arXiv subjects

M. Andersson

Publications and source records attributed to M. Andersson.

At least 19 recordsLinked to original sources

arXiv2026

Branching fraction measurements of $\eta$ and $\eta^\prime(958)$ anomalous decays

Branching fractions of $\eta$ and $\eta'(958)$ anomalous decays are measured using $pp$ collision data at a center-of-mass energy of $\sqrt{s}=13\,\text{TeV}$ collected by the LHCb experiment and corresponding to an integrated luminosity of $5.4\,\text{fb}^{-1}$. The measured values and upper limits are \begin{align*} \cal{B}(\eta\to\mu^+\mu^-) &= (5.02\pm0.14\pm0.18\pm0.26)\times10^{-6} , \\ \cal{B}(\eta'(958)\to\mu^+\mu^-) &<1.9\times10^{-7}\textrm{ at 90% CL} , \\ \cal{B}(\eta\to\pi^+\pi^-\mu^+\mu^-) &<5.4\times10^{-7}\textrm{ at 90% CL}, \\ \cal{B}(\eta'(958)\to\pi^+\pi^-\mu^+\mu^-) &= (2.73\pm0.14\pm0.13\pm0.14)\times10^{-5} , \end{align*} where the uncertainties are, respectively, statistical, systematic, and due to the external branching fraction of the $\phi(1020)\to\mu^+\mu^-$ decay, which is used for normalization. This marks the first experimental search for the $\eta'(958)$ decay, and the most precise measurement of the branching fraction $\cal{B}(\eta\to\mu^+\mu^-)$ to date.

hep-ex↗

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

Amplitude analysis of $B^+ \!\to K^+ \pi^+ \pi^-$ decays

The branching fractions and quasi-two-body $C\!P$-violating asymmetries of intermediate states obtained through an amplitude analysis of the charmless three-body decay $B^+ \!\to K^+ \pi^+ \pi^-$ are reported. The analysis is based on $pp$ collision data at centre-of-mass energies $\sqrt{s}=7$ and $8\,\text{TeV}$ recorded with the LHCb detector, corresponding to an integrated luminosity of $3\,\text{fb}^{-1}$. The most challenging aspect of the amplitude modelling lies in the description of the dominant $K^+ \pi^-$ and $\pi^+ \pi^-$ S-wave contributions. This is achieved by three complementary approaches based on a physically motivated analytic model built on the isobar approximation, the K-matrix formalism, and a quasi-model-independent procedure in which overlapping crossing partial waves are simultaneously studied. In addition, alternative sets of results are presented, considering the $\pi^+ \pi^-$ final state to manifest either through direct $\omega(782)$ decays or $\rho(770)^0\textrm{-}\omega(782)$ mixing. The most precise measurements of branching fractions and $C\!P$ asymmetries are obtained for the vast majority of intermediate states, establishing firmer reference points against which to cleanly probe model-independent physics beyond the Standard Model. The results from all three approaches agree and provide new insight into strong dynamics and the origin of $C\!P$-violation effects in $B^+ \!\to K^+ \pi^+ \pi^-$ decays.

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arXiv2026

Resolution of outstanding puzzles in $B^+ \!\to K^+ \pi^+ \pi^-$ decays

An amplitude analysis of $B^+ \!\to K^+ \pi^+ \pi^-$ decays is presented, based on $pp$ collision data recorded with the LHCb detector in 2011--2012, corresponding to an integrated luminosity of $3\,\text{fb}^{-1}$. Previous studies of the $B \!\to K \pi \pi$ sector have left key unresolved questions concerning the model of the S-wave contributions. A pivotal finding is that relaxing unitarity-based assumptions about the relation between the $K^*_0(1430)^0$ resonance and the slowly varying scalar part in $K^+\pi^-$ leads to considerably better agreement between the model and data. The $B^+ \!\to K^*_0(1430)^0 \pi^+$ branching fraction now challenges the experimental consensus that $B \!\to K^*_0(1430) \pi$ decays dominate the $B \!\to K \pi \pi$ phase space, aligning with the predictions of QCD factorisation rather than perturbative QCD, thus reversing the agreement found in previous measurements. With this increased flexibility, it also becomes possible to model the scalar $\pi^+ \pi^-$ amplitude using established states, eliminating the need for the ad-hoc ``$f_X(1300)$'' component included in previous analyses of the $B \!\to K\pi\pi$ sector. These advances facilitate the discovery of ten intermediate decays.

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arXiv2026

Observation of several sources of $C\!P$ violation in $B^+ \!\to K^+ \pi^+ \pi^-$ decays

An amplitude analysis of $B^+ \!\to K^+ \pi^+ \pi^-$ decays is presented in which six $C\!P$-violating phenomena are judged to be of significance for the first time. This analysis is based on $pp$ collision data recorded with the LHCb detector in 2011-2012, corresponding to an integrated luminosity of $3\,\text{fb}^{-1}$. Quasi-two-body $C\!P$ violation in $B^+ \!\to \rho(770)^0 K^+$ decays is discovered, while $C\!P$ violation at amplitude level is established in $B^+ \!\to f_2(1270) K^+$ decays. First evidence for $C\!P$ violation is reported in both the fully elastic S-wave $\pi\pi$-$\pi\pi$ rescattering region and also for any decay involving a spin-3 resonance. Additionally, significant $C\!P$-violation effects are identified in the interference between different $\pi\pi$ partial waves, with observation in S-P wave interference and evidence in S-D wave interference, both of which must be driven by long-distance interactions.

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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

New measurements of $B^+_c$ decays into single charm final states

Using proton-proton collision data corresponding to an integrated luminosity of $9 \,\textrm{fb}^{-1}$ collected by the LHCb experiment, searches are performed for $B^+_c$ mesons decaying to a charm and a charmless meson pair. Five products of branching fraction, ${\cal B}(B^+_c\!\to DX)$, and fragmentation ratio $f_c\big/f_u$ are reported, \begin{align*} R_{D^+ K^{*0}} &= ( 1.42 \pm 0.23 \pm 0.07 \pm 0.11 ) \times 10^{-6}, \\ R_{D^{*0} K^+} &= (1.46 \pm 0.30 \pm 0.11 \pm 0.05 ) \times 10^{-6}, \\ R_{D^+_s \phi}\ \ \ &= ( 4.0\pm 1.3 \pm 0.2 \pm 0.5) \times 10^{-7 }, \\ R_{D^0 K^+}\ &= ( 9.7 \pm 1.0 \pm 0.4 \pm 0.3 ) \times 10^{-7}, \\ R_{D^0 \pi^+}\ &<\ 1.4 \times 10^{-7}\ \text{at 95\% CL}. \end{align*} In each case, the first uncertainty is statistical, the second is systematic and the third includes external uncertainties. The first result is a first observation, the second and third exhibit clear evidence and the fourth improves the precision of previous measurements by a factor two. Additionally, the $CP$ asymmetry in $B^+_c\!\to D^0 K^+$ decays is measured and found to be compatible with zero.

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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.

nucl-ex↗

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

Associated $Z$ + $J/\psi$ production as a probe of multiparton interactions in the forward region

This letter reports a study of associated $Z$ boson and prompt $J/\psi$ production in proton-proton collisions at $\sqrt{s}=13$ TeV with the LHCb detector in the forward rapidity region, using a data sample taken during 2016 to 2018 corresponding to an integrated luminosity of $5.1 \mathrm{fb}^{-1}$. The measured fiducial cross-section is $5.5 \pm 1.5$ pb, significantly exceeding the single-parton scattering expectation of $0.10 \pm 0.08$ pb, indicating that multiparton interactions dominate this process in the explored phase space. Interpreted within the standard double-parton scattering framework, the data yields an effective cross-section $\sigma_{\mathrm{eff}} = 16.6 \pm 4.7$ mb. This provides a direct experimental constraint on the transverse spatial structure of the proton in a kinematic regime characterized simultaneously by small Bjorken-$x$ and an electroweak hard scale set by the $Z$ boson mass.

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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.

nucl-ex↗

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.

hep-ex↗

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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