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
DPG-Tagung Hamburg
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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