arXiv subjects

S. Blusk

Publications and source records attributed to S. Blusk.

At least 19 recordsLinked to original sources

arXiv2026

Search for $B$ meson decays to multimuon final states

A search for decays of $B$ mesons to final states with four or six muons using $pp$ collision data recorded by the LHCb experiment corresponding to an integrated luminosity of $5.4~\text{fb}^{-1}$ is presented. The decay modes of interest are $B_{(s)}^0 \rightarrow \mu^+\mu^-\mu^+\mu^-$, $B^+ \rightarrow K^+\mu^+\mu^-\mu^+\mu^-$, $B_{(s)}^0 \rightarrow \mu^+\mu^-\mu^+\mu^-\mu^+\mu^-$ and $B^+ \rightarrow K^+\mu^+\mu^-\mu^+\mu^-\mu^+\mu^-$, proceeding via both prompt and long-lived intermediate particles. No evidence for any of the signal modes is found, and upper limits spanning the range of $0.6\times10^{-9}$ to $5.4\times10^{-7}$ at the $95\%$ confidence level are set on their branching fractions, depending on the intermediate-particle masses and lifetimes. In addition, mass-integrated limits across the intermediate-particle lifetime ranges considered in this analysis are determined.

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arXiv2026

Improved measurement of $C\!P$ violation in $B^{0}_{s} \!\to J/\psi \pi^{+}\pi^{-}$ decays

The time-dependent $C\!P$ asymmetry in $B^{0}_{s} \!\to J/\psi \pi^{+}\pi^{-}$ decays is measured using proton-proton collision data, corresponding to an integrated luminosity of $6\,\text{fb}^{-1}$, collected with the LHCb detector at a centre-of-mass energy of $13\,\text{TeV}$ during $\mbox{2015--2018}$. The $C\!P$-violating phase, $\phi_{s}$, the direct $C\!P$-violation parameter, $\left|\lambda\right|$, and the decay width of the heavy mass eigenstate in the $B^{0}_{s}$ system, $\Gamma_{\mathrm{ H}}$, are measured respectively to be $\phi_{s} = -0.077 \pm 0.034 \pm 0.007\,\text{rad}$, $\left|\lambda\right| = 0.993 \pm 0.026 \pm 0.007$ and $\Gamma_{\mathrm{ H}} = 0.610 \pm 0.002 \pm 0.004\,\text{ps}^{-1}$, where the first uncertainties are statistical and the second systematic. These results are consistent with previous measurements and the expectation based on the Standard Model. The combination with previous measurements in $B^{0}_{s} \!\to J/\psi \pi^{+}\pi^{-}$ decays using $7\,\text{TeV}$ and $8\,\text{TeV}$ proton-proton collision data yields $\phi_{s} = -0.046 \pm 0.031\,\text{rad}$, $\left|\lambda\right| = 0.975 \pm 0.024$ and $\Gamma_{\mathrm{ H}} = 0.610 \pm 0.004\,\text{ps}^{-1}$, while the combination including all other LHCb measurements gives $\phi_{s} = -0.041 \pm 0.017\,\text{rad}$.

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arXiv2026

Study of muon-tagged $D_{s1}(2460)^+$ and $D_{s1}(2536)^+$ decays to the $D_s^{+}\pi^+\pi^-$ final state

Decays of the pseudovector $D_{s1}(2460)^+$ and $D_{s1}(2536)^+$ mesons to the three-body $D_{s}^+\pi^+\pi^-$ final state are studied. The data sample is based on decays of beauty hadrons into $D_{s1}^+$ states accompanied by a muon from the $b$-hadron decay chain collected by the LHCb detector during 2016--2018, corresponding to an integrated luminosity of 5.4 fb${}^{-1}$. The \mbox{$D_{s1}(2536)^+\to D_s^+\pi^+\pi^-$} branching fraction is measured for the first time, with the $D_{s1}(2536)^+\to D^+K^+\pi^-$ decay used as a reference. A simultaneous amplitude analysis of the $D_{s1}(2460)^+$ and $D_{s1}(2536)^+\to D_s^+\pi^+\pi^-$ decays is performed. The Dalitz-plot distributions of the two decays are found to be significantly different, suggesting differences in the internal structure of the two states, with evidence of exotic contributions to the $D_{s}^+\pi^{\pm}$ channel with the pole below the $DK$ threshold. Measurements of the masses of the $D_{s1}(2460)^+$ and $D_{s1}(2536)^+$ states are performed, and an upper limit on the $D_{s1}(2460)^+$ width is set.

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arXiv2026

Measurement of the average transverse momentum of forward prompt charged particles in $pp$ and $p\mathrm{Pb}$ collisions at $\sqrt{s_{NN}} = 5.02\; \mathrm{TeV}$

This letter presents the first measurements of the average transverse momentum of prompt charged particles in $pp$ and $p\mathrm{Pb}$ collisions as a function of collision multiplicity and pseudorapidity. The data were recorded at nucleon-nucleon centre-of-mass energy $\sqrt{s_{NN}} = 5.02\; \mathrm{TeV}$ with the LHCb experiment. The pseudorapidity dependence of the multiplicity distribution is also measured. The average transverse momentum results show a decreasing trend with pseudorapidity, more pronounced in high-multiplicity events, consistent with the collective behaviour of the produced matter. The measurements are reproduced by state-of-the-art (3+1D) hydrodynamic calculations, while saturation models are not compatible with the data.

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arXiv2026

Search for $C\!P$ violation in $D^+ \to \phi \pi^+$ decays

A search for charge-parity ($C\!P$) violation in the Cabibbo-suppressed $D^+ \to \phi \pi^+$ decay is presented, using proton-proton collision data corresponding to an integrated luminosity of $6\text{ fb}^{-1}$, collected at a centre-of-mass energy of $13\text{ TeV}$ with the LHCb detector during Run 2. An abundant sample of $D^+ \to K_{\rm S}^0 \pi^+$ decays is employed to correct for asymmetries arising from the production of the charmed meson and from detection effects associated with the charged pion accompanying the $\phi$ meson. Asymmetries induced by interference of multiple processes, such as neutral kaon mixing, regeneration of neutral kaons, and interference between the Cabibbo-favoured $D^+ \to \overline{K}^0 \pi^+$ and the doubly Cabibbo-suppressed $D^+ \to {K}^0 \pi^+$ decay, are subtracted. The direct $C\!P$ asymmetry in the $D^+ \to \phi \pi^+$ decay is measured to be \begin{equation*} a_{C\!P}(D^+ \to \phi \pi^+) = \left(0.1 \pm 4.9\text{ (stat)} \pm 2.4\text{ (syst)} \right)\times 10^{-4}. \end{equation*} For the first time at a hadron collider, the modulus of the ratio and the relative strong phase of the $D^+ \to {K}^0 \pi^+$ to $D^+ \to \overline{ K}^0 \pi^+$ decay amplitudes are also investigated. Two-dimensional confidence intervals are reported for these parameters.

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arXiv2026

Search for the decays $B^+_{(c)} \to \mu^+ \nu_\mu \gamma$

A search for the radiative leptonic decays $B^+\to\mu^+\nu_\mu\gamma$ and $B^+_c\to\mu^+\nu_\mu\gamma$ is performed using proton-proton collision data collected with the LHCb experiment at a center-of-mass energy of $13~{\rm TeV}$, corresponding to an integrated luminosity of $5.4~{\rm fb}^{-1}$. No evidence for an excess of events over background is observed for either signal decay. Upper limits at 90% confidence level are set on the branching fractions: \begin{align} {\cal{B}}(B^+\to\mu^+\nu_\mu\gamma)_{E_\gamma^\ast > 1\,\rm{GeV}} &< 4.0 \times 10^{-6},\\ {\cal{B}}(B_c^+\to\mu^+\nu_\mu\gamma)_{E_\gamma^\ast > 1\,\rm{GeV}} &< 1.6 \times 10^{-3}, \end{align} where the photon energy in the $B$-meson rest frame, $E_\gamma^\ast$, is required to be greater than $1~{\rm GeV}$. This constitutes the first search for these decays at a hadron collider and the first experimental investigation of the $B_c^+\to\mu^+\nu_\mu\gamma$ decay to date.

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arXiv2026

Observation of new excited $\it{\Sigma}_{c}^{0}$ states in the $B^-\rightarrow \it{\Lambda}^+_c\overline{p}\pi^{-}$ decay

An amplitude analysis of the $B^-\rightarrow \it{\Lambda}^+_c\overline{p}\pi^{-}$ decay is performed using proton-proton collision data, corresponding to an integrated luminosity of $9\text{ fb}^{-1}$, collected with the \mbox{LHCb} detector at center-of-mass energies of 7, 8, and 13$\text{ TeV}$. In the $\it{\Lambda}^+_c\pi^{-}$ invariant-mass spectrum, the known $\it{\Sigma}_c(2455)^0$, $\it{\Sigma}_c(2520)^0$, $\it{\Sigma}_c(2800)^0$ baryons are observed, along with two new states: $\it{\Sigma}_c(2900)^0$ and $\it{\Sigma}_c(3200)^0$. The masses and widths of the three high-mass states, modeled with Breit--Wigner line shapes, are determined. This result expands the landscape of charm baryon spectroscopy and provides crucial data for understanding charm baryon structure and dynamics.

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arXiv2026

Differential measurement of the branching fraction and $C\!P$ asymmetry of the decay $B^\pm\to \pi^\pm\mu^+\mu^-$

Differential measurements of the branching fraction and $C\!P$ asymmetry of the decay $B^\pm\to \pi^\pm\mu^+\mu^-$ are performed in intervals of the dimuon mass squared, $q^2$, using proton-proton collision data corresponding to an integrated luminosity of 9 fb$^{-1}$ recorded by the LHCb experiment at centre-of-mass energies of 7 TeV, 8 TeV and 13 TeV. The ratios of the branching fractions of $B^\pm\to \pi^\pm\mu^+\mu^-$ and $B^\pm\to K^\pm\mu^+\mu^-$ decays are also reported in the same $q^2$ intervals. The measured branching fractions are compatible with the predictions of the Standard Model, with consistency varying between 1.4$\sigma$ and 3.8$\sigma$ depending on the model assumptions.

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arXiv2026

Measurement of the ratio of branching fractions $\mathcal{B}(B_c^+ \to J/\psi \tau^+ \nu_{\tau})/\mathcal{B}(B_c^+ \to J/\psi \mu^+ \nu_{\mu})$

A measurement of the ratio of semileptonic branching fractions $\mathcal{R}(J\psi )$, defined as $\mathcal{R}(J/\psi ) \equiv \mathcal{B}(B_c^+ \to J/\psi \tau^+ \nu_{\tau})/\mathcal{B}(B_c^+ \to J/\psi \mu^+ \nu_{\mu})$, is reported using a sample of proton-proton collision data corresponding to an integrated luminosity of 5.4 fb$^{-1}$ recorded by the LHCb experiment in 2016--2018 at a center-of-mass energy of 13 TeV. The measured value is found to be $\mathcal{R}(J/\psi) = 0.51 \pm 0.12\text{(stat)} \pm 0.08\text{(syst)}$, which is within 1.8 standard deviations of the predictions from the Standard Model assuming lepton flavor universality.

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arXiv2026

Study of nuclear effects on charm production in light-ion collisions

The onset of nuclear effects in light-ion collisions is studied by measuring the ratio of $D^0$ meson production between NeNe and OO collisions at a center-of-mass energy per nucleon pair of $5.36\,{\rm TeV}$ recorded by the LHCb detector. The $D^0$ meson yields are measured differentially in transverse momentum ($p_{\rm T}$) for $0.5<p_{\rm T}<20\,{\rm GeV}$ in the rapidity ($y$) region $2.0<y<4.5$, and are normalized to the total number of recorded inelastic nucleus-nucleus collisions in each data sample. The resulting production ratio shows evidence of variation as a function of $p_{\rm T}$, which is inconsistent with predictions based on nuclear modification of nucleon structure alone. This measurement is consistent with the onset of quark-gluon plasma production with increasing nucleus size in high-energy nuclear collisions.

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arXiv2026

Evidence for the decay $B^0_s\to\phi\eta'$

Using a dataset corresponding to an integrated luminosity of $9 \,\textrm{fb}^{-1}$ collected in proton-proton collisions between 2011 and 2018 by the LHCb experiment, evidence is found for the decay $B^0_s\to\phi\eta'$ with $3.5 \sigma$ significance. The branching ratio relative to the $B^0_s\to\phi\phi$ decay is determined to be $R=(3.56 \pm 0.79\pm 0.18\pm 0.06)\times10^{-2}$. This corresponds to a branching fraction, $B(B^0_s\to\phi\eta')=(0.66 \pm 0.15 \pm 0.03 \pm 0.02) \times 10^{-6}$ where, in both cases, the first uncertainty is statistical, the second systematic, and the third due to external branching fractions.

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arXiv2026

$C\!P$ violation analysis of local and nonlocal amplitudes in the $\overline{B}^0 \to \overline{K}^{*0}\mu^+\mu^-$ decay

A search for $C\!P$ violation in the $\overline{B}^0 \to \overline{K}^{*0}\mu^+\mu^-$ decay is performed using proton-proton collision data collected by the LHCb experiment during Run 1 and Run 2, corresponding to an integrated luminosity of 8.4 fb$^{-1}$. The analysis exploits the full angular distribution of the decay, providing sensitivity to $C\!P$-violating effects in both vector and axial-vector contributions to this flavour-changing neutral-current process. The complex Wilson coefficients are determined within the Weak Effective Theory through an unbinned maximum-likelihood fit to the angular observables, incorporating nonlocal hadronic amplitudes across the full dimuon mass spectrum. The precision of the $C\!P$-violation observables is improved by an order of magnitude relative to previous measurements, with the imaginary parts of the Wilson coefficients now determined more precisely than the real parts. No significant $C\!P$ violation is observed, and the results are consistent with Standard Model.

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arXiv2026

Measurement of $\gamma$ using $B^{\pm}\rightarrow DK^{\pm}$ and $B^{\pm}\rightarrow D\pi^{\pm}$ decays with $D\rightarrow K_{\rm S}^{0}\pi^{+}\pi^{-}$ and $D\rightarrow K_{\rm S}^{0}K^{+}K^{-}$

A measurement of the CKM angle $\gamma$ using the decay channels $B^{\pm}\rightarrow DK^{\pm}$ and $B^{\pm}\rightarrow D\pi^{\pm}$, where the $D$ meson decays to $D\rightarrow K_{\rm S}^{0}\pi^{+}\pi^{-}$ or $D\rightarrow K_{\rm S}^{0}K^{+}K^{-}$, is performed with a data sample corresponding to an integrated luminosity of 5.8 fb$^{-1}$, collected during 2024 by the upgraded LHCb experiment. $C\!P$ violation is observed through a difference in the distributions of the Dalitz plot of the $D$ decay between the $B^{+}$ and $B^{-}$ mesons. The CKM angle $\gamma$ is determined to be $\gamma=(68.1\pm 6.7)^{\circ}$. Other parameters related to the examined $B$ meson decay modes are also measured. This is the first measurement of the CKM angle $\gamma$ using the upgraded LHCb detector.

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arXiv2026

Observation of the charmless purely baryonic decay $\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} p \overline{p}}$

A search for the charmless purely baryonic decay $\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} p \overline{p}}$ is performed using proton-proton collision data recorded by the LHCb experiment at a centre-of-mass energy of $\sqrt{s}=13\,\text{TeV}$ and corresponding to an integrated luminosity of $6.0\,\text{fb}^{-1}$. The signal decay is observed with a significance of 5.1 standard deviations. Its branching fraction is measured for the first time, relative to that of the topologically similar decay $\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} K^+ K^-}$. Contributions from intermediate charmonium resonances decaying to the $p \overline{p}$ and $K^+ K^-$ final states are explicitly excluded with a requirement on the invariant mass of the companion hadron system, $m(h\bar{h}) < 2.85\,\text{GeV}$, where $h$ stands for a proton or a charged kaon. The relative branching fraction is found to be $$ \frac{B(\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} p \overline{p}})}{B(\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} K^+ K^-})} = (5.1 \pm 1.3_{\text{(stat)}} \pm 0.3_{\text{(syst)}}) \times 10^{-2} \,. $$

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arXiv2026

Angular analysis of the $B^+\to\pi^+\mu^+\mu^-$ decay

This paper presents the first measurement of the forward-backward asymmetry, $A_{\rm FB}$, and the flat term, $F_{H}$, that parameterise the angular distribution of the $B^+\to\pi^+\mu^+\mu^-$ decay. The proton-proton collision dataset used in the analysis corresponds to an integrated luminosity of 9 fb$^{-1}$, collected with the LHCb experiment between 2011 and 2018. The analysis is performed in two intervals of dimuon mass squared, one above and one below the region containing the $J\mskip -3mu/\mskip -2mu\psi$ and $\psi(2S)$ narrow charmonium resonances. The Standard Model predictions lie within the obtained $68\%$ confidence level interval in the high-mass and within the $99\%$ interval in the low-mass region.

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arXiv2026

Search for the lepton-flavour violating decays $B^+ \to \pi^+ \mu^\pm e^\mp$

The first search for the lepton-flavour violating decays $B^+ \to \pi^+ \mu^{\pm} e^{\mp}$ in proton-proton collisions is presented, using data collected by the LHCb experiment between 2011 and 2018, corresponding to an integrated luminosity of 9 fb$^{-1}$. No significant signal is observed and an upper limit on the branching fraction is set at $\mathcal{B}(B^+ \to \pi^+ \mu^{\pm} e^{\mp}) < 1.8 \times 10^{-9}$ at the $90\%$ confidence level, two orders of magnitude more restrictive than the current world average. This is the first constraint on lepton-flavour violating $b \to d$ quark transitions at the LHC and also sets the most stringent upper limits to date on $b \to d \mu^{\pm} e^{\mp}$ transitions. Limits on left-handed and scalar scenarios beyond the Standard Model are also reported.

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arXiv2026

Observation of the doubly charmed baryon $\it{\Xi}_{cc}^+$ with the LHCb Run 3 detector

The first observation of the doubly charmed baryon $\it{\Xi}_{cc}^+$ is reported through its decay to the $\it{\Lambda}_c^+ K^-\pi^+$ final state, with a statistical significance exceeding seven standard deviations. The observation is made using proton-proton collision data collected in 2024 with the LHCb Run 3 detector at a center-of-mass energy of 13.6 TeV, corresponding to a total integrated luminosity of $6.9\,\mathrm{fb}^{-1}$. The $\it{\Xi}_{cc}^+$ mass is measured to be $3619.97 \pm 0.83 \pm 0.26 \,^{+1.90}_{-1.30}\,\mathrm{MeV}/c^2$, where the first uncertainty is statistical, the second is systematic, and the third is due to the unknown $\it{\Xi}_{cc}^+$ lifetime, which is assumed to lie in the range 15-160 fs with a baseline value of 45 fs. The difference between the masses of the $\it{\Xi}_{cc}^+$ and $\it{\Xi}_{cc}^{++}$ baryons is determined to be $-1.77 \pm 0.84 \pm 0.15 \,^{+1.90}_{-1.30}\,\mathrm{MeV}/c^2$. This is the first observation of a new particle made with the LHCb Run 3 detector.

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arXiv2025

Measurement of transverse $\Lambda$ and $\bar{\Lambda}$ hyperon polarization in $p$Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV

The transverse polarization of $\Lambda$ and $\bar{\Lambda}$ hyperons is measured in $p$Pb collisions collected by the LHCb experiment at a nucleon-nucleon center-of-mass energy of $5.02$ TeV. The polarization is averaged over hyperon transverse momentum in the ranges $0.15 < p_{T} < 6.00 $ GeV/$c$, and Feynman-$x$ in the ranges $0.005 < x_{F} < 0.040$ (forward region) and $-0.10 < x_{F} < -0.01$ (backward region) defined relative to the proton beam direction. The transverse polarization is found to be compatible with zero for both $\Lambda$ and $\bar{\Lambda}$ hyperons. The results are also measured as a function of $p_{T}$ and $x_{F}$ with no significant dependence on these variables observed. The results are compared with previous experimental measurements at different center-of-mass energies and collision environments.

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