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
