# Presentation: 661018

## Overview of Top Quarkand (Other) Standard Model PhysicsJohn ParsonsNevis Labs, Columbia UniversityUS ATLAS Software Mtg, BNL, August 28/2003

- ATLAS

## Why study the top quark?

**The top quark (still) exists!**
**m(top) is a fundamental parameter of the SM (and plays role in rad. corr’s)**
**want precision measurement ****	****	**
**large value of m(top) makes top quark unique**
**top is a sensitive window through which to look for new physics**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Top Quark Pair Production

**NLO prediction of ********(t-tbar)**** at LHC is ****833 pb****, which is ******** 100X larger than at FNAL**
**Producing 8 million t-tbar pairs per low lumi year, ****LHC will be a top factory**
**Production processes at LHC: **
**         ****90% gg + 10% q-qbar****   **
**                            ****(opposite at FNAL)**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Top Quark Event Yields

**NLO Xsect for t-tbar production = ****833 pb**
**8 million t-tbar pairs produced per 10 fb****-1**
**Example cuts to select ****single lepton (e/********) plus jets**** final state:**
**pT(lep) > 20 GeV,    pTmiss > 20 GeV**
**≥ ****4 jets with pT(j) > 40 GeV, incl. ≥ 2 jets with b-tag**
**selection effic. = 5% ******** ****126k events, with S/B = 65**
**Reconstruct t ******** Wb ******** (jj)b**
**In ****± 35 GeV window around mtop, have:**
**30k signal events**
**14k bkgnd events ****(dominated by t-tbar combo. bkgnd)**
**Example cuts to select ****dilepton**** final state:**
**pT(lep1, lep2) > 35, 25 GeV,    pTmiss > 40 GeV**
**≥ ****2 jets with pT(j) > 25 GeV**
**80k events, with S/B = 10**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Initial (t-tbar) Measurement

**Analysis of 100k DC1 events with no b-tagging info.**
**	****(< 2 days of running at 1E33 !!)**
**W ******** jj****	****	****	**** t ******** Wb ******** (jj)b **
**Fit of m(jjb) spectrum provides Xsect measurement with stat. error ******** 7%**
**Even with no b-tagging, can measure ********(t-tbar) to < 10% with few days of integrated luminosity at 1E33**
**Could be first sign of new physics at LHC??**
**Sample will be important for jet scale calibration and b-tagging commissioning**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Top Mass Measurement

**Top Mass Measurement**

**Fundamental parameter of SM**
**Precision measurement helps**
**constrain m(H) or over-constrain**
**SM once m(H) known**
**Many different samples and techniques have been investigated**
**Inclusive single lepton plus jets**
**High pT single lepton plus jets**
**Dileptons**
**All jets mode**
**Sample with J/******
**Stat. error negligible after few days!**
**Individual contributions to syst. errors < 1 GeV**

**Variety of signals and measurements provide valuable cross-checks for controlling overall error**
**Results so far collected into Scientific Note**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Leptonic final state with a J/

-  Correlation between
-   m(t) and m(lJ/)
-  Require a non-iso 
-   inside b-jet
-  BR=BR(ppttXb+lb)xBR(bJ/)xBR(b)=3.2x10-5
-  Channel can be used only with high L (2700 evts/year)
**Selection:** 1 iso lepton, pT > 30 GeV, || < 2.4
-       3 non-iso muons, pT > 3 GeV, || < 2.4, m() = m(lJ/)
-             ~16%  430 evts/ year at high L
-  **Background: **W/Z +jets, WW,WZ.   Wbb,  ~12%  S/B=55

****

**J/******

- 

- l

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Leptonic final state with a J/

-  m(lJ/) distributions for several
-   m(t) from MC compared with data
-  m(lJ/) ~0.58 m(t)
-  mstat(lJ/) ~0.5 GeV  (5y/high L)
- mstat(t) ~ 0.8-0.9 Gev
**               ****Uncertainties:** not sensitive to jet E
- 	          scale, ISR,PDF,b-quark fragmentation
- 	          msys(lJ/) ~0.5 GeV -> m(t)sys < 0.9 GeV
-                       **Strong pT(t) dependence of m(lJ/********)**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## 3 separate production processes: 

	   W-gluon fusion (250 pb)    W*/s-channel (10 pb)		Wt (60 pb)

ATLAS studies have shown:
Can measure cross-sections for all 3 processes separately
important since each is sensitive to different kinds of possible new physics
For 30 fb-1, can measure Vtb with stat. error of 0.4% – 2.7% (dep. on process)
For W-gluon fusion, can measure predicted W and top helicity
sensitive to possible V+A contribution at level of few per cent

More studies of sensitivity to possible new physics need to be done

**	****   ****W-gluon fusion (250 pb)    W*/s-channel (10 pb)****	****	****Wt (60 pb)**
**ATLAS studies have shown:**
**Can measure cross-sections for all 3 processes separately**
**important since each is sensitive to different kinds of possible new physics**
**For 30 fb****-1****, can measure Vtb with stat. error of 0.4% – 2.7% (dep. on process)**
**For W-gluon fusion, can measure predicted W and top helicity**
**sensitive to possible V+A contribution at level of few per cent**
**More studies of sensitivity to possible new physics need to be done**

**Electroweak Single Top Quark Production**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## trigger on isolated lepton with pT > 20 GeV

selection cuts
Njet  = 2, with pT > 30 GeV
Nbjet = 2 with pT > 75 GeV  
N(forward jet) = 1, pT > 50 GeV, | | > 2.5
Sum pt > 175 GeV (lepton plus jets)
Mtotal > 200 GeV (lepton plus jets)

after all cuts, have (for 30 fb-1)
1.1k signal events (effic. = 1.7%)
2.4k bkgnd events (mostly ttbar, plus Wg fusion single top, Wjj)

ie. S/B   0.46, S/B  23 

		 Vtb (stat.)  2.7%

**selection cuts**
**Njet  = 2, with p****T**** > 30 GeV**
**Nbjet = 2 with p****T**** > 75 GeV  **
**N(forward jet) = 1, p****T**** > 50 GeV, |******** | > 2.5**
**Sum pt > 175 GeV (lepton plus jets)**
**Mtotal > 200 GeV (lepton plus jets)**
**after all cuts, have (for 30 fb****-1****)**
**1.1k signal events (effic. = 1.7%)**
**2.4k bkgnd events (mostly ttbar, plus Wg fusion single top, Wjj)**
**ie. ****S/B  ******** 0.46, S/********B ******** 23 **
**	****	******** ********Vtb (stat.) ******** 2.7%**

**Example Analysis for W* Process**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Top Quark Couplings and Decays

**need to determine whether top quark behaves as described in the SM**
**Electric charge****	****	**
**Can use bjet charge tagging, or look at t tbar ******** events**** **
**Yukawa coupling can be measured to < 20% from t-tbar H production**
**according to SM, top decays are rather “uninteresting”**
**Br(t ********Wb) ******** 99.9%**
**Br(t ******** Ws) ******** 0.1%, Br(t ******** Wd) ******** 0.01%****	****	****(tough to measure!)**
**Many Beyond SM models involve anomalous top couplings**
**Several possible rare decay modes (eg. FCNC) have clear experiment signatures and, if observed at the LHC, would be evidence for new physics**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Top Quark Rare Decays

**In SM, FCNC top decays are **_**highly**_** suppressed ****	****(Br < 10****-13 ****- 10****-10****)**
**Several models of Physics Beyond SM can give HUGE enhancements**
**Sensitivities according to ATLAS studies:**
**t ******** Zq**** ****	**** ****	****	****(CDF Br < 33% @ 95%CL)**** ****	**
**Reconstruct t ******** Zq  ******** (l****+****l****-****)j **
**Sensitive to Br(t ******** Zq) = 1 X 10****-4****  ****	****(100 fb****-1****)**
**t ******** ********q**** ****	**** ****	****	****(CDF Br < 3.2% @ 95%CL)**** ****	**
**Sensitive to Br(t ******** ********q) = 1 X 10****-4****  ****	****(100 fb****-1****) **
**t ******** gq**** ****	**** ****	****	****	**
**Decay process overwhelmed by QCD bkgnd**
**Search instead for “like-sign” top production (ie. tt)**
**Sensitive to Br(t ******** gq) = 7 X 10****-3****  ****	****(100 fb****-1****)**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Top Quark Rare Decays (cont’d)

**t ******** Hq**
**Tblisi group has been studying various approaches to ****t ******** Hq**** **
**Earlier results for ****t tbar ******** Hq Wb ******** (b-bbar)j (l********b) ****for mH = 115 GeV**
**Sensitive to Br(t ******** Hq) = 4.5 X 10****-3**** ****	****(100 fb****-1****)**
**New results for ****t tbar ******** Hq Wb ******** WW*q Wb ******** (l******** l********j) (l********b)**
**≥ ****3 isolated leptons with ****pT(lep) > 30 GeV**
**pTmiss > 45 GeV**
**≥ ****2 jets with pT(j) > 30 GeV, incl. ≥ 1 jet with b-tag**
**kinematic cuts to take advantage of angular and**
**other correlations**
**Sensitive to Br(t ******** Hq) = 2.4 X 10****-3**
**	**** ****for mH = 160 GeV ****	****(100 fb****-1****)**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Top Quark and Exotica

**Due to its large mass, top is likely candidate to couple strongly to new massive particles**
**Clear experimental signature and ability to reconstruct top also make it a useful “tool” for studying exotica**
**Some examples include:**
**Resonances decaying to t tbar  ****(sensitive to few TeV)**
**Mass resolution ******** 6%**
**“****Heavy top” (T) in ****Little Higgs models****	**
**Top signatures in ****extra dimension models****	**

**Reconstructed M(tt) [GeV]**

**Reconstructed M(tt) [GeV]**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Heavy T in Little Higgs Models

**Main production mechanism is similar to “single top” production**

- Pair prod.

- Single T

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## T  tZ Analysis

- t

- T

- Z

- 1 TeV

*b*

- l

- 

- W

_*Cuts*_*:*
* **3 isolated leptons*
* **(2 of them with M*l l= MZ)
-  1 b –jet
-  ETmiss > 100 GeV
- Bkgnd (tZ, WZ) under study, but should be small

- M(T) = 1 TeV

**M(****tZ****)  (GeV)**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## T  th Analysis

**M(****th****)  (GeV)**

- h

*b*

- T

- t

- 1 TeV

- W

*b*

- _Signature:_

- 3 *b*-jets
- + 1 lepton
- + ETmiss

- l

- 

_*Cuts*_*:*
* *pT(3-jets) > 90 GeV
* *pT(lepton) > 70 GeV
-  100 < Mh < 140 GeV
- At least 1 b-tag
- Bkgnd due to t-tbar, W bb

- MT= 0.7, 1.0, 1.3  TeV

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## t-tbar Spin Correlations

**SM predicts substantial t-tbar spin correlations**
**Dilepton case has been studied, using the t-tbar rest frame angles of the  charged leptons as probe of correlations**
**Studies underway to determine sensitivity to various sources of anomalous couplings:**
**CP violation**
**New Higgs**
**Graviton effects in models with extra dimensions**

**SM prediction**

**No correlations**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Some Top Physics Topics for Further Studies

**Putting “more meat on the bones” **
**DC samples of fully simulated events**
**Impact of initial (staged) detector layout**
**Improving level of understanding of detector performance and commissioning both for and using top samples**
**Study of high pT isolated electrons and muons**
**Calibration of jet energy scale from W ******** jj **
**Measurement of missing E****T**
**Optimization and efficiency measurement for b-tagging**
**Triggering schemes and efficiency determinations**
**We must be ready for Day 1  (t-tbar production ~ 1 Hz @ 1E33!)**
**Single top sensitivity to new physics**
**Existence of W’, V+A, other anomalous couplings, ... **
**Impact of new physics on spin correlations**
**CP violation, models with extra dimensions, ... **

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Scope of ATLAS SM Group     (M. Dobbs, LBNL)

**heavy ion collisions**

**W mass**

**triple gauge-boson couplings**

**sin****2 ****θ****W**

**tau physics**

**Standard Model Working Group**
- born December 2001
**encompasses all SM physics except:**
- bottom
- top
- Higgs
**	****for which there are separate WG’s.**
**nominally includes heavy ions**

**Min. bias & **
**	****Underlying Event**

**Parton Density Functions**

**Q****C****D ****Dynamics**

**Strong Coupling Constant**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Triple Gauge-Boson Couplings

**TGC tests gauge structure of SM and provides sensitive probe for new physics at higher scale**
**can measure charged TGC couplings through ****WZ, Wγ**** production**
**Statistics will dominate LHC measurements (except for Δ g****1****)**
**sensitivity derived from a few events in the high P****T****(V) tail**

| -0.0035 < λγ < +0.0035 |
| --- |
| -0.0073 < λZ < +0.0073 |
| -0.075 < Δκγ < +0.076 |
| -0.11 < ΔκZ < +0.12 |
| -0.0086 < Δg1Z < 0.011  |

**95% Confidence Intervals**
**For  30 fb****-1****, systematics included.**
** ****typically factor 10 improvement over LEP/TeVatron**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Triple Gauge-Boson Couplings (cont’d)

**probe neutral couplings with ZZ, Zγ production**
**factor 10****3**** improvement**
**	****over LEP limits**

**Test for this coupling which **
**is Forbidden in the SM**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Precision EW: Measuring sin2θW with AFB

**pp******** l****+****l****-****    di-lepton signature is (almost) background free**
**asymmetry arises from interference between neutral currents**
**Statistical precision using 100 fb****-1****, near Z-pole (±6GeV)**
**Performance issue:**
**increasing forward lepton tagging acceptance greatly improves measurement**
****** *****ideal study for persons involved with FCAL, or e/γ performance group***

| Cuts | AFB (%) | Δ AFB (%) | Δ sin2θeff(MZ) |
| --- | --- | --- | --- |
| Both e±, |η|<2.5 | 0.774 | 0.020 | 0.00066 |
| One e±, |η|<2.5
 other e±,|η|<4.9 | 1.98 | 0.018 | 0.00014 |

- for comparison, **Δ**** sin****2****θ****eff****=0.00053** combining 4 **LEP** expts and e,μ,τ channels [CERN-EP/2001-098]

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Precision EW: W Mass Measurement

**60 million well measured W→*****l*****ν****  events per low L year**

| Source | CDF  Run Ib | ATLAS | W→ l ν , one lepton species |
| --- | --- | --- | --- |
|  | 30K evts, 84 pb-1 | 60M evts, 10fb-1 |  |
| Statistics | 65 MeV | < 2 MeV |  |
| Lepton scale | 75 MeV | 15 MeV | most serious challenge |
| Energy resolution | 25 MeV | 5 MeV | known to 1.5% from Z peak |
| Recoil model | 33 MeV | 5 MeV | scales with Z statistics |
| W width | 10 MeV | 7 MeV | ∆ГW≈30 MeV   (Run II) |
| PDF | 15 MeV | 10 MeV | LHC data further constrains |
| Radiative decays | 20 MeV | <10 MeV | (improved Theory calc) |
| PT(W) | 45 MeV | 5 MeV | PT(Z) from data, 
PT(W)/ PT(Z) from theory |
| Background | 5 MeV | 5 MeV |  |
| TOTAL | 113 MeV | ≤ 25 MeV  | Per expt, per lepton species |

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Some Examples of QCD Measurements

**Some Examples of QCD Measurements**

**LHC allows measurement to very large energy scales**
**Can use, for example, to measure the running of the strong coupling constant over a wide range of scales**
**with the single inclusive jet cross section**

**Other observables for measuring α****S**** have not yet been studied: **
**ratio of γ+jet to 2 jet production (Frixione)**
**ratio of 2 to 3 jet cross sections**
**ratio of  W+1 jet  to  W+0 jet  cross sections**
**jet shape observables**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Some Hot SM Topics for New Studies

**ideal topics will have impact on performance issues and utilize Data Challenges. **
**Some examples include:**
**Measuring Parton Density Functions**
**in particular the gluon **
**Measurement of the strong coupling constant**
** ****many channels not yet covered**
**W,Z production via vector boson fusion**
**performance issue: forward jet tagging (already much experience from Higgs group)**
**requires fancy Monte Carlos (Madison series)**
**Tau physics**
**Br( W********τ ν ) **
**limits on flavour violating τ********μμμ, & τ********μ γ**
**revisit Z********e****+****e****-****, forward backward asymmetry**
**Performance issue: tagging of electrons in forward region**
**DC samples already requested.**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Summary and Conclusions

**LHC will be a ****top quark factory**
**Studies of the top quark provide a ****sensitive window onto possible new physics beyond the SM**
**SM measurements at the LHC will be extended in precision (eg. TGC) and into new kinematic regimes**
**Some of the earliest ATLAS physics results, and earliest sensitivity to new physics, will come from top and SM physics**
**eg. ****can measure ********(t tbar) with stat. error < 10% with a few days of lumi**
**Early top/SM signals will also be critical to ****commissioning the detector**
**Study of high pT isolated electrons and muons**
**Calibration of jet energy scale **
**Measurement of missing E****T**
**Optimization and efficiency measurement for b-tagging**
**There is a lot of work to do, and volunteers are anxiously sought**
**Contact myself (top) or Matt Dobbs (SM) to get involved**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Backup Slides

## Dilepton channel 

** ****Indirect measurement of m(t)**
** ****For the TDR, correlations between **
**     ****some kinematical distributions **
**     ****(e.g.: m(lb) and m(t))**
** ****Also, trying to reconstruct the **entire event
- Selection:
-     **2 iso leptons, pT > 25, 35 GeV, |********| < 2.5 **
**      ****EmissT > 40 GeV****	****	****	****    ****	**
**      ****2 b-jets with pT > 25 GeV , |********| < 2.5**
- Background:
**    ****Negligible**

**80000 evts/year**
**at low L, S/B =10**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Single lepton channel 

**Clean sample, one top  **
**  ****directly reconstructed**
**Methods:   **
**	****	****	****- Hadronic part reconstruction**
**	****	****	****- Kinematic fit of the entire event**
**	****	****	****- Continuous jet definition**
**	****	****	****- High pT top sample**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Single lepton channel 

- Selection
- -   1 iso lepton, pT > 20 GeV, || < 2.5
- pTmiss > 20 GeV
- ≥ 4 jets with pT > 40 GeV, || < 2.5
- ≥ 2 jets with b-tag
**Selection effic. = 5% ******** ****126k events, with S/B  ~65**
**Reconstruct t ******** Wb ******** (jj)b**
**In ****± 35 GeV window around m(top), have:**
**30k signal events**
**14k bkgnd events ****(dominated by “wrong combinations” from t-tbar events)**

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

##  Reconstruction of the hadronic part

- W from jet pair with the closest invariant
- mass to m(W)
- Cut on |mjj-mW| < 20 GeV
- Association of  W with a b-tagged-jet
-  Cut on |mjjb-<mjjb>| < 20 GeV
-   _**Kinematic fit**_
- Using remaining l+b-jet, the leptonic
-  part is reconstructed
-  |mlb-<mjjb>| < 35 GeV
-  Kinematic fit to the ttbar hypothesis,
-  with m(top) and m(W) constraints
- Energy and direction of all objects
- (jets, l, ) can vary within their resolution

-  = 11,2 GeV

-  = 6.2 GeV

- m(jj)

- m(jjb)

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001

## Statistical Error

| Period
 | tt events |
| --- | --- |
| 1 year | 8x106 |
| 1 month | 2x106 |
| 1 week | 5x105 |

- In the **single lepton channel**, where we plan to measure m(top) with the best precision:

| Period | evts | Mtop(stat) |
| --- | --- | --- |
| 1 year | 3x105 | 0.1 GeV |
| 1 month | 7.5x104 | 0.2 GeV |
| 1 week | 1.9x103 | 0.4 GeV |

- L = 1x1033 cm-2s-1

- C. de La Taille         7th conference on LHC electronics   Stockholm

- 12 sept 2001