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CERN-ATS-Note-2012-026 PERF
BI DCCT Note
RESULTS OF THE LHC DCCT CALIBRATION STUDIES
C. Barschel1,2 , M. Ferro-Luzzi1 , J.J. Gras1 , M. Ludwig1 , P. Odier1 , and S. Thoulet1
1 CERN,
Geneva, Switzerland
2 RWTH Aachen University, III. Physikalisches Institut A, Aachen, Germany
May 24, 2012
Abstract
An important aspect of luminosity calibration measurements is the bunch population product normal-
ization. In the case of the LHC, the treatment of this normalization can be split into three subjects: the
total current measurement, the corrections from the non-perfect longitudinal distribution and the relative
amplitude of the individual bunch populations. In this note, we discuss the first item in details and in the
context of the 2010 and 2011 luminosity calibration measurements performed for each LHC Interaction
Point. Effects Internal to the DCCT, the sensitivity to external factors, uncertainty related to the abso-
lute calibration and comparison of two systems are all addressed. The DCCT uncertainty and numerical
examples are given.
1
CERN-ATS-Note-2012-026 PERF, May 24, 2012
Contents
1 Introduction 4
2 Description of the DCCT system 6
2.1 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3 Instrumental stability and linearity 9
3.1 Baseline subtraction method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Automatic baseline correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3 Fourier analysis of the noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.4 In-situ tunnel measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.4.1 Long term stability over 12 hours . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.4.2 Long term stability under load over 24 hours . . . . . . . . . . . . . . . . . . . . 22
3.4.3 DCCT Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.5 DCCT Linearity verified with alternate ADC . . . . . . . . . . . . . . . . . . . . . . . . 26
3.5.1 Reference response of NI ADC . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.5.2 DCCT Linearity compared with NI ADC . . . . . . . . . . . . . . . . . . . . . . 27
3.6 Absolute Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4 Sensitivity to beam conditions and other external factors 30
4.1 Cross talk between rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.2 Bunch pattern dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.2.1 Laboratory measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.2.2 Measurement with beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.2.3 Sensitivity to an injected RF sine wave . . . . . . . . . . . . . . . . . . . . . . . 36
4.3 Bunch position dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.4 Interference from Accelerator Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.4.1 Interference from magnetic field . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.4.2 Interference from RF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
5 Calibration Method 44
5.1 Current source accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
5.2 Position of the calibration rods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
5.3 Methodology and current leak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
6 Difference between systems A and B 48
7 Summary of uncertainties affecting total-intensity measurements 53
7.1 Example with a VDM fill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
7.2 Example with a typical high intensity fill . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
7.3 Example with a low intensity fill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
7.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
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CERN-ATS-Note-2012-026 PERF, May 24, 2012
A Appendices 59
A.1 Noise and baseline correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
A.2 Long term stability over 12 hours . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
A.3 Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
A.3.1 Linearity measurements with 12-bit ADC . . . . . . . . . . . . . . . . . . . . . . 66
A.3.2 Linearity measurements with 24-bit ADC . . . . . . . . . . . . . . . . . . . . . . 68
A.4 Cross talk between rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
A.5 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
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CERN-ATS-Note-2012-026 PERF, May 24, 2012
1 Introduction
Several luminosity calibration experiments were carried out in 2010 and 2011 at the LHC, with proton
collisions (p-p) and with ion collisions (Pb-Pb), to obtain physics cross section normalizations at each
Interaction Point (IP). Both the van der Meer (VDM) scan method and the beam-gas imaging (BGI) method
were used. The experiments were carried out at the zero-momentum frame energies s = 7 and 2.76 TeV
for p-p and s = 7 Z TeV for Pb-Pb. A summary of the most relevant conditions of each set of VDM scans
are listed in table 1.
The first measurements showed that one of the dominant uncertainties is introduced through the bunch
population product normalization. As a consequence, a detailed bunch population analysis was carried
out using data from the LHC Beam Current Transformers (BCTs) and from the LHC detectors (ALICE,
ATLAS, CMS and LHCb). An analysis procedure was defined and bunch population uncertainties were
quantified. The results of a first analysis for 2010 calibration measurements were documented in two bunch
current normalization notes [1, 2] where a detailed description of the procedure used to determine the bunch
populations and their associated uncertainties can be found. The precision was limited by the understanding
of the BCT data at that stage. Since then, a number of additional tests were carried out which significantly
improved the understanding of the bunch current measurements. The purpose of the present note and of two
companion notes [3, 4] is to review the bunch population measurements and their accuracy in the light of
these improvements.
Table 1: VDM luminosity calibration series for the LHC (2010 and 2011). The number of bunches in
brackets indicates the number of "pilot" proton bunches in addition to the number of "main" proton bunches.
Here, N is an indicative value of the main bunch charge in units of 1010 elementary charges.
Period / (m) Net angle s/Z LHC Nr of Colliding in scanned N
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