The production of $J/\psi$ pairs in proton-proton collisions at a centre-of-mass energy of 7 TeV has been observed using an integrated luminosity of $37.5 pb^{-1}$ collected with the LHCb detector. The production cross-section for pairs with both \jpsi in the rapidity range $2<y^{J/\psi}<4.5$ and transverse momentum $p_{T}^{J/\psi}<10 GeV/c$ is $$ \sigma^{J/\psi J/\psi} = 5.1\pm1.0\pm1.1 nb,$$ where the first uncertainty is statistical and the second systematic.
A search for new heavy particles manifested as resonances in two-jet final states is presented. The data were produced in 7 TeV proton-proton collisions by the Large Hadron Collider (LHC) and correspond to an integrated luminosity of 315 nb^-1 collected by the ATLAS detector. No resonances were observed. Upper limits were set on the product of cross section and signal acceptance for excited-quark (q*) production as a function of q* mass. These exclude at the 95% CL the q* mass interval 0.30 < mq* < 1.26 TeV, extending the reach of previous experiments.
We report on the first measurement of the triangular $v_3$, quadrangular $v_4$, and pentagonal $v_5$ charged particle flow in Pb-Pb collisions at 2.76 TeV measured with the ALICE detector at the CERN Large Hadron Collider. We show that the triangular flow can be described in terms of the initial spatial anisotropy and its fluctuations, which provides strong constraints on its origin. In the most central events, where the elliptic flow $v_2$ and $v_3$ have similar magnitude, a double peaked structure in the two-particle azimuthal correlations is observed, which is often interpreted as a Mach cone response to fast partons. We show that this structure can be naturally explained from the measured anisotropic flow Fourier coefficients.
The $p_{\rm T}$-differential inclusive production cross sections of the prompt charmed mesons $D^0$, $D^+$, and $D^{*+}$ in the rapidity range |y|<0.5 were measured in proton-proton collisions at $\sqrt{s} = 7$ TeV at the LHC using the ALICE detector. Reconstructing the decays $D^0\rightarrow K^-\pi^+$, $D^+\rightarrow K^-\pi^+\pi^+$, $D^{*+}\rightarrow D^0\pi^+$, and their charge conjugates, about 8,400 $D^0$, 2,900 $D^+$, and 2,600 $D^{*+}$ mesons with 1<$p_{\rm T}$<24 GeV/$c$ were counted, after selection cuts, in a data sample of 3.14x10$^8$ events collected with a minimum-bias trigger (integrated luminosity $L_{\rm int}$ = 5/nb). The results are described within uncertainties by predictions based on perturbative QCD.
The ratio between the B$_\mathrm{c}^+$$\to$ J/$ψ$$τ^+ν_τ$ and B$_\mathrm{c}^+$$\to$ J/$ψ$$μ^+ν_μ$ branching fractions is measured using a data sample of proton-proton collisions collected by CMS at a center-of-mass energy of 13 TeV in the years 2016$-$2018 and corresponding to an integrated luminosity of 138 fb$^{-1}$. The J$/ψ$ meson is identified through its J$/ψ$$\to$$μ^+μ^-$ decay and the tau lepton is reconstructed in the hadronic three-prong final state. The measured ratio of branching fractions in this tau decay mode, $\mathcal{R}^\text{had}_{\mathrm{J}/ψ}$ = 1.04$_{-0.44}^{+0.50}$, is combined with the previous analysis based on the $τ^+$$\to$$μ^+ν_μ\barν_τ$ leptonic decay channel, leading to $\mathcal{R}_{\mathrm{J}/ψ}$ = 0.49 $\pm$ 0.26. As this result is consistent with the standard model prediction of 0.258 $\pm$ 0.004, no evidence of lepton flavor universality violation is found.
Azimuthally anisotropic distributions of D$^0$, D$^+$ and D$^{*+}$ mesons were studied in the central rapidity region ($|y|<0.8$) in Pb-Pb collisions at a centre-of-mass energy $\sqrt{s_{\rm NN}} = 2.76$ TeV per nucleon-nucleon collision, with the ALICE detector at the LHC. The second Fourier coefficient $v_2$ (commonly denoted elliptic flow) was measured in the centrality class 30-50% as a function of the D meson transverse momentum $p_{\rm T}$, in the range 2-16 GeV/$c$. The measured $v_2$ of D mesons is comparable in magnitude to that of light-flavour hadrons. It is positive in the range $2 < p_{\rm T} < 6$ GeV/$c$ with $5.7\sigma$ significance, based on the combination of statistical and systematic uncertainties.
A measurement of the inclusive WW+WZ diboson production cross section in proton-proton collisions is reported, based on events containing a leptonically decaying W boson and exactly two jets. The data sample, collected at sqrt(s) = 7 TeV with the CMS detector at the LHC, corresponds to an integrated luminosity of 5.0 inverse femtobarns. The measured value of the sum of the inclusive WW and WZ cross sections is sigma(pp to WW+WZ) = 68.9 +/- 8.7 (stat.) +/- 9.7 (syst.) +/- 1.5 (lum.) pb, consistent with the standard model prediction of 65.6 +/- 2.2 pb. This is the first measurement of WW+WZ production in pp collisions using this signature. No evidence for anomalous triple gauge couplings is found and upper limits are set on their magnitudes.
The first measurements from proton-proton collisions recorded with the ATLAS detector at the LHC are presented. Data were collected in December 2009 using a minimum-bias trigger during collisions at a centre-of-mass energy of 900 GeV. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and charged-particle multiplicity are measured for events with at least one charged particle in the kinematic range |eta|<2.5 and pT>500 MeV. The measurements are compared to Monte Carlo models of proton-proton collisions and to results from other experiments at the same centre-of-mass energy. The charged-particle multiplicity per event and unit of pseudorapidity at eta = 0 is measured to be 1.333 +/- 0.003 (stat.) +/- 0.040 (syst.), which is 5-15% higher than the Monte Carlo models predict.
An angular analysis of the decay $J/\psi \rightarrow \mu^{+} \mu^{-}$ is performed to measure the polarization of prompt $J/\psi$ mesons produced in $pp$ collisions at $\sqrt{s}=7\; \mathrm{TeV}$. The dataset corresponds to an integrated luminosity of 0.37 fb$^{-1}$ collected with the LHCb detector. The measurement is presented as a function of transverse momentum, $p_\mathrm{T}$, and rapidity, $y$, of the $J/\psi$ meson, in the kinematic region $2 < p_\mathrm{T} < 15\; \mathrm{GeV}/c$ and $2.0<y<4.5$.
The first measurements of the invariant differential cross sections of inclusive $\pi^0$ and $\eta$ meson production at mid-rapidity in proton-proton collisions at $\sqrt{s}=0.9$ TeV and $\sqrt{s}=7$ TeV are reported. The $\pi^0$ measurement covers the ranges $0.4<p_T<7$ GeV/$c$ and $0.3<p_T<25$ GeV/$c$ for these two energies, respectively. The production of $\eta$ mesons was measured at $\sqrt{s}=7$ TeV in the range $0.4<p_T<15$ GeV/$c$. Next-to-Leading Order perturbative QCD calculations, which are consistent with the $\pi^0$ spectrum at $\sqrt{s}=0.9$ TeV, overestimate those of $\pi^0$ and $\eta$ mesons at $\sqrt{s}=7$ TeV, but agree with the measured $\eta/\pi^0$ ratio at $\sqrt{s}=7$ TeV.
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