1. The timing upgrade project of the TOTEM Roman Pot detectors
Mirko Berretti
CERN and Pisa INFN
The timing upgrade project of the TOTEM Roman Pot detectors
Proton Time-Of-Flight (TOF) detectors in the TOTEM vertical Roman Pots allow to reconstruct the longitudinal vertex position and thus to assign the vertex where the protons are generated to the proper vertex reconstructed by the CMS tracker.
The TOF detector is based on scCVD diamond sensors.
The upgrade will improve the detection capability of the TOTEM and CMS experiments for central diffraction (CD): Low mass resonances, glueballs, exclusive CD, missing mass searches..
After an extensive R&D on FE electronics, sensors were measured to have a time resolution between 80 and 110 ps (depending on the electrode capacitance). With the installation of 4 diamond planes per arm a time resolution of 50 ps on each side of the IP is obtained (sZ ~ 1 cm).
TOF detectors allow a reduction of the background due to the physics pile-up (single diffractive related) and of the machine background (beam halo).
MIP particle reconstruction efficiency in the diamond sensor as a function of the space Y-coordinate.
The Signal/Noise ratio for MIPs, obtained with our FE electronics is above 20. The efficiency is found > 98% in the bulk of the diamond, and is still >80% in the region not coverd by the metallization.
Track XY position of the protons in the vertical RP, for events with a double arm trigger. On the Top RP, an optimized layout of a diamond plane is reported (both TOP and BOTTOM RP will be equipped).

2. Waveform digitization SAMPIC CHIP SPECS
The Analog signal from the diamond is sampled by the SAMPIC chip, a waveform TDC developed in Saclay. It is used to acquire the full waveform shape of the detector signal, by sampling it through a 64 cell Delay Line Loop (DLL) based TDC and an ultrafast analog memory for fine timing extraction.
More details in the Dominique Breton contribution
SINGLE PLANE TIME RESOLUTION obtained from the arrival time difference of the same MIP particle in two close diamond detectors. The X axis shows the two pixels capacitances, in pF.
The effect of the waveform sampling is found to be negligible
Clock distribution and DAQ
Problem: the start and clock signal should arrive simultaneously in the two arms (more than 400m distant). In general this is not true due to different delays, additive noise in signal transmission etc..
TOTEM clock distribution is adapted from the “Universal Picosecond Timing System”, developed for FAIR at GSI [6]: TDC clock and start signals are modulated with different wavelength and optically sent from the CR to the detector. Another modulation is added on the same fiber and reflected back from the detector to a measuring unit in the CR. The delays of the two arms is therefore measured and a correction can be applied with a negligible worsening of the TOF measurement.
A radiation tolerant FPGA will be used to send the waveform (64 V-t points/arm) to the same RUN1 TOTEM DAQ. The maximum external trigger rate is <100 KHz. No active trigger on the timing detectors is foreseen for the data taking at the beginning of the LHC-RUN2.