RESULTADOS DE LAS ENCUESTAS APLICADAS A LOS

is able to reconstruct vertices and tracks for every processed event, while the reconstruction occurs in several steps.

First of all, the data read-out of the hardware is digitized and transformed into time information and signal amplitudes. For that purpose abstract elec- tronics identification numbers of all detector channels are mapped to real 3- dimensional coordinates. These informations shed light on particle trajecto- ries, using magnets (SM1, SM2) and the RICH detector[63].

Subsequently, the output is written into mDST (Mini Data Summary Tree) files which can be reconstructed by the analysis tool PHAST.

4.1.3

PHAST

PHAST(Physics Analysis Software Tool) is used to read the produced mDST files, which provides access to reconstructed event information and allows the proceeding and filtering of event subsamples[16].

PHAST is the framework for data analysis of the COMPASS experiment on the level of mDST. It provides:

1) access to reconstructed event information

2) environment for physics analysis code developments

3) tools for mDST processing and filtering of events’ sub-samples

PHAST also provides mDST output data stream at the stage of event re- construction. Moreover, PHAST allows to use standardized selection routines for analysis purposes and gives the opportunity to create reduced mDST files (called µDST) which contain preselected event samples.

4.2

Event selection

What can happen when an incoming negativeπbeam with 190 GeV energy(Eπ−)

hits the proton? After this chapter we should get some answers from here. For this analysis, which is presented in this thesis, the principle of particle identification and reconstruction is based on the knowledge, that neutral par- ticles (like the π0 _{and η) decay into γγ which are detected and charged pions}

An η in the final state π−_{p → π}−_π−_π+_{ηp rapidly decays into γγ, whose}

energy and position are assigned by the electromagnetic calorimeters of the COMPASS experiment.

The data selection occurs in several steps, which will be described in the following[11]:

(1)the trigger selection (2)the beam composition

(3)vertex selection (within the target) (4)Recoil Proton Detector

(5)cluster selection in ECAL1 and ECAL2 calorimeters

(6)kinematic fitting routines were integrated into the data selection to im- prove the reconstruction

The figure 4.1 show the process.

Figure 4.1: The process of event selection. Selected event topology in the reaction π−_p→π−_π0_{p orπ}−_{ηp is shown as an example.}

1) Trigger

The preselection is done with the DT0 Trigger. The DT0 is a hadron trig- ger which is used in 2008 data to select events where the beam particle (π−_{) is}

diffractively excited[21]. The composition of the DT0 trigger is the following (used for minimum selection criteria).

4.2 Event selection 45

The Beam Trigger is used to trigger on interactions of the beam in the target region[11]. To respect the geometrical properties of the target, such as its cylindrical shape, this trigger is provided.

A Proton Trigger is used to select recoiling protons emitted from the tar- get. These protons are detected by the Recoil Proton Detector (RPD). They are identified by time-of-flight and energy loss measurements[11].

The veto system is used to detect secondary particles produced along the beamline (hadronic interactions), halo particles, non-interacting particles and interactions leading to particles going outside the angular acceptance of the spectrometer and to reject such events.

2) The π− _Beam

The incoming beam from the M2 beam line is not a pure pion beam: It contains kaons (_≈ 4.5 percent) and anti-protons (_≈0.5 percent)[38].

To exclude negatively charged kaons of the incoming beam, a majority of hits in CEDAR 1 and CEDAR 2 less than 6 was claimed. The two CEDAR detectors were set to detect kaons. Majority < 6 correspond to a veto on Kaons[18]. This cut reduces the small fraction of kaons. Unfortunately, an exact percentage of the rejected kaons is unknown. One possible explanation for not knowing the exact value is that only 1/3 of the beam is tagged by the CEDARs[18].

3)Vertex Selection

For vertex selection we are asking for exactly one primary vertex. A primary vertex is a detected crossing point between the incoming beam and outgoing particles from the first(primary) interaction point of the beam hitting the tar- get (in a fixed target experiment)[11]. In collision experiments this interaction point is defined as the point where the particles collide.

Due to pileup events which often occur in the reconstruction, we lose statis- tics. Pileup are background events which are added to the observed signal[62]. They originate in multiple events that appear in the same time window as the signal of interest. Pileup frequently occurs in high luminosity experiments, such as colliding experiments, where multiple collisions can happen during a

single bunch crossing.

There are three outgoing negative charged tracks from every vertex in the reactionsπ−_p

→π−_π+_γγπ−_{por 5 charged tracks inπ}−_p

→π−_π+_π−_π+_γγπ−_p.

One incomingπbeam and 3 or 5 outgoing tracks are from the primary vertex. The primary vertex was located inside of the cylindrical hydrogen target: R [cm] < 1.57 and -67.5 <z (beam direction) [cm] < -29.5.

Every individual track can be defined by a Lorentz vector[62]. Then we can get information about the energy of each track. For the f1 final state 3

or 5 charged tracks are requested for every primary vertex. In the case of 3 or 5 outgoing particles, the π−_f

1 system fulfilled the energy balance given by

the incident beam particle: 186 < E[GeV]< 196 (The recoil proton carries a negligible amount of kinetic energy).

4) RPD Criteria

For the azimuthal correlation between the direction of the flight of a recoiling track (proton in this case) in the RPD with the direction of the flight of the negatively charged system X(πf1), the azimuthal angles were required to be

in the range of -0.3 < φf1-φπ− <0.3[61].

5) Cluster Selection in ECAL1 and ECAL2

γγ result from the π0 _{or η decay, therefore exactly 2 ”good” clusters were}

selected in ECAL1 and ECAL2, in accordance with the following criteria: Each cluster, which is detected by the electromagnetic calorimeters, correlates to an electromagnetic shower in a group of neighbouring crystals. These clus- ters are not pointed by a charged track[11].

A minimum of energy deposition in the calorimeter cells is essential, there- fore we use 1 GeV for ECAL1 clusters and 4 GeV for ECAL2[11]. Since all of the clusters must be in time with the beam, an additional time cut was per- formed: the difference of the beam time and the cluster time must be within t = -3 ns and t = 5 ns.

6) Kinematic Fitting

Experimental measurements of quantities like momentum, mass, time, 4-vectors etc. are always with errors. For this reason we need a procedure which is able