Glossary

8.4 B-physics and flavor-changing processes

5 min read•august 1, 2024

B-physics dives into the fascinating world of B-mesons, particles containing a . These mesons are like tiny labs, perfect for studying and . Their long lifetimes and hefty mass make them ideal for precise measurements and detailed studies.

B-meson decays help us measure the , which describes quark mixing in the Standard Model. act as sensitive probes for new physics, potentially revealing undiscovered particles. These studies could expose cracks in our current understanding of particle physics.

B-meson physics for CP violation

B-mesons as unique laboratories

Top images from around the web for B-mesons as unique laboratories

  • particle physics - Do matter and antimatter annihilate or release energy? - Physics Stack Exchange View original

    Is this image relevant?

  • particle physics - Do matter and antimatter annihilate or release energy? - Physics Stack Exchange View original

    Is this image relevant?

1 of 2

Top images from around the web for B-mesons as unique laboratories

  • particle physics - Do matter and antimatter annihilate or release energy? - Physics Stack Exchange View original

    Is this image relevant?

  • particle physics - Do matter and antimatter annihilate or release energy? - Physics Stack Exchange View original

    Is this image relevant?

1 of 2
  • B-mesons contain a bottom quark and a lighter antiquark, providing exceptional opportunities to study CP violation and flavor-changing processes due to their relatively long lifetimes (~1.5 ps) and large mass (~5.3 GeV/c^2)
  • CP violation manifests as asymmetry between matter and antimatter behavior plays a crucial role in explaining the observed matter-antimatter imbalance in the universe
  • B-meson systems exhibit significant CP violation effects allowing for precise measurements and detailed studies
  • Neutral B-meson oscillations involve transformations between particles and antiparticles (B0 ↔ B̄0) offer insights into flavor-changing processes and potential new physics beyond the Standard Model

CKM matrix and rare decays

  • B-meson decays enable precise measurements of Cabibbo-Kobayashi-Maskawa (CKM) matrix elements describing quark flavor mixing fundamental to the Standard Model's description of CP violation
  • CKM matrix represented as: [VCKM](https://www.fiveableKeyTerm:vckm)=(VudVusVubVcdVcsVcbVtdVtsVtb)[V_{CKM}](https://www.fiveableKeyTerm:v_{ckm}) = \begin{pmatrix} V_{ud} & V_{us} & V_{ub} \\ V_{cd} & V_{cs} & V_{cb} \\ V_{td} & V_{ts} & V_{tb} \end{pmatrix}
  • Rare B-meson decays act as sensitive probes for new physics occurring through loop-level processes potentially involving undiscovered particles
    • Examples include B0→K∗0μ+μ−B^0 \rightarrow K^{*0}\mu^+\mu^- and Bs0→μ+μ−B_s^0 \rightarrow \mu^+\mu^-
  • Precision measurements in B-physics constrain or potentially reveal inconsistencies in the Standard Model guiding theoretical developments in particle physics
    • Measurements of branching ratios, angular distributions, and CP asymmetries in rare decays provide stringent tests of the Standard Model

B-meson decay channels and the CKM matrix

Semileptonic and hadronic decays

  • Semileptonic decays determine |Vcb| and |Vub| CKM matrix elements describing coupling between b quarks and c or u quarks
    • B→D∗lνB \rightarrow D^*l\nu used for |Vcb|
    • B→πlνB \rightarrow \pi l\nu used for |Vub|
  • Hadronic decays involving b → c transitions provide complementary information on |Vcb| and study strong interaction effects
    • B→D+π−B \rightarrow D^+\pi^- serves as an example of such decays
  • Charmless hadronic decays involve b → u transitions sensitive to CKM angle α contributing to unitarity triangle measurements
    • B→π+π−B \rightarrow \pi^+\pi^- decay mode used for α determination

CP-violating decays and mixing

  • CP-violating decays allow measurement of CKM angle β providing direct evidence of CP violation in B-meson system
    • B→J/ψKSB \rightarrow J/\psi K_S decay known as the "golden mode" for β measurement
  • Mixing-induced CP violation in neutral B-meson decays observed through time-dependent asymmetries provides information on CKM matrix phase
    • Time-dependent CP asymmetry given by: ACP(t)=[Sf](https://www.fiveableKeyTerm:sf)sin⁡(Δmdt)−[Cf](https://www.fiveableKeyTerm:cf)cos⁡(Δmdt)A_{CP}(t) = [S_f](https://www.fiveableKeyTerm:s_f) \sin(\Delta m_d t) - [C_f](https://www.fiveableKeyTerm:c_f) \cos(\Delta m_d t) where SfS_f and CfC_f are CP violation parameters, and Δmd\Delta m_d represents B0-B̄0 oscillation frequency
  • These measurements test consistency of Standard Model CKM framework and search for new physics contributions

Flavor-changing neutral currents in the Standard Model

FCNC suppression and the GIM mechanism

  • change quark flavor without altering electric charge forbidden at tree level in Standard Model due to GIM (Glashow-Iliopoulos-Maiani) mechanism
  • FCNCs occur at loop level in Standard Model highly suppressed making them sensitive probes for new physics potentially enhancing these processes
  • cancellation illustrated by the following diagram for b → s transition: 
    b ---> s
 |     ^
 |     |
W    W
 |     |
 v     |
t,c,u  t,c,u
  • Suppression factor in FCNC processes approximately given by: GF216π2mt2MW2≈10−3\frac{G_F^2}{16\pi^2} \frac{m_t^2}{M_W^2} \approx 10^{-3}

FCNC processes as probes for new physics

  • Study of FCNC processes in B-meson decays provides stringent tests of Standard Model and constrains or potentially reveals new physics contributions
    • B→K∗l+l−B \rightarrow K^*l^+l^- and B→μ+μ−B \rightarrow \mu^+\mu^- serve as important examples
  • Rare FCNC decays play crucial role in searching for physics beyond Standard Model as new particles or interactions could significantly alter rates or kinematic distributions
  • Absence of tree-level FCNCs in Standard Model results from its flavor structure observation of enhanced FCNC processes would indicate departure from this structure
  • Theories beyond Standard Model (supersymmetry, extra dimensions) allow FCNCs to occur more readily potentially leading to observable deviations from Standard Model predictions in B-physics experiments

Experimental techniques in B-physics

B-factories and asymmetric colliders

  • B-factories operate as e+e- colliders at Υ(4S) resonance predominantly decaying to BB̄ pairs allowing clean and controlled production of B-mesons
    • BaBar at SLAC and Belle at KEK serve as prime examples
  • Asymmetric beam energies in B-factories produce boosted B-mesons enabling precise measurements of decay vertices and time-dependent CP violation studies
    • Typical boost factor β γ ≈ 0.55 results in average B-meson flight length of ~250 μm
  • High-precision vertex detectors reconstruct B-meson decay vertices crucial for time-dependent measurements and separating signal from background
    • achieve position resolutions of ~10-20 μm

LHCb and hadron collider experiments

  • at Large Hadron Collider designed specifically for B-physics takes advantage of high bb̄ production cross-section in proton-proton collisions
    • bb̄ cross-section at LHC (√s = 13 TeV) ≈ 500 μb compared to ~1 nb at B-factories
  • LHCb's forward geometry optimizes detection of B-hadrons produced in forward region where they are highly boosted facilitating precise lifetime and oscillation measurements
    • Detector covers pseudorapidity range 2 < η < 5
  • Advanced trigger systems in both B-factories and LHCb experiments essential for selecting rare B-decay events from large background of other particle interactions
    • LHCb employs a two-level trigger system reducing 40 MHz collision rate to ~10 kHz of events for offline analysis
  • Particle identification techniques (Cherenkov detectors, dE/dx measurements) crucial in B-physics experiments for distinguishing between different types of charged particles produced in B-meson decays
    • Ring Imaging Cherenkov (RICH) detectors in LHCb provide π/K separation up to ~100 GeV/c momentum

Key Terms to Review (29)

|v_{cb}|: |v_{cb}| is the magnitude of the CKM (Cabibbo-Kobayashi-Maskawa) matrix element that describes the strength of flavor-changing transitions between bottom quarks (b) and charm quarks (c) in weak decays. This parameter plays a crucial role in understanding B-physics and the mechanisms of flavor-changing processes, which are essential for exploring CP violation and the matter-antimatter asymmetry in the universe.
|v_{ub}|: |v_{ub}| represents the magnitude of the CKM matrix element that describes the transition between the up quark (u) and the bottom quark (b) through the process of flavor-changing charged current interactions. This value is crucial in understanding the mixing and decay of B mesons, as well as the violation of CP symmetry, providing insights into the nature of weak interactions and the overall flavor structure of the Standard Model.
Asymmetric Colliders: Asymmetric colliders are particle accelerators designed to collide beams of particles with different energies or masses, creating unique conditions for studying fundamental interactions. This setup allows researchers to investigate phenomena that may not be observable in symmetric collisions, offering insights into B-physics and flavor-changing processes, particularly in the context of quark mixing and CP violation.
B → d transitions: b → d transitions refer to flavor-changing processes in particle physics where a bottom (b) quark changes into a down (d) quark. This process is significant in understanding the interactions between quarks, as it involves the weak force and is crucial for studying CP violation, which is related to the differences in behavior between matter and antimatter.
B → d^*lν: The decay process b → d^*lν refers to the transition of a bottom quark (b) decaying into a charm quark (d^*), accompanied by a lepton (l) and a neutrino (ν). This process is significant in B-physics, particularly in understanding flavor-changing processes and the underlying mechanisms of particle interactions, as it involves the transformation of one type of quark into another while emitting specific particles.
B → d^+π^-: The decay process b → d^+π^- represents a transition of a bottom quark (b) into a down quark (d), accompanied by the emission of a positively charged pion (π^-). This process is significant in B-physics as it involves flavor-changing transitions and helps to explore the behavior of weak interactions and the properties of mesons.
B → j/ψ k_s: The decay process b → j/ψ k_s refers to a specific transition involving a bottom quark (b) decaying into a J/ψ meson and a K-short meson (k_s). This process is significant in studying flavor-changing processes, which are essential for understanding the behavior of quarks and the underlying mechanisms of the weak force, particularly in B physics.
B → s transitions: b → s transitions refer to flavor-changing processes where a bottom quark (b) transitions into a strange quark (s), typically via weak interactions. This process is crucial for understanding various phenomena in B-physics, particularly in the context of CP violation and the study of rare decays. These transitions provide insights into the underlying mechanisms of flavor physics and are vital for testing the Standard Model and exploring possible new physics beyond it.
B → π^+π^-: The decay process b → π^+π^- describes the transition of a bottom quark (b) into a pair of pions, specifically a positively charged pion ($$ ext{π}^+$$) and a negatively charged pion ($$ ext{π}^-$$). This process is significant in B-physics, particularly in flavor-changing processes, as it illustrates how quark flavors can change and how weak interactions facilitate these transformations.
B → πlν: The decay process represented by b → πlν describes the transition of a bottom quark (b) into a pion (π), a lepton (l), and a neutrino (ν). This process is significant in B-physics, as it highlights flavor-changing processes that involve weak interactions, playing a crucial role in understanding particle decays and the behavior of different flavors of quarks.
B factories: B factories are specialized particle accelerators designed to produce and study B mesons, which are crucial for exploring the physics of flavor-changing processes. These facilities enable high-energy collisions between electrons and positrons, resulting in the production of B mesons, which decay into various other particles. By analyzing these decay patterns, researchers can gain insights into CP violation and the underlying principles of the Standard Model of particle physics.
B meson: A b meson is a type of meson that contains a bottom (b) quark, which is one of the six types of quarks in the Standard Model of particle physics. B mesons are important in the study of flavor-changing processes because they can decay into lighter particles through weak interactions, enabling scientists to explore CP violation and the mechanisms of particle interactions.
B^0 → π^+ π^-: The process b^0 → π^+ π^- refers to the decay of a neutral B meson into a pair of charged pions. This decay mode is significant in the study of B-physics as it exemplifies flavor-changing processes, where the flavor of a quark changes during the interaction, allowing for deeper insights into the behavior of quarks and their interactions.
Babar Collaboration: The Babar Collaboration is a research collaboration focused on the study of B mesons and their properties, primarily using data collected from the PEP-II asymmetric B factory at SLAC. This collaboration plays a crucial role in understanding B-physics, especially flavor-changing processes, which are essential for testing the Standard Model of particle physics and exploring potential signs of new physics beyond it.
Belle Collaboration: The Belle Collaboration is a large international team of physicists who work together on the Belle experiment, which studies B meson decays at the SuperKEKB accelerator in Japan. This collaboration plays a critical role in investigating flavor-changing processes, particularly those involving bottom quarks, to explore the implications for the Standard Model and potential new physics beyond it.
Bottom quark: The bottom quark, also known as the beauty quark, is one of the six types of quarks in the Standard Model of particle physics. It carries a charge of -1/3 e and has a relatively high mass compared to other quarks, making it important in the study of particle interactions and flavor physics. Its role is essential in understanding the structure of hadrons and contributes to phenomena like quark mixing and flavor-changing processes.
C_f: In particle physics, c_f represents the 'color factor' associated with a specific process involving quarks and gluons. This factor plays a crucial role in calculating the probabilities of flavor-changing processes, particularly in B-physics, as it accounts for the contribution of the color charge in quantum chromodynamics (QCD). Understanding c_f is essential for analyzing interactions that involve flavor transitions between different quark types.
CKM matrix: The CKM (Cabibbo-Kobayashi-Maskawa) matrix is a complex unitary matrix that describes the mixing of the three generations of quarks in weak interactions. This matrix is crucial for understanding how quarks transform into one another during weak decays, and it plays a significant role in explaining CP violation, limitations of the Standard Model, and flavor-changing processes in B-physics.
Cp violation: CP violation refers to the phenomenon where the combined symmetries of charge conjugation (C) and parity (P) are not conserved in certain particle interactions, particularly in weak decays. This violation suggests that the laws of physics are not the same for particles and their antiparticles, leading to observable differences in behavior, which has profound implications for our understanding of the universe.
Flavor-changing neutral currents: Flavor-changing neutral currents (FCNC) are processes in which a particle changes its flavor without exchanging a charged particle, and this phenomenon is mediated by neutral force carriers like the Z boson. FCNC processes are rare due to their suppression in the Standard Model, making them crucial for understanding the limitations of current theories and potential extensions that might reveal new physics beyond the Standard Model.
Flavor-changing processes: Flavor-changing processes are interactions in particle physics where a quark changes from one type, or 'flavor', to another. These processes are crucial for understanding the behavior of particles such as mesons and baryons and play a significant role in the study of B-physics, particularly in phenomena like CP violation and mixing.
GIM Mechanism: The GIM mechanism, or Glashow-Iliopoulos-Maiani mechanism, is a theoretical framework used in particle physics to explain the suppression of flavor-changing neutral currents (FCNCs) in weak interactions. This mechanism is crucial in the context of the Standard Model as it provides a way to maintain consistency with experimental observations while allowing for the existence of multiple generations of quarks and leptons.
LHCb Experiment: The LHCb (Large Hadron Collider beauty) experiment is a particle physics experiment designed to study the properties of B mesons, which are particles containing a bottom quark. This experiment primarily focuses on understanding CP violation, flavor-changing processes, and potential new physics by analyzing the differences between matter and antimatter. The insights gained from LHCb contribute significantly to our knowledge of weak interactions and the fundamental symmetries of nature.
Rare decays: Rare decays are processes in particle physics where a particle transitions into different final states with a very low probability, making them infrequent occurrences. These decays are important for studying fundamental interactions and can provide insights into new physics beyond the Standard Model, particularly in the context of flavor-changing processes.
Ring imaging Cherenkov detectors: Ring imaging Cherenkov detectors are sophisticated particle detection devices that utilize the phenomenon of Cherenkov radiation to identify charged particles and measure their velocities. When a charged particle travels through a medium at a speed greater than the speed of light in that medium, it emits Cherenkov radiation in the form of a coherent ring of light. This technology is particularly useful in high-energy physics experiments for distinguishing between different types of particles, including those relevant to flavor-changing processes and B-physics.
S_f: In particle physics, s_f refers to the scale of flavor-changing processes, specifically related to B-meson decays and transitions between different quark flavors. This concept is crucial in understanding how certain interactions can change one type of quark into another, which is a key feature in B-physics studies. Flavor-changing processes are important for exploring the underlying symmetries and possible new physics beyond the Standard Model.
Silicon vertex trackers: Silicon vertex trackers are highly precise detector systems used in particle physics experiments to track the paths of charged particles and determine their interaction points with great accuracy. These trackers are crucial for studying B-physics and flavor-changing processes, as they provide detailed information about the decay of B mesons, allowing physicists to investigate the underlying mechanisms of flavor transitions and CP violation.
V_{ckm}: The CKM matrix, or Cabibbo-Kobayashi-Maskawa matrix, describes the mixing of quark flavors in particle physics. It is crucial for understanding flavor-changing processes, particularly in B-physics, as it provides insights into how different types of quarks transform into one another through the weak interaction. The matrix elements indicate the probability amplitudes for transitions between different quark flavors, playing a key role in explaining phenomena like CP violation and the hierarchy of quark masses.
δm_d: The term δm_d represents the mass difference between the two neutral B meson states, specifically the B_d meson. This mass difference is crucial for understanding B-physics and flavor-changing processes, as it provides insights into the mixing of B mesons and the effects of CP violation. The measurement of δm_d helps physicists study the underlying mechanisms that govern flavor transitions and contributes to our understanding of the Standard Model of particle physics.
© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Back
Practice QuizGlossary
Practice QuizGlossary
Next guide