Angles of particle tracks give hints of New Physics

August 10th 2013

Summary: A paper published today examines the angles at which particles are emitted in a rare decay and finds signs of a discrepancy with the Standard Model. A possible explanation by new physics has been suggested.

The black data points from the LHCb measurements are compared with the Standard Model prediction shown by the blue bands. The third data point from the left  is very different from the Standard Model prediction.

Image: .(Credit LHCb and CMS)

Tantalizing hints of phenomena that may not be described by the present theory of particle physics emerge from the LHCb experiment at CERN’s Large Hadron Collider.

Our current knowledge of fundamental particles and their interactions is embedded in the so-called Standard Model, which has been shown to be in excellent agreement with essentially all previous measurements of sub-atomic particles. However, the Standard Model has some serious shortcomings: it does not include gravity, nor can it explain the nature of the 95% of the Universe that is thought to be in the form of Dark Matter and Dark Energy.  New particles are therefore thought to exist to explain these and other problems of the model. While measurements at the LHC have discovered the Higgs Boson that was predicted by the Standard Model, no other new particles have yet been found.

A paper published today presents a new analysis of the decay of a B0 particle into a K*0 particle and two muons, B0→K*0μ+μ-, by the LHCb Collaboration. Only one in a million B0 particles decay in this way but the process is very sensitive to the presence of any new, non-Standard Model, particles. Previously published LHCb measurements of this decay were in good agreement with Standard Model predictions. However, the latest measurements show a tension in the quantity P5’ which is measured by looking at the angles of the particles produced in the decay. The image shows the distribution of the P5' observable as a function of the mass of the μ+μ- produced in the decay (denoted q2). The black data points from the LHCb measurements are compared with the Standard Model prediction shown by the blue bands. The third data point from the left, with q2 between 4.3 and 8.68 GeV2/c4, is very different from the Standard Model prediction. Taking into account that the LHCb data has been used to make a number of different measurements and this deviation is observed in one of 24 such q2 regions (the so-called look-elsewhere-effect –, such a measurement might happen by chance just 0.5% of the time. Dr Mitesh Patel, a researcher at Imperial College London, said “This is  an exciting hint of a disagreement with the Standard Model but it is not yet at the level that scientists require to say definitively that a new effect has been seen.”

The new LHCb results are stimulating theoretical work which makes a global analysis of the latest LHCb data and the previous measurements. One such analysis ( of the LHCb results suggests that the deviation in P5' and small discrepancies in other measurements for the B0→K*0μ+μ- decay follow a pattern that seen might happen by chance with odds of only around one in 4,500. This would make the results significant enough for scientists to claim evidence for something new. The same analysis claims that all the measurements can be explained with a single, simple mechanism involving a new particle. 

Dr. Patel commented “The LHCb results that have been presented so far are based on only a third of the data that have been recorded. Particle physicists are impatiently waiting for the rest of the data to be analysed, and for the next phase of the LHC operations to collect even larger data samples, to shed further light on the discrepancies that have been observed.”

More information can be found in the LHCb paper here (


Dr Mitesh Patel, Imperial College London, mitesh.patel ATNOSPAM


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.