Top-quark physics ─ Theoretical issues ─
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Top-quark physics─ Theoretical issues ─
Peter Uwer
Rencontre de Blois, 20.05.2014
GK1504
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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
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 3
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
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 4
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
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 5
Top-quark pair production ─ theory status
Incl. c
ross s
ectio
n NLO
Incl. c
ross s
ectio
n NNLO
1989
[Dawson, Ellis, Nason ’89, Beenakker et al ’89,91]
[Bernreuther, Brandenburg, Si, PU ’04
[Czakon,Mitov 08]
Spin de
pend
ent
cross
secti
on N
LO
2004
[Bonciani, Catani, Mangano,Nason ‘98, Kidonakis, Laenen, Moch, Vogt 01]
NLL re
summati
on
1998 2008
Analyt
ic res
ults N
LO
Steps t
oward
s NNLL
[Moch, PU 08]
Bound
state
effec
ts
[Kiyo,Kuhn,Moch,Steinhauser,PU 08Hagiwara, Sumion, Yokoya 08]
Full N
NLL re
summati
on
2010[Czakon,Fiedler,Mitov 13]
2013
Combin
ed N
NLL an
d 1/
[Ahrens, Beneke,Czakon,Ferroglia, Mitov, Schwinn…]
Impressive theoretical progress in last 25 yearsSimilar for single top-quark production
Many more results on differential distributions, add. jets,combination with parton shower, top decay…
Th. uncertaintybelow 5%
Further progress will require substantial effort from theory
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 6
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…
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 7
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 ttreal corr. to tt
QCD:
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 8
Forward-backward charge asymmetry (Tevatron)
Qualitative picture:
rapidity
Definitions used by CDF/D0:
Forward-backwardcharge asymmetry
Assuming CP invariance:
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 9
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 !
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 10
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)
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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
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Recent results from CDF
[CDF, Phys. Rev. D87 (2013) 092002]
9.4/fb
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Latest results from D0D0, arXiv 1405.0421
Now in agreement with SM
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 14
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
topanti-top
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 15
Charge asymmetry at LHC[CMS PAS TOP-14-006]
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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
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Top-quark mass: Recent results
Best known quark mass, shouldn’t we be happy ?
[arXiv:1403:4427]
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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:
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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
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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]
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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
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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)
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A related issue: color reconnection[Mangano, Top workshop,July 2012, CERN]
To avoid non perturbative effects, observable should not strongly rely on pt
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 24
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
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 25
Do we really care ?
[CMS-PAS-FTR-13-017]
See alsoJorgen’s talk
Yes, aiming for a precision of or even below 500 MeV
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 26
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
only exp. uncertainties
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 27
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
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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]
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 29
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
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Comparison pole mass versus MS mass
[Dowling,Moch 13]
LO, NLO, NNLO
Perturbative expansion using different mass schemes:
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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 ?
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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
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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
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Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, 20.05.2014 | page 34
Top-quark mass from jet rates
To enhance mass sensitivity study:
with
i.e. m0 = 170 GeV
[S. Alioli, P.Fernandez, J.Fuster, A. Irles, S. Moch, PU, M. Vos, to appear]
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Mass dependence
high energy threshold
Crossing due to normalization
[S. Alioli, P.Fernandez, J.Fuster, A. Irles, S. Moch, PU, M. Vos, to appear]
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Sensitivity
[S. Alioli, P.Fernandez, J.Fuster, A. Irles, S. Moch, PU, M. Vos, to appear]
25.5
17
8.5
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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
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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!
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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