Results are presented from a search for the rare decays Bs(0)→μ+ μ- and B(0)→μ+ μ- in pp collisions at sqrt[s]=7 and 8 TeV, with data samples corresponding to integrated luminosities of 5 and 20 fb(-1), respectively, collected by the CMS experiment at the LHC. An unbinned maximum-likelihood fit to the dimuon invariant mass distribution gives a branching fraction B(Bs(0)→μ+ μ-)=(3.0(-0.9)(+1.0))×10(-9), where the uncertainty includes both statistical and systematic contributions. An excess of Bs(0)→μ+ μ- events with respect to background is observed with a significance of 4.3 standard deviations. For the decay B(0)→μ+ μ- an upper limit of B(B(0)→μ+ μ-)<1.1×10(-9) at the 95% confidence level is determined. Both results are in agreement with the expectations from the standard model.
decay, with a statistical significance exceeding six standard deviations, and the best measurement so far of its branching fraction. Furthermore, we obtained evidence for the B 0 ? m 1 m 2 decay with a statistical significance of three standard deviations. Both measurements are statistically compatible with standard model predictions and allow stringent constraints to be placed on theories beyond the standard model. The LHC experiments will resume taking data in 2015, recording proton-proton collisions at a centre-of-mass energy of 13 teraelectronvolts, which will approximately double the production rates of B 0 s and B 0 mesons and lead to further improvements in the precision of these crucial tests of the standard model.Experimental particle physicists have been testing the predictions of the standard model of particle physics (SM) with increasing precision since the 1970s. Theoretical developments have kept pace by improving the accuracy of the SM predictions as the experimental results gained in precision. In the course of the past few decades, the SM has passed critical tests derived from experiment, but it does not address some profound questions about the nature of the Universe. For example, the existence of dark matter, which has been confirmed by cosmological data 3 , is not accommodated by the SM. It also fails to explain the origin of the asymmetry between matter and antimatter, which after the Big Bang led to the survival of the tiny amount of matter currently present in the Universe Fig. 1c, is forbidden at the elementary level because the Z 0 cannot couple directly to quarks of different flavours, that is, there are no direct 'flavour changing neutral currents'. However, it is possible to respect this rule and still have this decay occur through 'higher order' transitions such as those shown in Fig. 1d and e. These are highly suppressed because each additional interaction vertex reduces their probability of occurring significantly. They are also helicity and CKM suppressed. Consequently, the branching fraction for the B 0 s ?m z m { decay is expected to be very small compared to the dominant b antiquark to c antiquark transitions. The corresponding decay of the B 0 meson, where a d quark replaces the s quark, is even more CKM suppressed because it requires a jump across two quark generations rather than just one.The branching fractions, B, of these two decays, accounting for higher-order electromagnetic and strong interaction effects, and using lattice quantum chromodynamics to compute the B 8,9 , such as in the diagrams shown in Fig. 1f and g, that can considerably modify the SM branching fractions. In particular, theories with additional Higgs bosons 10,11 predict possible enhancements to the branching fractions. A significant deviation of either of the two branching fraction measurements from the SM predictions would give insight on how the SM should be extended. Alternatively, a measurement compatible with the SM could provide strong constraints on BSM theories. . Both CMS and LHCb later ...
The observation of a new b baryon via its strong decay into Ä À b þ (plus charge conjugates) is reported. The measurement uses a data sample of pp collisions at ffiffi ffi s p ¼ 7 TeV collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 5:3 fb À1 . The known Ä À b baryon is reconstructed via the decay chain Ä À b ! J=c Ä À ! þ À à 0 À , with à 0 ! p À . A peak is observed in the distribution of the difference between the mass of the Ä À b þ system and the sum of the masses of the Ä À b and þ , with a significance exceeding 5 standard deviations. The mass difference of the peak is 14:84 AE 0:74ðstatÞ AE 0:28ðsystÞ MeV. The new state most likely corresponds to the J P ¼ 3=2 þ companion of the Ä b . DOI: 10.1103/PhysRevLett.108.252002 PACS numbers: 14.20.Mr According to the well-established quark model and corresponding spectroscopy of baryons, there are several predicted baryons containing one strange and one beauty valence quark. These include the Ä b (ground state) and Ä 0 b , both with total angular momentum and parity J P ¼ 1=2 þ , a J P ¼ 3=2 þ state with angular momentum L ¼ 0 (often referred to, as will be done in this Letter, as Ä Ã b ), and two states with J P ¼ 1=2 À and 3=2 À , both with angular momentum L ¼ 1. These baryons can be neutral (valence quark content u À s À b) or negatively charged (d À s À b). At the Tevatron, baryons with masses and decay modes consistent with the theoretical predictions for the ground state Ä b baryons have been observed [1][2][3], although their quantum numbers have not yet been established. The allowed decays of the experimentally missing Ä b states should be analogous to the charmed sector [4][5][6]. In addition, theoretical calculations [7][8][9][10][11] predict the mass difference between the Ä 0 b and Ä b to be smaller than the mass of the pion, in which case the strong decay Ä 0 b ! Ä b is kinematically forbidden. The mass difference between the Ä Ã b and Ä b , however, is expected to be large enough to allow such a decay.This Letter presents a search for the decay . The CMS apparatus is described in detail in Ref. [12]. Its central feature is a superconducting solenoid, of 6 m internal diameter, providing a field of 3.8 T. The main subdetectors used in this analysis are the silicon tracker and the muon systems. The silicon tracker, composed of pixel and strip detector modules, is immersed in the magnetic field, and enables the measurement of charged particle momenta over the pseudorapidity range jj < 2:5, where ¼ À lnðtan=2Þ and is the polar angle of the track relative to the counterclockwise beam direction. Muons are identified in the range jj < 2:4 using gas-ionization detectors embedded in the steel return yoke of the magnet.The events used in this analysis were collected using the two-level trigger system of CMS. The first level consists of custom hardware processors and uses information from the muon systems to select events with two muons. The ''highlevel trigger'' processor farm further decreases the event rate befor...
and their charge conjugates.With respect to previous measurements in the same rapidity region, the coverage in transverse momentum ( p T ) is extended and the uncertainties are reduced by a factor of about two. The accuracy on the estimated total cc production cross section is likewise improved. The measured p Tdifferential cross sections are compared with the results of three perturbative QCD calculations.
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