This paper presents measurements of charged-particle distributions sensitive to the properties of the underlying event in events containing a $Z$ boson decaying into a muon pair. The data were obtained using the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 13 TeV with an integrated luminosity of 3.2 fb$^{-1}$. Distributions of the charged-particle multiplicity and of the charged-particle transverse momentum are measured in regions of the azimuth defined relative to the $Z$ boson direction. The measured distributions are compared with the predictions of various Monte Carlo generators which implement different underlying-event models. The Monte Carlo model predictions qualitatively describe the data well, but with some significant discrepancies.
Figure 09d, mean sumPt toward, toward region : Mean sum of transverse momenta ($\langle \Sigma p_{T} \rangle \pm stat. \pm syst.det. \pm syst.gen.[GeV]$)
transverse region : Mean sum of transverse momenta ($\langle \Sigma p_{T} \rangle \pm stat. \pm syst.det. \pm syst.gen.[GeV]$)
Figure 09c, mean sumPt transmin, transmin region : Mean sum of transverse momenta ($\langle \Sigma p_{T} \rangle \pm stat. \pm syst.det. \pm syst.gen.[GeV]$)
The average total energy as well as its hadronic and electromagnetic components are measured with the CMS detector at pseudorapidities $-$6.6 $<\eta<-$5.2 in proton-proton collisions at a centre-of-mass energy $\sqrt{s}=$ 13 TeV. The results are presented as a function of the charged particle multiplicity in the region $|\eta|< $2. This measurement is sensitive to correlations induced by the underlying event structure over a very wide pseudorapidity region. The predictions of Monte Carlo event generators commonly used in collider experiments and ultra-high energy cosmic ray physics are compared to the data.
Average total energy reconstructed in the region −6.6 < eta < −5.2 as a function of the number of reconstructed tracks for abs(eta)<2.
Average total energiy reconstructed in the region −6.6 < eta < −5.2 normalised to that in the first bin (Nch< 10) as a function of the number of reconstructed tracks for abs(eta)<2.
Average electromagnetic energy reconstructed in the region −6.6 < eta < −5.2 as a function of the number of reconstructed tracks for abs(eta)<2.
A study of excited $\Lambda_\mathrm{b}^0$ baryons is reported, based on a data sample collected in 2016-2018 with the CMS detector at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of up to 140 fb$^{-1}$. The existence of four excited $\Lambda_\mathrm{b}^0$ states: $\Lambda_\mathrm{b}$(5912)$^0$, $\Lambda_\mathrm{b}$(5920)$^0$, $\Lambda_\mathrm{b}$(6146)$^0$, and $\Lambda_\mathrm{b}$(6152)$^0$ in the $\Lambda_\mathrm{b}^0\pi^+\pi^-$ mass spectrum is confirmed, and their masses are measured. The $\Lambda_\mathrm{b}^0\pi^+\pi^-$ mass distribution exhibits a broad excess of events in the region of 6040-6100 MeV, whose origin cannot be discerned with the present data.
Measured masses
Measured mass differences
This paper presents a measurement of jet fragmentation functions in 0.49 nb$^{-1}$ of Pb+Pb collisions and 25 pb$^{-1}$ of $pp$ collisions at $\sqrt{s_{NN}} = 5.02$ TeV collected in 2015 with the ATLAS detector at the LHC. These measurements provide insight into the jet quenching process in the quark-gluon plasma created in the aftermath of ultra-relativistic collisions between two nuclei. The modifications to the jet fragmentation functions are quantified by dividing the measurements in Pb+Pb collisions by baseline measurements in $pp$ collisions. This ratio is studied as a function of the transverse momentum of the jet, the jet rapidity, and the centrality of the collision. In both collision systems, the jet fragmentation functions are measured for jets with transverse momentum between 126 GeV and 398 GeV and with an absolute value of jet rapidity less than 2.1. An enhancement of particles carrying a small fraction of the jet momentum is observed, which increases with centrality and with increasing jet transverse momentum. Yields of particles carrying a very large fraction of the jet momentum are also observed to be enhanced. Between these two enhancements of the fragmentation functions a suppression of particles carrying an intermediate fraction of the jet momentum is observed in Pb+Pb collisions. A small dependence of the modifications on jet rapidity is observed.
The D(z) distributions in different centrality intervals in PbPb and in pp for 126.00 < pTjet < 158.49 and 0.0 < eta < 0.3.
The D(z) distributions in different centrality intervals in PbPb and in pp for 126.00 < pTjet < 158.49 and 0.0 < eta < 2.1.
The D(pT) distributions in different centrality intervals in PbPb and in pp for 126.00 < pTjet < 158.49 and 0.0 < eta < 0.3.
A search for exotic decays of the Higgs boson to a pair of light pseudoscalar particles a$_1$ is performed under the hypothesis that one of the pseudoscalars decays to a pair of opposite sign muons and the other decays to b$\overline{\mathrm{b}}$. Such signatures are predicted in a number of extensions of the standard model (SM), including next-to-minimal supersymmetry and two-Higgs-doublet models with an additional scalar singlet. The results are based on a data set of proton-proton collisions corresponding to an integrated luminosity of 35.9 fb$^{-1}$, accumulated with the CMS experiment at the CERN LHC in 2016 at a centre-of-mass energy of 13 TeV. No statistically significant excess is observed with respect to the SM backgrounds in the search region for pseudoscalar masses from 20 GeV to half of the Higgs boson mass. Upper limits at 95% confidence level are set on the product of the production cross section and branching fraction, $\sigma_{\mathrm{h}}\mathcal{B}$(h $\to$ a$_1$ a$_1$ $\to$ $\mu^+\mu^-\mathrm{b}\bar{\mathrm{b}}$), ranging from 5 to 33 fb, depending on the pseudoscalar mass. Corresponding limits on the branching fraction, assuming the SM prediction for $\sigma_{\mathrm{h}}$, are (1$-$7)$\times$ 10$^{-4}$.
Observed and expected upper limits at 95% CL on the product of the Higgs boson production cross section and B(h->aa->mumubb)
Observed and expected upper limits at 95% CL on the branching fraction of (h->aa->mumubb)
A search is presented for a Higgs boson that is produced in association with a Z boson and that decays to an undetected particle together with an isolated photon. The search is performed by the CMS Collaboration at the Large Hadron Collider using a data set corresponding to an integrated luminosity of 137 fb$^{-1}$ recorded at a center-of-mass energy of 13 TeV. No significant excess of events above the expectation from the standard model background is found. The results are interpreted in the context of a theoretical model in which the undetected particle is a massless dark photon. An upper limit is set on the product of the cross section for associated Higgs and Z boson production and the branching fraction for such a Higgs boson decay, as a function of the Higgs boson mass. For a mass of 125 GeV, assuming the standard model production cross section, this corresponds to an observed (expected) upper limit on this branching fraction of 4.6 (3.6)% at 95% confidence level. These are the first limits on Higgs boson decays to final states that include an undetected massless dark photon.
Observed yields, background estimates after the fit to data, and signal predictions after the event selection in the signal region. The signal size corresponds to $0.1 \sigma_{\mathrm{\mathrm{ZH}}}$ for all three $m_{\mathrm{\mathrm{H}}}$ values shown. The combined statistical and systematic uncertainties are reported.
Observed yields, background estimates after the fit to data, and signal predictions after the event selection in the signal region. The signal size corresponds to $0.1 \sigma_{\mathrm{\mathrm{ZH}}}$ for all three $m_{\mathrm{\mathrm{H}}}$ values shown. The combined statistical and systematic uncertainties are reported.
Expected yields for different processes after several selection stages. The preselection requires two leptons and at least one photon with $\mathrm{p_\mathrm{T}}$ larger than 25, 20, and 25 GeV, respectively; in addition the dilepton $\mathrm{p_\mathrm{T}}$ must be larger than 60 GeV, and the $\mathrm{p_\mathrm{T}}^{\mathrm{miss}}$ larger than 70 GeV. The signal prediction corresponds to $0.1 \sigma_{\mathrm{\mathrm{ZH}}}$ at $m_{H}$ = 125 GeV.
A search is presented for pairs of light pseudoscalar bosons, in the mass range from 4 to 15 GeV, produced from decays of the 125 GeV Higgs boson. The decay modes considered are final states that arise when one of the pseudoscalars decays to a pair of tau leptons, and the other one either into a pair of tau leptons or muons. The search is based on proton-proton collisions collected by the CMS experiment in 2016 at a center-of-mass energy of 13 TeV that correspond to an integrated luminosity of 35.9 fb${-1}$. The 2$\mu$2$\tau$ and 4$\tau$ channels are used in combination to constrain the product of the Higgs boson production cross section and the branching fraction into 4$\tau$ final state, $\sigma\mathcal{B}$, exploiting the linear dependence of the fermionic coupling strength of pseudoscalar bosons on the fermion mass. No significant excess is observed beyond the expectation from the standard model. The observed and expected upper limits at 95% confidence level on $\sigma\mathcal{B}$, relative to the standard model Higgs boson production cross section, are set respectively between 0.022 and 0.23 and between 0.027 and 0.19 in the mass range probed by the analysis.
Expected and observed 95% CL upper limits on (sigma(pp->h)/sigma(pp->hSM)) * B(h -> aa -> tautautautau) as a function of m(a) obtained from the 13 TeV data, where h(SM) is the Higgs boson of the standard model, h is the observed particle with mass of 125 GeV, and (a) denotes a light Higgs-like state.
The differential cross section for isolated-photon production in $pp$ collisions is measured at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC using an integrated luminosity of 36.1 fb$^{-1}$. The differential cross section is presented as a function of the photon transverse energy in different regions of photon pseudorapidity. The differential cross section as a function of the absolute value of the photon pseudorapidity is also presented in different regions of photon transverse energy. Next-to-leading-order QCD calculations from JETPHOX and SHERPA as well as next-to-next-to-leading-order QCD calculations from NNLOJET are compared with the measurement, using several parameterisations of the proton parton distribution functions. The predictions provide a good description of the data within the experimental and theoretical uncertainties.
Measured cross sections for inclusive isolated-photon production as a function of $E_{\rm T}^{\gamma}$ for $|\eta^{\gamma}|<0.6$.
Measured cross sections for inclusive isolated-photon production as a function of $E_{\rm T}^{\gamma}$ for $0.6<|\eta^{\gamma}|<1.37$.
Measured cross sections for inclusive isolated-photon production as a function of $E_{\rm T}^{\gamma}$ for $1.56<|\eta^{\gamma}|<1.81$.
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 a light charged Higgs boson (H$^+$) decaying to a W boson and a CP-odd Higgs boson (A) in final states with e$\mu\mu$ or $\mu\mu\mu$ is performed using data from pp collisions at $\sqrt{s}=$ 13 TeV, recorded by the CMS detector at the LHC and corresponding to an integrated luminosity of 35.9 fb$^{-1}$. In this search, it is assumed that the H$^+$ boson is produced in decays of top quarks, and the A boson decays to two oppositely charged muons. The presence of signals for H$^+$ boson masses between 100 and 160 GeV and A boson masses between 15 and 75 GeV is investigated. No evidence for the production of the H$^+$ boson is found. Upper limits at 95% confidence level are obtained on the combined branching fraction for the decay chain t $\to$ bH$^+$ $\to$ bW$^+$A $\to$ bW$^+\mu^+\mu^-$, of 1.9 $\times$ 10$^{-6}$ to 8.6 $\times$ 10$^{-6}$, depending on the masses of the H$^+$ and A bosons. These are the first limits for these decay modes of the H$^+$ and A bosons.
Expected and observed upper limits at 95% CL on the branching fraction of the top quark, $\mathcal{B}(\mathrm{t}\to\mathrm{b}\mathrm{H^{+}})$, for the A boson masses ($\mathit{m}_{\mathrm{A}}$), with an assumption of the $\mathrm{H^{+}}$ boson mass $\mathit{m}_{\mathrm{H^{+}}}=\mathit{m}_{\mathrm{A}}$+85 GeV. In the calculation, the $t\overline{t}$ production cross section is set to 832 pb, and the branching fractions $\mathcal{B}(\mathrm{A}\to\mu^{+}\mu^{-})$ and $\mathcal{B}(\mathrm{H^{+}}\to\mathrm{W^{+}}\mathrm{A})$ are assumed to be $3\times10^{-4}$ and 1, respectively.
Expected and observed upper limits at 95% CL on $\mathcal{B}_{sig}=\mathcal{B}(\mathrm{t}\to\mathrm{b}\mathrm{H^{+}})\mathcal{B}(\mathrm{H^{+}}\to\mathrm{W^{+}}\mathrm{A})\mathcal{B}(\mathrm{A}\to\mu^{+}\mu^{-})$ for the A boson masses ($\mathit{m}_{\mathrm{A}}$). The $\mathrm{H^{+}}$ boson mass is assumed to be $\mathit{m}_{\mathrm{A}}$+85 GeV in the calculation.
Expected and observed upper limits at 95% CL on the branching fraction of the top quark, $\mathcal{B}(\mathrm{t}\to\mathrm{b}\mathrm{H^{+}})$, for the A boson masses ($\mathit{m}_{\mathrm{A}}$), with an assumption of the $\mathrm{H^{+}}$ boson mass $\mathit{m}_{\mathrm{H^{+}}}$ = 160 GeV. In the calculation, the $t\overline{t}$ production cross section is set to 832 pb, and the branching fractions $\mathcal{B}(\mathrm{A}\to\mu^{+}\mu^{-})$ and $\mathcal{B}(\mathrm{H^{+}}\to\mathrm{W^{+}}\mathrm{A})$ are assumed to be $3\times10^{-4}$ and 1, respectively.
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