Searching for extra $Z^{\prime }$ from strings and other models at the CERN LHC with leptoproduction

Discovery potentials for extra neutral interactions at the Large Hadron Collider in forthcoming experiments are analyzed using resonant leptoproduction. For this purpose we use high precision next-to-next-to-leading order determinations of the QCD background in this channel, at the tail of the Drell-Yan distributions, in the invariant mass region around $0.8<Q<2.5\mathrm{TeV}$. We focus our analysis primarily on a novel string-inspired $Z^{\prime }$, obtained in left-right symmetric free fermionic heterotic string models and whose existence at low energies is motivated by its role in suppressing proton decay mediation. We analyze the parametric dependence of the predictions and perform comparison with other models based on bottom-up approaches, that are constructed by requiring anomaly cancellation and enlarged Higgs structure. We show that the results are not particularly sensitive to the specific charge assignments. This may render quite difficult the extraction of significant information from the forward-backward asymmetries on the resonance, assuming that these are possible due to a sizeable width. The challenge to discover extra (nonanomalous) $Z^{\prime }$ in this kinematic region remains strongly dependent on the size of the new gauge coupling. Weakly coupled extra $Z^{\prime }$ will not be easy to identify even with a very good theoretical determination of the QCD background through next-to-next-to-leading order.

Figure 1

Plot of the LO, NLO, and NNLO cross section for the free fermionic model with $M_{Z^{\prime }}=800\mathrm{GeV}$.

Figure 2

Free fermionic model at the LHC $\tan \beta =40$.

Figure 3

Free fermionic model at the LHC $\tan \beta =40$ and $g_z=0.1$. Shown are also the SM results through the same perturbative orders.

Figure 4

Total cross section for the free fermionic model at NLO for three different values of $g_z$ and for $M_{Z^{\prime }}=1.2\mathrm{TeV}$. Here, we have chosen ${\mu }_F={\mu }_R=Q$ for simplicity, and we have integrated the mass invariant distribution on the interval $M_{Z^{\prime }}\pm 3{\Gamma }_{Z^{\prime }}$.