arXiv subjects
Publications and source records attributed to W. Barter.
Silicon detectors operating in the harsh mixed-hadron radiation environment of modern high-energy physics experiments suffer progressive bulk damage, manifesting macroscopically as a steady rise in leakage current.
Tests of fundamental symmetries, such as $C\!PT$ and Lorentz invariance, provide sensitive probes of physics beyond the standard quantum-field-theory framework. Possible violations of these symmetries are investigated through interference effects in decays of $B_s^0$ mesons to $D_s^-\pi^{+}$ states. The analysis uses proton-proton collision data collected with the LHCb detector in 2015-2018 at a centre-of-mass energy of $13~\mathrm{TeV}$, corresponding to an integrated luminosity of $6~\mathrm{fb}^{-1}$. The results are interpreted within the Standard Model Extension framework, yielding sensitivities to the Lorentz-violating parameter $\Delta a_{\mu}$ at the ${\cal O}(10^{-14})\,\mathrm{GeV}$ level. Without assuming Lorentz-invariance violation, the $C\!PT$-violating parameter $z$ in the $B_s^0$ system is measured to be $\mathcal{R}e(z) = 0.018 \pm 0.016 \pm 0.003$ and $\mathcal{I}m(z) = 0.003 \pm 0.004 \pm 0.002$, where the first uncertainties are statistical and the second systematic. These results represent the most precise constraints on $C\!PT$ violation in the $B_s^0$ system.
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.
A measurement of the $\psi(2S)$-to-$J/\psi$ production cross-section ratio is performed in proton-argon ($p\mathrm{Ar}$) and proton-proton ($pp$) collisions in fixed-target mode at $\sqrt{s_{\mathrm{NN}}}=113\,\mathrm{GeV}$. Data samples were collected by the LHCb experiment during argon and hydrogen gas injections in the SMOG2 storage cell, resulting in $p\mathrm{Ar}$ and $pp$ collisions, respectively. The $\psi(2S)$-to-$J/\psi$ production cross-section ratio is measured as a function of the charmonium transverse momentum, $p_{\mathrm{T}}$, and rapidity in the centre-of-mass system, $y^{*}$. The $\psi(2S)$-to-$J/\psi$ ratio in $p\mathrm{Ar}$ collisions over that in $pp$ collisions is measured to be $0.90 \pm 0.04 \pm 0.02$ for $-2.3<y^{*}<0.0$ and $0<p_{\mathrm{T}}<8\mathrm{GeV}/c$, indicating the emergence of nuclear effects in the $p\mathrm{Ar}$ system. This study acts as a baseline for the interpretation of future measurements with larger systems accessible by the LHCb experiment.
A search for the Cabibbo-suppressed decay $\Xi_c^0 \to pK^-$ is performed using $pp$ collision data corresponding to an integrated luminosity of $5.4\,\mathrm{fb}^{-1}$, collected by the LHCb experiment at a centre-of-mass energy of $13\,\mathrm{TeV}$. The decay is observed for the first time and its branching fraction measured to be $(4.5\pm0.5\pm0.2\pm0.9)\times10^{-5}$, where the uncertainties are statistical, systematic and from the branching fraction of the normalisation channel $\Xi_b^- \to \Xi_c^0 (\to p K^- K^- \pi^+) \pi^-$. Using the decay chain $\Xi_b^- \to \Xi_c^0(\to pK^-)\pi^-$, the decay asymmetry parameter of the $\Xi_c^0 \to pK^-$ decay is determined to be $\alpha_{\Xi_c^0}=0.32\pm0.15\pm0.01$.
The first angular analysis of ${\it \Lambda}_{\it b}^{0} \to {\it \Lambda}(1520){\it \mu^{+}\mu^{-}}$ decays is presented, using proton-proton collision data collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of 9 fb$^{-1}$. The leptonic forward-backward asymmetry, $A_\text{FB, 3/2}^\ell$, and the $CP$-averaged angular observable, $S_{1cc}$, are determined by fitting projections of the angular distributions in four intervals of the square of the dimuon invariant mass between 0.1 and 12.5 GeV$^2/c^4$. The results are in good agreement with predictions based on the Standard Model of particle physics.
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.
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 $\phi_{s} = -0.041 \pm 0.017\,\text{rad}$.
An analysis of the decay amplitudes of $B^0 \to K^{*0}(\to K^+\pi^-)\mu^+\mu^-$ is presented, using proton-proton collision data recorded by the LHCb experiment at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 8.4 fb$^{-1}$. The amplitudes are constructed from Legendre polynomials in the $\mu^+\mu^-$ invariant mass squared region $1.1<q^2<8.0$ GeV$^2/c^4$. $C\!P$-averaged observables are obtained from the amplitudes. Some of these observables present deviations with respect to the Standard Model, which can be interpreted as shifts in the effective Wilson coefficients. This model-independent approach enables tests of theoretical predictions that can help disentangle hadronic effects from potential contributions from physics beyond the Standard Model. This allows flexibility in the choice of $q^2$ binning for global analyses. Depending on the binning scheme, the deviation of the Wilson coefficient $C_9$ from its Standard Model expectation varies from $4.3\sigma$ to $4.8\sigma$.
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.
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.
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.
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.
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.
The production rates of $\mathit{\Upsilon}(2S)$ and $\mathit{\Upsilon}(3S)$ mesons relative to that of the $\mathit{\Upsilon}(1S)$ state are measured in $pp$, $p$O, and OO collisions by the LHCb collaboration. The ratios measured in $pp$ data are consistent with previous LHCb measurements at different center-of-mass energies. Only slight relative suppression of the $\mathit{\Upsilon}(2S)$ and $\mathit{\Upsilon}(3S)$ states is found in $p$O collisions, while in OO collisions the $\mathit{\Upsilon}(2S)$ is suppressed by a factor of $\sim2$, with evidence for suppression of the $\mathit{\Upsilon}(3S)$. The significant suppression in OO data, compared to the small effect in $p$O data, shows the emergence of additional suppression mechanisms in the relatively small OO collision system. Models incorporating quark-gluon plasma formation in OO collisions successfully describe the data. Implications for the interplay between cold nuclear matter effects and color screening in a deconfined quark-gluon plasma are discussed.
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.
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.
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.