Measurement of the φ meson radiative decays into scalar and pseudoscalar mesons with the KLOE detector

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  • 2s -- + -- !f!B SUPPLEMENTS ELSEVIER Nuclear Physics B (Proc. Suppl.) 126 (2004) 199-203

    Measurement of the qb meson radiative decays into scalar and pseudoscalar mesons with the KLOE detector The Kloe Collaboration* Presented by Camilla Di Donatoat

    aI.N.F.N. Sezione di Napoli, Complesso Universitario di Monte SantAngelo Via Cintia ed.G, 80126-Napoli, Italy

    The Kloe experiment has measured the radiative decays of the r$ meson into TO, 77 and q(958); these mea- surements are relevant to assess the mixing in the pseudoscalar nonet as well as to evaluate the gluon content in the q(958). Moreover also the radiative decays into #7ry and 17~~ have been measured. These decays are dominated by the final states fo(980) and a(980). The measurement of the branching ratios and of the 7r - r or 7 - no invariant mass spectrum helps to understand the controversial nature of the above scalar mesons.

    1. Introduction

    The Frascati &factory DA@NE is an eSe- col- lider with fi = 1020 MeV and the peak cross section for 4 production is u$ = 3pub. The KLOE data taking started on April 1999; in 2000 it has collected an integrated luminosity of 25 pb-I, cor- responding to 7.5 x 107$, 190 pb-l during the 2001, corresponding to 5.7 x 104 and 300 pb-I,

    *The Kloe Collaboration: A. Aloisio, F. Ambrosino, A. Antonelli, M. Antonelli, C. Bacci, G. Bencivenni, S. Bertolucci, C. Bini, C. Bloise, V. Bocci, F. Bossi, P. Branchini, S. A. Bulychjov, R. Caloi, P. Campana, G. Capon, T. Capussela, G. Carboni, M. Casarsa, V. Casavola, G. Cataldi, F. Ceradini, F. Cervelli, F. Cevenini, G. Chiefari, P. Ciambrone, S. Conetti, E. De Lucia, P. De Simone, G. De Zorzi, S. DellAgnello, A. Denig, A. Di Domenico, C. Di Donato, S. Di Falco, B. Di Micco, A. Doria, M. Dreucci, 0. Erriquez, A. Farilla, G. Felici, A. Ferrari, M. L. Ferrer, G. Finocchiaro, C. Forti, A. Franceschi, P. Franzini, C. Gatti, P. Gauzzi, S. Gio- vannella, E. Gorini, E. Graziani, S. W. Han, M. Incagli, W. Kluge, V. Kulikov, F. Lacava, G. Lanfranchi, J. Lee- Franzini, D. Leone, F. Lu, M. Martemianov, M. Mat- syuk, W. Mei, L. Merola, R. Messi, S. Miscetti, M. Moul- son, S. Miiller, F. Murtas, M. Napolitano, A. Nedosekin, F. Nguyen, M. Palutan, E. Pasqualucci, L. Passalacqua, A. Passeri, V. Patera, F. Perfetto, E. Petrolo, L. Pon- tecorvo, M. Primavera, F. Ruggieri, P. Santangelo, E. San- tovetti, G. Saracino, R. D. Schamberger, B. Sciascia, A. Sciubba, F. Scuri, I. Sfiligoi, A. Sibidanov, T. Spadaro, E. Spiriti, G. L. Tong, L. Tortora, P. Valente, B. Valeri- ani, G. Venanzoni, S. Veneziano, A. Ventura, S. Ventura, R. Versaci, G. W. Yu te-mail: Camilla.DiDonato@na.infn.it

    0920-5632/s - see front matter 0 2004 Published by Elscvier l3.V. doi: 10.1016/j.nuc1pbysbps.2003. I 1.033

    corresponding to 9.0 x 104, during the 2002. The analysis of 2001 and 2002 data is in progress, while results from 2000 have been published [l-3]

    2. Pseudoscalar mesons

    The mass eigenstates 77, $ a.re related to the SU(3) octet-singlet states through the pseu- doscalar mixing angle BP. Recent studies based on extended chiral perturbation theory and phe- nomenological analyses show the necessity to de- scribe the mixing with two angles. In the quark flavour basis the two mixing parameters differ only for small terms which violate the OZI rule [4], so it is still possible to describe the mixing with only one angle, (pp. We will use the ap- proach from Bramon[5] to extract the mixing an- gle from the ratio R = BR(4 -+ $y)/BR($ -+ TY). A precise BR($ -+ $7) measurement can probe a possible 7 gluon content, as the large BR(B -+ E(Q) value, observed by [6], seems to indicate. We measured R studying 4 + #-y and 4 -+ r]y de- cays in the same final state with r+r- and three photons ; BR(d + qy,~ -+ ~~+7r-,q + yy) M 1 x 10-5; BR(d + qy,l;l -+ n+7i?r0) M 3 x 10-3. The selection efficiency for 4 -+ $7 is 23%, while the one for 4 -+ qy is 37%; due to the ratio of the two branching ratio the Q decay is the main background for the q one. Other background comes from 4 -+ &~T-T and 4 -+ KLKs. The

  • 200 C. Di Donuto/Nucleur Physics B (Proc. Suppl.) 126 (2004) 199-203

    events selected after all backgrounds rejection are N $--tQ,y = 120f12&5 and N,++.,, = 50210*220, then R=(4.70 rt 0.47 f 0.31) x 10F3 is obtained. Our R extimation with the PDG value for the branching ratio BR(4 + ~7) = (1.297 f 0.003)%, yields BR(4 + ~7) = (6.10 f 0.61 f 0.43) x 10A5; in the figure (fig.1) we report the 77 invariant mass for the selected events. iFrom the ratio of the two branching ratios, we can extract our estimate for the mixing angle in the quark flavour basis: pp = (41.8+;:;). Up to know we neglected a possible gluonic content for the ~1; if we allow it, we have

    where the term with Z,, allows for gluonium. We performed a consistency check of our measure- ment with this hypothesis; if Z,I = 0, measuring cpp, we are measuring JYV, 1 = coscpp. Using con- straints from I($ + pr)/I(w -+ ~7) (4) and I($ + rr)/I(nO -+ 7~) (5), we find an overlap region of the three allowed bands in the XV,YVt plane (fig.2) which is consistent with a small value for ZVl: X,Z, + Yz = 0.94?::0,!.

    1920 930 940 950 960 970 980 990 1000

    M nw (MeV/c)

    Figure 1. Invariant mass for r], dots: data; yellow is expected Monte Carlo signal and blue is the background evaluated from side-band on data.

    Figure 2. Bounds on X,1 and YVl. The overlap region of the three allowed bands is consistent within one standard deviation with no gluonium contents in the $.

    A first observation of 4 + $7 with two tracks and seven photons final state produced a branch- ing ratio measurement consistent with the previ- ous one; this measurement has been performed looking at the two decays:

    The analysis on the 2000 data gives the following result BR(qS + $7) = (7.05 f 0.50?$;) x 10e5.

    3. Scalar mesons

    The scalar mesons (Jpc = O++) fe(980) and aa(980) are not easily interpreted as qq states. Other hypotheses are by Jaffe [7], who proposes a qgqr!j states and by Weinstein and Isgur [8] who suggest a KK bound state. The nature of these scalar mesons is not well established and the branching ratios of $J + for and 4 -+ UOY, as well as the fe and ae mass shapes, can give infor- mation about the structure of these particles[9]. KLOE has analysed the processes:

  • C. Di Donuto/Nuckau Physics B (Proc. Suppi.) 126 (2004) 199-203 201

    1. 4 + for, fo -+ 7r07ro

    For the first two decays the final state is fully neu- tral : five prompt photons in the detector; these two processes have been previously measured by SND and CMD-2 at VEPP-2M in Novosibirsk [lO].The third decay, which have two charged pi- ons and five prompt photons final state, has been observed by KLOE for the first time. The decays contributing to the five photon final state, besides the signals are:

    1. 4 -+ a(500)y, (T -+ 7ro7ro [ll]

    3. e+e- -+ w7r, w -+ 7ry

    A kinematic fit is performed, by requiring the 4-momentum conservation for the five prompt photons, to reject events from decays with three and seven prompt photons, wrongly recon- structed as five photon ones. A discrimination between $I -+ 7r07roy and #I + r/7ry is performed with a procedure of photon pairing, which asso- ciates pairs of photons to 7r and q. Events with 7ry invariant mass compatible with the w mass are recognised as w7r background and rejected. After all background subtraction the 4 --+ r7roy events selected are 3102 with a contamination at level of 20% as evaluated from Monte Carlo. The selection efficiency for the signal is 40% and by normalizing to the luminosity we find the follow- ing branching ratio:

    BR(q5 -+ rroy) = (1.08 i 0.03 i 0.05) x lo-*

    Concerning the 4 -+ q7ry decays the selection efficiency is 32% and the contamination at level of 30%; the number of selected events is 916. The branching ratio obtained is:

    BR($ -+ r/x07) = (8.51 f 0.51 i 0.57) x 1O-5

    both results are in agreement with the Novosi- birsk measurements [lo].

    The decay I$ -+ soy, as + nir with q -+ 7rr+r-7ro produces in the detector two charged tracks coming from the interaction point and five prompt photons. No other decay has the same final state, but events with two tracks and three ( 4 --+ 777, 77 -+ 7r+7r-7r), four (e+e- -+ w7r,w + 7r+n-7r; CjJ + KLKs, KL + rlv and KS -+ nrro) or six (4 + KLKS + 7r+7r-nro7ro, with prompt KL decay) prompt photons can mimic this final state. The selection of this decay has been done with a strategy similar to the one used for fully neutral final state: 197 events have been selected with 4 f 4 background events and with 19% efficiency. From this decay chain we find a branching ratio measurement in good agreement with the result of the five photon sample:

    BR(c#J + ?pry) = (7.96 zk 0.60 f 0.47) x 1O-5

    In order to disentangle the various contribu- tions, the data have been fitted to theoretical mass spectra obtained from the following model: (a) the coupling of 4 to S(O++)y is assumed to occur through a charged kaon loop [9], (b) the scalar propagator with finite width corrections is used, (c) the 4 -+ p7ro parametrization is taken from VDM calculations [12].

    We fit the 7r07ro mass spectrum, after back- ground subtraction, in two different hypothesis: Fit A, without and Fit B, with #J + 0(5OO)y con- tribution. The (T meson has been parametrized as a fixed width Breit-Wigner, with AJfl = 478 MeV and F,, = 324 MeV [ll], and a point-like 4 - ay coupling has been assumed. The free pa- rameters of the fits are the fa mass, the squared coupling g&KK/(4n), the ratio giOKK/g;Ors, the BR(q5 t pno + 7r07roy)) and g&ry (only for Fit B).

    In table 1 the results for Fit A and Fit B for fe meson are shown. Fit A gives a large x2, while Fit B shows a good

    agreement with experimental data. In fig.3 the differential decay rate is plotted, to- gether with the various contributions from Fit B.

    The depletion of the mass spectrum below 700 MeV is produced by a large negative interference between fs and cr. The huge amount of events

  • 202 C. Di Donato/Nucleav Physics B (Proc. Suppl.) 126 (2004) 199-203

    Table 1 Fit results for the fo meson

    f0 Fit A Fit B x2/ndf 109.5/34 42.2133 M.fo PeV) 962 f 4 973 f 1 g$KK/(4n) (Gev2) 1.29 f 0.14 2.79 rf: 0.12 SfoKKl&* 3.22 f 0.29 4.00 f 0.14 Pi% - 0.060 f 0.008

    Figure 3. Differential decay rate for r$ --+ 7r07roy. The solid line is the result of the Fit B, individual contributions are also shown.

    collected in 2001 and 2002 will allow to perform fit with more free parameters; for example we can release the constraint on the mass and width of the 0 meson.

    A similar study has been applyed to the 4 -+ v7ry decay. A combined fit of the two mass spectra, one from the fully neutral decay, and the other one from the charged and neutral fi- nal state, after background subtraction, has been performed. We fix the relative normalization to the ratio BR(q + yy)/BR(r) -+ 7r+7r-7r) from the PDG. Concernig this decay we have only one fit; the free parameters are the squared coupling doKKl(4~) (GeV2), th e ratio L7aovn/$?aoKK and the BR(4 + p7ro -+ ~7rr)). The fit results are listed in table 2.

    Table 2 Fit results for the an meson a0 Fit x2 /ndf 27.5125 doKK/(4T) (Gev2) 0.40 * 0.04 how %zoKK I 1.35 f 0.09 BR(4 + p7ro -+ qn$) (0.5 & 0.5) x 1O-5

    The mass of the ae has been fixed to the PDG value, M,, = 984.8 (MeV). The results are shown in fig.4

    50

    25

    0

    q-T--J7 od

    700 750 800 850 900 950 1000 M.~- IMeW

    !!l;,-,---ir_l 700 750 800 850 900 950 1000

    M,,% (MeV)

    Figure 4. qnO invariant mass spectra.top: five photon sample. middle: 7r+~- and five photons , sample; bottom: theoretical curve obtained from the combined fit

    The integration of the as contribution on the theoretical curve gives:

    BR(r$ -+ c&J-y -5 q7rO-f) = (7.4 f 0.7) x 1o-5

  • C. Di Donuto/Nuclenr Physics B (Pvoc. Suppl.) 126 (2004) 199-203 203

    Table 3 Comparison with KK- loop predictions [9]

    fa model KLOE sS(uG -I- dd)/JZ (m + dd)/JZ 5-s

    &KKI(W (Gev) 2.79 f 0.12 2.3 0.15 0.3

    g.fom I QfoKK 0.50 f 0.01 0.3 - 0.5 2 0.5

    BR($ + 7rnoy)) x lo4 1.09 zt 0.07 N 1 N 0.15 - 0.2

    a0 model KLOE squii - dd)/l.A (uii - dd)/&

    d,KK/(4r) (Gev) 0.40 f 0.04 2.3 0.15

    hoan I gaoKK 1.35 f 0.09 0.91 1.53 BR($ -+ aor)) x lo4 (0.74 f 0.07) - 2 N 0.2

    In tab.3 we compare the fit results to the pre- dictions of the K+K- loop model for some pos- sible structures of the scalar mesons. The fa pa-

    - - rameters are well compatible with the qqqq model, while the a0 ones disagree with that model.

    7. 8.

    9. 4. Conclusions

    10. Using year 2000 events we published our first

    papers on &radiative decays about the 77-q mix- ing and the scalar mesons. We have observed for the first time the $ -+ $7 decay with two tracks and seven photons final state and the 4 + a07 decay with a0 -+ q,lr and 77 -+ ~+Fx~. The integrated luminosity collected up to now is 20 times the 2000 one. The analysis are in progress.

    11.

    12.

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