top quark physics at collidersivicfa.uv.es/.../2012/12/topquarkphyatcolliders_grodrigo.pdf · 2012....

Top quark physics at colliders

Top quark physics at colliders

Germán Rodrigo

November 23, 2012

I F I CI N S T I T U T D E F I S I C A C O R P U S C U L A R

G. Rodrigo, top quark physics at colliders, IVICFA´s Fridays, 23 Nov 2012 2

the top quark: the gold(en) particle

● The heaviest known elementary particle

● Yukawa coupling to Higgs boson 1 : bridge to EWSB

● Special role in many BSM: a window to new physics that couples preferentially to top quarks


PrecisionLHC as a top factory for precision

physics: mass, cross-section, properties (charge asymmetry),

spin correlations


driving directions to precision in theory

[diagram by Schulze]

Higher orders in pQCDHigher orders in pQCD

Higher EnergyHigher Energy

Last but not leastLast but not least


the NLO revolution● 10 years ago 23 processes were at the border line● With the development of on-shell methods based on

analyticity/unitarity, which are a much more efficient than Feynmandiagrams, today 24 (and even 25) have become state-of-the art

● Many contributors, and many new results ready for LHCphenomenology

● Automated theoretical tools @ NLO: BlackHat+Sherpa, aMC@NLO(CutTools, MadLoop, HELAC-NLO), Rocket, SAMURAI, NGluon

● Give also new insight into structure and properties of scatteringamplitudes, not only in QCD


the NLO revolution● 10 years ago 23 processes were at the border line● With the development of on-shell methods based on

analyticity/unitarity, which are a much more efficient than Feynmandiagrams, today 24 (and even 25) have become state-of-the art

● Many contributors, and many new results ready for LHCphenomenology

● Automated theoretical tools @ NLO: BlackHat+Sherpa, aMC@NLO(CutTools, MadLoop, HELAC-NLO), Rocket, SAMURAI, NGluon

● Give also new insight into structure and properties of scatteringamplitudes, not only in QCD

● NLO revolution driven by LHC: higher order corrections in QCD athadron colliders is not only a question of improving systematicallythe precision of theoretical predictions, but

NLO is the first reliable estimate of central value

● NLO revolution driven by LHC: higher order corrections in QCD athadron colliders is not only a question of improving systematicallythe precision of theoretical predictions, but

NLO is the first reliable estimate of central value


the upcoming NNLO revolution● NNLO at hadron colliders (+ EW corrections at NLO) is the first

serious estimate of the theoretical error

● A very few NNLO results: e.g. →3jets (determination of ),→ (main background to Higgs [Catani et al.]), → ̅

[Baernreuther, Czazon, Mitov]

● On-shell methods at two-loops and beyond [Catani et al., Mastrolia,Ossola, Kosower, Gluza, … ] still at infancy phase


the top quark entering the precision era

● Close to use cross-section for determination of PDFs and/or αs [Huston]

CMS PAS TOP-12-0220.1178 0.0046 0.0040 consistent with world average,

although not competitive yet as a measurement


Charge asymmetry


Charge asymmetry in QCD [Kühn, GR, 1998]At O(αS2): top and antitop quarks have identical angular distributions

A charge asymmetry arises at O(αS3)

Interference of ISR with FSRLO for ̅+jet negative contribution to ̅+1 jet

Interference of box diagrams with Born (+soft) positive contribution to ̅+0 jet

Flavor excitation (qg channel) much smaller

● color factor dabc2 : pair in color singlet

● Loop (+soft) contribution larger than tree leveltop quarks are preferentially emitted in the direction of the incoming quark


Inclusive charge asymmetry at TevatronCharge conjugation symmetry* ( ̅ ) ⇨ equivalent to forward-backward [Kühn, GR,1998; arXiv:1109.6830]

.

̅∆ ∆∆ ∆

. ∆ ̅

■ mixed QCD-EW interference included: factor 1.2 [Kühn, GR / Hollik, Pagani]■ first contribution to the antisymmetric x-section is a loop effect, first contribution to the symmetric x-section is tree level: asymmetry normalized to LO x-section

■ stable to NLL threshold resummations (one per mille shift) [Almeida,Sterman,Vogelsang]■ NNLL threshold resummations [Ahrens,Ferroglia,Neubert,Pecjak,Yang]Not expanding the asymmetry in αS : the asymmetry decreases by 20% at NLO (K factor), but only by 5% at NLO+NNLL

■ Full NNLO on-going [Bonciani, Ferroglia, Anastasiou, …] but → ̅ [Baernreuther, Mitov, Czacon] enough for the asymmetry at NLO (normalized to symmetric NLO x-section)

* CP violation arising from electric or chromoelectric dipole moments do not contribute to the asymmetry

Invariantunder boost


Invariant mass dependent charge asymmetry

CDF [PRD83 (2011)112003 arXiv:1101.0034] ̅ rest frame 5.3 fb-1

̅ ̅ 450GeV = -0.116 ± 0.146 (stat) ± 0.047 (syst)

̅ ̅ 450GeV = 0.475 ± 0.101 (stat) ± 0.049 (syst)

■ below 450 GeV: negative asymmetry but still compatible with the SM within 1σ

■ above 450 GeV: positive asymmetry, disagrees with the SM at 3.4σ

■ The deviation from the SM in the lab frame is not as significant !!!

Since 2008, a systematic 1-2σ positivediscrepancy with the SM

(inclusive asymmetry)


Invariant mass dependent charge asymmetryThe slope in ̅ is positive in QCD: for

virtual corrections ̂ ̅, but for real corrections (negative asym.) ̂ ̅

That big ?

CDF arXiv:1

211.100

3

5.3 fb-1

9.4 fb-1Full dataset


From Tevatron to the LHC

q

t

q

cms rest frame

t

q q

q

t

q

LAB frame

q

t

q

LHC is symmetric ► no forward-backwardbut excess of tops (or antitops) in the forward and backward regions because incoming quarks carry more momenta than antiquarks

► gg dominated (symmetric): introduce cuts to enhance the sample, e.g. ̅, large rapidities (ggmore central)


Charge asymmetry at the LHC

Δ ̅

AySM = 0.0115 (6)[Kühn, GR, arXiv:1109.6830] @ 7 TeV

atlas (1203.4211)0.053 0.070 0.054 450 0.008 0.035 0.032 450

0.018 0.028 0.023atlas-conf-12-057 (dilepton)

0.057 0.024 0.015cms (1112.5100)

0.013 0.028 ..

cms-pas-top-11-0300.004 0.010 0.012

cms-pas-top-12-010 (dilepton)0.050 0.043 ..

* or ∆ ̅for ̅ fixed


Charge asymmetry at the LHC

■ scaling the statistical error20 fb-1 : 5 per mille100 fb-1 : 2-3 per mille

Δ ̅

AySM = 0.0115 (6)[Kühn, GR, arXiv:1109.6830] @ 7 TeV

atlas (1203.4211)0.053 0.070 0.054 450 0.008 0.035 0.032 450

0.018 0.028 0.023atlas-conf-12-057 (dilepton)

0.057 0.024 0.015cms (1112.5100)

0.013 0.028 ..

cms-pas-top-11-0300.004 0.010 0.012

cms-pas-top-12-010 (dilepton)0.050 0.043 ..


Tevatron versus LHC

axigluon (non‐universal) in the s‐channel

W´ and Z´ in the t‐channel withlarge flavour violating couplings


same-sign tops

Can be evaded in non-minimal models with several Z´ [Jung, Pierce Wells]


Top quark mass


stability of the EW vacuum● Could one extrapolate the SM to higher scales while keeping the

minimum of the scalar potential ? quartic Higgs coupling 0

● Current values cover all possibilities within 2σ● A precise assessment can only be made at the LC if ∆ 200MeV

(by threshold scan), and order of magnitude better than LHC - Tevatron

Critically depends on: ● The Higgs boson mass● The strong coupling ● The mass of the top quark:

value, MonteCarlo/pole/running mass, higher order QCD corrections

[Alekhin, Djouadi, Moch]


top quark mass and from energy scan and invariant mass

● CLIC above threshold (500 GeV) by reconstructing the invariant mass: 80 MeV statistical precision with 100 fb-1

● Threshold scan around 350 GeV at CLIC by fitting the 1S mass and αs: 34 MeV statistical precision of the mass, 0.0009 statistical precision of αs with 100 fb‐1 split across 10 equally spaced scan points

● 15% better at ILC due to different luminosity spectrum (threshold scan, invariant mass) and higher background (invariant mass)

ILC

[Simon]


Couplings


● Yukawa coupling to Higgs boson 1

● Indirect constraints at LHC from → and → assuming no

new particles in the loop

● Direct measurement from Higgs-strahlung from top quarks

[Zerwas]


and anomalous couplings

● Three observables (x 2 polarizations) to disentangle ̅ from ̅ couplings: cross-section, AFB and helicity angle

[Vos, Rouëné]

[Fiolhais]

● Effective operator framework allows to relate anomalous couplings in charge currents at LHC with ̅ and ̅couplings at LC: from polaziredobservables


Summary● The Tevatron, where the top was discovered, provided many

precision measurements of top quark and QCD (someanomalies persist, FB/charge asymmetries, single top)

● The LHC brings top quark and QCD physics to a new level ofprecision, rapidly improving many Tevatron results

● Higher precision on the strong coupling, cross-sections, topmass (~100MeV) and properties, anomalous couplings (

top quark physics at collidersivicfa.uv.es/.../2012/12/topquarkphyatcolliders_grodrigo.pdf · 2012....

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