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The top quark, effective operators, FCNC and the LHC. Rui Santos (U. Southampton). WHEPP XI, Ahmedabad 2010. the past:. - Indirect limits – cross sections converted to branching ratios - Direct limits – from top decays (Tevatron only). Do indirect limits have any meaning?.

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the top quark effective operators fcnc and the lhc

The top quark, effective operators, FCNC and the LHC

Rui Santos

(U. Southampton)

WHEPP XI, Ahmedabad 2010

slide2
the past:

- Indirect limits – cross sections converted to branching ratios

- Direct limits – from top decays (Tevatron only)

Do indirect limits have any meaning?

Can we do slightly better?

slide5

FCNC (top) in the SM and beyond

Any hint of FCNC welcome

Very old table from J. Aguilar-Saavedra– anyone has a new column to add?

slide6

Effective operator approach

Λ is the scale of new physics.

The X particle is from a new untested model (the same SM particles in the external lines) . It can have the same or a different Lorentz structure.

X

strong

electroweak

All terms are invariant under the SM gauge group.

slide7

Operators and observables

In a simple world, in the top FCNC case suppose we had only three observables and three operators! However…

…this is not the case. Therefore we have to find as many physical observables as possible. The number of operators we already know is huge.

Even in this simple case we need● Great precision from the theoretical side● Great precision from the experimental sideto look for the small deviations from the SM.

slide8

Different from the usual approach used by experimentalists (which will be different in top physics at the LHC):

a) write a piece of a lagrangian that contains your favorite interaction

b) search for some final state with a diagram that contains the anomalous vertex

c) further reduce the number of free parameters by setting for instance f=h

d) using the experimental result, set a bound on the remaining constant

slide9

Why is it different?

a) take the operators (there are much more)

b) contribution to FCNC top decays

c) contribution to single top production

slide10

Why is it different?

d) and to t tbar production

e) contribution to top + gauge boson

f) and to non-FCNC top decays

diy a minimal set of effective operators for top physics
DIY- a minimal set of effective operators for top physics

1. Write all possible dimension 6 operators that have at least one top quark

Complete list of dimension 6 effective operators:

Buchmüller e Wyler, Nucl. Phys. B268 (1986) 621.

2. Use equations of motion to further reduce the number of operators

results
Results

1. single top FCNC production (in pb) is not only direct production

DIRECT

GLUON-GLUON

GLUON-QUARK

Surprisingly the cross is proportional to the branching ratio

(these results are for "strong" operators only)

results1
Results

2. 4-fermion operators spoils the beautiful proportionality to the BR

results2
Results

3. Including the "electroweak" opeartors - It can be strong or electroweak

electroweak

strong

The electroweak sector can be probed even if the strong sector is negligible.

Excess in the cross section could be wrongly interpreted as coming from FCNC in the strong sector.

The numbers have to be added to the SM cross section.

results3
Results

3a. Tevatron LHC10 and LHC14

results4
Results

4. Electroweak or strong?

Look for observables that distinguish electroweak from strong effective couplings.

Sector where new physics comes from can (sometimes) be tested

results5
Results

5. relating observables

1. Use the Tevatron results for single top production.

2. B physics constraints from

"Deciphering top flavor violation at the LHC with B factories" Fox et al,Phys. Rev. D78 054008, 2008.

Tevatron bound

discussion
Discussion

This looks good but…

  • How to distinguish between the different operators?
  • How model independent is the approach? What is the scale of new physics we are probing?
  • Where do we find theories that predict large FCNC related to top?
  • Anomalous non-FCNC effective couplings can be used for the Tevatron asymmetry – what can we say with the effective theory approach?