April 22nd, 2013

TOTEM Experiment

The TOTEM experiment, composed of multiple “Roman Pot” units like this one (this is an actual detector removed during the shutdown for maintenance, not a mock-up), looks for protons deflected only slightly in collisions. It is the detector closest to the beam. Two arrays of four Roman Pot units are placed at distances of 147 and 220 metres from the interaction point within the CMS detector. Their sensors are inserted into the beam pipe with a bellows, and are positioned to within one millimetre of the beam. This allows them to detect protons which have undergone elastic scattering or diffractive dissociation, as opposed to the deep inelastic scattering (physicist-speak for a nearly head-on collision) at the interaction point. The position of the detectors permits them to sense protons deflected only by a few microradians at the interaction point.

Here you can see the edgeless silicon detector which allows detection of particles so close to the beam. The bottom of this sensor is positioned, with a precision of 20 μm, to within 1 mm of the beam. The edgeless design reduces the dead area on the side closest to the beam to 50 μm, compared to around ten times that for conventional silicon particle detectors. Study of these glancing interactions between protons allows measurement of how the proton-proton scattering cross-section varies with energy. This is poorly understood, and data from TOTEM should provide experimental data to theorists trying to model the phenomenon. By measuring the angular distribution of scattered protons, TOTEM may provide insight into the enigmatic pomeron interaction, first proposed in 1961 to explain the rising cross section of hadronic collisions with energy and, half a century later, still lacking a persuasive theory of the mechanism.

The physics objectives, design, and construction of the TOTEM experiment is discussed in a technical paper [PDF] published in 2008 by the TOTEM Collaboration.

by John Walker May 12th, 2013

This document is in the public domain.