Observation of the Bs2DstDst decay

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)

CERN-EP-2022-193

LHCb-PAPER-2022-023

July 13, 2023

Observation of the B0

s→D∗+D∗−

decay

LHCb collaboration†

Abstract

The ﬁrst observation of the

s→D∗+D∗−

decay and the measurement of its

branching ratio relative to the

B0→D∗+D∗−

decay are presented. The data

sample used corresponds to an integrated luminosity of 9

fb−1

of proton-proton

collisions recorded by the LHCb experiment at centre-of-mass energies of 7, 8 and

13 TeV between 2011 and 2018. The decay is observed with more than 10 standard

deviations and the time-integrated ratio of branching fractions is determined to be

B(B0

s→D∗+D∗−)

B(B0→D∗+D∗−)= 0.269 ±0.032 ±0.011 ±0.008 ,

where the ﬁrst uncertainty is statistical, the second systematic and the third due

to the uncertainty of the fragmentation fraction ratio

fs/fd

. The

s→D∗+D∗−

branching fraction is calculated to be

B(B0

s→D∗+D∗−) = (2.15 ±0.26 ±0.09 ±0.06 ±0.16) ×10−4,

where the fourth uncertainty is due to the

B0→D∗+D∗−

branching fraction. These

results are calculated using the average

meson lifetime in simulation. Correction

factors are reported for scenarios where either a purely heavy or a purely light

eigenstate is considered.

Published in JHEP 07 (2023) 119

†Authors are listed at the end of this paper.

arXiv:2210.14945v2 [hep-ex] 17 Jul 2023

1 Introduction

Decays of

mesons into two charm mesons can be used to probe elements of the

Cabibbo–Kobayashi–Maskawa (

CKM

) matrix [1, 2]. Measurements of

violation in

B0→D(∗)+D(∗)−

s→D(∗)+D(∗)−

and

s→D(∗)+

sD(∗)−

decays can be used to deter-

mine the

CKM

angles

–

8] and

βs

[9], although

violation in

s→D∗+D∗−

and

s→D∗+

sD∗−

has not yet been measured. These determinations are aﬀected by higher-

order Standard Model eﬀects [10

–

14], which can be constrained using measurements

violation parameters or branching ratios in additional decays of the

B→DD

family,

e.g.

the

s→D∗+D∗−

decay

[15, 16]. In the

s→D∗+D∗−

decay, tree and

penguin transitions cannot contribute. This makes these decays sensitive to the eﬀects of

-exchange and penguin-annihilation diagrams. The dominant Feynman diagrams of

the family of

B→DD

decays are shown in Fig. 1. The absolute branching fraction of

the

s→D∗+D∗−

decay is predicted to be (3

+1.3

−1.1

)

−4

using a perturbative QCD

approach based on kTfactorisation [17].

d d

W+

D(∗)+

D(∗)−

b d

d d

W+

u, c, t

D(∗)+

D(∗)−

d(s)

(s)

D(∗)+

D(∗)−

d(s)

(s)

D(∗)−

D(∗)+

Figure 1: Dominant Feynman diagrams for

B→DD

decays comprising (top left) tree, (top

right) penguin, (bottom left) W-exchange and (bottom right) penguin-annihilation transitions.

The

s→D∗+D∗−

decay can only occur via the

-exchange or penguin-annihilation diagram.

In this paper, the ﬁrst observation of the

s→D∗+D∗−

decay is presented. Its

branching fraction is measured relative to that of the

B0→D∗+D∗−

decay, thereby

canceling systematic uncertainties originating from the uncertainty on the integrated

luminosity and the

cross section. The measurement is performed with data collected

by the

LHCb

experiment, where proton beams collide at centre-of-mass energies up to

1Charge conjugation is implied throughout the paper.

TeV

. In 2011 and 2012 data samples corresponding to 1

fb−1

and 2

fb−1

were collected

at centre-of-mass energies of 7 and 8

TeV

, respectively, and from 2015 to 2018 a data

sample corresponding to 6

fb−1

was collected at 13

TeV

. Due to the diﬀerent centre-of-mass

energies and due to diﬀerent detector settings, the analysis is performed separately for

the two data-taking periods 2011–2012 (Run 1) and 2015–2018 (Run 2), combining both

results in the end.

The measurement of the ratio of branching fractions relies on the calculation of the

ratio of selection eﬃciencies and the determination of the signal yields for the two decays.

Therefore, the analysis begins with the selection of signal and control mode candidates in

Sec. 3 and continues with the extraction of the

and

yields using mass ﬁts in Sec. 4.

The systematic uncertainties are evaluated in Sec. 5 and the ﬁnal calculation of the ratio

of branching fractions is presented in Sec. 6.

2 Detector and simulation

The

LHCb

detector [18, 19] is a single-arm forward spectrometer covering the

pseudorapidity

range 2

< η <

5, designed for the study of particles containing

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

strip vertex detector surrounding the proton-proton (

) interaction region, a large-area

silicon-strip detector located upstream of a dipole magnet with a bending power of about

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

of the magnet. 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 (

the impact parameter (

), 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. Photons, electrons and hadrons are identiﬁed by a calorimeter system con-

sisting 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.

Simulation is used to calculate the selection eﬃciencies and to determine the shapes of

the mass distributions. In the simulation,

collisions are generated using

Pythia

[20]

with a speciﬁc

LHCb

conﬁguration [21]. The

EvtGen

package [22] is used to decay

unstable particles, with ﬁnal-state radiation generated using

Photos

[23]. The interaction

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

Geant4 toolkit [24, 25] as described in Ref. [26].

3 Selection of candidates

The online event selection is performed by a trigger [27,28], 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. At the hardware trigger stage, events are

required to have a muon with high

or a hadron, photon or electron with high transverse

energy in the calorimeters. The software trigger requires a two-, three- or four-track

secondary vertex with a signiﬁcant displacement from any primary

interaction vertex.

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摘要：

EUROPEANORGANIZATIONFORNUCLEARRESEARCH(CERN)CERN-EP-2022-193LHCb-PAPER-2022-023July13,2023ObservationoftheB0s→D∗+D∗−decayLHCbcollaboration†AbstractThefirstobservationoftheB0s→D∗+D∗−decayandthemeasurementofitsbranchingratiorelativetotheB0→D∗+D∗−decayarepresented.Thedatasampleusedcorrespondstoanintegr...

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Observation of the Bs2DstDst decay

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