CLNS 04/1895 CLEO 04-14 Search for e+e Λ0Λ0 Near Threshold − b b → D. Besson University of Kansas, Lawrence, Kansas 66045 T. K. Pedlar Luther College, Decorah, Iowa 52101 D. Cronin-Hennessy, K. Y. Gao, D. T. Gong, Y. Kubota, B. W. Lang, S. Z. Li, R. Poling, A. W. Scott, A. Smith, and C. J. Stepaniak 5 University of Minnesota, Minneapolis, Minnesota 55455 0 0 2 S. Dobbs, Z. Metreveli, K. K. Seth, A. Tomaradze, and P. Zweber n Northwestern University, Evanston, Illinois 60208 a J J. Ernst and A. H. Mahmood 9 1 State University of New York at Albany, Albany, New York 12222 4 K. Arms and K. K. Gan v 8 Ohio State University, Columbus, Ohio 43210 7 0 1 H. Severini 1 University of Oklahoma, Norman, Oklahoma 73019 4 0 / D. M. Asner, S. A. Dytman, W. Love, S. Mehrabyan, J. A. Mueller, and V. Savinov x e University of Pittsburgh, Pittsburgh, Pennsylvania 15260 - p e Z. Li, A. Lopez, H. Mendez, and J. Ramirez h University of Puerto Rico, Mayaguez, Puerto Rico 00681 : v i X G. S. Huang, D. H. Miller, V. Pavlunin, B. Sanghi, E. I. Shibata, and I. P. J. Shipsey r Purdue University, West Lafayette, Indiana 47907 a G. S. Adams, M. Chasse, M. Cravey, J. P. Cummings, I. Danko, and J. Napolitano Rensselaer Polytechnic Institute, Troy, New York 12180 C. S. Park, W. Park, J. B. Thayer, and E. H. Thorndike University of Rochester, Rochester, New York 14627 T. E. Coan, Y. S. Gao, F. Liu, and R. Stroynowski Southern Methodist University, Dallas, Texas 75275 M. Artuso, C. Boulahouache, S. Blusk, J. Butt, E. Dambasuren, O. Dorjkhaidav, J. Li, N. Menaa, R. Mountain, H. Muramatsu, R. Nandakumar, R. Redjimi, R. Sia, T. Skwarnicki, S. Stone, J. C. Wang, and K. Zhang Syracuse University, Syracuse, New York 13244 1 S. E. Csorna Vanderbilt University, Nashville, Tennessee 37235 G. Bonvicini, D. Cinabro, and M. Dubrovin Wayne State University, Detroit, Michigan 48202 A. Bornheim, S. P. Pappas, and A. J. Weinstein California Institute of Technology, Pasadena, California 91125 J. L. Rosner Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 R. A. Briere, G. P. Chen, T. Ferguson, G. Tatishvili, H. Vogel, and M. E. Watkins Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 N. E. Adam, J. P. Alexander, K. Berkelman, D. G. Cassel, V. Crede, J. E. Duboscq, K. M. Ecklund, R. Ehrlich, L. Fields, R. S. Galik, L. Gibbons, B. Gittelman, R. Gray, S. W. Gray, D. L. Hartill, B. K. Heltsley, D. Hertz, L. Hsu, C. D. Jones, J. Kandaswamy, D. L. Kreinick, V. E. Kuznetsov, H. Mahlke-Kru¨ger, T. O. Meyer, P. U. E. Onyisi, J. R. Patterson, D. Peterson, J. Pivarski, D. Riley, A. Ryd, A. J. Sadoff, H. Schwarthoff, M. R. Shepherd, S. Stroiney, W. M. Sun, J. G. Thayer, D. Urner, T. Wilksen, and M. Weinberger Cornell University, Ithaca, New York 14853 S. B. Athar, P. Avery, L. Breva-Newell, R. Patel, V. Potlia, H. Stoeck, and J. Yelton University of Florida, Gainesville, Florida 32611 P. Rubin George Mason University, Fairfax, Virginia 22030 C. Cawlfield, B. I. Eisenstein, G. D. Gollin, I. Karliner, D. Kim, N. Lowrey, P. Naik, C. Sedlack, M. Selen, J. J. Thaler, J. Williams, and J. Wiss University of Illinois, Urbana-Champaign, Illinois 61801 K. W. Edwards Carleton University, Ottawa, Ontario, Canada K1S 5B6 and the Institute of Particle Physics, Canada (Dated: Nov. 22, 2004) 2 Abstract UsingtheCLEOIIIdetector atCESRwestudye+e− collisions inthecenter-of-mass energyclose to, or above, Λ0Λ0 production threshold. We search for evidence of Λ0Λ0 resonance production b b b b and set upper limits based on inclusive hadron production as a barometer of Λ0Λ0 production. b b PACS numbers: 14.65.Fy, 13.66.Bc,13.30.Eg 3 I. INTRODUCTION The Λ0, consisting of b, u and d quarks, is the lowest-lying b-flavored baryon, about b which comparatively little is known. Recently the CDF collaboration reported an improved measurement of the Λ0 mass [1] of 5620.4 1.6 1.2 MeV. The lifetime has long been b ± ± measured to be somewhat lower than theoretical expectations [2]. There is, however, no measurement available onthe direct production of exclusive Λ0Λ0 in e+e− annihilation. Such b b events would be very useful for establishing absolute branching ratios and other properties. CLEO has accumulated data using e+e− collisions in the center-of-mass energy range from 11.227 to 11.383 GeV, close to or just above the Λ0Λ0 production threshold. It is possible to b b observe a resonant signal, similar to the Υ(4S) for B+ and B0 mesons, or just an increase in relative production above threshold. We report here limits on such resonant or non-resonant production. II. DATA AND MONTE CARLO SIMULATED SAMPLE The CLEO III detector is described in detail elsewhere [3] [4]. The inner part of the detector is surrounded by a 1.5 T solenoidal magnetic field. From the region near the e+e− interaction vertex radially outward it consists of a silicon strip based vertex detector and a drift chamber used to measure the momenta of charged tracks based on their curvature. Beyond the drift chamber is a Ring Imaging Cherenkov Detector, RICH, used to identify charged hadrons, followed by an Electromagnetic Calorimeter, EC, consisting of nearly 8000 CsI crystals. Next to the EC there is the solenoidal coil followed by an iron return path with wire chambers interspersed in 3 layers to provide muon identification. This study is based on the total 710 pb−1 data sample that was acquired at 3 MeV intervals between center-of-mass energies, E , of 11.227 GeV to 11.383 GeV, to be close to CM or above threshold for Λ0Λ0 production. The luminosity in each of these scan points varies b b from 14 to 20 pb−1. In addition, there are data points taken at a E of 11.150 and 11.203 CM GeV, respectively. The two data points with lowest and highest energies have integrated luminosities of 70 and 120 pb−1, respectively. We also use data taken in the four-flavor continuum below the Υ(4S) to measure the bb cross section above the Υ(4S). For the Monte Carlo, MC, study of the high energy data, we generated five times more hadronic qq events than at each beam energy contained in our data sample. Events were generated separately for “light” four-flavor continuum (c,s,u,d) and bb continuum events and then combined in the expected 10:1 ratio absent any resonance production. The decay channels and the branching fractions of the Λ are less well known than the B0 and B+ b mesons. We list the Λ decay modes and branching fractions we used for the signal Monte b Carlo in Table I. For the Λ0 Λ+ℓ−ν¯ branching fraction we re-scaled the B0 Xℓν¯ branching fraction by the ratibo →of lifectimes, τ(Λ )/τ(B0). The entries denoted by *q→q¯* indi- b cate that the processes are generated using a fragmentation process for the quark-antiquark pair. 4 TABLE I: Λ decay modes and branching fractions used in the Monte Carlo simulation. b Decay modes Branching fraction (%) Λ Λ+e−ν 8.4 b c e → Λ Λ+µ−ν 8.4 b c µ Λ → Λ+π− 4.2 b c Λ → Λ+ρ− 1.0 b c → Λ Λ+a1− 2.1 b c Λ → Λ+D− 2.1 b c s Λ → Λ+D∗− 4.2 b c s → Λ Λη 0.1 b c → Λ ΛJ/ψ 0.5 b → Λ Λ+π−π+π− 2.1 b c Λ → ΛK0π−π−π+π− 2.1 b Λ → p+D0π− 2.1 b → Λ Λ+ du 44.9 b c → ∗ ∗ Λ Σ+ du 8.4 b c → ∗ ∗ Λ Ω+ du 7.3 b cc → ∗ ∗ Λ p+ du 1.1 b Λ → Ξ′+∗ du∗ 1.0 b c → ∗ ∗ III. EVENT SELECTION ThemajorbackgroundstoΛ arenon-bbtypehadronicevents, two-photonevents(e+e− b e+e−X) and τ+τ− pairs. To suppress these backgrounds we require the following hadron→ic event selection criteria: (1)At least five chargedtracks; atrackcandidate isacceptable if itisa cosinewith respect to the beam line of less than 0.9 and has at least half of the potential tracking chamber hits along its length. This requirement rejects 81% of the τ+τ− pairs. (2) The total visible energy, E , is required to be greater than the beam energy, E . vis beam E receives contributions from both charged tracks and unmatched neutral energy clus- vis ters greater than 30 MeV. This requirement helps suppresses two-photon events. Fig. 1(a) shows the E /E distributions for data, five flavor Monte-Carlo continuum and simu- vis beam lated two-photon events [5]. Imposing the requirement E > E reduces the two-photon vis beam background by 75% with a small (3%) loss of hadronic events. (3) The ratio of the 2nd and 0th Fox-Wolfram moments, R , is less than 0.25 [6]. Fig. 1(b) 2 shows MC simulated distributions of R for both bb and non-bb continuum events. Both 2 areas are normalized to unity. Requiring R < 0.25 selects the more spherically shaped 2 events in momentum space and greatly enhances the bb fraction, by rejecting 65% of four- flavor continuum events while losing only 8% of the bb events. To subtract four-flavor continuum background we use data taken at a E 30 MeV below CM the Υ(4S) mass. Since we make a specific cut on R we need to take into account that the 2 shape of the R distribution can change when the E changes. The R distribution from 2 CM 2 below-Υ(4S) data is compared with the distribution using data taken in the Λ scan region b 5 FIG. 1: (a) E /E above Λ threshold data (triangle), five flavor continuum MC (solid) and vis beam b simulated two-photon events (circles). (b) R2 distribution for bb (dashed) and non-bb (solid) type events. in Fig. 2(a). The data are normalized by luminosity and 1/s, where s is the square of the center-of-mass energy. The distributions differ in two respects. The first is the obvious enhancement at small R values in the Λ scan region giving evidence for bb production. The 2 b second is the disagreement in shape at values of R > 0.5, where bb production is absent. 2 We confirm this change in shape with energy by comparing Υ(4S) “on-resonance” data and below-Υ(1S) resonance data (E =9.43 GeV) in Fig. 2(b). The subtracted spectra CM show an anomalous peak near R = 0.5. The number of events in this peak can be as large 2 as 30% of the total number of bb events in higher E region. Thus, it is important CM ∼ to transform correctly the below-Υ(4S) resonance data in order to correctly subtract the background when we apply a tight R requirement. Simple kinematic considerations suggest 2 ′ ′ ′ ′ that R (E )/R (E) E /E, where E > E. The boundary considerations that at R 2 2 2 ∼ values of both 0 and 1 the initial and corrected distributions be equal, result in a simple parameterization of the corrected, or “boosted” R distribution: 2 ′ ′ E E ′ ′ 2 R (E ) = R (E)+ 1 R (E) . (1) 2 2 2 E − E ! This expression describes the energy dependence of the R shape excellently. In Fig. 3 we 2 compare the boosted R distribution for below-Υ(4S) data, normalized by luminosity and 2 1/s, with the same distribution for the high energy data. The distributions match above R 2 of 0.5, as required. We have several strategies for observing the production of Λ0Λ0 events. One possibility b b is to look for enhancements in the (1) bb cross-section. Another is to look for an increase in (2) Λ or (3) anti-proton production. We don’t use protons because there is a large background rate from hadron interactions in the beam pipe and from residual beam gas collisions. Λ’s are promising because we expect that Λo Λ X has a large branching b → c 6 FIG. 2: The R2 distribution above Λb threshold compared with below-Υ(4S) data (a) and Υ(4S) on resonance data compared with below-Υ(1S) data (b). Circles show thesubtracted distributions. ratio, 96% and Λ+ ΛX is approximately 50%. Detecting anti-protons is very promising becaus∼e Λ0 decays calw→ays produce either one proton or neutron. In the case of non-resonant b Λ0Λ0 production we can expect that the cross-section will increase from zero at threshold b b to some constant fraction of the total bb cross-section. In order to ascertain an optimal search strategy, we assume this fraction is 7.9%, as predicted by the JETSET 7.3 Monte Carlo model [7]. This is consistent with the PDG value for bb baryon of 10% [8]. Further → FIG. 3: R2 Distribution at one energy point above Λb threshold compared with below-Υ(4S) after the boost (data). 7 support for this value comes from the ratio of Λ Λ to cc rates. As input to this estimate c c we use a measured value of (Λ+ pK−π+) σ(Λ+) = (10.0 1.5 1.5) pb [9], from B c → × c ± ± our below-Υ(4S) continuum data sample. We take the cc cross section as 4/10 of the total hadronic cross section, implying σ(cc) = 1.12 0.02 nb [10], and we use the PDG mean value for (Λ+ pK−π+) = (5.0 1.3)% [8], yield±ing the ratio or Λ Λ /cc = (8.9 3.0)%. TBhe cre→lative size of the Λ±0Λ0 component for our different scearcch strategi±es is shown in b b Fig. 4(a). Here we normalized the MC simulated five-flavor visible hadronic cross section to unity, defined here as “continuum” udsc and b, and then added the signal Λ0Λ0 to the total b b udscb cross section(i.e., the Λ0Λ0 enhancement here represents an additional 7.9% above b b expected inclusive bb hadronic cross-section, rather than simply presenting an additional channel available to bb hadronization). Λ’s have the highest relative yield closely followed by anti-protons. We optimize our search criteria by maximizing signal divided by square root of the background, S/√B, for our different search methods. The results are summarized in Fig. 4(b), where we show the statistical significance for signal we obtain for different analysis strategies for different Λ0Λ0 cross-sections (statistical errors only). b b FIG. 4: (a) Relative yield of the udsc (lower), b (middle) and Λ (upper) visible cross section for b the inclusive selection of bb , p and Λ assuming a 7.9% increase of the total bb cross section above Λ0Λ0 threshold. (b) Signal/√Background for different analysis strategies and cross-sections. b b Our studies indicate that baryon production (namely anti-protons and Λ’s) is the most sensitive measure of Λ0Λ0 . However, the systematic uncertainties in Λ protons and b b b → Λ Λ diminish their sensitivity relative to inclusive bb production. We also considered b → identifying Λ’s and protons with an additional lepton in the event but these methods offer less significance. The efficiencies for detecting hadronic events, and more importantly, for detecting events with one or more protons are listed in Table II; their evaluation will be discussed in more detail in the next section. We use both charged particle ionization loss in the drift chamber (dE/dx) and RICH information to identify anti-protons. The RICH is used for momenta larger than 1 GeV. Information on the angle of detected Cherenkov photons is translated into a likelihood of a 8 given photon being due to a particular particle. Contributions from all photons associated with a particular track are then summed to form an overall likelihood denoted as for i L each particle hypothesis. To differentiate between kaon and proton candidates, we use the difference: log( )+log( ). This cut is set at -4. To utilize the dE/dx information K proton − L L we calculate σ as the difference between the expected ionization loss for a kaon and the K measured loss divided by the measurement error. Similarly, σ is defined in the same proton manner using the expected ionization for a proton. We use both the RICH and dE/dx to select anti-proton candidates in the following manner: (a) If neither the RICH nor dE/dx information is available, then the track is rejected. (b) If dE/dx is available and RICH is not then we insist that proton candidates have PID σ2 σ2 < 0 (c) If RICH information is available and dE/dx is not avail- dE ≡ K − proton able, then we require that PID log( ) + log( ) < 4. (d) If both dE/dx RICH K proton ≡ − L L − and RICH information are available, we require that (PID +PID ) < 4. dE RICH − Λcandidates areformedfromapairofoppositelychargedtracks oneofwhich isconsistent with a proton or anti-proton hypothesis, with a looser criteria than that stated above, which are constrained to come from a single vertex. We also require that the invariant mass be within 5 times the width of the Λ mass peak, which has an r.m.s. width of 1.4 MeV. A. Efficiency Determinations To derive event selection efficiencies we simulated hadronic events using the JETSET 7.3 qq event generator [11], then followed through the full GEANT 3.21-based [12] CLEO-III detector simulation. For five-flavor hadronic and Λ0Λ0 events in the Λ scan region, we b b b generated Monte Carlo samples using the same generator with the properties described in section II. The efficiencies obtained from these simulations are presented in Table II, where we list the both the hadronic event selection efficiency and the efficiency for detecting a hadronic event with an anti-proton. These efficiencies include the branching ratios for the various processes into anti-protons in the second column. We take (Λo pX) = 0.50. The B b → row for bb includes only B meson production with additional pions allowed. As one would expect, the efficiencies for bb and Λ0Λ0 are very similar. The slightly lower efficiency for b b Λ0Λ0 arises from higher average jettiness for Λ0Λ0 events. b b b b TABLE II: Selection efficiencies for hadronic events and those with anti-protons. Data samples Selection efficiency for Selection efficiency for hadronic events (%) hadronic events with an p (%) Below-Υ(4S) continuum 25.5 0.2 0.8 2.1 0.1 0.1 ± ± ± ± Λ0Λ0 85.5 0.9 2.6 26.8 0.1 5.4 b b ± ± ± ± 4 flavor (udsc) continuum 21.9 0.4 0.7 1.8 0.2 0.1 ± ± ± ± at E m(Λ ) beam b ∼ bb 89.9 1.2 2.7 4.0 0.2 0.3 ± ± ± ± 5 flavor (udscb) continuum 28.1 2.5 0.8 2.0 0.3 0.2 ± ± ± ± ττ 0.024 0.005 0.001 < 10−5 ± ± 9 The errors listed in Table II are statistical and systematic, respectively. The systematic error for the hadronic event selection requirement is estimated from the variation in the number of hadronic events (corrected by efficiency and background) when changing selection requirements. The systematic error for the proton identification has been evaluated from proton efficiency measurements using reconstructed Λ events from data and then comparing with the equivalent MC estimation. Our simulations also give us the selection efficiency for detecting an event containing either a Λ or an Λ from Λ Λ decay of 16.6 0.1+1.0%, including the (Λ pπ−). Note that b b −0.0 ± B → thePDG worldaverage for (Λ p anything) is (50 16)%. Similarly (Λ Λ anything) c c B → ± B → is (35 11)% [8]. The errors on these rates will be included separately as systematic effects. ± B. Systematic Errors The systematic errors in determining Λ0Λ0 production are given in Table III. The largest b b error is due to the unknown branching fraction of (Λ pX) to which we assign a 32% c B → error. We also include errors on the hadron selection efficiency and the background in the hadronic event sample, evaluated by varying our selection criteria as well as taking into account the variation with E , the anti-proton identification efficiency evaluated by CM − examining a larger sample of Λ pπ data, and the luminosity measurement uncertainty → estimated as 1% [13]. The totalsystematic error foundby adding these elements in quadrature is2.7%, 32%and 31% on the determination of Λ0Λ0 production using bb, anti-protons and Λ’s, respectively. b b TABLE III: List of systematic errors in determining Λ0Λ0 production b b Source Error (%) Hadron efficiency 3 ± Λo Λ+X branching ratio 4 b → c ± Proton identification efficiency 4 ± Λ+ pX branching fraction 32 c → ± Λ+ ΛX branching fraction 31 c → ± Total background of hadronic events 2 ± Luminosity 1 ± IV. THE ESTIMATED bb CROSS SECTION The hadronic cross section is generally expressed in terms of its ratio R to the point cross section e+e− µ+µ−. To search for resonant or non-resonant production of Λ0Λ0 in e+e− → b b collisions we measure the bb cross section over the energy range of the scan. Theoretically, R can be expressed as follows: bb 0 2 3 R = R 1+α /π +C (α /π) +C (α /π) , (2) bb bb s 2 s 3 s h i 10