Natural Inflation with Natural Trans-Planckian Axion Decay Constant from Anomalous U(1)XU(1)_X

We propose a natural inflation model driven by an imaginary or axionic component of a K\"ahler modulus in string-inspired supergravity. The shift symmetry of the axion is gauged under an anomalous $U(1)_X$ symmetry, which leads to a modulus-dependent Fayet-Iliopoulos (FI) term. The matter fields are stabilized by F-terms, and the real component of the modulus is stabilized by the $U(1)_X$ D-term, while its axion remains light. Therefore, the masses of real and imaginary components of the modulus are separated at different scales. The scalar potential for natural inflation is realized by the superpotential from the non-perturbative effects. The trans-Planckian axion decay constant, which is needed to fit with BICEP2 observations, can be obtained naturally in this model.Comment: 14 pages, no figure, references added, version published in JHE

Helical Phase Inflation via Non-Geometric Flux Compactifications: from Natural to Starobinsky-like Inflation

We show that a new class of helical phase inflation models can be simply realized in minimal supergravity, wherein the inflaton is the phase component of a complex field and its potential admits a deformed helicoid structure. We find a new unique complex-valued index $\chi$ that characterizes almost the entire region of the $n_s-r$ plane favored by new Planck observations. Continuously varying the index $\chi$, predictions interpolate from quadratic/natural inflation parameterized by a phase/axion decay constant to Starobinsky-like inflation parameterized by the $\alpha$-parameter. We demonstrate that the simple supergravity construction realizing Starobinsky-like inflation can be obtained from a more microscopic model by integrating out heavy fields, and that the flat phase direction for slow-roll inflation is protected by a mildly broken global $U(1)$ symmetry. %, which is mildly broken at the inflation energy scale. We study the geometrical origin of the index $\chi$, and find that it corresponds to a linear constraint relating \kah moduli. We argue that such a linear constraint is a natural result of moduli stabilization in Type \MyRoman{2} orientifold compactifications on Calabi-Yau threefolds with geometric and non-geometric fluxes. Possible choices for the index $\chi$ are discrete points on the complex plane that relate to the distribution of supersymmetric Minkowski vacua on moduli space. More precise observations of the inflationary epoch in the future may provide a better estimation of the index $\chi$. Since $\chi$ is determined by the fluxes and vacuum expectation values of complex structure moduli, such observations would characterize the geometry of the internal space as well.Comment: 26 pages, 4 figures; 4+1 figure, discussion on several energy scales added, references added, to appear in JHE

The influence of reconstruction criteria on the sensitive probes of the symmetry potential

Different criteria of constructing clusters and tracing back $\Delta$ resonances from the intermediate-energy neutron-rich HICs are discussed by employing the updated UrQMD transport model. It is found that both the phase-space and the coordinate-density criteria affect the single and the double neutron/proton ratios of free nucleons at small transverse momenta, but the influence becomes invisible at large transverse momenta. The effect of different methods of reconstructing freeze-out $\Delta$s on the $\Delta^0/\Delta^{++}$ ratio is strong in a large kinetic energy region.Comment: 8 pages, 7 fig

Aligned Natural Inflation and Moduli Stabilization from Anomalous U(1)U(1) Gauge Symmetries

To obtain natural inflation with large tensor-to-scalar ratio in string framework, we need a special moduli stabilization mechanism which can separate the masses of real and imaginary components of K\"ahler moduli at different scales, and achieve a trans-Planckian axion decay constant from sub-Planckian axion decay constants. In this work, we stabilize the matter fields by F-terms and the real components of K\"ahler moduli by D-terms of two anomalous $U(1)_X\times U(1)_A$ symmetries strongly at high scales, while the corresponding axions remain light due to their independence on the Fayet-Iliopoulos (FI) term in moduli stabilization. The racetrack-type axion superpotential is obtained from gaugino condensations of the hidden gauge symmetries $SU(n)\times SU(m)$ with massive matter fields in the bi-fundamental respresentations. The axion alignment via Kim-Nilles-Pelroso (KNP) mechanism corresponds to an approximate $S_2$ exchange symmetry of two K\"ahler moduli in our model, and a slightly $S_2$ symmetry breaking leads to the natural inflation with super-Planckian decay constant.Comment: 17 pages, no figur

Helical Phase Inflation

We show that the quadratic inflation can be realized by the phase of a complex field with helicoid potential. Remarkably, this helicoid potential can be simply realized in minimal supergravity. The global $U(1)$ symmetry of the K\"ahler potential introduces a flat direction and evades the $\eta$ problem automatically. So such inflation is technically natural. The phase excursion is super-Planckian as required by the Lyth bound, while the norm of the complex field can be suppressed in the sub-Planckian region. This model resolves the ultraviolet sensitive problem of the large field inflation, besides, it also provides a new type of monodromy inflation in supersymmetric field theory with consistent field stabilization.Comment: 4 pages+references, 2 figures, phase monodromy in supersymmetric field theory is adde

Cosmological evolution of a D-brane

We study the cosmological evolution of a single BPS D-brane in the absence of potential, which is in the category of the Chaplygin gas cosmological model. When such a D-brane coupled to gravity moves in the bulk with a non-vanishing velocity, it tends to slow down to zero velocity via mechanisms like gravitational waves leakage to the bulk, losing its kinetic energy to fuel the expansion of the universe on the D-brane. If the initial velocity of the D-brane is high enough, the universe on the D-brane undergoes a dust-like stage at early times and an acceleration stage at late times, as observed in the original Chaplygin gas model. When the D-brane velocity is initially zero, the D-brane will always remain fixed at some position in the bulk, with the brane tension over the Plank mass squared as a cosmological constant. Interestingly, this kind of fixed brane universe can arise as defects from tachyon inflation on a non-BPS D-brane with one dimension higherWe study the cosmological evolution of a single BPS D-brane coupled to gravity in the absence of potential. When such a D-brane moves in the bulk with non-vanishing velocity, it tends to slow down to zero velocity via mechanisms like gravitational wave leakage to the bulk, losing its kinetic energy to fuel the expansion of the universe on the D-brane. If the initial velocity of the D-brane is high enough, the universe on the D-brane undergoes a dust-like stage at early times and an acceleration stage at late times, realising the original Chaplygin gas model. When the D-brane velocity is initially zero, the D-brane will always remain fixed at some position in the bulk, with the brane tension over the Plank mass squared as a cosmological constant. It is further shown that this kind of fixed brane universe can arise as defects from tachyon inflation on a non-BPS D-brane with one dimension higher.Comment: 13 page