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Supersymmetry Signals in Leptonic Decays of W and Z Bosons

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Supersymmetry Signals in Leptonic Decays of W and Z Bosons PHYSICAL REVIEW D VOLUME 35, NUMBER 3 1 FEBRUARY 1987 Supersymmetry signals in leptonic decays of 8' and Z bosons R. Arnowitt Department of Physics, Northeastern University, Boston, Massachusetts 02115 and Lyman Laboratory of Physics, Harvard University, Cambridge, Massachusetts 02138 Pran Nath Department of Physics, Northeastern University, Boston, Massachusetts 02115 (Received 16 July 1986) It is shown that opposite-sign dilepton events from Z decay represent an excellent signal for su- persymmetry if mrr(Mz/2 in certain kinematic domains where the standard-model background from Z z+r is absent. Excess of monoleptonic events from W 8'y consistent with UA1 cuts is also computed. Comparison with existing data is made. I. INTRODUCTION II. DILEPTON DISTRIBUTIONS Supergravity unified models' are of interest phenome- In calculating the dilepton distributions of Eq. (2) we nologically in that they predict the existence of a number approximately model the UA1 cuts for lepton detection. of low-lying superparticles accessible to present accelera- These include the "kinematic" requirements on the elec- tors or accelerators that will be on line in the near future. tron and muon transverse energy p, „, One of the strongest predictions of supergravity models which possess a light photino (y) is the existence of a W p, )15GeV, ~ri, ~ =2.5, (3a) below W' and a ly- gaugino (W) lying the Z gaugino (Z) ' )3G Ve, & p, ln. l (2. (3b) ing below the Z. Thus the vector-boson decays [where ri= —Intan(|)/2) is the pseudorapidity], as well as W~ W+ y, W~ W+Z, (la) various detection efficiencies. In calculating the number of electrons or muons the UA1 cuts, we have Z W+W passing (lb) neglected the energy dependence in decay matrix ele- are feasible provided the W and Z are not too heavy. ments, but have calculated all branching ratios using the rigorous matrix elements of Ref. 1. In this approximation [In addition, models based on renormalization-group — (RG) breaking of SU(2) x U(1) allow for Z Z(3) we may write for the number of e p events from W de- of the result +Z(3) where Z~3~ is a light superpartner of the axion. cays Z, N,„, Previous work has analyzed the hadronic decays N,~~ =Nz, +, -[I (Z WW)/I (Z e+e )] W,Z qqy into quarks and photinos. Those decays give rise to monojet and dijet events at the CERN SppS Col- x2B(W evy)B(W pvy)(P, „ /P„), (4) lider. Our analysis shows that the UA1 data does not where +, — is the number Z e+e events, rule out W masses mg, =-35-45 GeV, and indeed we find Nz, ' B(W Ivy) = —, is the branching ratio for W leptonic de- models in this mass range with Higgs-boson mixing angles ' + aH 45 consistent with the current UA1 data. cay, P,„ is the fraction of e p pairs from W decay In this Rapid Communication we study the effect of the passing the UA1 cuts, and P„ the fraction of e+e pairs from Z e+e the cuts. [Similar expres- leptonic decays of the W's, W l+ vI+ y. Equation decay passing (lb) then leads to the decays sions hold for the other processes of Eq. (2).] Since in the W leptonic decay the W energy (1Mz/2) is shared between it +PT P P +IIT e e +ItT (2) three particles, one qualitatively expects less than half the electrons to pass the UA I cuts of Eq. (3a), i e., where is the missing transverse energy from the escap- pr — ing v and y. The Z e p channel makes a striking signal for supersymmetry as it is expected to be hadroni- TABLE I. Number of dilepton events arising from Z WW relative to the number of Z e+e events as a function of the cally quiet. The only standard-model background for W mass m~. these decays is Z z+z followed by leptonic decays of the z' s. However~ as discussed below, certain kinematic — z+z domains of e p are forbidden to decays, and so (Gev) f.u even a single event outside the regions forbidden to the z 35 0.072 0.023 0.011 would be evidence for decay of a heavy object such as the 40 0.057 0.019 0.0085 W. We also investigate monoleptonic events arising from 45 0.031 0.118 0.0083 Wy. 1085 Oc 1987 The American Physical Society 1086 R. ARNOWI j I' AND PRAN NATH TABLE II. Number of monolepton events arising from W Wy relative to number of W e v events as a function of 3.0- the W mass m~. (GeV) 35 0.025 0.014 2.0- 40 0.022 0.012 45 0.019 0.0092 which will populate the low-energy side of the W Ivi distribution. (The W WZ channel closes for mg, ~ 37 GeV. ) Assuming again a constant matrix element in the phase-space integral over the UA1 cuts, one has, for the 0 number of W induced leptons, -10 -0.6 -0.2 0 0.2 1.0 ZT COS(ge f ) NI =N~-l, lr(w- wy)/r(w- tv) j — &B(W FIG. 1. Number Z p e p'T arising from Z WW as a Ivy)(P( /PI), (6) — 40-GeV function of Zr cos(p, p„) for a W. where PP is the fraction of leptons arising from W decay passing the UA1 cuts, and PI is the corresponding fraction for leptons from W l v decay. Here the effect is suppressed somewhat since in supersymmetry P,„=0.4. This is indeed confirmed in detailed Monte Carlo calculations. N,„ is enhanced, however, due to I (W Wy)/I (W lv)=0.3-0.4 . the fact I (Z WW)/I (Z e+e ) =4. The ratios The ratios =NI /N~ I„are given in Table II. For the f,„=N,„ /Nz—, +, —,etc. , for the three processes of Eq. fI (2) are given in Table I for W masses in the range con- earlier data, N~ „=172and N~ „„=47.One thus ex- sistent with the UA1 monojet data. For the earlier UA1 pects N, =4 events on the low shoulder of the W ev data of 399 nb ', Nz, +, -=18, and so one expects distribution and N„=2 events. Again these numbers about N,„=1.0 events for a 40-GeV W gaugino. (This should double when the more recent data is included. The number should double when the more recent data is in- earlier data can tolerate about +8 events on the low- cluded. ) In contrast, we estimate for the energy side of the W ev distribution, and so at present roughly— standard-model Z r+ r e p +p'T contribution this effect cannot be detected. With the inclusion of the that N,'„' = 0.5. The supersymmetry contribution thus ap- more recent data, however, the effect is on the edge of be- pears to be somewhat larger than that of the standard ing observable. model. Angle and energy distributions allow one to distinguish the z from W contributions to N,„. Thus the light mass of IV. CONCLUSION the z that all the e — events should be almost implies p Opposite-sign dilepton events from Z decay represents back to back in the transverse plane. The distribution of an excellent signal for W s if the W is sufficiently light for events expected for W's as a function of =cos(p, — ZT p„) the Z WW decay channel to be open. Such events are is given in Fig. 1. About one quarter of the W events lies expected to be hadronically quiet. By choosing the kine- in the first bin —1.0 ~ & —0. overlapping the z con- ZT 9, rnatic region, one can effectively veto the standard-model tribution. The region — 9 & & 0 contains about 50% 0. ZT background from Z z+z without significantly reduc- of the events, while the remaining events have ZT & 0. ing the supersymmetric signal. Furthermore, the z', a Thus there is a clear angular separation of W and z events. fourth sequential lepton with mass m, Mz/2, is The energy distribution gives a less clear separation of the ( distinguish- able from the W, as it couples to the Z with 8'and z. Thus for a z decay the maximum lepton energy strength = 20-25% of the W coupling and hence makes is about Mz/2, while a 40-GeV W yields leptons with ener- only a very small contribution to the dilepton events. Thus ~37 GeV. Thus very energetic leptons would arise gy observation of only a few such dilepton events would be only from z's. strong evidence for the decay of a heavy particle such as the W. Supersymmetry models predict about 1 event/400 nb ' at the CERN collider for W masses in the range con- III. MONOLEPTONIC DISTRIBUTION sistent with UA1 monojet data, which is on the edge of ob- The Wy channel of Eq. (la) will produce monoleptonic servability. With the increased luminosity at the collider events through arising from the antiproton accumulator, however, such events should be detectable by UA1. In addition, an ex- W+ y (l + v&+ y) + y, (5) cess of monoleptonic events from W Wy is predicted, SUPERSYMMETRY SIGNALS IN LEPTONIC DECAYS OF W. 1087 consistent with the current accuracy of the UA1 S'~ ev m~ =40 GeV, with correspondingly smaller N„ data. An increase in statistics, however, could make this and N„„. effect also observable. While dilepton events from 8'decays of the Z are barely ACKNOWLEDGMENTS observable at CERN in the current data sample, they should be copiously produced at the Stanford Linear Col- This work was supported by the National Science Foun- lider (SLC) and CERN LEP if 2mg, & Mz.
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