V. Niess

The LHCb trigger and its performance in 2011

The LHCb trigger and its performance based on data taken at the LHC in 2011 is presented. LHCb is designed to perform flavour physics measurements, and its trigger distinguishes charm and beauty decays from the light quark background. It uses a combination of lepton identification of particles, transverse momentum of particles and selects particles originating from hadrons which decay after a finite flight distance. The trigger reduces the

First evidence for the decay Bs -> mu+ mu-

\sim 11

Measurement of

MHz of bunch-bunch crossings with at least one non-elastic pp-interaction to 3 kHz of events which are written to storage in two trigger levels. The first level is implemented in hardware, while the next level is a software application which runs on all processors of a large computer farm. A data driven method is used to evaluate the performance of the trigger for several charm and beauty decay modes.

J/\psi

A search for the rare decays B(s)(0)→μ+ μ- and B(0)→μ+ μ- is performed with 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 B(s)(0)→μ+ μ- candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3×10(-4) corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of B(B(s)(0)→μ+ μ-)=(3.2(-1.2)(+1.5))×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 B(0)→μ+ μ- candidates is consistent with the background expectation, giving an upper limit of B(B(0)→μ+ μ-)<9.4×10(-10) at 95% confidence level.

production in

A bstract The production of J/ψ mesons is studied with the LHCb detector using data from pp collisions at

pp

\sqrt{s}=2.76

collisions at

TeV corresponding to an integrated luminosity of 71 nb−1. The differential cross-section for inclusive J/ψ production is measured as a function of its transverse momentum pT. The cross-section in the fiducial region 0 < pT  < 12 GeV/c and rapidity 2.0 <

\sqrt{s}=2.76

y

TeV

< 4.5 is measured to be 5.6 ± 0.1 (stat) ± 0.4 (syst) μb, with the assumption of unpolarised J/ψ production. The fraction of J/ψ production from b-hadron decays is measured to be (7.1 ± 0.6 (stat) ± 0.7 (syst))%.

Anatomy of new physics in B-B mixing

We analyse three different New Physics scenarios for Delta F=2 flavour-changing neutral currents in the quark sector in the light of recent data on neutral-meson mixing. We parametrise generic New Physics contributions to Bq - Bq-bar mixing (q=d,s), in terms of one complex quantity Delta_q, while three parameters Delta_K^{tt}, Delta_K^{ct} and Delta_K^{cc} are needed to describe K - Kbar mixing. In Scenario I, we consider uncorrelated New Physics contributions in the Bd, Bs, and K sectors. In this scenario, it is only possible to constrain the parameters Delta_d and Delta_s whereas there are no non-trivial constraints on the kaon parameters. In Scenario II, we study the case of Minimal Flavour Violation (MFV) and small bottom Yukawa coupling, where Delta=Delta_d=Delta_s=Delta_K^{tt}. We show that Delta must then be real, so that no new CP phases can be accomodated, and express the remaining parameters Delta_K^{cc} and Delta_K^{ct} in terms of Delta in this scenario. Scenario III is the generic MFV case with large bottom Yukawa couplings. In this case, the Kaon sector is uncorrelated to the B_d and B_s sectors. As in the second scenario one has Delta_d=Delta_s=Delta, however, now with a complex parameter Delta. Our quantitative analyses consist of global CKM fits within the Rfit frequentist statistical approach, determining the Standard Model parameters and the new physics parameters of the studied scenarios simultaneously. We find that the recent measurements indicating discrepancies with the Standard Model are well accomodated in Scenarios I and III with new mixing phases, with a slight preference for Scenario Ithat permits different new CP phases in the Bd and Bs systems. Within our statistical framework, we find that the Standard-Model hypothesis Delta_d=Delta_s=1 is disfavoured with p-values of 3.4 sigma and 3.1 sigma in Scenarios I and III respectively.