Light meson spectroscopy and gluonium searches in cand1S decays at BaBar Antimo Palano

2025-05-02 0 0 1010.55KB 20 页 10玖币

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Light meson spectroscopy and gluonium searches in ηcand Υ(1S) decays

at BaBar

Antimo Palano,

INFN Sezione di Bari, Italy

Abstract

We study the Υ

(1S)

radiative decays to

γπ+π−

and

γK+K−

using data recorded with the

BaBar detector operating at the SLAC PEP-II asymmetric-energy

e+e−

collider at center-of-

mass energies at the Υ

(2S)

and Υ

(3S)

resonances. The Υ

(1S)

resonance is reconstructed

from the decay Υ(

)

→π+π−

(1S)

3. We also study the processes

γγ →ηc→

η0K+K−

η0π+π−

, and

ηπ+π−

using a data sample of 519

fb−1

recorded with the BaBar

detector at center-of-mass energies at and near the Υ(

) (

4) resonances. A Dalitz

plot analysis is performed of

ηc

decays to

η0K+K−

η0π+π−

, and

ηπ+π−

. A new

(1700)

resonance is observed in the

ηπ±

invariant-mass spectrum from the

ηc→ηπ+π−

decay. We

compare

ηc

decays to

and

η0

ﬁnal states in association with scalar mesons as they relate to

the identiﬁcation of the scalar glueball.

On behalf of the BaBar Collaboration

Presented at the “Eighth Workshop on Theory, Phenomenology and Experiments in

Flavour Physics”-FPCapri2022

Jun 11 – 13, 2022, Villa Orlandi, Anacapri, Capri Island, Italy

arXiv:2210.00782v1 [hep-ex] 3 Oct 2022

1 Introduction

The existence of gluonium states is still an open issue for Quantum Chromodynamics (QCD).

Lattice QCD calculations predict the lightest gluonium states to have quantum numbers

JPC

and 2

and to be in the mass region below 2.5

GeV/c2

[1]. In particular, the

JPC

glueball

is predicted to have a mass around 1.7

GeV/c2

. The broad

(500),

(1370) [2],

(1500) [3, 4],

(1710) [5, 6] and possibly the

(2100) [7] have been suggested as scalar glueball candidates.

However, the identiﬁcation of the scalar glueball is complicated by the possible mixing with

standard q¯qstates.

Radiative decays of heavy quarkonia, in which a photon replaces one of the three gluons

from the strong decay of

J/ψ

or Υ

(1S)

, can probe color-singlet two-gluon systems that pro-

duce gluonic resonances.

J/ψ

decays have been extensively studied [8, 9]. In the ﬁrst BaBar

analysis [10] summarized in the present review, we study Υ

(1S)

decays, taking into account

that the experimental observation of radiative Υ

(1S)

decays is challenging because their rate is

suppressed by a factor of

≈

025 compared to

J/ψ

radiative decays, which are of order 10

−3

[11].

Decays of the

ηc

, the lightest pseudoscalar

c¯c

state, provide a window on light meson states. In

the second analysis [12] summarized in the present review, we consider the three-body

ηc

decays

η0K+K−

η0π+π−

, and

ηπ+π−

, using two-photon interactions,

e+e−→e+e−γ∗γ∗→e+e−ηc

. If

both of the virtual photons are quasi-real, the allowed

JPC

values of any produced resonances are

±+

, 2

±+

, 4

±+

... [13]. The possible presence of a gluonic component of the

η0

meson, due to the

so-called gluon anomaly, has been discussed in recent years [14, 15]. A comparison of the

and

η0

content of

ηc

decays might yield information on the possible gluonic content of resonances

decaying to π+π−or K+K−.

2 Study of Υ(1S)radiative decays to γπ+π−and γK+K−

2.1 Events reconstruction

We reconstruct the decay chains

Υ(2S)/Υ(3S)→(π+

sπ−

s)Υ(1S)→(π+

sπ−

s)(γπ+π−) (1)

and

Υ(2S)/Υ(3S)→(π+

sπ−

s)Υ(1S)→(π+

sπ−

s)(γK+K−),(2)

where we label with the subscript sthe slow pions from the direct Υ(2S) and Υ(3S) decays.

Events with balanced momentum are required to satisfy energy balance requirements. For

each combination of

π+

sπ−

candidates, we ﬁrst require both particles to be identiﬁed loosely as

pions and compute the recoiling mass

rec(π+

sπ−

s)=|pe++pe−−pπ+

s−pπ−

s|2,(3)

where

is the particle four-momentum. The distribution of

rec

(

π+

sπ−

) is expected to peak at

the squared Υ

(1S)

mass for signal events. Figure 1 shows the combinatorial recoiling mass

Mrec(π+

sπ−

s) for Υ(2S) and Υ(3S) data, where narrow peaks at the Υ(1S) mass can be observed.

) GeV/c

π(

rec

9.45 9.46 9.47

events/0.4 MeV/c

)S(2Υ(a)

) GeV/c

π(

rec

9.4 9.45 9.5

events/1.2 MeV/c

60 )S(3Υ(b)

Figure 1: Combinatorial recoiling mass

Mrec

π+

sπ−

candidates for (a) Υ

(2S)

and (b) Υ

(3S)

data. The

arrows indicate the regions used to select the Υ(1S) signal.

We select signal event candidates by requiring

|Mrec(π+

sπ−

s)−m(Υ(1S))f|<2.5σ, (4)

where

(Υ

(1S)

)

indicates the ﬁtted Υ

(1S)

mass value and

MeV/c2

and

σ=3.5 MeV/c2

for Υ

(2S)

and Υ

(3S)

data, respectively. To reconstruct Υ

(1S)→γπ+π−

or Υ

(1S)→γK+K−

decays, we require a loose identiﬁcation of both pions or kaons and isolate the two Υ

(1S)

decay

modes by requiring

9.1 GeV/c2<m(γh+h−)<9.6 GeV/c2,(5)

where h=π, K.

2.2 Study of the π+π−and K+K−mass spectra

The

π+π−

mass spectrum, for

(

π+π−

)

GeV/c2

and summed over the Υ

(2S)

and Υ

(3S)

datasets with 507 and 277 events, respectively, is shown in Fig. 2(Left). The spectrum shows

=0,

even++

resonance production, with low backgrounds above 1

GeV/c2

. We observe a

rapid drop around 1

GeV/c2

characteristic of the presence of the

(980), and a strong

(1270)

signal. The data also suggest the presence of additional weaker resonant contributions.

The

K+K−

mass spectrum, summed over the Υ

(2S)

and Υ

(3S)

datasets with 164 and 63

events, respectively, is shown in Fig. 2(Right) and also shows resonant production, with low

background. Signals at the positions of

(1525)

/f0

(1500) and

(1710) can be observed, with

further unresolved structure at higher mass.

We make use of a phenomenological model to extract the diﬀerent Υ

(1S)→γR

branching

fractions, where

is an intermediate resonance. We perform a simultaneous binned ﬁt to the

π+π−

mass spectra from the Υ

(2S)

and Υ

(3S)

datasets. We describe the low-mass region (around

the

(500)) using a relativistic

-wave Breit-Wigner lineshape having free parameters. We

describe the

(980) using the Flatt

e [16] formalism with parameters ﬁxed to the values from

ref. [17]. The

(1270) and

(1710) resonances are represented by relativistic Breit-Wigner

functions with parameters ﬁxed to PDG values [18]. In the high

π+π−

mass region we include a

) GeV/c

πm(

1 2

events/30 MeV/c

) GeV/c

m(K

1 1.5 2 2.5

events/60 MeV/c

Figure 2: (Left)

π+π−

mass distribution from Υ

(1S)→π+π−γ

for the combined Υ

(2S)

and Υ

(3S)

datasets.

The full (red) curves indicate the

-wave,

(1270), and

(1710) contributions. The shaded (gray) area

represents the estimated

(770)

background. (Right)

K+K−

mass distribution from Υ

(1S)→K+K−γ

for

the combined Υ

(2S)

and Υ

(3S)

datasets. The (red) curves show the contributions from

(1525)

/f0

(1500)

and f0(1710). Dashed (blue) lines indicate the background contributions.

single resonance

(2100) having a width ﬁxed to the PDG value (224

22) and unconstrained

mass. For the Υ

(3S)

data we also include

(770)

background with parameters ﬁxed to the

PDG values. The ﬁt is shown in Fig. 2. It has 16 free parameters and

χ2

=182 for ndf=152,

corresponding to a

-value of 5%. We note the observation of a signiﬁcant

-wave in Υ

(1S)

radiative decays. This observation was not possible in the study of

J/ψ

radiative decay to

π+π−

because of the presence of a strong, irreducible background from

J/ψ →π+π−π0

[19]. No

evidence is found for a Υ

(1S)→π+π−π0

decay in present data. We obtain the following

(500)

parameters:

m(f0(500)) =0.856 ±0.086 GeV/c2,Γ(f0(500)) =1.279 ±0.324 GeV,(6)

and

43 rad. The fraction of

-wave events associated with the

(500) is (27

3.1)%.

We perform a binned ﬁt to the combined

K+K−

mass spectrum using the following model.

The

(980) is parameterized according to the Flatt

e formalism. The

(1270),

(1525),

(1500),

and

(1710) resonances are represented by relativistic Breit-Wigner functions with parameters

ﬁxed to PDG values. We include an

(2200) contribution having parameters ﬁxed to the PDG

values. The ﬁt shown in Fig. 2(Right). It has six free parameters and

χ2

=35 for ndf=29,

corresponding to a

-value of 20%. The resonances yields and signiﬁcances are given in Table 1.

Systematic uncertainties are dominated by the PDG uncertainties on resonances parameters.

The eﬃciency distributions as functions of mass, for the Υ

(2S)

/Υ

(3S)

data and for the

π+π−γ

and

K+K−γ

ﬁnal states, are found to have an almost uniform behavior for all the ﬁnal states.

We deﬁne the helicity angle

θH

as the angle formed by the

, in the

h+h−

rest frame, and the

in the

h+h−γ

rest frame. We also deﬁne

θγ

as the angle formed by the radiative photon in

the

h+h−γ

rest frame with respect to the Υ

(1S)

direction in the Υ

(2S)

/Υ

(3S)

rest frame. We

iii

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Lightmesonspectroscopyandgluoniumsearchesincand(1S)decaysatBaBarAntimoPalano,INFNSezionediBari,ItalyAbstractWestudythe(1S)radiativedecaysto+andK+KusingdatarecordedwiththeBaBardetectoroperatingattheSLACPEP-IIasymmetric-energye+ecollideratcenter-of-massenergiesatthe(2S)and(3S)resonances.The(1S...

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Light meson spectroscopy and gluonium searches in cand1S decays at BaBar Antimo Palano

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