Evidence of a structure in consistent with a charged , and updated measurement of at Belle - Belle Collaboration

We report evidence for the charged charmed-strange baryon with a signal significance of 3.9 with systematic errors included. The charged is found in its decay to in the substructure of decays. The measured mass and width are  MeV/ and  MeV, respectively, and the product branching fraction is . We also measure with greater precision than previous experiments, and present the results of a search for the charmonium-like state Y (4660) and its spin partner, , in the invariant mass spectrum. No clear signals of the Y (4660) or are observed and the 90% credibility level (C.L.) upper limits on their production rates are determined. These measurements are obtained from a sample of pairs collected at the resonance by the Belle detector at the KEKB asymmetric energy electron-positron collider.

) upper limits on their production rates are determined. These measurements are obtained from a sample of (772 ± 11) × 10 6 BB pairs collected at the ϒ(4S) resonance by the Belle detector at the KEKB asymmetric energy electron-positron collider.
The study of the excited states of charmed and bottom baryons is important as they offer an excellent laboratory for testing the heavy-quark symmetry of the c and b quarks and the chiral symmetry of the light quarks. At present, the particle data group (PDG) lists ten charmed-strange baryons [1]. Among these, c (2930) and c (3123) are relatively less established and the evidence for them is poor [1]. For most of these excited c states the spin and parity (J P ) have not been determined by experiments due to limited statistics.
Theoretically, the mass spectrum of excited charmed baryons has been computed in many models, including quark potential models [2][3][4][5][6], the relativistic flux tube model [7,8], the coupled channel model [9], the Quantum Chromodynamics (QCD) sum rule [10][11][12][13][14], Regge phenomenology [15], the constituent quark model [16,17], and lattice QCD [18,19]. The strong decays of excited c baryons have also been studied in many models [20][21][22][23][24][25][26]. In these models, some possible J P assignments of these excited c have been performed. While many new excited charmed baryons have been discovered in experiments in recent years, and there has been dedicated theoretical work devoted to study the nature of charmed baryon such as the baryon internal structure and quark configuration, further cooperative efforts are needed from both experimentalists and theorists to make progress in this area.
Very recently, Belle reported the first observation of the c (2930) 0 charmed-strange baryon with a significance a e-mail: shencp@ihep.ac.cn greater than 5σ from a study of the substructure of B − → K − + c¯ − c decays [27]. The measured mass and width of the c (2930) 0 were found to be [2928.9 ± 3.0(stat.) +0 (2) Belle has also observed the Y (4660) in e + e − → γ ISR π + π − ψ with a measured mass and width of [4652 ± 10(stat.) ± 8(syst.)] MeV/c 2 and [68±11(stat.)±1(syst.)] MeV, respectively [31,32]. As the masses and widths of the Y (4630) and Y (4660) are close to each other, many theoretical explanations assume they are the same state [33][34][35]. In Refs. [36,37], the authors predicted a Y (4660) spin partnera f 0 (980)η c (2S) bound state denoted by the Y η -with a mass and width of (4613±4) MeV/c 2 and around 30 MeV, respectively, with the assumption that the Y (4660) is an f 0 (980)ψ bound state [35,37]. Belle has searched for these states in the substructure of B − → K − + c¯ − c decays, and no clear signals were observed [27]. The corresponding B 0 decay mode can also be used to study the + c¯ − c invariant mass. In this letter, we report an updated measurement ofB 0 → K 0 + c¯ − c and a search for the charged c (2930) + → K 0 + c state with a statistical significance of 4.1σ [38]. This analysis is based on the full data sample collected at the ϒ(4S) resonance by the Belle detector [39] at the KEKB asymmetric energy electron-positron collider [40,41].
The Belle detector is a large solid angle magnetic spectrometer that consists of a silicon vertex detector, a 50-layer central drift chamber (CDC), an array of aerogel threshold Cherenkov counters (ACC), a barrel-like arrangement of time-of-flight scintillation counters (TOF), and an electromagnetic calorimeter comprised of CsI(Tl) crystals located inside a superconducting solenoid coil that provides a 1.5 T magnetic field. An iron flux-return yoke located outside the coil is instrumented to detect K 0 L mesons and to identify muons. A detailed description of the Belle detector can be found in Ref. [39]. Simulated signal events with B meson decays are generated using EvtGen [42], while the inclusive decays are generated via PYTHIA [43]. These events are processed by a detector simulation based on GEANT3 [44].
GeV are used to check the backgrounds, corresponding to more than 5 times the integrated luminosity of the data.
In our analysis ofB 0 →K 0 + c¯ − c ,K 0 is reconstructed via its decay K 0 S → π + π − , and + c candidates are reconstructed in the + c → pK − π + , pK 0 S , and π + (→ pπ − π + ) decay channels. Then a + c and¯ − c are combined to reconstruct a B candidate, with at least one required to have been reconstructed via the pK − π + orpK + π − decay process.
For well reconstructed charged tracks, except for those from → pπ − and K 0 S → π + π − decays, the impact parameters perpendicular to and along the beam direction with respect to the nominal interaction point are required to be less than 0.5 cm and 4 cm, respectively, and the transverse momentum in the laboratory frame is required to be larger than 0.1 GeV/c. The information from different detector subsystems including specific ionization in the CDC, time measurements in the TOF and response of the ACC is combined to form the likelihood L i of the track for particle species i, where i = π , K or p [45]. Except for the charged tracks from → pπ − and K 0 S → π + π − decays, tracks with a likelihood ratio R π K = L K /(L K + L π ) > 0.6 are identified as kaons, while tracks with R π K < 0.4 are treated as pions. The kaon (pion) identification efficiency is about 94% (97%), while 5% (3%) of the kaons (pions) are misidentified as pions (kaons) with the selection criteria above. For proton identification, a track with R π p/p = L p/p /(L p/p + L π ) > 0.6 and R K p/p = L p/p /(L p/p + L K ) > 0.6 is identified as a proton/anti-proton with an efficiency of about 98%; less than 1% of the pions/kaons are misidentified as protons/antiprotons.
The K 0 S candidates are reconstructed from pairs of oppositely-charged tracks which are treated as pions, and identified by a multivariate analysis with a neural network [46] based on two sets of input variables [47]. Candidate baryons are reconstructed in the decay → pπ − and selected if the pπ − invariant mass is within 5 MeV/c 2 (5σ ) of the nominal mass [1].
A vertex fit to the B candidates is performed and the candidate with the minimum χ 2 vertex /n.d. f. from the vertex fit is selected as the signal B candidate if there is more than one B candidates in an event, where n.d. f. is the number of freedom of the vertex fit. Then χ 2 vertex /n.d. f. < 15 is required, which has a selection efficiency above 96%. As the continuum background level is very low, further continuum suppression is not necessary. The B candidates are identified using the beam-energy constrained mass M bc and the mass difference M B . The beam-energy constrained mass is defined as M bc ≡   2 is the assumed product branching fraction; L(B) is the corresponding maximized likelihood of the data; n Y is the number of Y signal events; and ε Y all = ε Y i × i / (pK − π + ) (ε Y i is the detection efficiency from MC simulation for mode i). To take the systematic uncertainty into account, the above likelihood is convolved with a Gaussian function whose width equals to the total systematic uncertainty discussed below.
The systematic uncertainties in the branching fraction measurements are listed below. The detection efficiency relevant (DER) uncertainties include those for tracking efficiency (0.35%/track), particle identification efficiency (1.0%/kaon, 0.9%/pion, 3.7%/proton and 3.4%/anti-proton), as well as (3.0%) and K 0 S (2.3%) selection efficiencies. Assuming all the above systematic uncertainty sources are independent, the DER uncertainties are summed in quadrature for each decay mode, yielding 5.8-8.6%, depending on the mode. For the four branching fraction measurements, the final DER uncertainties are summed in quadrature over the three c decay modes using weight factors equal to the product of the total efficiency and the c partial decay width. Systematic uncertainties associated with the fitting procedure are estimated by a changing the order of the background polynomial, changing the range of the fit, and by enlarging the mass resolution by 10% for all the fits; (b) adding the possible contributions from charged c (2815) and c (2970) states in the fit to M K 0