Abstract
The standard model of particle physics currently provides our best description of fundamental particles and their interactions. The theory predicts that the different charged leptons, the ...electron, muon and tau, have identical electroweak interaction strengths. Previous measurements have shown that a wide range of particle decays are consistent with this principle of lepton universality. This article presents evidence for the breaking of lepton universality in beauty-quark decays, with a significance of 3.1 standard deviations, based on proton–proton collision data collected with the LHCb detector at CERN’s Large Hadron Collider. The measurements are of processes in which a beauty meson transforms into a strange meson with the emission of either an electron and a positron, or a muon and an antimuon. If confirmed by future measurements, this violation of lepton universality would imply physics beyond the standard model, such as a new fundamental interaction between quarks and leptons.
Study of the doubly charmed tetraquark Tcc Abdelmotteleb, A. S. W; Ajaltouni, Z; Andreianov, A ...
Nature communications,
12/2022, Letnik:
13, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Quantum chromodynamics, the theory of the strong force, describes interactions of coloured quarks and gluons and the formation of hadronic matter. Conventional hadronic matter consists of baryons and ...mesons made of three quarks and quark-antiquark pairs, respectively. Particles with an alternative quark content are known as exotic states. Here a study is reported of an exotic narrow state in the D0D0π+ mass spectrum just below the D*+D0 mass threshold produced in proton-proton collisions collected with the LHCb detector at the Large Hadron Collider. The state is consistent with the ground isoscalar Tcc+ tetraquark with a quark content of ccu¯d¯ and spin-parity quantum numbers JP = 1+. Study of the DD mass spectra disfavours interpretation of the resonance as the isovector state. The decay structure via intermediate off-shell D*+ mesons is consistent with the observed D0π+ mass distribution. To analyse the mass of the resonance and its coupling to the D*D system, a dedicated model is developed under the assumption of an isoscalar axial-vector Tcc+ state decaying to the D*D channel. Using this model, resonance parameters including the pole position, scattering length, effective range and compositeness are determined to reveal important information about the nature of the Tcc+ state. In addition, an unexpected dependence of the production rate on track multiplicity is observed.The existence and properties of tetraquark states with two heavy quarks and two light antiquarks have been widely debated. Here, the authors use a unitarized model to study the properties of an exotic narrow state compatible with a doubly charmed tetraquark.
Abstract
Conventional, hadronic matter consists of baryons and mesons made of three quarks and a quark–antiquark pair, respectively
1,2
. Here, we report the observation of a hadronic state ...containing four quarks in the Large Hadron Collider beauty experiment. This so-called tetraquark contains two charm quarks, a
$$\overline{{{u}}}$$
u
¯
and a
$$\overline{{{d}}}$$
d
¯
quark. This exotic state has a mass of approximately 3,875 MeV and manifests as a narrow peak in the mass spectrum of
D
0
D
0
π
+
mesons just below the
D
*+
D
0
mass threshold. The near-threshold mass together with the narrow width reveals the resonance nature of the state.
A
bstract
Coherent production of
J/ψ
mesons is studied in ultraperipheral lead-lead collisions at a nucleon-nucleon centre-of-mass energy of 5 TeV, using a data sample collected by the LHCb ...experiment corresponding to an integrated luminosity of about 10 μb
−1
. The
J/ψ
mesons are reconstructed in the dimuon final state and are required to have transverse momentum below 1 GeV. The cross-section within the rapidity range of 2
.
0
< y <
4
.
5 is measured to be 4
.
45 ± 0
.
24 ± 0
.
18 ± 0
.
58 mb, where the first uncertainty is statistical, the second systematic and the third originates from the luminosity determination. The cross-section is also measured in
J/ψ
rapidity intervals. The results are compared to predictions from phenomenological models.
A
bstract
A precision measurement of the
Z
boson production cross-section at
s
= 13 TeV in the forward region is presented, using
pp
collision data collected by the LHCb detector, corresponding to an ...integrated luminosity of 5.1 fb
−
1
. The production cross-section is measured using
Z → μ
+
μ
−
events within the fiducial region defined as pseudorapidity 2
.
0
< η <
4
.
5 and transverse momentum
p
T
>
20 GeV
/c
for both muons and dimuon invariant mass 60
< M
μμ
<
120 GeV
/c
2
. The integrated cross-section is determined to be
σ
Z
→
μ
+
μ
−
=
196.4
±
0.2
±
1.6
±
3.9
pb
,
where the first uncertainty is statistical, the second is systematic, and the third is due to the luminosity determination. The measured results are in agreement with theoretical predictions within uncertainties.
A
bstract
The cross-sections of exclusive (coherent) photoproduction
J/ψ
and
ψ
(2S) mesons in ultra-peripheral PbPb collisions at a nucleon-nucleon centre-of-mass energy of 5
.
02 TeV are measured ...using a data sample corresponding to an integrated luminosity of 228 ± 10 μb
−
1
, collected by the LHCb experiment in 2018. The differential cross-sections are measured separately as a function of transverse momentum and rapidity in the nucleus-nucleus centre-of-mass frame for
J/ψ
and
ψ
(2S) mesons. The integrated cross-sections are measured to be
σ
J
/
ψ
coh
= 5
.
965
±
0
.
059
±
0
.
232
±
0
.
262 mb and
σ
ψ
2
S
coh
= 0
.
923
±
0
.
086
±
0
.
028
±
0
.
040 mb, where the first listed uncertainty is statistical, the second systematic and the third due to the luminosity determination. The cross-section ratio is measured to be
σ
ψ
2
S
coh
/
σ
J
/
ψ
coh
= 0
.
155
±
0
.
014
±
0
.
003, where the first uncertainty is statistical and the second is systematic. These results are compatible with theoretical predictions.
A
bstract
The CKM angle
γ
is determined from
CP
-violating observables measured in
B
±
→
D
K
∓
π
±
π
±
π
∓
h
±
, (
h
=
K, π
) decays, where the measurements are performed in bins of the decay ...phase-space of the
D
meson. Using proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7
,
8 and 13 TeV, corresponding to a total integrated luminosity of 9 fb
−
1
,
γ
is determined to be
γ
=
54.8
+
6.0
−
5.8
+
0.6
−
0.6
+
6.7
−
4.3
∘
,
where the first uncertainty is statistical, the second systematic and the third from the external inputs on the coherence factors and strong phases of the
D
-meson decays.
A
bstract
The production cross-section of
J/ψ
pairs in proton-proton collisions at a centre-of-mass energy of
s
= 13 TeV is measured using a data sample corresponding to an integrated luminosity of 4
....
2 fb
−
1
collected by the LHCb experiment. The measurement is performed with both
J/ψ
mesons in the transverse momentum range 0
< p
T
<
14 GeV/
c
and rapidity range 2
.
0
< y <
4
.
5. The cross-section of this process is measured to be 16
.
36 ± 0
.
28 (stat) ± 0
.
88 (syst) nb. The contributions from single-parton scattering and double-parton scattering are separated based on the dependence of the cross-section on the absolute rapidity difference ∆
y
between the two
J/ψ
mesons. The effective cross-section of double-parton scattering is measured to be
σ
eff
= 13
.
1
±
1
.
8 (stat)
±
2
.
3 (syst) mb. The distribution of the azimuthal angle
ϕ
CS
of one of the
J/ψ
mesons in the Collins-Soper frame and the
p
T
-spectrum of the
J/ψ
pairs are also measured for the study of the gluon transverse-momentum dependent distributions inside protons. The extracted values of ⟨cos 2
ϕ
CS
⟩ and ⟨cos 4
ϕ
CS
⟩ are consistent with zero, but the presence of azimuthal asymmetry at a few percent level is allowed.
A
bstract
A first search for the lepton-flavour violating decays
B
0
→
K
*0
τ
±
μ
∓
is presented. The analysis is performed using a sample of proton-proton collision data, collected with the LHCb ...detector at centre-of-mass energies of 7, 8 and 13 TeV between 2011 and 2018, corresponding to an integrated luminosity of 9 fb
−
1
. No significant signal is observed, and upper limits on the branching fractions are determined to be
B
B
0
→
K
∗
0
τ
+
μ
−
<
1.0
1.2
×
10
−
5
and
B
B
0
→
K
∗
0
τ
−
μ
+
<
8.2
9.8
×
10
−
6
at the 90% (95%) confidence level.
A measurement of
D
0
meson production by the LHCb experiment in its fixed-target configuration is presented. The production of
D
0
mesons is studied with a beam of 2.5
Te
V
protons colliding on a ...gaseous neon target at rest, corresponding to a nucleon–nucleon centre-of-mass energy of
s
NN
=
68.5
Ge
V
. The sum of the
D
0
and
D
¯
0
production cross-section in
p
Ne
collisions in the centre-of-mass rapidity range
y
⋆
∈
-
2.29
,
0
is found to be
σ
D
0
y
⋆
∈
-
2.29
,
0
=
48.2
±
0.3
±
4.5
μ
b
/
nucleon
where the first uncertainty is statistical and the second is systematic. The
D
0
-
D
¯
0
production asymmetry is also evaluated and suggests a trend towards negative values at large negative
y
⋆
. The considered models do not account precisely for all the features observed in the LHCb data, but theoretical predictions including 1
%
intrinsic charm and 10
%
recombination contributions better describe the data than the other models considered.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
