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diff --git a/include/HEJ/LorentzVector.hh b/include/HEJ/LorentzVector.hh
index 8190f92..1f4271d 100644
--- a/include/HEJ/LorentzVector.hh
+++ b/include/HEJ/LorentzVector.hh
@@ -1,25 +1,33 @@
/** \file
* \brief Auxiliary functions for Lorentz vectors
*
* \authors The HEJ collaboration (see AUTHORS for details)
* \date 2019
* \copyright GPLv2 or later
*/
#pragma once
#include "CLHEP/Vector/LorentzVector.h"
namespace HEJ {
inline
auto dot(
CLHEP::HepLorentzVector const & h1,
CLHEP::HepLorentzVector const & h2
) {
return h1.dot(h2);
}
inline
auto m2(CLHEP::HepLorentzVector const & h1) {
return h1.m2();
}
+
+ //! Split a single momentum into two lightlike momenta
+ /**
+ * @param p Momentum to be split
+ * @returns A pair (p1, p2) of momenta with p = p1 + p2 and p1^2 = p2^2 = 0
+ */
+ std::pair<CLHEP::HepLorentzVector, CLHEP::HepLorentzVector>
+ split_into_lightlike(CLHEP::HepLorentzVector const & p);
}
diff --git a/src/LorentzVector.cc b/src/LorentzVector.cc
new file mode 100644
index 0000000..4e241b3
--- /dev/null
+++ b/src/LorentzVector.cc
@@ -0,0 +1,38 @@
+/**
+ * \authors The HEJ collaboration (see AUTHORS for details)
+ * \date 2020
+ * \copyright GPLv2 or later
+ */
+#include "HEJ/LorentzVector.hh"
+
+#include <cmath>
+#include <utility>
+
+namespace HEJ {
+ std::pair<CLHEP::HepLorentzVector, CLHEP::HepLorentzVector>
+ split_into_lightlike(CLHEP::HepLorentzVector const & p) {
+ /*
+ * We take the ansatz
+ * p1 = (E, px, py, pz) = pt*(cosh(y), cos(phi), sin(phi), sinh(y))
+ * p2 = (E2, 0, 0, +-E2)
+ * and solve p1 + p2 = p for y and E2 (phi and pt are the same in p1 and p).
+ * The sign in p2 is chosen such that a real-valued solution exists
+ */
+ const double pt = p.perp();
+ if(p.plus() > 0){
+ const double y = std::log(p.plus()/pt);
+ const double E2 = (p.minus() - pt*pt/p.plus())/2.;
+ return std::make_pair(
+ CLHEP::HepLorentzVector{p.x(), p.y(), pt*std::sinh(y), pt*std::cosh(y)},
+ CLHEP::HepLorentzVector{0., 0., E2, -E2}
+ );
+ } else {
+ const double y = std::log(pt/p.minus());
+ const double E2 = (p.plus() - pt*pt/p.minus())/2.;
+ return std::make_pair(
+ CLHEP::HepLorentzVector{p.x(), p.y(), pt*std::sinh(y), pt*std::cosh(y)},
+ CLHEP::HepLorentzVector{0., 0., E2, +E2}
+ );
+ }
+ }
+}

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