There exists one experimental result that cannot be explained by the Standard
Model (SM), the current theoretical framework for particle physics: non-zero
masses for the neutrinos (elementary particles travelling close to light speed,
electrically neutral and weakly interacting). The SM assumes that they are
massless. Therefore, particle physicists are now exploring new physics beyond
the SM. There is strong anticipation that we are about to unravel it, in the
form of new matter and/or forces, at the Large Hadron Collider (LHC), presently
running at CERN. We discuss a minimal extension of the SM, based on a somewhat
larger version of its symmetry structure and particle content, that can
naturally explain the existence of neutrino masses while also predicting novel
signals accessible at the LHC, including a light Higgs boson, as evidenced by
current data.Comment: 5 pages: version to be published in Frontiers in High-Energy and
  Astroparticle Physics following a change of title and minor changes to the
  tex

Khalil, S.

Moretti, S.

Frontiers in Physics

English

arXiv

S. Khalil

S. Moretti

Crossref

A simple symmetry as a guide toward new physics beyond the Standard Model

Frontiers - Publisher Connector

MINI REVIEW ARTICLE
published: 18 September 2013
doi: 10.3389/fphy.2013.00010
A simple symmetry as a guide toward new physics beyond
the Standard Model
S. Khalil 1,2,3 and S. Moretti3,4*
1 Center for Theoretical Physics, Zewail City of Science and Technology, Giza, Egypt
2 Department of Mathematics, Faculty of Science, Ain Shams University, Cairo, Egypt
3 School of Physics and Astronomy, University of Southampton, Southampton, UK
4 Rutherford Appleton Laboratory, Particle Physics Department, Didcot, UK
Edited by:
German Rodrigo, Consejo Superior
de Investigaciones Científicas, Spain
Reviewed by:
German Rodrigo, Consejo Superior
de Investigaciones Científicas, Spain
Stefano Morisi, University of
Wurzburg, Germany
*Correspondence:
S. Moretti, School of Physics and
Astronomy, University of
Southampton, Highfield,
Southampton SO17 1BJ, UK
e-mail: stefano@soton.ac.uk
There exists one experimental result that cannot be explained by the Standard Model
(SM), the current theoretical framework for particle physics: non-zero masses for the
neutrinos (elementary particles traveling close to light speed, electrically neutral, and
weakly interacting). The SM assumes that they are massless. Therefore, particle physicists
are now exploring new physics beyond the SM. There is strong anticipation that we are
about to unravel it, in the form of new matter and/or forces, at the Large Hadron Collider
(LHC), presently running at CERN. We discuss a minimal extension of the SM, based on a
somewhat larger version of its symmetry structure and particle content, that can naturally
explain the existence of neutrino masses while also predicting novel signals accessible at
the LHC, including a light Higgs boson, as evidenced by current data.
Keywords: high energy physics, neutrino models, higgs bosons, collider physics, gauge theories
The fact that neutrinos have mass is now firm observational evi-
dence for new physics beyond the Standard Model (SM). In the
latter, neutrinos are strictly massless due to the absence of right-
handed neutrinos and exact global Baryon minus Lepton (B−L)
number conservation. This letter considers an extension of the
SM, based on the gauge group SU(3)C × SU(2)L × U(1)Y ×
U(1)B−L. Here, we show that it provides a viable and testable
solution to the neutrino mass mystery of contemporary parti-
cle physics. We also emphasize that it contains a SM-like Higgs
boson compatible with current data as well as predicts new parti-
cles: heavy neutrinos that contribute to light neutrino masses, an
extra gauge boson associated with the U(1)B−L gauge group, and
a new heavier Higgs boson that spontaneously breaks the B−L
symmetry at the TeV scale. We argue that all these new states can
promptly be probed at the Large Hadron Collider (LHC).
This machine, hosted at the European Organization for
Nuclear Research (CERN), with some supplemental help from the
Tevatron at the Fermi National Accelerator Laboratory (FNAL),
has apparently discovered a Higgs boson (ATLAS Collaboration,
2012; CDF and D0 Collaborations, 2012; CMS Collaboration,
2013) (which we label H), the only missing, and arguably most
important, piece of the current theoretical framework for par-
ticle physics, the so called SM. While this discovery represents
a milestone in the history of particle physics, it does very little
to answer several fundamental questions raised by experimen-
tal data. Amongst these, a pressing one emerges from neutrino
oscillations, which poses an unresolvable puzzle to the SM, as
they clearly provide evidence of non-zero masses for the neutri-
nos. The SM assumes that its (left-handed) neutrinos are strictly
massless due to the absence of (right-handed) neutrinos and an
exact (i.e., unbroken) global (i.e., space-time independent) B−L
number conservation. [However, at the non-renormalizable level,
neutrino masses can be generated within the SM via a dimension
5 operator (Weinberg, 1979): in fact, the lepton number is an acci-
dental symmetry in the SM at the renormalizable level, therefore
such an operator is not forbidden].
The need to find answers Beyond the SM (BSM) thus remains
clear. In exploring the possibility of some BSM scenario, two
key aspects should be borne in mind. Firstly, the very precise
experimental confirmations of the SM that have kept accumu-
lating throughout the last four decades make it mandatory for
any theory of new physics to exactly reproduce the SM up to the
Electro-Weak (EW) energy scale, of O(100GeV). Secondly, the
tremendous success of gauge symmetries in describing Nature,
which the SM relies upon, implies a general belief that any new
physics should be based on an enlarged gauge group.
Now, two further considerations, which will eventually inter-
twine, ought to be made. On the one hand, the most attrac-
tive dynamics that can naturally account for small yet sizable
neutrino masses is known as the seesaw mechanism (in vari-
ous forms) (Minkowski, 1977; Gell-Mann et al., 1979; Yanagida,
1979; Mohapatra and Senjanovic, 1980; Schechter and Valle,
1980, 1982; Lazarides et al., 1981; Mohapatra, 1986; Foot et al.,
1989; Khalil, 2008, 2010). In this case, three heavy singlet (right-
handed) neutrinos are invoked, with super-heavy masses, of
order 1013 Giga electron-Volt (GeV). Although this scenario
explains in a rather elegant way why neutrinos are much lighter
than the other elementary fermions, it is assumed that it can-
not have a direct low energy signature. On the other hand, the
aforementioned (B−L) symmetry needs be neither exact nor
global. In fact, if both such assumptions are dismissed at once,
a whole new dynamics is generated, for the mere purpose of self-
consistency of the new model. In practice, if the (B−L) symmetry
is locally gauged, then the existence of three SM singlet fermions
www.frontiersin.org September 2013 | Volume 1 | Article 10 | 1
PHYSICS
Khalil and Moretti (B−L)SSM
(the aforementioned right-handed neutrinos) is a quite natural
assumption to make in order to cancel the associated triangle
anomaly, which is a necessary condition for the self-consistency
of the ensuing model (Mohapatra, 1986).
Now, in general, the energy scale of (B−L) symmetry break-
ing is unknown, ranging from O(TeV) to much higher scales.
However, it has been proven in Khalil and Masiero (2008) that,
in a Supersymmetry (SUSY) framework (Martin, 2011), it is
naturally correlated with that of soft SUSY breaking, which is
indeed at TeV energies. Therefore, all such new dynamics does
not perturb establish physics at the EW scale, yet the predicted
new particles, from SUSY or not, will lead to novel signa-
tures at experiments currently probing the TeV energy regime
(i.e., the LHC). Further, after (B−L) gauge symmetry break-
ing has taken place, right-handed neutrinos acquire a mass MR,
which can be of O(TeV). Once standard EW Symmetry Breaking
(EWSB) has also occurred, at a lower energy, a Dirac neutrino
mass mD is finally generated. Therefore, the mass of the phys-
ical light neutrino is given by m2D/MR, which can account for
the measured experimental results on neutrino oscillation if mD
∼10−4 GeV (Mohapatra, 1986). While obviously small, the lat-
ter value is clearly not unnatural, as, e.g., the electron mass
is ∼0.5 × 10−3 GeV.
Despite the small mixing between light and heavy neutrinos,
new interaction terms between the physical heavy neutrinos (plus
the associated leptons) and the weak gauge bosons of the SM
(W and Z) are induced. Further, the model also contains an
extra gauge boson (hereafter denoted by Z′), corresponding to
a now broken (B−L) gauge symmetry, and an extra SM singlet
Higgs scalar (denoted by H′), responsible for it, which is heavier
than its SM counterpart. So, the phenomenological consequences
are numerous. Firstly, the lightest heavy neutrinos can now be
copiously (pair) produced via Z′ exchange at the LHC (Huitu
et al., 2008; Basso et al., 2009). Secondly, the Higgs sector of the
model (hence the specific light H signal accessible at the LHC,
hinting a mass of 125GeV) is now perturbed by the presence
of a heavy state, the H′ (Emam and Khalil, 2007; Basso et al.,
2010, 2011). Further, in this class of models, in addition to SM-
like decay channels, either or both Higgs bosons can decay in
genuine (B−L) final states, like heavy neutrino and/or Z′ pairs,
with sizable rates. This opens up then the intriguing possibil-
ity of all the new states predicted by such a (B−L) model being
simultaneously detected at the LHC (Basso et al., 2011) [see also
(Fileviez Perez et al., 2009; Aguilar-Saavedra, 2010; Majee and
Sahu, 2010)].
To stay with the Higgs sector, curiously enough, the same LHC
data that revealed a Higgs boson also hint at the possibility that
this state is not the SM one, because of a clear enhancement of the
di-photon production rate, over and above the SM predictions
(assuming a Higgs boson). Here, a version of our (B−L) model
supplemented by SUSY may have some light to shed. Firstly, the
current experimental hint of a SM-like Higgs boson with mass
of 125GeV is uncomfortably very near the absolute upper limit
predicted theoretically by the minimal SUSY model (i.e., the one
without additional heavy neutrinos, Z′ boson and singlet Higgs
state plus SUSY counterparts), of 130GeV or so, so as to seem
a very fine-tuned solution. Not so, though, in the SUSY ver-
sion of the (B−L) model, henceforth the (B−L)SSM. A striking
example here is the case of the (B−L)SSM with inverse see-
saw (Mohapatra and Valle, 1986), whereby, as shown in Elsayed
et al. (2012), the one-loop radiative corrections to the lightest
SM-like Higgs boson mass, due to the right-handed neutrinos
and sneutrinos (their SUSY counterparts) can give an absolute
upper limit on it at around 170GeV. (Needless to say, the pos-
sibility that the SM Higgs state had the observed mass would
be merely a coincidence, as such a mass is a free parameter).
Secondly, the (B−L)SSM can play a crucial role in the explana-
tion of such an enhanced di-photon decay rate, thanks to peculiar
contributions to it due to very light staus (the SUSY counterparts
of the tau leptons), whose origin is similar to the one in more
minimal SUSY models than the (B−L)SSM, yet it occurs under
conditions that would more naturally appear at the scale of a
Grand Unification Theory (GUT), which physicists are constantly
seeking and which they believe being underpinned by SUSY
(Basso and Staub, 2012).
In short, a symmetry structure deeply rooted in the SM could
well be the key to extend the latter into a credible new physics
scenario, embedding naturally the neutrino mass patterns mea-
sured by experiment and at the same time offering a wealth of
new physics signals, all promptly accessible at the LHC, as the
dynamics generating the new states occurring in the model can
emerge at the TeV scale (and particularly so in its SUSY version),
hence well within the reach of the CERN collider, and can easily
accommodate a light Higgs boson with its possible anomalies, as
evidenced by recent LHC data.
ACKNOWLEDGMENTS
S. Khalil is grateful to The Leverhulme Trust for financial support
in the form of a Visiting Professorship. S. Moretti is supported in
part through the NExT Institute.
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Conflict of Interest Statement: The
authors declare that the research
was conducted in the absence of any
commercial or financial relationships
that could be construed as a potential
conflict of interest.
Received: 29 June 2013; accepted: 22
August 2013; published online: 18
September 2013.
Citation: Khalil S and Moretti S (2013)
A simple symmetry as a guide toward
new physics beyond the Standard Model.
Front. Physics 1:10. doi: 10.3389/fphy.
2013.00010
This article was submitted to High-
Energy and Astroparticle Physics, a
section of the journal Frontiers in Physics.
Copyright © 2013 Khalil and Moretti.
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www.frontiersin.org September 2013 | Volume 1 | Article 10 | 3


There exists one experimental result that cannot be explained by the Standard Model (SM), the current theoretical framework for particle physics: non-zero masses for the neutrinos (elementary particles travelling close to light speed, electrically neutral and weakly interacting). The SM assumes that they are massless. Therefore, particle physicists are now exploring new physics beyond the SM. There is strong anticipation that we are about to unravel it, in the form of new matter and/or forces, at the Large Hadron Collider (LHC), presently running at CERN. We discuss a minimal extension of the SM, based on a somewhat larger version of its symmetry structure and particle content, that can naturally explain the existence of neutrino masses while also predicting novel signals accessible at the LHC, including a light Higgs boson, as evidenced by current data

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