Index: trunk/MagicSoft/Mars/mbase/MMath.cc
===================================================================
--- trunk/MagicSoft/Mars/mbase/MMath.cc	(revision 5883)
+++ trunk/MagicSoft/Mars/mbase/MMath.cc	(revision 5901)
@@ -70,5 +70,27 @@
 //
 //  Returns -1 if sum<0 or alpha<0 or the argument of sqrt<0
-//  Returns  0 if s+b==0 or s==0
+//  Returns  0 if s+b==0, s==0 or b==0
+//
+// Here is some eMail written by Daniel Mazin about the meaning of the arguments:
+//
+//  > Ok. Here is my understanding:
+//  > According to Li&Ma paper (correctly cited in MMath.cc) alpha is the
+//  > scaling factor. The mathematics behind the formula 17 (and/or 9) implies
+//  > exactly this. If you scale OFF to ON first (using time or using any other
+//  > method), then you cannot use formula 17 (9) anymore. You can just try
+//  > the formula before scaling (alpha!=1) and after scaling (alpha=1), you
+//  > will see the result will be different.
+//
+//  > Here are less mathematical arguments:
+//
+//  >  1) the better background determination you have (smaller alpha) the more
+//  > significant is your excess, thus your analysis is more sensitive. If you
+//  > normalize OFF to ON first, you loose this sensitivity.
+//
+//  >  2) the normalization OFF to ON has an error, which naturally depends on
+//  > the OFF and ON. This error is propagating to the significance of your
+//  > excess if you use the Li&Ma formula 17 correctly. But if you normalize
+//  > first and use then alpha=1, the error gets lost completely, you loose
+//  > somehow the criteria of goodness of the normalization.
 //
 Double_t MMath::SignificanceLiMa(Double_t s, Double_t b, Double_t alpha)
@@ -76,5 +98,5 @@
     const Double_t sum = s+b;
 
-    if (s==0 || sum==0)
+    if (s==0 || b==0 || sum==0)
         return 0;