tb ¯ NLO predictions on the ratio of t ¯ b and t ¯ tjj cross sections at the LHC Giuseppe Bevilacqua INFN - LNF HP2: High Precision for Hard Processes GGI, Florence - September 3, 2014 In collaboration with M. Worek (RWTH Aachen) Based on JHEP 1407 (2014) 135 , arXiv:1403.2046 [hep-ph] G. Bevilacqua HP2 Workshop 2014 1/26
Introduction and motivations After the discovery of a Higgs boson, the focus is now on the precision measurement of its couplings tH ( H → b ¯ pp → t ¯ b ) is a benchmark channel: • gives direct access to the top-Higgs and bottom-Higgs Yukawa couplings • benefits from new strategies to improve signal-to-background separation Biswah, Frederix, Gabrielli and Mele, arXiv:1403.1790 [hep-ph] � ` Experimental signature (semi-leptonic ch.) � W � - One isolated lepton + missing E T g t b - High jet multiplicity with multiple b -tags b t H t Challenges b g t b - Identification of b -jets ( b -tagging) 0 q - Reconstruction of top and H decays + W tb ¯ - Large QCD backgrounds: t ¯ b , t ¯ tjj q Accurate knowledge of dominant QCD backgrounds is fundamental G. Bevilacqua HP2 Workshop 2014 2/26
tb ¯ t ¯ b / t ¯ tjj backgrounds: what we learned b : large NLO QCD corrections ( ∼ 77% ) using µ 2 = m 2 pp → t ¯ tb ¯ t Dynamical scale choice improves stability: µ 2 = m t √ p T,b p T, ¯ b Bredenstein, Denner, Dittmaier and Pozzorini, 1001.4006 [hep-ph] pp → t¯ tb¯ pp → t¯ tb¯ b + X b + X σ [fb] σ [fb] 6000 6000 LO LO NLO NLO 5000 5000 R = m t √ p T , b p T , ¯ R = m t √ p T , b p T , ¯ µ 2 b ξ 2 µ 2 b ξ 2 4000 4000 µ 2 F = m t √ p T , b p T , ¯ b ξ 2 µ 2 F = m t √ p T , b p T , ¯ b /ξ 2 m b¯ b > 100 GeV m b¯ b > 100 GeV 3000 3000 2000 2000 1000 1000 0 0 0 . 125 0 . 25 0 . 5 1 2 4 8 0 . 125 0 . 25 0 . 5 1 2 4 8 ξ ξ d σ � fb � pp → t¯ tb¯ d σ � fb � pp → t¯ tb¯ b + X b + X d p T , b1 GeV d m b¯ GeV b 100 10 LO LO NLO NLO 10 1 1 m b¯ b > 100 GeV m b¯ b > 100 GeV 0 . 1 0 . 1 0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400 m b¯ b [GeV] p T , b 1 [GeV] G. Bevilacqua HP2 Workshop 2014 3/26
tb ¯ t ¯ b / t ¯ tjj backgrounds: what we learned tjj : NLO QCD corrections are fairly moderate using µ 2 = m 2 pp → t ¯ t G.B., Czakon, Papadopoulos and Worek, arXiv:1108.2851 [hep-ph] √ s = 7 TeV √ s = 7 TeV HELAC-NLO HELAC-NLO √ s = 7 TeV √ s = 7 TeV HELAC-NLO HELAC-NLO G. Bevilacqua HP2 Workshop 2014 4/26
tb ¯ t ¯ b / t ¯ tjj backgrounds: state of the art Fixed order @ NLO tb ¯ p ) → t ¯ • pp ( p ¯ b Bredenstein, Denner, Dittmaier and Pozzorini (2009, 2010) G.B, Czakon, Papadopoulos, Pittau, Worek (2009); Worek (2011) p ) → t ¯ • pp ( p ¯ tjj G.B, Czakon, Papadopoulos and Worek (2010, 2011) PS matching @ NLO • pp → t ¯ tb ¯ b Kardos and Trocsanyi (2013), Garzelli, Kardos and Trocsanyi (2014) Cascioli, Maierhoefer, Moretti, Pozzorini and Siegert (2013) PS + full jet merging @ NLO • pp → t ¯ t + 0 , 1 , 2 jets Hoeche, Krauss, Maierhoefer, Pozzorini, Schonherr and Siegert (2014) Residual scale uncertainties at the level of 20% G. Bevilacqua HP2 Workshop 2014 5/26
The cross section ratio Idea tb ¯ tb ¯ • Instead of extracting the cross section for pp → t ¯ b , measure the t ¯ b production rate normalized to the total t ¯ tjj sample: tb ¯ R = σ ( pp → t ¯ b ) σ ( pp → t ¯ tjj ) See studies in: CMS PAS TOP-12-024 and CMS PAS TOP-13-010 Advantages • More accurate measurement: common systematics are cancelled in the ratio (jet reconstruction efficiency, luminosity ...) • More accurate prediction[?]: theoretical uncertainties might be reduced in case the two processes are correlated tb ¯ How strong are correlations between t ¯ b and t ¯ tjj backgrounds? G. Bevilacqua HP2 Workshop 2014 6/26
Existing calculations are based on different setups, parameters, PDFs ... This makes a determination of the cross section ratio possible only at the price of introducing undesired additional theoretical uncertainties tb ¯ We want to perform a systematic NLO analysis of t ¯ b and t ¯ tjj backgrounds and extract predictions for the cross section ratio Our goals tb ¯ • analyse possible correlations between t ¯ b and t ¯ tjj • assess realistic theoretical uncertainties • assist LHC searches and compare with the available data (CMS) Caveat • assuming stable top quarks ( ∗ ) • this is a fixed-order analysis ( ∗ ) At LO, the impact of top quark decays on the ratio is less than 5% G. Bevilacqua HP2 Workshop 2014 7/26
Outline of the analysis • setup of the kinematical range tb ¯ • analysis of the t ¯ t system and jet activity in t ¯ b and t ¯ tjj • predictions on the ratio and its scale uncertainty • comparison with the available CMS data at √ s = 8 TeV G. Bevilacqua HP2 Workshop 2014 8/26
I. Setting up the range G. Bevilacqua HP2 Workshop 2014 9/26
As a preliminary step, we need to identify the kinematical region where our fixed-order predictions can be considered reliable A comparison with results matched to Parton Shower helps us to estimate which phase space regions can be safely investigated within our analysis Let’s focus on the benchmark process pp → t ¯ tjj and compare genuine fixed order (LO) predictions with results matched to PYTHIA 6.4 shower (LO+PS) Basic setup: √ s = 8 TeV p T ( j ) > 20 GeV | y ( j ) | < 2 . 5 ∆ R ( jj ) > 0 . 5 CT09MC1 PDF anti- k T algorithm µ R = µ F = m t = 173 . 5 GeV G. Bevilacqua HP2 Workshop 2014 10/26
pp → t ¯ tjj : LO vs LO+PS results G.B and M.Worek, arXiv:1403.2046 [hep-ph] p T ( j 1 ) p T ( j 2 ) p T ( t ¯ M ( jj ) tj 1 ) j 1 ( j 2 ) = 1 st (2 nd ) hardest jet G. Bevilacqua HP2 Workshop 2014 11/26
Interpretation: p T ( t ¯ p T ( j 2 ) tj 1 ) • LO: kinematics sets p T ( t ¯ tj 1 ) = p T ( j 2 ) ⇒ the two distributions coincide • LO+PS: correlation between the two observables is lost due to extra jet activity. Sudakov suppression starts below p T ( t ¯ tj ) ≃ 40 GeV • Dominant higher-order effects are likely to endanger perturbative stability at low p T ’s. Resummation of higher orders is needed Special restrictions on jet p T are required for a safe fixed-order analysis G. Bevilacqua HP2 Workshop 2014 12/26
Final setup Phase space cuts • p T ( j ) > 40 GeV , | y ( j ) | < 2 . 5 , ∆ R ( jj ) > 0 . 5 , anti- k T jet algorithm Scale choice F ≡ m t √ p T b p T ¯ • t ¯ tb ¯ µ 2 R = µ 2 b : arXiv:1001.4006 [hep-ph] b • t ¯ µ 2 R = µ 2 F ≡ m 2 tjj : arXiv:1002.4009 [hep-ph] t • scale uncertainty estimated by varying scales up and down by a factor 2 PDF set • CT09MC1 (LO), CT10 (NLO) Collider energies • √ s = 7 , 8 , 13 TeV NLO results obtained with the help of the package HELAC-NLO HELAC-NLO Collab., Comput.Phys.Commun. 184 (2013) 986-997, arXiv:1110.1499 [hep-ph] G. Bevilacqua HP2 Workshop 2014 13/26
II. Looking for correlations G. Bevilacqua HP2 Workshop 2014 14/26
Dominant production channels tb ¯ pp → t ¯ b gg channel pp → t ¯ tjj gg channel qg channel Interplay of different mechanisms: what’s the impact on correlations? G. Bevilacqua HP2 Workshop 2014 15/26
Differential cross sections Comparing NLO shapes: distributions normalized to unit 1. Transverse momentum of jets p T ( j 1 ) p T ( j 2 ) tb ¯ t ¯ tjj has (slightly) harder p T spectrum than t ¯ b G. Bevilacqua HP2 Workshop 2014 16/26
Differential cross sections Comparing NLO shapes: distributions normalized to unit 2. Rapidity of jets y ( j 1 ) y ( j 2 ) b -jets from t ¯ tb ¯ b prefer central regions of the detector G. Bevilacqua HP2 Workshop 2014 17/26
Differential cross sections Comparing NLO shapes: distributions normalized to unit 3. Invariant mass and ∆ R of two hardest jets M ( jj ) ∆ R ( jj ) tb ¯ Jet pairs from t ¯ b prefer small-angle emission G. Bevilacqua HP2 Workshop 2014 18/26
In summary • different production mechanisms dominate the two background processes tb ¯ • t ¯ b and t ¯ tjj show different properties in the jet activity, mainly in angular and invariant mass distributions What can be said about the underlying t ¯ t production? G. Bevilacqua HP2 Workshop 2014 19/26
Differential cross sections Comparing shapes at NLO: distributions normalized to unit 4. Invariant mass and p T of the t ¯ t system M ( t ¯ t ) p T ( t ) The underlying t ¯ t production shows a stronger correlation G. Bevilacqua HP2 Workshop 2014 20/26
III. tb ¯ NLO predictions for the ratio t ¯ b / t ¯ tjj G. Bevilacqua HP2 Workshop 2014 21/26
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