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Study of the baryonic B decay B- ->SigmaC++ p- pi- pi-

We report the measurement of the baryonic B decay B- ->SigmaC++ p- pi- pi-. Using a data sample of 467*10^6 B B-bar pairs collected with the BaBar detector at the PEP-II storage ring at SLAC, the measured branching fraction is (2.98 +/- 0.16 (stat) +/- 0.15 (syst) +/- 0.77 (LambdaC+))*10^{-4}, where the last error is due to the uncertainty in the branching fraction of the decay LambdaC+ ->p+ K- pi+. The data suggest the existence of resonant subchannels B- ->LambdaC(2595)+ p- pi- and, possibly, B- ->SigmaC++ anti-Delta-- pi-. We see unexplained structures in m(SigmaC++ pi- pi-) at 3.25 GeV/c^2, 3.8 GeV/c^2, and 4.2 GeV/c^2.


I. Introduction
The large mass of the B meson allows a wide spectrum of baryonic decays, which have, in total, a branching fraction of (6.8±0.6) % [1]. This makes B decays a good place to study the mechanisms of baryon production. One approach to investigate the baryonization process in B decays is to measure and compare their exclusive branching fractions and study the dynamic structure of the decay, i.e., the influence of resonant subchannels.
In this paper, we present a study of the decay B − → Σ ++ c pπ − π − [2]. This decay is a resonant subchannel of the five body final state B − → Λ + c p π + π − π − , which has the largest hitherto known branching fraction among all baryonic B decays and hence is a good starting point for further investigations. The analyzed decay can be compared with B − → Σ 0 c pπ + π − and B 0 → Σ ++ c pπ − , which have similar quark content and phase space.

II. The BABAR experiment
This analysis is based on a dataset of about 426 fb −1 , corresponding to 467 × 10 6 BB pairs. The sample was collected with the BABAR detector at the PEP-II asymmetric-energy e + e − storage ring, which was operated at a center-of-mass energy equal to the Υ (4S) mass. For generation of Monte Carlo (MC) simulated data we use EvtGen [5] for event generation and GEANT4 [6] for detector simulation.
The BABAR detector is described in detail elsewhere [7]. The selection of proton, kaon, and pion candidates is based on measurements of the energy loss in the silicon vertex tracker and the drift chamber, and of the Cherenkov radiation in the detector of internally reflected Cherenkov light [8]. The average efficiency for pion identification is approximately 95 %, with a typical misidentification rate of 10 % due to other charged particles such as muons and kaons, depending on the momentum and the polar angle of the particle. The efficiency for kaon identification is about 95 % with a misidentification rate less than 5 % due to protons and pions. The efficiency for proton and antiproton identification is about 90 % with a misidentification rate about 2 % due to kaons.

III. Decay reconstruction
For the reconstruction of the B candidate the entire decay tree is fitted simultaneously. A vertex fit is performed for B − , Σ ++ c , and Λ + c , and the χ 2 fit probability is required to exceed 0.1 %.
To suppress background, the invariant mass of the p K − π + combination is required to satisfy 2275 MeV/c 2 < m pK − π + < 2296 MeV/c 2 , i.e., compatible with coming from the decay Λ + c → p K − π + . This selection corresponds to 2.8 times the observed width of reconstructed Λ + c candidates which are centered at m pK − π + = 2285.4 MeV/c 2 . The separation of signal from background in the B-candidate sample is obtained using two kinematic variables, ∆E = E * B − √ s/2 and where √ s is the center-of-mass (CM) energy of the e + e − pair and E * B the energy of the B candidate in the CM system. (E i , p i ) is the four-momentum vector of the e + e − CM system and p B the B-candidate momentum vector, both measured in the laboratory frame. For correctly reconstructed B decays, m ES is centered at the B meson mass and ∆E is centered at zero. Throughout this analysis, B candidates are required to have m ES within 8 MeV (3.4σ) of the measured B mass of m ES = 5279.1 MeV. Figure 1 shows the distribution of ∆M ≡ m( Λ + c π + ) − m( Λ + c ) in data for candidates that satisfy the criteria described above for m ES and m pK − π + and for which ∆E is between −60 MeV and +40 MeV. We perform a binned minimum χ 2 fit using a second-order polynomial for the description of the background and the sum of a Voigt distribution (the convolution of a Breit-Wigner function with a Gaussian function) and a Gaussian to parametrize the Σ ++ c signal. A detailed explanation of the fit function is given in Sec. IV. The fitted Σ ++ c signal yield is N = 1020 ± 95.  Figure 2 shows the ∆E distribution in data for candidates that satisfy the criteria described above for m ES and m pK − π + and for which ∆M is between 0.157 GeV/c 2 and 0.178 GeV/c 2 . The latter is a selection of Σ ++ c candidates with an efficiency of 92% in signal MC.
In the binned minimum χ 2 fit we use the sum of two Gaussian functions for the signal and a linear function for the background. The second Gaussian accommodates B decays with missing energy due to final state radiation. Each Gaussian has a mean parameter (µ) and a standard deviation (σ). The joint normalization is described by N sig and the fraction of the first Gaussian is f 1 . We parametrize the background shape of the ∆E distribution as a first-order polynomial which provides a good description of the ∆E distribution for candidates in the m ES sideband in the range 5.20 GeV/c 2 < m ES < 5.26 GeV/c 2 . All parameters are permitted to vary during fitting. Table I presents the resulting signal parameters. The signal yield is 840 ± 55 events.

IV. Signal extraction
There are two sources of background that contribute to the signal in ∆E and ∆M . The first one is B decays that have the same final state, in particular B − → Λ + c p π + π − π − , and the other one is B decays that have a Σ ++ c among its decay products, e.g., B 0 → Σ ++ c p π − π 0 . To reject this background we make a binwise fit using ∆M as a discriminating variable to create a background-subtracted ∆E distribution from which we extract the true signal yield in order to determine The binwise fitting procedure is described in the following paragraph.
After applying the selection in m pK − π + and m ES (no selection in ∆M ), we divide the ∆E range (−105, 105) MeV into 14 equal slices and fit the ∆M distribution in each slice separately in the range 0.14 GeV/c 2 < ∆M < 0.2 GeV/c 2 . In the fits the Σ ++ c signal is represented by the sum of a Voigt function and a Gaussian function. The Voigt distribution has four parameters (N sig , µ, Γ, σ) and models the signal peak region, where µ is the mean of the Voigt and represents the Σ ++ c mass, which is fixed to the value obtained from an inclusive analysis of Σ ++ c → Λ + c π + candidates in the data. The parameter Γ is the intrinsic width of the Σ ++ c and is fixed to the 2010 Review of Particle Properties (RPP) value [1], and σ describes the detector resolution in ∆M for the Σ ++ c determined, independently for each ∆E slice, from the signal MC. The remaining parameter N i sig is the fitted Σ ++ c signal yield in each of the ∆E bins.
There is a correlation between ∆M and ∆E that is very prominent due to the inaccurate momentum measurement of the slow π + from the Σ ++ c decay. As a result the Σ ++ c signal has tails in the ∆M distribution that are modeled by the Gaussian function whose parameters are determined, independently for each ∆E slice, from the signal MC. The background is represented by a secondorder polynomial. This shape was determined from the sidebands |∆E| ∈ (50, 300) MeV and, compared to the other polynomials, gives the best χ 2 fit probability. The fits in ∆M determine the background level and the number of Σ ++ c baryons. Figure 3 shows the Σ ++ c signal yield as a function of ∆E. We fit this distribution with the same functions described in Sec. III and fix the signal parameters, except for N sig , to those determined there. The true signal yield is N sig = 787 ± 43 events.  Table I.

V. Efficiency
The efficiency is calculated from the simulated events. These events were generated uniformly in four-body phase space (PS), but the actual decay distribution is, a priori, unknown. Therefore, when calculating the efficiency, we weight the MC events so that we reproduce the distributions of the two-body invariant mass distributions for the decay products of the B candidates in data. The resulting efficiency is checked by repeating the procedure using the three-body masses and then again using the angles between the B daughters in the B rest frame. The different procedures give an average efficiency of (11.3 ± 0.2 (syst) ) %, which is used to determine the branching fraction. Out of the efficiencies from the different procedures, we use the maximum deviation from the average efficiency as systematic uncertainty. The statistical uncertainty, due to the use of the data, is negligible compared to the statistical uncertainty in the event yield. The efficiency calculated using unweighted events is 11.0 %.

VI. Systematic uncertainties
We estimate the uncertainty on the signal extraction in three different ways: (1) the fit to ∆E in Fig. 3 is repeated separately for each shape parameter in Table  I, while permitting this parameter to float. The absolute deviations (δN ) in the event yield to our true signal yield N sig = 787 add up to 23 (see Table II). (2) We use a second-order polynomial for the background while letting all other parameters fixed (δN = 5), and (3) we fit only the background with a first-order polynomial and subtract its integral from the histogram content in the range −60 MeV < ∆E < 45 MeV in order to obtain an alternative signal yield (δN = 3). The absolute values of the deviations in the event yields from all of these variations add up to 31. The resulting relative uncertainty on the signal yield is 4.0 %. Other systematic errors come from track reconstruction efficiency (2.4 %) [9], efficiency (1.8 %), and the number of produced BB pairs in the data sample (1.1 %). The total relative uncertainty on the branching fraction is 5.1 %.  Fig. 3 while the given parameter is allowed to float. δN is the absolute deviation to our true signal yield Nsig = 787.

VII. Branching fraction results
Using the results from the signal extraction, efficiency determination, and estimation of systematic errors we find = (1.49 ± 0.08 (stat) ± 0.08 (syst) ) × 10 −5 , and In Eq. 2 the last error is due to the uncertainty in B( Λ + c → pK − π + ).

VIII. Fraction of PS distributed decays
To compare the two-body and three-body invariant masses of the B decay products in data with PS, we determine an effective PS fraction of the total branching ratio. To do this, we assume that the resonant substructures are due to the intermediate states Λ * + c → Σ ++ c π − and ∆ −− → pπ − , and the remainder is distributed according to four-body PS. We investigate all two-dimensional planes that are spanned by the twobody invariant masses of the B decay products, e.g. m( Σ ++ c π − s ) against m(pπ − f ), to look for a range that is free from Λ * + c and ∆ −− resonances and hence can be described by a four-body PS distribution. The symbol π − s refers to the π − that has the lower momentum in the e + e − CM system. The other π − is denoted as π − f . We see no indication of ∆ −− and Λ * + c resonances for B candidates in the range 3.050 GeV/c 2 < m( Σ ++ c π − s ) < 3.450 GeV/c 2 , where the normalization of the PS distribution is determined by fitting the sideband-subtracted data (Fig. 4). From the ratio of the efficiency-corrected integrals of the distributions in Fig.4, we calculate an effective PS fraction: = 389 11.0 % · 11.3 % 816 = 49 % .
(3) This percentage will be used to normalize the PS projection in the two-body and three-body invariant mass distributions in Figs. 5-7.
We have calculated an effective PS fraction of 49 % for the observed decay, which may indicate the importance of resonant substructures in baryonic B decays. By comparing the data and four-body PS in the distributions of