Observation of the decay Λ b 0  → pK − μ + μ − and a search for CP violation

Journal of High Energy Physics, Jun 2017

A search for CP violation in the decay Λ b 0  → pK − μ + μ − is presented. This decay is mediated by flavour-changing neutral-current transitions in the Standard Model and is potentially sensitive to new sources of CP violation. The study is based on a data sample of proton-proton collisions recorded with the LHCb experiment, corresponding to an integrated luminosity of 3 fb−1. The Λ b 0  → pK − μ + μ − decay is observed for the first time, and two observables that are sensitive to different manifestations of CP violation are measured, \( \Delta {\mathcal{A}}_{CP}\equiv {\mathcal{A}}_{CP}\left({\Lambda}_b^0\to p{K}^{-}{\mu}^{+}{\mu}^{-}\right)-{\mathcal{A}}_{CP}\left({\Lambda}_b^0\to p{K}^{-}J/\psi \right) \) and a \( {a}_{CP}^{\widehat{T}\hbox{-} odd} \), where the latter is based on asymmetries in the angle between the μ + μ − and pK − decay planes. These are measured to be $$ \begin{array}{l}\Delta {\mathcal{A}}_{CP}=\left(-3.5\pm 5.0\left(\mathrm{stat}\right)\pm 0.2\left(\mathrm{syst}\right)\right)\times {10}^{-2},\hfill \\ {}{a}_{CP}^{\widehat{T}\hbox{-} odd}=\left(1.2\pm 5.0\left(\mathrm{stat}\right)\pm 0.7\left(\mathrm{syst}\right)\right)\times {10}^{-2},\hfill \end{array} $$ and no evidence for CP violation is found.

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Observation of the decay Λ b 0  → pK − μ + μ − and a search for CP violation

Accepted: May Observation of the decay search for CP violation A search for CP violation in the decay B physics; CP violation; FCNC Interaction; Hadron-Hadron scattering (ex- - ! pK HJEP06(217)8 The LHCb collaboration 0 b ! pK is presented. This decay is mediated by avour-changing neutral-current transitions in the Standard Model and is potentially sensitive to new sources of CP violation. The study is based on a data sample of proton-proton collisions recorded with the LHCb experiment, corresponding to an integrated luminosity of 3 fb 1 . The b0 ! pK time, and two observables that are sensitive to di erent manifestations of CP violation are measured, ACP ACP ( b0 ! pK ACP ( b0 ! pK J= ) and aTb-odd, where CP the latter is based on asymmetries in the angle between the and pK decay planes. + decay is observed for the rst These are measured to be ACP = ( 3:5 aTCbP-odd = ( 1:2 5:0 (stat) 0:2 (syst)) 5:0 (stat) 0:7 (syst)) and no evidence for CP violation is found. 1 Introduction 2 3 4 5 6 7 Detector and simulation Selection of signal candidates Asymmetry measurements Systematic uncertainties Conclusions The LHCb collaboration ter its discovery in the neutral kaon system [ 1 ]. Within the Standard Model of par ticle physics (SM), CPV is incorporated by a single, irreducible weak phase in the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix [ 2, 3 ]. However, the amount of CPV in the SM is insu cient to explain the observed level of matter-antimatter asymmetry in the Universe [4{6]. Therefore, new sources of CPV beyond the SM are expected to exist. Experimental observations of CPV remain con ned to the B- and K-meson systems. Recently, the rst evidence for CPV in b0 ! p + was found at the level of 3:3 standard deviations [7] and a systematic study of CPV in beauty baryon decays has now begun. Among dedicated heavy- avour physics experiments, the LHCb detector [8] is unique in having access to a wide range of decay modes of numerous b-hadron species. Beauty baryons are produced copiously at the LHC, and within the LHCb detector acceptance the production ratio of B0 : b0 : Bs0 particles is approximately 4 : 2 : 1 [9]. The LHCb collaboration has previously searched for CPV in as well as in charmless 0 b ! pKS0 , b0 ! 0 b ! p and J= and 0 b ! h+h transitions [11{13]. . The leading-order transition amplitudes in the SM are described 1The inclusion of charge-conjugate processes is implied throughout this paper, unless stated otherwise. { 1 { Λ0 u b b Vqs μ − μ + s u } u u d } K− p b u d W − Vb∗q νμ W + q Vqs K− p and q represents one of the three up-type quarks u, c or t, the t-quark contribution being dominant. The uu pairs originate from the hadronization process. by the loop diagrams shown in gure 1. In extensions to the SM, new heavy particles could contribute to the amplitudes with additional weak phases, providing new sources of CPV [14, 15]. The limited amount of CPV predicted for the decay and Areco(p) are the reconstruction asymmetries for kaons and protons, mainly due to the di erent interaction cross-sections of particles and antiparticles with the detector material. By measuring the di erence of raw asymmetries between the signal and the Cabibbo0 favoured control mode b ! pK J= (! + metries cancel to a good approximation. No signi cant CPV is expected in the latter decay, since its amplitude is dominated by tree-level CP -conserving diagrams, which leads to ), the production and reconstruction asymACP ACP ( b0 ! pK Araw( b0 ! pK + + ) ) ACP ( b0 ! pK J= ) Araw( b0 ! pK J= ): { 2 { (2.1) (2.2) (2.3) μ + Λ 0 b triple products of the nal-state particle momenta in the A pair of Tb-odd and P -odd observables, A T and AT , is obtained by de ning the Tb-odd b b b0 rest frame C C T b T b p~ + (p~p p~ (p~p p~K ); p~K+ ); and taking the asymmetries A T b N (CT > 0) b N (CT > 0) + N (CT < 0) b b N (CT < 0) b ; A T b N ( C N ( C T > 0) b T > 0) + N ( C b N ( C T < 0) b T < 0) b ; b0 rest frame, as shown in gure 2. sin the where N (N ) is the number of b0 ( b0) signal candidates. These asymmetries are measured from the angular distributions of the decay products, with C [18], where is the angle between the decay planes of the + b T being proportional to Following ref. [18], CP -odd and P -odd observables are de ned as The observables A T are P - and Tb-odd but are not sensitive to CPV e ects [17]. b ACP and aTb-odd are sensitive to di erent manifestations of CPV [17]. CP The CP asymmetry ACP depends on the interference of Tb-even amplitudes, de ned as (2.4) (2.5) (2.6) (2.7) aje exp hi( je + je)i, which have a relative CP -even strong phase 1e 2e and a relative CP ACP / ae1ae2 sin( 1e 2e) sin( e1 e2): tive CP -odd weak phase e1 o 1 , The convention used to de ne strong and weak phases is such that all CPV e ects are encoded in the CP -odd weak phases. phase di erence between the two amplitudes is large. Therefore, ACP is enhanced when the strong (...truncated)


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Lyu, F. Machefert. Observation of the decay Λ b 0  → pK − μ + μ − and a search for CP violation, Journal of High Energy Physics, 2017, pp. 1-17, Volume 2017, Issue 6, DOI: 10.1007/JHEP06(2017)108