What the PHOX codes cannot do - General WARNINGS

As any computer code, the PHOX codes obey Feynman's `GIGO' principle: "Garbage In, Garbage Out": if you ask the wrong question to a PHOX code, you may get a wrong answer.
Here we list the two most important warnings regarding the use of the PHOX codes:

The PHOX codes ARE NOT FULL EVENT GENERATORS.
The terminology "Monte Carlo program" refers here to the numerical integration technique used. The user should be careful not to be confused by this terminology.
- The PHOX codes do not provide a full, exclusive portrait of events which could for example be further processed through a detector simulation. This is not their purpose.
- The PHOX codes are not designed to be interfaced with parton showers and hadronisation models.
The PHOX codes are instead self contained programs designed to compute sufficiently inclusive observables with the possibility to account for various selection cuts which can be formulated at the parton level. The virtue of the PHOX codes is to provide predictions of Next to Leading Order (NLO) accuracy in the pertubative expansion in powers of alpha_s for such inclusive observables. This means that the PHOX codes give more accurate and quantitative answers than a full event generator regarding such inclusive quantites, but at the expense of a more restricted range of applicability. The PHOX codes and full event generators are complementary to each other.

The PHOX codes SHARE ALL THE LIMITATIONS INHERENT IN ANY FIXED ORDER PERTURBATIVE CALCULATION based on the QCD-improved parton model. In particular, the results provided by the PHOX codes should be considered with circumspection if
- the relevant partonic energy scale involved in the subprocess is not large enough compared to the typical hadronic scale O(1 GeV)
  (these codes involve no primordial k _T smearing nor any higher-twist/power correction).
- the relevant partonic energy scale is much smaller than the C.M.S. energy of the collision
  (these codes do not account for small-x physics).
- the observable considered is infrared sensitive
  (in the calculation of a distribution at any fixed order in perturbation theory, this infrared sensitivity is indicated by the appearence of large logarithms depending on the ratio of widely different scales. Such large logarithms are associated with soft gluon effects. These codes do not (yet) resum these multiple soft gluon effects. Therefore their predictions are not reliable whenever such large terms appear - example: transverse momentum (Q _T) distribution of a photon pair with a large invariant mass M, in the region where Q _T / M << 1).

In practice, the actual use of the various PHOX codes may not be affected by these restrictions, depending on the type of process concerned. For more detailed information please consult the warnings specific to the particular PHOX code which you intend to use.

Back to main page