An elementary particle has no resolved constituent substructure within the physical theory and experiments under discussion. In the Standard Model, examples include electrons, quarks and photons. A proton is instead a composite hadron, even though it often acts as one particle in a detector or an atomic model.

The distinction is tested through interactions rather than by picturing a smaller material ball inside a larger one. Scattering at increasing momentum transfer probes shorter spatial scales and can reveal internal response. A fundamental quantum field describes the particle's excitations; being elementary does not mean that all measured electromagnetic properties reduce to those of a classical point charge.

Particle decay is a spontaneous transformation of an unstable particle into allowed final particles. Conservation of energy, momentum, electric charge and the relevant interaction's other conserved quantities constrain the possibilities. A decay can be energetically allowed yet absent or weak because its transition amplitude is suppressed.

For an isolated state with an approximately constant rest-frame decay rate, the surviving population follows an exponential in proper time. The mean lifetime $\tau$ and energy decay width $\Gamma$ obey $\Gamma =\hslash /\tau$. A moving particle's laboratory flight-time distribution also includes time dilation.

Experimentally, detectors reconstruct decay products and sometimes a separated decay vertex. Correcting for acceptance and reconstruction efficiency is necessary before interpreting their counts as a lifetime or branching fraction. Different decay channels compete to make the total width.

A Haworth projection depicts cyclic monosaccharide connectivity and substituents above or below a reference ring plane; it does not assert a physically flat ring.

Spin accumulation is a nonequilibrium imbalance between spin populations, often described by a difference of spin-dependent electrochemical potentials $\delta {\mu }_s={\mu }_{\uparrow }-{\mu }_{\downarrow }$. A spin-polarized injected current can create such an imbalance even in a nonmagnetic conductor. Diffusion spreads it while spin relaxation tends to remove it, defining characteristic time and length scales. A transverse magnetic field makes the spin orientation precess and can change a detector signal, the principle of a Hanle spin-precession measurement. This nonequilibrium population is distinct from equilibrium magnetization or a charge-density accumulation.

Moseley's law describes an approximately linear relation between the square root of a characteristic X-ray frequency and the atomic number: $\sqrt{\nu }=a(Z-b)$ for a specified line series. The constants depend on which transitions are compared; $b$ represents an effective screening correction rather than a universal number for every line.

Henry Moseley bombarded samples with electrons and measured their characteristic X-ray wavelengths using a crystal spectrometer. Plotting the corresponding square-root frequencies revealed a systematic ordering by nuclear charge, including gaps where elements were missing. This helped distinguish the physical atomic number from ordering by atomic mass. Different line series and departures from exact straightness contain additional atomic-structure information; the simple relation is not an exact description of all X-ray transitions.^[ref]

Spectroscopy using X-rays, including X-ray emission spectroscopy.

Microwave spectroscopy measures a system's response in the microwave frequency range. Molecular rotation, atomic hyperfine structure and transitions shifted by external fields can occur at these frequencies; the physical transition must be identified rather than inferred from the word microwave alone.

A frequency-scanned source can drive a sample while a detector records absorption or a changed state population. The Lamb shift experiment provides a historical atomic example: a microwave field transfers metastable hydrogen atoms to a state that decays rapidly, allowing a downstream state-sensitive signal to reveal the resonance. The measured quantity is a change in detector response versus frequency, from which a transition energy is inferred.

Spectroscopy based on the energy distribution of electrons, including photoemission spectroscopy.

Atomic spectroscopy studies the emission, absorption and scattering associated with the energy levels of chemical atoms and ions. The positions of spectral lines identify differences of energy, while their strengths and polarization also depend on populations and selection rules.

A dilute-gas discharge can excite chemical atoms; a spectrometer then separates the emitted light by wavelength. Comparing the resulting lines with an independently calibrated reference can identify an element or reveal an isotope shift. Changing a magnetic field tests Zeeman splitting, and improving energy resolution can reveal fine structure. Different experimental changes therefore isolate different pieces of the atomic model.

Assigning wavelengths to detector positions using reference features, so a change in a spectral-line position can be interpreted as a physical wavelength shift.

Comparison of an observed spectrum with a shifted template or line mask as a function of trial Doppler velocity. The fitted correlation centre combines the position information from many spectral lines; its sign convention can produce a peak or a dip.

Energy resolution specifies the characteristic energy separation or broadening that an instrument can distinguish, with a stated width convention and measurement geometry. Resolving a phonon in inelastic scattering requires measuring a small energy transfer as well as momentum transfer. It is distinct from the incident beam's central energy.

Inverse photoemission spectroscopy injects an electron of known kinetic energy into a sample and detects the photon emitted as it enters an unoccupied electronic state. Conservation of energy, together with incident-angle information and an appropriate surface model, probes the unoccupied electronic band structure. Ordinary angle-resolved photoemission spectroscopy instead removes electrons from occupied states.

A software engineer designs, implements and maintains software systems, applying software engineering methods to requirements, interfaces and long-term change. The work can include selecting representations, writing and reviewing code, developing software testing, diagnosing failures and measuring performance. For example, a storage-format change requires reasoning about compatibility and migration as well as a serializer's local correctness. The role overlaps with programming but often includes responsibility for how components behave together over time.

Software testing executes a program under specified conditions and compares its behaviour with an expected result or property. A unit test may check one function; an integration test checks interacting parts; a regression test guards against recurrence of a known failure. For a sorting routine, useful properties include ordered output and preservation of the input multiset. Passing finitely many tests cannot generally prove correctness for every input. The choice of cases and the quality of the test oracle determine what evidence a test actually supplies.

The bandwidth is the width of a specified frequency interval, $\Delta \nu ={\nu }_h-{\nu }_l$. Its meaning depends on how the interval is selected: a signal bandwidth and an effective noise bandwidth can differ even for the same receiving system.

A linear filter that correlates data with a proposed signal, weighting frequencies according to their noise. In stationary Gaussian noise it maximizes the expected signal-to-noise ratio for that known signal.

A semiconductor heterostructure combines different semiconductors, using their interfaces and spatially varying band gaps to control device behavior.^[ref]

A semiconductor device uses controlled populations and motion of charge carriers in a semiconductor to rectify, switch, amplify, detect or emit signals. Its behavior depends on semiconductor doping, contacts, geometry and any heterojunctions, rather than only on the bulk material.

A diode illustrates the distinction: its two-terminal current-voltage characteristic comes from carrier transport through a junction and its contacts. A field-effect transistor instead uses a gate's electric field to control a conducting channel. Measuring a device therefore requires specified biasing and temperature; a quoted bulk electrical conductivity alone does not determine its terminal response.

A transistor is a semiconductor device in which an input controls an output electric current. Used with biasing and an electrical load, it can form an electronic amplifier or a switch. Its power gain comes from an external supply: the input signal controls the conversion of supplied energy into output signal power.

The point-contact transistor makes the idea tangible: two nearby contacts interact through one piece of semiconductor. A bipolar junction transistor instead uses two p-n junctions and a thin intervening region. The distinction matters both for the history and for choosing a physical model.