34th international conference on high energy physics

1Single Top Quark Studies, L. Li (UC Riverside) ICHEP 08, July 30 2008

Liang Li University of California, Riverside

for the CDF, DØ and H1 Collaborations

Single Top Quark Studies

34th International Conference on High Energy Physics

34th International Conference on High Energy Physics


Single Top Quark Production at Single Top Quark Production at TevatronTevatron

Single Top Quark Production at Single Top Quark Production at TevatronTevatron

Single Top: Electroweak Interaction• Study Wtb coupling

• Direct measurement of the |Vtb| CKM matrix element• Test of CKM unitarity• Anomalous Wtb couplings

• New physics, example:• s-channel is sensitive to W’, H+

• t(s)-channel is sensitive to FCNC• 4th quark generation?

• Study top properties:• Polarization, decay width, lifetime, CP violation …

• Background study helps other physics searches, e.g. SM Higgs

Tev : ~1 pb ~ 2pb ~ 0.25 pb s-channel(“tb”)

t-channel(“tqb”)

tW production(not yet seen)


CKM Matrix Element |VCKM Matrix Element |Vtbtb||CKM Matrix Element |VCKM Matrix Element |Vtbtb||Direct measurement of |Vtb| from cross section measurement:

• Calculate posterior in |Vtb|2 : |Vtb|2

Assuming standard model production:• Pure V-A and CP conserving Wtb interaction• |Vtd|2 + |Vts|2 << |Vtb|2

• Additional theoretical errors needed (top mass, scale, PDF etc…)No need to assume three quark generations or CKM unitarity

|Vtb| = 0.88 ± 0.14 (exp.) ± 0.07 (theory)|Vtb| = 1.00 + 0.00 (exp.+ theory)- 0.12


Anomalous Wtb CouplingsAnomalous Wtb CouplingsAnomalous Wtb CouplingsAnomalous Wtb Couplings

Top couplings to the W boson is a good place to look for deviations from SM

• Effective single top production cross section: = A (f1L)2 + B (f1

R)2 + C (f1

L + f2R)2 + D (f2

L + f1R)2

• f1: “Vector”-like, f2: “Tensor”-like• SM (left-handed vector coupling, LV ): f1

L=1, rest=0• Contributions also from left-handed left-handed tensor (LT), right-

handed vector (RV), right-handed tensor (RT) couplings

• Single top production directly sensitive to the Wtb interaction: rate, kinematics and angular distributions

Standard Model: ~ 2.9 pb

f2L(R)=1, rest=0 : ~ 10.4 pb


Anomalous Wtb CouplingsAnomalous Wtb CouplingsAnomalous Wtb CouplingsAnomalous Wtb Couplings• Use same multivariate analysis

technique and event selection as for DØ “single top evidence” analysis (Phys. Rev. Lett. 98,181802(2007))

• Simultaneous limit setting for two signals by calculating 2 dimensional posterior probability density

arXiv:0807.1692 [hep-ex]

|Vtbf1L| = 1.31 + 0.25

- 0.21

First direct constraints on Wtb tensor couplings• Data favor the standard model

left-handed vector hypothesis over the alternative hypotheses

• Compare with SM value


Heavy W’ ResonanceHeavy W’ ResonanceHeavy W’ ResonanceHeavy W’ ResonanceSingle top quark decay channel is a promising

searching background for heavy W’ with SM-like couplings• W’ decays leptonically into a single top

quark

• Same final states as single top production

• Concentrate on s-channel production• Enhancement from W’ resonance

• Consider purely left- and right-handed states• Left-handed couplings W’

• W-W’ interference

• DØ includes this term in signal model

• Right-handed W’R with M(W’R) > M(R)

• Decays to (l, ) and (q,q)

• Right-handed W’R with M(W’R) < M(R)

• Decays to (q,q) only


Heavy W’ ResonanceHeavy W’ ResonanceHeavy W’ ResonanceHeavy W’ Resonance• Four-vectors of all final state objects are added to obtain the invariant

mass of the W’ boson• Use the mass distribution to separate W’ from background and set limit


Charged Higgs ResonanceCharged Higgs ResonanceCharged Higgs ResonanceCharged Higgs ResonanceDiscovery of charged Higgs boson is an

unambiguous evidence for new physics: no “charged” Higgs boson in SM• Identical final state as s-channel single top

quark electroweak production• Three Two-Higgs Doublet Model (2HDM)

studied:• Type-I, II and III have different strategies to

give mass to fermions• tb decay channel dominates for large region

of parameter space

First direct search for heavy (mH+ > mtop ) H+ decaying to top• Reconstructed invariant mass M(jet1, jet2,

W) showed to be the single most sensitive variable, used to construct binned likelihood function

• Upper limits set close to predicted cross sections• Excluded parameter region for 2HDM Type-I:

180< mH+<184 GeV with 23 < tan< 7095%CL upper limit

on 2HDM type-I,III H+


Flavor-Changing Neutral CurrentsFlavor-Changing Neutral CurrentsFlavor-Changing Neutral CurrentsFlavor-Changing Neutral CurrentsAny evidence for FCNC will be a signal

of new physics beyond the SM • FCNC cross sections scale with

(gtu/)2 and (gtc/)2

• Use same event selection as in CDF standard model single top production measurement (CDF note 9251) except:• Require exactly one hadronic jet

(instead of two jets)

Assume no anomalous coupling


Anomalous Single Top Anomalous Single Top Production at HERAProduction at HERA

Anomalous Single Top Anomalous Single Top Production at HERAProduction at HERA

Looking for new physics at HERA: anomalous single top production via FCNC• Highest sensitivity to tu

• Coupling to Vtuz suppressed due to high Mz

tc suppressed due to low c-density

• Events with e + PTmiss in HERA I+II e+p data

• Form top/W discriminator (MVA method)

@ 95% CL

478 pb-1


SummarySummarySummarySummary

Single top quark production provides a rich testing ground for SM new physics• Five studies from three collaborations so far

• |vtb|, Anomalous Wtb coupling, W’/H+ tb resonances, FCNC production

• Many improved limits

• More studies and data to come, stay tuned!

34th international conference on high energy physics

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