1. PH599 Graduate Seminar presents: Discovery of Top Quark Karen Chen Stony Brook University November 1, 2010

2. Karen Chen2 Abstract  The third generation of quarks was predicted by Kobayashi and Maskawa to explain CP violation. When the bottom quark was discovered, the search to find its isospin partner began. The discovery of the top quark completes the family of six quarks in the Standard Model. Measurements of the top quark mass were conducted by the CDF and D0 experiments at the Fermilab Tevatron. The top quark mass was measured from events consistent with top pair production. The top pairs decay into a pair of bottom quarks and a pair of W bosons with a nearly 100% branching ratio. The experiments looked at events that result in either dilepton or lepton plus jets final states. It is possible for the W bosons to both decay into quarks but measurements based on events with all jets have low precision. More precise mass measurements were conducted after the top quark’s initial discovery. The experimental uncertainty associated with the top quark and W boson mass puts constraints on the mass of the Higgs boson. The high precision of the top quark mass may have important implications on the validity of the Standard Model.

3. Karen Chen3 Outline  Discovery 3 rd generation quarks to explain CP violation  Direct Measurement of Top Quark Mass Experiments at the Tevatron, detector basics Decay of top pairs  Possible final states: Dilepton, l+jets Event Selection  Signatures of signal and background processes Likelihood fits for m t  Top Quark Mass Relevance Today Constraints on Higgs mass

4. Karen Chen4 Discovery of the top quark  1964 - CP violation found in kaons  1973 – (Cabibbo), Kobayashi and Maskawa Need a third generation of quarks to explain CP violation  1977 – Bottom quark discovered! And thus begins the search for its isospin partner, the top quark

5. Karen Chen5 Discovery of the top quark  Tevatron – particle accelerator at Fermilab  Two experiments: Collider Detector at Fermilab D0 (or DZero)  Proton, antiproton collisions  CDF and D0 measured the top quark mass

6. Karen Chen6 Measurement of Top Quark Mass  Invariant Mass, m = m(E,p) E 2 = (pc) 2 + (mc 2 ) 2 Or more conveniently*:  m 2 = E 2 – p 2 = P 2, where P is four vector momentum,  P 2 = E 2 – p 2 = E 2 - p x 2 - p y 2 - p z 2  Example of a two body decay A  B+C m A 2 = (P B + P C ) 2  To find top quark mass, we need the energy and momentum of the decay products. Note: It is convenient to measure everything in units of GeV, so c is set to 1.

7. Karen Chen7

8. 8 Decays of top pairs  Top pair decay with branching ratio ~100%  W decayBranching ratios W  e ~1/9 W   ~1/9 W   ~1/9 W  qq ~2/3

9. Karen Chen9 Decays of top pairs  Top pair decay with branching ratio ~100%  Final decay products Both W’s decay into leptons  tt->bb + ll “Dilepton final state” One decays into a lepton, the other into quarks  tt->bb + l + qq“Lepton + jets” Both W’s decay into quarks  tt->bb + qqqq“All Jets”, “fully hadronic”

10. Karen Chen10 Decays of top pairs  What are “jets?”  Why don’t you see a single quark?  Quark confinement Gravity, EM ~ 1/r 2 Strong force increases with distance! 1. Collision produces quarks 2. Energy grows with distance 3. More quarks are created 4. Can combine to form hadrons

11. Karen Chen11 Measurement of Top Quark Mass  Dilepton (~4.5%) Muons or electrons Pure signal, low yield  l+jets (~30%) Moderate yield and bg  All jets (~44.5%) Large backgrounds  Tau channels (~21%) At least one W decays into a  Hard to identify  decays Short lifetime, hadronize quickly  Tevatron Average: m t = 173.1 ± 0.6 (stat.) ± 1.1(syst.) GeV/c 2 Hobbs et al.

12. Karen Chen12 Background sources  Signal (Dilepton)  Both have final states of ee pair and two jets.  What’s the difference? Neutrinos appear in the signal process. Problem: Neutrinos are weakly interacting, we can’t really see them!  Background (Diboson)

13. Karen Chen13 Background discrimination: E T  Total E T = 0, Missing E T must be from neutrinos.  Dilepton: E T > 35GeV  l+jets: E T > 15GeV Abazov 2009

14. Karen Chen14 Background sources  Signal  Both have W boson pair, so E T may be the same.  What’s the difference? The signal has two bottom quarks. You expect more jets in the signal than in the background. Can you check if the jet is from a bottom quark or a lighter quark?  Background

15. Karen Chen15 b-tagging  b-tagging efficiency ~50% per jet  Misidentification P(b-tag|q) = 1% P(b-tag|c) = 15% t t W+W+ W-W- C b  ~10 -12 s  ~10 -13 s Vertex is farther b-tagged jet

16. Karen Chen16 Background discrimination: # jets ≥ http://www-d0.fnal.gov/ l + jets final state with one b-tagged jet Dilepton final state Expect 2 jetsExpect 4 jets

17. Karen Chen17 Background discrimination: # jets CDF detector crack at η = 1.1 http://www-cdf.fnal.gov/physics/new/top/2004/jets/cdfpublic.html η = -ln(tan(θ/2)) θ: azimuthal angle from beam line

18. Karen Chen18 Measurement of Top Quark Mass  At this point: Have candidate tt events (using E T, b-tagging, and other cuts) with lowered background  Few unknowns Neutrino momentum Jet combinatorics  What you can do: A mix of template method and weighing methods that depend on the kinematic observables to determine a best fit for m t.

19. Karen Chen19 Measurement of Top Quark Mass  Example: l+jets  Consider jet combinatorics under these constraints: Likelihood function as a function of jet energy scale and M t. Energy = [1+ JES ] f(s) JES = 0 -> perfect calibration Hobbs et al.

20. Karen Chen20 Constraints on Higgs mass  Higgs boson explains why weak force carriers, W and Z, are massive.  The mass of the top quark is HUGE! compared to other elementary particles.  Higgs mass is related to W boson and top mass M W ~ log(M H ) M W ~ M t 2

21. Karen Chen21 Constraints on Higgs mass P. Renton 1995 Results from 1995Results from 2006 Heinemeyer et al., 2006

22. Karen Chen22 Summary  The main decay channel used for top quark mass measurement:  With appropriate cuts (E T, # of jets, b tagging), you can increase the purity of the tt signal.  Mass measurement of top quark was done with likelihood fits of m t using a combination of template and weighting methods.  Precision of top quark and W boson mass puts constraints on Higgs mass.

23. Karen Chen23 References  M. Kobayashi, T. Maskawa (1973). "CP-Violation in the Renormalizable Theory of Weak Interaction". Progress of Theoretical Physics 49 (2): 652– 657.  P Renton, arXiv:hep-ph/0206231v2 1 Aug 2002  P. Renton, Review of Experimental Results on Precision Tests of Electroweak Theories, Lepton-Photon 95, p35 (1995), published by World Scientific.  P. Renton, arXiv:0809.4566v1 [hep-ph] 26 Sep 2008  S. Heinemeyer, W. Hollik, D. St ockinger, A.M. Weber, G. Weiglein, arXiv:hep-ph/0604147v2 10 Oct 2006  V. Abazov et al. Measurement of the top quark mass in final states with two leptons. Phys. Rev., D80:092006,2009.  http://www-d0.fnal.gov/Run2Physics/WWW/results/summary.htm  John D. Hobbs, Mark S. Neubauer, Scott Willenbrock, Tests of the Standard Electroweak Model at the Energy Frontier, arXiv:1003.5733, 2010.