3.1 Production, Decays and Detection
49
The smallness of the couplings implies that the dark photons are also very longlived (up to 0.1 s) compared to the bulk of the SM particles. Hence: The decays
to SM particles can be optimally detected using experiments with long decay
volumes followed by spectrometers with excellent tracking systems and particle
identification capabilities.
– Missing momentum/energy techniques: invisible decay of dark photons can be
detected in fixed-target reactions as, for example, e
− Z → e
− ZA
(Z being the
nuclei atomic number) with A
→ χ χ and χ being a putative dark matter particle,
by measuring the missing momentum or missing energy carried away from the
escaping invisible particle or particles. The main challenge for this approach is the
very high background rejection that must be achieved, which relies heavily on the
detector being hermetically closed and, in some cases, on the exact knowledge of
the initial and final state kinematics.
These techniques guarantee an intrinsic better sensitivity for the same luminosity
than the technique based on the detection of dark photons decaying to visible final
states, as it is independent of the probability of decays and therefore scales only
as the SM-dark photon coupling squared, ε
2 .
– Missing mass technique:
This technique is mostly used to detect invisible particles (as DM candidates or
particles with very long lifetimes) in reactions with a well-known initial state, as
for example, at e
+ e
− collider experiments using the process e
+ e
−
→ A
γ , where
A
is on shell, using the single photon trigger.
Characteristic signature is the presence of narrow resonances emerging over a
smooth background in the distribution of the missing mass.
It requires detectors with very good hermeticity that allow to detect all the other
particles in the final state. Characteristic signature of this reaction is the presence
of a narrow resonance emerging over a smooth background in the distribution of
the missing mass. The main limitation of this technique is the required knowledge
of the background arising from processes in which particles in the final state escape
the apparatus without being detected.
3.2 Visible and Invisible Massive Dark Photon
In collecting the limits on the parameters of massive dark photon is important to
distinguish two cases accordingly on whether its mass is smaller or larger than twice
the mass of the electron, the lightest charged SM fermion.
The dark photon is visible if its mass is M A > 2m e 1 MeV because it can decay
into SM charged states which leave a signature in the detectors (Fig. 3.2, top). We
discuss the limits on the visible dark photon in Sect. 3.3.1.
In the same regime for which M A > 1 MeV, however, the massive dark photon
could also decay into dark sector states if their masses are light enough. In this case
we have a non-vanishing branching ratio into invisible final states. The invisible
decay into these states of the dark sector χ in given by (Fig. 3.2, bottom).
49
The smallness of the couplings implies that the dark photons are also very longlived (up to 0.1 s) compared to the bulk of the SM particles. Hence: The decays
to SM particles can be optimally detected using experiments with long decay
volumes followed by spectrometers with excellent tracking systems and particle
identification capabilities.
– Missing momentum/energy techniques: invisible decay of dark photons can be
detected in fixed-target reactions as, for example, e
− Z → e
− ZA
(Z being the
nuclei atomic number) with A
→ χ χ and χ being a putative dark matter particle,
by measuring the missing momentum or missing energy carried away from the
escaping invisible particle or particles. The main challenge for this approach is the
very high background rejection that must be achieved, which relies heavily on the
detector being hermetically closed and, in some cases, on the exact knowledge of
the initial and final state kinematics.
These techniques guarantee an intrinsic better sensitivity for the same luminosity
than the technique based on the detection of dark photons decaying to visible final
states, as it is independent of the probability of decays and therefore scales only
as the SM-dark photon coupling squared, ε
2 .
– Missing mass technique:
This technique is mostly used to detect invisible particles (as DM candidates or
particles with very long lifetimes) in reactions with a well-known initial state, as
for example, at e
+ e
− collider experiments using the process e
+ e
−
→ A
γ , where
A
is on shell, using the single photon trigger.
Characteristic signature is the presence of narrow resonances emerging over a
smooth background in the distribution of the missing mass.
It requires detectors with very good hermeticity that allow to detect all the other
particles in the final state. Characteristic signature of this reaction is the presence
of a narrow resonance emerging over a smooth background in the distribution of
the missing mass. The main limitation of this technique is the required knowledge
of the background arising from processes in which particles in the final state escape
the apparatus without being detected.
3.2 Visible and Invisible Massive Dark Photon
In collecting the limits on the parameters of massive dark photon is important to
distinguish two cases accordingly on whether its mass is smaller or larger than twice
the mass of the electron, the lightest charged SM fermion.
The dark photon is visible if its mass is M A > 2m e 1 MeV because it can decay
into SM charged states which leave a signature in the detectors (Fig. 3.2, top). We
discuss the limits on the visible dark photon in Sect. 3.3.1.
In the same regime for which M A > 1 MeV, however, the massive dark photon
could also decay into dark sector states if their masses are light enough. In this case
we have a non-vanishing branching ratio into invisible final states. The invisible
decay into these states of the dark sector χ in given by (Fig. 3.2, bottom).