From standard model premise to standard theory completion (in two pics and two comments)

A follow up to the former post, much shorter ;-)

Two pics to illustrate the begining and the end of the experimental evidenceof the standard model/theory of particle physics

The first sign of neutral current was an antineutrino-electron scattering event observed in December 1972 (figure above) in the Gargamelle bubble chamber. For the total exposure of 1.4*106 pictures 5 to 30 events were expected depending ...  finally 3 events were seen.

THE STANDARD MODEL 30 YEARS OF GLORY Jacques Lefrançois

Higgs event candidate in the ATLAS detector containing a pair of muons and a pair of electrons (recorded on June 30th 2011)

//Added on August 1st 2015:

A phenomenologist and an experimentalist point of view on this fantastic endeavour

The 1973 discovery of weak neutral currents in neutrino scattering by the Gargamelle bubble-chamber collaboration at CERN [9] made experimentalists, and the world at large, aware of Yang-Mills theories, much as the 1971 work of ’t Hooft [7] immediately attracted attention from (field) theorists to the same subject. Both lines of work [7,8,9] were monumental in their difficulty, and run against deep prejudices. In the case of neutral currents, the prejudices had various sources, among them [I have refreshed my memory on these issues by re-reading the talk by Bernard Aubert in [10], an interestingly uneven book; and their rendering by Peter Galison in [11], a thoroughly documented report, that reads like a good novel. I quote them freely in this section]: 

• The very strong limits on their strangeness-changing variant, which in the 70’s were at the branching ratio level of 10−6 (10−9 ) for K^± (K⁰ _L ) decays [12]. 

• The perception that the measurement of neutrino-induced weak neutral currents —at least in the semileptonic channels with larger cross-sections than the purely leptonic ones— was nearly impossible in its practical difficulty [13]. 

• The existence of severe (and incorrect) upper limits on strangeness-conserving neutral-current processes, such as the one by Ben Lee, stating that [The results of W. Lee [14]] rule out the existence of the neutral current predicted by Weinberg’s model... [15], or the one by John Bell, J. Løvseth and Tini Veltman: Thus the ratio of neutral-current “elastic events” is less than 3% [16]. 

• The fact that neutrino experiments at the time were primarily designed to look for sequential heavy leptons and for the Lee–Yang process [17] —ν_µ+N → W+µ+N— for light W’s, but not for neutral currents. 

Naturally, the neutral-current processes favoured by theorists were the ones whose cross sections could be calculated with confidence in the standard model: ν_l,¯ν_l elastic scattering on electrons, whose standard cross-sections were worked out by ’t Hooft as early as 1971 [18] (he may also have been the first to emphasize a trivial but important fact: a measurement of the weak-mixing angle, θ_W , would imply a prediction for the —then— enormous masses of the W and Z). By January 1973, the Aachen group of the Gargamelle collaboration had found a “picture book” event, with a single recoiling electron [19]. But it was just one event and —while it immediately had the effect of putting Gargamellers even harder at work on neutral currents— various cautions and dangerous backgrounds kept the team from publishing this result until July, right before they published their work on semileptonic neutral currents [9].

Fifty years of Yang-Mills Theories: a phenomenological point of view

Alvaro De Rujula

(Submitted on 23 Apr 2004 (v1), last revised 15 Jun 2004 (this version, v4))

... discovery does not only mean signal identification but also background analysis, cross check of the information , on the experimental aspect but also theoretical aspect. In the neutral current history, the possibility to check the neutron background and also to measure the rate in so many different channels as elastic, semi elastic, purely leptonic was a capital pieces in the process of building evidence. 

... More and more high energy particle physics calls for large collaboration. The Gargamelle experiment is the first large collaboration in modern physics. Announcing a discovery must first imply convincing evidence for the members of the collaboration. Each member has his own history and personal background . This is specially sensitive in what we calls an "evidence", for some of them more convincing is statistical analysis, for other it will be the presence of "golden platted" events. Clearly more active is the collaboration in an experiment, more solid must be the evidence, before to call for discovery.  

NEUTRAL CURRENTS, Bernard Aubert.