PHYSICS HIGHLIGHTS - PLAIN ENGLISH SUMMARIES

 

Beautiful  Matter Anti-matter Difference Limited

December 13th 2013

Summary: A new study by the LHCb experiment at CERN does not confirm the indications of an unexpectedly large matter-antimatter asymmetry seen in the decays of beauty particles that was reported by the D0 experiment at Fermilab in the US. This measurement sets tight limits on scenarios of New Physics beyond the Standard Model. measurements of matter-antimatter asymmetries 

Image: Measurements of the semileptonic asymmetries of the B_d (vertical bands) and B_s (horizontal bands) mesons. The black dot indicates the Standard Model prediction of close to zero for both asymmetries. The anomalous D0 measurement is sensitive to both B_s and B_d, and therefore makes a diagonal band, which is quite far (3.9 standard deviations) from the Standard Model. The blue hatched band shows the new measurement from LHCb, which agrees very well with the Standard Model prediction.

.(Credit LHCb)

A new study by the LHCb experiment at CERN does not confirm the indications of an unexpectedly large matter-antimatter asymmetry seen in the decays of beauty particles that was reported by the D0 experiment at Fermilab in the US. This measurement sets tight limits on scenarios of New Physics beyond the Standard Model.

Scientists working on the LHCb experiment at CERN have had accepted for publication in Phys. Lett. B a  measurement of the so called "Semileptonic asymmetry" of the Bs meson particle. The Bs meson is made of an "strange" quark and a "beauty" antiquark. A peculiarity of these particles is that they can spontaneously transform into their antiparticle (a beauty quark and a strange antiquark). In the Standard Model, this "mixing" occurs at almost the same rate in both directions: the difference is 1 in 100,000. However, the Standard Model is known to be incomplete, and many extensions predict much larger differences (or "CP-asymmetries") of up to 1%. Considerable excitement was generated In 2009 when the D0 experiment at Fermilab in the US reported an asymmetry of about 1% in a mixture of the decays of the Bs and its cousin the Bd (which has a down quark instead of a strange quark). If confirmed the result would indicate the presence of physics beyond the Standard Model such as large extra dimensions or new heavy bosons. Other measurements of these asymmetries are consistent with the Standard Model, but none are as sensitive as the "dimuon asymmetry" results from D0.

The large dataset collected by the LHCb experiment during the first LHC run has allowed to cast further light on this discrepancy.  The LHCb experiment  is specifically designed to study CP-asymmetries in beauty particles profiting from the fact that the LHC produces 1 billion Bs mesons per year. Dr. Mika Vesterinen, one of the lead analysts of this study commented, 
“Bs mesons are extremely difficult to detect , but thanks to the excellent design of the LHCb detector, we reconstructed around 200,000 Bs mesons decaying into a muon (a heavier cousin of the electron) and the Ds meson (a charm quark and a strange quark)  during the 2011 run of the LHC. The charge of the muon allows us to tag whether a Bs or Bsbar decayed and to study the asymmetry between matter
 and antimatter.”
Such measurements are challenging since fake asymmetries due the experiment being more efficient at detecting muons of one charge compared to the other must be carefully controlled and understood. The final measured semileptonic asymmetry agrees with the Standard Model prediction. 
“Though we do not confirm the D0 result our measurement narrows down the nature of new physics. We also have a factor of two more data already collected from the 2012 run to analyse and more to come when the LHC restarts in 2015. This will allow us to to probe further for New Physics”, added Dr Vesterinen.
There are four neutral mesons which allow particle-antiparticle transitions---mixing. These are ideal laboratories for studies of matter-antimatter asymmetries (CP violation) and indeed for three of these mesons, neutral kaons as well as B0 and Bs0, such an asymmetry has been observed. For neutral charm mesons (D0) such searches have thus far not revealed a positive result. Being the only of the four mesons to contain up-type quarks, D0 mesons provide a unique access to effects from physics beyond the Standard Model.

[1] LHCb-PAPER-2013-033, http://cds.cern.ch/record/1566754?ln=en, accepted by Phys. Lett. B.

Contacts:
Dr Mika Vesterinen,  mika.vesterinen ATNOSPAM cern.ch
LHCb-UK:
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.



http://cds.cern.ch/record/1566754?ln=enPrevious_Highlights.htmlshapeimage_2_link_0shapeimage_2_link_1