182 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.
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
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
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
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
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
Influence of the thermal factor on the composition of electron-beam high-entropy ALTiVCrNbMo coatings
This paper reports results of studying the element and phase compositions of electron-beam coatings based on the high-entropy alloy AlTiVCrNbMo, depending on the deposition temperature (in the range of 300...700 °С).
The high-entropy alloys were melted in an arc furnace in an atmosphere of high-purity argon. Vacuum condensates of the high-entropy alloy (AlTiVCrNbMo) with a thickness of 3–5 µm were obtained in the vacuum setup UVN-2M-1 at a working vacuum of 5·10-5 mТоrr. The alloy evaporation was performed from the water-cooled ingot mold using an electron-beam gun with a power of 5 kW. Condensation of vapors of all the elements of the alloy was performed onto copper substrates at temperatures of 300, 500, 700 °C. Based on analysis of the element composition of materials of the target made of the high-entropy six-element alloy AlTiVCrNbMo and electron-beam coatings, based on it, we established the critical parameter (specific heat of vaporization of an element) that defined a selective change in the element composition. In accordance with a characteristic change in the composition of coatings of the multi-element high-entropy alloy, 3 groups of elements were distinguished: with a specific heat of evaporation of 280...350 kJ/mol (group 1), 420…460 kJ/mol (group 2), and 590…680 kJ/mol (group 3). It was shown that the formation of a single-phase coating of the high-entropy alloy (based on BCC of the crystalline lattice) occurs at the higher deposition temperature of 500...700 °C when the coating consists of not less than 5 elements. It was established that based on the conditions for an electron-beam process of materials formation, the results obtained can be divided into two types: those determined by the condition of evaporation of the target and those determined by the conditions of coating deposition. The density of flows of elements, evaporated from the target, is determined by their specific heat of evaporation. However, the ratio of atoms in the flow, derived in this way, may not be retained in the formed coating due to the secondary evaporation of elements from the growth surface. The obtained results allow us to substantiate principles for the selection of components for achieving the optimal element and phase compositions of high-entropy alloys.На основі аналізу елементного складу матеріалів мішені з високоентропійного шестиелементного сплаву AlTiVCrNbMo і електронно-променевих покриттів на його основі встановлено критичний параметр (питома теплота випаровування елемента), який визначає селективну зміну елементного складу. Показано, що формування однофазного покриття високоентропійного сплаву відбувається, коли до складу покриття входить не менше 5 елементів. Отримані результати дозволяють обґрунтувати принципи підбору компонент для досягнення оптимальних елементного та фазового складу високоентропійних сплавів
Absence of evidence for pentaquarks on the lattice
We study the question of whether or not QCD predicts a pentaquark state. We
use the improved, fixed point lattice QCD action which has very little
sensitivity to the lattice spacing and also allows us to reach light quark
masses. The analysis was performed on a single volume of size with lattice spacing of fm. We use the
correlation matrix method to identify the ground and excited states in the
isospin 0, negative parity channel. In the quenched approximation where
dynamical quark effects are omitted, we do not find any evidence for a
pentaquark resonance in QCD.Comment: 17 pages, 5 figures replaced with revised versio
A possible explanation why the Theta+ is seen in some experiments and not in others
To understand the whole set of positive and null data on the
Theta+(1530)-production, we suggest the hypothesis that multiquark hadrons are
mainly generated from many-quark states, which emerge either as short-term
hadron fluctuations, or as hadron remnants in hard processes. This approach
allows us to describe both non-observation of the Theta+ in current null
experiments and peculiar features of its production in positive experiments.
Further, we are able to propose new experiments that might be decisive for the
problem of the Theta+ existence. Distributions of the Theta+ in such
experiments can give important information both on higher Fock components of
conventional hadrons and about structure and hadronization properties of hadron
remnants produced in hard processes. We also explain that description of
multiquark hadrons may require a modified form of the constituent quark model,
with quark masses and couplings being intermediate between their values for the
familiar constituent quarks and the current ones.Comment: 18 pages. Some changes in the text; experimental suggestions
collected in a special subsection, references added and refreshe
QCD sum rules for the anti-charmed pentaquark
We present a QCD sum rule analysis for the anti-charmed pentaquark state with
and without strangeness. While the sum rules for most of the currents are
either non-convergent or dominated by the continuum, the one for the
non-strange pentaquark current composed of two diquarks and an antiquark, is
convergent and has a structure consistent with a positive parity pentaquark
state after subtracting out the continuum contribution. Arguments are
presented on the similarity between the result of the present analysis and that
based on the constituent quark models, which predict a more stable pentaquark
states when the antiquark is heavy.Comment: 19 pages, 8 figures, REVTex, revised version,new figures added and
references update
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