PYTHIA 6.4 Physics and Manual

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• Switching off of generator parts not needed for toy simulations, e.g. fragmentationfor parton level studies.• Initialization of the event generation procedure. Here kinematics is set up,maxima of differential cross sections are found for future Monte Carlo generation,and a number of other preparatory tasks carried out. Initializationis performed by PYINIT, which should be called only after the switches andparameters above have been set to their desired values. The frame, the beamparticles and the energy have to be specified, e.g.CALL PYINIT(’CMS’,’p’,’pbar’,1800D0)• Any other initial material required by you, e.g. histogram booking.2. The generation loop. It is here that events are generated and studied. It includesthe following tasks:• Generation of the next event, withCALL PYEVNTor, for the new multiple interactions and showering model,CALL PYEVNW• Printing of a few events, to check that everything is working as planned, withCALL PYLIST(1)• An analysis of the event for properties of interest, either directly reading out informationfrom the PYJETS common block or making use of the utility routinesin Pythia.• Saving of events on disk or tape, or interfacing to detector simulation.3. The finishing step. Here the tasks are:• Printing a table of deduced cross sections, obtained as a by-product of theMonte Carlo generation activity, with the commandCALL PYSTAT(1)• Printing histograms and other user output.To illustrate this structure, imagine a toy example, where one wants to simulate theproduction of a 300 GeV Higgs particle. In Pythia, a program for this might looksomething like the following.C...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)COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)COMMON/PYDAT2/KCHG(500,4),PMAS(500,4),PARF(2000),VCKM(4,4)COMMON/PYDAT3/MDCY(500,3),MDME(8000,2),BRAT(8000),KFDP(8000,5)COMMON/PYSUBS/MSEL,MSELPD,MSUB(500),KFIN(2,-40:40),CKIN(200)COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)C...Number of events to generate. Switch on proper processes.NEV=1000MSEL=0MSUB(102)=1MSUB(123)=1MSUB(124)=1C...Select Higgs mass and kinematics cuts in mass.38
PMAS(25,1)=300D0CKIN(1)=290D0CKIN(2)=310D0C...For simulation of hard process only: cut out unnecessary tasks.MSTP(61)=0MSTP(71)=0MSTP(81)=0MSTP(111)=0C...Initialize and list partial widths.CALL PYINIT(’CMS’,’p’,’p’,14000D0)CALL PYSTAT(2)C...Book histogram.CALL PYBOOK(1,’Higgs mass’,50,275D0,325D0)C...Generate events. Look at first few.DO 200 IEV=1,NEVCALL PYEVNTIF(IEV.LE.3) CALL PYLIST(1)C...Loop over particles to find Higgs and histogram its mass.DO 100 I=1,NIF(K(I,1).LT.20.AND.K(I,2).EQ.25) HMASS=P(I,5)100 CONTINUECALL PYFILL(1,HMASS,1D0)200 CONTINUEC...Print cross sections and histograms.CALL PYSTAT(1)CALL PYHISTENDHere 102, 123 and 124 are the three main Higgs production graphs gg → h, ZZ → h,and WW → h, and MSUB(ISUB) = 1 is the command to switch on process ISUB. Fullfreedom to combine subprocesses ‘à la carte’ is ensured by MSEL = 0; ready-made ‘menus’can be ordered with other MSEL numbers. The PMAS command sets the mass of the Higgs,and the CKIN variables the desired mass range of the Higgs — a Higgs with a 300 GeVnominal mass actually has a fairly broad Breit–Wigner type mass distribution. The MSTPswitches that come next are there to modify the generation procedure, in this case toswitch off initial- and final-state radiation, multiple interactions among beam jets, andfragmentation, to give only the ‘parton skeleton’ of the hard process. The PYINIT callinitializes Pythia, by finding maxima of cross sections, recalculating the Higgs decayproperties (which depend on the Higgs mass), etc. The decay properties can be listedwith PYSTAT(2).Inside the event loop, PYEVNT is called to generate an event, and PYLIST(1) to listthe event. The information used by PYLIST(1) is the event record, stored in the commonblock PYJETS. Here one finds all produced particles, both final and intermediate ones, withinformation on particle species and event history (K array), particle momenta (P array)and production vertices (V array). In the loop over all particles produced, 1 through N,the Higgs particle is found by its code, K(I,2) = 25, and its mass is stored in P(I,5).After all events have been generated, PYSTAT(1) gives a summary of the number of39
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 and 44: original u and u. The parentheses e
Page 45: C...End of particle and event loops
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
Page 94 and 95: 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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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
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[Djo97] A. Djouadi, J. Kalinowski a
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[Gie04]S. Gieseke, A. Ribon, M.H. S
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[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
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Appendix B: Index of Subprograms an
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PYK function 429PYKCUT subroutine 1
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