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Searching for Low Mass Dark Portal at the LHC

Haipeng An, Ran Huo, Lian-Tao Wang

TL;DR

This work analyzes a light leptophobic $Z'$ as a dark portal connecting dark matter to the Standard Model, focusing on LHC8 sensitivity via associated production channels ($Z'+$jet, $Z'+\gamma$, $Z'+W$, $Z'+Z^0$). By reconstructing the $Z'$ resonance and exploiting lower SM backgrounds, the study shows that $M_{Z'}$ in the $\sim80$–$400$ GeV range can yield reach comparable to or better than monojet/dijet searches, particularly when $g_D<g_{Z'}$ and for which the thermal relic abundance favors $M_{Z'}\sim100$ GeV. The authors map collider constraints onto the DM direct detection cross section $\sigma_{SI}$ and relic density requirements, highlighting regions where associated production probes parameter space inaccessible to monojet searches, and emphasizing the complementarity between collider and direct-detection approaches. Overall, the paper demonstrates a viable, multi-channel search strategy for a dark portal at the LHC with significant implications for DM phenomenology and future collider studies.

Abstract

Light dark matter with mass smaller than about 10 GeV is difficult to probe from direct detection experiments. In order to have the correct thermal relic abundance, the mediator of the interaction between dark matter and the Standard Model (SM) should also be relatively light, $\sim 10^2$ GeV. If such a light mediator couples to charged leptons, it would already be strongly constrained by direct searches at colliders. In this work, we consider the scenario of a leptophobic light $Z'$ vector boson as the mediator, and study the the prospect of searching for it at the 8 TeV Large Hadron Collider (LHC). To improve the reach in the low mass region, we perform a detailed study of the processes that the $Z'$ is produced in association with jet, photon, $W^\pm$ and $Z^0$. We show that in the region where the mass of $Z'$ is between 80 and 400 GeV, the constraint from associated production can be comparable or even stronger than the known monojet and dijet constraints. Searches in these channels can be complementary to the monojet search, in particular if the $Z'$ couplings to quarks ($g_{Z'}$) and dark matter ($g_D$) are different. For $g_D < g_{Z'}$, we show that there is a larger region of parameter space which has correct thermal relic abundance and a light $Z'$, $M_{Z'} \sim 100 $ GeV. This region, which cannot be covered by the mono-jet search, can be covered by the resonance searches described in this paper.

Searching for Low Mass Dark Portal at the LHC

TL;DR

This work analyzes a light leptophobic as a dark portal connecting dark matter to the Standard Model, focusing on LHC8 sensitivity via associated production channels (jet, , , ). By reconstructing the resonance and exploiting lower SM backgrounds, the study shows that in the – GeV range can yield reach comparable to or better than monojet/dijet searches, particularly when and for which the thermal relic abundance favors GeV. The authors map collider constraints onto the DM direct detection cross section and relic density requirements, highlighting regions where associated production probes parameter space inaccessible to monojet searches, and emphasizing the complementarity between collider and direct-detection approaches. Overall, the paper demonstrates a viable, multi-channel search strategy for a dark portal at the LHC with significant implications for DM phenomenology and future collider studies.

Abstract

Light dark matter with mass smaller than about 10 GeV is difficult to probe from direct detection experiments. In order to have the correct thermal relic abundance, the mediator of the interaction between dark matter and the Standard Model (SM) should also be relatively light, GeV. If such a light mediator couples to charged leptons, it would already be strongly constrained by direct searches at colliders. In this work, we consider the scenario of a leptophobic light vector boson as the mediator, and study the the prospect of searching for it at the 8 TeV Large Hadron Collider (LHC). To improve the reach in the low mass region, we perform a detailed study of the processes that the is produced in association with jet, photon, and . We show that in the region where the mass of is between 80 and 400 GeV, the constraint from associated production can be comparable or even stronger than the known monojet and dijet constraints. Searches in these channels can be complementary to the monojet search, in particular if the couplings to quarks () and dark matter () are different. For , we show that there is a larger region of parameter space which has correct thermal relic abundance and a light , GeV. This region, which cannot be covered by the mono-jet search, can be covered by the resonance searches described in this paper.

Paper Structure

This paper contains 10 sections, 21 equations, 5 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Simulation and Event Selection
  3. Signal and Background Simulation
  4. Event Selection
  5. Reach of Different Channels
  6. $Z'$ as a Portal between the SM and Dark Matter
  7. Summary and discussions
  8. Acknowledgments
  9. Variations on jet energy thresholds
  10. Low $M_{Z'}$ detection thresholds for different channels

Figures (5)

  • Figure 1: Reach plots for $S/\sqrt{B}$ for the $Z'$ + jet (red), the $Z'+\gamma$ (green), the $Z'+ W^\pm$ (blue) and the $Z'+ Z^0$ (purple) channels, for 15 fb$^{-1}$ integrated luminosity and $S/\sqrt{B}=2$ which corresponds to 95% confidence level. All the channel is assuming $g_{Z'L}=g_{Z'R}=g_{Z'}$, and in the $Z'+ Z^0$ channel the reach from two charged leptons decay product and from two neutrinos are combined. Eventually their combination for exclusion is shown as the black thick line.
  • Figure 2: Comparison between the combined constraint of different channels shown in Fig. \ref{['fig:Z4channels']} and the constraints from dijet and monojet searches. We have assumed $r \equiv g_{D}/g_Z' =1$. The thick black curves shows the combined constraint as in Fig. \ref{['fig:Z4channels']}. The red curve corresponds to the 95% C.L. ATLAS monojet upper limit. The green and blue curves correspond to 95% C.L. upper limits from CDF and ATLAS dijet searches, respectively. The purple curve corresponds to 90% C.L. upper limits from UA2 dijet search.
  • Figure 3: Comparison of the $g_{Z'}=g_D$ reach of our $Z'$ association production (solid and dashed black curves for $2\sigma$ expected constraints and $5\sigma$ reach, respectively) with other experiments. The red curve is the constraint from ATLAS monojet search with 10 fb$^{-1}$ integrated luminosity at 8 TeV. The green curve is the bound from dijet resonance search by CDF with 1.13 fb$^{-1}$ integrated luminosity. The blue curve is the constraint from ATLAS dijet resonance search with 1 fb$^{-1}$ data set.
  • Figure 4: Comparison of the 5$\sigma$ reach for the discovery of a 5 GeV WIMP by the $Z'$ associated production search (all solid curves) and monojet search (all dash-dotted curves), for different ratios of $r\equiv g_D/g_{Z'}$. The region of parameter space above a certain curve either has been excluded or can be constrained by the corresponding search channel. All curves in red correspond to $g_D/g_{Z'} = 0.3$, all curves in green correspond to $g_D/g_{Z'} =0.7$, and all curves in blue correspond to $g_D/g_{Z'} =1$. In shaded region, the $\chi\bar{\chi}\leftrightarrow Z'$ process generates the correct relic abundance, namely requiring the cold dark matter density of the universe $\Omega_{\rm cdm} h^2 = 0.111\pm0.006$pdg.
  • Figure 5: Background distributions of dijet invariant mass in $Z'$+jet, $Z'+\gamma$ and $Z'+W$ channels. For $Z'+$jet channel, it is the invariant mass of the second and third hardest jets shown in the plot.