CERN Accelerating science

TOTEM: Moving Forward Through Consolidation and Upgrade

by Joachim Baechler on behalf of the TOTEM Collaboration

Building on the rich experience gathered during its very successful pre-LS1 operation that has already produced 9 publications (with more being in preparation), TOTEM profits from the two-year-long LHC break to start strengthening its very-forward Roman Pot (RP)
detector system, creating a leading-proton spectrometer with unprecedented performance and the flexibility to operate in a vast range of different beam conditions.

In June 2013, the proposal for an ambitious consolidation and upgrade programme was presented to the LHCC1.  In addition to pursuing its classic physics objectives, the measurement of total cross-section, elastic and soft diffractive scattering at all centre-of-mass energies accessible to the LHC, TOTEM extends its scope and – joining forces with its partner experiment CMS at the same interaction point – endeavours to explore the field of central diffractive production of particles with both initial protons surviving the collision and being detected in the Roman Pots. Thanks to the redundant determination of the process kinematics by both the CMS central detectors and TOTEM’s proton spectrometer, this class of processes has a unique discovery potential for exotic hadronic states such as glue balls and for new physics manifesting itself in ‘missing mass’ signatures.

One arm of a TOTEM T2 detector during its installation at interaction point 5. (Image@CERN Bulletin)

Starting in 2013 the entire RP spectrometer is being concentrated in the beam-optically advantageous regions between the quadrupoles Q5 and Q6 on both sides of IP5. The relocation of the former ±147 m RP stations into the free space between 200 and 210m enables the installation of the new collimators TCL4, a first step towards an optimised collimation system that will ultimately also comprise the collimators TCL6 intercepting possible showers produced by inserting the RPs close to high-intensity beams. To collect enough data of the low-cross-section hard diffractive processes, the Roman Pot system has to be adapted to cope with high luminosities. Impedance heating and the resulting vacuum deterioration by outgassing will be kept to a minimum by replacing all ferrites on the RPs with new elements made from a novel material and having undergone a bake-out at 1000 C. In addition, the horizontal RPs intended to operate at highest luminosities will be geometrically optimised in view of beam impedance by closing cavities with copper shields. High luminosities lead to event pileup and hence to reconstruction ambiguities. To enable multi-event resolution in the RP system, a two-fold strategy is pursued: (1) Measuring the time of flight of leading protons with a precision of a few tens of picoseconds allows resolving the longitudinal event vertex position and hence the attribution of a proton to the correct vertex given by the central CMS detectors. This time measurement will be realised by timing detectors to be installed in two additional RPs between the existing units of the 220 m RP station. These new RPs have been designed in cylindrical geometry minimising the beam impedance and offering enough space for 12 cm long

Cerenkov detectors, one of the technologies being explored for the time measurement. (2) The resolution of multiple tracks with the existing silicon strip detectors will be achieved by rotating one RP unit by 8o around the beam axis, thus creating a stereo effect, i.e. providing track measurements in two additional projections. Ultimately the replacement of the silicon strip detectors with pixel detectors will inherently provide multi-track resolution capability. The exchange of detector packages in RPs can be undertaken in any short technical stop after LS1 without breaking the primary vacuum.



1TOTEM Upgrade Proposal, CERN-LHCC-2013-009; LHCC-P-007