78 research outputs found
Measurement of the inelastic pp cross-sections using the LHCb experiment and development of a new vertex detector
In this thesis, measurements of the inelastic cross-sections of proton-proton collisions at
centre-of-mass energies of \sqrt{s} = 5 TeV and \sqrt{s} = 13 TeV with the LHCb detector are
presented. The cross-section measurements are performed using pp collisions with at least one
prompt long-lived charged particle in the pseudorapidity range 2 2
GeV/c. The extrapolation to the full phase-space is also performed using the official LHCb
simulation generated using the Pythia 8 event generator.
During the second long shut down of the LHC (2018-2019), the LHCb experiment will be upgraded,
and all its subdetectors will be modified. Specifically, the Vertex Detector (VELO) will move from
silicon strips to silicon pixels. The new readout ASIC will be the VeloPix, a successor of the Medipix
family of ASICs. The Single Event Upset robustness of the Medipix3 chip, an ancestor of the
VeloPix, is studied in order to evaluated the radiation hardness of the VeloPix. Different beam tests
at a cyclotron facility were designed and performed with the subsequent data analysis.
Using the Timepix3, a successor of the Medipix3, a telescope was developed. This telescope was
designed to test the sensors and readout electronics prototypes for the VELO upgrade. The design
and the performance of this telescope under very high particle incident rates are also presented
Measurement of forward Z â e+eâ production sâs=8 TeV
A measurement of the cross-section for Z-boson production in the forward region of pp collisions at 8 TeV centre-of-mass energy is presented. The measurement is based on a sample of Z â e+eâ decays reconstructed using the LHCb detector, corresponding to an integrated luminosity of 2.0 fbâ1. The acceptance is defined by the requirements 2.0 20 GeV for the pseudorapidities and transverse momenta of the leptons. Their invariant mass is required to lie in the range 60-120 GeV. The cross-section is determined to beS
Measurement of indirect CP asymmetries in D0 â KâK+ and D0 â ÏâÏ+ decays using semileptonic B decays
Time-dependent CP asymmetries in the decay rates of the singly Cabibbo-suppressed decays D0 â KâK+ and D0 â ÏâÏ+ are measured in pp collision data corresponding to an integrated luminosity of 3.0 fbâ1 collected by the LHCb experiment. The D0 mesons are produced in semileptonic b-hadron decays, where the charge of the accompanying muon is used to determine the initial state as D0 or DÂŻ0. The asymmetries in effective lifetimes between D0 and DÂŻ0 decays, which are sensitive to indirect CP violation, are determined to be
AÎ(KâK+)=(â0.134±0.077â0.026â0.034)%,AÎ(ÏâÏ+)=(â0.092±0.145â0.025â0.033)%,
where the first uncertainties are statistical and the second systematic. This result is in agreement with previous measurements and with the hypothesis of no indirect CP violation in D0 decays.S
Precise measurements of the properties of the B1(5721)0,+ and B *2(5747)0,+ states and observation of B+,0Ïâ,+ mass structures
Invariant mass distributions of B+Ïâ and B0Ï+ combinations are investigated in order to study excited B mesons. The analysis is based on a data sample corresponding to 3.0 fbâ1 of pp collision data, recorded by the LHCb detector at centre-of-mass energies of 7 and 8 TeV. Precise measurements of the masses and widths of the B1(5721)0,+ and B2(5747)0,+ states are reported. Clear enhancements, particularly prominent at high pion transverse momentum, are seen over background in the mass range 5850-6000 MeV in both B+Ïâ and B0Ï+ combinations. The structures are consistent with the presence of four excited B mesons, labelled B J (5840)0,+ and B J (5960)0,+, whose masses and widths are obtained under different hypotheses for their quantum numbersS
Measurement of CP asymmetries and polarisation fractions in B0sâKâ0KÂŻâ0 decays
An angular analysis of the decay B0sâKâ0KÂŻâ0 is performed using pp collisions corresponding to an integrated luminosity of 1.0 fbâ1 collected by the LHCb experiment at a centre-of-mass energy sâ=7 TeV. A combined angular and mass analysis separates six helicity amplitudes and allows the measurement of the longitudinal polarisation fraction f L = 0.201 ± 0.057 (stat.) ± 0.040 (syst.) for the B0sâKâ(892)0KÂŻâ(892)0 decay. A large scalar contribution from the K â0(1430) and K â0(800) resonances is found, allowing the determination of additional CP asymmetries. Triple product and direct CP asymmetries are determined to be compatible with the Standard Model expectations. The branching fraction B(BsâKâ(892)0KÂŻâ(892)0) is measured to be (10.8 ± 2.1 (stat.) ± 1.4 (syst.) ± 0.6 (f d /f s )) Ă 10â6.S
Observation of the B0âÏ0Ï0 decay from an amplitude analysis of B0â(Ï+Ïâ)(Ï+Ïâ)decays
Protonâproton collision data recorded in 2011 and 2012 by the LHCb experiment, corresponding to an integrated luminosity of 3.0fbâ1, are analysed to search for the charmless B0âÏ0Ï0decay. More than 600 B0â(Ï+Ïâ)(Ï+Ïâ)signal decays are selected and used to perform an amplitude analysis, under the assumption of no CP violation in the decay, from which the B0âÏ0Ï0decay is observed for the first time with 7.1 standard deviations significance. The fraction of B0âÏ0Ï0decays yielding a longitudinally polarised final state is measured to be fL=0.745+0.048â0.058(stat) ±0.034(syst). The B0âÏ0Ï0branching fraction, using the B0âÏKâ(892)0decay as reference, is also reported as B(B0âÏ0Ï0)=(0.94±0.17(stat)±0.09(syst)±0.06(BF))Ă10â6S
Study of the rare B0s and B0s decays into the Ï+ÏâÎŒ+ÎŒâfinal state
A search for the rare decays B0sâÏ+ÏâÎŒ+ÎŒâand B0âÏ+ÏâÎŒ+ÎŒâis performed in a data set corresponding to an integrated luminosity of 3.0fbâ1collected by the LHCb detector in protonâproton collisions at centre-of-mass energies of 7 and 8TeV. Decay candidates with pion pairs that have invariant mass in the range 0.5â1.3GeV/c2and with muon pairs that do not originate from a resonance are considered. The first observation of the decay B0sâÏ+ÏâÎŒ+ÎŒâand the first evidence of the decay B0âÏ+ÏâÎŒ+ÎŒâare obtained and the branching fractions, restricted to the dipion-mass range considered, are measured to be B(B0sâÏ+ÏâÎŒ+ÎŒâ) =(8.6 ±1.5(stat) ±0.7(syst) ±0.7 (norm)) Ă10â8and B(B0âÏ+ÏâÎŒ+ÎŒâ) =(2.11±0.51(stat)±0.15(syst)±0.16(norm))Ă10â8, where the third uncertainty is due to the branching fraction of the decay B0âJ/Ï(âÎŒ+ÎŒâ)Kâ(892)0(âK+Ïâ), used as a normalisation.S
Measurement of the CP-violating phase ÎČin B0âJ/ÏÏ+Ïâdecays and limits on penguin effects
Time-dependent CPviolation is measured in the (â)B0âJ/ÏÏ+Ïâchannel for each Ï+Ïâresonant final state using data collected with an integrated luminosity of 3.0fbâ1in ppcollisions using the LHCb detector. The final state with the largest rate, J/ÏÏ0(770), is used to measure the CP-violating angle 2ÎČeffto be (41.7 ±9.6+2.8â6.3)âŠ.This result can be used to limit the size of penguin amplitude contributions to CPviolation measurements in, for example, (â)B0sâJ/ÏÏdecays. Assuming approximate SU(3) flavour symmetry and neglecting higher order diagrams, the shift in the CP-violating phase Ïsis limited to be within the interval [â1.05âŠ, +1.18âŠ] at 95% confidence level. Changes to the limit due to SU(3) symmetry breaking effects are also discussed.S
Search for CP violation in D0âÏâÏ+Ï0 decays with the energy test
A search for time-integrated CPviolation in the Cabibbo-suppressed decay D0âÏâÏ+Ï0is performed using for the first time an unbinned model-independent technique known as the energy test. Using protonâproton collision data, corresponding to an integrated luminosity of 2.0fbâ1collected by the LHCbdetector at a centre-of-mass energy of âs=8TeV, the worldâs best sensitivity to CPviolation in this decay is obtained. The data are found to be consistent with the hypothesis of CPsymmetry with a p-value of (2.6 ±0.5)%S
Measurement of the inelastic pp cross-section at a centre-of-mass energy of âs = 7 TeV
The cross-section for inelastic proton-proton collisions, with at least one prompt long-lived charged particle of transverse momentum p T > 0.2GeV/c in the pseudorapidity range 2.0 < η < 4.5, is measured by the LHCb experiment at a centre-of-mass energy of âs = 7 TeV. The cross-section in this kinematic range is determined to be Ï accinel = 55.0 ± 2.4 mb with an experimental uncertainty that is dominated by systematic contributions. Extrapolation to the full phase space, using PYTHIA 6, yields Ï inel = 66.9 ± 2.9 ± 4.4 mb, where the first uncertainty is experimental and the second is due to the extrapolation.S
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