A new observable to measure the top-quark mass at hadron colliders. - - PowerPoint PPT Presentation

A new observable to measure the top-quark mass at hadron colliders.

Simone Alioli

LBNL & UC Berkeley Seattle, 2 July 2013 EF Snowmass Meeting

based on arXiv:1303.6415 SA, P . Fernandez, J. Fuster, A. Irles, S. Moch, P . Uwer, M. Vos

Motivations for precise mt measurements

◮ Fundamental parameter of SM Lagrangian ◮ The top sector might play a role in EWSB ◮ Important parameter in SM (and MSSM)

fits

168 170 172 174 176 178

mt [GeV]

80.30 80.40 80.50 80.60

MW [GeV]

MSSM MH = 123 GeV MH = 127 GeV SM Mh = 123 .. 127 GeV

MSSM, Mh = 123..127 GeV SM, MSSM Heinemeyer, Hollik, Stockinger, Weiglein, Zeune ’12

experimental errors 68% CL: LEP2/Tevatron: today LHC: future ILC/GigaZ

◮ Crucial for vacuum (meta-)stability of SM at NNLO DeGrassi et al. ’12 Alekhin, Djouadi, Moch ’12

102 104 106 108 1010 1012 1014 1016 1018 1020 0.04 0.02 0.00 0.02 0.04 0.06 0.08 0.10 RGE scale Μ in GeV Higgs quartic coupling ΛΜ Mh 125 GeV 3Σ bands in Mt 173.1 0.7 GeV ΑsMZ 0.1184 0.0007 Mt 171.0 GeV ΑsMZ 0.1163 ΑsMZ 0.1205 Mt 175.3 GeV

Instability 107 1010 1012 115 120 125 130 135 165 170 175 180 Higgs mass Mh in GeV Pole top mass Mt in GeV 1,2,3 Σ Instability Stability Metastability

Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 2

Theoretical issues in determination of top-quark mass

◮ Confinement = free quarks not observable = no pole in the S-matrix ◮ Parameters of the theory measured through their influence on hadronic

• bservables: fit Oexp(

x) with Oth(mt, x) and extract mt

Which mass are we measuring ? At least NLO required to fix the ren. scheme.

◮ Precise value depends on the mt definition: mpole t

, mMS

t

, etc. Which scheme ? Some show better convergence (e.g. mMS

t

), some ill-defined beyond PT (IR renormalons ∆mpole

t

∝ ΛQCD)

◮ Color reconnections Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 3

(Some) top-quark mass measurements at hadron colliders

◮ Template method - Ideogram method

✪ mMC

t

= mpole

t

(1 ± ∆), ∆ =? , LO

high-precision

◮ Matrix element method

✪ LO only, NLO under develop.

high-precision

◮ Cross section

theoretically clean, NLO, finite Γt

✪ reduced sensitivity, threshold eff. included

◮ J/ψ method

NLO, small sensitivity to JES unc. and top reco.

✪ finite Γt, very-high statistics required

◮ Dilepton-specific

NLO, JES unc., top reco., finite Γt

✪ reduced sensitivity, high statistics required

◮ Kinematic endpoint

NLO?, small sensitivity to top reco.

✪ JES, finite Γt?

Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 4

Current results

Tevatron LHC

◮ Dedicated studies of top-quark

mass wrt event kinematics show small dependence ⇒ mismodelling is small at current precision.

CMS-TOP-12-029 ATLAS-PHYS-PUB-2013-005 Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 5

New∗ proposal : top-quark mass from jet rates

∗cfr.mbfrom 3-jets rate @ LEP [Bilenky et al. ’95]

◮ Study t¯

t + 1-jet events : large rate at the LHC ( 30%), NLO and NLO+PS

available

◮ Experimentally accessible, errors reduced through normalization factor

R(mpole

t

, ρs) = 1 σt¯

t+1-jet

dσt¯

t+1-jet

dρs (mpole

t

, ρs) , ρs = 2m0 √st¯

tj

, m0 = 170 GeV

◮ Theoretically well defined, calculable at NLO, small uncertainties and small

NP corrections, R(mMS

t

, ρs) also possible. Low ρS control region st¯

tj reco.

)

s

ρ ,

pole t

(m

R

0.5 1 1.5 2 2.5 3 3.5 NLO LO

s

ρ

0.2 0.3 0.4 0.5 0.6 0.7 0.8

ratio

0.5 1 1.5

Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 6

Sensitivity on mt

◮ Linear approximation S(ρs) =

• η=±1

|R(mpole

t

, ρs) − R(mpole

t

+ η∆mpole

t

, ρs)| 2|∆|R(mpole

t

, ρs)

ρ

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

]

• 1

[GeV

)

ρ

( S

0.05 0.1 0.15

)

ρ

( S

×

t pole

m

25.5 17 8.5 +1Jet t t = 10 GeV

pole t

m ∆ = 5 GeV

pole t

m ∆ t t = 10 GeV

pole t

m ∆ = 5 GeV

pole t

m ∆

• ∆R

R

• ≈
• mpole

t

S

• ×
• ∆mpole

t

mpole

t

• ◮ Up to five times more sensitive than total xsec, ∆σ

σ ≈ −5∆mpole

t

mpole

t Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 7

Experimental viability study

◮ Event selection (lepton+jets):

• 1. one lepton (ℓ = e, µ) with pT > 25 GeV and

|η| < 2.5;

• 2. missing ET > 30 GeV
• 3. M W

T

> 35 GeV

• 4. ≥ 3 jets within |η| < 2.5, hardest with

pT > 50 GeV, other two pT > 25 GeV;

• 5. two additional identified b-jets
• 6. two light jets inv. mass compatible with mW

within 20%

• 7. two reconstructed top-jet system masses

within 20%

◮ Background contamination kept at the 5-10% level : : QCD (1,2) , single

top and W+jets (3,4,5)

◮ Preliminary study with no detector-specific tools.

Room for improvement when done by experimental collaborations.

◮ Dilepton channel also possible, but reduced statistics. Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 8

Theoretical uncertainties

◮ Scale and PDF uncertainties: ∆Rµ/R(mpole

t

,ρs) S(ρs)

,

∆RPDF/R(mpole

t

,ρs) S(ρs) ◮ Scale unc. ∆mpole t

≈ 0.2 GeV

PDF unc. ∆mpole

t

≈ 0.5 GeV

Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 9

Theoretical uncertainties

◮ Impact of higher-orders and parton showers: calculate RNLO and extract

top-quark mass that would fit the distribution from generated events

s

ρ

0.2 0.3 0.4 0.5 0.6 0.7 0.8

[GeV]

pole t

m

160 165 170 175 180 at NLO + Pythia8 t POWHEG t +1Jet at NLO + Pythia8 t POWHEG t +1Jet NLO (Pert. Calc.) t t

◮ POWHEG+Pythia vs. MC@NLO+Herwig gives ∆mpole t

≈ 0.2 GeV

Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 10

Theoretical uncertainties

◮ Colour reconnection effects: different CR models in Pythia6 vs. Pythia8 ◮ Switching CR on/off very conservative estimate: ∆mpole t

≤ 0.4 GeV

Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 11

Experimental uncertainties

◮ Jet Energy Scale uncertainty ±3% results in ∆mpole t

≈ 0.8 − 1.0 GeV

)

s

ρ ,

pole t

(m

R

0.5 1 1.5 2 2.5 3 3.5

=160 GeV

pole t

m =180 GeV

pole t

m =170 GeV Nominal

pole t

m 3% ± =170 GeV JES

pole t

m

s

ρ

0.2 0.3 0.4 0.5 0.6 0.7 0.8

ratio

1 2 3

Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 12

Experimental uncertainties

◮ Mass independent unfolding: associated unc. ∆mpole t

≈ 0.3 GeV (stat.)

)

s

ρ ,

pole t

(m

R

0.5 1 1.5 2 2.5 3 3.5 = 160 GeV

pole t

m True Unfolded

s

ρ

0.2 0.3 0.4 0.5 0.6 0.7 0.8

ratio

0.95 1 1.05

◮ Assuming final efficiency ≈ 1% and 5 fb−1 collected luminosity, expected

error is ≈ 1.4 GeV stat. in the ρs > 0.65 bin.

◮ Extrapolated to 20 fb−1 ∆mpole t

≈ 0.7 GeV

Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 13

Conclusions

◮ Top-quark physics is precision physics at the LHC ◮ Current precision O(1 GeV) already impressive. ◮ Theoretical interpretation not so well under control. ◮ At least NLO needed to fix renormalization scheme. ◮ Several methods availables for NLO top mass, important to take advantage

• f all of them.

◮ Observable proposed here complements existing approaches. NLO

top-quark mass definition, theoretical and experimental uncertainties evaluated at O(1 GeV) or below

Outlook:

◮ Analysis presented here being performed by ATLAS group in Valencia. ◮ LHC upgrade top-factory: 300 fb−1 at 13 TeV will produce ≈ 50M ttbar

events in the lepton+jet channel, 10M events in the dilepton channel, 15M single tops

◮ Extreme precision in e+e− : threshold scans at LC will reach O(0.1GeV)

with dedicated run, theoretically very clean (N3LO and NNLL).

Thank you for your attention!

Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 14

BACKUP

Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 15

Scale and PDF variations

)

s

ρ ,

pole t

(m

R

0.5 1 1.5 2 2.5 3 3.5

CTEQ6.6 MSTW2008nlo90cl

s

ρ

0.2 0.3 0.4 0.5 0.6 0.7 0.8

ratio

0.5 1 1.5

Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 16

Impact of NLO fixed-order corrections

s

ρ

0.2 0.3 0.4 0.5 0.6 0.7 0.8

[GeV]

pole t

m

160 165 170 175 180

LO NLO

Simone Alioli | Top mass | EF Snowmass Seattle 07/02/2013 | page 17