V0 cross-section measurement at LHCb. RIVET analysis module for Z boson decay to di-electron Outline of the presentation: 1. Introduction to LHCb physics and LHCb detector 2. RIVET plug-in for Z e+e- channel 3. V0 physics-numerical examples DUMITRIU ANA ELENA, IFIN-HH DFPE, LHCb Bucharest 1
1. Introduction to LHCb physics and LHCb detector The LHCb experiment http://lhcb-public.web.cern.ch/lhcb-public/ is situated at one of the four points around CERN s Large Hadron Collider where beams of protons are smashed together, producing an array of different particles. Other LHC experiments surround the entire collision point with layers of sub-detectors (onion like), the LHCb detector stretches for 20 metres along the beam pipe, with its sub-detectors stacked behind each other like books on a shelf. Each one of LHCb s sub-detectors specializes in measuring a different characteristic of the particles produced by colliding protons. Collectively: information about the identity, trajectory, momentum and energy of each particle generated - single out individual particles from the billions that spray2 out from the collision point.
LHCb detector at the Large Hadron Collider (LHC) at CERN (Geneva) High precision tracking system Particle identification: 2 ring-imaging 12/18/14 Cherenkov detectors https://lhcb.web.cern.ch IBWAP 2014 a single-arm forward spectrometer covering 10 mrad to 300 (250) mrad in the bending (non-bending) plane. 3 designed primarily to study particles containing b and c quarks.
Standard set of performance numbers Acceptance: 2.0 < η < 4.9 Resolutions: - momentum resolution (Δp/p) : 0.4% @ 5 GeV/c up to 0.6% @ 100 GeV/c - ECAL resolution (nominal): 1% + 10% / E [GeV1/2] - impact parameter resolution: 20 μm for high-pt tracks Stop particles as they pass through the detector, measuring the amount of energy lost as each one grinds to a halt. SPD/PS + Electromagnetic + Hadron Calorimeter Silicon-strip detectors recording tracks of charged particles Muon Chamber System Beam collision point where particles containing b and anti-b quarks are produced Dipole magnet with 4 Tm bending power - curves paths of charged particles; identify particles from curvature of the momentum/path Ring Imaging CHerenkov detectors built for particle identification by measuring characteristics of Cherenkov radiation.mainly identifying products of B meson decays: charged pions, kaons and protons. 12/18/14 IBWAP 2014 4
CALORIMETERS -designed to stop particles as they pass through the detector, measuring the amount of energy lost. ->electromagnetic calorimeter - measuring the energy of lighter particles (e and photons) ->hadron calorimeter- energy of protons, neutrons and other heavy particles The calorimeter -Scintillating Pad Detector (SPD), -Pre-Shower Detector (PS), - shashlik -type Electromagnetic Cal (ECAL) -scintillating tile iron plate Hadron Cal (HCAL) -The SPD - particles hitting the calorimeter system are charged or neutral -The PS -indicates the electromagnetic character of the particle (an electron, if charged, or a photon, if neutral). They are used at the trigger level in association with the ECAL to indicate the presence of electrons, photons and neutral pions. 12/18/14 IBWAP 2014 5
Measurement Review + σ ( pp Z e e )=76. 0±0. 8( stat )±2. 0( syst )±2. 6 ( lumi ) pb 1pb=10 40 Production of opposite sign charged lepton pair in hadron-hadron collisions In SM described by (Differential) cross-section determined at centre-of-mass energy of 7 TeV Θ=polar angle(beam,particle considered) Cross-section distributions measured with respect to: 12/18/14 IBWAP 2014 quantumdiaries.org Selection cuts: Daughter electrons: pt> 20 GeV/c, 2 < η < 4.5 Dilepton invariant mass interval: 60 120 GeV/c2 6/25
RIVET or Robust Independent Validation of Experiment and Theory (https://rivet.hepforge.org/) toolkit for validation of Monte Carlo (MC) event generators analysis code from the LHC and other high-energy collider experiments is preserved for comparison to and development of future theory models. provides a large set of experimental analyses useful for MC generator development, validation, and tuning Example of generator: Pythia, Herwig++
DATA POINTS JHEP 02(2013)106
Comparison between py8 diff event type and LHCb data 500,000 events, Pythia 8, PDF set: CTEQ6L1, 1 pp collision/event LbPy8Zee - 42122000 event type ( pure Z ee ) LbPy8Zgee - 42122001 event type ( Z/γ* ee; MZ > 40 GeV/c2) We are not sensitive to the differences between the 2 types of events, in good agreement with LHCb data.
Introduction V 0 hadrons ->named after the V -shaped track signature of their dominant decays: Λ pπ Λ pπ + KS π + π Date LHCb NoBias type (trigger on bunch + bunch crossing), "sqrt(s)"=2.76 TeV. Wrong mass hypothesis for Λ 0 /Ks: rejects Λ 0 /Ks candidates which lay close to the Ks/Λ 0 peek under wrong mass hypothesis for one of the daughter proton/pion. Only tracks with quality χ2 /ndf < 9 are considered, with the V 0 required to decay within the VELO and the daughter tracks to be reconstructed through the full spectrometer. Any oppositely-charged pair is kept as a potential V 0 candidate if it forms a vertex with χ2 < 9 (with one degree of freedom for a V 0 vertex). Λ, Λ and KS candidates are required to have invariant masses within ±50 MeV/c2 of the PDG values. This mass window is large compared to the measured mass resolutions of about 2 MeV/c2 for Λ (Λ) and 5 MeV/c2 for KS Combinatorial background reduced with a Fisher discriminant based on the impact parameters (IP) of the daughter tracks (d± ) and of the reconstructed V 0 mother ( IP = minimum distance of closest approach to the nearest reconstructed primary interaction vertex measured in mm) The Fisher discriminant: optimized for signal significance (S/ ( S + B)) on simulated events after the above quality criteria. 10/25
LHCb Unofficial Λ 0 Data Analysis Kshort Data Analysis 11
LHCb Unofficial Comparison between MC py8 with/without TRUE_ID=3122 cut applied Lambda LHCb Unofficial 12
LHCb Unofficial Comparison between MC py8 with/without TRUE_ID=3122 cut applied Kshort 13
Py 8: Mass distribution Without wrong mass hypothesis LHCb Unofficial 14
Py 8: With wrong mass hypothesis LHCb Unofficial 15
THANK YOU FOR YOUR ATTENTION! 16
Back-up slides 17
Cross section determination signal tot background - global event cut - trigger efficiency - track-finding efficiency (both e successfully reconstructed) - kinematic efficiency (reconstructed tracks pass the kinematic selection for and pt) - particle identification efficiency (reconstructed tracks fulfil calorimeter energy requirements for identified e) ƒfsr->final state elmg radiation ƒmz->events outside mass range 60-120 GeV + σ ( pp 12/18/14 Z e e )=76. 0±0. IBWAP 8(stat )±2. 0(syst )±2. 6( lumi)18/25pb 2014
Comparison between FEWZ with different PDF sets and LHCb data FEWZ (QCD approximation NNLO): good agreement with JHEP 02(2013) 106 data, no dependence on PDF set used.
Comparison between FEWZ with different PDF sets and LHCb data NNLO calculations tend to overestimate the data at low Φ* and underestimate the data at high Φ*.
PARTON DISTRIBUTION FUNCTIONS
mass fit for MC:Py 8 if we apply the cut Lambda0_TRUEID==3122 We used DSCB (http://en.wikipedia.org/wiki/cry stal_ball_function) to describe the signal and polynomial function of 3rd degree for the background 22/25
0 Λ analysis Date LHCb NoBias type (trigger on bunch + bunch crossing), "sqrt(s)"=2.76 TeV. Wrong mass hypothesis for Λ 0 Wrong mass hypothesis for Λ 0 : rejects lambda candidates which lay close to the K_short peek under wrong mass hypothesis for one of the daughter proton. 23/25
K-short analysis Date LHCb NoBias type (trigger on bunch + bunch crossing), "sqrt(s)"=2.76 TeV. Wrong mass hypothesis for Kshort Wrong mass hypothesis for K_short : rejects Kshort candidates which lay close to the Λ 0 peek under wrong mass hypothesis for one of the daughter pion. 24/25
Additional activities 25