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1 Electroweak Physics Lecture 4. 2 Physics Menu for Today Top quark and W boson properties at the Tevatron.

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Presentation on theme: "1 Electroweak Physics Lecture 4. 2 Physics Menu for Today Top quark and W boson properties at the Tevatron."— Presentation transcript:

1 1 Electroweak Physics Lecture 4

2 2 Physics Menu for Today Top quark and W boson properties at the Tevatron

3 3 Hadron-Hadron Collisions fragmentation parton distribution parton distribution Jet Underlying event Photon, W, Z, t, H etc. ISR FSR Hard scattering

4 4 Physics at Hadron Colliders Since hadron colliders collide composite objects – the extraction of the physics is often ''messy'' and not straight-forward. Need to understand: –underlying event, multiple interactions –proliferation of QCD radiation –high event rates Places a premium on: –real-time triggering (selection of interesting events) –accurate detectors with some redundancy –understanding QCD

5 5 Life at a Hadron Collider What happens when two hadrons collide: 1.~ 25% ELASTIC collisions – hadrons change direction/momenta but there is no energy loss : dull ! 2.~ 75% INELASTIC collisions – one or both of the hadrons have a change in energy and direction : rate ~ 1/Q 4 : Q is energy transfer – mostly dull ! In a collider we have bunches of hadrons circulating the accelerator –each bunch contains ~ 10 11 protons (anti-protons ~10 9 ) We can have more than one collision as the bunches pass through each other at the interaction region : ''Multiple Interaction'' 30  m 15cm BUNCH : 10 11 P: BUNCH

6 6 A typical (interesting) event For EWK physics: Try to extract the information about the subprocess

7 7 Hard Subprocesses The hadrons (protons and anti-protons) are made of quarks and gluons The momentum distribution of the quarks and gluons as a function of Feynman x: Effective energy of the collision: E cm x 1 x 2 –Not known on an event by event basis To make predictions (to compare with the Lagrangian) we need to know about the x distribution of the quarks and gluons. Parton Density Functions –This is known (to some precision) from lepton-nucleon experiments

8 8 Hard Subprocesses Three possible hard scattering processes: –qq: quark-quark, quark-antiquark, antiquark-antiquark –qg: quark-gluon, antiquark-gluon –gg: gluon-gluon at the Tevatron (2 TeV) quark-antiquark is dominant at the LHC (14 TeV) gluon-gluon is dominant … the LHC is really a gluon-gluon collider !

9 9 To relate what we want to know to what we want to measure define ''luminosity functions'' to determine what the important partonic sub-processes will be. - this is where HERA measurements are vital Knowledge of PDFs is Vital! PDFs = Particle Density Functions How many quarks and gluons are in proton and how much of each

10 10 Remember the TeVATRON! At Fermilab Proton anti-proton collider –Run 1 from 1987 to 1995: √s=1.8 TeV –Run 2 from 2000 to 2009: √s=1.96 TeV Two experiments: CDF and DØ

11 11 Current integrated luminosity: 1500 pb −1 Current Analysis: up to 400 pb −1 Analysis with 800 pb −1 underway Run II Luminosity

12 12 When protons & (anti-)protons collide Physics at proton collider is like… Drinking from a firehose –At TeVATRON: 1 collision every 396ns –1 to 2 interactions per collision Panning for gold –W, Z, top are rare events! –Need high luminosity –Use high momentum muons and electrons to select interesting events Collision Energy

13 13 Vital at hadron collider eg: - b quark was discovered with one b event per 10 10 collisions - top quark was discovered with one top per 10 12 collisions! by comparison, this is trivial at a lepton collider Needle in a haystack moving at 186,000 miles per second... 75 Hz Tape Robot ~ few Tb / day disks... CHALLENGES - ensuring high trigger efficiency & retaining purity - knowing what the trigger efficiency is (use pass-through triggers and rely on pre-scaled triggers with lower thresholds) Rejection factor of 1:20,000 after level-2 L1 : hardware 5 kHz 375 Hz L2 : firmware L3 : software 7.5 MHz Triggering at the Tevatron

14 14 Electroweak Lagrangian Important for M W Important for m top

15 15 Electroweak Lagrangian Higgs couples to all fermions in proportion to their mass Important for m top

16 16 Electroweak Lagrangian Higgs couples to W and Z WWH vertex ZZH vertex Important for M W

17 17 Electroweak Lagrangian Higgs quartic coupling to W and Z WWHH vertex ZZHH vertex

18 18 Putting it all Together W mass predicated in EWK Lagrangian –Corrections from interactions with Higgs boson and top quark Top corrections important for many processes –including those from LEP –Need accurate measurement of top quark mass to make comparisons between theory and experiment Top is by far the heaviest fundamental particle known (~175 GeV/c²) –Same scale as W & Z: it may offer insights into the nature of electroweak symmetry breaking (Higgs mechanism) –doesn’t have time to hadronise

19 19 Theory Experiment Now we know what physics to expect, let’s make some measurements For that we need…

20 20 Detector Coordinates Polar Angle: θ φ

21 21 CDF in Real Life Central tracking  η   1.0 Muon Chambers  η   1.5 Central+Plug Calorimetery  η   3.6 Silicon tracking  η   2.0

22 22 Transverse Quantities Colliding partons have small momentum transverse to beam We detect all interactions transverse to the beam Missing E T direction Any “missing momentum” in x,y plane is attributed to the neutrino –Or other non-interacting particles eg neutralinos –Transverse momentum:

23 23 An easy example: reconstructing Z → ℓ + ℓ − p T (μ + ) = 54.8 GeV/c p T (μ − ) = 39.2 GeV/c M(μ + μ − ) = 93.4 GeV/c² Select events with –2 leptons, –Opposite charge –momentum transverse to beam, p T >20 GeV/c 66 < M(ℓ + ℓ − )/GeVc −2 < 116

24 24 W Mass Current best single measurement of M W is ±58MeV World Average: (80.425±0.038) GeV/c²

25 25 Extracting W Mass Value of M W is sensitive to P T of lepton and Missing-E T The combination of both quantities in the Transverse Mass (M T ) has best sensitivity to M W Generate lots of MC samples with different M W Fit each one to date to find test M W value Transverse Mass of muon and neutrino. Invariant mass only using components of the momentum transverse to the beam

26 26 Largest W Mass Systematics How well do we understand energy scale of calorimeter? –Use Z → e+e− to calibrate detector How well do we understand hadronic recoil –Effects resolution of missing E T

27 27 Current & Predicted W Mass Measurements No Run II measurement yet! CDF Expected error for 200pb −1 is ±76 MeV/c²

28 28 Top Quark Production Main mode for top quark production at Tevatron is through two quarks fusing to form a gluon, which decays into top-antitop Gluon-gluon fusion too –All QCD production, no EWK involved Cross section decreases as m top increases Predicted cross section for m top =175 GeV/c²: (6.23-6.82) nb

29 29 Top Quark Decays CKM matrix: top decays 99% of the time into b-quark and W. Two tops: two b-quark jets + 2W –Two lepton channel Easy to identify  Small cross section  MET from 2 neutrinos –Lepton+jets 30% of cross section Only 1 neutrino –All jet channel  v. hard to reconstructed masses

30 30 Top Event in the Detector 2 jets from W decay 2 b-jets ℓ ± ν ℓ

31 31 Top Event Reconstruction

32 32 Tevatron Summary Hadron Colliders are great for discovery of new particles Need to use a trigger to select useful events Can also be used for precision physics: –Need to understand PDFs of colliding hadrons CDF and DØ have extensive physics programme Aim measure: –m top ±2.5 GeV/c 2 –M W to ±40 MeV/c 2 –Probably can do better –Other EWK tests possible too!

33 33 Backups: Other Tevatron EWK Measurements

34 34 Measurements of W and Z Cross Sections Test of QCD more than of EWK theory

35 35 A FB for pp → Z → e+e− Inverse of A fb measurement made at LEP At LEP e+e− θ is angle between incoming e− beam and outgoing f. At Tevatron: How do we choose θ *? P Z =0 Z0/*Z0/* Z0/*Z0/* Z-Axis lab frame Collin-Soper Frame

36 36 CDF results: A FB for pp → Z → e+e−

37 37 A FB for pp → Z → e+e− At Z pole measurement is sensitive to A f and V f couplings Measurement is not competitive with LEP results

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