Universit`a degli studi Roma Tre Measurement of the KL meson ...

50 CHAPTER 5. DATA ANALYSIS
bin-by-bin, in this analysis we do not modify the data distribution while both
efficiency and background are included in the fit function;
• the systematic errors are completely uncorrelated, with the exception of the
systematics due to the nuclear interactions.
From now on, the expressions “charged vertex” and “neutral vertex” will be used
respectively for a vertex reconstruction obtained by using tracks (drift chamber information)
and clusters not-associated-to-tracks (calorimeter information), like explained
in Sec. 3.6.2.
The two pions from KS → π + π − decay determine the KS direction in the φ rest
frame: the KL → π 0 π 0 π 0 neutral vertex is reconstructed along the opposite direction
using the calorimeter information, which provides the spatial coordinates, energies
and time of the photons.
The tagging KS → π + π − decays must satisfy the following requests:
1. there must be two opposite curvature tracks originating from a point C inside
a cylinder, centered at the IP (interaction point, see Chap. 3), of 10 cm radius
and |z| < 20 cm;
2. the KS momentum in the φ rest frame must satisfy 100 < p(KS) < 120 MeV/c;
3. the dipion invariant mass, M(ππ), has to lie in the interval
482.6 < M(ππ) < 512.6 MeV/c 2 ; (5.1)
4. at least one of the two tracks has to be associated to a calorimeter cluster of
E ≥ 100 MeV, to evaluate the event t0, evt (see Sec. 4.2).
The efficiency for finding KS → π + π − decays is ∼68%.
The previous KLOE lifetime measurement made use of the same tagging requests
at items n.1 and 2, while the dipion invariant mass cut in the item n.3 was tighter
(±5 MeV) and the energy threshold in the item n.4 was lower (E ≥ 50 MeV).
The event t0, evt for KS → π + π − decays is evaluated using the pion arrival time
on the calorimeter, the pion track length reconstructed by the DC, and its velocity
(see Sec. 4.2.1).
The t0, evt evaluation is used to correct all particles arrival times measured by
the calorimeter. The accuracy of this procedure is tested using the control sample
of KL → π + π − π 0 decays as explained in the following. The correct event bunch
crossing time assignment implies to satisfy the condition:
Tclu = LK
βc
Lγ
+ , (5.2)
c
where Tclu is the arrival time on the calorimeter of one of the two photons from π 0
decay, LK and β are the decay length and the velocity of the KL, Lγ is the photon
path length and c the speed of light. LK is evaluated using the charged vertex given
by π + π − , while Lγ is the distance between the KL charged vertex and the cluster
centroid. The (Tclu − LK/βc − Lγ/c) distribution for KL → π + π − π 0 control sample
is shown in Fig. 5.1: when a correct bunch crossing is assigned to the event, a peak