Study of W± bosons @ LHC energies Z Buthelezi, iThemba LABS for the ALICE Collaboration

Scope Introduction Motivation Objectives Status of W± →± production in pp collision @ 14 TeV Remarks Outlook

Introduction W± are intermediate vector bosons, 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) . W bosons are best known for their role in beta decay of a neutron into a proton, electron and anti-neutrino. . The fact that they have mass while other Gauge bosons e.g. photons are massless electromagnetism by U(1)) came about The Unified theory of electromagnetism and weak interaction the Glashow- Weinberg-Salam model (Nobel Prize 1979) and Higgs mechanism which postulated the necessity of W bosons to explain beta decay. Its existence was confirmed by the Buble Chamber Experiment at CERN

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 In the LO approximation gluon do not take part in the production of W. The most contribution to W+ total  is from the u-d scattering and about 17% from c-s scattering while other coupling contribute ~ 1%. For W- the contribution of c-s and other couplings is ~ 23% and 3% respectively at LHC energies.

 +  (< 8 x 10e-5 confidence level: 95%) 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%) In pp collisions @ LHC energies

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 pT distributions 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 In QCD parton model the structure functions fi (x,Q^2)dx is defined by the PDF. The quantity fi(x,Q^2)dx is the probability that a parton/quark of type i carries a momentum fraction between x and x+dx of the nucleon's momentum in a frame where the nucleon's momentum is large.

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)

Scale variations for LO, NLO & NNLO in: 2/M  M  2 M,

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 pp u valence quarks carry, on average, high amount of the proton’s incident energy (momentum) than d valence quarks HUMPS in W+ and flat mid rapidity cross section for W-: at high y W± are produced from initial valence q-interactions.

Predicted LO in pp → W + X @ LHC total for pp -> W+- + X @ LHC u + anti-d dominant c + anti-s ~ 17% (W+) and 23% (W-)‏ Other contributions ~ 1% (3%) from other coupling

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 is wider than that of W- due - Polarization effects Parity conservation: we expect + to be emitted in valence q direction BUT …. → Weak interactions only couple left-handed quarks with right-handed quarks → W± will be polarized in the direction of the anti-quark momentum. In pp W± will tend to polarize in the opposite direction to its momentum.

Polarization effects in W 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 Jz = -1 Jz = +1 W+ → +  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→  + 