w-boson production in association with jets at the atlas ...€¦ · lhc and atlas • large hadron...

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


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


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


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


• 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


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


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


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


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


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


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)


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~

qq

q

+W

e+

ν e

}

ET


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

w-boson production in association with jets at the atlas ...€¦ · lhc and atlas • large hadron...

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