# Presentation: 042298

## Quark Compositeness

- A search for Quark substructure at the LHC

- UC Berkeley Physics 290E;  Sarah Zalusky

- Lawrence Berkeley Labs

## Outline: 

- Outline:

**Definition**: what is compositeness?
**Motivation**: why  search for substructure?
**Method: **what are the detectable signatures?
**Current  Limits**: confinement limits from the Tevatron
**Predictions**: What  can/do we expect to see at the LHC 	granted compositeness is a real feature?
**Implications**:  What might it imply beyond SM physics?

- Phys 290E Sarah Zalusky LBNL

## I) DEFINITION

- Phys 290E Sarah Zalusky LBNL

## What is Compositeness? 

- What is Compositeness?

- Quarks may not be fundamental particles; but rather an agglomeration of smaller constituents called “preons.”
- These features are visible above a characteristic energy scale Λ  below which quarks appear point like.
- Λ characterizes both strength of preon coupling and physical size of composite scale.

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## II) MOTIVATION

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## Why Search?

- Simple answer: We have always found substructure in the past.
- Cells
- Molecules
- Atoms
- Protons/Neutrons
- Quarks
- But there always seems to be
-     something smaller!

*Democritus circa 400 BCE; believed all matter to be made of *
*Indivisible elements called “atoms.” *

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## More reasons to seek substructure.. 

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-  A composite model may  explain
- parameters such as particle mass,
- electric charge, and color charge which
- SM has not

- Possible explanation for three generations of Fermions (similar to isotopes? )

-  Effectively minimize the Particle Zoo!

## III) METHODS

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## Quick Recap: How can we see quarks? 

- Strong interaction dictates that quarks only appear in bound states, called hadrons.
- We see “jets” while
-     the  quark itself
-     remains confined
-     in the binding potential
-     of its hadron.

- Phys 290E Sarah Zalusky LBNL

## Possible Phenomenological Effects:

- I) Deviation from QCD  present as an excess of high energy jets in central detector regions.
- II) Angular Distributions

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- There are two main signatures of quark compositeness

## I) Signature as high energy jets                	with large transverse momentum: 

- I) Signature as high energy jets                	with large transverse momentum:

- Essentially Preons within quark would produce hard scattering!
- Benefits: Easily Measured Quantities
- Drawbacks:
- 			i) Admittedly simplistic view; discovery of 		contact interaction alone not necessarily 		enough to prove compositeness (could be 		indicative of other new phenomena.)
- 			ii) Requires good knowledge of detector 		specifics (energy of jets)

- Phys 290E Sarah Zalusky LBNL

## Rutherford Scattering; 

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- At small center of mass scattering angles,
- the dijet angular distribution predicted by
- the leading order QCD is proportional to
- the Rutherford cross-section.

- How angular distribution in dijet events indicate compositeness

- By convention the angular distribution is  measured in the flattened variable X

- *where n is the pseudorapidity of the two leading jets

## Rutherford scattering: Recall how RS demonstrated substructure within the atom 

- Phys 290E Sarah Zalusky LBNL

- - “Hard” nucleus within the atom
- causes deflections at large angles.

- _[http://www2.biglobe.ne.jp/~norimari/science/JavaApp/e-Scatter.html](http://www2.biglobe.ne.jp/~norimari/science/JavaApp/e-Scatter.html)_

## IV) CURRENT EXPERIMENTAL LIMITS

- Phys 290E Sarah Zalusky LBNL

## DØ Detector;  Fermilab Tevatron pp collisions

- Uses measurements of Dijet angular distributions to test predictions of QCD.
- *see [] for specific information regarding jet selection criteria

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## DØ Data Compared to LO and NLO QCD Predictions 

- JETRAD was used to generate
- 	predictions for LO and NLO
- Figure depicts DØ data for the
- 	four mass bins vs. LO and NLO
- 	predictions

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- Defines the renormalization scale (required by JETRAD) proportional  to the maximum transverse energy. Note that NLO predictions are dependent on the renormalization scale chosen.

## DØ Experimental Limits:

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- Dijet angular distributions for DØ data  compared to jetrad for LO for different Compositeness scales.
- NLO predictions found by comparing LO to finite values and multiplying NLO predictions by the same factional differences.

- DØ Experimental Limits:

- Dijet angular distributions studied at CM 1.8Tev (Highest Et at Tevatron 440GeV ~ 10-17 cm)

## Conclusion from DØ results: 

- Conclusion from DØ results:

- Thus far NO EVIDENCE of quark substructure has been found when compared to standard QCD predictions
- Limits place compositeness at a contact interaction scale above 2.2TeV.
- Compositeness below 2.2TeV ruled out at a confidence level of 95%

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## V) PREDICTIONS: WHAT WILL WE SEE AT THE LHC

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## Compositeness observed in Dijets 

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- QCD prediction of dijet angular distribution (light pink) compared to angular distributions Considering different compositeness scales in ATLAS.

- Xcut = 2.7

## Note that compositeness on a scale >10TeV will be undetectable by ATLAS. 

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## Dijet Ratio QCD vs QCD w/ quark contact interaction 

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-          The solid curve indicates ratio in CMS from QCD compared to QCD plus         	quark contact interaction at  15, 10, and 5 TeV.

## VI) IMPLICATIONS

## The Standard Model.. And Beyond

- Simplification of the “particle zoo”
- Possible “Higgs-less” extensions of SM
-  Candidates for Dark Matter?
- Who knows!?

- Phys 290E Sarah Zalusky LBNL