AROMA 2.2 – A Monte Carlo Generator for Heavy Flavour Events in ep Collisions

A program to simulate the production of heavy quarks through the boson-gluon fusion process in ep collisions is presented. The full electroweak structure of the electron–gluon interaction is taken into account as well as the masses of the produced heavy quarks. Higher order QCD radiation is treated using initial and final state parton showers, and hadronization is performed using the Lund string model. Physics and programming aspects are described in this manual. 1 Physics of included processes The production of heavy quarks in ep collisions is dominated by boson-gluon fusion (BGF) into a heavy quark-antiquark pair V (q) + g(p)→ Qf (pf ) + Q̄f ′(pf ′) , (1) occuring as O(αsα) parton level subprocess in the ep scattering process e(le) + p(P )→ l (l) +Qf (pf ) + Q̄f ′(pf ′) +X . (2) Here the symbols in brackets denote the corresponding four-momenta. In the charged current (CC) case V is the W-boson whereas in the neutral current (NC) case it corresponds to γ/Z exchange and the produced quark and antiquark have the same flavour f . Beside the normal DIS variables defined by Q ≡ −q = −(le − l ) , W 2 ≡ (P + q) , x ≡ Q 2P · q , y ≡ P · q P · le , (3) out of which only two are independent, three additional independent variables are needed to completely specify the process in eq. (2). These are here taken as the momentum fraction xg of the gluon relative to the proton, i.e. p = xgP , the variable z = P · pf P · q , (4)