Research Report On Intersection Of Collider And Dark Matter Phenomenology
As a particle physicist, my goal is to understand the fundamental building blocks of nature. Some of the questions that interest me are - what elementary particles constitute the universe and what symmetries govern their interactions? Why there are more particles than anti-particles in the Universe? How our cosmological history has been affected by fundamental laws of physics?
The Standard Model (SM) of particle physics, which uses the successful framework of quantum field theory, and is successful at describing the interactions of elementary particles at distance scales and energies (O (TeV)) probed by today’s experiments. The discovery of the elusive Higgs boson by the Large Hadron Collider (LHC) experiments completes the Standard Model (SM) of particle physics. However, the SM, although very effective, does not address many important phenomena observed in nature. Among other things, it fails to explain ∼ 80%of the matter density of the Universe (called dark matter), non-zero neutrino masses (neutrinos are massless in the SM), and the asymmetry between baryons and anti-baryons in the Universe (called baryogenesis). Besides, the SM suffers from serious conceptual difficulties that the SM Higgs boson mass is not stable under radiative quantum correction, and the enormous hierarchy between the electroweak (EW) scale and the Planck scale, where quantum gravity becomes important. All of these issues encourage us to expect physics beyond the SM (BSM) at higher energies. Moreover, any new fields present at higher energies will affect the evolution of the Universe and leave its imprint in experimentally measured cosmological parameters.
The quest to search for BSM physics has led us to construct gigantic colliders like the LHC, and hunt for DM in underground mines and outer space. In addition, there are various probes studying the structure of our Universe. We have recently detected gravitational waves as well, where physics pertaining to any phase transition during baryogenesis may show up. My research program spreads across these fields, trying to search for what lies beyond the SM, understanding the nature of DM, and to examine potential signals from various mechanisms that can produce baryon asymmetry in the Universe. The key insights of my research are to optimize our potential for discovery at the LHC and interpreting their results, along with DM direct and indirect detection and flavor physics experiment results, and build BSM physics models.
Collider phenomenology
In this section, I summarize some of the specific research directions that I would like to follow in collider phenomenology.
The discovery of the Higgs boson at the LHC has provided us a new window to search for BSM physics by exploring its connection with the new physics sector. My current research interests are significantly motivated by this newly opened gateway to BSM physics. Moreover, in the absence of any new physics signal, studying the properties of the Higgs boson more precisely may give us future directions to explore. Over the last few years, I have formed collaborations with theorists, experimentalists and machine learning researchers for exploring new physics searches and precision studies of the SM Higgs sector. In this collaboration, our goal is to employ cutting-edge data analysis techniques to high luminosity LHC to push the boundaries of what is possible for the LHC to explore.
In the first paper within the collaboration mentioned above, we investigated the potential for measuring the cubic self-coupling of the discovered Higgs at the LHC for possible signs that the Higgs sector is more complex than the SM. We developed a systemic approach to place cuts on kinematic variables using Bayesian optimization methods, and when this technique is used, in conjunction with any machine-learning algorithm (e. g. , boosted decision trees), it improves the sensitivity of a search at the LHC significantly. I can envisage several other precision studies of the Higgs sector where advanced data analysis tools will be helpful. Studying the Lorentz structure of Higgs-bottom quark Yukawa coupling using the polarization information of B-hadrons, and modification of the couplings of Higgs with weak gauge bosons due to potential higher dimensional operators are some examples. As we are on the cusp of entering the era of “big data” at the high luminosity LHC, I believe that my research will play an important role to find evidence of new physics by incorporating state-of-the-art machine learning algorithms into search strategies.
Precision study of the Higgs sector is also a major goal of the proposed e+e− machine, the International Linear Collider (ILC). In an ongoing project, I am investigating the CP property of the Yukawa coupling of a Higgs-like scalar (X) with b quarks and its possible impact in the threshold behavior of bb̄X production cross-section at the ILC. In the future, I plan to lead my research group into further precision studies of the EW sector at the ILC.
My past work in collider physics involved predicting signals at the LHC from well-motivated BSM models. As we enter the Run-III of the LHC, my research group will continue in this path, while exploring the possibilities for them in future colliders as well. In the high luminosity run of the LHC new particles may be discovered connecting new physics with the hierarchy problem, or gauge/Higgs bosons of some extended gauge group, or some exotics like vector-like fermions. These possibilities afford rich avenues for model building as well as innovative search strategies.
I have explored several of these avenues in the past. I studied the rich and complex Higgs sector of Left-Right symmetric models in the context of the LHC, analyzing the possibility of detecting exotic doubly-charged Higgs using weak vector boson fusion topology and estimating the rate of lepton flavor violating decays of the observed 125 GeV Higgs. I examined signatures of extra gauge bosons coming from Left-Right models, placing them within constraints coming from a variety of searches: the ATLAS study on WZ decaying into hadronic final states, and the CMS study in the Wh channel decaying into bb̄`ν and di-jet final states. More recently, I explored the phenomenology of a SM singlet scalar resonance at the LHC, produced in the presence of vector-like fermions. In particular, I, along with my collaborators, built a SU(6) grand unified theory model proposing a mechanism of generating the masses of vector-like fermions, protected by additional gauge symmetries. Previously, I have also worked extensively in searches for super symmetric particles. I highlighted the composition of the neutralinos that resolves the long-standing (g − 2)µ anomaly and probed the corresponding signal predictions of those scenarios at the high luminosity run of the LHC.
If the high luminosity LHC makes a discovery, it will be a priority of my research program to discern the underlying collider physics. On the other hand, in the unfortunate event of lack of any immediate discovery, I will direct my group to develop innovative analysis techniques to investigate any possible missed signatures, and pursue challenging kinematic configurations. I am continuing to design an array of algorithms in multivariate analysis with my collaborators that my group will keep using. I expect that our efforts will be fruitful in revealing signatures of new physics in the dark matter searches or the Higgs sector at the LHC. It is needless to say that if plans for future machines like the 100 TeV collider comes closer to reality, I will prepare my group to work on such colliders.
Dark matter phenomenology
In this section, I describe the direction in which my work in dark matter (DM) phenomenology is likely to go in the next few years.
The identity of DM is still elusive despite overwhelming evidence of its existence ranging from observations from galaxies and galactic clusters to the cosmic microwave background. The central focus of my work in DM physics in the past was to study the astrophysical signatures coming from DM annihilation or decay.
I examined the tension between the DM explanation of what is known as the Galactic Center Excess(an emission of diffuse gamma-rays distributed nearly spherically symmetric about the Galactic Center) with lack of gamma-ray signals from the dwarf galaxies discovered around Milky Way, which are quintessential targets for indirect DM searches. On the other hand, I interpreted Fermi-LAT dwarf galaxy data to give state-of-the-art results on the decaying dark matter lifetime. This study involved a bin-by-bin maximum likelihood analysis using the PASS-8 event class. In a recent study, I studied both cosmic-ray and gamma-ray signatures from a nearby hypothetical DM clump of angular size O(10◦) and with a dense core of size ∼ 10 pc. I searched for such DM clumps across all the sky outside the galactic plane using 10 years of Fermi-LAT data.
Going forward, I want to strengthen this line of research. In an age where increasingly more sensitive probes are sent to the space to study cosmic-rays, gamma rays and X-rays, if any hints of DM signal emerge from such future experiments, my experience with analyzing gamma-ray and cosmic ray data and collaborative experience with astrophysicists will make my group competitive.
The collider aspect of my DM research involves hunting for DM with subtle signals at the LHC. This requires designing creative search strategies to flush out tiny signals above the enormous SM background. In the course of several collider studies I investigated super-symmetric DM candidates with small production cross-section and kinematic features that are difficult to distinguish from the SM background due to compressed super-symmetric mass spectra. In my work, I advocated the use of one (mono-jet) or two (weak boson fusion) energetic jets to boost the system to enhance missing transverse energy and to produce detectable leptons at the LHC. In contrast, I built a model of SU(6) grand unified theory with a Majorana fermion DM candidate, where the fermion is a singlet under the SM gauge groups.
In future, I want to lead a DM research program on DM candidates that have non-thermal cosmological his-tory since the conventional thermal weakly interacting massive particle paradigm is being increasingly constrained by a host of experiments. In a different direction, I want to initiate a research program on the direct detection experiments of DM with the idea of using polarized detectors. With increasing sensitivity of direct-detection experiments, we expect to detect the atmospheric neutrino background very soon. If DM possesses a spin-independent scattering cross-section, similar to neutrinos, our next challenge will be how to distinguish between dark matter signal from neutrino background. To achieve that goal, I want to exploit the fact that neutrinos interact with ordinary matter, violating parity maximally, and using polarized detectors, I want to survey an extensive set ofnon-relativistic DM-nucleon interaction operators and various DM mediator models.
In the next few years, I expect DM searches to further move towards non-WIMP type paradigm (e. g. , axion-like DM, MeV scale DM, self-interacting DM). We will need new ideas of BSM physics to be incorporated in model-building and will require sophisticated analysis techniques to dig out small signal rates. Moreover, model-building will require an in-depth knowledge of cosmology as well. I believe my research program span across all these topics and is well equipped to play a leading role in our pursuit of discovering the particle nature of DM.
Baryon asymmetry and gravitational waves
In our quest to reconstruct the history of the Universe one of the critical missing pieces is the period between inflation and big bang nucleosynthesis (BBN). Several important events like generation of baryon asymmetry and neutrino masses, super-symmetry breaking, might have occurred during this period. My current research concentrates on un-earthing signatures from this period that can be tested in future experiments.
Particularly, the discovery of gravitational waves (GW) by LIGO opens up an avenue to probe pre-BBN Universe. One of the most obvious phenomena that can give rise to appreciable GW is a phase transition in the early Universe. We showed that EW baryogenesis (a popular mechanism to create baryon asymmetry) could produce a significant amount of GW energy density to be observable at future GW experiments like LISA, in the context of a model where the SM is extended by a singlet scalar. We take into account subtle issues with bubble wall velocity in the paper. Also, we examined the possibility of finding such a heavy scalar at high luminosity LHC to set-up a complementarity between GW and collider experiments. I am currently studying a number of mechanisms of baryon asymmetry that can produce detectable GW.
In coming decades, this line of research is imbued with possibilities. It will not only probe BSM models that generate strong first-order phase transition in the early Universe but also may offer us a way to study dark sectors, solutions to the hierarchy problem (e. g. , relaxion), as well as possible connections to super symmetry and its breaking. The potential of using future GW wave detectors to understand the evolution of the Universe will be an integral component of my research program.
Fermion mass hierarchy and neutrino mass generation
The SM does not offer an explanation of the observed flavor structure in either the lepton or the quark sector.
In fact, the Yukawa couplings of the recently discovered Higgs boson are the least understood part of the SM, and the coupling assignments appear arbitrary. Additionally, the neutrinos are predicted to be massless within the SM. From a cosmological perspective, we need a significant amount of charge-parity (CP) violation in the early Universe to produce the observed baryon asymmetry. The CP violation from the SM is too small, and a new source of CP violation beyond the SM must exist. One can accommodate adequate CP violation in neutrinomass model to account for observed baryon asymmetry via the mechanism called leptogenesis.
In the past year I have initiated a program on flavor physics by building a phenomenological model, where the SM fermions have democratic mass matrices (preserving a S(3)× S(3) global symmetry) at a high scale, that can explain the mass and mixing patterns in both lepton and quark sectors. We further outline the breaking scheme of the S(3)× S(3) symmetry necessary to fit all the fermion mass and mixing observables. However, such models are difficult to test in current experimental facilities since the breaking scale of the underlying symmetry is at a scale, far beyond the reach of the LHC. On the other hand, I examined the phenomenological prospects of a dimension-7 neutrino mass generation model, where the higher dimensional operator lowers the scale of new physics at O(TeV), rendering it testable at the LHC.
The future direction of my research in flavor physics is to continue building models that can explain fermion mass hierarchies and tiny neutrino masses, but testable at the LHC or GW detectors. Another direction in this topic that I want to pursue is to study the implication of using continuous gauge symmetries in the flavor sector and understanding their phenomenological consequences.
Conclusions
In conclusion, I plan to form a dynamic research group that will follow a comprehensive approach to research at the intersection of collider and dark matter phenomenology, and early Universe cosmology. I will lead a research program that will be equipped to propose new models and will search for new physics signals across energy scales and experimental facilities. I will continue to collaborate with experimentalists from collider physics, and astrophysics towards achieving my objectives as a researcher.
