Observation of a Broad Structure in the $\pi^+\pi^-J/\psi$ Mass Spectrum around 4.26~GeV/$c^2$ Page: 4 of 7
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75IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
76 Vanderbilt University, Nashville, Tennessee 37235, USA
7University of Victoria, Victoria, British Columbia, Canada V8W 3P6
78Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
79 University of Wisconsin, Madison, Wisconsin 53706, USA
80 Yale University, New Haven, Connecticut 06511, USA
(Dated: June 30, 2005)
We study initial-state radiation events, e+e- rISRr+7 with data collected with the BABAR
detector. We observe an accumulation of events near 4.26 GeV/c2 in the invariant-mass spectrum
of 7+7-J/0. Fits of the mass spectrum indicate that a broad resonance with a mass of about
4.26 GeV/c2 is required to describe the observed structure. The presence of additional narrow reso-
nances cannot be excluded. The fitted width of the broad resonance is 50 to 90 MeV/c2, depending
on the fit hypothesis.
PACS numbers: 14.40.Gx 13.66.Bc, 13.25.Gv
Recent observations of a heavy but narrow state, the
X(3872), decaying into 7r+7r--J/0 [1-4], have renewed
experimental interest in charmonium spectroscopy. We
have previously reported a search for direct X(3872) pro-
duction in e+e- annihilation through initial-state radi-
ation (ISR): e+e- - 'ISRX . No signal is observed,
suggesting that the X(3872) is not a 1-- state, just as
expected for a narrow state well above the DD thresh-
old. In this Letter we present a study of the e+e- -
1'ISR7r+7r1J process across the charmonium mass range.
We use data collected with the BABAR detector  at
the SLAC PEP-II asymmetric-energy e+e- storage ring.
These data represent an integrated luminosity of 211 fb-1
collected at fi = 10.58 GeV, near the peak of the Y(4S)
resonance, plus 22 fb-1 collected approximately 40 MeV
below this energy.
Charged-particle momenta are measured in a tracking
system consisting of a five-layer double-sided silicon ver-
tex tracker (SVT) and a 40-layer central drift chamber
(DCH), both situated in a 1.5-T axial magnetic field.
An internally reflecting ring-imaging Cherenkov detec-
tor (DIRC) with quartz bar radiators provides charged-
particle identification. A CsI electromagnetic calorime-
ter (EMC) is used to detect and identify photons and
electrons, while muons are identified in the instrumented
magnetic flux return system (IFR).
Electron candidates are identified by the ratio of the
shower energy deposited in the EMC to the momentum,
the shower shape, the specific ionization in the DCH, and
the Cherenkov angle measured by the DIRC. Muons are
identified by the depth of penetration into the IFR, the
IFR cluster geometry, and the energy deposited in the
EMC. Pion candidates are selected based on a likelihood
calculated from the specific ionization in the DCH and
SVT, and the Cherenkov angle measured in the DIRC.
Photon candidates are identified with clusters in the
EMC that have a shape consistent with an electromag-
netic shower but without an associated charged track.
Candidate J/ mesons are reconstructed via their de-
cays to e+e- and a+,--. The lepton tracks must be
well reconstructed, and at least one must be identified as
an electron or a muon. An algorithm to associate and
combine the energy from bremsstrahlung photons with
nearby electron tracks is used when forming J/0 -+ e+e-
candidates. An e+e- (+,--) pair with an invariant mass
within 1s (+4) MeV/c2 of the nominal J/0 mass is taken
as a J/0 candidate and is combined with a pair of oppo-
sitely charged tracks that are identified as pions.
Following an observation of an enhancement in the
r+7r--J/0 mass spectrum during an earlier search for
ISR X(3872) production in a 124fb-1- subsample of the
available data, we choose to exclude the mass region
from 4.2 to 4.4 GeV/c2 from consideration during opti-
mization of the selection criteria with the full sample
to avoid the introduction of statistical or other biases
in the analysis of this region. Radiative production of
the 0(2S) serves as a clean benchmark process  for
a data-driven optimization. Selection criteria are cho-
sen to maximize N/(3/2 + VB) , where N is the
total number of '1ISR0(2S),0(2S) - 7+7-J/0 candi-
dates in the 20 MeV/c2 7r+7r- J/ mass range that brack-
ets the 0(2S) mass, and B is the number of events in
the 7r+7r-J/0 mass regions [3.8, 4.2] GeV/c2 and [4.4,
4.8] GeV/c2, scaled to the width of the excluded region.
Simulated ISR events are validated with the 0(2S) data
and are used to extrapolate the selection criteria to the
excluded mass region as appropriate for small kinematic
differences due to the higher mass.
Radiative e+e- - 'ISR7r+7r- J/0 events are character-
ized by a small mass recoiling against the 7r+7r-J/0 sys-
tem and by low missing transverse momentum. These
properties are reflected in (1), (2), and (3) of the se-
lection criteria: (1) there must be no additional well-
reconstructed charged tracks in the event; (2) the trans-
verse component of the visible momentum in the e+e-
center-of-mass frame, including the ISR photon when
it is reconstructed, must be less than 2.5 GeV/c; (3)
the inferred value of the square of the mass recoil-
ing against the 7r+7r--J/ combination (m2ec) must be
within [-1.02,+3.27] GeV2/c4 for J/ -+ e+e- candi-
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Aubert, B.; Barate, R.; Boutigny, D.; Couderc, F.; Karyotakis, Y.; Lees, J. P. et al. Observation of a Broad Structure in the $\pi^+\pi^-J/\psi$ Mass Spectrum around 4.26~GeV/$c^2$, article, July 6, 2005; [Menlo Park, California]. (digital.library.unt.edu/ark:/67531/metadc881269/m1/4/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.