PYTHIA 6.4 Physics and Manual

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at 20 GeV energy. To do this, write a main programIMPLICIT DOUBLE PRECISION(A-H, O-Z)CALL PY2ENT(0,2,-2,20D0)CALL PYLIST(1)ENDand run this program, linked together with Pythia. The routine PY2ENT is specificallyintended for storing two entries (partons or particles). The first argument (0) is a commandto perform fragmentation and decay directly after the entries have been stored, thesecond and third that the two entries are u (2) and u (−2), and the last that the c.m.energy of the pair is 20 GeV, in double precision. When this is run, the resulting event isstored in the PYJETS common block. This information can then be read out by you. Nooutput is produced by PY2ENT itself, except for a title page which appears once for everyPythia run.Instead the second command, to PYLIST, provides a simple visible summary of theinformation stored in PYJETS. The argument (1) indicates that the short version shouldbe used, which is suitable for viewing the listing directly on an 80-column terminal screen.It might look as shown here.Event listing (summary)I particle/jet KS KF orig p_x p_y p_z E m1 (u) A 12 2 0 0.000 0.000 10.000 10.000 0.0062 (ubar) V 11 -2 0 0.000 0.000 -10.000 10.000 0.0063 (string) 11 92 1 0.000 0.000 0.000 20.000 20.0004 (rho+) 11 213 3 0.098 -0.154 2.710 2.856 0.8855 (rho-) 11 -213 3 -0.227 0.145 6.538 6.590 0.7816 pi+ 1 211 3 0.125 -0.266 0.097 0.339 0.1407 (Sigma0) 11 3212 3 -0.254 0.034 -1.397 1.855 1.1938 (K*+) 11 323 3 -0.124 0.709 -2.753 2.968 0.8469 p~- 1 -2212 3 0.395 -0.614 -3.806 3.988 0.93810 pi- 1 -211 3 -0.013 0.146 -1.389 1.403 0.14011 pi+ 1 211 4 0.109 -0.456 2.164 2.218 0.14012 (pi0) 11 111 4 -0.011 0.301 0.546 0.638 0.13513 pi- 1 -211 5 0.089 0.343 2.089 2.124 0.14014 (pi0) 11 111 5 -0.316 -0.197 4.449 4.467 0.13515 (Lambda0) 11 3122 7 -0.208 0.014 -1.403 1.804 1.11616 gamma 1 22 7 -0.046 0.020 0.006 0.050 0.00017 K+ 1 321 8 -0.084 0.299 -2.139 2.217 0.49418 (pi0) 11 111 8 -0.040 0.410 -0.614 0.751 0.13519 gamma 1 22 12 0.059 0.146 0.224 0.274 0.00020 gamma 1 22 12 -0.070 0.155 0.322 0.364 0.00021 gamma 1 22 14 -0.322 -0.162 4.027 4.043 0.00022 gamma 1 22 14 0.006 -0.035 0.422 0.423 0.00023 p+ 1 2212 15 -0.178 0.033 -1.343 1.649 0.93824 pi- 1 -211 15 -0.030 -0.018 -0.059 0.156 0.14025 gamma 1 22 18 -0.006 0.384 -0.585 0.699 0.00026 gamma 1 22 18 -0.034 0.026 -0.029 0.052 0.000sum: 0.00 0.000 0.000 0.000 20.000 20.000(A few blanks have been removed between the columns to make it fit into the formatof this text.) Look in the particle/jet column and note that the first two lines are the34
original u and u. The parentheses enclosing the names, ‘(u)’ and ‘(ubar)’, are there asa reminder that these partons actually have been allowed to fragment. The partons arestill retained so that event histories can be studied. Also note that the KF (flavour code)column contains 2 in the first line and −2 in the second. These are the codes actuallystored to denote the presence of a u and a u, cf. the PY2ENT call, while the names writtenare just conveniences used when producing visible output. The A and V near the end of theparticle/jet column indicate the beginning and end of a string (or cluster, or independentfragmentation) parton system; any intermediate entries belonging to the same systemwould have had an I in that column. (This gives a poor man’s representation of anup-down arrow, ↕.)In the orig (origin) column, the zeros indicate that u and u are two initial entries.The subsequent line, number 3, denotes the fragmenting uu string system as a whole, andhas origin 1, since the first parton of this string system is entry number 1. The particlesin lines 4–10 have origin 3 to denote that they come directly from the fragmentation ofthis string. In string fragmentation it is not meaningful to say that a particle comes fromonly the u quark or only the u one. It is the string system as a whole that gives a ρ + , aρ − , a π + , a Σ 0 , a K ∗+ , a p − , and a π − . Note that some of the particle names are againenclosed in parentheses, indicating that these particles are not present in the final stateeither, but have decayed further. Thus the π − in line 13 and the π 0 in line 14 have origin5, as an indication that they come from the decay of the ρ − in line 5. Only the namesnot enclosed in parentheses remain at the end of the fragmentation/decay chain, andare thus experimentally observable. The actual status code used to distinguish betweendifferent classes of entries is given in the KS column; codes in the range 1–10 correspondto remaining entries, and those above 10 to those that have fragmented or decayed.The columns with p x, p y, p z, E and m are quite self-explanatory. All momenta,energies and masses are given in units of GeV, since the speed of light is taken to be c = 1.Note that energy and momentum are conserved at each step of the fragmentation/decayprocess (although there exist options where this is not true). Also note that the z axisplays the rôle of preferred direction, along which the original partons are placed. The finalline is intended as a quick check that nothing funny happened. It contains the summedcharge, summed momentum, summed energy and invariant mass of the final entries at theend of the fragmentation/decay chain, and the values should agree with the input impliedby the PY2ENT arguments. (In fact, warnings would normally appear on the output ifanything untoward happened, but that is another story.)The above example has illustrated roughly what information is to be had in the eventrecord, but not so much about how it is stored. This is better seen by using a 132-columnformat for listing events. Try e.g. the following programIMPLICIT DOUBLE PRECISION(A-H, O-Z)CALL PY3ENT(0,1,21,-1,30D0,0.9D0,0.7D0)CALL PYLIST(2)CALL PYEDIT(3)CALL PYLIST(2)ENDwhere a 3-jet dgd event is generated in the first executable line and listed in the second.This listing will contain the numbers as directly stored in the common block PYJETSCOMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)For particle I, K(I,1) thus gives information on whether or not a parton or particle hasfragmented or decayed, K(I,2) gives the particle code, K(I,3) its origin, K(I,4) andK(I,5) the position of fragmentation/decay products, and P(I,1)–P(I,5) momentum,energy and mass. The number of lines in current use is given by N, i.e. 1 ≤ I ≤ N. The35
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: &XMI(2,240),Q2MI(240),IMISEP(0:240)
Page 45 and 46: C...End of particle and event loops
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
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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Page 134 and 135:
Page 136 and 137:
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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
Page 354 and 355:
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
Page 376 and 377:
are changed, some retuning should b
Page 378 and 379:
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
Page 384 and 385:
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) :
Page 406 and 407:
MSTU(4) : (D = 4000) number of line
Page 408 and 409:
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
Page 414 and 415:
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
Page 476 and 477:
[Lan00] K. Lane, T. Rador and E. Ei
Page 478 and 479:
[OPA91] OPAL Collaboration, M.Z. Ak
Page 480 and 481:
[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
show all

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