Heavy top production as a source of WW events

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HEAVY TOP PRODUCTION AS A SOURCE OF WW EVENTS P. COLAS and D. DENEGRI D@artement de Physique des Partwules Elbmentazres, CEN-Saclay, F-91191 Glf-sur- Yvette Cedex, France Received 24 November 1986

We discuss some consequences of the production of a top quark of mass comparable to Mw. Such a top quark decays into an on-shell W plus a b-quark For mtop ~ M w , the Q C D tt production cross section, already sizeable at SppS energies, increases rapidly with x ~ This, combined with the pecuhar kinematics of the top decay for mtop~Mw, results m the production of apparent W W pairs at a rate overwhelming the electroweak W W cross section. This W W production m e c h a m s m would also represent an important contribution to the inclusive hlgh-Pt W productton tad, even for masses of top (or hagher generahon t', b' quarks) substantially larger than Mw.

1. Introductton. Since the discovery of the ~ lepton [1] and the b quark [2], the sixth "top" quark is expected to complete the present three families of fermions. Experimental searches and theoretical arguments limit the top mass to the range 23 GeV
Moreover, there is almost no b-jet as the b comes from the weak decay of top with little Q-value. Furthermore, no top jet should be produced, as for massive top, the fragmentation is expected to be extremely hard [7]. The W decay electron is thus very well isolated. The top transverse momentum distribution itself is governed by the heavy top t-channel exchange gluon-gluon~tt diagram, which is the dominant QCD top production diagram at large g [ 8]. This gives an average p~OO of the order of rntop/2, i.e. ~ Mw/2 in this regime. The net result is an abundant QCD production of apparent WW pairs, the two soft b-decays being difficult to distinguish from the event background and the usual QCD gluon bremsstrahlung jets without an explicit b-jet signature. This QCD production cross section of order a2/g overwhelms the electroweak WW production of order ot2/g for a large domain of m o < 3 0 0 GeV as discussed below. However the confusion between the two mechanisms decreases rapidly with increasing mtop as the decay b-quarks become harder and develop recognizeable jets. At x/s= 2 TeV, for example, the QCD cross section for tt~WWbl3 still exceeds by an order of magnitude the electroweak WW cross section for m~op---140 GeV, and by more than two orders of magnitude at v/~= 10 TeV. This mechanism may therefore represent an additional large background to both the search for elec-

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troweak WW events and to a heavy Higgs (MH > 2Mw) search at hadron colhders as the LHC or SSC (the WZ and ZZ modes are not hampered, however). Of course this difficulty is not present in e + e - colliders.

2. Expected cross secttons and event rates. Fig. 1 shows the variatmn of the tt production cross section versus mtop at ,,/~=630 GeV and 2 TeV. Two QCD Monte Carlo programs, ISAJET [ 9 ] and EUROJET [ 10] have been used to estimate cross sections. To illustrate the theoretical uncertainties on this estimate, we show for example the EUROJET predictions for various choices of structure functions at x/~= 630 GeV. An additional variability comes from the choice of the Q2 scale parameter. We used Q2= (p~OV)+ mZop. The most convenient way to detect such f f ~ W W ( b b ) events is through W ( ~ e v ) W ( ~ j e t - j e t ) modes. At ~/s = 630 GeV for m t o , - 90 GeV we have aa ~ 10 pb. With branching ratios B R ( t ~ W b ) = 1, B R ( W ~ e v ) = I/9 (the W ~ t 6 channel is closed), 1000

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B R ( W ~ j e t - j e t ) - 0 . 7 , assummg for W ~ e v a detection efficiency of ~=0.6 as observed in UA1 [ 11 ], and with the two possible W ~ e v decays per event, this would lead to the observation of 0.9 such events for an experimental sensitivity f L d t = 1 p b - i. The right-hand scale in fig. 1 shows the expected number of events for this CERN SpaS regime. At Fermilab energies in comparable experimental conditions about 50 W(--,ev)W(~jet-jet) events would be expected.

3. Expected M(eu), p~°P, pb and pW dzstrtbuttons. Fig. 2 shows the expected evolution of the ev effective mass for the t ~ b e v decay as rntooapproaches and exceeds Mw (curves normalized to same number of events). As mtop--*Mw the M~v distribution varies rapidly until, switching from a three-body to a twobody decay regime, the W's are produced on mass shell. The distribution takes then its expected Breit-Wigner shape. Notice that in the calculation

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mr=p(5eV ) Fig 1. ff productmn cross sectmn as a function of the top mass' At x/s= 630 GeV for vinous structure funcUons(GHR= Gluck, Hoffman and Reya; DO=Duke and Owens; EHLQ--Eichten, Hmchhffe, Lane and Qulgg;references can be found in ref. [ 13] At x/s= 2 TeV with structure functions of EHLQA= 0.29 GeV. 296

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particularly striking at threshold for m t o p - M w + mb as can be seen in fig. 4 where the l a b o r a t o r y b quark transverse m o m e n t u m is shown for various mto p values. Notice that as rntop increases the pb d i s t r i b u t i o n first shrinks, reaches a m i n i m u m ( p ~ ) at rntop-~ 90 G e V a n d then b r o a d e n s again. These distributions are similar at x / s = 2 TeV, the average value o f p~ increasing by less than 15% as c o m p a r e d to 630 G e V

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the top was a s s u m e d t o be on-shell a n d the effects o f its finite width have been neglected. Fig. 3. shows the ( n o r m a l i z e d ) top transverse m o m e n t u m d i s t r i b u t i o n for rn~op=60, 80 a n d 120 GeV, as given by the E U R O J E T M o n t e Carlo. The average p~°V increases with mtop a n d is o f the o r d e r o f mtoo/2 as expected. It increases by 25% to 40% from x / s = 630 G e V to 2 TeV as mtop varies from 80 to 140 GeV, the k i n e m a t i c a l suppression being i m p o r t a n t at 630 G e V for large masses (see table 1 ). In the subsequent t--+Wb decay the W takes the larger fraction o f this transverse m o m e n t u m . This is

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Fig. 4. Transverse momentum distribution of the b, for varaous values of the top mass, at ,,/s= 630 GeV (sohd hnes) and x/s=2 TeV (dashed hne).

Table 1 Kanematlcal properties for pO~tt~WWbl~, at x/s= 630 GeV and 2 TeV mtop (GeV)

80 90 100 120

(p~°P)(GeV/c) (p~)(GeV/c)

FracUon of events with

(pW) (GeV/c)

630 GeV

2 TeV 630 GeV

2 TeV p~ <10, 630GeV

p~<20, 630 GeV 2 TeV

2 TeV

43 45 47 53

54 57 63 74

23 12 17 29

0 55 0.84 0.64 0 22

40 46 49 53

20 12 16 28

0 25 0.67 0.20 0.06

33 35 37 40

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Fig 5 InclusiveW transverse momentum distribution at (a) .,/]=630 GeV, (b) , ~ = 1.6 TeV, for: The usual QCD W production from ref. [13] (thlck curve) The process described m this paper, for varaous top masses (thin curves). For a W shghtly of-shell (dashed curve)

for 80
m~op---80, 90 and 100 GeV. The latter ptw contribution is clearly harder than for single W production and becomes comparable at p W > 4 0 GeV/c. At x/~= 1.6 TeV (fig. 5b), it is even more striking, as the single W total cross section increases by a factor of about 4 with ( p tw ) increasing by 60% [ 14], while the tt cross section increases by a factor of about 40 (fig. 1 ) and the corresponding (ptw ) by about 30%, when compared to x/~=630 GeV. Naturally the structure of the jets recoiling against the large-pt W's from these two mechanisms are different: a single large-pt jet predominantly in the former case and for tt a di-jet system from the hadronic shown the electroweak WW and WZ production cross

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are therefore kinematically compatible with the production of a pair of real or quasi-real W's. According to the mechanism described here we would expect ~0.3 events at pW > 4 0 GeV/c per leptonic decay mode for mtop=90 GeV and f L dt=0.8 pb -~, not too much below the experimental observation.

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Fig. 6 Expectedtt cross section as a funcUon of.~, for various values of mtop (sohd curves), compared to the expected electroweak WW and WZ production cross section (dashed curves) decay of the W with Mjet_jet~MW. The transverse energies of these two jets are expected to be rather unbalanced, as are the two leptons from W decays at large pW. In this respect it is worth noticing that UA1 reported the observation of two large-pt W events [ 13] (one in the W ~ e v mode and one in the W ~ t v mode), at pW~>80 GeV/c, which admittedly is somewhat high (fig. 5), but with large uncertainties on pW. These events are difficult to accommodate within standard QCD predictions [13,14]. It is remarkable that both have a hard di-jet recoil system with Mjet_jet compatible with a W mass. The e-v-jet-jet events C and D among the peculiar UA2 events [15] are of this type, with Mjet_jet~65 and ~ 90 GeV, respectively, and pW ~ 40 GeV for both events. In event C, there is also a third jet o f E t ~ 7 GeV which in this interpretation could be a manifestation of one of the decay b's (fig. 4). These events

4. t i ~ W W versus electroweak W W production cross secttons at htgher energies. In fig. 6 we show the expected variation of the tt production cross section with center-of-mass energy up to x/~=20 TeV, for various values of m~op> Mw. On the same figure are sections [8]. There is clearly an important uncertainty in these QCD cross sections; nonetheless for mtop~ 100 GeV this source of WW events clearly would exceed electroweak WW production by more than two orders of magnitude at ~ = 20 TeV. For top masses in this range it would be therefore very difficult to separate the electroweak production or a H l g g s ~ W W signal from this overwhelming QCD background. However, the electroweak WZ and ZZ processes do not suffer from this competition, therefore a H i g g s ~ Z Z ~ f o u r leptons search could still be possible with enough luminosity. With increasing top mass, however, the QCD background to electroweak WW production decreases since both the tt production cross section decreases and the t t ~ WWbb event structure with two clearly observable additional b-jets (Q-value becomes large) allows further background suppression. Clearly all the discussion made here for the top quark would be equally valid for a fourth generation b'-quark of mass mb,>Mw, provided mtop>mb, tO ensure dominant b ' - - , W + u/c quark decays. It would also be valid for a fourth-generation doublet (t', b ' ) with mr, - rob, > Mw and rob, << Mw, wath appropriately scaled cross sections. We wish to thank I. Ten Have and B. Van Eijk for their help in running the EUROJET Monte Carlo.

References [ 1]M.L Perl et al., Phys. Rev Lett. 35 (1975) 1489. [2] S.W. Herb et al, Phys. Rev. Lett. 39 (1977) 252 [3] F Halzen, Phys. Lett. B 182 (1986) 388; P. Colas, D Denegriand C. Stubenrauch, UA1 TN/87-18. [4] CELLO Collab., H.-J Behrend et al., Phys. Lett B 144 (1984) 297 299

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[5] L. Maaant, Proc 21st Intern. Conf on High energy physics (Paris, 1982), M Veltman, Nucl Phys B 123 (1977) 89 [6] UA1 Collab, G Armson et al, Phys Lett B 147 (1984) 493 [7] C. Peterson et al., Phys. Rev D27 (1983) 105 [ 8 ] E Elchten et al., Rev. Mod. Phys. 56 ( 1984 ) 579. [9] F E Paige and S.D Protopopescu, an: Supercolllder physics, ed. D. Soper (World Scientific, Singapore, 1986) p 41. [10] A. Ah, B Van E1jk and I Ten Have, Nuel. Phys B 291 (1987) 1 [ 11 ] UA1 Collab, G. Arnlson et al., Lett Nuovo Clmento 44 (1985) 1.

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[ 12] C Stubenrauch, Proc XXI6me Recontre de Monond (Les Arcs, 1986), D. Denegn, Proc. 6th Topical Workshop on Proton-antlproton colhder physics (Aachen, July 1986), eds. K. Eggert, H. Falssner and E. Radermacher. [ 13] UA1 Collab, C Albajar et al, Phys. Lett B 193 (1987) 389. [ 14] G. Altarelh et al, Z. Phys. C 27 (1985) 617. [15] UA2 Collab., P Bagnala et al, Phys Lett. B 139 (1984) 105, see also UA2 Collab., R Ansan et al., Phys Lett B 186 (1987) 422