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…) 

ee2    LZeZeAeNC    L00 L  L  1 2sin W   L 2sin cos 2sin cos  WW WW    e 2 eZeAeRWR 2sin 0 2sin cos WW   3 Weak Neutral current saga

SM construction

1960 1970 1980 1990 2000 2010 2020

Measurement of LEP era Deep investigation of FCNC sinW 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

 NX 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 sinW 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 sd 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 cosc  s sinc 

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 coscc s sin 

Couplings : u d GFcosc u s GF sinc ee

e e S=0 W W

u d   2 2 2 2 G = Gn = G cos c  0  +     e e pnee   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 coscc s sin  angle

Couplings : u d GFcosc u s GF sinc

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 ddcoscc 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 coscc d sin  22 cc sscoscc dd sin sd sd cos cc sin

22 uu cc dd  sscoscc  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

 1 5  jgqweak  qqq ab a2 b aL bL  u qL    dscos sin CCL

 0 1 u jg (/2)(,cos udCC s cos) 00dscosC  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 10u jgud (,C ) 01 dC 2 2 FCNC uu ddcoscc  ss sin  sd   ds cos cc sin c introducing   sd  sin s cos  CCCL Absence of FCNC  10uc 10  0   jgud(,CC ) gcs (, )   uuddsscc 01 dsCC  01  

0 10 jqLL3q ; 3   01   23   adding the charm in the charged currents c q   dssinCC cos

 01c jg (/2)(, cd sinCC s cos)  00dssinCC cos  (/2)singcdgcsCC  (/2)cos 

 01uc  01  jg /2(,) udCC g /2(,) cs   00dsCC  00 

dC d coscc sin V with V  ssC  sinc cosc

  d uc, 1 5 V s This rotation Matrix is  coscc sin  V  The Cabibbo Matrix sincc 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. The amount of cancellation depends on the mass of the new quark

22 2 2 ()cossinmmcu C C

0  For mc=mu It would be BR()0 K  A quark mass of 1.5GeV is necessary to get good agreement with the experimental data.

First “evidence” for Charm quark! and the

fact that mc is such that was not yet observed…

26 nd 2 2 MESSAGE

The coupling is not and this is codified in the CKM matrix. anymore universal

W u u u c c c t t t

d s b d s b d s b

The neutral currents stay universal, in the mass basis : we do not need extra parameters for their complete description

Z0 u d s c b t l 

u d s c b t l 

and this is completly included and comes out « naturally » from the Standard Model (I’ll not demonstrate during lecture, but is in Backup) 27 u d s c b t c s b t b b

u d s c b t u d s c b d

Flavour Changing Neutral Current (FCNC) NEUTRAL CURRENTS with Z0. occurs with W exchange. DO NOT CHANGE THE FLAVOUR THEY ARE SUPPRESSED IN THE SM SINCE OCCURS AT SECOND ORDER.

28 3 Weak Neutral Current saga

SM construction 1960 1970 1980 1990 2000 2010 2020

Measurement of LEP era Deep investigation of FCNC sinW 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 Dicovery

Charm discovery

Three families of neutrino 29 3 The SM Construction End of `70 years In two pages since all is in the lectures of Sébastien Descotes‐Genon

1,2,3 We have SU(2) et U(1) and so 4 field B et W  and we have 2 coupling constants g et g’. In the Lagrangian we want to avoid for instance that neutrino have electromagnetic interaction… 3 For that we introduce the field Z et A as an orthogonal combination of W et B….   WZA30cosWW sin   BZAsinWW0  cos Now if we associate to the photon te field A (the field Z acts both to neutrinos and on charged particles) we find the relation of the electric charge as a fonction of g and g’ gg ' e  gg'22

g ' sinW  We can also write gg'22 g cos  W  gg'22 e g  The two constants g et g’ can be written sinW in terms of « e » and of the Weinberg  . That implies W e ELECTROWEAK UNIFICATION g '  30 cosW ,W,Z In term of couplings

When you calculate ..

2 2 2 W g /2= e /2sin W W

 e2 

2 2 2 2 Z (g V + g A) (g/4cos W ) Z

Very predictive !! Vectorial constant Let’s me say the unexpected part of the SM… Axial constant

+ propagator… 2 1/M W

2 1/M Z 31 DETERMINATION OF THE WEINBERG ANGLE FROM NEUTRINO SCATTERING and use of Neutral Current Exemple againFigure ici-bas donne of Neutral un exemple current de courantes (NC) and Charge Currents neutres (CN) et des courantes chargées (CC) , (CC) describingdans la diffusion the diffusion of a muonic d’un neutrino muonique sur un Noyau. neutrinos on a Nucleus. Dans le cas des CC l’état final contient unIn case of CC the final state contain muon chargé + jets d’hadrons tandis que dans le cas des a muon + hadrons CN l’état final contiennent un

neutrino muoniquein case of NC not… but only +jet d’hadrons hadrons

    

+ W Z0

N hadrons N hadrons

 N  X courantNeutral neutre current  N  X courantCharge current chargé 32 2 2 (CC) ~ e /2sin W

2 2 2 2 (NC) ~ (g V + g A) (g/4cos W )

A POSSIBLE OBSERVABLE : THE RATIO of the TWO CROSS SECTIONS which means being able to count the number of events having or not a muon in a final state..

(NC)/ (CC) ~ f(W) In PDG today sin2 θ = 0.23155(5) ~1/4…

33 The SM Construction … continues…

FROM HIGGS MECHANISM   1 0 v is the vaccum expectation  0 0  value of the potential ,   2 v namely the value of the potential in its fondamental expandingen faisant l'expansion state. 1  0 ()x   2 vHx () gv gg2'2 MMvM 0 WZ22

MW M Z  cosW  2 2 MW ou sinW  1  M Z The value of the Z and W masses are linked through the Weinberg angle. It is a strong prediction of the SM ! ANY HINT ON THE W and Z masses ? 34 weak interaction Fermi EFFECTIVE theory

• 1932 : Fermi proposes a theory which is the analogous of electromagnetism to explain the decay by weak interaction) QED :  1 p Meuu pep --2  euu e q 2 e   MJJ 2  J current  (q) q p e 

e- ‐ + Fermi proposes for npee and pnee a local interaction :

 2 Gu n up  ue ue  Local interaction (no 1/q  term) [GeV-2]  pnN J Through the analogy with electromagnetism, the G constant should be of GeV-2 dimension

- e 35 e J e e 36  e  u 

  N 52 2 55  e   /M   -5 11 

  

 ~ 10  1.16 10 G   F 2 2 W 2 F G M ~ G GeV ; 8 From precise muon lifetime Mu u u  e - 2 - Gg  e 5 e  - 25 e e e  uu GE  1   decay for example)   2 e q 3 5    1 2 W

2    M Gm 192

 55  W   5 1  

 11    - 11 22         

 (which is the case for e 2 W 2 2 W 22 M g gg 8    e ~ ;   <

 if From dimensional arguments ~ : From calculations 1/2 1/2 2g 2 2 4 M  W 88sinGG2 FFW  1/2 1 37.28  [GeV ] sin sin 2GF WW

MW ~78 GeV The Masses of W and Z0 are SM predictions using the measured values of the couplings :  and GF and sinW

MW MZ  cosW 2  M MZ ~90 GeV2 W ou sinW  1  37 MZ The Nobel Prize in Physics 1979 The Nobel Prize in Physics 1979 was awarded jointly to Sheldon Lee Glashow, Abdus Salam and Steven Weinberg "for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including, inter alia, the prediction of the weak neutral current". Abdus Salam Steven Weinberg Sheldon Lee Glashow

The Nobel Prize in Physics 1999

The Nobel Prize in Physics 1999 was awarded jointly to Gerardus 't Hooft and Martinus J.G. Veltman "for elucidating the quantum structure of electroweak interactions in physics"

Gerardus 't Hooft Martinus J.G. Veltman

38 The Nobel Prize in Physics 2008 The Nobel Prize in Physics 2008 was divided, one half awarded to Yoichiro Nambu "for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics",the other half jointly to Makoto Kobayashi and Toshihide Maskawa "for the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature". Yoichiro Nambu Makoto Kobayashi Toshihide Maskawa

The Nobel Prize in Physics 2013

The Nobel Prize in Physics 2013 was awarded jointly to François Englert and Peter W. Higgs "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider" François Englert Peter W. Higgs

39 3 3rd MESSAGE NEUTRAL CURRENTS ARE PERFECTLY CODED IN THE STANDARD MODEL

SM is very predictive in neutral current sector : Coupling, Masses…

40 Weak Neutral Current saga

SM construction

1960 1970 1980 1990 2000 2010 2020

Measurement of LEP era Deep investigation of FCNC sinW GIM Look for NP 4 mechanism Cabibbo Z0, W discovery Theory End of LEP SPS at CERN The triumph of SM. Predictions of top and Higgs through precise measurments Neutral current Start of LEP Dicovery

Charm discovery

Three families of neutrino 41 4 THE DISCOVERY OF W+‐ and Z0 bosons The years `80 at SPS at CERN by UA1 and UA2 Coll.

p q W,Z 0 ud W+ p q uu,dd  Z0

+ ‐ + + p p  W X W  l l 0 0 0 + ‐ p p  Z X Z  l l 42 The accelerator complex

Antiproton Accumulator in 1980.

43 http://www.nobelprize.org/nobel_prizes/physics/laureates/1984/meer-lecture.pdf One of the first pictures of a 540 Ge V proton‐antiproton collision, as recorded in the big streamer chambers of the UA5 experiment at the CERN SPS 44 UA1

The UA1 detector, shown here in its "garage" position, was a multi-purpose detector. The main asset of the UA1 detector was a large-volume, high-resolution central tracking detector. It covered as large a solid angle as possible and could detect hadron jets, electrons and muons. 45 UA2

Particles emerging from the collisions are picked up in the inner vertex detector, equipped with interleaved proportional chambers and drift chambers. Surrounding this vertex detector are the central electromagnetic and hadronic calorimeters, segmented into 240 cells, each pointing towards the centre of the interaction region. Each of these cells is divided into electromagnetic (lead/scintillator) and hadronic (iron/scintillator) compartments. During its initial runs in 1981 and 1982, the UA2 central calorimeter had a 'wedge' removed to accommodate a magnetic 46 spectrometer which measured the level of neutral pion production. W

W‐ –> e‐ 

The decay of a W particle in the UA1 detector

showing the track of the high-energy electron towards the bottom. The yellow arrow marks the direction of the missing transverse energy and hence the path of the unseen neutrino. 47 Z e+e‐

CERN SppS, UA1 experiment, 30 April 1983

One of the first Z particles observed in UA1.

The two white tracks (towards the top right and almost directly downwards) reveal the Z's decay into an electron-positron pair that deposit their energy in the electromagnetic calorimeter. 48 Z0 discovery paper

UA1 collaboration

Phys.Lett. B126 (1983) 398-410

49 On 25 January 1983, CERN called a press conference to announce the discovery of the W particles.

Carlo Rubbia and Simon Van der Meer received the Nobel Prize in 1984

50 And the first mass measurements of W±, Z0

MW = 81 ± 5 GeV

2 MZ = 95.2 ± 2.5 GeV/c (UA1) = 91.9 ± 1.9 GeV/c2 (UA2)

51 And later…

UA1: Z0 e+e‐

UA2: Z0 e+e‐

52 4 4th MESSAGE Z0 (and W) DISCOVERY The Z0 (and W) where discovered as real particles (peaks in invariant mass) at the expected masses. It is a tremendous triumph of our vision of weak interactions (unified with electromagnetism) carried by intermediate bosons 53 Weak Neutral Current saga

SM construction

1960 1970 1980 1990 2000 2010 2020

Measurement of LEP era Deep investigation of FCNC sinW GIM Look for NP mechanism 5 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 Dicovery

Charm discovery

Three families of neutrino 54 The years `90 5

DETAILED STUDIES OF THE Z0 BOSON at LEP !

The TRIUMPH of the STANDARD MODEL

55 Large Electron–Positron Collider (LEP) at CERN. LEP collided electrons with positrons

starting at the energy of 90 GeV (for precise study of Z0)

 up to energies of 209 GeV.

56 As an example DELPHI DETECTOR

57 ,W,Z In term of couplings

When you calculate ..

2 2 2 W g /2= e /2sin W W

 e2 

2 2 2 2 Z (g V + g A) (g/4cos W ) Z

Very predictive !! Vectorial constant Let’s me say the unexpected part of the SM… Axial constant

2 + propagator… 1/M W

2 1/M Z 58 Z production

At pole :

Compared with

At Z0 pole/peak the weak cross section is 200 times larger than electomagnetism cross section !

59 60 61 62 63 64 65 66 SUMMARY +e‐

67 Z decay

68 Z0 prefers neutrinos to charged leptons

Z0 prefers d‐type quark to up‐down‐quark

69 NEUTRINO FAMILY COUNTING

1)

2)

1) + 2)  and

Using

70 SM Predictions with 3 neutrino

A famous PLOT :

71 The very famous plots !!

FROM FIRST MEASUREMENT

TO A PRECISE MEASURMENT

RESULT compatible with 3 families of neutrino with mass < MZ/2 72 Measurement of the mass of Z0

LEP 1 Phase 1989‐1995 •15 million Z’s

•MZ = 91187.52.1 MeV Precision of 2 10‐5 !

•Z=2495.22.3 MeV73 REMEMBER…

1/2 1/2 2g2 2 4 M  W 2 88sinGGFFW  1/2 1 37.28  [GeV ] sin sin 2GF WW And the measured masses MW ~78 GeV M M  W Z M = 91,18750.0021 GeV cosW Z 2  M ou sin2  1 W MZ ~90 GeVW  MZ

NOT BAD.. !!!..... BUT …74 COMPARISON BETWEEN MEASUREMENTS AND PREDICTIONS …

We have to do better predictions….

For instance we take here the value of the Weinberg angle mesured in neutrino diffiusion (measurement which is independent on W and Z mass ). 2 sin W = 0.22550.0021

MGeV78.51 0.36 W Predicted mass

MGeVZ 89.21 0.29

M = 91,18750.0021 GeV Z Measured masses

MW=80.398 0.025 GeV

prPredictedé dicte mesuréeMeasured Corresponding to MMWW  M W 1.88 soit -5.2

prédictePredicted mesuréeMeasured Corresponding to MMZZ  M Z 1.98 soit -6.8

???75 It is due to the radiative corrections

Electromagnitic radiative corrections

But also weak radiative corrections… correcting the propagator, the vertexes…

76 Exemple with MW

measured predicted MW=80.398 0.025 GeV

(Before was 78,51 0.36 GeV)

So the masses of the W and Z are sensitive to all the particles in the loops… top quark, Higgs… 77 Using all the electroweak measurements we got

M(Higgs) = 125.6 ± 0.4 GeV M(top) = 172.4 ± 1.2 GeV 78 5 5th MESSAGE SM TRIUMPH SMandinparticularneutralsector(Z0) has been studied at high precision (~per mill) and did not show sofar any deviations. So precise that we have seen radiative corrections… testing deeply the gauge structure of SM and opening the

possibility of testing New Physics. 79 Weak Neutral Current saga

SM construction

1960 1970 1980 1990 2000 2010 2020

6 Measurement of LEP era Deep investigation of FCNC sinW 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 Dicovery

Charm discovery

Three families of neutrino 80 The years `NOW 6 FCNC – a PRIVILIVIGIATE WAY FOR SEARCHING FOR NEW PHYSICS

e+ In general FCNC are rares since they pass through loops… second order effects e- 0 Z 0    9 BR(K   ) 710  108 s d    BR(K    ) 0.64

u u

K++ e+ e-

But particle beyond the SM could evenutally not share this ! Z0 could be an exotic Z0…

81 u d s c b t c s b t b b

u d s c b t u d s c b d

Flavour Changing Neutral Current (FCNC) NEUTRAL CURRENTS with Z0. occurs with W exchange. DO NOT CHANGE THE FLAVOUR THEY ARE SUPPRESSED IN THE SM SINCE OCCURS AT SECOND ORDER.

82 Example for B oscillations (FCNC-B=2)

FCNC porcesses are ideal place to look for NP effects because they are suppressed in SM

rcs esrmnsare needed. measurements Precise  W * b d VVtb tq 0 0 B t,c,u t,c,u B d SM d W 2 MW fet os1/ goes Effects d, b

BSM Local :  bq Dim 6  2 eff

In the language of The EFT, the operators have dimension 6  1/2 The measurements (in this case md) are modified wrt the predictions of the SM by the presence of BSM particles. modifications are important if couplings are larger and/or NP masses are lighter83  Very small branching ratio, but very well predicted in the Standard model: (3.54±0.30) x 10-9.

SM NP

 But possibly enhanced by new physics.

BUT RECENT RESULTS….

84 Other possibilities studying radiative b to s transitions…

85 Of course there are a lot of direct searches for direct FCNC

At pp Collider At ep Collider

At e+e‐ Collider

86 AND SOFAR… SM RESISTED TO ALL THE ATTACKS… All the measured quantities involving FCNC agree rather well with SM predictions !!

very large effects are excluded. so the extentions of the SM should be « corrections » to SM

 bq BUT IT IS NOT THE END OF THE STORY : 2 Couplings of NP small or NP at high masses eff

87 Weak Neutral Current saga NP NP NP… let’s be optimistic ! SM construction

1960 1970 1980 1990 2000 2010 2020

Measurement of LEP era Deep investigation of FCNC sinW 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 NP Dicovery

NP Charm discovery

Three families of neutrino 88 BACKUP

89 The Standard Model in the fermion sector

90 Flavour Physics in the Standard Model (SM) in the quark sector:

10 free parameters

6 quarks masses 4 CKM parameters ~ half of the Standard Model In the Standard Model, charged weak interactions among quarks are codified in a 3 X 3 unitarity matrix : the CKM Matrix.

The existence of this matrix conveys the fact that the quarks which participate to weak processes are a linear combination of mass eigenstates

The fermion sector is poorly constrained by SM + Higgs Mechanism mass hierarchy and CKM parameters 91 The Standard Model is based on the following gauge symmetry

SU(2)L  U(1)Y

Weak Isospin (symbol L because Weak Hypercharge : only the LEFT states are involved ) (LEFT and RIGHT states )

I I3 Q Y doublet L e ½ ½ 0 -1 e - ½ -½ -1 -1 L Idem for the Leptons singlet R - eR 0 0 -1 -2 other families 2/3 1/3 uL ½ ½ doublet L -1/3 1/3 dL ½ -½ singlet R 2/3 4/3 uR 0 0 singlet R -1/3 -2/3 92 quarks dR 0 0 Short digression on the mass

ur 2 1 Epm22  m 2 0 Lm    22 0    2 ()0im    Li  m

mmPPmPPPP     ()LR ( LLRR  )     mPP[(LL )( ) ( PP RR )( )] m  RLLR  The mass should appear in a LEFT-RIGHT coupling

R : SU(2) singlet The mass terms are not gauge invariant under

L : SU(2) doublet SU(2)L  U(1)Y R (I=0,Y=-2) leptoniR

Adding a doublet (I=0,Y=-2/3) quark dR (I=0,Y=4/3) quark u    1 R  0 I= Y=1  (I=1,Y=-1) leptoni  2 L L (I=1,Y=1/3) quark dL h (I=1/2,Y=1)

(I=1,Y=1/3) quark uL

Yukawa interaction :  L R 93 1  0    2 vH

 geR (LR  L )

(le deuxieme terme est l'hermitien conjuge du premier)

gv g ee()()     H After SSB 22LRRL LR RL

gev me  2 v/sqrt(2) ~natural mass (g~1) ou bien 2m g  e e v

m mee e eeH e v

gm ee couplage Hee 2 v

94 Int. u  g  aInta... IntLLii Int LQW  QWa1, 2, 3 Q L  2 Li LLLiiidl  LLii   Int..Int.. Int Int QQ QL 1ij Q universality of gauge interactions Lii LLij

u c t e   The SM quantum numbers are I3 and Y W GF  The gauge interactions are Flavour blind d s b e   01 % ** In this basis the Yukawa interactions has the following form : With:  i2   10 To be manifestly invariant under SU(2) dInt... Int... u Int% Int l Int Int LYQdYij YQu ij YLl ijLi Yij complex LLiiRRRjjj Two matrices are needed SSB*0  v/2;Re() 0  ( v  H 0 )/2to give a mass term to the u‐type and d‐type quarks

……….

dInt... Int... u Int Int l Int Int We made the choice of having the LMddMuuMllMijLLL ij ij Mass Interaction diagonal jjjRRRjjj ff uL dL e whereMv ( / 2) Y H ……. L …….

uR dR e * SSB=Spontaneous Symmetry Breaking R 95 f fff† To have mass matrices diagonal and real, we have defined: MdiagVMV() L R The mass eigenstates are: dInt.. dInt dVdLLij() ; dVd RRij () iLjj iR uInt.. uInt uVuL() L ij ; uVu R () R ij iLjj iR dInt.. dInt lVlLLij() ; lVl RRij () iLjj iR lInt.  LLijL ()V arbitrary (assumingv massless) iLij

In this basis the Lagrangian for the gauge interaction is: g  ud† a LWLLL uVVdWhcL  () .. 2 i j The coupling is not ud†  Unitary matrix anymore universal V()() CKM VLL V

W u u u c c c t t t

d s b d s b d s b 96 If a similar procedure is applied to the lepton sector vl†† ll Vleptons()()()1 VVLL VV LL

Since the neutrino are (were) massless the matrix which W e   GF change the basis from int-> mass is in principle arbitary We can always choose vl e   VVL  L

Now the neutrino have a mass, it exists a similar matrix in the lepton sector with mixing a CP violation

97 For the Z0 Facultatif

g Int.  aInta. LQW  QWa1, 2, 3 2 Li Li  121Int... Int Int LgQ'.[1 QInt  u 1 u Int .  d 1] dB Int . BijijijRRii 633Li LRRjjj 0' for theZZ cosWW W3 sin B ; tan W  gg /  in the mass basis (example ford ) L  g 11 2 † g 11 2 LdZW(sin)  L ()VV dZ (sin)W dL dZ i dL dL Li i Li cosW 2 3 cosW 2 3     The neutral currents stay universal, in the mass basis : we do not need extra parameters for their complete description

Z0 u d s c b t l 

u d s c b t l 

98 SUMMARY The mass is a LEFT-RIGHT coupling and has to respect the gauge invariance SU(2)L  U(1)Y      1  L R  L R 0 I= Y=1  2 h (I=1/2,Y=1) u c t e   W G F

d s b e  

dInt... Int... u Int Int l Int Int LMijMdLLL d Mu ij u Ml ij l jjjRRRj jj

To have mass matrices diagonal and real, we have defined: f fff† MdiagVMV() L R The mass eigenstates are: dInt.. dInt dVdL() L ij ; dVd R () R ij iLj iRj

The Lagrangian for the gauge interaction is: g  ud† a LWLLL uVVdWhcL  () .. 2 i j 99 