Observation of B_{s}^{0}\to\phi\gamma and Search for B_{s}^{0}\to\gamma\gamma Decays at Belle

Observation of and Search for Decays at Belle

J. Wicht École Polytechnique Fédérale de Lausanne (EPFL), Lausanne    I. Adachi High Energy Accelerator Research Organization (KEK), Tsukuba    H. Aihara Department of Physics, University of Tokyo, Tokyo    K. Arinstein Budker Institute of Nuclear Physics, Novosibirsk    V. Aulchenko Budker Institute of Nuclear Physics, Novosibirsk    T. Aushev École Polytechnique Fédérale de Lausanne (EPFL), Lausanne Institute for Theoretical and Experimental Physics, Moscow    A. M. Bakich University of Sydney, Sydney, New South Wales    V. Balagura Institute for Theoretical and Experimental Physics, Moscow    A. Bay École Polytechnique Fédérale de Lausanne (EPFL), Lausanne    K. Belous Institute of High Energy Physics, Protvino    V. Bhardwaj Panjab University, Chandigarh    U. Bitenc J. Stefan Institute, Ljubljana    A. Bondar Budker Institute of Nuclear Physics, Novosibirsk    A. Bozek H. Niewodniczanski Institute of Nuclear Physics, Krakow    M. Bračko University of Maribor, Maribor J. Stefan Institute, Ljubljana    T. E. Browder University of Hawaii, Honolulu, Hawaii 96822    P. Chang Department of Physics, National Taiwan University, Taipei    Y. Chao Department of Physics, National Taiwan University, Taipei    A. Chen National Central University, Chung-li    K.-F. Chen Department of Physics, National Taiwan University, Taipei    W. T. Chen National Central University, Chung-li    B. G. Cheon Hanyang University, Seoul    R. Chistov Institute for Theoretical and Experimental Physics, Moscow    I.-S. Cho Yonsei University, Seoul    Y. Choi Sungkyunkwan University, Suwon    J. Dalseno University of Melbourne, School of Physics, Victoria 3010    M. Dash Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061    A. Drutskoy University of Cincinnati, Cincinnati, Ohio 45221    S. Eidelman Budker Institute of Nuclear Physics, Novosibirsk    N. Gabyshev Budker Institute of Nuclear Physics, Novosibirsk    P. Goldenzweig University of Cincinnati, Cincinnati, Ohio 45221    B. Golob Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana J. Stefan Institute, Ljubljana    J. Haba High Energy Accelerator Research Organization (KEK), Tsukuba    K. Hayasaka Nagoya University, Nagoya    M. Hazumi High Energy Accelerator Research Organization (KEK), Tsukuba    D. Heffernan Osaka University, Osaka    Y. Hoshi Tohoku Gakuin University, Tagajo    W.-S. Hou Department of Physics, National Taiwan University, Taipei    Y. B. Hsiung Department of Physics, National Taiwan University, Taipei    H. J. Hyun Kyungpook National University, Taegu    T. Iijima Nagoya University, Nagoya    K. Inami Nagoya University, Nagoya    A. Ishikawa Saga University, Saga    H. Ishino Tokyo Institute of Technology, Tokyo    R. Itoh High Energy Accelerator Research Organization (KEK), Tsukuba    Y. Iwasaki High Energy Accelerator Research Organization (KEK), Tsukuba    D. H. Kah Kyungpook National University, Taegu    H. Kaji Nagoya University, Nagoya    J. H. Kang Yonsei University, Seoul    P. Kapusta H. Niewodniczanski Institute of Nuclear Physics, Krakow    H. Kawai Chiba University, Chiba    T. Kawasaki Niigata University, Niigata    H. Kichimi High Energy Accelerator Research Organization (KEK), Tsukuba    H. J. Kim Kyungpook National University, Taegu    H. O. Kim Kyungpook National University, Taegu    S. K. Kim Seoul National University, Seoul    Y. J. Kim The Graduate University for Advanced Studies, Hayama    K. Kinoshita University of Cincinnati, Cincinnati, Ohio 45221    S. Korpar University of Maribor, Maribor J. Stefan Institute, Ljubljana    P. Križan Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana J. Stefan Institute, Ljubljana    R. Kumar Panjab University, Chandigarh    C. C. Kuo National Central University, Chung-li    Y.-J. Kwon Yonsei University, Seoul    J. S. Lange Justus-Liebig-Universität Gießen, Gießen    J. Lee Seoul National University, Seoul    J. S. Lee Sungkyunkwan University, Suwon    S. E. Lee Seoul National University, Seoul    T. Lesiak H. Niewodniczanski Institute of Nuclear Physics, Krakow    A. Limosani University of Melbourne, School of Physics, Victoria 3010    S.-W. Lin Department of Physics, National Taiwan University, Taipei    Y. Liu The Graduate University for Advanced Studies, Hayama    D. Liventsev Institute for Theoretical and Experimental Physics, Moscow    F. Mandl Institute of High Energy Physics, Vienna    S. McOnie University of Sydney, Sydney, New South Wales    T. Medvedeva Institute for Theoretical and Experimental Physics, Moscow    W. Mitaroff Institute of High Energy Physics, Vienna    K. Miyabayashi Nara Women’s University, Nara    H. Miyake Osaka University, Osaka    H. Miyata Niigata University, Niigata    Y. Miyazaki Nagoya University, Nagoya    R. Mizuk Institute for Theoretical and Experimental Physics, Moscow    D. Mohapatra Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061    G. R. Moloney University of Melbourne, School of Physics, Victoria 3010    M. Nakao High Energy Accelerator Research Organization (KEK), Tsukuba    Z. Natkaniec H. Niewodniczanski Institute of Nuclear Physics, Krakow    S. Nishida High Energy Accelerator Research Organization (KEK), Tsukuba    O. Nitoh Tokyo University of Agriculture and Technology, Tokyo    T. Nozaki High Energy Accelerator Research Organization (KEK), Tsukuba    S. Ogawa Toho University, Funabashi    T. Ohshima Nagoya University, Nagoya    S. Okuno Kanagawa University, Yokohama    H. Ozaki High Energy Accelerator Research Organization (KEK), Tsukuba    P. Pakhlov Institute for Theoretical and Experimental Physics, Moscow    G. Pakhlova Institute for Theoretical and Experimental Physics, Moscow    H. Palka H. Niewodniczanski Institute of Nuclear Physics, Krakow    C. W. Park Sungkyunkwan University, Suwon    H. Park Kyungpook National University, Taegu    K. S. Park Sungkyunkwan University, Suwon    R. Pestotnik J. Stefan Institute, Ljubljana    L. E. Piilonen Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061    Y. Sakai High Energy Accelerator Research Organization (KEK), Tsukuba    O. Schneider École Polytechnique Fédérale de Lausanne (EPFL), Lausanne    J. Schümann High Energy Accelerator Research Organization (KEK), Tsukuba    A. J. Schwartz University of Cincinnati, Cincinnati, Ohio 45221    K. Senyo Nagoya University, Nagoya    M. E. Sevior University of Melbourne, School of Physics, Victoria 3010    M. Shapkin Institute of High Energy Physics, Protvino    H. Shibuya Toho University, Funabashi    J.-G. Shiu Department of Physics, National Taiwan University, Taipei    B. Shwartz Budker Institute of Nuclear Physics, Novosibirsk    J. B. Singh Panjab University, Chandigarh    A. Somov University of Cincinnati, Cincinnati, Ohio 45221    S. Stanič University of Nova Gorica, Nova Gorica    M. Starič J. Stefan Institute, Ljubljana    K. Sumisawa High Energy Accelerator Research Organization (KEK), Tsukuba    T. Sumiyoshi Tokyo Metropolitan University, Tokyo    F. Takasaki High Energy Accelerator Research Organization (KEK), Tsukuba    K. Tamai High Energy Accelerator Research Organization (KEK), Tsukuba    M. Tanaka High Energy Accelerator Research Organization (KEK), Tsukuba    G. N. Taylor University of Melbourne, School of Physics, Victoria 3010    Y. Teramoto Osaka City University, Osaka    K. Trabelsi High Energy Accelerator Research Organization (KEK), Tsukuba    T. Tsuboyama High Energy Accelerator Research Organization (KEK), Tsukuba    S. Uehara High Energy Accelerator Research Organization (KEK), Tsukuba    K. Ueno Department of Physics, National Taiwan University, Taipei    Y. Unno Hanyang University, Seoul    S. Uno High Energy Accelerator Research Organization (KEK), Tsukuba    Y. Ushiroda High Energy Accelerator Research Organization (KEK), Tsukuba    Y. Usov Budker Institute of Nuclear Physics, Novosibirsk    G. Varner University of Hawaii, Honolulu, Hawaii 96822    K. Vervink École Polytechnique Fédérale de Lausanne (EPFL), Lausanne    S. Villa École Polytechnique Fédérale de Lausanne (EPFL), Lausanne    C. H. Wang National United University, Miao Li    P. Wang Institute of High Energy Physics, Chinese Academy of Sciences, Beijing    X. L. Wang Institute of High Energy Physics, Chinese Academy of Sciences, Beijing    Y. Watanabe Kanagawa University, Yokohama    E. Won Korea University, Seoul    B. D. Yabsley University of Sydney, Sydney, New South Wales    Y. Yamashita Nippon Dental University, Niigata    M. Yamauchi High Energy Accelerator Research Organization (KEK), Tsukuba    Y. Yusa Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061    Z. P. Zhang University of Science and Technology of China, Hefei    V. Zhilich Budker Institute of Nuclear Physics, Novosibirsk    A. Zupanc J. Stefan Institute, Ljubljana    N. Zwahlen École Polytechnique Fédérale de Lausanne (EPFL), Lausanne
Abstract

We search for the radiative penguin decays and in a 23.6 data sample collected at the resonance with the Belle detector at the KEKB asymmetric-energy collider. We observe for the first time a radiative penguin decay of the meson in the mode and we measure . No significant signal is observed and we set a 90% confidence level upper limit of .

pacs:
13.25.Gv, 13.25.Hw, 14.40.Gx, 14.40.Nd
preprint: BELLE Preprint 2007-48 KEK Preprint 2007-63

The Belle Collaboration

Radiative penguin decays, which produce a photon via a one-loop Feynman diagram, are a good tool to search for physics beyond the Standard Model (SM) because particles not yet produced in the laboratory can make large contributions to such loop effects. The  chargeconj () mode is a radiative process described within the SM by a penguin diagram (Fig. 1 left); it is the strange counterpart of the  decay, whose observation by CLEO in 1993 b2kstg-cleo () unambiguously demonstrated the existence of penguin processes. In the SM, the  branching fraction has been computed with uncertainty to be about  bs2phigam-sm1 (); bs2phigam-sm2 (). The  mode is usually described by a penguin annihiliation diagram (Fig. 1 right), and its branching fraction has been calculated in the SM to be in the range  bs2gamgam-sm1 (); bs2gamgam-sm2 (); bs2gamgam-sm3 (). Neither  nor  has yet been observed, and the upper limits at the 90% confidence level (CL) on their branching fractions are, respectively,  bs2phigam-cdf () and  u5s-excl ().

Figure 1: Diagrams describing the dominant processes for the  (left) and  (right) decays.

A strong theoretical constraint on the  branching fraction is generally assumed due to good agreement between SM expectations and experimental results for rates, such as in  and  decays bs2phigam-sm1 (); bs2phigam-sm2 (); b2kstg-th (); PDG2007 () or inclusive decays b2xsg-th (); PDG2007 (). The  decay rate is constrained in a similar way bs2gamgam-xsg (), though various New Physics (NP) scenarios such as supersymmetry with broken -parity bs2gamgam-supersym (), a fourth quark generation bs2gamgam-4thquark () or a two Higgs doublet model with flavor changing neutral currents bs2gamgam-2higgsdoublet (), can increase the  branching fraction by up to an order of magnitude without violating constraints on the  branching fraction.

In this study, we use a data sample with an integrated luminosity () of 23.6  that was collected with the Belle detector at the KEKB asymmetric-energy (3.6 on 8.2 GeV) collider KEKB () operating at the  resonance (10.87 GeV).

The Belle detector is a large-solid-angle magnetic spectrometer that consists of a 4-layer silicon detector (SVD SVD2 ()), a central drift chamber (CDC), an array of aerogel threshold Cherenkov counters (ACC), a barrel-like arrangement of time-of-flight scintillation counters (TOF), and an electromagnetic calorimeter comprised of CsI(Tl) crystals (ECL) located inside a superconducting solenoid coil that provides a 1.5 T magnetic field. An iron flux-return located outside the coil is instrumented to detect mesons and to identify muons. The detector is described in detail elsewhere Belle ().

The variety of hadronic events at the  resonance is richer than at the . , and  mesons are all produced in  decay.  mesons are produced mainly via decays, with subsequent  low energy photon de-excitation. The production cross section at the , the fraction of  events in the events, and the fraction of  events among  events have been measured to be, respectively, nb u5s-incl (),  PDG2007 () and  u5s-excl (). The  and  decay fractions are small and not yet measured.

Charged tracks are reconstructed using the SVD and CDC detectors and are required to originate from the interaction point. Kaon candidates are selected from charged tracks with the requirement , where () is the likelihood for a track to be a kaon (pion) based on the response of the ACC and on measurements from the CDC and TOF. For the selected kaons, the identification efficiency is about 85% with about 9% of pions misidentified as kaons.

We reconstruct mesons in the decay mode  by combining oppositely charged kaons having an invariant mass within  () of the nominal mass PDG2007 ().

We reject photons from and decays to two photons using a likelihood based on the energy and polar angles of the photons in the laboratory frame and the invariant mass of the photon pair. To reject merged photons from decays and neutral hadrons such as neutrons and , we require an ECL shower shape consistent with that of a single photon: for each cluster, the ratio of the energy deposited in the central calorimeter cells to that of the larger array of cells has to be greater than 0.95. Candidate photons are required to have a signal timing consistent with originating from the same event. For the  mode, photons are selected in the barrel part of the ECL () and we require that the total energy of the event be less than 12 GeV.

 meson candidates are selected using the beam-energy-constrained mass and the energy difference . In these definitions,  is the beam energy and and are the momentum and the energy of the  meson, with all variables being evaluated in the center-of-mass (CM) frame. We select meson candidates with for both modes, and for the  mode and for the  mode. No events with multiple  candidates are observed in either data or Monte Carlo (MC) simulation.  mesons are not fully reconstructed due to the low energy of the photon from the  decay. Signal candidates coming from ,  and  are well-separated in , but they overlap in  u5s-excl ().

The main background in both search modes is due to continuum events coming from light-quark pair production (, , and ). Rejection of this background is studied and optimized using large signal MC samples and a continuum MC sample having about three times the size of the data sample. A Fisher discriminant based on modified Fox-Wolfram moments ( SFW ()) is used to separate signal from continuum background. The process is a source of high-energy photons with low polar angles and can thus be a background for radiative decays. Therefore, for the  mode, we apply a more restrictive  requirement when the candidate photon is recontructed outside the barrel part of the ECL. This procedure is not used for the  mode where photons are selected only in the barrel. For the  mode, the  requirement is chosen in order to maximize a figure of merit defined as , where and are the expected number of signal events coming from  events and continuum events, respectively. and are computed in the  signal window (, and ) and are normalized to an integrated luminosity of 23.6  assuming . The helicity angle  is the angle between the  and the in the rest frame. For signal events  should follow a distribution, while for continuum events the distribution is found to be flat. For the  mode, we optimize the  requirement to minimize the 90% CL upper limit on the branching fraction computed by the Feldman-Cousins method feldmancousins (). The upper limit calculation requires two inputs: the number of observed events () and the expected number of background events (). We assume and . and are computed in the  signal window ( and ) assuming that .

Inclusive backgrounds from  decays are studied using MC samples having about the same size as the data sample. Backgrounds coming from or decays are found to lie outside of the fit region. For  decays, no event is reconstructed in the  mode. The  decay is a potential background for the  mode and is studied using a dedicated MC sample. Assuming that its branching fraction is the same as its counterpart  PDG2007 (), we expect to reconstruct one  background event. Considering the large  branching fraction uncertainty, this background is treated as a source of systematic error.

For the  () mode, we perform a three-dimensional (two-dimensional) unbinned extended maximum likelihood fit to ,  and  ( and ) using the probability density functions (PDF) described below.

The signal PDFs for  and  are modeled separately for events coming from ,  and  with smoothed two-dimensional histograms built from signal MC events. The  () mean for the  signal is adjusted to the  mass (the - mass difference) obtained from  events reconstructed in the same  data sample. The  and  resolutions for the  () signal are corrected using a control sample of  events ( events) recorded on the resonance. Statistical uncertainties contained in these corrections are included in the systematic uncertainty. Continuum background is modeled with an ARGUS function argus () for  and a first-order polynomial function for . For the  mode, the signal (continuum) PDF for  is modeled with a (constant) function. The  background PDF is modeled using MC events as the product of a two-dimensional PDF for  and  and a one-dimensional histogram for . The likelihood is defined as

(1)

where runs over all events, runs over the possible event categories (signals or backgrounds), is the number of events in each category and is the corresponding PDF.

Both fits have six free fit variables: the yields for the ,  and  signals (,  and ), the continuum background normalization and PDF parameters, except the ARGUS endpoint which is fixed to . The branching fractions ( and ) are determined from the  signal yields with the relations

(2)
(3)

where ’s are the MC signal efficiencies listed in Table 1 and  is the number of  mesons evaluated as .

In the  mode we observe  signal events in the  region and no significant signals in the two other regions. These signal yields are compatible with  u5s-excl (). We measure and with a significance of , where the first uncertainty is statistical and the second is systematic. Systematic uncertainties and computation of the significance are detailed below. The measured branching fraction is in agreement with SM expectations bs2phigam-sm1 (); bs2phigam-sm2 () and with the measurements and  PDG2007 (). We observe no significant  signal and, including systematic uncertainties, determine a 90% CL upper limit of . This limit is about six times more restrictive than the previous one u5s-excl (), though still about one order of magnitude larger than SM expectations bs2gamgam-sm1 (); bs2gamgam-sm2 (); bs2gamgam-sm3 () and still above the predictions of NP models bs2gamgam-supersym (); bs2gamgam-4thquark (); bs2gamgam-2higgsdoublet (). The results are summarized in Table 1 and fit projections in the signal windows are shown in Figs. 2 and 3.

Mode (%)  () Sig.
Table 1: Efficiencies, signal yields, branching fractions and significances (Sig.) obtained from the fits described in the text. The first uncertainty is statistical and the second systematic. The upper limit is calculated at the 90% CL.
Figure 2: ,  and  projections together with fit results for the  mode. The points with error bars represent data, the thick solid curves are the fit functions, the thin solid curves are the signal functions, and the dashed curves show the continuum contribution. On the  figure, signals from ,  and  appear from left to right. On the  and  figures, due to the requirement  only the  signal contributes. The bottom right figure shows  versus  for selected data events. The dashed lines show the signal window.
Figure 3:  and  projections together with fit results for the  mode. The points with error bars represent data, the thick solid curves are the fit functions, the thin solid curves are the signal functions, and the dashed curves show the continuum contribution. On the  figure, signals from ,  and  appear from left to right. On the  figure, due to the requirement  only the  signal contributes.

Systematic uncertainties are listed in Table 2. The error on the signal reconstruction efficiency is dominated by uncertainty on the efficiency of the  requirement. This uncertainty is evaluated by comparing efficiencies in data and MC using the  control sample. For the  mode, we take as systematic uncertainty the  difference between the results of the nominal fit and the results of a fit where the continuum is parametrized with a second-order polynomial function for . For the  mode, the limit obtained with the nominal continuum parametrization is found to be conservative. For the  mode, systematic uncertainties on  are evaluated by repeating the fit with each parameter successively varied by plus or minus one standard deviation around its central value. The positive and negative uncertainty in  are obtained from the quadratic sum of the corresponding deviations from the  value returned by the nominal fit. The significance of the branching fraction measurement is defined as , where is the likelihood returned by the nominal fit and is the likelihood returned by the fit with  set to zero. Systematic uncertainties are included by choosing the lowest significance value returned by the fits used to evaluate the systematic uncertainty. The  background is the only source of systematic uncertainty having a non-negligible effect on the significance. For the  mode, the 90% CL limit, , is computed by likelihood integration, according to . Systematic uncertainties are included by convolving the likelihood function with Gaussian distributions for the parameters giving rise to systematic uncertainty.

Source
Photon reconstruction efficiency
Tracking efficiency
Kaon identification efficiency
requirement efficiency
MC statistics
(quadratic sum)
Signal shape negl.
 continuum shape negl.
backgrounds negl.
(quadratic sum)
Total (quadratic sum)
Table 2: Systematic uncertainties.

In summary, we observe for the first time a radiative penguin decay of the  meson in the  mode. We measure , which is in agreement with both the SM predictions and with extrapolations from measured  and  decay branching fractions. No significant signal is observed in the  mode and we set an upper limit at the 90% CL of . This limit significantly improves on the previously reported one and is only an order of magnitude larger than the SM prediction, providing the possibility of observing this decay at a future Super -factory superbelle (); superb ().

We thank the KEKB group for excellent operation of the accelerator, the KEK cryogenics group for efficient solenoid operations, and the KEK computer group and the NII for valuable computing and Super-SINET network support. We acknowledge support from MEXT and JSPS (Japan); ARC and DEST (Australia); NSFC (China); DST (India); MOEHRD, KOSEF and KRF (Korea); KBN (Poland); MES and RFAAE (Russia); ARRS (Slovenia); SNSF (Switzerland); NSC and MOE (Taiwan); and DOE (USA).

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