Measurement of theCPAsymmetry inBs0−B¯s0Mixing

Abstract

The CP asymmetry in the mixing of B Correcting the observed charge asymmetry for detection and background effects, the CP asymmetry is found to be a s sl ¼ ð0.39 AE 0.26 AE 0.20Þ%, where the first uncertainty is statistical and the second systematic. This is the most precise measurement of a s sl to date. It is consistent with the prediction from the standard model and will constrain new models of particle physics. DOI: 10.1103/PhysRevLett.117.061803 When neutral B mesons evolve in time they can change into their own antiparticles. This quantum-mechanical phenomenon is known as mixing and occurs in both neutral B meson systems, B 0 and B 0 s , where B is used to refer to either system. In this mixing process, the CP (chargeparity) symmetry is broken if the probability for a B meson to change into aB meson is different from the probability for the reverse process. This effect can be measured by studying decays into flavor-specific final states, B → f, such thatB → f transitions can only occur through the mixing processB → B → f. Such processes include semileptonic B decays, as the charge of the lepton identifies the flavor of the B meson at the time of its decay. The magnitude of the CP-violating asymmetry in B mixing can be characterized by the semileptonic asymmetry a sl . This is defined in terms of the partial decay rates, Γ, to semileptonic final states aswhere Δm (ΔΓ) is the difference in mass (decay width) between the mass eigenstates of the B system and ϕ 12 is a CP-violating phase [1]. In the standard model (SM), the asymmetry is predicted to be as small as a The high oscillation frequency Δm s reduces the effect of the small asymmetry in the production rates between B 0 s andB 0 s mesons in pp collisions by a factor 10 −3 [7,8]. Neglecting corrections, the untagged, time-integrated asymmetry is A raw ¼ a s sl =2, where the factor 2 reduction compared to the tagged asymmetry in Eq. (1) comes from the summation over mixed and unmixed decays. The tagged asymmetry would actually suffer from a larger reduction because of the tagging efficiency [9,10]. The * Full author list given at the end of the article.Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. PRL 117, 061803 (2016) P H Y S I C A L R E V I E W L E T T E R S week ending 5 AUGUST 20160031-9007=16=117(6)=061803 (9) 061803-1 © 2016 CERN, for the LHCb Collaboration unmixed decays have zero asymmetry due to CPT symmetry. The raw asymmetry is still affected by possible differences in detection efficiency for the two chargeconjugate final states and by backgrounds from otherwhere A det is the detection asymmetry, which is assessed from data using calibration samples, f bkg is the fraction of the b-hadron background, and A bkg the background asymmetry.The LHCb detector is a single-arm forward spectrometer designed for the study of particles containing b or c...

The CKM Parameters

“…This measurement is generally interpreted as a combination of the semileptonic asymmetries a d SL and a s SL in B 0 and B 0 s decays, respectively, which measure CP violation in mixing. Direct measurements of a d SL and a s SL at B factories [185][186][187], DØ [188,189], and LHCb [190,191] are consistent with the SM prediction and in tension with the DØ asymmetry. The origin of this discrepancy is still under investigation [192], as we discuss further in Section 4.2.1.…”

On the ultimate precision of meson mixing observables

“…The theory prediction uses conservative ranges for the input parameters -we will present a more aggressive estimate in Sec. 3 The experimental average for a s sl has been taken from [59]. Experiment and theory agree very well for the quantities ∆M q and ∆Γ s .…”

Averages of b-hadron, c-hadron, and $$\tau $$ τ -lepton properties as of summer 2016

“…Finally, direct determinations of A s SL , also shown in Table 20, are obtained by D0 [235] and LHCb [236] from the timeintegrated charge asymmetry of untagged B 0…”