Precision Measurements and New Physics at the Top

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Precision Measurements and New Physics at the Top

• Carlos E.M. Wagner
• EFI, University of Chicago
• HEP Division, Argonne National Lab.

Based, partially, on the following works D.
Choudhury, T. Tait and C.W., PRD65053002,
2002 D. Morrissey and C.W., PRD69053001, 2004
Pre-Susy 2005 Meeting, Univ. of Durham, England,
July 2005
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Standard Model

• Gauge Theory Based on the group
• All particle interactions of the three families
  of quarks, charged leptons and neutrinos well
  described by the Standard Model (SM)
• Excellent description of all experimental
  observables
• Includes heavy particles, like the top quark and
  the weak gauge bosons, as well as the almost
  massless neutrinos.

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Quantum numbers under SU(3) x SU(2) x U(1)


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Couplings to the Z-boson

• In the SM, the neutral gauge bosons are
  admixtures of the hypercharge gauge boson and the
  neutral weak boson
• Right and left-handed particles interact with the
  photon with a
• coupling equal to (Q e), and a charge equal
  to
• and to the Z with a coupling
• Since Q does not depend on the chirality of the
  particle, but T3
• does, the Z couples differently to right and
  left-handed components of a given Dirac particle.
  This is not a surprise, due to the chiral nature
  of weak interactions.

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Measuring the parameters of the SM

• The chiral properties of the couplings may be
  tested. Lets ignore fermion masses, and see how
  we can do this
• Imagine we have an electron, with a given
  helicity, and which collides
• with a positron, with the opposite
  helicity, producing an on-shell Z. Let us
• assume that the Z decay products go in the
  same direction as the electron-
• positron system and there is no orbital
  angular momentum.
• Once the Z decays, into, i.e. two leptons
  (quarks), the fermion and the
• antifermion must also have opposite
  helicities (angular momentum
• conservation).
• If the lepton (quark) from the decaying Z has the
  same helicity as
• the original electron, it should move
  forward (same direction as the electron)
• If, instead, the antilepton (antiquark) has
  the same helicity as the original
• Electron, then the lepton (quark) should
  move backwards.
• But since leptons and quarks of different
  helicities couple differently to
• the Z, then the amount of forward and
  backward leptons (quarks) will not
• be the same.

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Forward-Backward Asymmetries

• If the electrons are left- (right-) handed the
  difference between the
• forwardly produced quarks (leptons) is
  proportional to the difference
• between the production rate of left- (right-)
  and right (left-) handed
• fermions,
  .
• For a beam of unpolarized electrons colliding
  against protons, the forward-
• backward asymmetry, defined as the difference
  between forward and
• backward produced fermions, will be
  proportional to
• Although this picture is very naïve, it leads
  to the proper intiution. In
• chiral theories, like the weak interactions,
  there is an asymmetry
• between forward and backward produced
  fermions in lepton-antilepton
• (quark-antiquark) collisions.

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Technicalities

• Lets take as an example the LEP1 collider,
  working at energies close to the Z-pole ones.
  Then, the angular distribution of fermions
• Again, this result confirms our naïve result
  obtained before, by
• different methods.

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Help from Nature

• Measurement of the electron forward-backward
  asymmetry allows
• us to determine the electron left-right
  asymmetry . Now, since
• , then the right- and
  left-handed electrons to Z-bosons
• are close to each other and the electron
  left-right asymmetry is very
• small.
• On the other hand, since the bottom quark charge
  is 1/3, the left-
• right-asymmetry of the bottom quarks is
  close to 1. Numerically,
• calling

• Radiative corrections, mass corrections, as
  well as terms higher-order
• in delta must be included, to obtain
  information about . These
• corrections affect the value of the weak
  mixing angle at the few per mille level.
• The message is the following A good
  determination of the lepton and/or
• b-asymmetries allows a good determination of
  the weak mixing angle.

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Sensitivity to the Higgs Mass

• Once radiative corrections are included, the
  value of the weak mixing angle depends
  logarithmically on the Higgs mass.
• Since the weak mixing angles are well measured
  from the lepton and bottom quark asymmetries,
  this parameter provides the best
• indirect information about the possible
  value of the Higgs mass.
• Just to show how sensitive For a top quark mass
  of 175 GeV, values of
• A variation of the top quark mass of 10 GeV
  leads to a variation
• of

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Problem with Higgs mass fit

• The measurements of the weak mixing angle from
  the hadron and lepton
• asymmetries have become so precise, that
  they introduce a potential problem. The present
  values are
• The weighted average, of approximately 0.2315,
  leads to a Higgs mass
• of about 100 GeV, but this average comes
  from two very different
• determinations, one prefering very low
  Higgs masses and the other
• somewhat large Higgs masses !
• If we ignored the quark asymmetries, the Higgs
  mass would be shifted
• towards unacceptably low values, excluded
  by LEP ! Evidence for a light
• Higgs is weakened by these facts
  (Chanowitz01, Langacker and Erler00)

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Attitude Toward this problem

• Possible, sensible attitude Ignore it ! Looking
  for hints of new physics in experimental data,
  unless there is a very compelling hint, is not a
  well regarded activity.
• Assume that quark asymmetries are wrong and try
  to find new
• physics that leads to a shift of the Higgs
  mass. This was done
• by Altarelli et al01, and later by Carena,
  Tait, Ponton and C.W.02
• Only problem with this is how to justify
  that hadron data should be disregarded
• Assume that all experiments are correct and try
  to find new physics
• that leads to an explanation of the
  difference between the two values of the weak
  mixing angle. This is what well do, by trying to
• modify the effective couplings of the bottom
  quark to the Z.

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More on couplings to the Z boson

• In the SM fermions of different generations, with
  the same electric charge mix with each other.
• But since the gauge interactions preserve
  chirality, and fermions of the same chirality and
  charge couple in the same way to the Z, this
• mixing does not induce any flavor neutral
  current.
• This property may be changed in models that go
  beyond the SM description, in which there are
  left-handed fermions, which mix with the SM ones,
  and have with T3 0, or right-handed fermions
  with
• T3 different from zero.
• The part of the Z coupling proportional to Q will
  not be affected by
• the mixing, but the part proportional to T3
  will.

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b2,L and b2,R may be in any
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Left and right handed doublets
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Comment on Higgs Couplings

• In the SM, the masses proceed from Yukawa
  couplings to the Higgs
• field, which acquires a v.e.v.
• Once one diagonalize the mass matrix, the Yukawa
  couplings are
• automatically diagonalized and therefore,
  there are no flavor changing neutral currents
  associated with the neutral Higgs.
• In models like the one under consideration, there
  are masses that
• have nothing to do with the Higgs, and this
  is no longer true.
• In addition, the right-handed bottom has a large
  component on the
• new, new right-handed quark, which does not
  couple to the Higgs.
• This implies that the bottom coupling will be
  reduced and that there
• will be flavor changing neutral currents
  associated with the production of a bottom quark
  and a heavy down quark.

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Run I average )
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