Prof. Andrey Korytov


Higgs boson: search, discovery, and measurements of its properties

The standard model is the experimentally verifiable theory that encompasses our current understanding of the three fundamental forces (strong, electromagnetic, and weak) and the elementary constituents of matter (three generations of quarks and leptons). The model is build on a premise of a particular type of symmetry, so-called local gauge invariance, that, however, forbids particles to have masses, which we know they have. In 1964, Brout, Englert, and Higgs suggested a possible way out. The residual consequence of the suggested solution was a prediction of new type of a particle that has become known as a Higgs boson. The standard model has had an unprecedented success in describing the wealth of experimental data, except for one setback---the predicted Higgs boson has been nowhere to be seen, not until recently that is. The Large Hadron Collider at CERN, commissioned in 2009, was built to answer unambiguously the question of whether the putative Higgs boson exists or not.
    Since 2004, the main focus of my research program at LHC, as a member of the CMS Experiment, was on the Higgs boson physics: first, on preparations for the Higgs boson search; then, after the start-up of LHC in 2009, on the search itself, which, in 2012, culminated with the discovery of the Higgs boson with a mass near 125 GeV in a combination of searches in multiple Higgs boson decay modes; and now on measuring properties of the discovered boson. In 2009-2010, I was a convener of the CMS Higgs Physics Analysis Group, coordinating all Higgs boson searches in CMS. In 2011-2012, I was a convener of the CMS Higgs combination group. And all this time, I have also pursued an observation of the Higgs boson and now measurements of its properties in the so-called golden four-lepton decay channel H→ZZ→4l. This channel provides the most accurate measurement of the Higgs boson mass and allows for disentangling its spin-parity quantum numbers.
    Selected publications:
CMS Collaboration, "On the mass and spin-parity of the Higgs boson candidate via its decays to Z boson pairs", Phys. Rev. Lett. 110 (2013) 081803
P. Avery et al., "Precision studies of the Higgs boson decay channel H→ZZ→4l with MEKD", Phys. Rev. D 97 (2013) 055006
CMS Collaboration, "Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC", Phys. Lett. B 716 (2012) 30-61
CMS Collaboration, "Combined results of searches for the standard model Higgs boson in pp collisions at sqrt(s) = 7 TeV", Phys. Lett. B 710 (2012) 26-48
CMS Collaboration, "Search for the standard model Higgs boson in the decay channel H→ZZ→4l in pp collisions at sqrt(s) = 7 TeV", Phys. Rev. Lett. 108 (2012) 111804
CMS Collaboration, "Observation of Z→4l decays in pp collisions at sqrt(s) = 7 TeV", JHEP 12 (2012) 034

Search for supersymmetry

Considerations of symmetry have proven to be very instrumental in paving the way to understanding and formulating the fundamental laws of physics. By laws of quantum mechanics, all particles in nature can have an internal angular momentum, known as spin, which has to quantize and can be either an integer or half-integer number in units of the Plank constant. Particles with integer spin (e.g. photon) are called bosons; particles with half-integer spin (e.g. electron) are fermions. In 1970s, theories satisfying the new form of symmetry, now known as supersymmetry (SUSY), between bosons and fermions have emerged. SUSY theories predict that each particle we have observed in nature must have a twin sibling, a SUSY particle. The only difference between siblings in the limit of very high energies would be their spins. At low energy, the supersymmetry may be only approximate leading to a difference in masses between twin siblings.
    Besides its pure aesthetic appeal, SUSY has very important phenomenological consequences. First, it resolves the problem of the Higgs boson mass, which due to quantum fluctuations in the vacuum, without SUSY particles or any other new physics beyond of what we know now, is expected to be enormously high, of the order of 1019 GeV. With SUSY particles present in the quantum fluctuations, the Higgs boson becomes light; in fact, it has to be lighter than 130 GeV or so. The Higgs boson was discovered in 2012 (see above) and has a mass of 125 GeV. Second, the lightest SUSY particle is an excellent candidate for the dark matter, which cannot be explained within the standard model framework.
    From the start up of the LHC in 2009, I am actively involved in searches for SUSY particles, either produced in strong interactions, in events with same-sign dileptons, jets, and missing energy (see an example on the left); or produced in electroweak interactions, in events with multi-leptons and missing energy. Up to now, none of the SUSY searches revealed the existence of these "shadow" particles. The search continues…
    Selected publications:
CMS Collaboration, "Search for electroweak production of charginos and neutralinos using leptonic final states in pp collisions at sqrt(s) = 7 TeV", JHEP 1211 (2012) 147
CMS Collaboration, "Search for new physics with same-sign isolated dilepton events with jets and missing transverse energy", Phys. Rev. Lett. 109 (2012) 071803
CMS Collaboration, "Search for new physics with same-sign isolated dilepton events with jets and missing transverse energy at the LHC", JHEP 06 (2011) 077

Design, construction, and operation of the CMS Endcap Muon System detectors
The Large Hadron Collider at CERN provides head-on collisions of protons with unprecedented energy and intensity, which opens many exciting opportunities in a search for new particles and new physics phenomena, including Higgs boson, SUSY particles, sub-structure of what we now call "elementary" particles, extra spatial dimensions, and lots more. Many such searches rely on detecting high energy muons (heavy siblings of electrons).
    In 1994-2005, I led the design and construction of the Endcap Muon detectors for the CMS Experiment at LHC. The CMS Endcap Muon System, part of which is depicted on the left as it appeared on the cover page of Newsweek (Sep 15, 2008), is comprised of about 500 individual detector units of trapezoidal form. Each of these detectors is capable of registering a passage of a muon with precision of about 100 microns and a few nanosecond time resolution. The sensitive area of the system covers in total 6000 square meters, which is equivalent to the area of the American football field. The cost of the detectors (not counting electronics and infrastructure) was about $18M. The construction cost of the entire CMS Experiments was about $0.5B.
    Since the beginning of the LHC operation, our group continuously monitors the performance of the system to ensure high quality of the collected data.
    Selected publications:
A. Korytov, "Muon Detectors for Colliders", Nucl. Instr. and Meth. A 598 (2008) 175-182
CMS Collaboration, "Cathode strip chambers", in "The CMS experiment at the CERN Large Hadron Collider", 2008 JINST 3 S08004
D. Acosta et al., "Large CMS cathode strip chambers: design and performance", Nucl. Instr. and Meth. A 453 (2000) 182
M. Barmand et al., "Spatial resolution attainable with cathode strip chambers at the trigger level", Nucl. Instr. and Meth. A 425 (1999) 92-105

Probing the applicability of the perturbative Quantum Chromo-Dynamics to physics at large-distance scales

Protons (antiprotons) consist of three quarks (antiquarks) bound together by the so-called "strong" force, which unlike any other fundamental force, is infinitely large at large distances. This force is transmitted by massless particles called gluons. The theory describing the "strong" force is called Quantum Chromo-Dynamics, or QCD. When high-energy protons or proton-antiproton collide, often a pair of their elementary constituents (quarks, antiquarks, gluons) get knocked out. As these constituents fly apart, the force between them grows stronger and stronger, which results in an emission of numerous gluons and quark-antiquark pairs. On the time scale of 10-23 s, all these emitted gluons and quarks pair up to make jets of experimentally observable particles as shown in the picture on the left.
    The dynamics of the primary high-energy scattering is well understood in the framework of perturbative QCD, in which interactions are considered to be rare perturbations to freely propagating particles. The perturbative QCD methods, however, were thought to be inadequate for describing the jet fragmentation process, the main reasons being a bizarre nature of the strong force that grows infinitely large with distance. However, this perception was challenged by more advanced perturbative QCD calculations, in which multiple "strong" interactions responsible for the jet formation were successfully accounted for, albeit with a certain degree of approximations.
    In 1996-2008, my research program at the CDF Experiment at Fermilab was devoted to studies of jet properties and confronting the experimental results with the quantitative predictions arising from the advanced perturbative QCD calculations. The data showed a surprisingly good agreement with analytic perturbative QCD predictions in a large variety of observables (multiplicities of particles in jets, momentum distributions of particles in jets, correlations of momenta, etc.) traditionally thought to be off-limits for perturbative QCD.
    Selected publications:
CDF Collaboration, "Measurement of the kT-distribution of particles in jets produced in proton-antiproton collisions at sqrt(s)=1.96 TeV", Phys. Rev. Lett. 102 (2009) 232002
CDF Collaboration, "Two-particle momentum correlations in jets produced in proton-antiproton collisions at sqrt(s)=1.96 TeV", Phys. Rev. D 77 (2008) 092001
CDF Collaboration, "Measurements of charged particle multiplicities in gluon and quark jets in proton-antiproton collisions at sqrt(s)=1.96 TeV", Phys. Rev. Lett. 94 (2005) 171802
CDF Collaboration, "Momentum distribution of charged particles in jets in dijet events in proton-antiproton collisions at sqrt(s) =1.8 TeV and Comparisons to Perturbative QCD Predictions", Phys. Rev. D 68 (2003) 012003
CDF Collaboration, "Charged particle multiplicities in jets in proton-antiproton collisions at sqrt(s)=1.8 TeV", Phys. Rev. Lett. 87 (2001) 211804