Charm Decays Research Articles

Search for Rare or Forbidden Decays of the D^{0} Meson.

We present a search for nine lepton-number-violating and three lepton-flavor-violating neutral charm decays of the type D^{0}→h^{'-}h^{-}ℓ^{'+}ℓ^{+} and D^{0}→h^{'-}h^{+}ℓ^{'±}ℓ^{∓}, where h and h^{'} represent a K or π meson and ℓ and ℓ^{'} an electron or muon. The analysis is based on 468 fb^{-1} of e^{+}e^{-} annihilation data collected at or close to the ϒ(4S) resonance with the BABAR detector at the SLAC National Accelerator Laboratory. No significant signal is observed for any of the twelve modes, and we establish 90%confidence level upper limits on the branching fractions in the range (1.0-30.6)×10^{-7}. The limits are between 1 and 3 orders of magnitude more stringent than previous measurements.

Discovery of CP Violation in Charm Decays at the LHCb Experiment and Prospects for Run 3 and Run 4

Discovery of CP Violation in Charm Decays at the LHCb Experiment and Prospects for Run 3 and Run 4

The heavy quark expansion for inclusive semileptonic charm decays revisited

The Heavy Quark Expansion (HQE) has become an extremely powerful tool in flavor physics. For charm decays, where the expansion parameters αs(mc) and ΛQCD/mc are bigger than for bottom decays, it remains to be seen if the HQE can be applied with similar success. Nevertheless, to make optimal use of the plethora of data already available and coming in the near future, a better understanding of HQE for charm decays is crucial. This paper discusses in detail how the HQE for charm decays is set up, what is the role of four-quark (weak annihilation) operators and how this compares to the well understood bottom decays. Subtleties concerning radiative corrections and the charm mass scheme are briefly discussed. An experimental study of the relevant HQE hadronic matrix elements will then show if the HQE expansion for charm converges well enough. Besides serving as an important cross check for inclusive B decays, in the end, this study might open the road for inclusive |Vcs| and |Vcd| extractions.

D -wave charmonia

Inspired by the recent observations of the vector charmonium-like states at BES III Collaboration and $\psi(3842)$ at LHCb Collaboration, we comb the $D$ wave charmonium state in the present work. We first evaluate the possibility of $Y(4320)$ as $\psi(3^3D_1)$ by investigating its open charm decays in quark-pair creation model and we find the width of $Y(4320)$ can be reproduced in a reasonable parameter range. Moreover, we take $\psi(3770)$, $\psi(4160)$ and $Y(4320)$ as the scale of $1D$, $2D$ and $3D$ charmonia to estimate the open charm decays of other $D$ wave charmonia. The total and partial widths of $D$ wave charmonium states have been predicted, which could be tested by further measurements at LHCb and Belle II Collaborations.

Production of muons from heavy-flavour hadron decays in pp collisions at sqrt{s} = 5.02 TeV

Production cross sections of muons from semi-leptonic decays of charm and beauty hadrons were measured at forward rapidity (2.5 < y < 4) in proton-proton (pp) collisions at a centre-of-mass energy sqrt{s} = 5.02 TeV with the ALICE detector at the CERN LHC. The results were obtained in an extended transverse momentum interval, 2 < pT< 20 GeV/c, and with an improved precision compared to previous measurements performed in the same rapidity interval at centre-of-mass energies sqrt{s} = 2.76 and 7 TeV. The pT- and y-differential production cross sections as well as the pT-differential production cross section ratios between different centre-of-mass energies and different rapidity intervals are described, within experimental and theoretical uncertainties, by predictions based on perturbative QCD.

The emergence of the \u0394U = 0 rule in charm physics

We discuss the implications of the recent discovery of CP violation in two-body SCS D decays by LHCb. We show that the result can be explained within the SM without the need for any large SU(3) breaking effects. It further enables the determination of the imaginary part of the ratio of the ΔU = 0 over ΔU = 1 matrix elements in charm decays, which we find to be (0.65 ± 0.12). Within the standard model, the result proves the non-perturbative nature of the penguin contraction of tree operators in charm decays, similar to the known non-perturbative enhancement of ΔI = 1/2 over ΔI = 3/2 matrix elements in kaon decays, that is, the ΔI = 1/2 rule. As a guideline for future measurements, we show how to completely solve the most general parametrization of the D → P+P− system.

Measurement of charm and bottom production from semileptonic hadron decays in p+p collisions at s=200 GeV

Measurements of the differential production of electrons from open-heavy-flavor hadrons with charm- and bottom-quark content in $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV are presented. The measurements proceed through displaced-vertex analyses of electron tracks from the semileptonic decay of charm and bottom hadrons using the PHENIX silicon-vertex detector. The relative contribution of electrons from bottom decays to inclusive heavy-flavor-electron production is found to be consistent with fixed-order-plus-next-to-leading-log perturbative-QCD calculations within experimental and theoretical uncertainties. These new measurements in $p$$+$$p$ collisions provide a precision baseline for comparable forthcoming measurements in A$+$A collisions.

Understanding the ηcρ decay mode of Zc(′) via the triangle loop mechanism

Recently, the BESIII Collaboration reported a new measurement of the $\eta_c \rho$ decay mode of $Z_c^{(\prime)}$, which motivated us to study the inner structure of $Z_c^{(\prime)}$ via investigating the hidden charm decays of these two $Z_c$ states. We consider the {triangle loop mechanism} contribution in the hidden charm decays of $Z_c^{(\prime)}$. Our estimations indicate that the triangle loop mechanism plays an important role in the decays of the $Z_c^{(\prime)}$, where our results are in agreement with the experimental observations in a reasonable parameter range. Furthermore, we point out that the $Z_c^{(\prime)}$ can be interpreted as the hadronic molecules, while the tetraquark scenario is less favored.

Observation of the Decay D^{0}→K^{-}π^{+}e^{+}e^{-}.

We report the observation of the rare charm decay D^{0}→K^{-}π^{+}e^{+}e^{-}, based on 468 fb^{-1} of e^{+}e^{-} annihilation data collected at or close to the center-of-mass energy of the ϒ(4S) resonance with the BABAR detector at the SLAC National Accelerator Laboratory. We find the branching fraction in the invariant mass range 0.675<m(e^{+}e^{-})<0.875 GeV/c^{2} of the electron-positron pair to be B(D^{0}→K^{-}π^{+}e^{+}e^{-})=(4.0±0.5±0.2±0.1)×10^{-6}, where the first uncertainty is statistical, the second systematic, and the third due to the uncertainty in the branching fraction of the decay D^{0}→K^{-}π^{+}π^{+}π^{-} used as a normalization mode. The significance of the observation corresponds to 9.7 standard deviations including systematic uncertainties. This result is consistent with the recently reported D^{0}→K^{-}π^{+}μ^{+}μ^{-} branching fraction, measured in the same invariant mass range, and with the value expected in the standard model. In a set of regions of m(e^{+}e^{-}), where long-distance effects are potentially small, we determine a 90%confidence level upper limit on the branching fraction B(D^{0}→K^{-}π^{+}e^{+}e^{-})<3.1×10^{-6}.

Nuclear modification factor and flow of charm and bottom quarks in Au+Au collisions at [formula omitted] by the PHENIX Experiment

Experimental results at RHIC and at the LHC show a similar strong suppression for light and heavy quark probes at high pT, and a possible quark mass dependence of suppression at low pT. More high precision measurements of separated charm and bottom are needed to quantify the dependence of medium effects on the quark mass. The PHENIX experiment measured separated electrons from bottom and charm decays using displaced vertex distributions at mid-rapidity |y|<0.35. Azimuthal anisotropy of electrons from separated charm and bottom decays are obtained from high statistics Au+Au data taken in 2014. PHENIX also measures muons from heavy quark decays in d+Au collisions at forward rapidity to study collective effects on heavy flavor production in small systems. These proceedings report the azimuthal anisotropy of charm and bottom at mid-rapidity in minimum bias Au+Au collisions and the anisotropy of single muons from heavy quark decays in d+Au collisions.

Measurements of charm, bottom, and Drell-Yan via dimuons in p+p and p+Au collisions at [formula omitted] with PHENIX at RHIC

Dilepton spectra are a classic probe to study ultra-relativistic heavy ion collisions. At RHIC energies, the dimuon continuum is dominated by correlated pairs from semi-leptonic decays of charm and bottom hadrons and the Drell-Yan process. The dimuon spectra contain information on heavy flavor angular correlations, which can constrain the relative contributions from different heavy flavor production mechanisms. Studying heavy flavor correlations in p+Au collisions may provide further insight on cold nuclear matter effects. Measurements of the Drell-Yan cross-section can provide constraints to PDFs, as well as further our understanding in initial state interactions in p+Au collisions.In this talk, we report measurements of μμ pairs from charm, bottom, and Drell-Yan in p+p and p+Au collisions at sNN=200GeV. A further shape analysis is applied to the heavy flavor pair correlations to extract the relative contributions to heavy flavor production mechanisms.

Novel method for measuring charm-mixing parameters using multibody decays

We propose a novel method to measure flavor-oscillations and charge-parity (CP) violation in charm mixing. The approach uses multibody charm decays, such as $D^0\to K_S^0\pi^+\pi^-$, and avoids the need for a fit of the decay amplitudes while suppressing biases due to nonuniform signal-reconstruction efficiencies as functions of phase space and decay time. Data are partitioned in decay-time and Dalitz-plot regions (bins). The Dalitz-plot bins are symmetric with respect to the principal bisector and chosen to ensure nearly constant values of the strong-interaction phases in each. The ratios of signal yields observed in each symmetric bin pair are fit as functions of decay time, using independent auxiliary measurements of the strong-interaction phases as constraints, to determine the relevant physics parameters. Simulation shows that this approach improves the sensitivity to the normalized charm-eigenstate mass difference by 35% with respect to existing model-independent methods. In addition, we introduce a parametrization of oscillation and CP-violation effects in charm mixing that has attractive statistical properties and may find wider applicability.

Open Charm Physics at BESIII

The study of mesons and baryons containing at least one c quark is denoted as open charm physics. BESIII has collected large data samples at charm related thresholds which provide unique opportunities to study charm decays. BESIII has published many results in the past years. Among those we present the measurement of the CKM matrix elements |Vcd(s)|2 and the form factors fD(s) using the (semi-)leptonic decays Ds → µ+ νµ and D0 → K− µ+ νµ, a test for lepton flavour violation in the process D → Kπe+e+ and the study of intermediate states in the decays D → ππe+ν and $ {{\rm{D}}^{\rm{ + }}} \to {\rm{K}}_{\rm{S}}^0{\pi ^ - }{\pi ^ - }{\pi ^ + } $.

Real-time alignment and reconstruction: performance and recent developments at the LHCb experiment

The LHCb detector is a single-arm forward spectrometer designed for the efficient reconstruction decays of c- and b-hadrons. LHCb has introduced a novel real-time detector alignment and calibration strategy for LHC Run II. Data collected at the start of the fill are processed in a few minutes and used to update the alignment, while the calibration constants are evaluated for each run. This is one of the key elements which allow the reconstruction quality of the software trigger in Run-II, which fully includes the particle identification selection criteria, to be as good as the offline quality of Run-I. This approach greatly increases the efficiency, in particular for the selection of charm and strange hadron decays. We discuss strategy and performance of this novel approach, followed by a presentation of the recent developments implemented for the 2017 run of data taking, and with the performance and reconstruction quality achieved by the LHCb experiment in LHC Run-II.

Open charm and dileptons from relativistic heavy-ion collisions

Dileptons are considered as one of the cleanest signals of the quark-gluon plasma (QGP), however, the QGP radiation is masked by many 'background' sources from either hadronic decays or semileptonic decays from correlated charm pairs. In this study we investigate the relative contribution of these channels in heavy-ion collisions from $\sqrt{s_{\rm NN}}=$ 8 GeV to 5 TeV with a focus on the competition between the thermal QGP radiation and the semileptonic decays from correlated $D-$meson pairs. As a 'tool' we employ the parton-hadron-string dynamics (PHSD) transport approach to study dilepton spectra in Pb+Pb (Au+Au) collisions in a wide energy range incorporating for the first time a fully microscopic treatment of the charm dynamics and their semileptonic decays. We find that the dileptons from correlated $D-$meson decays dominate the 'thermal' radiation from the QGP in central Pb+Pb collisions at the intermediate masses (1.2 GeV $< M <$ 3 GeV) for $\sqrt{s_{\rm NN}} > $ 40 GeV, while for $\sqrt{s_{\rm NN}}=$ 8 to 20 GeV the contribution from $D,{\bar D}$ decays to the intermediate mass dilepton spectra is subleading such that one should observe a rather clear signal from the QGP radiation. We, furthermore, study the $p_T$-spectra and the $R_{AA}(p_T)$ of single electrons at different energies as well as the excitation function of the inverse slope of the $m_T$- spectra for intermediate-mass dileptons from the QGP and from charm decays. We find moderate but characteristic changes in the inverse slope parameter for $\sqrt{s_{\rm NN}} > $ 20 GeV which can be observed experimentally in high statistics data. Additionally, we provide detailed predictions for dilepton spectra from Pb+Pb collisions at $\sqrt{s_{\rm NN}} = $ 5.02 TeV.

Λc→N form factors from lattice QCD and phenomenology of Λc→nℓ+νℓ and Λc→pμ+μ− decays

A lattice QCD determination of the $\Lambda_c \to N$ vector, axial vector, and tensor form factors is reported. The calculation was performed with $2+1$ flavors of domain wall fermions at lattice spacings of $a\approx 0.11\:{\rm fm},\:0.085\:{\rm fm}$ and pion masses in the range $230\:{\rm MeV} \lesssim m_\pi \lesssim 350$ MeV. The form factors are extrapolated to the continuum limit and the physical pion mass using modified $z$ expansions. The rates of the charged-current decays $\Lambda_c \to n\, e^+ \nu_e$ and $\Lambda_c \to n\, \mu^+ \nu_\mu$ are predicted to be $\left( 0.405 \pm 0.016_{\,\rm stat} \pm 0.020_{\,\rm syst} \right)|V_{cd}|^2 \:{\rm ps}^{-1}$ and $\left( 0.396 \pm 0.016_{\,\rm stat} \pm 0.020_{\,\rm syst} \right)|V_{cd}|^2 \:{\rm ps}^{-1}$, respectively. The phenomenology of the rare charm decay $\Lambda_c \to p\, \mu^+ \mu^-$ is also studied. The differential branching fraction, the fraction of longitudinally polarized dimuons, and the forward-backward asymmetry are calculated in the Standard Model and in an illustrative new-physics scenario.

Low-mass dielectrons in pp, p–Pb and Pb–Pb collisions measured by the ALICE Experiment

Dielectrons produced in ultra-relativistic heavy-ion collisions at the LHC provide a unique probe of the system evolution as they are unperturbed by final-state interactions. The dielectron continuum is extremely rich in physics sources: on top of ordinary Dalitz and resonance decays of pseudoscalar and vector mesons, thermal black-body radiation is of particular interest as it carries information about the temperature of the hot and dense system created in such collisions. The dielectron invariant-mass distribution is furthermore sensitive to medium modifications of the spectral function of short-lived vector mesons that are linked to the potential restoration of chiral symmetry at high temperatures. Correlated electron pairs from semi-leptonic charm and beauty decays provide complementary information about the heavy-quark energy loss.

Strong decays of exotic and nonexotic heavy baryons in the chiral quark-soliton model

In the large $N_c$ limit both heavy and light baryons are described by the universal mean field, which allows us to relate the properties of heavy baryons to light ones. With the only input from the decays of light octet baryons (due to the universality of the chiral mean field), excellent description of strong decays of both charm and bottom sextets is obtained. The parameter-free prediction for the widths of exotic anti-decapentaplet ($\overline{\boldsymbol{15}}$) baryons is also made. The exotic heavy baryons should be anomalously narrow despite of large phase space available. In particular, the widths of $\Omega_c(3050)$ and $\Omega_c(3119)$, interpreted as members of $\overline{\boldsymbol{15}}$-plet, are very small: 0.48~MeV and 1.12~MeV respectivly. This result is in very good agreement with the measurements of the LHCb Collaboration and provides natural and parameter free explanation of the LHCb observation that $\Omega_c(3050)$ and $\Omega_c(3119)$ have anomalously small widths among five recently observed states.

PHENIX measurements of single electrons from charm and bottom decays at midrapidity in Au+Au collisions

Heavy quarks (charm and bottom) are a sensitive probe of the Quark-Gluon Plasma (QGP) created in high-energy nuclear collisions. The modification of their phase-space distribution in the QGP reflects the physical property of the QGP because they are generated in the early stage of collisions and subsequently propagate through the QGP. PHENIX has published the first measurement of the nuclear modification factors RAA of separated charm and bottom hadron decay electrons in minimum bias Au+Au collisions at sNN=200GeV using the 2011 data set. In these proceedings, we present RAA(c→e) and RAA(b→e) in most central Au+Au collisions at sNN=200GeV using 1/8 of our large 2014–2016 data set.

Measurements of the dielectron continuum in pp, p-Pb and Pb-Pb collisions with ALICE at the LHC

Dielectrons produced in ultra-relativistic heavy-ion collisions provide a unique probe of the whole system evolution as they are unperturbed by final-state interactions. The dielectron continuum is extremely rich in physics sources: thermal radiation is of particular interest as it carries information about the temperature of the hot and dense system created in such collisions. The dielectron invariant mass distribution is sensitive to medium modifications of the spectral function of vector mesons that are linked to the potential restoration of chiral symmetry. Correlated electron pairs from semi-leptonic charm and beauty decays provide information about the heavy-quark energy loss.A summary of the LHC Run-1 preliminary results in all three collisions systems (pp, p-Pb and Pb-Pb) is presented. Furthermore, the status of the ongoing Run-2 analyses is discussed with a focus on pp collisions collected with a high charged-particle multiplicity trigger, on new analysis methods to separate prompt from non-prompt sources, and on the usage of machine learning methods for background rejection.