Measurement of the ratio of branching fractions BRBc2BsPiBRBc2JpsiPi

2025-05-02 0 0 912.13KB 20 页 10玖币

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)

CERN-EP-2022-204

LHCb-PAPER-2022-034

July 20, 2023

Measurement of the ratio of

branching fractions

B(B+

c→B0

sπ+)/B(B+

c→J/ψπ+)

LHCb collaboration†

Abstract

The ratio of branching fractions of

B+

c→B0

sπ+

and

B+

c→J/ψπ+

decays is

measured with proton-proton collision data of a centre-of-mass energy of 13

TeV

The data were collected with the LHCb experiment during 2016–2018, corresponding

to an integrated luminosity of 5

fb−1

. The

mesons are reconstructed via the

decays

s→J/ψϕ

and

s→D−

sπ+

. The ratio of branching fractions is measured

to be

(

B+

c→B0

sπ+

)

(

B+

c→J/ψπ+

) = 91

3 where the ﬁrst uncertainty

is statistical, the second is systematic and the third is due to the knowledge of the

branching fractions of the intermediate state decays.

Published in JHEP 07 (2023) 066

†Authors are listed at the end of this paper.

arXiv:2210.12000v3 [hep-ex] 19 Jul 2023

1 Introduction

The

B(∗)+

family of mesons is the only one formed by two diﬀerent heavy ﬂavour quarks

(

). Both the

quark and the

quark can each decay with the other as a spectator,

leading to ﬁnal states such as

J/ψπ+

and

sπ+

. In addition, the

quark and the

quark

can annihilate via a W+boson, allowing pure leptonic ﬁnal states such as τ+ντ.

The

B+

c→B0

sπ+

decay

was ﬁrst observed in 2013 by the LHCb collaboration [1], and

can be used to tag the initial ﬂavour of the

meson with the charge of the accompanying

π+

meson. The

B+

c→B0

sπ+

decay is also the ﬁrst observed case of one

meson decaying

weakly into another

meson. This property makes the decay ideal for testing theoretical

models. The branching fraction of the

B+

c→B0

sπ+

decay should be large as it is a

Cabibbo-favoured decay. There are several predictions for the branching fraction of the

B+

c→B0

sπ+

decay based on QCD sum rules or quark-potential models, which range

between 2.5% and 16.4% [2

–

9]. A precise measurement of the branching fraction of the

B+

c→B0

sπ+decay will improve the understanding of the B+

ctheory models.

The

B+

c→τ+ντ

decay is highly sensitive to new physics eﬀects [10

–

12] but experimental

accessibility to this mode is limited. Due to the large branching fraction of the

B+

c→B0

sπ+

decay, its improved measurement contributes to a more stringent limit on the

B+

c→τ+ντ

decay via the

B+

total decay width, depending on the theoretical model assumed [11,13,14].

In this paper, the ratio between the branching fractions of the

B+

c→B0

sπ+

and

B+

c→

J/ψπ+

decays is measured, using proton-proton (

) collision data collected with the LHCb

experiment between 2016 and 2018 at a centre-of-mass energy of 13

TeV

, corresponding

to an integrated luminosity of 5

fb−1

. The ratio of branching fractions is measured

separately using the

s→J/ψ

(

→µ+µ−

)

(

→K+K−

) and

s→D−

(

→K+K−π−

)

π+

decays. These two measurements are then combined. The ratio of branching fractions is

evaluated as

RX≡B(B+

c→B0

sπ+)

B(B+

c→J/ψπ+)=NB+

c→B0

sπ+

NB+

c→J/ψπ+·ϵB+

c→J/ψπ+

ϵB+

c→B0

sπ+·1

int

,(1)

where

is the signal yield,

is the total eﬃciency to reconstruct these decays, and

int

is the ratio of branching fractions of the corresponding intermediate state decays, as

shown in Table 1. The

represents the ﬁnal state of the

decay,

J/ψϕ

D−

sπ+

. The

superscripts or subscripts

J/ψϕ

and

D−

sπ+

are used to indicate the

B+

c→B0

(

→J/ψϕ

)

π+

and B+

c→B0

s(→D−

sπ+)π+decay modes. The RX

int are deﬁned as

RJ/ψϕ

int ≡ B(B0

s→J/ψϕ)· B(ϕ→K+K−),(2)

RD−

sπ+

int ≡ B(B0

s→D−

sπ+)· B(D−

s→K+K−π−)/B(J/ψ →µ+µ−) (3)

with corresponding values of

RJ/ψϕ

int

= (5

20)

−4

and

RD−

sπ+

int

= (2

14)

−3

2 Detector and simulation

The

LHCb

detector [16, 17] is a single-arm forward spectrometer covering the

pseudorapidity

range 2

< η <

5, designed for the study of particles containing

1Inclusion of charge conjugate processes is implied throughout this paper.

Table 1: Branching fractions of intermediate-state decays [15].

Decay Branching fraction [%]

D−

s→K+K−π−5.38 ±0.10

s→D−

sπ+0.298 ±0.014

s→J/ψϕ 0.104 ±0.004

ϕ→K+K−49.1±0.5

J/ψ →µ+µ−5.961 ±0.033

quarks. The detector includes a high-precision tracking system consisting of a silicon-

strip vertex detector surrounding the

interaction region [18], a large-area silicon-strip

detector located upstream of a dipole magnet with a bending power of about 4 Tm, and

three stations of silicon-strip detectors and straw drift tubes placed downstream of the

magnet [19]. The tracking system provides a measurement of the momentum,

, of charged

particles with a relative uncertainty that varies from 0.5% at low momentum to 1.0% at

200

GeV/c

. The minimum distance of a track to a primary

collision vertex (PV), the

impact parameter (IP), is measured with a resolution of (15 + 29

/pT

)

µm

, where

the component of the momentum transverse to the beam, in

GeV/c

. Diﬀerent types of

charged hadrons are distinguished using information from two ring-imaging Cherenkov

detectors [20]. Photons, electrons and hadrons are identiﬁed by a calorimeter system

consisting of scintillating-pad and preshower detectors, an electromagnetic and a hadronic

calorimeter. Muons are identiﬁed by a system composed of alternating layers of iron

and multiwire proportional chambers [21]. The online event selection is performed by a

trigger [22], which consists of a hardware stage, based on information from the calorimeter

and muon systems, followed by a software stage, which applies a full event reconstruction.

Simulation is required to model the eﬀects of the detector acceptance and the imposed

selection requirements. In the simulation,

collisions are generated using

Pythia

8 [23]

with a speciﬁc

LHCb

conﬁguration [24]. A dedicated generator

Bcvegpy

[25] is used

to simulate the production of

B+

mesons. Decays of unstable particles are described

EvtGen

[26], in which ﬁnal-state radiation is generated using

Photos

[27]. The

interaction of the generated particles with the detector, and its response, are implemented

using the Geant4 toolkit [28] as described in Ref. [29].

3 Event selection

In the

B+

c→B0

sπ+

decay mode, the

meson is reconstructed using two decay modes,

s→J/ψϕ

and

s→D−

sπ+

, with the intermediate states selected based on the

D−

s→K+K−π−

J/ψ →µ+µ−

and

ϕ→K+K−

decays. The reconstructed

can-

didates are selected using the boosted decision tree (BDT) [30

–

32] classiﬁer used in

Refs. [1, 33], with the same working points. In the

B+

c→J/ψπ+

decay channel, the

J/ψ

mesons are reconstructed using a pair of oppositely charged muons. The selected

and

J/ψ

candidates are combined with a track identiﬁed as a pion to reconstruct

the

B+

candidates. These candidates and all intermediate states are required to have

good vertex-ﬁt quality. All ﬁnal state particles (pions, kaons and muons) are required

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EUROPEANORGANIZATIONFORNUCLEARRESEARCH(CERN)CERN-EP-2022-204LHCb-PAPER-2022-034July20,2023MeasurementoftheratioofbranchingfractionsB(B+c→B0sπ+)/B(B+c→J/ψπ+)LHCbcollaboration†AbstractTheratioofbranchingfractionsofB+c→B0sπ+andB+c→J/ψπ+decaysismeasuredwithproton-protoncollisiondataofacentre-of-massener...

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Measurement of the ratio of branching fractions BRBc2BsPiBRBc2JpsiPi

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