Energy Sqrt Research Articles

Search for mathrm{t}overline{mathrm{t}}mathrm{H} production in the mathrm{H}to mathrm{b}overline{mathrm{b}} decay channel with leptonic mathrm{t}overline{mathrm{t}} decays in proton-proton collisions at sqrt{s}=13 TeV

A search is presented for the associated production of a standard model Higgs boson with a top quark-antiquark pair ( mathrm{t}overline{mathrm{t}}mathrm{H} ), in which the Higgs boson decays into a b quark-antiquark pair, in proton-proton collisions at a centre-of-mass energy sqrt{s}=13 TeV. The data correspond to an integrated luminosity of 35.9 fb−1 recorded with the CMS detector at the CERN LHC. Candidate mathrm{t}overline{mathrm{t}}mathrm{H} events are selected that contain either one or two electrons or muons from the mathrm{t}overline{mathrm{t}} decays and are categorised according to the number of jets. Multivariate techniques are employed to further classify the events and eventually discriminate between signal and background. The results are characterised by an observed mathrm{t}overline{mathrm{t}}mathrm{H} signal strength relative to the standard model cross section, μ = σ/σSM, under the assumption of a Higgs boson mass of 125 GeV. A combined fit of multivariate discriminant distributions in all categories results in an observed (expected) upper limit on μ of 1.5 (0.9) at 95% confidence level, and a best fit value of 0.72 ± 0.24(stat) ± 0.38(syst), corresponding to an observed (expected) signal significance of 1.6 (2.2) standard deviations above the background-only hypothesis.

Prompt \u039bc+ production in pPb collisions at sqrt{s_{mathrm{NN}}}=5.02 TeV

The prompt production of Λc+ baryons is studied in proton-lead collisions collected with the LHCb detector at the LHC. The data sample corresponds to an integrated luminosity of 1.58 nb−1 recorded at a nucleon-nucleon centre-of-mass energy of sqrt{s_{mathrm{NN}}}=5.02 TeV. Measurements of the differential cross-section and the forwardbackward production ratio are reported for Λc+ baryons with transverse momenta in the range 2 < pT < 10 GeV/c and rapidities in the ranges 1.5 < y∗ < 4.0 and −4.5 < y∗ < −2.5 in the nucleon-nucleon centre-of-mass system. The ratio of cross-sections of Λc+ baryons and D0 mesons is also reported. The results are compared with next-to-leading order calculations that use nuclear parton distribution functions.

Search for the Higgs Boson Decaying to Two Muons in Proton-Proton Collisions at sqrt[s]=13 TeV.

A search for the Higgs boson decaying to two oppositely charged muons is presented using data recorded by the CMS experiment at the CERN LHC in 2016 at a center-of-mass energy sqrt[s]=13 TeV, corresponding to an integrated luminosity of 35.9 fb^{-1}. Data are found to be compatible with the predicted background. For a Higgs boson with a mass of 125.09GeV, the 95%confidence level observed (background-only expected) upper limit on the production cross section times the branching fraction to a pair of muons is found to be 3.0 (2.5) times the standard model expectation. In combination with data recorded at center-of-mass energies sqrt[s]=7 and 8TeV, the background-only expected upper limit improves to 2.2 times the standard model value with a standard model expected significance of 1.0 standard deviation. The corresponding observed upper limit is 2.9 with an observed significance of 0.9 standard deviation. This corresponds to an observed upper limit on the standard model Higgs boson branching fraction to muons of 6.4×10^{-4} and to an observed signal strength of 1.0±1.0(stat)±0.1(syst).

Elucidating the multiplicity dependence of mathrm {J}/psi production in proton\u2013proton collisions with PYTHIA8

A study of prompt and non-prompt mathrm {J}/psi production as a function of charged-particle multiplicity in inelastic proton–proton (pp) collisions at a centre-of-mass energy of sqrt{s} = 13 TeV based on calculations using the PYTHIA8 Monte Carlo is reported. Recent experimental data shows an intriguing stronger-than-linear increase of the self-normalized mathrm {J}/psi yield with multiplicity; several models, based on initial or final state effects, have been able to describe the observed behaviour. In this paper, the microscopic reasons for this behaviour, like the role of multiple parton interactions, colour reconnections and auto-correlations are investigated. It is observed that the stronger-than-linear increase and the transverse momentum (p_mathrm {T}) dependence, contrary to what is predicted by the other available models, can be attributed to auto-correlation effects only. In absence of auto-correlation effects, the increase of the yield of mathrm {J}/psi with multiplicity – and in general for all hard processes – is weaker than linear for multiplicities exceeding about three times the mean multiplicity. The possibility of disentangling auto-correlation effects from other physical phenomena by measuring the charged-particle multiplicity in different pseudo-rapidity and azimuthal regions relative to the mathrm {J}/psi direction is investigated. In this regard, it is suggested to extend the experimental measurements of mathrm {J}/psi production as a function of the charged-particle multiplicity by determining the multiplicity in several azimuthal regions and in particular in the Transverse region with respect to the direction of the mathrm {J}/psi meson.

Precise predictions for toverline{t}gamma /toverline{t} cross section ratios at the LHC

With the goal of increasing the precision of NLO QCD predictions for the ppto toverline{t}gamma process in the di-lepton top quark decay channel we present theoretical predictions for the mathrm{mathcal{R}}={sigma}_{toverline{t}gamma }/{sigma}_{toverline{t}} cross section ratio. Results for the latter together with various differential cross section ratios are given for the LHC with the Run II energy of sqrt{s}=13 TeV. Fully realistic NLO computations for toverline{t} and toverline{t}gamma production are employed. They are based on matrix elements for {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b} and {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b}gamma processes and include all resonant and non-resonant diagrams, interferences, and off-shell effects of the top quarks and the W gauge bosons. Various renormalisation and factorisation scale choices and parton density functions are examined to assess their impact on the cross section ratio. Depending on the transverse momentum cut on the hard photon a judicious choice of a dynamical scale allows us to obtain 1%–3% percent precision on mathrm{mathcal{R}} . Moreover, for differential cross section ratios theoretical uncertainties in the range of 1%-6% have been estimated. Until now such high precision predictions have only been reserved for the top quark pair production at NNLO QCD. Thus, mathrm{mathcal{R}} at NLO in QCD represents a very precise observable to be measured at the LHC for example to study the top quark charge asymmetry or to probe the strength and the structure of the t- overline{t} -γ vertex. The latter can shed some light on possible new physics that can reveal itself only once sufficiently precise theoretical predictions are available.

Searching for a charged Higgs boson with both H\xb1W\u2213Z and H\xb1tb couplings at the LHC

In certain new physics scenarios, a singly charged Higgs boson can couple to both fermions and W±Z at tree level. We develop new strategies beyond current experimental searches using pp → jjH±, H± → tb at the Large Hadron Collider (LHC). With the effective H±W∓Z and H±tb couplings we perform a model-independent analysis at the collision energy sqrt{s}=13 TeV with the integrated luminosity of 3 ab−1. We derive the discovery prospects and exclusion limits for the charged Higgs boson in the mass range from 200 GeV to 1 TeV. With |FWZ|, |At| ∼ 0.5–1.0 and 300 GeV ≲ {m}_{H^{pm }} ≲ 400 GeV, we point out that a discovery significance of 5σ can be achieved. The constraints and projected sensitivities are also discussed in a realistic model, i.e., the modified Georgi-Machacek model without custodial symmetry. Our proposed search would provide direct evidence for a charged Higgs boson H± that couples to W±Z and tb, and has better sensitivity to the couplings of H±W∓Z and H±tb than current searches.

Measurement of the Absolute Branching Fraction of the Inclusive Semileptonic Λ_{c}^{+} Decay.

Using a data sample of e^{+}e^{-} collisions corresponding to an integrated luminosity of 567 pb^{-1} collected at a center-of-mass energy of sqrt[s]=4.6 GeV with the BESIII detector, we measure the absolute branching fraction of the inclusive semileptonic Λ_{c}^{+} decay with a double-tag method. We obtain B(Λ_{c}^{+}→Xe^{+}ν_{e})=(3.95±0.34±0.09)%, where the first uncertainty is statistical and the second systematic. Using the known Λ_{c}^{+} lifetime and the charge-averaged semileptonic decay width of nonstrange charmed mesons (D^{0} and D^{+}), we obtain the ratio of the inclusive semileptonic decay widths Γ(Λ_{c}^{+}→Xe^{+}ν_{e})/Γ[over ¯](D→Xe^{+}ν_{e})=1.26±0.12.

J/\u03c8 polarization in the CGC+NRQCD approach

We compute the J/ψ polarization observables λθ, λϕ, λθϕ in a Color Glass Condensate (CGC) + nonrelativistic QCQ (NRQCD) formalism that includes contributions from both color singlet and color octet intermediate states. Our results are compared to low pT data on J/ψ polarization from the LHCb and ALICE experiments on proton-proton collisions at center-of-mass energies of sqrt{s}=7 TeV and 8 TeV. Our CGC+NRQCD computation provides a better description of data for pT ≤ 15 GeV relative to extant next-to-leading (NLO) calculations within the collinear factorization framework. These results suggest that higher order computations in the CGC+NRQCD framework have the potential to greatly improve the accuracy of extracted values of the NRQCD universal long distance matrix elements.

Search for Higgs boson pair production in the gamma gamma WW^* channel using pp collision data recorded at sqrt{s}=13 TeV with the ATLAS detector

Searches for non-resonant and resonant Higgs boson pair production are performed in the gamma gamma WW^* channel with the final state of gamma gamma ell nu jj using 36.1 hbox {fb}^{-1} of proton–proton collision data recorded at a centre-of-mass energy of sqrt{s}=13 TeV by the ATLAS detector at the Large Hadron Collider. No significant deviation from the Standard Model prediction is observed. A 95% confidence-level observed upper limit of 7.7 pb is set on the cross section for non-resonant production, while the expected limit is 5.4 pb. A search for a narrow-width resonance X decaying to a pair of Standard Model Higgs bosons HH is performed with the same set of data, and the observed upper limits on sigma (pprightarrow X)times B(Xrightarrow HH) range between 40.0 and 6.1 pb for masses of the resonance between 260 and 500 GeV, while the expected limits range between 17.6 and 4.4 pb. When deriving the limits above, the Standard Model branching ratios of the H{rightarrow }{gamma gamma } and H{rightarrow }WW^* are assumed.

Inelastic profiles of protons at 7 and 13 TeV

The shapes of the inelastic interaction region of protons with energies sqrt{s}=7 and 13 GeV are considered. The unitarity condition with different assumptions about the elastic scattering amplitude is used. It is shown that the sign of the imaginary part of the amplitude at large transferred momenta is especially important for the conclusion about this shape at small impact parameters (central collisions).

Transverse momentum spectra and nuclear modification factors of charged particles in pp, p-Pb and Pb-Pb collisions at the LHC

We report the measured transverse momentum (pT) spectra of primary charged particles from pp, p-Pb and Pb-Pb collisions at a center-of-mass energy sqrt{s_{mathrm{NN}}}=5.02 TeV in the kinematic range of 0.15 < pT< 50 GeV/c and |η| < 0.8. A significant improvement of systematic uncertainties motivated the reanalysis of data in pp and Pb-Pb collisions at sqrt{s_{mathrm{NN}}}=2.76 TeV, as well as in p-Pb collisions at sqrt{s_{mathrm{NN}}}=5.02 TeV, which is also presented. Spectra from Pb-Pb collisions are presented in nine centrality intervals and are compared to a reference spectrum from pp collisions scaled by the number of binary nucleon-nucleon collisions. For central collisions, the pT spectra are suppressed by more than a factor of 7 around 6–7 GeV/c with a significant reduction in suppression towards higher momenta up to 30 GeV/c. The nuclear modification factor RpPb, constructed from the pp and p-Pb spectra measured at the same collision energy, is consistent with unity above 8 GeV/c. While the spectra in both pp and Pb-Pb collisions are substantially harder at sqrt{s_{mathrm{NN}}}=5.02 TeV compared to 2.76 TeV, the nuclear modification factors show no significant collision energy dependence. The obtained results should provide further constraints on the parton energy loss calculations to determine the transport properties of the hot and dense QCD matter.

Coarse graining pi pi scattering

We carry out an analysis of pi pi scattering in the IJ=00, 11 and 20 channels in configuration space up to a maximal center-of-mass energy sqrt{s}=1.4 GeV. We separate the interaction into two regions marked by an elementarity radius of the system; namely, a long distance region above which pions can be assumed to interact as elementary particles and a short distance region where many physical effects cannot be disentangled. The long distance interaction is described by chiral dynamics, where a two-pion-exchange potential is identified, computed and compared to lattice calculations. The short distance piece corresponds to a coarse grained description exemplified by a superposition of delta-shell potentials sampling the interaction with the minimal wavelength. We show how the so constructed non-perturbative scattering amplitude complies with the proper analytic structure, allowing for an explicit N/D type decomposition in terms of the corresponding Jost functions and fulfilling dispersion relations without subtractions. We also address renormalization issues in coordinate space and investigate the role of crossing when fitting the scattering amplitudes above and below threshold to Roy-equation results. At higher energies, we show how inelasticities can be described by one single complex and energy dependent parameter. A successful description of the data can be achieved with a minimal number of fitting parameters, suggesting that coarse graining is a viable approach to analyze hadronic processes.

Search for dark matter in events with a hadronically decaying vector boson and missing transverse momentum in pp collisions at sqrt{s}=13 TeV with the ATLAS detector

A search for dark matter (DM) particles produced in association with a hadronically decaying vector boson is performed using pp collision data at a centre-of-mass energy of sqrt{s}=13 TeV corresponding to an integrated luminosity of 36.1 fb−1, recorded by the ATLAS detector at the Large Hadron Collider. This analysis improves on previous searches for processes with hadronic decays of W and Z bosons in association with large missing transverse momentum (mono-W/Z searches) due to the larger dataset and further optimization of the event selection and signal region definitions. In addition to the mono-W/Z search, the as yet unexplored hypothesis of a new vector boson Z′ produced in association with dark matter is considered (mono-Z′ search). No significant excess over the Standard Model prediction is observed. The results of the mono-W/Z search are interpreted in terms of limits on invisible Higgs boson decays into dark matter particles, constraints on the parameter space of the simplified vector-mediator model and generic upper limits on the visible cross sections for W/Z+DM production. The results of the mono-Z′ search are shown in the framework of several simplified-model scenarios involving DM production in association with the Z′ boson.

Angular analysis of Bd0\u2009\u2192\u2009K\u2217\u03bc+\u03bc\u2212 decays in pp collisions at sqrt{s}=8 TeV with the ATLAS detector

An angular analysis of the decay Bd0 → K∗μ+μ− is presented, based on proton-proton collision data recorded by the ATLAS experiment at the LHC. The study is using 20.3 fb−1 of integrated luminosity collected during 2012 at centre-of-mass energy of sqrt{s}=8 TeV. Measurements of the K* longitudinal polarisation fraction and a set of angular parameters obtained for this decay are presented. The results are compatible with the Standard Model predictions.

Hard photons in hadroproduction of top quarks with realistic final states

We present a complete description of top quark pair production in association with a hard photon in the dilepton channel. Our calculation is accurate to NLO in QCD. It is based on matrix elements for {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b}upgamma production and includes all resonant and non-resonant diagrams, interferences, and off-shell effects of the top quarks and the W gauge bosons. This calculation constitutes the first full computation for top quark pair production with a final state photon in hadronic collisions at NLO in QCD. Numerical results for total and differential cross sections are presented for the LHC at a centre-of-mass energy of sqrt{s}=13 TeV. For a few observables relevant for new physics searches, beyond some kinematic bounds, we observe shape distortions of more than 100%. In addition, we confirm that the size of the top quark off-shell effects for the total cross section is consistent with the expected uncertainties of the narrow width approximation. Results presented here are not only relevant for beyond the Standard Model physics searches but also important for precise measurements of the top-quark fiducial cross sections and top-quark properties at the LHC.

Search for beautiful tetraquarks in the \u03d2(1S)\u03bc+\u03bc\u2212 invariant-mass spectrum

The ϒ(1S)μ+μ− invariant-mass distribution is investigated for a possible exotic meson state composed of two b quarks and two overline{b} quarks, {X}_{boverline{b}boverline{b}} . The analysis is based on a data sample of pp collisions recorded with the LHCb detector at centre-of-mass energies sqrt{s}=7 , 8 and 13 TeV, corresponding to an integrated luminosity of 6.3 fb−1. No significant excess is found, and upper limits are set on the product of the production cross-section and the branching fraction as functions of the mass of the {X}_{boverline{b}boverline{b}} state. The limits are set in the fiducial volume where all muons have pseudorapidity in the range [2.0, 5.0], and the {X}_{boverline{b}boverline{b}} state has rapidity in the range [2.0, 4.5] and transverse momentum less than 15 GeV/c.

Search for a heavy resonance decaying into a Z boson and a Z or W boson in 2\u21132q final states at sqrt{s}=13 TeV

A search has been performed for heavy resonances decaying to ZZ or ZW in 2ℓ2q final states, with two charged leptons (ℓ = e, μ) produced by the decay of a Z boson, and two quarks produced by the decay of a W or Z boson. The analysis is sensitive to resonances with masses in the range from 400 to 4500 GeV. Two categories are defined based on the merged or resolved reconstruction of the hadronically decaying vector boson, optimized for high- and low-mass resonances, respectively. The search is based on data collected during 2016 by the CMS experiment at the LHC in proton-proton collisions with a center-of-mass energy of sqrt{s}=13 TeV, corresponding to an integrated luminosity of 35.9 fb−1. No excess is observed in the data above the standard model background expectation. Upper limits on the production cross section of heavy, narrow spin-1 and spin-2 resonances are derived as a function of the resonance mass, and exclusion limits on the production of W′ bosons and bulk graviton particles are calculated in the framework of the heavy vector triplet model and warped extra dimensions, respectively.

Measurement of charged particle spectra in minimum-bias events from proton\u2013proton collisions at sqrt{s}=13,text {TeV}

Pseudorapidity, transverse momentum, and multiplicity distributions are measured in the pseudorapidity range |eta | < 2.4 for charged particles with transverse momenta satisfying p_{mathrm {T}} > 0.5,text {GeV} in proton–proton collisions at a center-of-mass energy of sqrt{s} = 13,text {TeV} . Measurements are presented in three different event categories. The most inclusive of the categories corresponds to an inelastic mathrm {p}mathrm {p} data set, while the other two categories are exclusive subsets of the inelastic sample that are either enhanced or depleted in single diffractive dissociation events. The measurements are compared to predictions from Monte Carlo event generators used to describe high-energy hadronic interactions in collider and cosmic-ray physics.

Elliptic Flow of Charm and Strange Hadrons in High-Multiplicity p+Pb Collisions at sqrt[s_{NN}]=8.16 TeV.

The elliptic azimuthal anisotropy coefficient (v_{2}) is measured for charm (D^{0}) and strange (K_{S}^{0}, Λ, Ξ^{-}, and Ω^{-}) hadrons, using a data sample of p+Pb collisions collected by the CMS experiment, at a nucleon-nucleon center-of-mass energy of sqrt[s_{NN}]=8.16 TeV. A significant positive v_{2} signal from long-range azimuthal correlations is observed for all particle species in high-multiplicity p+Pb collisions. The measurement represents the first observation of possible long-range collectivity for open heavy flavor hadrons in small systems. The results suggest that charm quarks have a smaller v_{2} than the lighter quarks, probably reflecting a weaker collective behavior. This effect is not seen in the larger PbPb collision system at sqrt[s_{NN}]=5.02 TeV, also presented.

Measurement of the Absolute Branching Fraction of the Inclusive Decay Λ_{c}^{+}→Λ+X.

Based on an e^{+}e^{-} collision data sample corresponding to an integrated luminosity of 567 pb^{-1} taken at the center-of-mass energy of sqrt[s]=4.6 GeV with the BESIII detector, we measure the absolute branching fraction of the inclusive decay Λ_{c}^{+}→Λ+X to be B(Λ_{c}^{+}→Λ+X)=(38.2_{-2.2}^{+2.8}±0.9)% using the double-tag method, where X refers to any possible final state particles. In addition, we search for direct CP violation in the charge asymmetry of this inclusive decay for the first time, and obtain A_{CP}≡[B(Λ_{c}^{+}→Λ+X)-B(Λ[over ¯]_{c}^{-}→Λ[over ¯]+X)]/[B(Λ_{c}^{+}→Λ+X)+B(Λ[over ¯]_{c}^{-}→Λ[over ¯]+X)]=(2.1_{-6.6}^{+7.0}±1.6)%, a statistically limited result with no evidence of CP violation.