KIT – Universität des Landes Baden-Württemberg und nationales Forschungszentrum in der Helmholtz-Gemeinschaft

Matthias MozerInstitut für Experimentelle Kernphysik, Karlsruher Institut für Technologie

www.kit.edu

Standard Model Physics at the LHCPrecision Measurements at the Energy Frontier

DPG-Tagung Hamburg

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SM Lagrangian

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SM Lagrangian

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SM Lagrangian

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KIT – Universität des Landes Baden-Württemberg und nationales Forschungszentrum in der Helmholtz-Gemeinschaft

Matthias MozerInstitut für Experimentelle Kernphysik, Karlsruher Institut für Technologie

www.kit.edu

Standard Model Physics at the LHCPrecision Measurements at the Energy Frontier

DPG-Tagung Hamburg

KIT – Universität des Landes Baden-Württemberg und nationales Forschungszentrum in der Helmholtz-Gemeinschaft

Matthias MozerInstitut für Experimentelle Kernphysik, Karlsruher Institut für Technologie

www.kit.edu

QCD and EWK Physics at the LHCPrecision Measurements at the Energy Frontier

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A Rich Field

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ATLASCMS

LHCb

>150 publications(ATLAS+CMS+LHCb)

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Jet Production

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The dream analysis=> Basically background free=> Unlimited statistics

[CMS-SMP-15-007]also: [ATLAS-CONF-2015-034]

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Challenges with Jets

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Huge statistical precision: Dream or nightmare?

Systematic effects are everywhere:=> Jet energy scale/resolution=> Jet energy corrections depend on parton type/flavor=> Pileup effects=> …

Theory uncertainties not negligible=> QCD is hard to compute=> PDFs not precisely known=> Non-perturbative effects at low pT

Hard work=> Still some 8TeV analyses ongoing

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Jet Energy Corrections

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Huge effort:<1% scale uncertainty

Exploit:=> Z + jets (low pT)=> γ + jets (intermediate pT)=> Jet balance (high pT)=> Test beam (high pT)

Precision limited at=> Very low pT=> Very high pT=> Forward direction

[Eur. Phys. J. C (2015) 75:17]see also [CMS-JME-13-004]

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Jet Energy Corrections

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Very work intensive

Many systematicswith similar contributions=> No easy improvements

Pileup (mostly) not limitingin Run I=> More difficult in Run II=> Working on better

correction / subtraction

[Eur. Phys. J. C (2015) 75:17]see also [CMS-JME-13-004]

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Theory Uncertainties

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Large theory uncertainties at high jet pT

Large extrapolation from HERA data

Large x gluon density not that well constrained

[CM

S-S

MP

-14-001]

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Turning the tables

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Measurement interpretationlimited by theory uncertainties

Measurements constraintheory parameters

=> Parton densities (PDFs)=> Strong coupling (αs)

Systematics are critical=> some 7TeV studies still current

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αs: 3-jet mass

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More jets in the final state => higher power of αs

Tricky theory calculation (NLO available)

Correlated with PDFs => requires tuned PDF-sets

[Eur. P

hys. J. C 75 (2015) 186]

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αs: Results

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Z-kinematics directly relatedto PDF parameters

Rapidity ↔ x

Mll ↔ Q2

Vector Bosons in QCD

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Relatively simple to measure / predict

PDFs biggest theory uncertainty

Likely to enter PDF fits soon

EWK Bosons: Run II

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[CMS-SMP-15-004] [ATLAS-CONF-2015-039]

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W/Z + Jets

Behrends-Giele-Scaling:additional jets suppressedby factor ~αs

Important background tonew physics searches

Hard to calculate in NLOto high Njet

Staggered NLO/LOcomputations give reasonable results

Bosons and Jets

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[ATLAS-CONF-2015-041]see also [CMS-SMP-15-010]

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W charge asymmetry: => Exploit higher average momentum of valence quarks=> Sensitive to d/u-ratio, sea-antiquark densities

Flavor Sensitivity

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[CM

S-S

MP

-14-022]

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Use in external PDF Fits

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Stefano Forte

LHC measurementswell received by PDFcommunity

Not just QCD / EWK=> Also top quark

Uptake of some measurements limitedby theory issues:=> NNLO calculations=> EWK corrections

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Drell-Yan xsecPhysics with EWK bosons

≠EWK physics

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EWK Measurements

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Weinberg angle: dilution from unknown initial state

Z-mass: LEP-precision is better than 0.01%

[JHEP 1511(2015) 190]

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LEP vs LHC

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Many basic EWK parameters better measured at LEP:=> AFB=> MZ

Where can we do better?

MW=> Limited statistics at LEP=> Tevatron-style fit (MT, pT,l, MET) feasible at the LHC=> Some tricky theory uncertainties

(see ATL-PHYS-PUB-2014-015)

Multi-boson production=> Only WW easily accessible at LEP=> Allows study of EWK triple + quartic vertex

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W Mass Preparations

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Tevatron-style fit => MT, pT,l, MET

Needs excellent detector resolutions(especially MET)

Current precision:~ 0.02 GeV

Tricky theory uncertainties=> Intrinsic W pT spectrum?=> Radiation can be soft=> Hard to predict=> Check on Z pT spectrum

[arxiv:0906.4260]

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Z pT Measurements

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Theory computationsmost difficult at low pT

Lepton energy scale isgood, but can we do better

Use pT-correlated angularvariable instead: φ*

Only limited by angularresolution=> No energy scale uncertainties

Needs soft gluon resummationto describe low pT/φ* region

Could be improved at high pT

[arxiv:1512.02192]

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Multi-Boson Interactions

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Vector Boson Interactions

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QGC TGC Propagator W,W,W,W W,W,Z/γW,W,Z/γ,Z/γ Z/γ,Z/γ,Z/γZ/γ,Z/γ,Z/γ,Z/γ

⊕ ⊕

QGC: of special interest due to EWK symmetry breaking:

longitudinal scatteringviolating unitaritywithout interferencefrom Higgs diagrams

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Vector Boson Scattering

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QGC

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Vector Boson Scattering

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QGC

Double Brem

+ … (other α4)

TGC

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Vector Boson Scattering

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QGC

Double Brem

+ … (other α4)

TGC

+ … (other α2αs2)

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VBS - tagging

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Two tagging jets=> Large ∆η=> Large Mjj

Boson system between jets in rapidity

φ Angle between dijet and EWK system

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Same Sign WW

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QGC

Double Brem

+ … (other α4)

TGC

+ … (other α2αs2)

No gluon induced processes at LO

High purity

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Very pure, but low statistics => marginal cross section measurement

significance: 3.6 σ (expected: 2.8 σ)

Cross section: σfid= 1.3 ± 0.4 (stat.) ± 0.2 (syst.) fb

Same Sign WW

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[Phys. Rev. Lett. 113, 141803]

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Effective field theory

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Model independent search for BSM physics

Low energy approximationfor unknown full BSM model(~Fermi Theory)

New operators in the Lagrangian

new physics signature at => High mass=> High pT

New vertices break unitarity

Troublesome for studieswith the quartic vertex

… + 18 more

[PRL 114 (2015) 051801 ]

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same-sign WW prospects

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First evidence with 8TeV data

Will take more lumi for 5σ=> 2016?

Will take even more luminosityto separate longitudinal WW=> HL-LHC

Easier for exotic enhancements

Studies for HL-LHC:seems feasible even for pure SM

Looking also into WZ=> Full angular reconstruction=> Much more QCD processes

[CMS Phase II technical proposal]

[ATL-PHYS-PUB-2013-006]

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Other VBS final states

=> Zγ CMS-SMP-14-018

=> Wγ CMS-SMP-14-011

Multiboson Production

=> WVγ PRD 90 (2014) 032008 (CMS)

=> Wγγ Phys. Rev. Lett. 115, 031802 (2015)] (ATLAS)

Other Vertices

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Wγγ

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[Phys. R

ev. Lett. 115, 031802 (2015)]

Low cross section=> especially for heavy bosons

No branching ratio concernsfor photons

“Fake” photons are biggest background=> Requires tight identification

>3 σ cross section measurement

Interpret in EFT parameterization=> Only WWγγ contributions

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Low photon virtuality=> Protons don’t dissociate

Mostly EWK processes

Can be very pure for strict dissociation vetos

Thinking outside the box

3838

q’ pq

q’q p p’

p’

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Analysis Strategy

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Select e + µ (opposite charge)=> Suppress DY=> Suppress γγ→ll

pT(eµ) > 30 GeV=> Suppress γγ→ττ

Veto all tracks other than e,µ tracks from PV=> Suppress dissociative

processes

Worry about modeling the tiny corner of phase space left

Physics Letters B

749 (2015) 242-261

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Tricks of the Trade

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Excellent understanding of tracking efficiency, fakes

PU estimates, vertexing

Arcane hadronic physics effects:=> Pomerons=> Rescattering / gap-survival propability

Specialized MC generators => POMPYT (diffractive WW)=> Madgraph with Effective Photon Approximation=> LPAIR for EWK ll

Many uncommon features=> Can’t just trust background simulation=> Extensive studies of control regions

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Results

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Ntrack

p T

Signal region

13 events observed (8.8 expected)=> 3.6σ (2.4σ expected)

[CMS-FSQ-13-008]

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Summary

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8 TeV analyses coming to a close=> Many recent results=> Good understanding of Run I data → precision QCD=> Mapping out the quartic vertex of EWK interactions

13 TeV data opens up new possibilities:

=> Extended kinematic range for QCD measurements

=> Higher jet multiplicities

=> High statistics Z, W

=> High cross sections for multi-boson production

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Thank You!

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Multi-Production vs VBS

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triple vertex quartic vertex

VBS

Multi-Production

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2-Bosons + 2 Jets

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WVγ Phys. Rev. D 90, 032008 (2014)

Zγ + 2jets (a.k.a. ewk Zγ) SMP-14-018

exclusive WW (a.k.a γγ → WW) FSQ-13-008JHEP 07 (2013) 116

W+W+ + jets (a.k.a. ewk ss WW) PRL 114 (2015) 051801

=> covering the whole SM Lagrangian

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Track Mis-Association

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Mistaken association of PU tracks

check γγ→ll events

track veto enriches ewk process

look at very low acoplanarity:=> enriches elastic contribution

Data yield lower than MC=> low pT forward tracks common=> high chance of mis-association

Corrected with flat scale factor=> causes significant uncertainty

[FSQ-13-008]

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Rescattering

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Rescattering probability in elastic andquasi-elastic pp collisions not well understood

Long standing issue in comparisonsof Tevatron and HERA diffraction

Significantly different for elastic => little rescatteringsingle diss. => ???double diss. => high rescattering

Use γγ→ll to estimate survival eff.=> constrain to Mll > 2MW

Single largest systematic uncertainty (~10%)

[FSQ-13-008]

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Comparisons

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Similar Sensitivity ofssWW and Zγ

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W + charm:=> directly probes strange-quark density=> Charm tagged by D*/D-meson reconstruction

Flavor Sensitivity: W+c

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αs: 3-jet / 2-jet ratio

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only linear in αs => somewhat lower sensitivity

many systematic effects cancel

less dependent on PDFs

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Probing PDFs

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Wait for PDF collaborations?=> takes too long=> do PDF fits in CMS

using HERAFITTER frameworkfor DGLAP evolution and fitting

fastNLO parameterizationof NLO cross sections

Advantage: => very fast turn-around time

Disadvantage: => doesn’t include all relevant datasets