arXiv subjects

M. Andersson

Publications and source records attributed to M. Andersson.

At least 19 recordsLinked to original sources

arXiv2026

Measurement of the local and nonlocal amplitudes in $B^{+}\to K^{+}μ^{+}μ^{-}$ decays

This paper presents a thorough study of the local and nonlocal amplitudes in $B^+ \to K^+μ^+μ^-$ transitions through an amplitude analysis of the dimuon mass spectrum of the decay. The analysis is based on $pp$ collision data corresponding to an integrated luminosity of 8.4fb$^{-1}$ collected by the LHCb experiment. This measurement employs a model that describes both one-particle and two-particle nonlocal amplitudes across the entirety of the dimuon mass spectrum, enabling the determination of both short- and long-distance contributions to the decay. The compatibility of the Wilson coefficient combinations $C_9+C_9'$ and $C_{10}+C_{10}'$ with the Standard Model prediction is found to vary between $1.6\,σ$ and $4\,σ$, depending on the choice of local form factors.

hep-ex↗

arXiv2026

Evidence for the decay $B^0_s\toϕη'$

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 $B^0_s\toϕη'$ with $3.5 σ$ significance. The branching ratio relative to the $B^0_s\toϕϕ$ 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, $B(B^0_s\toϕη')=(0.66 \pm 0.15 \pm 0.03 \pm 0.02) \times 10^{-6}$ where, in both cases, the first uncertainty is statistical, the second systematic, and the third due to external branching fractions.

hep-ex↗

arXiv2026

Branching fraction measurement of the $B^0\to K^+π^-μ^+μ^-$ decay at high dihadron mass

This Letter presents the first measurement of the $B^{0} \to K^{+}π^{-}μ^{+}μ^{-}$ differential branching fraction in the region of the dihadron $K^+π^-$ mass, $m_{Kπ}$, $1000< m_{Kπ} < 1835\,\text{MeV}/c^{2}$. This decay mode is mediated by the quark-level transition $b \to s\ell^{+}\ell^{-}$, for which the measured effective couplings show tensions with Standard Model predictions. The analysis is performed using proton-proton collision data collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of $8.4\,\text{fb}^{-1}$. The differential branching fraction is determined across different regions of dimuon mass squared, $q^2$, in the interval $0.1 < q^2 < 18.0\,\text{GeV}^2/c^{4}$, as well as in different regions of $ m_{Kπ}$ in the interval $1.1 < q^2 < 6.0\,\text{GeV}^2/c^{4}$. The results represent the first differential measurement of the branching fraction across $m_{Kπ}$ within the selected dihadron-mass region, and the first observation of the decay within this region above the $J/ψ$ resonance.

hep-ex↗

arXiv2026

Measurement of the ratio of branching fractions $\mathcal{B}(B_c^+ \to J/ψτ^+ ν_τ)/\mathcal{B}(B_c^+ \to J/ψμ^+ ν_μ)$

A measurement of the ratio of semileptonic branching fractions $\mathcal{R}(Jψ)$, defined as $\mathcal{R}(J/ψ) \equiv \mathcal{B}(B_c^+ \to J/ψτ^+ ν_τ)/\mathcal{B}(B_c^+ \to J/ψμ^+ ν_μ)$, 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 $\mathcal{R}(J/ψ) = 0.51 \pm 0.12\text{(stat)} \pm 0.08\text{(syst)}$, which is within 1.8 standard deviations of the predictions from the Standard Model assuming lepton flavor universality.

hep-ex↗

arXiv2026

Angular analysis of the $B^+\toπ^+μ^+μ^-$ 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 $B^+\toπ^+μ^+μ^-$ 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 $J\mskip -3mu/\mskip -2muψ$ and $ψ(2S)$ 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.

hep-ex↗

arXiv2026

Search for $τ^-\to μ^-μ^+μ^-$ decays at the LHCb experiment with Run 2 data

A search for the lepton-flavour-violating decay $τ^-\to μ^-μ^+μ^-$ is carried out using data collected by the LHCb experiment between 2016 and 2018 in proton-proton collisions at the LHC at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb$^{-1}$. An upper limit of $1.9\,(2.3)\times 10^{-8}$ is set at the 90% (95%) confidence level on the branching fraction of the decay.

hep-ex↗

arXiv2026

Improved branching-fraction measurements of $B^0_{(s)} \to K_S^0 h^+ h^{'-}$ decays and first observation of $B^0_{s} \to K_S^0 K^+ K^-$

This paper presents a study of the charmless three-body decays ${B^0_{(s)} \to K_{\mathrm{S}}^0 h^+ h^{\prime -}}$ (where $h^{(\prime)} = π, K$), using a sample of $pp$ collision data collected by the LHCb experiment during Runs 1 and 2 of the LHC, corresponding to an integrated luminosity of $9\,\mathrm{fb}^{-1}$. The decay ${B^0_s \to K_{\mathrm{S}}^0 K^+ K^-}$ is observed for the first time, and the following ratios of branching fractions are measured: \begin{alignat*}{6} &\frac{{\cal B}(B^0 \to K^0_{\mathrm{S}} K^+ K^-)}{{\cal B}(B^0 \to K^0_{\mathrm{S}} π^+π^-)} &&= 0.578 &&\pm 0.007 &&\pm 0.017\,, &\frac{{\cal B}(B^0 \to K^0_{\mathrm{S}} K^\pmπ^\mp)}{{\cal B}(B^0 \to K^0_{\mathrm{S}} π^+π^-)} &&= 0.1363 &&\pm 0.0035 &&\pm 0.0051\,, &\frac{{\cal B}(B^0_s \to K^0_{\mathrm{S}} π^+π^-)}{{\cal B}(B^0 \to K^0_{\mathrm{S}} π^+π^-)} &&= 0.269 &&\pm 0.011 &&\pm 0.015 && \pm 0.008\,, &\frac{{\cal B}(B^0_s \to K^0_{\mathrm{S}} K^+ K^-)}{{\cal B}(B^0 \to K^0_{\mathrm{S}} π^+π^-)} &&= 0.0303 &&\pm 0.0041 &&\pm 0.0025 && \pm 0.0009\,, &\frac{{\cal B}(B^0_s \to K^0_{\mathrm{S}} K^\pmπ^\mp)}{{\cal B}(B^0 \to K^0_{\mathrm{S}} π^+π^-)} &&= 1.818 &&\pm 0.021 &&\pm 0.031 && \pm 0.056\,, \end{alignat*} where the uncertainties are statistical, systematic, and due to knowledge of the ratio of hadronisation fractions of the $B^0_s$ and $B^0$ mesons, respectively.

hep-ex↗

arXiv2026

Measurement of the $W$-boson production cross-sections in $pp$ collisions at $\sqrt{s}$ = 13 TeV in the forward region

A precision measurement of the $W$-boson production cross-section is performed using the $W \to μν$ decay channel, based on a sample of proton-proton collision data collected by the LHCb experiment at $\sqrt{s}$ = 13 TeV and corresponding to an integrated luminosity of 5.1 $fb^{-1}$. The cross-section is measured for muons with transverse momentum between 25 and 55 GeV and pseudorapidity between 2.0 and 4.5. The integrated production cross-sections of $W$ bosons are measured to be $$ \begin{array}{lcl} σ_{W^+ \to μ^+ν} &=& 1754.2 \pm 1.5 \pm 11.9 \pm 35.1\text{ pb} \\ σ_{W^- \to μ^-\barν} &=& 1178.1 \pm 1.3 \pm 9.7 \pm 23.6\text{ pb} \end{array} $$ where uncertainties are statistical, systematic, and due to the luminosity determination, respectively. Results are in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. This measurement is significantly more precise than previous results in this kinematic regime.

hep-ex↗

arXiv2026

Precision measurement of the muon charge asymmetry from $W$-boson decays in $pp$ collisions at $\sqrt{s}$ = 13 TeV in the forward region

A precision measurement of the muon charge asymmetry from $W$-boson decays in proton-proton collisions at $\sqrt{s}$ = 13 TeV is presented. The analysis utilizes data corresponding to an integrated luminosity of 5.1 $fb^{-1}$, recorded by the LHCb detector during 2016, 2017 and 2018. The asymmetry is measured for muons with transverse momentum between 25 and 55 GeV and pseudorapidity between 2.0 and 4.5. This result represents the most precise determination of the muon charge asymmetry in the forward region to date, exhibiting excellent agreement with next-to-next-to-leading-order predictions in perturbative quantum chromodynamics.

hep-ex↗

arXiv2026

Search for the decays $B_{(s)}^0\to J/ψγ$ at LHCb

A search for the rare decays $B_{(s)}^0\to J/ψγ$ is performed with proton-proton collision data collected by the LHCb experiment, corresponding to integrated luminosities of $3~\rm{fb}^{-1}$ at centre-of-mass energies of 7 and 8 TeV, and $6~\rm{fb}^{-1}$ at 13 TeV. Assuming no contribution from $B^0\to J/ψγ$ decay, an upper limit is set on the branching fraction $\mathcal{B}(B_{s}^0\to J/ψγ)<2.9\times10^{-6}$ at the 90% confidence level. If instead no contribution from $B_{s}^0\to J/ψγ$ decay is assumed, the limit is $\mathcal{B}(B^0\to J/ψγ)<2.5\times10^{-6}$ at the 90% confidence level. These results supersede the previous LHCb results, with the limit for $B_{s}^0\to J/ψγ$ improved by a factor of 2.5.

hep-ex↗

arXiv2026

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

Branching fractions of $η$ and $η'(958)$ 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}(η\toμ^+μ^-) &= (5.02\pm0.14\pm0.18\pm0.26)\times10^{-6} , \\ \cal{B}(η'(958)\toμ^+μ^-) &<1.9\times10^{-7}\textrm{ at 90% CL} , \\ \cal{B}(η\toπ^+π^-μ^+μ^-) &<5.4\times10^{-7}\textrm{ at 90% CL}, \\ \cal{B}(η'(958)\toπ^+π^-μ^+μ^-) &= (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 $ϕ(1020)\toμ^+μ^-$ decay, which is used for normalization. This marks the first experimental search for the $η'(958)$ decay, and the most precise measurement of the branching fraction $\cal{B}(η\toμ^+μ^-)$ to date.

hep-ex↗

arXiv2026

Polarization measurement of $Λ^+_c$ and $\overlineΛ{}^-_c$ baryons in $p$Ne collisions at $\sqrt{s_{NN}} = 68.6$ GeV

The first measurement of the polarization of charm baryons by the LHCb experiment recorded in fixed-target mode is presented. The polarization of $Λ_c$ baryons is studied in collisions of protons, at an energy of 2.51 TeV, incident on a gaseous target of neon, at a nucleon-nucleon center-of-mass energy of $68.6$ GeV. The world's first measurement of separate-charge polarizations for $Λ^+_c$ and $\overlineΛ{}^-_c$ baryons is performed, determining $$ P_{Λ^+_c} = ( 24 \pm 9 \pm 2 \, )\% , $$ $$ P_{\overlineΛ{}^-_c} = (-8 \pm 12 \pm 3 \, ) \% , $$ where the first uncertainty is statistical and the second systematic. The polarization is also measured in intervals of baryon transverse momentum and the Feynman-$x$ variable.

hep-ex↗

arXiv2026

First measurement of the decay-time-integrated $C\!P$ asymmetry in $B_s^0 \to D_s^- π^+$ decays

A measurement of the flavor-untagged decay-time-integrated ${C\!P}$ asymmetry in the flavor-specific decay ${B_s^0 \to D_s^-π^+}$, ${\langle A^s_{\rm untagged}\rangle}$, is performed using proton-proton collision data collected by the LHCb experiment between 2016 and 2018 at a center-of-mass energy of ${13\,{\rm TeV}}$, corresponding to a total integrated luminosity of ${5.4\,{\rm fb}^{-1}}$. The ${C\!P}$ asymmetry is measured in two $D_s^-$ meson decay modes, ${D_s^- \to K^-K^+π^-}$ and ${D_s^- \to π^-π^+π^-}$. The combined result, $\langle A^s_{\rm untagged}\rangle = ( -1.4 \pm 5.9\,\rm{(stat)} \pm 1.1\,\rm{(syst)}) \times 10^{-3}$, is consistent with the Standard Model expectation and provides a direct constraint on new physics in tree-level $b$-hadron decays.

hep-ex↗

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 $B$ 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 $B_{(s)}^0 \rightarrow μ^+μ^-μ^+μ^-$, $B^+ \rightarrow K^+μ^+μ^-μ^+μ^-$, $B_{(s)}^0 \rightarrow μ^+μ^-μ^+μ^-μ^+μ^-$ and $B^+ \rightarrow K^+μ^+μ^-μ^+μ^-μ^+μ^-$, 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.

hep-ex↗

arXiv2026

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

Decays of the pseudovector $D_{s1}(2460)^+$ and $D_{s1}(2536)^+$ mesons to the three-body $D_{s}^+π^+π^-$ 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{$D_{s1}(2536)^+\to D_s^+π^+π^-$} branching fraction is measured for the first time, with the $D_{s1}(2536)^+\to D^+K^+π^-$ decay used as a reference. A simultaneous amplitude analysis of the $D_{s1}(2460)^+$ and $D_{s1}(2536)^+\to D_s^+π^+π^-$ 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 $D_{s}^+π^{\pm}$ 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.

hep-ex↗

arXiv2026

Machine learning techniques for jet reconstruction at LHCb and application to the search for $H \to b \bar{b}$ and $H \to c \bar{c}$ in $\sqrt{s}=13$ TeV $pp$ collisions

Two machine learning techniques for jet measurements at the LHCb experiment are presented: a regression-based method for jet-energy calibration and a deep neural network algorithm for jet flavour tagging, distinguishing between $b$-quark, $c$-quark, and light parton jets. These techniques are applied to a search for inclusive $H \to \bbbar$ and $H \to c\barcc$ decays using a LHCb dataset corresponding to an integrated luminosity of 1.6\invfb. The observed (expected) 95\% confidence level upper limits correspond to 6.6 (11.1) times the SM cross-section for the $H \to b\bar b$ process, and 1003 (1834) times the SM cross-section for the $H \to c\bar c$ process.

hep-ex↗

arXiv2026

Amplitude analysis of $B^+ \!\to K^+ π^+ π^-$ 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 $B^+ \!\to K^+ π^+ π^-$ 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 $K^+ π^-$ and $π^+ π^-$ 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 $π^+ π^-$ final state to manifest either through direct $ω(782)$ decays or $ρ(770)^0\textrm{-}ω(782)$ 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 $B^+ \!\to K^+ π^+ π^-$ decays.

hep-ex↗