large loop-coupling enhancement of a 750 gev

PASCOS, Vietnam - 2016

Stefano Di Chiara

Large loop-coupling enhancement of a 750 GeV pseudoscalar from a light

dark sector

SD, A. Hektor, K. Kannike, L. Marzola, M. Raidal; arXiv:1603.07263

Motivations

2S. Di Chiara - PASCOS 2016

ATLAS-2015-081, CMS-15-004, Franceschini et al.’15

Latest LHC data at 13 TeV (3.3 fb�1) show diphoton events excess at750 GeV with local significance of 3.4� at CMS and 3.9� at ATLAS

Simplest interpretation: SM particle content extended by EW singletspin-zero field S, with mS = 750 GeV, coupled only to a vector quarkQv, with mQv ⇠ 700 GeV, but...

� (pp! S ! ��) ⇠ 10 fb ) ySQv⇠ 10 , ⇤UV = O(TeV)

This is unsatisfactory and increasing multiplicity Nf ⇥ df of Qv helpsonly up to a point: such large cross section might be a consequence ofstrong dyamics or...

Decay width to diphotonDecay widths of scalar H and pseudoscalar A to �� mediated by fermions

f :

�� =↵2

em3S

256⇡3v2w

��

X

f

afNfe2fFS

f

��

2

, af =yfvwp2mf

, S 2 {A, H}

with

FHf = �2⌧f [1 + (1� ⌧f ) f(⌧f )] , FA

f = �2⌧ff(⌧f ) , ⌧f =4m2

f

m2S

Pseudoscalar partial amplitude is a↵ected by a discontinuity

limmf!(mA/2)±

FAf =

⇡2

2, FA

f |mf=mA/2 = ⇡2 ,

3Ellis et al.’76

S. Di Chiara - PASCOS 2016

Decay width to diphotonLeft panel : Ratio of decay rate to �� of A to that of H mediated by

fermion of mass mf . Right panel: H decay rate to �� normalized to

mf = 700 GeV case

200 300 400 500 600 700 8001

2

3

4

5

6

mf êGeV

G ggAêG ggH

200 300 400 500 600 700 8001.0

1.5

2.0

2.5

mf êGeVG ggHêG ggH

mf=700GeV

Near the threshold: yAf /yH

f ' 1/3.5 ) �A�� = �H

��

4SD et al.’16


Experimental limits

5

Diphoton cross section at LHC in the narrow width approximation:

� (pp! A! ��) = Cgg�gg��

s mA�tot

, �gg =↵2

sm3

A

128⇡3v2

w

��

X

f

afFAf

��

2

,

withCgg = 2137 , mA = 750 GeV ,

ps = 13 TeV

Gluon-fusion process mediated by top quark not viable because of ttchannel 8 TeV LHC constraint & large contribution to �

tot

.

Experimental limits on vector quark masses range from 705 to 846 GeV,but relaxed to 690 GeV for decays to light quarks only. Vector chargedleptons must be heavier than 400 GeV, but limit relaxed to 104 GeV fordecays to nearly degenerate SU(2) neutral component.

Martin et al.’09,CMS-12-013,CMS-14-001,ATLAS PRD92(2015),Abbiendi et al.EPJC29(2003)


Model

Scalar and vector fermion content: Z2 symmetry to avoid lepton-number

violating couplings, with all the SM fermions but the 1st and 2nd quark

generation even under Z2. Bonus: N dark matter (DM) candidate.

Field SU(3)c SU(2)L U(1)Y Z2

H 1

✓�+

�vw + h + i�0

�/p

2

◆1/2 +

A 1 A 0 +

L 1

✓NE

◆�1/2 �

E01 E0 �1 �

Q 3

✓UD

◆1/6 �

U 03 U 0 2/3 �

6SD et al.’16


Lagrangian

7

• Vector fermions: gauge invariant mass terms, no anomalies

• Yukawa couplings to H generate SU(2) doublet mass splitting

• Yukawa couplings to A give (only) diboson widths

• CP symmetry: no terms odd in A (no A! hh decays), �AH > 0

• �A and �AH to stabilize SM vacuum

L �hyL

LLLHE0R + yR

L LRHE0L + yL

QQLHU 0R + yR

QQRHU 0L + H.c.

i

� iyLAL�5L� iyEAE0�5E0 � iyQAQ�5Q� iyUAU 0�5U 0

+ mLLL + mE0E0E0 + mQQQ + mU 0U 0U 0

�m2AA2 � �AA4 � �AHA2|H|2 .


LHC diphoton signal

Choice of initial conditions:

� (pp! A! ��) = 6 fb ,

yE = yU = �yL = �yQ ⌘ yv ,

mD = mT = 700 GeV , mT 0 = 705 GeV ,

Two DM scenarios:

Scenario mN (GeV) m�±1(GeV) m�±2

(GeV) yv

I 375 380 390 0.41

II 1200 1205 1210 1.09

Small EW doublet mass splitting imposed by T parameter constraint


LHC @ 8 TeV bounds

Complementary channels at 8 TeV LHC: prediction at 1 loop for the twoscenarios satisfies experimental upper bound for each channel

�th

I

(fb) �th

II

(fb) �exp

max

(fb)jj 38 199 2.5⇥ 103

WW 1.63 2.54 40ZZ 0.08 0.08 12Z� 0.04 0.14 11

9SD et al.’16, Franceschini et al.’15


UV cutoff scale

�H

�AH

�A

1000 104 105 106 107 108 1090

2

4

6

8

10

12

�/GeV

• scenario 1: ⇤UV = 109GeV


• Stable potential for both

scenarios

Run SM couplings from mt to 750 GeV within SM, use as initial condi-

tions, fix �A and �AH to maximize ⇤UV, defined as �AH (⇤UV) = 4⇡,

and avoid �H(µ) < 0:

10SD et al.’16


UV cutoff scale



• Stable potential for both

scenarios

Run SM couplings from mt to 750 GeV within SM, use as initial condi-

tions, fix �A and �AH to maximize ⇤UV, defined as �AH (⇤UV) = 4⇡,

and avoid �H(µ) < 0:

�H

�AH

�A

1000 2000 5000 1040

2

4

6

8

10

12

�/GeV


DM Abundance

��

��

��

�

�� / �

��/�

��

��

��

��

��=��

(�=��

��

)

��=��

��

(�=��

��

)

12

Vector neutrino N has only weak interaction = authentic WIMP: annihi-

lations to SM particles mediated by Z. Fraction of DM relic abundance

that N can account for, as a function of mN :


Conclusions

• Large diphoton excess can be produced via threshold e↵ects rather

than large tree level couplings

• Cuto↵ scale (without tuning) equal to 109GeV

• Byproduct: viable DM candidate

• SM vacuum stabilization


750 GeV Summary


750 GeV Summary

750 GeV


750 GeV Summary

750 GeV

VL fermionsDM

new interactionshidden valleys

...


SM background

statistical fluctuation

750 GeV BSM pheno


750 GeV Summary750 GeV Summary750 GeV Summary


750 GeV Summary

Which one of the two?For the answer wait for

ICHEP 2016!


750 GeV Summary

Which one of the two?For the answer wait for

ICHEP 2016!

S. Di Chiara - PASCOS 2016CẢM ƠN!

750 GeV Summary

Backup Slides


Decay width to diphoton�H!�� =

↵2em

3H

256⇡3v2w

��X

i

Nie2i Fi

��

2

,

where Ni is the number of colors, ei the electric charge, and

FV = [2 + 3⌧V + 3⌧V (2� ⌧V ) f(⌧V )] aV ,

Ff = �2⌧f [1 + (1� ⌧f ) f(⌧f )] af ,

FS = ⌧S [1� ⌧Sf(⌧S)] aS , ⌧i =

4m2i

m2H

,

with

f(⌧i) =

8><

>:

arcsin

2p

1/⌧i ⌧i � 1

�1

4

log

1 +

p1� ⌧i

1�p

1� ⌧i� i⇡

�2

⌧i < 1

.

In the limit of heavy particles: FV = 7 , Ff = �43 , FS = �1

3 .


large loop-coupling enhancement of a 750 gev

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