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Sequential 4 th Family Quarks at ATLAS

September 21, 2007 UCL ATLAS Physics. Sequential 4 th Family Quarks at ATLAS. V. E. Ö zcan University College London In collaboration with: G. Unel & S. Sultansoy. Sequential 4 th Family. SM itself does not make an argument on the number of generations Why 3 generations then?

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Sequential 4 th Family Quarks at ATLAS

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  1. September 21, 2007 UCL ATLAS Physics Sequential 4th Family Quarks at ATLAS V. E. Özcan University College London In collaboration with: G. Unel & S. Sultansoy

  2. Sequential 4th Family • SM itself does not make an argument on the number of generations • Why 3 generations then? • ~1973: KM point out 6 flavors in 3 generations would accommodate CP violation in the SM • ~1979: studies on abundances of light elements start to put constraints on # of light neutrinos • ~1989: SLC & LEP experiments establish 3 light neutrinos (with mass < mZ/2). • So we “naturally” assume that 3 is the number. • On the discovery of the muon, I. I. Rabi: “Who ordered that?” • Heavier quarks & leptons expected in many theories: t’ in Little Higgs models, iso-singlet & iso-triplet fermions in E6 GUT, some models of dynamical symmetry breaking, etc. • For this study, we look for a sequential 4th family – a full new generation of fermions within the SM, much like the first three.

  3. Signal • Search for 4th family quarks as predicted under the assumption of Flavor Democracy. • All flavors have comparable Yukawa couplings to start with, so the mass matrix is democratic. However this is “slightly” broken. • Prediction of this model: 4 families with “quasi-degenerate” 4th generation quarks, ie. |m(u4)-m(d4)|~few GeV • No fundamental reason to assume the 4x4 CKM follows the same trend of the 3x3 version: • ATLAS TDR : 4th family mixes predominantly with the 3rd family. • New study : 4th family mixes predominantly with 1st or 2nd. • Final state: pp => q4q4 => Wjjj + Wlnj ( 2 hard u,d,s,c jets & 2 Ws)

  4. Event Generation • 12k signal events with CompHep for 3 different choices of mass. (Later dropped 250 GeV due to recent upper limit from CDF.) • A total of 250k BG events generated with Madgraph: WWjj, WZjj, WWbb (tt), WWbbj (ttj)

  5. Reconstruction & Selection • Leptonic W : from missing ET & e/m • Hadronic W : from 3rd & 4th highest-PT jets • Combine W candidates with two hardest jets. • Do both combinations and choose the min |DmjW|=|m1q4-m2q4| • All 4 jets used have to be non b-tagged.

  6. Example Distributions The tail due to cases where W has high PT and ends up being a single jet. => Analysis can be improved.

  7. Reco. mq4 • Tricky part: Doing the fits…

  8. Fits • Finding the right fit function is difficult. • PT cuts on the hard jets effect the lower end of the BG mq4 distribution. • Even for cases which initially looked promising, problems were encountered when we went to Toy MC studies. • We want a fit that can run with as minimum human interaction as possible. • Finally settled with: • For signal, a Breit-Wigner – 3 parameters • For BG, a reverted Crystal Ball function (a Gaussian core and a power-law tail added together so that the function is not only contentious, but also smooth.) – 5 parameters

  9. Results • S = Integrate background function within ±2s of the signal peak • B = Integrate signal function within ±2s of the signal peak • The fit BG functions for the two masses are in agreement with each other (within statistics). Then, one can use these to generalize results to different q4 masses: • Compute the x-sections • Estimate BG around the new peaks by integrating. • Estimate cut efficiency by interpolating

  10. 5s Reach 1 fb-1 : mq4 <~ 650 GeV 30 fb-1 : mq4 <~ 850 GeV

  11. Conclusion • You can see the draft paper: ATL-COM-PHYS-2007-044 • All comments will be highly appreciated!

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