Triple gauge boson decay of heavy Higgs bosons

Volume 217, number 1,2

PHYSICS LETTERS B

19 January 1989

T R I P L E G A U G E B O S O N DECAY O F HEAVY H I G G S B O S O N S ¢* T h o m a s G. R I Z Z O

Ames Laboratory and Department of Physics, Iowa State University,Ames, Iowa 50011, USA Received 27 October 1988

The decay of heavy Higgs scalars into three gauge boson final states (e.g., W+W Z) is examined. We find that such final states can have significant ( > 0.03%) branching ratios for Higgs masses > 750 GeV. Such final states may provide an additional signal and probe (but with very low rate) for Higgs production at the SSC and provide information on the trilinear gauge couplings.

Interest [1] has been recently renewed [ 2 - 4 ] in the possibility o f p r o d u c i n g triple gauge boson final states at h a d r o n and e+e colliders. Such processes m a y allow for further tests o f the non-abelian nature of the gauge boson self-couplings. In this p a p e r we examine the possibility o f a significant triple gauge boson branching fraction in the decay of a heavy Higgs boson, H. As in well-known [ 5 ], once H is sufficiently heavy its m a j o r decay m o d e s involve pairs o f gauge bosons ( W + W - , 2Z) which are d o m i n a n t l y longitudinal especially in the large Higgs mass (mH) limit. This enhancement in the branching fraction for gauge boson pair final states is thus due to the enhanced coupling o f the longitudinal m o d e s by factors o f ~ mH/mw. We m a y expect, therefore, that H decay into three longitudinal gauge bosons might be even further enhanced ( ~ m H2/ m W"~) and lead to observable rates. It should be noted that in the G o l d stone limit the rate for this process is zero. The only purely longitudinal final state accessible from an initial neutral scalar is W ~-W E ZL which proceeds via the three graphs in fig. 1. The resulting matrix element for this process is somewhat complicated:

M=M~+Mb+M~,

(1)

•.

~=

Wx(Pz) Z~(p~)

H(p)

"-

(a)

W:(pt)

W~

Cb) "

WW~w,' Z~

w-~ u

H

~

(C)

Fig. 1. Graphs responsible for the decay H-,W+W-Z.

Ma =ig2cwMwe "+e~L~ X [g,,' --Mw2(p2 +P3)v(P2 +P3)"' ]

where (Cw-COS 0w) X [ (P2 + P 3 ) Z - - M w ] - l [ (P2 --t03).' g,o. This work was supported by the US Department of Energy, Contract No. W-7405-Eng-82, Office of Energy Research (KA01-01 ), Division of High Energy and Nuclear Physics. 0 3 7 0 - 2 6 9 3 / 8 9 / $ 03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing D i v i s i o n )

-- (2p2 + P 3 ) , g , ' a + (2p3 +P2)~gv'v], Mb=-Ma(P,~-'p2,

(2)

v~-+Z), 191

Volume 217, number 1,2

PHYSICS LETTERS B

M,. = + ig2Mz E~_e ~_e~

is in a AM bin 100 GeV on either side of MH, i.e., what is

X [g~u~,.--mz-2(pl -[-P2),,(Pl +P2),,, ] X [(p, + p 2 ) 2 - M z ] - ~ [ (2p, +P2)~g,,,

0"BK ~ -

- (P~ - P 2 ) , . g , ; - (2p2 +p~ ) , g , x].

(2 c o n t ' d )

In the limit of large M ~ / M w and taking only the longitudinal parts of the W +, W - and Z in the final state, eq. ( 2 ) simplifies greatly. We obtain

GFM~(x2 - x 3 ) c 2 ,

M a ----+ i , ~

Mb = -ix/-2 GFM~(x. -x3)Caw, M~ -- + i x / 2 GFM2(x. - x 2 ) ,

(2')

where x , = 2 P . p f f M ~ and Y~x,=2. Thus we obtain (Xw= sin20w) M = + i,,/2 GFMH(X, --X2)Xw,

(3)

SO that, with O<~x~<~1, 1 -x~ ~x2 <~1, we obtain &F(H~WL

+

-

WL Z c ) / d x l

dx 2

= (GvM~/128zr3)X2w(Xl - x 2 ) 2.

(4)

Now, in the same limit, for two gauge boson final (2GB) states we have F(H--, Wff W~ ) = 2F(H--* 2Zc)

= G v M 3 /8xf2 :~.

(5)

Thus, the branching fraction for the W ~-W ~-Zc final state is given by

F( H - , W~ WC Zc )/F(H--' 2GB ) = ( G v M ~ / 1 4 4 , f 2 ~z2)x 2 = 3 . 0 7 × 1 0 - 4 ( M ~ / 1 TeV) 2

(6)

which is the result advertised. Is such a triple gauge boson final state observable at the SSC? If we take M H = 0 . 8 TeV, rn,= 100 GeV and make a rapidity cut [Y l ~<2.5, the value of o.B for this process is [5] -~ 1.3× 10 -3 pb which is spread over an invariant mass bin of size AM-~ 100 GeV on either side of mH. With an integrated luminosity of 105 pb-~ at a high luminosity SSC this corresponds to only 130 events. The background from continuum W + W - Z production can be easily estimated although a complete calculation is surely necessary. Barger and Han [2] find that o ( W + W - Z ) at x/s-40 TeV is ~ 0.5 pb but how much of this background 192

19 January 1989

f

,44 -- A M

d M dowwz ~ 2AM ~d o w w z dM d

.~t..... (7)

For M H = 0 . 8 TeV, we estimate d a / d M I M _ ~ / , _~ 5 × 10 -5 pb/GeV, based on the kinematic behavior of the p p - , 2 Z , W + W - cross sections, and combining this with the results of ref. [4] leads to OBK--~9 × 10- 3 pb. Although somewhat crude, this estimate shows that this background is larger than the signal by ~ 7. The H production signal would thus appear as an excess of events over the background. The total sample (signal plus background) would still be only ~ 1030 events/yr at this luminosity. To reduce backgrounds from Q C D we consider the Z and one of the W's decaying leptonically while the second W decays to jets. Thus the signal is three charged leptons, two jets plus missing PT, with two leptons forming a Z and the 2 jets a W. Assuming x's are not identified this reduces our -~ 1030 events/yr to 23 events/ yr with the remaining Q C D background coming from W Z + 2 jets. One might expect this background to be small after cuts on 2-jet invariant mass around Mw are made. (This rate goes up by a factor of ~-2-45 events/yr if T's are identified with 100% efficiency. ) Although this signal rate is quite small it may be observable. With approximately 900 background and 130 signal events this would imply (statistically) a 4o increase in the number of observed events due to the H decay process. This situation should be contrasted with the more conventional case where the event rate for pp~H~W+W is large but one is swamped by the W + W - continuum unless very extensive cuts af~ made. Here, on the other hand, signal and background from continuum W + W - Z production are more comparable without any judicious choice of cuts and one looks for an excess of events. Before concluding, we have for completeness examined the decay H - , W ~ WL-7 ~1 which proceeds in

~J The decay H-*WW7 has been examined previously by Rizzo, and by Dicus et al. [ 6 ].

Volume 217, number 1,2

PHYSICS LETTERS B

a m a n n e r similar to that shown in fig. 1 for W+W~-ZL except that Z ~ 7 and fig. lc is absent. In the case of M~/M~v >> 1 and both W's longitudinal we find mwwv = - (ieg/ Mw ) ( P, -P2 )" ~,

(8)

and therefore F ( H - * W + W~ 7 ) = (a/z~)GvM~/8,~/2 zr = ( a / ~ z ) / ' ( H ~ WL~ WL )

(9)

which is infrared finite. This result should be contrasted with the more complete result in ref. [6] which includes transverse as well as longitudinal W's in the final state. We have considered the possibility of searching for the decay H ~ W ~ - W E ZL as a signal for heavy Higgs bosons and a probe of the trilinear gauge couplings at the SSC. Although the estimated signal plus background is reasonably small, the event rate ( -~ 23-45 e v e n t s / y r in the above c h a n n e l ) is sufficient so that the observation of the Higgs in this channel is difficult but may not be impossible. Serious Monte Carlo

19 January 1989

studies of this decay signature need to be made before one can come to stronger conclusions. The author would like to thank V. Barger, H. Georgi and J.L. Hewett for discussions related to this work.

Note added. After this work was completed we received a copy of refs. [4,7], where the possibility of the H - ~ W + W - Z decay has also been briefly discussed in the context o f e + e annihilation.

References [ 1] M. Golden and S. Sharpe, Nucl. Phys. B 261 ( 1985 ) 217. [2] V. Barger and T. Han, Phys. Len. B 212 (1988) 117. [3] C. Ahn et al., SLAC report-329 (1988). [4] V. Barger, T. Han and R.J.N. Phillips, University of Wisconsin report MAD/PH/420 ( 1988). [5] E. Eichten et al., Rev. Mod. Phys. 56 (1984) 579. [6] T.G. Rizzo, Phys. Rev. D 31 (1985) 2366; D. Dicus et al., Phys. Rev. D 34 ( 1986 ) 2157. [7] A. Tofighi-Niakiand J.F. Gunion, U.C. Davis report UCD88-24 (1988).

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