Forward-Backward Asymmetry of Top quark
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Forward-Backward Asymmetry of Top quark In Unparticle Physics. Choong Sun Kim (C.S. Kim). Department of Physics, Yonsei University, Korea. ICPP-Istanbul II. 2 nd International Conference on Particle Physics. Outline. Motivation  Observation at Tevatron

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Forward-Backward Asymmetry of Top quark

In Unparticle Physics

Choong Sun Kim (C.S. Kim)

Department of Physics, YonseiUniversity, Korea

ICPP-Istanbul II

2nd International Conference on Particle Physics


  • Motivation  Observation at Tevatron

  • Phenomenon & History over Q-FBA

  • Possible Models & Mechanisms

  • Our results in Unparticle Physics

  • Summary

1. Observations at TevatronD PRL100, 142002 (2008)

  • Process: p pbar→ t tbar + X;

    the first measurement of the

    integrated forward-backward (FB) charge asymmetry

  • D Observable:

  • Nf(b): number of events with a positive (negative) Δy

    y = yt– ytbar, y: rapidity

Observations at TevatronCDF: PRL101, 202001 (2008)

  • CDF observable:

  • With 1.9 fb−1

  • With 3.2 fb-1

2. Phenomenon

  • If CP is invariant,


  • Integral charge asymmetry

    Ni (j): # of particle i observed in

    the direction of particle j

  • Forward-backward asymmetry

History over Heavy Quark FBA

Brown et al, PRL43(1979)1069; FBA of c-quak production in analogy to QED.

[1] F. Halzen, P. Hoyer, C.S. Kim, PLB195(1987)74;

FBA of b,t-quark in QCD from (i) radiative corrections to q-qbar annihilation,

(ii) interference between different contributions to qg t+tbar+q, (and its CP conj.)

~~~ no more progress about 10 years

[2] J.H. Kuhn, G. Rodrigo, PRL81(1998)49; detailed

QCD calculation

[3] O. Antunano, J.H. Kuhn, G. Rodrigo, PRD77(2008)014003;

 FBA of t-quak within the SM = 0.050 +- 0.015

D0 and CDF measured FBA ~ 20%

 Now so many works beyond the SM

SM prediction Kuhn& Rodrigo PRD59(99); Antunanoetal PRD77(08);

  • Leading order (d): charge

    asymmetry vanishes

  • Next-to-leading order: nonzero AC will be induced by the interference between (a) and (b) and the

    interference between (c) and (d)

  • Since the measurements of D and CDF have 2 deviation from the SM, the observed large FBA strongly

    indicate the new Physics

3. Models by s-channelFerrario & Rodrigo PRD80(09); Frampton etal, arXiv:0911.2955

  • Axigluon: Color-octet gauge bosons (AGs) that couple to quarks with a

    nonvanishing axial-vector couplings

  • It could be generated by symmetry breaking from SU(3)RSU(3)L to


  • AG could be as heavy as TeV

Models by s-channelDjouadiMoreau, Richard,Singh, arXiv:0906.0604

  • KK excitations of gluons in

    warped extra dimensional


  • The couplings of KK to quarks

    have the axial-vector component

  • AtFB|RS~ few %

Models by t-channelJung, Murayama, Pierce, Wells, arXiv:0907.4112

  • New Z’ gauge boson

  • Nonuniversal Z’ model

Models by t-channelCheung,Keung, Yuan PLB682(09); Barger, Keung, Yu: arXiv:1002.1048

Model-independent approachD.W. Jung, P. Ko, J.S. Lee, S.H. Nam, arXiv:0912.1105

  • Effective interactions

SUSY R-parity violationby J. Chao, Z. Heng, L. Wu and J.M. Yang, arXiv:0912.1447

Diquark modelsShu,Tait, Wang :0911.3237, Arhrib, Benbrik,Chen:0911.4875, Dorsneretal :0912.0972

  • Diquark is the particle that

    couples to quarks by qaT C qbHab , where a, b are the color indices

  • The possible representations

    under SU(3)CSU(2)LU(1)Y :

    (6, 3)1/3 , (6,1) 4/3,1/3,-2/3 ,

    (3, 3)-1/3 ,, (3,1)2/3,-1/3,-4/3

  • We would like to pay attention to (3,1,-4/3) and (6,1,4/3)

  • The relevant interactions

  • The corresponding

    Feynman diagram

4. Unparticle Physics, s-channelC.H. Chen, Gorazd Cvetic, C.S Kim, PLB694(2011)393



Cacciapaglia, Marandella, Terning,


A stuff dictated by scale invariance, proposed by Georgi

Here, we considered colored unparticle, in which the effective interactions to quarks are proposed to be

Due to nonintegral scale dimension and no mass scale in the

propagator, it will be interesting to see the unparticle effects on the top-quark FBA



Scattering amplitude combined with the SM becomes

Spinand color averaged amplitude-square is

3 physical observables are:

3 independent parameters: ( , )

first we set

2 remaining dimensionless parameters are

from assuming flavor blind (colored) unparticle

and finally the scale dimension

 i.e. the flavor-blind and chirality-independent couplings

Experimental numerical constraints:

M. Cacciari et al., JHEP 0809, 127 (2008); N. Kidonakis and R. Vogt, Phys. Rev. D 78, 074005 (2008); S. Moch and P. Uwer, Phys. Rev. D 78, 034003 (2008)

 d_U=1.28+-0.22, lambda=2.05+-0.45

Phys.Rev.Lett.102:222003,2009 (CDF).

For detailed analysis, cosiderMtt-restricted FBA,

5. Summary

  • An unexpected large FBA of top-quark is observed by D and CDF: strongly indicate the new physics effects (NLO effects).

  • Various possible solutions to the “anomaly” are proposed, like axigluon, KK excitations of gluon, Z’, W’, colored scalars, etc

  • We investigated whether colored flavor-conserving unparticle

  • can explain the measured FBA of ttbar production at Tevatron.

  • With a natural assumption of quark flavor-blind and chirality-

  • independent unparticle, UP contribution can explain the

  • deviation.

Thank you for your attention

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