Lepton-Flavor-Violation in tau-Production at BaBar - A Search for e+ e- to l+ tau- Page: 2 of 7
This article is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to UNT Digital Library by the UNT Libraries Government Documents Department.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
S. Schenk
(SVT) and a 40-layer drift chamber (DCH) inside
a 1.5 T solenoidal magnet. Electrons and photons
are identified using an electromagnetic calorime-
ter (EMC). A ring-imaging Cherenkov detector
(DIRC) is used to identify charged hadrons and
provides additional electron identification infor-
mation. Muons are identified by an instrumented
magnetic flux return (IFR).
Monte Carlo (MC) simulation is used to eval-
uate the background contamination and selec-
tion efficiency. The simulated backgrounds are
also used to cross-check the selection optimization
procedure and for studies of systematic effects;
however, the final background yield estimation
relies solely on data. The signal c+e-- - +-
channels are simulated using EvtGen [11] in which
radiation is handled by the PHOTOS package [12]
to an accuracy better than 1%. The background
T-pair events are simulated using the KK2F MC
generator [13]. The T decays are modeled with
TAUOLA [14] according to measured rates with the
decay r- -> r7 +7r-v assuming an intermedi-
ate ai (1260) axial-vector state [4,15]. We also
generate light quark continuum events (c+e-_-
qq, q u, d, s), charm, dimuon, Bhabhas, BB
and two-photon events [11,16]. The detector re-
sponse is simulated with GEANT4 [17] and all simu-
lated events are reconstructed in the same manner
as data.
3. EVENT SELECTION
The signature of the signal process in the
CM frame is an isolated high-momentum muon
or electron recoiling against either one or three
charged pions and no neutral particles. The re-
constructed mass of the missing neutrino should
be consistent with a massless particle and the in-
variant mass of the recoiling pions and neutrino
consistent with that of the r.
We search for events with zero total charge and
either two or four well-measured charged tracks
originating from near the e+c- interaction point.
All charged tracks must be isolated from neutral
energy deposits in the EMC and be within the
acceptance of the EMC, DIRC and IFR to ensure
good particle identification. One track must be
identified as either an electron or muon with aCM momentum greater than 4.68 GeV/c and no
other track identified as a kaon or lepton. Events
with converted photons are also rejected, where a
converted photon is defined to be a pair of oppo-
sitely charged tracks assumed to have the electron
mass and coming from a vertex with a combined
mass of less than 150 MeV/c2.
In the CM system, the event topology must be
consistent with an e+/p+ recoiling against the re-
maining tracks. We calculate the thrust axis [18]
using all the charged and neutral deposits in the
event and define two hemispheres with respect to
the plane normal to the thrust axis. The e+/p+
and the other tracks are required to be in separate
hemispheres.
The r has a fixed CM energy and momentum:E* _V8+ (7 M2
2 2s
p* E*- M?(1)
where M and M are the masses of the T and
e+/p+, respectively [4]. We define the direction
of the r as opposite to that of the e+/p+ and
assign it the momentum from Equation 1. The
CM four-momentum of the missing neutrino from
the r decay, p*, is defined as p* p* -p*, where
p;, is the sum of the CM four-momenta of the
pions. The reconstructed mT mass is defined to
be mT X(E+ I p* 1)2 p 2 where E,* is the
CM energy of the pions.
The signal-background separation is mainly
performed by considering AE, the difference be-
tween the e /p+ CM energy and s/2, AE
El - Vi/2. Events are rejected if AE is less
than -0.5 GeV or greater than 0.2 GeV. True
signal events will have AE ~ -0.15 GeV while
c+e - +- or e+c- - e+e events will peak
at zero and c+e- - + +- background events
have large negative AE. The AE resolution is
approximately 50 MeV.
3.1. Optimization of the Signal Selection
We use a number of kinematic variables to fur-
ther suppress backgrounds. The missing event
energy in the CM frame, E* i, is defined as the
difference between 1i and the sum of the charged
track energies. It is distributed uniformly for
signal but peaks at zero or near </2 for the2
Upcoming Pages
Here’s what’s next.
Search Inside
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.
Schenk, S. & U., /Heidelberg. Lepton-Flavor-Violation in tau-Production at BaBar - A Search for e+ e- to l+ tau-, article, October 26, 2007; [Menlo Park, California]. (https://digital.library.unt.edu/ark:/67531/metadc877089/m1/2/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.