Observation and Properties of L = 1 B(1) and B*(2) Mesons Page: 4 of 8
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Excited B mesons B1 and B7 are observed directly for the first time as two separate states in
fully reconstructed decays to B+(*)r-. The mass of B1 is measured to be 5720.6+2.4+ 1.4 MeV/c2
and the mass difference AM between B* and B1 is 26.2 + 3.1 + 0.9 MeV/c2, giving the mass of the
BZ as 5746.8 + 2.4 + 1.7 MeV/c2. The production rate for B1 and BZ mesons is determined to be
a fraction (13.9 + 1.9 + 3.2)% of the production rate of the B+ meson.
PACS numbers: 12.15.Ff, 13.20.He, 14.40.NdTo date, the detailed spectroscopy of mesons contain-
ing a b quark has not been fully established. Only the
ground 0- states B+, B , B , Bf+ and the excited 1-
state B* are considered to be established by the PDG
[1]. Quark models predict the existence of two broad
(Bg [JP 0+] and B* [1+]) and two narrow (B1 [1+]
and B* [2+]) bound P states [2 7]. The broad states
decay through an S wave and therefore have widths of a
few hundred MeV/c2. Such states are difficult to distin-
guish, in effective mass spectra, from the combinatorial
background. The narrow states decay through a D wave
and therefore should have widths of around 10 MeV/c2
[3, 6, 7]. Almost all observations of B1 and Bz have been
made indirectly in inclusive or semi-inclusive decays [8
11], which prevents their separation and a precise mea-
surement of their properties. The measurement by the
ALEPH collaboration [12], although partially done with
exclusive B decays, was statistically limited and model
dependent. The masses, widths, and decay branching
fractions of these states, in contrast, are predicted with
good precision by various theoretical models [2 7]. These
predictions can be verified experimentally, and such a
comparison provides important information on the quark
interaction inside bound states, aiding further develop-
ment of non-perturbative QCD. This Letter presents a
study of narrow L 1 states decaying to B+(*>r-
with exclusively reconstructed B+ mesons using data col-
lected by the DO experiment during 2002 2006 and cor-
responding to an integrated luminosity of about 1.3 fb-1.
Throughout this Letter, charge conjugated states are im-
plied.
The DO detector is described in detail elsewhere [13].
The detector components most important for this anal-
ysis are the central tracking and muon systems. The DO
central tracking system consists of a silicon microstrip
tracker (SMT) and a central fiber tracker (CFT), both
located within a 2 T superconducting solenoidal mag-
net, with designs optimized for tracking and vertexing
at pseudorapidities y < 3 and y < 2.5, respectively
(where y -ln[tan(/2)], and 0 is the polar angle mea-
sured with respect to the beam line). The muon system
is located outside the calorimeters and has pseudorapid-
ity coverage y < 2. It consists of a layer of tracking
detectors and scintillation trigger counters in front of
1.8 T toroids, followed by two similar layers behind the
toroids [14].
The B1 and B mesons are studied by examining
B+(*r- candidates. This sample includes the followingdecays:
B1
B*B*+--. B*+
B*+ -; B*+
R+--.(1)
(2)
(3)The direct decay B1 -> B+-- is forbidden by conserva-
tion of parity and angular momentum. The B+ meson
is reconstructed in the exclusive decay B+ J/K+
with J/p decaying to p+p--. Each muon is required to
be identified by the muon system, have an associated
track in the central tracking system with at least two
measurements in the SMT, and a transverse momentum
pT > 1.5 GeV/c. At least one of the two muons is re-
quired to have matching track segments both inside and
outside the toroidal magnet. The two muons must form
a common vertex and have an invariant mass between
2.80 and 3.35 GeV/cz, to form a J/p candidate. An
additional charged track with pT > 0.5 GeV/c, with to-
tal momentum above 0.7 GeV/c and with at least two
measurements in the SMT, is selected. This particle is
assigned the kaon mass and required to have a common
vertex, with Xz < 16 for 3 degrees of freedom, with the
two muons. The displacement of this vertex from the pri-
mary interaction point is required to exceed three stan-
dard deviations in the plane perpendicular to the beam
direction. The primary vertex of the pp interaction was
determined for each event using the method described
in Ref. [15]. The average position of the beam-collision
point was included as a constraint.
From each set of three particles fulfilling these require-
ments, a B+ candidate is constructed. The momenta of
the muons are corrected using the J/p mass [1] as a con-
straint. To further improve the B+ selection, a likelihood
ratio method [16] is utilized. This method provides a sim-
ple way of combining several discriminating variables into
a single variable with increased power to separate signal
and background. The variables chosen for this analy-
sis include the smaller of the transverse momenta of the
two muons, the X2 of the B+ decay vertex, the B+ de-
cay length divided by its error, the significance (defined
below) SB of the B+ track impact parameter, the trans-
verse momentum of the kaon, and the significance SK of
the kaon track impact parameter.
For any track i, the significance 5, is defined as 5,
[cT/o(ET)]2 + [EL /(EL)]2, where ET (EL) is the projec-
tion of the track impact parameter on the plane perpen-
dicular to the beam direction (along the beam direction),
and u(ET) [u(EL)] is its uncertainty. The track of each
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Abazov, V. M.; Abbott, B.; Abolins, M.; Acharya, B. S.; Adams, M.; Adams, T. et al. Observation and Properties of L = 1 B(1) and B*(2) Mesons, article, May 1, 2007; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc890560/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.