We report on several studies of charm baryon production and decays by the
BaBar collaboration. We confirm previous observations of the $\Xi_c^{'0/+}$,
$\Xi_c(2980)^+$ and $\Xi_c(3077)^+$ baryons, measure branching ratios for
Cabibbo-suppressed $\Lambda_c^+$ decays and use baryon decays to study the
properties of the light-quark baryons, $\Omega^-$ and $\Xi(1690)^0$.Comment: 8 pages, 5 postscript figues, contributed to the Proceedings of
  ICHEP200

Petersen, Brian Aa.

English

arXiv

The authors report on several studies of charm baryon production and decays by the BABAR collaboration. They confirm previous observations of the {Xi}'{sub c}{sup 0/+}, {Xi}{sub c}(2980){sup +} and {Xi}{sub c}(3077){sup +} baryons, measure branching ratios for Cabibbo-suppressed {Lambda}{sub c}{sup +} decays and use baryon decays to study the properties of the light-quark baryons, {Omega}{sup -} and {Xi}(1690){sup 0}

UNT Digital Library

SLAC-PUB-12163BABAR-PROC-06/100hep-ex/0610049October, 2006Study of Charm Baryons with the BaBar ExperimentB. Aa. PetersenStanford UniversityStanford, California 94305, USA(for the BABAR Collaboration)AbstractWe report on several studies of charm baryon production and decays by the BABAR collabora-tion. We confirm previous observations of the Ξ′0/+c , Ξc(2980)+ and Ξc(3077)+ baryons, measurebranching ratios for Cabibbo-suppressed Λ+c decays and use baryon decays to study the propertiesof the light-quark baryons, Ω− and Ξ(1690)0.Contributed to the Proceedings of the 33rd International Conference on High Energy Physics,7/26/2006—8/2/2006, Moscow, RussiaStanford Linear Accelerator Center, Stanford University, Stanford, CA 94309Work supported in part by Department of Energy contract DE-AC02-76SF00515.p* (GeV/c)0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 yield per 0.5 GeV/c bin0 ’ cΞCorrected 05000100001500020000 BABARpreliminaryFigure 1: Efficiency-corrected, background-subtracted p∗ spectrum for Ξ′0c . The curve is the sim-ulated continuum distribution; it is fitted to the data for 2.0 < p∗ < 4.5GeV/c.1 IntroductionThe last few years have seen a revival of interest in charm spectroscopy with more than a dozen newstates being reported and hundreds of new theoretical investigations being published. The BABARexperiment[1] provides excellent opportunities to observe and study new and old charm hadrons.It records e+e− collisions at or just below the Υ (4S) resonance and with an integrated luminosityof 390 fb−1, the recorded data sample contains more than one billion charm hadron decays. About10% of these are charm baryons and we report here on several charm baryon studies based onsubsets of the BABAR data.2 Production of Ξ′c BaryonsThe isospin doublet (Ξ′0c ,Ξ′+c ) was first observed by CLEO in e+e− continuum events[2] and isidentified as the lightest JP = 12+csq baryon doublet with a symmetric light-quark wave-function.This observation has now been confirmed by BABAR and its production measured in both e+e−continuum events and B decays.[3]The BABAR analysis uses 232 fb−1 of data. The Ξ′0(+)c baryon is reconstructed in the onlykinematically allowed decay mode: Ξ′0(+)c → Ξ0(+)c γ. The Ξ0(+)c is reconstructed from the decaychain, Ξ0(+)c → Ξ−pi+(pi+), Ξ− → Λpi−, Λ → ppi−. Clear signals for Ξ′0(+)c are observed inthe Ξ0(+)c γ invariant mass spectrum and yields are extracted from fits to the mass spectrum in0.5GeV/c wide bins of p∗, the Ξ′0(+)c momentum in the e+e− center-of-mass frame. The resultsfor Ξ′0c are shown in Fig. 1 after correcting for reconstruction efficiency. Ξ′0(+)c baryons withp∗ > 2GeV/c come from continuum production, while most baryons with p∗ < 2GeV/c are due toB decays. Using a model of Ξ′0(+)c continuum production, the yield for p∗ > 2GeV/c is extrapolatedto the full p∗ range. From this the B component is separated to obtain the branching fractionsB(B → Ξ′+c X) × B(Ξ+c → Ξ−pi+pi+) = [1.69 ± 0.17 (exp.) ± 0.10 (model)] × 10−4 and B(B →2)2)+2.286 (GeV/c+pi-]-M(pK+pi-)K+pi-M[(pK2.92 2.94 2.96 2.98 3 3.02 3.04 3.06 3.08 3.1 3.12 3.14)2)+2.286 (GeV/c+pi-]-M(pK+pi)+pi-M[(pK2.452.52.552.62.65BABARpreliminaryFigure 2: Two-dimensional scatter plot of M(Λ+c pi+) vs. M(Λ+c K−pi+) for pK−pi+K−pi+ candi-dates.Ξ′0c X)×B(Ξ0c → Ξ−pi+) = [0.67± 0.07 (exp.)± 0.03 (model)]× 10−4. This is the first observationof the Ξ′c baryon in B decays. The measured continuum production cross sections are σ(e+e− →Ξ′+c X) × B(Ξ+c → Ξ−pi+pi+) = 141± 24 (exp.) ± 19 (model) fb and σ(e+e− → Ξ′0c X) × B(Ξ0c →Ξ−pi+) = 70±11 (exp.)±6 (model) fb. Comparing to a previous measurement of Ξ0c production,[4]one observes that about one third of the Ξ0c produced in B decays come from Ξ′0c decays, while incontinuum the fraction is only 18%.3 Ξc(2980)+ and Ξc(3077)+Two new charm-strange baryons, Ξc(2980)+ and Ξc(3077)+, were recently observed by BELLE indecays to Λ+c K−pi+.[5] These are the first baryon decays where the charm and strange quark arecontained in separate hadrons. BABAR has confirmed this observation and studied the resonantsubstructure in the decay.[6]For this study BABAR uses 316 fb−1 of data. The Λ+c baryons are reconstructed in the decaymode Λ+c → pK−pi+ and combined with a second kaon and pion candidate. The invariant mass ofthe Λ+c pi+ pair are plotted versus the mass of the Λ+c K−pi+ candidate in Fig. 2. Horizontal bandsfrom the Σc(2520)++ and Σc(2455)++ resonances are observed and enhancements around 2970 and3077MeV/c2 in M(Λ+c K−pi+) can also be seen.To extract the yields of the Ξc(2980)+ and Ξc(3077)+ baryons and determine how much decaysthrough a Σ++c resonance, a two-dimensional unbinned likelihood fit is performed on the eventsshown in Fig. 2. The fit contains both resonant Σ++c components and non-resonant components forboth the Ξc(2980)+ and Ξc(3077)+ signals and the background. Phase-space suppression factorsare including in the signal PDFs to account for the nearby kinematic thresholds.The fitted PDF is overlaid on the data in Fig. 3 and the fit yields are given in Table 1. Thestatistical significance of both the Ξc(2980)+ and the Ξc(3077)+ signals exceed 7σ. The Σ++cresonances are seen to be dominant in the Ξc(3077)+ decay, while only about 50% of the Ξc(2980)+decays are through Σc(2455)++. The mass and width of the two resonances are also obtained in3)2)+2.286 (GeV/c+pi-]-M(pK+pi-)K+pi-M[(pK2.92 2.94 2.96 2.98 3.00 3.02 3.04 3.06 3.08 3.10 3.12 3.14 )2Events / ( 0.005 GeV/c020406080100120140160180 )2Events / ( 0.005 GeV/c BABARpreliminaryFigure 3: Invariant mass distribution of Λ+c K−pi+ for data (points with error bars) and the fittedPDF (curves). The solid curve shows the total fit PDF. The dotted curve shows the sum of thebackground components, while the dashed curve on top shows the sum of the non-resonant signalcomponents.Table 1: Yields for the separate resonant and non-resonant (NR) decays.Yield (Events)Ξc(2980)+ → Σc(2455)++K− 132± 31± 5Ξc(2980)+ → Λ+c K−pi+ (NR) 152± 37± 45Ξc(3077)+ → Σc(2455)++K− 87± 20± 4Ξc(3077)+ → Σc(2520)++K− 82± 23± 6Ξc(3077)+ → Λ+c K−pi+ (NR) 35± 24± 16Table 2: Comparison of masses and widths for Ξc(2980)+ and Ξc(3077)+, measured by BABAR andBELLE in the Λ+c K−pi+ final state.Mass (MeV/c2) Width (MeV)BABAR 2967.1± 1.9± 1.0 23.6± 2.8± 1.3BELLE 2978.5± 2.1± 2.0 43.5± 7.5± 7.0BABAR 3076.4± 0.7± 0.3 6.2± 1.6± 0.5BELLE 3076.7± 0.9± 0.5 6.2± 1.2± 0.84the fit and are compared to the results from BELLE in Table 2. The parameters for the Ξc(3077)+baryon agree, but for the Ξc(2980)+ the BABAR mass and width are lower. This might be due todifferent treatment of the phase-space suppression.4 Cabibbo-Suppressed Λ+c DecaysOnly a few Cabibbo-suppressed Λ+c decays have been measured and most have poor precision.BABAR has performed a precise measurement of two Cabibbo-suppressed decay modes and searchedfor two new decays.[7]The analysis uses 125 fb−1 of data. It combines Λ and Σ0 candidates with K+ candidates toreconstruct Λ+c → ΛK+ and Λ+c → Σ0K+ decays. Large signals are seen in both decay modes andused together with signals from the Cabibbo-favored Λ+c → Λpi+ and Λ+c → Σ0pi+ decays to obtainthe branching ratios:B(Λ+c → ΛK+)B(Λ+c → Λpi+)= 0.044 ± 0.004 ± 0.003,B(Λ+c → Σ0K+)B(Λ+c → Σ0pi+)= 0.039 ± 0.005 ± 0.003,where the first uncertainty is statistical and the second systematic. These are a significant im-provement over previous measurements[8] and in agreement with quark-model predictions.[9]Searches for the four-body decays Λ+c → ΛK+pi+pi− and Λ+c → Σ0K+pi+pi− are also performed.After removing decays with intermediate resonances (ΛK+K0Sand Ξ−K+pi+), no significant signalsare observed and the following upper limits at 90% confidence level are obtained:B(Λ+c → ΛK+pi+pi−)B(Λ+c → Λpi+)< 4.1× 10−2,B(Λ+c → Σ0K+pi+pi−)B(Λ+c → Σ0pi+)< 2.0× 10−2.The measurements are the first limits on these decay modes.5 Measurement of the Ω− SpinThe quark model successfully predicted[10] the existence of the Ω− baryon. It also predicts itsspin to be 3/2, but until now experiments have only established that its spin is larger than 1/2.BABAR has studied its spin by using Ξ0c → Ω−pi+, Ω− → ΛK− decays reconstructed in 116 fb−1of data.[11] The distribution of the helicity angle θh, defined as the angle between the Λ and theΞ0c in the Ω− rest-frame, depends on the spin J of the Ω− and the Ξ0c . Assuming JΞ0c = 1/2, thedistributions for JΩ = 1/2, 3/2 and 5/2, respectively are:dN/d cos θh ∝ 1 (1)dN/d cos θh ∝ 1 + 3 cos2θh (2)dN/d cos θh ∝ 1− 2 cos2θh + 5 cos4θh (3)Parity violation in the Ξ0c and Ω− decays could give an additional term with odd powers of cos θh,but no evidence for that is seen in data.5)Λ(hθcos-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1Entries/0.202004006008001000Figure 4: The efficiency-corrected cosθh(Λ) distribution for Ξ0c → Ω−K+ data. The curves repre-sents the expected distribution for JΩ = 1/2 (dashed), JΩ = 3/2 (solid) and JΩ = 5/2 (dashed-dotted).Figure 4 shows the helicity angle distribution with the three spin hypothesis overlaid. TheJΩ = 3/2 hypothesis is in clear agreement with data, while the confidence levels for the JΩ = 1/2and JΩ = 5/2 hypotheses are 1 × 10−17 and 3 × 10−7, respectively. This confirms that the Ω−baryon has the expected spin.6 Study of the Ξ(1690)0 BaryonThe existence of the Ξ(1690)0 baryon has been known for many years, but relatively little infor-mation is available on it. BELLE has observed it as an intermediate resonance in Λc → (ΛK0S)K+decays.[8] BABAR uses this decay mode in a larger data sample (200 fb−1) to measure the mass,width and spin of the Ξ(1690)0 baryon.[12]BABAR reconstructs 2750 ± 300 Λc → ΛK0SK+ decays and the Ξ(1690)0 is clearly visible ininvariant mass of the ΛK0Spair (see Fig. 5). The remaining Λ+c decays cannot be described as anon-resonant contribution. Instead they appear to be decays through Λa0(980)+, where a0(980)+ →K0SK+. To measure the mass and width of the Ξ(1690)0, the mass projections of the ΛK0SK+ Dalitzplot are fitted to a coherent sum of Λa0(980)+ and Ξ(1690)0K+ decays. The fit result is overlaidin Fig. 5 and givesm(Ξ(1690)) = 1684.7±1.3 +2.2−1.6MeV/c2,Γ(Ξ(1690)) = 8.1+3.9−3.5+1.0−0.9MeV,where the first uncertainty is statistical and the second systematic, primarily related to the in-terference contribution. Both the mass and the width measurements are significant improvementsover previous results.The spin of the Ξ(1690)0 baryon can be studied analogously to the Ω− spin. Assuming the Λ+cbaryon has spin 1/2, the Ξ(1690)0 is found to be fully consistent with spin 1/2, while the spin 3/26)2)  (GeV/cS KΛm(1.6 1.62 1.64 1.66 1.68 1.7 1.72 1.74 1.76 1.78 1.82Entries/5 MeV/c-400-200020040060080010001200140016001800Figure 5: The ΛKS invariant mass projection in data (open circles) with the fit overlaid (blackline). Superimposed is the distribution for the Λa0(980)+ contribution (light-colored dots), theinterference term (stars) and their sum (dark-colored dots).and 5/2 hypotheses only have confidence levels of 0.02 and 0.01, respectively. This is the first spinmeasurement for the Ξ(1690)0 baryon.7 SummaryThe large sample of charm hadrons recorded by the BABAR experiment has been used to study theproperties of several charm baryons. Furthermore, the decays of charm baryons are used as a cleansource of light-quark baryons whose spin, mass and width are measured.References[1] B. Aubert et al., Nucl. Instr. Methods Phys. Res., Sect. A 479, 1 (2002).[2] C. P. Jessop et al., Phys. Rev. Lett. 82, 492 (1999).[3] B. Aubert et al., hep-ex/0607086.[4] B. Aubert et al., Phys. Rev. Lett. 95, 142003 (2005).[5] R. Chistov et al., Submitted to Phys. Rev. Lett., hep-ex/0606051.[6] B. Aubert et al., hep-ex/0607042.[7] B. Aubert et al., Submitted to Phys. Rev. D., hep-ex/0601017.[8] K. Abe et al., Phys. Lett. B524, 33 (2002).[9] T. Utpal, R. C. Verma and M. P. Khana, Phys. Rev. D49, 3417 (1994).7[10] M. Gell-Mann, Proceedings of the International Conference on High-Energy Physics, p. 805(1962).[11] B. Aubert et al., Phys. Rev. Lett. 97, 112001 (2006).[12] B. Aubert et al., hep-ex/0607043.8

Study of Charm Baryons with the BaBar Experiment

Abstract

Similar works

Full text

Available Versions

UNT Digital Library