TOTEM

The TOTEM collaboration has made the first measurement of the double diffractive cross-section in the very forward region at the LHC, in a range in pseudorapidity where it has never been measured before.

Double diffraction (DD) is the process in which two colliding hadrons dissociate into clusters of particles, the interaction being mediated by an object with the quantum numbers of the vacuum. Because the exchange is colourless, DD events are typically associated experimentally with a large "rapidity gap" – a range in rapidity that is devoid of particles.

The TOTEM experiment – designed in particular to study diffraction, total cross-sections and elastic scattering at the LHC – has three subdetectors placed symmetrically on both sides of the interaction point. Detectors in Roman pots identify leading protons, while two telescopes detect charged particles in the forward region. These two telescopes, T1 and T2, are the key to the measurement of double diffraction in the very forward region. T2 consists of gas-electron multipliers that detect charged particles with transverse momentum pT > 40 MeV/c at pseudorapidities of 5.3 < |η| < 6.5. The T1 telescope consists of cathode-strip chambers that measure charged particles with pT > 100 MeV/c at 3.1 < |η| < 4.7. (Pseudorapidity, η, is defined as –ln(tan θ/2), where θ is the angle of the outgoing particle relative to the beam axis, so a higher value corresponds to a more forward direction.)

For this novel measurement, TOTEM selected the DD events by vetoing T1 tracks and requiring tracks in T2. This is equivalent to selecting events that have two diffractive systems with 4.7 < |η|min < 6.5, where ηmin is the minimum pseudorapidity of all of the primary particles produced in the diffractive system. The measurement used data that were collected in October 2011 at a centre-of-mass energy of 7 TeV during a low pile-up run with special β* = 90 m optics and with the T2 minimum-bias trigger. After offline reconstruction, the DD events were selected by requiring tracks in both T2 arms and no tracks in either of the T1 arms. This allowed the extraction of a clean sample of double-diffractive events.

The analysis of these events led to a result for the double diffraction cross-section of σDD = (116±25) μb, for events where both diffractive systems have 4.7 < |η|min < 6.5. The measured values for the cross-section are between the predictions of the hadron-interaction models, Pythia and Phojet, for the corresponding ranges in η.