We present recent progress in the analysis of the Omega_c^0, the Xi_cc^+ and
the Xi_cc^++.Comment: Contribution to the proceedings of HQL06, Munich, October 16th-20th
  200

Engelfried, Jurgen

English

arXiv

SELEX (Fermilab Experiment 781) [1] employs beams of {Sigma}{sup -}, {pi}{sup -}, and protons at around 600 GeV/c to study production and decay properties of charmed baryons. It took data in the 1996/7 fixed target run and is currently analyzing those data. Here they focus on recently obtained results concerning the {Omega}{sub c}{sup 0} lifetime and the doubly-charmed baryons {Xi}{sub cc}{sup +} and {Xi}{sub cc}{sup ++}

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UASLP–IF–07–001FERMILAB–Conf–07/029–ESELEX: Recent Progress in the Analysis ofCharm-Strange and Double-Charm BaryonsContribution to the proceedings of HQL06,Munich, October 16th-20th 2006Ju¨rgen Engelfried1Instituto de F´ısica,Universidad Auto´noma de San Luis Potos´ı, Mexicofor the SELEX Collaboration1 IntroductionSELEX (Fermilab Experiment 781) [1] employs beams of Σ−, pi−, and protons ataround 600GeV/c to study production and decay properties of charmed baryons. Ittook data in the 1996/7 fixed target run and is currently analyzing those data.Here we will focus on recently obtained results concerning the Ω0c lifetime and thedoubly-charmed baryons Ξ+cc and Ξ++cc .2 New Results on the Ω0cSELEX observes the Ω0c in three decay modes, namely Ω0c → Ω−pi+, Ω0c → Ω−pi+pi+pi−,and Ω0c → Ξ−K−pi+pi+. The invariant mass distributions for these modes are shownin fig. 1. The total sample contains 107 ± 22 events, nearly half of them in Ω3pi.At this moment we are working on the systematics of the mass and branching ratiomeasurements of these modes [2].We use the Ωpi and Ω3pi channels to determine the lifetime of the Ω0c . We calculatethe reduced proper time ct, given by ct = (L−Nσ)/γ, requiring L/σ > N with N = 6,for each event within the mass region of the Ω0c . The proper lifetime resolution is∼ 20 fs. We make a maximum likelihood fit to a probability distribution having an1email: jurgen@ifisica.uaslp.mx1Figure 1: Invariant mass distributions for different decay modes of the Ω0c . Left:Ω−pi+, Signal: 35±12 events; center: Ω−pi+pi+pi−, 44±14 events; right: Ξ−K−pi+pi+,28± 12 eventsexponential decay for the signal and two exponentials for the fast and slow componentsof the background:Ns(1− α)f(t)τ−1e−t/τ + αNB(βτ−11 e−t/τ1 + (1− β)τ−12 e−t/τ2)where τ , α, β, τ1, τ2 are the fit parameters describing the lifetime and the relativecontributions of the background to the ct distribution, and f(t) is the acceptancefunction. We do this for each mode separately, and obtain for the Ωpi mode τ =62.6 ± 22.0 fs and for the Ω3pi mode τ = 65.8 ± 16.0 fs. Combining the two resultsyields τΩc = 65 ± 13 (stat)± 9 (sys) fs. More details can be found in [3]. This resultshould be compared to the current PDG average [4] of 69 ± 12 fs, using a total of175 events from three different experiments.3 Doubly Charmed Baryons3.1 The Discovery of Double Charm BaryonsIn 2002 the SELEX collaboration reported the first observation of a candidate for adouble charm baryon, decaying as Λ+c K−pi+ [5, 6]. The state had a mass of 3519 ±2MeV/c2, and its observed width was consistent with experimental resolution, lessthan 5MeV/c2. The final state contained a charmed baryon and negative strangeness(Λ+c and K−), consistent with the Cabibbo-allowed decay of a Ξ+cc configuration.In order to confirm the interpretation of this state as a double charm baryon, it isessential to observe the same state in some other way. Other experiments with largecharm baryon samples, e.g., the FOCUS [7] and E791 fixed target charm experimentsat Fermilab or the B-factories, have not confirmed the double charm signal. This is2not inconsistent with the SELEX results. The report in Ref. [5] emphasized that thisnew state was produced by the baryon beams (Σ−, proton) in SELEX, but not bythe pi− beam. It also noted that the apparent lifetime of the state was significantlyshorter than that of the Λ+c , which was not expected in model calculations [8]. Amore detailed discussion can be found in [9].3.2 Features and Problems in the Original Analysis, and Pos-sible SolutionsAll the signals observed so far are statistically significant, but have only a few signalevents. The signals are clean, e.g. there is very little background, but the backgrounditself is also difficult to estimate. SELEX only observes events from the baryon (Σ−,proton) beams, and the number of observed events is larger than some productionmodels (see for example [10, 11]) predict. As mentioned before, the lifetime seems tobe very short, and no other experiment has confirmed our observations.Another way to confirm the Ξ+cc is to observe it in a different decay mode that alsoinvolves a final state with baryon number and charm (not anti-charm). One such modeinvolving only stable charged particles is the channel pD+K−, another one Ξ+c pi+pi−.SELEX developed a new method for a more reliable background determination. Wealso improved the resolution on the secondary vertex position by including the single-charm track into the vertex fit, and we redid our full analysis chain to increase ourstatistics. In the following we will describe these step in details.3.3 New Analysis Features within SELEXThe Cabibbo-allowed decay of the Ξ+cc is shown in the following figure.ªªª®®®udc sW+d dc cIn the final state we expect a baryon, and the quarks csdud plus eventually somepairs from the sea. We also expect a cascaded decay chain, with the first, and laterthe second charm quark undergoing a weak decay.3For SELEX, the easily accessible decay modes for the different doubly charmedbaryons are: Ξ+cc → Λ+c K−pi+, Ξ+cc → pD+K−, Ξ+cc → Ξ+c pi−pi+, Ξ++cc → Λ+c K−pi+pi+,Ξ++cc → pD+K−pi+ (depending on the mass of the Ξ++cc ), Ξ++cc → Ξ+c pi+, Ξ++cc →Ξ+c pi+pi+pi−, Ω+cc → Ξ+c K−pi+, and Ω+cc → Ξ+c K−pi+pi+pi−. The first two modes arealready published [5, 12] by SELEX, and work on the other modes is in progress; herewe will report on a first observation of the third decay mode listed.For the background determination, we developed an event mixing method. Thefirst decay vertex is close to the primary vertex, and we assume that all the backgroundis purely combinatoric. We make combinatoric backgrounds by taking the first decayvertex from one event, and the second vertex from another event; to increase statistics,we use the single-charm vertex 25 times. The resulting combinatoric background isabsolutely normalized. We employed this method already in [12].3.4 Ξ+cc → Λ+c K−pi+ – New Analysis01002003004005006007008002.2 2.225 2.25 2.275 2.3 2.325 2.35 2.375ALLCHAN   7711.  34.03    /    35P1   2140.   55.07P2   2.287  0.2228E-03P3  0.7517E-02  0.2283E-03P4   138.1   2.083P5  -160.6   31.78Mass pK-pi+ [GeV/c2]Entries per 5 MeV/c2Figure 2: Λ+c → pK−pi+ data sets of original (left) and new (right) analysis.To increase our statistics, we re-analyzed our full data set with some softer cutsand with improved tracking software. In fig. 2 we show a comparison of the Λ+c dataset used for the analysis. The number of Λc → pK−pi+ candidates increased from1630 to 2140.We also improved the resolution of the decay vertex position of the Ξ+cc candidateby including the vector of the Λ+c into the vertex fit. This improved resolution reduces4the background when applying a cut in L/σ, while keeping more signal events. Italso increases the possibility of measuring the lifetime of double charm baryons.0246810123.4 3.45 3.5 3.55 3.6ALLCHAN   347.0L1/σ1>1SELEX PreliminaryEntries per 5MeV/c20123456789103.4 3.45 3.5 3.55 3.6ALLCHAN   206.0L1/σ1>1.6 SELEX Preliminary012345673.4 3.45 3.5 3.55 3.6ALLCHAN   107.0L1/σ1>2.4 SELEX Preliminary01234563.4 3.45 3.5 3.55 3.6ALLCHAN   73.00L1/σ1>3 SELEX PreliminaryMass Λc+K-pi+ [GeV/c2]Figure 3: Λ+c K−pi+ (Λ+c → pK−pi+) invariant mass distributions (blue) for variouscuts in L/σ on the first decay vertex. In green we show the estimated combinatoricbackground from the event mixing procedure described in the text.Figure 3 shows the results of our new analysis, for various cuts in L/σ of thefirst decay vertex. Re-analyzing and relaxing some cuts in the single charm sampleincreased the number of signal events, but also resulted in a somewhat higher back-ground level; but the background is nicely reproduced and well understood from thecombinatoric analysis. The improved secondary vertex resolution yields in cleanersignals and allows access to other decay modes, which we will pursue in the future.Measuring the lifetime now seems possible, but is still challenging. As seen from theyields for different cuts in L/σ, the lifetime seems to be around 1σ.3.5 Ξcc(3780)++ → Λ+c K−pi+pi+We also revisited with our re-analyzed data set the first double-charm baryon statewe found in SELEX [6], the Ξcc(3780)++. In fig. 4 is shown the Λ+c K−pi+pi+ in-variant mass distribution, restricting ourselves to Σ− induced events. The peak at3780MeV/c2 is statistically significant, and is wider than our experimental resolu-50246810123.4 3.5 3.6 3.7 3.8 3.9 4Λ+c K- pi+ pi+    Σ- Beampeak:     3780 MeV/c25 bin sig/bkg:   22/12Gaussian significance: 6.3 σPoisson Prob.:  < 1.0 x 10-7mixed event bkgabsolute normalizationM(Λ+c K- pi+ pi+) GeV/c2Events/15 MeV/c2Figure 4: The Λ+c K−pi+pi+ invariant mass distribution, for Σ− beam only.tion, as shown by Monte Carlo. The background is well described by our mixed eventprocedure. By removing the slower of the pi+’s, we observe that about half of theΞcc(3780)++ decay to Ξ+cc(3520). At this moment we are finishing up the analysis forthis state.64 First Observation of Ξ+cc → Ξ+c pi+pi−SELEX published [13] the first observation of the Cabibbo-suppressed decay of Ξ+c →pK−pi+; this is the same final state as we used before for the reconstruction of theΛ+c . Our sample of Ξ+c in the mode is much smaller than our Λ+c sample, but thebranching fraction of Ξ+cc → Ξ+c pi+pi− should be larger than to Λ+c K−pi+. We appliedthe same cuts and procedure as to the previously described analyzes, and obtained [14]the Ξ+c pi+pi− invariant mass distribution shown in fig. 5. A clear peak at about0204060801001202.4 2.45 2.5 2.55ALLCHAN   2458.  37.20    /    35P1   89.28   21.59P2   2.465  0.1312E-02P3  0.5551E-02  0.1215E-02P4   57.80   1.295P5  -102.0   20.69Mass pK-pi+ [GeV/c2]Entries per 5 MeV/c20123456783.4 3.45 3.5 3.55 3.6ALLCHAN   187.0L1/σ1>0 SELEX PreliminaryMass Ξc+pi+pi- [GeV/c2]Entries per 5MeV/c2Figure 5: Left: pK−pi+ invariant distribution and Ξ+c sample (yellow) used. Right:Ξ+c pi+pi− invariant mass distribution. The green histogram is our estimate of thecombinatoric background.3520MeV/c2 is seen in the figure. This constitutes the first observation of this decaymode of the Ξ+cc(3520).5 SummarySELEX is still the only experiment observing double charm baryons. We published ob-servations on two different decays modes, Ξ+cc → Λ+c K−pi+ [5] and Ξ+cc → pD+K− [12].After a re-analysis of our full data set, with improved efficiency and resolution, wepresented here a higher-statistics observation of Ξ+cc → Λ+c K−pi+, and a re-analysis ofthe Ξcc(3780)++. The new analysis also allows access to additional decay modes, andwe presented here the first observation of Ξ+cc → Ξ+c pi−pi+.SELEX will continue the line of analysis, by first publishing these preliminaryresults. We will try to measure the lifetime of the Ξ+cc. We will also seek the isospin-partner of the Ξ+cc, the Ξ++cc in all corresponding decay modes around 3500MeV/c.76 AcknowledgmentThe author thanks the organizers for the invitation to present these results at theconference. This work was supported in part by the Consejo Nacional de Ciencia yTecnolog´ıa (CONACyT), Mexico, and by a special research grant of UASLP.References[1] The SELEX (Fermilab E781) Collaboration: Ball State University, Bogazici Uni-versity, Carnegie-Mellon University, Centro Brasileiro de Pesquisas Fisicas, Fer-milab, Institute For High Energy Physics (Protvino), Institute of High EnergyPhysics (Beijing), Institute of Theoretical and Experimental Physics (Moscow),Max-Planck-Institute for Nuclear Physics, Moscow State University, PetersburgNuclear Physics Institute, Tel Aviv University, Universidad Auto´noma de SanLuis Potos´ı, Universidade Federal da Para´ıba, H. H. Wills Physics Laboratory,University of Bristol, University of Iowa, University of Michigan-Flint, Univer-sity of Rochester, University of Rome La Sapienza and INFN, University of Sa˜oPaulo, University of Trieste and INFN. http://www-selex.fnal.gov[2] SELEX Collaboration, to be published.[3] SELEX Collaboration, M.Iori, et al.: Measurement of the Ω0c lifetime. Submittedto Physical Review Letters. Preprint Fermilab-Pub-07/011-E, hep-ex/0701021.[4] Particle Data Group, W. M. Yao et al., Journal of Physics G 33, 1 (2006).[5] SELEX Collaboration, M. Mattson et al.: First observation of the dou-bly charmed baryon Ξ+cc. Phys. Rev. Letters 89 (2002) 112001, [arXiv:hep-ex/0208014].[6] M. Mattson, Ph.D. thesis, Carnegie Mellon University, 2002, FERMILAB-THESIS-2002-03.[7] S. Ratti, BEACH2002, Vancouver, B.C., see alsowww.hep.vanderbilt.edu/∼stenson/xicc/xicc focus.html[8] B. Guberina, B. Melic, and H. Stefancic: Phys. Lett. B484 (2000) 43.[9] J. Engelfried: The Experimental Discovery of Double-Charm Baryons. NuclearPhysics A 752 (2005) 121c-128c.[10] M. Moinester: Zeit. Phys. A355 (1996) 349.8[11] V. Kiselev and A. Likhoded: Phys. Usp. 45 (2002) 455-506, Usp.Fiz.Nauk 172(2002) 497-550, [arXiv:hep-ex/0103169], and references therein.[12] SELEX Collaboration, A. Ocherashvili, et al.: Confirmation of the double charmbaryon Ξ+cc(3520) via its decay to pD+K−. Physics Letters B 628 (2005) 18.[arXiv:hep-ex/0406033].[13] SELEX Collaboration, S.Y. Jun, et al.: Observation of the Cabibbo-suppresseddecay Ξ+c → pK−pi+. Phys. Rev. Letter84 (2000) 1857-1861. [hep-ex/9907062].[14] I. Torres, Ph.D. Thesis, Instituto de F´ısica, Universidad Auto´noma de San LuisPotos´ı, Mexico (2006).9

SELEX: Recent Progress in the Analysis of Charm-Strange and Double-Charm Baryons

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