Hunting down the X17 boson at the CERN SPS

Abstract

Recently, the ATOMKI experiment has reported new evidence for the excess of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msup><mml:mi>e</mml:mi> <mml:mo>+</mml:mo></mml:msup> <mml:msup><mml:mi>e</mml:mi> <mml:mo>-</mml:mo></mml:msup> </mml:mrow> </mml:math> events with a mass <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>∼</mml:mo></mml:math> 17 MeV in the nuclear transitions of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow></mml:mrow> <mml:mn>4</mml:mn></mml:msup> </mml:math> He, that they previously observed in measurements with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow></mml:mrow> <mml:mn>8</mml:mn></mml:msup> </mml:math> Be. These observations could be explained by the existence of a new vector <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>X</mml:mi> <mml:mn>17</mml:mn></mml:mrow> </mml:math> boson. So far, the search for the decay <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>X</mml:mi> <mml:mn>17</mml:mn> <mml:mo>→</mml:mo> <mml:msup><mml:mi>e</mml:mi> <mml:mo>+</mml:mo></mml:msup> <mml:msup><mml:mi>e</mml:mi> <mml:mo>-</mml:mo></mml:msup> </mml:mrow> </mml:math> with the NA64 experiment at the CERN SPS gave negative results. Here, we present a new technique that could be implemented in NA64 aiming to improve the sensitivity and to cover the remaining <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>X</mml:mi> <mml:mn>17</mml:mn></mml:mrow> </mml:math> parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>X</mml:mi> <mml:mn>17</mml:mn></mml:mrow> </mml:math> decay with the proposed method. To reach this goal an optimization of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>X</mml:mi> <mml:mn>17</mml:mn></mml:mrow> </mml:math> production target, as well as an efficient and accurate reconstruction of two close decay tracks, is required. A dedicated analysis of the available experimental data making use of the trackers information is presented. This method provides independent confirmation of the NA64 published results [1], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate show that the goal of the proposed search is feasible.