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at future colliders , many - particle final states will be accessible with much more statistics . among these , six fermion signals are of particular interest for their relevance in top - quark physics , higgs boson production in the intermediate mass range , vector boson scattering , and quartic gauge boson coupling analyses .
typically , multi - particle final states can come from the decay of intermediate unstable particles that are produced as resonances in subprocesses . relying on this , the most commonly used generators pythia @xcite and herwig @xcite
are entirely based on the _ production @xmath0 decay _ approach , i.e. narrow width ( nwa ) and effective vector boson approximation ( evba ) .
while the general expectation is that nwa ( with possible improvements by spin correlation between production and decay or by breit - wigner convolution ) can give an order ten percent unaccuracy , the evba is surely more problematic . in the basic leading - log formulation , it does not allow to freely implement any kind of kinematical cuts .
the parton coming out after the vector boson emission is in fact produced in a fully inclusive way . even in its improved realization , which accounts for the exact phase space
, the evba can not always reproduce the correct kinematics and event rate .
the result strongly depends on the applied cuts , and there is no unique recipe .
therefore , its reliability must be always cross - checked against a complete program implementing exact calculations .    a further reason for going towards full computations is connected to gauge invariance and the appearance of strong and extremely delicate cancellations at high energy among feynman diagrams contributing to the same final state .
these interferences can take place both within the signal diagram set and between signal and irreducible background diagrams .
thus exact matrix elements must be computed .
this means evaluating thousands of diagrams for a huge variety of possible processes . in the last decade , a big effort has been dedicated to the implementation of event generators for multi - parton production . at present , the following codes are available : amegic @xcite , comphep @xcite , grace @xcite , madgraph @xcite , phegas & helac @xcite , omega @xmath1 whizard @xcite . these are _ multi - purpose _ generators , which in principle can compute any tree level process .
a different approach is given by event generators _ dedicated _ to specific classes and topologies of final states .
a recent example of this kind of generator for lhc physics is alpgen @xcite .
in this section , we present the general features of the monte carlo event generator phase @xcite , which is fully _ dedicated _ to six - fermion physics . in its first version ,
the phase project is designed for all standard model processes @xmath2 in lowest order .    [ cols="<,<,<,<",options="header " , ]      +
we consider a typical channel including the subprocess @xmath3 at the lhc .
we analyze the process @xmath4 in the region where we have the distinctive two forward jets signature . to this aim
, we apply the following cuts :          the first effect we investigate is connected to gauge invariance .
it was already pointed out by kleiss and stirling in an old paper @xcite that selecting only the subset of vector boson signal diagrams from the full amplitude could give meaningless results .
this argument relies on the fact that the delicate gauge cancellations get destroyed by the off - shellness of the two initial vector bosons .
we have checked this issue in more detail . in table 2 , we present the total cross section and the pure vector boson scattering signal for different higgs masses , in unitary gauge . as one can see , the full cross section is two order of magnitude smaller than the signal .
this poses an important question on what kind of signal definition one can give at high energy .
this issue should receive more attention . in fig.1
we show the ww invariant mass distribution for different helicity configurations of the two reconstructed w s . starting from the left , from top to bottom we plot the uu , tt , tl and ll contributions , where u , t and l are for unpolarized , transverse and longitudinal gauge boson .
additional @xmath10 , @xmath1120 gev and @xmath12 10 gev cuts are included .          9 t. sjstrand
_ et al . _ , hep - ph/0308153 ; t. sjstrand _
et al . _ ,
phys . commun .
135:238 - 259 , 2001 . g. corcella _
et al . _ , hep - ph/0210213 ; g. corcella _
et al . _ , jhep 0101:010,2001 .
f. krauss , r. kuhn and g. soff , jhep 0202 ( 2002 ) 044 ; a. schalicke , f. krauss , r. kuhn and g. soff , jhep 0212 ( 2002 ) 013 .
boos , m.n .
dubinin , v.a .
ilyin , a.e .
pukhov , v.i .
savrin , hep - ph/9503280 ; a. pukhov _
et al . _ , hep - ph/9908288 .
t. ishikawa _
et al . _ , [ minami - tateya collaboration ] , kek-92 - 19 ; h. tanaka _ et al . _
[ minami - tateya collaboration ] , nucl .
instrum . meth .
a389 ( 1997 ) 295 ; f. yuasa _ et al .
138 ( 2000 ) 18 .
f. maltoni , t. stelzer , jhep 0302 ( 2003 ) 027 .
papadopoulos , comput .
137 ( 2001 ) 247 ; a. kanaki and c.g .
papadopoulos , comput .
132 ( 2000 ) 306 .
m. moretti , t. ohl and j. reuter , hep - ph/0102195 ; w. kilian , lc - tool-2001 - 039 , jan 2001 , in * 2nd ecfa / desy study 1998 - 2001 * 1924 - 1980 .
mangano , m. moretti , f. piccinini , r. pittau , a.d .
polosa , jhep 0307 ( 2003 ) 001 .
e. accomando , a. ballestrero and e. maina , in preparation .
a. ballestrero , hep - ph/9911318 .
a. ballestrero and e. maina , phys .
b350 ( 1995 ) 225 .
lepage , jour .
27 ( 1978 ) 192 .
e. accomando , a. ballestrero and e. maina , comput . phys
. commun . 150
( 2003 ) 166 .
et al . _ , hep - ph/0109068 .
r. kleiss , j. stirling , phys .
lett . 182b ( 1986 ) 75 .
is a monte carlo event generator , under construction , for all standard model processes with six fermions in the final state at the lhc . 
 it employs the full set of tree level feynman diagrams , taking into account fermion masses for b quarks . 
 the program can generate unweighted events for any subset of all six fermion final states in a single run , by making use of dedicated pre - samples . 
 an interface to hadronization is provided .