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

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

Branching fraction measurement of Λb0→J/ψΛ\Lambda_{b}^{0} \rightarrow J/\psi \Lambda

In this thesis, the first measurement of the branching fraction $\mathcal{B}(\Lambda_{b}^{0}\rightarrow J/\psi\Lambda)$ at the LHC is presented. The measurement uses the full Run 2 proton-proton data collected with the LHCb detector at $\sqrt{s}$ = 13 TeV. Using the decay $B^0\rightarrow J/\psi K^{0}_{S}$ as a normalisation channel, the measurement is reported to be $\mathcal{B}(\Lambda_{b}^{0}\rightarrow J/\psi\Lambda) = (2.67 \pm 0.03 \text{ (stat.)} \pm 0.20\text{ (external syst.)})\times 10^{-4}$, excluding experimental systematic uncertainties. This is the most precise measurement to date with an uncertainty improved by more than a factor three compared to previous estimations. The improved precision on $\mathcal{B}(\Lambda_{b}^{0}\rightarrow J/\psi\Lambda)$ will significantly reduce the uncertainties for the measurement of the branching fraction $\mathcal{B}(\Lambda_b^0\rightarrow\Lambda \mu^+ \mu^-)$ and search for the Lepton Flavour Violating decay $\mathcal{B}(\Lambda^0_b\rightarrow\Lambda e^{\pm}\mu^{\mp})$, for both of which $\Lambda_{b}^{0}\rightarrow J/\psi\Lambda$ is used as a normalisation channel

Extraction of polarization sensitivity in charm-baryon three-body decays in LHCb

The polarization P of a decaying particle can be measured via the following equation: dσ d cos β = N(1 + αP cos β) (1) This equation is a particular case when the decaying particle is only longitudinally polarized, i.e. has spin projection along or against the direction of motion. In practice, what can be measured is αP by measuring the distribution of the angle β. The asymmetry parameter α plays a crucial role and it is assumed to be known in order to obtain the polarization P. The α parameter is truly a universal property of the particle and is experiment independent, whereas the polarization P of a decaying particle depends on the production mechanism, i.e. the beam or the parent decay chain, if any. Thus, P is dependent on the conditions of the specific experiment where it is measured. We begin with the case of a two-body decay, such as Λ → pπ, where the decaying Λ baryon has spin- 1 2 and the proton and pion have spins 1 2 and 0, respectively. In this case, the asymmetry parameter α in Eq. (1) can be expressed as follows: α = |H1 2 | 2 − |H− 1 2 | 2 |H1 2 | 2 + |H− 1 2 | 2 , (2) where |Hi | 2 are helicity couplings and the ± 1 2 indices refer to the helicity of the proton. The complete derivation is given in Appendix A. Equation (2) shows α written in terms of helicity couplings. In order to get α different than 0, parity conserving and parity violating couplings are both needed. To see this and identify parity conserving and parity violating parts of the decay we change the helicity basis to the parity, or LS, basis. The expression for the α in terms of LS couplings clearly shows the importance of both parity conserving and parity violating couplings. The change of basis takes place linearly through Eq. (3), where the coefficients are given by the Clebsch-Gordan coefficients Hλ = X L s 2L + 1 2 · 1/2 + 1 1/2, λ; 0, 0|1/2, λ L, 0; 1/2, λ|1/2, λ HL, (3) H1 2 = r 3 2 HS − r 1 3 HP ! (4) H− 1 2 = r 3 2 HS + r 1 3 HP ! (5) In Eqs. (4) and (5), H1 2 and H− 1 2 (Hλ) are couplings in the helicity basis, while HS and HP (HL) are couplings in LS (parity) basis. The labels S and P refer to S- and P-wave couplings (see Section 3), both of which the spin-1/2 Λ particle can have, one being parity conserving while the other one parity violating. It is evident from Eq. (6) that both parity violating and parity conserving couplings are needed in order to obtain a non-zero α. α = − √ 3Re (HSH∗ P ) 3|HS| 2 + |HP | 2 . (6) This quantity is important since it is really a fundamental property of a baryonic decay and once measured, its value is used to measure the polarization. A recent example where an updated measurement of α(Λ → pπ) impacted many previous polarization measurements is given by [3, 4]. There are also LHCb measurements of polarization in certain decays such as Λb → ΛJ/ψ [1]. In this project, we explore polarization in charmed baryon (Ξc/Λc) decays. There is little knowledge on the asymmetry parameter α for these type of decays. Knowledge on the asymmetry parameter opens 3 up the possibility of using Λ + c decay angle to improve sensitivity of the angular analysis in searches for exotic hadrons in system with charm baryon in the final state. Also, measurements of the polarization of the Λ + c /Ξc are needed in searches for new physics using electromagnetic dipole moment (EDM) measurements as proposed by the SELDOM project [6]. The report is structured as follows: in Sec. 2 we present the general formalism of three-body decays which leads to the expression of the asymmetry parameter α in the polarized case. In Sec. 3 we construct general decay amplitude using Dalitz-plot decomposition (DPD) as well as provide the details of the toy model. In Sec. 4 the α observable is discussed. Sec. 5 discusses the model ambiguities and Sec. 6 discusses the fitting strategy to unpolarized data. Finally. Sec. 7 consists of conclusions and outlook

First measurement of the Z→μ+μ−Z\rightarrow \mu^+ \mu^- angular coefficients in the forward region of pppp collisions at s=13\sqrt{s}=13 TeV

The first study of the angular distribution of $\mu^+ \mu^-$ pairs produced in the forward rapidity region via the Drell-Yan reaction $pp \rightarrow \gamma^{*}/Z +X \rightarrow l^+ l^- + X$ is presented, using data collected with the LHCb detector at a centre-of-mass energy of 13TeV, corresponding to an integrated luminosity of 5.1 $\rm{fb}^{-1}$. The coefficients of the five leading terms in the angular distribution are determined as a function of the dimuon transverse momentum and rapidity. The results are compared to various theoretical predictions of the $Z$-boson production mechanism and can also be used to probe transverse-momentum-dependent parton distributions within the proton

Precision measurement of forward ZZ boson production in proton-proton collisions at s=13\sqrt{s} = 13 TeV

A precision measurement of the $Z$ boson production cross-section at $\sqrt{s} = 13$ TeV in the forward region is presented, using $pp$ collision data collected by the LHCb detector, corresponding to an integrated luminosity of 5.1 fb$^{-1}$. The production cross-section is measured using $Z\rightarrow\mu^+\mu^-$ events within the fiducial region defined as pseudorapidity $2.020$ GeV/$c$ for both muons and dimuon invariant mass $60<M_{\mu\mu}<120$ GeV/$c^2$. The integrated cross-section is determined to be \begin{equation*} \sigma(Z\rightarrow\mu^+\mu^-) = 195.3 \pm 0.2 \pm 1.5 \pm 3.9~pb, \end{equation*} where the first uncertainty is statistical, the second is systematic, and the third is due to the luminosity determination. The measured results are in agreement with theoretical predictions, including a prediction at next-to-next-to-leading order in perturbative quantum chromodynamics and a prediction with resummation

Measurement of the charm mixing parameter yCP−yCPKπy_{CP} - y_{CP}^{K\pi} using two-body D0D^0 meson decays

A measurement of the ratios of the effective decay widths of $D^0 \to \pi^-\pi^+$ and $D^0 \to K^-K^+$ decays over that of $D^0 \to K^-\pi^+$ decays is performed with the LHCb experiment using proton-proton collisions at a centre-of-mass energy of $13 \, \mathrm{TeV}$, corresponding to an integrated luminosity of $6 \, \mathrm{fb^{-1}}$. These observables give access to the charm mixing parameters $y_{CP}^{\pi\pi} - y_{CP}^{K\pi}$ and $y_{CP}^{KK} - y_{CP}^{K\pi}$, and are measured as $y_{CP}^{\pi\pi} - y_{CP}^{K\pi} = (6.57 \pm 0.53 \pm 0.16) \times 10^{-3}$, $y_{CP}^{KK} - y_{CP}^{K\pi} = (7.08 \pm 0.30 \pm 0.14) \times 10^{-3}$, where the first uncertainties are statistical and the second systematic. The combination of the two measurements is $y_{CP} - y_{CP}^{K\pi} = (6.96 \pm 0.26 \pm 0.13) \times 10^{-3}$, which is four times more precise than the previous world average

Measurement of the charm mixing parameter yCP−yCPKπy_{CP} - y_{CP}^{K\pi} using two-body D0D^0 meson decays

A measurement of the ratios of the effective decay widths of $D^0 \to \pi^-\pi^+$ and $D^0 \to K^-K^+$ decays over that of $D^0 \to K^-\pi^+$ decays is performed with the LHCb experiment using proton-proton collisions at a centre-of-mass energy of $13 \, \mathrm{TeV}$, corresponding to an integrated luminosity of $6 \, \mathrm{fb^{-1}}$. These observables give access to the charm mixing parameters $y_{CP}^{\pi\pi} - y_{CP}^{K\pi}$ and $y_{CP}^{KK} - y_{CP}^{K\pi}$, and are measured as $y_{CP}^{\pi\pi} - y_{CP}^{K\pi} = (6.57 \pm 0.53 \pm 0.16) \times 10^{-3}$, $y_{CP}^{KK} - y_{CP}^{K\pi} = (7.08 \pm 0.30 \pm 0.14) \times 10^{-3}$, where the first uncertainties are statistical and the second systematic. The combination of the two measurements is $y_{CP} - y_{CP}^{K\pi} = (6.96 \pm 0.26 \pm 0.13) \times 10^{-3}$, which is four times more precise than the previous world average