# Presentation: 098714

## Searching for  New Particles at the Fermilab Tevatron 

- Dave Toback
- University of Maryland
- Physics Department Colloquium
- Texas A&M University
- May, 2000

## Outline

- Why look for new particles?
- The standard way to search for new particles: 	A supersymmetric example
- Fermilab Tevatron, CDF and DØ detectors
- New methods:
- Searching for “Cousins” of interesting events
- Sherlock: A quasi-model independent method
- The future and conclusions

## The Domain of Particle Physics

- cm

- 10-10m

- 10-14m

- 10-15m

- u

- <10-18m

- 10-9m

- Matter

- Molecule

- Atom

- Nucleus

- Quark

- Baryon

- Electron

- <10-19m

- protons, neutrons,
- mesons, etc.
- 

- top, bottom,
- charm, strange,
- up, down

- Chemistry

- Atomic Physics

- Nuclear
- Physics

- Particle Physics

- Mass

- proton ~ 1 GeV/c2

- (Hadron)

- (Lepton)

**Notes:**
Give scale and relationship to the domain that high energy physicists work in.
Introduce the fundamental bits of matter loosely so that audience can point back to bulk matter.
Domain.

## Overview of the Standard Model

- The Standard Model of particle physics is a description of the known particles and their interactions
- An excellent description of physical processes at energies probed by experiments
- Accelerator experiments
- Measurements of, or bounds on, rare processes
- Consistent with all current data

## The Known Particles of the Standard Model and their Interactions

- Example Interaction:

## Particles of the Standard Model

- Why do we need so many different particles?
- Why are some so much heavier than the others?
- How do we know we aren’t missing any?

## How to attack the problem

- Theorists:
- Theoretical
-  Models

- Experimenters:
- Experimental
- Results

- Unexplained Phenomena

- Theoretical Parameters to Measure

- Results of Particle Searches

- New Particles to Look For

## An attractive theoretical solution

- One of the most promising theories is Supersymmetry which is an attempt to solve these (and other) problems
- Each Standard Model particle has a Supersymmetric partner

## Supersymmetric Particles?

- SM Particles  Superpartners

- Other New Particles: Higgs Boson

## How we might observe evidence of Supersymmetry in a laboratory

- Example* Final State:
- Two electrons, two photons and two Gravitinos
- *Gauge Mediated Supersymmetry Breaking

- _P

- P

- Proton Anti-Proton Collision
- (Actually the quarks inside)

## Typical Search for New Particles

- Look at the final state particles from a Proton Anti-Proton collision
- Use a computer (Monte Carlo) to simulate the interaction
- Probability a collision might produce Supersymmetric particles
- Properties of the final state particles
- Same for known Standard Model interactions which might produce similar results
- Compare

## Example with Supersymmetry 

- _P

- P

- _P

- P

- Supersymmetry:

- Standard Model:

- Supersymmetric Particles
- in Final State

- No Supersymmetric
- Particles in Final State

## Example with Supersymmetry 

- Example with Supersymmetry

- --Background Expectations
- from Standard Model
- --How the data might look

- Prediction from Supersymmetry

- Look for Regions
- where the backgrounds are small
- and the predictions for Supersymmetry are large

## Fermi National Accelerator Laboratory (Fermilab Tevatron)

- Proton Anti-Proton Collisions
- (An Event)
- Center of Mass energy is 1.8 TeV
- 1 collision every 3.5sec
- (300,000/sec)
- Data taking from 1992 to 1996
- Roughly 5x1012 pp collisions

## The Fermilab Accelerator

- ~4 Miles in
- Circumference

## The CDF and DØ Detectors

- Surround the collision point with a detector

**Beam **
**Direction**

- Collision point

## Identifying the Final State Particles

- Many particles in the final state
- Want to identify as many as possible
- Determine the 4-momentum
- Two types: short lived and long lived
- Long lived: electrons, muons, photons...
- Short lived: quarks, W, Z...“decay” into long lived particles
- Observe how long lived particles interact with matter
- Detection

## Short Lived Particles in the Detector

- mesons

- etc.

- etc.

- In the Detector

- _P

- P

- Jet

- Jet

**Notes:**
gluon self interaction means quark confinment
This means jet production. Very visual explaination. Use the word fragmentation.

## Long Lived Particles in the Detector

*e*

- 

- jet

- 

- 

- Long lived Supersymmetric particles do not interact in the detector
- Very much like neutrinos

- Beam Axis

- Tracking
- Chamber

- EM Layers

- Hadronic
- Layers

- Steel/Iron

- Calorimeter

- Muon Chamber

- Muon Chamber

## Missing ET : MET

- X

- Y

- e+

- e-

- X

- Y

- e+

- 

- Event with energy imbalance in transverse plane

- Event with energy balance in transverse plane

- Energy in direction transverse to the beam:
- ET = E sin()

- Missing ET : MET

- Event with
-  MET

## Search for Supersymmetry*

- _P

- P

- A number of models predict final states with +Missing Transverse Energy
- Look at a sample of events with two high energy photons in the final state
- Unbiased sample in which to search for evidence of Gravitinos (MET)

- *Work done at University of Chicago with Henry Frisch and Ray Culbertson on CDF Results published in PRL & PRD

- Leave detector causing MET

## Predictions and Comparisons

- Supersymmetric Predictions    Standard Model Predictions

- Select events above threshold            or
- Look for excess of events with large MET

- 1

- 2

- 3

- 1

- 2

- 3

- 1

- 2

- 3

- Background above threshold

- X

- MET

- MET

- MET

- MET

- Photon ET

- Photon ET

## Search for  events with large MET

- ET>12 GeV, MET>35 GeV
- Expect 0.5±0.1 Events  Observe 1 Event
- ET >25 GeV, MET>25 GeV
- Expect 0.5±0.1 Events
-  Observe 2 Events
- Our observations are consistent with background expectations with one possible exception (we’ll come back to this event.)

**-High Acceptance**
**-Large # of Background Events**

**-Lower Acceptance**
**-Smaller # of Background Events**

- 1

- 3

- MET

- MET

- MET

## Set limits on one of the models

- Since counting experiment is consistent with expectations we set limits on the new physics production at the 95% Confidence Level
- This constrains/excludes some theoretical models
- Gives feedback to theoretical community

- Example Limit

- Example Theory

- Excluded this side

- Unexcluded
- this side

- Lightest Chargino Mass

## The interesting event on the tail

- In addition to +MET the event has two high energy electron candidates
- Unexpected from Standard Model predictions
- Hint of new physics?
- Lots of discussion between theoretical and experimental communities

## How many of these did we expect?

- This is a difficult question
- Can’t estimate the probability of a single event (measure zero)
- “How many events of this ‘type’ did we expect to observe in our data set from known Standard Model sources?”
- Try to define a “reasonable” set of criteria to define ‘type’ after the fact

## Known Standard Model Sources

- Standard Model WW
- WWeeee8x10-7 Events
- All other sources: 5x10-7 Events
- Total: (1 ± 1) x 10-6 Events

## Interpreting the Event

- What is it?
- Statistical fluctuation?
- Lots of theoretical interpretations:
-  Supersymmetry?
-  Technicolor?
-  Others?

## What to do?

- As experimentalists we decided to do two things:
- Investigate the predictions of models which predict this type of event
- Previous search for +MET is a good example
- Also do something a little less standard just in case...

## Model Independent Search

- What if none of the theories are right?
- Use properties of the event to suggest a model independent search
- Look for “Cousins” of the event
- I.e, Others with “similar” properties, or of this ‘type’
- Possibility of looking for many models all at once
- This is a non-standard method of looking for new particles

## Unknown Interactions

- _P

- P

- Unknown
- Interaction

- _P

- P

- Similar
- Unknown
- Interaction

- Other final
- state particles

- Anything

**These two events would be Cousins**

## Example of a “Cousin” Search

*A priori* the event is unlikely to be Standard Model WW production (~10-6 Events)
- WWeeee
- Guess that the _unknown interaction_ is “Anomalous” WWproduction and decay
- _Similar unknown interaction_ could be with
- WW (qq)(qq)  jjjj
- Note: Br(WW  jjjj) >> Br(WW ee+MET)
- Look for “Cousins” of WWwhere
- WW (qq)(qq)   jjjj

## Quantitative Estimate

- Use a computer simulation of Standard Model WW production and decay
- Use known W decay branching ratios and detector response to the various decays of W’s
- Result: Given 1 +ll+MET event Expect ~30 jjj events

## + Jets Search at CDF

- Use same  data set and look for jets
- ET>12 GeV,  4 Jets
- Expect 1.6±0.4 Events
- Observe 2 Events
- ET >25 GeV,  3 Jets
- Expect 1.7±1.5 Events
- Observe 0 Events
- No Excess

- ~30 Event excess would show up here

**-High Acceptance**
**-Large # of Background Events**

**-Lower Acceptance**
**-Smaller # of Background Events**

- Number of Jets

- Number of Jets

## Generalized Cousin Search: X

- Generalize the Cousin Search
- Search for an excess of events in the X final state, where X is
- Gauge Boson
- W, Z, gluon ( jet) or extra 
- Quarks
- Light quarks (up, down, strange or charm  jet)
- b-quarks (jet with long lifetime)
- t-quarks (t  Wb)
- Leptons
- Electrons, muons, taus and neutrinos
- Leptons from W  l, Z  ll or Z  
- No rate predictions, just look for an excess

## Final Results of  Cousin Search

- Final Results of  Cousin Search

- All results are consistent with the Standard Model background expectations with our one possible exception

**High Acceptance,  Large # of Background Events**

**Lower Acceptance, Smaller # of Background Events**

## So where are we?

- We have one very interesting event
- Statistically unlikely to be from known Standard Model backgrounds
- No Cousins in the  +X final state
- What’s next?

## Take more data

- The Fermilab Tevatron is being upgraded
- The detectors are being upgraded
- Plans to start taking data in 2001
- Should be able to answer the question with 20 times the data:
- Scenario 1: We see more than a couple cousins
- Study the sample for more clues for its origins
- Scenario 2: We see very few or none
- Most likely a fluctuation (of whatever it was).

## Don’t want to get caught unprepared again

- Having to estimate the background for an interesting event *a posterori* is not good
- Need a systematic way of finding more interesting events
- Need a more systematic plan of what to do when we find them
- Need a systematic way of estimating the significance of unexpected events

## Towards a model independent solution

- Many believe Supersymmetry is correct, but what if we haven’t gotten the details right and we’re just looking at the wrong final states
- Looking for two photons in 1994  was not even considered as a Supersymmetry discovery channel
- Ought to be better prepared to search for new physics when we don’t know what we are looking for
- Design a system which should also find the kinds of things we know to look for

## Sherlock*

- *Work done at University of Maryland with Bruce Knuteson (Grad Student at Berkeley) on DØ
- Results to be submitted to PRD by the end of the spring

- A friend to help us look in our data for experimental clues

## Quasi-Model-Independent Search at DØ: Sherlock

- Assume nothing about the new particles except that they are high mass/ET
- If it were low mass, we most likely would have seen it already
- High Mass assumption  Quasi-Model Independent
- Systematically look at events by their final state particles
- Search for new physics by looking for excesses in multi-dimensional data distributions

## Overview of Sherlock

- Define final state signatures
- (which particles in the final state)
*A priori* prescription for defining variables and regions of space in those variables
- A systematic look for regions with an excess (more events than expected) with large ET
- Find most interesting region
- Compare with the expectations from hypothetical similar experiments using background expectations
- Take into account the statistics of the large number of regions searched

## Labeling Final State Signatures

- Final State particles:
- e, , j, b, c, MET, W or Z
- Each event is uniquely identified:
- All events which contain the same number of each of these objects belong to the same final state

## General rule for picking variables

- Looking for new high mass particles
- Mass-Energy Relationship
- Decay to known Standard Model particles
- light in comparison
- High energy long lived particles in final state
- High Mass  High ET
- Look at ET of the final state particles

## Sherlock Algorithm

- Look at various regions
- Find most interesting region (largest excess)
- Run _hypothetical similar experiments_ using background expectations
- Fraction of hypothetical similar experiments (from backgrounds alone) which have an excess more significant than the one observed

- Background expectation
- Example signal events

- ET of X

- ET of Y

## Using Sherlock on Run I Data

- Look in events with an electron and a muon for a excess which might indicate a new heavy particle(s)
- Why e? (why not?)
- Lots of theory models
- Supersymmetry? Anomalous Top quarks?
- Backgrounds include good example of heavy particles to look for
- Top quarks, W bosons

## tt and WW production

- _P

- P

- _P

- P

- _e____ + 0 Jets_

- _e____ + 2 Jets_

-  High ET relative to other backgrounds

## Testing Sherlock

- Both WW and top quark pair production are good examples of high ET events which might show up with Sherlock
- Run Mock Experiments pretending we don’t know about WW andtt production
- 4 Samples:
- e + 0 Jets WW
- e + 1 Jet
- e + 2 Jetstt
- e + 3 Jets

## Mock data with no signal

- Fraction of hypothetical similar experiments (from backgrounds alone) which have an excess more significant than the one observed

- _Probability is _
- _flat as expected_

- e + 1 Jets

- e + 0 Jets

- e + 3 Jets

- e + 2 Jets

- Small P is interesting Smallest bin is <5%
- No indication of anything interesting

## Sherlock with WW andtt

- e + 1 Jets

- e + 0 Jets

- e + 3 Jets

- e + 2 Jets

- Pretend we don’t know about WW andtt
- Mock experiments with WW andtt as part of the sample
- Observe an excess in        0 Jets (WW production)                  2 Jets (tt production)               in the mock trials Remember: 		      Small P is interesting Smallest bin is <5%

## Combining Results

- All overflows in last bin

- Significance of excess in standard deviations

- Excesses corresponding
- to WW andtt found

- DØ Preliminary

**/**
**/**
**/**
**/**

- Mock Experiments

- Bkg only

- Bkg +WW +tt

- Pick most significant excess
- Take into account that we looked in 4 datasets

## Sherlocktt

- Include WW as a background
- Expect an excess in 2 Jets only
- tt production

- e + 1 Jets

- e + 0 Jets

- e + 3 Jets

- e + 2 Jets

## Findingtt alone

- Findingtt alone

- All overflows in last bin

- Significance of excess in standard deviations

- Excess corresponding tott

- DØ Preliminary

- Use all backgrounds excepttt and look for excesses

**/**
**/**
**/**
**/**

- Mock Experiments

- Bkg only

- Bkg +tt

## Final Sherlock Results

- We see no evidence for new physics at high PT in the eX data

- DØ Preliminary

- Use all backgrounds and look for excesses

**/**
**/**
**/**
**/**

## Some thoughts on Sherlock

- Sherlock is sensitive to finding new physics when it is there to be found
- Would have pointed out either WW ortt if we didn’t know about them
- Would find events like the ee+MET naturally
- Should  be sensitive to many SUSY signatures
- While it can’t be as sensitive as a dedicated search, it may be our only shot if we guess wrong about where to look in our data.
- A natural complement to the standard searches

## The Future at Fermilab 

- Upgraded Fermilab Tevatron (Run II)
- Higher energy: 1.8 TeV  2.0 TeV
- Upgraded detectors
- Better acceptance, more data more quickly
- Start taking data in March 2001
- 20 times the data by 2003
- 200 times the data by 2005-2006

## The plan for the next few years

- Next Year:
- Finish off other final states from Run I (ee+Jets, +Jets etc)
- Prepare to take data (Run II)
- Next two years: Pursue best guesses for Run II
- Dedicated searches (Fermilab’s top priority)
- Higgs Boson, Supersymmetry
- Signature based “cousins” and Sherlock searches
- Lepton + Photon + X, Photon+Photon +X, Lepton+Lepton+X
- Next five years: Pursue best hints from Run II
- Higgs signal? Supersymmetry? Twenty eeMET events?
- Some other completely unexpected events?

## Conclusions

- The Fermilab Tevatron continues to be an exciting place to search for new particles
- Interesting hints in the Run I data
- “Cousins” Searches and Sherlock provide  powerful new search tools
- Provide an important complement to the standard searches
- Hopefully, between the two we’ll find something interesting in Run II