Recent cosmological observations have revealed growing tensions with the standard ΛCDM model, including indications of isotropic cosmic birefringence and deviations from <math altimg='si1.svg'><mi>w</mi><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mo linebreak='badbreak' linebreakstyle='after'>&#8722;</mo><mn>1</mn></math> in the dark energy equation of state, as suggested by DESI and supernova measurements. In this paper, we point out that such deviations can arise even from a subdominant energy density component. We then propose a unified framework based on a dynamical axion field that simultaneously accounts for both anomalies, providing a simple and natural extension of the standard &#923;CDM model. In our scenario, the axion field with <math altimg='si2.svg'><mn>2</mn><msub><mrow><mi>H</mi></mrow><mrow><mn>0</mn></mrow></msub><mo>&#8818;</mo><mi>m</mi><mo>&#8818;</mo><mn>7</mn><msub><mrow><mi>H</mi></mrow><mrow><mn>0</mn></mrow></msub></math>, where <math altimg='si3.svg'><msub><mrow><mi>H</mi></mrow><mrow><mn>0</mn></mrow></msub></math> is the current Hubble constant, induces a nonzero rotation of the CMB polarization plane and modifies the present-day dark energy equation of state. This framework accommodates recent observational data with natural parameter choices, even for a string axion with a decay constant of order 10 <sup>17</sup> <hsp sp='0.20' />GeV.

China

Shota Nakagawa

Yuichiro Nakai

Yu-Cheng Qiu

Japan

Masaki Yamada

Interpreting cosmic birefringence and DESI data with evolving axion in ΛCDM

<p>Recently, the Dark Energy Survey Collaboration presented evidence that the equation of state <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>w</mi></math> of the dark energy is varying or <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>w</mi><mo>≃</mo><mo>−</mo><mn>0.948</mn></math> if it is constant. In either case, the dark energy cannot be due to a cosmological constant alone. Here, we study an ultralight axion (or axionlike particle) with mass <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>ϕ</mi></msub><mo>≃</mo><mn>2</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>33</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></math> that has properties that can explain the new <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>w</mi></math> measurement. In particular, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>w</mi><mo>≥</mo><mo>−</mo><mn>1</mn></math> and a negative cosmological constant <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="normal">Λ</mi><mo>&lt;</mo><mn>0</mn></math> is preferred in this model. We also present a simple formula for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>w</mi></math> for the model to ease data fitting.</p>

Spain

Hoang Nhan Luu

United States

S.H. Henry Tye

Dynamical dark energy from an ultralight axion

The dilaton emerges as a pseudo-Nambu-Goldstone boson (pNGB) associated with the spontaneous breaking of scale invariance in a nearly conformal field theory (CFT). We show the existence of a wiggly dilaton potential that contains multiple vacuum solutions in a five-dimensional (5D) holographic formulation. The wiggly feature originates from boundary potentials of a 5D axion-like scalar field, whose naturally small bulk mass parameter corresponds to a marginally-relevant deformation of the dual CFT. Depending on the energy density of a boundary 3-brane, our model can provide a relaxion potential or generate a light dilaton. However, an extremely light dilaton requires fine-tuning.

Sudhakantha Girmohanta

Zhihao Zhang

Wiggly dilaton: A landscape of spontaneously broken scale invariance

We study the transverse momentum ( $p_T$ ) differential anisotropic flow and flow fluctuation in Pb+Pb collisions at $\sqrt{s_{_{NN}}}$ =5.02 TeV at the LHC. A (3+1)-dimensional CLVisc hydrodynamics framework with fluctuating TRENTO (or AMPT) initial conditions is utilized to simulate the space-time evolution of the quark-gluon plasma (QGP) medium. The effects of shear viscosity and the sub-nucleon structure on anisotropic flow and flow fluctuation are analyzed. Our result shows that shear viscosity tends to suppress both flow coefficients ( ${v_2\{2}\}$ , ${v_2\{4\}}$ , and ${\langle v_2\rangle}$ ) and flow fluctuation ( ${\sigma_{v_2}}$ ) owing to its smearing effect on local density fluctuation. The flow coefficients appear to be insensitive to the sub-nucleon structure, whereas the flow fluctuation ${\sigma_{v_2}}$ tends to be suppressed by the sub-nucleon structure in central collisions but enhanced in peripheral collisions. After taking into account the sub-nucleon structure effect, our numerical result can qualitatively describe the relative flow fluctuations ( ${v_2\{4\}/v_2\{2\}}$ , $F({v_2})$ ) measured by the ALICE Collaboration at the LHC. We further investigate the effects of shear viscosity, sub-nucleon structure, and initial condition model on the flow angle and flow magnitude decorrelations ( ${A_2^f}$ , ${M_2^f}$ ) using the four-particle correlation method. We find that the flow decorrelation effect is typically stronger in central collisions than in peripheral collisions. The flow angle decorrelation is found to be insensitive to the shear viscosity and sub-nucleon structure, whereas the flow magnitude decorrelation shows quite different behaviors when using the TRENTO and AMPT initial condition models. Our study sheds light on the anisotropic flow, transport properties, and initial structure of the QGP created in high-energy nuclear collisions.

Germany

Jie Zhu

Canada

Xiang-Yu Wu

Guang-You Qin

Anisotropic flow, flow fluctuation, and flow decorrelation in relativistic heavy-ion collisions: the roles of sub-nucleon structure and shear viscosity * Supported in part by the Natural Science Foundation of China (12225503, 11890710, 11890711, 11935007). Zhu Jie is supported in part by the China Scholarship Council (202306770009). Wu Xiang-Yu is supported in part by the National Science Foundation within the framework of the JETSCAPE collaboration (OAC-2004571) and in part by the Natural Sciences and Engineering Research Council of Canada. Some of the calculations were performed in the Nuclear Science Computing Center at Central China Normal University (NSC$^{3}$), Wuhan, Hubei, China.

The GLR-MQ equation is a nonlinear evolution equation that takes into account the shadowing effect, which tames the growth of the gluon at small-x. In this study, we analytically solve for the first time the nonlinear GLR-MQ equation using the homogeneous balance method. The definite solution of the GLR-MQ equation is obtained by fitting the MSTW2008LO gluon distribution data. We find that the geometric scaling is an inherent property of our analytical solution. Furthermore, the gluon distribution functions derived from our solution are capable of accurately reproducing the MSTW2008LO data across most regions. These results indicate that our analytical solution from the homogeneous balance method is valid to describe the gluon behavior at small-x and moderate <math><msup><mrow><mi>Q</mi></mrow><mrow><mn>2</mn></mrow></msup></math> regions. Moreover, the saturation scale <math><msub><mrow><mi>Q</mi></mrow><mrow><mi>s</mi></mrow></msub></math> has been extracted from our analytical solution, we find that the energy-dependent saturation scale obeys the exponential law <math><msubsup><mrow><mi>Q</mi></mrow><mrow><mi>s</mi></mrow><mrow><mn>2</mn></mrow></msubsup><mspace width="0.2em"></mspace><mo>∝</mo><mspace width="0.2em"></mspace><msubsup><mrow><mi>Q</mi></mrow><mrow><mn>0</mn></mrow><mrow><mn>2</mn></mrow></msubsup><msup><mrow><mi>e</mi></mrow><mrow><mi>λ</mi><mi>Y</mi></mrow></msup></math>. We also perform a numerical simulation using the analytical results at <math><msup><mrow><mi>Q</mi></mrow><mrow><mn>2</mn></mrow></msup><mo>=</mo><mn>5</mn><mspace width="0.25em"></mspace><msup><mrow><mtext>GeV</mtext></mrow><mrow><mn>2</mn></mrow></msup></math> as the initial condition. We find that our analytical results are consistent with the numerical results in small-x.

Yanbing Cai

Xiaopeng Wang

Xurong Chen

New analytical solution of the nonlinear Gribov-Levin-Ryskin-Mueller-Qiu equation

The <math><msub><mi>S</mi><mn>8</mn></msub><mo>/</mo><msub><mi>σ</mi><mn>8</mn></msub></math> tension in the large-scale structure can be explained by decaying dark matter with an almost degenerate spectrum and small enough decay width. Here we propose the Gemini dark matter model, which contains a heavy mother particle <math><msub><mi>χ</mi><mn>3</mn></msub></math> and two twins <math><msub><mi>χ</mi><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msub></math>, which are almost degenerate in mass and are produced at the same time. The dark sector is charged under the same Froggatt-Nielsen symmetry that can explain the hierarchy of the Standard Model Yukawa couplings. The slightly heavier <math><msub><mi>χ</mi><mn>2</mn></msub></math> decays into <math><msub><mi>χ</mi><mn>1</mn></msub></math> and the axionic component of the flavon, which washes out the small-scale structure and resolves <math><msub><mi>S</mi><mn>8</mn></msub><mo>/</mo><msub><mi>σ</mi><mn>8</mn></msub></math> tension. We present the production mechanism of Gemini dark matter and viable parameter regions. We find that, despite the preferred dark matter mass being <math><mrow><mi>O</mi><mo>(</mo><mn>1</mn><mo>)</mo><mi>–</mi><mi>O</mi><mo>(</mo><mn>100</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>keV</mi></mrow></math>, they constitute cold dark matter. The Gemini dark matter model predicts an abundance of dark radiation that will be probed in future measurements of the cosmic microwave background.

Andrew Cheek

Liang Tan

Gemini dark matter

The minimum seesaw model with two right-handed neutrinos is considered, where the lightest neutrino is naturally massless. Instead of adopting texture zeros in the lepton Yukawa matrices, which cause both theoretical and experimental troubles, here we propose two-<math><mrow><mi>ε</mi></mrow></math> textures, where <math><mi>ε</mi></math> is a small number. Combined with neutrino oscillation experimental data, we find that a large <math><mi>C</mi><mi>P</mi></math> angle is preferred for the normal neutrino mass order. In contrast, the <math><mi>C</mi><mi>P</mi></math> angle almost vanishes for the inverted order. This can be well tested in near-future experiments, such as Hyper-Kamiokande. Besides, the predicted effective Majorana neutrino mass <math><msub><mi>m</mi><mrow><mi>e</mi><mi>e</mi></mrow></msub></math> and the total neutrino mass <math><mo>∑</mo><msubsup><mi>m</mi><mi>i</mi><mi>ν</mi></msubsup></math> are also within reach of ongoing or future experiments.

Jin-Wei Wang

Tsutomu T. Yanagida

Large <math><mi>C</mi><mi>P</mi></math> violation from the minimum seesaw model

Atomic dark matter is usually considered to be produced asymmetrically in the early Universe. In this work, we first propose that the symmetric atomic dark matter can be thermally produced through the freeze-out mechanism. The dominant atom antiatom annihilation channel is the atomic rearrangement. It has a geometrical cross section much larger than that of elementary fermions. After the atomic formation, this annihilation process further depletes dark matter particles and finally freezes out. To give the observed dark matter relic, the dark atoms are naturally ultraheavy, ranging from <math><msup><mn>10</mn><mn>6</mn></msup></math> to <math><msup><mn>10</mn><mn>10</mn></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>.

Jie Sheng

Chuan-Yang Xing

Ultraheavy atomic dark matter freeze-out through rearrangement

The recent analysis of the Planck 2018 polarization data shows a nonzero isotropic cosmic birefringence (ICB) that is not explained within the ΛCDM paradigm. We then explore the question of whether the nonzero ICB is interpreted by the framework of the Standard Model Effective Field Theory (SMEFT), or at the energy scales of the cosmic microwave background, the low-energy EFT (LEFT) whose dynamical degrees of freedom are five SM quarks and all neutral and charged leptons. Our systematic study reveals that any operator in the EFT on a cosmological background would not give the reported ICB angle, which is observationally consistent with frequency independence. In particular, we estimate the size of the ICB angle generated by the effect that the cosmic microwave background photons travel through the medium of the cosmic neutrino background with parity-violating neutrino-photon interactions and find that it would be too small to explain the data. If the reported ICB angle should be confirmed, then our result would indicate the existence of a new particle that is lighter than the electroweak scale and feebly interacting with the SM particles.

Ryo Namba

Ippei Obata

Ryo Saito

Can we explain cosmic birefringence without a new light field beyond Standard Model?

The seesaw mechanism is the most attractive mechanism to explain the small neutrino masses, which predicts the neutrinoless double beta decay (<math><mn>0</mn><mi>ν</mi><mi>β</mi><mi>β</mi></math>) of the nucleus. Thus the discovery of <math><mn>0</mn><mi>ν</mi><mi>β</mi><mi>β</mi></math> is extremely important for future particle physics. However, the present data on the neutrino oscillation is not sufficient to predict the value of <math><msub><mi>m</mi><mrow><mi>e</mi><mi>e</mi></mrow></msub></math> as well as the neutrino mass <math><msubsup><mi>m</mi><mi>ν</mi><mi>i</mi></msubsup></math>. In this short article, by adopting a simple and consistent Froggatt-Nielsen model, which can well explain the observed masses and mixing angles of quark and lepton sectors, we calculate the distribution of <math><msub><mi>m</mi><mrow><mi>e</mi><mi>e</mi></mrow></msub></math> and <math><msubsup><mi>m</mi><mi>ν</mi><mi>i</mi></msubsup></math>. Interestingly, a relatively large part of the preferred parameter space can be detected in the near future.

Predictions of <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><mi>e</mi><mi>e</mi></mrow></msub></mrow></math> and neutrino mass from a consistent Froggatt-Nielsen model