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Searches for the Dark Photon at Fixed Target Experiments New Physics: Sae Mulli, Vol. 66, No. 8, August 2016, pp. 1036∼1044 http://dx.doi.org/10.3938/NPSM.66.1036 Searches for the Dark Photon at Fixed Target Experiments Suyong Choi∗ Department of Physics, Korea University, Seoul 02841, Korea (Received 3 July 2016 : revised 12 July 2016 : accepted 12 July 2016) Recent astrophysical observations provided indirect evidences for sub-GeV, light, dark-matter (DM). A new gauge particle in this mass range could exist as an extra U(1) gauge particle, called the dark photon. The dark photon can mix weakly with a ordinary photon and can mediate interactions of DM. If the dark photon is massive enough, it can decay into a pair of standard- model (SM) particles or DM particles. Light dark-matter particles in the MeV to GeV mass range, as well as in other mass ranges, have not been the subjects of intensive experimental researches. Large statistics is needed to search for dark-photon-mediated interactions. Therefore, high-power accelerators can be used to look for the dark photon. In this article, we review the current status of the experimental searches at various fixed-target experiments and the experiments planned in the near term future. PACS numbers: 12.60.-i, 14.70.Pw Keywords: Dark matter, Standard model, Dark photon, Accelerator, Fixed target experiment I. INTRODUCTION TO DARK PHOTON the regions of interest. Depending on the mass ranges, different experimental techniques are required. For the In 2008, PAMELA experiment reported on the rise WIMP DM, high-energy accelerators, such as the LHC, of the positron to electron ratio in cosmic rays start- are required to extend the mass reach and to have suf- ing from 10 GeV onwards, subsequently confirmed by ficiently high production cross-sections. Assuming the FERMI LAT and AMS-02 experiments [1–3]. In stan- WIMPs are still abundant in the universe today, they dard cosmic ray propagation models, it was expected could interact when they pass through the earth with that this ratio would decrease with increasing energy. small probability. Numerous deep underground experi- Also, the 511 keV γ rays from electron positron anni- ments look for a signature of WIMP recoiling in the de- hilation was seen to occur at galactic center [4]. These tector. In the absence of direct experimental detection, evidences point to a possibility of a light DM with MeV one must look elsewhere. to GeV mass light mediator of interaction with strong The model of dark photon has sparked the interest as coupling to DM could explain these experimental obser- a viable model and due to its simplicity. In a dark pho- vations and be consistent with the observed relic density ton model, an extra gauge symmetry UD(1) is assumed with a gauge particle that can mix with the SM photon as well as the (g − 2)µ anomaly [5]. through so-called “kinetic mixing”. Experimental and theoretical efforts in understanding ϵ the DM problem have mainly focused on the Weakly In- δL = F Y µν F D (1) 2 µν teracting Massive Particles (WIMPs) or axions. Both the axion DM and WIMP DM are theoretically well- This gauge particle is supposed to mediate interac- motivated models. The searches for WIMPs concentrate tions among DM particles. After the mixing, however, 0 on masses above 10 GeV, while for axions, below 1 eV are the dark photon (A ) can interact with the electrically charged SM particles weakly with coupling strength of ∗E-mail: [email protected] ϵe. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Searches for the Dark Photon at Fixed Target Experiments – Suyong Choi 1037 Important features of the dark photon, relevant for by a similar diagram, but whose cross section is sup- 2 2 experimental searches are as follows. pressed by ϵ /mA0 . In a Bremsstrahlung proess, photon is expected to • The dark photon is assumed to be massive and its carry almost all the energy of the incoming electron. mass mA0 is a free parameter of the theory. The Also the outgoing photon is collimated in the direction full theory should have a symmetry breaking mech- of the electron beam. Therefore, the dark photon can be anism to give the dark photon its mass. We assume considered almost mono-energetic and highly collimated. that such details occur at high enough masses to These characteristics allow one to design a compact ex- have little impact on phenomenology of dark pho- periment. ton interactions. In hadron accelerators, copious amounts of secondary hadrons are produced when the beam hits a fixed tar- • The dark photon can decay into a pair of SM get. Among these, π0, η, and η0 decay to γγ, hence a charged particles (leptons or hadrons). The source of photons. These neutral mesons can also de- branching fractions depend on the mass of the dark 0 cay into γA , as long as mA0 is smaller than the mass of photon mA0 . the meson. This practically limits the masses that can be covered to less than about 270 MeV. For very high • The dark photon mediates interaction between DM energy proton accelerators of a few hundred GeV, pro- particles. The mass of the dark matter particle ton Bremsstrahlung can be a source of A0, in which case, (m ), coupling to dark photon (α = g2 /4π) are χ D D larger mass A0 is accessible. free parameters. If mA0 > 2mχ, then dark photon can decay into a pair of DM particles. Details de- pend on whether the dark matter is a fermion or a 2. Decays of Dark Photons in Fixed-Target Ex- boson. periments 0 • The life time of A depends on (m 0 ; ϵ) if only SM A Design of the experiment has to take into account the decays are allowed. While, for the case DM decays particles to be detected. For me < mA0 /2 < mµ; mχ, are allowed, the life time becomes a function of 0 − A decays exclusively to e+e . Therefore, detection and (mA0 ; ϵ, mχ; αD). measurement of the momenta of e− and e+ is necessary. One must take into account the lifetime of A0, which is 2 proportional to 1/(ϵmA0 ) . The experiment would be II. PRODUCTION AND DECAYS OF THE sensitive to dark photons decaying after the target, but DARK PHOTON before the detector and whose decay products are in the 0 acceptance of the detector. For mA0 > 2mµ, A can de- The features summarized in the previous section al- + − cay into µ µ , however, such as massive mA0 would have lows for experimental searches using accelerators. Sub- smaller production cross section in a Bremsstrahlung GeV massive particles is kinematically accessible in many process, and muons would have wide opening angles, so, existing lepton and hadron accelerators. wider acceptance is needed. 0 If mA0 /2 > mχ, then A can decay into pair of (sta- ble) DM particles. The branching fraction to DM pair 1. Production of Dark Photon in Accelerators depends not only on the couping αD, but also ϵ, mχ and mA0 . However, since αD is not constrained yet from ex- Since the dark photon mixes with SM photon, A0 can isting data, it can even be large, αD ≫ αem, in which be produced by a similar mechanism that can generate case, this could be the dominant decay mode of A0. The −1 SM photons, but with reduced rate, determined by mA0 lifetime goes as αD , hence, the dark photons can be and ϵ. In electron accelerators, Bremsstrahlung photons rather short-lived. Even if the χ’s are produced inside can be produced when electrons are stopped in fixed- the target, they can reach the detector, since the χ has targets made of a high-Z material. A0 can be produced very small probability of interaction with matter. 1038 New Physics: Sae Mulli, Vol. 66, No. 8, August 2016 III. FIXED TARGET EXPERIMENTS FOR THE DARK PHOTON SEARCH In this section, we review some of the recent results that provide the most stringent limits. For a more com- prehensive review and discussions, consult [6]. 1. Dark Photon Search Experiments using Fixed Targets at Electron Accelerators – Visibly Decaying A0 case In fixed target experiments at electron accelerators, monoenergetic electrons are incident on a target, where the Bremsstrahlung process is the main production 0 0 mechanism of A . And the mass reach of A is set by Fig. 1. (Color online) Exclusion limits from various fixed − the incident electron energy. Production cross section of target experiments (see text) as well as e+e collider 0 0 2 2 (BaBar) in the (mA ; ϵ) parameter space of the dark pho- A through Bremsstrahlung is proportional to ϵ /mA0 . ton model, assuming there are no invisible decays [20– For the visible mode, only the A0 that decays into a 22]. pair of oppositely charged particles behind the target can be detected directly. The mean decay length is propor- Since these searches were model independent, reinter- 0 × 0 2 tional to Ee/mA 1/(ϵmA ) . All experiments in this pretations of the results in terms of the dark photon category detect and measure the momenta of the decay model have been done. In the (mA0 ; ϵ) plane, these ex- products. Since this would allow us to reconstruct the periments carve out the small mA0 and small ϵ (Fig. 1). mass of A0. Since the large ϵ and small mA0 is the region of high In experiments using lower-energy electron beams, production cross sections, that region is excluded first thin foils are used as targets followed by high-precision in an experiment and as data is accumulated, the reach spectrometers, to measure the momenta of decay prod- extends to the larger mA0 and smaller ϵ.
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