LEP Shines Light on Dark Matter

TL;DR

The paper analyzes dark matter production at LEP via mono-photon events to bound DM–electron couplings using an effective field theory with operators $O_V$, $O_S$, $O_A$, and $O_t$. It quantitatively translates limits on the suppression scale $\Lambda$ into DM–nucleon scattering and DM annihilation cross sections, comparing to direct and indirect searches, and explores renormalizable scenarios with light mediators. For $m_\chi \lesssim 80$ GeV, LEP bounds are competitive or superior to many astrophysical constraints, especially for spin-dependent scattering and for leptophilic scenarios where loop-induced nucleon couplings arise. The analysis shows that LEP results rule out simple leptophilic explanations for DAMA/CoGeNT and that the presence of light mediators can either weaken or strengthen the bounds depending on $M$ and $\Gamma$, underscoring the need to consider UV completions in collider-dark matter studies. Overall, LEP provides a complementary, robust probe of the dark sector that does not rely on astrophysical assumptions.

Abstract

Dark matter pair production at high energy colliders may leave observable signatures in the energy and momentum spectra of the objects recoiling against the dark matter. We use LEP data on mono-photon events with large missing energy to constrain the coupling of dark matter to electrons. Within a large class of models, our limits are complementary to and competitive with limits on dark matter annihilation and on WIMP-nucleon scattering from indirect and direct searches. Our limits, however, do not suffer from systematic and astrophysical uncertainties associated with direct and indirect limits. For example, we are able to rule out light (< 10 GeV) thermal relic dark matter with universal couplings exclusively to charged leptons. In addition, for dark matter mass below about 80 GeV, LEP limits are stronger than Fermi constraints on annihilation into charged leptons in dwarf spheroidal galaxies. Within its kinematic reach, LEP also provides the strongest constraints on the spin-dependent direct detection cross section in models with universal couplings to both quarks and leptons. In such models the strongest limit is also set on spin independent scattering for dark matter masses below ~4 GeV. Throughout our discussion, we consider both low energy effective theories of dark matter, as well as several motivated renormalizable scenarios involving light mediators.

Figures (9)

• Figure 1: Distribution of normalized photon energy in single-photon events at DELPHI. The agreement between the data (black dots with error bars) and both the full DELPHI Monte Carlo (solid yellow/light gray shaded histogram) as well as our CompHEP simulation (dotted histogram) is excellent. The blue shaded histogram shows what a hypothetical Dark Matter signal from $e^+ e^- \to \gamma\bar{\chi}\chi$ would look like. We have assumed vector-type contact interactions between electrons and dark matter, $m_\chi = 10$ GeV, and $\Lambda = 300$ GeV, see eq. \ref{['O1']}. The peak at $x_\gamma \sim 0.8$ corresponds to the process $e^+ e^- \to \gamma Z^0 \to \gamma \nu \bar{\nu}$, with an on-shell $Z^0$.
• Figure 2: DELPHI lower limits on the cutoff scale $\Lambda$ of the dark matter effective theory for the four operators eqs. \ref{['O1']}--\ref{['O3']} as a function of the dark matter mass. The wiggles in the plot are due to limited Monte Carlo statistics.
• Figure 3: DELPHI upper limits (thick lines) on the cross section for dark matter-nucleon scattering compared to results from direct detection experiments (thin lines and shaded regions). The left-hand plot is for spin-independent scattering, as would come from operators $\mathcal{O}_S$, $\mathcal{O}_V$, $\mathcal{O}_t$, and the right is for spin-dependent scattering through operator $\mathcal{O}_A$. The spin-independent limits of CDMS and XENON-100 are taken from Refs. Ahmed:2009zw and Aprile:2010um, respectively. The spin-dependent limits of DAMA, XENON-10, PICASSO, COUPP and SIMPLE are taken from Refs. Bernabei:2008yi, Angle:2008we, BarnabeHeider:2005pg, Behnke:2010xt and Girard:2011xc, respectively. The DAMA and CoGeNT-allowed regions are based on our own fit Kopp:2009qt to the data from Refs. Bernabei:2008yi and Aalseth:2010vx. Following Hooper:2010uy, we have conservatively assumed large systematic uncertainties on the DAMA quenching factors: $q_{\rm Na} = 0.3 \pm 0.1$ for sodium and $q_{\rm I} = 0.09 \pm 0.03$ for iodine. All limits are computed at the 90% confidence level, while the DAMA and CoGeNT allowed regions are shown at the 90% and $3\sigma$ confidence levels.
• Figure 4: Diagram for vector-type dark matter-proton scattering at the one-loop level.
• Figure 5: DELPHI upper limits on the cross section for spin-independent dark matter--nucleon scattering for the case of dark matter with tree level couplings only to electrons, but loop level couplings also to quarks, compared to results from the direct detection experiments DAMA Bernabei:2008yi, CoGeNT Aalseth:2010vx, CDMS Ahmed:2009zw, and XENON-100 Aprile:2010um. The DAMA and CoGeNT allowed regions are based on our own fit Kopp:2009qt to the data from refs. Bernabei:2008yiAalseth:2010vx. We conservatively assume $q_{\rm Na} = 0.3 \pm 0.1$ and $q_{\rm I} = 0.09 \pm 0.03$ for the DAMA quenching factors. All limits are computed at the 90% confidence level, while the DAMA and CoGeNT allowed regions are shown at the 90% and $3\sigma$ confidence levels.