Introduction

The AlpgenInterface is undergoing a complete restructuring, according to the new guidelines for GeneratorInterfaces. This page documents this restructuring

New Features

Validation

• What with using the SLHA mass line? Is this doing something bad? Not at all up till now. However, from the update notes from Pythia 6.420:

  PYSLHA: protected some internal PYTHIA particles from read-in of 
      SLHA mass and DECAY tables, in order to avoid incompatibilities with 
      Pythia's hadronization and hadron decay machinery. The SLHA interface is
      intended for read-in of non-SM physics, so we do not consider it a
      strong restriction to protect Pythia's special internal codes and
      all SM particles with masses less than 20 GeV from being
      overwritten by SLHA read-in. People who do wish to alter, say,
      the decay tables of B mesons, should not use the SLHA tables to
      accomplish this, but rather use Pythia's internal updating routine
      PYUPDA.  A modification of the b-quark mass, for example, would
      need to be handled through the PMAS array.
  

• W + jets
  • Basic mother-daughter relations validation.
  • Basic INORAD validation.
• Z + jets
  • Basic mother-daughter relations validation.
  • Basic INORAD validation.
    • Why do we get the complete event record seen by UPVETO?
    • I don't know, but Alpgen correctly disregards the six initial particles before passing them to the calorimeter.
    • It also correctly disregards the daughters of INORAD'd particles.
• ttbar + jets
  • Basic mother daughter relations validation.
  • Basic INORAD validation.
  • Why is it freezing? Why does it stop freezing when I use FourVectors instead of doing calculations manually.
  • Always bitten by FORTRAN/C-style array differences.

Issues

• The PYUPRE subroutine:

C...PYUPRE
C...Rearranges contents of the HEPEUP commonblock so that
C...mothers precede daughters and daughters of a decay are
C...listed consecutively.

In standalone Pythia, a call to UPEVNT is always followed by a call to PYUPRE. In order to mimic that behavior, we do the same in AlpgenInterface.

• What is the "phantom event" at the beginning of DBPART?
  • Solved: it was an extra call of DBPART in a wrong place.
• How to make sure the PDF sets are chosen consistently in ALPGEN and PYTHIA?
  • Answer: we don't. See what happened in the 2009 ALPGEN gamma+jets by Sushil.
• How to make sure the CM energy is consistently kept around?
  • Answer: we don't. There is no way to access the CM energy from within the JetMatching class.
• What ALPGEN uses for the jet clustering is the HEPEVT common block, NOT the HEPEUP.

ALPGEN processes

• For ALPGEN v2.13
• Notice that all of those processes can have + jets in the final state. For some processes this is explicitly mentioned.

IHRD	Prefix	Meaning
1	wqq	W + pair of heavy quarks
2	zqq	Z + pair of heavy quarks
3	wjet	W + jets
4	zjet	Z + jets
5	vbjet	Multi vector bosons (W,Z,H,photon)
6	2Q	Pair of heavy quarks
7	4Q	Two pairs of heavy quarks
8	QQh	Pair of heavy quarks + Higgs
9	njet	Pure multijets
10	wcjet	W + single charm
11	phjet	photon + jets
12	hjet	Higgs + jets
13	top	Single top
14	wphjet	W + photon + jets
15	wphqq	W + photon + pair of heavy quarks
16	2Qph	Pair of heavy quarks + photon

• Functions to fix the Event Record:

void fixEventWZ(lhef::HEPEUP &hepeup);
void fixEventMultiBoson(lhef::HEPEUP &hepeup);
void fixEventTTbar(lhef::HEPEUP &hepeup);
void fixEventHiggsTTbar(lhef::HEPEUP &hepeup);
void fixEventSingleTop(lhef::HEPEUP &hepeup, double mb, int itopprc);

FORTRAN routines

Here we describe the FORTRAN routines responsible for the matching in standalone matching.

UPINIT routine

• I/O
• Call ALSHCD - returns "PYT"
• I/O
  • Read until "****"
  • Read IHRD
  • Read six masses (mc, mb, mt, mw, mz, mh)
  • Read key-value pairs (PARVAL[NTMP] array)
  • Read until "****"
  • Read xsec, xsec_error, number of unw events, luminosity
• Call AHSPAR to setup parameters - this sets EBEAM.
  • This sets every parameter except IHRD, the masses and the jet clustering parameters.
• PBEAM1=DBLE(EBEAM), PBEAM2=DBLE(EBEAM), IH1=1 --> these are tramp variables to set up HEPRUP later.
• Call PDFCONVH (what does it do?)
• Call ALSHCD
• Prepares INORAD
• Gets jet matching criteria
• Sets up the HEPRUP
• Calls ALSHIN (passes the six masses to PYTHIA)
• Sets LPRUP (originally it was 661, now it has a meaning).

• UPINIT.odg: UPINIT

The HEPRUP has to be setup as:

Parameter	Value
BEAM	p,p,energy of CM
PDFGUP(1,2)	-1
IDWTUP	3
XSECUP(1)	AVGWGT
XERRUP(1)	ERRWGT
XMAXUP(1)	AVGWGT

UPEVNT routine

UPVETO routine

-- ThiagoTomei - 20 Mar 2009