The process
e
+
e
-
→
n
n
¯
is studied in the experiment at the VEPP-2000
e
+
e
-
collider with the SND detector. The technique of the time measurements in the multichannel NaI(Tl) electromagnetic ...calorimeter is used to select
n
n
¯
events. The value of the measured cross section in the center-of-mass energy range from 1.894 to 2 GeV varies from 0.5 to 0.35 nb. The effective neutron timelike form factor is derived from the measured cross section and compared with the proton form factor. The ratio of the neutron electric and magnetic form factors is obtained from the analysis of the antineutron polar angle distribution and found to be consistent with unity.
A search is made for potential cccover ¯cover ¯ tetraquarks decaying into a pair of charmonium states in the four muon final state using proton-proton collision data at sqrts=13 TeV, corresponding ...to an integrated luminosity of 140 fb^{-1} recorded by the ATLAS experiment at LHC. Two decay channels, J/ψ+J/ψ→4μ and J/ψ+ψ(2S)→4μ, are studied. Backgrounds are estimated based on a hybrid approach involving Monte Carlo simulations and data-driven methods. Statistically significant excesses with respect to backgrounds dominated by the single parton scattering are seen in the di-J/ψ channel consistent with a narrow resonance at 6.9 GeV and a broader structure at lower mass. A statistically significant excess is also seen in the J/ψ+ψ(2S) channel. The fitted masses and decay widths of the structures are reported.
Searches are performed for nonresonant and resonant di-Higgs boson production in the b¯bγγ final state. The dataset used corresponds to an integrated luminosity of 139 fb−1 of proton–proton ...collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No excess above the expected background is found and upper limits on the di-Higgs boson production cross sections are set. A 95% confidence-level upper limit of 4.2 times the cross section predicted by the Standard Model is set on pp→HH nonresonant production, where the expected limit is 5.7 times the Standard Model predicted value. The expected constraints are obtained for a background hypothesis excluding pp→HH production. The observed (expected) constraints on the Higgs boson trilinear coupling modifier κλ are determined to be −1.5,6.7 (−2.4,7.7) at 95% confidence level, where the expected constraints on κλ are obtained excluding pp→HH production from the background hypothesis. For resonant production of a new hypothetical scalar particle X (X→HH→b¯bγγ), limits on the cross section for pp→X→HH are presented in the narrow-width approximation as a function of mX in the range 251 GeV≤mX≤1000 GeV. The observed (expected) limits on the cross section for pp→X→HH range from 640 fb to 44 fb (391 fb to 46 fb) over the considered mass range.
The process
was studied in the energy range between 1 and 2 GeV to the best precision in the world. The data used correspond to an integrated luminosity of
accumulated with the SND detector at the ...VEPP-2000
collider. The statistical accuracy of the measured cross section is 2 to 22
, whereas the systematic uncertainty is 2.6 to 14
. The present results agree with the results of earlier measurements but have a better precision.
The process
is studied in the range of center-of-mass (c.m.) collision energies between 1.05 and 2.00 GeV on the basis of data corresponding to an integrated luminosity of 94.5 pb
that were ...accumulated in an experiment with the SND detector at the VEPP-2000
collider. The cross section for this process was measured for the first time and was found to receive a dominant contribution from the mechanism of a transition through an
intermediate state. The cross section measured for the process
agrees with the results of earlier measurements in the channel
. At a level of 5.7 standard deviations (5.7
), it is established that the process
is not fully described by hadronic intermediate states of the
type. The cross section was measured for this missing contribution, which may be generated by radiative processes—for example,
. It ranges from 15 to 20 pb over a broad energy interval extending from 1.3 to 1.9 GeV.
In an experiment with the SND detector at the VEPP-2000
collider, the cross sections for the processes
and
were measured at collision energies of
GeV in the center-of-mass frame. The results agree ...with the results of earlier measurements in the BABAR experiment and have a commensurate precision.
In the experiment with the SND detector at the VEPP-2000
collider, events of the process
were separated via measuring, in each counter of the multichannel calorimeter based on 1640 NaI(Tl) crystals, ...the time of delay of the signal from nonrelativistic antineutrons. The time resolution for events of the process
was 0.8 ns. The measured time spectrum of delays of the signal from antineutrons in the calorimeter at the c.m. energy of 1902 MeV agrees with the results of calculations.
Using the SND detector, the process
was studied in the energy range between 1.05 and 2.00 GeV. The respective measurement was performed on the basis of data accumulated in the experiments at the ...VEPP-2000 electron–positron collider throughout the runs of 2011 and 2012. Also, the invariant-mass spectra of the
and
combinations were analyzed in the energy range between 1.15 and 2 GeV. This resulted in finding an indication that the
excited state decayed predominantly through the
intermediate state.
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