My Research Interests



Theoretical Physics, Neutrino Physics, Quantum Optics


Some key words related to my research interests are :
- Neutrino mass and mixing
- Matter effects in neutrino oscillations and the issue of neutrino mass hierarchy
- Atmospheric and long baseline neutrinos
- Non-standard neutrino matter interactions and CPT Violation
- Origin of neutrino mass and mixing
- Geometric and topological phases in neutrino physics
- Geometric phases in polarised intensity interferometry
- Astrophysical neutrinos
- Neutrino decay
- Quantum decoherence


  You can find my papers if you search the hep-ph , quant-ph or astro-ph .



MY RESEARCH SUMMARY : pdf file and ps file


Astrophysical neutrinos and new physics :



(a) Interplay of energy dependent astrophysical neutrino flavor ratios and new physics effects :
We discuss the importance of flavor ratio measurements in neutrino telescopes, such as by measuring the ratio between muon tracks to cascades, for the purpose of extracting new physics signals encountered by astrophysical neutrinos during propagation from the source to the detector. The detected flavor ratios not only carry the energy information of specific new physics scenarios which alter the transition probabilities in distinctive ways, but also the energy dependent flavor composition at the source. In the present work, we discuss the interplay of these two energy dependent effects and identify which new physics scenarios can be distinguished from the detected flavor ratios as a function of astrophysical parameters. We use a recently developed self-consistent neutrino production model as our toy model to generate energy dependent source flavor ratios and discuss (invisible) neutrino decay and quantum decoherence as specific new physics examples. Furthermore, we identify potentially interesting classes of sources on the Hillas plot for the purpose of new physics searches. We find that sources with substantial magnetic fields $10^3$ Gauss $<= B <=$ $10^6$ Gauss, such as Active Galactic Nuclei (AGN) cores, white dwarfs, or maybe gamma-ray bursts, have, in principle, the best discrimination power for the considered new physics scenarios, whereas AGN jets, which typically perform as pion beam sources, can only discriminate few sub cases in the new physics effects. The optimal parameter region somewhat depends on the class of new physics effect considered. [21]



Geometric Phase in Quantum Optics :



(a) The Non-local Pacharatnam phase in Two-Photon interferometry :
The Pancharatnam phase was discovered in the context of amplitude interferometry of polarised light and anticipates Berry's discovery of the geometric phase. We propose a polarised intensity interferometry experiment which measures the nonlocal Pancharatnam phase acquired by a pair of Hanbury Brown-Twiss photons. The experimental setup involves two polarised thermal sources illuminating two polarised detectors. Varying the relative polarisation angle of the detectors introduces a geometric phase equal to half the solid angle traced out on the Poincare sphere by a pair of photons. Local measurements at either detector do not reveal the effects of the phase, which appears only in the coincidence counts of the two detectors and is a genuinely multiparticle and nonlocal effect. The phase is an optical analog of the multiparticle Aharonov-Bohm effect which has been measured in Quantum Hall systems. [19]


(a) Classical light analogue of the non-local Aharonov-Bohm effect :
We demonstrate the existence of a non-local geometric phase in the intensity-intensity correlations of classical incoherent light, that is not seen in the lower order correlations. This two-photon Pancharatnam phase was observed and modulated in a Mach-Zehnder interferometer. Using acousto-optic interaction, independent phase noise was introduced to light in the two arms of the interferometer to create two independent incoherent classical sources from laser light. The experiment is the classical optical analogue of the multi-particle Aharonov-Bohm effect. As the trajectory of light over the Poincare sphere introduces a phase shift observable only in the intensity-intensity correlation, it provides a means of deflecting the two-photon wavefront, while having no effect on single photons. [22]



Origin of neutrino masses and mixings :



(a) Maximal mixing as a 'sum' of small mixings :
In models with two sources of neutrino masses, we look at the possibility of generating maximal/large mixing angles in the total mass matrix, where both the sources have only small mixing angles. We show that in the two generation case, maximal mixing can naturally arise only when the total neutrino mass matrix has a quasi-degenerate pattern. The best way to demonstrate this is by decomposing the quasi-degenerate spectrum in to hierarchial and inverse-hierarchial mass matrices, both with small mixing. Such a decomposition of the quasi-degenerate spectra is in fact very general and can be done irrespective of the mixing present in the mass matrices. With three generations, and two sources, we show that only one or all the three small mixing angles in the total neutrino mass matrix can be converted to maximal/large mixing angles. The decomposition of the degenerate pattern in this case is best realised in to sub-matrices whose dominant eigenvalues have an alternating pattern. On the other hand, it is possible to generate two large and one small mixing angle if either one or both of the sub-matrices contain maximal mixing. We present example textures of this. With three sources of neutrino masses, the results remain almost the same as long as all the sub-matrices contribute equally. The Left-Right Symmetric model where Type I and Type II seesaw mechanisms are related provides a framework where small mixings can be converted to large mixing angles, for degenerate neutrinos. [16] [18]



Geometric phase and neutrino oscillations :



(a) Topological phase in two flavor neutrino oscillations :
We show that the phase appearing in neutrino flavor oscillation formulae has a geometric and topological contribution. We identify a topological phase appearing in the two flavor neutrino oscillation formula using Pancharatnam's prescription of quantum collapses between non-orthogonal states. Such quantum collapses appear naturally in the expression for appearance and survival probabilities of neutrinos. Our analysis applies to neutrinos propagating in vacuum or through matter. For the minimal case of two flavors with CP conservation, our study shows for the first time that there is a geometric interpretation of the neutrino oscillation formulae for the detection probability of neutrino species. [14] [17] [20]


(b) Geometric imprint of CP violation in two flavor neutrino oscillations :
In vacuum or constant density matter, the two flavor neutrino oscillation formulae are insensitive to the presence of CP violating phases owing to the fact that the CP phase can be gauged away. We show that there is a non-trivial geometrical aspect associated with the case of adiabatic evolution in presence of changing density which is very different from the constant density case. This distinction between the two cases can lead to visible consequences at the level of probability. We present a pure geometric visualization of this effect by exploiting Pancharatnam's prescription of cyclic quantum projections. [15]



NEUTRINO FACTORIES :



Neutrino factories will open up unprecedented opportunities to investigate the neutrino mixing parameters and at the same time also provide a physical laboratory for testing physics beyond the Standard Model. Our work has focussed on exploring the non-standard neutrino matter interactions (R-parity violating supersymmetry and theories with leptoquarks) in case of tau and wrong sign muon production [(a), (b)] and looking for heavy quark production via charged current and neutral current channels at a future Neutrino Factory [(c)].


(a) Signals of R-parity violating supersymmetry :
Neutrino oscillation signals at muon storage rings can be faked by supersymmetric interactions in an R-parity violating scenario. We have investigated the appearance of t and wrong sign m for both long-baseline and near site experiments, and concluded that the latter is of great use in distinguishing between oscillation and SUSY effects for tau production, while supersymmetry can cause a manifold increase in the event rate for wrong sign muons at a long-baseline setting. [1] [2]


(b) Leptoquark signals via n interactions :
The accurate prediction of neutrino beam produced in muon decays and the absence of opposite helicity contamination for a particular neutrino flavour makes a future neutrino factory based on a muon storage ring, the ideal place to look for the lepton flavour violating effects. In this work, we have addressed the contribution of mediating lepton flavour violating leptoquarks in n(anti-n)-N interactions leading to the production of t's and wrong sign m's at muon storage ring and investigate the region where leptoquark interactions are significant in the near-site and short baseline experiments. With the increase in baseline length, the leptoquark event rate falls off and neutrino oscillations are the main source of appearance of t and wrong sign m events. At near-site experiments, on the other hand, the events mainly arise from new interactions and can thus be used to constrain the theory. In particular, we obtained constraints on lepton flavour violating couplings between the first and the third generation, the bounds on which are generally not available. At near-site experiments, the event rate was found to be practically independent of baseline length. [3]


(c) Heavy quark production via Leptoquarks :
The proposed neutrino factory based on a muon storage ring is an ideal place to look for heavy quark production via neutral current and charged current interactions. In this work, we have addressed the issue of contributions coming from mediating LQ in n(anti-n)-N scattering, leading to the production of b (b bar) at a MSR, and investigate the region where leptoquark interactions are significant in the near-site experiments. [4]


ATMOSPHERIC NEUTRINOS :


Atmospheric neutrinos as probes of CPT and Lorentz Violation :
The CPT theorem is one of the few solid predictions of relativistic local quantum field theory in flat space time which respects locality, unitarity and Lorentz symmetry. It is argued that more fundamental theories may induce small violations of these sacred symmetries into the Standard Model naturally at low energies and this can be tested at levels accessible to high precision experiments. In string theory, the higher order field interactions due to the non-local nature of strings may modify the Lorentz properties of vaccuum. The general mechanism to model this effect at the level of SM is spontaneous symmetry breaking in which the tensor fields attain a non-vanishing expectation value in vaccuum at low energies. The CPT-odd interaction term is given by anti-nLa bmab gm nLb , where bm parametrizes the interaction of tensor fields in higher theories with the low energy SM fields and a,b are flavour indices. In addition there can be small violations of Lorentz symmetry in the neutrino sector coming from the fact that neutrinos can attain different limiting velocities.

In our work, we have shown that atmospheric neutrinos, in conjunction with a (currently planned) large mass magnetized iron calorimeter can provide a sensitive and robust probe of CPT violation. We have performed realistic eventrate calculations and study the variations of the ratio of total muon to antimuon survival rates with L/E and L (L is the baseline length, E is the neutrino energy) in a detector capable of identifying the muon charge. We have demonstrated that measurements of these ratios when coupled with the significant L and E range which characterizes the atmospheric neutrino spectrum provides a method of both detecting the presence of such violations and putting bounds on them which compare very favourably with those possible from a future neutrino factory. [5] [6]


MATTER EFFECTS IN NEUTRINO OSCILLATIONS :


(a) Large matter effects in nm to nt oscillations :
In this work, we show that matter effects change the nm to nt oscillation probability by as much as 70% for certain ranges of energies and pathlengths. Consequently, the nm to nm disappearance probability also undergoes large changes. We comment on
how these matter effects may be observed and the sign of D31 determined in atmospheric neutrino measurements and at neutrino factories and
how they lead to heightened sensitivity for small q13. [7]


(b) Earth matter effects at very long baselines and the neutrino mass hierarchy :
We study matter effects which arise in the probabilities Pme, Pmt, and Pmm relevant to atmospheric neutrinos and very long baseline (> 4000 km) beam experiments. We examine the inter-relations between the three probabilities which can result in a large and observable sensitivity to matter related phenomena and the neutrino mass hierarchy in Pmm. We emphasize that at these baselines, matter effects in Pmt are important under certain conditions and can be large [7] . The muon survival rates in experiments with very long baselines thus depend on matter effects in both Pmt as well as Pme. We indicate where these effects provide sensitivity to q13 and q23, and study in detail the effect of parameter degeneracies in the three probabilities. In the second part of the paper, we focus on using the matter effects studied in the first part as a means of determining the mass hierarchy of neutrinos. We perform realistic event rate calculations for a charge discriminating iron calorimeter which demonstrate the feasibility of realising this very important goal in neutrino physics using atmospheric neutrinos, within a timescale appreciably shorter than the one anticipated for neutrino factories. [8]


(c) Matter effects in atmospheric m- / m+ in magnetized iron calorimeters :
In this work, we present results for atmospheric neutrino measurements possible with an iron calorimeter detector for
determining the sign of Dm232,
matter effects, and
sensitivity to q13.
Since atmospheric neutrinos allow a large range of L and E that can be probed, the effects due to matter resonance etc can get washed away if we integrate over all possible L and all possible E. In order to see any measurable effect, one therefore requires narrowing down the L and E bins in a suitable way. Also, inorder to see any appreciable effect here, one needs a larger exposure of approximately 1000 kT-year than was required to see the oscillation dip which was the prime goal of atmospheric neutrino experiments until Super Kamiokande announced their result at NOON 04. We have performed realistic event-rate calculations to illustrate the above. [9]


(d) Probing the neutrino mass hierarchy via atmospheric nm + anti-nm survival rates in megaton water cerenkov detectors :
The neutrino mass hierarchy, presently unknown, is a powerful discriminator among various classes of unification theories. We show that the nm + anti-nm survival rate in atmospheric events can provide a new and useful method of determining the hierarchy in megaton water Cerenkov detectors. For baseline and energy ranges relevant to atmospheric neutrinos, this rate obtains significant matter sensitive variations not only from resonant matter effects in Pme but also from those in Pmt. By performing realistic event rate calculations, we identify energy and baseline ranges where these effects can be fruitfully observed in a statistically significant manner. [10]


(e) Mass Hierarchy Determination via future Atmospheric Neutrino Detectors :
We study the problem of determination of the sign of Dm231, or the neutrino mass hierarchy, through observations of atmospheric neutrinos in future detectors. We consider two proposed detector types :
Megaton sized water Cerenkov detectors, which can measure the survival rates of nm + anti-nm and ne + anti-ne and
100 kton sized magnetized iron detectors, which can measure the survival rates of nm and anti-nm.
For energies and path-lengths relevant to atmospheric neutrinos, these rates obtain significant matter contributions from Pme, Pmm and Pee, leading to an appreciable sensitivity to the hierarchy. We do a binned c2 analysis of simulated data in these two types of detectors which includes the effect of smearing in neutrino energy and direction and incorporates detector efficiencies and relevant statistical, theoretical and systematic errors. We also marginalize the c2 over the allowed ranges of neutrino parameters in order to accurately account for their uncertainties. Finally, we compare the performance of both types of detectors vis-a-vis the hierarchy determination. [11]


INDIAN-BASED NEUTRINO OBSERVATORY :


Physics Issues and Detector Simulations :
I was an active member of the India-based Neutrino Observatory (INO) Collaboration during the feasibility studies for a proposed neutrino observatory based in India. During 2002-2006, I have been involved in studying the physics potential of a large mass magnetized iron calorimeter type neutrino detector using atmospheric neutrinos [5] [6] [8] [9] [11] . For a comparative study of various detectors and their physics capabilities, see [12] . I have also been involved in simulations work. I have used NUANCE neutrino generator and GEANT package for detector simulations to carry out simulations studies for INO.

For more details, see the INO homepage [13]







Reference List

22. Classical light analogue of the non-local Aharonov-Bohm effect
Nandan Sathpathy, Deeepak Pandey, Poonam Mehta, Supurna Sinha, Joseph Samuel and Hema Ramachandran
[arXiv:1202.2685 [quant-ph] (2012), EPL 97, 50011 (2012)]

21. Interplay of energy dependent astrophysical neutrino flavor ratios and new physics effects
Poonam Mehta and Walter Winter
JCAP03(2011)041 [arXiv:1101.2673 [hep-ph] (2011).

20. Reply to the comment on "Topological phase in two flavor neutrino oscillations"
Poonam Mehta
[arXiv:1008.4543 [hep-ph] (2010)]
*** The decision by Phys. Rev. D on the related comment [ 1006.5935 ] by Rajendra Bhandari. ***

19. Non-local Pacharatnam phase in Two-Photon interferometry.
Poonam Mehta, Joseph Samuel and Supurna Sinha
[arXiv:1002.1547 [quant-ph] (2010), Phys. Rev. A 82, 034102 (2010)]
Highlighted in the Virtual Journal of Quantum Information

18. Quasi-degenerate neutrinos and Maximal mixing in hybrid seesaws.
Joydeep Chakrabortty, Anjan S. Joshipura, Poonam Mehta and Sudhir K. Vempati
[Proceedings of LP09]

17. The Pancharatnam phase in two flavor neutrino oscillations.
Poonam Mehta
[Proceedings of LP09]

16. Maximal mixing as a sum of small mixings.
Joydeep Chakrabortty, Anjan S. Joshipura, Poonam Mehta and Sudhir K. Vempati
[arXiv:0909.3116 [hep-ph] (2009)]

15. Geometric imprint of CP violation in two flavor neutrino oscillations.
Poonam Mehta
[arXiv:0907.0562 [hep-ph] (2009)]

14. Topological phase in two flavor neutrino oscillations.
Poonam Mehta
[arxiv:0901.0790 [hep-ph], Phys. Rev. D79, 096013 (2009)]

13. India-based Neutrino Observatory: Project Report. Volume I.
M. Sajjad Athar et al [INO Collaboration]
[INO-2006-01 (2006)]

12. Neutrino Detectors of the future : A Comparison table
Raj Gandhi, Poonam Mehta, S. Uma Sankar
[INO/HRI/2005/03 (INO Site)]  ,   [ps file]  ,  [pdf file (2005)]

11. Mass Hierarchy Determination via future Atmospheric Neutrino Detectors.
Raj Gandhi, Pomita Ghoshal, Srubabati Goswami, Poonam Mehta, S. Uma Sankar, S. Shalgar
[arxiv:0707.1723 [hep-ph], Phys. Rev. D76 , 073012 (2007)]

10. Probing the n mass hierarchy via atmospheric nm + anti-nm survival rates in megaton water Cerenkov detectors.
Raj Gandhi, Pomita Ghoshal, Srubabati Goswami, Poonam Mehta, S. Uma Sankar
[hep-ph/0506145, Archive (2005)]

9. SAWG Report
Raj Gandhi, Poonam Mehta, S. Uma Sankar
[ps file]  ,  [pdf file, HRI-P-04-10-001 (2004)]

8. Earth matter effects at very long baselines and the neutrino mass hierarchy.
Raj Gandhi, Pomita Ghoshal, Srubabati Goswami, Poonam Mehta, S. Uma Sankar
[hep-ph/0408361, Phys. Rev. D 73, 053001 (2006)]

7. Large matter effects in nm to nt oscillations.
Raj Gandhi, Pomita Ghoshal, Srubabati Goswami, Poonam Mehta, S. Uma Sankar
[hep-ph/0408361, Phys. Rev. Lett. 94, 051801 (2005)]

6. Atmospheric neutrinos as a probe of CPT violation.
Poonam Mehta
[pdf file, Poster presented at Neutrino 2004, Paris]  ,  [Nucl. Phys. B - Proc. Suppl. 143, 503 (2005)]

5. Atmospheric neutrinos as a probe of CPT violation.
Anindya Datta, Raj Gandhi, Poonam Mehta, S. Uma Sankar
[hep-ph/0312027, Phys. Lett. B 597, 356 (2004)]

4. Heavy quark production via leptoquarks at a neutrino factory.
Ashok Goyal, Poonam Mehta, Sukanta Dutta
[hep-ph/0209019, Phys. Rev. D 67, 053006 (2003)]

3. Leptoquark signals via neutrino interactions at neutrino factories.
Poonam Mehta, Sukanta Dutta, Ashok Goyal
[hep-ph/0107214, Phys. Lett. B 535, 219-228 (2002)]

2. Signals of R-parity violating supersymmetry at a muon storage ring.
Anindya Datta, Raj Gandhi, Biswarup Mukhopadhyaya, Poonam Mehta
[hep-ph/0105137, Talk presented at 14th DAE Symposium on High Energy Physics, Hyderabad, India (2001)]

1. Signals of R-parity violating supersymmetry in neutrino scattering at muon storage rings.
Anindya Datta, Raj Gandhi, Biswarup Mukhopadhyaya, Poonam Mehta
[hep-ph/0011375, MRI-P-001107, Phys. Rev. D 64, 015011 (2001)]

Poonam Mehta Back to