Cosmology and particle physics

The Standard Model (SM) of particle physics provides a very successful description of fundamental particles and their interactions but it is incomplete, as neutrino masses, dark matter and the baryon asymmetry of the Universe indicate. In addition, the origin of masses and of the approximate fundamental symmetries call out for deeper explanations. The quest for a New SM Theory, that extends the SM to a more general theory, is ongoing. For decades the main focus has been on the TeV scale, but despite an impressive theoretical and experimental effort, no hints of new physics at such scale has been found in experiments.Dark sectors provide an interesting alternative to TeV scale extensions of the SM to explain the open questions in particle and astroparticle physics. Going beyond minimal models, rich dark sectors extend the SM to a complex theory with multiple particles and interactions, in analogy to the SM itself. They have a wealth of theoretical and astrophysical/cosmological consequences and can lead to phenomenological signatures that can be markedly different to that of minimal ones. These include short-lived particles and semi-visible decay signatures, as opposed to minimal models where new states are typically long-lived and purely visible or invisible resonances. Given the experimental configurations and analysis strategies, current dark sector searches might miss such signatures. We advocate a dedicated programme of searches for rich dark sectors that overcomes the assumptions on minimality and on the long lifetime of particles and encompasses a broader range of possibilities. Here, we discuss a prototype model that includes a complex structure akin to the SM: multiple generations of fermions charged under a new spontaneously-broken gauge symmetry.

In these three lectures, I will review some of the major topics which concern both particle physics and cosmology. In the first lecture, I will give a general overview of the standard cosmological Big Bang model and briefly go over Big Bang nucleosynthesis as well as Big Bang baryosynthesis. In the second lecture, I will review the inflationary model with particular emphasis on supersymmetry. In the final lecture I will discuss the dark matter problems which confront cosmology today and their demands on elementary particle physics.

Elementary particle physics tries to find an answer to no minor question: What is our universe made of? To our current knowledge, the elementary constituents of matter are quarks and leptons, which interact via four elementary forces: electromagnetism, strong force, weak force and gravity. All forces, except gravity, can be described in one framework, the Standard Model of particle physics. The model's name reflects its exceptional success in describing all available experimental high energy physics data to high precision up to energies of about 100 GeV. An exception is given by the neutrino masses but even these can be integrated into the model. The Standard Model is based on the requirement of invariance of all physics processes under certain fundamental symmetry transformations. The consideration of these symmetries leads naturally to the correct description of the electromagnetic, weak and strong forces as the exchange of interaction particles, the gauge bosons. However, this formalism has the weakness that it only allows for massless particles. In order to obey the symmetries, a way to introduce the particle masses is given by the Higgs mechanism, which predicts the existence of the only particle of the Standard Model which has yet to be observed: the Higgs boson. In spite of the success of the Standard Model, it has to be considered as a low energy approximation of a more profound theory for various reasons. For example, the underlying theory is expected to allow for an integration of gravity into the framework and to provide a valid particle candidate for the dark matter in our universe. Furthermore, a solution has to be found to the problem that the Higgs boson as a fundamental scalar is sensitive to large radiative corrections driving its mass to the Planck scale of 1019 GeV. Several models have been proposed to address the remaining open questions of the Standard Model. Currently, the most promising extension of the Standard Model is Supersymmetry, which provides elegant solutions to the named problems by introducing a supersymmetric partner to each Standard Model particle. The superpartners of the matter particles are called squarks and sleptons, while the superpartners of the interaction particles are called gauginos. The mass eigenstates of the gauginos are referred to as charginos and neutralinos, according to their electric charge. Since the predicted supersymmetric particles have not yet been observed, Supersymmetry, if it exists in nature, has to be broken in such a way that the masses of Standard Model particles and of their superpartners differ. During the last decades, the energies accessible to experiments has steadily increased. The Tevatron Accelerator at the Fermi National Accelerator Laboratory, with the two multipurpose experiments D0 and CDF, provides currently the highest center-of-mass energy ever reached in experiments using collisions of protons and antiprotons (√s = 1.96 TeV). The study of the particle collisions allows probing of predictions of the Standard Model and its extensions, e.g. Supersymmetry.

Particle physics after the Higgs discovery: Philosophical perspectives

We investigate mixing of neutrinos in the $\nu$MSM (neutrino Minimal Standard Model), which is the MSM extended by three right-handed neutrinos. Especially, we study elements of the mixing matrix $\Theta_{\alpha I}$ between three left-handed neutrinos $\nu_\alpha$ ($\alpha = e,\mu,\tau$) and two sterile neutrinos $N_I$ ($I=2,3$) which are responsible to the seesaw mechanism generating the suppressed masses of active neutrinos as well as the generation of the baryon asymmetry of the universe (BAU). It is shown that $\Theta_{eI}$ can be suppressed by many orders of magnitude compared with $\Theta_{\mu I}$ and $\Theta_{\tau I}$, when the Chooz angle $\theta_{13}$ is large in the normal hierarchy of active neutrino masses. We then discuss the neutrinoless double beta decay in this framework by taking into account the contributions not only from active neutrinos but also from all the three sterile neutrinos. It is shown that $N_2$ and $N_3$ give substantial, destructive contributions when their masses are smaller than a few 100 MeV, and as a results $\Theta_{e I}$ receive no stringent constraint from the current bounds on such decay. Finally, we discuss the impacts of the obtained results on the direct searches of $N_{2,3}$ in meson decays for the case when $N_{2,3}$ are lighter than pion mass. We show that there exists the allowed region for $N_{2,3}$ with such small masses in the normal hierarchy case even if the current bound on the lifetimes of $N_{2,3}$ from the big bang nucleosynthesis is imposed. It is also pointed out that the direct search by using $\pi^+ \to e^+ + N_{2,3}$ and $K^+ \to e^+ + N_{2,3}$ might miss such $N_{2,3}$ since the branching ratios can be extremely small due to the cancellation in $\Theta_{eI}$, but the search by $K^+ \to \mu^+ + N_{2,3}$ can cover the whole allowed region by improving the measurement of the branching ratio by a factor of 5.

In recent years there has been considerable growth in research in so-called 'interface' areas where nuclear physics, particle physics and astrophysics meet; sharing common goals and language, as well as instrumentation techniques. Examples include: Probing the standard model by measuring elements of the CKM matrix The use of colliders and detectors with access to the highest energies to search for the underlying structure of matter and energy Fundamental interactions at the nuclear and particle level required to understand processes from solar burning to supernovae Neutrino physics; including oscillations, mass and interactions probing the standard solar model Hadronic physics dealing with the interactions, binding and structure of the strongly interacting particles under various conditions The increased use of 4 detectors in low-energy nuclear physics to select specific processes of interest. These are only a few examples of what has become a major meeting place for much of the exciting physics of the early 21st Century. In today's science, the spectrum of particles, the nucleus as well as the large-scale components of the universe, are our laboratories for testing and understanding nature.These exciting areas of cross-over require much further research and exploration. The Editorial Board and staff of Journal of Physics G: Nuclear and Particle Physics wish to strengthen their support for physicists working in such areas of intersection. To make our coverage of these interconnections more explicit, we have redrafted our scope statement as follows:Theoretical and experimental topics in the physics of elementary particles and fields, intermediate-energy physics and nuclear physics. The particle astrophysics section includes all aspects of experimental and theoretical research into cosmic rays, nuclear and particle astrophysics, gamma ray astronomy, neutrino astrophysics and dark matter. We also welcome articles from all areas of interface between these fields.We would like to invite all physicists working on these topics to publish their work in the pages of J.Phys.G. Naturally we shall continue to welcome outstanding research in our core nuclear and particle physics areas.Since its launch in 1975, J.Phys.G has had a broad scope covering all aspects of the physics of elementary particles, nuclear physics, and nuclear and particle astrophysics. We have always felt this provides the journal's authors and readers with a unique overview of these increasingly interconnected subjects.When you publish in J.Phys.G you'll benefit from fast and impartial peer review, with an average receipt-to-first decision time for papers of 58 days. We strive to provide outstanding author service with online article submission, tracking of manuscripts and electronic refereeing. Publishing with us provides your paper with wide international visibility in print and online at www.iop.org/journals/jphysg. As well as being accessible to our subscribers, each new paper is freely available to all for 30 days on our Website

We investigate the Q-ball decay in the gauge-mediated SUSY breaking. Q balls decay mainly into nucleons, and partially into gravitinos, while they are kinematically forbidden to decay into sparticles which would be cosmologically harmful. This is achieved by the Q-ball charge small enough to be unstable for the decay, and large enough to be protected kinematically from unwanted decay channel. We can then have right amounts of the baryon asymmetry and the dark matter of the universe, evading any astrophysical and cosmological observational constraints such as the big bang nucleosynthesis, which has not been treated properly in the literatures.

Spontaneous baryogenesis

Big bang nucleosynthesis (BBN) describes the production of the lightest elements in the first minutes of cosmic time. We review the physics of cosmological element production, and the observations of the primordial element abundances. The comparison between theory and observation has heretofore provided our earliest probe of the universe, and given the best measure of the cosmic baryon content. However, BBN has now taken a new role in cosmology, in light of new precision measurements of the cosmic microwave background (CMB). Recent CMB anisotropy data yield a wealth of cosmological parameters; in particular, the baryon‐to‐photon ratio η = nB/nγ is measured to high precision. The confrontation between the BBN and CMB “baryometers” poses a new and stringent test of the standard cosmology; the status of this test are discussed. Moreover, it is now possible to recast the role of BBN by using the CMB to fix the baryon density and even some light element abundances. This strategy sharpens BBN into a more powerful probe of early universe physics, and of galactic nucleosynthesis processes. The impact of the CMB results on particle physics beyond the Standard Model, and on non‐standard cosmology, are illustrated. Prospects for improvement of these bounds via additional astronomical observations and nuclear experiments are discussed, as is the lingering “lithium problem.”

Big Bang Nucleosynthesis (BBN) theory describes the formation of light isotopes such as D, 3 He, 4 He, 6 Li and 7 Li in the first minutes of cosmic time. Their abundance only depends on the baryon density, on particle physics and on nuclear astrophysics, through the competition between the universal expansion rate and the yields of the relevant nuclear reactions. As the expansion rate increases with the number of neutrino families (and any other relativistic species), the comparison between computed and observed abundances of light isotopes allows to constrain the number of neutrinos species, provided that the knowledge of the relevant nuclear processes is accurate enough. Starting from the present uncertainty of the relevant parameters (i.e. baryon density, observed abundance of isotopes and BBN nuclear cross sections), it will be shown that a renewed study of several nuclear reactions, possibly with existing or proposed underground accelerator facilities, is essential to improve the accuracy of computed abundances of light isotopes, providing the BBN theory a powerful probe of particle physics beyond the standard model. In particular, it will be shown that the accurate measurement of the D(p, ) 3 He reaction at BBN energies (40-400 keV), is of primary importance to constrain the number of active neutrinos and/or the lepton degeneracy in the neutrino sector.

We point out that the existence of metastable, τ >103 s, negatively charged electroweak-scale particles (X–) alters the predictions for lithium and other primordial elemental abundances for A > 4 via the formation of bound states with nuclei during Big-Bang nucleosynthesis (BBN). In particular, we show that the bound states of X– with helium, formed at temperatures of about T = 108 K, lead to the catalytic enhancement of 6Li production, which is eight orders of magnitude more efficient than the standard channel. In particle physics models, where subsequent decay of X– does not lead to large nonthermal BBN effects, this directly translates to the level of sensitivity to the number density of long-lived X– particles (τ > 105 s) relative to entropy of nX – / s [Formula: see text] 3 × 10–17, which is one of the most stringent probes of electroweak scale remnants known to date. It is also argued that unstable charged particles with lifetime of order ~2000 s may naturally lead to the depletion of 7Li by a factor of two, making it consistent with observationally determined abundances. PACS No.: 98.80.Ft

I both review and make the case for the current theoretical prejudice: a flat Universe whose dominant constituent is nonbaryonic dark matter, emphasizing that this is still a prejudice and not yet fact. The theoretical motivation for nonbaryonic dark matter is discussed in the context of current elementary-particle theory, stressing that (i) there are no dark-matter candidates within the "standard model" of particle physics, (ii) there are several compelling candidates within attractive extensions of the standard model of particle physics, and (iii) the motivation for these compelling candidates comes first and foremost from particle physics. The dark-matter problem is now a pressing issue in both cosmology and particle physics, and the detection of particle dark matter would provide evidence for "new physics." The compelling candidates are a very light axion (10(-6)-10(-4) eV), a light neutrino (20-90 eV), and a heavy neutralino (10 GeV-2 TeV). The production of these particles in the early Universe and the prospects for their detection are also discussed. I briefly mention more exotic possibilities for the dark matter, including a nonzero cosmological constant, superheavy magnetic monopoles, and decaying neutrinos.

The expansion rate of the Universe deviates from its standard value when the total energy density includes contribution from a new scalar field apart from the radiation energy density. The non-trivial modifications incurred in the Boltzmann equations render the well known analytical solutions unsuitable in non standard scenario. In the present study we derive analytical expressions for the efficiency factor (which is nothing but solution of set of Boltzmann equations) using certain legible approximations. A fair degree of accuracy of these formulas have been observed by juxtaposing the analytical results with that obtained through numerical solution of Boltzmann equations. Faster expansion of the Universe results in decrement of the effective decay parameter which brings down the amount of washout of asymmetry due to inverse decay. Thus in non-standard cosmology scenario, a larger fraction of the asymmetry (generated at early epoch) is expected to survive till present epoch. Alteration of the cosmology does not affect the underlying particle physics model responsible for the generation of the CP asymmetry. Therefore starting from an identical particle physics model we will end up with a larger final baryon asymmetry in the non-standard scenario. It hints towards the possible relaxation of the lower bound of the lightest right handed neutrino mass required to produce adequate asymmetry which is in agreement with current experimental data.

Probing the early universe: a review of primordial nucleosynthesis beyond the standard big bang

Many extensions of the Standard Model of particle physics, in particular scenarios based on supergravity or superstrings, predict a hidden sector of new particles interacting only very weakly with Standard Model particles. Such scenarios do not necessarily only contain Weakly Interacting Massive Particles (WIMPs ), new heavy states at the GeV-TeV scale and above some of which are candidates for the dark matter, but often also predict Weakly Interacting Sub-eV (or Slim) Particles (WISPs) that can couple to the photon field \(A_\mu \) [926]. The most well-known examples include pseudo-scalar axions and axion-like particles (ALPs) a and hidden photons that mix kinetically with photons. At the high end of the mass spectrum various particle physics models also predict the existence of non-elementary particle states that can be either one-dimensional topological defects such as monopoles or non-topological solitons such as condensations of bosonic states and so-called Q-balls . The particle physics aspects of many of these objects have been introduced in Chap. 2. In the present chapter we discuss them further and review some of their possible astrophysical and cosmological effects.