Searching a Dark Photon with HADES
HADES Collaboration, G. Agakishiev, A. Balanda, D. Belver, A. Belyaev, J. C. Berger-Chen, A. Blanco, M. Boehmer, J. L. Boyard, P. Cabanelas, S. Chernenko, A. Dybczak, E. Epple, L. Fabbbietti, O. Fateev, P. Finocchiaro, P. Fonte, J. Friese, I. Froehlich, T. Galatyuk, J. A. Garzon, R. Gernhaeuser, K. Goebel, M. Golubeva, D. Gonzalez-Diaz, F. Guber, M. Gumberidze, T. Heinz, T. Hennino, R. Holzmann, A. Ierusalimov, I. Iori, A. Ivashkin, M. Jurkovic, B. Kaempfer, T. Karavicheva, I. Koenig, W. Koenig, B. W. Kolb, G. Kornakov, R. Kotte, A. Krasa, F. Krizek, R. Kruecken, H. Kuc, W. Kuehn, A. Kugler, A. Kurepin, V. Ladygin, R. Lalik, S. Lang, K. Lapidus, A. Lebedev, T. Liu, L. Lopes, M. Lorenz, L. Maier, A. Mangiarotti, J. Markert, V. Metag, B. Michalska, J. Michel, C. Muentz, L. Naumann, Y. C. Pachmayer, M. Palka, Y. Parpottas, V. Pechenov, O. Pechenova, V. Petousis, J. Pietraszko, W. Przygoda, B. Ramstein, A. Reshetin, A. Rustamov, A. Sadovsky, P. Salabura, T. Scheib, H. Schuldes, A. Schmah, E. Schwab, J. Siebenson, Yu. G. Sobolev, S. Spataro, B. Spruck, H. Stroebele, J. Stroth, C. Sturm, A. Tarantola, K. Teilab, P. Tlusty, M. Traxler, R. Trebacz, H. Tsertos, T. Vasiliev, V. Wagner, M. Weber, C. Wendisch, J. Wuestenfeld, S. Yurevich, Y. Zanevsky
TL;DR
The paper addresses constraining a dark photon $U$ with mass in the range $M_U \in [0.02,0.55]$ GeV/$c^2$ that kinetically mixes with the Standard Model through $\epsilon$. It exploits inclusive dielectron spectra from HADES in $p+p$ and $\,^{40}$Ar+$\text{KCl}$ collisions, modeling the background from Dalitz decays of $\pi^0$, $\eta$, and $\Delta$ to search for a narrow $U \rightarrow e^+e^-$ signal, with the yield given by $N_{U \rightarrow ee}= \epsilon^2 \; BR_{ee} \; L(M_U)$. The analysis yields 90% CL upper limits on $\epsilon^2$ that improve previous bounds in the $0.03$--$0.1$ GeV/$c^2$ region and exclude much of the parameter space favored by the muon $g-2$ anomaly, along with an improved upper limit on ${\rm BR}(\eta \rightarrow e^+e^-)$ of $2.3\times 10^{-6}$ at 90% CL. These results provide important constraints on hidden-sector scenarios and complement other low-mass searches in this challenging mass range.
Abstract
We present a search for the e+e- decay of a hypothetical dark photon, also names U vector boson, in inclusive dielectron spectra measured by HADES in the p (3.5 GeV) + p, Nb reactions, as well as the Ar (1.756 GeV/u) + KCl reaction. An upper limit on the kinetic mixing parameter squared epsilon^{2} at 90% CL has been obtained for the mass range M(U) = 0.02 - 0.55 GeV/c2 and is compared with the present world data set. For masses 0.03 - 0.1 GeV/c^2, the limit has been lowered with respect to previous results, allowing now to exclude a large part of the parameter region favoured by the muon g-2 anomaly. Furthermore, an improved upper limit on the branching ratio of 2.3 * 10^{-6} has been set on the helicity-suppressed direct decay of the eta meson, eta-> e+e-, at 90% CL.