Charmed-Baryon Production in High-Energy Neutrino-Deuterium Interactions

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  • VOLUME 45, NUMBER 12 P H Y S I C A L R E V I E W L E T T E R S 22 SEPTEMBER 1980

    Charmed-Baryon Production in High-Energy Neutrino-Deuterium Interactions T. Kitagaki, S. Tanaka, H. Yuta, K. Abe, K. Hasegawa, A. Yamaguchi, K. Tamai,

    T. Hayashino, S. Kunori, Y. Ohtani, and H. Hayano Tohoku University, Sendai 980, Japan

    and

    R. A. Burns te in , J . Hanlon, and H. A. Rubin Illinois Institute of Technology, Chicago, Illinois 60616

    and

    C. Y. Chang, T. Dombeck, G. A. Snow, D. Son, and P. H. Steinberg University of Maryland, College Park, Maryland 20742

    and

    R. Engelmann, T. Kafka, and S. Sommars State University of New York at Stony Brook, Stony Brook, New York 11794

    and

    J. Callas, C. C. Chang, W. A. Mann, and J. Schneps Tufts University, Medford, Massachusetts 02155

    (Received 9 June 1980)

    In a sample of ~15 000 charged current v + d interactions from an exposure of the Fer-milab 15-ft bubble chamber to a high-energy, wide-band neutrino beam, the production of the Ac

    + charmed baryon decaying into Air+ and Kp is observed. The measured mass value is 2.2750.010 GeV. The products of the production rate and the branching ratio for these channels are (1.8 0.9) x io"3 and (3.5 2.0) x 10"3 , respectively.

    PACS numberst 13.15.+g, 14.20.-c

    Since the first observation1 of the charmed baryon, Ad

    + , in an exclusive neutrino reaction, additional evidence for Ac

    + production has been reported in photon interactions,2/)-/? interactions,3

    ande+e~ annihilation.4 In inclusive neutrino r e -actions the Ac

    + has been observed5 as the decay product of the S c

    + + . However, most of the Ac +

    properties have not been established. In this pa-per, we present results on Ac

    + production in high-energy inclusive neutrino-deuterium reac-tions, using a data sample of events with associ-ated neutral strange particles (vees). In particu-lar , we report the first observation of the Ac

    +

    peak in the inclusive channels of Avr+ and/f/>, in-cluding their production rates times branching ratios and a new measurement of the Ac

    + mass . The data sample used for this analysis is based

    on measurements of 90% of the vee events in a 328 000-frame exposure of the Fermilab 15-ft deuterium-filled bubble chamber, with a two-plane external muon identifier, to a wide-band single-horn focused neutrino beam produced by 350-GeV/c protons. This sample corresponds to a flux of 4.46xl01 8 protons on the target. The neutrino energies range from 10 to 250 GeV, with

    an average energy (E)~ 50 GeV. The vee-associated events are collected from

    a double scan for all neutral-induced events (^ 2 prongs) with one or more possible neutral strange decays. These events are measured and proc-essed through the TVGP-SQUAW program chain. The vees are classified into KS-~TT

    +TT~ and A *pir~ with use of the x2 probability of the fits to-gether with information about the transverse mo-menta of the decay products. The A/K ambigu-ous fits (~ 12% of the V decays) are now classi-fied: 10% into the A and 2% into the K sample from this selection. Examination of the TT+TT" mass distribution for the A decay sample (as-suming the proton as a TT + ) indicates - 3 % contam-ination from Ks decays in the A sample.

    To reduce background from low-energy neutral hadron events, we require that the sum of the longitudinal momenta for all visible secondaries including the V be greater than 5 GeV/c. In ad-dition, we restrict the fiducial volume to 14.2 m3

    corresponding to 2.0 tons of deuterium, to im-prove the momentum resolution. For these se-lected events, we find the Ks and A mass values of 497.7 0.2 and 1115.6 0.1 MeV/c2 with the

    1980 The American Physical Society 955

    14.20.-c

  • VOLUME 45, NUMBER 12 P H Y S I C A L R E V I E W L E T T E R S 22 SEPTEMBER 1980

    mass resolutions of a = 5.1 and 1.7 MeV/c2, r e -spectively. This Ks mass resolution is better than that obtained from the Ne-H2 mixture experi-ment6 by more than a factor of 2C

    The charged-current events are selected by applying the following kinematic method7 to the sample described above: Fi rs t , we take the M" to be the negative track having no visible interac-tions and the maximum value of the quantity JP , defined in Ref. 7. Then, we select the charged-current events as those with the M~ transverse momentum relative to the other total visible par-ticles, P T R , greater than 0.7 GeV/c. From this selection, we estimate a total of 15 000 charged-current events8 corresponding to the V sample used for this analysis. This number includes the corrections for the scanning and measuring effi-ciencies.

    Figures 1(a), 1(b), and 1(c) show, respectively, the invariant mass distributions for the An* ,Kp and the sum of these two channels, which are among the expected Cabibbo-favored weak decays of the Ac

    + . The charged multiplicity in these dis-tributions is restricted to be less than 8 prongs to reduce combinatorial background. With this r e -

    2.2 2.6 3.0 MA55,GeV

    FIG. 1. Invariant-mass distributions for (a) ATT+ , (b) Kfy f and (c) sum of the two channels. The shaded areas correspond to the events with the helicity decay angular cuts; (a) cosfl>-0.75, (b) cos0>-O.9, and (c) sum of these from (a) and (b).

    striction, we obtain 402 K and 339 A events. The corresponding number of charged-current events is estimated to be 13 000,9 of which 11 000 events have hadronic energy W> 2.2 GeV. For calculat-ing the effective masses , we take all hadron tracks except identified protons to be pions for events with an identified nonspectator proton or A. For events without an identified nonspectator proton or A, each positive track is taken to be both rr or proton.

    As seen in Fig. 1, we observe peaks at the Ac +

    mass region in the ATT+ ,Kp channels as well as in their sum0 We examine the effect of the A/K ambiguous events on the Ac

    + mass region and find that these events contribute a smooth back-ground [5 events in the mass range of 2.24 to 2.32 GeV/c2 in Fig. 1(c)]. The best fits to a polynomi-nal background plus a Gaussian, shown by the curves in Fig. 1, give the excesses of events 9.5 4.7, 9.8 5.7, and 19.3 7.3 above the back-ground curves for ATT + , Kp, and their sum in the mass interval of 2.24 to 2.32 GeV/c2, where the mass value and the resolution are fixed as m = 2.275 GeV/c2 and a =0.02 GeV/c2.

    In order to examine further this mass peak, we show in Figs. 2(a) and 2(b) the cos# distributions for the ATT+ andKp channels, where 9 is the

    FIG. 2. The decay angular distributions for (a) A7r+

    and (b) Kp system.

    956

  • VOLUME 45, NUMBER 12 P H Y S I C A L R E V I E W L E T T E R S 22 SEPTEMBER 1980

    helicity decay angle of the baryon relative to the momentum direction of ATT + (Kp) system in its rest frame. Apparent peaks are observed at cos# = - 1 for both systems. The peak for the Air* is broader than that for the Kp. To examine whether the effect at the Ac

    + mass is related to the peaks observed in Figs. 2, we remove events with cosO < - 0.75 for the ATT+ channel and cos0 < - 0.90 for the Kp channel and display the r e -remainder in Fig. 1 as the shaded events. These angular cuts should remove only 12.5% and 5% of the An + and Kp events, respectively, at the Ac

    +

    mass if the Ac + decay angular distribution is uni-

    form. As seen from the shaded events in Fig. 1, no apparent loss of events is observed from these angular cuts, indicating that the effect observed at the Ac

    + mass is not related to the peaks ob-served in Fig. 2. The best fits to a polynomial background plus a Gaussian for the shaded events in Fig. 1 give the signals 83.5, 114.5, and 19 5.7 events for the Atr+, Kp, and their sum, r e -spectively. The small peaks (< 3a) observed at 2.02 and 2.74 GeV/c2 in Fig. 1 are much reduced by these angular cuts indicating that these come primarily from the peak region observed in Fig. 2 There has been a F*(2030) reported,10 however.

    Since the sum of the two channels gives a peak of 3.3a and 2.6a for the shaded and unshaded events above the background at the previously measured values of the Ac

    + mass , we interpret this peak as the production of the charmed bary-on, Ac

    + , in inclusive neutrino reactions. In or-der to obtain the mass value of the Ac

    + , we show in Fig. 3(a) the ideogram11 for the sum of the An* and#/> channels fit to a distribution with a polynominal background plus a Gaussian. The mass value obtained from the fit is

    m = 2.275 0.010 GeV/c2,

    with a mass resolution of 20 MeV/c2 which is con-sistent, within the er ror , with the Ac

    + mass val-ues1"5 reported previously. We have examined the systematic shift of the ATT + and Kp masses. Examination of the KS-~IT

    +TT" peak position throughout the exposure indicates the variation of the magnetic field to be less than 0.7%; hence any shift of the ATT+ andif masses resulting from drifting of the field strength is less than 10 MeV/c2. Also, we have studied the masses of Ks for possible dependence upon position in the chamber and for possible variation with different sets of optical constants. We find that any varia-tion amounts to less than 1 MeV/c2.

    To estimate the production rates of the Ac+ fol-

    2.2 2.4 MASS, GeV

    2.6

    FIG. 3. Ideograms for (a) sum of the A7r+ and Kp mass distribution [unshaded area in Fig. 1(c)] and (b) for the ATT+ mass distribution for events fitting to the AS = AQ exclusive hypotheses. The inset is the A7r+ mass distribution with 40~MeV/c2 bins for these exclusive events.

    lowed by decays into Air + and Kp, we correct the observed events above the background for the A *pTT~ andK^-^TT^TT