W-boson production in association with jets at the ATLAS experiment at LHC

Transcription

1 W-boson production in association with jets at the ATLAS experiment at LHC Seth Zenz Qualifying Examination Talk January (Introductory section only; modified slightly for public distribution.)

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 2

3 ATLAS detector General-purpose detector for LHC collisions Trigger rate: 200 Hz 7000 tons, 44m long, collaborating physicists Concentric detectors for differentiating between high transverse momentum objects 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: cm 2 s 1 High luminosity: cm 2 s 1 z Beams along z 14 January 2008 S. Zenz 3 θ η = -ln(tan θ ) 2 (~rapidity)

4 ATLAS Sub-Detectors Transverse slice beams into/out of page φ 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 14 January 2008 S. Zenz 4

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 uarks Energy in hadronic and electromagnetic calorimeters May or may not have associated tracks Missing Transverse Energy (E T ): Use calorimeters, muon system, and conservation of momentum to determine total energy of noninteracting particle(s), e.g. neutrino, in the transverse plane 14 January 2008 S. Zenz 5

6 Parton distribution function f ( x) dx Probability of uark or gluon being found with momentum fraction between x and x+dx Valence uarks ~ ½ proton momentum Also gluons, sea uarks p X Proton constituents in high-pt scattering x 2 p x 1 X Y 14 January 2008 S. Zenz 6

7 Cross sections at the At right, total cross sections at the Tevatron and LHC Total cross section and jet crosssections are large compared to interesting physics LHC 14 January 2008 S. Zenz 7

8 The W Boson n n W * p e ν e p e ν e 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/c 2 Mass arises from SU(2)xU(1) breaking; simplest explanation is the Higgs Mechanism 14 January 2008 S. Zenz 8

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 uark and anti-uark 67% of the time 14 January 2008 S. Zenz 9

10 p p ' W Production and decay g + W e + 14 January 2008 S. Zenz 10 ν e (with jets) W bosons are produced via uark-antiuark interaction Limit ourselves to electron decay for now; muons also possible QCD background much too large to detect W uarks Gluons (and uark pairs) may also be radiated Fairly high probability, since α QCD ~ 0.1 High-momentum uarks 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 All particles except neutrino detected

11 ATLAS Trigger System Challenge: 4x10 7 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 11

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 uark production Decays to W boson plus b jet If one W from a top uark 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+jets background 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 12

13 New Physics (e.g. Supersymmetry) ~ g ~ g ~ R ~ L Jets χ ~ 2 χ ~0 1 ET e + ν e + W } + ET χ ~0 1 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 uark partner and standard model particle, which in turn decays into another uark plus another particle, and so on The lightest of the new particles is often stable (good for Dark Matter) missing energy 14 January 2008 S. Zenz 13

14 Goals of this analysis Present W+jet rate and related uantities 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 uantities that experimentalists can connect directly to data, and theorists can connect to models without knowledge of the detector 14 January 2008 S. Zenz 14