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Top Properties @ 4fb -1

Top Properties @ 4fb -1. Stop search Top charge asymmetry. Amnon Harel aharel@fnal.gov University of Rochester. DØ Collaboration Week, 19 th June 2007. Su-Jung Park. µ +. ν µ. µ +. b. W +. ν µ. W +. t. ~. -. χ 1 0. t. ~. -. χ 1 +. b. W -. b. ~. t 1. q. -. q’. ~.

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Top Properties @ 4fb -1

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  1. Top Properties @ 4fb-1 Stop search Top charge asymmetry Amnon Harelaharel@fnal.gov University of Rochester DØ Collaboration Week, 19th June 2007

  2. Su-Jung Park µ+ νµ µ+ b W+ νµ W+ t ~ - χ10 t ~ - χ1+ b W- b ~ t1 q - q’ ~ t1 - ~ b χ1- ~ W- χ10 q q’ A Stop Search The illustrated decay chain is relevant for: Searching in the l+jets channel, with at least one tight b-tag. The event signature is very similar to standard top l+jets decays. The MET will not be higher - the neutralinos are back to back. • The jets are softer: • achieved good data vs. MC agreement for 15GeV jets, used to get more signal acceptance • the b-tagged jet’s pT is a good separating variable • Subtler event kinematics are also different: • Use a kinematic fitter to identify top pair events. Amnon Harel

  3. Su-Jung Park A Stop Search Mass Points Studied: 0.9 fb-1 , l+jets Signal Expected Limits 0.9 fb-1 Bayesian approach to limit setting Includes partial systematics Note is in group review – aiming at summer conferences (SUSY07) Amnon Harel

  4. @ 1 fb-1 Limits a bit stronger when assuming the top cross section: 0.9 fb-1 Observed Expected Prospino NLO Amnon Harel

  5. @ 4 fb-1 Under the assumption that the newer data is exactly as useful as the current data 4.0 fb-1 Observed Expected Prospino NLO Amnon Harel

  6. Harel Several contributions to Afb, all are interference terms of at least NLO: Afb 4th jet pT NNLO terms, qgtt, etc. Top Charge Asymmetry Measuring the top charge asymmetry in the l+jets channel, using b-tagging and a kinematic fitter to reconstruct the tops. Probes new physics, e.g. On the face of it, it’s a trivial measurement: count and publish. The top asymmetry varies greatly throughout phase space MC@NLO • Keeps the acceptance simple: • hard cuts on pTs, • loose complicated cuts • fitting procedure • Statistics limited: • kinematic fitter • integrated measurement • uncorrected Afb Analysis Strategy Amnon Harel

  7. Top Charge Asymmetry A likelihood that discriminated between top and W+jets events without biasing |ΔY| 0.9 fb-1 0.9 fb-1 D D • MultijetAmount and asymmetry taken from data • W+jets • Asymmetric production • Reconstruction as a top pair washes out the asymmetry Analysis in EB review.Measured observable asymmetry: SM prediction: (1±1(syst))% Amnon Harel

  8. Z’ limits @ 1 fb-1 • Production through a Z’ resonance is asymmetric in most models • Assume Z-like couplings • Unique probe for a wide resonance 0.9 fb-1 Observed Expected Amnon Harel

  9. @ 4 fb-1 Just assumed 4 times the statistics. Ignoring 0.9 != 1, trigger issues, etc. Measurement still completely statistics dominated. Can we observe the difference between Afb in tt and Afb in ttj ? NLO predictions are 6% and -7%. This was my original motivation  Expect to measure Afb4 to within 5%, Afb5+ to within 7%, and Δ to within 8.5%. Of course, the 4 jet bin also contains ttj contributions, and so on. Z’ limits 4.0 fb-1 Expected (given 1 fb-1) Expected (MC@NLO) Amnon Harel

  10. Back up slides Amnon Harel

  11. Top Charge Asymmetry This one’s mine, so I’m probably going into too many details… Measuring the top charge asymmetry (Afb) in the l+jets channel, requiring an L4 b-tag (NN>0.2) and using HITFIT to reconstruct the tops. On the face of it, it’s a trivial measurement: count and publish. • Let’s slow down and first understand the top pair and W+jets asymmetries. • The top charge asymmetry has several origins, all at NLO interference terms: • An interference between a simple qq->g->tt diagram and the corresponding gluonic box diagram • An interference between that simple diagram with an added ISR gluon & with an FSR gluon • Interferences in qg->tt diagrams (e.g. Flavor creation) • Higher order terms (have not been evaluated) • Interferences with mixed (electroweak neutral current + gluon) box diagrams • Some of these effects tend to cancel out. The top asymmetry varies greatly throughout phase space (see next slide). Which “A” did we measure? The main background is also asymmetric! The W+jets asymmetry is much simpler and comes directly from the V-A nature of the W vertex and the proton anti-proton initial state (u&dPDFs). Only exception is Wc(+jets) production, which probes the sPDF and is thus almost symmetric. Amnon Harel

  12. Afb: The Asymmetries So the top asymmetry is unstable, and the W+jets asymmetry is stable. 4th Leading jet pT 5th Leading jet pT Not a surprise: The latest theory calculation is that the tt and ttj asymmetries are approximately 6% and -7% (jet acceptance cuts: pT>20GeV, |η|<3) Amnon Harel

  13. Afb: Analysis Strategy • From a hitfit study: the best observable is to combine the two top rapidities into ΔY. • This also gets the small benefit of reducing the effect of boosts along the z-axis. • As the phase space were the measurement is done must be specified: • Keep it simple • Jet pT>20GeV • Use only a loose cut on a likelihood discriminant that does no bias us towards low |ΔY|. • Study jet efficiencies from particle level to selected jets(jet smearing, JESμ, jet ID S.F., EM removal). • Closure test, using a dedicated PMCS on W+jets and signal MC@NLO events, showing that a factorized box cuts are sufficient. • There are also reconstruction biases: • Lepton charge misidentification and asymmetries are small and suppressed by using the entire effect for the sign of ΔY • Geometric misreconstruction described with event-by-event dilution factors parametrized in |ΔY|. • Time constraints: CDF Ph.D. thesis in July. Expected blessing 8/06, Moriond. Amnon Harel

  14. Afb: Fitting • Nsig and Afb are correlated in two ways: • A larger Nsig implies the observed asymmetry must be assigned to more events • Since Nsig=N++N-, a statistical fluctuation in, e.g., N+ would increase both. • Trying to untangle this by doing a simultaneous fit. • Currently doing a simultaneous fit over: • Likelihood discriminant –identified signal fraction • Leptonic W mass –cross checks matrix method’s determination of multijet fraction • Sign of events passing (loose) likelihood cut –gives Afb • Fit not as stable as expected: adding the asymmetry information changed fitted fractions significantly (higher multijet fraction). More fit stability tests needed. • Fit results: Nsig=261±20, NW=78±20, NQCD=39±4, Afb=0.16±0.08 Amnon Harel

  15. Afb: W+jets Asymmetry Breakdown needed for systematics (e.g. b-tagging RFs) A pleasant surprise was that the W+jets asymmetry is washed out by the reconstruction as a top pair. Some asymmetry remains and a detailed breakdown is required to evaluate systematics (e.g. TRFs). Reconstruction as a top pair washes it out  Amnon Harel

  16. The DØ Detector Amnon Harel

  17. Hard scatter partons Particles Matched MC Good at generating hard, large-angle processes(calculates interference) LO calculations for 2N hard processes Matrix element generator Alpgen / Sherpa Partons are matched to parton-shower jets to avoid double counting of equivalent phase space configurations. Weak on the “texture” of the QCD radiation Good at generating the details within a jet Only 2->2 hard processes Parton shower generator Pythia / Herwig Multijet events don’t describe data well (and are hard to generate) Resummed soft, collinear radiation. Detector simulation Amnon Harel

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