Recent advances in charm mixing and CPCP violation at LHCb

After playing a pivotal role in the birth of the Standard Model in the 70's, the study of charm physics has undergone a revival during the last decade, triggered by a wealth of precision measurements from the charm and $B$ factories, and from the CDF and especially the LHCb experiments. In this article, we sum up how the unique phenomenology of charmed hadrons can be used to test the Standard Model and we review the latest measurements performed in this field by the LHCb experiment. These include the historic first observations of $CP$ violation and of a nonzero mass difference between the charmed neutral-meson mass eigenstates, the most precise determination of their decay-width difference to date, and a search for time-dependent $CP$ violation reaching the unprecedented precision of $10^{-4}$. These results challenge our comprehension of nonperturbative strong interactions, and their interpretation calls for further studies on both the theoretical and experimental sides. The upcoming upgrades of the LHCb experiment will play a leading role in this quest.Comment: 39 pages, 13 figures. V2: fixed error in eq. 2.2; updated and improved list of references; added table of content

Measurements of time-dependent C ⁣PC\!P violation and mixing in charm at LHCb

The LHCb experiment has opened the possibility to test mixing and $C\!P$ violation in the charm sector with unprecedented precision thanks to the huge number of charm hadron decays collected, $\mathcal{O}(10^8)$. The first observation of $C\!P$ violation in the decay of charm quarks in March 2019 has been a fundamental achievement. The latest LHCb measurements in the complementary sectors of mixing and time-dependent $C\!P$ violation are illustrated in these proceedings. In particular, a new measurement of the $C\!P$ violation parameter $A_\Gamma$ with 2015--2016 data that was presented for the first time at this conference is described. In the last section, prospects are given for the improvements in precision expected in the next few years.Comment: 5 pages, 7 figures. Proceedings of the 2019 Conference on Flavor Physics and CP Violation (FPCP 2019

Mixing and CPCP violation in D0→K−π+D^0 \to K^- \pi^+ decays

The theoretical parametrisation of mixing and time-dependent $CP$ violation for $D^0$decays into two hadrons is reviewed, along with the experimental methods to measure them. While these phenomena are usually neglected for $D^0 \to K^- \pi^+$ decays, this approximation is not always justified at the current level of experimental precision. In particular, it is shown to produce a bias on the measurement of the parameter $y_{CP}$, when this is performed by relying on $D^0 \to K^- \pi^+$ decays as normalisation channel, whose size is around 40% of the precision of the current world average. The sensitivity to $CP$ violation in the mixing achievable by studying $D^0 \to K^- \pi^+$ and untagged $D \to K^- \pi^+$ decays, where $D$ stands for either of $D^0$ or $\bar{D}^0$ mesons, is also estimated.Comment: 19 pages, 2 figures; fitting code available at https://github.com/tpajero/charm-fitter; v2 prepared for submission to JHEP (added 2 figures, fixed minor typos in equations, updated the values of charm mixing parameters

Study of the B−→Λc+Λˉc−K−B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-} decay

The decay $B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-}$ is studied in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13$ TeV using data corresponding to an integrated luminosity of 5 $\mathrm{fb}^{-1}$ collected by the LHCb experiment. In the $\Lambda_{c}^+ K^{-}$ system, the $\Xi_{c}(2930)^{0}$ state observed at the BaBar and Belle experiments is resolved into two narrower states, $\Xi_{c}(2923)^{0}$ and $\Xi_{c}(2939)^{0}$, whose masses and widths are measured to be $$m(\Xi_{c}(2923)^{0}) = 2924.5 \pm 0.4 \pm 1.1 \,\mathrm{MeV}, \\ m(\Xi_{c}(2939)^{0}) = 2938.5 \pm 0.9 \pm 2.3 \,\mathrm{MeV}, \\ \Gamma(\Xi_{c}(2923)^{0}) = \phantom{000}4.8 \pm 0.9 \pm 1.5 \,\mathrm{MeV},\\ \Gamma(\Xi_{c}(2939)^{0}) = \phantom{00}11.0 \pm 1.9 \pm 7.5 \,\mathrm{MeV},$$ where the first uncertainties are statistical and the second systematic. The results are consistent with a previous LHCb measurement using a prompt $\Lambda_{c}^{+} K^{-}$ sample. Evidence of a new $\Xi_{c}(2880)^{0}$ state is found with a local significance of $3.8\,\sigma$, whose mass and width are measured to be $2881.8 \pm 3.1 \pm 8.5\,\mathrm{MeV}$ and $12.4 \pm 5.3 \pm 5.8 \,\mathrm{MeV}$, respectively. In addition, evidence of a new decay mode $\Xi_{c}(2790)^{0} \to \Lambda_{c}^{+} K^{-}$ is found with a significance of $3.7\,\sigma$. The relative branching fraction of $B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-}$ with respect to the $B^{-} \to D^{+} D^{-} K^{-}$ decay is measured to be $2.36 \pm 0.11 \pm 0.22 \pm 0.25$, where the first uncertainty is statistical, the second systematic and the third originates from the branching fractions of charm hadron decays.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-028.html (LHCb public pages

Measurement of the ratios of branching fractions R(D∗)\mathcal{R}(D^{*}) and R(D0)\mathcal{R}(D^{0})

The ratios of branching fractions $\mathcal{R}(D^{*})\equiv\mathcal{B}(\bar{B}\to D^{*}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(\bar{B}\to D^{*}\mu^{-}\bar{\nu}_{\mu})$ and $\mathcal{R}(D^{0})\equiv\mathcal{B}(B^{-}\to D^{0}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(B^{-}\to D^{0}\mu^{-}\bar{\nu}_{\mu})$ are measured, assuming isospin symmetry, using a sample of proton-proton collision data corresponding to 3.0 fb${ }^{-1}$ of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode $\tau^{-}\to\mu^{-}\nu_{\tau}\bar{\nu}_{\mu}$. The measured values are $\mathcal{R}(D^{*})=0.281\pm0.018\pm0.024$ and $\mathcal{R}(D^{0})=0.441\pm0.060\pm0.066$, where the first uncertainty is statistical and the second is systematic. The correlation between these measurements is $\rho=-0.43$. Results are consistent with the current average of these quantities and are at a combined 1.9 standard deviations from the predictions based on lepton flavor universality in the Standard Model.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-039.html (LHCb public pages

Multidifferential study of identified charged hadron distributions in ZZ-tagged jets in proton-proton collisions at s=\sqrt{s}=13 TeV

Jet fragmentation functions are measured for the first time in proton-proton collisions for charged pions, kaons, and protons within jets recoiling against a $Z$ boson. The charged-hadron distributions are studied longitudinally and transversely to the jet direction for jets with transverse momentum 20 $< p_{\textrm{T}} < 100$ GeV and in the pseudorapidity range $2.5 < \eta < 4$. The data sample was collected with the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 1.64 fb$^{-1}$. Triple differential distributions as a function of the hadron longitudinal momentum fraction, hadron transverse momentum, and jet transverse momentum are also measured for the first time. This helps constrain transverse-momentum-dependent fragmentation functions. Differences in the shapes and magnitudes of the measured distributions for the different hadron species provide insights into the hadronization process for jets predominantly initiated by light quarks.Comment: All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-013.html (LHCb public pages

Measurement of the CPCP violation parameter AΓA_\Gamma in D0→K+K−D^0\rightarrow K^+K^- and D0→π+π−D^0\rightarrow \pi^+\pi^- decays with LHCb Run 2 data

Time-dependent CP asymmetries in the decay rates of the D0 and D0bar mesons into the CP-symmetric, singly Cabibbo-suppressed final states K+K− and π+π− are measured in pp collisions collected by the LHCb experiment in 2015 and in 2016 at the centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 1.7 fb−1. The strong-interaction decay D∗+ → D0π+ is used to infer the flavour of the D0 meson at production. The asymmetry between the effective decay widths of the D0 and D0bar decays, sensitive to indirect CP violation, is measured: AΓ(D0 → K+K−) = (−60.9 ± 3.8 ± 0.6) · 10−4, AΓ(D0 → π+π−) = (−62.5 ± 6.7 ± 0.5) · 10−4, where the first uncertainty is statistical, the second one systematic and the central values are blinded pending the receipt of the approval of the analysis procedure by the LHCb collaboration. The obtained statistical precision is comparable to the previous, world-leading measurement of AΓ by the LHCb collaboration, whereas the systematic uncertainty is reduced by about a factor of 2

Search for time-dependent CPCP violation in D0→K+K−D^0 \to K^+ K^- and D0→π+π−D^0 \to \pi^+ \pi^- decays

A search for time-dependent violation of the charge-parity ($C\!P$) symmetry in $D^0 \to K^+ K^-$ and $D^0 \to \pi^+ \pi^-$ decays is performed at the LHCb experiment using proton-proton collision data recorded from 2015 to 2018 at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.7 fb$^{-1}$. The $D^0$ meson is required to originate from a $D^*(2010)^+ \to D^0 \pi^+$ decay, such that its flavour at production is identified by the charge of the accompanying pion. The slope of the time-dependent asymmetry of the decay rates of $D^0$ and $\overline{D}^0$ mesons into the final states under consideration is measured to be \begin{align*} \Delta Y_{K^+ K^-} &= (-2.3 \pm 1.5 \pm 0.3) \times 10^{-4},\\ \Delta Y_{\pi^+ \pi^-} &= (-4.0 \pm 2.8 \pm 0.4)\times 10^{-4}, \end{align*} where the first uncertainties are statistical and the second are systematic. Neglecting possible final-state dependent contributions, these results are combined yielding \begin{equation*} \Delta Y = (-2.7 \pm 1.3 \pm 0.3 )\times 10^{-4}, \end{equation*} which is compatible with the $C\!P$-invariance hypothesis at the level of 2 standard deviations. This result improves the precision of the world average of $\Delta Y$ by a factor of two, and sets the strongest bound on the value of the phase $\phi^M_2$, which quantifies dispersive $C\!P$-violating contributions to $D^0$ mixing

Mixing and indirect CP violation in two-body Charm decays at LHCb

The copious number of $D^0$ decays collected by the LHCb experiment during 2011--2016 allows the test of the violation of the $CP$ symmetry in the decay of charm quarks with unprecedented precision, approaching for the first time the expectations of the Standard Model. We present the latest measurements of LHCb of mixing and indirect $CP$ violation in the decay of $D^0$ mesons into two charged hadrons. All the results for $A_\Gamma$, mixing and $CP$ violation with $D^0 \rightarrow K^\pm\pi^\mp$ decays and $y_{CP}$ are the most precise to date and are still limited by the statistical uncertainty. No evidence of $CP$ violation has been found yet