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

Chapter 3: Luminosity Measurement at the LHC and in ATLAS 85
σtot, L, ρ and B can be determined from a fit.
This method has been applied by various collaborations to measure the total
elastic cross-section. Figure 3.10 shows measurements of dσel
dt
down to very small
values of |t| made by a number of experiments. The predicted measurements at the
LHC are shown as well, together with estimations of the number of events expected
per day at nominal luminosity in two different beam conditions [102].
10 2
10
1
10 −1
10 −2
10 −3
10 −4
10 −5
10 −6
10 −7
10 −8
10 −9
10 −10
10 −11
10 −12
10 −13
10 −14
E710/811,
CDF
UA4, CDF
Block model
10 −15
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
UA4
d σ/dt
[mb / GeV²]
Coulomb − nuclear interference
−B |t|
Pomeron exchange ~ e
diffractive structure
(1)
β∗= 1540 m
−8
pQCD ~ |t|
(3)
β∗= 18 m
Events / GeV² × day
pp at 14 TeV
(BSW model)
10 8
10 7
10 6
−t [GeV²]
Figure 3.10: Elastic scattering cross-section measurements as a function of the momentum
transfer t: (left) in proton-antiproton collisions in various experiments from
the ISR to the Tevatron; (right) prediction for proton-proton collisions at the LHC
at √ s = 7 TeV.
Access to the Coulomb-strong interference region requires measurement at very
small angles from the beam direction, down to ≈ 3 µrad, so that detector elements
must be placed very close to the beam. The measurement also demands low beam
intensity (L ≈ 10 27 − 10 28 cm −2 s −1 ) and zero crossing angle. In addition, the beam
divergence must be small, so that special runs with a high β ∗ at the IP are needed,
e.g., with β ∗ = 2630 m at IP1 (ATLAS) [16].
10 5
10 4
(1)
10 9
10 8
10 7
10 6
10 5
10 4
10 3
10 2
10
(3)