Search for the Decay $\tau^- \rightarrow 3\pi^- 2\pi^+2\pi^0 \nu_\tau$ Page: 4 of 8
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'9Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
s"University of Wisconsin, Madison, Wisconsin 53706, USA
8 Yale University, New Haven, Connecticut 06511, USA
A search for the decay of the r lepton to five charged and two neutral pions is performed using
data collected by the BABAR detector at the PEP-II asymmetric-energy e+e- collider. The analysis
uses 232 fb-i of data at center-of-mass energies on or near the Y(4S) resonance. We observe 10
events with an expected background of 6.514 events. In the absence of a signal, we set the limit
on the branching ratio B(T- - 3w--2m+2onv) < 3.4 x 10-6 at the 90% confidence level. This
is a significant improvement over the previously established limit. In addition, we search for the
decay mode r -> 2ww--v. We observe 1 event with an expected background of 0.410O events and
calculate the upper limit B(T -> 2ww--v7) < 5.4 x 10-7 at the 90% confidence level. This is the
first upper limit for this mode.
PACS numbers: 13.35.Dx, 13.66.De, 13.85.RmHadronic decays of T leptons provide an excellent lab-
oratory for the study of the strong interaction. Decays
of the T with one or three charged particles in the fi-
nal state have been well studied in the past [1]. Higher
multiplicity decays, however, have considerably lower
branching ratios [1], and high luminosity experiments
are needed to study their dynamics and search for new
modes. The BABAR experiment has recorded a large sam-
ple of e+- - +T- events suitable for detailed searches
for high multiplicity T decays.
The T- -> 3--2r+2rv mode [2] is of particular in-
terest. It has not been observed yet, and only an upper
limit B(- -> 3--27r+2zv7) < 1.1 x 10-4 at the 90%
confidence level (CL) has been set by the CLEO collabo-
ration [3]. The reason for the suppression of seven-pion T
decays is the limited phase space of this decay [4, 5]. For
the same reason, if this decay is observed with sufficient
statistics, it may lead to a more stringent limit on the 7
neutrino mass.
Since T decays to five charged pions and a Ar meson
most likely involve resonances (e.g., w or r/) [6], it is ex-
pected that the T- -> 3--2r +2rv decay may also
proceed through resonances. According to calculations
based on isospin symmetry [7], the decay r- -> 2c--v-
is expected to be the dominant mode.
This analysis is based on data recorded with the BABAR
detector at the PEP-II asymmetric-energy e+e- storage
ring operated at the Stanford Linear Accelerator Center.
The data sample consists of 232 fb-i recorded at center-
of-mass (CM) energies of 10.58 GeV and 10.54 GeV. With
an expected cross section for T pairs of o-11 (0.89
0.02) nb [8], the number of produced T-pair events is
NTT (206.5 4.7) x 106.
The BABAR detector is described in detail in Ref. [9],
and only a brief description is given here. Charged-
particle momenta are measured with a 5-layer double-
sided silicon vertex tracker (SVT) and a 40-layer drift
chamber (DCH) inside a solenoidal magnet with a 1.5 T
magnetic field. A calorimeter (EMC) consisting of 6580
CsI(Tl) crystals is used to measure the energy of elec-
trons, positrons, and photons. A ring-imaging Cherenkovdetector is used to identify charged hadrons, in combina-
tion with ionization energy loss measurements in the SVT
and the DCH. Muons are identified by an instrumented
magnetic-flux return (IFR).
Monte Carlo (MC) simulations are used to estimate
the T- -> 3--2r+2zv7 signal efficiency and background
contamination from other T decay modes. The produc-
tion of T pairs is simulated with the KK generator [10], and
non-signal T lepton decays are modeled with TAUOLA [11]
according to measured rates [1]. The background pro-
cesses eC+e - qq (q u, d, s, c, b) are simulated us-
ing the JetSet package [12]. Signal events are gener-
ated using phase space with a V - A interaction. We
find no significant variation in efficiency within the phase
space. The simulation of the BABAR detector is based on
GEANT 4 [13].
The principal backgrounds to our signal come from
e+C -> qq processes and multi-pion T decay modes
involving at least one Ar, namely r- -> 3--2r+VT,
w- -> 2r-r+2Arv and T- -> 2r-r+37zv7 modes. The
w- -> 3--2r+zvz contribution comes from reconstruct-
ing an additional (fake) Ar, while the three-prong modes
contribute through the Ar decay to a photon pair and
subsequent photon conversions in detector material.
The event selection criteria were developed to suppress
the background while maintaining high signal efficiency.
Events with six charged particle tracks and a net charge
of zero are first selected. To ensure well-reconstructed
tracks, each track is required to have a minimum trans-
verse momentum of 100 MeV/c, a distance of closest ap-
proach to the interaction point in the plane transverse
to the beam axis (DOCAxy) less than 1.5 cm, and a
distance of closest approach along the beam direction
less than 10 cm. Four or more tracks are required to
have hits in at least 12 DCH layers. Photons are recon-
structed from clusters in the EMC and are required to
have a minimum energy of 50 MeV, energy deposited in
at least three crystals, and a lateral energy profile con-
sistent with that of a photon. In addition, to suppress
background from backscattering in the EMC, the angle
between the position of a cluster and the impact point
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Aubert, B. Search for the Decay $\tau^- \rightarrow 3\pi^- 2\pi^+2\pi^0 \nu_\tau$, report, April 10, 2006; [Menlo Park, California]. (https://digital.library.unt.edu/ark:/67531/metadc885590/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.