Measurement of the Z boson cross-section in - Harvard University ...

Chapter 3: Luminosity Measurement at the LHC and in ATLAS 83
sections. Hence, calibration for absolute luminosity is not essential. The monitors
are used to estimate bunch-to-bunch variation in luminosity, so that they have the
bandwidth of individual bunch crossings, namely 40 MHz. The physics process to
monitor was chosen to be diffractive events in which at least one incoming proton
dissociates into a neutron and other secondary particles. The properties of this process
are well known, so that the cross-section at LHC design energies can be estimated
with a precision of about 10% [66].
The neutrons are emitted into a small solid angle in the forward direction. Forward-
going charged particles from the interaction are deflected away by the dipole magnets
D1L and D1R (Figure 3.9), but the neutrons continue in a straight line collinear with
the incoming bunches. Detectors placed in a direct flight path from the IP therefore
intercept neutral particles only.
The BRAN detectors consist of two designs. The BRAN-A, used at IP1 and IP5
(ATLAS and CMS), are pressurized gas ionization chambers installed inside the TAN
at a depth where the hadronic energy deposition is maximal (behind ≈ 30 cm of
copper [1]). The BRAN-B are cadmium-telluride solid-state detectors used at the
low-luminosity interaction points IP2 and IP8.
3.3 Luminosity monitoring in ATLAS
From 2011 onward, ATLAS is expected to use ALFA, a fiber tracker installed in
Roman Pots, for absolute luminosity calibration. Two other subdetectors, namely
LUCID and BCM, can deliver relative luminosity information, LUCID being the
detector used for this purpose during the 2010 run. Additionally, a number of methods