Confirmation of an old  Puzzle

March 20th 2015

Summary: An eagerly anticipated set of results were released today by the LHCb collaboration. These results confirm a discrepancy that was seen in the experiment’s 2011 dataset.

Image: The P5’ observable as a function of the invarinat mass of the two muons. A deviation is seen between the new data points (black) and the Standard Model (SM) prediction (orange) in one region of the plot.

(Credit LHCb)

Today at the 50th Rencontres de Moriond Electroweak, in La Thuile (Italy), the LHCb collaboration presented a complex analysis of a sample of around 2,500 B0 mesons decaying to a K* meson and a pair of muons. For the first time results were shown using all the data collected during run 1 of the LHC. This particular decay of B0 mesons is rare, happening only once in every million B mesons produced. The quantum mechanical process that control the decay make it sensitive to potential new physics effects. The results presented today describe a first full analysis of the angular distribution of the kaon, pion (from the K* meson) and the muons from the decay. One particularly interesting way to summarise this information is by looking at an observable called    “P5’ ”. The LHCb data for P5’ are shown by the black points as a function of the invariant mass of the two muons squared (“q2”) in the figure above. The data are also compared to a recent calculation based on the SM, show by the coloured rectangles, the height of which represents the theoretical uncertainty on the predictions. In low q2 region, there is a visible tension between the data and the SM prediction (at a statistical level of about 3.7 sigma). Dr Tom Blake (University of Warwick) describes further 

“Our result from the 2011 data generated a lot of interest in the particle physics community and everyone has been eagerly awaiting this new result ever since. We’re pleased to have been able to present it for the first time at Moriond. Given what we see in the data, we’re expecting a lot of renewed interest. The old discrepancies remain, in particular the modulation of the angular distribution that we refer to as P5’ appears a lot smaller in the data than predicted.” .

Dr Mitesh Patel (Imperial College London) is excited about the pattern that is emerging from the LHCb data, commenting that

“there has been some speculation in the community that the new P5' measurement and other discrepancies that we have seen in LHCb data, such as the differing rate of decays into electrons and muons, could be linked and could be explained by the existence of a new particle. More LHCb measurements will allow us to confirm if we are really seeing some new phenomena, or if the data show that we don't understand the Standard Model as well as we thought”.

LHCb physicists are currently preparing for the start of Run-2 of the LHC, which, by 2018, should see them collect 2-3 times as much data as they currently have. The analysis of this Run-2 sample will hopefully shed further light on this intriguing anomaly as the search for signs of physics beyond the Standard Model continues.

Further reading:

Conference note describing the result: LHCb-CONF-2015-002

LHCb webpage

Theoretical interpretation:


Dr Mitesh Patel , Mitesh.PatelATNOSPAM

Dr Tom Blake, Thomas.BlakeATNOSPAM


The UK participation in the international LHCb experiment is from eleven institutes.

University of Birmingham, University of Bristol, University of Cambridge, University of Edinburgh, University of Glasgow, Imperial College London, University of Liverpool, University of Manchester, University of Oxford, STFC Rutherford Appleton Laboratory, University of Warwick

UK participation in the experiment is funded by the Science and Technology Facillities Council (STFC), with contributions from the participating institutes, the Royal Society and European Union.