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Quark Compositeness

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Quark Compositeness

A search for Quark substructure at the LHC

UC Berkeley Physics 290E; Sarah Zalusky

Lawrence Berkeley Labs

- 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

Phys 290E Sarah Zalusky LBNL

- 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.

Phys 290E Sarah Zalusky LBNL

Phys 290E Sarah Zalusky LBNL

- Simple answer: We have always found substructure in the past.
- Cells
- Molecules
- Atoms
- Protons/Neutrons
- Quarks

something smaller!

Democritus circa 400 BCE; believed all matter to be made of

Indivisible elements called “atoms.”

Phys 290E Sarah Zalusky LBNL

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

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

- Effectively minimize the Particle Zoo!

Phys 290E Sarah Zalusky LBNL

Phys 290E Sarah Zalusky LBNL

- 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

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

II) Angular Distributions

There are two main signatures of quark compositeness

Phys 290E Sarah Zalusky LBNL

- 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

How angular distribution in dijet events indicate compositeness

At small center of mass scattering angles,

the dijet angular distribution predicted by

the leading order QCD is proportional to

the Rutherford cross-section.

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

*where n is the pseudorapidity of the two leading jets

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

Phys 290E Sarah Zalusky LBNL

Phys 290E Sarah Zalusky LBNL

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

Phys 290E Sarah Zalusky LBNL

- JETRAD was used to generate
predictions for LO and NLO

- Figure depicts DØ data for the
four mass bins vs. LO and NLO

predictions

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

Phys 290E Sarah Zalusky LBNL

DØ Experimental Limits:

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.

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

Phys 290E Sarah Zalusky LBNL

- 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%

Phys 290E Sarah Zalusky LBNL

Phys 290E Sarah Zalusky LBNL

Xcut = 2.7

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

Phys 290E Sarah Zalusky LBNL

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

Phys 290E Sarah Zalusky LBNL

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

Phys 290E Sarah Zalusky LBNL

VI) IMPLICATIONS

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

Phys 290E Sarah Zalusky LBNL