Charm Quark Jet Research Articles

Precise QCD predictions for W-boson production in association with a charm jet

The production of a W-boson with a charm quark jet provides a highly sensitive probe of the strange quark distribution in the proton. Employing a novel flavour dressing procedure to define charm quark jets, we compute W+charm-jet production up to next-to-next-to-leading order (NNLO) in QCD. We study the perturbative stability of production cross sections with same-sign and opposite-sign charge combinations for the W boson and the charm jet. A detailed breakdown according to different partonic initial states allows us to identify particularly suitable observables for the study of the quark parton distributions of different flavours.

Dead cone effect in charm and bottom quark jets

The evolution of a heavy quark initiated jet is mainly ruled by gluon bremsstrahlung. As a consequence of the dead-cone effect, this radiation is suppressed in the forward direction at angles smaller than that proportional to the heavy quark mass MQ, i.e. Θ0=MQ/EQ at energy EQ of the primary quark. In this paper, we unveil this effect in charm and bottom quark jets using DELPHI and OPAL data from Z0 boson decays in e+e− annihilation at center of mass energy 91.2 GeV. The analysis of the reconstructed heavy quark fragmentation function in momentum space shows the strong suppression of hadrons at high momenta in such events compared to light quark fragmentation by a factor ≲1/10. The amount of this suppression is well reproduced by perturbative QCD (pQCD) within the Modified Leading Logarithmic Aproximation and the compact scheme of Local Parton Hadron Duality (MLLA-LPHD). As a new result, we obtain an almost perfect agreement between the light quark fragmentation functions expected at W0∝MQ from DELPHI and OPAL data with Pythia8 and shed light on the reasons for the existence of the ultra-soft gluon excess at small momentum fraction in comparison with pQCD predictions.

Observation of the dead cone effect in charm and bottom quark jets and its QCD explanation

The production of a heavy quark is accompanied by gluon bremsstrahlung which is suppressed at small angles $\Theta\lesssim M_Q/E$ for mass $M_Q$ and high energy $E$ according to perturbative Quantum Chromo Dynamics (QCD) (``dead cone effect''). As particles at small angles typically have large momenta, the heavy quark mass also causes a suppression of high momentum particles. In this paper, we studied this effect in c- and b-quark events using data from Z boson decays in $e^+e^-$ annihilation. The heavy quark fragmentation function for charged particles is reconstructed in the momentum fraction variable $x$ or $\xi=\ln(1/x)$ by removing the decays of the heavy quark hadrons. Indeed, we find an increasing suppression of particles with rising $x$ down to a fraction of $\lesssim 1/10$ for particles with $x\gtrsim0.2$ in b-quark and $x\gtrsim0.4$ in c-quark jets in comparison to light quark fragmentation. The sensitivity to the dead cone effect in the present momentum analysis is considerably increased in comparison to the recently presented angular analysis. This amount of suppression and the differences between c- and b-quark fragmentation are in good quantitative agreement with the expectations based on perturbative QCD within the Modified Leading Logarithmic Approximation (MLLA) in the central kinematic region. The data also support a two parameter description in the MLLA of these phenomena (``Limiting Spectrum''). The sensitivity of these measurements to the heavy quark mass is investigated.

Heavy-Flavour Jets in High-Energy Nuclear Collisions

Reconstructed jets initiated from heavy quarks provide a powerful tool to probe the properties of the quark–gluon plasma (QGP) and to explore the mass hierarchy of jet quenching. In this article, we review the recent theoretical progresses on heavy-flavour jets in high-energy nuclear collisions at the RHIC and LHC. We focus on the yields and substructures of charm and bottom quark jets with jet-quenching effects, such as the nuclear modification factors, transverse momentum imbalance, angular correlation, radial profiles, fragmentation functions, the “dead-cone” effect, etc.

An Investigation of Charm Quark Jet Spectrum and Shape Modifications in Au+Au Collisions at \(\sqrt {s_{NN}} = 200\) GeV

An Investigation of Charm Quark Jet Spectrum and Shape Modifications in Au+Au Collisions at \(\sqrt {s_{NN}} = 200\) GeV

Measurements of the associated production of a W boson and a charm quark in proton–proton collisions at sqrt{s}=8,text {TeV}

Measurements of the associated production of a W boson and a charm ({text {c}}) quark in proton–proton collisions at a centre-of-mass energy of 8,text {TeV} are reported. The analysis uses a data sample corresponding to a total integrated luminosity of 19.7,text {fb}^{-1} collected by the CMS detector at the LHC. The W bosons are identified through their leptonic decays to an electron or a muon, and a neutrino. Charm quark jets are selected using distinctive signatures of charm hadron decays. The product of the cross section and branching fraction sigma (text {p}text {p}rightarrow text {W}+ {text {c}}+ text {X}) {mathcal {B}}(text {W}rightarrow ell upnu ), where ell = text {e} or upmu , and the cross section ratio sigma (text {p}text {p}rightarrow {{text {W}}^{+} + bar{{text {c}}} + text {X}}) / sigma (text {p}text {p}rightarrow {{text {W}}^{-} + {text {c}}+ text {X}}) are measured in a fiducial volume and differentially as functions of the pseudorapidity and of the transverse momentum of the lepton from the W boson decay. The results are compared with theoretical predictions. The impact of these measurements on the determination of the strange quark distribution is assessed.

On the interference of ggH and overline{mathrm{c}}mathrm{cH} Higgs production mechanisms and the determination of charm Yukawa coupling at the LHC

Higgs boson production in association with a charm-quark jet proceeds through two different mechanisms — one that involves the charm Yukawa coupling and the other that involves direct Higgs coupling to gluons. The interference of the two contributions requires a helicity flip and, therefore, cannot be computed with massless charm quarks. In this paper, we consider QCD corrections to the interference contribution starting from charm-gluon collisions with massive charm quarks and taking the massless limit, mc→ 0. The behavior of QCD cross sections in that limit differs from expectations based on the canonical QCD factorization. This implies that QCD corrections to the interference term necessarily involve logarithms of the ratio MH/mc whose resummation is currently unknown. Although the explicit next-to-leading order QCD computation does confirm the presence of up to two powers of ln(MH/mc) in the interference contribution, their overall impact on the magnitude of QCD corrections to the interference turns out to be moderate due to a cancellation between double and single logarithmic terms.

Measurement of the associated production of a Z boson with charm or bottom quark jets in proton-proton collisions at s=13 TeV

Ratios of cross sections, $\sigma$(Z+c jets) / $\sigma$(Z+jets), $\sigma$(Z+b jets) / $\sigma$(Z+jets), and $\sigma$(Z+c jets) / $\sigma$(Z+b jets) in the associated production of a Z boson with at least one charm or bottom quark jet are measured in proton-proton collisions at $\sqrt{s} =$ 13 TeV. The data sample, collected by the CMS experiment at the CERN LHC, corresponds to an integrated luminosity of 35.9 fb$^{-1}$, with a fiducial volume of $p_\mathrm{T}$ $>$ 30 GeV and $|\eta|$ $<$ 2.4 for the jets, where $p_\mathrm{T}$ and $\eta$ represent transverse momentum and pseudorapidity, respectively. The Z boson candidates come from leptonic decays into electrons or muons with $p_\mathrm{T}$ $>$ 25 GeV and $|\eta|$ $<$ 2.4, and the dilepton mass satisfies 71 $\lt$ $m_\mathrm{Z}$ $\lt$ 111 GeV. The measured values are $\sigma$(Z+c jets) / $\sigma$(Z+jets) = 0.102 $\pm$ 0.002 $\pm$ 0.009, $\sigma$(Z+b jets) / $\sigma$(Z+jets) = 0.0633 $\pm$ 0.0004 $\pm$ 0.0015, and $\sigma$(Z+c jets) / $\sigma$(Z+b jets) = 1.62 $\pm$ 0.03 $\pm$ 0.15. Results on the inclusive and differential cross section ratios as functions of jet and Z boson transverse momentum are compared with predictions from leading and next-to-leading order perturbative quantum chromodynamics calculations. These are the first measurements of the cross section ratios at 13 TeV.

A search for the standard model Higgs boson decaying to charm quarks

A direct search for the standard model Higgs boson, H, produced in association with a vector boson, V (W or Z), and decaying to a charm quark pair is presented. The search uses a data set of proton-proton collisions corresponding to an integrated luminosity of 35.9 fb−1, collected by the CMS experiment at the LHC in 2016, at a centre-of-mass energy of 13 TeV. The search is carried out in mutually exclusive channels targeting specific decays of the vector bosons: W → ℓν, Z → ℓℓ, and Z → νν, where ℓ is an electron or a muon. To fully exploit the topology of the H boson decay, two strategies are followed. In the first one, targeting lower vector boson transverse momentum, the H boson candidate is reconstructed via two resolved jets arising from the two charm quarks from the H boson decay. A second strategy identifies the case where the two charm quark jets from the H boson decay merge to form a single jet, which generally only occurs when the vector boson has higher transverse momentum. Both strategies make use of novel methods for charm jet identification, while jet substructure techniques are also exploited to suppress the background in the merged-jet topology. The two analyses are combined to yield a 95% confidence level observed (expected) upper limit on the cross section sigma left(mathrm{VH}right)mathrm{mathcal{B}}left(mathrm{H}to mathrm{c}overline{mathrm{c}}right) of 4.5 left({2.4}_{-0.7}^{+1.0}right) pb, corresponding to 70 (37) times the standard model prediction.

Constraints on the intrinsic charm content of the proton from recent ATLAS data

We investigate the possibility to constrain the intrinsic charm probability w_{mathrm{c} {bar{{mathrm{c}}}}}= P_{{mathrm{c}{bar{mathrm{c}}}} /mathrm{p}} using first ATLAS data on the associated production of prompt photons and charm-quark jets in pp collisions at sqrt{s} = 8 TeV. The upper limit w_{mathrm{c} {bar{{mathrm{c}}}}}< 1.93 % is obtained at the 68 % confidence level. This constraint is primarily determined from the theoretical scale and systematical experimental uncertainties. Suggestions for reducing these uncertainties are discussed. The implications of intrinsic heavy quarks in the proton for future studies at the LHC are also discussed.

Measurement of associated Z + charm production in proton\u2013proton collisions at sqrt{s} = 8,text {TeV}

A study of the associated production of a mathrm{Z} boson and a charm quark jet (mathrm{Z} + mathrm{c} ), and a comparison to production with a mathrm{b} quark jet (mathrm{Z} + mathrm{b} ), in mathrm {p}mathrm {p} collisions at a centre-of-mass energy of 8,text {TeV} are presented. The analysis uses a data sample corresponding to an integrated luminosity of 19.7,text {fb}^{-1}, collected with the CMS detector at the CERN LHC. The mathrm{Z} boson candidates are identified through their decays into pairs of electrons or muons. Jets originating from heavy flavour quarks are identified using semileptonic decays of mathrm{c} or mathrm{b} flavoured hadrons and hadronic decays of charm hadrons. The measurements are performed in the kinematic region with two leptons with p_{mathrm {T}} ^{ell } > 20,text {GeV} , {|eta ^{ell }|} < 2.1, 71< m_{ell ell } < 111,text {GeV} , and heavy flavour jets with p_{mathrm {T}} ^{text {jet}} > 25,text {GeV} and {|eta ^{ text {jet}}|} < 2.5. The mathrm{Z} + mathrm{c} production cross section is measured to be sigma (mathrm {p}mathrm {p}rightarrow mathrm{Z} + mathrm{c} + X) mathcal {B}(mathrm{Z} rightarrow ell ^+ell ^-) = 8.8 pm 0.5,text {(stat)} pm 0.6,text {(syst)} ,text {pb} . The ratio of the mathrm{Z} + mathrm{c} and mathrm{Z} + mathrm{b} production cross sections is measured to be sigma (mathrm {p}mathrm {p}rightarrow mathrm{Z} + mathrm{c} + X)/sigma (mathrm {p}mathrm {p}rightarrow mathrm{Z} + mathrm{b} + X) = 2.0 pm 0.2,text {(stat)} pm 0.2,text {(syst)} . The mathrm{Z} + mathrm{c} production cross section and the cross section ratio are also measured as a function of the transverse momentum of the mathrm{Z} boson and of the heavy flavour jet. The measurements are compared with theoretical predictions.

Measurements of the charm jet cross section and nuclear modification factor in pPb collisions at [formula omitted

The CMS Collaboration presents the first measurement of the differential cross section of jets from charm quarks produced in proton–lead (pPb) collisions at a nucleon–nucleon center-of-mass energy of sNN=5.02TeV, as well as results from charm quark jets in proton–proton (pp) collisions at s=2.76 and 5.02TeV. By comparing the yields of the pPb and pp collision systems at the same energy, a nuclear modification factor for charm jets from 55 to 400 GeV/c in pPb collisions at sNN=5.02TeV of RpA=0.92±0.07(stat)±0.11(syst) is obtained. This is consistent with an absence of final-state energy loss for charm quarks in pPb collisions. In addition, the fraction of jets coming from charm quarks is found to be consistent with that predicted by pythia 6 for pp collisions at s=2.76 and 5.02TeV, and is independent of the jet transverse momentum from 55 to 400 GeV/c.

Measurement of associated W + charm production in pp collisions at $ \sqrt{s} $ = 7 TeV

Measurements are presented of the associated production of a W boson and a charm-quark jet (W + c) in pp collisions at a center-of-mass energy of 7 TeV. The analysis is conducted with a data sample corresponding to a total integrated luminosity of 5 inverse femtobarns, collected by the CMS detector at the LHC. W boson candidates are identified by their decay into a charged lepton (muon or electron) and a neutrino. The W + c measurements are performed for charm-quark jets in the kinematic region pt[jet] > 25 GeV, abs(eta) < 2.5, for two different thresholds for the transverse momentum of the lepton from the W-boson decay, and in the pseudorapidity range abs(eta[ell]) < 2.1. Hadronic and inclusive semileptonic decays of charm hadrons are used to measure the following total cross sections: sigma(pp to W + c + X) times B(W to ell nu) = 107.7 +/- 3.3 (stat.) +/- 6.9 (syst.) pb (pt[ell] > 25 GeV) and sigma(pp to W + c + X) times B(W to ell nu) = 84.1 +/- 2.0 (stat.) +/- 4.9 (syst.) pb (pt[ell] > 35 GeV), and the cross section ratios sigma(pp to W+ + c + X)/sigma(pp to W- + c + X) = 0.954 +/- 0.025 (stat.) +/- 0.004 (syst.) (pt[ell] > 25 GeV) and sigma(pp to W+ + c bar + X)/sigma(pp to W- + c + X) = 0.938 +/- 0.019 (stat.) +/- 0.006 (syst.) (pt[ell] > 35 GeV). Cross sections and cross section ratios are also measured differentially with respect to the absolute value of the pseudorapidity of the lepton from the W-boson decay. These are the first measurements from the LHC directly sensitive to the strange quark and antiquark content of the proton. Results are compared with theoretical predictions and are consistent with the predictions based on global fits of parton distribution functions.

Measurement of Associated Production ofZBosons with Charm Quark Jets inpp¯Collisions ats=1.96 TeV

We present the first measurements of the ratios of cross sections σ(pp → Z+c jet)/σ(pp → Z+jet) and σ(pp → Z+c jet)/σ(pp → Z+b jet) for the associated production of a Z boson with at least one charm or bottom quark jet. Jets have transverse momentum ​pT(jet)​>20 GeV and pseudorapidity |​η(jet)​|<2.5. These cross section ratios are measured differentially as a function of jet and Z boson transverse momenta, based on 9.7 fb(-1) of pp collisions collected with the D0 detector at the Fermilab Tevatron Collider at √s=1.96 TeV. The measurements show significant deviations from perturbative QCD calculations and predictions from various event generators.

DETERMINATION OF CHARM QUARK MASS AND αs(MZ) FROM HERA DATA

Charm production data from HERA may be used to determine the charm quark mass and jet production data from HERA may be used to determine αs(MZ). Recent results are summarised.

Observation of the production of a W boson in association with a single charm quark.

The first observation of the production of a W boson with a single charm quark (c) jet in pp[over ¯] collisions at √s=1.96 TeV is reported. The analysis uses data corresponding to 4.3 fb(-1), recorded with the CDF II detector at the Fermilab Tevatron. Charm quark candidates are selected through the identification of an electron or muon from charm-hadron semileptonic decay within a hadronic jet, and a Wc signal is observed with a significance of 5.7 standard deviations. The production cross section σ(Wc)(p(Tc)>20 GeV/c,|η(c)|<1.5)×B(W→ℓν) is measured to be 13.6(-3.1)(+3.4) pb and is in agreement with theoretical expectations. From this result the magnitude of the quark-mixing matrix element V(cs) is derived, |V(cs)|=1.08±0.16 along with a lower limit of |V(cs)|>0.71 at the 95% confidence level, assuming that the Wc production through c to s quark coupling is dominant.

Charm Production in Association with an Electroweak Gauge Boson at the LHC

The production of charm quark jets in association with electroweak gauge bosons at the LHC can be used as a tool to constrain quark parton distribution functions (PDFs). Motivated by recent measurements at the Tevatron and LHC, we calculate cross sections for W/Z+c, comparing these to W/Z+jet, for various PDF sets. The cross-section differences can be understood in terms of the different underlying PDFs, with the strange quark distribution being particularly important for W+c production. We suggest measurements of appropriately defined ratios and comment on how these measurements at the LHC can be used to extract information on the strange and charm content of the proton at high Q(2) scales.

Why heavy and light quarks radiate energy with similar rates

The dead-cone effect has been predicted to reduce the magnitude of energy loss and jet quenching for heavy flavors produced with large ${p}_{T}$ in heavy-ion collisions. On the contrary, data from the Relativistic Heavy Ion Collider demonstrate a strong suppression of high-${p}_{T}$ electrons from charm and bottom decays. We show that vacuum radiation of a highly virtual quark produced at high ${p}_{T}$ with a stripped-off color field develops a much wider dead cone, which screens the one related to the quark mass. Lacking the field, gluons cannot be radiated within this cone until the color field is regenerated and the quark virtuality cools down to the scale of the order of the quark mass. However, this takes longer than is essential for the observed jet quenching, leading to similar nuclear effects for the light and charm quark jets. Open beauty is expected to radiate much less within the ${p}_{T}$ range studied so far in heavy-ion collisions.

Heavy-quarkonium production in high energy proton-proton collisions at RHIC

We update the study of the total Psi and Upsilon production cross section in proton-proton collisions at RHIC energies using the QCD-based Color-Singlet (CS) Model, including next-to-leading order partonic matrix elements. We also include charm-quark initiated processes which appear at leading order in alpha_s, but which have so far been overlooked in such studies. Contrary to earlier claims, we show that the CS yield is consistent with measurements over a broad range of J/Psi rapidities. We also find that charm-quark initiated processes, including both intrinsic and sea-like charm components, typically contribute at least 20% of the direct J/Psi yield, improving the agreement with data both for the integrated cross section and its rapidity dependence. The key signature for such processes is the observation of a charm-quark jet opposite in azimuthal angle phi to the detected J/Psi. Our results have impact on the proper interpretation of heavy-quarkonium production in heavy-ion collisions and its use as a probe for the quark-gluon plasma.

Υdecay to two charm-quark jets as a probe of the color-octet mechanism

We calculate the decay rate of bottomonium to two charm-quark jets $\ensuremath{\Upsilon}\ensuremath{\rightarrow}c\overline{c}$ at the tree level and one-loop level including color-singlet and color-octet $b\overline{b}$ annihilations. We find that the short-distance coefficient of the color-octet piece is much larger than the color-singlet piece, and that the QCD correction will change the end point behavior of the charm quark jet. The color-singlet piece is strongly affected by the one-loop QCD correction. In contrast, the QCD correction to the color-octet piece is weak. Once the experiment can measure the branching ratio and energy distribution of the two charm-quark jets in the $\ensuremath{\Upsilon}$ decay, the result can be used to test the color-octet mechanism or give a strong constraint on the color-octet matrix elements.