Single-top Physics at the Tevatron Matt Bowen with Matt Strassler, Steve Ellis University of Washington, Seattle

Matt Bowen 4/26/04 2 Outline 1.What single-top is 2.Why it is worthwhile to study 3.Research we’ve done 4.Future research directions 5.Conclusions

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

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

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

Matt Bowen 4/26/04 6 Why is single-top interesting? 1.Cross-section measurement yields a measurement of V tb 2.Is a background to other searches (Higgs, etc...) t-channel 1.98 pb s-channel 0.88 pb V tb Practical Reasons

Matt Bowen 4/26/04 7 “Extra” reasons 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: Z tc 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. Main reference: “Single Top Production as a Window to Physics Beyond the Standard Model” T. Tait hep-ph/ Affect s-channel Affect t-channel

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 M π± = 250 GeV, t R -c R 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/M π± 2 and that π ± doesn’t couple to light quarks reference: hep-ph/ time-like momentum allows for resonance

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

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 (W kk ) For M Wkk = 1TeV, s-channel amplitudes interfere destructively to reduce cross-section by 25% t-channel contributions are suppressed by 1/M W' 2 W kk

Matt Bowen 4/26/04 11 Extra quark generations: CKM constraints For 3 generations, the unitary of the CKM matrix constrains |V ts | < With >3 generations, one possibility is |V tb |=0.83 and |V ts |=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 Without imposing 3 family unitarity, these are the 90% CL direct constraints. V ts s s

Matt Bowen 4/26/04 12 Extra Couplings * : FCNC: Z-t-c Can argue that low energy constraints (κ Ztc < 0.3) may not apply in the presence of additional new physics For κ Ztc = 1, t-channel increases 60% These couplings change top decay structure κ Ztc recently constrained by LEP II data to be < ~ 0.5 (hep- ex/ ) c c Z *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 Plot from hep-ph/ t-channel CS has changed to 1.98pb ED from hep-ph/ SM prediction 3σ theoretical deviation Charged top-pion FCNC Z-t-c vertex 4 gen Top-flavor model Extra dimensions

Matt Bowen 4/26/04 14 Lessons 1)t-channel is affected by modifications to top quark couplings 2)s-channel is affected by heavy particles 3)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  1 non b-tagged jet (from light quark)  1 lepton  1 b-tagged jet  Missing Transverse Energy (from neutrino) b e+ ve not seen Detector Objects from t-channel

Matt Bowen 4/26/04 16 Numbers for a Sample Search Advanced Cuts: Same, except b-tagged jet PT>60GeV, other jet PT>30 GeV M top =invariant mass(blv): 160 GeV < M top < 190 GeV HT=PT lepton +MET+ Σ all jets (jet PT): 180 GeV < HT < 250 GeV Basic sig:bkg ratio is 1:15 Advanced sig:bkg is 1:4 Systematics prevent discovery ChannelsEvents for Basic Cuts Advanced cuts Systematic Uncertainty t-channel44332>15% s-channel19216>10% W+jets >10% tt320048>10%  Studies done with Madgraph + Pythia + Fast Detector Simulation for 4 fb -1 Basic Cuts : 1 lepton PT>15 GeV, |η| 15 GeV 1 b-tagged jet with PT>20 GeV, |η| 20 GeV, |η|<3.5

Matt Bowen 4/26/04 17 CP Invariance of the Tevatron PP 1.pp initial state at Tevatron is CP invariant, but not C or P invariant separately 2.At leading order, processes that proceed through an s- channel gluon “forget” that they are not separately C and P invariant (tt and QCD) 3.Processes with W’s “remember” that they are not separately C and P invariant (single top and W+jets) Initial State PP Under C or P transformation PP Under CP

Matt Bowen 4/26/04 18 What t-channel looks like P P u g t e+ b-tagged jet “the jet” b veve ~ 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 Usually don’t see the b! d Direction of jet is correlated with top spin, which is correlated with lepton direction b

Matt Bowen 4/26/04 19 t-channel: Q lepton *jet rapidity vs. Q lepton * lepton rapidity 1.Q lepton is the sign of the charge of lepton 2.jet: highest PT non b-tagged jet 3.Under P or C: x  -x, y  -y 4.t-channel is not P or C invariant 5.Under CP, the plot is invariant Q lepton * jet rapidity Q lepton * lepton rapidity Contours of Constant Cross-section CDF has looked at Q lepton * jet rapidity by itself

Matt Bowen 4/26/04 20 What W+jets looks like 1.Boosted in proton direction for W+ production, anti- proton direction for W- production 2.Final state is not P or C invariant 3.Jets from light quarks and gluons can be misidentified as b-jets P Q lepton * Jet Rapidity Q lepton * Lepton Rapidity e+ veve W+ P Contours of Constant Cross-section b-tag jet x > 0 y > 0

Matt Bowen 4/26/04 21 What tt looks like u u t t 1.Doesn’t tend to be boosted in either direction 2.Final state is both P and C invariant at leading order P P jet veve e+ u d Q lepton * Jet Rapidity Q lepton * Lepton Rapidity b-tag b x = 0 y = 0

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

Matt Bowen 4/26/04 23 Parity Even and Odd Combinations of R1 and R2 ChannelR1+R2R1-R2 t-channel6432 s-channel160 W+jets13523 tt960 1.R1+R2 R1+R2 under P 2.R1-R2 -(R1-R2) under P 3.tt and QCD are zero for P odd combination 4.Systematic errors in tt and QCD largely cancel 5.W+jets shape will have to be measured from data 6.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 M top = invariant mass(blv): 155 GeV < M top < 195 GeV HT= Pt lepton + MET + Σ all 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... Now our final state looks slightly different: 2 b quarks, 1 lepton and a neutrino q q'q' t b b e+ veve

Matt Bowen 4/26/04 26 Sample s-channel search There has been speculation (hep- ph/ ) that the s-channel cannot be found at the LHC – s- channel:tt ratio is ~ 1:220 ChannelBasicAdvanced s-channel8056 t-channel6852 W-jets tt Numbers of events for 4 fb -1 Advanced Cuts: M top =invariant mass(blv): 130 GeV < M top < 240 GeV HT=PT lepton +MET+ Σ all jets (jet PT): 170 GeV < HT < 370 GeV Basic Cuts : 1 lepton PT>15 GeV, |η| 15 GeV 2 b-tagged jets with PT>20 GeV, |η|<2.0 Same problem! Large backgrounds with large systematic uncertainties

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? PP P invariant C invariant CP invariant

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 P P jet mistag as b-jet mis-identified as lepton