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-n_{L}^{a
} b^{m}_{ab
}
g_{m}
n_{L}^{b}
, where b_{m
} 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 n_{m} to
n_{t} oscillations :
In this work, we show that matter effects change the n_{m} to
n_{t} oscillation probability by as
much as 70% for certain ranges of energies and pathlengths. Consequently, the
n_{m} to
n_{m} disappearance probability also undergoes large changes. We comment on
how these matter
effects may be observed and the sign of D_{31} determined in atmospheric neutrino
measurements and at neutrino factories and
how they lead to heightened sensitivity for small
q_{13}.
[7]
(b) Earth matter effects at very long baselines and the neutrino mass hierarchy :
We study matter effects which arise in the probabilities P_{me},
P_{mt},
and P_{mm} 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 P_{mm}.
We emphasize that at these baselines, matter effects in P_{mt}
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 P_{mt} as well as
P_{me}.
We indicate where these effects
provide sensitivity to q_{13} and q_{23},
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 Dm^{2}_{32},
matter effects, and
sensitivity to q_{13}.
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 n_{m} + anti-n_{m} 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 n_{m} + anti-n_{m}
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 P_{me} but
also from those in P_{mt}. 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 Dm^{2}_{31}, 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 n_{m} + anti-n_{m}
and
n_{e} + anti-n_{e} and
100 kton sized magnetized iron detectors, which can measure the survival rates of
n_{m} and anti-n_{m}.
For energies and path-lengths relevant to atmospheric neutrinos, these rates obtain significant
matter contributions from P_{me}, P_{mm} and P_{ee},
leading to an appreciable sensitivity to the hierarchy.
We do a binned c^{2} 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 c^{2}
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 n_{m} + anti-n_{m} 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 n_{m} to n_{t} 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)]