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Search for Z’ and New Particles Decaying to Z 0 +jets such as b’

Search for Z’ and New Particles Decaying to Z 0 +jets such as b’. Ye Li Graduate Student UW-Madison. Search for Z’ →e + e -. Make use of Dielectron Mass Angular Distribution SM based on SU(3) C ×SU(2) W ×U(1) Y Simplest extension to SM containing Z’: SU(3) C ×SU(2) W ×U(1) Y ×U(1) z

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Search for Z’ and New Particles Decaying to Z 0 +jets such as b’

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  1. Search for Z’ and New Particles Decaying to Z0+jets such as b’ Ye Li Graduate Student UW-Madison

  2. Search for Z’ →e+e- • Make use of • Dielectron Mass • Angular Distribution • SM based on SU(3)C×SU(2)W×U(1)Y • Simplest extension to SM containing Z’: SU(3)C×SU(2)W×U(1)Y×U(1)z • Z - Z’ mixing is small → perturbation • A couple of models MZ’>900 GeV → The analysis range extended from 200-920 GeV to 200-1050 GeV

  3. Z’ interaction with SM fermions: • Σf zf gz Z’μfγμf • f includes SM fermions and fermion doublet: • ejR, (ejL, νjL), ujR, djR, (ujL, djL) • Denote (ejL, νjL),=ljL, (ujL, djL)=qjL • Simplification of Z’ models • Generation-independent quark charges result in 11 parameters (MZ’, ΓZ’, zf) • Assume Z’ only decays to SM particles • Generation-independent lepton charges leads to 4 particular classes of solution of six anomaly cancellation equations • zf only depends on an arbitrary real number x

  4. For a given class, 3 parameters: MZ’, gZ, x • If the extension of the SM can be described by an effective U(1), the 4 classes is sufficient • Models not satisfying the above condition: • Little Higgs model • quark lepton compositeness model

  5. Background: • Dark grey: other background • Light grey: dijet background • Open: SM Drell-Yan • Inset: High mass data events • No events above 500 Gev/c2

  6. Left: distribution of higher mass region (Mee>200Gev/c2) • Histogram: prediction of Drell-Yan Monte Carlo • Right: forward and backward assymetry • Errors are statistical only • Dijet has at least 50% uncertainty; shifting 50% up • Given the large systematics, the agreement is good

  7. Z’ Exclusion Summary: expected and observed 95% C.L. lower limits on MZ’ for the sequential, the canonical E6 • In parentheses, 95% C.L. if all the decay channels to super-particles are open • Including superparticle decays enlarges the Z’ width, reducing the branching ratio to quark and lepton pairs; limit gets weaker (See examples attached)

  8. Result on Little Higgs Model MZ’ • Index 1,2,3,4 corresponds to cotθH = 0.3, 0.5, 0.7 and 1.0 respectively • In Little Higgs model, Z’ couples to Left-handed fermions only, and these couplings are parameterized as functions of the mixing angle cotangent cotθH

  9. Exclusion contours for the 4 classes • The dotted lines represent the exclusion boundaries derived from the LEP II results • The region below each curve is excluded by our data at 95% C.L. • Only models with MZ’ >200 GeV/c2 are tested, which explains the gap at small |x| for some models.

  10. Z’ constraints can be derived from contact interactions, if the collider energy is far below the Z’ pole • An effective Lagrangian for the qqee contact interaction: • ΣqΣi,j=L,R 4 πη eiγμei qjγμqj /Λij2 • Λ: the scale of the interaction • η = ±1 determines the interference structure with the Z /γ* amplitudes • Six helicity structure scenarios of Λ : • LL, LR, RL, RR, VV and AA • VV=LL+LR+RL+RR; AA = LL+RR-RL-LR

  11. Conclusion: • No significant evidence of Z’ has been found • 95% CL limit are set on these models • Exclusion contour for the generic Z’ model lines are mapped out • Constraints are also placed on contact interaction mass scale far above the Tevatron energy scale

  12. Search for New Particles → Z0+jets • A variety of new models predicting N.P. decaying to Z0+jets • Use the 4-th generation model • b’ may have a large BR to bZ0 via the following loop diagram

  13. Use 1.055 fb−1 of data collected with electron and muon triggers • To reject this background, this analysis requires the presence of high-ET jets • Variables used here: • N30jet = Number of jets in the event with ET > 30 GeV and |η| < 2 • J30T = Scalar sum of ET ’s of all jets in the event with ET > 30 GeV and |η| < 2

  14. The C.S. for new models are many orders of magnitude smaller than the C.S. for SM Z0 production • In the range 150<mb’<350 GeV, maximum sensitivity requires N30jet>3 and J30T > X, where X is scanned through in 50 GeV steps

  15. Background: • SM single-Z0 production with associated jets (Z0+jets) • SM WZ+jets, where the W decays to jets • SM ZZ+jets, where one of the Z’s decays to jets • SM tt-bar+jets, where both W’s decay to leptons • QCD multijet events, where two of the jets fake leptons • Multijet events occurring in conjunction with a cosmic ray

  16. Use jet ET distributions in the N30jet <=2 bins from the Z0+jets data itself to predict the number of background events expected with N30jet>=3 • The parameterization used:

  17. Estimate for the relative fractions of events in the N30jet >=3 bins using an exponential fit to the data in the N30jet<=2 bins • Then J30T distribution is obtained by a random sampling of the N30jet shape and the extrapolated jet ET shapes

  18. Results: • No significant excess in the data

  19. The b’ cross section is calculated at leading order using PYTHIA, with the assumption that BR(b’ → bZ0) = 100%. • With this assumption, the mass limit observed is mb0 > 270 GeV.

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