140 research outputs found

### Observing the Structure of the Landscape with the CMB Experiments

Assuming that inflation happened through a series of tunneling in the string
theory landscape, it is argued that one can determine the structure of vacua
using precise measurements of the scalar spectral index and tensor
perturbations at large scales. It is shown that for a vacuum structure where
the energy gap between the minima is constant, i.e. $\epsilon_i=i m_f^4$, one
obtains the scalar spectral index, $n_s$, to be $\simeq 0.9687$, for the modes
that exit the horizon 60 e-folds before the end of inflation. Alternatively,
for a vacuum structure in which the energy gap increases linearly with the
vacuum index, i.e. $\epsilon_i=\frac{i^2}{2} m_f^4$, $n_s$ turns out to be
$\simeq 0.9614$. Both these two models are motivated within the string theory
landscape using flux-compactification and their predictions for scalar spectral
index are compatible with WMAP results. For both these two models, the results
for the scalar spectral index turn out to be independent of $m_f$. Nonetheless,
assuming that inflation started at Planckian energies and that there had been
successful thermalization at each step, one can constrain $m_f<2.6069\times
10^{-5} m_P$ and $m_f<6.5396\times 10^{-7} m_P$ in these two models,
respectively. Violation of the single-field consistency relation between the
tensor and scalar spectra is another prediction of chain inflation models. This
corresponds to having a smaller tensor/scalar ratio at large scales in
comparison with the slow-roll counterparts. Similar to slow-roll inflation, it
is argued that one can reconstruct the vacuum structure using the CMB
experiments.Comment: v1: 8 pages, 2 figures; v2: grammatical typos corrected, results
unchanged v3: To be published in JCA

### Massless radiation from Strings: quantum spectrum average statistics and cusp-kink configurations

We derive general formulae for computing the average spectrum for Bosonic or
Fermionic massless emission from generic or particular sets of closed
superstring quantum states, among the many occurring at a given large value of
the number operator. In particular we look for states that can produce a
Bosonic spectrum resembling the classical spectrum expected for peculiar
cusp-like or kink-like classical configurations, and we perform a statistical
counting of their average number. The results can be relevant in the framework
of possible observations of the radiation emitted by cosmic strings.Comment: 13 pages, 4 figures, improved explanations, an appendix added on
rotating folded strin

### Search for the most stable massive state in superstring theory

In ten dimensional type II superstring, all perturbative massive states are
unstable, typically with a short lifetime compared to the string scale. We find
that the lifetime of the average string state of mass M has the asymptotic form
T < const.1/(g^2 M). The most stable string state seems to be a certain state
with high angular momentum which can be classically viewed as a circular string
rotating in several planes ("the rotating ring"), predominantly decaying by
radiating soft massless NS-NS particles, with a lifetime T = c_0 M^5/g^2.
Remarkably, the dominant channel is the decay into a similar rotating ring
state of smaller mass. The total lifetime to shrink to zero size is ~ M^7. In
the presence of D branes, decay channels involving open strings in the final
state are exponentially suppressed, so the lifetime is still proportional to
M^5, except for a D brane at a special angle or flux. For large mass, the
spectrum for massless emission exhibits qualitative features typical of a
thermal spectrum, such as a maximum and an exponential tail. We also discuss
the decay properties of rotating rings in the case of compact dimensions.Comment: 24 pages, 1 figure. Correction on lifetime of average stat

### Handbook on string decay

We explain simple semi-classical rules to estimate the lifetime of any given
highly-excited quantum state of the string spectrum in flat spacetime. We
discuss both the decays by splitting into two massive states and by massless
emission. As an application, we study a solution describing a rotating and
pulsating ellipse which becomes folded at an instant of time -- the ``squashing
ellipse''. This string interpolates between the folded string with maximum
angular momentum and the pulsating circular string. We explicitly compute the
quantum decay rate for the corresponding quantum state, and verify the basic
rules that we propose. Finally, we give a more general (4-parameter) family of
closed string solutions representing rotating and pulsating elliptical strings.Comment: 18 pages, 9 figures. Final version appeared in JHE

### Long Lived Large Type II Strings: decay within compactification

Motivated also by recent revival of interest about metastable string states
(as cosmic strings or in accelerator physics), we study the decay, in presence
of dimensional compactification, of a particular superstring state, which was
proven to be remarkably long-lived in the flat uncompactified scenario. We
compute the decay rate by an exact numerical evaluation of the imaginary part
of the one-loop propagator. For large radii of compactification, the result
tends to the fully uncompactified one (lifetime T = const M^5/g^2), as
expected, the string mainly decaying by massless radiation. For small radii,
the features of the decay (emitted states, initial mass dependence,....)
change, depending on how the string wraps on the compact dimensions.Comment: 32 pages, 24 text plus appendices, 4 figure

### Interactions of Cosmic Superstrings

We develop methods by which cosmic superstring interactions can be studied in
detail. These include the reconnection probability and emission of radiation
such as gravitons or small string loops. Loop corrections to these are
discussed, as well as relationships to $(p,q)$-strings. These tools should
allow a phenomenological study of string models in anticipation of upcoming
experiments sensitive to cosmic string radiation.Comment: 22 pages, 6 figures; v2: updated reference

### Quantum effects in gravitational wave signals from cuspy superstrings

We study the gravitational emission, in Superstring Theory, from fundamental
strings exhibiting cusps. The classical computation of the gravitational
radiation signal from cuspy strings features strong bursts in the special null
directions associated to the cusps. We perform a quantum computation of the
gravitational radiation signal from a cuspy string, as measured in a
gravitational wave detector using matched filtering and located in the special
null direction associated to the cusp. We study the quantum statistics
(expectation value and variance) of the measured filtered signal and find that
it is very sharply peaked around the classical prediction. Ultimately, this
result follows from the fact that the detector is a low-pass filter which is
blind to the violent high-frequency quantum fluctuations of both the string
worldsheet, and the incoming gravitational field.Comment: 16 pages, no figur

### On Tunnelling In Two-Throat Warped Reheating

We revisit the energy transfer necessary for the warped reheating scenario in
a two-throat geometry. We study KK mode wavefunctions of the full two-throat
system in the Randall--Sundrum (RS) approximation and find an interesting
subtlety in the calculation of the KK mode tunnelling rate. While wavepacket
tunnelling is suppressed unless the Standard Model throat is very long,
wavefunctions of modes localized in different throats have a non-zero overlap
and energy can be transferred between the throats by interactions between such
KK modes. The corresponding decay rates are calculated and found to be faster
than the tunnelling rates found in previously published works. However, it
turns out that the imaginary parts of the mode frequencies, induced by the
decay, slow the decay rates themselves down. The self-consistent decay rate
turns out to be given by the plane wave tunnelling rate considered previously
in the literature. We then discuss mechanisms that may enhance the energy
transfer between the throats over the RS rates. In particular, we study models
in which the warp factor changes in the UV region less abruptly than in the RS
model, and find that it is easy to build phenomenological models in which the
plane wave tunnelling rate, and hence the KK mode interaction rates, are
enhanced compared to the standard RS setup.Comment: 27 pages + appendices, 5 figures, latex. v2: Discussion of decay in
Section 4 changed: the most dangerous graviton amplitudes are zero, the
results are now more positive for the warped reheating scenario; typos fixed,
discussion cleaned up. v3:corrections in Section 5 (decay rates slowed down),
mild changes of overall conclusion

### Gravitational waves from first order phase transitions during inflation

We study the production, spectrum and detectability of gravitational waves in
models of the early Universe where first order phase transitions occur during
inflation. We consider all relevant sources. The self-consistency of the
scenario strongly affects the features of the waves. The spectrum appears to be
mainly sourced by collisions of bubble of the new phases, while plasma dynamics
(turbulence) and the primordial gauge fields connected to the physics of the
transitions are generally subdominant. The amplitude and frequency dependence
of the spectrum for modes that exit the horizon during inflation are different
from those of the waves produced by quantum vacuum oscillations of the metric
or by first order phase transitions not occurring during inflation. A moderate
number of slow (but still successful) phase transitions can leave detectable
marks in the CMBR, but the signal weakens rapidly for faster transitions. When
the number of phase transitions is instead large, the primordial gravitational
waves can be observed both in the CMBR or with LISA (marginally) and especially
DECIGO. We also discuss the nucleosynthesis bound and the constraints it places
on the parameters of the models.Comment: minor changes in the text and the references to match the published
versio

### On stable higher spin states in Heterotic String Theories

We study properties of 1/2 BPS Higher Spin states in heterotic
compactifications with extended supersymmetry. We also analyze non BPS Higher
Spin states and give explicit expressions for physical vertex operators of the
first two massive levels. We then study on-shell tri-linear couplings of these
Higher Spin states and confirm that BPS states with arbitrary spin cannot decay
into lower spin states in perturbation theory. Finally, we consider scattering
of vector bosons off higher spin BPS states and extract form factors and
polarization effects in various limits.Comment: 38 page

- âŠ