# Research

This page provides a brief overview of my current research work. You can find further details on my work on my INSPIRE profile.

For a brief introduction, have a look at the SCM 2000 presentation I gave in November 2022.

**Undergraduate Research Opportunities **

There are opportunities for undergraduate research projects. I can suggest several projects according to interest, based on my research themes outlined below. Interested students should contact me via e-mail at first instance.

My current **Research Themes** include:

- I: Imprints of Electro-Weak Symmetry Breaking at Colliders.
- II: Monte Carlo Event Generators and Quantum Chromo-Dynamics.
- III: Searching for New Phenomena at Colliders.

**Research Theme I: Imprints of Electro-Weak Symmetry Breaking at Colliders.**

One of my current research interests is to** understand the realization of electro-weak symmetry breaking (EWSB) in Nature** and what implications this carries on our theory of matter and its interactions.
I am engaging this from the point of view of high-energy particle colliders, with
the aim of amplifying their discovery potential through precision calculations and
simulations. This can have profound implications in the wider context of cosmology
and astrophysics.

**Figure 1.** The Higgs potential in the early Universe (left) versus today (right). You can imagine
the Higgs potential as existing "everywhere". The Higgs field has to "respond" to
it everywhere in space. In the early Universe, the Higgs potential has a minimum at
the origin. The Higgs field "sits" there and the symmetry of the potential is obvious
and explicit (left). Today the potential has the form shown on the right-hand side
figure: the potential itself is symmetric about the origin, but the Higgs field now
sits in a minimum away from the origin. We say that the Higgs field has a "vacuum expectation value". Thus
any physical observations we make today will seemingly hide or "break" the symmetry
that is inherent in the potential.

**The importance of EWSB to the fundamental theory of Nature cannot be overstated.** At the very least, it constitutes the mechanism that consistently explains the observed
masses of elementary particles. This is accomplished by introducing the scalar Higgs
field (H), which sits in a potential that enables it to acquire a non-zero value everywhere
in space: a vacuum expectation value (vev) [see Figure above]. The Higgs vev gracefully
breaks part of the symmetries of our best current theory of matter and energy, the
Standard Model (SM). The remnant of this field is a physical scalar (i.e. spin-less)
massive particle, the Higgs boson (h), discovered at the Large Hadron Collider (LHC),
a culmination of decades of collider experiments.

**Figure 2.** An illustration of the vacuum stability problem. The Higgs field may spontaneously
tunnel to another minimum that could be more stable than ours (green curve). This
would have catastrophic consequences for the Universe as we know it!

But electro-weak symmetry breaking, and consequently the Higgs field, can provide
*much more* than a neat way to introduce mass terms in the SM Lagrangian; they are very likely
to be **intricately connected to a multitude of important scientific questions.** For instance, calculations indicate that, at high values of the Higgs field, the
potential could become deeper than our electro-weak vacuum. We should therefore **worry about the Higgs field tunneling quantum-mechanically to such an instability
region**, thereby rendering the current state of the universe meta-stable or even unstable.
Studies indicate that our universe may currently lie in a meta-stable state (“the
vacuum stability problem”). Measuring the self-couplings of the Higgs boson – an endeavor
I have been pioneering for the better part of the last decade from a variety of aspects
– will allow us to map the Higgs potential and shed light on the question of vacuum
stability.

**Figure 3.** A form of non-luminous matter that permeates our Universe was first detected by observing
the rotation velocities of stars around their galaxies. This was expected to drop
off as A (dashed blue curve) but instead B (solid red curve) was observed, indicating
the presence of a previously-unknown form of matter. The nature of this so-called
"Dark Matter" remains entirely mysterious at present. It could, for example, consist
of one or many new particles that interact extremely weakly with those that make up
the known particles.

Furthermore, the Higgs boson could enable the existence of “portal” interactions with
dark matter, the form of non-luminous matter that dominates our universe. This comes
about through its unique property to interact, through the “singlet” Lagrangian operator
H^{†}H, to fields that otherwise would not communicate with the SM, except through gravitational
interactions (singlet fields). This portal to a whole hitherto-undetected matter could
be facilitated via new scalars in the “dark sector”, which themselves **may comprise part of dark matter.**

**Figure 4.** Matter and anti-matter are "mirror" versions of each other. For example, an electron
has an anti-particle called the positron that differs only in terms of its charge.
The interactions that we know of are symmetric with respect to matter and anti-matter:
a photon can break up into an electron-positron pair and these can then annihilate
back into a photon. These processes occur at equal rates. But somehow, most of the
things we see in the Universe, including ourselves, the planets and the stars, are
by far predominantly composed of matter. There has to be a mechanism to explain how
this gigantic asymmetry was generated in the early Universe.

Intriguingly, both the Higgs field and these new scalar particles **may also play a central rôle in generating the observed abundance of matter over
that of anti-matter in the universe** (“the baryon asymmetry”). On the one hand, the properties of the Higgs field and
potential determine the temperature scale at which the electro- weak phase transition
took place in the early universe, as well as the energy scale at which the non-perturbative “sphaleron”
processes which initiate the asymmetry occurred. On the other hand, the existence
of new singlet scalars may be necessary to prevent the “washout” of the generated
asymmetry, by acting as catalysts for a sufficiently “strong” first-order electro-weak
phase transition.

Τherefore, by **discovering new scalar particles, determining their intrinsic properties and discerning
their relation to the Higgs boson, we may simultaneously decipher the mysteries of
vacuum stability, dark matter and baryon asymmetry.** There are well-motivated theoretical arguments that such an accomplishment is well
within the reach of either the LHC or future collider experiments, such as a 27 TeV
high-energy LHC (HE-LHC) or a 100 TeV “Future Circular Collider” (FCC).

I have made first steps in this direction in a recent publication: arXiv:2010.00597. See a recent seminar I gave at the University of Sussex for a summarising exposé.

**Research Theme II: Monte Carlo Event Generators and Quantum ChromoDynamics.**

Monte Carlo Event Generators (MCs) are *absolutely essential tools* in particle physics. They provide detailed simulations that provide a realistic description
of particle collisions, employed both by experimentalists and theorists. This is accomplished
through high-precision calculations and phenomenological models, consisting of millions
of lines of computer code.

I have authored and maintain one of the most popular MC simulation tools used amongst
theorists and experimental collaborations, the **Herwig 7** framework. **Herwig 7** is robust and modular and provides high-precision simulations of particle collisions.

MCs incorporate various aspects of Quantum ChromoDynamics (QCD): the theory that describes the strong force. They contain two classes of calculations for this theory: "perturbative", where we can calculate many things by performing a series expansion in terms of the coupling constant of QCD, and "non-perturbative", where the coupling constant is too large and therefore expansions are not possible. The latter need to be fitted from data. The figure below shows how an event looks like in the Monte Carlo "picture". The red blobs and the incoming "hadrons" (e.g. protons at the LHC) represent parts of the event that are non-perturbative.

**Figure 5. **A view of a Monte Carlo event. The springy lines represent "gluons", carriers of the
strong force (QCD). The red blobs are so-called hadrons. The incoming hadrons (h)
could be protons or other composite particles. The circle in the middle could represent
the interaction that we are actually interested in!

**Research Theme III: Searching for New Phenomena at Colliders.**

Our best theory of fundamental particles and their interactions, the Standard Model (SM), is being put continuously to the test by the CERN Large Hadron Collider and other experiments. Thus far, there have been no stark signals of new phenomena. However, there are plenty of intriguing questions that remain unanswered (see above for some of them!). It is therefore important to sharpen our ability to search for new phenomena at colliders, that may go beyond the SM. These could be new interactions, new particles, changes in the behaviour of interactions at higher energies and so on.

One of my research interests is to examine such interesting models of new physics and develop techniques to detect them and comprehend their origin at colliders. For example, I've looked at new versions of the W-boson, a so-called W-prime, exotic Higgs bosons in super-symmetric theories, so-called "lepto-quarks", that interact with both leptons and quarks and I have developed techniques to look for deviations in the the way the Higgs boson interacts with itself. I'm also interested in exotic processes that are known to exist in the SM but have not been observed, such as electro-weak sphalerons or QCD instantons.

**Figure 6.** An example of how an electro-weak sphaleron would look like at a hadron collider:
initial-state up or down quarks (u, d) would interact and several quarks, leptons
and gauge bosons (wavy lines) would come out. These kind of events would light up
detectors in a spectacular fashion!