# Presentation: 935115

## Physics at Hadron CollidersSelected Topics: Lecture 3

**Boaz Klima**
**Fermilab**
**9****th**** Vietnam School of Physics**
**Dec. 30, 2002 – Jan. 11, 2003**
**Hue, Vietnam**
**http://d0server1.fnal.gov/users/klima/Vietnam/Hue/Lecture_3.pdf**

## 3 generations of fundamental fermions

**3 generations of fundamental fermions**

**leptons  q = 1****	****	****e **** ****	******** ****	******** ****	****     q = 0****	****	********e ****	**** ************	**** **********
**quarks  q = ****2****/****3****	****u****	****c****	****t****	****    q = –****1****/****3****	****d****	****s****	****b****	**
**The top quark was discovered at Fermilab in 1995**** **
**(and the tau neutrino was directly observed for the first time in 2000)**

**1975, Perl et al. **PRL 35, 1489 (1975)

**1977, Herb et al. **PRL 39, 252 (1977)
**No flavor changing neutral currents:**
**b must have a weak isospin partner = top**

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## Searches for top

**Direct searches**
**Indirect mass****determinations****as a function****of time**

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## Top quark production

**Top-antitop quark pair production**
**Single top quark production**

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## Top quark decays

**Standard Model (with m****t**** > m****W**** + m****b****)**
**expect t ******** Wb to dominate**

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- Boaz Klima (Fermilab)

## Event selection

**Requirements:**
**High p****T**** Leptons (leptonic W decay)**
**Large Missing E****T**** (neutrinos)**
**3 or more Jets with large E****T**
**Jets from b-quarks**
**Soft lepton tagged b-jets**
**Jet tagged in the SVX**
**Run I**
**~ 120 pb****-1****= handful of********tt events (********100/ experiment)**
**Run IIa : 2 fb****-1**
**Run IIb : 15 fb ****-1**

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## Top quark properties

**Top quark properties**

***p***

***p***

- Mass

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## Top Quark Mass

**Fundamental parameter of Standard Model (SM)**
**Affects predictions of SM via radiative corrections:**
**BB mixing**
**W and Z mass**
**measurements of M****W****, m****t**** constrain M****H**
**Large mass of top quark **
**Yukawa coupling ******** 1**
**may provide clues about electroweak symmetry breaking **

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## Lepton + Jets Channel

**1 unknown (*****p******z*********)**
**3 constraints**
***m*****(*****l***********) = *****m*****(*****qq*****) = *****m******W***
***m*****(*****l************b*****) = *****m*****(*****qqb*****)**
**2-constraint kinematic fit**
**up to 24-fold combinatoric ****ambiguity**
**compare to MC to measure *****m*****t**

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## Complications

**Combinatorics:**
**4 possible j**l****** pairings**
**there are 12 possible assignments of the 4 jets to the 4 quarks (bbqq) **
**only 6 if one of the jets is b-tagged**
**only 2 for events with double b-tagged jets**
**Gluon radiation can add extra jets **

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## Combinatorics

**Monte Carlo tests:**
**shaded plots show ****correct combinations**
**     ****(Herwig MC, ****m****t**** = 175 GeV)**
**The width and shape of the fitted mass distribution is due primarily to **
**jet combinatorics**
**QCD radiation**
**Double b-tag helps... but, too few events in RunI**

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## Basic Procedure

** ****In a sample of tt candidate events**
**For each candidate make a ****measurement of X = f(m****t****), ****where X is a suitable estimator ****for the top mass**
** ****e.g. result of the kinematic fit  **
**This distribution contains****signal and background.**
**From MC determine shape of X as ****a  function of m****t**
**Determine shape of X for ****background (MC & data). **
**Add these together and compare ****with data**

**likelihood  fit for m****t.**

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## Lepton + Jets channel (DØ)

***m*****t**** = 173.3±5.6±5.5 GeV**

- dominated by
- jet energy scale and
- gluon radiation

**Background-rich sample**

**Signal-rich sample**

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## Lepton + Jets channel (CDF)

**176.1±5.1±5.3 GeV**

**Signal-rich sample**

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## All-hadronic channel (CDF)

**Large background**
**3-constraint kinematic fit**

***m*****t**** = 186.0±10.0±5.7 GeV**

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## Dilepton Channel

**dilepton channel:**
**6 particles in the final state**
**We measure 14 quantities:**
**2 charged leptons**
**2jets **
**p****x****(********)+p****x****(********)**
**p****y****(********)+p****y****(********) **
**4 unknowns **
**only *********p******T********* known**
**3 constraints**
**underconstrained**
**dynamical likelihood analysis**

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## Dilepton channel

**Two different strategies ...**
**Find a kinematic variable with a strong mass dependence, OR, **_**calculate a weight as a function of m**__**t**_
**Assume m****t**
**18 degrees of freedom,**
** ****14 known quantities + 4 constraints**
**t and t momenta can be determined**
**Assign a weight **
**using parton distribution functions and lepton p****T****’s (an extension of ideas proposed by Kondo and Dalitz & Goldstein)   **
**Characterizes how likely this event originates from a top quark of mass m****t**
**Weight curve is determined for each assumed value of m****t **** for a given event**

- K. Kondo, J. Phys. Soc. Jpn. 57, 4126 (1988) and 60, 836 (1991).
-  R.H. Dalitz and G.R. Goldstein, Phys. Rev. D 51, 4763 (1995).

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## DØ dilepton sample weight curves

**For each event:**
**Assume a ****value of m****t**
**Reconstruct ****event**
**Calculate weight**
**Repeat for all m****t**
**Combine events**
**Fit the resulting****distribution to MC****samples as before**

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## DØ

**m****t**** =168.4±12.3±3.6 GeV**

**m****t**** = 167.4±10.3±4.8 GeV**

**DØ**

**CDF**

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## Top Quark mass

**150**

**160**

**170**

**180**

**190**

**200**

**m**

**t**

** ****(GeV)**

**168.4 ******** 12.8 GeV**

**173.3 ******** 7.8 GeV**

**172.1 ******** 7.1 GeV**

**167.4 ******** 11.4 GeV**

**176.1 ******** 7.4 GeV**

**186.0 ******** 11.5 GeV**

**176.1 ******** 6.6 GeV**

**174.3 ******** 5.1 GeV**

**D0 ll PRL 80, 2063 (1998)**

**D0 lj PRD 58, 52001 (1998)**

**D0 combined**

**CDF ll PRL 82, 271 (1999)**

**CDF lj PRL 80 2767 (1998)**

**CDF jj PRL 79, 1992 (1997)**

**CDF combined**

**Tevatron FERMILAB-TM-2084**

**Top Quark mass**

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**Top quark mass in lepton + jets channel using a method similar to the one used to measure it in the dilepton channel**
**Each event has its **_**own probability distribution**_
**The probability depends on (almost) **_**all measured quantities**_
**Each event’s contribution **_**depends on how well it is measured**_
**M****t****= ****179.9 ******** 3.6  ******** ****6.0**** ****GeV**
**Improvement in statistical error is equivalent to running the Tevatron for two more years...**

- New Result – presented for the first time at HCP (Karlsruhe, Sept. ’02)

**3.6**

- Was 5.6 GeV in publication

**Late Breaking News!!!**

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**M**w** can be measured in the same events where M**t** is measured!**
**Might be used for reducing the uncertainty in the jet energy scale, which is the single most dominant systematic one (****6** **to** **3-4 GeV?****)**

**Stay tuned...**

**conclusion - be efficient and be smart!**

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## Top quark production properties

**Top quark production properties**

***p***

***p***

- Production Cross Section
- Production Kinematics
- Resonance Production
- Spin Correlations

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## Cross Section

**Why?**
**test of QCD predictions**
**Any discrepancy indicates possible new physics:**
**production via a high mass intermediate state**
**Non Wb decay models**
**How?**
**Measurement performed using various final states**
**Dilepton channels**
**ee, e********, ******** and e******** final states**
**Lepton + jets channels**
**e+jets, ********+jets**
- topological analysis
- b-tagging (using soft leptons from semileptonic decays of the b)
**All-jets channel**
**Use topological variables**
**exploit b-tagging using soft leptons**
- Combine them using a Neural network technique

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## Cross Section Event Samples

**Dilepton channel: ee, e********, ******** + 2 jets + missing p****T**
**Lepton + jets channels: e, ******** + 3 or 4 jets + missing p****T**
**All jets: 5 or 6 jets, b-tag, neural networks**

- 11 events in common

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## Results

- D:  PRL _79_ 1203 (1997); CDF: PRL _80_ 2773 (1998)(+updates)

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## Cross section vs. mt

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## Prospects for Run IIa (2 fb-1)

**Precision on top cross section ~8%**
**Statistical Error : 4%**
**Systematic errors assumed to scale with statistics**** **
**errors from backgrounds:  decrease with increased statistics of control samples (2%)**
**jet energy scale (2%)**
**Radiation : **
- Initial state (2%),
- Final state (1%)
**Limiting Factors ?**
**error on geometric and kinematic acceptances  depend on differences between generators (Pythia, Herwig, Isajet) (4%)**
** ****luminosity error (4%)**

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## Constraints on the Higgs Mass

**m****W ****=  80.450****±****0.034 GeV (CDF, D****Ø, LEP2)**
**SM predictions for *****m******W***
**Degrassi *****etal*****, PL B418, 209 (1998)**
**Degrassi, Gambino, Sirlin, PL B394, 188 (1997)**

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## Mass: future prospects

**Lepton+jets channel: **
**	****	****	****	****	****	****	****	**
**Combine with dilepton channel (smaller systematics) **
**Improvements:**
**calibrate jet p****T**** scale using data**
**Z+jet, ********+jet, W********jj, Z********bb **
**constrain gluon radiation effects in MC with data**
**Maybe use stringent cuts to reduce effects of hard gluon radiation...**
**double b-tag ******** reduce combinatorics**
**Total uncertainty ******** 2-3 GeV  (per experiment)**
**combined with W mass uncertainty ******** 40 MeV **
****** ****constrain Higgs mass to 80%**

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- Boaz Klima (Fermilab)

## Single top production

**Electroweak process:**
**SM cross sections**
******(********pp ******** Wg******** t+X) =  1.7********0.2 pb ****	****(Stelzer et al.)**
******(********pp ******** W********* t+X) =  0.72********0.04 pb ****	****(Smith et al.)**
**direct access to Wtb vertex: measure top quark width and |V****tb****|**
******(qq ******** tb) ******** ******** (t ******** W+b) ******** |V****tb****|****2**
**Measure CKM element |V****tb****| without any assumptions on number  of generations**
**probe of anomalous couplings **
**large production rates**
**anomalous angular distributions**

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## Single top production

**Event topology**
**W decay products (lepton+neutrino) plus:**
**for the s-channel  (W*) process:**
**Two high P****T****, central b-jets**
**or for the t-channel (Wg) process:**
**One high P****T**** central b-jet (from top)**
**One soft, central b-jet **
**One high P****T****, forward light quark jet**
**Backgrounds:**
**Top pair production, W+jets,  multijets**
**Ability to extract signal depends on**
**b-tagging efficiency**
**fake lepton and fake b-quark jet reconstruction rates**
**Desirable to separately measure the two processes**
**different systematic errors for V****tb**** **
**different sensitivities to new physics**
**measure W and top helicities**
**sensitivity to V+A, anomalous couplings, CP violation etc**

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## DØ Run I search

**Search using 92 pb****-1**** data from Run I for s and t channel production of single top quarks**
**Optimize  S/********B for best significance **

**s channel: ******** < 39 pb  at 95% CL**
**t channel: ******** < 58 pb  at 95% CL**

** ****Not enough sensitivity during Run I to see a signal **

**D****Ø**** preliminary**

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## CDF Run I search

**Cross section ******** < 13.5 pb at 95% CL**

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## Single top prospects

**Efforts underway to improve DØ Run 1 analysis to increase efficiency and purity by using ****Neural Networks**** to discriminate between signal and background**
**Different backgrounds have very different kinematic properties.**
**Train networks (********20) for each background and signal type combination separately**
**Preliminary study:   **
**16% efficiency for signal**
**0.7% background survives**
**Application to D0 RunI data soon.**
**Run II:**
**Using 2 fb****-1****, expect to measure  **
******(qq ******** tb) to ******** 20%**
****** ****(t ******** W+b) to ******** 25%**
**V****tb**** to ********12%**
**Note single top will be a background for Higgs searches and many new physics signatures**

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## tt spin correlations

**Standard Model predicts:**
**90% of top quark pairs produced at ********s=1.8 TeV come from********qq annihilation via ****spin-1**** ****gluon  ******** source of spin correlation**
**width ******** t**** = ********(t ********bW) ******** 1.55 GeV**
**lifetime ******** ******** 4 ******** 10****-25**** s**
**QCD time scale 1/********QCD**** ******** few ******** 10****-24**** s**
**Top quark decays before losing the spin information at production**
**spin can be reconstructed, as decay products carry spin information.**
**Motivation**
**Experimental proof that top-quark lifetime is shorter than spin-decorrelation time scale**
**Lower bound on ********t**** and |V****tb****|**
**Direct probing of the properties of quark, free of QCD long-distance effects**
**Probe for non-standard interactions, both in production and decay**

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## Spin Configurations

**Optimal spin quantization basis is ****off-diagonal**
**Basis is determined once the velocity and scattering angle is known**
**Only like-spin combinations are produced in this optimized basis**
- G. Mahlon and S. Parke,  PLB 411, 173 (1997)

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## Decay of polarized top quark

**Differential decay rate of top quark with spin fully aligned:**
**To find the direction of spin:**
**measure angle between the off-diagonal basis and the **_**lepton **_**flight direction in  rest frame of the top ********-****, ********+**
**Spin correlation ******** correlation in ********+**** vs. ********-**** space**

**correlation parameter**
**SM value ******** 0.9**

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## Angular correlations

**No spin**
**correlation**
******=0**

**Off-diagonal**
**basis**
******=0.87**

**Helicity basis**
******=0.40**

**Beamline basis**
******=0.68**

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## DØ spin correlation analysis

**DØ spin correlation analysis**

**6 dilepton events (that’s all we have!); use binned 2D likelihood fit**

- > -0.25 @ 68% CL

**Fermilab-Pub-00/046-E**

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## Prospects for Run II

**Based on ensemble tests of 150 dilepton events (1.5 fb****-1****)**
**likelihood****	****	****	****             probability density estimator****	****	**
**One can distinguish ********=0 from ********=1 at greater than 2******

**RMS=0.42**

**RMS=0.43**

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## Another tool for investigating non-standard production mechanisms

Good agreement with QCD prediction

**Good agreement with QCD prediction**

**CDF preliminary**

**Top Quark Transverse Momentum**

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## tt resonances

**A general search for heavy objects decaying to top pairs**
**Predicted (for example) in dynamical models of Electroweak Symmetry Breaking where the “Higgs” is a bound state**
**color octet resonances ********tt **
**mass ******** several hundred GeV**
**Technicolor  **
** ****gg ********T******** (tt, gg)**
**Topcolor      **
**qq ******** V****8******** (tt, bb)**
****** ****peak  in********tt invariant mass**

**D****Ø**

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## tt resonances

**Use W+ ******** 4jet events**
**No evidence for a deviation from expectation (KS prob ********20%)**
**Use tt invariant mass spectrum to set limits on  narrow Z’ resonances in topcolor models**
**With 2fb****-1****, can probe Z’  resonances up to 1 TeV****.**

**CDF**

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## Top Quark Decay Properties

- Decay modes
- Branching ratios
- CKM matrix element |Vtb|
- Rare decays
- Non-SM decays

- W helicity

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## W helicity in top decays

**W helicity in top decays**

**Top quarks decay before they hadronize**
**polarization of W :  **
**non-standard top couplings may result in different W polarization**
**Charged lepton p****T**** &**
**angular distribution**
**              ******
**Longitudinal**** W vs.**
**Left Handed**** W’s**
******t********bW****long****) ******** 5%**

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## W helicity in top decays

**SM top (spin 1⁄2, V-A coupling)**
**top quark decays to longitudinal (h****W****=0) or left-handed (h****W****=-1) ****W bosons**
**Lepton p****T**** distribution in *****t**********bl*********** distinguishes the two helicity states.**
**h****W****= 0: hard p****T**
**h****W****=-1: soft p****T**
**Check for V+A component**
**F****+1**** determined by repeating fit with F****0**** constrained to SM value**
**should be zero in SM**

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## CDF analysis in Run I

**F****0****= 0.91 ± 0.37 (stat) ± 0.12 (sys)**
**F****+1****= 0.11 ± 0.15 (stat) ± 0.06 (sys)**
**CDF, PRL 84, 216 (2000)**

** ****Use dilepton and lepton+jets tt samples:**

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## |Vtb|

**|V****tb****| expected to be close to 1 (********, assuming 3 generations **
**if 4****th**** generation exists ********no constraints**
**Any departure of |V****tb****| from 1 ******** indication of non standard physics**
**Extract from**
**Measure R using b-tagging in********tt decays**
**Count events with zero, single and double tags in in l+jets and dilepton events.**
**CDF (RunI):  measure R = 0.94 +0.31/-0.24**
**|V****tb****| = 0.97+0.16/-0.12  or |V****tb****|  > 0.75 at 95% C.L.**
- Assuming 3 generations
**|V****tb****| > 0.046 at 95% CL**
- Without the 3 generation hypothesis
**Run II projections:  ********V****tb**** ********2% (with 2 fb****-1****) **
**benefits from improvements in b-tagging efficiency and reduced systematic errors**

- R  =

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## Rare decays: SM and beyond

_**Within the Standard Model**_
***t********** Wb + g*****/******
***t********** Wb + Z            ******	******	*****Near kinematic threshold***** ***** **
***t********** Wb + H******0******         ******	******	*****Beyond threshold**
***t********** W + s/d          ******	******	*****Measure CKM matrix element**
_**Beyond the SM**__**	**_** ****	****Run II sensitivity**
***t********** c/u + g*****/*********    (FCNC) ******	******< 1.4% / 0.3%***
***t********** c/u + Z       (FCNC)***** ****	****< 2%**
***t********** c/u + H******0******     (FCNC)***
**	****SM predictions for  FCNC decays ******** 10****-10**
**	****Observation of these decays would signal new physics**
***t********** H******+****** + b         (SUSY)******	****** < 11%***
**Current limits on rare decays (CDF)**
**BR(*****t**********Zq*****) < 33% @ 95% CL**
**BR(*****t****************q*****) < 3.2% @ 95% CL**
**Search for t********H****+****b (D****Ø****)**
**      **
**          **

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## Top Quark Yukawa Coupling

**In the SM, fermions acquire mass via Yukawa couplings to Higgs field (free parameters in the SM but set proportional to the fermion mass)**
** ****for the top quark**
**Large value of m****t**** has generated proposals for alternate mechanisms (e.g. topcolor) in which top plays a role in EW symmetry breaking**
**    ****	****	****A direct measurement of y****t**** is of extreme interest!**
**Measure y****t****  via associated Higgs production (ttH):**
**for m****H**** ******** 130 GeV, H ********bb is the dominant decay**
**look for events with W(********l********)W(********jj)+4b-jets**
**A recent feasibility study finds it may be possible to carry out this measurement at the Tevatron with large data samples in RunII **
**Goldstein et al., hep-ph/0006311**

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