Event generator tuning using Bayesian optimization

Monte Carlo event generators contain a large number of parameters that must be determined by comparing the output of the generator with experimental data. Generating enough events with a fixed set of parameter values to enable making such a comparison is extremely CPU intensive, which prohibits performing a simple brute-force grid-based tuning of the parameters. Bayesian optimization is a powerful method designed for such black-box tuning applications. In this article, we show that Monte Carlo event generator parameters can be accurately obtained using Bayesian optimization and minimal expert-level physics knowledge. A tune of the PYTHIA 8 event generator using e⁺e⁻ events, where 20 parameters are optimized, can be run on a modern laptop in just two days. Combining the Bayesian optimization approach with expert knowledge should enable producing better tunes in the future, by making it faster and easier to study discrepancies between Monte Carlo and experimental data.United States. Department of Energy (Grant DE-SC0010497)United States. Department of Energy (Grant DE-FG02-94ER40818

Measurement of the resonant and CP components in B¯0→J/ψπ+π− decays

The resonant structure of the reaction B¯0→J/ψπ+π− is studied using data from 3  fb−1 of integrated luminosity collected by the LHCb experiment, one third at 7 TeV center-of-mass energy and the remainder at 8 TeV. The invariant mass of the π+π− pair and three decay angular distributions are used to determine the fractions of the resonant and nonresonant components. Six interfering π+π− states, ρ(770), f0(500), f2(1270), ρ(1450), ω(782) and ρ(1700), are required to give a good description of invariant mass spectra and decay angular distributions. The positive and negative charge parity fractions of each of the resonant final states are determined. The f0(980) meson is not seen and the upper limit on its presence, compared with the observed f0(500) rate, is inconsistent with a model where these scalar mesons are formed from two quarks and two antiquarks (tetraquarks) at the eight standard deviation level. In the qq¯ model, the absolute value of the mixing angle between the f0(980) and the f0(500) scalar mesons is limited to be less than 17° at 90% confidence level.National Science Foundation (U.S.)United States. Dept. of Energ

Measurement of the forward W boson cross-section in pp collisions at √s = 7 TeV

A measurement of the inclusive W → μν production cross-section using data from pp collisions at a centre-of-mass energy of √s=7 TeV is presented. The analysis is based on an integrated luminosity of about 1.0 fb[superscript −1] recorded with the LHCb detector. Results are reported for muons with a transverse momentum greater than 20 [GeV over c] and pseudorapidity between 2.0 and 4.5. The W [superscript +] and W [superscript −] production cross-sections are measured to be σW[superscript +]→μ[superscript +]ν=861.0±2.0±11.2±14.7pb, σW[superscript −]→μ[superscript −] [bar over ν] = 675.8±1.9±8.8±11.6pb, where the first uncertainty is statistical, the second is systematic and the third is due to the luminosity determination. Cross-section ratios and differential distributions as functions of the muon pseudorapidity are also presented. The ratio of W [superscript +] to W [superscript −] cross-sections in the same fiducial kinematic region is determined to be σW [superscript +] → μ [superscript +] ν [over σW [superscript −] → μ [superscript −] [bar over ν]] = 1.274±0.005±0.009, where the uncertainties are statistical and systematic, respectively. Results are in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics.National Science Foundation (U.S.

Measurement of the Z+b-jet cross-section in pp collisions at √s = 7 TeV in the forward region

The associated production of a Z boson or an off-shell photon γ * with a bottom quark in the forward region is studied using proton-proton collisions at a centre-of-mass energy of 7 TeV. The Z bosons are reconstructed in the [Z over γ] * → μ [superscript +] μ [superscript −] final state from muons with a transverse momentum larger than 20 GeV, while two transverse momentum thresholds are considered for jets (10 GeV and 20 GeV). Both muons and jets are reconstructed in the pseudorapidity range 2.0 < η < 4.5. The results are based on data corresponding to 1.0 fb [superscript −1] recorded in 2011 with the LHCb detector. The measurement of the Z+b-jet cross-section is normalized to the Z+jet cross-section. The measured cross-sections are 0.1 σ( [Z over γ] *(μ[superscript+] μ[superscript −]) + b-jet) = 295 ± 60 (stat) ± 51 (syst) ± 10 (lumi) fb for p [subscript T] (jet) > 10 GeV, and 0.2 σ( [Z over γ]*(μ[superscript +] μ[superscript −]) + b-jet)=128±36 (stat) ±22 (syst) ±5 (lumi) fb for p [subscript T](jet) > 20 GeV.National Science Foundation (U.S.

Precision Measurement of the Mass and Lifetime of the Ξ0b Baryon

Using a proton-proton collision data sample corresponding to an integrated luminosity of 3  fb[superscript −1] collected by LHCb at center-of-mass energies of 7 and 8 TeV, about 3800 Ξ0b→Ξ+cπ−, Ξ+c→pK−π+ signal decays are reconstructed. From this sample, the first measurement of the Ξ0b baryon lifetime is made, relative to that of the Λ0b baryon. The mass differences M(Ξ0b)−M(Λ0b) and M(Ξ+c)−M(Λ+c) are also measured with precision more than 4 times better than the current world averages. The resulting values are τΞ0bτΛ0b=1.006±0.018±0.010,M(Ξ0b)−M(Λ0b)=172.44±0.39±0.17  MeV/c2,M(Ξ+c)−M(Λ+c)=181.51±0.14±0.10  MeV/c2, where the first uncertainty is statistical and the second is systematic. The relative rate of Ξ0b to Λ0b baryon production is measured to be fΞ0bfΛ0bB(Ξ0b→Ξ+cπ−)B(Λ0b→Λ+cπ−)B(Ξ+c→pK−π+)B(Λ+c→pK−π+)=(1.88±0.04±0.03)×10−2, where the first factor is the ratio of fragmentation fractions, b→Ξ0b relative to b→Λ0b. Relative production rates as functions of transverse momentum and pseudorapidity are also presented.National Science Foundation (U.S.

Effective lifetime measurements in the Bs0→K+K−, B0→K+π− and Bs0→π+K− decays

Measurements of the effective lifetimes in the B[0 over s]→K+K−, B0→K+π−B[superscript 0]→K+π− and B[0 over s]→π+K− decays are presented using 1.0 fb−11.0 fb[superscript −1] of pp collision data collected at a centre-of-mass energy of 7 TeV by the LHCb experiment. The analysis uses a data-driven approach to correct for the decay time acceptance. The measured effective lifetimes are τBs0→K+K−=1.407±0.016(stat)±0.007(syst) ps, τB0→K+π−=1.524±0.011(stat)±0.004(syst) ps, τBs0→π+K−=1.60±0.06(stat)±0.01(syst) ps. This is the most precise determination to date of the effective lifetime in the Bs0→K+K− decay and provides constraints on contributions from physics beyond the Standard Model to the Bs0 mixing phase and the width difference ΔΓs.National Science Foundation (U.S.

Observation of Overlapping Spin-1 and Spin-3 D¯0K− Resonances at Mass 2.86 GeV/c2

The resonant substructure of B_{s}^{0}→D[over ¯]^{0}K^{-}π^{+} decays is studied using a data sample corresponding to an integrated luminosity of 3.0  fb^{-1} of pp collision data recorded by the LHCb detector. An excess at m(D[over ¯]^{0}K^{-})≈2.86  GeV/c^{2} is found to be an admixture of spin-1 and spin-3 resonances. Therefore, the D_{sJ}^{*}(2860)^{-} state previously observed in inclusive e^{+}e^{-}→D[over ¯]^{0}K^{-}X and pp→D[over ¯]^{0}K^{-}X processes consists of at least two particles. This is the first observation of a heavy flavored spin-3 resonance, and the first time that any spin-3 particle has been seen to be produced in B decays. The masses and widths of the new states and of the D_{s2}^{*}(2573)^{-} meson are measured, giving the most precise determinations to date.National Science Foundation (U.S.)United States. Dept. of Energ

Study of the kinematic dependences of Λ b 0 production in pp collisions and a measurement of the Λ b 0 → Λ c + π − branc

The kinematic dependences of the relative production rates, fΛ[0 over b]/fd , of Λ [0 over b] baryons and B 0 mesons are measured using Λ [0 over b] → Λ [+ over c] π − and B¯¯0 →D+π− decays. The measurements use proton-proton collision data, corresponding to an integrated luminosity of 1 fb−1 at a centre-of-mass energy of 7 TeV, recorded in the forward region with the LHCb experiment. The relative production rates are observed to depend on the transverse momentum, p T, and pseudorapidity, η, of the beauty hadron, in the studied kinematic region 1.5 < p T < 40 GeV/c and 2 < η < 5. Using a previous LHCb measurement of fΛ0b/fd in semileptonic decays, the branching fraction ℬ(Λ [0 over b]  → Λ [+ over c] π −) = (4.30 ± 0.03 − 0.11 + 0.12  ± 0.26 ± 0.21) × 10− 3 is obtained, where the first uncertainty is statistical, the second is systematic, the third is from the previous LHCb measurement of fΛ0b/fd and the fourth is due to the b B¯¯0 →D+π− branching fraction. This is the most precise measurement of a Λ [0 over b] branching fraction to date.National Science Foundation (U.S.

Observation of two new Ξb−\Xi_b^- baryon resonances

Two structures are observed close to the kinematic threshold in the $\Xi_b^0 \pi^-$ mass spectrum in a sample of proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb$^{-1}$ recorded by the LHCb experiment. In the quark model, two baryonic resonances with quark content $bds$ are expected in this mass region: the spin-parity $J^P = \frac{1}{2}^+$ and $J^P=\frac{3}{2}^+$ states, denoted $\Xi_b^{\prime -}$ and $\Xi_b^{*-}$. Interpreting the structures as these resonances, we measure the mass differences and the width of the heavier state to be $m(\Xi_b^{\prime -}) - m(\Xi_b^0) - m(\pi^{-}) = 3.653 \pm 0.018 \pm 0.006$ MeV$/c^2$, $m(\Xi_b^{*-}) - m(\Xi_b^0) - m(\pi^{-}) = 23.96 \pm 0.12 \pm 0.06$ MeV$/c^2$, $\Gamma(\Xi_b^{*-}) = 1.65 \pm 0.31 \pm 0.10$ MeV, where the first and second uncertainties are statistical and systematic, respectively. The width of the lighter state is consistent with zero, and we place an upper limit of $\Gamma(\Xi_b^{\prime -}) < 0.08$ MeV at 95% confidence level. Relative production rates of these states are also reported.Comment: 17 pages, 2 figure