Highlights from the LHCb experiment

We report recent results by the LHCb collaboration in heavy-ion collisions in collider and fixed-target mode at the LHC. A large variety of measurements show the potential of LHCb in nuclear collisions

Direct photon production at LHCb

At small Bjorken-x, the large gluon number density in the nucleon leads to gluon recombination competing with gluon splitting, which could result in saturation of the gluon PDF. This gluon saturation has yet to be conclusively observed. Direct photon production provides sensitivity to gluon densities in protons and nuclei, and the forward acceptance of LHCb detector allows for measurements of this process at low Bjorken-x, providing an ideal probe of saturation effects. Progress towards the measurement of forward direct photon production using the LHCb detector is presented

Evidence for an nc(1S)ff- resonance in B0 yc(1S)K+ decays

A Dalitz plot analysis of B0→ηc(1S)K+π- decays is performed using data samples of pp collisions collected with the LHCb detector at centre-of-mass energies of s=7,8 and 13TeV , corresponding to a total integrated luminosity of 4.7fb-1 . A satisfactory description of the data is obtained when including a contribution representing an exotic ηc(1S)π- resonant state. The significance of this exotic resonance is more than three standard deviations, while its mass and width are 4096±20-22+18MeV and 152±58-35+60MeV , respectively. The spin-parity assignments JP=0+ and JP=1- are both consistent with the data. In addition, the first measurement of the B0→ηc(1S)K+π- branching fraction is performed and gives B(B0→ηc(1S)K+π-)=(5.73±0.24±0.13±0.66)×10-4, where the first uncertainty is statistical, the second systematic, and the third is due to limited knowledge of external branching fractions

Measurement of Z -> tau(+)tau(-) production in proton-proton collisions at root s=8 TeV

A measurement of Z → τ$^{+}$τ$^{−}$ production cross-section is presented using data, corresponding to an integrated luminosity of 2 fb$^{−1}$, from pp collisions at $\sqrt{s}=8$ TeV collected by the LHCb experiment. The τ$^{+}$τ$^{−}$ candidates are reconstructed in final states with the first tau lepton decaying leptonically, and the second decaying either leptonically or to one or three charged hadrons. The production cross-section is measured for Z bosons with invariant mass between 60 and 120 GeV/c$^{2}$, which decay to tau leptons with transverse momenta greater than 20 GeV/c and pseudorapidities between 2.0 and 4.5. The cross-section is determined to be ${\sigma}_{pp}{{}_{\to Z\to {\tau}^{+}}}_{\tau^{-}}=95.8 \pm 2.1 \pm 4.6 \pm 0.2 \pm 1.1$ pb, where the first uncertainty is statistical, the second is systematic, the third is due to the LHC beam energy uncertainty, and the fourth to the integrated luminosity uncertainty. This result is compatible with NNLO Standard model predictions. The ratio of the cross-sections for Z → τ$^{+}$τ$^{−}$ to Z → μ$^{+}$μ$^{−}$ (Z → e$^{+}$e$^{−}$), determined to be 1.01 ± 0.05 (1.02 ± 0.06), is consistent with the lepton-universality hypothesis in Z decays.A measurement of $Z\rightarrow\tau^+\tau^-$ production cross-section is presented using data, corresponding to an integrated luminosity of 2 fb$^{-1}$, from $pp$ collisions at $\sqrt{s}=8$ TeV collected by the LHCb experiment. The $\tau^+\tau^-$ candidates are reconstructed in final states with the first tau lepton decaying leptonically, and the second decaying either leptonically or to one or three charged hadrons. The production cross-section is measured for $Z$ bosons with invariant mass between 60 and 120 GeV/$c^2$, which decay to tau leptons with transverse momenta greater than 20 GeV/$c$ and pseudorapidities between 2.0 and 4.5. The cross-section is determined to be $\sigma_{pp\rightarrow{}Z\rightarrow{}\tau^+\tau^-} = 95.8 \pm 2.1 \pm 4.6 \pm 0.2 \pm 1.1 \mathrm{pb}$, where the first uncertainty is statistical, the second is systematic, the third is due to the LHC beam energy uncertainty, and the fourth to the integrated luminosity uncertainty. This result is compatible with NNLO Standard model predictions. The ratio of the cross-sections for $Z\rightarrow\tau^+\tau^-$ to $Z\rightarrow\mu^+\mu^-$ ($Z\rightarrow{}e^+e^-$), determined to be $1.01 \pm 0.05$ ($1.02 \pm 0.06$), is consistent with the lepton-universality hypothesis in $Z$ decays

First Observation of the Doubly Charmed Baryon Decay Ξcc + + → Ξc+ π+

The doubly charmed baryon decay Ξcc++→Ξc+π+ is observed for the first time, with a statistical significance of 5.9σ, confirming a recent observation of the baryon in the Λc+K-π+π+ final state. The data sample used corresponds to an integrated luminosity of 1.7 fb-1, collected by the LHCb experiment in pp collisions at a center-of-mass energy of 13 TeV. The Ξcc++ mass is measured to be 3620.6±1.5(stat)±0.4(syst)±0.3(Ξc+) MeV/c2 and is consistent with the previous result. The ratio of branching fractions between the decay modes is measured to be [B(Ξcc++→Ξc+π+)×B(Ξc+→pK-π+)]/[B(Ξcc++→Λc+K-π+π+)×B(Λc+→pK-π+)]=0.035±0.009(stat)±0.003(syst)

Measurement of Antiproton Production in p-He Collisions at root S-NN=110 GeV

The cross section for prompt antiproton production in collisions of protons with an energy of 6.5 TeV incident on helium nuclei at rest is measured with the LHCb experiment from a data set corresponding to an integrated luminosity of 0.5 nb-1. The target is provided by injecting helium gas into the LHC beam line at the LHCb interaction point. The reported results, covering antiproton momenta between 12 and 110 GeV/c, represent the first direct determination of the antiproton production cross section in p-He collisions, and impact the interpretation of recent results on antiproton cosmic rays from space-borne experiments

Prompt Lambda(+)(c) baryons and D-0 meson production cross-section and nuclear modification in pPb collisions at root S-NN=5.02 TeV with the LHCb detector

A(c)(+) baryons and D-0 mesons are studied in pPb collisions at root S-NN = 5.02 TeV. The nuclear modification factor and forward-backward cross-section asymmetry are measured in order to study the cold nuclear matter effects. The prompt A(c)(+) production cross-section is compared to that of the prompt D-0 mesons, providing insights into the hadronisation mechanism of charmed hadrons

Measurement of the branching fractions of the decays D+ -> K-K+K+, D+ -> pi-pi(+) K+ and D-s(+) -> pi-K+K+

The branching fractions of the doubly Cabibbo-suppressed decays D + → K − K + K + , D + → π − π + K + and D s+ → π − K + K + are measured using the decays D + → K − π + π + and D s+ → K − K + π + as normalisation channels. The measurements are performed using proton-proton collision data collected with the LHCb detector at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 2.0 fb −1 . The results areℬ(D+→K−K+K+)ℬ(D+→K−π+π+)=(6.541±0.025±0.042)×10−4,ℬ(D+→π−π+K+)ℬ(D+→K−π+π+)=(5.231±0.009±0.023)×10−3,ℬ(Ds+→π−K+K+)ℬ(Ds+→K−K+π+),=(2.372±0.024±0.025)×10−3, where the uncertainties are statistical and systematic, respectively. These are the most precise measurements up to date. [Figure not available: see fulltext.]

Study of coherent J/psi production in lead-lead collisions at root s(NN)=5 TeV with the LHCb experiment

Coherent production of J/ψ mesons is studied in lead-lead collision data at a nucleon-nucleon centre-of-mass energy of 5 TeV collected by the LHCb experiment. The data set corresponds to an integrated luminosity of about 10μb−1. The J/ψ mesons are reconstructed in the dimuon final state, where the muons are detected within the pseudorapidity region 2.0 < η < 4.5. The J/ψ mesons are required to have transverse momentum pT < 1 GeV and rapidity 2.0 < y < 4.5. The cross-section within this fiducial region is measured to be σ = 5.3 ± 0.2 (stat) ± 0.5 (syst) ± 0.7 (lumi) mb. The differential cross-section is also measured in five bins of J/ψ rapidity. The results are compared to predictions from phenomenological models