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Search for a new 17 MeV resonance via $e^+e^-$ annihilation with the PADME Experiment

F. Bossi, R. De Sangro, C. Di Giulio, E. Di Meco, D. Domenici, G. Finocchiaro, L. G. Foggetta, M. Garattini, P. Gianotti, M. Mancini, I. Sarra, T. Spadaro, C. Taruggi, E. Vilucchi, K. Dimitrova, S. Ivanov, Sv. Ivanov, K. Kostova, V. Kozhuharov, R. Simeonov, F. Ferrarotto, E. Leonardi, P. Valente, E. Long, G. C. Organtini, M. Raggi, A. Frankenthal

Abstract

The PADME Experiment at the Frascati DA$Φ$NE LINAC has searched for a hypothetical particle with mass around 17 MeV, commonly referred to as the X17, using a positron beam incident on a fixed target. The beam energy was varied between 262 and 296 MeV, corresponding to center-of-mass energies $\sqrt{s}$ between 16.4 and 17.4 MeV. The X17 should be produced resonantly via $e^+e^-$ annihilation when $\sqrt{s}$ approaches its mass, inducing an excess of events with a two-body final state over the background expectation. The beam energy spacing was fixed to less than half the expected width of the resonance's line shape. Uncertainties below 1% per $\sqrt{s}$ point were achieved. A blind analysis has been performed. The data are consistent with the expected background in most of the explored energy range, and limits are set in previously unexplored regions of the available parameter space. The most significant deviation is found for $\sqrt{s} \approx 16.90$ MeV, corresponding to a global significance of approximately 2 standard deviations over the null hypothesis expectation.

Search for a new 17 MeV resonance via $e^+e^-$ annihilation with the PADME Experiment

Abstract

The PADME Experiment at the Frascati DANE LINAC has searched for a hypothetical particle with mass around 17 MeV, commonly referred to as the X17, using a positron beam incident on a fixed target. The beam energy was varied between 262 and 296 MeV, corresponding to center-of-mass energies between 16.4 and 17.4 MeV. The X17 should be produced resonantly via annihilation when approaches its mass, inducing an excess of events with a two-body final state over the background expectation. The beam energy spacing was fixed to less than half the expected width of the resonance's line shape. Uncertainties below 1% per point were achieved. A blind analysis has been performed. The data are consistent with the expected background in most of the explored energy range, and limits are set in previously unexplored regions of the available parameter space. The most significant deviation is found for MeV, corresponding to a global significance of approximately 2 standard deviations over the null hypothesis expectation.

Paper Structure

This paper contains 24 sections, 8 equations, 24 figures, 2 tables.

Table of Contents

  1. Introduction
  2. The PADME setup
  3. Beam line
  4. Detector
  5. Run III analysis strategy
  6. The Run III dataset
  7. Event selection
  8. Analysis-level corrections
  9. Determination of B
  10. Reconstruction efficiency
  11. Reliability of the B estimate
  12. Production yield
  13. Determination of NPOT
  14. Energy-loss correction
  15. Radiation-induced losses
  16. ...and 9 more sections

Figures (24)

  • Figure 1: Sketch of the PADME beam line and the BTF-1 complex at LNF.
  • Figure 2: Top-view layout of the PADME detector in the Run III configuration.
  • Figure 3: Chronological ID of the energy points taken during the resonance scan vs. CoM energy. Filled (open) points refer to Scan 1 (Scan 2).
  • Figure 4: Distribution of $N_{\mathrm{POT}}$ vs. CoM energy. Blue points represent the scan region, and green and red ones the out-of-resonance data. The latter are used for the $N_{\mathrm{POT}}$ calibration and for scaling the beam flux. The $N_{\mathrm{POT}}$ variation is due to fluctuations in data-taking efficiency and in machine uptime.
  • Figure 5: Radial positions (top) and energies (bottom) of the final-state particles allowed by the geometric acceptance of the ECal and the event selection, shown as a function of CoM energy. Solid lines denote the maximum and dashed lines the minimum values. The maximum radial position is imposed as a constant value (270$\,\text{mm}$) to avoid edge effects. The double-headed arrow in each panel shows the range used for the energy scan in the analysis.
  • ...and 19 more figures