Nuclear Physics B (Proc . Suppl .) 16 (1990) 640-642 North-Holland
640
Effects
of Cascade Decays on Gluino and Squark Signals from the Tevatron *
Howard Baerl), Xerxes Tataa , and Je$rey Woodside3) 1) Physics Department, Florida State University, Tallahasee, FL 32306 2) Department of Physics & Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822 3) Physics Department, Oklahoma State University, Stillwatér, OK 74078 If squarks (q) and gluinos (9) are in the mass range currently accessible at the Tevatron, the branching :actions for the direct decays of qL and g' to the LSP may well be less than 50% within the minimal model. We compare the Tevatron signals from squarks and gluinos when all their decays as given by minimal supersymmetry are incorporated with the corresponding signals obtained under the (usual) assumption that they can only decay to the LSP. The v^DF Collaboration at the Fermiiab Tevatron have recently announced' 90% CL lower limits, mi > 74 GeV,
mi > 73 GeV
(1)
on the masses of the squarks (q) and the gluino (g) of supersymmetry (SUSY). Although these limits have been derived from just about 0.5% o£ their current data sample of 4.7pb-1, they already represent a significant improvement over the previous boundsa m4 > 45 GeV, m9 > 50 - 53 GeV obtained by the UA1 and UA2 experiments at CERN. The analyses leading to these bounds all assume that (i) there are ten species of (approximately) degenerate squarks, (ii) the produced gluinos (squarks) de cay via g` -> q q 9r1 (q --> q 91) where 91 the lightest neutralino, is assumed to be the lightest SUSY particle (LSP) and escape detection and, (iii) the LSP mass is smaller than ( 2 - 3) that of the parent. Assumption (i) which only affects the squark mass is expected to be valid in all models with a common SUSY-breaking scalar mass (eg. supergravity modes) since the quark masses are known to be small. It has been known for some time" that if charginos (Wi) and other neutralinos (ZZ, Z3 .,-) are kinematically accessible in q and â decals, the brancl .:zg fraction for * Presented by Xeraes Tata.
0920-5632/90/$3 .50 © Elsevier Science Publisher;, B.V. North-Holland
the direct decay to the LSP can be rather small . The decay patterns _ of W_(q) depend on the masses and mixings of those W and Z accessible in their decays and so are model-dependent. To estimate these, we use the minimal SUSY model as a guide . In this case the charginoneutralino sector is completely parameterized in terms of of supersymmetry Higgsino mass 2m1, the ratio that occur in all SUSY the vevs of the two Higgs fields models and a gaugino mass parameter which we choose
v
to be m9. Shown in Fig. 1 is the branching fraction for the gluino to directly decay to the LSP as a function of 2ml for several gluino masses with = 3 . We see that while assumption (ii) is valid for the range of m9 probed at CERN, it has already started to break down for values of my near the bound (1) . This is dominantly due to the fact that the decay 9 --+ q q W which proceeds via the large SU(2)L coupling (in contrast to the decay to the LSP which proceeds via the U(1) coupling) rapidly becomes important once m9 > mW. Although we have not shown this here4, the same general pattern holds for qL decays whereas qR which are SU(2)L singlets continue to dominantly decay via the LSP mode for most values of parameters . These general features are insensitive to and also to the details of the model. For larger values of mi (and also mqL) depicted by the
v
H. Baer et al./ Cascade decays on gluino and squark signals
160 GeV curve in Fig. 1, the direct decays to the LSP occur 20-50% ofthe time so that only 4-25% of 9g pairs are amenable to this analysis .
0.8 06 0.4 0 .2 0.0
-400
-200
0 2m, (GeV)
200
400
Fig. 1 The branching fraction for the direct decay of the gluino into the LSP within the minimal model . Here, - 23 as stated in the text . 9
The main effect of the cascade decays on the analysis leading to the bounds (1) which are based on the non-observation of an excess (over standard model (SM) expectations) of events with large PT(>40 GeV) comes from the fact the secondary LSP's are typically softer . The resulting reduction in the fraction ofevents passing the P7, cut leads to a smaller value for the bounds on mi and mg as compared with an analysisl ,2 where only their direct decays to the LSP are allowed . In a recent papér, 5 we studied the effect of incorporating the cascade decays as given by the minimal model on the CDF bounds (1) . We found that a gluino as light as 45 GeV may be allowed whereas for the squarks, because qR primarily has only direct decays, the reduction in the CDF bound is <10 GeV unless mLSF > 20-25 GeV in which case there is no bound . At this conference, 6 the CDF collaboration from an analysis of4.7 pb-1 of their data have announced a preliminary limit mi > 150 GeV (m9 >> mQ) again assum ing that only the direct - : .says to the LSP are allowed . This is in keeping with earlier theoretical expectations .? Since this can be translated to a limit on cross-section for )°T events at the Tevatron, we are able to convert this to an excluded region in the mi - m9 plane .8 For mQ ô m9, gg, qq as well as 9q pair production lead to )° T events so that larger masses can be excluded
-whereas for mg < mi gg is the only BUSY source of PT events . The results of our analysis (assuming the cuts in Ref. 1) with and without the cascade decays incorporated are shown by the solid and dashed lines in Fig . 2, where for the solid line, we have taken 2m1 = Mw and !~ = 3 . The dip in the curve near mg = 0.8 TeV is due to the opening up of the 12 decay channel and is special to our choice of parameters. We see that the "typical reduction" of the bounds due to the incorporation of the cascade decays and the LSP mass is 10-20 GeV though larger values are possible. We have checked that altering the PT cut to 60 GeV does not significantly alter the results . A systematic analysis of the dependence on model parameters is in progress.9 1000
Boo 800
200
0
200
400 800 Glulno Mass (GeV)
800
1000
Fig. 2
A translation of the preliminary CDF bound m4 > 150 GeV into the excluded region of the m4- mâ plane as discussed in the text with (solid line) and without (dotted line) the cascade decays incorporated.
Shown in Fig. 3 are the topological cross-sections for n jet + 0 leptons -F- PT >- 50 GeV events expected from gluino at the Tevatron for both the cascade and di rect decay cases.10 We see that the cascades reduce the APT signal by a factor - 3 (which causes the reduction of the bound discussed above). Note also that events with higher jet multiplicities are favoured for larger values of m9 as can be seen from our SM background estimates shown by the arrows on the right. In 5- jet event S/B - 1 is obtained for m9 â 180 GeV though a large (100 pb-1) integrated luminosity is needed to see this signal.
H. Baer et al ./ Cascade decays on gluino and squark signals
642
To conclude we note (i) the incorporation of cascade decays in BUSY analyses at the Tevatron and other4 high energy colliders is a must (ü) .a confirmation of BUSY necessitates seeing a signal in several channels and (iii) the observation of the cascade decays may provide the first evidence for charginos and neufralinos if they are too heavy to be produced in Z decays at SLC or LEP. Fig. 3
Topological n jet + PT cross-section from gg production at the Tevatron after acceptance cuts of Ref. 10, along with backgrounds from SRId sources, including a 75 GeV top quark.
Since the gluino is a Majorana particle, it is equally likely to decay into a positive as a negative chargino so that half the g 9' which decay into charginos contain same sign W's. If both these charginos decay leptonically, this leads to spectacular eventsli ,lo with hard isolated dilepton pairs together with jets with a crosssection of a few x 10_Z pb as shown in Fig . 4 provided m9 < 130 GeV. We have been unable to find any significant background for this signal. The observation of even 2-5 of these striking events could provide a spectacular confirmation of a SUSY signal in other channels at an upgraded Tevatron. A study of the rates for like sign dilepton events from qg and qq production is in progress .' It has been notedli the gluino masses as low as 180 GeV can be searched for at the SSC in this channel so that is appears that there is no window ofmg that cannot be probed either at an upgraded Tevatron or the SSC . loo l0 -1
0
10_ 2
REFERENCES 1. F. Abe et al., Phys. Rev. Lett. 62, (1989), 1825 2. C. Aljabar et al., Phys. Rev. Lett. 1988, (1987), 261 ; R. Ansari, et al., Phys. Lett. 195B, (1987), 613 . 3. H. Baer et al., Phys. Lett. 161B, (1985), 75; G. (7amberiri, Z. Phys . C30 , (1986), 605 . 4. H. Baer et al., Phys. Rev. D36, (198`T), 96. 5. H. Baer, X. Tata and J. Woodside, Phys. Rev. Lett. 63, (1989) 352. 6. L. Nodulman, Talk in these Proceedings . 7. H. Baer and E.L. Berger, Phys. Rev . D34 , (1986), 1361 ; E. Reya and D.P. Roy, Z. Phys. C32 , (1986), 615 . 8. We are aware (see eg. U. Ellwanger, Phys. Lett. 141B, (1984) 435) that if mi is much smaller than m9, renormalization group evolution drives m9 Z negative below the unification scale unless there are new Yukawa couplings . Partly due to our ignorance of physics beyond the 100 GeV scale and partly for phenomenological completeness we have shown our results in the whole mi - mg plane. 9. H. Baer, X. Tata and J. Woodside, paper in preparation .
10 _3 10 _4
This research was supported, in part, by the U.S. Dept. of Energy under Contract DE-AM03-765F00235 and DE-FG05-85ER40215 .
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150 ml (CeV) Fig. 4
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Same-sign dilepton topologies from gg production at a 2 TeV Tevatron, using the acceptance tâta of Ref. 10 .
10. H.Baer, X.Tata and J.Woodside, Hawaii Preprint, (1989), UH-511-677-89 . 11. R.M. Barneti,t, J . Gunion and H. Haber, Ravis preprint, (1988) UCD-88-30 .