1 / 18

The Large Hadron Collider Machine, Experiments, Physics Top Physics (at the LHC)

The Large Hadron Collider Machine, Experiments, Physics Top Physics (at the LHC). Johannes Haller Thomas Schörner-Sadenius Hamburg University Summer Term 2009. TOP PHYSICS.

hamilton-haley
Download Presentation

The Large Hadron Collider Machine, Experiments, Physics Top Physics (at the LHC)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The Large Hadron ColliderMachine, Experiments, PhysicsTop Physics (at the LHC) Johannes HallerThomas Schörner-Sadenius Hamburg UniversitySummer Term 2009

  2. TOP PHYSICS Remember: The top quark was discovered in 1994in proton-antiproton collisions at the Tevatron. It proved to be very heavy: This makes the top quark unique in many respects: – Mass: New physics phenomena might couple to high masses. Remember for example the Higgs coupling to fermions (Yukawa couplings): – The top quark decays as a quasi-free quark  no hadronisation phase  chance to study free quark and its properties (spin!). Hadronisation time from QCD considerations: Hadronisation takes place at energy scale ΛQCD of the order of 250 MeV. Using Heisenberg this cor- responds to about 3x10-24 s: – Top mass might decay to other, yet undiscovered particles. – Top quark allows for very precise tests of SM via its connection to the W and Higgs masses. UHH SS09: LHC

  3. TOP PHYSICS Remember direct and indirect measurements of W, t, H masses at LEP and Tevatron: – In addition, top constitutes an important background for new physics like Higgs and SUSY!  Many good reasons to measure and understand top quark production and top properties! • – Top quark currently only produced at Tevatron (extremely small cross-section at HERA). It is pro- duced via strong interactions: • Cross-sections Tevatron: ~pb. LHC: ~nb !!! • More gluon-induced processes at LHC: centre-of-mass energy, parton luminosities and PDFs. • LHC is a top factory  precision measurements! Current status top mass from Tevatron: UHH SS09: LHC

  4. TOP PHYSICS: GOALS From full events like the following one would liketo extract a number of things: – The ttbar production cross-section, spin correlations and charge asymmetries.– Top mass and charge.– Cross-section for single top production (FCNC?).– Top decays (W helicity, FCNC, tH+b, …) Top quark decays are weak processes! In cross sectionsquare of CKM matrix element: Therefore decays tWs and twd that are allowed byall quantum numbers are practically not realised!  BR(tWb)~100%! UHH SS09: LHC

  5. TOP EVENTS: CLASSIFICATION All tops decay into Wb. The W can then decay leptonically or hadronically – this allows a simpleclassification of top-antitop events: – Dileptons:- easy to identify (at least e,μ channels)- small cross-sections for e,μ. - missing energy of two neutrinos  difficult! – Lepton + Jets:- cross-section about 30%.- only one neutrino. – Purely hadronic:- difficult to separate from QCD background – will be ignored here! Note: All events contain 2 b jets  possible identification using “b tagging” – mostly via secondary vertex reconstruction! Define strategies to enrich and refine clean samples: • Exploit kinematic and topological variables! • b jet identification with lifetime information. • b jet identification with leptons from blνX. UHH SS09: LHC

  6. “B TAGGING” B hadrons (here from tWb) have typically largerlifetimes (of the order of some picoseconds) than for example D mesons or other decaying hadrons  Will decay in flight after ~mm (cτ~500 μm) distance from vertex. Try to reconstruct this “secondary vertex”  b jet identification! Combination of various observables like decay length,decay length significance, lepton pT, invariant mass etc. allows efficient and pure b tag. UHH SS09: LHC

  7. TOP PAIRS: DILEPTON CHANNEL Typical selection: – >1 central hard jets (pT>20 GeV) – 2 charged central leptons with high transverse momentum > 15 GeV and opposite charge.– missing ET > 35 GeV (accounts for neutrinos).– specific cuts against background. Distribution of selected events: data versus signal and background Monte Carlo simulations. Before 2-jet cut: Low jet multiplicity well de-scribed by MC without top! At high jet multiplicities topcontribution needed to de-scribe the data  “Top discovery” !!! After 2-jet cut: Adjust top signal MC con-tribution until data are described. Best descriptiongives cross-section for top-antitop production! Dominated by statistics: only28 events selected here! UHH SS09: LHC

  8. TOP PAIRS: LEPTON + JETS CHANNEL – >2 central hard jets (pT>20 GeV) – 1 hard charged central lepton.– large missing ET.– specific cuts against background. Distribution of selected events: data versus signal and background Monte Carlo simulations as function of transverse momentum of all objects in event.  B tag improves purity of top sample! 2 b tags 1 b tag • 2 jets • 2 b jets • lepton • ETmiss UHH SS09: LHC

  9. TOP PAIRS: SUMMARY TEVATRON – Theoretical predictions are derived in the usual way: factorised ansatz: Comparison of different cross-section measurements from D0 and CDF and theory prediction: • Lepton+jets best! • Agreement theory-data. • No indications for BSM. • Large errors! UHH SS09: LHC

  10. TOP MASS MEASUREMENTS – Can derive mass information from all three event classes (dilepton, lepton+jets, hadronic), but - dileptons difficult because of two neutrinos. - hadronic channel suffers from large background lepton+jets channel most promising! Again selection: - 1 hard charged lepton. - ETmiss > 20 GeV - at least 4 hard jets! Basic assumption for these event candidates: ttbar!– 24 possibilities to attribute jets to quarks (com- binatorics!!).– Perform kinematic fit for each possibility: – First term: Variation of momenta within their errors (otherwise χ2 becomes big). – Other terms constrain masses of W and top quark, ensure that momentum conservation is obeyed! – For each attribution of jets to quarks one obtains one χ2 and one mt,reco. Choose the one with the smallest χ2. – Use MC-simulated events to check validity of method: How well do we reconstruct the mass? • More correct attributions with 2 b tags (less combi- natorics). • For 2 b tags 50% correct! • Correct combinations have better mass resolution! UHH SS09: LHC

  11. TOP MASS MEASUREMENTS UHH SS09: LHC

  12. TOP MASS MEASUREMENTS • – Determination of top mass from comparison of distributions of mt,reco in MC and data (“template method”). • Best single top mass measurement: But also other methods are used in the lepton+jetschannel. Summary of measurements: Prospects of top mass measurements at the LHC: UHH SS09: LHC

  13. TOP MASS MEASUREMENTS – Consistency of direct and indirect determinations: Data suggest low mass Higgs boson of 80 GeV (at least less than 144 GeV!) Development of to mass measurements (direct and indirect) over the times: Development of top mass uncertainty with time: UHH SS09: LHC

  14. TOP: W HELICITY IN TOP EVENTS – Top quarks decay as fermions in V-A theory: – V-A coupling at tbW+ vertex requires left-handed b quark. Neglecting b quark mass, b quarks must also have negative helicity. – But then also W+ boson only with negative helicity (numbers f: SM predictions for relative fractions): – SM test: Measurement of f+. Value different from 0 would mean new physics at tbW+ vertex. W helicity experimentally accessible! Angle θ* between negative top direction and direction of charged lepton in W rest system: Assuming F0=0.70 (SM):  Limit on F+ < 0.10! UHH SS09: LHC

  15. TOP: SPIN CORRELATIONS – Top quark pairs from pp collisions are basically unpolarized, but the two top spins are correlated: Either parallel (qq events) or antiparallel (gg). – Test of the top quark spin: - Search for new physics, i.e. CP violating interactions, Higgs with undefined parity, properties of s-channel resonance - Test of top decays as a quasi free quark - precise test of production and decay mechanism - Top spins affect the angular distributions of decay products  important for event selections. – Define observables C and D (a and b are quantisation axes, for example reconstructed top flight directions). … but how to measure the spin of the top quark? Spin of top quark from angular distribution of decay products:  Studies at ATLAS of observables A, AD that are closely correlated to C, D (in various channels): precision 4-5% ! Currently achieved at TEVATRON: 15-20%  large potential at LHC! UHH SS09: LHC

  16. TOP: SPIN CORRELATIONS: REAL LIFE • Dilepton analysis: • For parton and detector level. • With and without correlation of top quark spins. •  Experimentally extremely difficult to achieve precision! UHH SS09: LHC

  17. SINGLE TOP PRODUCTION • – Possibility to produce single top quarks in pp: • Small cross-sections, and also experimentally more demanding (fewer b jets and kinematic constraints, …) • Use multi-variate methods to select events: - likelihoods - neural networks - decision trees … one number between 0 and 1 for each event. Latest result from CDF: SM: 1.98±0.25 pb SM: 0.88±0.11 pb CDF: σ = 2.3 ± 0.5 pb 5σ statistical significance  discovery ! UHH SS09: LHC

  18. TOP: FURTHER TOPICS – Search for flavour changing neutral currents. – Search for fourth-generation quarks: Additional generations might effectively reduce Vtb (which is only indirectly known!) and thus the single-top cross-section. Do we observe that? Also: Are the unitarity relations fulfilled? – Measurement of tt+jets events  test of SM couplings. – Measurement of tt+photon events  measurement of top quark electric charge.– …  Lots to do for LHC! UHH SS09: LHC

More Related