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The rise and fall of the fourth quark-lepton generation
M.I.Vysotsky ITEP QFTHEP, Repino, June 26, 2013

M.I. Vysotsky

The rise and fall of the fourth quark-lepton generation

June 26, 2013

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Prehistory

1974, J / (cc) discovery, "November Revolution" Ї c quark - the last member of the 2nd family (c, s, µ, µ ) 1976, -lepton, 1978; (bЇ) b the 3rd family 1994 - t-quark, but already in the 1980s - why only 3 generations? where is the fourth generation? Special conference in mid 1980s on the 4th generation. How heavy are U, D, E ?

M.I. Vysotsky

The rise and fall of the fourth quark-lepton generation

June 26, 2013

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1989: SLC, LEP

Z invisible width: (invisible) = 499 ± 1.5MeV Theory: 166 3 = 498 - no space for extra neutrinos; ng = 3 - the only discovery made at SLC and LEP. BUT: mN > MZ /2

M.I. Vysotsky

The rise and fall of the fourth quark-lepton generation

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Electroweak precision data
Since the fourh generation quarks and leptons contribute to the W and Z polarization operators and since these contributions do not decouple in the limit of heavy new generation (the essence of electroweak theory; opposite to the case of QED, where (g - 2)µ (mµ /mt )2 ) one can get constraints on the 4th generation from the precision measurements of MW , mt and Z -boson parameters. Indeed: RPP, 2000 year edition, Erler, Langacker: "An extra generation is excluded at the 99.6%CL (97%) by the analysis based on S,T and U parametrization of New Physics contributions into electroweak observables." The same 2000 year, Maltoni, Novikov, Okun, Rozanov, Vysotsky: "One extra generation is still allowed".
M.I. Vysotsky The rise and fall of the fourth quark-lepton generation June 26, 2013 4 / 17


What was missed by Erler and Langacker
Comment for specialists: 1. S, T and U are applicable only for M >> MZ ; 2. Instead of making global fit they studied S, T and U separately, while they are correlated.

Alexander Lenz, CERN-PH-TH/2012 - detailed description of 1998-2010 PDG extra generation story.

M.I. Vysotsky

The rise and fall of the fourth quark-lepton generation

June 26, 2013

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What we had before LHC
mN (GeV)
550

500

450

400

350

300

250

200

150

100

50 -200

-150

-100

-50

0

50

100

150

200

MH = 120 GeV, mE = 200 GeV, mU + mD = 600 GeV, /d.o.f . = 17.7/11, the quality of fit is the same as in SM.
2
M.I. Vysotsky The rise and fall of the fourth quark-lepton generation June 26, 2013 6 / 17

mU-mD (GeV)


mN (GeV)

550

500

450

400

350

300

250

200

150

100

50 -200

-150

-100

-50

0

50

100

150

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MH = 600GeV, mE = 200 GeV, mU + mD = 600 GeV, /d.o.f . = 18.4/11, the quality of fit is the same as in SM.
2

mU-mD (GeV)

M.I. Vysotsky

The rise and fall of the fourth quark-lepton generation

June 26, 2013

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LHC direct bounds
ATLAS: mt > 656 GeV at 95% CL (t t - W bW b, Phys. Lett. B 718 (2013) 1284); mb > 480 GeV (b b - W tW t). CMS has similar bounds. These bounds push heavy quarks out from the perturbative unitarity domain: mq < 500 GeV, strong dynamics. t = mt /( / 2) = 172/(246/ 2) 1 However these bounds depend on the pattern of heavy quarks decay and are not universal. Much more interesting bounds follow from higgs boson production and decays.
M.I. Vysotsky The rise and fall of the fourth quark-lepton generation June 26, 2013 8 / 17


Higgs data, µ /S
ATLAS - conf H - : H - Z Z : H - W W : H - : V H - V bb - 2013 - 034 1.6 ± 0.3 1.5 ± 0.4 1.0 ± 0.3 0.8 ± 0.7 : -0.4 ± 1.0

M

CMS-PAS-HIG-13-005, 13-012 H - : 0.77 ± 0.27 H - Z Z : 0.92 ± 0.28 H - W W : 0.68 ± 0.20 H - : 1.10 ± 0.41 V H - V bb : 1.00 ± 0.49 I am grateful to Ilya Tsukerman for proper references.
M.I. Vysotsky The rise and fall of the fourth quark-lepton generation June 26, 2013 9 / 17


H production cross section

p t t t p H

t - t, t , b (g g - H )S
M4

9 (g g - H )S

M3

M.I. Vysotsky

The rise and fall of the fourth quark-lepton generation

June 26, 2013

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H V V decay rates
Mainly because of H - g g enhancement: B r(H - Z Z , W W )
SM 4

0.6B r(H - Z Z , W W )S

M3

Taking into account EW loop corrections (GF m2 ) (Passarino, t Denner,... arXiv:1111.6395): 0.6 - 0.2 and for H - Z Z , W W


B r(S M 4) 2 B r(S M 3), which is definetly excluded by CMS data on higgs production (slide #9).
M.I. Vysotsky The rise and fall of the fourth quark-lepton generation June 26, 2013 11 / 17


light N
There is a possibility to diminish B r(H - Z Z , W W )S by choosing MH /2 > mN > MZ /2 since H N N becomes a dominant H decay mode. From the ATLAS study of Z H ll+invisible 95% CL upper bound B r(H invisible)< 0.65 follows (ATLAS-CONF-2013-011). So, we can make B r(H - visible) up to three times smaller than in SM3.
M.I. Vysotsky The rise and fall of the fourth quark-lepton generation June 26, 2013 12 / 17

M4


m , ml 600GeV

Up to now we present the result of the 4th generation loop corrections for moderate values of the masses of new leptons. If their masses approach 600 GeV then factor 0.2 in the suppression of H - V V decays becomes 0.15, and the product B r approaches its value for the 3 generation case (Djouadi, Lenz, arXiv 1204.1252).

M.I. Vysotsky

The rise and fall of the fourth quark-lepton generation

June 26, 2013

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H , SM
W t H H t W

A 7 - 4/3 3 (2/3)2 = 7 - 16/9, in the limit MH << 2MW , 2mt . These 7 and 16/9 are the QED - function coefficients; the signs correspond to asymptotic freedom and zero charge behavior respectively. "7" for the first time appears in the 1965 paper of M.V.Terentiev and V.S.Vanyashin. Now: 7 = 22/3 - 1/6 - 1/6, 22/3 = 11/3 2, factors 1/6 originate from the higgs doublet contributions into running of g and g .
M.I. Vysotsky The rise and fall of the fourth quark-lepton generation June 26, 2013 14 / 17


H , 4 generations
For MW = 80.4 GeV 7 should be substituted by 8.3, while for mt = 172GeV 16/9 has 3% accuracy. So, SM: A 8.3 - 16/9 = 6.5 4 gen: A 8.3 - 16/9 - 16/9 - 4/9 - 4/3 = 3.0 and taking into account the enhancement of the H - g g decay in 4 gen case we obtain the same B r as in Standard Model.

BUT (2) loop corrections in case of 4 generations greatly diminish B r(H 2 ); according to Denner et al, arXiv: 1111.6395 it equals 1/3 of 3 generations result (or even less), while the average of ATLAS and CMS data is 1.2 ± 0.2, so the 4th generation is excluded at 4 - 5 level. Would be good to calculate 3 loops.
M.I. Vysotsky The rise and fall of the fourth quark-lepton generation June 26, 2013 15 / 17


H , bb
B r for mode at tree level equals approximately 9 (H production) * 0.6(enhancement of H decay into gluons) 5 and electroweak loop corrections make the decay width larger by 30%. The experimental data on H (slide #9) exclude this huge enhancement (though light N helps to avoid contradiction).

Consideration differs for bb mode: it is seen only in associative higgs boson production V H V bb, which unlike gluon fusion is not enhanced in the 4th generation case, and there is no contradiction with the LHC experimental data.

M.I. Vysotsky

The rise and fall of the fourth quark-lepton generation

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Conclusions
LHC data on 126 GeV Higgs boson production and decays exclude Standard Model with the sequential fourth generation in perturbation domain: too small g g - H - , too big g g - H - . (Light N?) If we are out of perturbation domain (m4 1TeV) extra generation can not be excluded, but we loose the understanding of why all µ's are close to 1 and SM3 works so well. In two Higgs doublets model the fourth generation is still allowed (e.g. Geller, Bar-Shalom, Eilam, Soni, arXiv:1209.4081). Vector generation - another story...
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