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Physical Review Letters 110 (2013) 021801
First evidence for the decay Bs -> mu+ mu-
R. Aaij, S. Barsuk1, O. Callot1, J. He1, B. Jean-Marie1, J. Lefrançois1, F. Machefert1, A. Martin Sanchez1, M. Nicol1, P. Robbe1, M.-H. Schune1, B. Viaud1, I. Videau1, E. Ben-Haim2, M. Benayoun2, P. David2, L. Del Buono2, A. Martens2, Z J Ajaltouni3, O. Deschamps3, E. Cogneras3, I. El Rifai3, P. Henrard3, M. Hoballah3, M. Jahjah Hussein3, R. Lefèvre3, L. Li Gioi3, S. Monteil3, V. Niess3, Pascal Perret3, D.A. Roa Romero3, K. Sobczak3, C. Adrover4, E. Aslanides4, J. Cogan4, R. Legac4, O. Leroy4, G. Mancinelli4, E. Maurice4, M. Perrin-Terrin4, M. Sapunov4, J. Serrano4, A. Tsaregorodtsev4, I. De Bonis5, D. Decamp5, Ph. Ghez5, P. Hopchev5, J.-P. Lees5, I.V. Machikhiliyan5, M.-N. Minard5, B. Pietrzyk5, W. Qian5, S. T'Jampens5, V. Tisserand5, E. Tournefier5
For the LHCb collaboration(s)

A search for the rare decays Bs->mu+mu- and B0->mu+mu- is performed using data collected in 2011 and 2012 with the LHCb experiment at the Large Hadron Collider. The data samples comprise 1.1 fb^-1 of proton-proton collisions at sqrt{s} = 8 TeV and 1.0 fb^-1 at sqrt{s}=7 TeV. We observe an excess of Bs -> mu+ mu- candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3 x 10^-4 corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of BR(Bs -> mu+ mu-) = (3.2^{+1.5}_{-1.2}) x 10^-9, where the statistical uncertainty is 95% of the total uncertainty. This result is in agreement with the Standard Model expectation. The observed number of B0 -> mu+ mu- candidates is consistent with the background expectation, giving an upper limit of BR(B0 -> mu+ mu-) < 9.4 x 10^-10 at 95% confidence level.
1:  LAL - Laboratoire de l'Accélérateur Linéaire
2:  LPNHE - Laboratoire de Physique Nucléaire et de Hautes Énergies
3:  LPC - Laboratoire de Physique Corpusculaire [Clermont-Ferrand]
4:  CPPM - Centre de Physique des Particules de Marseille
5:  LAPP - Laboratoire d'Annecy le Vieux de Physique des Particules
Physics/High Energy Physics - Experiment
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