The Top Quark Charge Asymmetry at the LHC Recent ATLAS measurements and strategies for the LHC F. Rubbo (IFAE, Barcelona) Status aer LHC@7-8 TeV Null results from direct The Higgs discovery completed the beyond-SM searches Standard Model at the ElectroWeak scale up to ~1 TeV - The Top Charge Asymmetry at the LHC - 2 The Top Quark – gateway towards New Physics Important role in EW • Heaviest parWcle, m ~173 GeV top symmetry breaking • Yukawa coupling ~1 and BSM models. Measurements of top quark properWes, such as the charge asymmetry in the producWon of top quark pairs, might provide hints of new physics beyond the Standard Model. - The Top Charge Asymmetry at the LHC - 3 The Top Quark – Pair producWon at the LHC Large rate: ~5M of top pair events in Run-I Main producWon modes: σ pp →tt ≈ 200pb - gluon fusion gg →tt ( ) - quark-anWquark annihilaon qq →tt € € € ~85% of σ _ ~15% of σ Relevant events to measure asymmetry - The Top Charge Asymmetry at the LHC - 4 Top Quark Pair Charge Asymmetry What is the Top Charge Asymmetry: Measure anisotropy in (anW-)top producWon wrt NO ASYMMETRY incoming (anW-)quark direcWon. t q q € top direcWons anW-top direcWons q q € ASYMMETRY € t € C.M. reference frame € q q € N.B. requires asymmetric inial state (e.g. quark-an,quark annihila,on€ ) € - The Top Charge Asymmetry at the LHC - 5 Outline • MoWvaon – the A FB puzzle • The asymmetry at the LHC - A C • Early measurements at the LHC: ATLAS lepton+jets 1.04 g-1 at √s=7 TeV Eur.Phys.j. C72 (2012) 2039 • BoosWng the charge asymmetry Phys.Le. B707 (2012) 92-98 • Charge asymmetry at ATLAS: lepton+jets 4.7 g-1 at √s=7 TeV jHEP 02 (2014) 107 • A C at √s=14 TeV with a photon handle arxiv:1402.3598 - The Top Charge Asymmetry at the LHC - 6 Outline • MoWvaon – the A FB puzzle • The asymmetry at the LHC - A C • Early measurements at the LHC: ATLAS lepton+jets 1.04 g-1 at √s=7 TeV Eur.Phys.j. C72 (2012) 2039 • BoosWng the charge asymmetry Phys.Le_. B707 (2012) 92-98 • Charge asymmetry at ATLAS: lepton+jets 4.7 g-1 at √s=7 TeV jHEP 02 (2014) 107 • A C at √s=14 TeV with a photon handle arxiv:1402.3598 - The Top Charge Asymmetry at the LHC - 7 AFB @ Tevatron Afb of the Top Quark In 2011 both CDF and D0 collaboraons V. Ahrens et. al., July 2011 arXiv:1106.6051v1 (2011) reported a non-zero forward-backward (** submitted to a journal) W. Hollik and D. Pagani, asymmetry in top quark pair producWon arXiv:1107.2606 (2011) (* preliminary) defined as: CDF LJ 0.158 ± 0.074 (±0.072 ± 0.017) -1 N(Δy > 0) − N(Δy < 0) ( 5.3 fb ) AFB = N(Δy > 0) + N(Δy < 0) CDF DIL* 0.420 ± 0.158 (±0.150 ± 0.050) -1 rapidity ( 5.1 fb ) with Δy = ytop – yanW-top CDF combined* 0.201 ± 0.067 (±0.065 ± 0.018) (± stat ± syst) € D0 LJ** 0.196 0.060 +0.018 • Triggered theoreWcal acWvity for ± -0.026 -1 ( 5.4 fb ) precise SM predicWons and BSM hypotheses (axigluons,Z’,..). 0 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 Afb - The Top Charge Asymmetry at the LHC - 8 The SM charge asymmetry 1. 2. • SM top quark pair producWon is charge asymmetric at NLO. ⊕ ⊕ • Main source of asymmetry is the QCD quark-anWquark € annihilaon process. € asymmetric QCD The SM QCD asymmetry in originates from: qq →tt 1. Interference between tree and box diagram AFB>0 2. Interference between gluon ISR and FSR diagrams AFB<0 € - The Top Charge Asymmetry at the LHC - 9 BSM asymmetry Z’ / W’ Axigluon ( ) d € (W’) ⊕ ( ) d € t-channel: s-channel: exoWc flavor changing vector bosons interference between SM QCD and € exoWc gluons with axial coupling - The Top Charge Asymmetry at the LHC - 10 AFB @ Tevatron • iniWal state pp forward-backward asymmetry • qq → dominant process (~85%)tt small diluWon -1 • updated results with full datasets (~10 g ) sWll report AFB consistently larger than SM predicWon. € AFB 2.7σ from SM € t q q € updated 9.7 -1 p p € € t € Lab reference frame € j.A. Aguilar-Saavedra et al. - arxiv:1302.6618 € - The Top Charge Asymmetry at the LHC - 11 Outline • MoWvaon – the A FB puzzle • The asymmetry at the LHC - A C • Early measurements at the LHC: ATLAS lepton+jets 1.04 g-1 at √s=7 TeV Eur.Phys.j. C72 (2012) 2039 • BoosWng the charge asymmetry Phys.Le_. B707 (2012) 92-98 • Charge asymmetry at ATLAS: lepton+jets 4.7 g-1 at √s=7 TeV jHEP 02 (2014) 107 • A C at √s=14 TeV with a photon handle arxiv:1402.3598 - The Top Charge Asymmetry at the LHC - 12 Overview of the LHC W.j. SWrling, arxiv: 0901.0002 WJS 2010 1000 ratios of parton luminosities at 7 TeV LHC and Tevatron 100 gg • p-p collisions: charge symmetric iniWal state luminosity ratio 10 • dominated by gluon-gluon fusion: ~85% at √s=7,8 TeV Yqq >90% at √s=14 TeV MSTW2008NLO • average momentum imbalance 1 101 102 103 in qq events (anW-quarks from sea) MX (GeV) - The Top Charge Asymmetry at the LHC - 13 € Asymmetry @ LHC • More diluted: qq →tt only ~15% at 8 TeV • (anW)top is preferenWally produced along the direcWon of the incoming (anW)quark. t € t € q q q q € CM reference frame Lab reference frame € € € € • DirecWon of the incoming quark not known in pp collisions - BUT - • Quarks more likely to be valence quarks ( larger fracWon of momentum) • only sea anW-quarks available • top quarks more forward than anWtops - The Top Charge Asymmetry at the LHC - 14 Asymmetries Tevatron - collisions pp LHC - collisions pp N(Δy > 0) − N(Δy < 0) N(Δ | y |> 0) − N(Δ | y |< 0) AFB = AC = €N (Δy > 0) + N(Δy < 0) € N(Δ | y |> 0) + N(Δ | y |< 0) with Δ|y| = |y | - |y | with Δy = ytop – yanW-top top anWtop € € figures from G.Rodrigo, arxiv:1207.0331 - The Top Charge Asymmetry at the LHC - 15 Outline • MoWvaon – the A FB puzzle • The asymmetry at the LHC - A C • Early measurements at the LHC: ATLAS lepton+jets 1.04 g-1 at √s=7 TeV Eur.Phys.j. C72 (2012) 2039 • BoosWng the charge asymmetry Phys.Le_. B707 (2012) 92-98 • Charge asymmetry at ATLAS: lepton+jets 4.7 g-1 at √s=7 TeV jHEP 02 (2014) 107 • A C at √s=14 TeV with a photon handle arxiv:1402.3598 - The Top Charge Asymmetry at the LHC - 16 Charge asymmetry – l+jets |y| 0.4 6 ATLAS ! + * 4 jets (* 1 b tag) /d m -1 data d 0.35 L dt = 1.04 fb m MC@NLO 1/ 0 0.3 ATLAS - l+jets Eur.Phys.j. C72 (2012) 2039 -1 0.25 √s = 7 TeV, 1.04 g 0.2 AC = -0.019 ± 0.028(stat.) ± 0.024(syst.) 0.15 0.1 Theory: AC = 0.0123 (5) W. Bernreuther et al., Phys. Rev. D 86 (2012) 034026 0.05 0 -3 -2 -1 0 1 2 3 6|y| • First measurements at LHC7 compable with 0 asymmetry (and SM) • Large stasWcal and systemac uncertainWes • The expected SM asymmetry is very small (~1%). - The Top Charge Asymmetry at the LHC - 17 Experimental challenge • Small fracWon of quark-anWquark annihilaon small AC~1% • DirecWon of the incoming quark not known in pp collisions • Measured A C distorted by resoluWon and acceptance. Need to extrapolate to parton-level asymmetry to compare with theory. • Large systemac effects from parton-level extrapolaon and from background modelling. - The Top Charge Asymmetry at the LHC - 18 Outline • MoWvaon – the A FB puzzle • The asymmetry at the LHC - A C • Early measurements at the LHC: ATLAS lepton+jets 1.04 g-1 at √s=7 TeV Eur.Phys.j. C72 (2012) 2039 • BoosWng the charge asymmetry Phys.Le. B707 (2012) 92-98 • Charge asymmetry at ATLAS: lepton+jets 4.7 g-1 at √s=7 TeV jHEP 02 (2014) 107 • A C at √s=14 TeV with a photon handle arxiv:1402.3598 - The Top Charge Asymmetry at the LHC - 19 Boosted AC • Expected A C is very small. Hard to measure. Phys.Le. B707 (2012) 92-98 • FracWon of qq →tt events increases with the velocity of the top quark pair along the beam axis € “valence” quark tt large xq q q “sea” anW-quark small xq Lab reference frame € € € - The Top Charge Asymmetry at the LHC - 20 Outline • MoWvaon – the A FB puzzle • The asymmetry at the LHC - A C • Early measurements at the LHC: ATLAS lepton+jets 1.04 g-1 at √s=7 TeV Eur.Phys.j. C72 (2012) 2039 • BoosWng the charge asymmetry Phys.Le_. B707 (2012) 92-98 • Charge asymmetry at ATLAS: lepton+jets 4.7 g-1 at √s=7 TeV jHEP 02 (2014) 107 • A C at √s=14 TeV with a photon handle arxiv:1402.3598 - The Top Charge Asymmetry at the LHC - 21 Overview of ATLAS Efficiently collected ~20 g-1 of p-p collision data energy of jets, electrons precision measurement of Muon detecWon momenta of charged parWcles in toroidal magneWc field - The Top Charge Asymmetry at the LHC - 22 Outline • MoWvaon – the A FB puzzle • The asymmetry at the LHC - A C • Early measurements at the LHC: ATLAS lepton+jets 1.04 g-1 at √s=7 TeV Eur.Phys.j. C72 (2012) 2039 • BoosWng the charge asymmetry Phys.Le_.