Scientific Goals of Feynman@Home

in preparation

For the LHC as well as for the linear collider program, the computation of higher precision and high multiplicity processes has become a must.

For QCD, next-to-leading order computations mitigate scale uncertainties, provide a first estimate of the observable normalization and include additional sub-processes that may change observable distribution shapes. Resummation techniques can be checked and improved.

But most importantly many QCD processes contribute to the background to Higgs or “new” physics searches; If the background is large, it has to be known precisely (st least better than signal/background ratio), otherwise extracting the signal will become intractable. Such a precision can only be obtained by including NLO calculations.

An experimenter’s wish list has been presented a few years ago, in the frameworks of the Tevatron Run II, it still apply to the LHC energy. One can see that the multiplicity is quite high, from 6 to 8 still keeping the boson undecayed. Performing these calculations at tree level is already a substantial effort, at NLO it is a challenge.

Due to these extreme difficulties, the “les Houches 2005” the wish list was less ambitious. But it still requires computing power Feynman_at_Home could provide:

1. 2 --> 4 processes
   • pp --> V V jet,
   • pp --> V V V

1. 2 --> 4 processes
   • pp --> 4 jets
   • pp --> t t-bar b b-bar
   • pp --> t t-bar + 2 jets,
   • pp --> t t-bar H + jet
   • pp --> V + 3 jets
   • pp --> V V + 2 jets
   • pp --> V V V + jet

"New Physics" models (MSSM, extra-dimensions) generate many more particles and therefore many more diagrams translating into large memory size and computing time increases.

For example: e+e- nu-e nu-e-bar b b-bar b b-bar goes from 5720 diagrams in the SM to 19508 in MSSM. So the signal is also more and more complex even at tree level.

Today achievement for one loop amplitudes processes are presented in “Les Houches 2005” as ‘state of the art for amplitudes, although only a few results are available for 2--> 3 and 2-->4 are considered as frontier calculations (see. Gudrun Heinrich).

1. Status NLO 2 --> 3 (LHC)
   • pp --> 3 jets
   • pp --> V jj (V = γ;Z;W)
   • pp --> γγj
   • pp --> V bb-bar
   • pp --> tt-barH; b_b-barH
   • pp --> tt-bar j
2. Status NLO 2 --> 3 (ILC)
   • e+e- --> 4jets
   • e+e- --> νν-barH
   • e+e- --> e+e-H
   • e+e- --> ννγ
   • e+e- --> tt-bar H
   • e+e- --> ZHH
   • γγ --> t t-bar H
3. Status NLO 2 -> 4
   • e+e --> 4 fermions : in progress

A lot of effort is directed toward theoretical issues, but often process cannot be computed because they involve too much computing although most if not all theoretical issues a well under control. The Feynman_at_Home collaboration will be active in both sectors: Theory and computing.

-- DenisPerretGallix - 26 Oct 2005