Extending the discovery potential for inelastic-dipole dark matter with FASER

Open Access

Extending the discovery potential for inelastic-dipole dark matter with FASER

Keith R. Dienes, Jonathan L. Feng, Max Fieg, Fei Huang, Seung J. Lee, and Brooks Thomas

Phys. Rev. D 107, 115006 – Published 12 June 2023

Abstract

Neutral particles are notoriously difficult to observe through electromagnetic interactions. As a result, they naturally elude detection in most collider detectors. In this paper, we point out that neutral particles that interact through a dipole interaction can nevertheless be detected in far-forward detectors designed to search for long-lived particles (LLPs). In contrast to previous analyses that focused on neutral particles with elastic interactions, we consider inelastic interactions. This naturally leads to LLPs, and we demonstrate that FASER (and future experiments at the Forward Physics Facility) will be able to probe substantial regions of the associated parameter space. In particular, we find that FASER is capable of probing the region of parameter space wherein thermal freeze-out gives rise to an $O (GeV)$ dark-matter candidate with the appropriate relic abundance, as well as regions of parameter space that are difficult to probe at fixed-target experiments. FASER and its successor experiments may therefore play a critical role in the discovery of such a dark-matter candidate.

Received 9 February 2023
Accepted 9 May 2023

DOI:https://doi.org/10.1103/PhysRevD.107.115006

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Dark matter Electric moment Magnetic moment

Colliders

Particles & Fields

Authors & Affiliations

Keith R. Dienes ^1,2,*, Jonathan L. Feng ^3,†, Max Fieg ^3,‡, Fei Huang ^4,§, Seung J. Lee^5,∥, and Brooks Thomas ^6,¶

¹Department of Physics, University of Arizona, Tucson, Arizona 85721 USA
²Department of Physics, University of Maryland, College Park, Maryland 20742 USA
³Department of Physics and Astronomy, University of California, Irvine, California 92697 USA
⁴Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 7610001, Israel
⁵Department of Physics, Korea University, Seoul 136-713, Korea
⁶Department of Physics, Lafayette College, Easton, Pennsylvania 18042 USA

^*dienes@arizona.edu
^†jlf@uci.edu
^‡mfieg@uci.edu
^§fei.huang@weizmann.ac.il
^∥sjjlee@korea.ac.kr
^¶thomasbd@lafayette.edu

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Issue

Vol. 107, Iss. 11 — 1 June 2023

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Images

Figure 1
Effective cross-sections $σ_{χ_{1} M}^{(eff)}$ for the production of $χ_{1}$ particles in the direction of FASER from the decay of several relevant meson species $M$ , shown as functions of $m_{0}$ for $η_{F} = 9.2$ . In order to display our results in as model-independent a manner as possible, we scale each cross-section by a factor of $(Λ_{O} / GeV)^{2}$ to cancel the overall factor of $Λ_{O}^{- 2}$ common to all of the $σ_{χ_{1} M}^{(eff)}$ . The solid curves correspond to the case of a MDM operator, while the dashed curves correspond to the case of an EDM operator. The results shown here correspond to the case in which $Δ = 0.01$ ; however, we note that these results are not particularly sensitive to the value of $Δ$ , provided that $Δ ≪ 1$ .
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Figure 2
Contours within the $(Δ, E_{γ, \min})$ -plane of the fraction $f_{γ} (E_{γ, \min})$ of signal events that would be detected at FASER for a given photon-energy threshold $E_{γ, \min}$ , relative to the number that would be detected for $E_{γ, \min} = 0$ . The solid, dashed, and dotted contours appearing in each panel correspond to different combinations of the parameters $m_{0}$ and $Λ_{O}^{- 1}$ . These combinations are specified in the legend using the notation $(m_{0}, Λ_{O}^{- 1})$ , where $m_{0}$ and $Λ_{O}^{- 1}$ are given in units of MeV and ${GeV}^{- 1}$ , respectively. Two such contours are included for each parameter combination. The upper contour corresponds to $f_{γ} (E_{γ, \min}) = 0.1$ , while the lower contour corresponds to $f_{γ} (E_{γ, \min}) = 0.5$ . The three panels of the figure simply display different regions of the $(Δ, E_{γ, \min})$ -plane in order to illustrate the shapes of these contours for the three $m_{0}$ and $Λ_{O}$ combinations shown therein.
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Figure 3
The integrated luminosity required to observe a monophoton signal of inelastic-dipole dark matter at 95% C.L. at FASER (red curves) and FASER2 (blue curves) within the $(Λ_{O}^{- 1}, m_{0})$ -plane for a variety of different choices of $Δ$ . The top, middle, and bottom panels in each column respectively correspond to the choices $Δ = 0.001$ , $Δ = 0.01$ , and $Δ = 0.05$ , while the left and right columns respectively correspond to the cases in which the dark-sector particles couple to the visible sector via an MDM interaction and via an EDM interaction. The dashed curve in each panel indicates the contour along which the present-day relic abundance of $χ_{0}$ accords with the observed dark-matter abundance. The gray regions in each panel are excluded by existing constraints. These include constraints from searches at BABAR, constraints from Nu-Cal and CHARM-II limits on $χ_{1}$ decay, and constraints from probes of dipole-interacting LLPs that are insensitive to the value of $Δ$ and thus effectively identical to those obtained for an elastic-dipole interaction [30]. The exclusion region labeled “ $CHARM − II + LEP$ ( $Δ$ -Ins.)” in each panel is excluded by this last set of constraints.
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Figure 4
The evolution of the comoving number densities $Y_{i}$ of the two dark-sector fields $χ_{i}$ , shown as functions of $m_{0} / T$ for the parameter assignments specified in the legend. The solid red and dashed blue curves respectively represent $Y_{0}$ and $Y_{1}$ , whereas the solid orange and dashed cyan curves represent the corresponding equilibrium comoving number densities $Y_{0}^{eq}$ and $Y_{1}^{eq}$ . The dotted gray line indicates the value of $Y_{i}$ that yields the correct present-day dark-matter abundance for this choice of model parameters. The yellow curve represents the quantity $Y_{0} (Y_{1}^{eq} / Y_{0}^{eq})$ , which, in accord with Eq. (4.4), effectively coincides with $Y_{1}$ across the entire range of $m_{0} / T$ shown.
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Figure 5
Similar to Fig. 3, but including contours representing the discovery reach of SHiP (dashed orange curves) for different monophoton energy thresholds. Each such contour corresponds to the observation of three signal events for a total of $2 \times 10^{20}$ protons on target. Contours are shown for the thresholds $E_{γ} \geq 0.3 GeV$ , $E_{γ} \geq 1 GeV$ , $E_{γ} \geq 3 GeV$ , and $E_{γ} \geq 5 GeV$ . We observe that SHiP has a comparable reach to FASER2 for large $Δ$ , even for larger detection thresholds, but has far less sensitivity than FASER2 for small $Δ$ , even for a detection threshold $E_{γ} \geq 0.3 GeV$ .
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