A brief history of Timing

Abstract

This review traces the evolution of precision timing in particle physics experiments, from the first large-scale applications of scintillator and photomultiplier tube (PMT) systems in the 1990s to the picosecond-precision detectors of future colliders. Four technological generations are identified: (i) the era of discrete electronics (NIM, CAMAC, VME) and PMTs, which established the three canonical uses of timing -- particle identification via time-of-flight, background and pile-up rejection, and directionality triggering; (ii) the silicon revolution enabled by Silicon Photomultipliers (SiPMs), Low-Gain Avalanche Diodes (LGADs), and Application-Specific Integrated Circuits (ASICs); (iii) the current transition to ubiquitous four-dimensional (4D) tracking, in which time is a coordinate measured at every point along a particle trajectory. Under-construction systems at the HL-LHC (CMS MTD, ATLAS HGTD, CMS HGCAL) demonstrate the maturity of 30--50\,ps silicon timing at the million-channel scale. The EIC, LHCb VELO Upgrade~II, and ALICE~3 push this technology into new regimes of radiation hardness, material budget, and granularity. (iv) The Far-future facilities such as the Muon Collider and FCC require a further leap to $\sim$10\,ps, setting the agenda for the next decade of detector R\&D.

Figures (11)

• Figure 1: The three canonical uses of timing in particle physics. (1) Time-of-flight: a particle traverses a known distance between a start counter (T0) and a stop counter (T1); the transit time determines its velocity and, combined with momentum, its mass. (2) Pile-up and veto: multiple particles arrive at the same detector at different times; the out-of-time track (red) is rejected by a timing window. (3) Directionality: two stacked detectors measure $T_1$ and $T_2$; a downward-going particle satisfies $T_2 > T_1$, while an upward-going particle satisfies $T_2 < T_1$.
• Figure 2: Left: The CDF-II Time-of-Flight detector, showing the cylindrical arrangement of scintillator bars (length 279.5 cm) and fine-mesh PMTs at both ends, positioned between the outer tracking volume and the solenoid at a radius of 140 cm Acosta2005. Right: One of the four AMS-02 TOF planes, illustrating the scintillator paddles with light guides and PMTs.
• Figure 3: Interior of the Super-Kamiokande detector, showing the array of 50 cm diameter PMTs covering the cylindrical walls, floor, and ceiling of the 50 kt water volume. Vertex and direction reconstruction rely entirely on the relative arrival times of Cherenkov photons across the PMT array, making Super-Kamiokande one of the earliest examples of timing as the primary reconstruction tool rather than an auxiliary measurement Fukuda1998.
• Figure 4: Schematic cross-section of a Multigap Resistive Plate Chamber (MRPC). Multiple high-resistivity glass plates (green) are interleaved with sub-millimetre gas gaps. The uniform electric field across all gaps simultaneously amplifies the avalanche initiated by a traversing particle. Differential signals from the anode and cathode pickup electrodes are fed directly into the NINO ASIC discriminators (shown at left and right), followed by an HPTDC for time digitisation. This readout chain enabled the ALICE TOF to achieve $\sim$80 ps resolution across 140 m$^2$ of active area ALICE_TOF.
• Figure 5: Three-dimensional cross-section of a KETEK SiPM microcell, showing the individual SPAD structure: the P$^+$ window (active region) on N-silicon bulk, the trench isolation between cells, and the bias/quenching resistor network on the surface. Each microcell fires independently in Geiger mode; the parallel sum of all cells gives an analog output proportional to the number of incident photons. Modern SiPMs integrate $10^3$--$10^4$ such cells per mm$^2$.