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

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

3. Introduction
W± are intermediate vector bosons, MW = ± 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 CERN in 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

4. 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.

5.  +  (< 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 LHC energies

6. Motivation Background study:
W production in pp, PbPb & pPb 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 /01, arXiv: v1 [hep-ph] 1 Dec 2007 and PhD thesis, 2007

7. 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

8. Status of W → production in pp @ 14TeV
1. Simulation
PYTHIA version 6.2, AliRoot v
QCD process: 2 → 1
Decay channel: W± → ± + 
PDF: CTQ4L
Nevents =
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.

9. 2. Analysis:
Differential cross section determined using eqtn by Frixione and Mangano
(Ref. Hep-ph/ )
Muon Spectrometer Acceptance (AW) = y = 0.1, NObs = 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/ v2, 10 Aug 1996)

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

11. 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+ yhigh while W- 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.

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

13. 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.

14. 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 mid rapidity & high rapidity
Jz = -1
Jz = +1
W+ → +  W

15. C. Production cross section ratios in the whole rapidity range
W+ / W-
+ / -

16. Projection of ALICE Muon Spectrometer cuts in W+ and W- Rapidity distributions

17. D. W→ pT distributions:
Whole rapidity range
± pT = 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 a higher rate in the muon spectrometer than m-
Muon+
Muon-
ALICE Muon Spectrometer Acceptance: 2<<9

18. 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 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

19. 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.

20. Outlook 1. pp collisions W→ c + X → …→  + Y
Generate pT distribution for different channels
dHLT analysis: 2 GeV/c < pT < 20 GeV/c
2. Pb – Pb 5.5 TeV
W→  + 