchannel plates for the measurement of minimum ionising tracks and for electromagnetic showers, with efficiencies and time resolutions suitable for the realisation of a precision timing layer. The achievement of a resolution of a few 10 ps, which is one order of magnitude better than the one at the current LHC experiments, represents, therefore, a major progress in the research field; the possibility to use MCPs as secondary emission device, as investigated in this project, presents advantages in terms of easier and cost effective detector assembly. Also, although this project focused on applications in high energy physics, a wider impact is expected with interesting applications in other fields like in medical imaging for the realisation of TOF-PET (Time Of Flight Positron Emission Tomography) systems.
The ionization MCP operation concept.
Experimental setup with MCPs at the test beam facility.
Illustration of how time-aware vertexing separates vertices in the time domain.