Observation of the Bottomonium Ground State in the Decay Y(3S) to ynb Page: 4 of 7
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We report the results of a search for the bottomonium ground state r6(1S) in the photon energy
spectrum with a sample of (109+1) million of Y(3S) recorded at the Y(3S) energy with the BABAR
detector at the PEP-II B factory at SLAC. We observe a peak in the photon energy spectrum at
E, = 921.2}10(stat) + 2.4(syst) MeV with a significance of 10 standard deviations. We interpret
the observed peak as being due to monochromatic photons from the radiative transition Y(3S) -
y qb(1S). This photon energy corresponds to an 06(1S) mass of 9388.9214(stat) +2.7(syst) MeV/c2.
The hyperfine Y(1S)-06(1S) mass splitting is 71.4+I(stat) + 2.7(syst) MeV/c2. The branching
fraction for this radiative Y(3S) decay is estimated to be (4.8 + 0.5(stat) + 1.2(syst)) x 10-4.
PACS numbers: 13.20.Gd, 14.40.Gx, 14.65.Fy
Thirty years after the discovery of the narrow T(nS)
resonances , no evidence has been reported for the
spin-singlet pseudoscalar partners Yb(nS) of these states.
Measurement of the hyperfine mass splittings between
the triplet and singlet states in quarkonium systems is
of key importance in understanding the role of spin-spin
interactions in quarkonium models and in testing QCD
calculations . Theoretical estimates of the mass split-
ting between the 15 singlet and triplet states vary from
36 MeV/c2 to 100 MeV/c2 .
In this letter, we report the observation of the radiative
transition T(3S) -- %Yb(1S), where the Yb(1S), hereafter
referred to as the Yb, is the pseudoscalar partner of the
triplet state T(1S), and corresponds to the ground state
of the bottomonium system. Theoretical predictions of
the decay branching fraction range from 1 to 20 x 10-4 ,
where the unknown Yb mass is a major source of the un-
certainties. The current limit from the CLEO III exper-
iment, B[T(3S) -- ryb] < 4.3 x 10-4 at 90% confidence
level, is based on 1.39 fb-i1 of T(3S) data .
The data sample used in this study was collected with
the BABAR detector  at the PEP-II asymmetric-energy
C+e- storage rings. It consists of 28.0 fb-1 of integrated
luminosity collected at a e+e center-of-mass (CM) en-
ergy of 10.355 GeV, corresponding to the mass of the
T(3S) resonance. Additional samples of 2.4 fb-1 and
43.9 fb-1 were collected 30 MeV below the T(3S) [below-
T(3S)] and 40 MeV below the T(4S) [below-T(4S)] res-
onances, respectively and are used for background and
calibration studies. The trajectories of charged particles
are reconstructed using a combination of five layers of
double-sided silicon strip detectors and a 40-layer drift
chamber, all operated inside the 1.5-T magnetic field of
a superconducting solenoid. Photons are detected using
a CsI(Tl) electromagnetic calorimeter (EMC), which is
also inside the coil. The energy resolution for photons
varies from 2.9% (at 600 MeV) to 2.5% (at 1400 MeV).
The signal for T(3S) -- ryn is extracted from a fit to
the inclusive photon energy spectrum in the CM frame.
Any reference to photon energy hereafter will be in the
CM frame, unless otherwise noted.
The monochromatic photon from the decay appears
as a peak on top of a smooth non-peaking background
from continuum (e+e -> qq with q u, d, s, c) events
and bottomonium decays. Two other processes pro-
duce peaks in the photon energy spectrum close to
the signal region. Double radiative decays T(3S) -
%XbJ(2P); XbJ(2P) -> -T(1S), J 0,1, 2, produce a
broad peak centered at 760 MeV due to photons from
decays of the XbJ(2P) states. The peaks from the three
XbJ(2P) transitions appear merged due to photon en-
ergy resolution and the Doppler broadening that arises
from the motion of the XbJ(2P) in the CM frame. This
XbJ(2P) photon peak is well separated from the signal
region of interest (around E,= 900 MeV). We use the
peak as a tool to verify the optimization of the selection
criteria and to determine signal reconstruction efficiencies
and the absolute photon energy scale. The other process
leading to a peak near 860 MeV in the photon energy
spectrum is the radiative production of the T(1S) via
initial state radiation (ISR) e+e -> ryS T(1S). Knowl-
edge of the magnitude and photon energy line shape of
this background is crucial in extracting the nb signal.
We employ a simple set of selection criteria to suppress
the backgrounds while retaining a high signal efficiency.
Decays of the nb via two gluons, expected to be a large
component of its decay modes, have high track multi-
plicity. Hadronic events are selected by requiring four or
more charged tracks in the event and that the ratio of the
second to zeroth Fox-Wolfram moments  be less than
Photon candidates are required to be isolated from all
charged tracks. To ensure that their shapes are con-
sistent with an electromagnetic shower, the lateral mo-
ments  are required to be less than 0.55. The signal
photon candidate is required to lie in the central angu-
lar region of the EMC, -0.762 < cos(O2,LAB) < 0.890,
where 02,LAB is the angle between the photon and the
beam axis in the laboratory frame. This requirement
ensures high reconstruction efficiency and good energy
resolution, and reduces the contributions of ISR photons
from e+e -> yISRT(1S) events.
Due to the fact that there is no preferred direction in
the decay of the spin-zero Yb, the correlation of the di-
rection of the photon momentum in the CM frame with
the thrust axis  of the nb is small. In contrast, there
is a strong correlation between the photon direction and
thrust axis in continuum events. The thrust axis is com-
puted with all charged tracks and neutral calorimeter
clusters in the event, with the exception of the signal
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Aubert, B. Observation of the Bottomonium Ground State in the Decay Y(3S) to ynb, article, July 11, 2008; [Menlo Park, California]. (digital.library.unt.edu/ark:/67531/metadc895669/m1/4/: accessed October 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.