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Weak Neutral Current the Discovery of the Z0 Boson

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Weak Neutral Current the Discovery of the Z0 Boson Weak Neutral Current the discovery of the Z0 boson. and…future discoveries.. Achille Stocchi LAL-Orsay CNRS (IN2P3) and Université Paris Sud [email protected] TESHEP 2018 ‐ Poltava 1 1 DISCOVERY of the Weak Neutral current in neutrino interactions - `70 Andante 2 WEAK NEUTRAL current IN THE ‘60/’70. THE FCNC PUZZLE Allegro 3 STANDARD MODEL CONSTRUCTION – END OF THE `70. Andante a piacere 4 THE DISCOVERY OF W, and Z0 bosons at SPS at CERN by UA1 and UA2 Presto accellerando 5 DETAILED STUDIES OF Z0 BOSON at LEP. THE TRIUMPH of SM Vivace animato 6 FCNC – a PRIVILIVIGIATE WAY FOR SEARCHING FOR NEW PHYSICS Andante senza tempo 2 e Charged fermions e Electromagnetic Interaction e (eL A eL ) Neutral current Coupling : charge Interaction : vector field Photon SPIN JP = 1‐ , CHARGE = 0 Neutral current in the SM (see later…) ee2 LZeZeAeNC L00 L L 1 2sin W L 2sinWW cos 2sin WW cos e 2 eZeAeRWR 2sin 0 2sinWW cos 3 Weak Neutral current saga SM construction 1960 1970 1980 1990 2000 2010 2020 Measurement of LEP era Deep investigation of FCNC sinW GIM Look for NP mechanism Cabibbo Z0, W discovery Theory SPS at CERN End of LEP The triumph of SM. Predictions of top and Higgs through precise measurments Neutral current Start of LEP 1 Discovery Charm discovery Three families of neutrino 4 1 The years `70 DISCOVERY OF WEAK neutral currents in NEUTRINO INTERECTIONS In the following I discuss neutrino interactions; There were studied/discovered before the Z0 , W establishment, discovery… Often I’ll use them in my graph In effective / Fermi‐like theory In SM with interaction bosons Charged 0 +1 W+ N hadrons N hadrons Neutral 0 0 Z0 ?? N hadrons N hadrons 5 DISCOVERY OF WEAK NEUTRAL currents in NEUTRINO INTERECTIONS iron rock target 1012p 550 m 50m 230m NX charged current n.b. Here for illustration we use already the N X neutral current Feynman diarams with W, Z… Consider that at that tim W, Z where not discovred yet Because the + 0 W are neutral… Z ?? N N hadrons N hadrons hadrons6 Gargamelle was a bubble chamber at CERN designed to detect neutrinos. It operated from 1970 to 1976 with a muon‐neutrino beam produced by the CERN Proton Synchrotron, before moving to the Super Proton Synchrotron (SPS) until 1979. Gargamelle was 4.8 metres long and 2 metres in diameter. It weighed 1000 tonnes and held nearly 12 cubic metres of heavy‐liquid freon (CF3Br). 7 A typical neutrino event as observed in the Big European Bubble Chamber (BEBC) filled with neon (CERN 450 GeV Super Proton Synchrotron (SPS)) W+ N hadrons The muon can be seen on the left. It has been tagged by an external muon identifier. The many‐particle hadron shower is visible on the right. 8 Z0 N hadrons hadronic neutral current event, where the interaction of the neutrino coming from the left produces three secondary particles, all clearly identifiable as hadrons, as they interact with other nuclei in the liquid. There is no charged lepton (muon or electron). 9 MORE diagrams concerning neutrino interaction with electrons + e e e e + + e e e e Z0 W+ e+ e+ e+ e 2 diagrams with W and Z e e e e W‐ - - Z0 e̅ e e̅ e ‐ ‐ e e‐ e‐ e ( ) ( ) GOLDEN PLATED DIAGRAM!! - - ̅ e ̅ e Z0 No diagram with a W ! (would violate the leptonic number) e‐ e‐ 10 Gargamelle : Phys. Lett. B46, 138-140 (1973) Over a total of 1.4 million pictures: 3 events (data taking : 2 ans) Z0 - e- e GOLDEN PLATED EVENT Almost background free !! The electron is projected forward with an energy of 400 MeV at an angle of 1.5 ± 1.5° to the beam Kinematical analysis : direction close from the direction of the incoming beam 11 In 2009 EPS High Energy and Particle Physics Prize is this year awarded to the Gargamelle collaboration, for the "observation of the weak neutral current interaction" in 1973. Papers signed by 55 physicists, from Aachen, Brussels, CERN, Paris, Milano, Orsay and London. A. Pullia from CERN (now in Milano) and J.P. Vialle (Orsay ) 12 1 1st MESSAGE NEUTRAL WEAK CURRENTS EXIST « eventually » brought by a neutral boson … Z0 13 Weak Neutral Current saga SM construction 1960 1970 1980 1990 2000 2010 2020 Measurement of LEP era Deep investigation of FCNC sinW GIM Look for NP mechanism Cabibbo Z0, W discovery End of LEP Theory SPS at CERN The triumph of SM. 2 Predictions of top and Higgs through precise measurments Neutral current Start of LEP Discovery Charm discovery Three families of neutrino 14 In the following I still discuss « facts » which happened before the Z0 , W establishment, discovery… I’ll use them in my graph… and these type of graphs… u d W s u u u p d d ALL THE SAME IN CASE YOU HAVE A NEUTRAL CURRENT 15 WEAK NEUTRAL CURRENTS. years `60 – `70 2 THE PUZZLE OF THE FCNC Strange particles were discovered in the late ‘40 / early ’50 Today we know that strange particles contain strange quark d How do we see the decay of a strange particle ? W s u u u p dd We have used a simplied vision which works well, called spectator model, were the heaviest quark decay first ! Doing some calculation we expect that the ~ Lifetime of heaviest particle is shorter that those of lighter particle So strange particle are expected to have shorter lifetime than pions for example. 16 TWO « PUZZLING » FACTS S = 1 transition. e+ e+ Strangness changes 1 0 e- 0 e Z W + s d s u u u u u + 0 + + + + - K e e K e e su transition/coupling sd transition/coupling 1 The existence of neutral weak interaction and When Lifetime was measured : the existence of strange quarks implie naturally Strange particle have the possibilities of having sd transitions… long lifetime of ~10-10 s (longer that But no observation of : ~ 10-8 s ; ~10-6 s ) – K0+ ‐ 2 – K++ e+ e‐ – K++ ̅ 17 No Flavour Changing Neutral Currents (FCNC) • What can explain the lifetimes of the strange particles ~ 10‐10 s? ( ~ 10‐8 s) The transition rates S=1 are ~20 times smaller than S=0 rates What is the difference between weak processes with S=1 and S=0 ? Cabibbo’s hypothesis : the d and s quarks involved in the weak processes are mixed. The mixing angle is c : the Cabibbo’s angle. Quarks are organized in doublets : uu ddc cosc s sinc An additional parameter … in which the components get transformed by weak interaction. 18 c : the Cabibbo angle Cabibbo The quarks d e s involved in weak uu Theory : processes are « rotated » by an angle ddc coscc s sin Couplings : u d GFcosc u s GF sinc ee e e S=0 W W u d 2 2 2 2 G = Gn = G cos c 0 + e e pnee e e e Many measurements S=1 explained with : e 2 2 W GF sin c sin ~0.22 s u c K 0 e e u u Weak interaction among quarks Non universal 19 c : the Cabibbo angle Cabibbo The quarks d e s involved in weak uu Theory : processes are « rotated » by an ddc coscc s sin angle Couplings : u d GFcosc u s GF sinc S=1 + Purely leptonic decays (e.g. muon decay) do not contain the Cabibbo factor: cos or sin c c W+ u d, s ‐8 1.2 10 s (~0.63) 2 + + 2 2 ()K BR(K v) m 1- (m/m ) = sin c k k + + 2 m 2 () BR( v) cos c 1- (m/m) ‐8 (~1) 2.6 10 s 8.5 e e G 2 sin2 W F c s u 0 K e e u u 20 But the theory predicts flavour changing neutral transition : sd 22 uu ddcoscc ss sin sd sd cos cc sin 1970 : Glashow, Iliopoulos et Maiani (GIM) proposed the introduction of a fourth quark : the quark c (of charge 2/3) : cc Term added to the neutral coupling ssc coscc d sin 22 cc sscoscc dd sin sd sd cos cc sin 22 uu cc dd sscoscc dd ss sin uu cc dd ss 1) Strange particles have a longer lifetime introduction of Cabibbo theory. 2) The neutral current does not change flavour : absence of FCNC prediction of the existence of the charm quark ! 21 More formally. If we write the weak charged current gaa 0 ggud (/2)cos C ggus (/2)sin C 5 1 jgqweak qqq ab a2 b aL bL u qL dscos sin CCL 0 1 u jg (/2)(,cos udCC s cos) 00dscosC sin C (/2)cosg udguCC (/2 )ssn i 01 j qLL q ; 0 0 22 The interaction comes from a gauge group. From the previous page it seems to be clear that for the weak interactions the group is the weak isospin. are the matrices which increase(decrease) of one unity the weak isospin. But to form an algebra we also need 0 10u jgud (,C ) 01 dC 2 2 FCNC uu ddcoscc ss sin sd ds cos cc sin c introducing sd sin s cos CCCL Absence of FCNC 0 10uc 10 jgud (,CC ) gcs (, ) uuddsscc 01 dsCC 01 0 10 jq LL 3q ; 3 01 23 adding the charm in the charged currents c q dssinCC cos 01c jg (/2)(, cd sinCC s cos) 00dssinCC cos (/2)singcdgcsCC (/2)cos 01uc 01 jg /2(,) udCC g /2(,) cs 00dsCC 00 dC d coscc sin V with V ssC sinc cosc d uc, 1 5 V s This rotation Matrix is coscc sin V The Cabibbo Matrix sincc cos 24 25 These two diagrams cancel out the divergence More details It remains a non zero contribution (which is infrared divergent) for momentum lower than the mc, which does not cancel out.
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