New features for V2.1 (for the features of V2.0X relative to V1.X, see the version-update history (V2.01->V2.06):

Makefiles, compilation:
To help the user select the compilers and compilation options most suitable to his platform, these are now collected in a single file located in the main directory: compile.mk

The Makefile for the various processes refer to compile.mk for the compiler options, and to alplib/alpgen.mk for the list of files to be compiled. The executable can now be generated within each working directory issuing the command:

• For the F77 version: > make gen
• For the F90 version: > make gen90

Updates to jet-parton matching:
The use of jet-parton matching is described in the version history of V2.0 . A complete review of the MLM matching scheme is being documented in a forthcoming publication.

Few changes to the matching are implemented in V2.1:

1. The matching and vetoing for events with heavy quarks (charm, b and top) has been improved. For these processes the matching now proceeds in two steps. First the matching of the light partons with the jets is performed. As in previous versions, radiation off the heavy quarks is not used to cluster the jets used in the matching. The radiation emitted by the heavy quarks is then subsequently included in the recostruction of possible additional jets, and is used to veto against extra-jet emissions when generating exclusive samples.
2. The treatment of the highest-multiplicity jet sample, which is to be generated in "inclusive mode (iexc=0) has been slightly modified. Events with extra jets are kept only if the jets which do not match partons have pt smaller than the minimum pt of the matched jets.
3. The default parameters of the matching algorithm (the minimum et of the clusters, their rapidity range) have been modified. The proposed defaults appear to be better than the previous ones.
4. The scale choice has been slightly modified. In the case of processes that at LO start with several powers of alphas (like jet production, 2Q  or wqq), we separate the powers of alphas that accompany the LO process from those that accompany the additional light partons. It is only to these extra powers that the CKKW scale is applied. By construction, this means that for a LO process now ickkw=0 or =1 should give the same result as ickkw=0 before.
   
5. The matching algorithm for some processes (most notably vbjet with vector boson fusion) is still being optimzied, and will likely be updated in forthcoming releases
   

The Herwig version provided with V2.1 is Herwig6510, which properly compiles under gfortran. The matching has still not been updated to be used with the new shower of the latest versions of Pythia. 

Other notable features: 

• A photon-photon minimal separation parameter (drphmin) has been introduced. So far the minimal separation between photon pairs was taken to be equal to the minimal dR separaiotn between light jets. This, among other changes, called for an addition to the alpgen.inc common blocks, which are therefore now incompatible with those of V2.0X.
• The default value of the charm mass has been set to 1.5 gev (was 0) for all processes with an explicit final-state charm (e.g. Wc cbar+jets, Wc+jets, cc+jets, etc)
• In the process Wc, it is now possible to place separate generation cuts for the charm (ptcmin and etacmax) and for the light jets (ptjmin, etajmax).
  

Remarks/Bugs for V2.1. This section contains comments and/or bug reports. The source library linked above contains the most up-to-date version, with all bugs reported below fixed. It is advised to check this section from time to time, to ensure you are running the most up-to-date version. Only the program elements explicitly listed have been updated with respect to the previous version.

• The code for hjet incorrectly allowed more than 1 Higgs bosons being produced. The effective-action vertex used by Alpgen only allows for one. Now nh=1 is hard-wired in:  alplib/alpgen.f
• A bug affected the flavour selection in one of the WQQ+3jet subprocesses (jproc=23). The affected events (about 1%) of the total were rejected by the parton shower. No significant bias is introduced, and there is no reason to regenerate samples or distributions obtained with earlier versions. We thank Juan Antonio Aguilar-Saavedra for pointing out this problem. Fix implemented in: wqqlib/wqq.f
• A bug was found in the vbjet processes affecting the assignement of particle IDs to gauge bosons when more than one photon was present in the final state. Fixed in: vbjetlib/vbjet.f

• Decay BRs for vbjet processes with 2 or more Ws were wrong. This has been fixed in the file:  vbjetlib/vbjet.f
• The sampling of DY lepton pair masses for masses below ~60 GeV was incorrect after unweighting in the zjet and zqq processes. This only affects event samples generated within a mass range containing the Z mass, and in imode=2. The bulk of the events around the Z peak is OK. Therefore a typical analysis based on Z final states (as opposed to a study of DY pairs well below the Z peak) is unaffected. The bug has been fixed in the files:  zjetlib/zjet.f and zqqlib/zqq.f. We thank G. Grenier for pointing out this issue.
• A bug was found in the matching for the associated production of W and a single charm (wcjet, with Njet=0) . This has been fixed in the file:  alplib/alpgen.f

• A bug affecting the leptonic decay rates of Z bosons in the vbjet processes with 2 or more Zs, and one of them selected with the decay option zfstate=2 (namely inclusive charged-lepton decays). We thank C.Mariotti for pointing this out to us. Affected routines: vbjetlib/vbjet.f
• A bug affecting the processing by PYTHIA of tt+jets final states in 2Q when using the ickkw=1 and iexc=1 (exclusive matching) mode. The "improved" (hmmm...) matching for heavy quarks, which is described above under point 1. of Section "Updates to jet/parton matching", was mishandling – in the case of PYTHIA evolution – the radiation off the decay products of the top quarks in the reconstruction of jets. This resulted in an unphysical removal of events due to the presence of the extra jets from top decay. The bug was introduced in V21, does not affect the event processing with HERWIG, and was not present before V21. We thank D. Tucker-Smith for promptly bringing this serious bug to our attention.  Affected routines: alplib/alpsho.f  

• Several bugs in the Wc process have been fixed. They had a minor impact on the partonic generation (an artificial asymmetry in c vs cbar production), and affected the application of the matching algorithm
• The cut on a minimal separation between light jets and top quarks has been removed from the WQQ+jets process

Forthcoming features:

• Improved treatment of matching for the vbjet processes (including vector boson fusion)
• Framework to allow users to incorporate new BSM processes

Notice for CDF users: Claudio Ferretti (Univ. of Michigan) prepared an interface to handle all of the ALPGEN processes, from generation to showering, including conversion to HEPG. Scripts are available to save as ROOT objects weighted, unweighted and showered events, generation cuts and topdraw plots. This package and its documentation are available from this link

Notice for CMS users: The Alpgen-CMS interface is described at this URL

Michelangelo L. MANGANO
TH Division, CERN, CH-1211 Geneva 23, Switzerland
Building 4, Room 4-2.046
Tel: +41 22 767 2820
Fax: +41 22 767 3850
Michelangelo.Mangano@cern.ch