221 research outputs found
Further evidence for formation of a narrow baryon resonance with positive strangeness in K+ collisions with Xe nuclei
We have continued our investigation of the charge-exchange reaction K^+ Xe
--> K^0 p Xe' in the bubble chamber DIANA. In agreement with our previous
results based on part of the present statistics, formation of a narrow p K^0
resonance with mass of 1537+-2 MeV/c^2 is observed in the elementary transition
K^+ n --> K^0 p on a neutron bound in the Xenon nucleus. Visible width of the
peak is consistent with being entirely due to instrumental resolution and
allows to place an upper limit on its intrinsic width: \Gamma < 9 MeV/c^2. A
more precise estimate of the resonance intrinsic width, \Gamma = 0.36+-0.11
MeV/c^2, is obtained from the ratio between the numbers of resonant and
non-resonant charge-exchange events. The signal is observed in a restricted
interval of incident K^+ momentum, that is consistent with smearing of a narrow
p K^0 resonance by Fermi motion of the target neutron. Statistical significance
of the signal is some 7.3, 5.3, and 4.3 standard deviations for the estimators
S/sqrt{B}, S/sqrt{S+B}, and S/sqrt{S+2B}, respectively. This observation
confirms and reinforces our earlier results, and offers strong evidence for
formation of a pentaquark baryon with positive strangeness in the
charge-exchange reaction K^+ n --> K^0 p on a bound neutron.Comment: 13 pages, 8 figures, some chenges in text and references, more
precise estimate of Theta(1540) to add, submitted to Phys.Atom.Nucl(Yad.Fiz.
Observation of a baryon resonance with positive strangeness in K+ collisions with Xe nuclei
The status of our investigation of low-energy Xe collisions in the Xenon
bubble chamber DIANA is reported. In the charge-exchange reaction the spectrum of effective mass shows a resonant enhancement
with MeV/c and ^24.4\sigma$. The mass and width of the
observed resonance are consistent with expectations for the lightest member of
the anti-decuplet of exotic pentaquark baryons, as predicted in the framework
of the chiral soliton model.Comment: 9 pages, 4 figure
Small size pentaquark width: calculation in QCD sum rules
The pentaquark width is calculated in QCD sum rules. The higher dimension
operators contribution is accounted. It is shown, that should
be very small, less than .Comment: 10 peges, 7 figure
Influence of a Z+(1540) resonance on K+N scattering
The impact of a (I=0, J^P=1/2^+) Z^+(1540) resonance with a width of 5 MeV or
more on the K+N (I=0) elastic cross section and on the P01 phase shift is
examined within the KN meson-exchange model of the Juelich group. It is shown
that the rather strong enhancement of the cross section caused by the presence
of a Z^+ with the above properties is not compatible with the existing
empirical information on KN scattering. Only a much narrower Z^+ state could be
reconciled with the existing data -- or, alternatively, the Z^+ state must lie
at an energy much closer to the KN threshold.Comment: 9 pages, RevTeX, 3 eps figure
Pentaquark Masses in Chiral Perturbation Theory
Heavy baryon chiral perturbation theory for pentaquarks is applied beyond
leading order. The mass splitting in the pentaquark anti-decuplet is calculated
up to NNLO. An expansion in the coupling of pentaquarks to non-exotic baryons
simplifies calculations and makes the pentaquark masses insensitive to the
pentaquark-nucleon mass difference. The possibility of determining coupling
constants in the chiral Lagrangian on the lattice is discussed. Both positive
and negative parities are considered.Comment: 11 pages; reference added, minor changes in wordin
Baryon Exotics in the Quark Model, the Skyrme Model and QCD
We identify the quantum numbers of baryon exotics in the Quark Model, the
Skyrme Model and QCD, and show that they agree for arbitrary colors and
flavors. We define exoticness, E, which can be used to classify the states. The
exotic baryons include the recently discovered qqqq qbar pentaquarks (E=1), as
well as exotic baryons with additional q qbar pairs (E >=1). The mass formula
for non-exotic and exotic baryons is given as an expansion in 1/N, and allows
one to relate the moment of inertia of the Skyrme soliton to the mass of a
constituent quark
Diquarks and Exotic Spectroscopy
We propose that the recently discovered \Theta baryon is a bound state of
four quarks and an antiquark, containing two highly correlated ud-pairs. If so,
the \Theta baryon has positive parity, and it lies in an near-ideally mixed
SU(3)_{f} \mathbf{\bar{10}}_{f} oplus \mathbf{8}_{f}. The Roper resonance and
the P_{11}(1710) fit naturally into this classification. We predict an isospin
3/2 multiplet of \Xi's (S=-2) with J^{\Pi}=\half^{+} around 1750 MeV. A search
for manifestly exotic \Xi^{+} and \Xi^{--} in this mass range could provide a
sharp test of our proposal. We predict that charm and bottom analogues of the
\Theta baryon are stable against strong decays.Comment: 5 pages, 2 figures, revtex 4, minor corrections and revisions for
journal publicatio
Experimental search for radiative decays of the pentaquark baryon \Theta^+(1540)
The data on the reactions K^+Xe --> K^0 \gamma X and K^+Xe --> K^+ \gamma X,
obtained with the bubble chamber DIANA, have been analyzed for possible
radiative decays of the \Theta^+(1540) baryon: \Theta^+ --> K^0 p \gamma and
\Theta^+ --> K^+ n \gamma. No signals have been observed, and we derive the
upper limits \Gamma(\Theta^+ --> K^0 p \gamma) / \Gamma(\Theta^+ --> K^0 p) <
0.032 and \Gamma(\Theta^+ --> K^+ n \gamma) / \Gamma(\Theta^+ --> K^+ n) <
0.041 which, using our previous measurement of \Gamma(\Theta^+ --> KN) =
(0.39+-0.10) MeV, translate to \Gamma(\Theta^+ --> K^0 p \gamma) < 8 keV and
\Gamma(\Theta^+ --> K^+ n \gamma) < 11 keV at 90% confidence level. We have
also measured the cross sections of K^+ -induced reactions involving emission
of a neutral pion: \sigma(K^+n --> K^0 p \pi^0) = (68+-18) \mub and \sigma(K^+N
--> K^+ N \pi^0) = (30+-8) \mub for incident K^+ momentum of 640 MeV.Comment: 8 page
Production of Theta+ in gamma + D -> Lambda + Theta+ and gamma + D -> Sigma + Theta+ reactions
The gamma + D -> Lambda + Theta+ and gamma + D -> Sigma + Theta+ reactions
can be used to determine the width of Theta+ almost model-independently. We
calculate the differential cross sections of the gamma +D -> Lambda + Theta+,
gamma + D -> Sigma + Theta+ and relevant background reactions in the photon
energy range 1.2 < E_{\gamma} < 2.6 GeV. We determine the most favorable
kinematic conditions and observables for the experimental studies of Theta+ in
the considered processes. We argue that a comparison of the gamma + D -> Lambda
+ Theta+ and gamma + D -> Sigma + Theta+ cross sections should unambiguously
determine isospin of Theta+.Comment: 22 pages, 9 figures. Final published versio
Negative parity pentaquarks in large Nc QCD and quark model
Recently, the 1/Nc expansion has been applied to the study of exotic baryons
containing both quarks and antiquarks. We extend this approach to exotic states
with mixed symmetric spin-flavor symmetry, which correspond in the quark model
to negative parity pentaquarks, and discuss the large Nc predictions for their
mass spectrum. The heavy exotics Qbar q^4 transform as 3, 6bar,15 and 15' under
flavor SU(3), while the light states include the exotic multiplets 10bar, 27,
35. We give mass relations among these multiplets following from the 1/Nc
expansion. In the quark model, the mass splittings between these states are
given by color-spin interactions. Using the observation of an anticharmed
exotic by the H1 collaboration, we give predictions for the masses of other
expected heavy pentaquarks.Comment: 18 pages, one figure, uses youngtab.sty; v3: eq.(40) corrected, final
version to appear in PR
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