precision qcd] physics input to european strategy update...[precision qcd] physics input to european...

[Precision QCD] Physics Input to European Strategy UpdateGavin Salam*  

University of Oxford and All Souls College

1

* on leave from CERN TH department and from CNRS (LPTHE)

Joint Snowmass 2021 EF05 / EF06 / EF07 Topical Group Meeting (Precision QCD / Hadronic Structure & Forward QCD / Heavy Ions)

https://indico.fnal.gov/event/43488/

European Particle Physics Strategy Update (EPPSU), 2018 – 2020➤ http://europeanstrategyupdate.web.cern.ch/welcome

➤ Major preparatory work on approved and proposed future colliders, e.g.

➤ Report on the Physics at the HL-LHC, and Perspectives for the HE-LHC

➤ FCC physics and design reports

➤ Submitted input: https://indico.cern.ch/event/765096/contributions/ (10pp/doc)

➤ Open Symposium (May 2019), including strong interactions session & summary

➤ Physics briefing book: arXiv:1910.11775 (strong interactions: section 4)

➤ January 2020: final closed meeting of European Strategy Group

➤ Strategy Approval: still ongoing

2

http://europeanstrategyupdate.web.cern.ch/welcomehttps://inspirehep.net/literature/1768569https://fcc-cdr.web.cern.ch/https://indico.cern.ch/event/765096/contributions/https://indico.cern.ch/event/808335/timetable/#all.detailedhttps://indico.cern.ch/event/808335/sessions/306788/#allhttps://indico.cern.ch/event/808335/contributions/3365132/attachments/1845449/3028389/Summary-Strong-Interactions-Granada-May2019.pdfhttps://arxiv.org/abs/1910.11775

two broad roles for QCD

4

QCD in service of broad particle physics goals (Higgs, EW, DM/BSM, etc.)

QCD as a fascinating subject in its own right

colliders

5

Where do we stand?

• QCD at high energies: weak coupling and asymptotic freedom• Perturbative QCD as quantitative framework• Dynamics of quarks and gluons• Jet observables were early test of QCD• Factorization separates weak from strong coupling effects

• Quantitative predictions• Multi-loop calculations for inclusive quantities• Higher orders (NLO, NNLO, …), resummation and parton shower simulation• Strong coupling dynamics parametrized in parton distributions, hadronization

ESPP Update Open Symposium GranadaThomas Gehrmann

from Thomas Gehrmann’s EPPSU talk

6

Where do we stand?

• Precision tests of the Standard Model• Measurements of masses and couplings

• Interplay of calculations and measurements• Accuracy on most cross sections ≳5%• Limited by PDFs, QCD corrections

• Perturbative QCD as analysis tool• Jet substructure techniques• Data-driven background predictions

pp

total (2x)

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Status: March 2019

ATLAS Preliminary

Run 1,2ps = 5,7,8,13 TeV

Theory

LHC ppps = 5 TeV

Data 0.025 fb�1

LHC ppps = 7 TeV

Data 4.5 � 4.9 fb�1

LHC ppps = 8 TeV

Data 20.2 � 20.3 fb�1

LHC ppps = 13 TeV

Data 3.2 � 79.8 fb�1

Standard Model Production Cross Section Measurements



7

Where do we stand?

• QCD at strong coupling: diverse research program• Hadron physics, low-energy dynamics, heavy ions• Precision spectroscopy of light hadrons ⟷ lattice QCD at high precision• Determination of hadron properties

• Proton radius• Form factors• Nucleon structure

• Demands and drives new quantitative approaches• Understanding non-perturbative dynamics of QCD

proton charge radius [fm]0.83 0.84 0.85 0.86 0.87 0.88 0.89 0.9

s5.6 CODATA-2014

H spectroscopy

e-p scatt

p 2013µ

p 2010µ

1706.00696



8

Where do we stand?

• Crucial interplay between QCD at strong and at weak coupling• Non-perturbative effects on precision collider observables• Parton distributions• Intrinsic transverse momentum• Soft underlying event and hadronization

• Hadronic input to SM tests and BSM searches• Form factors in flavor physics• Hadronic cross sections in neutrino and astroparticle physics• Hadronic effects in QED precision observables: α(MZ), (g-2)μ 0811.4622



9

Where do we stand?

• Feed-in and feed-back between strong and weak coupling QCD• Example: photon content of the proton (photon PDF)• Important ingredient to EW corrections of collider processes• Required for precision predictions at highest energies• Previously ad-hoc models with large uncertainty• LUXqed

• relate to elastic and inelastic form factors• Exploit low-energy data• Combine with perturbative QCD evolution

• Different motivation to address similar questions1607.04266



to maximally exploit HL-LHC

10

QCD theory is workhorse of LHC experiments

110

0.2

0.4

0.6

0.8

1Papers commonly cited by ATLAS and CMS (since 2017)

as of 2019-02-13, excluding self-citations; all papers > 0.2

Plotby

GPSalambasedon

datafromInspireHEP

fractionofAT

LAS&CMSpapersthatcitethem

GEANT4

Anti-k tjetalg.

MG5aMCatNLO

FastJetManual

POWHEGBox

NNPDF30PDFs

Pythia8.1

CL(s) technique(A.Read)

POWHEG(2007)

Likelihoodtests

for new

physics

POWHEG(2004)

Pythia6.4MC

Pythia8.2

CT10PDFs

Sherpa1.1

CL(s) technique(T.Junk)

NNPDF23PDFs

top++

CTEQ

6PDFs

MSTW2008PDFs

PDF4LHC-RunII

Herwig++MC

ReviewofParticle

Physics

2016

POWHEGsingle-top(2010)

FxFx

EvtGen(2001)

(red papers ≡ QCD)

Need for precision @ HL-LHC➤ illustrated in the case of Higgs physics

➤ theory uncertainty (PDF + strong coupling + missing higher orders) dominates in 7/9 channels

➤ this is with the assumption of reduction by x2 in today’s uncertainties

➤ depending on channel, it can be the uncertainties for the signal or the background that dominates.

12

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14Expected uncertainty

γZκµκτκbκtκ

gκZκ

Wκγκ

9.8

4.3

1.9

3.7

3.4

2.5

1.5

1.7

1.8

6.4 7.2 1.7

1.7 3.8 1.0

1.5 0.9 0.8

3.2 1.3 1.3

3.1 0.9 1.1

2.1 0.9 0.8

1.2 0.7 0.6

1.3 0.8 0.7

1.3 0.8 1.0 Tot Stat Exp ThUncertainty [%]

CMS and ATLASHL-LHC Projection

per experiment-1 = 14 TeV, 3000 fbs

TotalStatisticalExperimentalTheory

2% 4%

Figure 1. Projected uncertainties on ki, combiningATLAS and CMS: total (grey box), statistical (blue),experimental (green) and theory (red). From Ref. [2].

These coupling measurements assume the absence of sizableadditional contributions to GH . As recently suggested, the patternsof quantum interference between background and Higgs-mediatedproduction of photon pairs or four leptons are sensitive to GH .Measuring the off-shell four-fermion final states, and assumingthe Higgs couplings to gluons and ZZ evolve off-shell as in theSM, the HL-LHC will extract GH with a 20% precision at 68% CL.Furthermore, combining all Higgs channels, and with the soleassumption that the couplings to vector bosons are not larger thanthe SM ones (kV 1), will constrain GH with a 5% precision at95% CL. Invisible Higgs boson decays will be searched for atHL-LHC in all production channels, VBF being the most sensitive.The combination of ATLAS and CMS Higgs boson coupling mea-surements will set an upper limit on the Higgs invisible branchingratio of 2.5%, at the 95% CL. The precision reach in the mea-surements of ratios will be at the percent level, with particularlyinteresting measurements of kg/kZ, which serves as a probe ofnew physics entering the H ! gg loop, can be measured with anuncertainty of 1.4%, and kt/kg, which serves as probe of newphysics entering the gg ! H loop, with a precision of 3.4%.

A summary of the limits obtained on first and second gen-eration quarks from a variety of observables is given in Fig. 2(left). It includes: (i) HL-LHC projections for exclusive decays ofthe Higgs into quarkonia; (ii) constraints from fits to differentialcross sections of kinematic observables (in particular pT); (iii)constraints on the total width GH relying on different assumptions(the examples given in the Fig. 2 (left) correspond to a projected limit of 200 MeV on the total width from the mass shiftfrom the interference in the diphoton channel between signal and continuous background and the constraint at 68% CL on thetotal width from off-shell couplings measurements of 20%); (iv) a global fit of Higgs production cross sections (yielding theconstraint of 5% on the width mentioned herein); and (v) the direct search for Higgs decays to cc using inclusive charm taggingtechniques. Assuming SM couplings, the latter is expected to lead to the most stringent upper limit of kc / 2. A combination ofATLAS, CMS and LHCb results would further improve this constraint to kc / 1.

The Run 2 experience in searches for Higgs pair production led to a reappraisal of the HL-LHC sensitivity, including severalchannels, some of which were not considered in previous projections: 2b2g , 2b2t , 4b, 2bWW, 2bZZ. Assuming the SM Higgs

Figure 2. Left: Summary of the projected HL-LHC limits on the quark Yukawa couplings. Right: Summary of constraints onthe SMEFT operators considered. The shaded bounds arise from a global fit of all operators, those assuming the existence of asingle operator are labeled as "exclusive". From Ref. [2].

2

gluon fusion Higgs theory uncertainties

13

2.2.1.1 Gluon fusion

In this section we document cross section predictions for a standard model Higgs boson produced throughgluon fusion in 27 TeV pp collisions. To derive predictions we include contributions based on pertur-bative computations of scattering cross sections as studied in Ref. [47]. We include perturbative QCDcorrections through next-to-next-to-next-to-leading order (N3LO), electroweak (EW) and approximatedmixed QCD-electroweak corrections as well as effects of finite quark masses. The only modificationwith respect to YR4 [45] is that we now include the exact N3LO heavy top effective theory cross sectionof Ref. [48] instead of its previous approximation. The result of this modification is only a small changein the central values and uncertainties. To derive theoretical uncertainties we follow the prescriptionsoutlined in Ref. [47]. We use the following inputs:

ECM 27 TeVmt(mt) 162.7 GeVmb(mb) 4.18 GeV

mc(3 GeV) 0.986 GeV↵S(mZ) 0.118

PDF PDF4LHC15_nnlo_100 [49]

(5)

All quark masses are treated in the MS scheme. To derive numerical predictions we use the programiHixs [50].

Sources of uncertainty for the inclusive Higgs boson production cross section have been assessedrecently in refs. [47, 51, 52, 45]. Several sources of theoretical uncertainties were identified.

0 20 40 60 80 100

0

2

4

6

8

10

12

Collider Energy / TeV

i/total×100%

(scale)

(PDF-TH)

(EW)(t,b,c)

(1/mt)

(PDF+ s)

Fig. 1: The figure shows the linear sum of the different sources of relative uncertainties as a functionof the collider energy. Each coloured band represents the size of one particular source of uncertainty asdescribed in the text. The component �(PDF+↵S) corresponds to the uncertainties due to our impreciseknowledge of the strong coupling constant and of parton distribution functions combined in quadrature.

– Missing higher-order effects of QCD corrections beyond N3LO (�(scale)).– Missing higher-order effects of electroweak and mixed QCD-electroweak corrections at and be-

yond O(↵S↵) (�(EW)).– Effects due to finite quark masses neglected in QCD corrections beyond NLO (�(t,b,c) and �(1/mt)).

17

LHC

QCD theory anticipated / needed for full exploitation of HL-LHC(1) Fixed-order / resummed calculations ➤ Core processes at high accuracy (2→1 and 2→2): 1%, N3LO ➤ Splitting functions at N3LO (also needed for potential ep machines) ➤ Complex processes at few percent accuracy ➤ Accuracy at high pT ➤ Technical requirements for NLO multi-particle precision ➤ Multi-variate analyses / observables: performance and uncertainties ➤ Non-perturbative effects ➤ Resummation (incl. SCET) ➤ Accurate predictions for BSM effects

14

QCD theory anticipated / needed for full exploitation of HL-LHC(2) General purpose Monte Carlo event-generator tools ➤ Perturbative improvements for Matching and Merging (e.g. generalisation of

approaches for parton shower + NNLO merging,)

➤ Understanding & exploiting relation between parton-shower algorithms and resummation

➤ Phenomenological Models (hadronisation, underlying event, also connects with HI physics, neutrino programmes, low energy QCD, various “beyond colliders” experiments, cosmic-ray physics)

15

projected improvements in PDFs & strong coupling

➤ plot illustrates use of pseudodata with HL-LHC stats to obtain estimates of expected PDF uncertainties at HL-LHC

➤ PDF extractions will need to move to N3LO once available

➤ strong coupling remains contentious

➤ tensions between different groups’ extractions (PDFs, event shapes, and to a lesser extent lattice QCD)

➤ what ultimate accuracy on 10-15 year timescale?

16

[GeV]ttm310

[fb/

GeV

]tt

/dm

σd

3−10

2−10

1−10

1

10

210

310

PDF4LHC15

ttPDF4LHC15+ HL-LHC mHL-LHC pseudo-data, F = 0.2

Lumi error = 1.5 %

mass datatProjected invariant t

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Rat

io0.60.70.80.9

11.11.21.31.4

x5−10 4−10 3−10 2−10 1−10

orig

inal

g/g

0.7

0.8

0.9

1

1.1

1.2

1.3

PDF4LHC15tPDF4LHC15 + HLLHC t

F = 0.2

Q = 100 GeV

datatgluon PDF with projected t

Figure 3.5. Left: As in Fig. 3.2, now for the mtt̄ distribution in top quark pair production. Right: Asin Fig. 3.3, now for the gluon PDF after including the HL–LHC tt̄ pseudo–data in the fit.

x5−10 4−10 3−10 2−10 1−10

ρ

1−

0.8−

0.6−

0.4−

0.2−

0

0.2

0.4

0.6

0.8

1

TPDF4LHC15 + HLLHC p

to maximally exploit proposed future colliders

(ee, eh, hh)

17

future e+e- colliders➤ 3-loop and partial 4-loop calculations of Zff vertex for Tera-Z for EW pseudo-observables

➤ precision for decays, e.g. in Higgs physics and top-quark physics

➤ new generations of MC programs for QED and EW effects, understanding two-photon physics

18

5. Numerically dominant effects due multi-photon emission constitute another key problem;their complexity will be often comparable to that of the electroweak loop calculations. Themethodology of joint treatment of electroweak and QCD loop corrections with the photoniccorrections is essentially at hand (AB from the LHC?). In practice, however, constructionof new Monte Carlo programs, which can handle efficiently the above problems, will needspecial efforts [6].

The elementary techniques for higher order perturbative SM corrections are basically under-stood, but their use in practical calculations at the level of computer programs will be highly non-trivial and will require considerable effort. The understanding of all sources of possible theoreticaluncertainties will be fundamental for success of the FCC-ee data analysis.

��Z [MeV] �Rl [10�4] �Rb [10�5] � sin2,leff

✓ [10�6]Present EWPO theoretical uncertainties

EXP-2018 2.3 250 66 160TH-2018 0.4 60 10 45

EWPO theoretical uncertainties when FCC-ee will startEXP-FCC-ee 0.1 10 2÷ 6 6TH-FCC-ee 0.07 7 3 7

Table 1: Comparison for selected precision observables of present experimental measurements(EXP-2018), current theory errors (TH-2018), FCC-ee precision goals at the end of the Tera-Z

run (EXP-FCC-ee) and rough estimates of the theory errors assuming that electroweak 3-loop

corrections and the dominant 4-loop EW-QCD corrections are available at the start of FCC-ee

(TH-FCC-ee). Based on discussion in [2].

Table 1 shows the current experimental and theoretical errors (EXP-2018, TH-2018) for somebasic Z-physics EWPOs, and the prospective measurement errors at FCC-ee (EXP-FCC-ee) to-gether with the corresponding estimate for theoretical uncertainties after the leading 3/4-loop re-sults become available (TH-FCC-ee). The entry TH-2018 takes into account recent completionof the 2-loop electroweak calculations [7, 8], so the error estimate comes solely from an estimateof magnitudes of missing 3-loop and 4-loop EW and mixed EW-QCD corrections. The estimatedTH-FCC-ee error stems from remaining 4-loop and higher effects. These are rather difficult toestimate presently, however, a rough conservative upper bound on them has been provided in [2].They are denoted in Tab. 1 as TH-FCC-ee. As we can see, the uncertainties of the TH-FCC-eescenario are comparable to the corresponding EXP-FCC-ee experimental errors and will not

limit the FCC-ee physics goals.An illustration of complexity of future perturbative calculations is provided in Table 2, where

the numbers of distinct topologies of diagrams to be calculated and the numbers of diagrams andvarious categories at the 1-, 2- and 3-loop order are shown for the most complicated Z ! bb̄ decay.According to Ref. [2], the calculation of fermionic 3-loop corrections, which will be the largestpart of them, are within reach, already with present methods and tools.

Another important issue in electroweak fits are uncertainties from the existing (often experi-

5

future pp colliders

➤ combination of higher energies and luminosities will continue to push potential for precision

➤ need for precision will extend to high transverse momenta → requires improved treatment of EW corrections, including mixed QCD-EW effects

19

FCC Physics Opportunities

Figure 5.4: High-pT jet rates (left) and luminosity evolution of the minimum pT thresholds leading to 1and 10% statistical uncertainties (right).

Figure 5.5: Left plot: combined statistical and 1% systematic uncertainties, at 30 ab�1, vs pT threshold;these are compared to the rate change induced by the presence of 4 or 8 TeV gluinos in the running of↵S . Right plot: the gluino mass that can be probed with a 3� deviation from the SM jet rate (solid line),and the pT scale at which the corresponding deviation is detected.

perturbative calculations and in the knowledge of the PDFs. For measurements sensitive to the shapeof the distribution (e.g. the running of ↵S or the search of shape anomalies due to higher-dimensionoperators), the absolute luminosity determination does not contribute to the systematics. The jet energyresolution will only lead to a predictable smearing of the pT spectrum. Furthermore, the energy resolu-tion itself will be very small, limited in the multi-TeV region by the 2.6% constant term, as reported inVolume 3, Section 7.5.2. The stochastic contribution, even in presence of 1000 pile-up events, scales inthe region |⌘| < 1.3 like 104%/

ppT /GeV, and drops below the % level for pT>⇠10 TeV. The leading

experimental systematics will most likely be associated with the determination of the absolute jet-energyscale (JES). A great deal of experience is being accumulated at the LHC on the JES calibration [143,144],combining, among others, test-beam data, hardware monitoring via sources, and data-driven balancingtechniques. The latter rely on events such as Z[! e+e�]+jet, g+jet or multijets. The precise measure-ment of EM energy deposit of photons and electrons allows the calibration of the jet energy, up to energylevels where there is sufficient statistics. At larger energies, leading jets are calibrated against recoil sys-tems composed of two or more softer jets. In their final calibration of approximately 20 fb�1 of 8 TeVdata from run 1, CMS [144] achieved a JES uncertainty for central jets of about 0.3% in the 200-300 GeVrange. ATLAS [143], using 3.2 fb�1 of 13 TeV data, determined a more conservative uncertainty of lessthan 1% for central jets with pT in the range 100-500 GeV. A naive rescaling of the statistics based on the

54PREPRINT submitted to Eur. Phys. J. C

jet rate  @ 100 TeV pp

10k events (1% stat.prec) for jets with pT > 15 TeV

➤ very high-multiplicity final states, possibly involving multiple scales → needs understanding of regions of validity of perturbation theory, interplay with parton showers, etc., including for EW objects

2018/27Strong Interactions, EPPS Update, May'19 David d'Enterria (CERN)

PDFs: Still work to do for FCC...PDFs: Still work to do for FCC...

NEW

PHYSICS

PRECISION

PHYSICS

LOW-X

PHYSICS

High-x

Low-x

■ Still large PDF uncertainties in pp at 100 TeV in key (x,Q2) regions:

gg

■ FCC-ep required to reach O(1%)

uncertainty for s(W,Z,H) at FCC-pp

Mid-x

g uncert.

g uncert.

from David d’Enterria’s EPPSU talk

21

The ep Physics at the Energy Frontier

HiggsBoson

-four

-mom

entu

m tr

ansf

er s

quar

ed

Bjorken

RPV SUSY, LQSubstructure ?

High PrecisionQCD &EW (t, W, Z)Physics

Extensions of both x and Q2 ranges are crucial for pp experiments and

HEP theory developments;

HERA established the validity of

pQCD down to x > 10-4

(DGLAP) due to a very high lever arm in Q2:

high luminosity colliders

with high c.m.s. energy of

1.3 3.5 TeVsmall x for UHE Neutrinos & FCC-hh

H HH

and unfold hadron sub-structure for LHC and FCC-hh unambiguously

New ep colliders beyond HERA c.m.s.

Large xGluon &valence quarks

High Density MatterSaturation? New form ofgluonic matter?

from Uta Klein’s

EPPSU talk


High-precision gluon & quark jet studies (FCC-ee)High-precision gluon & quark jet studies (FCC-ee)

■ Exploit FCC-ee H(gg) as a ”pure gluon” factory:

H→gg (BR~8% accurately known) provides

O(100.000) extra-clean digluon events.

■ Multiple handles to study gluon radiation & g-jet properties:

➧Gluon vs. quark via H→gg vs. Z→qq

(Profit from excellent g,b separation)

➧ Gluon vs. quark via Z→bbg vs. Z→qq(g)

(g in one hemisphere recoiling

against 2-b-jets in the other).

➧ Vary Ejet

range via ISR: e+e-→Z*,g*→jj(g) ➧ Vary jet radius: small-R down to calo resolution

■ Multiple high-precision analyses at hand: – BSM: Improve q/g/Q discrimination tools

– pQCD: Check NnLO antenna functions. High-precision QCD coupling.

– non-pQCD: Gluon fragmentation: Octet neutralization? (zero-charge gluon

jet with rap gaps). Colour reconnection? Glueballs ? Leading h's,baryons?

LH angularities

[G.Soyez et al.]



Highly-boosted jets, multijets (FCC-pp)Highly-boosted jets, multijets (FCC-pp)

■ Proton-proton collisions at 100 TeV provide unique conditions to

produce & study highly-boosted objects: top, W, Z, H, RBSM

(jj),...

Resolving small angular dijet sep. ∆R ≈ 2M(jj)/pT(j).

■ Jet substructure: key to separate dijets from QCD & (un)coloured

resonance decays, e.g.

R10-TeV

Æ tt,qq,gg,WW

■ Diffs. in MC generators for

quark vs. gluon jets

(& jet radius).

■ Also unique multijet (N>>10)

BSM,

QCD

studies

[FCC-pp, arXiv:1607.01831]


resources & the next generation

24

incoming / early-stage researchers and subsequent career development➤ early-stage researchers need recognition for a variety of types of contribution (e.g.

including the technical work that simply “makes things work” but that comes neither with glory nor even necessarily papers)

➤ how do we ensure recognition for early-stage researchers working within the medium-sized teams (O(10) researchers) that are increasingly common?

➤ specialisation v. broad training

➤ successful projects need skills that span interface with maths (incl. computer algebra), interface with computing, machine-learning, and a range of physics/pheno applications → individuals specialise

➤ at same time we need to ensure future generation can combine specific expertise with broad physics ability within the field

25

issues of long-term support ➤ funding for projects that last longer than typical funding cycles ➤ support for codes:

➤ state-of-the-art physics codes often developed in small groups, but subsequent long-term maintenance & user-support of successful codes often requires substantial dedicated expert time, which can be a substantial burden

➤ the “glue” codes (e.g. LHAPDF, HepMC): may not be seen as physics by funding agencies, but support (people/resources) & evolution essential for long-term smooth operation of the field

➤ “mechanisms need to be developed to share the effort between event generator projects and their user communities” [Id114] (& we need to ensure that conditions are attractive for those who do this well, e.g. in terms of career recognition)

➤ computing aspects ➤ adapting codes to new architectures ➤ availability of state-of-the-art hardware (e.g. hundreds of GPUs, very high-memory machines) ➤ many university groups can’t afford to keep up with disparate landscape of hardware. How

best to share nationally and internationally?26

Summary

27

QCD theory summary➤ Advances in QCD theory are essential to exploit HL-LHC and future colliders (and already

built into some projections!)

➤ They will involve a wide range of topics, spanning calculations of amplitudes to Monte Carlo event generations, including phenomenological work to connect with data

➤ Theory advances can bring light also on many topics of intrinsic interest in QCD, including proton structure, exotic hadrons, connections with “theorists’s theories” like N=4 SUSY

➤ Continued support of QCD theory is essential for success of European collider programme, and community needs to keep in mind

➤ recognition of contributions of early-stage researchers as teams grow larger

➤ funding structure for increasingly long-term theory projects

➤ positions and career development for individuals who provide essential “support” roles (maintenance of widely used tools, interfacing with & support for users, …)

➤ computing (access to hardware and expertise)28

backup

29

gluon fusion Higgs theory uncertainties

30

2.2.1.1 Gluon fusion

In this section we document cross section predictions for a standard model Higgs boson produced throughgluon fusion in 27 TeV pp collisions. To derive predictions we include contributions based on pertur-bative computations of scattering cross sections as studied in Ref. [47]. We include perturbative QCDcorrections through next-to-next-to-next-to-leading order (N3LO), electroweak (EW) and approximatedmixed QCD-electroweak corrections as well as effects of finite quark masses. The only modificationwith respect to YR4 [45] is that we now include the exact N3LO heavy top effective theory cross sectionof Ref. [48] instead of its previous approximation. The result of this modification is only a small changein the central values and uncertainties. To derive theoretical uncertainties we follow the prescriptionsoutlined in Ref. [47]. We use the following inputs:

ECM 27 TeVmt(mt) 162.7 GeVmb(mb) 4.18 GeV

mc(3 GeV) 0.986 GeV↵S(mZ) 0.118

PDF PDF4LHC15_nnlo_100 [49]

(5)

All quark masses are treated in the MS scheme. To derive numerical predictions we use the programiHixs [50].

Sources of uncertainty for the inclusive Higgs boson production cross section have been assessedrecently in refs. [47, 51, 52, 45]. Several sources of theoretical uncertainties were identified.

0 20 40 60 80 100

0

2

4

6

8

10

12

Collider Energy / TeV

i/total×100%

(scale)

(PDF-TH)

(EW)(t,b,c)

(1/mt)

(PDF+ s)

Fig. 1: The figure shows the linear sum of the different sources of relative uncertainties as a functionof the collider energy. Each coloured band represents the size of one particular source of uncertainty asdescribed in the text. The component �(PDF+↵S) corresponds to the uncertainties due to our impreciseknowledge of the strong coupling constant and of parton distribution functions combined in quadrature.

– Missing higher-order effects of QCD corrections beyond N3LO (�(scale)).– Missing higher-order effects of electroweak and mixed QCD-electroweak corrections at and be-

yond O(↵S↵) (�(EW)).– Effects due to finite quark masses neglected in QCD corrections beyond NLO (�(t,b,c) and �(1/mt)).

17

Theoretical path for QCD physics: main inputs

31

Id100 Precision calculations for high-energy collider processes

Id101 Theory Requirements and Possibilities for [ee colliders]

Id114 MC event generators for HEP physics event simulation

Id163 Quantum Chromodynamics: Theory
https://indico.cern.ch/event/765096/contributions/3295741/https://indico.cern.ch/event/765096/contributions/3295742/https://indico.cern.ch/event/765096/contributions/3295772/https://indico.cern.ch/event/765096/contributions/3296021/

precision qcd] physics input to european strategy update...[precision qcd] physics input to european...

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