We study collider signatures of a class of dark matter models with a GeV-scale dark
Article funded by SCOAP3
500$\,GeV and $M_{Z^\prime}=1$\,GeV, the separation of the tracks is on the order of $10^{-3}$ which is similar to the angular resolution of the ATLAS inner tracker~\cite{Aad-ml-2008zzm,Aad-ml-2010bx}. Although this presents an experimental challenge, the variables considered here are primarily sensitive to the distribution of the radiation and can be applicable even if individual tracks are difficult to resolve. Distributions of the observables above for $Z'$-jets and QCD jets are shown in figure~\ref{fig:jetsub}, where there is a clear difference in the properties of the two objects. (For $\tau_h$, the distributions are very similar to that of $Z'$-jets, except that events primarily have $N_{\rm track} = 1$ or $N_{\rm track} = 3$.) To account for the experimental resolution of tracks, we have applied a simple smearing of track $p_T$ with $\delta p_T/p_T = 0.05$~\cite{Aad-ml-2010bx}. This has the largest effect on the track jet mass and the track radius, while barely affecting $f_\textrm{core}$. Note that since the primary distinguishing observables are track-based, and since the observables are highly correlated, we neglect the effects of pileup and calorimeter energy resolution in $f_\textrm{core}$. \begin{table} \renewcommand{\arraystretch}{1.25} \centering \begin{tabular}{ | c | c | c | c | c | c | } \hline $f_\textrm{core} \ge$ & $m_\textrm{track} \le $ & $R_\textrm{track}\le$ & $N_\textrm{track}\le$ & Signal Eff. & Background Eff. \\ \hline 0.95 & 10\,GeV & 0.01 & 4 & 0.36 & 0.001 \\ \hline 0.9 & 20\,GeV & 0.01 & 4 & 0.40 & 0.002 \\ \hline 0.9 & 20\,GeV & 0.02 & 2 & 0.48 & 0.001 \\ \hline 0.9 & 20\,GeV & 0.02 & 4 & 0.60 & 0.005 \\ \hline 0.9 & 20\,GeV & 0.02 & 6 & 0.63 & 0.019 \\ \hline \end{tabular} ]]>
500$\,GeV. The black and lower lines are using the traditional mono-jet analysis. The red and upper lines are based on a jet-substructure analysis for the mono-$Z^\prime$ jet. The systematic error is assumed to be 10\% (5\%) for the solid (dashed) line. The mono-$Z^\prime$ tag efficiency is chosen to be 50\% and the background mistag efficiency is 2\%. Right panel: the same as the left one but for the axi-vector interaction operator.]]>
500$\,GeV. We require the leading jet to have $p_T$ above 200\,GeV and veto events with a second jet with $p_T>60$\,GeV or with a lepton with $p_T>20$\,GeV. The total background cross section, $W/Z$ + jets, is simulated to be 142\,fb at leading order and after applying detector-level cuts. For the mono-$Z^\prime$ signature, there is also a potential background from $W(\tau \nu)$+jets with hadronic-$\tau$ decay. We neglect this background, since it can be suppressed by a veto on $\tau_h$. The total cross section for $W(\tau_h \nu)$+jets is 1.1\,fb after the $E_T^{\rm miss} >$ 500\,GeV cut and 0.8\,fb after imposing our $Z^\prime$-jet tagging. An additional veto on events with one or three tracks in the inner $\Delta R=0.2$ ``isolation cone'' can reduce this background to 0.05\,fb, which is negligible. In figure~\ref{fig:cutoff-constraints}, we show the 90\% C.L. constraints on the cutoff of the effective operators for two different assumptions of systematic errors. Compared to the current limits, which give $\Lambda \gtrsim 1030$\,GeV for light dark matter at the 8\,TeV LHC, the 14\,TeV LHC mono-jet analysis can improve the limits by around 30\%. By including the contribution of FSR from mono-$Z'$, the limits can be improved by another 50$\%$. On the other hand, if one performs a dedicated analysis for the mono-$Z^\prime$ jet signature, the constraints on the cutoff can be improved by another factor of two. This is because the current mono-jet searches are limited by the systematic errors and the unique characteristics of the mono-$Z^\prime$ jet can dramatically reduce the background events. For a long-lived $Z^\prime$, the signature is so peculiar such that the SM background is expected to be negligibly small. The existing search on displaced dijets has focused on heavier particle masses above 50\,GeV~\cite{CMS-ml-2014wda,Han-ml-2007ae}. The light $Z^\prime$ should behave more like a $\tau$-lepton, with smaller vertex track multiplicity and smaller jet mass. We do not perform a detailed analysis for the long-lived $Z^\prime$ case in this paper. For light dark matter and still requiring $p_T(Z^\prime) > 500$\,GeV, one can obtain a constraint on the cutoff as large as $\Lambda > 8$\,TeV by assuming negligible backgrounds and allowing up to five signal events. A dedicated analysis with a less stringent $p_T$ cut on $Z^\prime$ is very likely to have even better sensitivity. \begin{figure} \centering \includegraphics[width=0.46\textwidth]{sip.pdf} \hspace{3mm} \includegraphics[width=0.46\textwidth]{sdp.pdf} ]]>
500$\,GeV, the two-body productions become important only for a light dark matter mass because of the need of a large boost for $Z^\prime$. If the excited state is light enough, then it can be produced on-shell with a large boost $O(100)$, which in turn allows the $Z'$ produced in the decay to have a large enough momentum $p_T(Z^\prime) = O(100 M_{Z^\prime})$ and pass the $p_T$ cut. For heavy dark matter mass, the three-body process dominates and the small mass splitting of the two states can be ignored, such that the production cross sections follow the same behavior as in the elastic case. For $\Lambda=5$\,TeV, we show the production cross section at the 14\,TeV LHC in the left panel of figure~\ref{fig:secluded-zprime-prod-idm}. As one can see, for dark matter mass below roughly 10\,GeV there is an enhanced production cross section due to the two-body process. In the right panel of figure~\ref{fig:secluded-zprime-prod-idm}, we show the potential constraints on the cutoff from the standard mono-jet analysis and the jet-substructure based mono-$Z^\prime$ analysis. Comparing to the limits for the elastic dark matter model in figure~\ref{fig:cutoff-constraints}, one can see that for the inelastic dark matter model the constraints for light dark matter below 10\,GeV are more stringent. ]]>
120$\,GeV to search for dark matter together with a light $Z^\prime$ at the 14\,TeV LHC. We also show the limits from applying jet-substructure cuts, assuming the default 50\% of the signal tag efficiency and 2\% background mistag efficiency. One can see that even though the mono-$Z^\prime$ signature has a small production cross section, the jet-substructure analysis can improve on the sensitivity beyond the mono-jet analysis. ]]>