Tuesday, February 4, 2025

Q-Balls As Dark Matter?

A new preprint, entitled "Can Q-balls describe cosmological and galactic dark matter?" by Susobhan Mandal and S. Shankaranarayanan proposes "Q-Balls" as a dark matter candidate (hat tip to neo). The abstract and introduction to the preprint, excerpts of which explains the hypothesis (but contradict each other on the question of Q-ball mass, a probably that may be fixed in edits to the pre-print). I have interlineated my commentary in square brackets.
Abstract 
Q-balls, which are localized, non-topological solitons, can be a bridge between the two hypotheses. Q-balls formed in the early Universe can mimic CDM at cosmological scales. Interestingly, Q-balls can exhibit MOND-like behavior in the late Universe at galactic scales, providing a unified framework. Specifically, we demonstrate that millicharged composite Q-balls formed from complex scalar fields, decoupled from the background radiation, can naturally arise during the radiation-dominated epoch. From the matter-radiation equality, we also obtain the mass of Q-balls to be 1 eV [Ed. the body text says 41 eV], which are much smaller than the electron mass. Using the constraints from the invisible decay mode of ortho-positronium, we obtain Q<3.4×10^−5. We also establish an upper bound on the number density of Q-balls, which depends on the charge of the Q-ball and the small initial charge asymmetry. . . .  
Introduction 
The most convincing evidence for dark matter (DM) is found at the cosmological scales. The Λ-Cold-Dark-Matter (ΛCDM) model, in which DM is composed of collisionless particles, fits the temperature fluctuations of the Cosmic Microwave Background (CMB), the matter power spectra, and the abundance and mass function of galaxy clusters extraordinarily well. The foundation of the conventional cold dark matter hypothesis is the existence of non-relativistic, collision-free dark matter particles. Also, dark matter constituents cannot possess an electric charge comparable to that of electrons unless they are extremely heavy. 

However, this does not rule out the intriguing possibility of electrically millicharged particles. Millicharged particles have an electric charge e′ = εe, where e is the electron charge and ε ≪ 1. These particles can be either bosons or fermions, and they naturally arise in a wide range of models. [Ed. the exquisite sensitivity of physics measurements to even slight electromagnetic charges makes an EM millicharged particle hypothesis highly unlikely. For example, this would impact muon g-2 measurements in ways that are not observed and are at parts per billion levels.]
This naturally raises the question: Is there a framework where the DM behaves as a CDM in the cosmological scales and can mimic MOND in the galaxy scales? In other words, is there a mechanism where CDM and MOND share a common ancestry?  
This work proposes an alternative mechanism with the same origin but behaves differently in the cosmological and galactic scales. We demonstrate that non-topological solitons — the Q-balls — can be formed in the early Universe and mimic CDM at the cosmological scales. Specifically, we show that Q-balls from complex scalar field decoupled from the background radiation can naturally form in the radiation-dominated epoch. In contrast, they mimic MONDat galactic scales in the late Universe. 
Q-balls can naturally form in a wide range of particle physics models and can be produced in the early Universe, and can be stable. Recently, the existence of Q-balls in the dark sector of the Universe and its astrophysical consequences have gained interest. [Ed. actually, Q-ball models are far out of the mainstream in particle physics, involve Byzantine contortions to produce a questionable physics model disfavored by Occam's Razor, and aren't well-motivated.]

For a class of complex scalar field theory, we show that Q-ball configurations exist in the recombination epoch during the radiation-dominated (RD) era in the early Universe. We show that the Q-balls formed satisfy the two conditions — stability and existence. We show that the density perturbations that reenter during the recombination epoch increase the production of Q-balls and compute the number density of the thin-wall Q-balls in the Universe. We also compute the upper bound on the number density of Q-balls based on its global U(1) charge and small primeval charge asymmetry, which might be created due to primordial anomaly due to helical magnetic fields. It is also possible to generate charge asymmetry due to the helicity of the gravitational waves. From the matter-radiation equality, we also obtain the mass of Q-balls to be 41 eV, which are much smaller than the electron mass. Combined with the invisible decay mode of ortho-positronium leads to Q < 3.4×10^−5. Suggesting that the millicharged Q-balls are DM candidates responsible for the early structure. 

Since the Q-balls decay as 1/a3(η) in the early epoch, they behave like CDM in the cosmological scales. Later, we look at the possibility of forming Bose-Einstein condensate (BEC) and superf luidity by these Q-balls in the present Universe (dominated by dark energy). 
We find the Q-balls can indeed form these phases of matter, which eventually lead to the law predicted by Modified Newtonian dynamics at the galactic scale. Thus, we show that the cosmological and galactic dark matter share a common ancestry, representing distinct phases of a single background fluid. 

The possibility that galactic DM can be a superfluid has recently attracted attention. This is based on two key ideas: The first notion that the DM forms a superfluid within galaxies with a coherence length equal to the size of the galaxies is widespread. Second, the DM superfluidity phenomenon occurs frequently if the DM particle is sufficiently light and has a significant self-interaction. A superfluid is a state of matter where impurity particles flow without dissipation as long as they remain below a critical velocity, a phenomenon known as Landau’s criterion. Superfluidity and Bose-Einstein condensation are intimately related phenomena. In order to acquire a superfluid phase, BoseEinstein condensation must first occur; however, the converse is not true, as the superfluidity property disappears in the absence of interactions. Thus, we show that Q-balls can be considered a DM candidate, which acts as a composite object at the cosmological scale and behaves as collective excitation on the galactic scale.

The background analysis of why other solutions to explaining dark matter phenomena don't work in the introduction is also interesting, although not very rigorous: 

On cosmological scales, CDM accurately predicts the formation of structure. At smaller distance scales, however, the accurate picture is nebulous. As galaxy simulations and measurements have advanced, the CDM paradigm has encountered several challenges. Contradictory predictions concerning structure on galactic and sub-galactic scales, however, seem to result from it. Local group dwarf satellite galaxies present the main obstacles. Dwarf satellites are excellent candidates for in-depth studies of DM microphysics due to their DM dominance. With the discovery of ultra-faint dwarfs, the old ”missing satellite” problem has been gradually resolved, and other, more pressing problems have emerged. Recent attempts to match populations of simulated subhaloes and observed Milky Way (MW) dwarf galaxies uncovered a “too big to fail” issue. The most massive black halos are too dense to contain the brightest MW satellites. Even more perplexing is that most MW and Andromeda (M31) satellites co-rotate within immense planar structures. This can not be explained within the ΛCDM model. Also, recently, it has been suggested that the growth rate of perturbations is higher than predicted by the ΛCDM model. These issues raise the possibility or implication that dark matter is not cold or collision free. 

Weakly interacting massive particles (WIMPS) or axions, the two generally accepted possibilities for dark matter, would be excluded in that case. 

The defining feature of the thermal WIMP is its relic abundance naturally explained by the freeze-out process with a weak-scale cross-section. This cross-section would account for the elusive non-gravitational interactions of dark matter (DM). WIMP candidates naturally appear around the weak scale in many theories beyond the standard model (BSM). While a simple thermal WIMP is not the only possibility for DM, it is a compelling scenario that demands definitive testing. If WIMPs constitute the DM in the Universe, including our Galaxy’s DM halo, they should be ubiquitous, even in our immediate vicinity. This raises the question of directly detecting these WIMPs in the laboratory. This possibility was first explored by M Goodman and E Witten in 1985. They proposed that WIMPs elastically scattering off nuclei of chosen detector materials might leave recoiling nuclei with detectable kinetic energies. Goodman and Witten suggested that coherent elastic scattering of WIMPs off nuclei, with the cross-section proportional to A (A is the mass number of the nucleus), could yield a detectable rate of scattering events. Subsequently, numerous experiments worldwide, employing diverse detection techniques and materials, have sought WIMPs. However, to date, none of these experiments have definitively claimed WIMP detection. 

In contrast, Modified Newtonian Dynamics (MOND) provides a radical alternative to DM by modifying the Newtonian force law. According to MOND, the modification to Newtonian force law occurs at low acceleration. Thus, MOND can be understood as either a change to the Poisson equation that modifies gravity or a shift in inertia that modifies inertia by breaking the inertial and gravitational mass equivalence. This empirical force law has astonishingly explained a broad range of galactic events. It predicts asymptotically flat rotation curves for spiral galaxies and provides an excellent fit to exact rotation curves. The only free parameter is the critical acceleration a(0), whose best-fit value is the Hubble constant. Interestingly, the Baryonic Tully Fisher Relation (BTFR) is a direct result of this force law deep within the MOND regime. In ΛCDM, galaxies are surrounded by extensive DM halos, so a merger cannot be avoided. In contrast, in MOND, there is only stellar dynamical friction so that a merger can be avoided. According to MOND, tidal dwarf galaxies should have flat rotation curves and reside on the BTFR, consistent with NGC5291’s dwarfs. On extragalactic scales, however, MOND faces more severe problems. 

MOND and CDM are, therefore, effective in almost mutually exclusive regimes. The Λ-CDM model can explain the expansion and linear growth histories and the abundance of clusters, but on a galactic scale, it has certain limitations. MOND explains the observable features of galaxies reasonably well in general, notably the empirical scaling relations. However, it is highly improbable that it can be consistent with the complex shape of the CMB and matter power spectra. [Ed. this speculation is false.] 

To our knowledge, it is difficult to have CDM and MOND behaviors through particle dark matter models. As mentioned earlier, WIMPS or Axions cannot connect these theories in their respective mutually exclusive domains. More than that, WIMPs and axions cannot explain the low energy theory of phonon modes at the galactic scale as their rest mass energy is very high. [Ed. WIMPS are indeed largely ruled out. This paper has no solid support for the claims it makes about axion-like particle dark matter, which relies on mechanism similar to Q-balls to come closer to reproducing MOND than CDM does.]

Recently, the interest in Primordial Blackholes (PBHs) as a dark matter candidate over particle dark matter candidates is to fill this gap. Also, PBHs are not subject to the Big Bang nucleosynthesis (BBN) constraints of Baryons, making them non-baryonic entities that exhibit similar characteristics to CDM particles. [Ed. the paper fails to note that PBH is largely ruled out as the primary source of DM, but dismisses this hypothesis anyway without explanation. "For reasonable assumptions on those PBH binaries' properties before their evolution inside dark matter halos, we get that fraction to be in the range of 5×10^−3 to 0.1, for PBH masses of 5-80M⊙." Of course, the linked paper itself has issues, because PBH's are largely ruled out for masses other than asteroid masses, by micro-lensing for larger PBH's and by evaporation via Hawking radiation for smaller ones. There is also strong suggestive evidence from other sources that asteroid sized PBH's don't exist. PBH's of 5-80⊙ aren't what the DM candidate PBH's are talking about.]

Tentative DM Signal Not Replicated

The DAMA/LIBRA modulation signal is proposed to be a signal of dark matter annihilation. But, the ANAIS-112 experiment has failed to replicate this signal, casting doubt upon it as evidence of dark matter.
This article presents the results of the annual modulation analysis corresponding to six years of ANAIS-112 data. Our results, the most sensitive to date with the same target material, NaI(Tl), are incompatible with the DAMA/LIBRA modulation signal at a 4σ confidence level. Such a discrepancy strongly challenges the DAMA/LIBRA dark matter interpretation and highlights the need to address systematic uncertainties affecting the comparison, particularly those related to the response of detectors to nuclear recoils, which may require further characterization of the DAMA crystals.
From here.

Monday, February 3, 2025

Constraints On Ultra-High Energy New Physics From Cosmic Rays

Deligny cleverly uses ulta-high energy cosmic rays as a natural experiment to discern limits on possible new high energy physics beyond the Standard Model of Particle Physics (BSM) at energies well beyond those that can be explored in existing, and even next generation, particle colliders.

Various phenomena of physics beyond that of the Standard Model could occur at high scale. Ultra-high energy cosmic rays are the only particles available to explore scales above a few dozens of TeV. Although these explorations are much more limited than those carried out with colliders, they provide a series of constraints in several topics such as tests of Lorentz invariance, dark matter, phase transitions in the early universe or sterile neutrinos. Several of these constraints are reviewed in these proceedings of UHECR2024 based on searches for anomalous characteristics in extensive air showers or searches for ultra-high energy gamma rays and neutrinos.
O. Deligny, "Various constraints on BSM physics from extensive air showers and from ultra-high energy gamma-ray and neutrino searches" arXiv:2501.19322 (January 31, 2025).

The body text discusses the following constraints, most of which are rather obscure:

1. Lorentz Violation.

If Lorentz violation (i.e. deviations from special relativity) exists, a certain parameter the suppresses neutral pion decay would be negative, which would also allow individual photons to decay without interacting with other particles. This parameter is observationally constrained by be not smaller than negative 6 parts per 10^21.

In general, the observational constraints on Lorentz violation from this source, as well as from other methodologies, are extremely strict, but can't entirely rule out extremely slight Lorentz violations.

2.  Superheavy dark matter particles

Constraints on superheavy dark matter particles including "inflatons" and spin 3/2 gravitinos can be established:

Other observational astronomy constraints not mentioned also disfavor ultra-heavy dark matter particle candidates, even in the parts of the parameters space where cosmic ray observations alone allow them.

3. Early Universe Phase Transitions

Cosmic strings "are regions of space-time that remain in a symmetry unbroken phase due to boundary conditions that topologically restrict their decay." These would spontaneously decay in the very early universe resulting in a phase transition as the U(1) symmetry and force that holds these strings together completely disappears as the universe cools decaying into still very high energy beyond the Standard Model particles that decay into Standard Model particles.

The energies at which such a phase transition could occur are constrained by ultra-high energy cosmic ray observations.


4. Anomalous Ultra-High Energy Sterile Neutrinos

The body text notes that:
In the SM, the neutrino-nucleon scattering cross-section increases with the energy of the incoming neutrino. Consequently, ultra-high-energy neutrinos may only propagate through the Earth for relatively short distances of the order of O(100) km.

Since the path through Earth to upwards to a neutrino detector is more than 100 km for all but the most grazing angles of approach relative to the surface of the Earth, ultra-high energy neutrinos aren't expected from that direction. But "two “anomalous” radio pulses [have been] observed with the ANITA instrument compatible with EASs developing in the upward direction and inconsistent with SM expectations[.]" 

Efforts have been made to devise SM and BSM explanations for these two outlier data points. An explanation of these anomalies with BSM neutrino physics that could both produce these anomalies, and not produce significantly more anomalies than were observed by ANITA, however, is quite constrained.

One of the less radical BSM neutrino physics possibilities is a sterile neutrino the evades interaction with nucleons in the Earth because it has no weak force interactions until it oscillates into an active high energy neutrino shortly before it reaches the detector. The allowed parameter space of such a sterile neutrino in terms of the oscillation probability U to a sterile neutrino, and the sterile neutrino mass has been established subject to certain assumptions and is shown in the chart below. This mostly limit a sterile neutrino explanation to sterile neutrinos with more than 4 GeV in mass but less than 16 GeV in mass (compared to significantly less than 1 eV for the most massive active neutrino mass, which is ten orders of magnitude smaller), and a probability of transitioning to from sterile neutrino to an active neutrino that is probably in the range of 10^-5 to 10^-6. These parameters are far outside the range that has been suggested by weak anomalies in other searches for sterile neutrinos (which are themselves mutually inconsistent with each other).

Less frequent transition probabilities for a heavy sterile neutrino are discouraged by a lack of close enough sources to produce two ultra-high energy sterile neutrino events, while laboratory based sterile neutrino searches rule out more frequent transition probabilities.

These transition probabilities are four to six orders of magnitude smaller than any of the PMNS matrix neutrino oscillation probabilities, and are two to four orders of magnitude smaller than the smallest CKM matrix probabilities of W boson mediated transitions between first and third generation quark flavors (which differ in mass by five orders of magnitude). Sterile neutrinos of less than 4 GeV masses are also disfavored as an explanation.

This is the only experimental data suggestive of sterile neutrinos with these parameters.

Ancient Demographic Change In China

A new review paper sums up the demographic history of China from hunter-gatherers who were there before the Last Glacial Maximum (ca. 20,000 years ago) to the modern era, using recent ancient DNA discoveries and other data. Unfortunately, it appears to be a closed access paper, so I can't glean much from it beyond the abstract.

Over the past decade, the continuous development of ancient genomic technology and research has significantly advanced our understanding of human history. Since 2017, large-scale studies of ancient human genomes in East Asia, particularly in China, have emerged, resulting in a wealth of ancient genomic data from various time periods and locations, which has provided new insights into the genetic history of East Asian populations over tens of thousands of years. Especially since 2022, there emerged a series of new research progresses in the genetic histories of the northern and southern Chinese populations within the past 10,000 years. However, there is currently no systematic review focused on these recent ancient genomic studies in East Asia. Therefore, this article emphasizes the study of ancient human genomes in China and systematically reviews the genetic patterns and migration history of populations in East Asia since the Late Paleolithic.  

Existing research indicates that by at least 19,000 years ago, there was a north-south differentiation among ancient East Asian populations, leading to different genetic lineages divided by the Qinling-Huaihe line. Gene flow and interactions between northern and southern East Asians began in the Early Neolithic and were further strengthened from the Mid-Neolithic. By the historical period, northern East Asian ancestry played a profound role in the genetic components of southern populations, shaping the genetic structure of present-day Chinese populations. 
Throughout this process, ancient populations in northern and southern China also engaged in extensive interactions through coastal and inland routes with populations from surrounding regions, including Siberia, Japan, Korea, Southeast Asia, and Pacific islands, playing a crucial role in the formation of different linguistic groups. These studies have charted the evolutionary and interaction history of East Asian populations over tens of thousands of years; yet, many unresolved mysteries remain. Further exploration is needed through ancient genomic data from additional time periods and broader geographic areas to facilitate a more comprehensive and detailed investigation, thereby advancing related scientific questions.
Ping WJ, Xue JY, Fu QM. "Ancient DNA elucidates the migration and evolutionary history of northern and southern populations in East Asia." 47(1) Yi Chuan.18-33. (January 2025) doi: 10.16288/j.yczz.24-224.