off-shell effects in t + /z production at the lhc · arxiv:1907.09359 [hep-ph] arxiv:1912.09999...
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Off-shell effects in tt + γ/Zproduction at the LHC
Giuseppe Bevilacqua
MTA-DE Particle Physics Research Group, Debrecen
LHCP2020
May 25, 2020
In collaboration with H. B. Hartanto, M. Kraus, T. Weber and M. Worek
Based on: arXiv:1803.09916 [hep-ph] arXiv:1809.08562 [hep-ph]
arXiv:1907.09359 [hep-ph] arXiv:1912.09999 [hep-ph]
Introduction
In the absence of convincing evidence for new resonances effects, precisemeasurements of SM observables are key to look for effects of New Physics(NP) at the LHC
This is especially true for the top quark sector, where NP effects are expectedto be more prominent due to the large mass scale
https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsCombined
tt, ttj
ttγ
ttZ
As luminosity increases, LHCallows to probe associated ttchannels with more precision
We’ll focus on tt+ γ andtt+ Z(Z → νν) productionat Run II of the LHC
G. Bevilacqua LHCP2020 2/22
Motivationsttγ
- Direct probe of top quark electric charge Melnikov et al. ’11 ...
↪→ σttγ ∼ Q2t , while indirect through other processes
- Probe of the effective ttγ interaction Baur et al. ’05 Aguilar Saavedra ’09
↪→ - dimension-six SM EFT Schulze et al. ’16 Maltoni et al. ’16 ...
- top-quark anomalous couplings ...
- Irreducible background to direct BSM searches
ttZ
- Direct probe of ttZ coupling Rontsch and Schulze ’15 ...
↪→ search for deviations from SM expectations
- Dark Matter searches at LHC (tt + EmissT ) Arina et al. ’16 Haisch et al. ’16
↪→ in several models, DM couples preferentially to top quarks
- Background to searches for rare processes↪→ e.g. search of same-sign leptons + jets + EmissT
G. Bevilacqua LHCP2020 3/22
Predictions for ttγ/Z at LHC: state of the artttγ
- NLO QCD & EW: on-shell, inclusive Duan et al. ’09,’11,’16 Maltoni et al. ’16
↪→ top quarks undecayed: general idea about size of NLO corrections
- NLO QCD: on-shell, with radiative decays Melnikov, Scharf and Schulze ’11
↪→ QCD corrections to production and decays, with spin correlations and γ radiation from decays
- NLO QCD: on-shell, with PS effects Kardos and Trocsanyi ’14
↪→ top decays in PS, no spin correlations, only direct-photon contribution
- NLO QCD: complete off-shell G.B, Hartanto, Kraus, Weber and Worek ’18
↪→ resonant and non-resonant diagrams, interferences and finite-width effects (dilepton channel)
ttZ
- NLO QCD: on-shell, inclusive Lazopoulos et al. ’08
- NLO QCD: on-shell, with NLO decays Rontsch and Schulze ’14
- NLO QCD & EW: on-shell, with PS effects Kardos et al. ’12 Frixione et al. ’15
- NLO QCD & EW + NNLL Kulesza et al. ’18, ’20 Broggio et al. ’17, ’19
- NLO QCD: complete off-shell G.B, Hartanto, Kraus, Weber and Worek ’19
G. Bevilacqua LHCP2020 4/22
Narrow Width Approximation
Based on the narrow-width limit:
• cross section factorizes into on-shell tt (+X) production ⊗ decays↪→ only double-resonant diagrams are retained
• non-factorizable contributions are suppressed by powers of Γt/mt = O(1%)for sufficiently inclusive observables
• huge simplification, but several contributions have to be put together to accountfor all resonant structures...
e.g. ttγ
G. Bevilacqua LHCP2020 5/22
Beyond NWARepresentative Feynman diagrams for the case of ttZ(Z → νν):
double-resonant (DR) single-resonant (SR) non-resonant (NR)
• all resonant and non-resonant contributions are taken into account
• most complete calculation of ME’s at fixed perturbative order
• computationally demanding: O(10) increase in complexity w.r.t. NWA
”Off-shell effects” = finite-width effects for tops and W ’s
+ SR + NR
+ interferences among DR, SR, NR
G. Bevilacqua LHCP2020 6/22
The HELAC-NLO framework
→ C++/ROOT analysis framework for event Ntuples- reweighting for multiple scale/PDFs
- kinematical selection of events
- histogramming tools
G. Bevilacqua LHCP2020 7/22
Predictions for ttγ at√s = 13 TeV
G. Bevilacqua LHCP2020 8/22
Predictions for off-shell ttγ
10−2
10−1
dσ/dpT,γ
[fb/GeV
] LO
NLO
0 50 100 150 2000.60.81
1.21.4
pT,γ [GeV]
NLO
/LO
pp→ tt(γ)→ bbe+νeµ−νµγ @ 13 TeV-dilepton channel-
µ = mt/2
HELAC-NLO
G.B., Hartanto, Kraus, Weber and Worek, arXiv:1803.09916 [hep-ph]
10−2
10−1
dσ/dpT,γ
[fb/GeV
] LO
NLO
0 50 100 150 2000.60.81
1.21.4
pT,γ [GeV]
NLO
/LO
µ = HT /4
HELAC-NLO
Dynamical scale µR = µF = HT /4 improves perturbative stabilityHT =
∑i={b,b,`±,γ} pTi + 6pT
Comparative analysis of different approaches of ttγ modeling in progressATLAS-CONF-2019-042
G. Bevilacqua LHCP2020 9/22
ttγ: off-shell vs on-shell
10−5
10−4
10−3
10−2
10−1
100
dσ/p
T(b
avg)
[fb/G
eV]
off-shell NWA (HT /4)
NWA LOdec (HT /4) NWA (mt/2)
NWA LOdec (mt/2)
0.6
0.8
1
Rat
ioto
off-s
hel
l
0 100 200 300 400 500 600
0.6
0.8
1
pT (bavg)
Rati
oto
off-s
hel
lImpact of off-shell effects
and radiative decays
pp→ tt(γ)→ bbe+νeµ−νµγ @ 13 TeV-dilepton channel-
G.B., Kraus, Hartanto, Weber and Worek, arXiv:1912.09999 [hep-ph]
HELAC-NLO
10−4
10−3
10−2
10−1
100
dσ/M
(bl+
) min
[fb/G
eV]
off-shell NWA (HT /4)
NWA LOdec (HT /4) NWA (mt/2)
NWA LOdec (mt/2)
0.4
0.6
0.8
1
1.2
Rat
ioto
off
-shel
l
0 20 40 60 80 100 120 140 160 180 200
0.5
1
M(bl+)min
Rat
ioto
off-s
hel
l
HELAC-NLO
σNLO, NWA = 7.33 fb ↔ σNLO, NWA (LO decays) = 4.63 fb
G. Bevilacqua LHCP2020 10/22
Predictions for off-shell ttγ
10−6
10−5
10−4
10−3
10−2
10−1
100
101
dσ/pT(b
avg)[fb/G
eV]
full NWADR SRNR
0 100 200 300 400 500 6000
0.25
0.50
0.75
1
pT (bavg)
Ratio
tofull
Analysis of Double-, Single- andNon-Resonant phase space regionsKauer and Zappenfeld ’01 ...
pp→ tt(γ)→ bbe+νeµ−νµγ @ 13 TeV-dilepton channel-
G.B., Kraus, Hartanto, Weber and Worek, arXiv:1912.09999 [hep-ph]
HELAC-NLO
10−7
10−5
10−3
10−1
101
dσ/M
(bl+) m
in[fb/G
eV]
full NWADR SRNR
0 20 40 60 80 100 120 140 160 180 2000
0.25
0.50
0.75
1
M(bl+)min
Ratio
tofull
HELAC-NLO
DR → |M(t)−mt| < nΓt ∧ |M(t)−mt| < nΓt (n = 15)
NR → |M(t)−mt| > nΓt ∧ |M(t)−mt| > nΓt
SR → full− (DR + NR)
G. Bevilacqua LHCP2020 11/22
Predictions for ttZ(Z → νν) at√s = 13 TeV
G. Bevilacqua LHCP2020 12/22
tt+ EmissT
While leading to the same visible final state, off-shell tt and ttZ(Z → νν)have pretty different kinematics
0 100 200 300 400 500
10−5
10−4
10−3
10−2
10−1
pT,miss [GeV]
1/σ·dσ/dpT,m
iss
[1/G
eV]
tt ttZHELAC-NLO
200 400 600 800 1000 1200 1400
10−6
10−5
10−4
10−3
10−2
HT [GeV]
1/σ·dσ/dH
T[1
/GeV
]
tt ttZHELAC-NLO
G.B, Hartanto, Kraus, Weber and Worek, arXiv:1907.09359 [hep-ph]
[distributions normalized to one]
ttZ populates high pmissT regions which are important for BSM studies.However tt dominates over ttZ(Z → νν) by several orders of magnitude (atinclusive level).
↪→ Is it important to strive for higher accuracy in ttZ? Yes!
G. Bevilacqua LHCP2020 13/22
Determining the CP nature of spin-0 mediators in tt + DM productionHaisch, Pani and Polesello, arXiv:1611.09841 [hep-ph]
(tt, tW , ttW ) (WW,WZ,ZZ...)
(ttZ)−→
- Distribution of events in the(EmissT ,mT2) plane for thedifferent backgrounds andfor one example of DM signal[Mφ = 100 GeV , Mχ = 1 GeV ]
- The area in the upper rightcorner above the black lineis the region selected in theanalysis
- With 300 fb−1, assuming 20% systematics for SM backgrounds, it should be possibleto resolve the (pseudo-)scalar nature of the mediator up to mass ≈ 200 GeV
- Discovery reach depends on syst. uncertainty of SM backgrounds, dominated by ttZ
G. Bevilacqua LHCP2020 14/22
Off-shell ttZ(Z → νν): differential cross sections
10−1.5
10−1
dσ/d
cosθ l
l[fb]
mt +mZ/2 HT /3 ET /3
E′T /3 E′′T /3
1
1.1
1.2
1.3
NLO
/LO
0 0.2 0.4 0.6 0.8 1
1
1.5
cos θll
scaledep
enden
ce LO mt +mZ/2 HT /3
ET /3 E′T /3 E′′T /3
HELAC-NLO
G.B, Hartanto, Kraus, Weber and Worek, arXiv:1907.09359 [hep-ph]
cos θll = tanh(∆yll/2)
- Sensitive to the nature of DM mediatorHaisch et al. ’16
- Differential K-factors far from constant!
- µ = mt +mZ/2 : +4%↔ 27%
- µ = HT /3 : −5%↔ 10%
- µ = E′′T /3 : −1%↔ 14%
G. Bevilacqua LHCP2020 15/22
Summary
• Achieving the highest precision for tt+ V requires to include non-resonant,finite-width and interference effects.
• Systematic comparisons between predictions at different levels of accuracy(on-shell vs off-shell) are important to assess the reliability of variousapproximations
• We have computed predictions for a variety of off-shell tt+X processes(dilepton channel) with the package HELAC-NLO:
- pp→ tt+ γ - pp→ tt+ Z
- pp→ tt+ j - pp→ tt+W± NEW! arXiv:2005.09427 [hep-ph]
• Our predictions are available in the form of ROOT Ntuples. We can provide themalong with a user-friendly software to extract physical results for your analysis.If interested, contact us!
A Zoom meeting room is available for further discussions.Please find the details on next slide →
G. Bevilacqua LHCP2020 16/22
Details of Zoom Meeting room (May 25, 16:30-17:00 CERN time)
Topic: Giuseppe Bevilacqua’s Zoom Meeting
Date, Time: May 25, 2020 04:30 PM Paris
Click here to join
(same password of this parallel session)
In case of problems, just email me( giuseppe.bevilacqua AT science.unideb.hu )
G. Bevilacqua LHCP2020 17/22
Backup slides
G. Bevilacqua LHCP2020 18/22
Off-shell ttγ: fixed vs dynamical scalesA collection of distributions based on the scale choice µR = µF = mt/2
0
2
4
6
dσ/d∆Rb1,γ
[fb
]
LO
NLO
0 1 2 3 4 5
1
2
3
∆Rb1,γ
NL
O/
LO
G.B., Hartanto, Kraus, Weber and Worek, arXiv:1803.09916 [hep-ph]
0
2
4
6
dσ/d∆Rl 1,γ
[fb
]
LO
NLO
0 1 2 3 4 5
1
2
3
∆Rl1,γ
NL
O/
LO
0
2
4
6
dσ/d∆Rl 2,γ
[fb
]
LO
NLO
0 1 2 3 4 5
123
∆Rl2,γ
NL
O/
LO
10−3
10−2
10−1
dσ/dpT,b
1[fb/GeV
] LO
NLO
0 100 200 300 400
0.5
1
1.5
pT,b1 [GeV]
NLO
/LO
10−3
10−2
10−1
dσ/dpT,b
2[fb/GeV
] LO
NLO
0 50 100 150 200
0.5
1
pT,b2 [GeV]
NLO
/LO
10−3
10−2
10−1
dσ/dpT,l1[fb/GeV
] LO
NLO
0 100 200 300
0.5
1
pT,l1 [GeV]
NLO
/LO
G. Bevilacqua LHCP2020 19/22
Off-shell ttγ: fixed vs dynamical scalesA collection of distributions based on the scale choice µR = µF = HT /4
0
2
4
6
dσ/d∆Rb1,γ
[fb
]
LO
NLO
0 1 2 3 4 5
1
2
3
∆Rb1,γ
NL
O/
LO
G.B., Hartanto, Kraus, Weber and Worek, arXiv:1803.09916 [hep-ph]
0
2
4
6
dσ/d∆Rl 1,γ
[fb
]
LO
NLO
0 1 2 3 4 5
1
2
3
∆Rl1,γ
NL
O/
LO
0
2
4
6
dσ/d∆Rl 2,γ
[fb
]
LO
NLO
0 1 2 3 4 5
123
∆Rl2,γ
NL
O/
LO
10−3
10−2
10−1
dσ/dpT,b
1[fb/GeV
] LO
NLO
0 100 200 300 400
0.5
1
1.5
pT,b1 [GeV]
NLO
/LO
10−3
10−2
10−1
dσ/dpT,b
2[fb/GeV
] LO
NLO
0 50 100 150 200
0.5
1
pT,b2 [GeV]
NLO
/LO
10−3
10−2
10−1
dσ/dpT,l1[fb/GeV
] LO
NLO
0 100 200 300
0.5
1
pT,l1 [GeV]
NLO
/LO
G. Bevilacqua LHCP2020 20/22
Off-shell ttγ: comparisons to Run II data”Theory NLO” = off-shell ttγ
(HELAC-NLO)”Madgraph5” = double-resonant ttγ (LO)
ATLAS-CONF-2019-042
σexp = 44.2± 0.9 (stat)+2.6−2.4 (syst) fb = 44.2± 2.6 fb
σTheory NLO = 39.50+0.56−2.18 (scale)+1.04
−1.18 (PDF) fb
G. Bevilacqua LHCP2020 21/22
Off-shell ttZ(Z → νν): differential cross sections
Checks on dimensionful observables
G.B, Hartanto, Kraus, Weber and Worek, arXiv:1907.09359 [hep-ph]
10−4
10−3
dσ/dm
ll[fb/G
eV]
mt +mZ/2 HT /3 ET /3
E′T /3 E′′T /3
1
1.2
NLO
/LO
0 100 200 300 400 500
1
1.5
mll [GeV]
scaledep
endence LO mt +mZ/2 HT /3
ET /3 E′T /3 E′′T /3
HELAC-NLO
dilepton inv. mass
10−5
10−4
10−3
10−2
dσ/dpT,l[fb/G
eV]
mt +mZ/2 HT /3 ET /3
E′T /3 E′′T /3
0.8
1
1.2
NLO
/LO
0 50 100 150 200 250 300 350 4000.5
1
1.5
2
pT,l [GeV]
scaledep
endence LO mt +mZ/2 HT /3
ET /3 E′T /3 E′′T /3
HELAC-NLO
charged lepton’s pT
10−6
10−5
10−4
10−3
dσ/dH
T[fb/G
eV]
mt +mZ/2 HT /3 ET /3
E′T /3 E′′T /3
0.8
1
1.2
NLO
/LO
200 400 600 800 1000 1200 1400
0.5
1
1.5
2
HT [GeV]
scaledep
endence LO mt +mZ/2 HT /3
ET /3 E′T /3 E′′T /3
HELAC-NLO
transverse energy (HT )
- µ = mt + mZ/2 → NLO gets outside LO uncertainties
- µ = HT/3, ET/3, ... → improves perturbative convergence
G. Bevilacqua LHCP2020 22/22