Testability of the Axion-Gauge Sector at Cosmological Observations

概要

The Standard Model of elementary particles (SM) is consistent with the results of all the accelerator experiments that have been run to date. However, it cannot explain phenomena such as ”dark matter, neutrino oscillations, baryon number asymmetry, and inflation”. Therefore, the SM is a low-energy effective theory on the energy scale reachable by current accelerators, and theories describing the above unsolved phenomena are likely to exist in the higher energy regime. Inflation provides significant information for identifying the energy scale at which such new physics appears.

Inflation is the accelerated expansion of the universe in the extremely early universe, and detailed analysis of the polarization of the cosmic microwave background radiation (CMB) has confirmed that such an expansion has actually occurred. Most simply, a scalar field with a flat potential (inflaton) can cause inflation. However, there are many unknown details of the potential. Since the structure of the potential is related to models and scenarios of new physics, it is important to derive information about its details through observations. In particular, if the energy scale can be determined, it can be an important benchmark for studying new physics, both theoretically and experimentally.

The information on the inflaton potential is mainly investigated from the polarization of the CMB. In particular, primordial gravitational waves (GW) from vacuum fluctuations during inflation are expected to be detected as B-mode polarization of the CMB. Since the power spectrum of such GW is corresponding one-to-one with the energy scale during inflation, the B-mode polarization is the most remarkable observable in the context of inflation. Currently, projects such as CMB-S4 and Lite Bird are underway to make more precise observations of the CMB.

However, some models have been proposed that include GW sources in addition to vacuum fluctuations during inflation. Each of these sources has a different scale dependence. Therefore, in order to determine the energy scale during inflation, in addition to detecting gravitational waves, means to identify their source are necessary. In this paper, we particularly investigate observational constraints on the axion gauge field, which is a candidate source of GW.

For ordinary scalar and vector fields, the decomposition theorem bans the generation of GW by linear perturbations. Therefore, the contributions from these are neglected compared to those from the vacuum fluctuations. However, the axion gauge field has homogeneous isotropic vacuum expectation value, which can produce GW at linear order. Therefore, scenarios are allowed in which GW from the axion gauge field are the main contribution to observed it.

In order to test the consistency of such scenarios, we first investigate the constraints from current observations and theoretical requirements. Then, within the constraints, we estimate the abundances of the particles in this model, and show the testability of the model by future experiments.

First, we consider the backreaction. In this model, the temporary instability of the gauge field generates large amplitude GW. Since the energy of the GW is provided by the background field, the stability of the background is broken in extreme cases. In such a case, the GW is not generated stably and its spectrum is not appropriate for observation. Therefore, such a scenario is excluded.

In addition, the gauge field can give a non-negligible contribution to the curvature fluctuations in this model. Therefore, it is necessary to choose parameters that can reproduce the observed value of curvature fluctuations.

Finally, we review the constraints from the non-Gaussianity of CMB. The coupling between the axion and inflaton perturbations is amplified by the non-dynamical gauge field perturbations. This implies that a large non-Gaussianity ⟨ζhh⟩ can be generated from the coupling of ⟨χtt⟩ modes. Therefore, this contribution should be taken small enough to avoid the current observational bound on ⟨ζhh⟩.

Note these constraints, we analyze the power spectrum of the GW produced by the gauge field and the abundance introduced by the hidden axion-gauge sector. The axion and the gauge field in this model have large vacuum expectation values due to their interaction with each other. Because of this property, these fields may leave a large abundance without diluting the energy density during inflation.

At first, we analyze the abundance in the case where the GW enhanced by the gauge field are observed as B-mode polarization of CMB. In this case, we show that there is no parameter region where the GW from the gauge field are dominant but remain observable abundance. However, in the case with subdominant contribution to the observed GW, observable abundances can remain. We show that this is testable by future experiments such as CMB-S4.

Next, we check whether the gravitational waves enhanced by the gauge field can be directly detected by NANOGrav. We also check whether the abundance is consistent with the current observation with a setup that yields observable gravitational waves.