Hasil untuk "Elementary particle physics"

V. L. Chernyak, A. Zhitnitsky

Benjamin Koch, Ali Riahinia

We present a canonical quantization framework for static spherically symmetric spacetimes described by the Einstein–Hilbert action with a cosmological constant. In addition to recovering the classical Schwarzschild–(Anti)-de Sitter solutions via the Ehrenfest theorem, we investigate the quantum uncertainty relations that arise among the geometric operators in this setup. Our analysis uncovers an intriguing relation to black hole thermodynamics and opens a new angle towards generalized uncertainty relations. We further obtain an upper and a lower limit of the mass that is allowed in our model, for a given value of the cosmological constant. Both limits, when evaluated for the known value of the cosmological constant, have a stunning relation to observed bounds. These findings open a promising avenue for deeper insights into how quantum effects manifest in spacetime geometry and gravitational systems.

G. Gelmini, E. Roulet

Even if neutrino masses are unknown, we know neutrinos are much lighter than the other fermions we know, and we do not have a good explanation for it. In the Standard Model of elementary particles neutrinos are exactly massless, although this is not insured by any basic principle. Non-zero neutrino masses arise in many extensions of the Standard Model. Massive neutrinos and their associated properties, such as the Dirac or Majorana character of neutrinos, their mixings, lifetimes and magnetic or electric moments, may have very important consequences in astrophysics, cosmology and particle physics. Here we explore these consequences and the constraints they already impose on neutrino properties, as well as the large body of experimental and observational efforts currently devoted to elucidate the mystery of neutrino masses. Several hints for non-zero masses in solar and atmospheric neutrinos, that will be confirmed or rejected in the near future, make this field of research particularly exciting at present.

Elizabeth P. Tito, Vadim I. Pavlov

We analytically obtain a relativistic generalization of the classical Larmor formula for the power of the radiation friction force <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>P</mi><mo>=</mo><mi>m</mi><msup><mi>c</mi><mn>3</mn></msup><msub><mi>r</mi><mi>e</mi></msub><mrow><mo>(</mo><mo>−</mo><msub><mi>w</mi><mi>i</mi></msub><msup><mi>w</mi><mi>i</mi></msup><mo>)</mo></mrow></mrow></semantics></math></inline-formula> for the case where a relativistic charged particle moves in the vicinity of a rotating Kerr black hole.

A. Ali, Saurya Das, E. Vagenas

Various approaches to Quantum Gravity (such as String Theory and Doubly Special Relativity), as well as black hole physics predict a minimum measurable length, or a maximum observable momentum, and related modifications of the Heisenberg Uncertainty Principle to a so-called Generalized Uncertainty Principle (GUP). We propose a GUP consistent with String Theory, Doubly Special Relativity and black hole physics, and show that this modifies all quantum mechanical Hamiltonians. When applied to an elementary particle, it implies that the space which confines it must be quantized. This suggests that space itself is discrete, and that all measurable lengths are quantized in units of a fundamental length (which can be the Planck length). On the one hand, this signals the breakdown of the spacetime continuum picture near that scale, and on the other hand, it can predict an upper bound on the quantum gravity parameter in the GUP, from current observations. Furthermore, such fundamental discreteness of space may have observable consequences at length scales much larger than the Planck scale.

D. Morris, D. Tennant, D. Tennant et al.

Magnetic Monopoles Magnets come with a north and a south pole. Despite being predicted to exist, searches in astronomy and in high-energy particle physics experiments for magnetic monopoles (either north or south on their own) have defied observation. Theoretical work in condensed-matter systems has predicted that spin-ice structures may harbor such elusive particles (see the Perspective by Gingras). Fennell et al. (p. 415, published online 3 September) and Morris et al. (p. 411, published online 3 September) used polarized neutron scattering to probe the spin structure forming in two spin-ice compounds—Ho2Ti2O7 and Dy2Ti2O7—and present results in support of the presence of magnetic monopoles in both materials. Neutron scattering measurements on two spin-ice compounds show evidence for magnetic monopoles. Sources of magnetic fields—magnetic monopoles—have so far proven elusive as elementary particles. Condensed-matter physicists have recently proposed several scenarios of emergent quasiparticles resembling monopoles. A particularly simple proposition pertains to spin ice on the highly frustrated pyrochlore lattice. The spin-ice state is argued to be well described by networks of aligned dipoles resembling solenoidal tubes—classical, and observable, versions of a Dirac string. Where these tubes end, the resulting defects look like magnetic monopoles. We demonstrated, by diffuse neutron scattering, the presence of such strings in the spin ice dysprosium titanate (Dy2Ti2O7). This is achieved by applying a symmetry-breaking magnetic field with which we can manipulate the density and orientation of the strings. In turn, heat capacity is described by a gas of magnetic monopoles interacting via a magnetic Coulomb interaction.

The CMS collaboration, A. Hayrapetyan, A. Tumasyan et al.

Abstract Results are presented from a search for new physics in high-mass diphoton events from proton-proton collisions at s $$ \sqrt{s} $$ = 13 TeV. The data set was collected in 2016–2018 with the CMS detector at the LHC and corresponds to an integrated luminosity of 138 fb −1. Events with a diphoton invariant mass greater than 500 GeV are considered. Two different techniques are used to predict the standard model backgrounds: parametric fits to the smoothly-falling background and a first-principles calculation of the standard model diphoton spectrum at next-to-next-to-leading order in perturbative quantum chromodynamics calculations. The first technique is sensitive to resonant excesses while the second technique can identify broad differences in the invariant mass shape. The data are used to constrain the production of heavy Higgs bosons, Randall-Sundrum gravitons, the large extra dimensions model of Arkani-Hamed, Dimopoulos, and Dvali (ADD), and the continuum clockwork mechanism. No statistically significant excess is observed. The present results are the strongest limits to date on ADD extra dimensions and RS gravitons with a coupling parameter greater than 0.1.

Ricardo A. C. Cipriano, Nailya Ganiyeva, Tiberiu Harko et al.

In this work, we present a review of Energy-Momentum Squared Gravity (EMSG)—more specifically, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>f</mi><mo>(</mo><mi>R</mi><mo>,</mo><msub><mi>T</mi><mrow><mi>μ</mi><mi>ν</mi></mrow></msub><msup><mi>T</mi><mrow><mi>μ</mi><mi>ν</mi></mrow></msup><mo>)</mo></mrow></semantics></math></inline-formula> gravity, where <i>R</i> represents the Ricci scalar and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>T</mi><mrow><mi>μ</mi><mi>ν</mi></mrow></msub></semantics></math></inline-formula> denotes the energy-momentum tensor. The inclusion of quadratic contributions from the energy-momentum components has intriguing cosmological implications, particularly during the Universe’s early epochs. These effects dominate under high-energy conditions, enabling EMSG to potentially address unresolved issues in General Relativity (GR), such as the initial singularity and aspects of big-bang nucleosynthesis in certain models. The theory’s explicit non-minimal coupling between matter and geometry leads to the non-conservation of the energy-momentum tensor, which prompts the investigation of cosmological scenarios through the framework of irreversible thermodynamics of open systems. By employing this formalism, we interpret the energy-balance equations within EMSG from a thermodynamic perspective, viewing them as descriptions of irreversible matter creation processes. Since EMSG converges to GR in a vacuum and differences emerge only in the presence of an energy-momentum distribution, these distinctions become significant in high-curvature regions. Therefore, deviations from GR are expected to be pronounced in the dense cores of compact objects. This review delves into these facets of EMSG, highlighting its potential to shed light on some of the fundamental questions in modern cosmology and gravitational theory.

Michael Bordag

The chromomagnetic vacuum of SU(2) gluodynamics is considered in the background of a finite radius flux tube (center vortex) with a homogeneous field inside and a zero field outside. In this background, there are tachyonic modes. These modes cause an instability. It is assumed that the self-interaction of these modes stops the creation of gluons, and it is assumed that a condensate will be formed. For constant condensates, the minimum of the effective potential is found at the tree level. In the background of these condensates, all tachyonic modes acquire non-zero real masses, which will result in a real effective potential of this system. Considering only the tachyonic modes and adding the energy of the background field, the total energy is found to have a minimum at some value of the background field, which depends on the coupling of the initial SU(2) model. For small coupling, this dependence is polynomial in distinction from the Savvidy vacuum where it is exponentially suppressed. The minimum of this energy will deepen with a shrinking radius of the flux tube. It can be expected that this process can be stopped by adding quantum effects. Using the high-temperature expansion of the effective potential, it can be expected that the symmetry, which is broken by the condensate, will be restored at sufficiently high temperatures.

Kartik Saini, Khaznah Alshammari, Shah Muhammad Hamdi et al.

Solar flares are characterized by sudden bursts of electromagnetic radiation from the Sun’s surface, and are caused by the changes in magnetic field states in active solar regions. Earth and its surrounding space environment can suffer from various negative impacts caused by solar flares, ranging from electronic communication disruption to radiation exposure-based health risks to astronauts. In this paper, we address the solar flare prediction problem from magnetic field parameter-based multivariate time series (MVTS) data using multiple state-of-the-art machine learning classifiers that include MINImally RandOm Convolutional KErnel Transform (MiniRocket), Support Vector Machine (SVM), Canonical Interval Forest (CIF), Multiple Representations Sequence Learner (Mr-SEQL), and a Long Short-Term Memory (LSTM)-based deep learning model. Our experiment is conducted on the Space Weather Analytics for Solar Flares (SWAN-SF) benchmark data set, which is a partitioned collection of MVTS data of active region magnetic field parameters spanning over nine years of operation of the Solar Dynamics Observatory (SDO). The MVTS instances of the SWAN-SF dataset are labeled by GOES X-ray flux-based flare class labels, and attributed to extreme class imbalance because of the rarity of the major flaring events (e.g., X and M). As a performance validation metric in this class-imbalanced dataset, we used the True Skill Statistic (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>T</mi><mi>S</mi><mi>S</mi></mrow></semantics></math></inline-formula>) score. Finally, we demonstrate the advantages of the MVTS learning algorithm MiniRocket, which outperformed the aforementioned classifiers without the need for essential data preprocessing steps such as normalization, statistical summarization, and class imbalance handling heuristics.

D. d’Enterria, D. Le

We perform an extensive survey of rare and exclusive few-body decays ---defined as those with branching fractions $\mathcal{B} \lesssim 10^{-5}$ and two or three final particles--- of the Higgs, Z, W bosons, and the top quark. Such rare decays can probe physics beyond the Standard Model (BSM), constitute a background for exotic decays into new BSM particles, and provide precise information on quantum chromodynamics factorization with small nonperturbative corrections. We tabulate the theoretical $\mathcal{B}$ values for almost 200 rare decay channels of the four heaviest elementary particles, indicating the current experimental limits in their observation. Among those, we have computed for the first time ultrarare Higgs boson decays into photons and/or neutrinos, H and Z radiative decays into leptonium states, radiative H and Z quark-flavour-changing decays, and semiexclusive top-quark decays into a quark plus a meson, while updating predictions for a few other rare H, Z, and top quark partial widths. The feasibility of measuring each of these unobserved decays is estimated for p-p collisions at the high-luminosity Large Hadron Collider (HL-LHC), and for $e^+e^-$ and p-p collisions at the future circular collider (FCC).

Y. Semertzidis, S. Youn

The axion is a highly motivated elementary particle that could address two fundamental questions in physics—the strong charge-parity (CP) problem and the dark matter mystery. Experimental searches for this hypothetical particle started reaching theoretically interesting sensitivity levels, particularly in the micro–electron volt (gigahertz) region. They rely on microwave resonators in strong magnetic fields with signals read out by quantum noise limited amplifiers. Concurrently, there have been intensive experimental efforts to widen the search range by devising various techniques and to enhance sensitivities by implementing advanced technologies. These orthogonal approaches will enable us to explore most of the parameter space for axions and axion-like particles within the next decades, with the 1- to 25-gigahertz frequency range to be conquered well within the first decade. We review the experimental aspects of axion physics and discuss the past, present, and future of the direct search programs.

Y. Motome, J. Nasu

A Majorana fermion is a fermionic particle that is its own antiparticle. Since the theoretical discovery in 1937, the exotic particle has long been searched in particle physics. In the last few decades, however, it has attracted renewed interest in condensed matter physics, where it can be realized as an elementary excitation (quasiparticle) in quantum states of matter. In this review, we discuss another platform for Majorana fermions, the quantum spin liquid, in which interacting magnetic moments remain disordered down to the lowest temperature under strong quantum fluctuations. They are characterized by topological entanglement and fractional excitations, whose possible application to topological quantum computation is recently discussed intensively. As a prime candidate for such exotic states, we here focus on the Kitaev magnets, a subgroup of the spin-orbit Mott insulators. After a brief overview of the Kitaev model and the fractionalization of spins in the exact ground state, we review recent explosive development in this rapidly growing field, with a focus on numerical solutions of the Kitaev model at finite temperatures and the comparison with experiments. The key concept is thermal fractionalization --- two types of fractional excitations manifest themselves at largely different temperatures. This leads to distinct thermodynamics and spin dynamics in a variety of experimentally measurable quantities. We discuss such peculiar behaviors as the signatures of fractional quasiparticles, in careful comparison with the available experimental data for the candidate materials of the Kitaev magnets. Our review gives an overview of the current status of the identification of Majorana fermions in the Kitaev magnets, which would serve as a basis for further experimental and theoretical studies toward the manipulation of the exotic particles for topological quantum computation.

Sonali Patnaik, Rajeev Singh

Behind succeeding measurements of anomalies in semileptonic decays at LHCb and several collider experiments hinting at the possible violation of lepton flavor universality, we undertake a concise review of theoretical foundations of the tree- and loop-level <i>b</i>-hadron decays, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>b</mi><mo stretchy="false">→</mo><mi>c</mi><mi>l</mi><msub><mi>ν</mi><mi>l</mi></msub></mrow></semantics></math></inline-formula> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>b</mi><mo stretchy="false">→</mo><mi>s</mi><msup><mi>l</mi><mo>+</mo></msup><msup><mi>l</mi><mo>−</mo></msup></mrow></semantics></math></inline-formula> along with experimental environments. We revisit the world averages for <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>R</mi><mrow><mi>D</mi><mo>(</mo><msup><mi>D</mi><mo>*</mo></msup><mo>)</mo></mrow></msub></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>R</mi><mrow><mi>K</mi><mo>(</mo><msup><mi>K</mi><mo>*</mo></msup><mo>)</mo></mrow></msub></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>R</mi><mrow><mi>J</mi><mo>/</mo><mi>ψ</mi></mrow></msub></semantics></math></inline-formula>, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>R</mi><msub><mi>η</mi><mi>c</mi></msub></msub></semantics></math></inline-formula>, for the semileptonic transitions and provide results within the framework of the relativistic independent quark model in addition to the results from model-independent studies. If the ongoing evaluation of the data of LHC Run 2 confirms the measurements of Run 1, then the statistical significance of the effect in each decay channel is likely to reach 5 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>σ</mi></semantics></math></inline-formula>. A confirmation of these measurements would soon turn out to be the first remarkable observation of physics beyond the Standard Model, providing a wider outlook on the understanding of new physics.

M. Agostini, A. Bakalyarov, M. Balata et al.

Looking for an exotic decay Neutrinos—elementary fermionic particles with no electrical charge—defy the standard model of particle physics by having a tiny, but nonzero mass. One explanation for their properties is that they are Majorana fermions, which are particles equal to their antiparticles. If neutrinos were Majorana fermions, a process called neutrinoless double-β decay would become possible: an unstable nucleus could decay by turning two of its neutrons into protons with the emission of two electrons but no antineutrinos. The GERDA Collaboration searched for this decay in a particular isotope of germanium. Housed deep underground to reduce the background signal, the experiment did not detect the elusive process but did place improved boundaries on its half-life. Science, this issue p. 1445 A germanium detector–based experiment places improved boundaries on the half-life of an exotic nuclear decay. A discovery that neutrinos are Majorana fermions would have profound implications for particle physics and cosmology. The Majorana character of neutrinos would make possible the neutrinoless double-β (0νββ) decay, a matter-creating process without the balancing emission of antimatter. The GERDA Collaboration searches for the 0νββ decay of 76Ge by operating bare germanium detectors in an active liquid argon shield. With a total exposure of 82.4 kg⋅year, we observe no signal and derive a lower half-life limit of T1/2 > 0.9 × 1026 years (90% C.L.). Our T1/2 sensitivity, assuming no signal, is 1.1 × 1026 years. Combining the latter with those from other 0νββ decay searches yields a sensitivity to the effective Majorana neutrino mass of 0.07 to 0.16 electron volts.

W. L. Tan, P. Becker, F. Liu et al.

Particles subject to confinement experience an attractive potential that increases without bound as they separate. A prominent example is colour confinement in particle physics, in which baryons and mesons are produced by quark confinement. Confinement can also occur in low-energy quantum many-body systems when elementary excitations are confined into bound quasiparticles. Here we report the observation of magnetic domain-wall confinement in interacting spin chains with a trapped-ion quantum simulator. By measuring how correlations spread, we show that confinement can suppress information propagation and thermalization in such many-body systems. We quantitatively determine the excitation energy of domain-wall bound states from the non-equilibrium quench dynamics. We also study the number of domain-wall excitations created for different quench parameters, in a regime that is difficult to model with classical computers. This work demonstrates the capability of quantum simulators for investigating high-energy physics phenomena, such as quark collision and string breaking. Long-range Ising interactions present in one-dimensional spin chains can induce a confining potential between pairs of domain walls, slowing down the thermalization of the system. This has now been observed in a trapped-ion quantum simulator.

G. L. Trigg, S. J. Rothman, R. Benedek et al.

Claus Kiefer, Patrick Peter

Time in quantum gravity is not a well-defined notion despite its central role in the very definition of dynamics. Using the formalism of quantum geometrodynamics, we briefly review the problem and illustrate it with two proposed solutions. Our main application is quantum cosmology—the application of quantum gravity to the Universe as a whole.

Soham Sen, Sukanta Bhattacharyya, Sunandan Gangopadhyay

The Heisenberg uncertainty principle is modified by the introduction of an observer-independent minimal length. In this work, we have considered the resonant gravitational wave detector in the modified uncertainty principle framework, where we have used the position momentum uncertainty relation with a quadratic order correction only. We have then used the path integral approach to calculate an action for the bar detector in the presence of a gravitational wave and then derived the Lagrangian of the system, leading to the equation of motion for the configuration-space position coordinate in one dimension. We then find a perturbative solution for the coordinate of the detector for a circularly polarized gravitational wave, leading to a classical solution of the same for the given initial conditions. Using this classical form of the coordinate of the detector, we finally obtain the classical form of the on-shell action describing the harmonic oscillator–gravitational wave system. Finally, we have obtained the free particle propagator containing the quantum fluctuation term considering gravitational wave interaction.

Viacheslav Khruschov, Sergey Fomichev

To study the oscillations of active neutrinos in the framework of the model with three active and three sterile neutrinos, the analytical expressions are obtained for the appearance and survival probabilities of different neutrino flavors taking into account the decaying sterile neutrinos contributions. In the framework of the considered phenomenological neutrino model, we make an interpretation of the experimentally detected XENON1T-excess of electronic recoil events in the energy range of 1–7 keV as a result of the radiative decay of a sterile neutrino with a mass of about 7 keV. Estimations of the decay parameters for the radiative decay of Majorana sterile neutrinos due to the magnetic dipole transitions into the active neutrino states are made. The value of the parameter of active and sterile neutrinos mixing has been derived from the Baksan Experiment on Sterile Transitions (BEST) experimental data. The graphical dependences for the probabilities of appearance and survival of muonic and electron neutrinos at short baseline (SBL) are presented with the use of that gained from the experimental data estimations of the model parameters.