New avenues for |∆B| = 2 processes beyond neutron-antineutron oscillations

A bstract We explore baryon-number-violating (|∆ B | = 2) processes beyond the well-known neutron-antineutron ( $$ n-\overline{n} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>n</mml:mi> <mml:mo>−</mml:mo> <mml:mover> <mml:mi>n</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> ) oscillations, focusing on the $$ \Lambda -\overline{\Lambda} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Λ</mml:mi> <mml:mo>−</mml:mo> <mml:mover> <mml:mi>Λ</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> system. The presence of a strange quark in the Λ baryon introduces a new set of six-quark operators roughly of the form ( uds ) 2 , which are different from the ( udd ) 2 operators responsible for $$ n-\overline{n} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>n</mml:mi> <mml:mo>−</mml:mo> <mml:mover> <mml:mi>n</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> oscillations. Using the Standard Model Effective Field Theory (SMEFT), we classify all dimension-9 operators that cause |∆ B | = 2 transitions and study their UV completions mediated by exotic scalar fields with trilinear interactions. We demonstrate that in these models, $$ \Lambda -\overline{\Lambda} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Λ</mml:mi> <mml:mo>−</mml:mo> <mml:mover> <mml:mi>Λ</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> oscillations can occur at tree level, with $$ n-\overline{n} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>n</mml:mi> <mml:mo>−</mml:mo> <mml:mover> <mml:mi>n</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> mixing potentially appearing at higher loop levels. We employ a chiral effective theory to constrain the effective mass mixing δm Λ , deriving bounds from current experimental limits on $$ n-\overline{n} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>n</mml:mi> <mml:mo>−</mml:mo> <mml:mover> <mml:mi>n</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> oscillations and dinucleon decays such as pp → K + K + . These bounds indicate that $$ \Lambda -\overline{\Lambda} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Λ</mml:mi> <mml:mo>−</mml:mo> <mml:mover> <mml:mi>Λ</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> oscillations probe a complementary parameter space, sensitive to baryon-number violation at scales up to 10 2 − 10 3 TeV. We show that the existing indirect bounds make it challenging to provide a competitive bound on δm Λ at BESIII.