1. SuperB 2 nd June1 Experience in migrating the code to a modern framework Peter Richardson IPPP, Durham University Changing HERWIG  Herwig++

2. Summary Motivation Early Years Mid-Life Crisis Current Lessons Conclusions SuperB 2 nd June2

3. Motivation The HERWIG project began 25 years ago and was intended to provide a good description of perturbative QCD. Improvements and developments over the years. Mainly made by generations of PhD students/postdocs modifying (hacking) the code. SuperB 2 nd June3

4. Motivation In 2000 it became clear that: –The structure was difficult to maintain; –Making certain changes could led to unstable results; –Necessary improvements to the physics modelling were impossible to implement with the current code structure. SuperB 2 nd June4 Decided to write a new program based on the same physics ideas but with improvements wherever possible to the modelling.

5. Early Years Having made that decision the logical choice was an object-oriented program in C++. This was in an environment where: –other generator projects, e.g. PYTHIA, were also being rewritten in C++; –and new projects, e.g. SHERPA, were being developed in C++. When we started the PYTHIA rewrite had already made significant progress on the infrastructure code. SuperB 2 nd June5

6. Early Years Decided to join forces and separate the infrastructure from the physics of the event generator. Split the infrastructure from PYTHIA7 into ThePEG. Intended that the physics modules of both programs would then be built on this infrastructure. Seemed like a good idea at the time, would have allowed pick and mix for physics models. SuperB 2 nd June6

7. Early Years As there was nobody who understood both the physics and C++ applied for funding to PPARC/STFC for 4 years of postdoc funding to write the new program. Employed people who were particle physicists with C++ expertise, but not event generator authors. Hoped to increase the number of experts in the field in this way, again it seemed like a good idea at the time. SuperB 2 nd June7

8. Early Years One postdoc started October 2000 for two years, the second a year later eventually for three years. After 4 man years of effort we had a basic version for e + e - collisions: –Basic cluster hadronization model with minor improvements for baryon and bottom/charm hadron production; –Improved parton shower for final-state radiation; –Simple model of hadron decays as in the FORTRAN program. SuperB 2 nd June8

9. LEP Event Shapes SuperB 2 nd June9 Herwig++ compared to ALEPH Phys.Lett.B512:30-48,2001 Herwig++ compared to SLD Phys.Rev.D65:092006,2002

10. Mid-Life Crisis After this version we were more reliant on the authors of the original FORTRAN program for further developments, no dedicated manpower although more recently dedicated C++ expert Took a further two years for a basic version for hadron-hadron collisions. Two years after that for the inclusion of multiple parton-parton interactions, improved hadron decays and BSM physics. So about 8 years to produce a program suitable for simulating LHC collisions! SuperB 2 nd June10

11. Underlying Event SuperB 2 nd June11 Herwig++ compared to CDF data

12. SuperB 2 nd June12 POWHEG method for Drell-Yan CDF Run I Z p T D0 Run II Z p T Herwig++ POWHEG MC@NLO JHEP 0810:015,2008 Hamilton, PR, Tully

13. p T of jets in W+jets at the Tevatron SuperB 2 nd June13 Herwig++ compared to data from CDF Phys.Rev.D77:011108,2008 All Jets 3 rd Hardest Jet

14. QED Radiation SuperB 2 nd June14

15. SuperB 2 nd June15 UED Look at the decay q* L e* R Z* ** q e - near e + far e - near e + near e - far e + far Herwig++ compared to hep- ph/0507170 Smillie and Webber

16. Hadron Decays The current simulation of hadron decays –includes spin correlations; –detailed simulation of tau decays; –good simulation of light meson and baryon decays; –QED radiation using the YFS approach ; –Sophisticated treatment of off-shell effects; –Reasonable description of inclusive B decays. Not focussed on B mixing or rare B decays, but can be interfaced to EvtGen. SuperB 2 nd June16

17. SuperB 2 nd June17 Hadron Decays Tau Decays,     B   e

18. Current The current version Herwig++ 2.4 is a better simulation of lepton-lepton, lepton-hadron and hadron-hadron collisions than its FORTRAN predecessor HERWIG6.5. However its taken a long time to get there and if the LHC had been finished on time we would have been too late. So what have we learnt? SuperB 2 nd June18

19. Lessons: Structure Using ThePEG as our structure has a number of pros –Memory handling –Parameter setting and cons –Steep learning curve –Some things weren’t the way we’d have done them With hindsight I still think this was the right decision and some of the advantages were lost when PYTHIA withdrew. SuperB 2 nd June19

20. Lessons: Structure In C++ getting the structure right is the key. Many features we intended to add, which were impossible in the FORTRAN, we very easy in the C++. Conversely changes we hadn’t intended proved painful. Equally for some things like the parton shower it took a number of iterations to get the structure right. SuperB 2 nd June20

21. Lessons: Structure In the B-physics community tend to rely on EvtGen for all decays, with PHOTOS for QED radiation. However both Herwig and SHERPA now have good internal modelling for hadron decays. In any new event simulation code need to think how bits of this can be used. Equally need to avoid the problems which led to different experiments using different versions of EvtGen. SuperB 2 nd June21

22. Lessons: People In hindsight it would have been easier to teach the people who knew the physics C++ than teach the people who knew C++ the physics: –Learning event generator physics 5-10 years; –Learning C++ 5-10 weeks. Many things would have been faster if we’d realised what the bugs were earlier. The money spent on postdocs would have been better used to buy out the teaching of the generator experts. SuperB 2 nd June22

23. Lessons: People In practice a lot of the recent code has been written by physicists with input/review by a C++ expert. Or even with two people in the same room doing the key pieces together which has worked very well in practice. SuperB 2 nd June23

24. Lessons: Collaboration As a group of theorists we weren’t really ready for the level of management that was probably needed. In the early stages this was a major problem with too little oversight of the C++ experts and too little involvement of the event generator experts. In the later stages most of the work has been done by a small number of people in close collaboration of those with physics and C++ expertise. SuperB 2 nd June24

25. SuperB 2 nd June25 Summary Herwig++ is now provides a sophisticated simulation of hadron collisions. However it’s been a long slow and often painful process. Any rewrite will certainly take longer than you expect. Good luck!