# Finding data in the Bookkeeping

Knowing how data flows through the various Gaudi applications is crucial for knowing where to look for your data.

Data are catalogued in ‘the bookkeeping’, and are initially sorted in broad groups such as ‘real data for physics analysis’, ‘simulated data’, and ‘data for validation studies’. After this, a tree of various application and processing versions will eventually lead to the data you need.

So, before we can run our first DaVinci job we need to locate some events. In this tutorial we will use the decay ${\displaystyle D^{*+}\to D^{0}\pi ^{+}}$ as an example, where the ${\displaystyle D^{0}}$ decays to ${\displaystyle K^{-}\pi ^{+}}$.

Learning Objectives
• Find MC in the bookkeeping
• Find data in the bookkeeping
• Find the decay you want
• check reco
• ignore what (flagged) stripping is used
• run over those files with the strip21 module you want
• nTuples!

Navigate to the bookkeeping which lets you find both simulated and real data.

At the bottom of the "Bookkeeping tree" tab there is a drop-down menu labelled Simulation Condition, open it and change it to Event type.

We will analyse 2012 data, and correspondingly use simulation for 2012 data. To navigate to the simulation, expand the folder icon in the "Bookkeeping tree" window. Navigate to the MC/2012 folder. This will give you a very long list of all possible decay types for which there is simulated data. We are looking for a folder which is named 27163003 (Dst_D0pi,Kpi=DecProdCut). The number is a numerical representation of the event type. The text is the human readable version of that.

This sample of simulated events will only contain events where a ${\displaystyle D^{*+}\to D^{0}(\to K^{-}\pi ^{+})\pi ^{+}}$ was generated within the LHCb acceptance, although the decay might not have been fully reconstructed. (Not all simulated samples have the same requirements made on the signal decay.)

If you expand the 27163003 (Dst_D0pi,Kpi=DecProdCut) folder you will find several different subfolders to choose from. The names of these subfolders correspond to different data-taking conditions, such as magnet polarity (MagDown and MagUp), as well as different software versions used to create the different samples that are available. We will use Beam4000GeV-2012-MagDown-Nu2.5-Pythia8.

So much choice!
Often there are only one or two combinations of data-taking conditions and software versions to choose from, but sometimes there can be very many. Generally newer versions are the best bet, but you should always ask the Monte Carlo liason of your working group for advice on what to use if you're not sure.

Next we need to choose what version of the simulation you want to use. Usually the latest available version is best, which is Sim08e in this case. We also have to choose the version of the digitisation and what configuration of the trigger and reconstruction we want to have in the simulated sample. Usually there is only one choice for these, which makes choosing easier.

We also have to select a version of the stripping. Choose any as long as it contains the word Flagged.

Flagged and filtered samples
In the usual data-taking flow, the trigger and stripping are run in filtering mode, whereby events that don't pass any trigger line or any stripping line are thrown away. In the simulation, it's often useful to keep such events so that the properties of the rejected events can be studied. The trigger and stripping are then run in flagging mode, such that the decisions are only recorded for later inspection. Filtered Monte Carlo can be produced for analyses that need lots of events.

After all this, you will be presented with a ALLSTREAMS.DST entry. By clicking on it we finally see a list of files that we can process. At the bottom right of the page there is a “Save” button which will let us download a file specifying the inputs that we'll use for running our DaVinci job. Click it and select “Save as a python file”. Clicking “Save” once again in the pop-up menu will start the download. Save this file somewhere you can find it again.

Once you get a bit of experience with navigating the bookkeeping you can take a shortcut! At the bottom of your browser window there is a text field next to a green "plus" symbol. You can directly enter a path here to navigate there directly. For example you could go straight to: evt+std://MC/2012/27163003/Beam4000GeV-2012-MagDown-Nu2.5-Pythia8/ by typing this path and pressing the Go button.
Think of a decay and try to find a Monte Carlo sample for it. You could use the decay that your analysis is about, or if you don't have any ideas you could look for the semileptonic decay ${\displaystyle \Lambda _{b}^{0}\to \Lambda _{c}^{+}\mu ^{-}{\bar {\nu }}_{\mu }}$, where the ${\displaystyle \Lambda _{c}^{+}}$ decays to ${\displaystyle pK^{-}\pi ^{+}}$.