void ArcDiff(void)
{
	gROOT->Reset();
	
	double p = 1.0; // GeV/c
	double B1 = 2.0; // Tesla
	double B2 = 2.2; // Tesla
	double theta = 10.0*TMath::DegToRad(); // radians
	double qBr2p = 0.003; // convert from Tesla-cm to GeV/c
	double R1 = p*sin(theta)/(qBr2p*B1);
	double R2 = p*sin(theta)/(qBr2p*B2);
	double delta_z = 200.0; // cm
	double phi = tan(theta)/R1 * delta_z;

	phi/=2.0;
	
	double A = cos(phi) - (1.0 - B1/B2);
	double B = sin(phi);
	double delta = R1*(sqrt(A*A + B*B) - B1/B2);
		
	cout <<"phi = "<<phi/TMath::Pi()<<"pi  or "<<phi*TMath::RadToDeg()<<" degrees"<<endl;
	cout << "delta = "<<delta<<endl;
	cout << "delta/sqrt(12) = "<<delta/sqrt(12.0)<<endl;
}