Several extensions of the Standard Model predict the production of dark matter particles at the LHC. A search for dark matter particles produced in association with a dark Higgs boson decaying into $W^{+}W^{-}$ in the $\ell^\pm\nu q \bar q'$ final states with $\ell=e,\mu$ is presented. This analysis uses 139 fb$^{-1}$ of $pp$ collisions recorded by the ATLAS detector at a centre-of-mass energy of 13 TeV. The $W^\pm \to q\bar q'$ decays are reconstructed from pairs of calorimeter-measured jets or from track-assisted reclustered jets, a technique aimed at resolving the dense topology from a pair of boosted quarks using jets in the calorimeter and tracking information. The observed data are found to agree with Standard Model predictions. Scenarios with dark Higgs boson masses ranging between 140 and 390 GeV are excluded.
Global polarizations ($P$) of $\Lambda$ ($\bar{\Lambda}$) hyperons have been observed in non-central heavy-ion collisions. The strong magnetic field primarily created by the spectator protons in such collisions would split the $\Lambda$ and $\bar{\Lambda}$ global polarizations ($\Delta P = P_{\Lambda} - P_{\bar{\Lambda}} < 0$). Additionally, quantum chromodynamics (QCD) predicts topological charge fluctuations in vacuum, resulting in a chirality imbalance or parity violation in a local domain. This would give rise to an imbalance ($\Delta n = \frac{N_{\text{L}} - N_{\text{R}}}{\langle N_{\text{L}} + N_{\text{R}} \rangle} \neq 0$) between left- and right-handed $\Lambda$ ($\bar{\Lambda}$) as well as a charge separation along the magnetic field, referred to as the chiral magnetic effect (CME). This charge separation can be characterized by the parity-even azimuthal correlator ($\Delta\gamma$) and parity-odd azimuthal harmonic observable ($\Delta a_{1}$). Measurements of $\Delta P$, $\Delta\gamma$, and $\Delta a_{1}$ have not led to definitive conclusions concerning the CME or the magnetic field, and $\Delta n$ has not been measured previously. Correlations among these observables may reveal new insights. This paper reports measurements of correlation between $\Delta n$ and $\Delta a_{1}$, which is sensitive to chirality fluctuations, and correlation between $\Delta P$ and $\Delta\gamma$ sensitive to magnetic field in Au+Au collisions at 27 GeV. For both measurements, no correlations have been observed beyond statistical fluctuations.
We measure the ratio of cross section times branching fraction, $R_p \equiv \sigma_{\chi_{c2}} {\cal B}(\chi_{c2} \to J/\psi \gamma)/ \sigma_{\chi_{c1}} {\cal B}(\chi_{c1} \to J/\psi \gamma)$, in 1.1 fb$^{-1}$ of $p\bar{p}$ collisions at $\sqrt{s} =$ 1.96 TeV. This measurement covers the kinematic range $p_T(J/\psi)>4.0$ GeV/$c$, $|\eta(J/\psi)| < 1.0$, and $p_T(\gamma)>1.0$ GeV/$c$. For events due to prompt processes, we find $R_p = 0.395\pm0.016(stat.)\pm0.015(sys.)$. This result represents a significant improvement in precision over previous measurements of prompt $\chi_{c1,2}$ hadroproduction.
We report on a study of the dijet invariant-mass distribution in events with one identified lepton, a significant imbalance in the total event transverse momentum, and two jets. This distribution is sensitive to the possible production of a new particle in association with a $W$ boson, where the boson decays leptonically. We use the full data set of proton-antiproton collisions at 1.96 TeV center-of-mass energy collected by the Collider Detector at the Fermilab Tevatron and corresponding to an integrated luminosity of 8.9 fb$^{-1}$. The data are found to be consistent with standard-model expectations, and a 95$\%$ confidence level upper limit is set on the cross section for a $W$ boson produced in association with a new particle decaying into two jets.
We present a measurement of the differential cross section as a function of transverse momentum of the Z boson in ppbar collisions at sqrt{s}=1.8 TeV using data collected by the D0 experiment at the Fermilab Tevatron Collider during 1994--1996. We find good agreement between our data and the NNLO resummation prediction and extract values of the non-perturbative parameters for the resummed prediction from a fit to the differential cross section.
The global topologies of inclusive three-- and four--jet events produced in $\pp$ interactions are described. The three-- and four--jet events are selected from data recorded by the D\O\ detector at the Tevatron Collider operating at a center--of--mass energy of $\sqrt{s} = 1800$ GeV. The measured, normalized distributions of various topological variables are compared with parton--level predictions of tree--level QCD calculations. The parton--level QCD calculations are found to be in good agreement with the data. The studies also show that the topological distributions of the different subprocesses involving different numbers of quarks are very similar and reproduce the measured distributions well. The parton shower Monte Carlo generators provide a less satisfactory description of the topologies of the three-- and four--jet events.
This Letter describes the current most precise measurement of the $W$ boson pair production cross section and most sensitive test of anomalous $WW\gamma$ and $WWZ$ couplings in $p \bar p$ collisions at a center-of-mass energy of 1.96 TeV. The $WW$ candidates are reconstructed from decays containing two charged leptons and two neutrinos, where the charged leptons are either electrons or muons. Using data collected by the CDF II detector from 3.6 fb$^{-1}$ of integrated luminosity, a total of 654 candidate events are observed with an expected background contribution of $320 \pm 47$ events. The measured total cross section is $\sigma (p \bar p \to W^+ W^- + X) = 12.1 \pm 0.9 \textrm{(stat)} ^{+1.6}_{-1.4} \textrm{(syst)}$ pb, which is in good agreement with the standard model prediction. The same data sample is used to place constraints on anomalous $WW\gamma$ and $WWZ$ couplings.
We present the first measurement of the production cross section of a W boson with a single charm quark (c) in p-pbar collisions at sqrt(s)=1.96 TeV, using soft muon tagging of c jets. In a data sample of ~1.8 fb-1, recorded with the CDF II detector at the Fermilab Tevatron, we select events with W+1 or 2 jets. We use the charge correlation between the W and the muon from the semileptonic decay of a charm hadron to extract the Wc signal. We measure sigma_{Wc}(p_{Tc}>20 GeV/c, |\eta_c|<1.5)\times BR(W->\ell\nu) = 9.8+/-3.2 pb, in agreement with theoretical expectations.
We report measurements of the inclusive transverse momentum pT distribution of centrally produced kshort, kstar(892), and phi(1020) mesons up to pT = 10 GeV/c in minimum-bias events, and kshort and lambda particles up to pT = 20 GeV/c in jets with transverse energy between 25 GeV and 160 GeV in pbar p collisions. The data were taken with the CDF II detector at the Fermilab Tevatron at sqrt(s) = 1.96 TeV. We find that as pT increases, the pT slopes of the three mesons (kshort, kstar, and phi) are similar, and the ratio of lambda to kshort as a function of pT in minimum-bias events becomes similar to the fairly constant ratio in jets at pT ~ 5 GeV/c. This suggests that the particles with pT >~ 5 GeV/c in minimum-bias events are from soft jets, and that the pT slope of particles in jets is insensitive to light quark flavor (u, d, or s) and to the number of valence quarks. We also find that for pT <~ 4 GeV relatively more lambda baryons are produced in minimum-bias events than in jets.
We report a measurement of the differential cross section, d{\sigma}/d(cos {\theta}t), for top-quark-pair production as a function of the top-quark production angle in proton-antiproton collisions at sqrt{s} = 1.96 TeV. This measurement is performed using data collected with the CDF II detector at the Tevatron, corresponding to an integrated luminosity of 9.4/fb. We employ the Legendre polynomials to characterize the shape of the differential cross section at the parton level. The observed Legendre coefficients are in good agreement with the prediction of the next-to-leading-order standard-model calculation, with the exception of an excess linear-term coefficient, a1 = 0.40 +- 0.12, compared to the standard-model prediction of a1 = 0.15^{+0.07}_{-0.03}.
Forum