Homepage of Yevgeny Kats

Senior Lecturer in the Department of Physics at Ben-Gurion University

Research positions for graduate students and postdocs are available.

Email:  katsye@bgu.ac.il
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Yevgeny Kats

Welcome! I do research in theoretical high-energy physics. I am interested in a broad range of topics, and especially the phenomenology of particle physics beyond the Standard Model. Currently I focus on studies relevant to the experiments at the Large Hadron Collider (LHC).

Bar-Ilan University (B.Sc., M.Sc.)
Harvard University (A.M., Ph.D.)

Postdoctoral experience
2010–2013: Rutgers
2013–2016: Weizmann Institute
2016–2017: CERN

Selected research topics

Methods for ATLAS and CMS to measure quark polarizations

While it's easy for the LHC detectors to reconstruct the momentum of an energetic quark by measuring the jet it produces, there is no straightforward way to determine the quark's polarization. We have pointed out that it is actually possible. For the bottom and charm quarks, which are heavy relative to the QCD scale, the polarization is expected to be largely preserved when they hadronize into the Λb and Λc baryons, respectively. With collaborators from CMS, we analyzed how such measurements can be done using various decays of these baryons.

The most interesting application would be characterization of new physics processes producing bottom or charm quarks. While new physics is yet to be discovered, we motivated a set of Standard Model analyses for ATLAS, CMS, LHCb, BaBar and Belle that would help calibrate the polarization measurements.

    Heavy baryons as polarimeters at colliders
    Mario Galanti, Andrea Giammanco, Yuval Grossman, Yevgeny Kats, Emmanuel Stamou, Jure Zupan
    JHEP 1511 (2015) 067; arXiv:1505.02771 [hep-ph]

    Measuring c-quark polarization in W+c samples at ATLAS and CMS
    Yevgeny Kats
    JHEP 1611 (2016) 011; arXiv:1512.00438 [hep-ph]

For the strange quark, the heavy-quark approximation cannot be used. However, it is known from experiments at LEP that Λ baryons in fact preserve much of the strange-quark polarization. We argued that this allows measuring the polarization of strange quarks at the LHC.

Furthermore, there are reasons to believe that up and down quarks hadronizing into a Λ may also be transferring some of their polarization. This may open the way for measuring their polarization as well. We motivated studies in top-quark samples in ATLAS and CMS that would provide additional information about the polarization transfer from the strange, up and down quarks to the Λ.

    Measuring polarization of light quarks at ATLAS and CMS
    Yevgeny Kats
    Phys. Rev. D 92 (2015) 071503(R); arXiv:1505.06731 [hep-ph]

A brief summary of the proposed quark polarization measurements in top-quark samples is available in:

    Measuring polarizations of bottom, charm, strange, up and down quarks in top decays
    Yevgeny Kats
    PoS (TOP2015) 028; arXiv:1601.07462 [hep-ph]

Implications of results from colliders on theoretical models

Measurements and new-physics searches from the LHC can be re-used for examining the viability of new physics scenarios other than those for which they were originally designed. Determining the status of those scenarios involves simulating the relevant physical process, the resulting signatures in the detectors, and the analysis done in the experimental studies. As a welcome by-product, we identify general gaps in the experimental coverage of potential new physics signatures.

Visiting the CMS detector (left) and the ATLAS detector (right) of the Large Hadron Collider at CERN (center).

In one such project, our goal was to clarify the experimental status of supersymmetry as a natural explanation for the electroweak symmetry breaking scale. Besides requiring the existence of a light superpartner (as at least the higgsino is expected to be light), and a gluino within the kinematic range of the 8 TeV LHC, we have kept our analysis quite general, allowing for arbitrary departures from any minimal model of supersymmetry. We were able to argue that gluino decays always give rise to either a significant amount of missing energy and/or frequently produce top quarks and/or large jet multiplicity, to the extent that they are covered by a certain class of LHC searches in each case. We have found that gluinos are almost always excluded up to masses above 1 TeV. We have also identified several (rather contrived) classes of scenarios in which the limits are weaker, and proposed strategies for addressing these gaps.

    Toward Full LHC Coverage of Natural Supersymmetry
    Jared A. Evans, Yevgeny Kats, David Shih, Matthew J. Strassler
    JHEP 1407 (2014) 101; arXiv:1310.5758 [hep-ph]

In an earlier work, motivated by the lack of any signals in supersymmetry searches based on missing energy, we have addressed the status of more general models of supersymmetry, those that do not assume R-parity. As a compromise between minimizing the fine-tuning of the electroweak symmetry breaking scale, and the apparent absence of significant production of colored superpartners, we considered scenarios in which the only light colored superpartners are the third-generation squarks (and in particular, one of the stops). We constructed a set of simplified models that span the parameter space of the R-parity violating couplings and the mediators through which the stop may decay. We derived limits on these models using a complete set of potentially relevant recent LHC searches. We then looked into the least constrained scenarios in more detail and suggested several ideas for search methods that may allow addressing many of them.

    LHC Coverage of RPV MSSM with Light Stops
    Jared A. Evans and Yevgeny Kats
    JHEP 1304 (2013) 028; arXiv:1209.0764 [hep-ph]

In another project, we surveyed multiple representative slices of the parameter space of supersymmetry (with conserved R-parity) broken via gauge mediation. Using a set of analyses based on 1/fb of 7 TeV LHC data we derived limits on the superpartner masses in the various scenarios. In several cases we suggested ways in which the reach of the LHC searches could be improved. Among our most interesting results were constraints, or the possibility to constrain, scenarios with direct production of sbottoms or stops, or electroweak production.

    The status of GMSB after 1/fb at the LHC
    Yevgeny Kats, Patrick Meade, Matthew Reece, David Shih
    JHEP 1202 (2012) 115; arXiv:1110.6444 [hep-ph]

Yet earlier, we have studied the possibility for the stop to be very light, even lighter than the top quark, in a scenario where it is the NLSP that decays to a W, b and gravitino. Based on Tevatron and LHC top-quark cross section measurements in various channels and searches in top-like samples, we have found that the stop could still be as light as 150 GeV.

    Light stop NLSPs at the Tevatron and LHC
    Yevgeny Kats and David Shih
    JHEP 1108 (2011) 049; arXiv:1106.0030 [hep-ph]

Discovering or characterizing new colored particles using their QCD bound states
including the simplest explanation for the 750 GeV diphoton excess (RIP)

In any extension of the Standard Model with pair-produced colored particles, they would sometimes form a QCD bound state with each other when produced close to threshold. As long as they have no unsuppressed 2-body decays, the bound particles will typically annihilate into a pair of Standard Model particles rather than decay in their usual way. Such processes give rise to resonances in the invariant mass distributions of the annihilation products, which often include easily measurable objects such as isolated photons or leptons. These signals can provide detailed information about the properties of the new particles. This approach is very general and to a large extent model-independent: it retains its full power regardless of how obscure the usual decay signatures of the particles might be.

A few years ago, we have shown how the mass, spin, electric charge, color representation and additional properties of the new particles can be determined unambiguously by combining information from the dijet, photon+jet, diphoton and dilepton channels of the bound states annihilation. We have also derived limits on new colored particles based on the non-observation of such signals at the 7 TeV LHC. We noted that in some cases, even when dedicated searches for the particles' decays existed, the bound state signals had greater reach.

    Probing Colored Particles with Photons, Leptons, and Jets
    Yevgeny Kats and Matthew J. Strassler
    JHEP 1211 (2012) 097; arXiv:1204.1119 [hep-ph]

More recently, we found that the diphoton excesses reported by ATLAS and CMS near the mass of 750 GeV in the early 13 TeV LHC data could indeed be a bound state annihilation signal of a new colored particle whose direct decays would not have been observed yet.

    Resonances from QCD bound states and the 750 GeV diphoton excess
    Yevgeny Kats and Matthew J. Strassler
    JHEP 1605 (2016) 092; arXiv:1602.08819 [hep-ph]

This explanation was very attractive since in the simplest case it involved just a single beyond-the-Standard-Model particle (with mass around 375 GeV) and only renormalizable couplings. This was appealing in view of the general difficulty to account for the excesses in not-overly-contrived versions of the most popular weakly-coupled frameworks beyond the Standard Model, some of which we explored in

    Interpreting a 750 GeV Diphoton Resonance
    Rick S. Gupta, Sebastian Jäger, Yevgeny Kats, Gilad Perez, Emmanuel Stamou
    JHEP 1607 (2016) 145; arXiv:1512.05332 [hep-ph]

While it seems that the 750 GeV excess is not being confirmed by the more recent data, our results on the bound state signals are sufficiently general to be applicable to interpreting other resonant excesses that might appear in the future. My earlier works on the topic are:

    Annihilation decays of bound states at the LHC
    Yevgeny Kats and Matthew D. Schwartz
    JHEP 1004 (2010) 016; arXiv:0912.0526 [hep-ph]

    Distinguishing spins at the LHC using bound state signals
    Dilani Kahawala and Yevgeny Kats
    JHEP 1109 (2011) 099; arXiv:1103.3503 [hep-ph]

Unparticle physics

The term "unparticle physics" refers to an extension of the Standard Model that contains a hidden sector that is conformally invariant (or one that effectively becomes conformal at an infrared fixed point). If the anomalous dimensions are far from integers, the excitations of the conformal sector produced in collisions of Standard Model particles cannot be described in terms of particles.

In our work, we studied how to describe and compute processes generated by the self-interactions of the unparticle sector, whose physics is encoded in higher correlation functions of the CFT. We argued that the production of "unparticle stuff" in Standard Model-initiated processes involving unparticle self-interactions can be decomposed using the conformal partial wave expansion into a sum over contributions from the production of various kinds of unparticle stuff, corresponding to the various primary operators in the CFT, often different from those to which the Standard Model couples directly. We discussed inclusive and exclusive techniques for computing these processes and exemplified our methods by computing processes involving unparticle self-interactions in the 2D Thirring model which is exactly solvable.

    Unparticle self-interactions
    Howard Georgi and Yevgeny Kats
    JHEP 1002 (2010) 065; arXiv:0904.1962 [hep-ph]

In another context, we have analyzed a more general but still exactly solvable 2D model, the Sommerfield model, that is a theory of a massless fermion coupled to a massive vector boson which flows to the Thirring model at low energies. There we could see in detail how the production of the unparticle stuff from ordinary particles proceeds between the high-energy particle-like behavior of the unparticle sector and the low-energy unparticle behavior. As is well-known, in ordinary QCD the short-distance perturbative physics of quarks and gluons turns into the physics of hadrons at large distances. We have found that analogously to QCD there is a massive "hadron" in the spectrum of the Sommerfield model, but in sharp contrast with QCD there is also the unparticle stuff.

    Unparticle Example in 2D
    Howard Georgi and Yevgeny Kats
    Phys. Rev. Lett. 101 (2008) 131603; arXiv:0805.3953 [hep-ph]

Shear viscosity in strongly-coupled theories via AdS/CFT

The properties of strongly-coupled large-N CFTs are accessible via the AdS/CFT correspondence. We studied how the shear viscosity of such a theory (at finite temperature) is affected by R-squared corrections to the AdS action. We presented an example of a 4D theory in which the conjectured lower bound on the viscosity-to-entropy ratio, 1/4π, is violated by 1/N corrections. The existence of such examples may be relevant to the QCD quark-gluon plasma whose η/s is close to 1/4π.

    Effect of curvature squared corrections in AdS on the viscosity of the dual gauge theory
    Yevgeny Kats and Pavel Petrov
    JHEP 0901 (2009) 044; arXiv:0712.0743 [hep-th]

My complete list of publications is available here.

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