Measurement of the Tau Lepton Lifetime with BaBar Page: 2 of 8
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photon and radiative Bhabha events are studied
and estimated on data.
In the following, unless otherwise noted, all
particles attributes are evaluated in the e+e-
center-of-mass (CM) frame. Charged particles
momenta are boosted from the laboratory (LAB)
frame to the CM frame assuming that they have
the pion mass. Photon candidates are recon-
structed from neutral EMC clusters with a de-
posited energy above 100 MeV that are not asso-
ciated to tracks. Their 4-momenta are set (and
boosted) assuming they are photons that origi-
nate from the nominal interaction point.
3.1. Candidate Selection
Candidate signal events are required to have
a "1-3 topology", where one tau decay yields
three charged particles (3-prong), while the sec-
ond tau decay yields one charged particle (1-
prong). Therefore, four tracks with zero net
charge are required, pointing to the luminous re-
gion, as expected for decay products of a short
lived particle. The opening angle between the
highest momentum track and each of the remain-
ing three tracks must be larger than 90g. The
plane perpendicular to the highest momentum
track divides the surviving events into two hemi-
spheres: one hemisphere contains one track while
the other contains exactly three, defining the 1-
3 topology. In the 3-prong hemisphere, all three
tracks must have at least 6 hits in the SVT, and
all pairs of oppositely charged tracks must have
an e+e invariant mass above 150 MeV/c2, in or-
der to reject radiative Bhabha events. In order to
remove events at the edge of the detector accep-
tance, the cosine of the polar angle of the 3-prong
tracks total momentum vector is required to be
between -0.9 and 0.8.
The 1-prong track must be identified as an elec-
tron candidate, according to the ratio of the en-
ergy detected in the EMC to the track momen-
tum, the cluster shape in the EMC, the energy
loss in the DCH, and the DIRC Cherenkov an-
gle, if available. On the other side, all 3-prong
tracks must fail the electron identification crite-
ria. In order to select the - -> e-vz- decay
model, the 1-prong hemisphere must not contain
'Throughout this paper, charge conjugate decay modes
any photon candidates.
To reduce background further, several require-
ments on the event's kinematic properties are im-
* The event thrust computed using the four
tracks must be larger than 0.92 (to suppress
hadronic and two-photon background) and
smaller than 0.995 (to suppress Bhabha
* The energy of the four tracks ("charged en-
ergy") is required to be larger than 4GeV
in order to reject two-photon events.
* We define the visible energy as the sum of
the energies of the photon candidates and
charged particle candidates. The missing
energy is computed by subtracting the vis-
ible energy from the event CM energy, de-
termined from the beam energies, and is re-
quired to be larger than 1.5 GeV, in order
to account for undetected neutrinos in tau
* The ratio sin a PT/ (ECM ECH), where
PT is the net transverse momentum of the
four tracks, ECM is the event CM energy,
and ECH is the charged energy, must be
larger than 0.05.
* The momentum of the 1-prong track must
be between 1.72 GeV/c and 4.25 GeV/c, in
order to suppress two-photon and Bhabha
events, which are expected to populate the
low and high momenta regions of the 1-
prong momentum distribution, respectively.
Although Bhabha and two-photon events are not
included in the Monte Carlo samples, the simu-
lation predicts the number of selected candidates
with an accuracy better than 5%. The amount
of Bhabha and two-photon events in the selec-
tion has been estimated to be lower than 1% (see
3.2. Tau Decay Length
The tau decay length is reconstructed in the
plane transverse to the boost direction ("trans-
verse plane"). The tau production point is taken
also are implied.
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Lusiani, A. & /Pisa, Scuola Normale Superiore /INFN, Pisa. Measurement of the Tau Lepton Lifetime with BaBar, article, June 27, 2005; [Menlo Park, California]. (digital.library.unt.edu/ark:/67531/metadc876973/m1/2/: accessed October 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.