Volume 105B, number 2,3 PHYSICS LETTERS 1 October 1981

SEARCH FOR RARE DECAYS OF ~1- AND 77'-MESONS, AND FOR LIGHT HIGGS PARTICLES

R.I. DZHELYADIN, S.V. GOLOVKIN, A.S. KONSTANTINOV, V.F. KONSTANTINOV,

V.P. KUBAROVSKI, L.G. LANDSBERG, V.A. MUKHIN, V.F. OBRAZTSOV, Yu.D. PROKOSHKIN,

V.A. VICTOROV and A.M. ZAITSEV Institute for High Energy Physics, Serpukhov, USSR

Received 21 July 1981

The upper limits for the branching ratios of rare r/, r/' decays have been obtained: BR(r~ ~ ~r0~+# -) < 6 X 10 -6, BR (r~' --+ r/~+~:) < 1.5 X 10 -s , BR (r/ '~ ~r0~+U -) < 7 X 10 -s and BR (n--* 7r°ta+/~-'r) < 3 X 10 -6 (90% c.1.). These are several orders of magnitude lower than the previous limits. The analysis of the decay 77' ~ r/~z+~ - excludes the existence of light Higgs particles with a mass lower than 409 MeV/c 2.

Below we describe the search made for rare decays of ~7- and ~'-mesons:

~7 -+ rr0/a+/~ - , (1)

r/' -+ rr0/a+tt - , (2)

n'-, nu+ - , (3)

r / -* zr°t~+/~-7. (4)

The decays ( 1 ) - ( 3 ) may proceed through a two- photon approximation (the single-photon process is forbidden by C-parity) according to the diagram in fig. la . The decay width depends on the P' ~ P77 ver- tex structure (P is a pseudoscalar meson). To describe this structure one may use, for example, a vector- meson dominance model (fig. l b ) [1] or 8(e) domi- nance model (fig. lc) , close to the one used in ref. [2]. A two-photon mechanism leads to very small branch- ing ratios of the decays ( 1 ) - ( 3 ) (BR ~ 10-6) , which makes these decays rather sensitive to exotic processes.

For instance, if there exist light H/ggs particles H, with quantum numbers jPC = 0++, they might be ob- served through the decay into a muon pair

lY-~P+ H • (5)

Up to now there is only one experimental l imitation for the Higgs-boson mass M H obtained in the search

p' p' - -

a c

P

P ' p' p-

¢=q

d

Fig 1 Feynman diagrams for the decay P'--* P~+ - ' (a) the # • general form of the two-photon vertex; (b) the two-photon vertex in the vector-meson dominance model; (c) the two- photon vertex in the 8@) dominance model; (d) the one- photon process with C-violation (due to the negative C-parity of the photon).

for the decay K + ~ 7r+H -~ 7r+/g+/~ - (M H > 350 MeV/c) 2 Another exotic example is a possible C-violation in electromagnetic interactions (as in the model of ref. [3]). In this case the processes ( 1 ) - ( 3 ) might develop by a one-photon approximation (fig. 1 d) with a prob- abili ty much higher than given by the two-photon esti- mations.

The present experiment has been performed in the framework of a series of investigations for rare electro-

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Volume 105B, number 2,3 PHYSICS LETTERS 1 October 1981

magnetic decays of light mesons [4 -7 ] . The measure- ments were made in the negative pion beam of " 3 0 GeV/c of the IHEP 70 GeV accelerator using the Lepton-G set-up [8]. The binary reactions rr-p -+ ~n [9,10] and rr-p -+ r/'n [11 ] were used as r/and r/' sources, similar to the work in refs. [4,5,7]. During the running time a total effective flux of 5 X 1011 7r- passed through the set-up, resulting in a production of 2 X 107 r? and 107 r/' in the set-up target.

The measurement technique and data processing procedure are similar to those used in earlier experi- ments on observation and study of rare co -+ 7r0#+/a - decay [5,12]. The events corresponding to the exclu- sive reactions

rr-p -+ #+/~- 3'7n, (6)

rr-p -~/~+/~- 3"77n, (7)

were selected. In these reactions were studied the mass spectra of the rr0/~+t* - , 7/#+/1 - and rr0/l+/a - 7 systems. We see no evidence of any enhancement in the r /and r/' mass region.

Thus, the decays (1 ) - (4 ) have not been observed in the present experiment. High experimental sensi- tivity allows one to put low limits for the branching ratios of these processes, improving the limits obtain- ed earlier by some orders of magnitude (see table 1). For comparison the same table contains also the data on the decays with emission of an electron pair.

When calculating the set-up efficiency for the de- cays (1) - (4) several assumptions on the decay mecha- nism were used: vector-meson dominance, 8(e) domi-

Table 1 The upper limits for the branching ratios of the r/and r/' decays

Decay The upper Earlier The upper limits for the upper limits for branching ratio limits b) decay into BR a) (90 c.l., an electron present work) pair b), c)

r/ + *r°/a+ta- 5 X 10 -6 5 X 10 --4 4 X 10 -s r/'-+ *r01a+~- 6 X 10 -s 10 -2 r/' -+ r/ta+tz - 1.5 X 1W s 10 -2 r/ ~*rO/a+ju-'~, 3 X 10 -6

a) BR(rl ~ *r°ta+t~-) = F(r/-+ *r°#+ta-)/P(n -+ all) and similar for other decays, b) Ref. [13]. c) ,7 -+ *r°e+e -, r/' ~ rr° e+e -, r/' -+ rl e+C decays, respectively.

nance, phase-space model. The efficiency values ob- tained with the various models turn out to be the same within some percent. This shows that the boundary es- timates obtained are model independent.

As was noted above, the decays (1) - (3) may be used in the search for light Higgs bosons. Though the non-existing ideas concerning the nature of weak in- teractions are in favour of heavy Higgs particles, still it should be noted that there are in the models with several doublets of Higgs bosons no theoretical limita- tions for their mass. Therefore a search for Higgs par- ticles is to be carried out in the whole available mass range.

The theoretical situation has been treated in detail in papers [14,15]. A characteristic feature of Higgs- boson interaction with fermion fields is the growth of the coupling constant with fermion mass. Therefore, at masses M r / < 2 m-r, Higgs particles would decay mainly into a muon pair

H -+/~+/a-. (8)

At higher masses a new decay channel H -+ rrrr is open- ed, but when the mass M H is not very large mode (8) remains dominating:

BR(H -+/a+~ - ) ~> L 1 + \m---~H !

(1 2 2 1/2 - 4m~r/M~i)l/z l - 1 X (9)

(1 a~2/~/t2 a3/2]

(~> 0.9 at M H ~ 0.5 GeV/c2). In this, the lifetime of Higgs particles is r H < 10 -15 s.

It is natural to search for light Higgs bosons in pro- cesses of the type (5):

r/ -+ rr0H (10)

r/' -+ rr0H (11)

+_ . (12)

The decays (10)- (12) are semiweak, their branching ratio should be BR "" 10 -5 . Thus, our experiment has a sufficient sensitivity for the search for light Higgs particles.

More detailed calculations of the decay (10) [14,

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Volume 105B, number 2,3 PHYSICS LETTERS 1 October 1981

H Q)

n: °

H b)

-r I r~ °

• / H c)

~l ~ o

Fig. 2. Feynman diagram for the decay P ' ~ PH: (a) is the di- rect production of Higgs particles; (b), (c) are pole graphs of Higgs particle production.

15] showed that this decay seems to be highly sup- pressed by compensation of the Higgs-boson direct production diagrams and pole diagrams (fig. 2). Ac- cording to Vainshtein [15] in the decays (11), (12) the pole term contribution is small, and the diagram of di- rect interaction is dominating (fig. 2a). This is con- nected with the peculiarity o f the rl '-meson, whose mass is not proport ional to the quark masses. The cor- responding estimates for the branching ratios are

BR(rt ' ~ PH) - * 2 2 , - PH g~'eH/81rm~'P07 ~ al l ) . (13)

Here P = 7r0,rT; p h is the Higgs-boson momentum in

10

~8

r r t,o

2

o

250

theory

300 350 400

M H [MeV/c 2 ]

Fig. 3. The data on the decay n' ~ r~Ht~.+ _ . _ _ , , The upper curve

is the theoretical estimate of the branching ratio with formula (13). The lower curve shows our experimental upper limits (90% c.1.).

the r/' rest flame. The interaction constants are equal to

g,,IrOH = (2/3)1/2'm 2 [(m d -- mu)/(m d + mu) ]

X (2-1/2G) 1/2, (14)

, H = - m 2) ( 2 - 1 / 2 a ) 1 / 2 , ( 1 5 )

mu, d are the masses of the u and d quarks. 2-1 /2G is the weak interaction constant.

The calculation results on the decay (12) are pre- sented in fig. 3 together with our experimental upper limits. As is clear from the figure the experiment al- lows us to exclude the existence of Higgs bosons on the whole kinematical range available in the decay (12), i.e.

M H > 409 MeV/c 2

[the interaction constants being accepted to be as in (15)]. The data on the decay (11) are not sensitive enough for the search for Higgs particles.

The authors would like to thank V.V. Bazhanov, G.P. Pron'ko, M.A. Shifman, L.D. Soloviev, A.I. Vainshtein and O.L. Zorin for the discussions o f the results.

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