Detection of W bosons in the ALICE Muon Spectrometer

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Detection of W bosons in the ALICE Muon
Spectrometer

• Zaida Conesa del ValleSUBATECH, Nantes
  (France)Torino, 14 May 2005
• International School on QGP and HIC past,
  present, future

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Outlook

• Motivation and detection method
• W Production Physics Processes
• LO and NLO processes
• Individual quark contribution
• Differences pp - PbPb collisions
• W Pythia Generation Results
• pp collisions at 14 TeV
• PbPb collisions at 5.5 TeV
• Background Studies
• Charm and Beauty analysis in pp collisions at
  14TeV
• Conclusions and perspectives

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Motivation and detection method

• Why?
• Intrinsic interest to observe, measure and study
  W? bosons
• W? are produced in initial hard collisions they
  do not interact with surrounding medium
• Evaluate Glauber model validity
• Study the influence of effects as energy loss of
  heavy quarks
• How?
• Through their muonic decay
• W ? ? ?? W- ? ?- ???
• Where?
• In the ALICE Muon Arm Spectrometer

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ALICE Experiment
ALICE Experiment
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ALICE Muon Spectrometer
ALICE Experiment
Dipole Magnet
Trigger Chambers
Absorber
Tracking Chambers
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W Production Physics processes
W? Generation Process

• Leading Order diagram
• Higher Order diagrams ? ?
  ? ?
• ? ? ?
• Whats their contribution to cross section?
  ?12 Frixione Mangano (hep-ph/0405130)

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W Production Physics processes
W? Generation Process

• Leading Order Decay
• Quarks contribution to W? production
• LHC Energies, MW Bjorken-x (y0) ?
  710-3 ? Valence ? Sea quarks
• Charge conservationW
• W-
• Valence quarks contribution in pp PbPb
  collisions
• pp collisions p(uud) ? 2 x 2u d
  valence quarks ? More W than W- could be
  produced
• PbPb collisions p(uud) n(udd) ? 2 x (2Z
  N)u (Z 2N)d valence quarks

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W Generation with Pythia
W? Generation Process

• pp Collisions
• Pythia itself generates pp collisions at given
• PbPb Collisions
• Pythia does not take into account that Pb nuclei
  has n and p, and that their quark constituents
  are different
• On the other hand, pythia is able to make
  difference between p and n, and can generate pp,
  nn, pn np collisions
• We decided to generate PbPb collisions as a
  combination of weighted pp, nn, np pn
  collisions

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W distributions at pp Collisions
W? Pythia Generation Results

• pp 14 TeV

• pp 5.5 TeV

More W than W- are produced ? theres more u
than d valence quarks W are peaked at
high-rapidity and W- are peaked at mid-rapidity ?
u valence quarks carry almost 30 of the proton
energy When CMS energy decreases, W distributions
become narrower
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Muons from W at pp Collisions
W? Pythia Generation Results
Pythia Values BR?? ?W (14TeV) ? 17. nb ?W
(5.5TeV) ? 5.8 nb
? 510.000 ?? /run in the ALICE IP
pp 14 TeV
pp 5.5 TeV
Acceptance -4.0 lt ? lt -2.5 p gt 4 GeV/c pT gt 1
GeV/c
? 14 ? 71.000 ?? /runin the Muon Spectrometer
Acceptance
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W at pp, pn, np and nn Collisions
W? Pythia Generation Results
pp 5.5 TeV
nn 5.5 TeV
W distributions at pp (pn) collisions are
similar to those of W- at nn (np) collisions, and
vice versa ? these are effects of the proton and
neutron valence quarks constituents
pn 5.5 TeV
np 5.5 TeV
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Muons from W at pp, pn, np and nn Coll.
W? Pythia Generation Results
pp 5.5 TeV
nn 5.5 TeV
? distributions are narrower and more peaked
than those of ?- ? these could be due to spin
correlation effects of the weak interaction (W-?
polarization)
pn 5.5 TeV
np 5.5 TeV
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Comparison of pp, nn, pn np Coll.
W? Pythia Generation Results

• Muons from W decay distributions
• ? distributions are narrower and more peaked
  than those of ?-
• This could be due to spin correlation effects in
  weak interaction (W? ?? polarization)We know
  that neutrinos are left-handed and anti-neutrinos
  are right-handed? ? from W decay (W ? ? ??)
  have to be right-handed, and ?- have to be
  left-handed
• Then, muons are polarized!The same argument can
  be used to observe that also W? are polarized,
  cause W only couples with left-handed quarks and
  right-handed anti-quarks. Their polarization
  gives us a plain explanation for the different
  behaviour in ?? rapidity distributions.

s 1
s 1
Right handed
Left handed
Right handed
Left handed
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W at PbPb Collisions
W? Pythia Generation Results

• PbPb at 5.5 TeV

? 125.000 ?? /run in the ALICE IP
Acceptance? 10.8 13.000 ?? /run
BR?? ?W (5.5TeV) ? 5.8 nb
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Beauty and Charm Production
Background Studies

• Quarkonia production Processes
• Lowest Order processes
• Gluon fusion
• qqbar annihilation
• Higher Order processes
• Gluon splitting
• Flavor excitation
• Gluon radiation
• Quarkonia spectra
• Single muons from Charm and Beauty are the most
  important source of muon background for W studies
  (at high pT)

R. Guernane et al ALICE Note
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Composed Charm, Beauty and W Spectra
Background Studies

• Single muon spectra from charm, beauty and W
  pythia generation

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Composed Charm, Beauty and W Spectra
Background Studies

• ? in Acceptance

• ?- in Acceptance

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Conclusions and perspectives

• Remarks
• W production in numbers
• In order to generate W at pp PbPb collisions we
  have to bear in mind
• Valence quark composition of the colliding
  nuclei
• Nature of the weak-interaction spin correlation
  effects (polarization effects)
• Beauty and Charm production are the main source
  of single muon background in the studied pT
  range.
• Beauty and Charm production contribution to
  single muon cross section in the analyzed pT
  range is small compared to those of Ws.
• Forthcoming tasks
• Normalize Spectra to phenomenological predictions
• Realistic response function of the spectrometer
  (slow simulation)
• Study of the influence of effects as energy loss
  of heavy quarks