Top polarisation at colliders Top polarisation: what physics can - PowerPoint PPT Presentation

Top polarisation at colliders. Rohini M. Godbole Top polarisation at colliders ♦ Top polarisation: what physics can it probe Probes of the top polarisation and effects of ♦ anomalous coupling on them. ♦ Example of use of angular distribution as probe of top spin – For a t ¯ t resonance. – t ¯ t spin spin correlation in t ¯ tH and t ¯ tjj produc- tion. • Polarisation measures using energies of decay products October 1, 2010. Grenoble

Top polarisation at colliders. Rohini M. Godbole Based in part on 1)RG, Rindani and SinghJHEP 0612, 021 (2006), 2)D. Choudhury, R. G. , S.D. Rindani, R. Singh and K. Wagh, in hep-ph/0602198 3) RG, S.D. Rindani, Kumar Rao, Ritesh Singh.arXiv:1010.XXXX 4)D. Choudhury, RG, Pratishruti Saha (in preparation) arXiv: 10YY:XXXX October 1, 2010. Grenoble

Top polarisation at colliders. Introduction Top quarks at the LHC: • Copious production of t ¯ t pairs at LHC (SM c.s. ≈ 800 pb at 14 TeV) • Large single top production (seen at Tevatron) • Important role in new physics signatures: Top quarks can also arise in the decays of new particles – resonances, new gauge bosons, Higgs bosons, squarks, gluinos . . . • Template for issues in new physics : example of determination of spin and mass! • Most important background to a lot of new physics. What features can be used effectively to de lineate SM from BSM tops! • Polarisation can be one important handle. October 1, 2010. Grenoble

Top polarisation at colliders. Production mechanisms and top polarization • Top polarization can give more information about the production mechanism than just the cross section does. • Top partners with the different spin (SUSY) or same spin UED/Little Higgs.. Shelton : PRD 79, Nojiri et al JHEP, Perelstein. Produce t in cascade de- cays of top partners and top polarisation can carry information on the model parameters. Polarisation measurement can provide model parameter information, mdoel descrimination, kinematic features due to polarisation effects can be used efffectively to isolate signal from background in searches. • Non zero polarisation requires parity violation, and hence measures left-right mixing. R-parity violating SUSY can give rise to nonzero top polarisation (Hikasa PRD, 1999). • It can give a clue to CP violation through dipole couplings. October 1, 2010. Grenoble

Top polarisation at colliders. Specific models One example is t ¯ t resonance with Parity violating couplings. Look for illustration at an extra Z model. Little Higgs model has an extra massive gauge boson Z H with (left) right-handed couplings to fermions depending on one parameter ( θ ) g u V = ( − ) g u A = g cot θ g d V = ( − ) g d A = − g cot θ t production and decay via γ, Z, Z ′ depends only on two new param- t ¯ eters: m Z ′ and cot θ . SM t ¯ t production through QCD. t distribution for total (unpolarised) c.section and polarised ( dσ R /dm t ¯ m t ¯ t − dσ L /dm t ¯ t ) (only the new physics contribution). October 1, 2010. Grenoble

t ¯ Top polarisation at colliders. t invariant mass distribution 0.25 Unpolarized Polarized SM, Unpolarized (X100) SM, Polarized 0.2 0.15 (pb/GeV) 0.1 − tt d σ /dM 0.05 0 −0.05 900 1000 1100 1200 1300 1400 M (GeV) − tt The model can be tested using the t ¯ t invariant mass distribution Polarization can be a further more sensitive test and also tool to get information on the couplings. October 1, 2010. Grenoble

Top polarisation at colliders. Top longitudinal polarization P t ≡ σ R − σ L σ R + σ L Can be enhanced using cuts on m t ¯ t 0.25 θ = 1.2 cot θ = 1.6 cot θ = 2.0 cot 0.2 0.15 Polarization 0.1 0.05 0 600 800 1000 1200 1400 M Z’ (GeV) October 1, 2010. Grenoble

Top polarisation at colliders. R-parity violating SUSY Hikasa PRD 60, 114041, 99 Expected polarisation at Tevatron: d d R R t t L R i i ~ ~ e d L R � � � t � t L R d d R R ( a ) ( b ) October 1, 2010. Grenoble

Top polarisation at colliders. R-parity violating SUSY Expected poalrisation at the Tevatron: October 1, 2010. Grenoble

Top polarisation at colliders. BSM characterisation using top polarisation CDF and D0 reported FB asymmetry in t ¯ t production D0:PRL 100, 142002 (2008), CDF: PRL 101, 202001 (2008) . A t FB = 0 . 193 ± 0 . 0065 ± 0 . 024 CDF published result, newer value somewhat lower SM expectation (NLO : Rodrigo/Kuehn) : 0.051 October 1, 2010. Grenoble

Top polarisation at colliders. BSM characterisation using top polarisation A host of new physics models: Examples of some which explain most of the observed features ’sat- isfactorily’ 1) t -channel colour triplet (sextet) scalar object: (generalisation of RPV case above, but not necessarily chiral couplings ) J. Shu, T. M. P. Tait, K. Wang, PRL D81, 034012 (2010) 2) t -channel colour singlet vector exchange: S. Jung, H. Murayama, A. Pierce et al., PR D81, 015004 (2010) Chiral couplings. 3) s -channel strongly interacting vector exchange: (axigluon: (pre)dicted : D. Choudhury, RG, Singh and Wagh, PLB 657 (2007) 69 : alas wrong sign!) Generalisation: flavour non-universal axigluon P. Frampton, J. Shu and K. Wang, PLB 683 (2010) 294. Non-chiral couplings. October 1, 2010. Grenoble

Top polarisation at colliders. BSM characterisation using top polarisation The Forward backward top asymmetry origniates due to different reasons in different model explanations. The chirality structure is also different. Expected top polarisation can be different. October 1, 2010. Grenoble

Top polarisation at colliders. BSM characterisation using top polarisation In all the three different models expected top po- 0.3 larisation quite different 0.2 for different physics ex- planations. 0.1 A P Corrleation between 0 top polarisation and FB asymmetry quite -0.1 Φ Z ′ different. A -0.2 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Exploring Measurement A FB of top polarisation a use- Φ : Tait et al colour triplet/sextet scalar ful tool to get informa- Z ′ : Murayama, Wells t –channel vector tion on production mech- A : Flavour nonuniversal axigluons. anism. October 1, 2010. Grenoble

Top polarisation at colliders. Top spin correlation vs. single top polarization When t and ¯ t are produced, a useful observable is top spin correlation: 1 dσ = 1 4(1 + B 1 cos θ a + B 2 cos θ b − C cos θ a cos θ b ) σ d cos θ a d cos θ b t ¯ tH , t ¯ This has been very well studied theoretically (for example: t produced in RS Graviton decay etc.) Needs reconstruction of both t and ¯ t rest frames. It is conceivable that single top polarization can give better statistics. October 1, 2010. Grenoble

Top polarisation at colliders. Measuring Polarisation Polarisation can be measured by studying the decay distribution of a decay fermion f in the rest frame of the top: 1 d Γ = 1 � � 1 + P t κ f cos θ f , Γ d cos θ f 2 θ f is the angle between the f momentum and the top momentum, P t is the degree of top polarization, κ f is the “analyzing power” of the final-state particle f . κ f depends on the weak isospin and the mass of decay product f . October 1, 2010. Grenoble

Top polarisation at colliders. Analyzing power for various channels The analyzing power k f for various channels is given by: κ b = − m 2 t − 2 m 2 W ≃ − 0 . 4 m 2 t + 2 m 2 W κ W = − κ b ≃ 0 . 4 κ ℓ + = κ d = 1; κ u = κ ν l = − 0 . 31 • The charged lepton or d quark has the best analysing power • d -quark jet cannot be distinguished from the u -quark jet. • In the top rest frame the down quark is on average less energetic than the up quark. Thus the less energetic of the two light quark jets can be used. Net spin analyzing power is κ j ≃ 0 . 5 October 1, 2010. Grenoble

Top polarisation at colliders. Corrections to the analyzing power Leading QCD corrections to κ b and κ j are of order a few per cent. QCD corrections decrease | κ | [Brandenburg,Si,Uwer 2002] κ also affected by corrections to the form of the tbW coupling (“anoma- lous couplings”) It is useful to have a way of measuring polarization independent of such corrections. Also useful is distribution in lab. frame, rather than in top rest frame. κ f for ℓ + and down-quark is unchanged by anom. tbW vertex (RG, Rindani, Singh) October 1, 2010. Grenoble

Top polarisation at colliders. Lab observables? Angular distribution of the decay lepton l in the rest frame of the top is the most efficient polarisation observable. Which of the kinematic observables of the decay lepton as measured in the lab frame carry this polarisation information faithfully? What are the special issues here since LHC is a pp machine. For highly boosted tops : what about rest frame reconstruction and angle measurements? October 1, 2010. Grenoble