Physics Letters B 757 (2016) 97–120 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb Measurement of the CP-violating weak phase φs and the decay width 0 → difference √s using the Bs J/ψ φ(1020) decay channel in pp collisions at s = 8TeV .CMS Collaboration CERN, Switzerland a r t i c l e i n f o a b s t r a c t Article history: 0 0 The CP-violating weak phase φs of the Bs meson and the decay width difference s of the Bs Received 27 July 2015 light and heavy mass eigenstates are measured with the CMS detector at the LHC using a data sam- Received in revised form 11 March 2016 0 → → + − + − ple of Bs J/ψ φ(1020) μ μ K K decays. The analysed data set corresponds to an integrated Accepted 16 March 2016 − luminosity of 19.7 fb 1 collected in pp collisions at a centre-of-mass energy of 8 TeV. A total of Available online 23 March 2016 0 49 200 reconstructed Bs decays are used to extract the values of φs and s by performing a time- Editor: M. Doser + − + − dependent and flavour-tagged angular analysis of the μ μ K K final state. The weak phase is mea- Keywords: sured to be φs =−0.075 ± 0.097 (stat) ± 0.031 (syst) rad, and the decay width difference is s = −1 CMS 0.095 ± 0.013 (stat) ± 0.007 (syst) ps . Physics © 2016 CERN for the benefit of the CMS Collaboration. Published by Elsevier B.V. This is an open access B-physics 3 article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP . CP violation 0 1. Introduction heavy Bs mass eigenstates BL and BH, assuming no new physics in 0 0 = − = ± −1 Bs –Bs mixing, is s L H 0.087 0.021 ps [4]. While no direct evidence of physics beyond the standard model The weak phase φs was first measured by the Tevatron exper- 0 iments [5–8], and then at the LHC by the LHCb and ATLAS ex- (SM) has yet been found at the CERN LHC, the Bs meson provides 0 → 0 → a rich source of possibilities to probe its consistency. In this Let- periments [9–13], using Bs J/ψ φ(1020), Bs J/ψ f0(980), and 0 0 → + − + − + − ter, a measurement of the weak phase φs of the Bs meson and the Bs J/ψ π π decays to h h , where denotes a muon in 0 decay width difference s between the light and heavy Bs mass the present analysis and h stands for a kaon or a pion. Final states eigenstates is presented, using the data collected by the CMS ex- that do not have a single CP eigenvalue require an angular analysis periment in pp collisions at the LHC with a centre-of-mass energy to disentangle the CP-odd and CP-even components. The time- −1 of 8 TeV, corresponding to an integrated luminosity of 19.7 fb . dependent angular analysis can be performed by measuring the + − + − The CP-violating weak phase φs originates from the interfer- decay angles of the final-state particles h h and the proper 0 0 ence between direct Bs meson decays into a CP eigenstate ccss decay time of the Bs multiplied by the speed of light [14], re- 0 0 0 → and decays through B –B mixing to the same final state. Ne- ferred to as ct in what follows. In this Letter, the Bs J/ψ φ(1020) s s + − + − glecting penguin diagram contributions [1,2], φs is related to the decay to the final state μ μ K K is analysed, and possible ad- elements of the Cabibbo–Kobayashi–Maskawa quark mixing matrix ditional contributions to the result from the nonresonant decay ∗ ∗ 0 + − − = − B → J/ψ K K are taken into account by including a term for an by φs 2βs, where βs arg( V ts V tb/V cs V cb). The prediction for s 2βs, determined via a global fit to experimental data within the additional amplitude (S-wave) in the fit. +0.0016 SM, is 2βs = 0.0363− rad [3]. Since the value predicted by In this measurement the transversity basis is used [14]. The 0.0015 = the SM is very precise, any significant deviation of the measured three angles (θT, ψT, ϕT) of the transversity basis are illus- value from this prediction would be particularly interesting, as it trated in Fig. 1. The angles θT and ϕT are the polar and azimuthal + would indicate a possible contribution of new, unknown particles angles, respectively, of the μ in the rest frame of the J/ψ where 0 the x axis is defined by the direction of the 1020 meson in the to the loop diagrams describing Bs mixing. The theoretical pre- φ( ) diction for the decay width difference s between the light and J/ψ rest frame, and the x–y plane is defined by the decay plane of + − + the φ(1020) → K K . The helicity angle ψT is the angle of the K in the φ(1020) rest frame with respect to the negative J/ψ mo- E-mail address: [email protected]. mentum direction. http://dx.doi.org/10.1016/j.physletb.2016.03.046 0370-2693/© 2016 CERN for the benefit of the CMS Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. 98 CMS Collaboration / Physics Letters B 757 (2016) 97–120 Table 1 Angular and time-dependent terms of the signal model. igi (θT,ψT, ϕT) Ni ai bi ci di 2 2 2 2 12cosψT(1 − sin θT cos ϕT) |A0(0)| 1 DC−S 2 2 2 2 2sinψT(1 − sin θT sin ϕT) |A(0)| 1 DC−S 2 2 2 3sinψT sin θT |A⊥(0)| 1 −DCS 2 4 − sin ψT sin 2θT sin ϕT |A(0)||A⊥(0)| C sin(δ⊥ − δ) S cos(δ⊥ − δ) sin(δ⊥ − δ) D cos(δ⊥ − δ) √1 2 5 sin 2ψT sin θT sin 2ϕT |A0(0)||A(0)| cos(δ − δ0) D cos(δ − δ0) C cos(δ − δ0) −S cos(δ − δ0) 2 √1 6 sin 2ψT sin 2θT sin ϕT |A0(0)||A⊥(0)| C sin(δ⊥ − δ0) S cos(δ⊥ − δ0) sin(δ⊥ − δ0) D cos(δ⊥ − δ0) 2 7 2 (1 − sin2 θ cos2 ) |A (0)|2 1 −DCS 3 √ T ϕT S 2 8 1 6sinψ sin θ sin 2 |A (0)||A(0)| C cos(δ − δ ) S sin(δ − δ ) cos(δ − δ ) D sin(δ − δ ) 3 √ T T ϕT S S S S S 9 1 6sinψ sin 2θ cos |A (0)||A⊥(0)| sin(δ⊥ − δ ) −D sin(δ⊥ − δ ) C sin(δ⊥ − δ ) S sin(δ⊥ − δ ) 3 √ T T ϕT S S S S S 4 − 2 2 | || | − − − − 10 3 3cosψT(1 sin θT cos ϕT) A S (0) A0(0) C cos(δ0 δS ) S sin(δ0 δS ) cos(δ0 δS ) D sin(δ0 δS ) simplified as λ f = η f λ, where η f is the CP eigenvalue. The amount of CP violation in mixing is assumed to be negligible [17]. Thus, no | | 0 q/p terms are used in Eq. (1) when going from the Bs model to 0 the Bs model. Since direct CP violation is expected to be small the- oretically [3] and is measured to be small [9], |λ| is set to 1.0. 2. The CMS detector The central feature of the CMS apparatus is a 13 m long super- conducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Within the solenoid volume are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter, Fig. 1. Definition of the three angles θT, ψT, and ϕT describing the decay topology 0 → and a brass and scintillator hadron calorimeter, each composed of of Bs J/ψ φ(1020). See text for details. a barrel and two endcap sections. Muons are measured in gas- 0 → The differential decay rate of Bs J/ψ φ(1020) is represented ionisation detectors embedded in the steel flux-return yoke outside using the function f ( , ct, α) as in Ref. [15]: the solenoid. Extensive forward calorimetry complements the cov- erage provided by the barrel and endcap detectors. 4 0 10 d Bs The main subdetectors used for the present analysis are = f ( , ct, α) ∝ O i(ct, α) gi( ), (1) d d(ct) the silicon tracker and the muon detection system. The sili- i=1 con tracker measures charged particles within the pseudorapidity 2 where O i are time-dependent functions, gi are angular functions, range |η| < 2.5. It consists of 66 million 100×150 μm silicon pix- and α is a set of physics parameters. els and more than 9 million silicon strips. For nonisolated particles The functions O (ct, α) are: of transverse momentum 1 < p < 10 GeV and |η| < 1.4, the track i T resolutions are typically 1.5% in pT and 25–90 (45–150) μm in the −ct/cτ st st O i(ct, α) = Nie ai cosh + bi sinh transverse (longitudinal) impact parameter [18]. 2 2 Muons are measured in the pseudorapidity range |η| < 2.4, with detection planes made using three technologies: drift tubes, + ci cos (mst) + di sin (mst) , cathode strip chambers, and resistive plate chambers.