Measurements of the weak φs phase
The observed difference between properties of matter and antimatter is governed by violation of the Charge-Parity (CP) symmetry. The Standard Model (SM) of particle physics predicts such violation, however the predicted size is to small to explain our observations. Therefore it is expected that some new phenomena are responsible for the excessive CP violation.
Measurement of the difference between properties of matter and antimatter for the strange beauty Bs mesons is one of promising ways to verify such assumption. The size of this difference is controlled by the parameter φs, which is predicted to be small in the Standard Model. However, effects of new particles not predicted by the Standard Model can make the measured value much larger. The measurement of φs is one of the most important goals of LHCb experiment.
The decay of the strange-beauty particle B0s, composed of a beauty antiquark (b) bound with a strange quark (s), into a J/ψ meson and a φ meson is used for this measurement. The J/ψ meson decays in turn into a μ+μ– or e+e– pair, and the φ decays to K+K– pair. In order to make this difficult measurement one has to analyze the Bs decay particles in 3 dimensions as well as to measure precisely the fast oscillations of strange beauty. In the world of quantum mechanics the strange-beauty meson Bs matter antimatter system is alternatively described as a heavy and light mass Bs meson system. The mass difference Δms between the heavy and light states leads to matter antimatter oscillations namely B0s changing back and forth into anti-B0s.
Our group is involved in measurements of the weak phase φs with the reaction B0s→J/ψ(e+e–)φ(K+K–). The analysis is ongoing.
Search for the CPT symmetry breaking
The CPT transformation is the combined operation of charge conjugation (C), parity (P) and time reversal (T) transformations. We say that the system has the CPT symmetry if it is invariant under the transformation of C, P and T applied together. The discrete CPT symmetry is believed to be strictly conserved in the nature. This statement is strengthened by the general theorem formed in 50′ by Pauli, Luders, Schwinger and Jost. The theorem states that any local, Lorentz-invariant quantum field theory must be also CPT-invariant. Hence, any tests of the CPT validity are the searches for the New Physics phenomena.
In LHCb we test the CPT symmetry in the so-called neutral flavour meson systems. The main idea is to perform precise experimental tests of indirect CPT symmetry violation by exploiting the neutral-meson oscillation phenomena, one of the fascinating effects predicted by Quantum Mechanics, in which the meson – bound state of quark and antiquark – oscillates back and forth between particle and antiparticle states. LHCb has published the article “Search for violations of Lorentz invariance and CPT symmetry in B0(s)” (Phys. Rev. Lett. 116, 241601), in which the best in the world upper limits for the CPT violating parameters in B0 and B0s systems are determined.
Our group is involved in the research in the charm sector with the reaction
D*+ → D0 + π+ , D0 → K– + π+, by performing the time-dependent asymmetry studies both in the classical and Alan Kostelecky‘s Standard Model Extension framework. The analysis is ongoing.
Two connected analysis are performed in search for exotic hadrons:
search for resonances in χc1π+π–, and to determine upper limit for production cross-section in case of non-observation,
search for Z tetraquarks states in B+→χc1π+π–K+ decay
The search of resonances in χc1π+π– decay is motivated by theoretical indication that this final state can be favorable for a lowest charmonium hybrid with exotic quantum numbers JPC=1-+. The mass prediction for this state varies from 3700 MeV (QCD sum rules) up to 4370 MeV (lattice QCD). There is no direct calculation of charmed hybrid production in proton-proton collision, however the production cross-section is expected to be close to conventional charmonium.
The search for a resonant structure in B+→χc1π+π–K+ decay is motivated by observation made by Belle experiment of two resonances decaying to χc1π+ in the decay B0→χc1π+K– decay named Z(4050)+ and Z(4250)+. These resonances were not confirmed later by Babar experiment so independent confirmation is important. They represent particular interest being charged and therefore they are explicitly do not belong to conventional charmonium states.