Found and fixed a trivial bug in merging programs

present in the W and all the single-top processes
(formula for total combined cross section was wrong,
a **2 was missing).



git-svn-id: svn://powhegbox.mib.infn.it/trunk/POWHEG-BOX@459 9e129b38-c2c0-4eec-9301-dc54fda404de

Author: re

ST_sch/merge_t_tb.f

@@ -138,7 +138,7 @@ subroutine merge(total_ev)
       errXsec_tb = xerrup(1)
 
       Xsec_tot=Xsec_t+Xsec_tb
-      errXsec_tot=sqrt(errXsec_t**2+errXsec_tb)
+      errXsec_tot=sqrt(errXsec_t**2+errXsec_tb**2)
 
       nev_t=nint(1.05*(Xsec_t/Xsec_tot)*total_ev)
       nev_tb=nint(1.05*(Xsec_tb/Xsec_tot)*total_ev)

ST_tch/merge_t_tb.f

@@ -138,7 +138,7 @@ subroutine merge(total_ev)
       errXsec_tb = xerrup(1)
 
       Xsec_tot=Xsec_t+Xsec_tb
-      errXsec_tot=sqrt(errXsec_t**2+errXsec_tb)
+      errXsec_tot=sqrt(errXsec_t**2+errXsec_tb**2)
 
       nev_t=nint(1.05*(Xsec_t/Xsec_tot)*total_ev)
       nev_tb=nint(1.05*(Xsec_tb/Xsec_tot)*total_ev)

ST_wtch_DR/merge_t_tb.f

@@ -138,7 +138,7 @@ subroutine merge(total_ev)
       errXsec_tb = xerrup(1)
 
       Xsec_tot=Xsec_t+Xsec_tb
-      errXsec_tot=sqrt(errXsec_t**2+errXsec_tb)
+      errXsec_tot=sqrt(errXsec_t**2+errXsec_tb**2)
 
       nev_t=nint(1.05*(Xsec_t/Xsec_tot)*total_ev)
       nev_tb=nint(1.05*(Xsec_tb/Xsec_tot)*total_ev)

ST_wtch_DS/merge_t_tb.f

@@ -138,7 +138,7 @@ subroutine merge(total_ev)
       errXsec_tb = xerrup(1)
 
       Xsec_tot=Xsec_t+Xsec_tb
-      errXsec_tot=sqrt(errXsec_t**2+errXsec_tb)
+      errXsec_tot=sqrt(errXsec_t**2+errXsec_tb**2)
 
       nev_t=nint(1.05*(Xsec_t/Xsec_tot)*total_ev)
       nev_tb=nint(1.05*(Xsec_tb/Xsec_tot)*total_ev)

W/merge_wp_wm.f

@@ -138,7 +138,7 @@ subroutine merge(total_ev)
       errXsec_wm = xerrup(1)
 
       Xsec_tot=Xsec_wp+Xsec_wm
-      errXsec_tot=sqrt(errXsec_wp**2+errXsec_wm)
+      errXsec_tot=sqrt(errXsec_wp**2+errXsec_wm**2)
 
       nev_wp=nint(1.05*(Xsec_wp/Xsec_tot)*total_ev)
       nev_wm=nint(1.05*(Xsec_wm/Xsec_tot)*total_ev)