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In 1962, Riccardo Giacconi and colleagues flew three Geiger counters to search for lunar X-ray fluorescence. Different mica thicknesses tested penetration; lampblack blocked ultraviolet. Each counter sat in an anticoincidence detector using a scintillation counter to reject cosmic rays. There was no mechanical collimator. Rotation scanned the sky; optical sensors supplied orientation.^[ref]

Counts were sorted by azimuth; a faulty counter and tumbling descent data were excluded. A directional peak lay above a diffuse component. Window, altitude and angular responses tested X-rays against charged particles; the Moon's nondetection checked optical leakage. The authors argued for an origin outside the Solar system, while residual cosmic ray background was not directly measured and source size, position and spectrum remained assumption-dependent.^[ref]

A simple original forward problem makes the distinction precise. Let $\varphi$ be the scan azimuth, let $g(\varphi )$ be a dimensionless directional response with maximum one, let $F$ be a source's band-integrated photon arrival rate per unit area, let $A_0$ be its on-axis X-ray effective area, and let $b$ be a constant accepted background rate. The expected rate isFor a broad illustrative response $g(\varphi )=\max (0,\cos \varphi )$, the full width at half maximum is $120^{\circ }$: $\cos \varphi =1/2$ at $\varphi =\pm 60^{\circ }$. For a one-dimensional ideal collimator model $g(\varphi )=\max (0,1-|\varphi |/{\theta }_c)$, the full width at half maximum is ${\theta }_c$. Choosing ${\theta }_c=20^{\circ }$ narrows the response sixfold without focusing any additional photons. These are teaching response functions, not reconstructions of the rocket's window transmission or geometry. A narrower scan peak can reflect the instrument response even when the source is unresolved.

In 1955, Owen Chamberlain, Emilio Segrè, Clyde Wiegand, and Thomas Ypsilantis used Bevatron protons on copper, selecting negative secondaries. Magnetic fields selected $p=1.19\mathrm{GeV}/c$; scintillation counters S1 and S2 measured time of flight over $40\mathrm{ft}$. Cherenkov detector C1 flagged $\beta >0.79$; C2 accepted $0.75<\beta <0.78$ after material losses. S3 checked traversal. The cuts rejected negative pions.^[ref]

Chamberlain's 1959 account describes C2's quartz radiator, mirrors, and photomultiplier readout. The trigger combined delayed scintillation counter signals with C2; photographed oscilloscope traces exposed fast particles and accidental coincidences through C1 pulses or extra pulses. Reversing the magnetic fields provided a positive-proton control and revealed an alignment error. Changing selected momentum at fixed velocity acceptance tested for a proton-mass peak.^[ref]

Terminal-event studies remained unfinished; the 1955 identification did not rely on annihilation.^[ref]

The logic is useful far beyond this experiment. A magnetic field separates electric charge signs and magnetic rigidity, but particles of different rest masses can share both. A time of flight signal adds velocity, while a Cherenkov detector independently checks whether that velocity is plausible. An accidental coincidence can imitate the delay of a slow particle without being one particle traversing the apparatus. Agreement between independent measurements is therefore part of the inference, rather than a redundant decoration.

James H. Christenson, James Watson Cronin, Val Logsdon Fitch and René Turlay reported the result in 1964. At Brookhaven's AGS, 30 GeV protons struck beryllium; collimators selected a neutral particle beam at 30 degrees. A downstream helium decay volume reduced interactions.^[ref]

The flight distance suppressed the short-lived kaon component. Each arm of a two-arm magnetic spectrometer had spark chambers before and after its magnet. Cherenkov detectors and scintillation counters triggered photographic recording in coincidence.^[ref]

The helium sample yielded $45\pm 9$ signal events; $(2.0\pm 0.4)\times 10^{-3}$ was the ratio to all charged decay channels.^[ref] That denominator must be retained when comparing this result with a total branching fraction.

The evidence was kinematic. Later regenerated-beam quantum interference established that the short- and long-lived states reached the same two-pion final state, excluding alternatives such as distinct pion-like particles.^[ref] The 1964 observation established evidence for CP violation; it did not determine every parameter of the CKM matrix. Inferring time-reversal symmetry violation from CP violation invokes CPT symmetry, rather than a direct comparison of time-reversed transitions in that experiment.

Counterrotating Electron and Positron beams at SPEAR collided inside the Mark I magnetic particle detector. Lead–scintillator scintillation counters identified Electrons through large shower pulses; small pulses plus hits beyond the iron return identified muons.^[ref]

The selection required opposite-charge Electron–muon tracks, no detected photons, momenta above $0.65\mathrm{GeV}/c$, and transverse noncoplanarity above $20^{\circ }$. Multitrack hadron samples calibrated momentum-dependent misidentification; collinear lepton pairs checked identification losses. Photon and charge control samples tested backgrounds. Acceptance corrections depended on the proposed signal.^[ref]

The 1975 claim was anomalous events without a conventional explanation; new-particle pair production was a possible interpretation.^[ref] Martin Lewis Perl's lecture describes subsequent evidence for a charged lepton.^[ref]

The particle identification problem matters as much as the apparent missing four-momentum in electron-positron collisions. A penetrating hadron can imitate a muon; a particle shower can imitate an Electron; an undetected photon or a detector gap can imitate invisible recoil. An absence of additional recorded signals is conditional on detection efficiency. In a proposed signal model, the selected count has the schematic formwhere $\mathcal{L}$ is integrated collider luminosity, $\sigma$ a cross section, and $\epsilon$ the combined acceptance and detection efficiency. A single count cannot determine all three factors. A control sample must probe the relevant momentum and angular range; its abundant events do not automatically calibrate a differently distributed signal.