w-boson production in association with jets at the atlas ...€¦ · lhc and atlas • large hadron...
TRANSCRIPT
W-boson production in
association with jets at the
ATLAS experiment at LHC
Seth Zenz
Qualifying Examination Talk
January 14 2008
(Introductory section only; modified slightly for public distribution.)
14 January 2008 S. Zenz 2
Outline
• LHC and ATLAS
– Subdetectors and physics objects
• High-momentum scattering
• W boson
– History
– Production and decay
• W+jets and the ATLAS physics program
14 January 2008 S. Zenz 3
LHC and ATLAS• Large Hadron Collider (LHC)
– Proton-proton collider at CERN
– 27 km in diameter
– Center-of-mass energy 14 TeV
– Time between collisions: 25 ns
– Low luminosity: 1033 cm–2s–1
– High luminosity: 1034 cm–2s–1
• ATLAS detector
– General-purpose detector for
LHC collisions
– Trigger rate: 200 Hz
– 7000 tons, 44m long, 2000+
collaborating physicists
– Concentric detectors for
differentiating between high
transverse momentum objects
z
θ
)2
-ln(tanθη =
(~rapidity)
Beams
along z
14 January 2008 S. Zenz 4
ATLAS Sub-Detectors• Inner Detector
– Measures track curvature in 2T B field to give charged particle momentum
– -2.5 < η < 2.5
• Electromagnetic calorimeter– Absorbes and measures
electromagnetic energy– Absorbs mostly electrons and
photons– Hermetic to contain missing
energy (-5.0 < η < 5.0)
• Hadronic Calorimeter– Absorbs and measures hadronic
energy– Protons, neutrons, pions, kaons– Hermetic to contain missing
energy (-5.0 < η < 5.0)
• Muon system– Toroidal magnetic field– Detects and measures
momentum of muons – only interacting stable particle that passes through calorimeters
φ
Transverse slice – beams into/out of page
14 January 2008 S. Zenz 5
Objects in this analysis
• Electrons: energy deposited in EM calorimeter
with associated isolated track
• Hadronic jets: collection of stable hadrons
produced as a manifestation of outgoing quarks
– Energy in hadronic and electromagnetic calorimeters
– May or may not have associated tracks
• Missing Transverse Energy (ET): Use
calorimeters, muon system, and conservation of
momentum to determine total energy of non-
interacting particle(s), e.g. neutrino, in the
transverse plane
14 January 2008 S. Zenz 6
• Parton distribution function– Probability of quark or gluon being
found with momentum fraction between x and x+dx
– Valence quarks ~ ½ proton momentum
– Also gluons, sea quarks
–
Proton constituents
in high-pT scattering
dxxf )(
Y
p
p
2x
1x
X
X
14 January 2008 S. Zenz 7
Cross sections at the
LHC• At right, total cross
sections at the Tevatron and LHC
• Total cross section and jet cross-sections are large compared to interesting physics
14 January 2008 S. Zenz 8
The W Boson
• W boson initially postulated as a charged analogue to the photon, which would account for β-decay
• Weinberg-Salam SU(2)xU(1) model of the electroweak interactions allowed prediction of W (and Z) mass and other properties from already-known weak interactions– Weak force is weak because W is very
heavy: ~80.4 GeV/c2
– Mass arises from SU(2)xU(1) breaking; simplest explanation is the Higgs Mechanism
pn
e−
ν e
e−
ν e
W*−
pn
14 January 2008 S. Zenz 9
W Discovery
• Found at SppS at CERN in 1983 with precisely the predicted properties– Signature: High transverse
momentum isolated lepton, plus missing energy
• Decays to each species of lepton 11% of the time, to quark and anti-quark 67% of the time
14 January 2008 S. Zenz 10
W Production and decay
q
g
q
+W e+
ν e
p
p
'q
q
(with jets)• W bosons are produced via
quark-antiquark interaction
– Limit ourselves to electron decay for now; muons also possible
– QCD background much too large to detect W �quarks
• High-momentum quarks and gluons hadronize, producing separate hadronic jets
– Non-perturbative phenomenon
– Can only be modeled, not calculated
– No theoretically-rigorous prescription is known for separating radiation and hadronization; this introduces uncertainty in W + jet cross section calculations
• Gluons (and quark pairs) may also be radiated
– Fairly high probability, since αQCD ~ 0.1
• All particles except neutrino detected
14 January 2008 S. Zenz 11
ATLAS Trigger System
• Challenge: 4x107 beam crossings / sec � 200 events / sec on tape
• Three stage trigger system to identify physically interesting events – First stage is “on-detector”, identifying regions of interest
– Second stage is on computer farms
– Third stage uses reconstruction code; final decision to record the event made within seconds
• Most events are low-pT
• Jet cross-section also very large, which is why I need to look for isolated leptons in order to identify the W
• Trigger simulation is not incorporated in this talk, but I would use the 15 GeV isolated electron trigger which will (probably) be included in early running
14 January 2008 S. Zenz 12
W+Jets and the ATLAS
physics program• Production of W boson with jets produced by Quantum
Chromodynamics is a background to several ATLAS measurements– Top quark production
• Decays to W boson plus b jet
• If one W from a top quark pair decays to an electron and neutrino, while the other decays to jets, the signal is an electron, four jets, and missing energy
– Beyond-the-Standard-Model (e.g. Supersymmetry)– Higgs physics
• These signals are much larger compared to the W+jetsbackground than at the Tevatron
• W+jets cross section is also a test of perturbative QCD and hadronization models– Presence of W guarantees high momentum transfer
(perturbative)
14 January 2008 S. Zenz 13
New Physics
(e.g. Supersymmetry)
• Many models of new physics have cascade decays into jets, leptons and missing energy
• Some new symmetry (e.g. sypersymmetry) implies partners for all Standard Model particles
• Partner of gluon decays into quark partner and standard model particle, which in turn decays into another quark plus another particle, and so on
• The lightest of the new particles is often stable (good for Dark Matter) � missing energy
g~
q~L
q~R
q
χ~0
1
χ~2
+ ET
Jets
χ~0
1
g~
q
+W
e+
ν e
}
ET
14 January 2008 S. Zenz 14
Goals of this analysis
• Present W+jet rate and related quantities detector-independent way• Quantities:
– Rate for W + n or more jets, for n = 0,1,2,… as a function of minimum jet transverse energy (ET), and σ(W+≥(n+1) jets)/ σ(W+≥n jets)
– ET rate of leading and second jet
• Detector-independence– Correct for object reconstruction efficiency and fake rates
– Correct jet ET to truth jet level, i.e. the ET that would be measured by reconstructing a jet using all stable particles
– Minimize extrapolation based on theory and parton distribution functions• Report cross sections only for η range of detector and minimum jet energy
• Do not correct jet energy to parton level
– Goal is to have quantities that experimentalists can connect directly to data, and theorists can connect to models without knowledge of the detector