\ / I N S T I T U T E FOR HIGH ENERGY PHYSICS И Ф В Э

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1 \ / I N S T I T U T E FOR HIGH ENERGY PHYSICS И Ф В Э ОЭФ SERP-E-134 R.I.Dzhelyadin, S.V.Golovkin, A.S.Konstantinov, V.F.Konstantinov, V.P.Kubarovski, L.G.Landsberg, V.A.Mukhin, V.F.Obrastsov, Yu.D.Prokoshkin, V.A.Victorov, A.M.Zaitsev SEARCH FOR RARE DECAYS OF - AND rj'- MESONS v AND FOR LIGHT HIGGS PARTICLES Serpukhov 1980

2 R.I.Dzhelyadin, S.V.Golovkin, A.S.Konstantinov, V.P.Konstantinov, V.P.Kubarovski, L.G.Landsberg, V.A.Mukhin, V.P.Obrastsov, Yu.D.Prokoshkin, V.A.Victorov, A.M.Zaitsev SEARCH FOR RARE DECAYS OF tj- AND 77'- MESONS AND FOR LIGHT HIGGS PARTICLES Submitted to Yad. Fiz.

3 M-24 Abstract Dzhelyadin R.I., Golovkin S.V., Konstantinov A.S., Konstantinov V.F., Kubarovskl V.P., Landsberg L.G., Mukhin V.A., Obrastsov V.F., Prokoehkln TU.D., Victorov V.A., Zaitsev A.M. Search for Xare Decays of ij-and ц- Mesons and for Light Hlggs Particles. Serpukhov, p. И. (1НИ> ). Kefs. IS. The upper limits for the branching ratios of rare IJ, V -decays have been obtained: В»(п- т> + и~)< б-ю" 6, BR(ij'- irve") <7.10-5, BH(ij'- гщ + 1л-)< " 5 and BRO) - <>,»+«~ у)< < 3-10" 6 (90% C.L.). This la ieveral orders of Magnitude lower than the previous limit». The analysis of the decay IJ' ->i7fi' l '((~excludes the exiatence of light Higgs particles with a Bass lower than 409 MoY/c2. Викторов В.А., Головкин СВ., Джеладин Р.И., Зайцев А.М., Константинов А.С., Константинов В.Ф., Кубароасхий В.П., Ландсберг Л.Г., Мухин В.А., Образцов В.Ф, Прокошкин Ю.Д. Поиски редких распадов г)-* ч' -мезонов и легких хиггсовскихчастиц. Серпухов, стр. с рис. (ИФВЭОЭФ ). Библиогр. IS. Получены верхние граничные оценки относительных вероятностей редких распадов 1-ц' (80% уровень достоверности). Это на несколько порядков величины ниже ранее существовавших пределов. На основании анализа распада т{- ур + р~ исключено существование легких хиггсовских частиц с массой ниже 409 МэВ/с2.

4 q' mesons: Below we describe the search made for rare decays of i\ and,.», JL +ll - t (I) v'-> *v r, / + _ Tj * T] Ц (I, <2) Decays (l)-(3) may proceed through two-photon approximation (single-photon process is forbidden by 0-conservation) according to the diagram in fig. la. The decay width depends on the P 1 -»Pyу vertex structure (P is a pseudoscalar meson). To describe this structure one may use,for example, a vector meson dominance (fig. lb)' 'or8(s)dominance model (fig. lc), close to' 2 '. A model twophoton mechanism leads to very small branching ratios of decays (l)-(3) (BR«10~ 6 ), which makes these decays rather sensitive to exotic processes.

5 a) О b) c=*1 d) Fig. 1. М ш п ш diagrams for the p 1 -> p+ _ decay*: a) the general form of the two-photon vertex; b) the two-photon vertex In the vector aeaon dominance model; c) the twophoton vertex in the «(<) dominance model; d) one- photon ргосвж» with O-vlolatlon (due to negative O-parity of photon), Рог instance, if there exist light Higgs particles H, with quantum numbers jw = 0 ++, they might be observed through the decay into a muon pair P'-* P + H, (5) Up to now there is only one experimental limitation for the Higgs boson mass U^ obtained in the search for K + -+ it+ H -» n + ц+ц~ decay (M H > 350 MeV/c 2 ). Another exotic example is a possible С viola- /3/ tion in electromagnetic interactions (as in the model ). In this case processes (l)-(3) might develope in a one-photon approximation (fig. Id) with the probability much higher than given by twophoton estimations.

6 The present experiments were performed in a framework of the series of investigations for rare electromagnetic decays of light /4-7/ mesons. The measurements were made in the ~30 GeV/c negative pion beam of the IHEP 70 GeV accelerator using Lepton-G set-up /8/ The binary reactions jr~p-> qn ' and w~p * i\ 'n' 11 ' were used as 7j and TI' sources, similar to' 4» 5» 7 /. During the running time a total effective flux of 5-10 a production of r/ n~ passed through the set-up, resulting in and 10 IJ' in the set-up target. The measurement technique and data processing procedure are similar to those used in earlier experiments on observation and study of rare a-* /r /x /i~decay '. The events corresponding to the exclusive reaction TT~ p * fi + ii~yy n (6) with muon and photon energies above 4,5 GeV and 1.4 GeV,respectively, were selected. Fig. 2 presents the two-dimensional distribution for all the events of type (6) over the effective masses of ц (i~yy and yy systems, и уу and m. The concentration of the events in the region of m sm and m «m corresponds to the «a * п O /*+ ц~ decay. Region III, which corresponds to decay (3) (with the account for the set-up resolution) contains only one event. In regions I and II there is no any enhancement above the smooth background.

7 Ю 20 N Fit. 2. Two-dimenaional diltributlon of type (6) «venti over the effective u n n of ц + ц~уу and yy ay*test.the domain» I-III correspond to decay* (l)-(3), with the account for the experimental reaolution. Thin hlatofraa la for all eventa, thick hiatograb la for the eventa with the selection of «,,. The arrow* point to the table aeaon вааа values. Thus,decays(l)-(3) have not been observed in the present experiment. High experimental sensitivity allows one to put low limits for the branching ratios of this processes, improving by some orders of magnitude the limits obtained earlier (see Table). For comparison the same Table contains also the data on the decays with emission of an electronic pair. When calculating the set-up efficiency for decays (l)-(4) several assumptions on the decay mechanism were used; vector meson dominance, 5(0 dominance,phase space model. The efficiency values obtained with various models turn out to be the same within some per cent.this shows that the boundary estimates obtained are model independent. The Table presents also the results on the search for decay(4). To select this decay we used the effective mass spectrum for тг p + p~y systems produced in exclusive reaction (7) 6

8 Table The upper limits for the branching ratios of -q and 77' decays Decay The upper limits for the branching ratio BH*> (90% C.L. (the present work) /13/ Earlier upper limits The upper limits for the decays into an electron pair/"/**) r\ -» n (i + (t~ 5-10"" " " 5 З-Ю" 6 5-Ю" " 5 10" 2 Д0" Я,- >г% + /Г) = T( v - noftiri/rij) ->all) and similar for other decays. **>Decays rj -я о е + вг, 4'-»ir e + er, v' -*г, е + в~? respectively. No events of this type were observed in the TJ region of the mass spectrum. Whereof a low limit for the branching ratio of decay (4) has been obtained which might be of a certain interest in connection with the problem of a large branching ratio for the decay As was noted above, decays (1) and (3) may be used in the search for light Higgs bosons. Though the now-existing ideas concerning the nature of weak interactions are in favour of heavy Higgs particles,still it should be noted that in the models with several doublets of Higgs bosons there are no theoretical limita-

9 tions for their mass. Therefore a search for Higgs particles is to be carried out in the whole available mass range. The theoretical situation has been treated in detail in papers» 15 ' # д characteristic feature of Higgs boson interaction with fermion fields is a growth of the coupling constant with the fermion mass. Therefore, at masses Мц < 2m, Higgs particles would decay mainly into a muon pair H ->,f ft. (8) At higher masses a new decay channel H->77-77 is opened, but when mass Иц is not very large the mode (8) remains dominating : KHV,-Mi-}< if /* H (1 - _) M 2 S-T7 r ' ( 5,0.9 at MJJ^O.5 GeV/c 2 ). In this, the lifetime of Higgs particles is r H <io~15 s> It is natural to search for light Higgs bosons in processes of the type (5): 4 - * H + _, do) Decays (10)-(12) are semiweak, their branching ratio should be BR ~10~. Thus, our experiment has a sufficient sensitivity for the search for light Higgs particles.

10 More detailed calculations of the decay (10)' 14 > 15 ' showed that this decay seems to be highly suppressed by the compensation of the Higgs boson direct production diagrams and the pole diagrams (fig. 3). According to A.I.Vainstein, in decays (11), (12) the pole terms contribution is small, and the diagram of direct interaction is dominating (fig. 3a). This is connected with the peculiarity of ' meson, whose mass is not proportional to the quark masses. corresponding estimates for the branching ratios are»* «2 8,7 mlj(n' - all) The (13) a) b) Fig; 3. Feinmann diagram for the P' PH d«- caya: a) la the direct production of Higga particlea; b),c) are pole grapha of Higgs particle production. T\ Tt e

11 Here p»^,»?;pg is Higgs boson momentum in the 77' rest frame. The interaction constants are equal to (14) (15) m u,d are masses of u and d quarks. G/v " is the weak interaction constant. The calculation results on decay (12) are presented in fig. 4 together with our experimental upper limits. As it is clear from the figure the experiment allows to exclude the existence of Higgs bosons on the whole kinematical range available in decay (12),i.e. %> 409 MeV/c 2. (the interaction constants being accepted to be (15)). The data on decay (11) are not sensitive enough for the search for Higgs particles. I 1 I JL upptr limit (nptrlirwnt) Fig. 4. The data on ц " чч_, + - decays. Curve I it the theoretical estimitei of the branching ratio with formula (13). Curve 2 «howa our experimental upper limita (90% C.L.). 10

12 In conclusion the authors would like to thank V.V.Bazhanov, G.P.Pron'ko, M.A.Shifman, L.D.Soloviev, A.I.Vainstein, O.L.Zorin for the discussions of the results. R E F E R E N C E S 1. T.P.Cheng. Phys. Rev., 162_, 1734 (1967). 2. C.H.Llewellyn Smith. Nuovo Cim., 48A, 234 (1967). 3. J.Bernstein et al. Phys. Rev,, 139B, 1650 (1965). 4. Yu.B.Bushnin et al. Yad. Piz.,28, 1507 (1978); phys. Lett.,79B, 147 (1978). 5. V.A.Viktorov et al. Yad. Fiz., 29_, 1513 (1979); phys. Lett., 84B, 143 (1Э79). 6. V.A.Viktorov et al. Piz'ma v JETP, 30, 387 (1979), Phys.Lett,, 88B, 379 (1979). 7. V.A.Viktorov et al. Yad. Fiz., 32_, 998, 1002, 1005 (1980); Phys. Lett., 94B, 548 (1980). 8. Yu.B.Bushnin et al. Preprint IHEP 80-57, Serpukhov, O.I.DaM et al. phys. Rev. Lett., 37_, 80 (1976). 10. W.D.Apel et al.yad. Fiz., 29_, 1519 (1979); Nucl. Phys., B152.1 (1979). 11. W.D.Apel et al. Yad. Fiz., J30_, 366 (1979); Phys. Lett., 83B.131 (1979). 12. V.A.Viktorov et al. Preprint IHEP , Serpukhov, C.Bricman et al. Rev. of Part. Properties. Rev. Mod. Phys., 52_, 1 (1980). 14. J.Ellis et al.nucl. Phys., B106, 292 (1976). 15. A.I.Vainstein et al. UFN, 131, 537 (1980) (here see the references). Received 12 December

13 Цена 6 коп. - Институт физики высоких энергий, Издательская группа И Ф В Э Заказ 95. Тираж ,5 уч.изд.-л. Т Январь Редактор А.А. Антипова.