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In article <10APR199210204620@deimos.ucc.umass.edu>, s5169180@deimos.ucc.umass.edu (S5169180) writes...
>>Brahm at Caltech wrote :
>>Mass is the pole of the propagator. ...
> 
>I absolutely agree with that argument. Therefore finding mass is same to
>finding bump in the cross section. It has a very useful meaning
>phenomenologically. However it can't give the reason how or why a particle has
>a specific physical(or numerical) value. My question is if we can predict
>masses of quarks or leptons purely in theoretical way, instead of experiments.

Sure.  Check out Physical Review Letters or Physical Review D on any typical
week.  You'll find many theoretical explanations of the mass spectrum of the
quarks and/or leptons.  The problem is that we don't know which, if any, 
are correct explanations.  In the absence of additional experimental evidence
for which theories are correct, it's just speculation.  

By the way,

I would say that the pole of the propagator is the mass, not the other way
around.  You write a wave-equation and solve it for the Green's function.  It
turns out that the propagator has a pole when the particle is on-shell.  I
suppose you could just postulate things in the reverse order, but you don't
learn anything by doing that.  

-Scott

s5169180@deimos.ucc.umass.edu (S5169180) asks:
> [Can we] predict masses of quarks or leptons purely in theoretical way,
> instead of experiments?

No, there is no accepted theory of the fermion masses.  On the other hand
the gauge boson masses (W,Z) are predictable (M_W = gv/2), and their
detection at CERN with the predicted masses was a great triumph for the
Standard Model.  Of the Standard Model's 19 free parameters, most are
related to masses:
  9 quark and lepton masses (top mass still unknown)
  4 CKM matrix parameters (3 angles, 1 phase), related to quark masses
  2 Higgs parameters: vev is known, Higgs mass is unknown
  3 gauge couplings
  1 Theta_QCD = 0 but nobody's sure why

--
Staccato signals of constant information,  |  -- David Brahm
A loose affiliation of millionaires and    |  (brahm@coil.caltech.edu)
  billionaires and Baby ...                |---------------------------
These are the days of miracle and wonder,  | I will not post 4 copies
And don't cry, Baby, don't cry, don't cry. |   of the same article...

In article <1992Apr14.024815.14039@cco.caltech.edu>, brahm@cco.caltech.edu (David E. Brahm) writes...

>Of the Standard Model's 19 free parameters, most are
>related to masses:
>  9 quark and lepton masses (top mass still unknown)
>  4 CKM matrix parameters (3 angles, 1 phase), related to quark masses
>  2 Higgs parameters: vev is known, Higgs mass is unknown
>  3 gauge couplings
>  1 Theta_QCD = 0 but nobody's sure why
> 

Although not strictly part of the Standard Model, we should also add to this
list:
   3 neutrino masses
   3 neutrino mixing angles 

I don't know what name we should give to the Standard Model when you allow 
for neutrino mass.  Probably just "Standard Model" all over again.  But
most people allow for this possibility today.  

-Scott

In article <22637@dog.ee.lbl.gov> sichase@csa1.lbl.gov writes:
>>>Brahm at Caltech wrote :
>>>Mass is the pole of the propagator. ...
>> 
>I would say that the pole of the propagator is the mass, not the other way
>around.  You write a wave-equation and solve it for the Green's function.  It
>turns out that the propagator has a pole when the particle is on-shell.  
>-Scott

  The advantage of Scott's viewpoint is that it highlights the hidden
  assumption that there is a "particle" to talk about.  For example,
  you cannot prove that the full QED electron satisfies the Dirac
  equation... in perturbation theory one assumes in-out states which
  do, but that's just a working paradigm, probably incorrect.