This document is written as a contribution to the European Strategy of Particle Physics (ESPP) update. We offer a detailed overview of current developments and future directions for the CODEX-b detector, which aims to detect long-lived particles beyond the Standard Model. We summarize the scientific motivation for this detector, advances in our suite of simulation and detector optimization frameworks, and examine expected challenges, costs, and timelines in realizing the full detector. Additionally, we describe the technical specifications for the smaller-scale demonstrator detector (CODEX-β) we have installed in the LHCb experimental cavern.

We report measurements of the e(+)e(-) -> B (B) over bar, B (B) over bar*, and B*(B) over bar* cross sections at four energies, 10653, 10701, 10746 and 10805 MeV, using data collected by the Belle II experiment. We reconstruct one B meson in a large number of hadronic final states and use its momentum to identify the production process. In the first 2 - 5 MeV above B* B* threshold, the e(+)e(-) -> B*(B) over bar* cross section increases rapidly. This may indicate the presence of a pole close to the threshold.

Abstract We report results from a study of B±→ DK± decays followed by D decaying to the CP-even final state K+K− and CP-odd final state $$ {K}_S^0{\pi}^0 $$ K S 0 π 0 , where D is an admixture of D0 and $$ {\overline{D}}^0 $$ D ¯ 0 states. These decays are sensitive to the Cabibbo-Kobayashi-Maskawa unitarity-triangle angle ϕ3. The results are based on a combined analysis of the final data set of 772 × 106$$ B\overline{B} $$ B B ¯ pairs collected by the Belle experiment and a data set of 198 × 106$$ B\overline{B} $$ B B ¯ pairs collected by the Belle II experiment, both in electron-positron collisions at the Υ(4S) resonance. We measure the CP asymmetries to be $$ \mathcal{A} $$ A CP+ = (+12.5 ± 5.8 ± 1.4)% and $$ \mathcal{A} $$ A CP− = (−16.7 ± 5.7 ± 0.6)%, and the ratios of branching fractions to be $$ \mathcal{R} $$ R CP+ = 1.164 ± 0.081 ± 0.036 and $$ \mathcal{R} $$ R CP− = 1.151 ± 0.074 ± 0.019. The first contribution to the uncertainties is statistical, and the second is systematic. The asymmetries $$ \mathcal{A} $$ A CP+ and $$ \mathcal{A} $$ A CP− have similar magnitudes and opposite signs; their difference corresponds to 3.5 standard deviations. From these values we calculate 68.3% confidence intervals of (8.5° < ϕ3 < 16.5°) or (84.5° < ϕ3 < 95.5°) or (163.3° < ϕ3 < 171.5°) and 0.321 < rB < 0.465.

We report measurements of the e+e− → BB , BB^∗ , and B^∗B^∗ cross sections at four energies, 10653, 10701, 10746 and 10805 MeV, using data collected by the Belle II experiment. We reconstruct one B meson in a large number of hadronic final states and use its momentum to identify the production process. In the first 2 – 5 MeV above B^∗B^∗ threshold, the e+e− → B^∗B^∗ cross section increases rapidly. This may indicate the presence of a pole close to the threshold.

We report a measurement of decay-time dependent charge-parity (CP) asymmetries in B^0 → K^0_S K^0_S K^0_S decays. We use 387 × 10^6 BB̅ pairs collected at the Υ(4S) resonance with the Belle II detector at the SuperKEKB asymmetric-energy electron-positron collider. We reconstruct 220 signal events and extract the CP-violating parameters S and C from a fit to the distribution of the decay-time difference between the two B mesons. The resulting confidence region is consistent with previous measurements in B^0 → K^0_S K^0_S K^0_S and B^0 → (cc̅)K^0 decays, and with predictions based on the standard model.

Abstract We report measurements of the e + e − → B B ¯ $$ B\overline{B} $$ , B B ¯ ∗ $$ B{\overline{B}}^{\ast } $$ , and B ∗ B ¯ ∗ $$ {B}^{\ast }{\overline{B}}^{\ast } $$ cross sections at four energies, 10653, 10701, 10746 and 10805 MeV, using data collected by the Belle II experiment. We reconstruct one B meson in a large number of hadronic final states and use its momentum to identify the production process. In the first 2 – 5 MeV above B ∗ B ¯ ∗ $$ {B}^{\ast }{\overline{B}}^{\ast } $$ threshold, the e + e − → B ∗ B ¯ ∗ $$ {B}^{\ast }{\overline{B}}^{\ast } $$ cross section increases rapidly. This may indicate the presence of a pole close to the threshold.

We report measurements of the branching fractions and direct $\it{CP}$ asymmetries of the decays $B^0 \to K^+ \pi^-$, $B^+ \to K^+ \pi^0$, $B^+ \to K^0 \pi^+$, and $B^0 \to K^0 \pi^0$, and use these for testing the standard model through an isospin-based sum rule. In addition, we measure the branching fraction and direct $\it{CP}$ asymmetry of the decay $B^+ \to \pi^+\pi^0$ and the branching fraction of the decay $B^0 \to \pi^+\pi^-$. The data are collected with the Belle II detector from $e^+e^-$ collisions at the $\Upsilon(4S)$ resonance produced by the SuperKEKB asymmetric-energy collider and contain $387\times 10^6$ bottom-antibottom meson pairs. Signal yields are determined in two-dimensional fits to background-discriminating variables, and range from 500 to 3900 decays, depending on the channel. We obtain $-0.03 \pm 0.13 \pm 0.04$ for the sum rule, in agreement with the standard model expectation of zero and with a precision comparable to the best existing determinations.

We present measurements of the first to fourth moments of the lepton mass squared q(2) of B -> X(c)l (nu) over bar (l) decays for l = e, mu and with X-c a hadronic system containing a charm quark. These results use a sample of electron-positron collisions at the Upsilon(4S) resonance corresponding to 62.8 fb(-1) of integrated luminosity and collected by the Belle II 2 experiment in 2019 and 2020. To identify the X-c system and reconstruct q(2), one of the B mesons from an Upsilon(4S) -> B (B) over bar decay is fully reconstructed in a hadronic decay mode using a multivariate B tagging algorithm. We report raw and central moments for q(2) > 1.5 GeV2/c(4) up to q(2) > 8.5 GeV2/c(4), probing up to 77% of the accessible B -> X(c)l (nu) over bar (l) phase space. This is the first measurement of moments in the experimentally challenging range of [1.5, 2.5] GeV2/c(4). The results can be used for a new determination of vertical bar V-cb vertical bar using inclusive B -> X(c)l (nu) over bar (l) decays.

We determine the Cabibbo-Kobayashi-Maskawa matrix-element magnitude vertical bar V-cb vertical bar using (B) over bar (0) -> D*(+)l(-)(nu) over bar (l) decays reconstructed in 189 fb(-1) of collision data collected by the Belle II experiment, located at the SuperKEKB e(+)e(-) collider. Partial decay rates are reported as functions of the recoil parameter w and three decay angles separately for electron and muon final states. We obtain vertical bar V-cb vertical bar using the Boyd-Grinstein-Lebed and Caprini-Lellouch-Neubert parametrizations, and find vertical bar V-cb vertical bar(BGL) = (40.57 +/- 0.31 +/- 0.95 +/- 0.58) x 10(-3) and vertical bar V-cb vertical bar(CLN) = (40.13 +/- 0.27 +/- 0.93 +/- 0.58) x 10(-3) with the uncertainties denoting statistical components, systematic components, and components from the lattice QCD input, respectively. The branching fraction is measured to be B((B) over bar (0) -> D*(+)l(-)(nu) over bar (l)) = (4.922 +/- 0.023 +/- 0.220)%. The ratio of branching fractions for electron and muon final states is found to be 0.998 +/- 0.009 +/- 0.020. In addition, we determine the forward-backward angular asymmetry and the D*(+) longitudinal polarization fractions. All results are compatible with lepton-flavor universality in the Standard Model.

We present a measurement of the tau-lepton mass using a sample of about 175 million e(+) e(-) -> tau(+) tau(-) events collected with the Belle II detector at the SuperKEKB e(+) e(-) collider at a center-of-mass energy of 10.579 GeV. This sample corresponds to an integrated luminosity of 190 fb(-1). We use the kinematic edge of the tau pseudomass distribution in the decay tau(-) -> pi(-) pi(+) pi(-) nu(tau) and measure the t mass to be 1777.09 +/- 0.08 +/- 0.11 MeV/c(2), where the first uncertainty is statistical and the second systematic. This result is the most precise to date.

We present measurements of the branching fractions for the singly Cabibbo-suppressed decays D+ -K+K-x-+x-0 and D+s -K+x--x-+x-0, and the doubly Cabibbo-suppressed decay D+ -K+x--x-+x-0, based on 980 fb-1 of data recorded by the Belle experiment at the KEKB e+e- collider. We measure these modes relative to the Cabibbo-favored modes D+ -K-x-+x-+x-0 and D+s -K+K-x-+x-0. Our results for the ratios of branching fractions are B(D+ -K+K-x-+x-0)/B(D+ -K-x-+x-+x-0) = (11.32 +/- 0.13 +/- 0.26)%, B(D+ -K+x--x-+x-0)/B(D+ -K-x-+x-+x-0) = (1.68 +/- 0.11 +/- 0.03)%, and B(D+s -K+x--x-+x-0)/ B(D+s -K+K-x-+x-0) = (17.13 +/- 0.62 +/- 0.51)%, where the uncertainties are statistical and systematic, respectively. The second value corresponds to (5.83 +/- 0.42) x tan4 BC, where BC is the Cabibbo angle; this value is larger than other measured ratios of branching fractions for a doubly Cabibbo-suppressed charm decay to a Cabibbo-favored decay. Multiplying these results by world average values for B(D+ -K-x-+x-+x-0) and B(D+s -K+K-x-+x-0) yields B(D+ -K+K-x-+x-0) = (7.08 +/- 0.08 +/- 0.16 +/- 0.20) x 10-3, B(D+ -K+x--x-+x-0) = (1.05 +/- 0.07 +/- 0.02 +/- 0.03) x 10-3, and B(D+s -K+x--x-+x-0) = (9.44 +/- 0.34 +/- 0.28 +/- 0.32) x 10-3, where the third uncertainty is due to the branching fraction of the normalization mode. The first two results are consistent with, but more precise than, the current world averages. The last result is the first measurement of this branching fraction.

Using (771.6 +/- 10.6)x106 B over line B meson pairs recorded by the Belle experiment at the KEKB e+e- collider, we report the branching fractions B( over line B0 -> D*+pi-)=(2.62 +/- 0.02 +/- 0.09)x10-3 and B( over line B0 -> D*+K-)=(2.22 +/- 0.06 +/- 0.08)x10-4; the quoted uncertainties are statistical and systematic, respectively. A measurement of the ratio of these branching fractions is also presented, RK/pi=B( over line B -> D*+K-)/B( over line B -> D*+pi-)=(8.41 +/- 0.24 +/- 0.13)x10-2, where systematic uncertainties due to the D*+ meson reconstruction cancel out. Furthermore, we report a new QCD factorization test based on the measured ratios for over line B -> D*+h- and over line B -> D*+l-nu decays at squared momentum transfer values equivalent to the mass of the h=pi or K hadron. The parameters |a1(h)| are measured to be |a1(pi)|=0.884 +/- 0.004 +/- 0.003 +/- 0.016 and |a1(K)|=0.913 +/- 0.019 +/- 0.008 +/- 0.013, where the last uncertainties account for all external inputs. These values are approximately 15% lower than those expected from theoretical predictions. Subsequently, flavor SU(3) symmetry is tested by measuring the ratios for pions and kaons, |a1(K)|2/|a1(pi)|2=1.066 +/- 0.042 +/- 0.018 +/- 0.023, as well as for different particle species. The ratio is consistent with unity and therefore no evidence for SU(3) symmetry breaking effects is found at the 5% precision level.

We study the decay B+ -> K+K- =pi(+) and investigate the angular distribution of K+K- pairs with invariant mass below 1.1 GeV/c(2). This region exhibits both a strong enhancement in signal and very large direct CP violation. We construct a coherent sum model for the angular distribution of the S- and P-wave, and report the ratio of their amplitudes, the relative phase and the forward-backward asymmetry. We also report absolute differential branching fractions and direct CP asymmetry for the decay in bins of MK+K- and the differential branching fractions in bins of MK+pi-. The results are based on a data sample that contains 772 x 10(6) B (B) over bar pairs collected at the (sic)(4S) resonance with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. The result favors the presence of S- and D-waves in low MK+K- region to the detriment of a P-wave.

We report the first measurement of the inclusive e+e− → bb → D_s^± X and e+e− → bb → D0/ D^0 X cross sections in the energy range from 10.63 to 11.02 GeV. Based on these results, we determine σ(e+e− → B_s^0B_s^0 X) and σ(e+e− → BB X) in the same energy range. We measure the fraction of B_s^0 events at Υ(10860) to be fs = ( 22.0_-2.1^+2.0 ) B_s^0 inclusive branching fractions ℬ ( B_s^0 → D0/ D^0 X)/ ℬ ( B_s^0 → D_s^± X) = 0.416 ± 0.018 ± 0.092. The results are obtained using the data collected with the Belle detector at the KEKB asymmetric-energy e+e− collider.

We report the first measurement of the inclusive e(+)e(-) -> b (b) over bar -> D-s(+/-) X and e(+)e(-) -> b (b) over bar -> D-0/(D) over bar (0) X cross sections in the energy range from 10.63 to 11.02 GeV. Based on these results, we determine sigma(e(+)e(-) -> B-s(0)(B) over bar (0)(s) X) and sigma(e(+)e(-) -> B (B) over barX) in the same energy range. We measure the fraction of B-s(0) events at Upsilon(10860) to be f(s) = (22.0(-2.1)(+2.0))%. We determine also the ratio of the B-s(0) inclusive branching fractions B(B-s(0) -> D-0/(D) over bar (0) X)/B(B-s(0) -> D-s(+/-) X) = 0.416 +/- 0.018 +/- 0.092. The results are obtained using the data collected with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider.

We report the first measurement of the inclusive e^+e^- → bb̅→ D_s^±X and e^+e^- → bb̅→ D^0/D̅^0X cross sections in the energy range from 10.63 to 11.02. Based on these results, we determine σ(e^+ e^- → B_s^0 B̅_s^0 X) and σ(e^+ e^- → BB̅X) in the same energy range. We measure the fraction of B_s^0 events at Υ(10860) to be f_ s=(22.0^+2.0_-2.1)%. We determine also the ratio of the B_s^0 inclusive branching fractions ℬ(B_s^0 → D^0/D̅^0X)/ℬ(B_s^0 → D_s^± X)=0.416 ± 0.018 ± 0.092. The results are obtained using the data collected with the Belle detector at the KEKB asymmetric-energy e^+e^- collider.

This paper presents a search for the rare flavor-changing neutral current process B-0 -> K*(0)tau(+)tau(-) using data taken with the Belle detector at the KEKB asymmetric energy e(+)e(-) collider. The analysis is based on the entire Gamma(4S) resonance data sample of 711 fb(-1), corresponding to 772 x 10(6) B (B) over bar pairs. In our search we fully reconstruct the companion B meson produced in the process e(+)e(-) -> Gamma(4S)-> B (B) over bar from its hadronic decay modes, and look for the decay B-0 -> K-*0 tau(+)tau(-) in the rest of the event. No evidence for a signal is found. We report an upper limit on the branching fraction B(B-0 -> K*(0)tau(+)tau(-) ) < 3.1 x 10(-3) at 90% confidence level. This is the first direct limit on B(B-0 -> K*(0)tau(+)tau(-) ).

Additional spin-0 particles appear in many extensions of the standard model. We search for long-lived spin-0 particles $S$ in $B$-meson decays mediated by a $b\to s$ quark transition in $e^+e^-$ collisions at the $\Upsilon(4S)$ resonance at the Belle II experiment. Based on a sample corresponding to an integrated luminosity of $189 \mathrm{\,fb}^{-1}$, we observe no evidence for signal. We set model-independent upper limits on the product of branching fractions $\mathrm{Br}(B^0\to K^*(892)^0(\to K^+\pi^-)S)\times \mathrm{Br}(S\to x^+x^-)$ and $\mathrm{Br}(B^+\to K^+S)\times \mathrm{Br}(S\to x^+x^-)$, where $x^+x^-$ indicates $e^+e^-, \mu^+\mu^-, \pi^+\pi^-$, or $K^+K^-$, as functions of $S$ mass and lifetime at the level of $10^{-7}$.

We report the first search for a nonstandard-model resonance decaying into τ pairs in e^{+}e^{-}→μ^{+}μ^{-}τ^{+}τ^{-} events in the 3.6-10  GeV/c^{2} mass range. We use a 62.8  fb^{-1} sample of e^{+}e^{-} collisions collected at a center-of-mass energy of 10.58 GeV by the Belle II experiment at the SuperKEKB collider. The analysis probes three different models predicting a spin-1 particle coupling only to the heavier lepton families, a Higgs-like spin-0 particle that couples preferentially to charged leptons (leptophilic scalar), and an axionlike particle, respectively. We observe no evidence for a signal and set exclusion limits at 90% confidence level on the product of cross section and branching fraction into τ pairs, ranging from 0.7 to 24 fb, and on the couplings of these processes. We obtain world-leading constraints on the couplings for the leptophilic scalar model for masses above 6.5  GeV/c^{2} and for the axionlike particle model over the entire mass range.

We report a study of A+c , E+pi 0, A+c , E+eta, and A+c , E+ eta i using the data sample correspon-ding to an integrated luminosity of 980 fb-1 collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. The branching fractions relative to A+c , E+pi 0 are measured as BA+c,E+eta=BA+c,E+pi 0 = 0.25 +/- 0.03 +/- 0.01 and BA+c,E+eta i=BA+c,E+pi 0 = 0.33 +/- 0.06 +/- 0.02. Using BA+c,E+pi 0 = (1.25 +/- 0.10)%, we obtain BA+c,E+eta = (3.14 +/- 0.35 +/- 0.17 +/- 0.25) x 10-3 and BA+c,E+eta i = (4.16 +/- 0.75 +/- 0.25 +/- 0.33) x 10-3. Here the uncertainties are statistical, systematic, and from BA+c,E+pi 0, respectively. The ratio of the branching fraction of A+c , E+eta i with respect to that of A+c , E+eta is measured to be BA+c,E+eta i=BA+c,E+eta = 1.34 +/- 0.28 +/- 0.08. We update the asymmetry parameter of A+c , E+pi 0, alpha E+pi 0 = -0.48 +/- 0.02 +/- 0.02, with a considerably improved precision. The asymmetry parameters of A+c , E+eta and A+c , E+eta i are measured to be alpha E+eta = -0.99 +/- 0.03 +/- 0.05 and alpha E+eta i = -0.46 +/- 0.06 +/- 0.03 for the first time.