looking for a higgs boson of intermediate mass

Volume 229, number 1,2 PHYSICS LETTERS B 5 October 1989

LOOKING FOR A HIGGS BOSON OF INTERMEDIATE MASS

Pankaj AGRAWAL and Stephen D. ELLIS Department of Physics, FM-15, University of Washington, Seattle, WA 98195, USA

Received 17 July 1989

It is pointed out that the ability to identify and reconstruct bottom jets will be important for the identification of the Higgs boson at hadron colliders, if its mass is in the intermediate mass range (2Mw>Mn>Mz). We examine a particular tagging scheme.

At present, there are no established disagreements between the standard model and the experimental data [1 ]. Still the standard model is not yet completely substantiated. In particular, one of the most important elements of the standard model, the Higgs mechanism, has no direct empirical support. The Higgs mechanism is necessary for the standard model to be renormalizable while still describing the ob- served symmetry breaking. One implication of the Higgs mechanism is the existence of a particle, the Higgs boson. Since there is no compelling and exper- imentally viable alternative to this scenario [ 2 ], one must look for the Higgs boson. The stumbling blocks in the search for the Higgs boson have been twofold: the absence of significant bounds on its mass and its small coupling to the copiously available particles. Although a light Higgs boson (MH < Mz) can be detected at LEP, for a higher mass Higgs boson one has to resort to either the SSC or the LHC [ 3 ]. In fact, one of the prime motivations for the SSC and the LHC is to search for the Higgs boson. However, it is not certain that the Higgs boson can be detected there, even if it is produced. If the Higgs boson is heavy (MH > 2Mw) it can presumably be detected, but there are problems for the case of the intermediate mass Higgs boson (2Mw > MH > Mz) [ 3 ]. In this letter, we focus on this intermediate mass range and look at a particular detection scheme.

The dominant production mechanism in the intermediate mass range is the gluon fusion process, gg--, H [ 4 ]. However, this mechanism does not help with the identification of the Higgs boson. The Higgs boson,

produced through the gluon fusion mechanism, will give rise to 2jets in the final state. This is because the Higgs boson primarily decays into 2 quarks. This constitutes the signal. The background due to usual strong interaction inclusive 2 jet production is much larger (at least 6 orders of magnitude larger) [4]. Prospects for the use of the rare decay channels of the Higgs boson have been explored and it has been found that H--,ZZ* (Z* is an off mass shell Z boson) may be useful if MH > 130 GeV [5 ]. However, the number of such events will be quite small with the ex- pected luminosity after the experimental cuts are applied. Also this scheme will not be useful ifMH < 130 GeV.

Here we consider a different production mechanism for the intermediate mass Higgs boson. This mechanism involves the production of a Higgs boson in association with a W, i.e., through the process pp-~HWX. We show that it will be important to identify and reconstruct bottom jets in the search for the Higgs boson using this mode of production. We assume m t > Mw SO that the Higgs boson, in this mass range, prominently decays into bb. This is a plausible assumption. Furthermore, this is necessary only if we take our results to be valid for the entire intermediate mass range. Since we shall be presenting results only up to MH= 120 GeV, we are assuming only mt>60 GeV ~1

In such a scheme, as mentioned above, we are

~ On the basis of an analysis ofe+-~t + events at the Tevatron the CDF Collaboration finds a preliminary limit mt> 60 GeV [ 6 ].

0370-2693/89/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )

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looking at the process p p ~ H W X . The Higgs boson principally decays into bb and the W decays into ~ -+ v~ or 2 jets. The decay of the W into 2 jets will yield an event with 4 jets in the final state. The rate o f such events cannot complete with the strong interaction 4 jet production rate even if we could identify bot tom jets. Therefore the final state that one must look at is an isolated e ÷ or Ix ÷ and missing PT (from the decay of the W) plus 2 jets. Unfortunately the background rate is still at least 2 orders o f magnitude larger than the signal. However, now we can use the fact that the jets due to the signal are produced by bot tom quarks. Therefore, if we look for final states with an isolated e -+ or Ix-+ and missingpT plus 2 bottom jets, we achieve a significant reduction in the background. The problem in this case is flavour tagging.

One way to tag flavour is to look at the semileptonic decay channel of the (anti)quark. Then, if we denote by p~q the momentum of the charged lepton (e -+ or tx -+ ), in the plane perpendicular to the motion of the (anti)quark, we have p~fq < mq/2. The distin- guishing feature o f this charged lepton, as compared to that from the decay of the W, will be that it is within the jet. We note that although flavour tagging through a IX ÷ is quite clean, an e -+ within a jet can be faked by an overlapping pair of n o and n ÷ . Therefore the experimental efficiency of tagging flavour with an e -+ will likely be much lower than with a IX ÷. I f we also reject events with an isolated e -+ or Ix ÷ and missing Px plus 2 jets in which pgfq < 1 GeV, we will be left with a fairly clean sample of events with W plus 2 bottom jets. Unfortunately, when we use the semileptonic decay channel o f the bot tom quark to tag it, there is a neutrino in its decay products. This means that one cannot reconstruct the bot tom jet completely using this kind of tagging. There exists an- other method to tag bot tom jets which involves hadronic decays and has been employed successfully at e+e - colliders [7]. This method exploits the rela- tively long life time of the B meson. The identification of the secondary vertex structure associated with the B meson decay is used as a tag. If this method can also be used at pp colliders, it may allow us to reconstruct bot tom jets more completely.

To see the potential usefulness o f tagging the bottom quark we compute the cross sections for both the signal and the background employing a tag on the semileptonic decay channel. The background to the

Higgs boson signal comes from the process pp->Wbl3X when the pair mass M ( b b ) is n e a r M H.

The diagrams contributing to both the signal and the background are displayed in figs. la and lb respectively. These diagrams lead to lowest order cross sections for the signal and the background. We use the amplitude level spinor techniques to calculate the cross sections [ 8 ].

We present results for three values o f MH (our con- clusions will be independent o f this choice). We use the structure functions of Duke and Owens [ 9 ] eval- uated at the factorization scale p = MH. We also choose Mw = 81 GeV and rob--5 GeV. For the SSC we use x/~= 40 TeV while for the LHC x//s= 17 TeV. For both the SSC and the LHC we use an integrated luminosity of LP= l04 pb-~. Although the LHC is ex- pected to have larger operational luminosity that the SSC, we use identical integrated luminosity in the two cases for comparison purposes. To define an event sample we apply the c u t s mb/2>p~q> 1 GeV and I M H - M ( b b ) I < 5 GeV. These are intended to tag the bot tom quarks and focus on the Higgs boson. There are also other more generic experimental cuts to en- sure observable and identifiable leptons and jets and to flag the presence o f a W. The cuts that we apply, in addition to those mentioned above, to simulate these "typical" experimental cuts are

]~/(£) I < 3 . 0 , I~/(j) I < 3 . 0 ,

Pr(~,J, v) > 10 G e V ,

R(I~,j) > 0.4, R ( j , j ) > 0 . 6 .

Here R ( k, l ) = x/(At/) 2 + (A~) 2, where A~ and At/are the differences in azimuthal angle ~ and pseudorap- idity ~/( = - I n tan 0 /2) between the pair k, l. The in these cuts refers to the e ± or ix ± due to the decay of W. No cuts other than those on p~q as outlined above are applied to the lepton from the bottom quark decay. The PT of the neutrino is, o f course, the missing PT in the event. Typical numbers of events ob- tained with this leptonic tagging scheme are given in table 1. These results are only intended to give an in- dication of the possible size o f event samples, i.e., o f order hundreds o f events. We see that the event rate at the SSC is approximately twice the rate at the LHC. For all values of MH we observe a signal to background ratio of order 1 to 1. This is also illustrated in figs. 2a and 2b where we plot the M ( b b ) distribu-

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a)

W

b)

Fig. 1. (a) The lowest order Feynman diagram contributing to the production of Higgs boson, H, in association with a W boson plus decays. The wavy lines signify W bosons, the solid lines fermions and the broken line represents the Higgs boson. (b) The lowest order Feynman diagrams contributing to the production of a W boson with bl~ quarks plus decays. The wavy lines signify W bosons, the solid lines fermions and the coiled line represents a gluon.

Table 1 Number of events with the integrated luminosity and kinematic cuts specified in the text at the SSC and LHC energies

x/s(TeV) M H ( G e V ) Signal Background

40 90 553 474 40 120 268 244 17 90 294 252 17 120 139 130

tions for the SSC energy and for MH = 90 and 120 GeV respectively. Thus, i f we can both tag and reconstruct the bo t t om quark jets, we will see a " b u m p " in the corresponding 2 je t M ( b b ) dis t r ibut ion. The M ( b b ) d is t r ibut ions for the energy o f the LHC exhibit essentially identical shapes but with fewer events. For compar ison we repeated the same calculat ion for x/s = 40 TeV with the structure funct ions o f ref. [4] for M H = 120 GeV. Although both the signal and the background decrease by approximate ly 10%, the signal to background rat io remains essentially the same. We note that , due to the very small width of the Higgs boson in this mass range (of the order of a few MeV) ,

an improvemen t in the exper imenta l resolut ion for M(bl~) can yield a larger signal to background ratio. This improvemen t will arise from employing a bin size smaller than 10 GeV so that the " b u m p " will be more distinct. By the same token, worse resolut ion for M ( b b ) will degrade the signal to background ratio. The accuracy of the measurement of the posi t ion of the " bump" , the Higgs boson mass, will also de- pend on the exper imental resolut ion of the energy of the bo t tom jets. To s imulate the uncertaint ies in the measurement o f the je t energies we smeared the je t energies with respect to the pa t ton energies using gaussian distr ibut ions with a = 5 GeV and 8 GeV. The results are displayed in figs. 2a and 2b as dashed and dot ted histograms respectively. Although the "bump" has become less distinct, it should still be detectable. It is impor tan t to note that the presence of a " b u m p " in the M ( b b ) d is t r ibut ion is not dependent on the par t icular me thod of tagging the bo t tom quark. Since the signal and the background (as def ined here ) both involve bo t tom quarks, the ratio of the two is independent o f the details of the tagging scheme as long as the resulting event sample has good puri ty (i.e.,

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1 0 0 0

1500

R a=

~ 5 0 0

' ' i I i i i , I ~ ' ' '

a c]g ENERGY = 40 TeV

- - a= 0 Gev

. . . . . . ~ = s GeV

. . . . . . # = 8 GeV

\ t _

L . _

l O G 1 5 o 2 0 0 2 5 0 U (bT)) [GeV]

I ' ' ' ' I ' ' ' ' I r , r - , [ ,

600 b CM ENERGY = 40 TeV

- - a = o CeV

- - - ~ = s cev

.... • ..... a= •

o400

=

2O0

1 0 0 1 5 0 2 0 0 2 5 0

U(b-b) [C-,,V]

Fig. 2. (a) Histograms illustrating the distribution of events for the process pp--,Wbl3X~±vcel~+~-vgX versus the invariant mass M(bla) arising from the sum of the signal and the background with the various kinematic cuts described in the text. Here MH = 90 GeV and x/s= 40 TeV. The solid histogram corresponds to no smearing while the dashed and dotted histograms corre- spond to gaussian smearing of the bottom jet energies with dis- tributions of widths a= 5 and 8 Gev, respectively. (b) Identical to (a) except MH= 120 GeV.

contains mostly events with bo t tom quarks) . The efficiency o f the tagging scheme will serve to de te rmine the absolute numbers o f events in the sample. How- ever, it is impor tan t to be able to reconstruct M ( b b ) . As an example, the hadronic tagging scheme of ref. [ 7 ] appears to fulfill these constraints.

Unfortunately, as noted earlier, there is a serious problem with the specific leptonic tagging scheme implemented here. Because o f the use of the semileptonic decay channel of the bo t tom quarks, we have 2 neutr inos in the decay products that will go unde-

tected. Therefore, instead o f measuring the mass o f the bb system, M ( b b ) , we can realist ically measure only the mass of the ce~+Q - system, M ( c e ~ + ~ - ) . Unl ike the M ( b b ) dis t r ibut ion, where the signal is peaked around the Higgs boson mass, there is no such peak in the M(c~J~+~ - ) dis t r ibut ion. Thus the signal to background rat io is seriously degraded. Whatever cut we apply on the M ( c ~ + ~ - ) the signal is only 20% or less of the background. This is evident from the fig. 3 where we have plot ted the M(ce~+£ - ) distri- but ion for the SSC with MH = 90 GeV. The solid histogram corresponds to the background while the dashed his togram is for the sum of the signal and the background. The plot of this distr ibution for the LHC is similar. Since exper imental and theoret ical uncer- ta int ies will be larger than this signal to background ratio, the Higgs boson will not be detectable in this way.

In conclusion, we have demons t ra ted the impor- tance o f exper imental ly identifying and reconstruct- ing bo t tom jets in the search for the Higgs boson, i f its mass is in the in termedia te mass range. We have also shown that it is impor tan t to be able to measure the mass o f the bl~ system fairly well. We have exam- ined a par t icular tagging scheme, using the semileptonic decay channel of the bo t tom quark, and shown that it is unlikely to work. Therefore the tagging

I ' ' ' '

~ 1 0 o o

5 0 0


I ' ' ' '

- L - - - L ~ L

o I ~ ~ ~ , I ~ ~ ~ ~ I ~ ~ J " ' , l - - S 0 1 0 0 1 5 0 2 0 0

u(eaT) [G,,v]

I

Fig. 3. Histograms illustrating the distribution of events for the process pp~WblaX-~(~ ± vc~+~-vgX versus the invariant mass M(c0~+~ - ) with the various kinematic cuts described in the text. Here MH = 90 GeV and ~ = 40 TeV. The solid histogram corresponds to the background while the dashed histogram corresponds to the sum of the signal and the background.

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scheme mus t be such as to explo i t the h a d r o n i c decay

channe l s o f the b o t t o m q u a r k [ 7 ].

Th i s work was suppor t ed in par t by the U S De-

p a r t m e n t o f Energy, C o n t r a c t No . DE-AS06-

88ER40423 . We thank ou r e x p e r i m e n t a l and theo-

re t ica l col leagues for useful discussions.

References

[ 1 ] P. Langacker, in: Proc. XXIV Intern. Conf. on High energy physics (Munich), eds. R. Kotthaus and J.H. Kiihn (Springer, Berlin, 1989) p. 190.

[ 2 ] L.J. Hall, in: Proc. XXIV Intern. Conf. of High energy physics (Munich), eds. R. Kotthaus and J.H. KiJhn (Springer, Berlin, 1989) p. 335.

[ 3 ] V. Barger, in: Proc. XXIV Intern. Conf. of High energy physics (Munich), eds. R. Kotthaus and J.H. Kiihn (Springer, Berlin, 1989 ) p. 1265, and references therein.

[4] E. Eichten et al., Rev. Mod. Phys. 56 (1984) 579. [ 5 ] J.F. Gunion, G.L. Kane and J. Wudka, Nucl. Phys. B 299

(1988) 231. [ 6 ] H. Frisch, private communication. [ 7 ] TASSO Collab., W. Braunschweig et al., Z. Phys. C 42 (1989)

17. [8] R. Kleiss and W.J. Stirling, Nucl. Phys. B 262 (1985) 235. [9] D.W. Duke and J.F. Owens, Phys. Rev. D 30 (1984) 49.

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looking for a higgs boson of intermediate mass

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