1 #ifndef AliHMPIDHelix_h
2 #define AliHMPIDHelix_h
4 #include <TObject.h>              //base class
5 #include <TVector3.h>             //used extensively
7 class AliHMPIDHelix: public TObject
8 {
9 public:
10   AliHMPIDHelix(                                                            ):TObject(),fX0(TVector3(0,0,0)),fP0(TVector3(0,0,0)),fQ(0),fBz(0 )  {}
11   AliHMPIDHelix(const TVector3 &x0,const TVector3 &p0,Int_t q=1,Double_t b=2):TObject(),fX0(x0             ),fP0(p0             ),fQ(q),fBz(b )  {}
12   AliHMPIDHelix(Double_t p,Double_t theta,Double_t phi,Double_t bz=2        ):TObject(),fX0(TVector3(0,0,0)),fP0(TVector3(0,0,0)),fQ(0),fBz(bz)
14   virtual ~AliHMPIDHelix()                                                                                                                   {}
16          void     Draw          (const Option_t *opt=""              );                              //from TObject, draw helix
17          void     Print    (const Option_t *opt=""              )const;                         //from TObject, print status
18 //private part++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
19   inline Bool_t   Intersect(            TVector3 &pnt,TVector3 &norm );          //intersection with plane given by point and normal vector
20   inline void     Propagate(Float_t len,TVector3 &x,  TVector3 &p    );          //propogate helix by given length along it
21 protected:
22   TVector3 fX0;            //helix position in point of definition, [cm]    in MARS
23   TVector3 fP0;            //helix momentum in point of definition, [GeV/c] in MARS
24   Int_t    fQ;             //sign of track charge
25   Float_t  fBz;            //magnetic field along z, [kGaus]
26   ClassDef(AliHMPIDHelix,0) //General helix
27 };//class AliHMPIDHelix
29 typedef AliHMPIDHelix AliRICHHelix; // for backward compatibility
31 //++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
32 void AliHMPIDHelix::Propagate(Float_t len,TVector3 &x,TVector3 &p)
33 {
34 // Propogates the helix from inintial point by a given distance along helix. Assumes uniform magnetic field along z direction.
35 // Arguments: len - distance to propagate by, [cm]
36 //   Returns: none
37   if(fBz==0){//no magnetic field->straight line
38     x=fX0+fP0.Unit()*len;
39     p=fP0;
40   }else{
41     const Float_t c = 0.000299792458;//this speed of light value provides that coordinates are in [cm] momentum in [GeV/c] field in [kGaus]
42     Float_t a = -c*fBz*fQ;
43     Float_t rho = a/fP0.Mag();
44     x.SetX( fX0.X()+fP0.X()*TMath::Sin(rho*len)/a-fP0.Y()*(1-TMath::Cos(rho*len))/a  );
45     x.SetY( fX0.Y()+fP0.Y()*TMath::Sin(rho*len)/a+fP0.X()*(1-TMath::Cos(rho*len))/a  );
46     x.SetZ( fX0.Z()+fP0.Z()*len/fP0.Mag()                                            );
47     x.SetX( fP0.X()*TMath::Cos(rho*len)-fP0.Y()*TMath::Sin(rho*len)                  );
48     p.SetY( fP0.Y()*TMath::Cos(rho*len)+fP0.X()*TMath::Sin(rho*len)                  );
49     p.SetZ( fP0.Z()                                                                  );
50   }
51 }//Propagate()
52 //++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
53 Bool_t AliHMPIDHelix::Intersect(TVector3 &pnt,TVector3 &norm)
54 {
55 // Finds point of intersection (if exists) of the helix with the plane. Stores result in fX and fP.
56 // Arguments: pnt  - arbitrary point of the plane, [cm] in MARS
57 //            norm - vector, normal to the plane, [cm] in MARS
58 //   Returns:      - kTrue if helix intersects the plane, kFALSE otherwise.
59 //                 - pnt contains the point of intersection, [cm] in MARS
60   TVector3 x,p;                                                    //current helix position and momentum
61   Double_t s=(pnt-fX0)*norm,dist=99999,distPrev=dist;              //estimates initial distance to plane
62   while(TMath::Abs(dist)>0.00001){                                 //loop while the distance is less then precision
63     Propagate(s,x,p);                                              //calculates helix at the distance s from x0 ALONG the helix
64     dist=(x-pnt)*norm;                                             //distance between current helix position and plane
65     if(TMath::Abs(dist) >= TMath::Abs(distPrev)) { return kFALSE;} //if distance increases then no intersection
66     distPrev=dist;
67     s-=dist;
68   }
69   norm=p;
70   pnt=x;
71   return kTRUE;
72 }//Intersect()
73 //++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
74 #endif