**************************** * * * Update notes * * * * PYTHIA version 6.4 * * * **************************** (Last updated 31 August 2006) PYTHIA version 6.4 is a direct continuation of version 6.3; actually 6.400 is identical with 6.327. Therefore it should not be a big operation for the normal user to run the program. PYTHIA 6.4 comes with an updated big manual, reflecting the status as of version 6.400. These update notes document what is new in each subsequent subversion. ----------------------------------------------------------------------- 6.400 : 25 March 2006 - This version is identical with Pythia 6.327. Readers should therefore turn to the Pythia 6.3 update notes to trace the evolution of the program. - The only changes are updates of character strings, 6.3 -> 6.4. 6.401 : 16 April 2006 - Bug fix in PYRAND: when the Les Houches Accord interface is used with more than 50 processes, some of them would be mapped into a range of PYTHIA process numbers assigned to SUSY processes, where KFPR(ISUB,1) and KFPR(ISUB,2) would be overwritten. - Modification of string fragmentation, introducing new tolerance parameter. PARU(14): (D=2.) when passing string corners, the (mis)match of transverse momentum directions may need to be compensated by using momentum fractions x outside the allowed range 0 < x < 1, by having a slightly negative x. Occasionally the x can become quite negative, and then rarely give strange results. The new parameter sets limits how far outside the allowed range one may go before rejecting the current try and restarting the fragmentation of the current string. - The size of the /PYPART/ commonblock has been expanded, by replacing the current MAXNUP size parameter by a new MAXNUR one with size 1000. This should avoid some problems with overflows. - Bug correction: for the MSTP(95) > 3 options the colour reconnection in PYFSCR was not called from PYEVNW. 6.402 : 2 May 2006 - PYFSCR: bugfix and updates (performing colour reconnections in the new underlying-event framework). A bug for MSTP(95)=2,3 caused an unintended suppression of connections between free colour octets. This has now been fixed, so the scenario works as described in the manual. In addition, a new option has been added, by MSTP(95)=6,7, where only partons with one and only one free colour tag are allowed to initiate a string piece. This amounts to saying that the string collapse is driven by free triplet charges. Finally, the parameter PARP(78) now also works for MSTP(95)>1, and controls the strength of the colour reconnections. In addition, the reconnection probability grows as a function of the number of multiple interactions, to reflect the possibility of a higher amount of reconnections occurring in collisions with a higher amount of strings. The full description of the PYFSCR parameters is thus now: MSTP(95) : CR Type = 1 : old cut-and-paste reconnections, handled in PYMIHK = 2 : Type I(no gg loops); hadron-hadron only = 3 : Type I(no gg loops); all beams = 4 : Type II(gg loops) ; hadron-hadron only = 5 : Type II(gg loops) ; all beams = 6 : Type S ; hadron-hadron only = 7 : Type S ; all beams Types I and II are described in Sandhoff+Skands, in hep-ph/0604120. Type S is driven by starting only from free triplets, not octets. PARP(78) : CR Strength The probability to keep a given colour-dipole pair depends on PARP(78) and the number of interactions, MINT(31): PKEEP = (1D0-PARP(78))**(MINT(31)) ; PARP(78) -> 1D0 : Full strength. PARP(78) -> 0D0 : Zero strength. No reconnections. NOTE: New min-bias/UE tunes with these new models are also available from the web page. - Speeded-up machinery for minimum-bias events, ISUB = 95. Specifically, the simultaneous choice of impact parameter b and pT for the hardest interaction has been optimized. The key trick is to split the b range in two, roughly where the probability for an interaction of two hadrons passing through each other has dropped to 1/2. With such a split it is feasible to pick b once and for all, and then to use two different formalisms for the pT selection. At small b, one includes the "Sudakov" exponent to generate a trial sequence of falling pT's until accepted. If the sequence reaches pT = 0 the generation is restarted at maximum. At large b the trial pT's are selected unordered over the whole pT range, and then the "Sudakov" exponent is used as (part of) the acceptance weight. This technical change should not affect physics. Internally, two new variables are used: MINT(39) : region of b value in current event. = 0 : not defined (e.g. for MSTP(82) = 1 or 2). = 1 : low-b region. = 2 : high-b region. VINT(139) : current b value. Note that no rescaling to physical units is performed, but only to an average b of 1 in minimum-bias events, and thereby below 1 for high-pT events. (Related to the jet pedestal effect.) Thus it only has a relative meaning in the context of comparing different events inside the same sample (i.e. generated with the same parameter values). - Initialization of multiple interactions did not take into account that the old and new multiple interactions scenarios use somewhat different factorization scale choices. This gave partly misleading initialization information for the new scenarios, but hardly affects the events actually generated for Tevatron or LHC applications. It could be more visible if tunes are extended to lower energies - PYSLHA: update. When reading in SLHA decay tables for Higgs bosons, a message is printed to alert the user to the fact that the h/H/A->gg decay width is also used to compute the cross section for the inverse process: gg -> h/H/A - PYFEYN: update. The previous interface to FeynHiggs became obsolete with FeynHiggs version 2.3, where a number of parameters in the relevant FeynHiggs call routines were changed from DOUBLE PRECISION to DOUBLE COMPLEX. The corresponding change has now been introduced in PYFEYN as well, so that earlier Pythia versions are compatible with FeynHiggs 2.2, while Pythia versions from 6.402 onward are compatible with FeynHiggs 2.3. - PYSUGI: small bug fix, removed a CLOSE statement that could lead to crashes if no file had previously been opened. - Production channels g g and gamma gamma -> Higgs rely on knowledge of the Higgs -> g g and gamma gamma partial widths. If SLHA is used to replace the internal decay tables the program would be looking in the wrong place. This has now been corrected. Note, however, that it is not possible to study e.g. g g -> Higgs if the new decay tables do not provide the Higgs -> g g decay channel. 6.403 : 7 June 2006 - The Pythia 6.4 Physics and Manual is now published in T. Sjostrand, S. Mrenna and P. Skands, JHEP05 (2006) 026 [hep-ph/0603175]. From now on, this is the only correct reference to the Pythia 6 program. - PYFEYN: further bug correction in this interface. - Typo in the speeded-up multiple-interactions treatment in 6.402 may affect results for the impact-parameter profile MSTP(82) = 5 when PARP(83) < 2, and is now corrected. - Insert Planck scale in PYSUGI in appropriate units: AMPL = 2.4 E18 GeV = {8 pi G_newton}^{1/2}. - Remove superfluous comma in PYSUGI printout statement. - Move default initialization of the PYLH3P and PYLH3C commonblocks to BLOCK DATA PYDATA for consistency with Fortran standard. - PYSIGH: An additional possible classification of basic scattering processes now exists, for the special case of Weak Boson Fusion and g g or q qbar -> Q Qbar Higgs processes: ISJETS = 2: Processes for which the matrix element does contains jets at leading order, but which are not included in ISJETS=1 since they are not of a QCD origin, as for instance the forward jets in WW/ZZ fusion to Higgs, qq -> qqH. The processes classified as ISJETS=2 are currently ISUB=121-124, 173, 174, 178, 179, 181, 182, 186, 187. The maximum scale for initial state radiation in these processes is set equal to the factorisation scale, see MSTP(32) and MSTP(39). 6.404 : 10 August 2006 - Changes to handle the new proposed Les Houches Event Files (LHEF) format [in preparation]. 1) The existing two sample routines UPINIT and UPEVNT, which read the PYTHIA old separate initialization and event files, have been commented out (so the code is still there, just in case...). 2) The new (uncommented) UPINIT and UPEVNT routines handle an LHEF, for initialization and event read-in. As before, the file to be used must be opened by the user in the main program, and the file number set in MSTP(161) and MSTP(162). Previously, with two separate input files, MSTP(161) and MSTP(162) were different. Now they are the same, but both must still be set. (As before, the routines that come with PYTHIA are only suggestions. You are free to supply your own routines and link them instead. The difference is that now there will be default code that eventually will work right away with several different parton-level generators.) 3) A new routine PYLHEF has been added to produce an LHE file from a parton-level PYTHIA run output. Recall that, previously, PYUPEV had to be used instead of PYEVNT to produce parton-level-only events and write them to file MSTP(161), and a call to PYUPIN had to be added at the end to write initialization information to file MSTP(162), both files to be opened by the user. (The PYUPIN call has to come at the end, since it is only then that cross sections are known.) By adding a call to PYLHEF after the PYUPIN call, the two files will be combined into one single LHE file. MSTP(163) : (D=0) file unit number on which the LHE file will be written. The corresponding file must be opened by the user in the main program. MSTP(164) : (D=0) fate of input files when an LHE file is created by PYLHEF. = 0: the input files, associated with units MSTP(161) and MSTP(162), are removed. = 1: the input files are kept unchanged, although the information is now duplicated in the new file. 4) Note: it would have been possible to avoid the use of two intermediate files, by a judicious use of dummy cross sections when the beginning of the file is first written, with the correct ones substituted after a REWIND. This seemed like overkill, given the small use of PYTHIA as a parton-level-only generastor. The current facility to write LHE files is mainly intended to test the LHEF structure. 5) The precision with which lifetime and spin are written in PYUPEV has been reduced, in particular since spin is not assigned by PYTHIA and thus always defaults to 9. = unassigned,. and lifetime is usually assumed 0. = vanishing. - The processes 381-388 for QCD processes modified by technicolor interactions are reclassified as being QCD processes in PYSIGH (except for top production), i.e. multiple interactions and initial-state radiation are restricted to be softer than the hard process itself. - The colour octet wavefunction-at-the-origin default values for charmonia and bottomonia in PARP(141)-PARP(150) are now set according to the values in P. Nason et al., in "Standard model physics (and more) at the LHC", eds. G. Altarelli and M.L. Mangano, CERN 2000-004, p. 231 [hep-ph/0003142] as described in talk by M. Bargiotti in the HERA4LHC workshop, CERN, 8 June 2006. - The values of the parton densities, xf(x,Q2) of the two incoming partons to the hard interaction (if any) are now stored in PARI(29) and PARI(30), respectively. (And internally also in VINT(39) and VINT(40). The XSFX array, which contains this kind of information, is overwritten for each new multiple interaction, and is therefore not so useful.) Since before the related flavours are stored in MSTI(15), MSTI(16), the x values in PARI(33) and PARI(34), and the Q scale in PARI(23). - The size NMXHEP of the HEPEVT commonblock is now stored in MSTU(8) (for informational purposes). - The two dummy subroutines FHSETPARA and FHIGGSCORR have been updated to take complex variables, cf. update note 6.402 for PYFEYN. Irrelevant in practice, but introduced for consistency. 6.405 : 31 August 2006 - Mistake in 6.404 when processes 381-388 were to be classified as QCD processes (an .OR. instead of an .AND.) lead to all processes being classified as QCD-type, and thereby, in some cases, to the maximum scale for ISR and MI being set incorrectly. - Several processes involving photons in the initial, intermediate or final state have been assigned ISJETS=3 in PYSIGH. This mean that the maximum shower scale is set equal to the hard-process scale, typically pT, rather than the full energy. Thereby (in most cases) the old behaviour from Pythia 6.2 should be recovered for these processes. The ones relevant for hadron colliders are 10, 18, 19, 20, 83, 106 and 114. The ones for incoming photon beams are 33 - 36, 54, 58, 69, 70, 80, 84, 85, 107, 108, 110 and 131 - 140. - A few processes have been added to the ISJETS = 2 class introduced in 6.403, i.e. WW/ZZ fusion: 5, 8, 71, 72, 73, 76, 77, 351 and 352. - Showering and PDF scales for elastic and diffractive processes explicitly set zero, for cosmetics reasons only. - New option MSTP(123) = 3 gives same bahaviour as = 2 but does not print out warnings. -----------------------------------------------------------------------