Highlights
Selected highlights of my research are presented below (in reverse chronological order).
Main research page is HERE and for a complete overview please see my publication page.
A deuteron is a bound state of two baryons, a proton and a neutron, and it is made of six light valence quarks. In the early Universe, deuterons were created and their stability is responsible for the creation of other elements. Interestingly, the strong interactions between quarks, which bring stability to deuterons, also allow various other six-quark combinations leading to the possible formation of many other dibaryons. However, no such dibaryons, though speculated over many years have been observed yet. Using a state-of-the-art first principles calculation of lattice quantum chromodynamics (QCD), here we report, for the first time, a definite prediction of the existence of other deuteron-like spin-1 dibaryons. We expect that prediction from this work will aid in discovering these new subatomic particles at experimental facilities, such as the Large Hadron Collider. In fact this study opens up the possibility of the existence of many other exotic nuclei, which can be formed through the fusion of heavy baryons, similar to the formation of nuclei of elements in the Periodic Table. Formation of these hadrons also enhances the possibility of a quark level analogue of nuclear fusion. Further study of these exotic states can also provide information on the strong interaction dynamics at multiple scales.
A schematic quark structure of these predicted dibaryons are shown in the left along with the deuteron. The other figure represents the predicted binding energies of these dibaryons with respect to their closest two-particle strong decay thresholds.
Published in Physical Review Letters (with P. Junnarkar, 2019).
See TIFR Science News, Current Science for popular coverage.
Mesons and baryons are the usual subatomic particles that we observe in nature. An example of meson is a pion and the more familiar proton is a baryon. Mesons are made of a pair of valence quark and anti-quark while a baryons are made of three quarks. However, besides the usual mesons and baryons, the theory of quantum chromodynamics allows the existence of other states, such as multiquark (> 3 quarks) states, hybrid mesons and baryons (bound by excited gluons) along with glueballs. In fact very recently tetra and pentaquark states have been discovered in the Large Hadron Collider at CERN and Belle experiment at KEK.
Lattice QCD calculations provide the best way to study these exotic states. In this article we have shown that there is a real possibility for the discovery of more tetraquark hadrons. Using a detail lattice QCD calculation we predicted the possible binding energies of few such states which are stable under strong and electromagnetic interactions. Among those the most stable one is a combination made of two light quarks with two bottom antiquarks, that is "b-bar b-bar ud" (or its charge conjugation). Varying quark masses over a large range (light to bottom) on lattices with three different spacings we were able to predict strong bindings of various tetraquark states. We believe some of those states will be discovered in the near future at high energy laboratories, such as at the Large Hadron Collider and Belle II, KEK.
Published in Physical Review D (with P. Junnarkar and M. Padmanath, 2018)
A baryon is made of three valence quarks in a color singlet state. Our familiar proton is one of them and is made of one down-type and two up-type quarks. There are six flavours of quarks and Quantum chromodynamics, the theory of strong interactions of our universe, allows any such combinations of three quarks. One such combination is a baryon made of two heavy quarks (charm or bottom) and a light quark (up or down). However, the first doubly heavy baryon was discovered as late as in 2017. It is made of two charm quarks and a light quark and is called Cascade(cc). Using Lattice QCD we had predicted this state as far back as 2001, at its now measured mass.
After the first discovery of such subatomic particles it is natural to ask when would the next one be discovered as QCD also allows many such particles. In this work we showed that the next doubly heavy baryon to be discovered is a combination of two charm and a strange quark, called Omega(ccs) and predicted its mass with an accuracy of 0.5%! We also predicted masses of other doubly charmed baryons. We believe Omega(ccs) will be discovered soon at the Large Hadron Collider at CERN.
Published in Physical Review D Rapid Communication (with M. Padmanath, 2018)
Precise predictions of the properties of subatomic particles using ab-initio calculations have been a big challenge for physicists. In an article published in Physical Review Letters we predicted masses of several new subatomic particles with both charm and beauty quarks as their constituents. Using the large-scale computational facility of the Indian Lattice Gauge Theory Initiative (ILGTI), we predicted those charming-beautiful subatomic particles with per mille accuracy!
These new subatomic particles are yet to be discovered at various high-energy physics laboratories around the world, particularly at the Large Hadron Collider (LHC) at CERN. In fact, a large set of data that has already been accumulated at LHC may contain signatures of some of these particles and it is anticipated that they may well be discovered soon. This first-principles calculation leading to precious predictions of the masses of these particles, provides crucial input for their discovery. The predicted energy spectra of these particles will be quite helpful to gain important insights into the strong dynamics of heavy hadrons at multiple scales. Understanding their interactions and decays may also provide significant information on the physics beyond the standard model that particle physicists have been envisaging for a long time.
Published in Physical Review Letters (with M. Padmanath and S. Mondal, 2018).
See TIFR Science News, Current Science for popular coverage.
In this work we predicted the quantum numbers of the newly discovered Ωc-0 baryons which were otherwise unknown experimentally. In fact, my student M. Padmanath’s thesis work predicted the masses of these particles four year before their discovery. Using state-of-the-art methods of LQCD and computational resources of the Department of Theoretical Physics and the Indian Lattice Gauge Theory Initiative (ILGTI), we performed a precise and systematic determination of energies and quantum numbers for the tower of excited states of Ωc-0 baryons. In this study our predicted results are compared with experimental findings (see figure on the right). This result also shows the predictive power of lattice calculations where the excitations of subatomic particles are predicted from first principles which is a very non-trivial task. Predicted quantum numbers of these particles will help to understand the properties of these particles which in turn will help to understand the nature of strong interactions.
Published in Physical Review Letters (with M. Padmanath, 2018)
See also TIFR Science News, Current Science for popular coverage.
Hadrons are composite particles made of quarks and gluons. Beside the ground state a hadron has energy excitations similar to the excited states of atoms. Extraction of the excited energy spectra of hadrons through first principles calculations is a non-trivial task and lattice QCD provides the best way to calculate those states. However, unlike the ground state energy which dominates in large Euclidean time, extraction of the excited states are not that simple. Here one first needs to build a large set of interpolating operators within various irreducible representations of octahedral group. After that, a large correlation matrix is constructed out of those operators from which one can get various energy eigenstates through the solution of generalized eigenvalue problem. Identification of continuum quantum numbers of a physical state from the eigenstates thus obtained is also a complicated issue.
Overcoming all the above mentioned difficulties, in this work for the first time we calculated the excited states of singly, doubly and triply charmed baryons. In a series of article we predicted the excited spectra of these hadrons most of which are still unknown. An an example, in the left side we show the excited spectra on doubly charmed baryon containing two charm and a light quark. We would like to mention here that our predictions on Omega(c) baryons were later confirmed by LHCb experiment at the Large Hadron Collider, CERN. We believe many of other predictions will also be confirmed in future experiments. The above calculations can be repeated in future with the availability of more realistic lattices for more realistic quantitative predictions of these states.
Published in Physical Review D, Physical Review D and Charm proceedings (with M. Padmanath, R. Edwards and M. Peardon, 2013-15).
Also included in The Review of Particle Physics of the Particle Data Group
We know that a proton is a spin 1/2 particle. Naively a proton is made of two up and a valence quarks. However, in reality it is a very complicated system of strongly interacting quarks (including sea quarks) and gluons. To understand the structure of the proton it is important to know, in a given energy scale, the decomposition of its spin among its constituents. It is natural to ask in a given energy scale is it possible to define and extract the contributions from quarks and gluons to the proton's spin? Is there any orbital angular momentum contribution from quarks? It has been shown that it is possible to decompose proton's spin, in a gauge invariant way, into quark's spin, quark orbital angular momentum and gluon's angular momentum. However, EM.C experiments showed us that the quark spin contribution is only about 25%. It is thus important to understand what is the contribution from quark orbital angular momentum and how much spin gluons carry to make proton as a spin 1/2 particle. Lattice QCD is the best theoretical tool to answer this questions.
We performed the first lattice calculation on this problem where tried to understand the contribution from the orbital angular momentum of quarks as well as the angular momentum of gluons from first principles. In 2000 we published first paper on this showing with quantitative predictions on these quantities. Subsequently many other calculations followed our works. We improved our results later again with better control over systematics. Eventually some of the other members of the ChiQCD collaborations made a solid predictions on the proton spin decomposition which could be measured experimentally.
Published in Physical Review D, Physical Review D, Physical Review D (with ChiQCD collaboration, 2000 and 2009, 2015)
Published in Physical Review D, Physical Review D (with ChiQCD collaboration and S. Prelovsek et al, 2006, 2010)
How strange is a proton?
Naively a proton is made of two up and a valence quarks. However, in reality it is a very complicated many body system of strongly interacting quarks and gluons where any number of sea quarks get generated through gluonic interactions between quarks and gluons. A schematic picture is shown on the right. Though a proton does not have a valence strange quark any number of sea strange quarks get generated through this process.
A possible way to estimate the strangeness content of proton is through calculating different matrix elements, such as scalar, vector and tensor form factors which are related to scalar content, electromagnetic form factors and quark orbital momentum of the proton. Since the strange quarks enters as a sea quark in these matrix elements, one needs to calculate the so called disconnected diagrams. However, it is very difficult to extract signals from the disconnected diagrams and one needs special techniques to estimate them. Over many years within the ChiQCD collaborations we developed these techniques and have estimated these form factors. In the right side we show here the strangeness magnetic form factors where a non-zero value signify the presence of strange quarks inside the proton.
Published in Physical Review D and Nuclear Physics B (with ChiQCD collaboration)
Published in Physical Review D (with ChiQCD collaboration, 2006)
Also included in The Review of Particle Physics of the Particle Data Group since 2007.
