[Show abstract][Hide abstract] ABSTRACT: For the periodic sphaleron potential in the electroweak theory, we find the
one-dimensional time-independent Schr\"{o}dinger equation with the Chern-Simons
number as the coordinate, construct the Bloch wave function and determine the
corresponding conducting (pass) band structure. We show that the baryon-lepton
number violating processes can take place without the exponential tunneling
suppression (at zero temperature) at energies around and above the barrier
height (sphaleron energy) at 9.0 TeV. Phenomenologically, probable detection of
such processes at LHC is discussed.
Physical Review D 05/2015; 92(4). DOI:10.1103/PhysRevD.92.045005 · 4.64 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: At its very beginning, the universe is believed to have grown exponentially in size via the mechanism of inflation. The almost scale-invariant density perturbation spectrum predicted by inflation is strongly supported by cosmological observations, in particular the cosmic microwave background (MB) radiation. However, the universe's precise inflationary scenario remains a profound problem for cosmology and for fundamental physics. String theory, the most-studied theory as the final physical theory of nature, should provide an answer to this question. Some of the proposals on how inflation is realized in string theory are reviewed. Since everything is made of strings, some string loops of cosmological sizes are likely to survive in the hot big bang that followed inflation. They appear as cosmic strings, which can have intricate properties. Because of the warped geometry in flux compactification of the extra spatial dimensions in string theory, some of the cosmic strings may have tensions substantially below the Planck or string scale. Such strings cluster in a manner similar to dark matter leading to hugely enhanced densities. As a result, numerous fossil remnants of the low tension cosmic strings may exist within the galaxy. They can be revealed through the optical lensing of background stars in the near future and studied in detail through gravitational wave emission. We anticipate that these cosmic strings will permit us to address central questions about the properties of string theory as well as the birth of our universe.
International Journal of Modern Physics D 02/2015; 24(03):1530010. DOI:10.1142/S0218271815300104 · 1.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: At its very beginning, the universe is believed to have grown exponentially
in size via the mechanism of inflation. The almost scale-invariant density
perturbation spectrum predicted by inflation is strongly supported by
cosmological observations, in particular the cosmic microwave background
radiation. However, the universe's precise inflationary scenario remains a
profound problem for cosmology and for fundamental physics. String theory, the
most-studied theory as the final physical theory of nature, should provide an
answer to this question. Some of the proposals on how inflation is realized in
string theory are reviewed. Since everything is made of strings, some string
loops of cosmological sizes are likely to survive in the hot big bang that
followed inflation. They appear as cosmic strings, which can have intricate
properties. Because of the warped geometry in flux compactification of the
extra spatial dimensions in string theory, some of the cosmic strings may have
tensions substantially below the Planck or string scale. Such strings cluster
in a manner similar to dark matter leading to hugely enhanced densities. As a
result, numerous fossil remnants of the low tension cosmic strings may exist
within the galaxy. They can be revealed through the optical lensing of
background stars in the near future and studied in detail through gravitational
wave emission. We anticipate that these cosmic strings will permit us to
address central questions about the properties of string theory as well as the
birth of our universe.
[Show abstract][Hide abstract] ABSTRACT: Recent BICEP2 detection of low-multipole B-mode polarization anisotropy in
the cosmic microwave background radiation supports the inflationary universe
scenario and suggests a large inflation field range. The latter can be achieved
with axion fields in the framework of string theory. We present such a helical
model which reduces to the (quadratic) chaotic inflation model in the limit.
Its slightly smaller tensor/scalar ratio $r$ provides a signature of the
periodic nature of axion potentials. As axions are intimately related to
strings/vortices and strings are ubiquitous in string theory, we explore the
possibility that cosmic strings may be contributing to the B-mode polarization
anisotropy observed.
[Show abstract][Hide abstract] ABSTRACT: Recent BICEP2 detection of low-multipole B-mode polarization anisotropy in the cosmic microwave background radiation supports the inflationary universe scenario and suggests a large inflaton field range. The latter feature can be achieved with axion fields in the framework of string theory. We present such a helical model which naturally becomes a model with a single cosine potential, and which in turn reduces to the (quadratic) chaotic inflation model in the super-Planckian limit. The slightly smaller tensor/scalar ratio $r$ of models of this type provides a signature of the periodic nature of an axion potential. We present a simple way to quantify this distinctive feature. As axions are intimately related to strings/vortices and strings are ubiquitous in string theory, we explore the possibility that cosmic strings may be contributing to the B-mode polarization anisotropy observed.
[Show abstract][Hide abstract] ABSTRACT: We study a racetrack model in the presence of the leading alpha'-correction
in flux compactification in Type IIB string theory, for the purpose of getting
conceivable de-Sitter vacua in the large compactified volume approximation.
Unlike the K\"ahler Uplift model studied previously, the alpha'-correction is
more controllable for the meta-stable de-Sitter vacua in the racetrack case
since the constraint on the compactified volume size is very much relaxed. We
find that the vacuum energy density \Lambda for de-Sitter vacua approaches zero
exponentially as the volume grows. We also analyze properties of the
probability distribution of \Lambda in this class of models. As in other cases
studied earlier, the probability distribution again peaks sharply at \Lambda=0.
We also study the Racetrack K\"ahler Uplift model in the Swiss-Cheese type
model.
Journal of High Energy Physics 05/2013; 2013(7). DOI:10.1007/JHEP07(2013)052 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study the probability distribution P(\Lambda) of the cosmological constant
\Lambda in a specific set of KKLT type models of supersymmetric IIB vacua. We
show that, as we sweep through the quantized flux values in this flux
compactification, P(\Lambda) behaves divergent at \Lambda =0^- and the median
magnitude of \Lambda drops exponentially as the number of complex structure
moduli h^{2,1} increases. Also, owing to the hierarchical and approximate
no-scale structure, the probability of having a positive Hessian (mass squared
matrix) approaches unity as h^{2,1} increases.
Physics Letters B 11/2012; 723(4). DOI:10.1016/j.physletb.2013.05.027 · 6.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Based on the properties of probability distributions of functions of random
variables, we proposed earlier a simple stringy mechanism that prefers the
meta-stable vacua with a small cosmological constant \Lambda. As an
illustration of this approach, we study in this paper particularly simple but
non-trivial models of the K\"ahler uplift in the large volume flux
compactification scenario in Type IIB string theory, where all parameters
introduced in the model are treated either as fixed constants motivated by
physics, or as random variables with some given uniform probability
distributions. We determine the value w_0 of the superpotential W_0 at the
supersymmetric minima, and find that the resulting probability distribution
P(w_0) peaks at w_0=0; furthermore, this peaking behavior strengthens as the
number of complex structure moduli increases. The resulting probability
distribution P(\Lambda) for meta-stable vacua also peaks as \Lambda -> 0, for
both positive and negative \Lambda. This peaking/divergent behavior of
P(\Lambda) strengthens as the number of moduli increases. In some scenarios for
\Lambda > 0, the likely value of \Lambda decreases exponentially as the number
of moduli increases. The light cosmological moduli issue accompanying a very
small \Lambda is also mentioned.
Journal of Cosmology and Astroparticle Physics 09/2012; 2013(02). DOI:10.1088/1475-7516/2013/02/006 · 5.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Based on the probability distributions of products of random variables, we
propose a simple stringy mechanism that prefers the meta-stable vacua with a
small cosmological constant. We state some relevant properties of the
probability distributions of functions of random variables. We then illustrate
the mechanism within the flux compactification models in Type IIB string
theory. As a result of the stringy dynamics, we argue that the generic
probability distribution for the meta-stable vacua typically peaks with a
divergent behavior at the zero value of the cosmological constant. However, its
suppression in the single modulus model studied here is modest.
Journal of Cosmology and Astroparticle Physics 04/2012; 2012(08). DOI:10.1088/1475-7516/2012/08/032 · 5.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study Coleman–de Luccia tunneling in some detail. We show that, for a single scalar field potential with a true and a false vacuum, there are four types of tunneling, depending on the properties of the potential. A general tunneling process involves a combination of thermal (Gibbons–Hawking temperature) fluctuation part way up the barrier followed by quantum tunneling. The thin-wall approximation is a special limit of the case (of only quantum tunneling) where inside the nucleation bubble is the true vacuum while the outside reaches the false vacuum. Hawking–Moss tunneling is the (only thermal fluctuation) limit of the case where the inside of the bubble does not reach the true vacuum at the moment of its creation, and the outside is cut off by the de Sitter horizon before it reaches the false vacuum. A typical tunneling process is a combination of thermal and quantum tunnelings. We estimate the tunneling rate for this case and find that the corrections to the Hawking–Moss formula can be large. In all cases, we see that the Euclidean action of the bounce decreases rapidly as the vacuum energy density increases, signaling that the tunneling is not exponentially suppressed. This phenomenon may be interpreted as a finite temperature effect due to the Gibbons–Hawking temperature of the de Sitter space. As an application, we discuss the implication of this tunneling property to the cosmic landscape.
International Journal of Modern Physics A 01/2012; 25(05). DOI:10.1142/S0217751X10047981 · 1.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The spin structure construction of four-dimensional fermionic string models of the heterotic type is extended by considering a generalized form of the world-sheet super-current. The rules for model building are given and illustrated with two sets of examples: the original spin structure construction and the Z3 asymmetric orbifold.
International Journal of Modern Physics A 01/2012; 03(01). DOI:10.1142/S0217751X88000102 · 1.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We review the construction and classification of three-family grand unified models within the framework of asymmetric orbifolds in perturbative heterotic superstring. We give a detailed survey of all such models which is organized to aid analysis of their phenomenological properties. We compute tree level superpotentials for these models. These superpotentials are used to analyze the issues of proton stability (doublet–triplet splitting and R-parity-violating terms) and Yukawa mass matrices. To have agreement with phenomenological data all these models seem to require a certain degree of fine-tuning. We also analyze the possible patterns of supersymmetry breaking in these models. We find that the supersymmetry breaking scale comes out either too high to explain the electroweak hierarchy problem, or below the electroweak scale unless some degree of fine-tuning is involved. Thus, none of the models at hand seem to be phenomenologically flawless.
International Journal of Modern Physics A 01/2012; 13(15). DOI:10.1142/S0217751X98001323 · 1.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The search for classically stable Type IIA de-Sitter vacua typically starts
with an ansatz that gives Anti-de-Sitter supersymmetric vacua and then raises
the cosmological constant by modifying the compactification. As one raises the
cosmological constant, the couplings typically destabilize the classically
stable vacuum, so the probability that this approach will lead to a classically
stable de-Sitter vacuum is Gaussianly suppressed. This suggests that
classically stable de-Sitter vacua in string theory (at least in the Type IIA
region), especially those with relatively high cosmological constants, are very
rare. The probability that a typical de-Sitter extremum is classically stable
(i.e., tachyon-free) is argued to be Gaussianly suppressed as a function of the
number of moduli.
Journal of High Energy Physics 12/2011; 2012(4). DOI:10.1007/JHEP04(2012)026 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The A phase and the B phase of superfluid He-3 are well studied, both theoretically and experimentally. The decay time scale of the A phase to the B phase of a typical supercooled superfluid 3He-A sample is calculated to be 1020,000 years or longer, yet the actual first-order phase transition of supercooled A phase happens very rapidly (in seconds to minutes) in the laboratory. We propose that this very fast phase transition puzzle can be explained by the resonant tunneling effect in field theory, which generically happens since the degeneracies of both the A and the B phases are lifted by many small interaction effects. This explanation predicts the existence of peaks in the A→B transition rate for certain values of the temperature, pressure, and magnetic field. Away from these peaks, the transition simply will not happen.
[Show abstract][Hide abstract] ABSTRACT: We argue that classical transitions can be the key to explaining the long
standing puzzle of the fast A-B phase transition observed in superfluid Helium
3 while standard theory expects it to be unobservably slow. Collisions between
domain walls are shown to be capable of reaching phases inaccessible through
homogenous nucleation on the measured timescales. We demonstrate qualitative
agreements with prior observations and provide a definite, distinctive
prediction that could be verified through future experiments or, perhaps, a
specific analysis of existing data.
[Show abstract][Hide abstract] ABSTRACT: We comment on Weinberg’s interesting analysis of asymptotically safe inflation [ S. Weinberg Phys. Rev. D 81 083535 (2010)]. We find that even if the gravity theory exhibits an ultraviolet fixed point, the energy scale during inflation is way too low to drive the theory close to the fixed point value. We choose the specific renormalization group flow away from the fixed point towards the infrared region that reproduces the Newton’s constant and today’s cosmological constant. We follow this renormalization group flow path to scales below the Planck scale to study the stability of the inflationary scenario. Again, we find that some fine-tuning is necessary to get enough e folds of inflation in the asymptotically safe inflationary scenario.
[Show abstract][Hide abstract] ABSTRACT: We propose a new way to implement an inflationary prior to a cosmological
dataset that incorporates the inflationary observables at arbitrary order. This
approach employs an exponential form for the Hubble parameter $H(\phi)$ without
taking the slow-roll approximation. At lowest non-trivial order, this $H(\phi)$
has the unique property that it is the solution to the brachistochrone problem
for inflation.
Journal of Cosmology and Astroparticle Physics 12/2010; 4(04). DOI:10.1088/1475-7516/2011/04/026 · 5.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently, Bjerrum-Bohr, Damgaard, Feng, and Sondergaard derived a set of new interesting quadratic identities of the Yang-Mills (YM) tree scattering amplitudes, besides Bern-Carrasco-Johansson (BCJ) identities. Here we comment that these quadratic identities of YM amplitudes actually follow directly from the KLT (Kawai-Lewellen-Tye) relation for graviton-dilaton-axion scattering amplitudes (in four-dimensional spacetime). This clarifies their physical origin and also provides a simpler version of the new identities. We also comment that the recently discovered BCJ identities of YM helicity amplitudes, at least for the maximal helicity-violating case, can be verified by using (repeatedly) the Schouten identity. We also point out additional quadratic identities that can be written down from the KLT relations.