Search for high-mass dilepton resonances in $pp$ collisions at $\sqrt{s}=8\mathrm{TeV}$ with the ATLAS detector

The ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to dielectron or dimuon final states. Results are presented from an analysis of proton-proton ($pp$) collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of $20.3{\mathrm{fb}}^{-1}$ in the dimuon channel. A narrow resonance with Standard Model $Z$ couplings to fermions is excluded at 95% confidence level for masses less than 2.79 TeV in the dielectron channel, 2.53 TeV in the dimuon channel, and 2.90 TeV in the two channels combined. Limits on other model interpretations are also presented, including a grand-unification model based on the $E_6$ gauge group, $Z^{*}$ bosons, minimal $Z\prime$ models, a spin-2 graviton excitation from Randall-Sundrum models, quantum black holes, and a minimal walking technicolor model with a composite Higgs boson.

Figure 1

Product of acceptance and efficiency for the dielectron (upper distribution) and dimuon (lower distribution) selections as a function of the $Z_{\mathrm{SSM}}^{\prime }$ pole mass.

Figure 2

Dielectron (top) and dimuon (bottom) invariant mass ($m_{\ell \ell }$) distributions after event selection, with two selected $Z_{\mathrm{SSM}}^{\prime }$ signals overlaid, compared to the stacked sum of all expected backgrounds, and the ratios of data to background expectation. The bin width is constant in $\log m_{\ell \ell }$. The green band in the ratio plot shows the systematic uncertainties described in Sec. 9.

Figure 3

Median expected (dashed line) and observed (solid red line) 95% C.L. upper limits on cross section times branching ratio ($\sigma B$) in the combined dilepton channel, along with predicted $\sigma B$ for $Z_{\mathrm{SSM}}^{\prime }$ production. The inner and outer bands show the range in which the limit is expected to lie in 68% and 95% of pseudoexperiments, respectively. The thickness of the $Z_{\mathrm{SSM}}^{\prime }$ theory curve represents the theoretical uncertainty from the PDF error set and ${\alpha }_S$, as well as the choice of PDF.

Figure 4

Median expected (dashed line) and observed (solid line) 95% C.L. upper limits on cross section times branching ratio ($\sigma B$) for $Z_{\mathrm{SSM}}^{\prime }$ production for the exclusive dimuon and dielectron channels, and for both channels combined. The width of the $Z_{\mathrm{SSM}}^{\prime }$ theory band represents the theoretical uncertainty from the PDF error set, the choice of PDF, as well as ${\alpha }_S$.

Figure 5

Observed upper cross section times branching ratio ($\sigma B$) limits at 95% C.L. for $Z_{\mathrm{SSM}}^{\prime }$, $E_6$-motivated $Z\prime$ and $Z^{*}$ bosons using the combined dilepton channel. In addition, theoretical cross sections on $\sigma B$ are shown for the same models. The stars indicate the lower mass limits for each considered model. The width of the $Z_{\mathrm{SSM}}^{\prime }$ band represents the theoretical uncertainty from the PDF error set, the choice of PDF, as well as ${\alpha }_S$. The width of the $Z_{\mathrm{SSM}}^{\prime }$ band applies to the $E_6$-motivated $Z\prime$ curves as well.

Figure 6

Expected (dotted and dashed lines) and observed (filled area and lines) limits at 95% C.L. on the strength of the $Z\prime$ boson coupling relative to that of the SM $Z$ boson ($\gamma \prime$) for the combined dielectron and dimuon channels as a function of the $Z_{\min }^{\prime }$ mass in the minimal $Z\prime$ models parametrization. Limit curves are shown for three representative values of the mixing between the generators of the B-L and the weak hypercharge Y gauge groups (${\theta }_{\min }$). These are $\tan {\theta }_{\min }=0$, $\tan {\theta }_{\min }=-2$, and $\tan {\theta }_{\min }=-0.8$, which correspond respectively to the $Z_{\mathrm{B}\text{−}\mathrm{L}}^{\prime }$, $Z_{3\mathrm{R}}^{\prime }$, and $Z_{\chi }^{\prime }$ models at specific values of $\gamma \prime$. The region above each line is excluded. The gray band envelops all observed limit curves, which depend on the choice of ${\theta }_{\min }\in [0,\pi ]$. The corresponding expected limit curves are within the area delimited by the two dotted lines.

Figure 7

Expected (empty markers and dashed lines) and observed (filled markers and lines) limits at 95% C.L. on the strength of the $Z\prime$ boson coupling relative to that of the SM $Z$ boson ($\gamma \prime$) for the combined dielectron and dimuon channels as a function of the mixing between the generators of the B-L and the weak hypercharge Y gauge groups (${\theta }_{\min }$) in the minimal $Z\prime$ models parametrization. The limits are set for several representative values of the mass of the $Z\prime$ boson, $M_{Z_{\min }^{\prime }}$. The region above each line is excluded.

Figure 8

Expected and observed 95% C.L. limits in the plane of the coupling strength of the Randall-Sundrum $G^{*}$ to SM particles ($k/{\overline{M}}_{\mathrm{Pl}}$) versus $G^{*}$ mass for the combination of the dielectron and dimuon channels. The region above the curve is excluded at 95% C.L.

Figure 9

Expected and observed 95% C.L. upper limits on cross section times branching ratio ($\sigma B$) for quantum black hole production in the extra-dimensional model proposed by ADD and RS for the combined dilepton channel.

Figure 10

Exclusion contours at 95% C.L. in the plane of the minimal walking technicolor parameter space defined by the bare axial-vector mass versus the strength of the spin-1 resonance interaction ($M_A$, $\tilde{g}$). Electroweak precision measurements exclude the green area in the bottom left corner. The requirement to stay in the walking regime excludes the blue area in the right corner. The red area (black dashed line) shows the observed (expected) exclusion for both channels combined. The upper region is excluded due to nonreal axial and axial-vector decay constants.