Measurement of the Tau Lepton Lifetime with BaBar Page: 2 of 8
This article is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to Digital Library by the UNT Libraries Government Documents Department.
The following text was automatically extracted from the image on this page using optical character recognition software:
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.
Here’s what’s next.
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
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 December 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.