2,975 research outputs found

### Brane Supersymmetry Breaking

We show how to construct chiral tachyon-free perturbative orientifold models,
where supersymmetry is broken at the string scale on a collection of branes
while, to lowest order, the bulk and the other branes are supersymmetric. In
higher orders, supersymmetry breaking is mediated to the remaining sectors, but
is suppressed by the size of the transverse space or by the distance from the
brane where supersymmetry breaking primarily occurred. This setting is of
interest for orbifold models with discrete torsion, and is of direct relevance
for low-scale string models. It can guarantee the stability of the gauge
hierarchy against gravitational radiative corrections, allowing an almost exact
supergravity a millimeter away from a non-supersymmetric world.Comment: 15 pages, LaTe

### Split Supersymmetry in String Theory

Type I string theory in the presence of internal magnetic fields provides a
concrete realization of split supersymmetry. To lowest order, gauginos are
massless while squarks and sleptons are superheavy. For weak magnetic fields,
the correct Standard Model spectrum guarantees gauge coupling unification with
\sin^2{\theta_W}=3/8 at the compactification scale of M_{\rm GUT}\simeq 2
\times 10^{16} GeV. I discuss mechanisms for generating gaugino and higgsino
masses at the TeV scale, as well as generalizations to models with split
extended supersymmetry in the gauge sector.Comment: 7 pages, 3 figures, prepared for the Proceedings of PASCOS-05 and of
CORFU2005 Summer Institut

### Topics on String Phenomenology

These lectures present some topics of string phenomenology and contain two
parts. In the first part, I review the possibility of lowering the string scale
in the TeV region, that provides a theoretical framework for solving the mass
hierarchy problem and unifying all interactions. The apparent weakness of
gravity can then be accounted by the existence of large internal dimensions, in
the submillimeter region, and transverse to a braneworld where our universe
must be confined. I review the main properties of this scenario and its
implications for observations at both particle colliders, and in
non-accelerator gravity experiments. In the second part, I discuss a simple
framework of toroidal string models with magnetized branes, that offers an
interesting self-consistent setup for string phenomenology. I will present an
algorithm for fixing the geometric parameters of the compactification, build
calculable particle physics models such as a supersymmetric SU(5) Grand Unified
Theory with three generations of quarks and leptons, and implement low energy
supersymmetry breaking with gauge mediation that can be studied directly at the
string level.Comment: 42 pages, 15 figures, Lectures given at Les Houches 2007 Summer
School, Added reference

### The Physics of Extra Dimensions

Lowering the string scale in the TeV region provides a theoretical framework
for solving the mass hierarchy problem and unifying all interactions. The
apparent weakness of gravity can then be accounted by the existence of large
internal dimensions, in the submillimeter region, and transverse to a
braneworld where our universe must be confined. I review the main properties of
this scenario and its implications for observations at both particle colliders,
and in non-accelerator gravity experiments. Such effects are for instance the
production of Kaluza-Klein resonances, graviton emission in the bulk of extra
dimensions, and a radical change of gravitational forces in the submillimeter
range. I also discuss the warped case and localization of gravity in the
presence of infinite size extra dimensions.Comment: 29 pages, 11 figures. Lectures to appear in the proceedings of the
Third Aegean Summer School, Karfas, Chios, Greece, 26 September-1 October
200

### Mass scales in string and M-theory

I review the relations between mass scales in various string theories and in
M-theory. I discuss physical motivations and possible consistent realizations
of large volume compactifications and low string scale. Large longitudinal
dimensions, seen by Standard Model particles, imply in general that string
theory is strongly coupled unless its tension is close to the compactification
scale. Weakly coupled, low-scale strings can in turn be realized only in the
presence of extra large transverse dimensions, seen through gravitational
interactions, or in the presence of infinitesimal string coupling. In the
former case, quantum gravity scale is also low, while in the latter,
gravitational and string interactions remain suppressed by the four-dimensional
Planck mass. There is one exception in this general rule, allowing for large
longitudinal dimensions without low string scale, when Standard Model is
embedded in a six-dimensional fixed-point theory described by a tensionless
string. Extra dimensions of size as large as TeV$^{-1}\simeq 10^{-16}$ cm are
motivated from the problem of supersymmetry breaking in string theory, while
TeV scale strings offer a solution to the gauge hierarchy problem, as an
alternative to softly broken supersymmetry or technicolor. I discuss these
problems in the context of the above mentioned string realizations, as well as
the main physical implications both in particle accelerators and in experiments
that measure gravity at sub-millimeter distances.Comment: 32 pages, LaTeX, 2 eps-figures, uses sprocl.sty Lectures given at the
Trieste Spring Workshop, ICTP, Italy, 22-30 March 1999, and at the Advanced
School on "Supersymmetry in the Theories of Fields, Strings and Branes",
Sandiago de Compostela, Spain, 26-31 July 1999. A short version was given as
an invited talk at Strings 99, Potsdam, Germany, 19-24 July 1999 and at the
European Program meeting on "Quantum Aspects of Gauge Theories, Supersymmetry
and Unification", Paris, France, 1-7 September 199

### String and D-brane Physics at Low Energy

1. Preliminaries.
2. Heterotic string and motivations for large volume compactifications;
2.1 Gauge coupling unification; 2.2 Supersymmetry breaking by
compactification.
3. M-theory on S^1/Z_2 \times Calabi-Yau.
4. Type I/I' string theory and D-branes;
4.1 Low-scale strings and extra-large transverse dimensions; 4.2 Relation
type I/I' -- heterotic.
5. Type II theories;
5.1 Low-scale IIA strings and tiny coupling; 5.2 Large dimensions in type
IIB; 5.3 Relation type II -- heterotic.
6. Theoretical implications;
6.1 U.V./I.R. correspondence; 6.2 Unification ; 6.3 Supersymmetry breaking
and scales hierarchy ; 6.4 Electroweak symmetry breaking in TeV-scale strings.
7. Scenarios for studies of experimental constraints.
8. Extra-dimensions along the world brane: KK excitations of gauge bosons;
8.1 Production at hadron colliders; 8.2 High precision data low-energy
bounds; 8.3 One extra dimension for other cases; 8.4 More than one extra
dimension.
9. Extra-dimensions transverse to the brane world: KK excitations of
gravitons;
9.1 Signals from missing energy experiments; 9.2 Gravity modification and
sub-millimeter forces.
10. Dimension-eight operators and limits on the string scale.
11. D-brane Standard Model;
11.1 Hypercharge embedding and the weak angle; 11.2 The fate of U(1)'s and
proton stability.
12. Appendix: Supersymmetry breaking in type I strings;
12.1 Scherk-Schwarz deformations; 12.2 Brane supersymmetry breaking.Comment: 53 pages, Latex, 7 eps-figures, references and acknowledgments added.
Based on lectures given at Centre Emile Borel during the semester
"Supergravity, Superstrings and M-theory", at the "LNF-INFN Spring School in
Nuclear, Subnuclear and Astropartcle Physics", Frascati, at the Glasgow
"Workshop on Phenomenology of Extra Dimensions", at the "NATO ASI school on
Recent Developments in Particle Physics and Cosmology", Portugal, at the
"38th Course on Theory and Experiment Heading for New Physics", Erice, and at
the "RTN Workshop on the Quantum Structure of Spacetime", Berli

### Direct collider signatures of large extra dimensions

The realization of low (TeV) scale strings usually requires the existence of
large (TeV) extra dimensions where gauge bosons live. The direct production of
Kaluza-Klein excitations of the photon and Z-boson at present and future
colliders is studied in this work. At the LEPII, NLC and Tevatron colliders,
these Kaluza-Klein modes lead to deviations from the standard model
cross-sections, which provide lower bounds on their mass. At the LHC the
corresponding resonances can be produced and decay on-shell, triggering a
characteristic pattern in the distribution of the dilepton invariant mass.Comment: 14 pages, LateX, 5 figure

### A closer look at string resonances in dijet events at the LHC

The first string excited state can be observed as a resonance in dijet
invariant mass distributions at the LHC, if the scenario of low-scale string
with large extra dimensions is realized. A distinguished property of the dijet
resonance by string excited states from that the other "new physics" is that
many almost degenerate states with various spin compose a single resonance
structure. It is examined that how we can obtain evidences of low-scale string
models through the analysis of angular distributions of dijet events at the
LHC. Some string resonance states of color singlet can obtain large mass shifts
through the open string one-loop effect, or through the mixing with closed
string states, and the shape of resonance structure can be distorted. Although
the distortion is not very large (10% for the mass squared), it might be able
to observe the effect at the LHC, if gluon jets and quark jets could be
distinguished in a certain level of efficiency.Comment: 12 pages, 8 figure

### Open string topological amplitudes and gaugino masses

We discuss the moduli-dependent couplings of the higher derivative F-terms
(\Tr W^2)^{h-1}, where $W$ is the gauge N=1 chiral superfield. They are
determined by the genus zero topological partition function $F^{(0,h)}$, on a
world-sheet with $h$ boundaries. By string duality, these terms are also
related to heterotic topological amplitudes studied in the past, with the
topological twist applied only in the left-moving supersymmetric sector of the
internal $N=(2,0)$ superconformal field theory. The holomorphic anomaly of
these couplings relates them to terms of the form $\Pi^n({\rm Tr}W^2)^{h-2}$,
where $\Pi$'s represent chiral projections of non-holomorphic functions of
chiral superfields. An important property of these couplings is that they
violate R-symmetry for $h\ge 3$. As a result, once supersymmetry is broken by
D-term expectation values, (\Tr W^2)^2 generates gaugino masses that can be
hierarchically smaller than the scalar masses, behaving as $m_{1/2}\sim m_0^4$
in string units. Similarly, $\Pi{\rm Tr}W^2$ generates Dirac masses for
non-chiral brane fermions, of the same order of magnitude. This mechanism can
be used for instance to obtain fermion masses at the TeV scale for scalar
masses as high as $m_0\sim{\cal O}(10^{13})$ GeV. We present explicit examples
in toroidal string compactifications with intersecting D-branes.Comment: 57 pages, 6 figures; Abstract and references correcte

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