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Particle & Nuclear Physics Collaboration with CERN

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Particle & Nuclear Physics Collaboration with CERN Beyond The Standard Model (General Overview) Shaaban Khalil Center for Fundamental Physics Zewail City of Science and Technology 1 The Standard Model • Standard Model is defined by – 4-dimension QFT (Invariant under Poincare group) – Symmetry: Local SU(3)C x SU(2)L x U(1)Y – Particle content (Point particles): » 3 fermion (quark & Lepton) Generations » No Right-handed neutrinos → Massless Neutrinos – Symmetry breaking: one Higgs doublet • No candidate for Dark Matter • SM does not include gravity. 2 Evidence for Physics beyond SM • Three firm observational evidences of new physics BSM: 1. Neutrino Masses. The discovery of the neutrino oscillations in the nineties of the last century in Super-Kamiokande experiment implies that neutrinos are massive. ne, nm, nt are not mass eigenstates Mass states are n1, n2, and n3 Lepton number not conserved 2. Dark Matter Most astronomers, cosmologists and particle physicists are convinced that 90% of the mass of the Universe is due to some non-luminous matter, called `Dark Matter/Energy'. The explanation for these flat rotation curves is to assume that disk galaxies are immersed in extended dark matter halos • The Big-Bang nucleosynthesis, which explains the origin of the elements, sets a limit to the number of baryons that exists in the Universe: Ωbaryon <0.04 • Dark Matter must be non-baryonic. • The properties of a good Dark Matter candidate: – stable (protected by a conserved quantum number), – relic abundance compatible to observation, – electrically neutral, no color, – weakly interacting (i.e., WIMP) • No such candidate in the Standard Model • SM describes the interactions between quarks, leptons & the force carriers very successfully. • NP beyond SM (SUSY) provides this type of candidate for dark matter. 5 3. Baryon Asymmetry (Matter- Antimatter Asymmetry) • Why is our universe made of matter and not antimatter? • Neither the standard model of particle physics, nor the theory of general relativity provides an obvious explanation • In 1967, A. Sakharov showed that the generation of the net baryon number in the universe requires: • Baryon number violation • Thermal non-equilibrium • C and CP violation All of these ingredients were present in the early Universe! • Do we understand the cause of CP violation in particle interactions? • Can we calculate the BAU from first principles? (n - n )/ n = 6.1 x 10-10 B 푩 γ There are a number of questions we hope will be answered: Electroweak symmetry breaking, which is not explained within the SM. Why is the symmetry group is SU(3) x SU(2) x U(1)? Can forces be unified? Why are there three families of quarks and leptons? Why do the quarks and leptons have the masses they do? Can we have a quantum theory of gravity? Why is the cosmological constant much smaller than simple estimates would suggest? DIRECTIONS BEYOND THE STANDARD MODEL 1. Extension of gauge symmetry 2. Extension of Higgs Sector 3. Extension of Matter Content 4. Extension with Flavor Symmetry 5. Extension of Space-time dimenstions (Extra-dimensions) 6. Extension of Lorentz Symmetry (Supersymmetry) 7. Incorporate Gravity (Supergravity) 8. One dimension object (Superstring) 1. Extension of gauge symmetry • The idea of the Grand Unified Theories (GUTs) is to embed the SM gauge groups into a large group G and try to interpret the additional resultant symmetries. • Currently the most interesting candidates for G are SU(5), SO(10), E6 . • The SU(5) model of Georgi and Glashow is the simplest and one of the first attempts in which the SM gauge are combined into a single gauge group. • In SU(5) leptons and quarks are combined into single irreducible representations. elds Aμ comes in SU(5) in the 24-dimensional adjoint representation. Since the 24ﰀ The gauge representation decomposes under the SM subgroup as following We can identify 8 gauge bosons, G8; transform as (8,1)0, which are the 8 gluons of SU(3)C. + − 3 Similarly, we have 3 gauge bosons (W , W ,W ) transforming as (1,3)0, which correspond to the weak gauge bosons. The adjoint representation of Higgs scalars Φ breaks SU(5) to SUc(3) × SUL(2) × UY (1). The most general Lagrangian is U(1)B-L Extension of the SM • The minimal extension is based on the gauge group GB−L ≡ SU(3)C × SU(2)L × U(1)Y × U(1)B−L This model accounts for the exp. results of the light neutrino masses New particles are predicted: − Three SM singlet fermions (right handed neutrinos) (cancellation of gauge anomalies) − Extra gauge boson corresponding to B−L gauge symmetry − Extra SM singlet scalar (heavy Higgs) These new particles have Interesting signatures at the LHC U(1)B-L Model . Under U(1)B−L we demand: iYBL (x) iYBL (x) L e L , R e R , . Derivatives are covariant if a new gauge field Cμ is introduced: ig ig ig D ( W r YB Y C ) L 2 r 2 2 BL L ig ig D ( YB Y C ) R 2 2 BL R . Lagrangian: fermionic and kinetic sectors LBL Lleptons Lgauge 1 1 1 ilD l ie D e i D W r W r B B C C R R R R 4 4 4 U(1)B-L Symmetry Breaking . The U(1)B−L gauge symmetry can be spontaneously broken by a SM singlet complex scalar field χ: v 2 . The SU(2)L×U(1)Y gauge symmetry is broken by a complex SU(2) doublet of scalar field φ: v 2 . Lagrangian: Higgs and Yukawa sectors LHiggsYukawa (D)(D ) (D )(D ) V (, ) ~ 1 ( le l c h.c.) e R R 2 R R R . Most general Higgs potential: 2 2 V (, ) m1 m2 2 2 1( ) 2 ( ) 3 ( )( ) U(1)B-L Symmetry Breaking (Cont.) . For V(φ,χ) bounded from below, we require: 3 2 12 , 2 ,1 0 . For non-zero local minimum, we require 2 3 412 . Non-zero minimum: 4 m2 2 m2 2(m2 v2 ) 2 2 1 3 2 2 1 1 v 2 , v 3 412 3 . Two symmetry breaking scenarios depending on λ3: λ3 0 v v : Two stages symmetrybreaking at different scales λ3 0 v v : low scale v of order the electroweak . Interesting scale: 0 3 2 12 . After the B−L gauge symmetry breaking, the gauge field Cμ acquires mass: 2 2 2 M z 4g v . Strongest Limit on Mz’/g’’ comes from LEP II: M z O(TeV ), g O(1) v O(TeV ) g ZB-L Discovery at LHC The interactions of the Z′ boson with the SM fermions are described by YBL g Z' f f f Branching ratios Y 2 g2 (Z l l ) l M 24 Z Y 2 g2 (Z bb,cc, ss) q M (1 s ) 8 Z Y 2 g2 m2 4m2 m2 q t t 1/ 2 s s t (Z tt ) M Z (1 2 )(1 2 ) [1 O( 2 ))] 8 M Z M Z M Z Branching ratios of Z’ → l+l- are relatively high compared to Z’ → qq: BR(Z l l ) 30%, BR(Z qq) 10% Search for Z’ at LHC via dilepton channels are accessible at LHC. 2. Extension of Higgs Sector • Why one Higgs doublet only in SM …. (just economically ) • Most of theories BSM include more than one Higgs doublet. • In SM • SM + Singlet scalar The Higgs sector of this model is given by Two physics Higgs bosons are obtained: With • Two Higgs doublets • In the SU(2)×U(1) gauge theory, if there are n scalar multiplets φi, with weak isospin Ii, weak hypercharge Yi, and vev vi, then the parameter ρ is defined as • Experimentally ρ is very close to one. • Both SU(2) singlets with Y =0 and SU(2) doublets with Y =±1 give ρ=1. • The most general scalar potential for two doublets Φ1 and Φ2 with hypercharge +1 is • The minimization of this potential gives • With two complex scalar SU(2) doublets there are eight fields: • Three of those get ‘eaten’ to give mass to the W± and Z0 gauge bosons; the remaining five are physical scalar (‘Higgs’) fields: H± , H, h and A • Fermions can couple to both Φ1 or Φ2 in principle • Depending on that several types of 2HDM are possible • We take Type-II, where down-type quarks and leptons couple to Φ1 and up-type quarks couple to Φ2 3. Extension of Matter Content 1. SM + νR • SM predicts massless neutrinos. Gauge symmetry of e.m. interaction massless photons. For massless ν no such symmetry. • Neutrino oscillations confirmed massive neutrinos. • We can introduce a Dirac mass term if νR exists in addition to νL • The neutrino mass matrix • Then 2. 4th Generation • SM describes the presence of three fermion families. • Experimental Measurements are in good consistence with the three family but don’t not exclude a neutrino of a fourth family with mν4 > mZ . • The existence of a fourth generation neutrino would also mean the presence of two additional quarks and a charged lepton in the same family The current mass limits on fourth generation fermions at a 95% confidence limit. 4. Extension with Flavor Symmetry The problem of flavour: the problem of the undetermined fermion masses and mixing angles (including neutrino masses and mixing angles) together with the CP violating phases SM with S3 flavor symmetry The smallest non-Abelian discrete symmetry is the group S3 of the permutation of three objects. It has six elements, and is isomorphic to the symmetry group of the equilateral triangle (identity, rotations by ±2π/3, and three reflections) It has three irreducible representations 1, 1′, 2, with the multiplication rules: Let us assign the quarks as follows: 0 − Also assume three Higgs doublets Φi = (φ i , φ i ) with assignments: In this case, the c-t and s-b quark Yukawa interactions are given by: The 3 × 3 quark mass matrices are given by 5.
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