DELPHI week, Uppsala15 September 2000
RecentQCD GeneratorDevelopments
(A Subjective Selection)
Torbjorn SjostrandLund University
IntroductionImproved parton showers
a) Final state showersb) Initial state showers
Hadronizationγ∗γ∗ physicsOther topics
http://www.thep.lu.se/∼torbjorn/talks/uppsala00.ps
Goodbye LEP!
. . . and thanks for making life exciting4 September 1979: my first LEP workshop13 August 1989: first LEP events1 November 2000 (?): closedown of LEP
LEP part of a High Energy Physics Landscape:
e+e−: . . .→ PETRA/PEP → TRISTAN→ LEP 1 → LEP 2→ LC (TESLA/NLC/CLIC)
ep: fixed target → HERA → LEP+LHC?pp: . . .→ SppS → Tevatron → LHCµ+µ−: ?AA: fixed target → RHIC → LHC
+ fixed target, underground detectors, . . .
Complexity:
e+e− < ep < pp < AA< γp < γγ
= ep = e+e−
Building blocks of understanding⇒ unity of observable particle physics
Event physics overview
Structure of the basic generation process:
1) Hard subprocess:dσ/dt, Breit-Wigners.
2) Resonance decays:includes correlations.
3) Final-stateparton showers:(or matrix elements).
4) Initial-stateparton showers:(or matrix elements).
5) Multipleparton–partoninteractions.
q
q Z0 Z0
h0
Z0
µ+
µ−
h0
W−
W+
ντ
τ−s
c
q → qg
g → gg
g → qq
q → qγ
g
q
Z0
6) Beam remnants:colour-connectedto rest of event
7) Hadronization(PYTHIA: string;HERWIG: cluster;ISAJET: independent).
8) Normal decays:hadronic, τ , charm, . . .
p
p
b
b
ud
ud
u
u
�
�
�
�
�
q
g
g
q
hadrons
ρ+
π0
π+
γ
γ
9) QCD interconnection effects:
e−
e+
W−
W+
q3
q4
q2
q1
�
�
π+
π+
�� BE
a) colour rearrangement (⇒ rapidity gaps?);b) Bose-Einstein.
10) The forgotten/unexpected: a chain isnever stronger than its weakest link!
The role of exploration, be it experimental, theoretical,or phenomenological:
• to predict: too late for LEP⇒ LHC/LC/. . .
• to explain/understand: never too late
LEP will provide the reference for
• final-state QCD showers
• hadronization
• γγ physics (?)
• some electroweak parameters: mZ . . .
• limits of non-observation (?)
cf. PETRA: QCD vs. top/SUSY limits
Improved models and new ideas coming along⇒ important to preserve physics data in easily acces-sible formHZTOOL: correct concept
wrong implementation
⇒ need clean, minimal interface
QCD Radiation off Heavy Particles(E. Norrbin & TS, in preparation)
Shower: effective resummation ofmultiple-gluon-emission effects.
Evolution variable Q2:
PYTHIA: m2
HERWIG: E2θ2
ARIADNE: p2⊥z: energy/momentum sharing in branching
Rbl3 (yc) =
Rb3(yc)
Ru+d+s3 (yc)
=σ(bb → 3jets)/σ(bb)
σ(qq → 3jets)/σ(qq)
0.9
0.92
0.94
0.96
0.98
1
1.02
0 0.02 0.04 0.06 0.08 0.1 0.12
R3bl - DURHAM algorithm
Data
NLO, O(αs2)
LO, O(αs)
PYTHIA 6.125 with mass effects
PYTHIA 6.125 without mass effects
PYTHIA 6.131 with mass effects
mb= 3 GeV
mb= 3 GeV
mb= 5 GeV
yc
R3bl
DELPHI
0
1 (q)
i
2 (q)
3 (g)
0
1 (q)
i
2 (q)
3 (g)
xj = 2Ej/ECM ⇒ x1 + x2 + x3 = 2
mq = 0:
1
σ0
dσME
dx1 dx2=
αs
2πCF
≈2︷ ︸︸ ︷
x21 + x22(1 − x1)(1 − x2)
1
σ0
dσPS
dQ2 dz=
αs
2πCF
dQ2
Q2
≈2︷ ︸︸ ︷
1 + z2
1 − zdz · (Sudakov)
Q21 = m2
i = (p0 − p2)2 = (1 − x2)E
2CM
z1 =p0p1p0pi
=E1
Ei=
x1x1 + x3
=x1
2 − x2,
⇒ dQ21
Q21
dz11 − z1
=dx2
1 − x2
dx1x3
⇒ 1
σ0
dσPS
dx1 dx2∝ 2
(1 − x2)x3+
2
(1 − x1)x3
=2
(1 − x1)(1 − x2)
since (1−x1)+(1−x2)=x3
⇒ ME/PS < 1, i.e. good MC starting point
r = mq/ECM > 0:
1
σ0
dσME
dx1 dx2∝ x21 + x22 − r2(. . .)
(1 − x1)(1 − x2)
Q21 = m2
i = (p0 − p2)2 = (1 − x2 + r2)E2
CM
z more messy but dz/(1 − z) unchanged
⇒ 1
σ0
dσPS
dx1 dx2∝ 2
(1 − x1 + r2)x3+
2
(1 − x2 + r2)x3
restore by
Q21 = m2
i −m2q = (p0 − p2)
2 − p21 = (1 − x2)E2CM
Q2j = m2
j −m2j,onshell is relevant propagator;
generalized for r1 6= r2, rj = mj/ECM:
Q21 = (1 + r22 − r21 − x2)E
2CM
Q22 = (1 + r21 − r22 − x1)E
2CM
1
σ0
dσME
dx1 dx2=
(. . .)
Q21Q
22
− (. . .)
Q41
− (. . .)
Q42
Also radiation from decaying particle:
0 (t)
1 (b)
2 (W+)
i
3 (g)
0 (t)
1 (b)
2 (W+)
i 3 (g)
Q20 = |m2
i −m20| = |(p0 − p3)
2 −m20| = x3E
2CM
ME 1Q2
0Q21
matches PS b → bg
⇒ can match PS to generic a→ bcg ME
• subsequent branchings: also matched to ME,with reduced energy of system
• angular ordering• αs(p2⊥)
• secondary heavy flavours by gluon splitting• widths of unstable particles: for the future
Calculate for 1 → 2 processes in SM + MSSM:1
σ(a → bc)
dσ(a → bcg)
dx1 dx2
Depends on• mass ratios r1 = mb/ma and r2 = mc/ma
• colour and spin structure• vector vs. axial vector etc. (γ5)
when mb,mc 6= 0
colour spin γ5 example
1 → 3 + 3 — — (eikonal)
1 → 3 + 3 1 → 12+ 1
21, γ5,1 ± γ5 Z0 → qq
3 → 3 + 1 12→ 1
2+ 1 1, γ5,1 ± γ5 t → bW+
1 → 3 + 3 0 → 12+ 1
21, γ5,1 ± γ5 H0 → qq
3 → 3 + 1 12→ 1
2+ 0 1, γ5,1 ± γ5 t → bH+
1 → 3 + 3 1 → 0 + 0 1 Z0 → qq
3 → 3 + 1 0 → 0 + 1 1 q → q′W+
1 → 3 + 3 0 → 0 + 0 1 H0 → qq
3 → 3 + 1 0 → 0 + 0 1 q → q′H+
1 → 3 + 3 12→ 1
2+ 0 1, γ5,1 ± γ5 χ→ qq
3 → 3 + 1 0 → 12+ 1
21, γ5,1 ± γ5 q → qχ
3 → 3 + 1 12→ 0 + 1
21, γ5,1 ± γ5 t → tχ
8 → 3 + 3 12→ 1
2+ 0 1, γ5,1 ± γ5 g → qq
3 → 3 + 8 0 → 12+ 1
21, γ5,1 ± γ5 q → qg
3 → 3 + 8 12→ 0 + 1
21, γ5,1 ± γ5 t → tg
Universal gluon radiation patterns (= no spin depen-dence) for small gluon energies . . .
(with textbook dead cone)
. . . but very process-dependent for large gluon ener-gies . . .
(and no dead cone except for spin 0 → 0 + 0)
. . . results in process-dependent jet rates
r1 = r2 = 0.2
colour spin γ5 Eg 3 jet 3 jet’1 → 3 + 3 1 → 1
2+ 1
21 1.000 1.000 1.000γ5 1.056 1.112 1.133
0 → 12+ 1
21 1.134 1.293 1.376γ5 1.093 1.207 1.271
1 → 0 + 0 1 1.073 1.205 1.3100 → 0 + 0 1 0.875 0.758 0.72012→ 1
2+ 0 1 0.953 0.918 0.916
γ5 1.057 1.132 1.1791 → 3 + 3 eikonal – 0.802 0.695 0.659
eikonal +x23 – 1.201 1.518 1.670
3 → 3 + 1 12→ 1
2+ 1 1 0.323 0.306 0.287
γ5 0.356 0.365 0.34912→ 1
2+ 0 1 0.312 0.284 0.258
γ5 0.357 0.363 0.3440 → 0 + 1 1 0.287 0.242 0.2180 → 0 + 0 1 0.279 0.224 0.1940 → 1
2+ 1
21 0.359 0.379 0.375γ5 0.347 0.354 0.346
12→ 0 + 1
21 0.294 0.257 0.239γ5 0.314 0.302 0.298
3 → 3 + 8 0 → 12+ 1
21 1.634 1.833 1.922γ5 1.574 1.712 1.775
12→ 0 + 1
21 1.385 1.320 1.291γ5 1.549 1.664 1.675
8 → 3 + 3 12→ 1
2+ 0 1 0.561 0.493 0.445
γ5 0.621 0.607 0.574
0.9
0.91
0.92
0.93
0.94
0.95
0.96
0.97
0.98
0.99
1
0 0.02 0.04 0.06 0.08 0.1
R3bl
yc
Pythia 6.153Pythia 6.152Pythia 6.129
DELPHIALEPH
Rbl3 (yc)
ECM = 91 GeV
mb = 4.8 GeV
0.94
0.95
0.96
0.97
0.98
0.99
1
0 0.02 0.04 0.06 0.08 0.1
R3bl
yc
VectorZ0 mixture
Axial vectorDELPHIALEPH
0.94
0.95
0.96
0.97
0.98
0.99
1
0 0.02 0.04 0.06 0.08 0.1
R3bl
yc
Default pythia 6.153Modified pT
Minimal change of coherenceNo coherence (unrealistic)
DELPHIALEPH
0.75
0.8
0.85
0.9
0.95
0 0.01 0.02 0.03 0.04 0.05
R4bl
yc
Pythia 6.153Pythia 6.152Pythia 6.129
DELPHI
Rbl4 (yc)
ECM = 91 GeV
mb = 4.8 GeV
0.82
0.84
0.86
0.88
0.9
0.92
0.94
0.96
0.98
0 0.01 0.02 0.03 0.04 0.05
R4bl
yc
VectorZ0 mixture
Axial vectorDELPHI
0.82
0.84
0.86
0.88
0.9
0.92
0.94
0.96
0.98
0 0.01 0.02 0.03 0.04 0.05
R4bl
yc
Default pythia 6.153Modified pT
Minimal change of coherenceNo coherence (unrealistic)
DELPHI
HERWIG shower developments
(B. Webber, http://home.cern.ch/webber/)
Combine ME and PS for multijet production,e.g. e+e− → 2 jet + 3 jet + 4 jet + . . .
Define jet separation e.g. by Durham p⊥yij = 2min(E2
i , E2j )(1 − cos θij)/E
2CM ≈ p2⊥/E
2CM
For yij > yc: exclusive multijet states byME · SudakovSudakov: needs reconstructed shower history, byDurham clustering
For yij < yc: further evolution by vetoed PSVeto: Durham p2⊥ 6= HERWIG Q2
⇒ do evolution from Qmax ∼ ECM
but reject branchings that would giveextra jet by Durham p⊥ definition
Correct to NLL = next-to-leading logarithmbut not to NLO = next-to-leading order⇒ further development
Photon ISR in Z0 production(G. Miu & TS, PLB449 (1999) 313)
By-product of a study on W± production in hadroncolliders, attempting to combine matrix-element (ME)and parton-shower (PS) strengths.
Merging strategy: correct hardest emissions in show-ers so as to reproduce one order higher matrix ele-ments.
2 → 1 process e+(1) + e−(2) → Z0(0) startingpoint for backwards shower evolution:
34
15
6
20
2 → 2 process e+(3) + e−(2) → γ(4) + Z0(0):
s = (p3 + p2)2 =
(p1 + p2)2
z=m2
Z
zt = (p3 − p4)
2 = p21 = −Q2
u = m2Z − s− t = Q2 − 1 − z
zm2
Z
Relate ME and PS rates:
dσ
dt
∣∣∣∣ME
=σ0s
αem
2π
t2 + u2 + 2m2Zs
tu
Q2→0−→ σ0αem
2π
1 + z2
1 − z
1
Q2=
dσ
dQ2
∣∣∣∣∣PS1
dσ
dt
∣∣∣∣PS1
=σ0s
αem
2π
s2 +m4Z
t(t+ u)
Add mirror e+(1) + e−(5) → γ(6) + Z0(0):
dσ
dt
∣∣∣∣PS
=dσ
dt
∣∣∣∣PS1
+dσ
dt
∣∣∣∣PS2
=σ0s
αem
2π
s2 +m4Z
tu
Ree→γZ(s, t) =(dσ/dt)ME
(dσ/dt)PS=t2 + u2 + 2m2
Zs
s2 +m4Z
1
2< Ree→γZ(s, t) ≤ 1
Improve PS:
• Q2max = s, not Q2
max ≈ m2Z (intermediate)
• MC correction by R(s, t) for first (≈ hardest)emission on each side (new)
Now default.
pp → W± at 1.8 TeV:
1e-06
1e-05
0.0001
0.001
0.01
0.1
1
0 10 20 30 40 50 60 70 80 90 100 110
(1/ N
)(d
N/d
pW
)(G
eV-1)
p W (GeV)
PS,k =0.44 GeVPS,k =4 GeV
D0 data
e+e− → Z0 at 500 GeV,80 GeV <
√s < 100 GeV:
Hadronization
Lund string successful . . .
even HERWIG adopts ‘stringy’ description:• large-mass clusters split in two• small-mass clusters decay anisotropicallyalong ‘string’ direction
. . . but many murky issues
• perturbative ⇒ nonperturbative transition
• ambiguities of gluon structure
• origin of flavour composition(hadron vs. quark masses, wave functions)
• baryon production mechanism(diquark, popcorn, ‘curtain’ quark)
• role of higher resonances: tensor etc.(poorly known, nonisotropic decays,duality with string description)
• spin/polarization phenomena
• interconnection(colour rearrangement, Bose–Einstein)
Small-mass string ⇒ ‘cluster’,by g → qq in shower
(E. Norrbin & TS, PLB442 (1998) 407,LUTP 00-16 (hep-ph/0005110)
Improved description of when collapse occurs(mass spectrum ⇐ constituent quark masses)
example:charmstringin πpcollision
2 2.5 3 3.50
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Msinglet
(1/N
)dN
/dM
(a)
1-body
2-body
cluster
2-body
string
3-body
and1-body collapse: energy-momentum shuffling2-body decay: smoother joining to stringpicture (matched anisotropic decay)
A(xF) =
#D−−#D+
#D−+#D+
in π−p
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
A(x
F)
xF
(a)Pair production
All channelsWA92, 350 GeVWA82, 340 GeVE791, 500 GeVE769, 250 GeV
Virtual photon processes(C. Friberg & TS, Eur. Phys. J. C13 (2000) 151,LUTP 00-29 (hep-ph/0007314), LUTP 00-31 (hep-ph/0009003))
The photon flux
k1
k′1θ1
k2k′2
θ2
q1
q2
X
W2 = (q1 + q2)2
Q2i = −q2i
yi =qikj
kikjj=2(1) for i=1(2)
xi =qi(k1 + k2)
ki(k1 + k2)
yi ≈ xi +Q2i
s
Q2i ≈ x2i
1 − xim2i + (1 − xi)s sin
2(θi/2)
dσ(ee → eeX) =∑
ξ1,ξ2=T,L
∫
dy1 dQ21 dy2 dQ2
2
× fξ1γ/e
(y1, Q21) f
ξ2γ/e
(y2, Q22) dσ(γ∗ξ1γ
∗ξ2
→ X)
with
fTγ/e(y,Q
2) =αem
2π
(
(1 + (1 − y)2
y
1
Q2− 2m2
ey
Q4
)
fLγ/e(y,Q
2) =αem
2π
2(1 − y)
y
1
Q2
Photoproduction / γγ
Direct VMD Anomalous
Direct: point-like Resolved: hadronic state
γγ: 3 × 3 = 9 combinations (+ subdivisions)
Spectrum of fluctuations γ ↔ qq ∝ dk2⊥/k2⊥
alt. m ' 2k⊥; dm2/m2
? k⊥ < k0 ' 0.5 GeV: nonperturbative γ → qq
hadronic physics ⇒ VMD(Vector Meson Dominance)
parameterized couplings to ρ0, ω, φ, J/ψσγ→ρtot = P(γ → ρ) · σρtot
PDF fγ→ρi (x, µ2), σγ→ρ
jet = . . .
beam remnants, multiple interactions, . . .
? k⊥ > k0: perturbative γ → qq
PDF calculable: anomalous part of γbut σqq
tot not ⇒ GVMD (Generalized VMD)
geometric scaling ansatz σqqtot ∝ k2V /k
2⊥,
kV ' mρ/2 for light quarksagain hadronic character: beam remnants, . . .
Photon ‘high-p⊥’ processes
Three main ‘high-p⊥’ jet process classes:1. direct×direct
γ∗Tγ∗T → qq
γ∗Tγ∗L → qq
γ∗Lγ∗L → qq
γ∗
γ∗
2. direct×resolved , resolved = VMD or anomalous
γ∗Tg → qq
γ∗Lg → qq
γ∗Tq → qg
γ∗Lq → qg
γ∗
γ∗
γ∗
γ∗
etc.
3. resolved×resolved
qq′ → qq′
qq → q′q′
qq → gg
qg → qg
gg → gg
gg → qq
γ∗
γ∗
γ∗
γ∗
etc.
dσ(γ∗γ∗ → X) =
(∫
dx1 fγ∗i (x1, µ
2, Q21)
)
×(∫
dx2 fγ∗j (x2, µ
2, Q22)
) ∫
dtdσ
dt(s = x1x2W
2)
Deeply Inelastic Scattering / γ∗γ
Virtual photon: γ∗q → q, e.g. q in (VMD) ρ0
σγ∗ρtot ' 4π2αem
Q2Fρ2(x,Q2) ' 4π2αem
Q2
∑
q,q
e2q xq(x,Q2)
but F2 → 0 for Q2 → 0 by gauge invariance,+ limit doublecounting with photoproduction
σγ∗ρDIS '
(
Q2
Q2 +m2ρ
)24π2αem
Q2
∑
q,q
e2q xq(x,Q2)
where q(x,Q2) frozen for Q2 < Q20;
and prefactor ensures σDIS → 0 for Q2 → 0
O(αs) DIS =
{
QCDC γ∗q → qgBGF γ∗g → qq
}
= dir × res
σγ∗ρLO DIS = σ
γ∗ρDIS − σ
γ∗ρdir×res → σ
γ∗ρDIS exp
−
σγ∗ρdir×res
σγ∗ρDIS
corresponds to Sudakov form factor
γγ: 9 combinations = (dir+VMD+GVMD)2
γ∗γ∗: + 4 combinations = 2 sides×(VMD+GVMD)
13 !!
From Real to Virtual Photons
Direct photon: Q2 in ME expression
Resolved photon:total cross section σγ→i
tot dampened by dipole(
m2
m2 +Q2
)2
(fewer fluctuations, smaller size)
VMD: m = mρ,mω,mφ,mJ/ψ
GVMD: m ' 2k⊥; in total∫
k20
dk2⊥k2⊥
k2Vk2⊥
(
4k2⊥4k2⊥ +Q2
)2
fγ∗Ti (x, µ2, Q2): SaS 1D (also dipole-based)
fγ∗Li (x, µ2, Q2): simple multiplicative factor
or Chyla (hep-ph/0006232)
Putting it all together, res = VMD + GVMD:
σγ∗γ∗
tot (W 2, Q21, Q
22) = σγ
∗γ∗
DIS×res exp
(
−σγ
∗γ∗
dir×res
σγ∗γ∗
DIS×res
)
+ σγ∗γ∗
dir×res
+ σγ∗γ∗
res×DIS exp
(
−σγ
∗γ∗
res×dir
σγ∗γ∗
res×DIS
)
+ σγ∗γ∗
res×dir
+ σγ∗γ∗
dir×dir +
(W 2
Q21 +Q2
2 +W 2
)3
σγ∗γ∗
res×res
(1 − x)3 reduces doublecounting at large x
Process composition
1E-02
1E-01
1E+00
1E+01
1E+02
1E+03
0.01 0.1 1 10 100 1000 10000
σ totγ∗ γ (
nb)
Q12 (GeV2)
Q22 = 0 GeV2
DISDirect1-res2-resTotal r1Total r2
Wγ∗γ∗ = 100 GeV
Direct = dir×dir1-res = dir×res2-res= res×res
fγ∗L
i /fγ∗T
i :
r1 =2m2Q2
(m2 +Q2)2
r2 =2Q2
m2 +Q2
1E-02
1E-01
1E+00
1E+01
1E+02
0.01 0.1 1 10 100 1000 10000
Q12 (GeV2)
Q22 = 10 GeV2
DISDirect1-res2-resTotal r1Total r2
Energy dependence
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
Photopr. DataOPALTPC
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
PLUTOTOPAZCELLO
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
L3ALEPH
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1 10 100
σ totγ∗ γ (
nb)
W (GeV)
×2-2
×20
×22
×24
×26
×28
×210
×212
9.90
7.25
5.15
4.04
2.13
1.31
0.71
0.00
σtotγ∗γ×
Q12 / GeV2
Total Transv.Total r1Total r2
fγ∗L
i /fγ∗T
i :
r1 =2m2Q2
(m2 +Q2)2
r2 =2Q2
m2 +Q2
1E-02
1E-01
1E+00
1E+01
1E+02
10 100W (GeV)
Q12 = 9.90 GeV2
1E-02
1E-01
1E+00
1E+01
1E+02
10 100W (GeV)
Q12 = 9.90 GeV2
DISDirect1-res2-resTotal r1
Event properties
Wγ∗γ∗ = 100 GeV
average charged multiplicity:
0
5
10
15
20
25
0.01 0.1 1 10 100 1000 10000
⟨ nch
⟩
Q12 (GeV2)
Q22 = 1 GeV2
pp
γ∗p
γ∗γ
γ∗γ∗
0
5
10
15
20
25
30
0.01 0.1 1 10 100 1000 10000
⟨ nch
⟩
Q12 (GeV2)
Q22 = 1 GeV2
γ∗γ∗
1-res
2-res
DIS
Direct
probability to have a jet with E⊥ > 5 GeV
inside a cone R =√
(∆η)2 + (∆ϕ)2 < 1:
1E-03
1E-02
1E-01
1E+00
0.01 0.1 1 10 100 1000 10000
P(N
jet >
0)
Q12 (GeV2)
Q22 = 1 GeV2
pp
γ∗p
γ∗γ
γ∗γ∗ 1E-02
1E-01
1E+00
0.01 0.1 1 10 100 1000 10000
P(N
jet >
0)
Q12 (GeV2)
Q22 = 1 GeV2
γ∗γ∗
1-res
2-res
DIS
Direct
Effects of longitudinal photons
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0.01 0.1 1
f iγ L(x
i, µ2 ,Q
2 ) /
fiγ T
(xi,
µ2 ,Q2 )
xi
uL(xi, µ2=10, Q2=0.5) / uT
uL(xi, µ2=10, Q2=2.0) / uT
gL(xi, µ2=10, Q2=0.5) / gT
gL(xi, µ2=10, Q2=2.0) / gT
γ∗L: Chyla
γ∗T: SaS 1D
0
0.5
1
1.5
2
2.5
-4 -3 -2 -1 0 1 2 3 4
dσ /
dη
(µb)
η
Q2 = 1.0 GeV2 ETjet > 4 GeV Rjet < 1
Transverse Resolvedr1r2RPDFDirect
r1 =2m2Q2
(m2 +Q2)2
r2 =2Q2
m2 +Q2
RPDF =fγ∗L
i (x, µ2, Q2)
fγ∗T
i (x, µ2, Q2)
Program particulars
To access new γ∗ flux convolution:CALL PYINIT(’cms’,’gamma/e-’,’gamma/e+’,200D0)
Also possible to have γ∗γ∗ collisions directly:CALL PYINIT(’five’,’gamma’,’gamma’,100D0)
with P(1,J) and P(2,J) defining momenta and virtu-alities (P(I,5) < 0 for spacelike ones).
Photon character regulated by MSTP(14):= 10 : mix direct/VMD/anomalous for real
photons; d×V = V×d etc. ⇒ 6 classes.= 30 : (new default) mix dir/VMD/GVMD
and DIS for virtual photons; ⇒ 13 classes.= other numbers : individual classes.Warning : γ∗γ∗ → `+`− included in dir×dir if
not switched off (automatic when mixing).
MSEL = 1, CKIN(3) > 2 : jets with p⊥ >CKIN(3)MSEL = 1, CKIN(3) = 0 : jet + low-p⊥MSEL = 2, CKIN(3) = 0 : + diffractive + elastic
Possible to specify cuts on xi, yi, Q2i , θi, W2
in CKIN(61) - CKIN(78).Phase space sampled according to∏
i
(dQ2i /Q
2i ) (dxi/xi) dϕi
⇒ full efficiency for xi and Q2i cuts.
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PYTHIA
lh hh
statusAuthors: TS, P. Eden,C. Friberg, L. Lonnblad,G. Miu, S. Mrenna, E. Norrbin
JETSET 7.4PYTHIA 5.7SPYTHIA
4 March 1997 : PYTHIA 6.1
Currently PYTHIA 6.152 of 17 August 2000∼ 51,900 lines Fortran 77
Code, manuals, sample main programs:http://www.thep.lu.se/∼torbjorn/Pythia.html
PYTHIA 6.1 main news:
• JETSET routines renamed:LUxxxx → PYxxxx + some more
• All real variables in DOUBLE PRECISION
• New SUSY processes and improved SUSY simu-lation; new PDG codes for sparticles
• New processes for Higgs, technicolour, . . .
• Many improved resonance decays
• Several initial- and final-state showers matched tomatrix elements
• New machinery to handle real and virtual photonfluxes and cross sections
• Newer parton distributions (but . . . )
• QED radiation off an incoming muon
• Energy-dependent p⊥min in multiple interactions
• Colour rearrangement options for W+W−
• Expanded Bose-Einstein algorithm
• New baryon production scheme (optional)
• One-dimensional histograms (GBOOK)
2–, 4– and 6–fermion standard interfaces for showersand hadronization:CALL PY2FRM(IRAD,ITAU,ICOM)
CALL PY4FRM(ATOTSQ,A1SQ,A2SQ,ISTRAT,
&IRAD,ITAU,ICOM)CALL PY6FRM(P12,P13,P21,P23,P31,P32,PTOP,
&IRAD,ITAU,ICOM)
qqgg and qqq′q′ QCD 4–jet standard interface forshowers and hadronization:CALL PY4JET(PMAX,IRAD,ICOM)
HERWIG status
Authors: G. Marchesini, B.R. Webber,G. Abbiendi, G. Corcella, I. Knowles,S. Moretti, K. Odagiri, P. Richardson,M. Seymour, L. Stanco.
Currently HERWIG 6.1 of 16 December 1999∼ 32,000 lines Fortran 77
Code, manuals, related programs:http://hepwww.rl.ac.uk/theory/seymour/herwig/
6.1 release notes: hep-ph/9912396
• SUSY production and decay (in hadron collisions)including R-parity violation
• matrix element corrections to top decay(and W/Z production in hadron collisions)
• e+e− → 4 jets matrix element option• new soft underlying event options• new particles and updated particle data• enhanced nonisotropic decay of clusters• beamstrahlung (interface to CIRCE)
HERWIG 6.2 will follow soon, including e.g.full simulation of MSSM Higgs physics.
Subprocess survey
Process PYTH HERW ISAJ
QCD & relatedSoft QCD ? ? ?Hard QCD ? ? ?Heavy flavour ? ? ?Top threshold — — —γγ physics ? ? —DIS ? ? —γ∗γ∗ physics ? (?) —
Electroweak SMSingle γ∗/Z0/W± ? ? ?
(γ/γ∗/Z0/W±/f/g)2 ? ? ?Light SM Higgs ? ? ?Heavy SM Higgs (?) ? ?
SUSY BSMh0/H0/A0/H± ? ? ?SUSY ? ? ?R/ SUSY — ? —
Other BSMTechnicolor ? — ?New gauge bosons ? — —Compositeness ? — —Leptoquarks ? — —H±± (from LR-sym.) ? — —Extra dimensions — (?) (?)
? = yes, (?) = partial/in progress, — = no
On to C++(L. Lonnblad, CPC 118 (1999) 213;
M. Bertini, L. Lonnblad & TS, LUTP 00-23 (hep-ph/0006152);
input to [email protected])
Why Fortran → C++?
• SLAC →, FNAL →, CERN → LHC era.
• Industrial standard.
• Educational and professional continuity for stu-dents.
• Better to program – for experts.
• User-friendy interfaces – for the rest of us.
PYTHIA 7 milestones:
• January 1998: project formally started.
• June 2000: “proof of concept” version, withgeneric event generation machinery, some pro-cesses and string fragmentation.
• 2001–2002: useful version (?), but limited inscope.
• ??: more and better than current PYTHIA.
HERWIG++ progress:PPARC funds 2 dedicated “postdocs”,3-year work recently started.
Summary
? Renewed interest in shower evolution
• mass effects
• process dependence (spin, colour, . . . )
• multijet composition & topologies
Needed for future physics @ LHC/LC⇒ LEP data needed for calibration
? Hadronization
• strings best first approximation
• slow but steady progress
⇒ LEP data needed as inspiration & reference
? γ∗γ∗ physics
• not for the weak of heart
• new ‘complete’ framework . . .
• . . . but still at level of ‘strawman’
⇒ LEP will provide important tests
? Plea:plan for easy comparison with final QCD data⇒ will keep DELPHI alive in the LHC/LC era!
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