A new look at new physics
Nikhef physicists design new tests for the Standard Model of particle physics
Researchers from the FOM Institute Nikhef and the VU University Amsterdam have proposed a new method to test the Standard Model of particle physics. For many years 'new physics' has been sought in precision studies into the rare decay of very specific particles. So far no minimum deviations from the Standard Model have been found. The researchers have now devised a new method to extend the search for new physics in future measurements of the LHCb experiment at CERN. The results of the research were published on 25 July 2012 in the renowned journal Physical Review Letters
Precision studies of the decay of Bs mesons, which consist of a 'beauty' quark b and a 'strange' quark s, result in unprecedented tests of the Standard Model of particle physics. In particular, the decay to two muons, Bs → μ+μ- , is highly promising in this context. For example, extra Higgs particles that might exert an influence on this decay could exist. Therefore this research might ultimately reveal a fingerprint of a discovery beyond the Standard Model of particle physics.
The Bs → μ+μ- decay has not yet been observed but the most recent experimental upper limit measured by the LHCb experiment at CERN reveals that less than 1 in 222 million Bs mesons decay into two muons (i.e. an upper limit of the so-called branching ratio of 4.5 x 10-9). This approaches the expectation of the Standard Model that 1 in 312 million Bs mesons decay into two muons, (a branching ratio of (3.2 ± 0.2) x 10-9). As a result of this a large proportion of the models for new physics can already be excluded.
In the article the scientists point out that the Bs → μ+μ- decay can be investigated in a new manner due to a fascinating characteristic of the Bs mesons: the time-dependent oscillations between Bs mesons and their anti-mesons. Thanks to this quantum mechanical phenomenon a decay width difference ΔΓs arises which was indeed recently measured in the LHCb experiment. The impact of ΔΓs has not yet been included in the analyses of the decay. In the article, however, this factor is taken into consideration for the first time. It was found to increase the reference value of the Standard Model for this branching ratio by almost 10% to (3.5 ± 0.2) x 10-9. Moreover, it provides access to a new observable: the effective Bs → μ+μ- lifetime that is theoretically pure and contains extra information not found in the branching ratio measurement. If the Bs → μ+μ- branching ratio concurs with the expectation of the Standard Model, and therefore reveals no new physics, a measurement of the lifetime, which appears to be feasible with a future upgrade of the LHCb experiment, might still reveal significant new physics effects.
The research was carried out by a team of scientists from the Theoretical Physics and LHCb research groups at Nikhef (National Institute for Subatomic Physics) and the VU University Amsterdam. The Netherlands Organisation for Scientific Research (NWO) and the Foundation for Fundamental Research on Matter (FOM) supported the research.
Further information
Prof. Robert Fleischer +31 (0)20 592 2172 – Nikhef & VU University Amsterdam
Vanessa Mexner +31 (0)20 592 5075 – Science Communication Department Nikhef
Reference
'Probing New Physics via the Bs → μ+μ- Effective Lifetime'
K. de Bruyn, R. Fleischer, R. Knegjens, P. Koppenburg, M. Merk, A. Pellegrino, N. Tuning
http://arXiv.org/abs/arXiv:1204.1737
Phys. Rev. Lett. 109, 041801 (2012)