arXiv subjects

Amarjit Soni

Publications and source records attributed to Amarjit Soni.

At least 73 records · Page 4Linked to original sources

arXiv2016

Lepton flavor violating Z' explanation of the muon anomalous magnetic moment

We discuss a minimal solution to the long-standing $(g-2)_μ$ anomaly in a simple extension of the Standard Model with an extra $Z'$ vector boson that has only flavor off-diagonal couplings to the second and third generation of leptons, i.e. $μ, τ, ν_μ, ν_τ$ and their antiparticles. A simplified model realization, as well as various collider and low-energy constraints on this model, are discussed. We find that the $(g-2)_μ$-favored region for a $Z'$ lighter than the tau lepton is totally excluded, while a heavier $Z'$ solution is still allowed. Some testable implications of this scenario in future experiments, such as lepton-flavor universality-violating tau decays at Belle 2, and a new four-lepton signature involving same-sign di-muons and di-taus at HL-LHC and FCC-ee, are pointed out. A characteristic resonant absorption feature in the high-energy neutrino spectrum might also be observed by neutrino telescopes like IceCube and KM3NeT.

hep-ph↗

arXiv2016

Chiral heavy fermions in a two Higgs doublet model: 750 GeV resonance or not

We revisit models where a heavy chiral 4th generation doublet of fermions is embedded in a class of two Higgs doublets models (2HDM) with a discrete $Z_2$ symmetry, which couples the "heavy" scalar doublet only to the 4th generation fermions and the "light" one to the Standard Model (SM) fermions - the so-called 4G2HDM introduced by us several years ago. We study the constraints imposed on the 4G2HDM from direct searches of heavy fermions, from precision electroweak data (PEWD) and from the measured production and decay signals of the 125 GeV scalar, which in the 4G2HDM corresponds to the lightest CP-even scalar h. We then show that the recently reported excess in the $γγ$ spectrum around 750 GeV can be accommodated by the heavy CP-even scalar of the 4G2HDM, H, resulting in a unique choice of parameter space: negligible mixing (sinα~ O(0.001)) between the two CP-even scalars h,H and heavy 4th generation quark and lepton masses m_t',m_b' < 400 GeV and $m_{ν'},m_{τ'}$ > 900 GeV, respectively. Whether or not the 750 GeV γγresonance is confirmed, interesting phenomenology emerges in q' - Higgs systems (q'=t',b'), that can be searched for at the LHC. For example, the heavy scalar states of the model, S=H,A,H^+, may have BR(S -> q'q') ~ O(1), giving rise to observable q'q' signals on resonance, followed by the flavor changing q' decays t'->uh (u=u,c) and/or b'->dh (d=d,s,b). This leads to distinct high jet-multiplicity signatures, with or without charged leptons, of the form q'q' -> (nj + mb + lW)_S (j and b being light and b-quark jets, respectively), with n+m+l =6-8 and unique kinematic features. It is also shown that the 4G2HDM can easily accommodate the interesting recent indications of a percent-level branching ratio in the lepton-flavor-violating (LFV) decay $h \to τμ$ of the 125 GeV Higgs, if confirmed.

hep-ph↗

arXiv2016

Emerging lattice approach to the K-Unitarity Triangle

It has been clear for past many years that in low energy observables new physics can only appear as a perturbation. Therefore precise theoretical predictions and precise experimental measurements have become mandatory. Here we draw attention to the significant advances that have been made on the lattice in recent years in $K\to ππ$, $ΔM_K$, the long-distance part of $\varepsilon$ and rare K-decays. Thus, in conjunction with experiments, the construction of a unitarity triangle purely from Kaon physics should soon become feasible. Along with the B-unitarity triangle, this should allow for more stringent tests of the Standard Model and tighter constraints on new physics.

hep-ph↗

arXiv2016

Hidden SU(N) Glueball Dark Matter

We investigate the possibility that the dark matter candidate is from a pure non-abelian gauge theory of the hidden sector, motivated in large part by its elegance and simplicity. The dark matter is the lightest bound state made of the confined gauge fields, the hidden glueball. We point out this simple setup is capable of providing rich and novel phenomena in the dark sector, especially in the parameter space of large N. They include self-interacting and warm dark matter scenarios, Bose-Einstein condensation leading to massive dark stars possibly millions of times heavier than our sun giving rise to gravitational lensing effects, and indirect detections through higher dimensional operators as well as interesting collider signatures.

hep-ph↗

arXiv2015

Neutron and proton electric dipole moments from $N_f=2+1$ domain-wall fermion lattice QCD

We present a lattice calculation of the neutron and proton electric dipole moments (EDM's) with $N_f=2+1$ flavors of domain-wall fermions. The neutron and proton EDM form factors are extracted from three-point functions at the next-to-leading order in the $θ$ vacuum of QCD. In this computation, we use pion masses 0.33 and 0.42 GeV and 2.7 fm$^3$ lattices with Iwasaki gauge action and a 0.17 GeV pion and 4.6 fm$^3$ lattice with I-DSDR gauge action, all generated by the RBC and UKQCD collaborations. The all-mode-averaging technique enables an efficient and high statistics calculation. Chiral behavior of lattice EDM's is discussed in the context of baryon chiral perturbation theory. In addition, we also show numerical evidence on relationship of three- and two-point correlation function with local topological distribution.

hep-lat↗

arXiv2015

Unraveling flavor & naturalness from RUN II to 100 TeV

The importance of incorporating flavor constraints, when providing bounds on new physics is stressed. As is well known it is very difficult for models of new physics to have scales lighter than about 10 TeV once flavor constraints are built in. Although, in the conventional sense, this higher scale means more tuning, it may well make the underlying theory simpler as illustrated with one example. Direct production of new particles heavier than about 5 TeV becomes very difficult at LHC though indirect signals such as deviations in the Higgs to gamma gamma branching ratio may still be possible. Also of course precision studies at low energy facilities can be useful for detecting new phenomena. For direct production of new particles over 5-10 TeV a new high energy collider perhaps at 100 TeV is highly desirable. Indeed, a very strong physics case can be made for such a machine. It can be used to significantly improve many bounds as well as allow a multitude of direct searches in many channels without relying on specific models. Naturalness can be tested by another factor of about 50 past LHC to O($10^{-4}$), a valuable achievement in itself.

hep-ph↗

arXiv2015

A Possible Two-component Flux for the High Energy Neutrino Events at IceCube

Understanding the spectral and flavor composition of the astrophysical neutrino flux responsible for the recently observed ultra-high energy events at IceCube is of great importance for both astrophysics and particle physics. We perform a statistical likelihood analysis to the 3-year IceCube data and derive the allowed range of the spectral index and flux normalization for various well-motivated physical flavor compositions at source. While most of the existing analyses so far assume the flavor composition of the neutrinos at an astrophysical source to be (1:2:0), it seems rather unnatural to assume only one type of source, once we recognize the possibility of at least two physical sources. Bearing this in mind, we entertain the possibility of a two-component source for the analysis of IceCube data. It appears that our two component hypothesis explains some key features of the data better than a single-component scenario, i.e it addresses the apparent energy gap between 400 TeV to about 1 PeV and easily accommodates the observed track to shower ratio. Given the extreme importance of the flavor composition for the correct interpretation of the underlying astrophysical processes as well as for the ramification for particle physics, this two-component flux should be tested as more data is accumulated.

hep-ph↗

arXiv2015

Neutral $B$ meson mixings and $B$ meson decay constants with static heavy and domain-wall light quarks

Neutral $B$ meson mixing matrix elements and $B$ meson decay constants are calculated. Static approximation is used for $b$ quark and domain-wall fermion formalism is employed for light quarks. The calculations are carried out on $2+1$ flavor dynamical ensembles generated by RBC/UKQCD Collaborations with lattice spacings $0.086$fm ($a^{-1}\sim 2.3$GeV) and $0.11$fm ($1.7$GeV), and a fixed physical spatial volume of about $(2.7{\rm fm})^3$. In the static quark action, link-smearings are used to improve the signal-to-noise ratio. We employ two kinds of link-smearings, HYP1 and HYP2, and their results are combined in taking the continuum limit. For the matching between the lattice and the continuum theory, one-loop perturbative $O(a)$ improvements are made to reduce discretization errors. As the most important quantity of this work, we obtain SU(3) breaking ratio $ξ=1.208(60)$, where the error includes statistical and systematic one. (Uncertainty from infinite $b$ quark mass is not included.) We also find other neutral $B$ meson mixing quantities $f_B\sqrt{\hat{B}_B}=240(22)$MeV, $f_{B_s}\sqrt{\hat{B}_{B_s}}=290(22)$MeV, $\hat{B}_B=1.17(22)$, $\hat{B}_{B_s}=1.22(13)$ and $B_{B_s}/B_B=1.028(74)$, $B$ meson decay constants $f_B=219(17)$MeV, $f_{B_s}=264(19)$MeV and $f_{B_s}/f_B=1.193(41)$, in the static limit of $b$ quark, which do not include infinite $b$ quark mass uncertainty.

hep-lat↗

arXiv2015

Lepton-Flavored Dark Matter

In this work, we address two paradoxes. The first is that the measured dark-matter relic density can be satisfied with new physics at O(100 GeV - 1 TeV), while the null results from direct-detection experiments place lower bounds of O(10 TeV) on a new-physics scale. The second puzzle is that the severe suppression of lepton-flavor-violating processes involving electrons, e.g. mu->3e, tau->e mu mu, etc., implies that generic new-physics contributions to lepton interactions cannot exist below O(10 - 100 TeV), whereas the 3.6sigma deviation of the muon g-2 from the standard model can be explained by a new-physics scale < O(1 TeV). Here, we suggest that it may not be a coincidence that both the muon g-2 and the relic density can be satisfied by a new-physics scale < 1 TeV. We consider the possibility of a gauged lepton-flavor interaction that couples at tree level only to mu- and tau-flavored leptons and the dark sector. Dark matter thus interacts appreciably only with particles of mu and tau flavor at tree level and has loop-suppressed couplings to quarks and electrons. Remarkably, if such a gauged flavor interaction exists at a scale O(100 GeV - 1 TeV), it allows for a consistent phenomenological framework, compatible with the muon g-2, the relic density, direct detection, indirect detection, charged-lepton decays, neutrino trident production, and results from hadron and e+e- colliders. We suggest experimental tests for these ideas at colliders and for low-energy observables.

hep-ph↗

arXiv2015

EFT naturalness: an effective field theory analysis of Higgs naturalness

Assuming the presence of physics beyond the Standard Model (SM) with a characteristic scale M ~ O(10) TeV, we investigate the naturalness of the Higgs sector at scales below M using an effective field theory (EFT) approach. We obtain the leading 1-loop EFT contributions to the Higgs mass with a Wilsonian-like hard cutoff, and determine the constraints on the corresponding operator coefficients for these effects to alleviate the little hierarchy problem up to the scale of the effective action Λ< M, a condition we denote by "EFT-naturalness". We also determine the types of physics that can lead to EFT-naturalness and show that these types of new physics are best probed in vector-boson and multiple-Higgs production. The current experimental constraints on these coefficients are also discussed.

hep-ph↗

arXiv2015

B-meson decay constants from 2+1-flavor lattice QCD with domain-wall light quarks and relativistic heavy quarks

We calculate the B-meson decay constants f_B, f_Bs, and their ratio in unquenched lattice QCD using domain-wall light quarks and relativistic b-quarks. We use gauge-field ensembles generated by the RBC and UKQCD collaborations using the domain-wall fermion action and Iwasaki gauge action with three flavors of light dynamical quarks. We analyze data at two lattice spacings of a ~ 0.11, 0.086 fm with unitary pion masses as light as M_pi ~ 290 MeV; this enables us to control the extrapolation to the physical light-quark masses and continuum. For the b-quarks we use the anisotropic clover action with the relativistic heavy-quark interpretation, such that discretization errors from the heavy-quark action are of the same size as from the light-quark sector. We renormalize the lattice heavy-light axial-vector current using a mostly nonperturbative method in which we compute the bulk of the matching factor nonperturbatively, with a small correction, that is close to unity, in lattice perturbation theory. We also improve the lattice heavy-light current through O(alpha_s a). We extrapolate our results to the physical light-quark masses and continuum using SU(2) heavy-meson chiral perturbation theory, and provide a complete systematic error budget. We obtain f_B0 = 199.5(12.6) MeV, f_B+ = 195.6(14.9) MeV, f_Bs = 235.4(12.2) MeV, f_Bs/f_B0 = 1.197(50), and f_Bs/f_B+ = 1.223(71), where the errors are statistical and total systematic added in quadrature. These results are in good agreement with other published results and provide an important independent cross check of other three-flavor determinations of $B$-meson decay constants using staggered light quarks.

hep-lat↗

arXiv2014

Constraining the flavor changing Higgs couplings to the top-quark at the LHC

We study the flavor-changing couplings of the Higgs-boson with the top-quark using the processes: (a) pp --> tt, (b) pp --> t_bar j, and, (c) pp --> t_bar j h at the LHC in light of current discovery of a 126 GeV Higgs-Boson. Sensitivities for the flavor-changing couplings are estimated using the LHC data that was collected until spring 2013. It is found that the process (c) is the most capable of yielding the best upper bound on the flavor-changing couplings with 2 sigma level sensitivities of {|xi_{tc}^2 + xi_{tu}^2|}^{1/2} <= 4.2 x 10^{-3} and <= 1.7 x 10^{-3} resulting from t --> b l nu_l, h --> jj with the 7 TeV and 8 TeV centre-of-mass energies respectively using existing data from the LHC. The corresponding bounds from h --> b b_bar are worse by a factor of about 1.8.

hep-ph↗

arXiv2014

Calculating the $K_L-K_S$ mass difference and $ε_K$ to sub-percent accuracy

The real and imaginary parts of the $K_L-K_S$ mixing matrix receive contributions from all three charge-2/3 quarks: up, charm and top. These give both short- and long-distance contributions which are accessible through a combination of perturbative and lattice methods. We will discuss a strategy to compute both the mass difference, $ΔM_K$ and $ε_K$ to sub-percent accuracy, looking in detail at the contributions from each of the three CKM matrix element products $V_{id}^*V_{is}$ for $i=u, c$ and $t$ as described in Ref. [1]

hep-lat↗

arXiv2014

The 125 GeV Higgs in the context of four generations with 2 Higgs doublets

We interpret the recent discovery of a 125 GeV Higgs-like state in the context of a two Higgs doublets model with a heavy 4th sequential generation of fermions, in which one Higgs doublet couples only to the 4th generation fermions, while the second doublet couples to the lighter fermions of the 1st-3rd families. This model is designed to accommodate the apparent heaviness of the 4th generation fermions and to effectively address the low-energy phenomenology of a dynamical electroweak symmetry breaking scenario. The physical Higgs states of the model are, therefore, viewed as composites primarily of the 4th generation fermions. We find that the lightest Higgs, h, is a good candidate for the recently discovered 125 GeV spin-zero particle, when tanβ~ O(1), for typical 4th generation fermion masses of M_{4G} = 400 -600 GeV, and with a large t - t' mixing in the right-handed quarks sector. This, in turn, leads to BR(t' -> t h) ~ O(1), which drastically changes the t' decay pattern. We also find that, based on the current Higgs data, this two Higgs doublet model generically predicts an enhanced production rate (compared to the SM) in the pp -> h -> tau tau channel and a reduced VV -> h -> gamma gamma and pp -> V -> Vh -> Vbb ones. Finally, the heavier CP-even Higgs is excluded by the current data up to m_H ~ 500 GeV, while the pseudoscalar state, A, can be as light as 130 GeV. These heavier Higgs states and the expected deviations from the SM in some of the Higgs production channels can be further excluded or discovered with more data.

hep-ph↗

arXiv2014

Higgs-radion unification: radius stabilization by an SU(2) bulk doublet and the 126 GeV scalar

We investigate a Randall-Sundrum model with an SU(2) doublet propagating in the bulk. Upon calculating its gravitational effect we find that a stabilized radius can be generated without the use of an additional scalar, as needed for example in the Goldberger-Wise (GW) mechanism, and with no additional fine-tuning other than the inescapable one due to the cosmological constant; similar tuning is also present in the GW mechanism. The lowest scalar excitation in this scenario, the counterpart of the radion of the GW mechanism, has both radion-like and Higgs-like couplings to the SM fields. It, thus, plays a dual role and we, therefore, denote it as the "Higgs-radion" ($h_r$). As opposed to the GW radion case, our Higgs-radion is found to be compatible with the 126 GeV scalar recently discovered at the LHC, at the level of $1σ$, with a resulting $95\%$ CL bound on the KK-gluon mass of: $4.48~TeV<M_{KKG}< 5.44~TeV$. An important consequence of our setup should be accentuated: the radion of the traditional RS scenarios simply does not exist, so that our Higgs-radion is not the conventional mixed state between the GW radion and the Higgs.

hep-ph↗

arXiv2014

Standard Model Explanation of the Ultra-high Energy Neutrino Events at IceCube

The recent observation of two PeV events at IceCube, followed by an additional 26 events between 30 - 300 TeV, has generated considerable speculations on its origin, and many exotic New Physics explanations have been invoked. For a reliable interpretation, it is however important to first scrutinize the Standard Model (SM) expectations carefully, including the theoretical uncertainties, mainly due to the parton distribution functions. Assuming a new isotropic cosmic neutrino flux with a simple unbroken power-law spectrum, $Φ\propto E^{-s}$ for the entire energy range of interest, we find that with $s=1.5$ - 2, the SM neutrino-nucleon interactions are sufficient to explain all the observed events so far, without the need for any beyond the SM explanation. With more statistics, this powerful detector could provide a unique test of the SM up to the PeV scale, and lead to important clues of New Physics.

hep-ph↗

arXiv2013

Hybrid dynamical electroweak symmetry breaking with heavy quarks and the 125 GeV Higgs

Existing models of dynamical electroweak symmetry breaking (EWSB) find it very difficult to get a Higgs of mass lighter than $m_t$. Consequently, in light of the LHC discovery of the ~125 GeV Higgs, such models face a significant obstacle. Moreover, with three generations those models have a superheavy cut-off around $10^{17}$ GeV, requiring a significant fine-tuning. To overcome these twin difficulties, we propose a hybrid framework for EWSB, in which the Higgs mechanism is combined with a Nambu-Jona-Lasinio mechanism. The model introduces a strongly coupled doublet of heavy quarks with a mass around 500 GeV, which forms a condensate at a compositeness scale $Λ$ about a few TeV, and an additional unconstrained scalar doublet which behaves as a "fundamental" doublet at $Λ$. This "fundamental"-like doublet has a vanishing quartic term at $Λ$ and is, therefore, not the SM doublet, but should rather be viewed as a pseudo-Goldstone boson of the underlying strong dynamics. This setup is matched at the compositeness scale $Λ$ to a tightly constrained hybrid two Higgs doublet model, where both the composite and unconstrained scalars participate in EWSB. This allows us to get a good candidate for the recently observed 125 GeV scalar which has properties very similar to the Standard Model Higgs. The heavier (mostly composite) CP-even scalar has a mass around 500 GeV, while the pseudoscalar and the charged Higgs particles have masses in the range 200 -300 GeV.

hep-ph↗

arXiv2013

Neutral B meson mixing with static heavy and domain-wall light quarks

Neutral B meson mixing matrix elements and B meson decay constants are calculated. Static approximation is used for b quark and domain-wall fermion formalism is employed for light quarks. The calculations are done on 2+1 flavor dynamical ensembles, whose lattice spacings are 0.086 fm and 0.11 fm with a fixed physical spatial volume of about (2.7 fm)^3. In the static quark action, link-smearings are used to improve the signal-to-noise ratio. We employ two kinds of link-smearings and their results are combined in taking a continuum limit. For the matching between the lattice and the continuum theory, one-loop perturbative calculations are used including O(a) improvements to reduce discretization errors. We obtain SU(3) braking ratio ξ=1.222(60) in the static limit of b quark.

hep-lat↗