Chuan-Ren ChenIPMU, Univ. of Tokyo

2nd Workshop of LHC Topic Program @ NCTS, Dec. 8~10, 2010

In collaboration with Ed. Berger, Q-H Cao, G. Shaughnessy, H. Zhang

Top Quarks as A Probe of Heavy Resonances at the LHC

Outline

why s-channel heavy resonances

why top quark

color sextet scalar, W’, Z’

Conclusion

e.g. Carena, Daleo, Dobrescu, Tait, hep-ph/0408098 (Z’); Rizzo, 0704.0235 (W’); Petriello and Quackenbush, 0801.4389 (Z’); Diener, Godfrey, Martin, 1006.2845 (Z’); Gopalakrishna, Han, Lewis, Si, Zhou, 1008.3508 (W’); Godbole, Rao, Rindani, Singh, 1010.1458(Z’) talk this afternoon

SU(2)LXU(1)Y

G

Heavy s-channel Resonances generic in models beyond the SM

Extra SU(2)

Extra U(1)

W’ and Z’

Simple topology and large discovery potential q

q’

f

f’

W’/Z’

Z’

can serve as model discriminators (W’/Z’) 400 600 800 1000 1200 1400 (GeV)

CDF, dijet search, 0812.4036e.g.

Z’ → e+e- (CMS NOTE 2006-083)

1008.24611010.0203

Tevaton limits on W’/Z’ are stronger

Top quarkThe heaviest particle

Bare quark

~173 GeV,

decay quickly before hadronization (~10-25 sec)

Spin information is kept

~100% decay into W, b

(PDG 2010)

(PDG 2010)

main production at hadron colliders – QCD (top pair,~800 pb), EW(single-top, ~300 pb)

V-A coupling

Semileptonic decay mode

Two-fold solution

top quark reconstruction in (1)

break the 2-fold ambiguity by selection

(1)

(2)

break the 2-fold ambiguity by selection

Z’

Barger, Han, Walker, hep-ph/0612016

if < 0

purely leptonic decay

two missing neutrinos

two neutrinos and two charged leptons are from two Ws

two neutrinos, two charged leptons and two (b) jets are from two quarks

two top quarks are from a heavy resonance

using on-shell conditions, momentum of 2 neutrinos can be solved

MT2

top quark reconstruction in (2)

Z’

Sonnenshein, hep-ph/0603011

HOWEVER, correct paring is crucial!

MT2 methodC. G. Lester and D. J. Summers, hep-ph/9906349

knowing the correct mx,mT2 is correct mass mother particle

MT2 method Cho, Choi, Kim, Park, 0804.2185; CDF, 0911.2956

Z’missing

visible

10 20 30 40 50 60 70 80 90 100

0 50 100 150 200 250 300 350 400 450 500

Reconstructionschoose the pair whose MT2 is smller

Top polarizationTait, Yuan, hep-ph/0007298

charged lepton from top (anti-top) decay prefers to move along (against) the direction of top (anti-top) polarization

(longitudinal W)

(longitudinal W)

(transverse W)

(transverse W)

Probing Heavy Resonances

Color sextet

same-sign right-handed top-quark pair

Mohapatra, Okada, Yu, 0709.1486; CRC, Klemm, Vikram, Wang, 0811.2105; Han, Lewis, McElmurry, 0909.2666; Arnold, Pospelov, Trott, Wise, 0911.2225Gogoladze, Mimura, Okada, Shafi, 1001.5260

background

collider signature Berger, Cao, CRC, Shaughnessy and Zhang, 1005.2622

cuts

mis-taggingb-tagging

signal >> background

distributions after all cutsMΦ = 600 GeV

clearly, it is right-handed!

W’ and Z’

Z’ Z’

W’ and Z’

Z’ Z’

here, for simplicity, only couple to quarks

in Mt 0 limit, Br(W’ tb) = 1/3, Br(Z’ t tbar) = 1/6

narrow width approximation still holds

W’ and Z’

main background:single-top, WjjW’

Z’ top pair

cross section is not small!

0 200 400 600 800 1000 1200 1400 1600 1800 2000

HT

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Mtrans

(GeV)(GeV)

Cuts:

gv’ = 0.4

after cuts

discovery potential:~9σ (10 fb-1) for 1 TeV mass window~3σ (10 fb-1) for 1.5 TeV mass window

cos distributions after reconstructions, all cuts

Gopalakrishna, Han, Lewis, Si, Zhou, 1008.3508W’

W’ signal: ~ 0.1 pb >> background (~ 10-3 pb)

in analysis, only need to consider signal!!

cos distributions after reconstructions, all cuts

0 10 20 30 40 50 0 10 20 30 40 50 xL (%) xL (%)

Can we extract the information of couplings??fitting with left-handed and right-handed Z’ distributions

true: 30 %

due to Th error

(statistical only)

combine W’ and Z’

true: (30, 30)

true: (70, 70)

expect W’⊕Z’ in SSM, LH models

expect W’⊕Z’ in R-L symm. models

in progress

Berger, Cao, CRC and Zhang, 1012.****

Conclusion Heavy resonances produced in S-channel will be soon discovered/strongly constrained at LHC.

top quark final state is complementary to dijet and and leptonic state searches.

Z’ (1 TeV) -> t tbar can be discovered with > 5σ statistical significance, W’ -> t b >> background.

top quark momentum can be reconstructed, spin information study through charged lepton can be done.

W’/Z’ couplings to top quark serve as a model discriminator. (in progress) 5% th error, stat. in 10 fb-1 data -> both cause ±10% in xL

Top polarization