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This study focuses on the production of W± bosons in proton-proton collisions at 14 TeV at the LHC. The aim is to analyze the rapidity and transverse momentum distributions of W± bosons and their decay modes, particularly into muons. The results provide valuable insights into weak interactions and quark distribution functions.
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Study of W± bosons @ LHC energiesZ Buthelezi, iThemba LABSfor the ALICE Collaboration
Scope • Introduction • Motivation • Objectives • Status of W±→± production in pp collision @ 14 TeV • Remarks • Outlook
Introduction • W± are intermediate vectorbosons, MW = 80.398± 0.25 GeV/c2, electric charge: ±1 e, spin = 1 • Carrier particles for weak interactions can change generation of a particle (quark flavour change) • Best known for role in n beta decay to p, e and • First observed experimentally @ CERN in 1983 - Carlo Rubbia and Simon van der Meer (Nobel prize 1984)
Introduction continues…. • In pp collisions W± are produced in initial hard collisions between quarks • Lowest order process: • Highest order processes: g and initial & final state radiation
Intro continues... W± decay modes: • leptonic: l + l(10.80 ± 0.09%), + (< 8 x 10e-5 confidence level: 95%) • Electronic: e + e(10.75 ± 0.13%) • Muonic: + (10.57 ± 0.15%) • Charm: c + X( 33.4 ± 2.6%), c +(31 ± 13%) • Light unflavored meson: + (11.25 ± 0.2%) • Charmed meson: + (< 1.3 x 10e-3 confidence level: 95%)
Motivation Background study: W production in pp, PbPb & pPb collisions @ LHC energies: W± detection in the Muon Spectrometer. Z Conesa del Valle et al. Scientific motivation: • Probe PDF in the Bjorken-x range: x (10-4 – 10-3) -4.0 < y < -2.5 for Q2 ~ MW2 • Validate binary scaling, study nuclear modification of quark distribution function • W as reference for observing GQP induced effects on other probes, e.g. suppression of high pT heavy q Analysis: - Fast simulation (without ALICE detector configuration) - Rapidity (y) and pTdistributions for W→ (10.57 ± 0.15%) , W→cX→…→ ( 33.4 ± 2.6%) Ref: ALICE-INT-2006-021/01, arXiv:0712.0051v1 [hep-ph] 1 Dec 2007 and PhD thesis, 2007
Objectives • ±pT ~ MW / 2 = 30 – 50 GeV/c • Full simulation (ALICE detector configuration) in whole rapidity range and in in the ALICE Muon Spectrometer Acceptance: 2 < < 9 -4.0 < y < -2.5 • Reproduce y and pT distributions • Used dHLT to cut background
Status of W → production in pp @ 14TeV 1. Simulation • PYTHIA version 6.2, AliRoot v4-15-01 • QCD process: 2 → 1 • Decay channel: W±→± + • PDF: CTQ4L • Nevents = 500 000 • Total cross section
2. Analysis: • Differential cross section determined using eqtn by Frixione and Mangano (Ref. Hep-ph/0405130) • Muon Spectrometer Acceptance (AW) = y = 0.1, NObs = 500 000 events, (W)PYTHIA x BRW→ = 17.3 nb Note: Spectra normalised to NLO theoretical calculations: th(W)NLO x BRW→ = 20.9 nb (Ref: Lai et al, arXiv:hep-ph/9060399v2, 10 Aug 1996)
3. Results:A. W± rapidity distributions in pp @ ECMS = 14 TeV for whole rapidity range • More W+ than W-: W± are produced in by charge conservation - W+ produced by and - W- produced and BUT there are more u than d in pp, i.e. Nu~ 2Nd NW+ > NW- • W+ peaks @ yhigh while W- peaks @ ymid: In ppu valence quarks carry, on average, high amount of the proton’s incident energy (momentum) than d valence quarks
B. W±→rapidity distributions in pp @ ECMS = 14 TeV in whole rapidity range • More + than - because NW+ > NW- • +distribution is narrower than that of W+ while -distribution iswider than that of W- due - Polarization effects Parity conservation: we expect + to be emitted in valence q direction BUT …. →
W- → - Jz = -1 W+ → + Jz = +1 Jz = -1 Polarization effects in W • - will be emitted in the valence q direction, i.e. P and J are conserved. • Due violation total J conservation + will be produced in opposite direction of valence quark momentum + could be produced @ mid rapidity & - @ high rapidity W
C. Production cross section ratios in the whole rapidity range W+ / W- + / -
Projection of ALICE Muon Spectrometer cuts in W+ and W- Rapidity distributions
D. W→ pT distributions: Whole rapidity range • ± peaks @ pT = 30-40 GeV/c • m± Differential x-section is reduced by factor ~7 in the muon spectrometer acceptance range • Significant effect of muon spectrometer in the shape of pT distribution, i.e. peak has a pronounced structure to it. • m+ is produced @ a higher rate in the muon spectrometer than m- Muon+ Muon- ALICE Muon Spectrometer Acceptance: 2<<9
E. Ratio of single m+ / m- yields as functions of pT • m+ / m- is higher in the muon spectrometer acceptance than in the whole rapidity range. • At pT < 40 GeV/c the m+ / m- is consistently below 1.5 in the whole rapidity range and @ pT > 40 GeV/c it increases from 1.5 up 3.4 • In the Muon spectrometer Acceptance the m+ / m- is constantly below 2 at pT up to 20 GeV/c and slowly increases to ~ 10 at pT > 30 GeV/c
Remarks: • W+ generation is greatly favoured in pp collisions due to Nu > Nd. • Charge asymmetry on W • W parity violation has an important effect in the m+ and m- rapidity distributions • Single ± pT distribution is significantly reduced in the ALICE Muon Spectrometer Acceptance need to double statistics for efficient dHLT analysis.
Outlook 1. pp collisions • W→ c + X → …→ + Y • Generate pT distribution for different channels • dHLT analysis: cuts @ 2 GeV/c < pT < 20 GeV/c 2. Pb – Pb collisions @ 5.5 TeV • W→ + • W→ c + X → …→ + Y