The present report describes the status of the LHCb experiment at the start-up of the LHC data taking at the center-of-mass energy of √s = 7 TeV

Marconi, U.

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DOI 10.1393/ncc/i2011-10764-6Colloquia: IFAE 2010IL NUOVO CIMENTO Vol. 33 C, N. 6 Novembre-Dicembre 2010Status of LHCbU. MarconiINFN, Sezione di Bologna - Bologna, Italy(ricevuto l’ 8 Ottobre 2010; pubblicato online il 2 Febbraio 2011)Summary. — The present report describes the status of the LHCb experiment atthe start-up of the LHC data taking at the center-of-mass energy of√s = 7 TeV.PACS 13.20.He – Decays of bottom mesons.PACS 29.40.-n – Radiation detectors.1. – Heavy flavour physics at LHCIn the last ten-fifteen years experiments have been successful in demonstrating theeffectiveness of the Standard Model (SM) in describing heavy quark phenomenology.Most notably, the measurements of CP violation in B-decays, across a wide range ofdecay modes, have been found to be consistent with the CKM mechanism describing thequark mixing.Experiments of the LHC collider, which are now starting their data taking, will be col-lecting unprecedented statistics of both rare and extremely rare processes, and hopefullywill reveal discrepancies of measurement results with respect to SM predictions.The LHCb experiment in particular focuses on using precision heavy flavour physicswith the main goal of looking for indirect evidence of New Physics in CP violation andrare decays of beauty and charm hadrons. To perform its research program LHCb willexploit the large proton-proton beauty cross section of LHC (expected to be of the orderof 500μb at the beam energy of 14 TeV), to collect large data sample of B0, Bs, Bc andb-baryons (such as the Λb) decays.LHCb aims to test the SM in five main areas:– Measuring the size of CP violation in B0s ↔ B0s mixing.– Measuring branching fractions and kinematic properties of several dimuonB-decays, which have been identified as having high sensitivity to NP contribu-tions in loop diagrams. The most promising of these modes being B0s → μ+μ− andB0 → K∗μ+μ−.c© Societa` Italiana di Fisica 1314 U. MARCONIFig. 1. – LHCb event display. First event recorded at√s = 7 TeV.– In precision measurements of the CKM parameter γ, where tree processes willyield results independent of New-Physics couplings, while loop processes will provesensitive to them.– Sensitivity to anomolous right-handed currents can be tested by measuring thepolarization of photons produced in radiative B-decays, as in the Penguin processesB0s → φγ and B0 → K∗γ.– LHCb has a rich charm programme, which includes a measurement of CP violationin D mixing and searches for rare muonic D decays.With respect to the other LHC experiments LHCb requires a modest instantaneousluminosity of 2–5× 1032 cm−2 s−1, which favours events dominated by single interactionper bunch crossing, thus easing reconstruction and analysis(1). The integrated luminosityis expected to be of the order of 2 fb−1 per nominal year of data taking.According to plans, the LHC experiments will collect 1 fb−1 before the foreseen LHClong term shutdown. Feasibility studies on key measurements, relying on Monte Carlosimulated events, indicate that LHCb will already provide interesting results with 1 fb−1integrated luminosity. For instance, LHCb is expected to be competitive in measuringthe weak phase φs of the B0s ↔ B0s oscillation, in the B0s → J/Ψ(μ+μ−)φ(K+K−) goldenmode, with an order of 200 pb−1 exploiting its high resolution vertex detector and alarger boost compared to previous experiments.A detailed review of the LHCb physics programme and performance on key channelscan be found in [1].2. – Detector overviewThe LHCb detector (visible in fig. 1) is a single arm spectrometer with detector ele-ments placed along one of the proton beam lines of the LHC, covering the forward region(1) Luminosity can be tuned at the LHCb interaction point IP8, independently of the otherexperiments.STATUS OF LHCb 15Fig. 2. – φ coordinates of tracks matching vertex segments. Vertex detector in open position.within 10 mrad to 300 (250) mrad in the bending (non-bending) plane, correspondingto the pseudo-rapidity range 1.9 < η < 4.9 [2]. Detector geometry optimally containsb- and b-hadrons, which at LHC are predominantly produced in the same forward (orbackward) cone(2).The tracking system of LHCb is comprised of: the vertex detector (VELO), a sil-icon strip device that surrounds the proton-proton interaction region; a silicon striptracking station, located before the magnet; and three tracking stations, located be-hind the magnet, each one equipped with silicon strips in the regions close to thebeam-pipe (Internal Tracking, IT, to cope with the high particle current density), andwith drift-time detectors in the external regions (Outer Tracker, OT) further from thebeam-pipe.VELO provides precise measurements of the track coordinates close to the interac-tion region(3), for precise lifetime determination, with an expected resolution on propertime measurements of about 30 fs. Relative momentum resolution in the foreseen mo-mentum range 5–100 GeV/c is expected to be in average of the order of δp/p ∼ 0.4%,resulting in an invariant mass resolution of about 20 MeV/c2 on the reconstructed Bmass.Particle identification to separate pions from kaons is ensured by two RICH detectors.Due to the use of three radiators, the RICH detectors can resolve similar hadronic finalstates over the whole momentum range. The upstream RICH1 uses C4F10 and Aero-gel to cover the lower momentum range, and RICH2, downstream of the spectrometermagnet, contains CF4 to provide particle ID for higher momentum tracks. LHCb is alsoequipped with a calorimeter system, consisting of a Scintillator Pad Detector (SPD),a Pre-Shower (PS), a sampling Electromagnet Calorimeter (ECAL) and a HadronicCalorimeter (HCAL) (used only for triggering on hadronic B-decay modes). Identifi-cation of muons is provided by a dedicated MUON system built using MWPC, andtriple-GEM detectors in a region close to the beam-pipe.In order to cope with the high rate production of b-hadrons in the harsh LHC environ-ment, the LHCb detector requires a selective and flexible trigger system which is sensitive(2) bb pairs are produced in gluon-gluon fusion by strongly asymmetric colliding partons. As aconsequence pairs are boosted along the direction of the higher momentum gluon and both theb-hadrons are produced in the same forward (or backward) direction.(3) Module halves have to be retracted from the data taking working positions during the LHCbeams injection phase for safety reasons.16 U. MARCONIFig. 3. – X coordinate of primary vertices vs. number of tracks.to many different final states and a data acquisition system with a high bandwidth andpowerful online data processing capability. The trigger system has been structured intotwo layers which are designed to achieve the reduction of the bunch crossing frequencyof 40 MHz to the frequency of 2 kHz, at which events are recorded. The first level trig-ger (L0) reduces the bunch crossing rate to 1 MHz, selecting high-pT objects using thecalorimeter and muon systems, operating on custom electronics with a constant latencyof 4μs. The second level trigger (HLT) is a software trigger, based on selection algo-rithms running on the online cluster, which is highly adaptable and which increases withknowledge. The first task of the HLT is to confirm L0-objects before using the vertexdetector to search for a displaced vertex. The trigger then uses the full spectrometerinformation to accept decay modes of interest at the foreseen write-to-tape frequency.An HLT mean processing time of the order of 10 ms, at the input rate of 1 MHz, setsthe size of the computing cluster to the order of 104 CPU cores.3. – Commissioning and first signalsThe horizontal layout of the LHCb detector is not well suited to detect cosmic rays.Nevertheless, by relaxing the trigger thresholds on pT and ET a data rate of about1Hz has been achieved, allowing to collect several million useful cosmic events in a cou-ple of years. Cosmic-ray sample permitted to perform time and spatial alignment ofthe large sub-detectors. Later commissioning with beams dumped on TED (Transfer-line External Dump), located 340 m behind LHCb, were used to improve the align-ment of sub-detectors, especially smaller ones. The resulting showers (of approximatelyFig. 4. – Invariant mass of track pairs reconstructed downstream.STATUS OF LHCb 17Fig. 5. – Tracks matching segments reconstructed by the vertex detector10 particle/cm2) were used to test the VELO and silicon tracker data-acquisitions systemsfor the first time. Over 99% of the detector channels have been seen to be functioningcorrectly. The alignment procedure performed using tracks from the TED showed sen-sors were displaced by less than 10μm with respect to their surveyed positions. The hitresolutions after alignment for VELO sensors showed very good agreement with a binaryperformance.Data collected at the proton beam energy of 450 GeV with a minimum bias triggerbased on the calorimeters and muon system was then used to complete the commissioningof the sub-detectors and of the trigger. Tracking efficiency above 95% has been measuredfor particles traversing the entire detector, with momenta greater than 5 GeV/c, with acontamination of ghost tracks at the level of 10%.In order to avoid possible damages to the vertex detector during the early commission-ing of the accelerator, module halves have been kept in safe position at ±15 mm from thebeam line. The distribution of the φ coordinates of tracks matched to vertex segmentsis visible in fig. 2. Resolution in the primary vertex reconstruction as a function of thenumber of tracks attached to the vertex is shown in fig. 3. In this configuration vertexresolution in the transverse plane is above 100μm, in reasonable agreement (within 10%)with the results of Monte Carlo simulations. This resolution is five to ten times worsethan the foreseen design performance, when the detector halves are closer to the beamat ±8 mm, in the configuration foreseen to run at higher energies.With protons colliding at the energy of 900 GeV LHCb observed the first signals ofK0s and φ. The mass spectra of the reconstructed K0s → π+π− are presented in fig. 4 andfig. 5. The two distributions show how the mass resolution improves when the tracksare required to match the vertex segments. Figure 6 shows the two-body invariant massFig. 6. – K+K− invariant mass without RICH particle ID.18 U. MARCONIFig. 7. – K+K− invariant mass distribution requiring the RICH particle ID.assuming the kaon hypothesis for both tracks. In order to observe the φ signal we needthe RICH particle ID to reject the background as shown in fig. 7. The invariant massdistribution of the muon pairs is shown in fig. 8. One can notice here the strong agreementbetween the observed distribution and the Monte Carlo simulations. Figure 9 shows theefficiency of the L0 muon trigger as a function of the pT compared to the Monte Carlo.The agreement between the two distributions again is quite remarkable.During data taking the spectrometer is monitored with a fast reconstruction of thereference signal J/Ψ → μ+μ−, together with K0s → π+π− and Λ0 → pπ−. In order toprovide the calibration samples of pions and kaons for the RICH system the decay chainD∗+ → D0(K−π+)π+ is reconstructed, where a selection based on the mass differencebetween D∗+ and D0 leaves a clean sample of D0. The kaons and pions from the D0are used to calibrate the ring size and number of photons per track observed in eachRICH detector, while Λ0 decays provide a clean set of both protons and pions. Muonsare identified by hits in a window around the extrapolated track direction in the muonsub-detector. Window sizes are tuned on data using samples from J/Ψ→ μ+μ− decays,while Λ0 → pπ− decays provide backgrounds of both pions and protons to evaluatemisidentification probabilities.In the next two years LHC will provide proton-proton collisions at a center-of-massenergy of 7 TeV. For this reason the beauty quark production cross section, whichlinearly depends on√s, is expected to be reduced by 50% with respect to the LHCdesign operating conditions. On the other hand, the instantaneous luminosity is foreseento increase to 1032 cm−2 s−1 as the number of circulating bunches increases approachingthe preferred LHCb luminosity range. In this scenario LHCb should operate withoutFig. 8. – Muon pairs invariant mass.STATUS OF LHCb 19Fig. 9. – The L0 trigger efficiency as a function of pT .severe limitations. In particular the physics program can be enriched by collecting largesamples of charmed mesons operating with low pT and ET trigger thresholds, until theinteraction rate remains well below the L0 maximum output frequency of 1 MHz.One of the important goals of the program is to measure the beauty quark productioncross section at a centre-of-mass energy of√s = 7 TeV. This should already be possiblewith the first 10 nb−1 using the J/Ψ→ μ+μ− sample. J/Ψ from b-hadron decays can beseparated from prompt J/Ψ using the J/Ψ proper time. From a fit to the proper timedistribution it is possible to determine the total cross sections for all J/Ψ and for thedelayed b → J/Ψ in the pT acceptance 0.5–10 GeV/c and rapidity range 2.5 < y < 5.0.Measurement precision is expected to be limited by systematic error on the luminositymeasurement and on tracking efficiency. Luminosity can be determined using Van derMeer scan and with the beam gas interaction events (only possible at LHCb) with anexpected precision at the level of 10%.REFERENCES[1] Adeva B. et al. (LHCb Collaboration), Roadmap for selected key measurements ofLHCb, arXiv:0912.4179.[2] Adeva B. et al. (LHCb Collaboration), JINST, 3 (2008) S08005.

Status of LHCb

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Status of LHCb

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