The $B_\mathrm{c}^+$ meson is observed for the first time in heavy ion collisions. Data from the CMS detector are used to study the production of the $B_\mathrm{c}^+$ meson in lead-lead (PbPb) and proton-proton (pp) collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}} =$ 5.02 TeV, via the $B_\mathrm{c}^+ \to (J/\psi\to\mu^+\mu^-)\mu^+\nu_\mu$ decay. The $B_\mathrm{c}^+$ nuclear modification factor, derived from the PbPb-to-pp ratio of production cross sections, is measured in two bins of the trimuon transverse momentum and of the PbPb collision centrality. The B$_\mathrm{c}^+$ meson is shown to be less suppressed than quarkonia and most of the open heavy-flavor mesons, suggesting that effects of the hot and dense nuclear matter created in heavy ion collisions contribute to its production. This measurement sets forth a promising new probe of the interplay of suppression and enhancement mechanisms in the production of heavy-flavor mesons in the quark-gluon plasma.
The $B_c$ meson production (pp-equivalent) cross-section times branching fraction of the $B_c\rightarrow (J/\psi \rightarrow \mu\mu) \mu \nu_{\mu}$ decay in pp and PbPb collisions. The used kinematic variables correspond to those of the trimuon final state. The two $p_T$ bins correspond to different rapidity ranges. The total uncertainty is decomposed in an uncertainty from the fit and one representing all other sources. The markers of the $p_T^{\mu\mu\mu}$ bins are placed according to the Lafferty-Wyatt prescription.
The $B_c$ meson nuclear modification factor in PbPb collisions, in $p_T^{\mu\mu\mu}$ bins corresponding to different trimuon rapidity ranges. The total uncertainty is decomposed in a bin-to-bin-uncorrelated uncertainty and one fully correlated along the two bins. The markers of the $p_T^{\mu\mu\mu}$ bins are placed at the average of the Lafferty-Wyatt prescriptions applied to the pp and PbPb spectra.
The $B_c$ meson nuclear modification factor in PbPb collisions, in centrality bins, integrated over the studied kinematic range. The cut on the trimuon rapidity depends on the trimuon $p_T$. The total uncertainty is decomposed in a bin-to-bin-uncorrelated uncertainty and one fully correlated along the two bins. The centrality bin markers are placed at the minimum bias average number of participants $N_{part}$.
A search is reported for high-mass hadronic resonances that decay to a parton and a Lorentz-boosted resonance, which in turn decays into a pair of partons. The search is based on data collected with the CMS detector at the LHC in proton-proton collisions at $\sqrt{s} =$ 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. The boosted resonance is reconstructed as a single wide jet with substructure consistent with a two-body decay. The high-mass resonance is thus considered as a dijet system. The jet substructure information and the kinematic properties of cascade resonance decays are exploited to disentangle the signal from the large quantum chromodynamics multijet background. The dijet mass spectrum is analyzed for the presence of new high-mass resonances, and is found to be consistent with the standard model background predictions. Results are interpreted in a warped extra dimension model where the high-mass resonance is a Kaluza-Klein gluon, the boosted resonance is a radion, and the final state partons are all gluons. Limits on the production cross section are set as a function of the Kaluza-Klein gluon and radion masses. These limits exclude at 95% confidence level models with Kaluza-Klein gluon masses in the range from 2.0 to 4.3 TeV and radion masses in the range from 0.20 to 0.74 TeV. By exploring a novel experimental signature, the observed limits on the Kaluza-Klein gluon mass are extended by up to about 1 TeV compared to previous searches.
Observed upper limits on the product of signal cross section and branching fraction, as a function of the mass ratio $m(\mathrm{R}_{2}) / m(\mathrm{R}_{1})$ vs. $m(\mathrm{R}_{1})$, for a resonance model with three gluons in the final state. The excluded regions from this search (black hatched) are optimized for the decay of a Kaluza--Klein gluon ($\mathrm{G}_{\mathrm{KK}}$) to a gluon ($\mathrm{g}$) and a radion ($\phi$), which in turns decays to two gluons, leading to a final state with three gluons ($\mathrm{ggg}$). The couplings of the model are set to the values $g_{\mathrm{grav}} = 6.0$ and $g_{\mathrm{GKK}} = 3.0$. These excluded regions are compared with those obtained from a reinterpretation of the inclusive CMS dijet resonance search (JHEP 05 (2020) 033), which is more sensitive to the decay channel of $\mathrm{G}_{\mathrm{KK}}$ to a quark-antiquark pair (red hatched). The vertical band between the $m(\mathrm{R}_{1})$ values of $\sim$3.0 and $\sim$3.1 TeV, for $m(\mathrm{R}_{2}) / m(\mathrm{R}_{1}) \lesssim 0.19$, is not excluded by the dijet search because of an upward statistical fluctuation in the observed limit. The white, dashed lines represent a sample of curves corresponding to fixed $m(\mathrm{R}_{2})$ values.
Observed upper limits on the product of signal cross section and branching fraction, as a function of the mass ratio $m(\mathrm{R}_{2}) / m(\mathrm{R}_{1})$ vs. $m(\mathrm{R}_{1})$, for a resonance model with three gluons in the final state. The excluded regions from this search (black hatched) are optimized for the decay of a Kaluza--Klein gluon ($\mathrm{G}_{\mathrm{KK}}$) to a gluon ($\mathrm{g}$) and a radion ($\phi$), which in turns decays to two gluons, leading to a final state with three gluons ($\mathrm{ggg}$). The couplings of the model are set to the values $g_{\mathrm{grav}} = 6.0$ and $g_{\mathrm{GKK}} = 3.0$. These excluded regions are compared with those obtained from a reinterpretation of the inclusive CMS dijet resonance search (JHEP 05 (2020) 033), which is more sensitive to the decay channel of $\mathrm{G}_{\mathrm{KK}}$ to a quark-antiquark pair (red hatched). The vertical band between the $m(\mathrm{R}_{1})$ values of $\sim$3.0 and $\sim$3.1 TeV, for $m(\mathrm{R}_{2}) / m(\mathrm{R}_{1}) \lesssim 0.19$, is not excluded by the dijet search because of an upward statistical fluctuation in the observed limit. The white, dashed lines represent a sample of curves corresponding to fixed $m(\mathrm{R}_{2})$ values.
Expected 95% CL upper limits on signal cross section times branching fraction, as a function of the ratio $m(\mathrm{R}_{2})/m(\mathrm{R}_{1})$ vs. $m(\mathrm{R}_{1})$, for a trijet resonance model with 3 gluons in the final state. The limits are optimized for the decay of a Kaluza-Klein gluon ($\mathrm{G}_{\mathrm{KK}}$) to a radion ($\phi$) and a gluon ($\mathrm{g}$) where the radion itself decays to 2 gluons, leading to a final state with 3 gluons ($\mathrm{ggg}$).
A search for Kaluza-Klein excited vector boson resonances, $W_\mathrm{KK}$, decaying in cascade to three W bosons via a scalar radion $R, W_\mathrm{KK}\to WR \to WWW$, with two or three massive jets is presented. The search is performed with proton-proton collision data recorded at $\sqrt{s} =$ 13 TeV, collected by the CMS experiment at the CERN LHC, during 2016-2018, corresponding to an integrated luminosity of 138 fb$^{-1}$. Two final states are simultaneously probed, one where the two W bosons produced by the R decay are reconstructed as separate, large-radius, massive jets, and one where they are merged in a single large-radius jet. The observed data are in agreement with the standard model expectations. Limits are set on the product of the $W_\mathrm{KK}$ resonance cross section and branching fraction to three W bosons in an extended warped extra-dimensional model and are the first of their kind at the LHC.
Distribution of $m_{\mathrm{jj}}$ for preselected events with $\mathrm{N}_{j}$ = 2
Distribution of $m_{\mathrm{j}}$ for preselected events with $\mathrm{N}_{j}$ = 2
Distribution of the deep-WH value of the highest-mass jet with $m_{\mathrm{j}}$ > 100 GeV for preselected events with $\mathrm{N}_{j}$ = 2
Multiparticle azimuthal correlations of prompt D$^0$ mesons are measured in PbPb collisions at a nucleon-nucleon center-of-mass energy of $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV. For the first time, a four-particle cumulant method is used to extract the second Fourier coefficient of the azimuthal distribution ($v_2$) of D$^0$ mesons as a function of event centrality and the D$^0$ transverse momentum. The ratios of the four-particle $v_2$ values to previously measured two-particle cumulant results provide direct experimental access to event-by-event fluctuations of charm quark azimuthal anisotropies. These ratios are also found to be comparable to those of inclusive charged particles in the event. However, hints of deviations are seen in the most central and peripheral collisions. To investigate the origin of flow fluctuations in the charm sector, these measurements are compared with models implementing fluctuations of charm quark energy loss via collisional or radiative processes in the quark-gluon plasma. These models cannot quantitatively describe the data over the full transverse momentum and centrality ranges, although the calculations with collisional energy loss provide a better description of the data.
Prompt $D^0+\overline{D}^0$ meson $v_2\{2\}$, $v_2\{4\}$, and $v_2\{4\}/v_2\{2\}$ as a function of $p_T$ from PbPb collisions at $\sqrt{s_{NN}} = 5.02$ TeV.
Prompt $D^0+\overline{D}^0$ meson $v_2\{2\}$, $v_2\{4\}$, and $v_2\{4\}/v_2\{2\}$ as a function of $p_T$ from PbPb collisions at $\sqrt{s_{NN}} = 5.02$ TeV.
Prompt $D^0+\overline{D}^0$ meson $v_2\{2\}$, $v_2\{4\}$, and $v_2\{4\}/v_2\{2\}$ as a function of centrality from PbPb collisions at $\sqrt{s_{NN}} = 5.02$ TeV.
The first collider search for dark matter arising from a strongly coupled hidden sector is presented and uses a data sample corresponding to 138 fb$^{-1}$, collected with the CMS detector at the CERN LHC, at $\sqrt{s} =$ 13 TeV. The hidden sector is hypothesized to couple to the standard model (SM) via a heavy leptophobic Z' mediator produced as a resonance in proton-proton collisions. The mediator decay results in two "semivisible" jets, containing both visible matter and invisible dark matter. The final state therefore includes moderate missing energy aligned with one of the jets, a signature ignored by most dark matter searches. No structure in the dijet transverse mass spectra compatible with the signal is observed. Assuming the Z' has a universal coupling of 0.25 to the SM quarks, an inclusive search, relevant to any model that exhibits this kinematic behavior, excludes mediator masses of 1.5-4.0 TeV at 95% confidence level, depending on the other signal model parameters. To enhance the sensitivity of the search for this particular class of hidden sector models, a boosted decision tree (BDT) is trained using jet substructure variables to distinguish between semivisible jets and SM jets from background processes. When the BDT is employed to identify each jet in the dijet system as semivisible, the mediator mass exclusion increases to 5.1 TeV, for wider ranges of the other signal model parameters. These limits exclude a wide range of strongly coupled hidden sector models for the first time.
The normalized distribution of the characteristic variable $R_{\text{T}}$ for the simulated SM backgrounds and several signal models. The requirement on this variable is omitted, but all other preselection requirements are applied. The last bin of each histogram includes the overflow events.
The normalized distribution of the characteristic variable $\Delta\phi_{\text{min}}$ for the simulated SM backgrounds and several signal models. The requirement on this variable is omitted, but all other preselection requirements are applied. The last bin of each histogram includes the overflow events.
The normalized distributions of the BDT input variable $m_{\text{SD}}$ for the two highest $p_{\text{T}}$ jets from the simulated SM backgrounds and several signal models. Each sample's jet $p_{\text{T}}$ distribution is weighted to match a reference distribution (see text). The last bin of each histogram includes the overflow events.
The top quark pair production cross section is measured in proton-proton collisions at a center-of-mass energy of 5.02 TeV. The data were collected in a special LHC low-energy and low-intensity run in 2017, and correspond to an integrated luminosity of 302 pb$^{-1}$. The measurement is performed using events with one electron and one muon of opposite charge, and at least two jets. The measured cross section is 60.7 $\pm$ 5.0 (stat) $\pm$ 2.8 (syst) $\pm$ 1.1 (lumi) pb. To reduce the statistical uncertainty, a combination with the result in the single lepton + jets channel, based on data collected in 2015 at the same center-of-mass energy and corresponding to an integrated luminosity of 27.4 pb$^{-1}$, is then performed. The resulting measured value is 63.0 $\pm$ 4.1 (stat) $\pm$ 3.0 (syst+lumi) pb, in agreement with the standard model prediction of 66.8 $^{+2.9}_{-3.1}$ pb.
Leading lepton ${p}_{T}$ in the selected events. The hatched band correspond to systematic and statistical uncertainties. The lower panels show the data-to-prediction ratio.
Subleading lepton ${p}_{T}$ in the selected events. The hatched band correspond to systematic and statistical uncertainties. The lower panels show the data-to-prediction ratio.
Leading jet ${p}_{T}$ in the selected events. The hatched band correspond to systematic and statistical uncertainties. The lower panels show the data-to-prediction ratio.
Evidence is reported for electroweak (EW) vector boson scattering in the decay channel $\ell\nu$qq of two weak vector bosons WV (V = W or Z), produced in association with two parton jets. The search uses a data set of proton-proton collisions at 13 TeV collected with the CMS detector during 2016-2018 with an integrated luminosity of 138 fb$^{-1}$. Events are selected requiring one lepton (electron or muon), moderate missing transverse momentum, two jets with a large pseudorapidity separation and a large dijet invariant mass, and a signature consistent with the hadronic decay of a W/Z boson. The cross section is computed in a fiducial phase space defined at parton level requiring all parton transverse momenta $p_\mathrm{T}$$\gt$ 10 GeV and at least one pair of outgoing partons with invariant mass $m_\mathrm{qq}$$\gt$ 100 GeV. The measured and expected EW WV production cross sections are 1.90 $^{+0.53}_{-0.46}$ pb and 2.23 $^{+0.08}_{-0.11}$ (scale) $\pm$ 0.05 (PDF) pb, respectively, where PDF is the parton distribution function. The observed EW signal strength is $m_\mathrm{EW}$ = 0.85 $\pm$ 0.12 (stat) $^{+0.19}_{-0.17}$ (syst), corresponding to a signal significance of 4.4 standard deviations with 5.1 expected, and it is measured keeping the quantum chromodynamics (QCD) associated diboson production fixed to the standard model prediction. This is the first evidence of vector boson scattering in the $\ell\nu$qq decay channel at LHC. The simultaneous measurement of the EW and QCD associated diboson production agrees with the standard model prediction.
Expected and observed cross sections for EW and EW+QCD WV production in association with 2 jets. Two separate maximum likelihood fits are performed: the measurement of the purely EW signal strength μ_EW keeping the QCD WV production contribution fixed to the SM prediction μQCD = 1; the measurement of the signal strength considering as signal the EW and QCD WV processes together.
Invariant mass of the pair of the VBS tag jets in the resolved category. The last bin contains overflow events.
Invariant mass of the pair of the VBS tag jets in the boosted category. The last bin contains overflow events.
In high-energy heavy-ion collisions, partonic collectivity is evidenced by the constituent quark number scaling of elliptic flow anisotropy for identified hadrons. A breaking of this scaling and dominance of baryonic interactions is found for identified hadron collective flow measurements in $\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au collisions. In this paper, we report measurements of the first- and second-order azimuthal anisotropic parameters, $v_1$ and $v_2$, of light nuclei ($d$, $t$, $^{3}$He, $^{4}$He) produced in $\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au collisions at the STAR experiment. An atomic mass number scaling is found in the measured $v_1$ slopes of light nuclei at mid-rapidity. For the measured $v_2$ magnitude, a strong rapidity dependence is observed. Unlike $v_2$ at higher collision energies, the $v_2$ values at mid-rapidity for all light nuclei are negative and no scaling is observed with the atomic mass number. Calculations by the Jet AA Microscopic Transport Model (JAM), with baryonic mean-field plus nucleon coalescence, are in good agreement with our observations, implying baryonic interactions dominate the collective dynamics in 3 GeV Au+Au collisions at RHIC.
The rapidity and $p_{T}$ dependencies of $v_{1}$ for $p$ in 10-40% mid-central Au+Au collisions at 3 GeV.
The rapidity and $p_{T}$ dependencies of $v_{1}$ for $d$ in 10-40% mid-central Au+Au collisions at 3 GeV.
The $p_{T}$ dependencies of $v_{1}$ within $-0.1<y<0$ for $t$ in 10-40% mid-central Au+Au collisions at 3 GeV.
Measurements of the associated production of a W boson and a charm (c) quark in proton-proton collisions at a centre-of-mass energy of 8 TeV are reported. The analysis uses a data sample corresponding to a total integrated luminosity of 19.7 fb$^{-1}$ collected by the CMS detector at the LHC. The W bosons are identified through their leptonic decays to an electron or a muon, and a neutrino. Charm quark jets are selected using distinctive signatures of charm hadron decays. The product of the cross section and branching fraction $\sigma$(pp $\to$ W + c + X) $\mathcal{B}$(W $\to$$\ell\nu$), where $\ell$ = e or $\mu$, and the cross section ratio $\sigma$(pp $\to$ W$^+$ + c + X) / $\sigma$(pp $\to$ W$^-$ + $\mathrm{\bar{c}}$ + X) are measured inclusively and differentially as functions of the pseudorapidity and of the transverse momentum of the lepton from the W boson decay. The results are compared with theoretical predictions. The impact of these measurements on the determination of the strange quark distribution is assessed.
Signal yields after background subtraction, efficiency*acceptance correction factors, and cross section measurements for the four channels (W decay to muon or electron and charm identification via muon or secondary vertex inside a jet).
Measured production cross sections $\sigma(W^+ + \overline{c})$, $\sigma(W^- + c)$ and their ratio.
Measured diferential cross sections $\sigma(W^- + c) + \sigma(W^+ + \overline{c})$ as a function of the absolute value of the pseudorapidity of the lepton from the W decay.
We report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at 3.0 GeV, measured by the STAR experiment in the kinematic acceptance of rapidity ($y$) and transverse momentum ($p_{\rm T}$) within $-0.5 < y<0$ and $0.4 < p_{\rm T} <2.0 $ GeV/$c$. In the most central 0--5% collisions, a proton cumulant ratio is measured to be $C_4/C_2=-0.85 \pm 0.09 ~(\rm stat.) \pm 0.82 ~(\rm syst.)$, which is less than unity, the Poisson baseline. The hadronic transport UrQMD model reproduces our $C_4/C_2$ in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in $C_4/C_2$ is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3 GeV.
$\sqrt{s_{NN}}$ = 3.0 GeV data (black markers), GM (red histogram), and single and pile-up contributions from unfolding. Vertical lines on markers represent statistical uncertainties. Single, pile-up and single+pile-up collisions are shown in solid blue markers, dashed green and dashed magenta curves, respectively. Analysis is performed on 0–5% central events, indicated by a black arrow.
$\sqrt{s_{NN}}$ = 3.0 GeV data (black markers), GM (red histogram), and single and pile-up contributions from unfolding. Vertical lines on markers represent statistical uncertainties. Single, pile-up and single+pile-up collisions are shown in solid blue markers, dashed green and dashed magenta curves, respectively. Analysis is performed on 0–5% central events, indicated by a black arrow.
Centrality dependence of the proton cumulant ratios for Au+Au collisions at $\sqrt{s_{NN}}$ = 3.0 GeV. Protons are from $-0.5 < y < 0$ and $0.4 < p_{T} < 2.0$ GeV/$c$. Systematic uncertainties are represented by gray bars. Statistical uncertainties are smaller than marker size. CBWC is applied to all cumulant ratios. While open squares represent the data without the VFC correction, blue triangles and red circles are the results with VFC using the $\langle N_{\rm{part}} \rangle$ distributions from the UrQMD and Glauber models, respectively. UrQMD model results are represented as gold dashed line.
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