3.2 LHCb
3.3.3 Analysis
The final stage in the LHCb software chain is DaVinci [41]. It is a Gaudi-based
framework providing all the information needed to perform an analysis such as kinematic information about the particles, PID information and overall event infor- mation. Starting from a list of the particles in an event it can find decay chains and create aROOT [42] ntuple containing the relevant events.
Part II
4
Introduction
I was involved with the measurement of the time-dependentCP-violation observables in Bs0 → DsK decays. While that analysis focused on Bs0 → DsK, it included
Bs0 → Dsπ as a cross-check channel and so a full event selection, mass fit and systematic uncertainty study was performed for that channel. Since much of the work performed for theB0
s→DsK study feeds directly into the main analysis topic presented in this thesis I will here give an overview of the analysis, particularly as it pertains to the main analysis on Bs0→ Dsπ. A full internal analysis note was written up as Ref [1] with further details and was submitted as a conference paper as Ref [2].
The purpose of the analysis is to measure C, Sf, Sf¯, Df and Df¯on Bs0→
DsK decays from 2011 data from LHCb over a dataset of integrated luminosity
∫
L = 1.0fb−1 of pp collisions recorded at a centre-of-mass energy of√s= 7 TeV. TheseCP parameters are related to the physics parametersrDsK,∆andγ−2βsby
C= 1−r 2 DsK 1 +r2 DsK , (4.1) Df = 2rDsKcos(∆−(γ−2βs)) 1 +r2DsK , (4.2) Df¯= 2rDsKcos(∆ + (γ−2βs)) 1 +r2DsK , (4.3) Sf = 2rDsKsin(∆−(γ−2βs)) 1 +r2DsK , (4.4) Sf¯= 2rDsKsin(∆ + (γ−2βs)) 1 +r2DsK , (4.5)
whererDsK =
A(B0s→D−sK+)/A(Bs0→Ds−K+)is the ratio of the magnitudes of the decay amplitudes and ∆is the strong phase difference. The Bs0 mixing phase,
βs is predicted by the Standard Model to be small and so from this it is possible to constrain the CKM angle γ.
5
Data selection
5.1
Data sample
This analysis uses data from the 2011 run of LHCb. This comprises an integrated luminosity ∫ L = 1.0 fb−1 of pp collisions recorded at a centre-of-mass energy of
√
s= 7 TeV.
5.2
Simulated data
Several samples of simulated data were created for the analysis, primarily for the use in event selection and background studies. In each sample, aB hadron is forced to decay to a specific final state as listed in Table 5.1 along with the number of events generated for each channel.
5.3
Reconstruction
TheBs0→DsK decay mode is reconstructed in two stages, first theDs candidate is created from its daughter particles and then aK is added to make the B0
s meson. TheDscandidates are reconstructed in three separate final states,Ds+→K+K−π+,
Ds+→K+π−π+andD+s →π+π−π+each of which are selected as independent sam- ples based on particle identification requirements. The invariant mass of the com- bination of the three Ds meson daughters is fixed to the nominal value of the Ds meson when reconstructing the mass of theB0s meson and, conversely, when calcu- lating the decay time of theB0
s meson, the mass of theDs meson is not constrained but the momentum vector of theBs0is required to point from theppprimary vertex.
Sample Sample size B0s→Dsπ Ds+→K+K−π+ 1052495 B0s→DsK Ds+→K+K−π+ 1887293 Bs0→D∗sπ Ds+→K+K−π+ 524098 Bs0→D∗−s K+ Ds+→K+K−π+ 206000 B0s→Dsρ Ds+→K+K−π+ 2019391 Bs0→D∗sρ Ds+→K+K−π+ 1019191 Λb→D−sp Ds+→K+K−π+ 539994 Λb→D∗−s p Ds+→K+K−π+ 630598 Λb→Λcπ Λ+c →pK−π+ 2033496 Λb→ΛcK− Λ+c →pK−π+ 519495 B0→Dρ D+→K−π+π+ 2054494 B0→D∗π D∗ →Dπ0,D+→K−π+π+ 1046498 B0→Dπ D+→K−π+π+ 1016198 B0→D−K+ D+→K−π+π+ 958393 B0→D−sK+ Ds+→K+K−π+ 517198
Table 5.1: Simulated samples used during the analysis for selection and background studies.
The sample is further divided based on the polarity of the magnet (up or
down) as well as the flavour tagging information of theBs0meson candidate (Bs0,B0s
or untagged). Thus there are 18 sub-samples of data with no events being present in more than one data set.
5.4
Event selection
The event selection is performed in a four-step process, defined partially by the LHCb experimental considerations. The steps of the selection process are:
1. trigger,
2. experiment-wide offline selection (stripping), 3. analysis-specific offline selection,
4. particle identification.
5.5
Trigger
The first level of selection is performed by the LHCb trigger system described in Section 3.2.7. All events for this analysis are required to be those which contained the particles which the trigger used to make its decision. This means that the track which activated the trigger is required to be used in the reconstruction of the signal candidate. For an event to be considered, two independent trigger algorithms must have fired. First, in the HLT1, a region of interest is defined by a straight line track in the VELO and then a single detached high-momentum track is required to be within that region. This trigger (internally known as1TrackAllL0) is detailed in a dedicated note at Ref [43]. Secondly, the HLT2 trigger is required to have fired on the detection of a single, high-momentum track, displaced from theppcollision point and to have found a single similarly displaced vertex containing the detected track and 1–3 other tracks. This trigger algorithm (called the 2-, 3- or 4-bodyTopoBBDT) is described by a public note at Ref [44].
5.6
Stripping selection
Stripping is performed centrally within the LHCb collaboration and the results of it are made available to all through the standard LHCb book-keeping processes. Its primary purpose is to provide a number of data sets, each defined by a set of relatively loose selection criteria, to be used by numerous analyses within the LHCb collaboration. While the stripping selection is performed offline, after the events have been stored to disk, it is treated as a separate step to theoffline selection.
The selection (called astripping line within LHCb) used to select the initial set ofBs0 candidates for this analysis is theStrippingB02DPiD2HHHBeauty2Charm- Line and it is performed as a two-step process. First a loose pre-selection is made based on the kinematics of the particles and their displacement from the primary interaction. All charged particles which are used to make theBs0meson are required to have a trackχ2/ndof<4,p
T> 100 MeV/cand p > 1 GeV/c. Finally, each track
used to reconstruct the Bs0 meson is, in turn, artificially combined with the tracks used to create the primary vertex. If theχ2 of this vertex combination is small (⩽4) then the given track is not used in theB0
s reconstruction.
To speed up the processing, additional requirements are placed on the Ds meson candidate before its decay vertex is created: the scalar sum of the pT of
the particles used to create it must be> 1.8 GeV/c, the largest distance of closest
) [MeV] s m(B 5000 5200 5400 5600 Events / 6 MeV 10000 20000 30000
)
sDistribution of m(B )
sDistribution of m(B
[ns] τ 0 0.002 0.004 0.006 0.008 Events / 0. 00001 ns 0 100 200 300 400 3 10×
Distribution of Proper Times
Figure 5.1: Distributions for Bs0 → DsK candidates in data after the stripping selection. ) [MeV] s m(B 5000 5200 5400 5600 Events / 6 MeV 0 4000 8000
)
sDistribution of m(B )
sDistribution of m(B
[ns] τ 0 0.002 0.004 0.006 0.008 Events / 0.00001 ns 0 2000 4000Distribution of Proper Times
Figure 5.2: Distributions for simulatedBs0→DsK decays after the stripping selec- tion.
than 0.5 mmand the reconstructed invariant mass must be within 100 MeV/c2 of the nominalD+orDsmeson mass. After the vertex has been formed, final requirements of a vertex χ2/ndof < 10 and that the vertex is well separated from the primary collision vertex are imposed.
After the initial pre-selection, remaining events are passed through abagged
boosted decision tree(BBDT) [45]. It is trained using thepTof theBs0 meson candi-
date, the separation of its decay vertex from the primary vertex and a combination of theχ2/ndof of theBs0 meson and Ds meson vertices. The BBDT response value is required to be >0.05to give the distributions shown in Figures 5.1 and 5.2.
5.7
Offline selection
The offline selection is run over the output of the stripping selection and is composed of a number of parts. First a boosted decision tree selection is used which is trained on kinematic and topological information. Then PID requirements are applied for the Ds daughters and bachelor pion and finally a set of vetoes for D, Λc and J/ψ decays are set.