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Register a new userResonant production of dark photons in positron beam dump experiments
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Positrons beam dump experiments have unique features to search for very narrow resonances coupled superweakly to $e^+ e^-$ pairs. Due to the continue loss of energy from soft photon bremsstrahlung, in the first few radiation lengths of the dump a positron beam can continuously scan for resonant production of new resonances via $e^+$ annihilation off an atomic $e^-$ in the target. In the case of a dark photon $A$ kinetically mixed with the photon, this production mode is of first order in the electromagnetic coupling $alpha$, and thus parametrically enhanced with respect to the $O(alpha^2)$ $e^+e^- to gamma A$ production mode and to the $O(alpha^3)$ $A$ bremsstrahlung in $e^--$nucleon scattering so far considered. If the lifetime is sufficiently long to allow the $A$ to exit the dump, $A to e^+e^-$ decays could be easily detected and distinguished from backgrounds. We explore the foreseeable sensitivity of the Frascati PADME experiment in searching with this technique for the $17,$MeV dark photon invoked to explain the $^8$Be anomaly in nuclear transitions.
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High energy positron annihilation is a viable mechanism to produce dark photons ($A^prime$). This reaction plays a significant role in beam-dump experiments using experiments using multi-GeV electron-beams on thick targets by enhancing the sensitivity to $A^prime$ production. The positrons produced by the electromagnetic shower can produce an $A^prime$ via non-resonant ($e^+ + e^- to gamma + A^prime$) and resonant ($e^+ + e^- to A^prime$) annihilation on atomic electrons. For visible decays, the contribution of resonant annihilation results in a larger sensitivity with respect to limits derived by the commonly used $A^prime$-strahlung in certain kinematic regions. When included in the evaluation of the E137 beam-dump experiment reach, positron annihilation pushes the current limit on $varepsilon$ downwards by a factor of two in the range 33 MeV/c$^2<m_{A^prime}<120$ MeV/c$^2$.
A wealth of new physics models which are motivated by questions such as the nature of dark matter, the origin of the neutrino masses and the baryon asymmetry in the universe, predict the existence of hidden sectors featuring new particles. Among the possibilities are heavy neutral leptons, vectors and scalars, that feebly interact with the Standard Model (SM) sector and are typically light and long lived. Such new states could be produced in high-intensity facilities, the so-called beam dump experiments, either directly in the hard interaction or as a decay product of heavier mesons. They could then decay back to the SM or to hidden sector particles, giving rise to peculiar decay or interaction signatures in a far-placed detector. Simulating such kind of events presents a challenge, as not only short-distance new physics (hard production, hadron decays, and interaction with the detector) and usual SM phenomena need to be described but also the travel has to be accounted for as determined by the geometry of the detector. In this work, we describe a new plugin to the {sc MadGraph5_aMC@NLO} platform, which allows the complete simulation of new physics processes relevant for beam dump experiments, including the various mechanisms for the production of hidden particles, namely their decays or scattering off SM particles, as well as their far detection, keeping into account spatial correlations and the geometry of the experiment.
We consider the light $Z$ explanation of the muon $g-2$ anomaly. Even if such a $Z$ has no tree-level coupling to electrons, in general one will be induced at loop-level. We show that future beam dump experiments are powerful enough to place stringent constraints on$-$or discover$-$a $Z$ with loop-suppressed couplings to electrons. Such bounds are avoided only if the $Z$ has a large interaction with neutrinos, in which case the scenario will be bounded by ongoing neutrino scattering experiments. The complementarity between beam dump and neutrino scattering experiments therefore indicates that there are good prospects of probing a large part of the $Z$ parameter space in the near future.
A novel mechanism to produce and detect Light Dark Matter in experiments making use of GeV electrons (and positrons) impinging on a thick target (beam-dump) is proposed. The positron-rich environment produced by the electromagnetic shower allows to produce an $A^prime$ via non-resonant ($e^+ + e^- to gamma + A^prime$) and resonant ($e^+ + e^- to A^prime$) annihilation on atomic electrons. The latter mechanism, for some selected kinematics, results in a larger sensitivity with respect to limits derived by the commonly used $A^prime-strahlung$. This idea, applied to Beam Dump Experiments and {it active} Beam Dump Experiments pushes down the current limits by an order of magnitude.
The review of using of compton backscattering method for determination of the beam energy in collider experiments is given.
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