Search for Second-Class Currents in tau- -> omega pi- nu_tau
aa r X i v : . [ h e p - e x ] J u l B A B AR -CONF-08/015SLAC-PUB-13339arXiv:0807.4900July 2008 Search for Second-Class Currents in τ − → ωπ − ν τ The B A B AR Collaboration
Abstract
We report on an analysis of τ − decaying into ωπ − ν τ with ω → π + π − π using data containingnearly 320 million tau pairs collected with the BABAR detector at the PEP-II asymmetric energy B -Factory. We find no evidence for second-class currents and set an upper limit at . at a 90%confidence level for the ratio of second- to first-class currents.Submitted to the 34 th International Conference on High-Energy Physics, ICHEP 08,30 July—5 August 2008, Philadelphia, Pennsylvania.
Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309
Work supported in part by Department of Energy contract DE-AC02-76SF00515. he B A B AR Collaboration,B. Aubert, M. Bona, Y. Karyotakis, J. P. Lees, V. Poireau, E. Prencipe, X. Prudent, V. Tisserand
Laboratoire de Physique des Particules, IN2P3/CNRS et Universit´e de Savoie, F-74941 Annecy-Le-Vieux, France
J. Garra Tico, E. Grauges
Universitat de Barcelona, Facultat de Fisica, Departament ECM, E-08028 Barcelona, Spain
L. Lopez ab , A. Palano ab , M. Pappagallo ab INFN Sezione di Bari a ; Dipartmento di Fisica, Universit`a di Bari b , I-70126 Bari, Italy G. Eigen, B. Stugu, L. Sun
University of Bergen, Institute of Physics, N-5007 Bergen, Norway
G. S. Abrams, M. Battaglia, D. N. Brown, R. N. Cahn, R. G. Jacobsen, L. T. Kerth, Yu. G. Kolomensky,G. Lynch, I. L. Osipenkov, M. T. Ronan, K. Tackmann, T. Tanabe
Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA
C. M. Hawkes, N. Soni, A. T. Watson
University of Birmingham, Birmingham, B15 2TT, United Kingdom
H. Koch, T. Schroeder
Ruhr Universit¨at Bochum, Institut f ¨ur Experimentalphysik 1, D-44780 Bochum, Germany
D. Walker
University of Bristol, Bristol BS8 1TL, United Kingdom
D. J. Asgeirsson, B. G. Fulsom, C. Hearty, T. S. Mattison, J. A. McKenna
University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
M. Barrett, A. Khan
Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom
V. E. Blinov, A. D. Bukin, A. R. Buzykaev, V. P. Druzhinin, V. B. Golubev, A. P. Onuchin, S. I. Serednyakov,Yu. I. Skovpen, E. P. Solodov, K. Yu. Todyshev
Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia
M. Bondioli, S. Curry, I. Eschrich, D. Kirkby, A. J. Lankford, P. Lund, M. Mandelkern, E. C. Martin,D. P. Stoker
University of California at Irvine, Irvine, California 92697, USA
S. Abachi, C. Buchanan
University of California at Los Angeles, Los Angeles, California 90024, USA
J. W. Gary, F. Liu, O. Long, B. C. Shen, G. M. Vitug, Z. Yasin, L. Zhang
University of California at Riverside, Riverside, California 92521, USA
V. Sharma
University of California at San Diego, La Jolla, California 92093, USA Deceased . Campagnari, T. M. Hong, D. Kovalskyi, M. A. Mazur, J. D. Richman University of California at Santa Barbara, Santa Barbara, California 93106, USA
T. W. Beck, A. M. Eisner, C. J. Flacco, C. A. Heusch, J. Kroseberg, W. S. Lockman, A. J. Martinez, T. Schalk,B. A. Schumm, A. Seiden, M. G. Wilson, L. O. Winstrom
University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, California 95064, USA
C. H. Cheng, D. A. Doll, B. Echenard, F. Fang, D. G. Hitlin, I. Narsky, T. Piatenko, F. C. Porter
California Institute of Technology, Pasadena, California 91125, USA
R. Andreassen, G. Mancinelli, B. T. Meadows, K. Mishra, M. D. Sokoloff
University of Cincinnati, Cincinnati, Ohio 45221, USA
P. C. Bloom, W. T. Ford, A. Gaz, J. F. Hirschauer, M. Nagel, U. Nauenberg, J. G. Smith, K. A. Ulmer,S. R. Wagner
University of Colorado, Boulder, Colorado 80309, USA
R. Ayad, A. Soffer, W. H. Toki, R. J. Wilson
Colorado State University, Fort Collins, Colorado 80523, USA
D. D. Altenburg, E. Feltresi, A. Hauke, H. Jasper, M. Karbach, J. Merkel, A. Petzold, B. Spaan, K. Wacker
Technische Universit¨at Dortmund, Fakult¨at Physik, D-44221 Dortmund, Germany
M. J. Kobel, W. F. Mader, R. Nogowski, K. R. Schubert, R. Schwierz, A. Volk
Technische Universit¨at Dresden, Institut f ¨ur Kern- und Teilchenphysik, D-01062 Dresden, Germany
D. Bernard, G. R. Bonneaud, E. Latour, M. Verderi
Laboratoire Leprince-Ringuet, CNRS/IN2P3, Ecole Polytechnique, F-91128 Palaiseau, France
P. J. Clark, S. Playfer, J. E. Watson
University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
M. Andreotti ab , D. Bettoni a , C. Bozzi a , R. Calabrese ab , A. Cecchi ab , G. Cibinetto ab , P. Franchini ab ,E. Luppi ab , M. Negrini ab , A. Petrella ab , L. Piemontese a , V. Santoro ab INFN Sezione di Ferrara a ; Dipartimento di Fisica, Universit`a di Ferrara b , I-44100 Ferrara, Italy R. Baldini-Ferroli, A. Calcaterra, R. de Sangro, G. Finocchiaro, S. Pacetti, P. Patteri, I. M. Peruzzi, M. Piccolo, M. Rama, A. Zallo
INFN Laboratori Nazionali di Frascati, I-00044 Frascati, Italy
A. Buzzo a , R. Contri ab , M. Lo Vetere ab , M. M. Macri a , M. R. Monge ab , S. Passaggio a , C. Patrignani ab ,E. Robutti a , A. Santroni ab , S. Tosi ab INFN Sezione di Genova a ; Dipartimento di Fisica, Universit`a di Genova b , I-16146 Genova, Italy K. S. Chaisanguanthum, M. Morii
Harvard University, Cambridge, Massachusetts 02138, USA Now at Temple University, Philadelphia, Pennsylvania 19122, USA Now at Tel Aviv University, Tel Aviv, 69978, Israel Also with Universit`a di Perugia, Dipartimento di Fisica, Perugia, Italy . Adametz, J. Marks, S. Schenk, U. Uwer Universit¨at Heidelberg, Physikalisches Institut, Philosophenweg 12, D-69120 Heidelberg, Germany
V. Klose, H. M. Lacker
Humboldt-Universit¨at zu Berlin, Institut f ¨ur Physik, Newtonstr. 15, D-12489 Berlin, Germany
D. J. Bard, P. D. Dauncey, J. A. Nash, M. Tibbetts
Imperial College London, London, SW7 2AZ, United Kingdom
P. K. Behera, X. Chai, M. J. Charles, U. Mallik
University of Iowa, Iowa City, Iowa 52242, USA
J. Cochran, H. B. Crawley, L. Dong, W. T. Meyer, S. Prell, E. I. Rosenberg, A. E. Rubin
Iowa State University, Ames, Iowa 50011-3160, USA
Y. Y. Gao, A. V. Gritsan, Z. J. Guo, C. K. Lae
Johns Hopkins University, Baltimore, Maryland 21218, USA
N. Arnaud, J. B´equilleux, A. D’Orazio, M. Davier, J. Firmino da Costa, G. Grosdidier, A. H ¨ocker,V. Lepeltier, F. Le Diberder, A. M. Lutz, S. Pruvot, P. Roudeau, M. H. Schune, J. Serrano, V. Sordini, A. Stocchi, G. Wormser
Laboratoire de l’Acc´el´erateur Lin´eaire, IN2P3/CNRS et Universit´e Paris-Sud 11, Centre Scientifique d’Orsay, B. P.34, F-91898 Orsay Cedex, France
D. J. Lange, D. M. Wright
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
I. Bingham, J. P. Burke, C. A. Chavez, J. R. Fry, E. Gabathuler, R. Gamet, D. E. Hutchcroft, D. J. Payne,C. Touramanis
University of Liverpool, Liverpool L69 7ZE, United Kingdom
A. J. Bevan, C. K. Clarke, K. A. George, F. Di Lodovico, R. Sacco, M. Sigamani
Queen Mary, University of London, London, E1 4NS, United Kingdom
G. Cowan, H. U. Flaecher, D. A. Hopkins, S. Paramesvaran, F. Salvatore, A. C. Wren
University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom
D. N. Brown, C. L. Davis
University of Louisville, Louisville, Kentucky 40292, USA
A. G. Denig M. Fritsch, W. Gradl, G. Schott
Johannes Gutenberg-Universit¨at Mainz, Institut f ¨ur Kernphysik, D-55099 Mainz, Germany
K. E. Alwyn, D. Bailey, R. J. Barlow, Y. M. Chia, C. L. Edgar, G. Jackson, G. D. Lafferty, T. J. West, J. I. Yi
University of Manchester, Manchester M13 9PL, United Kingdom
J. Anderson, C. Chen, A. Jawahery, D. A. Roberts, G. Simi, J. M. Tuggle
University of Maryland, College Park, Maryland 20742, USA Also with Universit`a di Roma La Sapienza, I-00185 Roma, Italy . Dallapiccola, X. Li, E. Salvati, S. Saremi University of Massachusetts, Amherst, Massachusetts 01003, USA
R. Cowan, D. Dujmic, P. H. Fisher, G. Sciolla, M. Spitznagel, F. Taylor, R. K. Yamamoto, M. Zhao
Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139, USA
P. M. Patel, S. H. Robertson
McGill University, Montr´eal, Qu´ebec, Canada H3A 2T8
A. Lazzaro ab , V. Lombardo a , F. Palombo ab INFN Sezione di Milano a ; Dipartimento di Fisica, Universit`a di Milano b , I-20133 Milano, Italy J. M. Bauer, L. Cremaldi R. Godang, R. Kroeger, D. A. Sanders, D. J. Summers, H. W. Zhao
University of Mississippi, University, Mississippi 38677, USA
M. Simard, P. Taras, F. B. Viaud
Universit´e de Montr´eal, Physique des Particules, Montr´eal, Qu´ebec, Canada H3C 3J7
H. Nicholson
Mount Holyoke College, South Hadley, Massachusetts 01075, USA
G. De Nardo ab , L. Lista a , D. Monorchio ab , G. Onorato ab , C. Sciacca ab INFN Sezione di Napoli a ; Dipartimento di Scienze Fisiche, Universit`a di Napoli Federico II b , I-80126 Napoli, Italy G. Raven, H. L. Snoek
NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, TheNetherlands
C. P. Jessop, K. J. Knoepfel, J. M. LoSecco, W. F. Wang
University of Notre Dame, Notre Dame, Indiana 46556, USA
G. Benelli, L. A. Corwin, K. Honscheid, H. Kagan, R. Kass, J. P. Morris, A. M. Rahimi, J. J. Regensburger,S. J. Sekula, Q. K. Wong
Ohio State University, Columbus, Ohio 43210, USA
N. L. Blount, J. Brau, R. Frey, O. Igonkina, J. A. Kolb, M. Lu, R. Rahmat, N. B. Sinev, D. Strom, J. Strube,E. Torrence
University of Oregon, Eugene, Oregon 97403, USA
G. Castelli ab , N. Gagliardi ab , M. Margoni ab , M. Morandin a , M. Posocco a , M. Rotondo a , F. Simonetto ab ,R. Stroili ab , C. Voci ab INFN Sezione di Padova a ; Dipartimento di Fisica, Universit`a di Padova b , I-35131 Padova, Italy P. del Amo Sanchez, E. Ben-Haim, H. Briand, G. Calderini, J. Chauveau, P. David, L. Del Buono,O. Hamon, Ph. Leruste, J. Ocariz, A. Perez, J. Prendki, S. Sitt
Laboratoire de Physique Nucl´eaire et de Hautes Energies, IN2P3/CNRS, Universit´e Pierre et Marie Curie-Paris6,Universit´e Denis Diderot-Paris7, F-75252 Paris, France Now at University of South Alabama, Mobile, Alabama 36688, USA . Gladney University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
M. Biasini ab , R. Covarelli ab , E. Manoni ab , INFN Sezione di Perugia a ; Dipartimento di Fisica, Universit`a di Perugia b , I-06100 Perugia, Italy C. Angelini ab , G. Batignani ab , S. Bettarini ab , M. Carpinelli ab , A. Cervelli ab , F. Forti ab , M. A. Giorgi ab ,A. Lusiani ac , G. Marchiori ab , M. Morganti ab , N. Neri ab , E. Paoloni ab , G. Rizzo ab , J. J. Walsh a INFN Sezione di Pisa a ; Dipartimento di Fisica, Universit`a di Pisa b ; Scuola Normale Superiore di Pisa c , I-56127Pisa, Italy D. Lopes Pegna, C. Lu, J. Olsen, A. J. S. Smith, A. V. Telnov
Princeton University, Princeton, New Jersey 08544, USA
F. Anulli a , E. Baracchini ab , G. Cavoto a , D. del Re ab , E. Di Marco ab , R. Faccini ab , F. Ferrarotto a , F. Ferroni ab ,M. Gaspero ab , P. D. Jackson a , L. Li Gioi a , M. A. Mazzoni a , S. Morganti a , G. Piredda a , F. Polci ab , F. Renga ab ,C. Voena a INFN Sezione di Roma a ; Dipartimento di Fisica, Universit`a di Roma La Sapienza b , I-00185 Roma, Italy M. Ebert, T. Hartmann, H. Schr ¨oder, R. Waldi
Universit¨at Rostock, D-18051 Rostock, Germany
T. Adye, B. Franek, E. O. Olaiya, F. F. Wilson
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom
S. Emery, M. Escalier, L. Esteve, S. F. Ganzhur, G. Hamel de Monchenault, W. Kozanecki, G. Vasseur,Ch. Y`eche, M. Zito
CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, France
X. R. Chen, H. Liu, W. Park, M. V. Purohit, R. M. White, J. R. Wilson
University of South Carolina, Columbia, South Carolina 29208, USA
M. T. Allen, D. Aston, R. Bartoldus, P. Bechtle, J. F. Benitez, R. Cenci, J. P. Coleman, M. R. Convery,J. C. Dingfelder, J. Dorfan, G. P. Dubois-Felsmann, W. Dunwoodie, R. C. Field, A. M. Gabareen,S. J. Gowdy, M. T. Graham, P. Grenier, C. Hast, W. R. Innes, J. Kaminski, M. H. Kelsey, H. Kim, P. Kim,M. L. Kocian, D. W. G. S. Leith, S. Li, B. Lindquist, S. Luitz, V. Luth, H. L. Lynch, D. B. MacFarlane,H. Marsiske, R. Messner, D. R. Muller, H. Neal, S. Nelson, C. P. O’Grady, I. Ofte, A. Perazzo, M. Perl,B. N. Ratcliff, A. Roodman, A. A. Salnikov, R. H. Schindler, J. Schwiening, A. Snyder, D. Su, M. K. Sullivan,K. Suzuki, S. K. Swain, J. M. Thompson, J. Va’vra, A. P. Wagner, M. Weaver, C. A. West, W. J. Wisniewski,M. Wittgen, D. H. Wright, H. W. Wulsin, A. K. Yarritu, K. Yi, C. C. Young, V. Ziegler
Stanford Linear Accelerator Center, Stanford, California 94309, USA
P. R. Burchat, A. J. Edwards, S. A. Majewski, T. S. Miyashita, B. A. Petersen, L. Wilden
Stanford University, Stanford, California 94305-4060, USA
S. Ahmed, M. S. Alam, J. A. Ernst, B. Pan, M. A. Saeed, S. B. Zain
State University of New York, Albany, New York 12222, USA Also with Universit`a di Sassari, Sassari, Italy . M. Spanier, B. J. Wogsland University of Tennessee, Knoxville, Tennessee 37996, USA
R. Eckmann, J. L. Ritchie, A. M. Ruland, C. J. Schilling, R. F. Schwitters
University of Texas at Austin, Austin, Texas 78712, USA
B. W. Drummond, J. M. Izen, X. C. Lou
University of Texas at Dallas, Richardson, Texas 75083, USA
F. Bianchi ab , D. Gamba ab , M. Pelliccioni ab INFN Sezione di Torino a ; Dipartimento di Fisica Sperimentale, Universit`a di Torino b , I-10125 Torino, Italy M. Bomben ab , L. Bosisio ab , C. Cartaro ab , G. Della Ricca ab , L. Lanceri ab , L. Vitale ab INFN Sezione di Trieste a ; Dipartimento di Fisica, Universit`a di Trieste b , I-34127 Trieste, Italy V. Azzolini, N. Lopez-March, F. Martinez-Vidal, D. A. Milanes, A. Oyanguren
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
J. Albert, Sw. Banerjee, B. Bhuyan, H. H. F. Choi, K. Hamano, R. Kowalewski, M. J. Lewczuk, I. M. Nugent,J. M. Roney, R. J. Sobie
University of Victoria, Victoria, British Columbia, Canada V8W 3P6
T. J. Gershon, P. F. Harrison, J. Ilic, T. E. Latham, G. B. Mohanty
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
H. R. Band, X. Chen, S. Dasu, K. T. Flood, Y. Pan, M. Pierini, R. Prepost, C. O. Vuosalo, S. L. Wu
University of Wisconsin, Madison, Wisconsin 53706, USA INTRODUCTION
Weak currents can be classified as either first- or second-class depending on the J P G of the decaycurrent [1], where G -parity is a combination of charge conjugation and an isospin rotation, ˆ G =ˆ Ce iπ ˆ I , and is a multiplicative quantum number. In the Standard Model, first-class currents (FCC),where P G ( − J = +1 ( J P G = 0 ++ , −− , + − , − + , . . . ), are expected to dominate decays whilesecond-class currents (SCC), where P G ( − J = − ( J P G = 0 + − , − + , ++ , −− , . . . ), are expectedto be small and to vanish in the limit of perfect isospin symmetry. An example of such a decayis τ − → ωπ − ν τ , which is expected to proceed through FCC mediated by the ρ resonance. Thisdecay may also potentially proceed through SCC, such as b (1235) [2] with τ − → b − ν τ → ωπ − ν τ ,producing final state particles with J P G = 1 ++ and − + .Since the decay b − → ωπ − occurs through S- and D-waves, as compared to a P-wave for FCC,different polarizations of ω spin result in different angular distributions of the final state particles.The expected distributions of cos θ ωπ for all possible spin-parity states of the final state particles arelisted in Table 1, where θ ωπ is the angle between the normal to the ω decay plane and the directionof the remaining π in the ω rest frame, as shown in Figure 1. The existing measurement of theangular distribution of τ − → ωπ − ν τ is consistent with having only P-wave contribution, and thepresent limit is 5.4% for the ratio of SCC to FCC contributions, N ωπ (non-vector current) / N ωπ (vector current) ,at 90% confidence level [3]. This paper presents a search for SCC in τ − → ωπ − ν τ decays with ω → π + π − π by studying the angular distributions of final state particles. + ωπ n π πππ θω decay plane Figure 1: Illustration of the angle θ ωπ : the angle between the normal to the ω decay plane and thedirection of the remaining π in the ω rest frame. A B AR DETECTOR AND DATASET
This analysis is based on data recorded by the B A B AR detector [4] at the PEP-II asymmetric-energy e + e − storage rings operated at the Stanford Linear Accelerator Center. The data sample consistsof . fb − recorded at the center-of-mass energy of .
58 GeV . With a cross section for τ pairsof σ ττ = (0 . ± . nb [5], this data sample contains nearly 320 million pairs of tau decays.The B A B AR detector is described in detail in Ref. [4]. Charged-particle momenta are measuredwith a 5-layer double-sided silicon vertex tracker (SVT) and a 40-layer drift chamber (DCH) insidea 1.5-T superconducting solenoidal magnet. An electromagnetic calorimeter (EMC) consisting of Charge-conjugate reactions are implied throughout this paper. F L (cos θ ωπ ) , for possible spin-parity states in the decay of τ − → ωπ − ν τ . L is the orbital angular momentum. J P L F F CC F SCCL (cos θ ωπ )1 − F F CC ∝ (1 − cos θ ωπ )1 + F SCC ∝ + F SCC ∝ (1 + 3 cos θ ωπ )0 − F SCC ∝ cos θ ωπ dE/dx ) in the SVT and the DCH. Muons are identified by an instrumented magnetic-flux return(IFR).Monte Carlo (MC) simulations are used to estimate the signal efficiencies and background con-tamination. The production of τ pairs is simulated with the KK2f generator [6], and the decays ofthe τ lepton are modeled with Tauola [7]. Continuum qq events are simulated using JETSET [8].Final state radiative effects are simulated for all decays using
Photos [9]. The detector response issimulated with
GEANT4 [10], and the simulated events are then reconstructed in the same manneras data.
Since τ pairs are produced back-to-back in the e + e − center-of-mass frame, each event is dividedinto two hemispheres according to the thrust axis [11], calculated using all reconstructed chargedparticle tracks. Candidate events in this analysis are required to have a “1-3 topology,” where onetrack is in one hemisphere (tag hemisphere) and three tracks are in the other hemisphere (signalhemisphere). Events with four well-reconstructed tracks and zero net charge are selected. Thepolar angles, in the laboratory frame, of all four tracks and the neutrals used in π reconstructionare required to be within the calorimeter acceptance range. Events are rejected if the invariantmass of pairs of oppositely charged tracks, assuming electron mass hypotheses, is less than 90MeV/ c , as these tracks are likely to be from photon conversions in the detector material.The charged particle found in the tag hemisphere must be either an electron or a muon can-didate. Electrons are identified using the ratio of calorimeter energy to track momentum ( E/p ),the shape of the shower in the calorimeter, and dE/dx . Muons are identified by hits in the IFRand small energy deposits in the calorimeter consistent with expectation for a minimum-ionizingparticle. Muons with momentum less than 0.5 GeV/ c cannot be identified in this manner as theydo not penetrate far enough into the IFR. Charged particles found in the signal hemisphere mustbe identified as pion candidates using dE/dx . The π candidates are reconstructed from two sep-arate EMC clusters with energies above 100 MeV that are not not associated with charged tracksand are required to have invariant masses between 100 and 160 MeV/ c . Events are required tohave a single π in the signal hemisphere. The τ candidates are reconstructed in the signal hemi-sphere using the three tracks and the π candidate, and the invariant mass of the τ candidate, m (2 π − π + π ) is required to be less than the nominal mass of the τ lepton, m τ = 1 .
777 GeV /c [12].After the event selection process, from the MC it is found that 14% of the events remaining are9 ) [GeV/c π - π + π m(0.65 0.7 0.75 0.8 0.85 0.9 ) E n t r i es / ( M e V / c × ] ) [GeV/c π - π + π m(0.65 0.7 0.75 0.8 0.85 0.9 ) E n t r i es / ( M e V / c SSB SB × data τ ν π + π - π → - τ decays τ other udscc B B B + B Figure 2: ω candidate mass spectra for selected events in data and expected Monte Carlo back-ground (colored histograms). The background histograms do not include the non-resonant τ − → π − π + π ν τ decays. The signal (S) and sideband (SB) regions are indicated in the figure. τ -pair events that do not contain a τ − → π − π + π ν τ decay, and 1.3% are e + e − → qq events.For each selected event with m (2 π − π + π ) < m τ two ω candidates are reconstructed from π + π − π combinations. The mass of the ω candidates, m ( π + π − π ) , is required to be between 670MeV/ c and 890 MeV/ c ; within this range, the signal region is defined between 760 MeV/ c and800 MeV/ c with mass regions of width 60 MeV/ c on each side of the peak used as sidebandregions for background studies, as shown in Figure 2. For each ω candidate in the signal region,the angle θ ωπ is calculated and is used in the SCC measurement, after background subtraction.There are three background types to be considered in this analysis. The first type is com-binatoric background, which is expected to have an angular distribution that is independent of m ( π + π − π ) , and is thus subtracted from the signal region using the sideband regions. The num-ber of combinatoric events lying within the signal region is obtained by fitting the m ( π + π − π ) spectrum with a smeared relativistic Breit-Wigner for the ω resonance and a polynomial for thecombinatoric background. The polynomial is integrated over the signal region to find the numberof continuum events in the signal region. The second type of background comes from e + e − → qq events that contain ω → π + π − π decays. While the event selection process significantly reducesthe number of qq events, approximately 0.3% of the events in the signal region are expected tobe of qq origin. This type of background is studied using events with m (2 π − π + π ) well abovethe τ mass ( > . /c ). In this region, where all events are considered to be of qq origin, acomparison of the numbers of events in MC and data is used to obtain a scaling factor for the qq background events.After subtracting combinatoric and qq background events, approximately 4.6% of the remain-10ng ω candidates in the signal region are expected to be background events from non-signal τ decays. The dominant of these, comprising 99% of these background events, is τ − → ωπ − π ν τ ,where one π has not been reconstructed. The decay τ − → ωπ − π ν τ has not been well measuredand is incorrectly modeled in the MC. Both the branching fraction and decay angular distributionneed to be corrected, as shown in Figures 3(a) and 3(b). To correct for the differences betweendata and MC, events with an additional π candidate in the signal hemisphere are selected, usingthe same cuts discussed above. Using these events, the MC branching fraction of τ − → ωπ − π ν τ is corrected by comparing the numbers of fitted ω candidates in data and MC. The fit functionused for this is a smeared relativistic Breit-Wigner with a polynomial background. The branchingfraction obtained using this correction technique is found to be consistent with existing measure-ments [12]. To correct the angular distribution of τ − → ωπ − π ν τ in the signal region, backgrounds,consisting of combinatorics, qq events and τ − → ωπ − ν τ decays, are subtracted from the two π data sample, and the remaining cos θ ωπ distribution, shown in Figure 3(c), is used to correct the τ − → ωπ − π ν τ distribution in the MC. ] ) [GeV/c π - π + π m( ) E n t r i es / ( M e V / c × ] ) [GeV/c π - π + π m( ) E n t r i es / ( M e V / c × data τ ν π + π - π → - τ τ ν π + π - π → - τ decays τ other udscc B B B + B πω θ cos -1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1 E n t r i es / ( . ) πω θ cos -1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1 E n t r i es / ( . ) (a) (b) πω θ cos -1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1 E n t r i es / ( . ) (c)Figure 3: (a) m ( π + π − π ) and (b) cos θ ωπ distributions when requiring an additional π in thesignal hemisphere, before background subtraction. These are used to correct the τ − → ωπ − π ν τ MC. (c) The cos θ ωπ distribution obtained from data after subtracting background.11o account for any variation in efficiency as a function of cos θ ωπ , the generated and recon-structed MC cos θ ωπ distributions of τ − → ωπ − ν τ in the signal region are compared. The ratioof the two distributions, shown in Figure 4, is used as an efficiency function to correct the back-ground subtracted data. πω θ cos-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1 E ff i c i e n cy ( % ) Figure 4: Efficiency as a function of cos θ ωπ obtained from τ − → π − π + π ν τ MC.
After subtracting background events and applying efficiency corrections, a binned fit to the re-maining cos θ ωπ distribution is carried out using F (cos θ ωπ ) = N × [ ǫF SCC (cos θ ωπ ) + (1 − ǫ ) F F CC (cos θ ωπ )] , (1)where N is a normalization factor, the parameter ǫ is the fraction of τ − → ωπ − ν τ decays thatproceed through SCC, and F F CC and F SCC are normalized angular functions described in Ta-ble 1. The parameter ǫ is related to N ωπ (non-vector current) / N ωπ (vector current) by the equation ε/ (1 − ε ) = N ωπ (non-vector current) / N ωπ (vector current) . In Eq.1, only F SCC is used for the function describing the SCCcontribution since the shape of this function gives the most conservative (largest) estimate of ǫ .This method is tested by adding various amounts of S -wave decays to the standard MC andfitting for the levels of SCC. As the results of this test indicate, as shown in Table 2, in all casesthe measured fractions of SCC, ǫ , are consistent with the fractions added to the MC. The errorslisted in Table 2 are not necessarily indicative of the expected uncertainties in the data; they do notcontain systematic uncertainties, and statistical correlations exist among the MC samples used inthe studies.The largest contributions to systematic uncertainties on ǫ are scaling and modeling of the MCbackground. The correction applied to the branching fraction of τ − → ωπ − π ν τ has an errorassociated with it, determined by the available statistics. The correction factor is adjusted by ± σ to obtain the uncertainty in ǫ while the errors associated with correcting the angular distribution12able 2: Test of SCC measurements in MC with various amounts of S -wave decays added.Fraction of L = 0 τ − → ωπ − ν τ decays added ǫ none (0 . ± . (1 . ± . (2 . ± . are folded into the statistical uncertainty. In addition, there are τ decays that may be present in thefinal event sample but which are not simulated in the MC. The largest of these are expected to be τ − → ωK − ν τ , τ − → ωπ − π ν τ and τ − → ω π − π + ν τ decays, which when combined can add up to0.2% of the final event sample. Since the effect that these decays have on the angular distributionis unknown, the extreme cases are taken to obtain the uncertainty. These cases correspond tothese decays having either entirely − cos θ ωπ or entirely cos θ ωπ distributions. The scaling of qq events can also affect the measurement of ǫ , and the uncertainty is obtained by adjusting thescaling factor by ± σ . These systematic uncertainties are summarized in Table 3.Table 3: Summary of systematic uncertainties on ǫ Source Uncertainty ( σ ǫ ) B ( τ − → ωπ − π ν τ ) ± . un-simulated τ decays +0 . − . qq scaling ± . Total +0 . − . To estimate the sensitivity of this analysis without the effect of statistical correlations in theMC samples used, a toy MC study is conducted. In this study, angular distributions are generatedfor the signal and sideband regions to simulate the statistics available in the data and variousMC samples used in the analysis. After subtracting background samples from the toy data, theangular distribution is corrected for efficiency and fitted using Eq.1. The statistical uncertaintyon ǫ obtained from the fit is . , which combined with the systematic uncertainties leads to anestimated uncertainty of σ ǫ = +0 . − . .With the MC studies completed, the angular distribution in the data is obtained by subtractingestimated background events as described above. The remaining distribution is corrected forefficiency and fitted using Eq. 1 as shown in Figure 5. The fit has χ /dof = ǫ in the data is ( − . ± . (stat) +0 . − . (syst) ) × − , which is consistent with no SCCcontribution to τ − → ωπ − ν τ decays. For the upper limit on N ωπ (non-vector current) / N ωπ (vector current) , aBayesian approach [13] is used as negative values of ǫ are non-physical. Using only the positiveportion of the probability distribution for the value of the SCC contribution, ǫ true , the distributionfor ǫ true is a bifurcated Gaussian with mean ǫ = 5 . × − and errors σ ǫ = +0 . − . . The limitsobtained from this method are . at 90% C.L. and . at 95% C.L.13 ω θ cos-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1 E n t r i es / ( . ) × Figure 5: The fitted cos θ ωπ distribution for the data. The fitted curve is described in the text. A search for second-class currents in the decay τ − → ωπ − ν τ is conducted with the B A B AR detector.No evidence for second-class currents is observed, and a 90% confidence level Bayesian upperlimit for N ωπ (non-vector current) / N ωπ (vector current) is set at . . This limit is an order of magnitude lowerthan the limit set by the CLEO collaboration [3]. We are grateful for the extraordinary contributions of our PEP-II colleagues in achieving theexcellent luminosity and machine conditions that have made this work possible. The successof this project also relies critically on the expertise and dedication of the computing organiza-tions that support B A B AR . The collaborating institutions wish to thank SLAC for its support andthe kind hospitality extended to them. 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