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.
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).
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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..
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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).
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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.
Waveform digitization
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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.
Clock distribution and DAQ
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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.
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