Exploring the doubly charmed baryon

The doubly charmed baryon,like the proton and neutron,is formed by three quarks.However,they are different,as such a baryon consists of two heavy quarks(charm-quark)and one light quark,while the proton and neutron respectively contain three light quarks.Therefore the doubly charmed baryon is expected to have different internal structures and properties.For example,its mass is 3-4 times higher than those of the proton and neutron.Studying the doubly charmed baryons can provide important information to understand the three-body bound system,the hadronic interaction and the quark model.     

Hunting the doubly charmed baryon with strangeness

正In the standard model of particle physics, the baryon is formed by three quarks. Unlike the well-known proton and neutron that are composed of three light quarks, the doubly charmed baryon consists of two heavy charm quarks and a light quark.Therefore, it is expected to have different internal structures and properties, and to serve as a unique system for understanding the three-body bound system and testing the mechanism of quantum chromodynamics (QCD).     

First observation of the doubly charmed baryon Xi(+)(cc)

We observe a signal for the doubly charmed baryon Xi(+)(cc) in the charged decay mode Xi(+)(cc)-->Lambda(+)(c)K-pi(+) in data from SELEX, the charm hadroproduction experiment at Fermilab. We observe an excess of 15.9 events over an expected background of 6.1+/-0.5 events, a statistical significance of 6.3sigma. The observed mass of this state is 3519+/-1 MeV/c(2). The Gaussian mass width of this state is 3 MeV/c(2), consistent with resolution; its lifetime is less than 33 fs at 90% confidence.     

How to search for doubly charmed baryons and tetraquarks

Possible experimental searches of doubly charmed baryons and tetraquarks at fixed target experiments with high energy hadron beams and a high intensity spectrometer are considered here. The baryons are:Ξ _cc ⁺ (ccd),Ξ _cc ⁺⁺ (ccu), andΩ _cc ⁺ (ccs); and the tetraquark is T (ccūd). Estimates are given of masses, lifetimes, internal structure, production cross sections, decay modes, branching ratios, and yields. Experimental requirements are given for optimizing the signal and minimizing the backgrounds. This paper is designed as an experimental and theoretical review. It may therefore be of assistance in the planning for a future state-of-the-art high statistics charm experiment, in the spirit of the aims of the recent CHARM2000 workshop.     

On the lifetime of charged charmed particles first direct observation of a charmed baryon decay

Two new examples of production by neutrinos and subsequent decay of charged charmed hadrons are reported. Together with two similar events reported previously they show that the lifetime of charged charmed particles is in the neighbourhood of 5 × 10^?13 s, as expected from current theoretical models. One of the new events is identified as a baryon λ_c⁺ of mass 2.295 ± 0.015 GeV/c² which undergoes the decay process λ_c⁺ → pπ⁺K^? with a proper decay time (7.3 ± 0.1) × 10^?13 s.     

How to search for doubly charmed baryons and tetraquarks

Possible experimental searches of doubly charmed baryons and tetraquarks at fixed target experiments with high energy hadron beams and a high intensity spectrometer are considered here. The baryons are: , and Ω _cc ⁺ ; and the tetraquark is T (ccud). Estimates are given of masses, lifetimes, internal structure, production cross sections, decay modes, branching ratios, and yields. Experimental requirements are given for optimizing the signal and minimizing the backgrounds. This paper is designed as an experimental and theoretical review. It may therefore be of assistance in the planning for a future state-of-the-art high statistics charm experiment, in the spirit of the aims of the recent CHARM2000 workshop.     

Belle实验发现新超子和新粲重子

过去一年里,运行在日本高能加速器研究机构的Belle实验在重子研究,尤其是在超子和粲重子研究上取得了一系列重要实验进展,包括发现了两个超子Ξ(1620)和Ω(2012)、一个粲重子Ξ_c(2930)等新的重子,并首次测量了粲重子Ξ_c衰变的绝对分支比等,为完善重子谱和深入理解强相互作用性质提供了关键的实验数据。     

Remarks on the lifetime of charmed baryonΛ c +

The interference andW-exchange contributions toΛ _c ⁺ inclusive nonleptonic decay rate are expressed in terms of matrix elements of effective fourquark operators and evaluated in the MIT bag model. It is found that although theW-exchange is not necessarily subject to helicity and color suppression, it does not suffice to explain the measured lifetime ofΛ _c ⁺. The observed semileptonic branching ratio ofΛ _c ⁺, however, requires that its inclusive decay rate be dominated by the nonspectator contribution and thus theW-exchange be substantially enhanced by some nonperturbative effects. The implication for exclusive two-body decays ofΛ _c ⁺ is briefly discussed.     

Diffractive production of the charmed baryon Λ+c at the CERN ISR

In a sample of diffractive events of high multiplicity a sharp five standard deviation signal is observed at M = 2255 MeV/c² in the K^?pπ⁺ mass distribution and, although with less statistical strength, at the same mass in the Λ⁰π⁺π⁺π^? channel. These signals are identified as being due to the decay of the charmed baryon Λ⁺_c which is produced with a cross section times branching ratio σ_cB in the range 0.7?1.8 μb for the K^?π⁺p decay and 0.3?0.7 μb for the Λ^-π⁺π⁺ π^? system.     

Hadronic production of doubly charmed baryons and intrinsic charm in the proton

The production of baryons involving two charmed quarks (Ξ _cc ^* or Ξ_cc) in hadron interactions at high energies and high transverse momenta is considered. It is assumed that a Ξ_cc baryon is formed in the nonperturbative fragmentation of a (cc) diquark produced in the hard partonic process of the scattering of charmed quarks from colliding hadrons: c+c → (cc)+g. It is shown that, upon the inclusion of this mechanism, the cross section for the production of doubly charmed baryons becomes approximately twice as large as that which is expected at the Tevatron and LHC colliders according to the predictions based on the model of gluon-gluon production of a (cc) diquark and obtained in the leading order of perturbative QCD.     

Analysis of the scalar doubly charmed hexaquark state with QCD sum rules

In this article, we study the scalar-diquark–scalar-diquark–scalar-diquark type hexaquark state with the QCD sum rules by carrying out the operator product expansion up to the vacuum condensates of dimension 16. We obtain a lowest hexaquark mass of \(6.60^{+0.12}_{-0.09}\,\mathrm {GeV}\), which can be confronted with the experimental data in the future.     

Evidence for charmed baryon “elastic” production in neutrino reactions at the CERN SPS

Evidence for charmed baryon “elastic” production is found in a sample of 55 000 charged-current neutrino interactions in the bubble chamber Gargamelle exposed to the SPS wide-band neutrino beam. The product of the average cross section times the branching ratio into modes including a Λ⁰ or a $\text{K}$⁰p system is found to be α·B=(14.3 ± 7.4) × 10^?40 cm² for ν+n reactions and α · B<3.3×10^?40 cm² (90% CL) for ν+p reactions.