Guido Haefeli

A bstract: A search for charge-parity ( CP ) violation in the Cabibbo-suppressed D ⁺ → ϕπ ⁺ decay is presented, using proton-proton collision data corresponding to an integrated luminosity of 6 fb ^{− 1} , collected at a centre-of-mass energy of 13 TeV with the LHCb detector during Run 2. An abundant sample of $$ {D}^{+}\to {K}_{\mathrm{S}}^0{\pi}^{+} $$ D + → K S 0 π + 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 ϕ meson. Asymmetries induced by interference of multiple processes, such as neutral kaon mixing, regeneration of neutral kaons, and interference between the Cabibbo-favoured $$ {D}^{+}\to {\overline{K}}^0{\pi}^{+} $$ D + → K ¯ 0 π + and the doubly Cabibbo-suppressed D ⁺ → K ⁰ π ⁺ decay, are subtracted. The direct CP asymmetry in the D ⁺ → ϕπ ⁺ decay is measured to be $$ {a}_{CP}\left({D}^{+}\to \phi {\pi}^{+}\right)=\left(0.1\pm 4.9\left(\mathrm{stat}\right)\pm 2.4\left(\mathrm{syst}\right)\right)\times {10}^{-4}. $$ a CP D + → ϕ π + = 0.1 ± 4.9 stat ± 2.4 syst × 10 − 4 . For the first time at a hadron collider, the modulus of the ratio and the relative strong phase of the D ⁺ → K ⁰ π ⁺ to $$ {D}^{+}\to {\overline{K}}^0{\pi}^{+} $$ D + → K ¯ 0 π + decay amplitudes are also investigated. Two-dimensional confidence intervals are reported for these parameters.

Di-muon events emanating from a replica of the SHiP target at the CERN SPS (with a 400 GeV/c proton beam) contain a clear signal of $${J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi \hspace{1.111pt}}$$ J / ψ production. The target consisted of 13 interaction lengths of molybdenum and tungsten, followed by a 2.4m iron hadron absorber. The di-muon rate is in reasonable agreement with Monte Carlo simulation based on Pythia v8. However, the $${J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi \hspace{1.111pt}} $$ J / ψ rate observed in the data is much lower than predicted by the Monte Carlo simulation, which is advantageous for SHiP since muons with a large $$p_\textrm{T} $$ p T are more difficult to deflect by the active muon shield. For the center of mass rapidity ( $$y_{\textrm{cm}} $$ y cm ) interval with the largest overlap with the NA50 measurement, $$0.3< y_{\textrm{cm}} < 0.6$$ 0.3 < y cm < 0.6 , we obtain the production cross-section per nucleon $$\sigma ({J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi \hspace{1.111pt}})/A$$ σ ( J / ψ ) / A including the branching ratios of $${J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi \hspace{1.111pt}} $$ J / ψ decays into muons, $$B_{\mu ^+\mu ^-}$$ B μ + μ - : $$B_{\mu ^+\mu ^-}\sigma ({J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi \hspace{1.111pt}})/A=(1.18~\pm ~0.04~\pm ~0.10)~\textrm{nb}$$ B μ + μ - σ ( J / ψ ) / A = ( 1.18 ± 0.04 ± 0.10 ) nb /nucleon. This is to be compared to the NA50 extrapolated result (using a much thinner target): $$B_{\mu ^+\mu ^-}\sigma ({J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi \hspace{1.111pt}})/A=(0.99~\pm ~0.04)~\textrm{nb}$$ B μ + μ - σ ( J / ψ ) / A = ( 0.99 ± 0.04 ) nb /nucleon. Within the systematic errors, no significant enhancement due to secondary production of $${J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi \hspace{1.111pt}}$$ J / ψ inside the target is observed. An upper limit at 90% confidence level of possible contributions from secondary collisions of $$<32\%$$ < 32 % is obtained.

A bstract: Using a dataset corresponding to an integrated luminosity of 9 fb ^{− 1} collected in proton-proton collisions between 2011 and 2018 by the LHCb experiment, evidence is found for the decay $$ {B}_s^0\to {\phi \eta}^{\prime } $$ B s 0 → ϕη ′ with 3 . 5 σ significance. The branching ratio relative to the $$ {B}_s^0\to \phi \phi $$ B s 0 → ϕϕ decay is determined to be ℛ = (3 . 56 ± 0 . 79 ± 0 . 18 ± 0 . 06) × 10 ^{− 2} . This corresponds to a branching fraction, $$ \mathcal{B}\left({B}_s^0\to {\phi \eta}^{\prime}\right)=\left(0.66\pm 0.15\pm 0.03\pm 0.02\right)\times {10}^{-6} $$ B B s 0 → ϕη ′ = 0.66 ± 0.15 ± 0.03 ± 0.02 × 10 − 6 where, in both cases, the first uncertainty is statistical, the second systematic, and the third due to external branching fractions.

The SND@LHC experiment is currently taking data at the Large Hadron Collider (LHC), exploring the unique forward region at pseudorapidities $$7.2<\eta <8.4$$ 7.2 < η < 8.4 . Its physics programme covers neutrinos originating from heavy-flavour decays and feebly interacting particles produced in proton–proton collisions. Building upon the successful operation of the present detector, this paper presents the physics reach of the approved SND@LHC upgrade for Run 4 of the LHC, and compares it with an alternative installation scenario. Lowering the detector by approximately 40 cm and shifting it horizontally by about 30 cm, while keeping it off-axis, increases the total neutrino interaction rate by a factor of five. The paper describes the design of the upgraded detector and compares the physics performance in both installation scenarios.

A bstract: This paper presents a thorough study of the local and nonlocal amplitudes in B ⁺ → 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 8.4 fb ^{− 1} collected by the LHCb experiment during Run1 and Run2. 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 $$ {C}_9+{C}_9^{\prime } $$ C 9 + C 9 ′ and $$ {C}_{10}+{C}_{10}^{\prime } $$ 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.

The SND@LHC experiment investigates neutrinos in the $$7.2< \eta < 8.4$$ 7.2 < η < 8.4 forward pseudorapidity range. The detector consists of a veto system, a scintillating fiber tracker interleaved with emulsion cloud chambers, and a downstream muon system. Muons originating from collisions at ATLAS (IP1) constitute the primary background for CC neutrino interactions and determine the replacement frequency of the emulsion target. A precise characterization of this flux is therefore essential. In this work, we report the muon flux measured in the central $$31 \times 31$$ 31 × 31 $$\textrm{cm}^2$$ cm 2 fiducial area of the detector using data from 2023 through 2025. The measured fluxes for proton collisions are: (1.90±0.04) $$\times 10^{-2}$$ × 10 - 2 $$\text {nb/cm}^{2}$$ nb/cm 2 (2023), (3.76 ± 0.09) $$\times $$ × $$\text {10}^{-2}$$ 10 - 2 $$\text {nb/cm}^{2}$$ nb/cm 2 (2024), and (2.48 ± 0.05) $$\times 10^{-2}$$ × 10 - 2 $$\text {nb/cm}^{2}$$ nb/cm 2 (2025). A 2024 reference proton run at $$\sqrt{s} = 5.36\text { TeV}$$ s = 5.36 TeV yielded (4.21 ± 0.14) $$\times $$ × $$\text {10}^{-2}$$ 10 - 2 $$\text {nb/cm}^{2}$$ nb/cm 2 , providing a direct baseline for the heavy-ion energy regime. The measured fluxes for heavy-ion collisions are (3.11±0.12) $$\times \text {10}^{4}$$ × 10 4 $$\text {nb/cm}^{2}$$ nb/cm 2 , (5.53±0.22) $$\times 10^{4}$$ × 10 4 $$\text {nb/cm}^{2}$$ nb/cm 2 , and (3.24±0.13) $$\times \text {10}^{4}$$ × 10 4 $$\text {nb/cm}^{2}$$ nb/cm 2 in 2023, 2024, and 2025, respectively. Uncertainties are dominated by systematic effects, with the statistical component contributing $$\lesssim $$ ≲ 0.1% to the total uncertainty. These results are in agreement with Monte Carlo predictions.

A bstract: Measurements of the combined CP asymmetries in $$ {\overline{B}}^0\to {D}_s^{-}{D}^{+} $$ B ¯ 0 → D s − D + and $$ {\overline{B}}_s^0\to {D}_s^{-}{D}^{-} $$ B ¯ s 0 → D s − D − decays are made using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 9 fb ^{− 1} . The measurements are found to be $$ {\displaystyle \begin{array}{l}{A}_{CP}\left({\overline{B}}^0\to {D}_s^{-}{D}^{+}\right)=0.0009\pm 0.0053\pm 0.0040,\\ {}{A}_{CP}\left({\overline{B}}_s^0\to {D}_s^{-}{D}^{-}\right)=0.103\pm 0.053\pm 0.010,\end{array}} $$ A CP B ¯ 0 → D s − D + = 0.0009 ± 0.0053 ± 0.0040 , A CP B ¯ s 0 → D s − D − = 0.103 ± 0.053 ± 0.010 , where the first and second uncertainties are statistical and systematic, respectively. This is the first measurement of this asymmetry in $$ {\overline{B}}_s^0 $$ B ¯ s 0 decays, and the most precise measurement to date for $$ {\overline{B}}^0 $$ B ¯ 0 decays. Both measurements are found to be consistent with CP symmetry.

A bstract: A search for the charmless purely baryonic decay $$ {\Lambda}_b^0\to \Lambda p\overline{p} $$ Λ b 0 → Λ p p ¯ is performed using proton-proton collision data recorded by the LHCb experiment at a centre-of-mass energy of $$ \sqrt{s}=13 $$ s = 13 TeV and corresponding to an integrated luminosity of 6 . 0 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 $$ {\Lambda}_b^0\to \Lambda {K}^{+}{K}^{-} $$ Λ b 0 → Λ K + K − . Contributions from intermediate charmonium resonances decaying to the $$ p\overline{p} $$ p p ¯ and K ⁺ K ⁻ final states are explicitly excluded with a requirement on the invariant mass of the companion hadron system, $$ m\left(h\overline{h}\right)<2.85 $$ m h h ¯ < 2.85 GeV, where h stands for a proton or a charged kaon. The relative branching fraction is found to be $$ \frac{\mathcal{B}\left({\Lambda}_b^0\to \Lambda p\overline{p}\right)}{\mathcal{B}\left({\Lambda}_b^0\to \Lambda {K}^{+}{K}^{-}\right)}=\left(5.1\pm {1.3}_{\left(\mathrm{stat}\right)}\pm {0.3}_{\left(\mathrm{syst}\right)}\right)\times {10}^{-2}. $$ B Λ b 0 → Λ p p ¯ B Λ b 0 → Λ K + K − = 5.1 ± 1.3 stat ± 0.3 syst × 10 − 2 .

A bstract: A measurement of the CKM angle γ using the decay channels B ^± → DK ^± and B ^± → Dπ ^± , where the D meson decays to $$ D\to {K}_{\mathrm{S}}^0{\pi}^{+}{\pi}^{-} $$ D → K S 0 π + π − or $$ D\to {K}_{\mathrm{S}}^0{K}^{+}{K}^{-} $$ D → K 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. CP 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 γ is determined to be γ = (68 . 1 ± 6 . 7) ° . Other parameters related to the examined B meson decay modes are also measured. This is the first measurement of the CKM angle γ using the upgraded LHCb detector.