Data taking

Since the LHC has started to deliver pp collisions at a centre-of-mass energy of√s =

900 GeVin 2009, the LHCb detector collected a huge amount of data in different beam

configurations and at various energies. This section summarises the main data taking periods of the proton-proton runs in 2010-2012 and the proton-ion run in 2013.

4.5.1. Proton-proton period

The first proton-proton collisions with a centre-of-mass energy of √s = 7 TeV were

delivered and recorded in 2010. During the early data taking period after the commis- sioning, the LHC beams were filled with less than 10 proton-bunches per beam. The

instantaneous luminosity at this time was only around Linst = 2· 1028cm−2s−1 with

an average number of interactions per bunch-crossing of less than 0.1. The recorded data samples of this period contain basically no pile-up interactions. These are ideal conditions to perform particle multiplicity measurements. Hence, data of this early period are used for the multiplicity analysis presented in the first part of this thesis.

Figure 4.17.:Summary of the delivered (dark lines) and recorded (light lines) luminosity at

the LHCb detector during the pp runs in the years 2010 − 2012. Figure is taken from Ref. [58].

4.5. Data taking

With an increasing number of bunches (up to ≈ 350), also the interaction rate and

luminosity increased up to Linst = 1.2·1032cm−2s−1by the end of the year. The collected

data for the entire year 2010 amounts to an integrated luminosity of L = 0.04 fb−1_.

In 2011, the delivered instantaneous luminosity went up to Linst = 3.8· 1032cm−2s−1

with an average number of pp interactions of 1.5. The number of filled bunches in the LHC reached 1380 which is almost half of the design value of 2808 bunches. The size of

the collected data sample in 2011 corresponds to L = 1.11 fb−1_.

In 2012, the energy of the proton beams could be increased, leading to a centre-of-mass

energy of√s = 8 TeV. The running conditions with around 1380 filled bunches and an

average interaction rate of 1.7 are comparable to those of 2011. The stable conditions in 2012 were aiming at collecting as much data as possible. The recorded luminosity by

LHCb in this year amounts to L = 2.08 fb−1_.

The size of the data samples for each year are summarised in Fig. 4.17.

4.5.2. Proton-ion period

After a short pilot run of proton-ion collisions in 2012, where a small data sample

with a luminosity of L ≈ 0.6 µb−1 _{was collected, a period of four weeks dedicated to}

proton-ion data taking followed in January/February of 2013. During that time, the

LHCb detector collected L ≈ 1.7 nb−1 _{of proton-ion collisions in different configurations.}

The instantaneous luminosity during that time was on average Linst = 3· 1027cm−2s−1.

In proton-ion collisions, two different beam configurations have to be distinguished. Depending on the orientation of the beams, either the proton or the lead remnant is travelling through the LHCb detector after the collision. The two configurations are visualised in Fig. 4.18. In the forward configuration the proton beam is pointing downstream the LHCb detector. Proton-lead collisions in this setup are further referred to as p+Pb collisions. In the opposite backward configuration the lead ion is pointing downstream the LHCb detector. This results in a larger particle density accompanied by larger detector occupancies. Collisions in this beam configuration are referred to as

Pb+pcollisions. Beam 1: protons E𝒑= 𝟒 𝑻𝒆𝑽 Beam 2: Pb-ions EPb = 𝟏. 𝟓𝟖 𝑻𝒆𝑽

p+Pb configuration (forward)

p

Pb

Pb+p configuration (backward)

p

Pb

Figure 4.18.: Visualisation of the forward (left) and backward (right) beam configuration

for proton-ion collisions. The nucleon-nucleon centre-of-mass energy in both

4. The LHCb experiment at the LHC

Pb+p

p+Pb

Figure 4.19.:Delivered and recorded integrated luminosity of the proton-ion run in 2013.

Further indicated are the two different beam configurations and the reversal of the LHCb dipole. Original figure is taken from Ref. [58].

The asymmetric beam configuration of the proton and the lead beam results in a boost of the nucleon-nucleon centre-of-mass system in the direction of the proton, as shown in Chap. 3.2. As a result, two different (pseudo)rapidity regions in the nucleon-nucleon centre-of-mass system are probed with the LHCb detector. In the forward configuration

the coverage is 1.5 < ηcms < 4.5, in the backward configuration 2.5 < ηcms < 5.5.

During the data taking, also the magnetic field has been reversed. The collected integrated luminosity of the entire proton-ion data sample is given in Fig. 4.19, indicating the different periods of p+Pb and Pb+p collisions together with the corresponding magnet polarities. The sizes of the data samples are listed in Tab. 4.1. Except for a larger amount of data collected in p+Pb configuration with magnet down polarity, the data samples have a comparable size. For the analysis of angular correlations in proton-ion collisions presented in the second part of this thesis, smaller equally sized sub-samples are used. Details are given in Chap. 14.

beam magnet number of data size integrated

configuration configuration recorded events (reconstructed) luminosity L

p+Pb down 2248× 106 _{120.0Tbytes 769 µb}−1

p+Pb up 533_{× 10}6 _{27.9Tbytes 298 µb}−1

Pb+p down 675_{× 10}6 _{51.2Tbytes 303 µb}−1

Pb+p up 591× 106 _{47.2Tbytes 263 µb}−1

Table 4.1.:Overview of the proton-ion data samples recorded with the LHCb detector. The

number events corresponds to all recorded events, the quoted integrated luminosity only considers data taking periods of good quality data, in which the detector was fully operational.

CHAPTER

5

From particles to tracks - Event and track reconstruction at LHCb

Particles produced in high-energetic pp or pPb collisions traverse the LHCb detector and leave signals in various components. These signals have to be first converted and prepared, before the properties of the original collision can be reconstructed in different software steps.

The focus of this thesis are charged particles whose trajectories can be reconstructed as

tracks by exploiting the information of the tracking detectors. By knowing the position

of a particle at a few different points in space the full flight path can be calculated. With the information of the traversed magnetic field also the momentum of a particle is determined. The specific particle species is not of interest for the presented analyses.

The performance of the track reconstruction is limited by experimental precision. The reconstruction is always affected by inefficiencies and artefacts related to the detector or the applied reconstruction software. These effects have an impact on the physics analyses and need to be accounted for.

The reconstructed tracks are further used to determine the primary interaction vertex (PV) of the collision. This allows separating primary produced particles that originate

from the PV and secondary particles that are produced in decays.

This chapter focusses on the event reconstruction which is performed at LHCb. Firstly, the LHCb software framework is presented. It comprises software used to reconstruct the recorded data, to prepare event samples for the physics analyses, but also to simulate particle collisions within the detector. Secondly, an overview of the different track types is given. They are defined by the sub-detectors which are involved in the reconstruction process. This is followed by a discussion of the relevant reconstruction algorithms used in this thesis. The last part is dedicated to reconstruction artefacts and inefficiencies which have an impact on the physics analyses.