The interpretation of cosmic antiproton flux measurements from space-borne experiments is currently limited by the knowledge of the antiproton production cross-section in collisions between primary ...cosmic rays and the interstellar medium. Using collisions of protons with an energy of 6.5
Te
V
incident on helium nuclei at rest in the proximity of the interaction region of the LHCb experiment, the ratio of antiprotons originating from antihyperon decays to prompt production is measured for antiproton momenta between 12 and
110
Ge
V\!/
c
. The dominant antihyperon contribution, namely
Λ
¯
→
p
¯
π
+
decays from promptly produced
Λ
¯
particles, is also exclusively measured. The results complement the measurement of prompt antiproton production obtained from the same data sample. At the energy scale of this measurement, the antihyperon contributions to antiproton production are observed to be significantly larger than predictions of commonly used hadronic production models.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Results are presented on hyperon and antihyperon production in Pb–Pb, pPb and pBe collisions at 158 GeV/c per nucleon. Λ, Ξ and Ω yields have been measured at central rapidity and medium transverse ...momentum as functions of the centrality of the collision. Comparing the yields in Pb–Pb to those in pBe interactions, strangeness enhancement is observed. The enhancement increases with the centrality and with the strangeness content of the hyperons, reaching a factor of about 20 for the Ω in the central Pb–Pb collisions
A
bstract
This article presents the first measurement of the differential
Z
-boson production cross-section in the forward region using proton-lead collisions with the LHCb detector. The dataset was ...collected at a nucleon-nucleon centre-of-mass energy of
s
NN
= 8
.
16 TeV in 2016, corresponding to an integrated luminosity of 30
.
8 nb
−
1
. The forward-backward ratio and the nuclear modification factors are measured together with the differential cross-section as functions of the
Z
boson rapidity in the centre-of-mass frame, the transverse momentum of the
Z
boson and a geometric variable
ϕ
*
. The results are in good agreement with the predictions from nuclear parton distribution functions, providing strong constraining power at small Bjorken-
x
.
The measurement of charmonium states produced in proton-neon (
p
Ne
) collisions by the LHCb experiment in its fixed-target configuration is presented. The production of
J
/
ψ
and
ψ
(
2
S
)
mesons is ...studied with a beam of 2.5
Te
V
protons colliding on gaseous neon targets at rest, corresponding to a nucleon-nucleon centre-of-mass energy
s
NN
=
68.5
Ge
V
. The data sample corresponds to an integrated luminosity of
21.7
±
1.4
nb
-
1
. The
J
/
ψ
and
ψ
(
2
S
)
hadrons are reconstructed in
μ
+
μ
-
final states. The
J
/
ψ
production cross-section per target nucleon in the centre-of-mass rapidity range
y
⋆
∈
-
2.29
,
0
is found to be
506
±
8
±
46
nb/nucleon
. The ratio of
J
/
ψ
and
D
0
cross-sections is evaluated to
(
1.06
±
0.02
±
0.09
)
%
. The
ψ
(
2
S
)
to
J
/
ψ
relative production rate is found to be
(
1.67
±
0.27
±
0.10
)
%
in good agreement with other measurements involving beam and target nuclei of similar sizes.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The OPERA experiment was designed to search for ν_{μ}→ν_{τ} oscillations in appearance mode, i.e., by detecting the τ leptons produced in charged current ν_{τ} interactions. The experiment took data ...from 2008 to 2012 in the CERN Neutrinos to Gran Sasso beam. The observation of the ν_{μ}→ν_{τ} appearance, achieved with four candidate events in a subsample of the data, was previously reported. In this Letter, a fifth ν_{τ} candidate event, found in an enlarged data sample, is described. Together with a further reduction of the expected background, the candidate events detected so far allow us to assess the discovery of ν_{μ}→ν_{τ} oscillations in appearance mode with a significance larger than 5σ.
A
bstract
A study of B
+
→
J
/
ψηK
+
decays, followed by J
/
ψ
→
μ
+
μ
−
and η
→
γγ, is performed using a dataset collected with the LHCb detector in proton-proton collisions at centre-of-mass ...energies of 7, 8 and 13 TeV, corresponding to an integrated luminosity of 9 fb
−
1
. The J
/
ψη mass spectrum is investigated for contributions from charmonia and charmonium-like states. Evidence is found for the B
+
→
(ψ
2
(3823)
→
J
/
ψη)K
+
and B
+
→
(ψ(4040)
→
J
/
ψη)K
+
decays with significance of 3.4 and 4.7 standard deviations, respectively. This constitutes the first evidence for the ψ
2
(3823)
→
J
/
ψη decay.
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
A
bstract
The production of prompt
D
+
and
D
s
+
mesons is studied in proton-lead collisions at a centre-of-mass energy of
s
NN
= 5
.
02 TeV. The data sample corresponding to an integrated luminosity ...of (1
.
58 ± 0
.
02)nb
−
1
is collected by the LHCb experiment at the LHC. The differential production cross-sections are measured using
D
+
and
D
s
+
candidates with transverse momentum in the range of 0
< p
T
<
14 GeV
/c
and rapidities in the ranges of 1
.
5
< y
∗
<
4
.
0 and –5
.
0
< y
∗
<
–2
.
5 in the nucleon-nucleon centre-of-mass system. For both particles, the nuclear modification factor and the forward-backward production ratio are determined. These results are compared with theoretical models that include initial-state nuclear effects. In addition, measurements of the cross-section ratios between
D
+
,
D
s
+
and
D
0
mesons are presented, providing a baseline for studying the charm hadronization in lead-lead collisions at LHC energies.
A
bstract
The associated production of prompt
J/ψ
and
Υ
mesons in
pp
collisions at a centre-of-mass energy of
s
= 13 TeV is studied using LHCb data, corresponding to an integrated luminosity of 4 fb
...−
1
. The measurement is performed for
J/ψ
(
Υ
) mesons with a transverse momentum
p
T
<
10 (30) GeV/
c
in the rapidity range 2
.
0
< y <
4
.
5. In this kinematic range, the cross-section of the associated production of prompt
J/ψ
and
Υ
(1
S
) mesons is measured to be 133 ± 22 ± 7 ± 3 pb, with a significance of 7
.
9
σ
, and that of prompt
J/ψ
and
Υ
(2
S
) mesons to be 76 ± 21 ± 4 ± 7 pb, with a significance of 4
.
9
σ
. The first uncertainty is statistical, the second systematic, and the third due to uncertainties on the used branching fractions. This is the first observation of the associated production of
J/ψ
and
Υ
(1
S
) in proton-proton collisions. Differential cross-sections are measured as functions of variables that are sensitive to kinematic correlations between the
J/ψ
and
Υ
(1
S
) mesons. The effective cross-sections of the associated production of prompt
J/ψ
and
Υ
mesons are obtained and found to be compatible with measurements using other particle productions.
The first measurement of
J
/
ψ
and
D
0
production in PbNe collisions by the LHCb experiment in its fixed-target configuration is reported. The production of
J
/
ψ
and
D
0
mesons is studied with a ...beam of lead ions with an energy of 2.5
TeV
per nucleon colliding on gaseous neon targets at rest, corresponding to a nucleon-nucleon centre-of-mass energy of
s
NN
=
68.5
GeV
. The
J
/
ψ
/
D
0
production cross-section ratio is studied as a function of rapidity, transverse momentum and collision centrality. These data are compared with measurements from
p
Ne
collisions at the same energy and show no difference in the observed
J
/
ψ
suppression trend when comparing
p
Ne
and PbNe peripheral collisions with PbNe central collisions.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
PDF
