# Presentation: 408878

## Introduction to Flavor Physics in and beyond the Standard Model

- Enrico Lunghi
- References:
- The BaBar physics book, http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-r-504-TOC-pdf.pdf
- B physics at the Tevatron: Run II and beyond, hep-ph/0201071
- The Discovery potential of a Super B Factory, hep-ph/0503261

*Lecture 4/4*

## Outline

-  Key points from the previous lecture on CP violation
-  General considerations on extending the SM
-  Various New Physics models

## CP violation in a nutshell

## B meson mixing

- Schrodinger equation for two states that at t = 0 are pure       and       :
- The mass eigenstates are (                          ):
- For the B system we have                        and  we can write:

- mass matrix

- decay matrix

## Time evolution

- The evolution of pure interaction eigenstates is:
-       where
- In the =0 limit:

## CP violation in mixing

- if                  , it is possible to measure CPV in decays to final states accessible only to       or
- An example is the neutral semileptonic decay asymmetry to wrong sign leptons:

## CP violation in mixing

- Their magnitude is the same:
- The time dependent rates are:
- The asymmetry is:

- small
- time independent
- large hadronic uncertainties

## CP violation in decay

- It stems from the difference between the rates for               and
- We need two amplitudes with different weak and strong phases:
- We A1, A2 and 1-2: all these quantities are sensitive to nonperturbative QCD

## Interference in decays with and without and mixing

- We look at the time dependent CP asymmetries of neutral B decays to final states that are CP eigenstate:
- Same as CPV in decay, the only difference is that
*We need two amplitudes, with different weak and strong phases!*
- More amplitudes usually implies non-perturbative trouble...
- Even if                                                we can use                 and

## Interference in decays with and without and mixing

- In formulae:
- If                 we get:

## The three types of CP violation

- There are three quantities that drive CPV effects:
- CPV in mixing:
- CPV in decay:
- CPV in the interference between decay and mixing:

## Calculation of M12

- B=2 transitions need to go through a W loop
- As usual we write an effective Hamiltonian that captures the SD physics:

## This process is mediated by both tree and penguin operators:

- The last term is doubly suppressed:

- penguin pollution

## This process is mediated by penguin operators only:

- The last term is only CKM suppressed:

## This process is mediated by both tree and penguin operators:

- Is the last term suppressed?

- penguin pollution

- O(1)

## Extraction of CKM angles

- For                             we have:
- For                we have:

## New Physics effects

## General considerations

- The Standard Model is the most general renormalizable theory given its particle content
- New physics enters via new particles:
- Higgs bosons
- fermions
- scalars
- gauge bosons
- In order to escape present bounds, new particles can be
- very heavy with large couplings (e.g. stop)
- relatively lighter with smaller couplings (e.g. neutralino)
- If masses are larger than energies presently accessible we have to look for virtual effects
- In order for these effects to be visible we have to look for processes that are suppressed within the Standard Model

## General considerations

- Interesting processes:
- rare decays
- CP asymmetries
- proton decay (accidentally protected in the SM)
- neutrino oscillations
- dark matter
- muon anomalous magnetic moment
- ....
- Hadronic uncertainties might be large enough to hide a possible new physics signal

## Example: a fourth generation

- We have now eight quarks:
-  Using this U(4)3 symmetry we can write:
- The eight quark masses are:

- 4x4 CKM matrix

## Example: a fourth generation

- Mass eigenstate basis:
- There are no Flavor Changing Neutral Currents
-  Flavour Changing Charged Currents follow a pattern similar to the SM:

## Example: a fourth generation

- Some entries have been measured directly (tree level decays proportional to a single coupling). Since the fourth generation is necessarily heavy, these direct determinations are still valid.
- On Vtd and Vts we have only indirect constraints (i.e. mB, bs, ...)
- Note that loop processes are always proportional to combination of CKM entries:
- Before the measurement of Vtb, it was possible to arrange the masses and couplings of the 4th generation in such a way that Vtb as small as 0.1 was possible.

## Example: a fourth generation

- Physical parameters in the 4x4 CKM matrix:
-      Separating angles and phases in V we get:
- What part of the phenomenology might drastically change?
- CP asymmetries
- Unitarity triangle fits (the 3x3 u-c-t submatrix is not unitary anymore)
- Smoking gun: direct production and detection of 4th generation quarks

## More on loop effects

- Given a coupling, we can get direct or indirect information:
- Kinematically forbidden flavor-conserving interactions:

## More on loop effects

- Kinematically forbidden flavor-changing interactions:
- The SM contribution is loop and CKM suppressed
- CKM-like new physics contributions are sizable even with heavy new particles. If the t-s coupling is not suppressed, this new physics diagram easily outguns the SM one.

## The delicate structure

- The SM pattern of Flavor Changing Neutral Currents and CP violation follows directly from the choice of using one single Higgs doublet to give masses to all the fermions
- The two Yukawas (that contain 36 real parameters) break almost completely the flavor symmetry of the SM: U(3)xU(3)xU(3)U(1). Because of the latter, 26 of the 36 parameters are unphysical.
- The introduction of additional fields capable of interacting with quarks, lead often to new flavor changing interactions. The latter cannot be usually rotated away since we already used all the freedom allowed by the U(3)3 symmetry to simplify the Yukawas.
*The SM has a very delicate structure: almost any addition to it will result in a proliferation of flavor changing interactions and CP violating phases. *

## Why one doublet?

- For *phenomenological* reasons we *needed* to introduce LR interactions among fermion fields:
- The hypercharges of the two Higgs doublets are opposite to each other and we decided to use one doublet only:
- This choice does not follow from any symmetry or principle, it is just the minimal option
- We are completely free to add more doublets (not so for triplets)

## The Two-Higgs Doublet Model (2HDM)

- Let us introduce two Higgses with opposite Hypercharge: 1 and 2
- After electroweak symmetry breaking 3 of their 8 degrees of freedom will be eaten by the W and Z, the other 5 remain physical.
- The Higgs spectrum contains now a charged Higgs (       ), a pseudo-scalar Higgs (     ), and two scalar Higgses (        )
- The neutral components of *both* Higgs doublets acquire a vacuum expectation value:
- Additional parameters up to now:
- mH§, mA, mH
- tan
-   (mixing angle between H and h)

## The Two-Higgs Doublet Model (2HDM)

- Now we have to decide how to couple the Higgs doublets to the quarks
- Type-I  : 1 couples to up and down quarks, 2 doesn’t couple to quarks
- Type-II : 1 couples to the up quarks, 2 to the down quarks
- Type-III: both doublets couple to up and down quarks

## The Two-Higgs Doublet Model: type-II

- the CKM matrix is again the only source of flavor changing interactions
- there are no Flavor Changing Neutral Currents
*we have new Flavor Changing Charged Currents (charged Higgs)*
- Charged Higgs FC interactions are CKM-like
- Charged Higgs loops contribute to several rare decays:

## The Two-Higgs Doublet Model: type-II

- Rare decays put a strong constraint on the charged Higgs mass:

*[Misiak et al., 06]*

## The Two-Higgs Doublet Model: type-III

- The (non-diagonal) quark mass matrices are:
- In the mass eigenstate basis:
- The unitary transformations that diagonalize the mass matrices do not diagonalize any of the four Yukawa matrices
- Unitarity again leads to the cancellation of flavor changing neutral currents involving gauge bosons
- The CKM matrix again appears in the W vertices:

## The Two-Higgs Doublet Model: type-III

- The delicate flavor structure of the SM is completely destroyed and phenomenology imposes strong constraints on the various FC and CPV parameters
- We have new flavor changing neutral and charged currents involving the neutral and charged Higgses, respectively.

## The Minimal Supersymmetric Standard Model (MSSM)

- The MSSM is the minimal SUSY theory that contains the SM gauge group and matter content
- To each SM particle is associated a sparticle
- SUSY predicts sparticles degenerate in mass with the respective particles; hence we expect SUSY to be broken at low energies
- We do not know the mechanism of SUSY breaking and we parametrize it with soft breaking terms
- this rises the number of parameters to about 124
- A truly fundamental theory would fix most of them

## The Minimal Supersymmetric Standard Model (MSSM)

- For deep reasons it is necessary to couple up and down quarks to distinct Higgs doublets (as in the 2HDM type-II)
- Particles and sparticles interact via gauge bosons (and their partners, the gauginos). As in the SM these interactions are, initially, flavor diagonal:
- The soft breaking terms introduce new flavor structures.
- 	Here are the terms involving the squarks:

## The Minimal Supersymmetric Standard Model (MSSM)

- After diagonalizing the quark Yukawas (i.e. switch to quark mass eigenstates), particle-sparticle interactions are no-longer flavor diagonal:
- Unless a miracle happens, the diagonalization of the sdown-left mass matrix will require a different matrix (    )
- In the quark and squark mass eigenstate basis, we have
- This is the supersymmetric flavor problem

## Minimal Flavor Violation (MFV)

- In the SM the quark Yukawas are:
-       and the three unitary matrices Su, Ru and Rd are unobservable
- In several new physics models (type-III 2HDM, MSSM, ...) these matrices become observable
*The MFV principle requires these matrices to remain unobservable*
- In the MSSM this implies              , hence the absence of extra sources of flavor changing

## Minimal Flavor Violation (MFV)

- This means that all flavor changing interactions are controlled by the CKM matrix
- For instance:

- General MSSM:

- MSSM with MFV:

## MFV: Model Independent Analysis

- Once we decide to walk the MFV road, the structure of new physics contributions to rare decays and CP violating observables is very strongly constrained
- With generally, MFV contributions can be absorbed in the following parameters:
- Some predictions for several observables change:

## Model Independent Analysis: bd sector (Bd)

## Model Independent Analysis: bs sector (Bs)

- Model Independent Analysis: bs sector (Bs)

## Model Independent Analysis: sd sector (K)

## The UT fit

## What do we learn?

- The Unitarity Triangle fit is a tremendous check of the CKM mechanism
- Arbitrary new physics is amazingly constrained
- If new physics is MFV (i.e. shows proper respect to the CKM) the UT fit constraints are partially lifted: in these models the scale at which new physics enters (read: the masses of the new particles) can be reasonably low.
- Complementarity between direct and indirect searches

*- NP scale is identified*
*- little information on FC structure*

*- NP scale is poorly constrained*
*- completely explore FC interactions*

*Both necessary to pin down the New Standard Model*