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Single-top Physics at the Tevatron

Single-top Physics at the Tevatron. Matt Bowen with Matt Strassler, Steve Ellis University of Washington, Seattle. Outline. What single-top is Why it is worthwhile to study Research we’ve done Future research directions Conclusions. Chicago. CDF. Batavia, Illinois. DØ. DØ. DØ. CDF.

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Single-top Physics at the Tevatron

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  1. Single-top Physics at the Tevatron Matt Bowenwith Matt Strassler, Steve Ellis University of Washington, Seattle

  2. Outline • What single-top is • Why it is worthwhile to study • Research we’ve done • Future research directions • Conclusions Matt Bowen 4/26/04

  3. Chicago CDF Batavia, Illinois DØ DØ DØ CDF p p s =1.96 TeV t = 396 ns Booster p Tevatron p Run I 1992-95 Run II 2001-09(?) p source Main Injector & Recycler Fermilab Gordon Borrowed from John Womersley’s talk 1/12/04 Matt Bowen 4/26/04

  4. Detectors at the Tevatron Detector Objects • Electrons • Muons • Jets • B-tagged Jets • MET vacation time! Gordon Matt Bowen 4/26/04

  5. What is single-top? Two single-top channels are classified by W momentum s-channel t-channel time-like space-like • The top quark was discovered in Run I through qq tt • Neither single-top channel has been discovered in Run II yet Matt Bowen 4/26/04

  6. Why is single-top interesting? Practical Reasons • Cross-section measurement yields a measurement of Vtb • Is a background to other searches (Higgs, etc...) Vtb t-channel 1.98 pb Vtb s-channel 0.88 pb Matt Bowen 4/26/04

  7. “Extra” reasons The two single-top channels are sensitive to a variety of new physics models. Here are just a few. Because top mass is of order EWSB scale, top plays a “special” role in a number of models. • Extra Scalar Bosons – top-color • Extra Gauge Bosons – top flavor • Extra Dimensions – 5D with gauge bosons in bulk • Extra Generations of Quarks - will change unitarity constraints on CKM elements • Extra couplings (Modified) – top interaction with SM particles. ex: Ztc Affect s-channel Affect t-channel Main reference: “Single Top Production as a Window to Physics Beyond the Standard Model” T. Tait hep-ph/0007298 Matt Bowen 4/26/04

  8. Extra (Pseudo-)Scalar Bosons: Top-color models • Scalars (such as Higgs) exist as bound states of top and bottom quarks • For Mπ± = 250 GeV, tR-cR mixing of ~20% s-channel cross-section doubles • No interference as SM is from left-handed light quarks • t-channel contribution is suppressed by 1/Mπ±2 and that π± doesn’t couple to light quarks time-like momentum allows for resonance reference: hep-ph/9810367 Matt Bowen 4/26/04

  9. Extra Gauge Bosons: Top-flavor models SU(3)C x SU(2)h x SU(2)l x U(1)Y • Postulate a larger gauge group which reduces to the SM gauge group at low energies to explain top mass • 1st and 2nd gen quarks transform under SU(2)l, and 3rd under SU(2)h, add heavy doublet of quarks • SU(2)h gauge couplings mix with SU(2)l according to sin2φ • For MW‘ = 1 TeV, sin2φ =0.05 s-channel increases ~20% • t-channel contribution suppressed by 1/MW‘2 W' Matt Bowen 4/26/04

  10. Extra Dimensions:5-D Gauge Bosons • Allow only SM gauge bosons to propagate in compactified extra dimension • Permits Kaluza-Klein modes of W (Wkk) • For MWkk= 1TeV, s-channel amplitudes interfere destructively to reduce cross-section by 25% • t-channel contributions are suppressed by 1/MW'2 Wkk Matt Bowen 4/26/04

  11. Extra quark generations:CKM constraints • For 3 generations, the unitary of the CKM matrix constrains |Vts| < 0.043 • With >3 generations, one possibility is |Vtb|=0.83 and |Vts|=0.55 • Because gluons split to ss far more than bb, the t-channel cross-section rises by 60% • s-channel produces as many tops as before, but less with an additional b quark – so the observable cross-section goes down a little. • Changes decay structure of top s s Vts Without imposing 3 family unitarity, these are the 90% CL direct constraints. Matt Bowen 4/26/04

  12. Extra Couplings*:FCNC: Z-t-c • Can argue that low energy constraints (κZtc < 0.3) may not apply in the presence of additional new physics • For κZtc = 1, t-channel increases 60% • These couplings change top decay structure • κZtc recently constrained by LEP II data to be < ~ 0.5 (hep-ex/0404014) Z c c *there’s nothing “extra” about these couplings; the appropriate title would be “Modifications to Top Couplings” Matt Bowen 4/26/04

  13. Shifted cross-sections plot SM prediction 3σ theoretical deviation Charged top-pion FCNC Z-t-c vertex 4 gen Top-flavor model Extra dimensions Plot from hep-ph/0007298 t-channel CS has changed to 1.98pb ED from hep-ph/0207178 Matt Bowen 4/26/04

  14. Lessons • t-channel is affected by modifications to top quark couplings • s-channel is affected by heavy particles • Many other models to consider Therefore, measuring the t- and s-channels separately is important and could potentially be a “Window to Physics Beyond the SM” Matt Bowen 4/26/04

  15. Research on t-channel Detector Objects from t-channel • 1 non b-tagged jet (from light quark) • 1 lepton • 1 b-tagged jet • Missing Transverse Energy (from neutrino) e+ b ve not seen Matt Bowen 4/26/04

  16. Numbers for a Sample Search Basic Cuts :1 lepton PT>15 GeV, |η|<2.0 MET > 15 GeV 1 b-tagged jet with PT>20 GeV, |η|<2.0 1 other jet with PT>20 GeV, |η|<3.5 Advanced Cuts: Same, except b-tagged jet PT>60GeV, other jet PT>30 GeV Mtop=invariant mass(blv): 160 GeV < Mtop < 190 GeV HT=PTlepton+MET+ Σall jets (jet PT): 180 GeV < HT < 250 GeV • Studies done with Madgraph + Pythia + Fast Detector Simulation for 4 fb-1 • Basic sig:bkg ratio is 1:15 • Advanced sig:bkg is 1:4 • Systematics prevent discovery Matt Bowen 4/26/04

  17. CP Invariance of the Tevatron • pp initial state at Tevatron is CP invariant, but not C or P invariant separately • At leading order, processes that proceed through an s-channel gluon “forget” that they are not separately C and P invariant (tt and QCD) • Processes with W’s “remember” that they are not separately C and P invariant (single top and W+jets) Initial State P P Under C or P transformation P P Under CP P P Matt Bowen 4/26/04

  18. What t-channel looks like b • ~ ¾ of the time, top quarks are formed from ug initial state and boosted in proton direction, ¼ from gd and are boosted in anti-proton direction “the jet” b-tagged jet d e+ P P g t u • Direction of jet is correlated with top spin, which is correlated with lepton direction b ve Usually don’t see the b! Matt Bowen 4/26/04

  19. t-channel: Qlepton*jet rapidity vs. Qlepton* lepton rapidity Qlepton is the sign of the charge of lepton jet: highest PT non b-tagged jet Under P or C: x  -x, y-y t-channel is not P or C invariant Under CP, the plot is invariant Contours of Constant Cross-section Qlepton * lepton rapidity CDF has looked at Qlepton* jet rapidity by itself Qlepton * jet rapidity Matt Bowen 4/26/04

  20. What W+jets looks like Contours of Constant Cross-section Boosted in proton direction for W+ production, anti-proton direction for W- production Final state is not P or C invariant Jets from light quarks and gluons can be misidentified as b-jets Qlepton * Lepton Rapidity Qlepton* Jet Rapidity W+ e+ P x > 0y > 0 ve b-tag P jet Matt Bowen 4/26/04

  21. What tt looks like Doesn’t tend to be boosted in either direction Final state is both P and C invariant at leading order Qlepton * Lepton Rapidity b-tag e+ u P t Qlepton* Jet Rapidity ve u t x = 0y = 0 jet u P b d Matt Bowen 4/26/04

  22. What can be done with this? • Fit, likelihood methods are possible • We have pursued another approach Region 1 (R1) Qlepton * Lepton Rapidity Under Parity, R1  R2 Region 2 (R2) Qlepton * Jet Rapidity Matt Bowen 4/26/04

  23. Parity Even and Odd Combinations of R1 and R2 R1+R2 R1+R2 under P R1-R2 -(R1-R2) under P tt and QCD are zero for P odd combination Systematic errors in tt and QCD largely cancel W+jets shape will have to be measured from data For P odd combination, sig:bkg is better than 1:1! Numbers of Events for 4 fb-1 of data Cuts: b-tagged jet PT>45GeV, jet PT>35 GeV Mtop= invariant mass(blv): 155 GeV < Mtop < 195 GeVHT= Ptlepton + MET + Σall jets (jet PT): 180 GeV < HT < 250 GeV Matt Bowen 4/26/04

  24. t-channel conclusions • t-channel discovery is challenging because of large backgrounds and large systematic uncertainties • The jet direction – lepton direction correlation provides powerful discrimination between signal and background for t-channel production of single-top • Parity odd combinations of regions connected by parity transformations yield sig:bkg ratios better than 1:1, with systematics in tt and QCD largely cancelling • W+jets is the challenge for this method, not tt or QCD. It will require collaboration between theory and experiment to model the W+jets jet rapidity vs. lepton rapidity shape • Finding better cuts and better regions (R1,R2) will increase significance Matt Bowen 4/26/04

  25. Quick look at s-channel We haven’t yet worked on the s-channel, but let’s quickly look at the challenges ahead... b q t ve q' e+ b Now our final state looks slightly different: 2 b quarks, 1 lepton and a neutrino Matt Bowen 4/26/04

  26. Sample s-channel search Basic Cuts :1 lepton PT>15 GeV, |η|<2.0 MET > 15 GeV 2 b-tagged jets with PT>20 GeV, |η|<2.0 Advanced Cuts: Mtop=invariant mass(blv): 130 GeV < Mtop < 240 GeV HT=PTlepton+MET+ Σall jets (jet PT): 170 GeV < HT < 370 GeV Same problem! Large backgrounds with large systematic uncertainties Numbers of events for 4 fb-1 There has been speculation (hep-ph/9807340) that the s-channel cannot be found at the LHC – s-channel:tt ratio is ~ 1:220 Matt Bowen 4/26/04

  27. Future Directions • Think about strategies for s-channel at the Tevatron • Examine how new physics will alter the parity-odd, parity-even combinations • Single-top at the LHC. Now we have a P invariant initial-state. What can we do with it? • P invariant C invariant CP invariant P P Matt Bowen 4/26/04

  28. Final Conclusions • s-channel and t-channel are affected differently by new physics – measuring both is important • We’ve presented a t-channel correlation and method that will be useful in single-top discovery • Still more work to do on s-channel and LHC • Single-top discovery will be the “Flagship Measurement of Run II” – Z. Sullivan Thanks to Gordon Watts, Andy Haas, Henry Lubatti Matt Bowen 4/26/04

  29. Buffer Matt Bowen 4/26/04

  30. What QCD looks like • Light quark and gluon jets are sometimes identified as leptons • Energy mis-measurements can fake missing transverse energy (neutrino signature) • Final state is both P and C invariant mis-identified as lepton P P jet mistag as b-jet Matt Bowen 4/26/04

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