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

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.]

LHCb

The LHCb detector is designed to study CP violation and other rare phenomena in decays of hadrons with heavy flavours, in particular $\rm B_s$ mesons. Interest in CP violation comes not only from elementary particle physics but also from cosmology, in order to explain the dominance of matter over antimatter observed in our universe, which could be regarded as the largest CP violation effect ever seen. The LHCb experiment will improve significantly results from earlier experiments both quantitatively and qualitatively, by exploiting the large number of different kinds of b hadrons produced at LHC. This is done by constructing a detector which has \begin{enumerate} \item Good trigger efficiencies for b-hadron final states with only hadrons, as well as those containing leptons. \item Capability of identifying kaons and pions in a momentum range of $\sim 1$ to above 100 GeV/$c$. \item Excellent decay time and mass resolution. \end{enumerate} The LHCb spectrometer shown in the figure consists of the following detector components: \begin{itemize} \item Beam Pipe\\ A 1.8 m-long section of the beam pipe around the interaction point has a large diameter of approximately 120 cm. This accommodates the vertex detector system with its retraction mechanics, and has a thin forward window made of aluminium over the full detector acceptance. This part is followed by two conical sections; the first is 1.5 m long with 25 mrad opening angle, and the second is 16 m long with 10 mrad opening angle. The entire first and most of the second section are made of beryllium in order to reduce the production of the secondary particles. \item Magnet\\ A dipole magnet with the normal conductive Al coil provides a high field integral of 4 Tm. The polarity of the field can be changed to reduce systematic errors in the CP-violation measurements that could result from a left-right asymmetry of the detector. \item Vertex Locator\\ A total of 21 stations made from two layers of silicon detector are used as a vertex detector system (VELO). Additional two stations with only one Si layer are dedicated to the detecting bunch crossings with more than one pp interaction as a part of Level-0 trigger. The closest distance between the active silicon area and the beam is 8 mm. The silicon detectors are placed in Roman pots with 300 $\mu$m thick aluminium windows, which act as a shield against RF pickup from the circulating beam bunches. In order to avoid collapse of the windows, a secondary vacuum is maintained inside the Roman pots. During the injection and acceleration, the Roman pot system will be moved away from the beam to avoid interference with the machine operation and accidental irradiation of the detectors. \item Tracking\\ The LHCb tracking system consists of four stations; one upstream of the magnet (TT) and three just behind the magnet (T1 to T3). No tracking device is positioned in the magnet and most of the tracks are reconstructed by combining the VELO and tracking system. The first station is made of silicon detectors. The stations behind the magnet are split into Inner Tracker (IT) and Outer Tracker (OT) systems due to the high particle density close to the beam pipe. The IT system is made of Si, and drift chambers based on straw technology are used for the OT system. \item Ring Imaging Cherenkov Detectors\\ The RICH system of the LHCb detector consists of two detectors with three different radiators in order to cover the required momentum range, 1-100 GeV/$c$ . The first detector uses aerogel and $\rm C_4 F_ {10}$ gas as radiators. The second detector, used for high momentum particles, is placed after the magnet and has $\rm C F_4$ gas as radiator. The Cherenkov light is detected with planes of Hybrid Photon Detectors (HPD's) placed outside the spectrometer acceptance. \item Calorimeters\\ The calorimeter system consists of a preshower detector followed by electromagnetic and hadronic calorimeters. It also serves as the initial part of the muon filter system. The cells of the Preshower detector are made up from 12 mm-thick lead plates sandwiched by square scintillators, 15 mm thick. For the electromagnetic part a Shashlik calorimeter is used since only modest energy resolution is required. The hadron calorimeter is based on a scintillating tile design similar to that developed for the ATLAS experiment. \item Muon System\\ The Muon system consists of tracking stations and absorber layers. The first tracking station is in front of the calorimeter system, which acts as the first absorber. Behind the calorimeter system, there are four tracking stations with Fe absorber walls in between. An additional Fe absorber is placed after the last tracking station against the muon background from the accelerator tunnel. Multi Wire Proportional Chambers are used everywhere except in the region close to the beam pipe of the first station where Triple-GEM chambers are used. \item Trigger\\ The LHCb trigger has two decision levels. Using custom made electronics, the first decision is made based on high transverse momentum hadrons or electrons found in the calorimeter system, or muons found in the muon system, at the bunch crossing rate of 40 MHz. All data from the detector are then read out at a rate of 1 MHz and sent to a CPU farm for further event reduction. For this purpose, all the detector information is available. With a rate of 2 kHz, events which include calibration data are stored for offline analysis. \end{itemize} Due to the large production cross section for b-$\rm \overline{ b}$ pairs (500~$\rm \mu b$) and efficient trigger, the LHCb experiment requires only a much lower luminosity ($2 \times 10^{32}$~$\rm cm^{-2} s^{-1}$) than the nominal LHC luminosity ($10^{34}$~$\rm cm^{-2}s^{-1}$) for its physics programme. The experiment therefore can reach its full physics potential from the beginning of LHC operation. The luminosity at the LHCb interaction point can be locally tuned so that the experiment is able to continue its physics programme when the machine reaches the nominal operating condition. \end{document

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)

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 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