DOCSLIB.ORG

Charged Current Weak Interaction of Polarized Muons

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Charged Current Weak Interaction of Polarized Muons - 55 - CHARGED CURRENT WEAK INTERACTION OF POLARIZED MOONS G. Smadja and G. Vesztergambi*) DPhPE, CEN-Saclay, 91191 Gif-sur-Yvette Cedex, France (Paper was presented by G. Vesztergombi) ABSTRACT The polarization of the muon beam can be used to test the presence of right-handed couplings in charged current interaction of muons in process /*.+Jf-*v+ X. The experimental feasibility and the limits which can be obtained on the mass of right-handed inter• mediate boson are discussed. 1. INTRODUCTION At first glance it seems to be hopeless to detect charged current weak interaction of muons, because the typical electromagnetic cross-sections are about a million times larger even at an incoming muon energy of several hundred GeV. One should not forget, however, some remarkable merits of the muon beam. a) Energy Due to the kinematic properties of pion decay the average energy of muons is 3 times higher than the one of neutrinos. This may turn out to be rather important. On one hand, the weak cross-sections are linearly proportional to the beam energy and on the other hand the energy can be critical in presence of thresholds. In addition, the incoming muon energy can be preselected which provides more flexibility in the study of threshold phenomena. b) Helicity Due to their extremely small (if any) mass the helicity of neutrinos is uniquely pre• determined. In case of muons one can select helicity at will. This provides an unique possibility to look for such type of weak interactions which are impossible in case of neutrinos as it was noticed by K. Winter^. Namely, if the pion momentum is denoted by pv then the decay muon with momentum pFORW ^ (forward decay in tr-system) will have "unnatural" helicity (i.e. opposite to the helicity of the neutrino with the same lepton number). One can get "natural" helicity selecting muon momenta BACK 2 2 p^ » (m^/m^-p^ (backward decay in ir-system). Thus if one is interested in normal left-handed interactions then the muon beam energy should be lowered relative to the parent pion energy. Even in this case it remains about a factor two higher than the average neutrino energy from the same pion decay. c) Beam handling Beam phase space can be measured easily. The well collimated muon beam spot can reduce by order(s) of magnitude the required lateral size of the target and the interaction vertex is well identified. Severe difficulties compensate the flexibility of muon beam. x) On leave from the Central Research Institute for Physics, Budapest, Hungary. - 56 - i) One needs ah extremely good (~10"<') hardware rejection against events which have a muon in the final state in order to catch weak fishes from the QED ocean. ii) The final state is rather complicated. In absence of neutrino detection the relevant kinematic information should be extracted from the analysis of hadronic shower. With modern experimental techniques these problems can be overcome in most of the kinematical range, thus at least in some areas muon experiments can challenge and/or complement neutrino experiments. 2. RIGHT-HANDED CURRENTS The theoretical interest for weak effects with "unnatural" right-handed polarization can be summarized in the following way: a) The dominantly V-A structure of weak interactions is well established at present ener• gies but the search for deviations should still proceed. b) The standard SU^(2) x U(1) model is highly asymmetric and unaesthetic. c) Right-handed neutrinos and intermediate bosons seem to be one of the simplest candidates 21 which can "bloom" in the desert between 100 GeV and the grand unification mass 1. In the following we discuss the experimental possibilities to search for right-handed neutrinos by muon beam in the framework of SUL(2) x SUR(2) x U(1) model. We start with a "manifestly right-left symmetric" Lagrangian where WL and WR are coupled to V-A and V+A currents, respectively: L W= CV+A)-WR + H.C.] where e. g. CV + Al^v^^ijf,)^ or (V + A)^ ü^ft+^d. The combinations fV + hA)lep and (V + hA)^^ represent leptonic and hadronic currents having non-vanishing. matrix ele• ments between only définit helicity states. For simplicity, here and in the next we neglect masses and Cabbibo or KM angles ( h = +_ 1 ). That is in a given vertex the transition is prohibited, for example between )UR(h =+1) and VL(h =-1). The left-right symmetry of however is broken by an asymmetric vacuum. The Higgs potencial and the vacuum expectation values of the Higgs-fields are arranged to yield to mass eigenstates which are linear combinations of unmixed chiral eigenstates, W.. = W.-cosi* - W„-sin$ \ ' L K . > for definiteness M„ > M„ = w sin "2' "T W + 2 L' í WR-cosl} The modified Lagrangian reads = A cos h ^$z{ [N- í t -CV+A)siniî] W1 + [CV-A)sin4 +(V+A)cos^ ] W2 + H.C. | . In case of left-handed v£ neutrinos (and also ju^) the (V+A) leptonic current does not give any contribution, in contrast with the hadronic current because (u, d) quarks are occuring always in left-right symmetric combinations inside the nucléons. Thus one gets the "left-handed" effective four-fermion Lagrangian in the form: J 2 LLEFT= g f{V-A)i«.-cosé [(V-AX.rfCos^- (V+AWsing ]+ CV-A)tep-sintj [CV-A)<..^in4+(V+A)n.)|Coslt11 8 I M^ i x) At present energies Q dependence is negligible. - 57 - After some rearrangements one obtaines .LEFT jcosj^sin |CV-A)¡ CV-A^CV+A^staiMsC lep^had 2 8 M: M: M " ^| "1 "'2 Í This interaction can be studied by standard neutrino scattering because by default left- handed neutrino (and right-handed antineutrino) beams are available. Assuming however the existence of right-handed neutrinos the situation will be re• versed and one should retain only (V+A)^ leptonic currents which provide the effective four-fermion Lagrangian for right-handed neutrino (orp."^) scattering: sinlj ,cos é 1 1 CV+A) CV ^CV ^siitfcos* had +A -A M: M: M2""M¡. 2 J The cross-section calculations are reduced to the evaluation of matrix elements which has the general form: A h C V + )+ C V + h A C lePf "had ) =< I T" W had A > In this notation the differential cross-section can be written as v 22 2 2 = |A (-1;-1) |?cJL + |A C-1,+D| -C = O" c LR LL °LEFT a(+1 +1) C a 2 I ' I'RR + I (+I'-I)|2-CRL RIGHT where the meaning of cr„ is self-explanatory and 2 2 • 2x 2, cos ¿ sin ^ sxn cos c = *» + Ç . _c sin£ cosÇ "LL "RL * M: RR »»2 »»2 ' IJR M: M1 M2 1 Some representative diagrams corresponding to different terms are shown in Figure 1. After trace calculations one can get <r^. In general case they consist of two parts which are i *•* j symmetric in helicities: a 2 h h + 1+h 2 °ij • I cvVl • i j W f ?)ci+h ) K2(Pk) where and K2 represent définit kinematic functions of particle momenta. Due to the fact that in our calculations the masses are neglected and K2 are depending only on the CM- energy and CM-scattering angle which is equivalent io the scaling variable y. Thus there remain only two possibilities i) h.h. =+1 yields da / dy »«• 1 1 J 2 ii) hjh. =-1 yields d» / dy oí (1-y) . Of course, one has x s 1 for "elastic scattering" on quarks. From these the usual proce• dure gives for charged current scattering on isoscalar nuclei Cx.y) q to + (1-y)2 q (x) (x,y) = <T+_ (x,y) oc q (x) + (1-y) q (x) where q (x) and q (x) represent the quark and anti-quark distribution inside target nuclei. In the general case the muon beam consists of yu£ and yU^ components. Thus the charged - 58 - current cross-section for negative muon beam with polarization ¿ can be written as a sum = ~2~ LEFT + 2 RIGHT = ~~a-CLL + -J"0^ -CRL ' It is worth to remark that the left-right mixing term is independent of muon polarization, in accordance with the fact, that it is parity conserving. Of course, this left-right sym- metry can be broken by mass terms and for too heavy the ^pjgjp part would be zero due to energy conservation. For completeness, we write down the differential cross-sections in a more explicit way: 2 + ~7 ff + a 2 + 2 CRL oxdy - — «L " R RL°° 2 q+O-y) ^ + ^ [q+ 0 -yâj) 4 ^+Ci-y3 q} 2 d *** 1 + * _+ A 1 x _+ _ 1+J. axd7= —°L+—RL — where the transition ju." *—» )X is formally achieved by substituting — à and ' q(x) <-* q(x). One gets the corresponding neutrino cros-sections simply by setting X = + 1: 2 + AW= *l + 0 + «Sl ~ H^Kl M-^Kl 2 = + 0 + 2 AW < °k ~ [^N-y) q]ÍL * [q+N-y) q]4 - In summary, the muon cross-sections consist generally of 3 parts: the parity violating piece includes pure left- and right-handed contributions, the parity conserving part contains their mixing which is within our approximation independent of A. In the case of neutrino scattering the pure right-handed term is lost. The remaining terms contain li^ in sinC/J)^ and sin2(L5R/M2 combinations. Thus it is practically impossible to deduce any limit on the right-handed intermediate boson mass, M~ from neutrino scattering. 3. PRESENT LIMITS ON RIGHT-HANDED PHENOMENA. 3-71 A number of partial reviews have been published on this topic .
Load more

Recommended publications
  • The Story of Large Electron Positron Collider 1
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Publications of the IAS Fellows GENERAL ç ARTICLE The Story of Large Electron Positron Collider 1. Fundamental Constituents of Matter S N Ganguli I nt roduct ion The story of the large electron positron collider, in short LEP, is linked intimately with our understanding of na- ture'sfundamental particlesand theforcesbetween them. We begin our story by giving a brief account of three great discoveries that completely changed our thinking Som Ganguli is at the Tata Institute of Fundamental and started a new ¯eld we now call particle physics. Research, Mumbai. He is These discoveries took place in less than three years currently participating in an during 1895 to 1897: discovery of X-rays by Wilhelm experiment under prepara- Roentgen in 1895, discovery of radioactivity by Henri tion for the Large Hadron Becquerel in 1896 and the identi¯cation of cathode rays Collider (LHC) at CERN, Geneva. He has been as electrons, a fundamental constituent of atom by J J studying properties of Z and Thomson in 1897. It goes without saying that these dis- W bosons produced in coveries were rewarded by giving Nobel Prizes in 1901, electron-positron collisions 1903 and 1906, respectively. X-rays have provided one at the Large Electron of the most powerful tools for investigating the struc- Positron Collider (LEP). During 1970s and early ture of matter, in particular the study of molecules and 1980s he was studying crystals; it is also an indispensable tool in medical diag- production and decay nosis.
    [Show full text]
  • 12 from Neutral Currents to Weak Vector Bosons
    12 From neutral currents to weak vector bosons The unification of weak and electromagnetic interactions, 1973{1987 Fermi's theory of weak interactions survived nearly unaltered over the years. Its basic structure was slightly modified by the addition of Gamow-Teller terms and finally by the determination of the V-A form, but its essence as a four fermion interaction remained. Fermi's original insight was based on the analogy with elec- tromagnetism; from the start it was clear that there might be vector particles transmitting the weak force the way the photon transmits the electromagnetic force. Since the weak interaction was of short range, the vector particle would have to be heavy, and since beta decay changed nuclear charge, the particle would have to carry charge. The weak (or W) boson was the object of many searches. No evidence of the W boson was found in the mass region up to 20 GeV. The V-A theory, which was equivalent to a theory with a very heavy W , was a satisfactory description of all weak interaction data. Nevertheless, it was clear that the theory was not complete. As described in Chapter 6, it predicted cross sections at very high energies that violated unitarity, the basic principle that says that the probability for an individual process to occur must be less than or equal to unity. A consequence of unitarity is that the total cross section for a process with 2 angular momentum J can never exceed 4π(2J + 1)=pcm. However, we have seen that neutrino cross sections grow linearly with increasing center of mass energy.
    [Show full text]
  • The Standard Model Part II: Charged Current Weak Interactions I
    Prepared for submission to JHEP The Standard Model Part II: Charged Current weak interactions I Keith Hamiltona aDepartment of Physics and Astronomy, University College London, London, WC1E 6BT, UK E-mail: [email protected] Abstract: Rough notes on ... Introduction • Relation between G and g • F W Leptonic CC processes, ⌫e− scattering • Estimated time: 3 hours ⇠ Contents 1 Charged current weak interactions 1 1.1 Introduction 1 1.2 Leptonic charge current process 9 1 Charged current weak interactions 1.1 Introduction Back in the early 1930’s we physicists were puzzled by nuclear decay. • – In particular, the nucleus was observed to decay into a nucleus with the same mass number (A A) and one atomic number higher (Z Z + 1), and an emitted electron. ! ! – In such a two-body decay the energy of the electron in the decay rest frame is constrained by energy-momentum conservation alone to have a unique value. – However, it was observed to have a continuous range of values. In 1930 Pauli first introduced the neutrino as a way to explain the observed continuous energy • spectrum of the electron emitted in nuclear beta decay – Pauli was proposing that the decay was not two-body but three-body and that one of the three decay products was simply able to evade detection. To satisfy the history police • – We point out that when Pauli first proposed this mechanism the neutron had not yet been discovered and so Pauli had in fact named the third mystery particle a ‘neutron’. – The neutron was discovered two years later by Chadwick (for which he was awarded the Nobel Prize shortly afterwards in 1935).
    [Show full text]
  • Charged Current Anti-Neutrino Interactions in the Nd280 Detector
    CHARGED CURRENT ANTI-NEUTRINO INTERACTIONS IN THE ND280 DETECTOR BRYAN E. BARNHART HIGH ENERGY PHYSICS UNIVERSITY OF COLORADO AT BOULDER ADVISOR: ALYSIA MARINO Abstract. For the neutrino beamline oscillation experiment Tokai to Kamioka, the beam is clas- sified before oscillation by the near detector complex. The detector is used to measure the flux of different particles through the detector, and compare them to Monte Carlo Simulations. For this work, theν ¯µ background of the detector was isolated by examining the Monte Carlo simulation and determining cuts which removed unwanted particles. Then, a selection of the data from the near detector complex underwent the same cuts, and compared to the Monte Carlo to determine if the Monte Carlo represented the data distribution accurately. The data was found to be consistent with the Monte Carlo Simulation. Date: November 11, 2013. 1 Bryan E. Barnhart University of Colorado at Boulder Advisor: Alysia Marino Contents 1. The Standard Model and Neutrinos 4 1.1. Bosons 4 1.2. Fermions 5 1.3. Quarks and the Strong Force 5 1.4. Leptons and the Weak Force 6 1.5. Neutrino Oscillations 7 1.6. The Relative Neutrino Mass Scale 8 1.7. Neutrino Helicity and Anti-Neutrinos 9 2. The Tokai to Kamioka Experiment 9 2.1. Japan Proton Accelerator Research Complex 10 2.2. The Near Detector Complex 12 2.3. The Super-Kamiokande Detector 17 3. Isolation of the Anti-Neutrino Component of Neutrino Beam 19 3.1. Experiment details 19 3.2. Selection Cuts 20 4. Cut Descriptions 20 4.1. Beam Data Quality 20 4.2.
    [Show full text]
  • The Weak Interaction
    The Weak Interaction April 20, 2016 Contents 1 Introduction 2 2 The Weak Interaction 2 2.1 The 4-point Interaction . .3 2.2 Weak Propagator . .4 3 Parity Violation 5 3.1 Parity and The Parity Operator . .5 3.2 Parity Violation . .6 3.3 CP Violation . .7 3.4 Building it into the theory - the V-A Interaction . .8 3.5 The V-A Interaction and Neutrinos . 10 4 What you should know 11 5 Furthur reading 11 1 1 Introduction The nuclear β-decay caused a great deal of anxiety among physicists. Both α- and γ-rays are emitted with discrete spectra, simply because of energy conservation. The energy of the emitted particle is the same as the energy difference between the initial and final state of the nucleus. It was much more difficult to see what was going on with the β-decay, the emission of electrons from nuclei. Chadwick once reported that the energy spectrum of electrons is continuous. The energy could take any value between 0 and a certain maximum value. This observation was so bizarre that many more experiments followed up. In fact, Otto Han and Lise Meitner, credited for their discovery of nuclear fission, studied the spectrum and claimed that it was discrete. They argued that the spectrum may appear continuous because the electrons can easily lose energy by breamsstrahlung in material. The maximum energy observed is the correct discrete spectrum, and we see lower energies because of the energy loss. The controversy went on over a decade. In the end a definitive experiment was done by Ellis and Wooseley using a very simple idea.
    [Show full text]
  • Sam Treiman Was Born in Chicago to a First-Generation Immigrant Family
    NATIONAL ACADEMY OF SCIENCES SAM BARD TREIMAN 1925–1999 A Biographical Memoir by STEPHEN L. ADLER Any opinions expressed in this memoir are those of the author and do not necessarily reflect the views of the National Academy of Sciences. Biographical Memoirs, VOLUME 80 PUBLISHED 2001 BY THE NATIONAL ACADEMY PRESS WASHINGTON, D.C. Courtesy of Robert P. Matthews SAM BARD TREIMAN May 27, 1925–November 30, 1999 BY STEPHEN L. ADLER AM BARD TREIMAN WAS a major force in particle physics S during the formative period of the current Standard Model, both through his own research and through the training of graduate students. Starting initially in cosmic ray physics, Treiman soon shifted his interests to the new particles being discovered in cosmic ray experiments. He evolved a research style of working closely with experimen- talists, and many of his papers are exemplars of particle phenomenology. By the mid-1950s Treiman had acquired a lifelong interest in the weak interactions. He would preach to his students that “the place to learn about the strong interactions is through the weak and electromagnetic inter- actions; the problem is half as complicated.’’ The history of the subsequent development of the Standard Model showed this philosophy to be prophetic. After the discovery of parity violation in weak interactions, Treiman in collaboration with J. David Jackson and Henry Wyld (1957) worked out the definitive formula for allowed beta decays, taking into account the possible violation of time reversal symmetry, as well as parity. Shortly afterwards Treiman embarked with Marvin Goldberger on a dispersion relations analysis (1958) of pion and nucleon beta decay, a 3 4 BIOGRAPHICAL MEMOIRS major outcome of which was the famed Goldberger-Treiman relation for the charged pion decay amplitude.
    [Show full text]
  • The Standard Model Lagrangian
    The Standard Model Lagrangian Elementary Particle Physics Strong Interaction Fenomenology Diego Bettoni Academic Year 2011-12 Dirac Formalism i m 0 j Conserved Current 0 0 1 1 0 i i 0 5 i 0 1 0 0 1 R L i m L D. Bettoni Fenomenologia Interazioni Forti 2 Gauge Invariance U i 1 1 igA A U U UA U Abelian D g D U D 1 W W D ig2 W i i ijk j k Non 2 g2 Abelian Gauge invariance requires the introduction of vector bosons, which act as quanta of new interactions. In gauge theories the symmetries prescribe the interactions. D. Bettoni Fenomenologia Interazioni Forti 3 The Symmetries of the Standard Model • U(1) invariance. All particles appear to have this kind of invariance, related to electromagnetism. It requires a vector boson, B, whose connection with the photon will be determined later. • SU(2) invariance. Non abelian gauge invariance (electroweak isospin). It requires three vector bosons, Wi , one for each generator of SU(2). The physical W particles have definite electromagnetic charges. 1 2 1 2 0 3 W W iW 2 W W iW 2 W W • SU(3) invariance. It requires eight vector bosons, Ga , the gluons, the quanta of the strong interaction, described by Quantum ChromoDynamics (QCD). D. Bettoni Fenomenologia Interazioni Forti 4 The Lagrangian • In order to obtain the Standard Model Lagrangian we start from the free particle Lagrangian and replace the ordinary derivative by the convariant derivative. It will contain two parts: Lgauge kinetic energies of the gauge fields Lferm covariant derivative fermion kinetic energies • Next we must specify the particles and their transformation properties under the three internal symmetries.
    [Show full text]
  • Weak Interactioninteraction
    WeakWeak InteractionInteraction OutlineOutline Introduction to weak interactions Charged current (CC) interactions Neutral current (NC) Weak vector bosons W± and Z0 Weak charged interactions Beta decay Flavour changing charged current W± boson propagator Fermi coupling constant Parity violation Muon decay Decay rate /lifetime Lepton Universality W± boson couplings for leptons Tau decays Weak quark decays W± boson couplings for quarks Cabibbo angle, CKM mechanism Spectator model Nuclear and Particle Physics Franz Muheim 1 IntroductionIntroduction Weak Interactions Account for large variety of physical processes Muon and Tau decays, Neutrino interactions Decays of lightest mesons and baryons Z0 and W± boson production at √s ~ O(100 GeV) Natural radioactivity, fission, fusion (sun) Major Characteristics Long lifetimes Small cross sections (not always) “Quantum Flavour Dynamics” Charged Current (CC) Neutral Current (NC) mediated by exchange of W± boson Z0 boson Intermediate vector bosons MW = 80.4 GeV ± 0 W and Z have mass MZ = 91.2 GeV Self Interactions of W± and Z0 also W± and γ Nuclear and Particle Physics Franz Muheim 2 BetaBeta DecayDecay Weak Nuclear Decays See also Nuclear Physics Recall β+ = e+ β- = e- Continuous energy spectrum of e- or e+ Î 3-body decay, Pauli postulates neutrino, 1930 Interpretation Fermi, 1932 Bound n or p decay − n → pe ν e ⎛ − t ⎞ N(t) = N(0)exp⎜ ⎟ ⎝ τ n ⎠ p → ne +ν (bound p) τ n = 885.7 ± 0.8 s e 32 τ 1 = τ n ln 2 = 613.9 ± 0.6 s τ p > 10 y (p stable) 2 Modern quark level picture Weak charged current mediated
    [Show full text]
  • Field Theory and the Standard Model
    Field theory and the Standard Model W. Buchmüller and C. Lüdeling Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany Abstract We give a short introduction to the Standard Model and the underlying con- cepts of quantum field theory. 1 Introduction In these lectures we shall give a short introduction to the Standard Model of particle physics with empha- sis on the electroweak theory and the Higgs sector, and we shall also attempt to explain the underlying concepts of quantum field theory. The Standard Model of particle physics has the following key features: – As a theory of elementary particles, it incorporates relativity and quantum mechanics, and therefore it is based on quantum field theory. – Its predictive power rests on the regularization of divergent quantum corrections and the renormal- ization procedure which introduces scale-dependent `running couplings'. – Electromagnetic, weak, strong and also gravitational interactions are all related to local symmetries and described by Abelian and non-Abelian gauge theories. – The masses of all particles are generated by two mechanisms: confinement and spontaneous sym- metry breaking. In the following chapters we shall explain these points one by one. Finally, instead of a summary, we briefly recall the history of `The making of the Standard Model' [1]. From the theoretical perspective, the Standard Model has a simple and elegant structure: it is a chiral gauge theory. Spelling out the details reveals a rich phenomenology which can account for strong and electroweak interactions, confinement and spontaneous symmetry breaking, hadronic and leptonic flavour physics etc. [2, 3]. The study of all these aspects has kept theorists and experimenters busy for three decades.
    [Show full text]
  • Deep-Learning in Search of Light Charged Higgs
    Deep-Learning in Search of Light Charged Higgs G. K. Demir ∗a, N. Sönmezb, and H. Do˜gana aDepartment of Electrical and Electronics Engineering, Dokuz Eylül University, 35390, İzmir, Turkey bDepartment of Physics, Ege University, 35040, İzmir, Turkey Abstract In this work, we deep-learn light charged Higgs signal in top quark decays which poses difficulties due to strong W boson contamination. We construct Deep Neural Networks (DNN) with appropriate architecture and determine signal extraction efficiency by consid- ering various features (kinematical and human engineered parameters). Results show that DNN gives better performance than the classical neural networks and has ability to find regions of high efficiency even the input features are not human-engineered. In a sense, human-engineered high-level features are offset by DNNs with different combinations of the low-level kinematical features. Additionally, it is shown that increasing the number of pro- cessing units in DNNs does not necessarily cause an increase in efficiency due mainly to increased complexity. Our method and results can set an example of signal extraction from strong backgrounds. 1 Introduction Charged Higgs boson, frequently arising in models with more than one Higgs doublet, takes part in charged current interactions. The two-Higgs doublet model (2HDM) [1] or minimal supersymmetric model (MSSM) are perhaps the simplest models that can have a charged Higgs boson. The recent review volume [2] gives a detailed discussion of the charged Higgs boson H± in terms of possible models (according to quarks and leptons) and experimental bounds. In all searches, the goal is to disentangle the charged Higgs effects from the background, and this becomes a challenging task for light (mH± & 100 GeV), and weakly-coupled charged Higgs particles.
    [Show full text]
  • Fermi's Theory of Weak Interactions
    FERMI AND THE THEORY OF WEAK INTERACTIONS G. Rajasekaran Institute of Mathematical Sciences, Chennai 600113 and Chennai Mathematical Institute, Chennai 603103 e-mail: [email protected] Abstract The history of weak interactions starting with Fermi’s creation of the beta decay the- ory and culminating in its modern avatar in the form of the electroweak gauge theory is described. Discoveries of parity violation, matter-antimatter asymmetry, W and Z bosons and neutrino mass are highlighted. Introduction Sun gives us light and heat that makes life possible on the Earth. How do the Sun and stars produce energy and continue to shine for billions of years? Thermonuclear fusion is the answer as Eddington proposed in 1920 and Bethe demonstrated explicitly in 1939. Through a series of nuclear reactions, four protons (which are Hydrogen nuclei) in the core of the Sun combine to form a Helium nucleus emitting two positrons and two neutrinos and releasing 27 MeV of energy: 4 + + p + p + p + p He + e + e + νe + νe + 27MeV → This can be regarded as the most important reaction for all life, for without it there can be no life on Earth! The above reaction is caused by one of the basic forces of Nature, called weak interaction. Beta decays of nuclei and in fact the decays of most of the elementary particles are now known to be due to weak interaction. Enrico Fermi formulated the theory of weak interactions in 1934 and his theory has stood the ground very successfully with appropriate amendments and generalizations and finally served as a core part of the Standard Model of High Energy Physics, which is now known as the basis of almost ALL of Physics, except for gravitation.
    [Show full text]
  • 9. the Weak Force Particle and Nuclear Physics
    9. The Weak Force Particle and Nuclear Physics Dr. Tina Potter Dr. Tina Potter 9. The Weak Force 1 In this section... The charged current weak interaction Four-fermion interactions Massive propagators and the strength of the weak interaction C-symmetry and Parity violation Lepton universality Quark interactions and the CKM Dr. Tina Potter 9. The Weak Force 2 The Weak Interaction The weak interaction accounts for many decays in particle physics, e.g. − − − − µ ! e ν¯eνµ τ ! e ν¯eντ + − − π ! µ ν¯µ n ! pe ν¯e Characterised by long lifetimes and small interaction cross-sections Dr. Tina Potter 9. The Weak Force 3 The Weak Interaction Two types of weak interaction Charged current (CC): W ± bosons Neutral current (NC): Z bosons See Chapter 10 The weak force is mediated by massive vector bosons: mW = 80 GeV mZ = 91 GeV Examples: (The list below is not complete, will see more vertices later!) Weak interactions of electrons and neutrinos: − νe − e e νe Z W − W + Z + ν¯e e+ e ν¯e Dr. Tina Potter 9. The Weak Force 4 Boson Self-Interactions In QCD the gluons carrycolour charge. In the weak interaction the W ± and Z bosons carry the weak charge W ± also carry the electric charge ) boson self-interactions W − W − Z γ W + W + Z W + Z W + γ W + W + W + W − W − Z W − γ W − γ W − (The list above is complete as far as weak self-interactions are concerned, but we have still not seen all the weak vertices. Will see the rest later) Dr.
    [Show full text]