Quark Compositeness

1. Quark Compositeness A search for Quark substructure at the LHC UC Berkeley Physics 290E; Sarah Zalusky Lawrence Berkeley Labs

2. 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

3. I) DEFINITION Phys 290E Sarah Zalusky LBNL

4. 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

5. II) MOTIVATION Phys 290E Sarah Zalusky LBNL

6. 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

7. 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

8. III) METHODS Phys 290E Sarah Zalusky LBNL

9. 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

10. 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

11. 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

12. 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

13. 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

14. IV) CURRENT EXPERIMENTAL LIMITS Phys 290E Sarah Zalusky LBNL

15. 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

16. 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

17. 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

18. 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

19. V) PREDICTIONS: WHAT WILL WE SEE AT THE LHC Phys 290E Sarah Zalusky LBNL

20. 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

21. Note that compositeness on a scale >10TeV will be undetectable by ATLAS. Phys 290E Sarah Zalusky LBNL

22. 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

23. VI) IMPLICATIONS

24. 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