Re: Class LorentzVector Discussion
Nick van Eijndhoven (Nick@phys.uu.nl)
Wed, 14 Jul 1999 11:33:17 +0200
Jeff Templon wrote:
>
> Hi,
>
> I saw Pasha's message of a few days ago about the new LorentzVector
> class and thought I'd weigh in with a couple of concerns.
>
> First, I am surprised about the lack of "interknowledge" of a
> LorentzVector with its three-vector as well. I am prototyping a
> Monte-Carlo program for electron scattering just to evaluate ROOT/C++
> as a platform for a further, bigger project. I have four-vector
> classes I wrote (in Python) before, and the ones in ROOT operate
> totally differently.
>
> The tack I took was that the invariant square of the vector was
> _invariant_ so that this, not the fourth component (energy or time)
> was the fourth data member. This has the advantage that if one
> changes the momentum of a vector (through a process such as ionization
> energy loss), you can simply reset the momentum and let the class
> figure out the energy by itself that is necessary to keep the mass
> identical. It sees to me that with the current ROOT classes, lots of
> extra work is necessary if you worry about keeping the invariant
> square really invariant under operations on a four-vector.
>
> Given the design decision made by the ROOT team, there should be other
> convenience functions. Perhaps this is what is meant by the function
>
> void SetVectM(TVector3& spatial, Double_t mass)
>
> but it was not too obvious from the documentation. And again not
> preserving the invariant mass under operations has led to
> floating-point roundoff difficulties for me in the past; in my first
> version of four-vector classes I took the same approach as the ROOT
> classes did.
>
> JAT
Hi Jeff,
I indeed followed a similar track for Ali4Vector & co. for the ALICE reco.
software. Therefore having TLorentzVector & co. coexisting with Ali4Vector & co.
in our AliRoot framework gives us the full functionality and benefits of the
both approaches.
Furthermore, as I mentioned earlier, I stumbled over some more rounding problems
when I tried the stuff out on high energy neutrino events (a la AMANDA) where
one needs boosts from one frame to the other. Also here having both 'packages'
available enabled me overcoming all problems.
--
Cheers,
Nick.
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Dr. Nick van Eijndhoven Department of Subatomic Physics
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