Charm Quark Research Articles

Improved indirect limits on charm and bottom quark EDMs

We derive indirect limits on the charm and bottom quark electric dipole moments (EDMs) from paramagnetic AMO and neutron EDM experiments. The charm and bottom quark EDMs generate CP-odd photon-gluon operators and light quark EDMs at the c- and b-quark mass thresholds. These CP-odd operators induce the CP-odd semi-leptonic operator CS and the neutron EDM below the QCD scale that are probed by the paramagnetic and neutron EDM experiments, respectively. The bound from CS is |dc| < 1.3 × 10−20e cm for the charm quark and |db| < 7.6 × 10−19e cm for the bottom quark, with its uncertainty estimated as 10 %. The neutron EDM provides a stronger bound, |dc| < 6 × 10−22e cm and |db| < 2 × 10−20e cm, though with a larger hadronic uncertainty.

Slight excess at 130 GeV in search for a charged Higgs boson decaying to a charm quark and a bottom quark at the Large Hadron Collider

Searches for a charged Higgs boson (H ±) decaying to a charm quark and a bottom quark (H ± → cb) have been carried out at the Large Hadron Collider (LHC) in the decay of top quarks (t → H ± b). In a recent search by the ATLAS collaboration (with all run II data, 139 fb−1) a local excess of around 3σ has been observed, which is best fitted by a charged Higgs boson with a mass of around 130 GeV and a product of branching ratios (BRs) given by BR(t → H ± b) × BR(H ± → cb) = 0.16% ± 0.06%. In the context of two-Higgs-doublet models (2HDM) with independent Yukawa couplings for H ± we present the parameter space for which this excess (assuming it to be genuine) can be accommodated, taking into account the limits from LHC searches for H ± → cs and H ± → τν at = 130 GeV and the constraint from b → sγ. It is then shown that such an excess cannot be explained in 2HDMs with natural flavour conservation, but can be accommodated in the flipped three-Higgs-doublet model (3HDM) and in the aligned 2HDM (A2HDM). Upcoming searches with 139 fb−1 in the channels H ± → cb (CMS), H ± → cs (ATLAS/CMS) and H ± → τν (ATLAS/CMS) will determine if the excess is the first sign of an H ± with GeV.

Heavy quark dynamics in a strongly magnetized quark-gluon plasma

We present a calculation of the heavy quark momentum diffusion coefficients in a quark-gluon plasma under the presence of a strong external magnetic field, within the Lowest Landau Level (LLL) approximation. In particular, we apply the Hard Thermal Loop (HTL) technique for the resummed effective gluon propagator, generalized for a hot and magnetized medium. Using the derived effective HTL gluon propagator and the LLL quark propagator we analytically derive the full results for the longitudinal and transverse momentum diffusion coefficients as well as the energy losses for charm and bottom quarks beyond the static limit. We also show numerical results for these coefficients in two special cases where the heavy quark is moving either parallel or perpendicular to the external magnetic field.

Higher-order corrections to the kinetic mass definition for the bottom and the charm quarks

In these proceedings we discuss the relation between the kinetic and the on-shell schemes for the bottom and the charm quarks and present the methods for the calculation of the mass relation to higher orders in perturbative QCD. The bottom mass in the kinetic scheme is a pivotal input parameter in the inclusive determination of |V_{cb}||Vcb| from B\to X_c \ell \nu_\ellB→Xcℓνℓ decays. By combining the relation between the kinetic and the on-shell mass with well-know results for the \overline{\mathrm{MS}}MS¯-on-shell conversion, we obtain a prediction for m_b^\mathrm{kin}mbkin based on precise determinations of \overline{m}_b(\overline{m}_b)m¯b(m¯b).

Constraining the Charm-Yukawa coupling at the Large Hadron Collider

We present theoretical results for the sensitivity of charm Yukawa coupling measurements in future high-luminosity LHC runs in three channels: Vector Boson Fusion (VBF), W Higgs-strahlung and Z Higgs-strahlung production of a Higgs boson and its subsequent decay into charm quarks. To reduce the overwhelmingly large backgrounds and to reduce false positives, we apply a set of simple kinematic and jet feature cuts and feed neural network data structures of three types; jet features, jet images and particle level features. To facilitate straightforward comparison with experimental studies [1,2], we express our results in terms of signal strengths [3].

Observation of an exotic narrow doubly charmed tetraquark

Conventional, hadronic matter consists of baryons and mesons made of three quarks and a quark–antiquark pair, respectively1,2. Here, we report the observation of a hadronic state containing four quarks in the Large Hadron Collider beauty experiment. This so-called tetraquark contains two charm quarks, a overline{{{{{u}}}}} and a overline{{{{{d}}}}} quark. This exotic state has a mass of approximately 3,875 MeV and manifests as a narrow peak in the mass spectrum of D0D0π+ mesons just below the D*+D0 mass threshold. The near-threshold mass together with the narrow width reveals the resonance nature of the state.

Theoretical study of the process Ds+→π+KS0KS0 and the isovector partner of f0(1710)

We present a theoretical study of ${a}_{0}(1710)$, the isovector partner of ${f}_{0}(1710)$, in the process ${D}_{s}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{K}_{S}^{0}{K}_{S}^{0}$. The weak interaction part proceeds through the charm quark decay process: $c(\overline{s})\ensuremath{\rightarrow}(s+\overline{d}+u)(\overline{s})$, while the hadronization part takes place in two mechanisms, differing in how the quarks from the weak decay combine into $\ensuremath{\pi}{K}^{*}$ with a quark-antiquark pair $q\overline{q}$ with the vacuum quantum numbers. In addition to the contribution from the tree diagram of the ${K}^{*+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{K}_{S}^{0}$, we have also considered the ${K}^{*}{\overline{K}}^{*}$ final-state interactions within the chiral unitary approach to generate the intermediate state ${a}_{0}(1710)$, then it decays into the final states ${K}_{S}^{0}{K}_{S}^{0}$. We find that the recent experimental measurements on the ${K}_{S}^{0}{K}_{S}^{0}$ and ${\ensuremath{\pi}}^{+}{K}_{S}^{0}$ invariant mass distributions can be well reproduced, and the proposed mechanism can provide valuable information on the nature of scalar ${f}_{0}(1710)$ and its isovector partner ${a}_{0}(1710)$.

Search for charged-lepton flavor violation in top quark production and decay in pp collisions at sqrt{s} = 13 TeV

Results are presented from a search for charged-lepton flavor violating (CLFV) interactions in top quark production and decay in pp collisions at a center-of-mass energy of 13 TeV. The events are required to contain one oppositely charged electron-muon pair in the final state, along with at least one jet identified as originating from a bottom quark. The data correspond to an integrated luminosity of 138 fb−1, collected by the CMS experiment at the LHC. This analysis includes both the production (q → eμt) and decay (t → eμq) modes of the top quark through CLFV interactions, with q referring to a u or c quark. These interactions are parametrized using an effective field theory approach. With no significant excess over the standard model expectation, the results are interpreted in terms of vector-, scalar-, and tensor-like CLFV four-fermion effective interactions. Finally, observed exclusion limits are set at 95% confidence levels on the respective branching fractions of a top quark to an eμ pair and an up (charm) quark of 0.13 × 10−6 (1.31 × 10−6), 0.07 × 10−6 (0.89 × 10−6), and 0.25 × 10−6 (2.59 × 10−6) for vector, scalar, and tensor CLFV interactions, respectively.

Addendum to: Towards next-to-next-to-leading-log accuracy for the width difference in the Bs− {overline{B}}_s system: fermionic contributions to order (mc/mb)0 and (mc/mb)1

We calculate the three-loop master integrals of Ref. [1] [arXiv:1709.02160] in analytic form. This allows us to present the fermionic contributions to the $\Delta B=2$ Wilson coefficients of the $B$-$\bar B$ decay matrix in next-to-next-to-leading order of QCD with full analytic dependence on the mass of the charm quark in the fermionic loops.

Current bounds on baryogenesis from complex Yukawa couplings of light fermions

We calculate the contribution to the baryon asymmetry of the Universe (BAU) from a $\mathcal{CP}$-violating source of the light quarks (charm, strange, down, up) and the electron, resulting from a dimension-six effective field theory term. We derive relevant bounds from the electric dipole moments of the electron and neutron to estimate the maximal contribution from each single flavor modification. Current bounds show that the charm quark can generate at most $\mathcal{O}(1\%)$ of the BAU, while the lighter quarks and the electron contribute at much lower levels.

Charm production and hadronisation in ALICE

Heavy-flavour production is an active and highly interesting field of research. Since charm and beauty quarks are predominantly produced in the hard scattering process of colliding nuclei, they serve as unique probes to study the quark-gluon plasma in \PbPb{} collisions. In addition, recent investigations of hadronisation mechanisms have revealed interesting and unexpected features when comparing measurements in \ee{} and in hadronic collisions as in \pp{}, \pPb{} and \PbPb{}. This article summarises the latest results of charm production measured by the ALICE Collaboration. For the first time, \Scnullpp{} and \Onull{} were measured in hadronic collisions and production measurements of the \Lc{} and \Xcnull{} are now included in the computation of the charm production cross section.

Direct observation of the dead-cone effect in quantum chromodynamics

In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD)1. These partons subsequently emit further partons in a process that can be described as a parton shower2, which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass mQ and energy E, within a cone of angular size mQ/E around the emitter3. Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques4,5 to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics.

Magnetic field induced hair structure in the charmonium gluon dissociation

We study the electromagnetic field effect on charmonium gluon dissociation in quark-gluon plasma. With the effective Hamiltonian derived from QCD multipole expansion under an external electromagnetic field, we first solve the two-body Schr\"odinger equation for a pair of charm quarks with mean field potentials for color and electromagnetic interactions and obtain the charmonium binding energies and wave functions, and then calculate the gluon dissociation cross section and decay width by taking the color electric and magnetic dipole interactions as perturbations above the mean field and employing Fermi's golden rule. Considering the charmonium deformation in a magnetic field, the discrete Landau energy levels make the dissociation cross section grow hair, and the electric dipole channel is significantly changed, especially for the $P$-wave states ${\ensuremath{\chi}}_{c0}$ and ${\ensuremath{\chi}}_{c\ifmmode\pm\else\textpm\fi{}}$. From our numerical calculation, the magnetic field strength $eB=5{m}_{\ensuremath{\pi}}^{2}$ already changes the gluon dissociation strongly, which may indicate measurable effects in high-energy nuclear collisions.

Revival of H− interpretation of RD(*) anomaly and closing low mass window

Thanks to the recent careful revisit of the theoretical prediction of the $B_c$ meson lifetime, the conservative upper bound on the branching ratio (BR) of $\tau \nu$ mode is found to be $\simeq 63\%$ due to the large charm quark mass uncertainty. Although it is well known that a charged Higgs ($H^-$) interpretation of the $R_{D^{(*)}}$ anomaly is excluded by the previously proposed bounds, BR$(B_c\to\tau\nu)\le 30\%$ and $\le10\%$, $H^-$ can still explain the anomaly within $1\sigma$ if we adopt the 63$\%$ one. The scalar contribution is also favored by the polarization data $F_L^{D^*}$ measured at the Belle. Since the implied NP scale is within the reach of the Large Hadron Collider (LHC), collider searches are powerful tools to test the scenario. For instance, the $\tau\nu$ resonance search has already put the more stringent bound for $m_{H^-}\ge 400\,$GeV. In this work we revisit the further lighter mass range, $180\,$GeV$\le m_{H^-}\le 400\,$GeV which has not been covered yet. We will see that a combination of the conventional stau search and low mass flavor inclusive and bottom flavored di-jet resonance searches can place a new limit on the interpretation. We summarize the current status of the low mass region and discuss the future sensitivity in the high luminosity (HL)-LHC based on the existent collider constraints.

Explaining the b → sℓ+ℓ− anomalies in Z′ scenarios with top-FCNC couplings

Motivated by the recent anomalies in b→sℓ+ℓ− transitions, we explore a minimal Z′ scenario, in which the Z′ boson has a flavour-changing coupling to charm and top quarks and a flavour-conserving coupling to muons. It is found that such a Z′ boson can explain the current b→sℓ+ℓ− anomalies, while satisfying other flavour and collider constraints simultaneously. The Z′ boson can be as light as few hundreds GeV. In this case, the t→cμ+μ− decay and the tZ′ associated production at the LHC could provide sensitive probes of such a Z′ boson. As a special feature, the Z′ contributions to all rare B- and K-meson processes are controlled by one parameter. This results in interesting correlations among these processes, which could provide further insights into this scenario. In addition, an extended scenario, in which the Z′ boson interacts with the SU(2)L fermion doublets with analogous couplings as in the minimal scenario, is also investigated.

Numerov and phase-integral methods for charmonium

This paper applies the Numerov and phase-integral methods to the stationary Schrödinger equation that studies bound states of charm anti-charm quarks. The former is a numerical method well suited for a matrix form of the second-order ordinary differential equations, and can be applied whenever the stationary states admit a Taylor-series expansion. The latter is an analytic method that provides, in principle, even exact solutions of the stationary Schrödinger equation, and well suited for applying matched asymptotic expansions and higher-order quantization conditions. The Numerov method is found to be always in agreement with the early results of Eichten et al., whereas an original evaluation of the phase-integral quantization condition clarifies under which conditions the previous results in the literature on higher-order terms can be obtained.

Determination of the spectroscopic parameters of beauty-partners of Tcc from QCD

Motivated by the recent discovery of a new tetraquark ${T}_{cc}^{+}$ state with two charm quarks and two light quarks by LHCb Collaboration, we calculate the spectroscopic parameters; namely, the mass and residues of beauty partners of ${T}_{cc}^{+}$ within QCD sum rules. The obtained results are compared with the predictions of different approaches in the literature.

Nonleptonic two-body decays of Λb→Λcπ , ΛcK in the perturbative QCD approach

We study the color-allowed $\Lambda_b\rightarrow \Lambda_c \pi, \Lambda_cK$ decays in the perturbative QCD approach (PQCD) to lowest order in strong coupling constant $\alpha_s$. Both the factorizable and nonfactorizable contributions are taken into account in our calculations. It is found that these processes are dominated by the factorizable contributions, while the important nonfactorizable contributions can enhance the branching ratios by about 30$\%$. The decay branching ratios are predicted to be $\mathcal{B}(\Lambda_b\rightarrow \Lambda_c \pi)=6.7^{+4.1}_{-3.8}\times 10^{-3}$ and $\mathcal{B}(\Lambda_b\rightarrow \Lambda_c K)=5.0^{+4.0}_{-3.0}\times 10^{-4}$, where the uncertainties arise from the baryon light-cone distribution amplitudes (LCDAs), the heavy charm quark and charmed baryon masses, and the hard scales. It is shown that the asymmetry parameters in the two decays are approximately equal to -1, which is expected as in the heavy quark limit and the soft meson limit. Our predictions are consistent with the recent experimental data within errors. The obtained numerical results are also compared to those in the other theoretical approaches when they are available.

Signals of quark combination at hadronization in pp collisions at s=200 GeV

We find signals of quark combination at hadronization from the experimental data of ${p}_{T}$ spectra of hadrons at midrapidity in $pp$ collisions at $\sqrt{s}=200\text{ }\text{ }\mathrm{GeV}$. The first is the constituent quark number scaling property for ${p}_{T}$ spectra of ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}}$ and $\ensuremath{\phi}$ and that for ${p}_{T}$ spectra of $p$ and ${\ensuremath{\rho}}^{0}$. The second is that ${p}_{T}$ spectra of $\mathrm{\ensuremath{\Lambda}}$, ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}$, and ${K}^{*0}$ can be self-consistently described using the spectrum of strange quarks from $\ensuremath{\phi}$ data and that of up/down quarks from $p$ data in the equal-velocity combination mechanism. The third is that experimental data for ${p}_{T}$ spectrum of ${D}^{*+}$ are also well described by the spectrum of up/down quarks from $p$ data and a spectrum of charm quarks which is consistent with theoretical calculations of the fixed-order next-to-leading-logarithmic approach. These results indicate a similarity between hadron production in $pp$ collisions at $\sqrt{s}=200\text{ }\text{ }\mathrm{GeV}$ and that at LHC energies. We predict ${p}_{T}$ spectra of single-charm hadrons and their spectrum ratios. We suggest systematic measurements in $pp$ collisions at $\sqrt{s}=200\text{ }\text{ }\mathrm{GeV}$ in the future so as to better understand the property of the small parton system created in $pp$ collisions at different collision energies.

Collectivity of J/ψ Mesons in Heavy-Ion Collisions.

The production of J/ψ mesons in heavy-ion collisions at the Large Hadron Collider is believed to be dominated by the recombination of charm and anticharm quarks in a hot QCD medium. However, measurements of the elliptic flow (v_{2}) of J/ψ mesons in these reactions are not well described by existing calculations of J/ψ recombination for transverse momenta p_{T}≳4 GeV. We revisit these calculations in two main aspects. Employing the resonance recombination model, we implement distribution functions of charm quarks transported through the quark-gluon plasma using state-of-the-art Langevin simulations and account for the space-momentum correlations of the diffusing charm and anticharm quarks in a hydrodynamically expanding fireball. This extends the relevance of the recombination processes to substantially larger momenta than before. We also revisit the suppression of primordially produced J/ψ's by propagating them through the same hydrodynamic medium, leading to a marked increase of their v_{2} over previous estimates. Combining these developments into a calculation of the p_{T}-dependent nuclear modification factor and v_{2} of inclusive J/ψ production in semicentral Pb-Pb collisions at the LHC, we find a good description of the experimental results by the ALICE Collaboration. Our results thus resolve the abovementioned v_{2} puzzle and imply the relevance of recombination processes for p_{T}'s of up to ∼8 GeV.