challenges for qcd theory - luhome.thep.lu.se/~torbjorn/talks/nobel13.pdf · bose-einstein hδm wi...

Challenges for QCD Theory– some personal reflections –

Torbjörn Sjöstrand

Department of Astronomy and Theoretical PhysicsLund University, Lund, Sweden

Nobel Symposium on LHC ResultsKrusenberg, Uppsala, 13 – 17 May 2013

Motivation

��

��

At the LHC everything is QCD: “signal” and “background”

Torbjörn Sjöstrand Challenges for QCD Theory slide 2/24

The Three Frontiers of QCD

QCD L not an issue:well tested by now!

gg→ H0

UnderstandingconfinementQGP hadronization

small-x MPI col.recon.

Precision

NnLOαs(Q2)

PDF’smatchingshowers Discovery

signal vs.backgroundBNV, R-had.

new SU(N)

p spinjets

σ(pp→X)

mt(mX)


Perturbative QCD

Healthy community of calculators and program authors.Influx of superstring people. Challenges more math than physics.

LO: solved for all practical applications.

NLO: in process of being automatized.

NNLO: the current calculational frontier.

Another bottleneck: efficient phase space sampling.

(Calculational techniques beyond scope.)

(Results belong with respective physics topic.)

gg → H0 at crossroads of frontiers:• Need high-precision calculations• to search for BSM physics,• but limited by poorly-understood

slow convergence.


Parton Distributions

Also several groups of PDF providers, offering cross-checks.

x

-510 -410 -310 -210 -110 1

)2xg

(x,Q

0

5

10

2 = 2 GeV2Q

x

-510 -410 -310 -210 -110 1

)2xg

(x,Q

0

5

10

MSTW 2008 LO

MSTW 2008 NLO

MSTW 2008 NNLO

x

-510 -410 -310 -210 -110 1

)2xg

(x,Q

0

5

10

15

2 = 5 GeV2Q

x

-510 -410 -310 -210 -110 1

)2xg

(x,Q

0

5

10

15

x

-510 -410 -310 -210 -110 1

)2xg

(x,Q

0

10

20

30

2 = 20 GeV2Q

x

-510 -410 -310 -210 -110 1

)2xg

(x,Q

0

10

20

30

x

-510 -410 -310 -210 -110 1

)2xg

(x,Q

0

10

20

30

40

50

2 = 100 GeV2Q

x

-510 -410 -310 -210 -110 1

)2xg

(x,Q

0

10

20

30

40

50

Figure 57: The gluon distribution at LO, NLO and NNLO including the one-sigma PDF uncer-tainty bands.

123

But higher orders ⇒ increasingregion at low x and Q2 beyondperturbative control!

Adds to instability ofreal − virtual ME’s.

What use (−ME)⊗(−PDF) = +σ?

Open question 1:

new scheme with ”resummed”ME’s and PDF’s?

(complementing/replacing MS)


Multiparton Interactions

MPI’s driving force forstructure of minbiasand underlying events.

n0 20 40 60 80 100 120 140 160 180

nP-610

-510

-410

-310

-210

-110

1

10

210

310 CMS DataPYTHIA D6TPYTHIA 8PHOJET

)47 TeV (x10

)22.36 TeV (x10

0.9 TeV (x1)

| < 2.4η| > 0

Tp

(a)CMS NSD

New dampening scale p⊥0 ∼ 2− 3 GeV gives 2− 3 MPI’s/event,with large fluctuations from pp impact parameter.

(In perturbative regime: LO OK, NLO fails?)

Open question 2: p⊥0 effect of colour screening or what?


Multiple Colour Sources

Lund string model:colour confinement fields stretchedfrom q (qq) end to q (qq) endvia intermediate g’s(8 ≈ 3⊗ 3, cf. NC →∞). q

gg

q

Introduction(V.A. Khoze & TS, PRL72 (1994) 28, ZPC62 (1994) 281,EPJC6 (1999) 271;L. Lönnblad & TS, PLB351 (1995) 293, EPJC2 (1998) 165)

ΓW,ΓZ,Γt ≈ 2 GeVΓh > 1.5 GeV for mh > 200 GeVΓSUSY ∼ GeV (often)

τ =1

Γ≈

0.2GeV fm

2GeV= 0.1 fm # rhad ≈ 1 fm

⇒ hadronic decay systems overlap,between pairs of resonances⇒ cannot be considered separate systems!

Three main eras for interconnection:1. Perturbative: suppressed for ω > Γ by propaga-

tors/timescales⇒ only soft gluons.2. Nonperturbative, hadronization process:

colour rearrangement.

B0d

bc

W− c

s

!

"

!

"

B0d

b

c

W−c

sg

!

"

K0S

!

"

J/ψ

3. Nonperturbative, hadronic phase:Bose–Einstein.

Above topics among unsolved problems of strong in-teractions: confinement dynamics, 1/N2C effects, QMinterferences, . . . :

• opportunity to study dynamics of unstable parti-cles,

• opportunity to study QCD in new ways, but• risk to limit/spoil precision mass measurements.

So far mainly studied for mW at LEP2:

1. Perturbative: 〈δmW〉

Colour Reconnection

MPI ⇒ high force-field density!

or reduced string length

chN 0 50 100

[G

eV]

〉 T p〈

0.6

0.8

1

1.2

1.4

1.6ATLASPythia 8Pythia 8 (no CR)

7000 GeV pp Soft QCD (mb,diff,fwd)

mcp

lots

.cer

n.ch

200

k ev

ents

≥R

ivet

1.8

.2,

Pythia 8.175

ATLAS_2010_S8918562

> 0.5 GeV/c)T

> 1, pch

(Nch vs NTAverage p

0 50 1000.5

1

1.5Ratio to ATLAS

Open question 3: mechanisms at play for colour (re)structuring?

and how to model them correctly?


The Mass of Unstable Coloured Particles

MC: close to pole mass, in the sense of Breit–Wigner mass peak.t, W, Z: cτ ≈ 0.1 fm < rp .

t

t

W

b

At the Tevatron: mt = 173.20± 0.51± 0.71 GeV = PMAS(6,1)At the LHC: mt = 173.4± 0.4± 0.9 GeV (CMS) = 6:m0 ?Open question 4: better mass definition for coloured particles?


The Mass of Unstable Coloured Particles – 2

Dependence*of*Top*Mass*on*Event*

Kinema2cs*

10*

!  First#top#mass#measurement#binned#in#kinema3c#observables.#!  Addi2onal*valida2on*for*the*top*mass*measurements.**!  With*the*current*precision,*no*mis^modelling*effect*due*to*

"  color*reconnec2on,*ISR/FSR,**b^quark*kinema2cs,*difference*

between*pole*or*MS~*masses.*

color*recon.*

ISR/FSR*

b^quark*kin.*

Global*χ2/ndf*=*0.9*based*on*

mt1D*and*JES*which*are*

independent*(comparing*data*

and*MadGraph)*neglec2ng*

correla2ons*between*

observables.*

CMS^PAS^TOP^12^029*

NEW*

0 50 100 150 200 250 300>

[G

eV

]2

Dt

- <

m2

Dt

m-4

-2

0

2

4

)-1Data (5.0 fbMG, Pythia Z2MG, Pythia P11MG, Pythia P11noCRMC@NLO, Herwig

= 7 TeV, lepton+jetssCMS preliminary,

[GeV

]

[GeV]T,t,had

p0 50 100 150 200 250 300

da

ta -

MG

Z2

-5

0

5

E. Yazgan(Moriond 2013)


Hadronization: the Lund String Model

Field lines compressed totubelike regions: strings!Linear confinement:V (r) ≈ κr , κ ≈ 1 GeV/fmLorentz invariant.

String breaking by tunneling:

P ∝ exp(−πm2⊥q/κ)with adjacent pairs formingmesons (and baryons).

dn/dy flat, but with short-range (anti)correlations.Common Gaussian p⊥ spectrum.

Varied hadron production, with suppression of heavy quarks,and short-range flavour (anti)correlations.

Generally supported by LEP, but some unresolved flavour issues.Torbjörn Sjöstrand Challenges for QCD Theory slide 11/24

Hadronization: the Lund Gluon Picture

Force ratio g/q = 2, cf. QCD NC/CF = 9/4, → 2 for NC →∞.

Verified at PETRA/. . . /LEP!


Hadronization

Only current alternative to string is cluster:

cluster model fails unless “tweaked” to mimic stringsneither gives fully satisfactory description of flavour datadense LHC environment by reconnection, not new modelslittle new original work in > 30 years

Open question 5: better hadronization models

for simple (e+e−) and complex (pp) environments?


Parton Showers

Traditional DGLAP picture a → bc (coherence by θ ordering):

Lund string ⇒ St. Petersburg dipole ⇒ Lund dipole shower:ab → cde ≈ radiator + recoiler (coherence by p⊥ ordering)

Nowadays most common shower type.(Also NLO calculations: “Catani–Seymour dipoles”.)


Parton Showers – 2

Holy Grail of showers: unitarity by Sudakov factors:

dPa =dp2⊥p2⊥

αs2π

Pa→bc(z) dz

× exp

(−∫ p2⊥max

p2⊥

dp′2⊥p′2⊥

αs2π

∫Pa→bc(z)dz

)

(but topology changed ⇒ implicit cross section change)

Shower challenges:

full phase space coverage (sector showers)(recoils ⇒ ambiguous p⊥ ordering; soft coherence overdone)NLL, NNL, . . . accuracy (in practice close to NLL)

complete colour structure (each nonleading ∝ 1/N2C but many!)W±,Z0 emission in showers (competition)Attach as well as possible to ME behaviour


ME–PS Matching: Legs

Basic idea of CKKW(-L)/MLM:use ME for real emissionsand PS for virtual corrections(to ∼ restore unitarity).

CKKW-L:

1 generate n-body by MEmixed in proportions

∫dσn

above p⊥min cut

2 reconstruct fictitiousp⊥-ordered PS

p⊥0 p⊥1 p⊥2 p⊥3 p⊥min

4 reject from fix αs = αs(p⊥min) to αs(p⊥i )

5 run trial shower between each p⊥i and p⊥i+16 reject if shower branching ⇒ Sudakov factor7 regular shower below p⊥min (or below p⊥n for n = nmax)


ME–PS Matching: Loops

Master formula for meaningful NLO implementations:

dσ = dσR,hard +(σB +σR,soft +σV )[dσR,soft

σBexp

(−∫

dσR,softσB

)]ordered in “p⊥”, with shower from selected “p⊥” downwardsPOWHEG: σR,hard = 0MC@NLO: σR,soft = σR,MC (warning: positivity?)“Best” choice process-dependent (guess NLO behaviour of σR)

S.Alioli, P. Nason, C. Oleari, E. Re, JHEP 00904 (2009) 002Torbjörn Sjöstrand Challenges for QCD Theory slide 17/24

ME–PS Matching: Legs and Loops

Issue: ? multileg does not guarantee∑

n=0 σH+n jet = σH;

? loop only guarantees σH to NLO and σH+1 jet to LO.

Recent progress: σH to NLO, σH+1 jet to NLO, σH+≥2 jet to LO.

Requires careful unitarity considerations, expansion of PDF’s, . . .⇒ lengthy formulae and even lengthier procedures:

this means that once we want to add a one-jet NLO calculation, we have to subtract its

integrated version from zero-jet events. Similarly we need to remove the O(α0s (µR)

)and

O(α1s (µR)

)in the UMEPS tree-level weights, not only for the one-jet events but also for

the corresponding subtracted zero-jet events. In this way we ensure that the inclusive

zero-jet cross section is still given by the NLO calculation and we will also improve the

O(α2s )−term of exclusive zero-jet observables.The UNLOPS prediction for an observable O, when simultaneously merging inclusive

NLO calculations for m=0, . . . ,M jets, and including up to N tree-level calculations, is

given by

〈O〉 =M−1∑

m=0

∫dφ0

∫· · ·∫

O(S+mj)

{

Bm +[B̂m]

−m,m+1

−M∑

i=m+1

∫

sBi→m −

M∑

i=m+1

[ ∫

sB̂i→m

]

−i,i+1

−N∑

i=M+1

∫

sB̂i→m

}

+

∫dφ0

∫· · ·∫

O(S+Mj)

{

BM +[B̂M

]

−M,M+1−

N∑

i=M+1

∫

sB̂i→M

}

+N∑

n=M+1

∫dφ0

∫· · ·∫

O(S+nj)

{

B̂n −N∑

i=n+1

∫

sB̂i→n

}

(3.10)

Here we see, in the first line, the addition of the Bm and the removal of the O (αms (µR))and O

(αm+1s (µR)

)terms of the original UMEPS B̂m contribution. On the second line we

see the subtracted integrated Bm+1 term to make the m-parton NLO-calculation exclusive

and the corresponding O (αms (µR))- and O(αm+1s (µR)

)-subtracted UMEPS term together

with subtracted terms from higher multiplicities where intermediate states in the clustering

were below the merging scale. The third line is the special case of the highest multiplicity

corrected to NLO, and the last line is the standard UMEPS treatment of higher multiplic-

ities.

The full derivation of this master formula is given in appendix D, where we also discuss

the case of exclusive NLO samples and explain the necessity for subtraction terms from

higher multiplicities. We will limit ourselves to including only zero- and one-jet NLO

calculations in the results section. For the sake of clarity we will thus only discuss this

special case here. For this case, the UNLOPS prediction (when including only up to two

tree-level jets) is

〈O〉 =∫

dφ0

{

O(S+0j)

(

B0 −∫

sB1→0 −

[ ∫

sB̂1→0

]

−1,2

−∫

sB̂2→0

)

+

∫O(S+1j)

(B1 +

[B̂1]

−1,2−∫

sB̂2→1

)

+

∫ ∫O(S+2j)B̂2 (3.11)

– 16 –

(UNLOPS, L. Lönnblad, S. Prestel)

Open question 6: generic, robust, reliable matching procedure?

(back to open question 1: new scheme with ”resummed” ME’s and PDF’s?)


Event Generators

Daunting complexity of LHC event.General-purpose event generators: currently only wayto break down the problem into manageable subtasks:

Open question 7: better (alternative to) event generators?


The Workhorses: What are the Differences?

HERWIG, PYTHIA and SHERPA offer convenient frameworksfor LHC physics studies, but with slightly different emphasis:

PYTHIA (successor to JETSET, begun in 1978):• originated in hadronization studies: the Lund string• leading in development of MPI for MB/UE• pragmatic attitude to showers & matching

HERWIG (successor to EARWIG, begun in 1984):• originated in coherent-shower studies (angular ordering)• cluster hadronization & underlying event pragmatic add-on• large process library with spin correlations in decays

SHERPA (APACIC++/AMEGIC++, begun in 2000):• own matrix-element calculator/generator• extensive machinery for CKKW ME/PS matching• hadronization & min-bias physics under development

MCnet: combined projects, meetings & summer schools


Other Relevant Software

Some examples (with apologies for many omissions):Other event/shower generators: PhoJet, Ariadne, Dipsy, Cascade, Vincia

Matrix-element generators: MadGraph/MadEvent, CompHep, CalcHep,Helac, Whizard, Sherpa, GoSam, aMC@NLO

Matrix element libraries: AlpGen, POWHEG BOX, MCFM, NLOjet++,VBFNLO, BlackHat, Rocket

Special BSM scenarios: Prospino, Charybdis, TrueNoir

Mass spectra and decays: SOFTSUSY, SPHENO, HDecay, SDecay

Feynman rule generators: FeynRules

PDF libraries: LHAPDF

Resummed (p⊥) spectra: ResBos

Approximate loops: LoopSim

Jet finders: anti-k⊥ and FastJet

Analysis packages: Rivet, Professor, MCPLOTS

Detector simulation: GEANT, Delphes

Constraints (from cosmology etc): DarkSUSY, MicrOmegas

Standards: PDF identity codes, LHA, LHEF, SLHA, Binoth LHA, HepMC

Can be meaningfully combined and used for LHC physics!


QCD and BSM physics

BNV ⇒ junction topology⇒ special handling ofshowers and hadronization

Hidden valleys:showers potentially interleavedwith normal ones;hadronization in hidden sector;decays back to normal sector

R-hadron formation

Squarkfragmenting tomeson or baryon

Gluinofragmenting tobaryon or glueball

Most hadronization properties by analogy with normalstring fragmentation, butglueball formation new aspect, assumed ⇠ 10% of time (or less).

Torbjörn Sjöstrand QCD Aspects of BSM Physics slide 12/18

R-hadrons: long-lived g̃ or q̃;new: hadronization of massive object “inside” the string


Apologies

Many areas not covered, e.g.

σtot and dσel/dt.Diffraction and forward physics.

Small-x evolution.

p spatial structure.

Spin physics.

Pre-QGP in pp.

Underlying events vs. minbias.

Jet algorithms.

Jet properties, q vs. g.

Prompt γ production.

Quarkonium production.

Ridge effect.

Klaus Rabbertz La Thuile, Italy, 10.03.2013 Moriond QCD 16

Dijets separated in Rapidity

CMS, EPJC72, 2012.ATLAS, JHEP09, 2011

Quantities sensitive to potential deviations from DGLAP evolution at small xSome MC event generators run into problems … but also BFKL inspired ones!Large y coverage needed, also useful for WBF tagging jets.

BFKL inspired

Most forward-backward dijet selectionAll possible dijet pair distances overleading dijet pair distance

DGLAP

η∆-4

-20

24

φ∆0

2

4)

φ∆,

η∆

R( -2

-101

Summary and outlook

Much progress, but also many unresolved issues:1 New scheme with ”resummed” ME’s and PDF’s?2 p⊥0 effect of colour screening or what?3 Mechanisms at play for colour (re)structuring?4 Better mass definition for coloured particles?5 Better hadronization models for simple (e+e−)

and complex (pp) environments?6 Generic, robust, reliable matching procedure?7 Better (alterative to) event generators? (Make me unemployed!)


challenges for qcd theory - luhome.thep.lu.se/~torbjorn/talks/nobel13.pdf · bose-einstein hδm wi...

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