Transcript Document

Supersymmetric Models with extra singlets: a review

DJ Miller, University of Glasgow Plenary talk, Whepp-9, Bhubaneswar, India 3-14 January 2006

This talk will lean heavily on the review

Models with extra singlets written

for the CPNSH Report by J. Gunion, DJM, A. Pilaftsis


: This talk presents of a personal viewpoint of the supersymmetric models with extra singlets. It does not claim to be complete, and if your favourite model/paper is omitted, please accept my apologies.

3rd January 2006 1


What’s wrong with the MSSM?

Solving the

-problem with an extra singlet Breaking the Peccei-Quinn symmetry The Domain Wall Problem and its solution


The NMSSM parameters and mass spectrum Two interesting NMSSM scenarios Neutralinos in the NMSSM

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What’s wrong with the MSSM?

Supersymmetry has many motivations, but here are the two that I find most convincing:

Only possible extension of the Poincar é symmetry of space-time

To me, this alone tells me that SUSY exists, but it is no argument for low energy SUSY.

The hierarchy problem

If you ever want to extend the SM by including new physics, we will need to explain why the Higgs boson is so light. Supersymmetry tells us why.

Note that the hierarchy problem is not a problem at all if you new physics.

don’t include 3rd January 2006 3

Less convincing (but still good) arguments: The unification of gauge couplings at the GUT scale SUSY has a good

dark matter

candidate – the neutralino

This isn’t really an argument for SUSY – it is an argument for a

Z 2

symmetry This could equally well be part of an alternative theory, e.g. extra dimensions 3rd January 2006 4

So the Minimal Supersymmetric Standard Model (MSSM) has a lot going for it.

Why would we want to extend it?

MSSM Superpotential: Yukawa terms  term The superpotential has mass dimensions [mass] 3 , so the


parameter 


in the superpotential must have dimensions of



What mass should we use?

The natural choices would be


(forbidden by some symmetry) or

M Planck

(or M GUT ) 3rd January 2006 5

Notice that this mass-parameter is in the supersymmetry breaking (via soft terms).

about the electroweak scale


superpotential , so it is present before

Therefore, it should know nothing

If  = 0 then there is no mixing between the two Higgs doublets. Any breaking of electroweak symmetry generated in the up-quark sector (by M H 2 < 0) could not be communicated to the down-quark sector ) the down-type quarks and leptons would remain massless.

If  = M Planck then the Higgs bosons and their higgsino partners would gain Planck scale masses, in contradiction with upper bounds from triviality and precision electroweak data.

For phenomenologically acceptable supersymmetry, the of order the electroweak scale.

-parameter must be This contradiction is known as the


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Solving the

-problem with an extra singlet

One way to link the

expectation value


 -parameter with the electroweak scale is to make it a


Introduce a new iso-singlet neutral colorless chiral superfield usual two Higgs doublet superfields. The scalar part of this is , coupling together the If S gains a vacuum expectation value we generate and



with We must also modify the

supersymmetry breaking terms

to reflect the new structure 3rd January 2006 7

The new scalar naturally picks up a VEV of order the SUSY breaking parameters, just as for the usual Higgs doublets.

Writing: the minimization equations for the VEVs become:

So is of the electroweak/SUSY scale, as desired.

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So our superpotential so far is Yukawa terms effective  term But this too has a problem – it has an extra U(1) Peccei-Quinn symmetry [Peccei and Quinn, Phys.Rev.Lett.


(1977) 1440; Phys.Rev.D


(1977) 1791] Setting U(1) charges for the states as: the Lagrangian is invariant under the (global) transformation This extra U(1) is broken with electroweak symmetry breaking (by the effective  -term)

massless axion

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This Peccei-Quinn axion can be very useful, since it solves the

strong CP problem


The most general QCD Lagrangian contains the  -term with Even if this is set to zero by hand in the Lagrangian, it is regenerated by instantons.

However, this term is CP violating, and would lead to a dipole moment . Experimental limits imply non-zero neutron electric

So why is this so small?

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The axion also contributes a  -term through an axion-gluon triangle anomaly diagram axion decay constant axion field As a result the potential is a function of and the axion naturally relaxes to a value thereby cancelling the  -term and solving the strong CP problem

Unfortunatetly a (nearly) massless axion (actually a psudoscalar Higgs boson) has not been found, so this is a bit of a wild goose chase …..or is it?

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Removing the Peccei-Quinn axion

While the Peccei-Quinn axion would be nice to have around, we do not see it, so we have another problem. There are (at least) three possible ways out, all of which introduce more problems.

Decouple the axion

We could just make  very small, thereby decoupling the axion so that it would not have been seen in colliders.

Unfortunately there are rather severe astrophysical constraints on the cooling rate of stars in globular clusters, which constrain  from .

There is (to my knowledge) no good reason why  should be so small.

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Since the restriction on  ) Note that this is not an


problem since it is  eff which is naturally of the EW scale e.g.

Any mechanism which makes  small also makes large.

So we have solved the strong CP problem and the introducing a new “  problem”.

 -problem at the expense of Unfortunately (fortunately?) we have a further, if rather mild,

fine tuning problem

: It is difficult to keep the vacuum stable since the lightest scalar mass-squared tends to become negative. It is only positive for a restricted parameter range.

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In order to keep the lightest Higgs boson (from the extra singlet) mass real, we need to enforce [where M A and tan in the MSSM]  are as defined 3rd January 2006 This is a mild fine-tuning, but also provides a


for this model at the LHC [DJM, R Nevzorov, hep-ph/0309143] 14

Finally, there may be a problem with Dark Matter in this model. The higgsino partner has a very small mass, typically .

Since this is the lightest supersymmetry particle (LSP) it is stable and contributes to dark matter.

Usually such light particles are disastrous because they are very hard to annihilate, and therefore give too much dark matter. However, this LSP is so decoupled that it may never have come into equilibrium in the first place…..

Eat the axion

Making the U(1) Peccei-Quinn symmetry a

gauge symmetry

introduces a new gauge boson which will eat the PQ-axion when the PQ symmetry breaks and become massive (a Z 0 ). Searches for a Z 0 provide rather model dependent results but generally indicate that it must be heavier than a few hundred GeV.

To cancel anomalies one needs new chiral quark and lepton states too, so I discuss these models further here.

won’t 3rd January 2006 15

Explicity break the PQ symmetry

In principle, one can add

extra terms

into the superpotential of the form S n with n 2 Z but only for n =3 will there be a break the PQ symmetry, giving the dimensionless coefficient . Any such term will “axion” a mass so that it can escape experimental constraints.

The superpotential of the



N ext-to M inimal S upersymmetric S tandard M odel

3rd January 2006 Yukawa terms effective  term PQ breaking term 16

We also need soft supersymmetry breaking terms in the Lagrangian: [Higgs sector SUSY breaking terms only] This model has the same particle content as the MSSM except one extra scalar Higgs boson one extra pseudoscalar Higgs boson one extra higgsino for a total of 3 scalar Higgs bosons 2 pseudoscalar Higgs bosons 5 neutralinos The charged Higgs bosons and charginos are the same as in the MSSM.

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The Domain Wall Problem

Unfortunately we have yet another problem.

The Lagrangian above has a (global) Z

3 symmetry

) the model has

3 degenerate vacua

separated by potential barriers [This was an unavoidable consequence of having dimensionless couplings.] vacuum 1 vacuum 2 We expect causally disconnected regions to choose different vacua and when they meet a

domain wall

form between the two phases.

will These domain walls are unobserved (they would be visible in the CMBR) so we need to remove them.

3rd January 2006 domain wall [Y.B.Zeldovich, I.Y.Kobzarev and L.B.Okun, Zh.Eksp.Teor.Fiz.


(1974) 3].


The degeneracy may be broken by the unification with gravity at the Planck scale.

Introducing new higher dimensional operators raises the vacuum energies unequally, resulting in a preferred vacuum.

However, the same operators give rise at the loop level to

tadpole terms

of the form

quadratically divergent

where n is the loop order they appear.

[S.A.Abel, S.Sarkar and P.L.White, Nucl.Phys.B


(1995) 663; S.A.Abel, Nucl.Phys.B


(1996) 55.] If such operators do break the degeneracy, then they must be suppressed to a high enough loop order that they don’t cause a new hierarchy problem.

Use symmetries to suppress then to high loop order.

Example of a 6-loop tadpole contribution 3rd January 2006 [C.Panagiotakopoulos and K.Tamvakis, Phys.Lett.B


(1999) 224; [ C.Panagiotakopoulos and A.Pilaftsis, Phys.Rev.D


(2001) 055003.] 19

There are many different choices of symmetries to do this. Which you choose, changes the model.

The 2 most studied are:

Next-to-Minimal Supersymmetric Standard Model (NMSSM)

Choose symmetries to forbid divergent tadpoles to a high enough loop order to make them phenomenologically irrelevant but still large enough to break the degeneracy.

Minimal Non-minimal Supersymmetric Standard Model (MNSSM)

Choose symmetries to forbid also the S^3 term, but allow tadpoles which have a coefficient of the TeV scale.

3rd January 2006 radiatively induced tadpole 20

The NMSSM mass spectrum

[DJM, R. Nevzorov, P.M. Zerwas, Nucl.Phys.B681 (2004) 3] The NMSSM Higgs sector has the potential V = V F + V D + V soft +  V


Replace soft masses with vacuum expectation values using minimization condistions: 3rd January 2006 21

Parameters:   A  A  m H d m Hu m S   A  v d v u v s The MSSM limit is  !

0,  !

0, keeping  /  and  fixed.

3rd January 2006 Top left entry of CP-odd mass matrix. Becomes MSSM M A in MSSM limit.

M Z , tan  ,  eff Will also sometimes use 22

 and  are forced to be reasonably small due to

renormalisation group running


To stop them blowing up, we need to insist that 3rd January 2006 23

Lightest Higgs mass bound

In the MSSM In the NMSSM The extra contribution from the new scalar raises the lightest Higgs mass bound, but only by a little.

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Approximate masses

The expressions for the Higgs masses are rather complicated and unilluminating, even at tree level, but we can make some approximations to see the general features.

Regard both M EW /M A and 1/tan  as small and expand as a power series.

CP-odd Higgs masses 2 :

heavy pseudoscalar one pseudoscalar whose mass depends on how well the PQ symmetry is broken

CP-even Higgs masses 2 :

heavy scalar Notice the different signs for A  3rd January 2006 intermediate mass scalar one scalar whose mass depends on how well the PQ symmetry is broken 25

Computer codes for NMSSM phenomenology: NMHDECAY

by Ellwanger, Gunion & Hugonie

A (hopefully!) soon to be released code by Djouadi, Kalinowski, King, DJM, Moretti. 3rd January 2006 26

Two interesting scenarios

PQ symmetry only “slightly” broken

[DJM, S Moretti, hep-ph/0403137] Most of the M A range is excluded (at 95%) by LEP2 higgs-strahlung but there is still a substantial region left.

Notice the rather light Higgs boson!

In the allowed region, the couplings of the lightest Higgs to gauge bosons is switching off, which is why LEP would not have seen it.

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Branching ratios of lightest Higgs: This Higgs decays mostly hadronically, so it will be difficult to see at the LHC, due to huge SM backgrounds.

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LHC production rates are quite high, but many channels switch off.

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A 500GeV linear e + e collider would be able to narrow the gap, but not close it. 3rd January 2006 30

A very light pseudoscalar

[Ellwanger, Gunion & Hugonie JHEP 0507:041,2005] For these parameters, The lightest pseudoscalar is now so light that » pseudoscalar pairs: 100% of H 1 decays are into ) the lightest scalar could be significantly lighter than 114GeV and have been missed by LEP 3rd January 2006 31

F It is claimed that this model is less fine tuned too.

Taking and scanning over parameter space [From J. Gunion’s talk at SUSY05] 3rd January 2006 £ have M H1 > 114GeV


have M H1 < 114GeV Points with high H 1 !

A 1 A 1 branching ratio have smaller fine tuning If the pseudoscalar is heavy enough, it may be observable through decays to tau pairs: 32

Recently (2 weeks ago) a paper by Schuster & Toro, hep-ph/0511344 , pointed out that this point has fine tunings with respect to other observables, e.g. the pseudoscalar mass with respect to A  However, it was not clear how different fine-tunings should be


to form one measure of how fine tuned a model is.

To my mind, all this shows is that we really

need a better measure of fine tuning.

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Neutralinos in the NMSSM

[Choi, DJM, Zerwas, Nucl.Phys. B711 (2005) 83 ] The extra higgsino means that we have an extra neutralino in the spectrum.

The neutralino mass matrix: Again, the expressions for the masses helpful but we can make aren’t very approximations in particular limits to help understand what is going on.

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Large gaugino mass

Two neutralinos at the gaugino masses Two neutralinos split around  One neutralino with mass strongly dependent on   This is primarily a “singlino” e.g.

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Large higgsino mass

This is very similar to the previous scenario.

Large singlino mass

Now with other masses as in the MSSM The extra neutralino decouples, making this difficult to distinguish from the MSSM.

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Neutralino production

If the singlino mass is small (ie. small) as indicated by RGEs, then in the LHC the singlino will be mainly produced through decays of heavier neutralinos, charginos and sleptons.

Some possble decay channels are: (three body) 3rd January 2006 Which of these channels is open or dominant for the singlino is highly dependent on the scenario.


e.g. for Huge variations in partial widths with   .

Only for   tiny is the neutralino decoupled enough to have observable displaced vertices 3rd January 2006 38

Conclusions & Summary

The MSSM has a serious flaw

: it phenomenologically requires a parameter of order the electroweak scale with no justification.

 to be This problem can be circumvented by

introducing a new scalar field

the vacuum expectation value of this new field to the  -parameter.

and linking The resulting model contains a

broken U(1) Peccei-Quinn symmetry which must be

with extra interactions. Extra symmetries must be enforced to prevent quadratically divergent tadpoles. Which symmetries are imposed determine which model we have, with examples being the NMSSM and the MNSSM.

The NMSSM spectrum contains the particles of the MSSM with an

scalar, and extra Higgs pseudoscalar and an extra neutralino


extra Higgs

The mass of these extra particles strongly depends on

symmetry is broken


how severely the PQ

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interesting scenario

is where the extra Higgs boson is » 90GeV or so and has suppressed couplings to the gauge bosons. This would have prevented it being seen at LEP. Its decays are primarily hadronic, making its observation challenging at the LHC.

A second interesting scenario has a

very light pseudoscalar

. The lightest scalar then decays predominantly into a pseudoscalar pair and is difficult to observe.

This scenario requires less fine-tuning to obtain the correct Z-boson mass, but may have fine-tuning to obtain other observables.

The NMSSM is a very interesting model, and more studies should be done to investigate its phenomenology at future colliders. Experimental studies are particularly needed.

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CP studies and Non-Standard Higgs physics

Aiming to produce a “

hitchhiker’s guide to non-standard Higgs physics


Editor: Sabine Kraml Topics include: The CP-violating two Higgs doublet model The MSSM with CP phases

SUSY models with an extra singlet

MSSM with R-parity violation Extra gauge groups Little Higgs models Large EDims: ADD Randall-Sundrum: Radion Higgsless models Chiral symmetry breaking Technicolour, topcolour Deadline for contributions: 31 st March 2006 .

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