The production ofπ0 andη mesons has been studied in the reactions20Ne +Al at 350 MeV/u and40Ar + Ca at 1.0 GeV/u. Rapidity distributions and transverse momentum spectra have been measured and are compared to thermal distributions.
Transverse momentum spectra of η mesons have been measured near the free nucleon-nucleon production threshold in the heavy ion reactions Ar40+ Canat, Kr86 + Zrnat, and Au197+ Au197 at 1.0A GeV and also in Ar40+ Canat at 1.5A GeV. The measured transverse momentum distributions are compared to model calculations. The relative abundance of Δ(1232) and N(1535) resonances excited in the collision is deduced. A comparison to pion data reveals scaling with the transverse mass of the emitted meson.
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Extrapolation to full solid angle.
We present results from an experimental study of reabsorption effects in subthreshold π0 production in the reaction Xe129+197Au at 44 MeV/nucleon. Within the picture of pion generation in nucleon-nucleon scattering we deduce, from our data and from a comparison with the systematics of production cross sections available for lighter reaction systems, information on the π0 absorption length in nuclear matter. For the π0 kinetic-energy range ≃5–100 MeV the energy-averaged λabs and its momentum dependence are obtained, and compared with optical-model calculations.
This Letter describes a model-independent search for the production of new resonances in photon + jet events using 20 inverse fb of proton--proton LHC data recorded with the ATLAS detector at a centre-of-mass energy of sqrt(s) = 8 TeV. The photon + jet mass distribution is compared to a background model fit from data; no significant deviation from the background-only hypothesis is found. Limits are set at 95% credibility level on generic Gaussian-shaped signals and two benchmark phenomena beyond the Standard Model: non-thermal quantum black holes and excited quarks. Non-thermal quantum black holes are excluded below masses of 4.6 TeV and excited quarks are excluded below masses of 3.5 TeV.
Invariant mass of the photon+jet pair for events passing the final selections. The number of observed events and the fit background estimates are given in each bin, where the fit estimates are rounded to the nearest integer.
The 95% CL upper limits on SIG*BR*A*EPSILON for a hypothetical signal with a Gaussian-shaped M(GAMMA JET) distribution as a function of the signal mass M(G) for four values of the relative width SIGMA(G) / M(G).
Acceptance (A), efficiency (EPSILON), cross-section (SIG) and limits in number of events for the quantum black hole (QBH) benchmark model, as a function of the threshold mass M(th). Uncertainties on the cross section are on the order of 1%. The limits include statistical uncertainties only. Expected limits include the 68% uncertainty band. Acceptance was calculated using parton-level quantities by imposing criteria that apply directly to kinematic selections (photon/jet |eta|, photon/jet transverse momentum, Delta(eta), Delta(R)). All other selections, which in general correspond to event and object quality criteria, were used to calculate the efficiency based on the events included in the acceptance.
A search for flavour-changing neutral current (FCNC) events via the coupling of a top quark, a photon, and an up or charm quark is presented using 81 fb$^{-1}$ of proton-proton collision data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Events with a photon, an electron or muon, a $b$-tagged jet, and missing transverse momentum are selected. A neural network based on kinematic variables differentiates between events from signal and background processes. The data are consistent with the background-only hypothesis, and limits are set on the strength of the $tq\gamma$ coupling in an effective field theory. These are also interpreted as 95% CL upper limits on the cross section for FCNC $t\gamma$ production via a left-handed (right-handed) $tu\gamma$ coupling of 36 fb (78 fb) and on the branching ratio for $t\rightarrow \gamma u$ of $2.8\times 10^{-5}$ ($6.1\times 10^{-5}$). In addition, they are interpreted as 95% CL upper limits on the cross section for FCNC $t\gamma$ production via a left-handed (right-handed) $tc\gamma$ coupling of 40 fb (33 fb) and on the branching ratio for $t\rightarrow \gamma c$ of $22\times 10^{-5}$ ($18\times 10^{-5}$).
Post-fit distributions of a background-only fit to the signal region (SR) and the control regions (CRs) of the NN output in the SR. In addition, the expected signal is overlaid for an effective coupling strength corresponding to the observed limit multiplied by a factor of ten.
Observed (expected) 95 % CL limits on the effective coupling strengths for different vertices and couplings, the production cross section, and the branching ratio. For the former, the energy scale is assumed to be $\Lambda$ = 1 TeV.
Post-fit distributions of a background-only fit to the SR and the CRs of the NN output in the SR for the $tu\gamma$ right-handed coupling. In addition, the expected signal is overlaid for an effective coupling strength corresponding to the observed limit multiplied by a factor of ten.
A search for excited electrons produced in $pp$ collisions at $\sqrt{s} = 13$ TeV via a contact interaction $q\bar{q} \to ee^*$ is presented. The search uses 36.1 fb$^{-1}$ of data collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider. Decays of the excited electron via a contact interaction into an electron and a pair of quarks ($eq\bar{q}$) are targeted in final states with two electrons and two hadronic jets, and decays via a gauge interaction into a neutrino and a $W$ boson ($\nu W$) are probed in final states with an electron, missing transverse momentum, and a large-radius jet consistent with a hadronically decaying $W$ boson. No significant excess is observed over the expected backgrounds. Upper limits are calculated for the $pp \to ee^* \to eeq\bar{q}$ and $pp \to ee^* \to e\nu W$ production cross sections as a function of the excited electron mass $m_{e^*}$ at 95% confidence level. The limits are translated into lower bounds on the compositeness scale parameter $\Lambda$ of the model as a function of $m_{e^*}$. For $m_{e^*} < 0.5$ TeV, the lower bound for $\Lambda$ is 11 TeV. In the special case of $m_{e^*} = \Lambda$, the values of $m_{e^*} < 4.8$ TeV are excluded. The presented limits on $\Lambda$ are more stringent than those obtained in previous searches.
A search for the production of three massive vector bosons in proton--proton collisions is performed using data at $\sqrt{s}=13\,TeV$ recorded with the ATLAS detector at the Large Hadron Collider in the years 2015--2017, corresponding to an integrated luminosity of $79.8\,\text{fb}^{-1}$. Events with two same-sign leptons $\ell$ (electrons or muons) and at least two reconstructed jets are selected to search for $WWW\to\ell\nu\ell\nu qq$. Events with three leptons without any same-flavour opposite-sign lepton pairs are used to search for $WWW\to\ell\nu\ell\nu\ell\nu$, while events with three leptons and at least one same-flavour opposite-sign lepton pair and one or more reconstructed jets are used to search for $WWZ\to\ell\nu qq \ell\ell$. Finally, events with four leptons are analysed to search for $WWZ\to\ell\nu\ell\nu\ell\ell$ and $WZZ\to qq \ell\ell\ell\ell$. Evidence for the joint production of three massive vector bosons is observed with a significance of 4.0 standard deviations, where the expectation is 3.1 standard deviations.
Two--kaon production in proton--deuteron collisions has been studied at three energies close to threshold using a calibrated magnetic spectrograph to measure the final $^3$He and a vertex detector to measure the $K^+K^-$ pair. Differential and total cross sections are presented for the production of $\phi$--mesons, decaying through $\phi\to K^+K^-$, as well as for prompt $K^+K^-$ production. The prompt production seems to follow phase space in both its differential distributions and in its energy dependence. The amplitude for the $pd\to ^3${He}$ \phi$ reaction varies little for excess energies below 22 MeV and the value is consistent with that obtained from a threshold measurement. The angular distribution of the $K^+K^-$ decay pair shows that near threshold the $\phi$--mesons are dominantly produced with polarization $m=0$ along the initial proton direction. No conclusive evidence for $f_0(980)$ production is found in the data.
Total cross section for K+ K- production.
Total cross section for PHI production.
Differential cross section as a function of the excitation energy of the K+ K- pair.
This paper presents a search for exotic decays of the Higgs boson into a pair of new pseudoscalar particles, $H\rightarrow aa$, where one pseudoscalar decays into a $b$-quark pair and the other decays into a $\tau$-lepton pair, in the mass range $12\leq m_{a}\leq 60$ GeV. The analysis uses $pp$ collision data at $\sqrt{s} = 13$ TeV collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 140 ${fb}^{-1}$. No significant excess above the Standard Model (SM) prediction is observed. Assuming the SM Higgs boson production cross-section, the search sets upper limits at 95% confidence level on the branching ratio of Higgs bosons decaying into $b\bar{b}\tau^+\tau^-$, $\mathcal{B}(H \rightarrow aa \rightarrow b\bar{b}\tau^+\tau^-)$, between 2.2% and 3.9% depending on the pseudoscalar mass.
Visible mass $m^{\mathrm{vis}}(\mu\tau_{\mathrm{had}})$ and distribution for signal and the expected background. In order to compare the shapes, the expected signal distribution is shown assuming ten times the production cross section of the Higgs boson and a 100% branching ratio to $b\bar{b}\tau^+\tau^-$. Overflow events are included in the last bins.
Sum of the transverse mass $\Sigma m_T$ distributions for signal and the expected background. Events with high $m^{\mathrm{vis}}(\mu\tau_{\mathrm{had}})$ and high $\Sigma m_T$ are included in the $t\bar{t}$ region. In order to compare the shapes, the expected signal distribution is shown assuming ten times the production cross section of the Higgs boson and a 100% branching ratio to $b\bar{b}\tau^+\tau^-$. Overflow events are included in the last bins.
The production of a $W$ boson in association with a single charm quark is studied using 140 $\mathrm{fb}^{-1}$ of $\sqrt{s} = 13\,\mathrm{TeV}$ proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. The charm quark is tagged by a charmed hadron, reconstructed with a secondary-vertex fit. The $W$ boson is reconstructed from an electron/muon decay and the missing transverse momentum. The mesons reconstructed are $D^{\pm} \to K^\mp \pi^\pm \pi^\pm$ and $D^{*\pm} \to D^{0} \pi^\pm \to (K^\mp \pi^\pm) \pi^\pm$, where $p_{\text{T}}(e, \mu) > 30\,\mathrm{GeV}$, $|\eta(e, \mu)| < 2.5$, $p_{\text{T}}(D) > 8\,\mathrm{GeV}$, and $|\eta(D)| < 2.2$. The integrated and normalized differential cross-sections as a function of the pseudorapidity of the lepton from the $W$ boson decay, and of the transverse momentum of the meson, are extracted from the data using a profile likelihood fit. The measured fiducial cross-sections are $\sigma^{\mathrm{OS-SS}}_{\mathrm{fid}}(W^{-}{+}D^{+}) = 50.2\pm0.2\,\mathrm{(stat.)}\,^{+2.4}_{-2.3}\,\mathrm{(syst.)}\,\mathrm{pb}$, $\sigma^{\mathrm{OS-SS}}_{\mathrm{fid}}(W^{+}{+}D^{-}) = 48.5\pm0.2\,\mathrm{(stat.)}\,^{+2.3}_{-2.2}\,\mathrm{(syst.)}\,\mathrm{pb}$, $\sigma^{\mathrm{OS-SS}}_{\mathrm{fid}}(W^{-}{+}D^{*+}) = 51.1\pm0.4\,\mathrm{(stat.)}\,^{+1.9}_{-1.8}\,\mathrm{(syst.)}\,\mathrm{pb}$, and $\sigma^{\mathrm{OS-SS}}_{\mathrm{fid}}(W^{+}{+}D^{*-}) = 50.0\pm0.4\,\mathrm{(stat.)}\,^{+1.9}_{-1.8}\,\mathrm{(syst.)}\,\mathrm{pb}$. Results are compared with the predictions of next-to-leading-order quantum chromodynamics calculations performed using state-of-the-art parton distribution functions. The ratio of charm to anti-charm production cross-sections is studied to probe the $s$-$\bar{s}$ quark asymmetry and is found to be $R_c^\pm = 0.971\pm0.006\,\mathrm{(stat.)}\pm0.011\,\mathrm{(syst.)}$.
Measured fiducial cross-sections times the single-lepton-flavor W boson branching ratio.
Measured cross section ratios for the W+D production. The $R_{c}(D^{(*)})$ observable is obtained by combining the individual measurements of $R_{c}(D^{+})$ and $R_{c}(D^{*+})$ as explained in the text. The displayed cross sections are integrated over each differential bin.
Measured $p_{\mathrm{T}}(D^{+})$ differential fiducial cross-section times the single-lepton-flavor W boson branching ratio in the $W^{-}+D^{+}$ channel. The last $p_{\mathrm{T}}$ bin has no upper bound. The displayed cross sections are integrated over each differential bin.
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