Vector boson pairs from top-antitop-Higgs production

Physics Letters B 287 ( 1992 ) 231-236 North-Holland P H YS I C $ L E TT E R $ 8

Vector boson pairs from top-antitop-Higgs production

Alessandro Ballcstrero a and Ezio Maina a,b a INFN, Sezione di Torino, I-I0125 Turin, Italy b Dipartimento di Fisica Teorica, Universitt~ di Torino, 1-10125 Turin, Italy

Received 15 April 1992

We study the possibility of detecting the Higgs in association with energetic and isolated leptons. These rare final states could be useful in dctcrmining the structure of the Higgs sector. The reactions ttH--,~77, ttH--,WW~3~ or 4L ttH--.l~Z),--,~7, ttH ~ ZZ-. 5~ and ttH--. ~tttt are discussed. We compute the irreducible as well as some of the reducible backgrounds.

I f the Higgs particle exists the next generation of colliders will discover it, measuring some increase in a suitable cross section, or will prove that the Higgs is too heavy to fit in our present perturbative under- standing of electroweak physics. Several studies [ 1 ] have shown that LEP I and II will explore the mass range mH ~< 80 GeV using e - e +--,ZH. The best channel at hadron coUiders in the mass region 80 ~ rnH ~< 130 GeV is the associated production of the Higgs with a W boson [ 2 ] or a t( pair [ 3 ] followed by the decay H-, ' /7. I f detectors have good enough electro- magnetic resolution the intermediate mass region will be satisfactorily covered. For mH >/130 GeV inclusive production of Higgs bosons decaying to two, possibly virtual, Z's will allow detection in the four- lepton channel up to masses of several hundreds GeV, the precise reach depending on machine energy and luminosity [ 4,5 ].

Once a charge zero, spin zero object is found one would like to determine its properties in as much detail as possible. In particular one would like to know whether it can be adequately described by the standard model or whether it requires a modified theory, Probably the best way to answer this kind of question is to try to measure the couplings of the Higgs, since their proportionality to the particle masses in the standard model is in general altered by some sort of mixing mechanism in more complicated theories.

* Work supported in part by Ministero dell' Universit/l e della Ricerca Scientifica.

This in turn requires detecting the Higgs in as many channels as feasible, but we will see that only in very small mass intervals it is possible to detect the Higgs in more than one reaction. I f SSC will operate solely at design luminosity this goal will be extremely difficult to achieve. At LHC prospects are even worse. I f a luminosity substantially higher than 1033 cm-2 s - 1 cannot be achieved and dealt with by the detectors, it is doubtful whether the whole Higgs mass region around 100 GeV can be explored. On the other hand, it has been shown that in the intermediate mass region, with high luminosity (1034 cm -2 s - l ) at SSC, both WH and ttH production mechanisms yield enough events for discovery in the two-photon channel. It has been suggested by Gunion [ 3 ] that the two contributions could be separated on the basis of the different jet structure and that, therefore, the ratio of the coupling of the Higgs to the top and to the W could be measured. IfmH is not much above 130 GeV these two measurements could be combined with the inclusive four-lepton-mode result, allowing further checks of the theory. It is interesting that the preci- sion of experiments probing the structure of electroweak interactions has improved to the point that global fits are starting to put limits on the Higgs mass, and that rather low values mH < 160 GeV (68% con- fidence level) appear to be favored [ 6 ].

The mass region between 80 and 130 GeV, being the most difficult range, has been explored in great detail and all relevant decay modes have been con-

0370-2693/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved. 231

Volume 287, number 1,2,3 PHYSICS LETTERS B 6 August 1992

sidered in the literature. This is not yet true for smaller and larger masses.

At hadron colliders, since rn~> mw + mb, most of the large decay channels, H ~ b 6 and H - ~ z for mH<2mz and H ~ t t above threshold, are over- whelmed, in the inclusive mode, by tt and bb production and decay. H - . W W may be observed for mr> 150 GeV [7]. H ~ Y t has been shown to be visible, at design luminosity, only with extremely high photon-photon mass resolution and with very good photon-jet discriminating power [4,5 ].

It might be therefore advantageous to search for events in which the Higgs is associated with energetic and isolated charged leptons from heavy quark or W decay. This requirement greatly reduces the background and more than compensates the lower event rate, as the analysis of the intermediate mass region has demonstrated. For masses above 130 GeV and below 500 GeV the dominant Higgs production mechanism is expected to be gluon fusion through a top loop. The event rate could be modified by a non- standard Higgs-top coupling or by additional strongly interacting heavy particles circulating in the loop. I f it were possible to detect at the same time the Higgs in association with a top this would check both possibilities.

In this paper we systematically study the decay channels of the Higgs, looking for observable signatures from Higgs production in association with a tt pair at hadron supercolliders. All original calcula- tions have been performed at the amplitude level fol- lowing the method of ref. [8 ]. The HMRSB set of distribution functions has been used [ 9 ] throughout. The strong coupling constant and the distribution functions have consistently been evaluated at a scale equal to the subprocess total invariant mass. We have used the two-loop expression for as with A ~ = 190 MeV. Changing the scale and/or the distribution functions should not affect our absolute predictions by more than a factor of two.

For m , = 150 GeV the ttH cross section is about 1.5X 103 fb at SSC decreasing to 103 fb for m n = 2 0 0 GeV, mt = 100 GeV. For m n = 400 GeV, m, = 100 GeV it is still approximately 3.5 X 10 2 lb. The corresponding figures at LHC are 200 fb, 120 fb and 40 lb. WH production is in general lower at SSC than for ttH, while it is slightly higher at LHC. In this region only the Higgs decay to WW, ZZ and tt ( if kinemat- ically allowed) have branching ratios larger than 10- 4

and produce a sizable number of events per year. The five-lepton mode t tH~ttZZ-- ,5£ with a com-

bined branching ratio of about 3.6X10 -4 for mH = 200 GeV produces only approximately 35 (3.5) raw events per SSC year at high (standard) luminosity. At LHC the rate is about seven times lower with the same luminosity. On the other hand it is very difficult to imagine a QCD reaction capable of deliver- ing or faking five leptons, four of them with a total invariant mass in the neighbourhood of the already known Higgs and grouped in two £+£- pairs, both in the Z mass window. The irreducible background pp--.ttZZ and p p ~ b 6 Z Z are the only processes which could give a competing signal. We have computed the ZZ mass distribution for these two reactions, obtaining dty/dMzz(ttZZ) ~ 5 X 10-= fb /GeV at Mzz = 200-250 GeV at SSC. This background turns out to be totally negligible and probably undetectable even at high luminosity once the appropriate branching ratios are folded in. The cross section for pp--,b6ZZ is about two orders of magnitude larger but still unimportant if the large reduction factor which can be obtairled with standard cuts on the lepton from b decay is taken into account. The acciden- tal coincidence of a Higgs decaying to ZZ and of one additional W or Z producing five leptons can be estimated at less than one event per year. Therefore, t tH-qtZZ-- ,5£ is free or background and despite its small rate it could provide, given enough luminosity, a very clean signal.

The Higgs decay to WW or tt can produce final states with three or four isolated and energetic charged leptons. In this case the Higgs mass cannot be reconstructed and it is necessary to consider the production of several leptons over a wide range in mass. The main background processes capable of producing three isolated leptons are WZ, tt, ttZ and ZZ with one lepton undetected. The prompts background ttWW cannot be much larger than ttZZ and therefore is of no concern. The contribution from four-heavy- quark production tttt, and ttbb and bbb6 has not been evaluated yet. The WZ, ZZ and ttZ background can be eliminated requiring events in which no lepton pair has an invariant mass in an appropriate interval centered around the Z mass. This has a small impact on the signal as will be discussed later in more detail. The most dangerous background is therefore tt production.

232


A detailed analysis of t t events at LHC is presented in ref. [ 10 ] and therefore we will concentrate on LHC energies. The branching ratio for the three- and four- lepton channel are at least 0 . 7 × 4 × ( 0 . 2 ) 3 × 0 . 7 ~ 1.6× 10 -2 and 0 .7× (0 .2)4~ 1.1 × 10 -3 respectively for Higgs masses above 150 GeV. The cross section a(t(H--,4W--,3~) is 3.2 fb for m n = 150 GeV, and 2 (1.2) fb for mr= 100 (200) GeV and mH----- 200 GeV. For a( t tH--*4W~4~) we have 0.22 fb for mH= 150 GeV, and 0.11 (0.07) fb for mt = 100 (200) GeV and rnn = 200 GeV. The cross section a( t t~(WW--.£v~v)bfi) with £=e or ~t, requiring p~r>/50 GeV/c and l~l~<l.5 is about 2×104 ( 8 × 10 3) fb for rnt= 100 (200) GeV. Requiring an additional lepton from one of the b quarks reduces the cross section by a semileptonic BR= 2 × 2/9 and by an additional factor depending on the PT, t/ and isolation cuts. The probability of producing a third high Pa- lepton depends quite strongly on the top mass (fig. 5.2, ref. [ 10] ). Moreover, the effectiveness of the isolation requirement depends on the PT threshold (fig. 3.9, ref. [ 10] ). A possible strategy for max- imizing the t tH to tt ratio is to impose the same set of cuts on all three leptons. Therefore we have studied the signal to background ratio requiring I r/~l ~< 1.5 and p~ >1 50 GeV/c. We have estimated that about one half of the leptons from b decay pass the PT threshold for mt= 200 GeV compared with only about 0.5% for rn~= 100 GeV. Provided the rejection factors obtained for bl~ production [ 10 ] can be applied to secondary b quarks in tt events, further isolation cuts (XET< 10 GeV in a cone AR<0.4) could re- duce the background cross section by a factor of about 55. Applying these reduction factors leads to a final estimate for the t t - , 3~ rate of 0.8 fb for m t = 100 GeV and 12.8 fb for mr=200 GeV. The same set of cuts have been imposed on t r ( t tH- ,4W-,3~) obtaining about 0.2 fb for both top masses and rnn=200 GeV. Requiring no lepton pair with ] r n ~ - mz I < 10 GeV less than 15% of the signal is lost. These numbers do not look very encouraging at first sight but there are several factors to be taken into account. First of all the results of ref. [ 10 ] include full next-to-leading corrections which are known to increase the cross section by about 50%. Moreover, the scale used in the calculation is QE=m2 +p2 which is much smaller than the scale Q2=g adopted in this paper. It is ob-

vious that the higher order corrections and the scale dependence for the two processes should be very similar. As a comparison, the cross section gg--,tt at Born level, with the parametrization used in the ttH calculation, is equal to 0.27 nb for mr---200 GeV, about ( 15-20)% of the estimate of ref. [ 10]. Second, the set of cuts applied in ref. [ 10 ] have been selected in order to maximize the tt signal compared to the bb background and not in order to suppress tt. It is clear for instance that a stronger isolation cut on one of the three leptons would have a much larger effect on tt events than on ttWW ones. Therefore we conclude that our exercise shows that with 100 fb - ~ at LHC it might be possible, though very difficult, to detect a signal of the H ~ W W decay, particularly if the top is not too heavy, with an appropriate combination of cuts. We believe a realistic re-evaluation of the tt background is required, including hadronization and pile-up effects, with a special stress on the efficiency of isolation cuts on events with several leptons. The cross section at SSC is about six times larger and would yield a good signal at high luminosity. Let us finally recall that the WH mechanism does increase the signal even though it complicates its interpretation.

In the four lepton channel the total cross section times branching ratio is 1.5 fb at SSC and only about 0.22 fb at LHC with mH = 150 GeV and no cuts. The tt background is large [ 11 ]. It can be substantially reduced by the imposition of high Pa- and isolation cuts on all four leptons, but it is doubtful whether enough of the handful of signal events survives after, even relatively mild, cuts.

Below threshold the two most promising reactions are t tH~£77 and t t H ~ Z ~ / ~ £ ~ ' / . The first channel has already been shown to be the best possibility for discovering the Higgs for 80~<mn~< 130 GeV. Our previous study has been extended to higher and lower masses in order to determine in what range of mH and rnt the signal is visible above background. Notice that at LEP II the expected rate is so low that only the dominant H--.bl3 decay can be seen [ 1 ], therefore it is important to look for signature of the Higgs in this mass range at hadron colliders. In figs. 1 and 2 we compare the expected number of prompt photon ttT7 events in a window of 6 GeV centered around the Higgs mass, with the number of events from pp--,ttH--,ttqq for Higgs masses from 50 GeV, just

233


20 ,

mr=200 X/S=40 TeV

"~ 7 j "- 120 " ' . ,

~ ~ . - . . . . . . . . . . . . . . . . . . . ,%' , , ] , ,o ..:::';; . . . . ; ; . .

150

200

I I I I ~ I ' I

50 70 90 110 130 150 170

M~ (GeV) Fig. I. Expected number of t{ (--*~X)'~ events at SSC, in one year at the design luminosity, with the cuts described in the text, from prompt photons (continuous curves) and from ttH (dashed) in a window AM= 6 GeV centered around the Higgs mass.

mr=200 ~/s=16 TeV IO0 _ . ~ . . ~ ' - ' - - - - - - - - - . [ [ " - . .

. . . . . . . . "-:?.. 2o 17o -: .f;.f? . ~ 10o ---.-...,,

o . . ; Y : : " ;',, 2

200 " ~

I J I I I I I , I

50 70 90 110 130 150 170

M:,r (GeV) Fig. 2. Expected number of tt(--,~X)?7 events at LHC, in one year at high luminosity, with the cuts described in the text, from prompt photons (continuous curves) and from ttH (dashed) in a window AM= 6 GeV centered around the Higgs mass.

above the present LEP limit, to 170 GeV at SSC and LHC respectively. The cuts AR(7~, 72 )>0 .4 , AR(£, 7 ) > 0 . 4 , PT(L ~ / )>20 GeV, Ir/(~, y) 1<2.5 and IPx(7] ) +PT (72) I > 30 GeV have been appl ied to both signal and background. S tandard luminosi ty L = 10 f b - ~ at SSC and high luminos i ty L = 100 f b - l at LHC have been assumed. The branching rat io BR (H-* )7 )

includes Q C D correct ions to the H ~ b 6 width which increase the lowest order result by a factor o f two. QCD correct ions to the H - , ?y width are neglected but have been shown in ref. [12 ] to be very small. The branching rat io B R ( t L ~ v X ) =0.4 , ~ = e or It, is included. The results presented in figs. 1 and 2 differ slightly from our previous ones [ 3 ]. This is due to

234


the different choice for ors, the different [I~r(T~)+PT(T2) I CUt and, for a few percent, to an error in the old version of the routine implementing the cuts, which has been now eliminated. To the prompt background computed in this paper one must add the QCD background studied by Mangano [2], for which we have taken the intermediate estimate with photon-jet rejection factor R~j~ 5× 10 - 4 , and the b6"/7 and the W?7 background discussed by Kunszt et al. [ 3 ]. In the low mass range the least fa- vorable combination of masses is mH= 50 GeV and m t = 100 GeV. Rescaling all backgrounds to a 6 GeV window and adding about two signal events for the WH mechanism leads to a signal to background ratio at SSC of S/x/~= 8 / x / / ~ = 1.33. Therefore with an integrated luminosity L = 50 f b - ~ a ratio S/xfB.~ 3 could be achieved even for this extreme case. The expected number of signal events at SSC is higher than six up to masses of order mrs= 150 GeV while the background rates slowly decrease with increasing mH. Therefore with a few times the assumed luminosity this channel can be detected for all top masses com- patible with present limits within the standard model and for all Higgs masses between 50 and 150 GeV. Beyond mH = 150 GeV the signal drops rapidly and even with ten times the design luminosity it becomes very difficult to obtain a sufficient number of events. Similar conclusions are expected to hold for LHC, but a complete analysis of the QCD background is not available. We have tried different combinations of cuts in order to get a larger signal and/or a larger S/B ratio. Let us only mention the substitution of the Ipw(Y~)+pw(T2) I >30 GeV cut, proposed by Gun- ion [ 3 ] in order to overcome possible double brems- strahlung events, with a cut on the maximum A¢ or AR the photon pair can have. The S/x/~ ratio is not significantly modified but, particularly for low Higgs masses the number of surviving events can be in- creased by almost 20% for A¢ < 165 ° or AR < 3. With 100 fb - ~ per year at SSC, over most of the range studied in this paper, the tt (H-+ 77 ) process yields enough events that both top quarks can be required to decay semileptonically since BR ( tt - , ~ v£vX ) = 4.8 × 10- 2. This would completely eliminate any background from WH production.

The reaction p p - * t t H - - , Z y ~ y has a production cross section times branching ratio of about 0.1 fb for mH= 130-160 GeV at SSC. Clearly only if high lu-

minosity is available there is any hope of detecting it. The corresponding prompt background distribution is da/dMvz ( t tyZ) ~ 0.9 tb /GeV at Mvz = 150 GeV. The combined branching ratio to three leptons is 2.4× 10 -2. Therefore the cross section in a window of 6-10 GeV centered around the Higgs mass is of the order of 0.1-0.2 fb, roughly equal to the signal. We have not tried to simulate the effect of experimental cuts on such a tiny signal. Our experience for the t~r/ case suggests that asking for pseudorapidities less than 2.5 and transverse momenta of order 20 GeV should cut the signal by about 30% and the background by 50%. The presence of such a background together with the very small production rate makes detection of pp--, t tH ~ Zy practically hopeless.

Finally, we want to mention the possibility of detecting the decay of the Higgs to two muons. Taking QCD corrections to Higgs decay into account, and in particular the running mass effects, one obtains a branching ratio which is relatively large, BR(H--, l l lx)=2.6× 10 -4 or, in other words, BR(H--,~t~t)/ B R ( H - , ~ ) = 0 . 8 (0.3) for mH=50 (80) GeV. The inclusive signal pp--, H--, ~t~t is buried under ordinary Drell-Yan events, t tH production results in about 8 (5) events per year at LHC with the set of cuts adopted for the two-photon channel. The same number of events can be accumulated at SSC in two years at standard luminosity. The main backgrounds are WZ and tt production. The first one only matters if I m H -- m z I < 10 GeV, a region which is difficult in any case because of the small rate. The second one is not so worrisome in this channel since the Higgs mass can be reconstructed. Again, only a careful simulation can decide on the observability of this decay mode.

In this paper we have systematically studied the decay channels of the Higgs, looking for observable signatures from Higgs production in association with tt pairs at hadron supercolliders. Only in limited small mass intervals it is possible to detect the Higgs in more than one reaction. In general several years of data have to be accumulated, or a luminosity higher than presently planned is required. The most dangerous background usually comes from tt production which should, in our opinion, be re-examined.

235


References

[ l ] P.J. Franzini and P. Taxil, in: Proc. Workshop on Z physics at LEP I, eds. G. Altarelli, R. Kleiss and C. Verzegnassi, CERN report CERN 89-08; S.L. Wu et al., in: Proc. ECFA Workshop on LEP 200, eds. A. Bohm and W. Hoogland, report CERN 87-08, ECFA 87/ 108.

[2] R. Kleiss, Z. Kunszt and W.J. Stirling, Phys. Lett. B 253 (1991) 269; M.L. Mangano, SDC Collaboration Note SSC-SDC-90- 00113.

[3] J.F. Gunion, Phys. Lett. B 261 ( 1991 ) 510; W.J. Marciano and F.E. Paige, Phys. Rev. Lett. 66 (1991) 2433; A. Ballestrero and E. Maina, Phys. Lett. B 268 ( 1991 ) 437; Z. Kunszt, Z. Tr6cs~myi and W.J. Stirling, Phys. Lett. B 271 (1991) 247; D.J. Summers, Phys. Lett. B 277 (1992) 366.

[4] S. Jensen, ed., Proc. 1988 Snowmass Workshop (World Scientific, Singapore, 1989);

J.F. Gunion et al., Overview and recent progress in Higgs boson physics at the SSC, in: Proc. 1990 Snowmass Workshop, to appear.

[5] G. Jarlskog and D. Rein, eds., Proc. ECFA Large Hadron Collider Workshop (Aachen, 1990).

[6] J. Ellis, G.L. Fogli and E. Lisi, Phys. Lett. B 274 (1992) 456.

[ 7 ] V. Barger, G. Bhattacharya, T. Han and B.A. Kniehl, Phys. Rev. D 43 (1991) 779.

[ 8 ] K. Hagiwara and D. Zeppenfeld, Nucl. Phys. B 274 ( 1986 ) 1.

[9 ] P.N. Harriman, A.D. Martin, W.J. Stirling and R.G. Roberts, Phys. Rev. D 42 (1990) 798.

[ 10 ] F. Cavanna, D. Denegri and T. Rodrigo, in: Proc. ECFA Large Hadron Collider Workshop (Aachen, 1990 ), eds. G. Jarlskog and D. Rein.

[ 11 ] A. Nisati, in: Proc. ECFA Large Hadron Collider Workshop (Aachen, 1990), eds. G. Jarlskog and D. Rein.

[ 12 ] A. Djouadi, M. Spira, J.J. van der Bij and P.M. Zerwas, Phys. Lett. B 257 (1991) 187.

236

Documents

Vector boson pairs from top-antitop-Higgs production