Color-singlet Processes Research Articles

Jet-veto resummation at N3LLp + NNLO in boson production processes

Vetoing energetic jet activity is a crucial tool for suppressing backgrounds and enabling new physics searches at the LHC, but the introduction of a veto scale can introduce large logarithms that may need to be resummed. We present an implementation of jet-veto resummation for color-singlet processes at the level of N3LLp matched to fixed-order NNLO predictions. Our public code MCFM allows for predictions of a single boson, such as Z/γ*, W± or H, or with a pair of vector bosons, such as W+W−, W±Z or ZZ. The implementation relies on recent calculations of the soft and beam functions in the presence of a jet veto over all rapidities, with jets defined using a sequential recombination algorithm with jet radius R. However one of the ingredients that is required to reach full N3LL accuracy is only known approximately, hence N3LLp. We describe in detail our formalism and compare with previous public codes that operate at the level of NNLL. Our higher-order predictions improve significantly upon NNLL calculations by reducing theoretical uncertainties. We demonstrate this by comparing our predictions with ATLAS and CMS results.

Fiducial qT resummation of color-singlet processes at N3LL+NNLO

We present a framework for qT resummation at N3LL+NNLO accuracy for arbitrary color-singlet processes based on a factorization theorem in SCET. Our implementation CuTe-MCFM is fully differential in the Born kinematics and matches to large-qT fixed-order predictions at relative order {alpha}_s^2 . It provides an efficient way to estimate uncertainties from fixed-order truncation, resummation, and parton distribution functions. In addition to W±, Z and H production, also the diboson processes γγ, Zγ, ZH and W±H are available, including decays. We discuss and exemplify the framework with several direct comparisons to experimental measurements as well as inclusive benchmark results. In particular, we present novel results for γγ and Zγ at N3LL+NNLO and discuss in detail the power corrections induced by photon isolation requirements.

A toolbox for q_{T} and 0-jettiness subtractions at hbox {N}^3hbox {LO}

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}^3hbox {LO}, which are an important ingredient for matching hbox {N}^3hbox {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}^3hbox {LL}' and hbox {N}^4hbox {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}^3hbox {LO} in the zrightarrow 1 threshold limit, and we also estimate the size of the unknown terms beyond threshold.

The studies on Z rightarrow Upsilon (1S)+g+g at the next-to-leading-order QCD accuracy

In this paper, we carry out the next-to-leading-order (NLO) studies on Z rightarrow Upsilon (1S)+g+g via the color-singlet (CS) b{bar{b}} state. We find the newly calculated NLO QCD corrections to this process can significantly influence its leading-order (LO) results and greatly improve the dependence on the renormalization scale. By including the considerable feeddown contributions, the branching ratio {mathcal {B}}_{Z rightarrow Upsilon (1S)+g+g} is predicted to be (0.56–0.95) times 10^{-6}, which can reach up to 19–31% of the LO predictions given by the CS dominant process Z rightarrow Upsilon (1S)+b+{bar{b}}. Moreover, Z rightarrow Upsilon (1S)+g+g also seriously affects the CS predictions on the Upsilon (1S) energy distributions, especially when z is relatively small. In summary, for the inclusive Upsilon (1S) productions in Z decay, besides Z rightarrow Upsilon (1S)+b+{bar{b}}, the gluon radiation process Z rightarrow Upsilon (1S)+g+g can provide indispensable contributions as well.

Study on the inclusive hadroproduction of ϒ+γ at next-to-leading order

In this paper we investigate the inclusive associated production of {\Upsilon} + {\gamma} at the hadron collider. We calculate the color-singlet sub-processes and all three color-octet sub-processes at the next-to-leading order. Seven sets of long distance matrix elements(LDMEs), which are extracted from the studies on the prompt production of {\Upsilon} at hadron colliders, are used to give the numerical results and we find that there are three sets of LDMEs give the unphysical results on the yield and the polarization for this process. The yield is enhanced by several or two orders times comparing to that of color-singlet process and the polarization changes from longitudinal to slightly transverse or even mainly transverse in the large pt region. The estimation results indicate that to study the process at the large hadron collider (LHC) is difficult and it may be well investigated at the future Super Proton-Proton Collider(SPPC).

Jet veto resummation with jet rapidity cuts

Jet vetoes are widely used in experimental analyses at the LHC to distinguish different hard-interaction processes. Experimental jet selections require a cut on the (pseudo)rapidity of reconstructed jets, |ηjet| ≤ ηcut. We extend the standard jet-pT (jet-veto) resummation, which implicitly works in the limit ηcut → ∞, by incorporating a finite jet rapidity cut. We also consider the case of a step in the required pTcut at an intermediate value of |η| ≃ 2.5, which is of experimental relevance to avoid the increased pile-up contamination beyond the reach of the tracking detectors. We identify all relevant parametric regimes, discuss their factorization and resummation as well as the relations between them, and show that the phenomenologically relevant regimes are free of large nonglobal logarithms. The ηcut dependence of all resummation ingredients is computed to the same order to which they are currently known for ηcut → ∞. Our results pave the way for carrying out the jet-veto resummation including a sharp cut or a step at ηcut to the same order as is currently available in the ηcut → ∞ limit. The numerical impact of the jet rapidity cut is illustrated for benchmark qoverline{q} and gg initiated color-singlet processes at NLL′+NLO. We find that a rapidity cut at high ηcut = 4.5 is safe to use and has little effect on the cross section. A sharp cut at ηcut = 2.5 can in some cases lead to a substantial increase in the perturbative uncertainties, which can be mitigated by instead using a step in the veto.

Safe jet vetoes

Central jet vetoes are powerful tools for reducing QCD background in measurements and searches for electroweak and colorless, new physics processes in hadron collisions. In this letter, we report the key findings of a new philosophy to designing searches for such phenomena at hadron colliders, one designed and centered around a dynamical jet veto instead of a static veto applied independently of other selection criteria. Specifically, we investigate the theoretical and phenomenological consequences of setting the jet veto scale to the transverse momentum (pT) of the leading charged lepton ℓ in multi-lepton processes on an event-by-event basis. We consider the case of a TeV-scale heavy neutrino N decaying to the trilepton final state and find the following: (i) Perturbative uncertainties associated with the veto greatly reduce due to tying the veto scale to the hard process scale. (ii) The signal efficiency for passing the veto jumps to ≳95% and exhibits little-to-no dependence on the neutrino mass scale. (iii) Top quark and “fake” lepton rejection capabilities also improve compared to only vetoing heavy flavor-tagged jets above a fixed pT. This results in an increased sensitivity to active–sterile neutrino mixing by approximately an order of magnitude over the LHC's lifetime. For a Dirac neutrino with mass mN=150–1000 GeV and the representative active–sterile mixing hypothesis |Ve4|=|Vτ4| with |Vμ4|=0, we find that LHC experiments can probe |Ve4|2,|Vτ4|2≲6×10−4–8×10−3, surpassing the global upper limit for mN<450 GeV, with L=3 ab−1 of data at s=14 TeV. Due to the color structures of the heavy N production mechanisms considered, we argue that our results hold broadly for other color-singlet processes.

Factorization and resummation for massive quark effects in exclusive Drell-Yan

Exclusive differential spectra in color-singlet processes at hadron colliders are benchmark observables that have been studied to high precision in theory and experiment. We present an effective-theory framework utilizing soft-collinear effective theory to incorporate massive (bottom) quark effects into resummed differential distributions, accounting for both heavy-quark initiated primary contributions to the hard scattering process as well as secondary effects from gluons splitting into heavy-quark pairs. To be specific, we focus on the Drell-Yan process and consider the vector-boson transverse momentum, qT, and beam thrust, mathcal{T} , as examples of exclusive observables. The theoretical description depends on the hierarchy between the hard, mass, and the qT (or mathcal{T} ) scales, ranging from the decoupling limit qT ≪ m to the massless limit m ≪ qT. The phenomenologically relevant intermediate regime m ∼ qT requires in particular quark-mass dependent beam and soft functions. We calculate all ingredients for the description of primary and secondary mass effects required at NNLL′ resummation order (combining NNLL evolution with NNLO boundary conditions) for qT and mathcal{T} in all relevant hierarchies. For the qT distribution the rapidity divergences are different from the massless case and we discuss features of the resulting rapidity evolution. Our results will allow for a detailed investigation of quark-mass effects in the ratio of W and Z boson spectra at small qT, which is important for the precision measurement of the W-boson mass at the LHC.

A comprehensive framework for studying W\u2032 and Z\u2032 bosons at hadron colliders with automated jet veto resummation

The production of high-mass, color-singlet particles in hadron collisions is universally accompanied by initial state QCD radiation that is predominantly soft with respect to the hard process scale Q and/or collinear with respect to the beam axis. At TeV-scale colliders, this is in contrast to top quark and multijet processes, which are hard and central. Consequently, vetoing events with jets possessing transverse momenta above pTVeto in searches for new color-singlet states can efficiently reduce non-singlet backgrounds, thereby increasing experimental sensitivity. To quantify this generic observation, we in-vestigate the production and leptonic decay of a Sequential Standard Model W′ boson at the 13 TeV Large Hadron Collider. We systematically consider signal and background processes at next-to-leading-order (NLO) in QCD with parton shower (PS) matching. For color-singlet signal and background channels, we resum Sudakov logarithms of the form αsj(pTVeto) logk(Q/pTVeto) up to next-to-next-to-leading logarithmic accuracy (NNLL) with NLO matching. We obtain our results using the MadGraph5_aMC@NLO and MadGraph5_aMC@NLO-SCET frameworks, respectively. Associated Universal Feyn-Rules Output model files capable of handling NLO+PS- and NLO+NNLL-accurate computations are publicly available. We find that within their given uncertainties, both the NLO+PS and NLO+NNLL(veto) calculations give accurate and consistent predictions. Consequently, jet vetoes applied to color-singlet processes can be reliably modeled at the NLO+PS level. With respect to a b-jet veto of pTVeto = 30 GeV, flavor-agnostic jet vetoes of pTVeto = 30 − 40 GeV can further reduce single top and toverline{t} rates by a factor of 2-50 at a mild cost of the signal rate. Jet vetoes can increase the signal-to-noise ratios by roughly 10% for light W′ boson masses of 30 − 50 GeV and 25%-250% for masses of 300-800 GeV.

Fully automated precision predictions for heavy neutrino production mechanisms at hadron colliders

Motivated by TeV-scale neutrino mass models, we propose a systematic treatment of heavy neutrino $(N)$ production at hadron colliders. Our simple and efficient modeling of the vector boson fusion (VBF) $W^\pm\gamma\rightarrow N\ell^\pm$ and $N\ell^\pm+nj$ signal definitions resolve collinear and soft divergences that have plagued past studies, and is applicable to other color-singlet processes, e.g., associated Higgs $(W^\pm h)$, sparticle $(\tilde{\ell}^\pm\tilde{\nu_\ell})$, and charged Higgs $(h^{\pm\pm}h^{\mp})$ production. We present, for the first time, a comparison of all leading $N$ production modes, including both gluon fusion (GF) $gg\rightarrow Z^*/h^*\rightarrow N\overset{(-)}{\nu_\ell}$ and VBF. We obtain fully differential results up to next-to-leading order (NLO) in QCD accuracy using a Monte Carlo tool chain linking FeynRules, N{\small LO}CT, and MadGraph5\_aMC@NLO. Associated model files are publicly available. At the 14 TeV LHC, the leading order GF rate is small and comparable to the NLO $N\ell^\pm+1j$ rate; at a future 100 TeV Very Large Hadron Collider, GF dominates for $m_N={300-1500}$ GeV, beyond which VBF takes lead.

Recent forward physics and diffraction results from CMS

Recent CMS results on forward physics and diffraction are reviewed. The differential diffractive cross section is measured as a function of ξ = M 2 X / S in the region dominated by single dissociation (SD) and double dissociation (DD), where M X is the mass of one of the two final-state hadronic systems separated by the largest rapidity gap in the event. The total SD and DD cross sections are extracted. The observation of a hard color-singlet exchange process in events with a large rapidity gap between two leading jets (jet-gap-jet) is reported. The fraction of jet-gap-jet to all dijet events is measured as a function of the second leading jet transverse momentum and the size of the pseudorapidity gap. The measured fractions are compared with predictions as well as Tevatron data.

Rapidity-dependent jet vetoes

Jet vetoes are a prominent part of the signal selection in various analyses at the LHC. We discuss jet vetoes for which the transverse momentum of a jet is weighted by a smooth function of the jet rapidity. With a suitable choice of the rapidity-weighting function, such jet-veto variables can be factorized and resummed allowing for precise theory predictions. They thus provide a complementary way to divide phase space into exclusive jet bins. In particular, they provide a natural and theoretically clean way to implement a tight veto on central jets with the veto constraint getting looser for jets at increasingly forward rapidities. We mainly focus our discussion on the 0-jet case in color-singlet processes, using Higgs production through gluon fusion as a concrete example. For one of our jet-veto variables we compare the resummed theory prediction at NLL'+NLO with the recent differential cross section measurement by the ATLAS experiment in the $H\to\gamma\gamma$ channel, finding good agreement. We also propose that these jet-veto variables can be measured and tested against theory predictions in other SM processes, such as Drell-Yan, diphoton, and weak diboson production.

J/Ψ inclusive production in χbJ(nP) decays

We study J/Ψ inclusive production in the decays of P-wave bottomonium χbJ(1P, 2P) for J = 0,1,2. Within the framework of the non-relativistic QCD (NRQCD) factorization method we calculate the contributions coming from four relevant processes including one color-singlet process and three color-octet ones, which are and . Our calculation shows that the color-octet processes, especially the gluon fragmentation one, contribute the most in the decays of χb0 and χb1. However, the J/Ψ production in the χb2 decay is dominated by the color-singlet process. Furthermore, the gluon fragmentation process gives a δ function in the energy fraction distribution of J/Ψ, which can be considered to be another characteristic for its identification. From our estimation the branching ratio for these processes is about 10−4 – 10−5, which indicates that J/Ψ inclusive production is detectable at B-factories. Studying these processes would help us to gain a deeper understanding of the color-octet mechanism.

ηb decay into charmonium in association with cc̄ pair

We calculate the inclusive decay rates of ηb into charmonium via double cc̄ pairs for S- and P-wave states ηc, J/Ψ and χcJ within the framework of non-relativistic QCD (NRQCD) factorization at leading order in αs. Besides calculating the contributions of the color-singlet channels ηb → cc̄[2s+1SL(1)] + cc̄, the effects of cc̄ pair in the color-octet configurations are also considered. We find that ηb → cc̄[3S1(8)] + cc̄ make a small contribution to Br(ηb → J/ϕ(ηc) + cc̄). While in the ηb → χcj + cc̄ case, the color octet contributions are significant, for they are of the same α4Sν5c order as the color-singlet processes. We predict Br(ηb → J/ϕ(ηc) + cc̄) = 2.99(2.75) × 10−5 for S-wave states J/Ψ and ηc, and Br(ηb → χcj) + cc̄) = (4.37,3.40,2.83) × 10−5 (for J = 0,1,2) for P-wave states χcJ. In the end, we also find Br(ηb → cc̄cc̄) + cc̄) is almost saturated by ηb decay into charmonium in association with cc̄ pair from the point of view of duality.

Relativistic correction toe+e−→J/ψ+ggatBfactories and constraint on color-octet matrix elements

We calculate the relativistic correction to $J/\psi$ production in the color-singlet process $e^{+}e^{-}\to J/\psi+gg$ at B-factories. We employ the non-relativistic QCD (NRQCD) factorization approach, where the short-distance coefficients are calculated perturbatively and the long-distance matrix elements are extracted from the decays of $J/\psi$ into $e^{+}e^{-}$ and light hadrons. We find that the $O(v^2)$ relativistic correction can enhance the cross section by a factor of 20-30%, comparable to the enhancement due to the $O(\alpha_s)$ radiative correction obtained earlier. Combining the relativistic correction with the QCD radiative correction, we find that the color-singlet contribution to $e^{+}e^{-}\to J/\psi+gg$ can saturate the latest observed cross section $\sigma(e^{+}e^{-}\to J/\psi+X_{\mathrm{non-c\bar{c}}})=0.43 \pm0.09\pm0.09$ pb by Belle, thus leaving little room to the color-octet contributions. This gives a very stringent constraint on the color-octet contribution, and may imply that the values of color-octet matrix elements are much smaller than expected earlier by using the naive velocity scaling rules or extracted from fitting experimental data with the leading-order calculations.

QED contribution to the production ofJ/ψ+cc¯+Xat the Tevatron and LHC

We calculate ${\ensuremath{\alpha}}^{2}{\ensuremath{\alpha}}_{s}^{2}$ order QED contribution to $J/\ensuremath{\psi}$ production in the $pp(\overline{p})\ensuremath{\rightarrow}J/\ensuremath{\psi}+c\overline{c}$ color-singlet process at the Tevatron and LHC in the framework of nonrelativistic QCD. The contribution of the interference between the ${\ensuremath{\alpha}}^{2}{\ensuremath{\alpha}}_{s}^{2}$ QED and ${\ensuremath{\alpha}}_{s}^{4}$ QCD is also taken into account. The $J/\ensuremath{\psi}$ production associated with a charm-quark pair could be a measurable signal at hadron collider experiment. Our calculations show that by including the QED contribution, the ${p}_{t}$ distribution is enhanced by a factor of 1.5 (1.9) at the Tevatron (LHC) at ${p}_{t}=50(100)\text{ }\text{ }\mathrm{GeV}$. In addition, the polarization of $J/\ensuremath{\psi}$ turns from unpolarized in all regions to increasingly transverse when ${p}_{t}$ becomes larger.

Inclusive charm production inχbdecays

We calculate the inclusive decay rate of the spin-triplet bottomonium states ${\ensuremath{\chi}}_{bJ}$ into charm hadrons, including the leading-order color-singlet and color-octet $b\overline{b}$ annihilation mechanisms. We also calculate the momentum distribution of the charm quark from the decay of ${\ensuremath{\chi}}_{bJ}$. The infrared divergences from the color-singlet process $b\overline{b}\ensuremath{\rightarrow}c\overline{c}g$ are factored into the probability density at the origin for a $b\overline{b}$ pair in a color-octet state. That probability density can be determined phenomenologically from the fraction of decays of ${\ensuremath{\chi}}_{bJ}$ that include charm hadrons. It can then be used to predict the partial widths into light hadrons for all four states in the $P$-wave bottomonium multiplet.

J/ψ+c+c¯photoproduction ine+e−scattering

We investigate the $J/\ensuremath{\psi}+c+\overline{c}$ photoproduction in ${e}^{+}{e}^{\ensuremath{-}}$ collision at CERN LEP II energies. The physical motivations for this study are the following: (1) such a process was not considered in previous investigations of $J/\ensuremath{\psi}$ photoproduction in ${e}^{+}{e}^{\ensuremath{-}}$ interaction, and we show in this work that it is worthwhile to do so in order to make sound predictions for experimental comparison; (2) from recent Belle experiment results, the process with the same final states at the B factory has a theoretically yet unexplainable large fraction; hence it is interesting to see what may happen at other colliders; (3) the process can be measured with a high accuracy at the planned linear colliders; (4) it is necessary to take this process into consideration in elucidating the quarkonium production mechanism, especially in testing the universality of nonrelativistic QCD nonperturbative matrix elements. We find that the process concerned is really important at LEP experimental energies; within the theoretical uncertainties, it is of similar magnitude to the other color-singlet processes when the transverse momentum ${p}_{T}&gt;1\mathrm{GeV}.$ Nevertheless, to explain the recent DELPHI experimental result, the color-octet mechanism is still necessary, but with a shrunken contribution compared to previous analysis. It is found that the $J/\ensuremath{\psi}+c+\overline{c}$ photoproduction process cannot be mimicked by the simple fragmentation scheme.

Color-singlet and color-octetJ/ψproduction in top quark rare decays

$J/\ensuremath{\psi}$ production in top quark rare decays is investigated under the framework of the nonrelativistic QCD factorization formalism. Various production channels are studied, and we find that the contributions from the color-singlet quark fragmentation and the color-octet gluon fragmentation are both over $3$ orders larger than that from the leading order color-singlet process. The numerical results show that the branching ratio $B(\stackrel{\ensuremath{\rightarrow}}{t}c+J/\ensuremath{\psi}+X)$ is about ${10}^{\ensuremath{-}14}$ in the standard model, and ${10}^{\ensuremath{-}10}$ in the minimal supersymmetric standard model.

Color-singlet quark-pair transmutation: Helicity amplitudes and partial-wave analysis.

Quark-pair transmutation via two gluons is the lowest-order, color-singlet process. In this paper the helicity amplitudes for this process are evaluated and a partial-wave analysis performed. The amplitudes for these annihilations are infrared divergent and this divergence is removed by giving the gluons a confinement mass. The results are generally dependent on this mass. Many of the features are similar to those of quark-pair annihilation into two gluons. Somewhat surprisingly, there is a large proportion of a ${1}^{+}$ partial wave. Threshold effects are understood in terms of threshold effects in pair annihilation into two gluons. However, there is a ``competition'' effect that depends on the ratio of masses of the initial- and final-quark pairs. For similar masses, singlet production dominates as expected, but for very dissimilar masses, singlet annihilation of the heavier pair competes with triplet production of the lighter pair. Some comments about the possible applications and extensions of this work are made. Obvious applications are to the Okubo-Zweig-Iizuka-rule-suppressed decays of quarkonia. Other applications include development of a quark-pair-creation model that differs from the usual $^{3}\mathrm{P}_{0}$ and $^{3}\mathrm{S}_{1}$ models.