Searching for dilepton resonances below the $Z$ mass at the LHC

We consider LHC searches for dilepton resonances in an intermediate mass range, $\sim 10–80\mathrm{GeV}$. We adopt a kinetically mixed $Z^{\prime }$ as an example of weakly coupled new physics that might have evaded detection at previous experiments but which could still be probed by LHC dilepton spectrum measurements in this mass range. Based on Monte Carlo simulations, we estimate that existing data from the 7 and 8 TeV LHC could be used to test values of the kinetic mixing parameter $\epsilon$ several times smaller than precision electroweak upper bounds, provided an appropriate analysis were carried out by one of the experimental collaborations.

Figure 1

The 95% C.L. upper bounds on $\epsilon$ from ATLAS [35] (dashed blue line) and CMS [36] (thick green line) searches for high-mass dilepton resonances, along with the precision electroweak constraint taken from Ref. [25] (thin red line).

Figure 2

Left: with $\sqrt{s}=7\mathrm{TeV}$ and an integrated luminosity of $4.5{\mathrm{fb}}^{-1}$, a comparison between the Drell-Yan dimuon invariant-mass distributions predicted by CMS simulations (red squares) and by our simulations (blue circles). The muons are required to have ${(p_T)}_1>14\mathrm{GeV}$, ${(p_T)}_2>9\mathrm{GeV}$, and $|\eta |<2.4$. We rescale our $m_{\mu \mu }$ distribution to give the same total counts as the CMS simulation in the 60–120 GeV mass range. Right: discrepancies between our simulated bin counts and those of CMS.

Figure 3

With $\sqrt{s}=7\mathrm{TeV}$ and an integrated luminosity of $4.5{\mathrm{fb}}^{-1}$, simulated signal plus background for the benchmark parameter point $M_{Z^{\prime }}=50\mathrm{GeV}$ and $\epsilon =0.02$. The muons are required to have ${(p_T)}_1>14\mathrm{GeV}$, ${(p_T)}_2>9\mathrm{GeV}$, and $|\eta |<2.4$. We normalize background to data and apply a 75% efficiency to signal.

Figure 4

For $\sqrt{s}=7\mathrm{TeV}$ and an integrated luminosity of $4.5{\mathrm{fb}}^{-1}$, our simulated Drell-Yan $m_{\mu \mu }$ distributions with and without the ${(p_T)}_{\mu \mu }$ cut, along with best-fit fifth-order polynomial curves. The muons are required to have ${(p_T)}_1>14\mathrm{GeV}$, ${(p_T)}_2>9\mathrm{GeV}$, and $|\eta |<2.4$.

Figure 5

Estimated 95% C.L. upper bounds on $\epsilon$ based on Ref. [11], the $\sqrt{s}=7\mathrm{TeV}$ CMS study of the dimuon invariant mass distribution. For the thick blue line, $\kappa$ is set according to the procedure described in Sec. 3c. The dashed lines show results for other $\kappa$ values. The thin red line is the precision electroweak constraint taken from Ref. [25].

Figure 6

Exclusion sensitivity estimates for 7 TeV LHC data based on an integrated luminosity of $4.5{\mathrm{fb}}^{-1}$, with the cut $(p_T{)}_{\mu \mu }>23\mathrm{GeV}$ (left), and without a $(p_T{)}_{\mu \mu }$ cut (right). For the thick blue line, $\kappa$ is set according to the procedure described in Sec. 3c. The dashed lines show results for other $\kappa$ values. The thin red line is the precision electroweak constraint taken from Ref. [25].

Figure 7

For $\sqrt{s}=8\mathrm{TeV}$ and an integrated luminosity of $20{\mathrm{fb}}^{-1}$, our simulated Drell-Yan $m_{\mu \mu }$ distributions with and without the ${(p_T)}_{\mu \mu }$ cut, along with best-fit fifth-order polynomial curves. The muons are required to have ${(p_T)}_1>20\mathrm{GeV}$, ${(p_T)}_2>10\mathrm{GeV}$, and $|\eta |<2.4$.

Figure 8

Exclusion sensitivity estimates for 8 TeV LHC data based on an integrated luminosity an integrated luminosity of $20{\mathrm{fb}}^{-1}$, with the cut $(p_T{)}_{\mu \mu }>30\mathrm{GeV}$ (left), and without a $(p_T{)}_{\mu \mu }$ cut (right). For the thick blue line, $\kappa$ is set according to the procedure described in Sec. 3c. The dashed lines show results for other $\kappa$ values. The thin red line is the precision electroweak constraint taken from Ref. [25].

Figure 9

Estimated upper bounds on $\epsilon$ based on the CMS analysis of Ref. [11] (black, solid), and exclusion sensitivity estimates for 7 TeV LHC data (blue, short dashed), for 8 TeV LHC data (green, long dashed), and, under the assumptions stated in the text, for $3000{\mathrm{fb}}^{-1}$ of 14 TeV data (orange, dot-dashed). Results for the low-mass analyses incorporating a $(p_T{)}_{\mu \mu }$ cut are indicated by square markers. The $\kappa$ parameters are set according to the procedure described in Sec. 3c. The thin red line is the precision electroweak constraint taken from Ref. [25].