Search for dark photons produced in 13 TeV $pp$ collisions

Abstract: Searches are performed for both prompt-like and long-lived dark photons, $A^{\prime}$, produced in proton-proton collisions at a center-of-mass energy of 13 TeV, using $A^{\prime}\to\mu^+\mu^-$ decays and a data sample corresponding to an integrated luminosity of 1.6 fb$^{-1}$ collected with the LHCb detector. The prompt-like $A^{\prime}$ search covers the mass range from near the dimuon threshold up to 70 GeV, while the long-lived $A^{\prime}$ search is restricted to the low-mass region $214<m(A^{\prime})<350$ MeV. No evidence for a signal is found, and 90% confidence level exclusion limits are placed on the $\gamma$-$A^{\prime}$ kinetic-mixing strength. The constraints placed on prompt-like dark photons are the most stringent to date for the mass range $10.6 < m(A^{\prime}) < 70$ GeV, and are comparable to the best existing limits for $m(A^{\prime}) < 0.5$ GeV. The search for long-lived dark photons is the first to achieve sensitivity using a displaced-vertex signature.

• The paper reports that LHCb achieved the most stringent dark photon constraints between 10.6 and 70 GeV, extending previous limits.
• The study employed both prompt-like and long-lived searches using 1.6 fb⁻¹ of data and dimuon decay signatures.
• The findings underscore LHCb’s potential for probing dark sector physics and pave the way for enhanced sensitivity in future runs.

Search for Dark Photons at LHCb

This paper presents the findings from a search for dark photons, $A'$, conducted by the LHCb experiment at CERN. The study is motivated by the hypothesis that dark matter might interact via forces that are weakly coupled to Standard Model (SM) particles, potentially mediated via a dark photon. The dark photon scenario considers a massive particle that couples to the SM electromagnetic current through kinetic mixing, characterized by a parameter $\varepsilon$.

Methodology

The LHCb collaboration conducted searches for both prompt-like and long-lived dark photons produced in proton-proton collisions at a center-of-mass energy of 13 TeV. The dataset used corresponds to an integrated luminosity of 1.6 fb$^{-1}$. The research focused on dimuon decays, capitalizing on the LHCb detector's capabilities.

• Prompt-like Search: This aspect of the study aimed to discover dark photons with masses near the dimuon threshold extending up to 70 GeV, a range dominated by the $Z$ boson at higher masses. In this mass window, the kinetic mixing strengths were systematically probed, yielding the most stringent constraints to date for dark photon masses between 10.6 and 70 GeV.
• Long-lived Search: For this search, focus was given to the mass range between 214 MeV and 350 MeV. The study employed a unique approach by utilizing a displaced-vertex signature, marking the first such analysis to attain sensitivity via this method.

Results and Interpretations

In both the prompt-like and long-lived dark photon searches, no evidence for a signal exceeding the background noise was identified. The results allow for the exclusion of regions within the $(m_{A'}, \varepsilon^2)$ parameter space at a 90% confidence level. Notably, the constraints on prompt-like dark photons extend the previously known limits and set new standards for the mass range of interest.

Key findings include:

• For prompt-like dark photons: Constraints were most stringent for masses between 10.6 and 70 GeV, surpassing former experimental limits.
• For long-lived dark photons: Although the parameter space exclusion was limited, the study demonstrated significant potential as more data are expected to be collected, which would enhance sensitivity.

Implications and Future Prospects

The results reinforce the LHCb experiment's utility in exploring dark sector physics. With enhancements in trigger efficiencies, especially targeting low-mass regions as indicated by this analysis, substantial improvements in sensitivity are anticipated in future runs (Run 3 and beyond). This optimizes the potential to explore still uncharted domains of kinetic mixing strength relevant to dark photon models, thereby elucidating new pathways in dark matter research.

The paper fundamentally strengthens the narrative that while no signals were found, the methodological advancements and limits set pave a pivotal trajectory for forthcoming explorations in particle physics, especially concerning dark matter constituent interactions. By broadening the operational parameter space accessible to experiments like LHCb, further refinements and data acquisitions could uncover new phenomena associated with the dark sector.