# Presentation: 506769

## The Top Quark

- Cecilia E. Gerber
- UIC
- Feb 20, 2002

## What is the World Made of?

- Anaximenes of Miletus (6BC)

**ELEMENTARY CONSTITUENTS**

**Water**

**Fire**

**Earth**

**Air**

**INTERACTIONS**

- “All forms of Matter are obtained from rarifying Air”

-  Simple: few constituents and interactions.
-  Wrong: No experimental confirmation.

## What is the World Made of?

- Standard Model  (~1970 AC)

**ELEMENTARY CONSTITUENTS**

**INTERACTIONS**

- Electromagnetic     10-2

- Strong                    1

- Weak		     10-6

**H**

- Higgs

- Gravity 	    10-40

## Worldwide discoveries that led to the Standard Model

-  up/down	1968	SLAC		1990 Nobel Prize
-  strange	1964	BNL  		1980 Nobel Prize
-  charm		1974    SLAC/BNL	1976 Nobel Prize
-  bottom	1977	Fermilab
-  Top		1995	D0/CDF
- Photon		1905 	Planck/Einstein	  1918/1921 Nobel Prizes
-  Gluon		1979     DESY
-  W/Z		1983	CERN		  1984 Nobel Prize

- QUARKS

- Force Carriers

## Worldwide discoveries that led to the Standard Model (cont.)

## The Fermilab Tevatron Accelerator

**1992-96**
**   ****Run 1: 100pb****-1****, 1.8TeV**
**2001-2007(?)**
**   ****Run 2: ~15fb****-1****, 1.96TeV**

**Next in line: CERN LHC**
**~2007 (pp) ****14TeV**

**p anti-p collider:**

## How do we produce particles?

- Accelerate and collide (anti)protons: the collision takes place between the constituents:

## Proton-anti Proton Collision

- Small x, products boosted along beam direction

- Large x, can create massive objects that decay to secondaries with large momentum component transverse to the beam

- For every proton there is a the probability that a single quark (or gluon)  carries a fraction “x*”* of the proton momentum

- Good way of telling that a hard collision occurred.

## A generic HEP detector

## Slide 10

## The D0 and CDF detectors at Fermilab

## D0 Detector

## Slide 13

**Notes:**
550 physicists
>75 institutions
18 countries (50% US)

## Top physics at the Tevatron collider

- Top spin
- polarization

- Cross section

- Production kinematics

- Top-antitop
-  resonance states

- Top mass

**Physics beyond**
**SM?**

## Top quarks are not yet well understood

- Discovered in 1995 by D0 and CDF at Fermilab Run 1 (1992-1996)
- Integrated Luminosity ~120pb-1
-  <100 t-tbar events per experiment
- Everything we know about the top comes from these events!
- Cross Section = 5.9+-1.7 pb (D0, PRL79,1203,1997)
- Mass = 174.3+-5.1 GeV (D0/CDF, Fermilab-TM-2084)
-  Run 2 started March 1, 2001. Expect to double the Run 1 data set by the end of the year.

**Notes:**
5 dilepton
11 lepton+jet b-tag
19 lepton+jets topological
41 all jets
4 e-nu (e+jets or e+mu or e+e)

## Top quark production at hadron colliders

- Top anti-Top pair production (via strong interaction)

-  Run1(1.8TeV) 		Run2(2TeV)               LHC(14TeV)
-         90%		      85%		    5%
-          10%		       15%	               95%
-           5                                7                            800

- x-sec(pb)

## Top quark production at hadron colliders

- (Drell-Yan)

- (W-gluon fusion)

- Single Top  production
- (not observed yet)

-  Run1(1.8TeV) 		Run2(2TEV)               LHC(14TeV)
-           0.7		        0.9               	     10
-            1.7		         2.4	               250

- x-sec(pb)

## Importance of studying the Top quark

- Measurement of  (tt)
-  test of QCD predictions
-  any discrepancy indicates possible new physics
- Measurement of top mass
-  fundamental parameter of the SM
- (SM predicts all top properties
- given its mass)
-  Affects predictions of the
- SM via radiative corrections
- (measuring top and W mass
- constrains the mass of the
- Higgs Boson)

## Constraining the Higgs Mass

## Top quark events are rare!

** **Top production is a rare process, in Run 1 one collision in every 3109 produced a top-anti top quark pair.
** **Small cross sections require high luminosity,
- and the ability to detect and filter out t t-bar events from a large number of other processes (backgrounds)

## Event Selection

- ~100% of the time, a top quark decays  into a bottom quark and a W boson.
- A W boson can decay into two quarks or into a charged lepton and a neutrino.
- So, an event in which top quarks are  produced should have either:
- 6 quarks
- 4 quarks, a charged lepton
-     and a neutrino
- 2 quarks, 2 charged leptons
-     and 2 neutrinos
- In all cases, 2 of the jets
-     originate from b-quarks

## Top-quark decay

- Standard Model: For m t>mW+m b
- Expect tWb to dominate

- The quarks of the fist two generations (*u,d,s,c*)  appear as a shower of particles called a JET, and cannot be separated from each other.

## Identifying electrons

## Identifying electrons

## Identifying Muons

## Identifying Muons

## Identifying Neutrinos

- Neutrinos do not interact with the detector

- We infer the presence of a neutrino from the imbalance in the transverse momentum

## Identifying Neutrinos

## Identifying Quarks

- Quarks (and Gluons) do not exist as free particles.  q q-bar pairs are pulled from the vacuum to produce stable particles : mesons, baryons

**Quarks ``hadronize’’**** **
**single quark appears as a ``jet’’ (spray) of hadrons in the detector**

## Identifying Quarks

## Identifying b-quarks

- Semileptonic decays of the b-quark
- example: B(b  + X)  20%  detect muons in jets

- 1

## Identifying b-quarks

- precise tracking close to primary collision point
-  silicon microstrip detectors

- life time  1.5 ps  c  0.5 mm (short, but not too short)

- 2

## Identifying b-jets

## Top challenges 

- Different processes can create top-like collisions. We call these “background” processes.
- We work hard on identifying the unique features of top which will separate it from background.
- Our detector and event selection are not perfect. We need to estimate how many Top events do not make it into our samples
- We optimize event selection for high signal efficiency.
- Need to solve ambiguities – Statistically sophisticated methods
- Neutrinos show up as missing energy. If there are 2 in one event, they cannot be disentangled.
- We do not know which jet comes from which quark
- Quarks emit gluons, which give rise to extra jets in the events.

## t t-bar Cross Section – Run 1

- Dilepton (ee, e******, ******) + 2 Jets + Met
- Lepton (e, ****) + 3 or 4 Jets + Met
- All Jets (5 or 6 Jets, b-tag, NN)

## Cross Sections

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

## Top Quark Mass

- lepton+jets channel:
- 1 unknown (*p**z*)
- 3 constraints
*m*(*l***) = *m*(*qq*) = *m**W*
*m*(*l****b*) = *m*(*qqb*)
- 2-constraint kinematic fit
- compare to MC to measure *m*t
- Gluon radiation can add extra jets

- up to 24-fold combinatoric ambiguity
-  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

## The Basic Procedure

- In a sample of t t-bar candidates, for each event make a measurement of X = f(mt),    where X is a suitably selected estimator for the top mass, e.g. result of the kinematic fit

- From MC determine shape of X as a  function of mt.
- Determine shape of X for background (MC & data).
- Add these together and compare with data

## Top Quark Mass

*m*t = 173.3±5.6±5.5 GeV

## Run 1Top Mass Summary

## Run 2A Prospects: number of events

| dilepton
 | 200 events
(9 in Run 1) |
| --- | --- |
| lepton+4jets
 | 1800 events
(19 in Run 1) |
| lepton+ 3jets/b-tag | 1400 events
(11 in Run 1) |
| lepton+ jets/2 b-tags | 450 events |

- Run 2A (2001-2004)
-  Integrated Luminosity ~ 2fb-1

- Run 2B (2004-2007?)
-  Integrated Luminosity ~ 15fb-1

## Run 2A Prospects: Cross Section

- 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%)

## Run2A Prospects: Mass

|  | Run I (DØ) | Run IIa (2 fb-1) |
| --- | --- | --- |
| statistics | 5.6 GeV | 1.3 GeV |
| jet pT scale | 4.0 GeV | 2.2 GeV |
| MC generator | 3.1 GeV | 0.7 GeV |
| MC model | 1.6 GeV | 0.4 GeV |
| fit procedure | 1.3 GeV | 0.3 GeV |
| Total syst | 5.5 GeV | 2.3 GeV |
| Total | 7.8 GeV | 2.7 GeV |

- Total uncertainty  2-3 GeV  (per experiment)
- W mass uncertainty  40 MeV
- 	 **constrain Higgs mass to 80%**

## Accelerator Performance Run2A

- Accelerator: long list of problems to improve luminosity   Run 2a (~2fb-1) will take through  2004.

## Future Top physics and beyond

- We have gone a long way, but many questions still have to be addressed:
-  do we understand the properties of top? its mass ?
-  is there a Higgs particle? is this a good theory to
-            explain all fundamental particles masses?
- 2 Strategies:
- Look Harder		  Precision
- Get a Bigger Hammer           Energy
- The Tevatron is well suited to both of these strategies. The next 5 years will hopefully lead to findings which may change the course of particle physics.... 

## For More Information

- http://www.fnal.gov/pub/inquiring/index.html

- http://particleadventure.org

- Fermilab inquiring minds page

- LBL page on Particle Physics