Jonathan A. Bagger
My research interests are illustrated in the following selection of recent papers.
Index:
QCD corrections to flavor changing neutral currents in the supersymmetric
standard model
Jonathan
A. Bagger, Konstantin
T. Matchev, Ren-Jie
Zhang
Eprint hep-ph/9707225,
Phys. Lett. B412 (1997) 77.
We compute the leading QCD corrections to K-Kbar mixing in the supersymmetric
standard model with general soft supersymmetry-breaking parameters. We construct
the \Delta S=2 effective Lagrangian for three hierarchies of supersymmetric
particle masses, namely, when the gluino mass is comparable to, much greater
than, or much less than the masses of the first two generation squarks.
We find that the QCD corrections tighten the limits on squark mass splittings
by more than a factor of two.
PDF or Compressed
Postscript
Gauge and Yukawa unification in models with gauge-mediated supersymmetry
breaking
Jonathan
A. Bagger, Konstantin
T. Matchev, Damien
M. Pierce, Ren-Jie
Zhang
Eprint hep-ph/9611229,
Phys. Rev. Lett. 78 (1997) 1002; (E) 78 (1997) 2497.
We examine gauge and Yukawa coupling unification in models with gauge-mediated
supersymmetry breaking. We work consistently to two-loop order, and include
all weak, messenger and unification-scale threshold corrections. We find
that successful unification requires unification-scale threshold corrections
that are in conflict with the minimal SU(5) model, but are consistent with
the modified missing doublet SU(5) model for small tan beta, and large tan
beta with mu>0.
PDF or Compressed
Postscript
Weak-scale phenomenology of models with gauge-mediated supersymmetry
breaking
Jonathan
A. Bagger, Konstantin
T. Matchev, Damien
M. Pierce, Ren-Jie
Zhang
Eprint hep-ph/9609444,
Phys. Rev. D55 (1997) 3188.
We study in some detail the spectral phenomenology of models in which supersymmetry
is dynamically broken and transmitted to the supersymmetric partners of
the quarks, leptons and gauge bosons, and the Higgs bosons themselves, via
the usual gauge interactions. We elucidate the parameter space of what we
consider to be the minimal model, and explore the regions which give rise
to consistent radiative electroweak symmetry breaking. We include the weak-scale
threshold corrections, and show how they considerably reduce the scale dependence
of the results. We examine the sensitivity of our results to unknown higher-order
messenger-sector corrections. We compute the superpartner spectrum across
the entire parameter space, and compare it to that of the minimal supergravity-inspired
model. We delineate the regions where the lightest neutralino or tau slepton
is the next-to-lightest supersymmetric particle, and compute the lifetime
and branching ratios of the NLSP. In contrast to the minimal supergravity-inspired
model, we find that the lightest neutralino can have a large Higgsino component,
of order 50%. Nevertheless, the neutralino branching fraction to the gravitino
and the light Higgs boson remains small, < 10^{-4}, so the observation
of such a decay would point to a non-minimal Higgs sector.
PDF or Compressed
Postscript
Precision corrections in the minimal supersymmetric standard model
Jonathan
A. Bagger, Konstantin
T. Matchev, Damien
M. Pierce, Ren-Jie
Zhang
Eprint hep-ph/9606211,
Nucl. Phys. B491 (1997) 3.
In this paper we compute one-loop corrections to masses and couplings
in the minimal supersymmetric standard model. We present explicit formulae
for the complete corrections and a set of compact approximations which hold
over the unified parameter space associated with radiative electroweak symmetry
breaking. We illustrate the importance of the corrections and the accuracy
of our approximations by scanning over the unified parameter space. We calculate
the supersymmetric one-loop corrections to the W-boson mass, MW, the effective
weak mixing angle, sin^2 theta_{eff}^{lept}, and the quark and lepton masses,
and discuss implications for gauge and Yukawa coupling unification. We also
compute the one-loop corrections to the entire superpartner and Higgs-boson
mass spectrum. We find significant corrections over much of the parameter
space, and illustrate that our approximations are good to O(1%) for many
of the superparticle masses.
PDF or Compressed
Postscript
Precision corrections to supersymmetric unification
Jonathan
A. Bagger, Konstantin
T. Matchev, Damien
M. Pierce
Eprint hep-ph/9501277,
Phys. Lett. B348 443 (1995).
We compute the full set of weak-scale gauge and Yukawa threshold corrections
in the minimal supersymmetric standard model, including all finite (non-logarithmic)
corrections, which we show to be important. We use our results to examine
the effects of unification-scale threshold corrections in the minimal and
missing-doublet SU(5) models. We work in the context of a unified mass spectrum,
with scalar mass M_0 and gaugino mass M_1/2, and find that in minimal SU(5)
with squark masses less than one TeV, successful gauge and Yukawa coupling
unification requires M_1/2 << M_0 and M_0 \simeq 1 TeV. In contrast,
we find that the missing-doublet model permits gauge and Yukawa unification
for a wide range of supersymmetric masses.
PDF or Compressed
Postscript
Supersymmetry Theory
The tensor Goldstone multiplet for partially broken supersymmetry
Jonathan
Bagger, Alexander
Galperin
Eprint hep-th/9707061,
Phys. Lett. B412 (1997) 296.
We show that the tensor gauge multiplet of N=1 supersymmetry can serve as
the Goldstone multiplet for partially broken rigid N=2 supersymmetry. We
exploit a remarkable analogy with the Goldstone-Maxwell multiplet of hep-th/9608177
to find its nonlinear transformation law and its invariant Goldstone action.
We demonstrate that the tensor multiplet has two dualities. The first transforms
it into the chiral Goldstone multiplet; the other leaves it invariant.
PDF or Compressed
Postscript
New Goldstone multiplet for partially broken supersymmetry
Jonathan
Bagger, Alexander
Galperin
Eprint hep-th/9608177,
Phys. Rev. D55 1091 (1997).
The partial spontaneous breaking of rigid N=2 supersymmetry implies the
existence of a massless N=1 Goldstone multiplet. In this paper we show that
the spin-(1/2,1) Maxwell multiplet can play this role. We construct its
full nonlinear transformation law and find the invariant Goldstone action.
The spin-1 piece of the action turns out to be of Born-Infeld type, and
the full superfield action is duality invariant. This leads us to conclude
that the Goldstone multiplet can be associated with a D-brane solution of
superstring theory for p=3. In addition, we find that N=1 chirality is preserved
in the presence of the Goldstone-Maxwell multiplet. This allows us to couple
it to N=1 chiral and gauge field multiplets. We find that arbitrary Kahler
and superpotentials are consistent with partially broken N=2 supersymmetry.
PDF or Compressed
Postscript
Matter couplings in partially broken extended supersymmetry
Jonathan
Bagger, Alexander
Galperin
Eprint hep-th/ 9406217,
Phys. Lett. B336 25 (1994).
We use nonlinear realizations to describe the spontaneous breaking of N=2
supersymmetry to N=1 in four dimensions. We identify the Goldstone multiplet
with an N=1 chiral superfield, and show that chiral N=1 matter is consistent
with the partially broken N=2 supersymmetry. We find that the chiral matter
can be in any representation of the gauge group; no mirror particles are
required. We present the Goldstone action and the general couplings to N=1
matter to the first nontrivial order in the scale of symmetry breaking.
PDF or Compressed
Postscript
Destabilizing divergences in supergravity theories at two loops
Jonathan
Bagger, Erich
Poppitz, Lisa
Randall
Eprint hep-ph/9505244,
Nucl. Phys. B455 59 (1995).
We examine the stability of the mass hierarchy in hidden-sector supergravity
theories. We show that a quadratically divergent tadpole can appear at two
loops, even in minimal supergravity theories, provided the theory has a
gauge- and global-symmetry singlet with renormalizable couplings to the
visible fields. This tadpole can destabilize the hierarchy. We also find
a quadratically divergent two-loop contribution to the field-dependent vacuum
energy. This result casts doubt on the efficacy of the "LHC mechanism"
for controlling quadratic divergences. We carry out the two-loop calculation
in a manifestly supersymmetric formalism, and explain how to apply the formalism
in the presence of supersymmetry breaking to derive radiative corrections
to the supersymmetric and soft supersymmetry-breaking operators. Our approach
greatly simplifies the calculation and guarantees consistency of our results
with the underlying supergravity framework.
PDF or Compressed
Postscript
The R axion from dynamical supersymmetry breaking
Jonathan
Bagger, Erich
Poppitz, Lisa
Randall
Eprint hep-ph/9405345,
Nucl. Phys. B426 3 (1994).
All generic, calculable models of dynamical supersymmetry breaking have
a spontaneously broken R symmetry and therefore contain an R axion. We show
that the axion is massive in any model in which the cosmological constant
is fine-tuned to zero through an explicit R-symmetry-breaking constant.
In visible-sector models, the axion mass is in the 100 MeV range and thus
evades astrophysical bounds. In nonrenormalizable hidden-sector models,
the mass is of order of the weak scale and can have dangerous cosmological
consequences similar to those already present from other fields. In renormalizable
hidden- sector models, the axion mass is generally quite large, of order
10^7 GeV. Typically, these axions are cosmologically safe. However, if the
dominant decay mode is to gravitinos, the potentially large gravitino abundance
that arises from axion decay after inflation might affect the successful
predictions of big-bang nucleosynthesis. We show that the upper bound on
the reheat temperature after standard inflation can be competitive with
or stronger than bounds from thermal gravitino production, depending on
the model and the gravitino mass.
PDF or Compressed
Postscript
Electroweak Phenomenology
LHC analysis of the strongly interacting WW system: gold-plated modes
J.
Bagger, V.
Barger, K.
Cheung, J.
Gunion, T.
Han, G.
A. Ladinsky, R.
Rosenfeld, C.-P.
Yuan
Eprint hep-ph/9504426,
Phys.Rev. D52 3878 (1995).
We study the gold-plated purely leptonic signal and background rates at
the LHC for the ZZ, W^+W^-, W^\pm Z and W^\pm W^\pm final states associated
with strongly interacting electroweak symmetry breaking. We work at an energy
of \sqrt s = 14 TeV, and develop a combination of back-to-back leptonic,
central-jet-vetoing and forwaring cuts that suppresses the Standard-Model
backgrounds. We find that the LHC with an annual luminosity of 100 fb^{-1}
will achieve a reasonably good sensitivity to the physics of strongly interacting
electroweak symmetry breaking.
PDF or Compressed
Postscript
Higgs mechanism in string theory
J.
Bagger, I.
Giannakis
Eprint hep-th/9703202,
Phys. Rev. D56 (1997) 2317.
In first-quantized string theory, spacetime symmetries are described by
inner automorphisms of the underlying conformal field theory. In this paper
we use this approach to illustrate the Higgs effect in string theory. We
consider string propagation on M^{24,1} \times S^1, where the circle has
radius R, and study SU(2) symmetry breaking as R moves away from its critical
value. We find a gauge-covariant equation of motion for the broken-symmetry
gauge bosons and the would-be Goldstone bosons. We show that the Goldstone
bosons can be eliminated by an appropriate gauge transformation. In this
unitary gauge, the Goldstone bosons become the longitudinal components of
massive gauge bosons.