Charge violation in fragmentation of partonic hadron decays

[Frank Siegert]

In roughly 1e-5 of the B decays we seem to have charge violation in the fragmentation process following a partonic decay. Here is one example hadronisation outcome:

  Blob [32]( 4, Cluster Formation       , 4 -> 5 @ (0,0,0,0)
  Incoming particles :
  [F] 4 d                     5 (   3 ->    4) [( 3.1289e-01, 6.5179e-02,-1.1239e-01,-5.1163e-02), p^2= 7.8400e-02, m= 2.8000e-01] (601,  0)
  [F] 4 db                    6 (   3 ->    4) [( 1.1050e+00,-6.1580e-01, 8.7003e-01, 7.9827e-02), p^2= 7.8400e-02, m= 2.8000e-01] (  0,601)
  [F] 4 d                     7 (   3 ->    4) [( 1.5387e+00,-4.5153e-01, 9.3543e-01, 1.1001e+00), p^2= 7.8400e-02, m= 2.8000e-01] (602,  0)
  [F] 4 sb                    8 (   3 ->    4) [( 2.3227e+00, 1.0022e+00,-1.6931e+00,-1.1288e+00), p^2= 2.5000e-01, m= 5.0000e-01] (  0,602)
  Outgoing particles :
  [P] 1 a_{1}(1260)           9 (   4 ->     ) [( 1.4539e+00,-4.5552e-01, 6.2678e-01, 2.3714e-02), p^2= 1.5129e+00, m= 1.2300e+00] (  0,  0)
  [P] 1 K_{1}(1400)          10 (   4 ->     ) [( 1.5471e+00,-1.5753e-01, 3.5951e-01, 5.2325e-01), p^2= 1.9656e+00, m= 1.4020e+00] (  0,  0)
  [P] 1 P                    11 (   4 ->     ) [( 1.7293e-02, 1.2711e-02,-7.0344e-03, 9.3793e-03), p^2= 2.8658e-16, m= 0.0000e+00] (  0,  0)
  [P] 1 pi                   12 (   4 ->     ) [( 5.2025e-01,-5.6073e-03, 4.0163e-01, 3.0184e-01), p^2= 1.8219e-02, m= 1.3498e-01] (  0,  0)
  [P] 1 pi+                  13 (   4 ->     ) [( 1.7407e+00, 6.0594e-01,-1.3809e+00,-8.5818e-01), p^2= 1.9480e-02, m= 1.3957e-01] (  0,  0)

I am attaching a debug log file showing the full debug output of a B decay event where this happens. If you have any ideas for the cause and a fix I can quickly test whether it's fixed.

This happens both in rel-2-1-1 and in trunk.

PS: This was found during ATLAS validation by Jackie Brosamer (thanks!).