September 21, 2007 UCL ATLAS Physics

Sequential 4

th

Family Quarks at ATLAS

V. E. Özcan

University College London

In collaboration with: G. Unel & S. Sultansoy 1

Sequential 4

th

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 < m Z /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 4 th family – a full new generation of fermions within the SM, much like the first three.

2

Signal

• Search for 4 th 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” 4 th generation quarks, ie. |m(u 4 )-m(d 4 )|~few GeV • No fundamental reason to assume the 4x4 CKM follows the same trend of the 3x3 version: – ATLAS TDR : 4 th family mixes predominantly with the 3 rd family.

– New study : 4 th family mixes predominantly with 1 st or 2 nd .

• Final state: pp => q 4 q 4 => W jj j + W l n j ( 2 hard u,d,s,c jets & 2 Ws) 3

Event Generation

• 12k signal events with CompHep for 3 different choices of mass.

(Later dropped 250 GeV due to recent upper limit from CDF.) M d4 G (GeV) (MeV) s (pb) 250 0.01

99.8

500 0.08

2.63

750 0.28

0.250

• A total of 250k BG events generated with Madgraph: WWjj, WZjj, WWbb (tt), WWbbj (ttj) 4

Reconstruction & Selection

• Leptonic W : from missing E T & e/ m • Hadronic W : from 3 rd & 4 th highest-P T jets • Combine W candidates with two hardest jets.

– Do both combinations and choose the min | D m jW |=|m 1 q4 -m 2 q4 | • All 4 jets used have to be non b-tagged.

5

Example Distributions

The tail due to cases where W has high P T and ends up being a single jet. => Analysis can be improved.

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Reco. m

q4

• Tricky part: Doing the fits…

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Fits

• Finding the right fit function is difficult.

• P T m q4 cuts on the hard jets effect the lower end of the BG 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 8

Results

• S = Integrate background function within ±2 s • B = Integrate signal function within ±2 s of the signal peak 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 9

1 fb -1 : m q4 <~ 650 GeV 30 fb -1 : mq4 <~ 850 GeV

5

s

Reach

10

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

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