Adding TLorentzVector::EtaPhiVector()

beojan · February 2, 2019, 10:16am

TLorentzVector::EtaPhiVector() returns a TVector2 whose X component is eta, and whose Y component is phi. Is there a function in ROOT that would allow adding these in a way that correctly treats phi?

moneta · February 4, 2019, 9:02am

Hi

I have not understood what do you mean a way that correctly treats phi. Can you please explain better your problem ?
Thank you

Lorenzo

beojan · February 4, 2019, 9:16am

Phi is an angle, so the result of an addition or subtraction should be moved into that range.

moneta · February 4, 2019, 9:37am

HI,

If you want to add two Lorenz vectors, you should add them and then you ask for the resulting phi. Otherwise, I don;t think there is a public special function to treat phi, but you can copy and use this from here:

github.com

root-project/root/blob/2fcb08218fc7a8e3630a677beda0055782d5486c/math/genvector/inc/Math/GenVector/Polar3D.h#L160




/**
    set all values using cartesian coordinates
*/
void SetXYZ(Scalar x, Scalar y, Scalar z);




private:
inline static Scalar pi()  { return M_PI; }
inline void Restrict() {
   if (fPhi <= -pi() || fPhi > pi()) fPhi = fPhi - floor(fPhi / (2 * pi()) + .5) * 2 * pi();
}


public:


/**
    scale by a scalar quantity - for polar coordinates r changes
*/
void Scale (T a) {
   if (a < 0) {
      Negate();

Lorenzo

beojan · February 4, 2019, 10:11am

In detectors, we often use (pₜ, η, ϕ, E) coordinates, where ϕ is an angle. TLorentzVector lets you set these with SetPtEtaPhiE. The (η, ϕ) vector can be extracted as a TVector2 using EtaPhiVector. η and ϕ are the x and y component of this TVector2, so adding or subtracting TVector2s simply adds and subtracts the η and ϕ components. This can take ϕ outside the 0 - 2π (or -π to π) range it should always be in. I wanted to know if there were a convenience function that deals with this automatically, rather than having to manually add the components and call TVector2::Phi_0_2pi or TVector2::Phi_mpi_pi on the ϕ result.

moneta · February 4, 2019, 10:28am

HI,

For your use case, I would then recommend to use the the ROOT class ROOT::Math::PtEtaPhiEVector

see https://root.cern/doc/master/classROOT_1_1Math_1_1PtEtaPhiE4D.html

beojan · February 4, 2019, 10:33am

That would be a 4-vector. What I’m looking for is essentially a 2-vector with a ROOT::Math::CylindricalEta2D<T> coordinate system (this coordinate system isn’t actually available).

I have no problem doing the additions manually, but I think it would be useful for EtaPhiVector to return something that can be added and subtracted naturally.

moneta · February 4, 2019, 10:38am

Hi,

Yes this is not available, because normally in 2D you have always the radius. But cannot you use ROOT::Math::CylindricalEta3D with rho=1 ?

Lorenzo

beojan · February 4, 2019, 10:54am

I certainly could, yes. That isn’t what EtaPhiVector() returns though.

system · February 18, 2019, 11:03am

This topic was automatically closed 14 days after the last reply. New replies are no longer allowed.