# Baryon number violation talk background material

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Searches for the baryon- and lepton-number violating decays $B^0\rightarrow\Lambda_c^+\ell^-$, $B^-\rightarrow\Lambda\ell^-$, and $B^-\rightarrow\barΛ\ell^-$

None of this material is required to follow the talk. I will pitch the talk to the level of the audience, assuming that people have not read this webpage.

# The paper

Searches for B mesons decaying to final states containing a baryon and a lepton are performed, where the baryon is either Λc or Λ and the lepton is a muon or an electron. These decays violate both baryon and lepton number and would be a signature of physics beyond the standard model. No significant signal is observed in any of the decay modes, and upper limits in the range $(3.2-520)\times 10^{-8}$ are set on the branching fractions at the 90% confidence level.

# References for the interested reader

## Physics background

### Baryon asymmetry

• From the paper:

Observations show that the universe contains much more matter than antimatter (Coppi, Steigman:1976ev). This suggests that there are processes that violate CP-symmetry and baryon-number conservation (Sakharov). However, experimentally observed CP violation, combined with the baryon-number violating processes that are allowed by the standard model (Kuzmin), cannot explain the observed matter-antimatter asymmetry.

• Andre Sakharov's paper, Violation of CP Invariance, C Asymmetry and Baryon Asymmetry of the Universe.
• JETP Letters
• This is generally considered the canonical paper that established the three Sakarov conditions", necessary to explain the baryon asymmetry of the universe.
• This paper is pitched at a higher level and may not be terribly enlightening to non-graduate-level experts and up. Having said that, it is referenced so often that it is worth reading through at least once.

### Particles

• While this is not a comprehensive list, this analysis primarily made use of the following particles. I have provided links to the relevant PDG pages that detail the properties of these particles.
• It is not necessary to know everything about these states, but familiarizing yourself with the masses and quark content (where applicable) will give you more intuition for these processes.
• You may also be interested in exploring the particle properties in this visualization I made, using data from the [PDG http://pdg.lbl.gov] and the Google Visualization API.

# Exercises for motivated potential-audience members

• The PEP-II collider, used by the BaBar experiment, collided electrons and positrons at sufficient energy to produce bottom and anti-bottom quarks.

$e^+ e^- \rightarrow b \bar{b}$ At this energy, what other fermion-anti-fermion pairs are created by the electron-positron annihilation?

• Some of the particles analyzed in this study decay weakly and can be relatively long-lived. What is cτ for the
1. B0 meson?
2. Λ baryon?
• In 1980, James Cronin and Val Fitch won the Nobel Prize for observing CP-violation in neutral kaons. In Cronin's acceptance speech he discussed proton decay and how it would proceed if it was mediated by a fractionally charged boson. You can find a diagram of this process on p. 583 of his speech. Using the same mediating bosons (1/3 and 2/3 charge), can you draw tree-level diagrams for all of the following reactions? Some of them?
1. $B^0 \rightarrow \Lambda_c^+ \mu^-$
2. $B^- \rightarrow \Lambda \mu^-$
3. $B^- \rightarrow \bar{\Lambda} \mu^-$