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. Outline:. Definition : what is compositeness? Motivation : why search for substructure? Method: what are the detectable signatures?

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Quark compositeness

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

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.

Phys 290E Sarah Zalusky LBNL


Ii motivation

II) MOTIVATION

Phys 290E Sarah Zalusky LBNL


Why search

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.”

    Phys 290E Sarah Zalusky LBNL


    More reasons to seek substructure

    More reasons to seek substructure..

    • 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


    Iii methods

    III) METHODS

    Phys 290E Sarah Zalusky LBNL


    Quick recap how can we see quarks

    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

    Possible Phenomenological Effects:

    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


    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

    Rutherford Scattering;

    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


    Rutherford scattering recall how rs demonstrated substructure within the atom

    Rutherford scattering: Recall how RS demonstrated substructure within the atom

    - “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


    Iv current experimental limits

    IV) CURRENT EXPERIMENTAL LIMITS

    Phys 290E Sarah Zalusky LBNL


    D detector fermilab tevatron pp collisions

    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

    Phys 290E Sarah Zalusky LBNL


    D data compared to lo and nlo qcd predictions

    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

    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


    Quark compositeness

    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


    Conclusion from d 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%

    Phys 290E Sarah Zalusky LBNL


    V predictions what will we see at the lhc

    V) PREDICTIONS: WHAT WILL WE SEE AT THE LHC

    Phys 290E Sarah Zalusky LBNL


    Compositeness observed in dijets

    Compositeness observed in Dijets

    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


    Quark compositeness

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

    Phys 290E Sarah Zalusky LBNL


    Dijet ratio qcd vs qcd w quark contact interaction

    Dijet Ratio QCD vs QCD w/ quark contact interaction

    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

    VI) IMPLICATIONS


    The standard model and beyond

    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


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