Due to an excellent mass resolution and a large amount of available data, the
CDF experiment, located at the Tevatron proton-antiproton accelerator, allows
the precise measurement of spectroscopic properties, like mass and decay width,
of a variety of states. This was exploited to examine the first orbital
excitations of the Lambda_c baryon, the resonances Lambda_c(2595) and
Lambda_c(2625), in the decay channel Lambda_c^+ pi^+ pi^-, as well as the
Lambda_c spin excitations Sigma_c(2455) and Sigma_c(2520) in its decays to
Lambda_c^+ pi^- and Lambda_c^+ pi^+ final states in a data sample corresponding
to an integrated luminosity of 5.2 invfb. We present measurements of the mass
differences with respect to the Lambda_c and the decay widths of these states,
using significantly higher statistics than previous experiments.Comment: submitted to Proceedings of the International Workshop on Charm
  Physics (Charm 2010

Wick, Felix

English

arXiv

 Due to an excellent mass resolution and a large amount of available data, the CDF experiment, located at the Tevatron proton-antiproton accelerator, allows the precise measurement of spectroscopic properties, like mass and decay width, of a variety of states. This was exploited to examine the first orbital excitations of the Λc baryon, the resonances Λc(2595) and Λc(2625), in the decay channel [Formula: see text], as well as the Λc spin excitations Σc(2455) and Σc(2520) in its decays to [Formula: see text] and [Formula: see text] final states in a data sample corresponding to an integrated luminosity of 5.2 fb-1. We present measurements of the mass differences with respect to the Λc and the decay widths of these states, using significantly higher statistics than previous experiments. </jats:p

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FELIX WICK

Crossref

International Journal of Modern Physics Conference Series

RESULTS ON CHARM BARYON SPECTROSCOPY FROM TEVATRON

Due to an excellent mass resolution and a large amount of available data, the CDF experiment, located at the Tevatron proton-antiproton accelerator, allows the precise measurement of spectroscopic properties, like mass and decay width, of a variety of states. This was exploited to examine the first orbital excitations of the {Lambda}{sub c} baryon, the resonances {Lambda}{sub c}(2595) and {Lambda}{sub c}(2625), in the decay channel {Lambda}{sub c}{sup +} {pi}{sup +}{pi}{sup -}, as well as the {Lambda}{sub c} spin excitations {Sigma}{sub c}(2455) and {Sigma}{sub c}(2520) in its decays to {Lambda}{sub c}{sup +} {pi}{sup -} and {Lambda}{sub c}{sup +} {pi}{sup -} final states in a data sample corresponding to an integrated luminosity of 5.2 fb{sup -1}. We present measurements of the mass differences with respect to the {Lambda}{sub c} and the decay widths of these states, using significantly higher statistics than previous experiments

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International Journal of Modern Physics: Conference Seriesc© World Scientific Publishing CompanyRESULTS ON CHARM BARYON SPECTROSCOPY FROMTEVATRONFELIX WICK∗Institut fuer Experimentelle Kernphysik, Karlsruhe Institute of TechnologyWolfgang-Gaede-Str. 1, Karlsruhe, 76131, Germanywick@fnal.govDue to an excellent mass resolution and a large amount of available data, the CDFexperiment, located at the Tevatron proton-antiproton accelerator, allows the precisemeasurement of spectroscopic properties, like mass and decay width, of a variety ofstates. This was exploited to examine the first orbital excitations of the Λc baryon, theresonances Λc(2595) and Λc(2625), in the decay channel Λ+c π+π−, as well as the Λcspin excitations Σc(2455) and Σc(2520) in its decays to Λ+c π− and Λ+c π+ final statesin a data sample corresponding to an integrated luminosity of 5.2 fb−1. We presentmeasurements of the mass differences with respect to the Λc and the decay widths ofthese states, using significantly higher statistics than previous experiments.PACS numbers: 14.20.Lq, 14.20.Gk1. IntroductionHeavy quark baryons provide an interesting laboratory for studying and testingQuantum Chromodynamics (QCD), the theory of strong interactions. The heavyquark states test regions of the QCD where perturbation calculations cannot beused, and many different approaches to solve the theory were developed. In thiswrite-up we focus on Λc(2595), Λc(2625), Σc(2455) and Σc(2520) baryons. All fourstates were observed before and some of their properties measured.1 However, differ-ent mass measurements for Σc(2520) and Λc(2625) are inconsistent and the statis-tics for Λc(2595) and Λc(2625) is rather low. In addition, Blechman and co-workersshowed that a more sophisticated treatment, which would take into account theproximity of the threshold in the Λc(2595) decay, yields a Λc(2595) mass which is2− 3MeV/c2 lower than the one observed.2 Σc states were observed and studied inΛcπ decays, while excited Λc states decay mainly to a Λcππ final state and decaysthrough intermediated Σc resonances are possible. One peculiarity of the experi-mental studies of these baryons is in their cross talks, which requires special carein the treatment of the background due to different kinematic regions allowed fordifferent sources.∗On behalf of the CDF Collaboration.1FERMILAB-CONF-10-572-E-PPD2 Felix WickIn this analysis we exploit a large sample of Λ+c→ pK−π+ decays collectedby the CDF detector to perform the measurement of the masses and widths ofthe discussed charmed baryons. We take into account all cross-talks and thresholdeffects expected in the decays under study.3 Throughout this document the use ofa specific particle state implies the use of the charge-conjugate state as well.2. Analysis StrategyThe employed dataset corresponds to an integrated luminosity of 5.2 fb−1 and wascollected using the displaced track trigger which requires two charged particles com-ing from a single vertex with transverse momenta larger than 2GeV/c and impactparameters in the plane transverse to the beamline in the region of 0.1 to 1mm.The selection of the candidates is done in two steps. As all final states feature aΛc daughter, as first step we perform a Λc selection. In the second step we performa dedicated selection of the four states under study. In each step we use neuralnetworks to distinguish signal from background. All neural networks4 are trainedusing data only by means of the sPlot technique5. As we use only data for the neuralnetwork trainings, for each case we split the sample to two parts (even and odd eventnumbers) and train two networks. Each of them is applied to the complementarysubsample in order to maintain a selection which is trained on a sample independentfrom the one to which we apply it.In order to determine the mass differences relative to the Λc and the decaywidths of the six studied states, we perform binned maximum likelihood fits ofthree separate mass difference (∆M) distributions. The first two are Λ+cπ+ andΛ+cπ− where the states Σc(2455)++,0 and Σc(2520)++,0 are studied. The last oneis Λ+cπ+π− for Λc(2595)+ and Λc(2625)+. In each of the distributions we need toparametrize two signals and several background components.Both Σc(2455)++,0 and Σc(2520)++,0 are described by a nonrelativistic Breit-Wigner function convolved with a resolution function. The resolution functionis parametrized with three Gaussians centered in zero and other parameters de-rived from simulated events. The mean width of the resolution function is about1.6MeV/c2 for Σc(2455)++,0 and about 2.6MeV/c2 for Σc(2520)++,0. We considerthree different types of background, namely random combinations without real Λc,combinations of real Λc with a random pion and events due to the decay of excitedΛc to Λ+cπ+π−. Thereby, random combinations without a real Λc dominate and aredescribed by a second-order polynomial with shape and normalization derived ina fit to the ∆M distribution from Λc mass sidebands. The apparent difference be-tween doubly-charged and neutral combinations is due to D∗(2010)+ mesons withmultibody D0 decays, where not all D0 decay products are reconstructed. In orderto describe this reflection, an additional Gaussian function is used. The second back-ground source, consisting of real Λc combined with a random pion, is modeled bya third-order polynomial. As we do not have an independent proxy for this source,all parameters are left free in the fit. The last source, originating from nonresonantResults on Charm Baryon Spectroscopy from Tevatron 3a)150 200 250 3002Candidates per 0.5 MeV/c05001000150020002500 DataFit Function+π +cΛ → ++(2455)cΣ+π +cΛ → ++(2520)cΣ 13800≈) ++(2455)cΣN( 8800≈) ++(2520)cΣN(-1CDF Run II preliminary, L = 5.2 fb]2) [MeV/c+cΛ)-Mass(+π +cΛMass(fit(data - fit)-2024b)150 200 250 3002Candidates per 0.5 MeV/c050010001500200025003000 DataFit Function-π +cΛ → 0(2455)cΣ-π +cΛ → 0(2520)cΣ 15900≈) 0(2455)cΣN( 9000≈) 0(2520)cΣN(-1CDF Run II preliminary, L = 5.2 fb]2) [MeV/c+cΛ)-Mass(-π +cΛMass(fit(data - fit)-4-202Fig. 1. The (a) M(Λ+c π+)−M(Λ+c ) and (b) M(Λ+c π−)−M(Λ+c ) distribution obtained from data(points with error bars) together with the fit projection (black line). The brown and purple linescorrespond to the two signal contributions, the green line represents the combinatorial backgroundwithout real Λc, the blue line shows real Λc combined with a random pion and the red dotted linerepresents a reflection from excited Λc decays. The red dashed line corresponds to the sum of allthree background contributions.Λc(2625) decays, is described by the projections of the flat Λ+cπ+π− Dalitz ploton the appropriate Λ+cπ+ respective Λ+cπ− axes. The measured data distributionstogether with the fit projections can be found in figure 1.In the fit for Λc(2595) and Λc(2625), an additional complication compared to theΣc case arises from the fact that previous measurements of the Λc(2595) propertiesindicate that it decays dominantly to Σc(2455)π, which has its threshold very closeto the Λc(2595) mass. Ref. 2 shows that taking into account the resulting strongvariation of the natural width yields a lower Λc(2595) mass than observed by formerexperiments. With CDF statistics we are much more sensitive to the details of theΛc(2595) line shape than previous analyses. The Λc(2595) parametrization followsRef. 2. The state is described by a nonrelativistic Breit-Wigner function with amass-dependent decay width which is calculated as integral over the Dalitz plotcontaining the Σc(2455) resonances. The pion coupling constant h2 is related to theΛc(2595) decay width and represents the quantity we measure instead of the naturalwidth. The shape is then numerically convolved with a three Gaussian resolutionfunction determined from simulation, which has a mean width of about 1.8MeV/c2.The signal function for the Λc(2625) is a nonrelativistic Breit-Wigner which is againconvolved with a three Gaussian resolution function determined from simulation.The mean width of the resolution function is about 2.4MeV/c2. The backgroundconsists of three different sources which are combinatorial background without realΛc, real Λc combined with two random pions and real Σ++,0ccombined with arandom pion. The combinatorial background without real Λc is parametrized bya second order polynomial whose parameters are determined in a fit to the ∆Mdistribution of candidates from Λc mass sidebands. The second source, consisting4 Felix Wick300 350 4002Candidates per 0.5 MeV/c02004006008001000DataFit Function-π +π +cΛ → +(2595)cΛ-π +π +cΛ → +(2625)cΛ 3500≈) +(2595)cΛN( 6200≈) +(2625)cΛN(-1CDF Run II preliminary, L = 5.2 fb]2) [MeV/c+cΛ)-Mass(-π +π +cΛMass(fit(data - fit)-202Fig. 2. The M(Λ+c π+π−) − M(Λ+c ) distribution obtained from data (points with error bars)together with the fit projection (black line). The brown and purple lines correspond to the twosignal contributions, the green line represents the combinatorial background without real Λc, theblue line shows real Λc combined with two random pions and the red dotted line represents realΣc(2455) combined with a random pion. The red dashed line corresponds to the sum of all threebackground contributions.of real Λc combined with two random pions, is parametrized by a second orderpolynomial with all parameters allowed to float in the fit. The model of the finalbackground source, real Σc(2455) combined with a random pion, is based on auniform function defined from the threshold to the end of the fit range, where thenatural widths as well as the resolution effects of the Σc(2455)++,0 are taken intoaccount for the threshold determination. The size of this contribution is constrainedto the Σc(2455) yield obtained from fits to the projections of the Λ+cπ+π− Dalitzplot from the upper Λc(2625) mass sideband on the appropriate Λ+cπ+ respectiveΛ+cπ− axes. The measured data distribution together with the fit projection can befound in figure 2.We investigate several systematic effects which can affect our measurements.Generally, they can be categorized as imperfect modeling by the simulation, imper-fect knowledge of the momentum scale of the detector, ambiguities in the fit modeland uncertainties on the external inputs to the fit of the Λc(2595) signal. The mostcritical point is that we need to understand the intrinsic resolution of the detector inorder to properly describe the signal shapes, because an uncertainty in the resolu-tion has a large impact on the determination of the natural widths. As we estimateit using simulated events, it is necessary to substantiate that the resolution obtainedfrom simulation agrees with the one in real data. We use D∗(2010)+ → D0π+ withD0 → K−π+ and ψ(2S) → J/ψπ+π− with J/ψ → µ+µ− for this purpose andcompare the resolution in data and simulated events as a function of the transversemomentum of the pions added to D0 or J/ψ. We also compare the overall resolutionscale between data and simulated events and find that all variations are below 20%which we assign as uncertainty on our knowledge of the resolution function.Results on Charm Baryon Spectroscopy from Tevatron 53. ResultsWe select about 13800 Σc(2455)++, 15900 Σc(2455)0, 8800 Σc(2520)++, 9000Σc(2520)0, 3500 Λc(2595)+ and 6200 Λc(2625)+ signal events and obtain the fol-lowing results for the mass differences relative to the Λc mass and the decay widths:∆M(Σc(2455)++) = 167.44± 0.04 (stat.)± 0.12 (syst.)MeV/c2,Γ(Σc(2455)++) = 2.34± 0.13 (stat.)± 0.45 (syst.)MeV/c2,∆M(Σc(2455)0) = 167.28± 0.03 (stat.)± 0.12 (syst.)MeV/c2,Γ(Σc(2455)0) = 1.65± 0.11 (stat.)± 0.49 (syst.)MeV/c2,∆M(Σc(2520)++) = 230.73± 0.56 (stat.)± 0.16 (syst.)MeV/c2,Γ(Σc(2520)++) = 15.03± 2.12 (stat.)± 1.36 (syst.)MeV/c2,∆M(Σc(2520)0) = 232.88± 0.43 (stat.)± 0.16 (syst.)MeV/c2,Γ(Σc(2520)0) = 12.51± 1.82 (stat.)± 1.37 (syst.)MeV/c2,∆M(Λc(2595)+) = 305.79± 0.14 (stat.)± 0.20 (syst.)MeV/c2,h22(Λc(2595)+) = 0.36± 0.04 (stat.)± 0.07 (syst.),∆M(Λc(2625)+) = 341.65± 0.04 (stat.)± 0.12 (syst.)MeV/c2.For the width of the Λc(2625) we get a value consistent with zero and thereforecalculate an upper limit using a Bayesian approach with a uniform prior restrictedto positive values. At the 90% C.L. we obtain Γ(Λc(2625)+) < 0.97MeV/c2. Foreasier comparison to previous results, h22 corresponds to Γ(Λc(2595)+) = 2.59 ±0.30± 0.47MeV/c2.Except of ∆M(Λc(2595)+), all measured quantities are in agreement with previ-ous measurements. For ∆M(Λc(2595)+) we observe a value which is by 3.1MeV/c2smaller than the existing world average. This difference is of the same size as foundin Ref. 2. The precision for the Σc states is comparable to the precision of theworld averages. In the case of excited Λc states, our results provide a significantimprovement in precision against previous measurements.References1. C. Amsler et al. (Particle Data Group), Review of Particle Physics, Phys. Lett. B667,1 (2008).2. A. Blechman, A. Falk and D. Pirjol, Threshold effects in excited charmed baryon decays,Phys. Rev. D67, 074033 (2003).3. CDF Public Note 10260:http://www-cdf.fnal.gov/physics/new/bottom/100701.blessed-Charm Baryons/cdf10260 CharmBaryons.pdf4. M. Feindt and U. Kerzel, The NeuroBayes neural network package, Nucl. In-strum. Meth. A559, 190-194 (2006).5. M. Pivk and F. Le Diberder, sPlot: a statistical tool to unfold data distributions, Nucl.Instrum. Meth. A555, 356-369 (2005).

Results on Charm Baryon Spectroscopy from Tevatron

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