Search

We formulate PanScales parton showers for hadron collisions so as to achieve next-to-leading logarithmic (NLL) accuracy across a broad set of observables. We do so specifically for colour singlet production. Relative to the existing PanScales final-state showers, the main new question is that of how to redistribute momentum imbalances from initial-state branching across the remainder of the event. We presents tests of the showers at fixed order, including the treatment of full colour for soft-collinear emissions and of spin correlations in both the soft and collinear domains. We also include comparisons to a formulation of a standard dipole shower, the current leading-logarithmic state of the art. A forthcoming companion paper will explore all-order tests of the new showers., 65 pages, 15 figures; v2 includes additional clarifications and comparisons, as accepted for publication in JHEP

We study the associated production of prompt $J/\psi$ mesons and $W$ or $Z$ bosons within the factorization approach of nonrelativistic QCD (NRQCD) at next-to-leading order in $\alpha_s$, via intermediate color singlet ${^3}S_1^{[1]}$ and ${^3}P_J^{[1]}$ and color octet ${^1S}_0^{[8]}$, ${^3S}_1^{[8]}$ and ${^3P}_J^{[8]}$ states. Requiring for our predictions to be compatible with recent ATLAS measurements yields stringent new constraints on charmonium long-distance matrix elements (LDMEs) being nonperturbative, process-independent input parameters. Considering four popular LDME sets fitted to data of single $J/\psi$ inclusive production, we find that one is marginally compatible with the data, with central predictions typically falling short by a factor of three, one is unfavored, the factor of shortfall being about one order of magnitude, and two violate cross section positivity for direct $J/\psi+W/Z$ production. The large rate of prompt $J/\psi$ plus $W$ production observed by ATLAS provides strong evidence for the color octet mechanism inherent to NRQCD factorization, the leading color singlet contribution entering only at $\mathcal{O}(G_F\alpha_s^4)$, beyond the order considered here.

We consider a particular composite Higgs model which contains SU(3) color octet top partners besides the usually considered triplet representations. Moreover, color singlet top partners are present as well which can in principle serve as dark matter candidates. We investigate the LHC phenomenology of these unusual top partners. Some of these states could be confused with gluinos predicted in supersymmetric models at first glance., 29 pages, 10 figures + appendices; v2: matches published version

We present state-of-the-art predictions for transverse observables relevant to colour-singlet production at the LHC, in particular the transverse momentum of the colour singlet in gluon-fusion Higgs production and in neutral Drell-Yan lepton-pair production, as well as the $\phi^*_\eta$ observable in Drell Yan. We perform a next-to-next-to-next-to-leading logarithmic (N$^3$LL) resummation of such observables in momentum space according to the RadISH formalism, consistently including in our prediction all constant terms of relative order $\alpha_s^3$ with respect to the Born, thereby achieving N$^3$LL$^\prime$ accuracy. The calculation is fully exclusive with respect to the Born kinematics, which allows the application of arbitrary fiducial selection cuts on the decay products of the colour singlet. We supplement our results with a transverse-recoil prescription, accounting for dominant classes of subleading-power corrections in a fiducial setup. The resummed predictions are matched with fixed-order differential spectra at next-to-next-to-leading order (NNLO) accuracy. A phenomenological comparison is carried out with 13 TeV LHC data relevant to the Higgs to di-photon channel, as well as to neutral Drell-Yan lepton-pair production. Overall, the inclusion of ${\cal O}(\alpha_s^3)$ constant terms, and to a lesser extent of transverse-recoil effects, proves beneficial for the comparison of theoretical predictions to data, leaving a residual theoretical uncertainty in the resummation region at the 2-5% level for Drell-Yan observables, and 5-7% in Higgs production., Comment: 35 pages, 9 figures. Journal version

The European physical journal / Plus 136(2), 214 (1-52) (2021). doi:10.1140/epjp/s13360-021-01155-y, We derive the leading-power singular terms at three loops for both $q_T$ and 0-jettiness, $\mathcal {T}_0$, for generic color-singlet processes. Our results provide the complete set of differential subtraction terms for $q_T$ and $\mathcal {T}_0$ subtractions at $\hbox {N}^3\hbox {LO}$, which are an important ingredient for matching $\hbox {N}^3\hbox {LO}$ calculations with parton showers. We obtain the full three-loop structure of the relevant beam and soft functions, which are necessary ingredients for the resummation of $q_T$ and $\mathcal {T}_0$ at $\hbox {N}^3\hbox {LL}'$ and $\hbox {N}^4\hbox {LL}$ order, and which constitute important building blocks in other contexts as well. The nonlogarithmic boundary coefficients of the beam functions, which contribute to the integrated subtraction terms, are not yet fully known at three loops. By exploiting consistency relations between different factorization limits, we derive results for the $q_T$ and $\mathcal {T}_0$ beam function coefficients at $\hbox {N}^3\hbox {LO}$ in the $z\rightarrow 1$ threshold limit, and we also estimate the size of the unknown terms beyond threshold., Published by Springer, Heidelberg

We study reported deviations between observations and theoretical predictions associated with the production of a pair of di-leptonically decaying top quarks at the LHC, and we examine the possibility that they reflect a signal of toponium formation. We investigate the production by gluon fusion of a color-singlet spin-0 toponium $\eta_t$ bound state of a top and anti-top quark, that then decays di-leptonically ($gg\to\eta_t \to \bar\ell \ell b \bar{b} \nu \bar{\nu}$). We find strong correlations favoring the production of di-lepton systems featuring a small angular separation in azimuth and a small invariant mass. Although toponium production only contributes to 0.8% of the total top-quark pair-production cross section at the 13 TeV LHC, there is a possibility that it can account for observed excesses in the narrow edges of phase space. We propose a method to discover toponium formation by `reconstructing' both its top and anti-top quark constituents in the di-lepton channel., Comment: 6 pages, 2 figures and 2 tables; matches published version

I describe MiNNLO$_{\rm PS}$, a novel method to match NNLO QCD computations with Parton Showers, and present a selection of results for color-singlet production at the LHC., Comment: 5 pages, 2 figures. Contribution to the proceedings of the 55th Rencontres de Moriond on QCD and High Energy Interactions, 27 March-3 April 2021

We present the combination of fully differential cross sections for colour-singlet production processes at next-to-next-to-leading order (NNLO) QCD obtained with MATRIX and all-order resummation through RadISH. This interface allows us to achieve unprecedented accuracy for various transverse observables in $2 \rightarrow 2$ production processes. As an important application we consider $W^+ W^-$ production at the LHC, more precisely the full leptonic process $pp\to \ell^+\ell^{\prime\, -}\nu_{\ell}{\bar\nu}_{\ell^\prime}+X$ with $\ell'\neq\ell$, and we present resummed predictions for differential distributions in presence of fiducial selection cuts. In particular, we resum the transverse-momentum spectrum of the $W^+ W^-$ pair at next-to-next-to-next-to-leading logarithmic (N$^3$LL) accuracy and match it to the integrated NNLO cross section. The transverse-momentum spectrum of the leading jet in $W^+ W^-$ production is calculated at NNLO+NNLL accuracy. Finally, the joint resummation for the transverse-momentum spectrum of the $W^+ W^-$ pair in the presence of a jet veto is performed at NNLO+NNLL. Our phenomenological study highlights the importance of higher-order perturbative and logarithmic corrections for precision phenomenology at the LHC., Comment: 47 pages, 7 figures, 2 tables; corresponds to the version published on JHEP

The nuclear $p_\perp$-broadening of Drell-Yan and quarkonium ($J/\psi$, $\Upsilon$) production in $\pi$A and pA collisions is investigated. The world data follow a simple scaling from SPS to LHC energies, once the process-dependent color factors are properly taken into account, which allows for the extraction of the transport coefficient in cold nuclear matter. We find that $\hat{q}(x)\propto x^{-\alpha}$ with $\alpha=0.25$-$0.30$. The magnitude of the transport coefficient at $x=10^{-2}$ is $\hat{q}_0=0.051$ GeV$^2$/fm and $\hat{q}_0=0.075$ GeV$^2$/fm, whether $Q\bar{Q}$ pairs are assumed to be produced as color octet or color singlet states, respectively. The relation between nuclear broadening data and the (CT14) gluon density is also investigated., Comment: 22 pages, 5 figures

In view of the good agreement between the LHCb prompt-eta(c) data at sqrt(s)=7 and 8 TeV and the NLO colour-singlet model predictions --i.e. the leading v^2 NRQCD contribution--, we provide predictions in the LHCb acceptance for the forthcoming 13 TeV analysis bearing on data taken during the LHC Run2. We also provide predictions for sqrt(s)=115 GeV for proton-hydrogen collisions in the fixed-target mode which could be studied during the LHC Run3. Our predictions are complemented by a full theoretical uncertainty analysis. In addition to cross section predictions, we elaborate on the uncertainties on the p bar-p branching ratio --necessary for data-theory comparison-- and discuss other usable branching fractions for future studies., 10 pages, 4 figures, 1 table, LaTeX uses elsarticle

Soft threshold factorization has been used extensively to study hadronic collisions. It is derived in the limit where the momentum fractions $x_{a,b}$ of both incoming partons approach $x_{a,b}\to 1$. We present a generalized threshold factorization theorem for color-singlet processes, which holds in the weaker limit of only $x_a \to 1$ for generic $x_b$ (or vice versa), corresponding to the limit of large rapidity but generic invariant mass of the produced color singlet. It encodes the complete soft and/or collinear singular structure in the partonic momentum fractions to all orders in perturbation theory, including in particular flavor-nondiagonal partonic channels at leading power. It provides a more powerful approximation than the classic soft threshold limit, capturing a much larger set of contributions. We demonstrate this explicitly for the Z and Higgs rapidity spectrum to NNLO, and we use it to predict a nontrivial set of its N3LO contributions. Our factorization theorem provides the relevant resummation of large-$x$ logarithms in the rapidity spectrum required for resummation-improved PDF fits. One of our factorization ingredients is a new beam function closely related to the N-jettiness beam function. As a byproduct, we identify the correct soft threshold factorization for rapidity spectra among the differing results in the literature.

The perturbative description of certain differential distributions across a wide kinematic range requires the matching of fixed-order perturbation theory with resummation of large logarithmic corrections to all orders. We present precise matched predictions for transverse-momentum distributions in Higgs boson (H) and Drell-Yan pair (DY) production as well as for the closely related ϕ η * distribution at the LHC. The calculation is exclusive in the Born kinematics, and allows for arbitrary fiducial selection cuts on the decay products of the colour singlets, which is of primary relevance for experimental analyses. Our predictions feature very small residual scale uncertainties and display a good convergence of the perturbative series. A comparison of the predictions for DY observables to experimental data at 8 TeV shows a very good agreement within the quoted errors., Journal of High Energy Physics, 2018 (12), ISSN:1126-6708, ISSN:1029-8479

Physical review / D covering particles, fields, gravitation, and cosmology 93(9), 094012(2016). doi:10.1103/PhysRevD.93.094012, In the framework of the $k_T$-factorization approach, the production of prompt ψ(2S) mesons in pp collisions at the LHC energies is studied. Our consideration is based on the off-shell amplitudes for hard partonic subprocesses g*g*→χ$_{cJ}$ and nonrelativistic QCD formalism for bound states. The transverse-momentum-dependent (unintegrated) gluon densities in a proton were derived from the Ciafaloni-Catani-Fiorani-Marchesini evolution equation or, alternatively, were chosen in accordance with the Kimber-Martin-Ryskin prescription. Taking into account both color-singlet and color-octet contributions, we deduce the corresponding nonperturbative long-distance matrix elements from the fits to the latest ATLAS data on χ$_{c1}$ and χ$_{c2}$ transverse-momentum distributions at $\sqrt{s} =$ 7 TeV. We find that these distributions at small and moderate pT are formed mainly by the color-singlet components. We successfully described the data on the relative production rates σ(χ$_{c2}$)/σ(χ$_{c1}$) presented by the ATLAS, CMS, and LHCb Collaborations. We find that the fit points to unequal wave functions of χ$_{c1}$ and χ$_{c2}$ states., Published by APS, Woodbury, NY

In the framework of the $k_T$-factorization approach, the production of prompt χ$_c$ mesons in pp collisions at the LHC energies is studied. Our consideration is based on the off-shell amplitudes for hard partonic subprocesses g*g*→χ$_{cJ}$ and nonrelativistic QCD formalism for bound states. The transverse-momentum-dependent (unintegrated) gluon densities in a proton were derived from the Ciafaloni-Catani-Fiorani-Marchesini evolution equation or, alternatively, were chosen in accordance with the Kimber-Martin-Ryskin prescription. Taking into account both color-singlet and color-octet contributions, we deduce the corresponding nonperturbative long-distance matrix elements from the fits to the latest ATLAS data on χ$_{c1}$ and χ$_{c2}$ transverse-momentum distributions at $\sqrt{s}=$ 7 TeV. We find that these distributions at small and moderate pT are formed mainly by the color-singlet components. We successfully described the data on the relative production rates σ(χ$_{c2}$)/σ(χ$_{c1}$) presented by the ATLAS, CMS, and LHCb Collaborations. We find that the fit points to unequal wave functions of χ$_{c1}$ and χ$_{c2}$ states.

In quantum chromodynamics (QCD), the binding energy of an infinitely heavy quark--antiquark pair in a color singlet state can be calculated as a function of the distance. We investigate this static potential of QCD perturbatively and calculate the full two-loop coefficient, correcting an earlier result. Beyond this order, the perturbative expansion breaks down., 4 pages, LaTeX, 1 figure. Invited talk given at at QCD Euroconference 99, Montpellier, 7-13 July 1999

The inclusive J/ψJ/ψ production cross-section and fraction of J/ψJ/ψ mesons produced in B -hadron decays are measured in proton–proton collisions at √s = 7 TeV with the ATLAS detector at the LHC, as a function of the transverse momentum and rapidity of the [J over ψ], using 2.3 pb[superscript −1] of integrated luminosity. The cross-section is measured from a minimum pT of 1 GeV to a maximum of 70 GeV and for rapidities within |y, European Organization for Nuclear Research, United States. Dept. of Energy, National Science Foundation (U.S.), Brookhaven National Laboratory

We present results on J/psi production in muon interactions with tin and carbon targets at incident muon energies of 200 and 280 GeV. The ratio of cross sections per nucleon for J/psi production on tin and carbon, R(Sn/C), is studied as a function of p(T)2, z and x. We find an enhancement for coherent J/psi production R(coh)(Sn/C) = 1.54 +/- 0.07, a suppression for quasielastic production R(qe)(Sn/C) = 0.79 +/- 0.06 and for inelastic production R(in)(Sn/C) = 1.13 +/- 0.08. The inelastic cross section ratio can be interpreted within the Colour Singlet model as an enhancement ot the gluon distribution in tin with respect to that in carbon. The dependence of the ratio on z and p(T)2 can explain the discrepancy between the results obtained in previous experiments.

Universität Hamburg, Diss., 2009; 102 pp. (2009). = Universität Hamburg, Diss., 2009

We study the free energy of static three quark systems in singlet, octet, decuplet and average color channels in the quenched approximation and in 2-flavor QCD at finite temperature. We show that in the high temperature phase singlet and decuplet free energies of three quark systems are well described by the sum of the free energies of three diquark systems plus self energy contributions of the three quarks. In the confining low temperature phase we find evidence for a Y-shaped flux tube in SU(3) pure gauge theory, which is less evident in 2-flavor QCD due to the onset of string breaking. We also compare the short distance behavior of octet and decuplet free energies to the free energies of single static quarks in the corresponding color representations., 11 pages, 17 figures

38 pp. (2003)., The cross sections for inelastic photoproduction of J/psi and psi' mesons have been measured in ep collisions with the ZEUS detector at HERA, using an integrated luminosity of 38.0 pb-1. The events were required to have 0.1 < z < 0.9 and 50 < W < 180 GeV, where z is the fraction of the incident photon energy carried by the J/psi in the proton rest frame and W is the photon-proton centre-of-mass energy. The psi' to J/psi cross-section ratio was measured in the range 0.55 < z < 0.9. The J/psi data, for various ranges of transverse momentum, are compared to theoretical models incorporating colour-singlet and colour-octet matrix elements. Predictions of a next-to-leading-order colour-singlet model give a good description of the data, although there is a large normalisation uncertainty. The J/psi helicity distribution for z > 0.4 is compared to leading-order QCD predictions.

Universität Hamburg, Diss., 2003; 154 pp., (2003). doi:10.3204/DESY-THESIS-2003-040, Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg

Universität Hamburg, Diss., 2001; 136 pp. (2001). = Universität Hamburg, Diss., 2001

Universität Hamburg, Diss., 1999; 79 pp. (1999). = Universität Hamburg, Diss., 1999

160 pp. (1997).

McGill Univerity, Diss., 1997; 125 pp. (1997). = McGill Univerity, Diss., 1997

McGill University, Diss., 1996; 141 pp. (1996). = McGill University, Diss., 1996

Measurements are presented of the inclusive distributions of theJ/Ψ meson produced by muons of energy 200 GeV from an ammonia target. The gluon distribution of the nucleon has been derived from the data in the range 0.04

Journal of high energy physics 2013(2), 071(2013). doi:10.1007/JHEP02(2013)071, The cross sections for inelastic photoproduction of J/ψ and ψ′ mesons have been measured in ep collisions with the ZEUS detector at HERA, using an integrated luminosity of 468 pb(−1) collected in the period 1996–2007. The ψ′ to J/ψ cross section ratio was measured in the range 0.55 < z < 0.9 and 60 < W < 190 GeV as a function of W, z and p ( )T( ). Here W denotes the photon-proton centre-of-mass energy, z is the fraction of the incident photon energy carried by the meson and p ( )T( ) is the transverse momentum of the meson with respect to the beam axis. The J/ψ cross sections were measured for 0.1 < z < 0.9, 60 < W < 240 GeV and p ( )T( ) > 1 GeV. Theoretical predictions within the non-relativistic QCD framework including NLO colour-singlet and colour-octet contributions were compared to the data, as were predictions based on the k ( )T( )-factorisation approach., Published by Springer, Berlin