PYTHIA 6.4 Physics and Manual

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V matrix contains decay vertices; to view those PYLIST(3) has to be used. NPAD is adummy, needed to avoid some compiler troubles. It is important to learn the rules forhow information is stored in PYJETS.The third executable line in the program illustrates another important point aboutPythia: a number of routines are available for manipulating and analyzing the eventrecord after the event has been generated. Thus PYEDIT(3) will remove everything exceptstable charged particles, as shown by the result of the second PYLIST call. More advancedpossibilities include things like sphericity or clustering routines. Pythia also containssome simple routines for histogramming, used to give self-contained examples of analysisprocedures.Apart from the input arguments of subroutine calls, control on the doings of Pythiamay be imposed via many common blocks. Here sensible default values are always provided.A user might want to switch off all particle decays by putting MSTJ(21) = 0 orincrease the s/u ratio in fragmentation by putting PARJ(2) = 0.40D0, to give but twoexamples. It is by exploring the possibilities offered here that Pythia can be turned intoan extremely versatile tool, even if all the nice physics is already present in the defaultvalues.As a final, semi-realistic example, assume that the p ⊥ spectrum of π + particles is tobe studied in 91.2 GeV e + e − annihilation events, where p ⊥ is to be defined with respectto the sphericity axis. Using the internal routines for simple histogramming, a completeprogram might look likeC...Double precision and integer declarations.IMPLICIT DOUBLE PRECISION(A-H, O-Z)IMPLICIT INTEGER(I-N)INTEGER PYK,PYCHGE,PYCOMPC...Common blocks.COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)C...Book histograms.CALL PYBOOK(1,’pT spectrum of pi+’,100,0D0,5D0)C...Number of events to generate. Loop over events.NEVT=100DO 110 IEVT=1,NEVTC...Generate event. List first one.CALL PYEEVT(0,91.2D0)IF(IEVT.EQ.1) CALL PYLIST(1)C...Find sphericity axis.CALL PYSPHE(SPH,APL)C...Rotate event so that sphericity axis is along z axis.CALL PYEDIT(31)C...Loop over all particles, but skip if not pi+.DO 100 I=1,NIF(K(I,2).NE.211) GOTO 100C...Calculate pT and fill in histogram.PT=SQRT(P(I,1)**2+P(I,2)**2)CALL PYFILL(1,PT,1D0)36
C...End of particle and event loops.100 CONTINUE110 CONTINUEC...Normalize histogram properly and list it.CALL PYFACT(1,20D0/NEVT)CALL PYHISTENDStudy this program, try to understand what happens at each step, and run it to checkthat it works. You should then be ready to look at the relevant sections of this reportand start writing your own programs.3.6 Getting Started with the Event Generation MachineryA run with the full Pythia event generation machinery has to be more strictly organizedthan the simple examples above, in that it is necessary to initialize the generation beforeevents can be generated, and in that it is not possible to change switches and parametersfreely during the course of the run. A fairly precise recipe for how a run should bestructured can therefore be given.Thus, the nowadays normal usage of Pythia can be subdivided into three steps.1. The initialization step. It is here that all the basic characteristics of the cominggeneration are specified. The material in this section includes the following.• Declarations for double precision and integer variables and integer functions:IMPLICIT DOUBLE PRECISION(A-H, O-Z)IMPLICIT INTEGER(I-N)INTEGER PYK,PYCHGE,PYCOMP• Common blocks, at least the following, and maybe some more:COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)COMMON/PYSUBS/MSEL,MSELPD,MSUB(500),KFIN(2,-40:40),CKIN(200)COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)• Selection of required processes. Some fixed ‘menus’ of subprocesses can beselected with different MSEL values, but with MSEL=0 it is possible to compose‘à la carte’, using the subprocess numbers. To generate processes 14, 18 and29, for instance, one needsMSEL=0MSUB(14)=1MSUB(18)=1MSUB(29)=1• Selection of kinematics cuts in the CKIN array. To generate hard scatteringswith 50 GeV ≤ p ⊥ ≤ 100 GeV, for instance, useCKIN(3)=50D0CKIN(4)=100D0Unfortunately, initial- and final-state radiation will shift around the kinematicsof the hard scattering, making the effects of cuts less predictable. One thereforealways has to be very careful that no desired event configurations are cut out.• Definition of underlying physics scenario, e.g. Higgs mass.• Selection of parton-distribution sets, Q 2 definitions, showering and multipleinteractions parameters, and all other details of the generation.37
Page 1 and 2: hep-ph/0603175LU TP 06-13FERMILAB-P
Page 4 and 5: Contents1 Introduction 11.1 The Com
Page 6 and 7: 8.6.3 Left-Right Symmetry and Doubl
Page 8 and 9: 13 Particles and Their Decays 38413
Page 10 and 11: another sense, the overall energy f
Page 13 and 14: p ⊥ -ordered parton showers and a
Page 15 and 16: you expect to happen if you do not
Page 17 and 18: The history of the Pythia program i
Page 19 and 20: andom, according to the relative co
Page 21 and 22: Of course, the above ‘resonance
Page 23 and 24: 2.2.1 Matrix elementsMatrix element
Page 25 and 26: analysis strongly suggests the scal
Page 27 and 28: y calling PYEVNW instead of PYEVNT,
Page 29 and 30: charge Casimir operators, N C /C F
Page 31 and 32: 3 Program OverviewThis section cont
Page 33 and 34: Table 2: The main versions of Pythi
Page 35 and 36: • A new master event-generation r
Page 37 and 38: A test program, PYTEST, is included
Page 39 and 40: argument is used with a value that
Page 41 and 42: &XMI(2,240),Q2MI(240),IMISEP(0:240)
Page 43: original u and u. The parentheses e
Page 47 and 48: PMAS(25,1)=300D0CKIN(1)=290D0CKIN(2
Page 49 and 50: third the particle-flavour code (wh
Page 51 and 52: 4 Monte Carlo TechniquesQuantum mec
Page 53 and 54: different g i ’s, it is possible
Page 55 and 56: The last equality is most easily se
Page 57 and 58: Purpose: to generate a (pseudo)rand
Page 59 and 60: 5 The Event RecordThe event record
Page 61 and 62: Table 4: Gauge boson and other fund
Page 63 and 64: Table 7: Meson codes, part 1.KF Nam
Page 65: Table 10: QCD effective states.KF P
Page 68 and 69: is then required. Normally this mea
Page 70 and 71: quarks. Note that the positions nee
Page 72 and 73: original entries appear with pointe
Page 74 and 75: consistent summary, but then in div
Page 76 and 77: = 3 : a documentation line, defined
Page 78 and 79: 6 The Old Electron-Positron Annihil
Page 80 and 81: 6.1.2 First-order QCD matrix elemen
Page 82 and 83: 6.1.4 Second-order three-jet matrix
Page 84 and 85: evaluated and compared with a param
Page 86 and 87: The angular orientation of a 3- or
Page 88 and 89: these offer unique possibilities to
Page 90 and 91: 6.3.3 Common-block variablesThe sta
Page 92 and 93: 3-jet cross section, so large that
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fraction of events containing a rad
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! PYSPHE to work on it (unusual)CAL
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Pdflib has been frozen in recent ye
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modelling of γp and γγ events, s
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One can obtain the positron distrib
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7.2 Kinematics and Cross Section fo
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7.3 Resonance ProductionThe simples
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y a factor 1/N C = 1/3. Secondly, t
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This choice certainly is not unique
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The phase-space points tested at in
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spin-1 resonance the expression is,
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probabilities.In some processes, su
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where decay channels are chosen acc
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and for a resonance-antiresonance p
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The Regge formulae above for single
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emnant, but this remnant has a larg
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called direct, our direct×VMD and
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are also covered in other places in
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Table 17: Subprocess codes, part 2.
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Page 134 and 135:
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Table 25: Subprocess codes, part 10
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8.2 QCD ProcessesObviously most pro
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1. if MSEL = 4 - 8 then the flavour
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into the cross section. However, cu
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Uncertainties come from a number of
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If you are only concerned with stan
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for MSTP(14) = 1 and the VMD part o
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vertex. MSTP(66) offer some alterna
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calculation is preferred. Some caut
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contains a full two-Higgs-multiplet
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8.5.2 Heavy Standard Model HiggsISU
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parton-distribution-function approa
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The advantage of the explicit pair
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handed neutrinos. The Higgs fields
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former and ŝ for the latter. To ma
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mode is therefore tt, if kinematica
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through a vector-dominance mechanis
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where α a = ga/4π.2The phenomenol
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has two scalar partners, one associ
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above. To obtain proper linking wit
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or (ii) the masses of the first- an
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mSUGRA model input parameters shoul
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stable. If this is not the case, MW
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is no a priori generic prediction f
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tools exist. Therefore the producti
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’gamma/lepton’ machinery. There
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numerical precision may suffer; if
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call, e.g. at the end of the run, o
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on so as to give the full (paramete
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MSEL = 10) (ISUB = 19, 20, 22, 23,
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CKIN(7), CKIN(8) : (D = −10., 10.
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For two resonances that are identic
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= 1 : you should set couplings for
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or GVMD; we will use both interchan
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MSTP(17) : (D = 4) possibility of a
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h 0 and H 0 . It is mainly intended
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correct only in the Standard Model
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for three technicolors, based on sc
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esponsibility on you.Note 2: when P
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= 1 : the recommended standard for
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where a reconnection is actually ma
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MSTP(145) : (D = 0) choice of polar
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PARP(32) : (D = 2.0) special K fact
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PARP(121) : (D = 1.) the maxima obt
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Q2 : the momentum transfer scale Q
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1-PARU(136) corresponding to the sa
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cot θ 3 .ITCM(5) : (D = 0) presenc
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are SUGRA, SSMSSM and VISAJE, locat
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IMSS(22) : (D = 0) Read-in of SLHA
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it is the constant of proportionali
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k = 1 : only possibility.ISUB = 13,
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number is obtained as a by-product
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vanishing if no splitting is needed
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CALL PYINIT(’CMS’,’p’,’pb
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C...Histograms.CALL PYBOOK(1,’dn_
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Of course, the separation of which
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does imply an infinite total cross
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Pythia. The solution adopted here
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est strategy would vary, depending
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tered during the course of the run,
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NPRUP entries of the XSECUP, XERRUP
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should be denoted by using the valu
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more information than is provided b
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9.9.3 An exampleTo exemplify the ab
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DOUBLE PRECISION EBMUP,XSECUP,XERRU
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momenta and masses can be reconstru
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quantity, the effect of such events
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9.10 Interfaces to Other Generators
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= 2 : assign the interference contr
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value before the routine is called.
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• PYSGQC : normal QCD processes,
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afterwards.)MINT(17), MINT(18) : fl
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= 0 : normal initialization.= 1 : i
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agrees with the Bjorken definition,
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action; used to find what is left f
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VINT(319) : photon flux factor in P
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COMMON/PYINT4/MWID(500),WIDS(500,5)
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Purpose: to store information on to
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10 Initial- and Final-State Radiati
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for the original parton a. On the o
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180 ◦ and therefore the emission
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anching activity of the gluon. (Wit
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anching of both the q and the q, in
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10.2.5 Other final-state shower asp
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wide selection of generic colour an
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e a function of the transverse mome
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That way we hope to achieve the mos
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step by step one moves ‘backwards
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in hadronic interactions, with the
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happens it is almost always for one
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need be.The scheme presented above
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with the new PYPTIS one. The advant
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of the system.)CALL PYPTFS(NPART,IP
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Note:a few non-standard options hav
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g → gg and reduced angle for g
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(MSTP(42) = 2) and MSTJ(46) = 3).PA
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= 3 : the k ⊥ of the anomalous/GV
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to the cut implied by PARP(65).PARP
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outlines where the intermediate and
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11.1.4 Primordial k ⊥It is custom
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mended first to read this section,
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In general, there is quite a strong
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• Scatterings of the gg → gg ty
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where the denominator comes from th
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hard processes in pp or pp collisio
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proton net flavour content, and ene
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can be used here, from sharing the
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There is also the matter of a lower
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The program needs to know the assum
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Λ FSR scale: PARJ(81) = 0.14D0;Bea
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‘correct’ total cross section.M
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generated with the new model (in PY
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e attached to the remnant, PARP(80)
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Determine physical PT2MAX and PT2MI
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COMMON/PYCTAG/NCT, MCT(4000,2)Purpo
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string is assumed to have no transv
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Only two baryon multiplets are incl
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Additional parameters include the r
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or the q one, ‘left-right symmetr
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are changed, some retuning should b
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end in the initial stages of the st
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The fact that the hadron should be
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(including the one between the two
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the ratio of rescaled jet momentum
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duly randomized, but properly it wo
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Other models include a simplified i
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The negative sign is exactly what w
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13 Particles and Their DecaysPartic
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multiplet m 0 kpseudoscalars and ve
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toIn Dalitz decays, π 0 or η →
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fragmentation description. This doe
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14 The Fragmentation and Decay Prog
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Initially the partons must be given
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SUBROUTINE PYZDIS(KFL1,KFL3,PR,Z) :
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MSTU(4) : (D = 4000) number of line
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full. Is reset at each PYEXEC call.
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= 4 : transverse momenta are compen
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included in the event. If the showe
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allowed to shower, is 0 if no pair
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an independent gluon jet generated
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• Increase PARJ(1) by about a fac
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With five possible flavours for q 1
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LFN :is intended for the program au
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where both KTAB1 and KTAB3 are know
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a fourth generation is small.Remark
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code it is possible to define chann
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to be borne out by comparisons with
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CALL PYEXEC! particles decay, excep
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15 Event Study and Analysis Routine
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it can be further specified to +z a
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= 8 : KF code (K(I,2)) for a remain
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15.2 Event ShapesIn this section we
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To find the thrust value and axis t
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Hence rapidity, azimuthal angle and
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as a starting point for the iterati
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may be performed in E rather than i
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Here n = ∑ j n j is the total mul
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MSTU(63) and PARU(61) - PARU(63). I
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H40 : H 4 /H 0 .Remark: the number
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V(I,2) - V(I,5) = statistical error
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PYTHRU, PYCLUS and PYCELL.= 1 : sto
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MSTU(53)).PARU(57) : (D = 3.2) maxi
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Purpose: to dump the contents of ex
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References[Abb87]A. Abbasabadi and
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[Bai83] R. Baier and R. Rückl, Z.
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[Bra64] S. Brandt, Ch. Peyrou, R. S
Page 470 and 471:
[Djo97] A. Djouadi, J. Kalinowski a
Page 472 and 473:
[Gie04]S. Gieseke, A. Ribon, M.H. S
Page 474 and 475:
[Ing80] G. Ingelman and T. Sjöstra
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[Lan00] K. Lane, T. Rador and E. Ei
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[OPA91] OPAL Collaboration, M.Z. Ak
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[Sjö92][Sjö92a][Sjö92b]T. Sjöst
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Appendix A: Subprocess Summary Tabl
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No. Subprocess No. SubprocessNo. Su
Page 486 and 487:
Appendix B: Index of Subprograms an
Page 488 and 489:
PYK function 429PYKCUT subroutine 1
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