References - Bogoliubov Laboratory of Theoretical Physics - JINR

for final l + l − events (l = μ, τ); 35% and 60% for c¯c and b ¯ b, respectively. The major
systematic uncertainties are found to originate from uncertainties on beam polarizations
and on the time-integrated luminosity: we assume δP − /P − = δP + /P + = 0.2% and
δLint/Lint =0.5%, respectively.
As theoretical inputs, for the SM amplitudes we use the effective Born approximation
[5]. Concerning the O(α) QED corrections, the (numerically dominant) effects from
initial-state radiation for Bhabha scattering and the annihilation processes in (2) are
accounted for by a structure function approach including both hard and soft photon
emission [6]. Effects of radiative flux return to the s-channel Z exchange are minimized
by the cut Δ ≡ Eγ/Ebeam < 1 − M 2 Z /s on the radiated photon energy, with Δ = 0.9.
The expected sensitivity bounds on MZ ′ are assessed by means of “conventional” χ2
analysis and by assuming a situation where no deviation from the SM predictions is
observed within the experimental uncertainty. Accordingly, the corresponding limits on
MZ ′ are determined by the condition χ2 (O) ≤ χ2 CL ,andwetakeχ2CL =3.84 for a 95%
C.L. Also, in deriving limits on MZ ′ we combine all the final states of processes (2). In
Table 1 we present the numerical results for the Z ′ sensitivity bounds obtained from the
processes (2) at the ILC with √ s =0.5 TeVandLint = 500 fb−1 .
Table 1: Discovery reaches (in TeV) on Z ′ bosons (at 95% C.L.) at the ILC with polarized (pol) and
unpolarized (unp) beams at √ s =0.5 TeVandLint = 500 fb −1 .
Model Z ′ ψ Z′ η Z ′ χ Z ′ LRS Z′ SSM Z′ ALR
ILC unp 3.7 3.6 6.2 5.4 8.0 8.4
ILC pol 4.5 4.8 7.7 7.5 9.4 10.1
In distinction from the consideration above, where the “discovery reach” on MZ ′ was
based on the assumption that no corrections are observed and the data are consistent with
the SM predictions, we here make the hypothesis that a Z ′ signal is effectively observed
(so that the SM is excluded at a certain C.L.) and the data is consistent with one of
the Z ′ models. We want to assess the level at which this Z ′ model, that we call “true”
model, can be expected to be distinguishable from the others, that may compete with
it as sources of the deviations from the SM and that we call “tested” models, for any
values of their mass MZ ′. Quantitatively, this amounts to determining the foreseeable
“identification reach” on the “true” model.
In conclusion, we have explored in some detail how the Z ′ discovery reach at the ILC
depends on the c.m. energy, on the available polarization, as well as on the model actually
realized in Nature. The lower part of this range, up to MZ ′ � 5 TeV, will also be covered
by the LHC, but the identification reach at the LHC is only up to MZ ′ < 2.2 TeV.
In this LHC discovery range, the cleaner ILC environment, together with the availability
of beam polarization, allow for an identification of the particular Z ′ version realized.
Actually, this ILC identification range extends considerably beyond the LHC identification
range. Specifically, the ILC with polarized beams at √ s =0.5 TeV allows to identify
all considered Z ′ bosons if MZ ′ <
∼ 4.5 TeV, substantially improving the LHC reach.
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