Las formas de lucha por la vivienda en Colombia, décadas de 1960, 1970 y

We thus confirm the presence of an enhancement at∼1250 MeV/c2_{, with}

a width of∼300 MeV/c2_{, broader than for correspondent results of previous}

photoproduction experiments.

4.1.7 Alternative selection procedures

An alternative selection is conceivable and has also been tried, even if only for a small subsample of the whole data: period W42, year 2006.

This second try runs parallel to our main selection until the event sam- ple with at least two π0_{’s. At this point, events has been further selected}

requiring two and only two distinct π0_{’s found in the event, to univoquely}

have the signature

µ+N →µ0+π+π−π0π0+X. (4.8) As usual, X is the undetected hadronic system recoiling off the 4π system. This sample corresponds to about the 75% of the events with at least two π0_{’s. This cannot be evinced from Figure 4.2, due to the previously discussed}

ambiguities concerning cluster pairs.

If we further search events with at least an identified ω, the sample reduces to 15% of that of reaction (4.8).

Of these events,∼17% havemore than oneω-candidate. Thus events of the type

µ+N →µ0+ω(π+π−π0)π0+X, (4.9) withone and only one identifiedω, correspond to∼13% of those in reaction (4.8). If we proceed further and select the exclusive sample, we obtain 105 counts, which is a∼19% gain in statistics compared with what obtained in the same period in our analysis.

The observed increase is indeed expected, due to the better selection of events concerning π0 _{pair identification. It is also most probable, that}

an even better selection of good events already at the ECAL2 cluster level, meaning better cluster/cell quality selection criteria, may help to further enhance the statistics. We reserve to do so in a future, better optimized analysis work.

4.2

Events within the target cells

In COMPASS, the right-handed coordinate system is defined to have the z-axis directed along the incoming beam direction, they-axis oriented verti- cally upwards, andx=y×z. Figure 4.10 shows the coordinate distributions for the reconstructed best primary vertices for the 2004 and 2006 ωπ0 ex- clusive samples. As already explained, in 2006 the new three-cell target was employed for the first time, bringing an enhanced geometrical acceptance and increased number of reconstructed vertices per event. It can be seen

48 Exclusive ωπ0 _{leptoproduction with muons} Entries 929 Z [cm] -150 -100 -50 0 50 100 150 events / 3 cm 0 10 20 30 40 50 Entries 929 Year 2004 X [cm] -3 -2 -1 0 1 2 3 Y [cm] -3 -2 -1 0 1 2 3 Entries 929 Entries 929 Year 2004 Entries 711 Z [cm] -150 -100 -50 0 50 100 150 events / 3 cm 0 5 10 15 20 25 30 35 40 Entries 711 Year 2006 X [cm] -3 -2 -1 0 1 2 3 Y [cm] -3 -2 -1 0 1 2 3 Entries 711 Entries 711 Year 2006

Figure 4.10: Distribution of the b.p.v. along the beam direction z (left) and

xy section (right) for 2004 (top) and 2006 (bottom) of exclusiveωπ0 _events.

that for both targets the majority of reconstructed ωπ0 _{events originates}

in the downstream cell. Unfortunately, for 2006 data a bigger amount of exclusive events is reconstructed outside the target cells, as compared with previous years.

As can be seen from Figure 4.10, some of the reconstructed exclusiveωπ0

events are found to be produced outside the target cells. This is particularly evident for the year 2006 data: the three cell structure can be seen, together with a shift of the whole target cell position downstream along thezaxis with respect to the position in year 2004. The presence of a comparatively much bigger sample ofωπ0 _{events which are produced in the window layer of the}

target vessel or, expecially for 2006 data, in a tracking detector plane atz≈

120 cm is clearly visible. In the same Figure, the primary vertex distribution in thexy-plane is shown, together with the nominal section of the target (the red circle). A small amount of events are found to be produced outside the

4.2 Events within the target cells 49

target cell. In addition, one sees that the vertex distribution is somewhat “tilted” towards opposite directions for the two years, finding more events in the lower (upper) part in 2004 (2006) data.

Nothing forbids the production of exclusiveωπ0_{from interaction of beam}

muons with nucleons and/or nuclei belonging to materials other than the target cells. On the other side, some kinematical distributions like, e.g., the transverse momentum squaredp2

t of theωπ0 with respect to the virtual

photon direction ~q, may depend on the nature of the target. To avoid confusion and enhance coherence with other COMPASS analysises on meson production, we have used the standard geometrical cuts on vertex position applied by the COMPASS Collaboration: the geometrical limit along the z-axis are listed in Table 4.4, whereas for both the cut on the cell diameter

rv =

p

x2

v+y2v <1.4 cm (4.10)

applies; x_v and y_v are the respective transversal coordinate of the recon- structed b.p.v.

Target cell [cm] 2004 2006

uspstream z∈[−100,−40] z∈[−63,−33]

central - z∈[−28,32]

downstream z∈[−30,30] z∈[37,67] Table 4.4: Target cell geometrical dimensions along thez-axis.

The b.p.v. selection has been performed with the aid of a routine already available within PHAST.

Small deviations from the nominal geometrical dimensions of target seen in the vertex distribution plots are explained with the (observed) incomplete filling of target cells with the solid target material (6_{LiD), which does not}

form a unique sample, but an aggregate of small crystal grains, thus leaving some part of the cells empty.

The final statistic for events within the target cells is shown in Table 4.5. We see that requiring the b.p.v. to be in a target cell results in a statistic loss of 30.8% for year 2004, 35.5% for year 2006, and a total 32.8% loss for the sum of the two years.

In the following, if not explicitely different stated, we will refer to the

Year Reconstructed ωπ0 2004 (E_γ>2 GeV) 643

2006 (Eγ>2 GeV) 459

2004+2006 1102

50 Exclusive ωπ0 _{leptoproduction with muons}

exclusive ωπ0 _{sample with b.p.v. within a target cell as the (ωπ}0_{) final}

Chapter 5

Results

Having illustrated, in the previous Chapter, the selection procedure for ex- clusiveωπ0_{events within a target cell, we now study the kinematical aspects}

of the reaction.