Top-quark physics ─ Theoretical issues ─ Peter Uwer Rencontre de Blois, 20.05.2014 GK1504.

Top-quark physics─ Theoretical issues ─

Peter Uwer

Rencontre de Blois, 20.05.2014

GK1504

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 2

Outline

1. Motivation and introduction

2. Issues:

- Charge asymmetry

- Top-quark mass

3. Conclusion


Introduction: Why are we interested in top-quarks

1) Top-quark = one building block of the Standard Model

Want to measure/know all properties as precise as possible

2) Top-quark physics as window to new physics

could decay in new heavy particles ( resonances in tt) Very sensitive to EWSB, strong coupling to Higgs Important correction to Higgs mass Affects the running of the quadratic Higgs coupling (vac .stability)

Top-quark mass important input parameter

1) and 2) are related through precision physics


Introduction: New Physics searches SM physics

[Degrassi, Di Vita, Elias-Miro,Spinosa,Giudici ’12, Alekhin, Djouadi, Moch ’12]

Vacuum stability Consistency of the SM

Precise theoretical predictions required


Top-quark pair production ─ theory status

Incl.

cros

s sec

tion

NLO

Incl.

cros

s sec

tion

NNLO

1989

[Dawson, Ellis, Nason ’89, Beenakker et al ’89,91]

[Bernreuther, Brandenburg, Si, PU ’04

[Czakon,Mitov 08]

Spin d

epen

dent

cros

s sec

tion

NLO

2004

[Bonciani, Catani, Mangano,Nason ‘98, Kidonakis, Laenen, Moch, Vogt 01]

NLL re

sum

mat

ion

1998 2008

Analyt

ic re

sults

NLO

Steps

towar

ds N

NLL

[Moch, PU 08]

Bound

stat

e ef

fects

[Kiyo,Kuhn,Moch,Steinhauser,PU 08Hagiwara, Sumion, Yokoya 08]

Full N

NLL re

sum

mat

ion

2010

[Czakon,Fiedler,Mitov 13]

2013

Combin

ed N

NLL a

nd 1

/

[Ahrens, Beneke,Czakon,Ferroglia, Mitov, Schwinn…]

Impressive theoretical progress in last 25 years

Similar for single top-quark production

Many more results on differential distributions, add. jets,combination with parton shower, top decay…

Th. uncertainty

below 5%

Further progress will require substantial effort from theory


Current situation

No smoking gun so far

Most measurements in good agreement with predictions

Precision physics with top-quarks has just started

more precise measurements more involved observables

Future directions:

Issues (= something which could become a problem…):

Forward-backwardcharge asymmetry

Understandingthe top-quark mass

or may disappear…


Charge asymmetry

Remember: Furry’s theorem

+Holds also for more complicated diagrams

Interference term does not contribute to total cross section Asymmetric contribution if t-t phase space is un-integrated

[Berends et al ’73, Kuhn, Rodrigo ‘98]

+

= 0

= 0= 0+ = 0+t t

virt. corr. to tt

real corr. to tt

QCD:


Forward-backward charge asymmetry (Tevatron)

Qualitative picture:

rapidity

Definitions used by CDF/D0:

Forward-backwardcharge asymmetry

Assuming CP invariance:


Charge asymmetry: Theory predictions

[Kühn, Rodrigo ´11]

Coherent picture of theoretical predictions,Theoretical uncertainties based on scale variations,

possibly underestimates higher order effects (ratios!)

Soft gluonresummation

QCD+EW

QCD+EW

QCD

0.088

OnlyLO !


Measurements from Tevatron

[1] CDF, arXiv:1101.0034, [2] D0, arXiv:1107.4995, [7] CDF note 10807

At most 2.4 deviation“Some tension”

[Bernreuther, Si, PRD86 (2012) 034026]

O(100) theory papers refined experimental studies (full data sample, lepton

asymmetries)


Recent results on the leptonic charge asymmetries

[CDF, 1404.3698]

CDF results:

D0 results:

[D0, 1403.1294]

[PRD84 (2011) 112005]5.4/fb

9.7/fb

9.1/fb

Note: Lepton asymmetries depend on top polarization, ind. confirmation of Pt 0


Recent results from CDF

[CDF, Phys. Rev. D87 (2013) 092002]

9.4/fb


Latest results from D0

D0, arXiv 1405.0421

Now in agreement with SM


Charge asymmetry at the LHC

No forward backward charge asymmetry at LHC due toP symmetric initial state

However: t tend to follow initial q, while tb tend to follow initial qb initial state is not symmetric with respect to q,qb q tend to be more energetic

should be broader w.r.t

y

top

anti-top


Charge asymmetry at LHC

[CMS PAS TOP-14-006]


Summary on AFB and lessons to be learned

Charge asymmetry = just another subtle quantum effect

We should measure these effects even if they look un-spectacular or out of reach as far as the

SM predictions are concerned

The signal which could have been the first indication of new physics seems to have disappeared

Nothing particular to learn…

…apart from understanding the quantum level !!!

Important to probe theory at quantum level


Top-quark mass: Recent results

Best known quark mass, shouldn’t we be happy ?

[arXiv:1403:4427]


How do we measure a quark mass (in theory) ?

Top-quark is not stable, even if it would be, confinement wouldprevent us from seeing free top-quarks

What is the meaning of the top-quark mass ?

Formal answer:

Top-quark mass / Yukawa coupling just a parameter of the underlying theory (e.g SM) Value depends on renormalization scheme used to define the parameters in theor. predictions

Measure mass in specific scheme through comparison/fit:


Requirements for a good observable

Observable should show good sensitivity to m

Observable must be theoretically calculable, at least predictions at NLO accuracy required

Theory uncertainties should be small

Method should employ well defined mass scheme

small perturbative and non-perturbative corrections


Different mass definitions

Pole mass scheme

MS mass

Chose constants minimal to cancel 1/ poles in

1S mass

Potential subtracted mass

Each scheme well defined in perturbation theory conversion possible

Position of would-be 1S boundstate in e+e- tt

[Hoang, Teubner 99]

[Beneke 98]


Conversion between schemes

Pole mass MS mass:

Example:

Difference is formally of higher order in coupling constant

Which scheme shall we use?

Scheme should be well defined, should lead to small perturbative corrections


Intrinsic uncertainty of the pole mass

Renormalon ambiguity in pole mass

Pole mass has intrinsic uncertainty of order QCD

[Bigi, Shifman, Uraltsev, Vainshtein 94 Beneke, Braun,94 Smith, Willenbrock 97]

Qualitatively: Expect non-perturbative corrections since full S-matrix has no pole

Quantitative understanding:

(recently confirmed by lattice studies)


A related issue: color reconnection

[Mangano, Top workshop,July 2012, CERN]

To avoid non perturbative effects, observable should not strongly rely on pt


What is currently done in experiment

ATLAS-CDF-CMS-D0 combination: [arXiv 1403.4427]

“The systematic uncertainty related to the specific MC choice is found to be marginal with respect to the possible intrinsic difference between the top-quark mass implemented in any MC and the pole mass definition”

Related uncertainty


Do we really care ?

[CMS-PAS-FTR-13-017]

See alsoJorgen’s talk

Yes, aiming for a precision of or even below 500 MeV


Alternatives: MS mass from cross section

[Langenfeld, Moch, PU 09]

Tevatron, D0

Drawback: Limited sensitivity to mt

Mass scheme well defined,higher orders can be included

Mass scheme well defined,higher orders can be included

only exp. uncertainties


Alternative Methods

1. Invariant mass of J/+ lepton in top decay / MlB

2. Top-quark mass from jet rates

3. “Endpoint method”

4. e+e- theshold scan


Towards a “global fit”…

Include top-quark cross section in PDF analysis and fitmt together with as and PDF’s in particular gluon distribution

Idea:

Correlations with as and PDF’s are automatically taken into account

Result: [Alekhin,Blümlein,Moch ’13]


Summary

Precise theoretical predictions for top-quark physics

available

So far no significant deviations from SM predictions

found

Precision physics has just started!

Very precise measurements for top-quark mass

available

Further improvements require to put more theory in


Comparison pole mass versus MS mass

[Dowling,Moch 13]

LO, NLO, NNLO

Perturbative expansion using different mass schemes:


Top mass in leptonic final states with J/

[A. Kharchilava, CMS-Note 1999-065,Phys. Lett. B476 (2000) 73,Corcella, Mangano, Seymour ’00,Chierici, Dierlamm, CMS-Note 2006-058]

Advantages: Experimentally very clean Independent from production Good sensitivity

Disadvantages: Small branching fraction Relies on Monte Carlo modeling Which mass do we measure ?


Top mass in leptonic final states with J/

Recent progress: [Biswas, Melnikov, Schulze ‘10]

linear fit

Slope difference of 0.01 compared to MC results 3 GeV shift

NLO correctionsare important

Further studies required


Top-quark mass from jet-rates (ttj)

Similar to b-quark mass measurement at LEPusing 3-jet rates [Bilenky, Fuster, Rodrigo, Santarmaria ‘95]

Use tt+1-jet events

[S. Alioli, P.Fernandez, J.Fuster, A. Irles, S. Moch, PU, M. Vos, to appear]

Large event rates (~30 % of inclusive tt events)

NLO corrections available

NLO+shower available

[Dittmaier, PU, Weinzierl ´07,´08, Melnikov, Schulze ’10, Melnikov, Scharf, Schulze ´12]

[Alioli, Moch, PU ´11,Kardos, Papadopoulos, Trocsanyi ‘11]

Less sensitive to color reconnection Mass parameter fixed through NLO calculation MS mass in principle possible


Top-quark mass from jet rates

To enhance mass sensitivity study:

with

i.e. m0 = 170 GeV



Mass dependence

high energy threshold

Crossing due to normalization



Sensitivity


25.5

17

8.5


New physics scenarios

[JHEP02(2014)107]

No evident explanation in terms of new physics

New physics further constraint if additional observables are included

[Delaunay, Gedalia, Hochberg,Perez, Soreq `11]EFT approach


Uncertainty estimates I

Non-perturbative effects at the LHC

Simulate top mass measurement using different models/tunesfor non-perturbative physics / colour reconnection

[Skands,Wicke ‘08]

Non-perturbativeeffects result in uncertainty

of the order of 500 MeV

blue: pt-ordered PSgreen: virtuality ordered PSoffset from generated mass

different offset for different tunes!


Uncertainty estimates II

Suppose top-quark form T-mesons and would not decay:

HQET:

do not depend on mt and are calculable in HQET

Estimate from B-physics / QCD sum rules:

Identifying

would lead to a systematic (calculable) shift of 500 MeV

Top-quark physics ─ Theoretical issues ─ Peter Uwer Rencontre de Blois, 20.05.2014 GK1504.

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Transcript of Top-quark physics ─ Theoretical issues ─ Peter Uwer Rencontre de Blois, 20.05.2014 GK1504.