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 SM based on SU(3)C×SU(2)W×U(1)Y
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 GeV to 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. Inset: High mass data events No events above 500 Gev/c2
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
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

12. 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.

13. An effective Lagrangian for the qqee contact interaction:
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 /Λij 2
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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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:

20. 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

21. Results:
No significant excess in the data

22. 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.