| Summary: | The NA62 experiment at CERN Super Proton Synchrotron (SPS) is currently taking data to measure the ultra-rare decay <inline-formula> <math display="inline"> <semantics> <mrow> <msup> <mi>K</mi> <mo>+</mo> </msup> <mo>→</mo> <msup> <mi>π</mi> <mo>+</mo> </msup> <mi>ν</mi> <mover accent="true"> <mi>ν</mi> <mo>¯</mo> </mover> </mrow> </semantics> </math> </inline-formula>. This decay, whose Branching Ratio (BR) is predicted with high precision within the Standard Model (SM), is one of the best candidates to reveal the indirect effects of New Physics (NP) at the highest mass scales. The NA62 experiment is designed to measure BR (<inline-formula> <math display="inline"> <semantics> <mrow> <msup> <mi>K</mi> <mo>+</mo> </msup> <mo>→</mo> <msup> <mi>π</mi> <mo>+</mo> </msup> <mi>ν</mi> <mover accent="true"> <mi>ν</mi> <mo>¯</mo> </mover> <mrow> <mo>)</mo> </mrow> </mrow> </semantics> </math> </inline-formula> with a decay-in-flight technique, novel for this channel. NA62 took data in 2016, 2017 and 2018; statistics collected in 2016 allows NA62 to reach the SM sensitivity for this decay, reaching the single event sensitivity (SES) and showing the proof of principle of the experiment. The preliminary result on BR (<inline-formula> <math display="inline"> <semantics> <mrow> <msup> <mi>K</mi> <mo>+</mo> </msup> <mo>→</mo> <msup> <mi>π</mi> <mo>+</mo> </msup> <mi>ν</mi> <mover accent="true"> <mi>ν</mi> <mo>¯</mo> </mover> <mrow> <mo>)</mo> </mrow> </mrow> </semantics> </math> </inline-formula> from the analysis of the 2016 data set is described.
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