Flavorful Z ′ signatures at LHC and ILC Shao

There are lots of new physics models which predict an extra neutral gauge boson, referred as Z -boson. In a certain class of these new physics models, the Z -boson has flavor-dependent couplings with the fermions in the Standard Model (SM). Based on a simple model in which couplings of the SM fermions in the third generation with the Z -boson are different from those of the corresponding fermions in the first two generations, we study the signatures of Z -boson at the Large Hadron Collider (LHC) and the International Linear Collider (ILC). We show that at the LHC, the Z -boson with mass around 1 TeV can be produced through the Drell-Yan processes and its dilepton decay modes provide us clean signatures not only for the resonant production of Z -boson but also for flavor-dependences of the production cross sections. We also study fermion pair productions at the ILC involving the virtual Z -boson exchange. Even though the center-of-energy of the ILC is much lower than a Z -boson mass, the angular distributions and the forward-backward asymmetries of fermion pair productions show not only sizable deviations from the SM predictions but also significant flavor-dependences. The search for new physics beyond the Standard Model (SM) is one of the most important issues of particle physics today. In a class of new physics models, the SM gauge group is embedded in a larger gauge group and such a model often predicts an electrically neutral massive gauge boson, referred as Z -boson, associated with the original gauge symmetry breaking into the SM one. There are many example models such as the left-right symmetric model [1], Grand Unified Theories based on the gauge groups SO(10) [2] and E6 [3], and string inspired models [4] (for a review, see, for example, [5]). It will be very interesting if a Z -boson is discovered at future collider experiments such as the LHC and ILC. Current limits for the direct production at the Tevatron and indirect effects from LEP experiments imply that the Z -boson is rather heavy and has a very small mixing with the SM Z-boson. No evidence of a signal has been found, and the lower limits on Z ′ mass at 95% confidence level are set to be in the range from 650 to 900 GeV, depending on the considered theoretical models [6]. Recently studies about measurement of the Z -boson at the LHC have been performed [7]. Through the Drell-Yan process, pp → Z X → llX, a Z -boson could be discovered at the LHC if its mass lies around TeV scale with typical electroweak scale couplings to the SM fermions. Once a Z -boson resonance is observed at the LHC, the Z -boson mass can be precisely measured. The next task is to precisely measure other properties of the Z -boson, such as couplings to the SM particles, its spin, etc. Future ee linear colliders, such as the ILC, will be capable of such a task, even if the collider energy is not sufficiently high to produce the Z -boson. For example, the precision goal of the ILC can allow us to indicate the existence of Z -boson with mass up to 6 times of center-of-mass energies of the collider [8]. In general, the coupling of Z -boson with the SM fermions can be flavor-dependent. In fact, such a class of models has been proposed by many authors [9, 10, 11, 12]. If this is the case, the signature of Z -boson should show flavor-dependences and the collider phenomenology of Z -boson would be more interesting. In this Letter, we take a simple model recently proposed [12], where the SM fermions in the third generation have couplings with the Z -boson different from those of the corresponding fermions in the first two generations, and study (flavor-dependent) Z -boson signatures at the LHC and ILC. Let us first give a brief review on a recently proposed “top hypercharge” model [12]. This model is based on the gauge group SU(3)C× SU(2)L× U(1)1× U(1)2 and the SM fermions in the first two generations have hypercharges only under U(1)1 while the third generation fermions have charges only under U(1)2. A complex scalar field, Σ, in the representation (1, 1, 1, 1/2,−1/2) is introduced, by whose vacuum expectation value (VEV) (〈Σ〉 = u/ √ 2) the gauge symmetry U(1)1× U(1)2 is broken down to the SM U(1)Y . Associated with this gauge symmetry breaking, the mass eigenstates of two gauge bosons are described as