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\begin{center}
{\Large\bf Shifman's Profile}
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\begin{center}
{\bf From the Theoretical Physics Institute Newsletter, Volume 2, Issue 2, Fall
2001}
 \end{center}
\vspace{0.1cm}


The most fundamental components of the universe are protons and neutrons.  
By 1970 it was clear that protons and neutrons are just two representatives of a huge
family of hadrons, which include, among dozens of other particles, $\pi ,\, K, \, \rho$,
etc.  At that time, the dynamical laws governing the hadronic family were a total
mystery.  This changed, overnight, in 1973 with the revolutionary discovery of
{\em quantum chromodynamics} (QCD), the theory of hadrons.  Experimental puzzles
that had been accumulating for decades began unfolding one after another. This
exciting time coincided with the beginning of Mikhail Shifman's research career. 

     According to QCD, hadrons are not elementary, rather they are complicated 
conglomerates composed of quarks and gluons.  The gluons are responsible for the
binding force between the quarks.  This binding has a very special nature �-- the
further apart the quarks are split, the stronger the binding potential, so that ``isolated"
quarks do not exit.  This feature was named color confinement, a dynamical regime
with no analogs in other bound systems that had been studied previously. 

      The impact of the gluon ``medium" and color confinement on the quark degrees of
freedom became the focus of Shifman's research for years to come.  QCD opened new
possibilities for the solution of long-standing mysteries.  A dramatic issue was a huge
enhancement of the decay $K_S \to  \pi^+\pi^-$  over a 
similar $K^+\to  \pi^+\pi^0$ and analogous
unexplained enhancements in a variety of the hyperon decays.  Shifman (with
Vainshtein and Zakharov) discovered a class of graphs (later dubbed $penguins$),
shown to be responsible for this enhancement.  This was one of the early triumphs of
quantum chromodynamics.  The penguin graphs proved to be instrumental in a wide
range of phenomena �-- from CP nonconservation to rare heavy flavor decays. 

     At short distances  $(< 10^{ �14}$ cm) quarks and gluons fully describe QCD
dynamics.  At larger distances $(\sim 10^{ �13}$ cm) these degrees of freedom become
irrelevant because of the color confinement.  The hadronic degrees of freedom take
over.  The transition occurs at strong coupling where familiar methods of dealing with
field theory fail.  The hope for understanding laws of the hadronic world lay with the
development of new methods.  Two such methods exist today, the inception of both
can be traced back to 1978. 

     One of them, lattice QCD, is purely numerical.  Space and time are discretized 
and are represented by a finite number of isolated points, each of which is endowed
with a number of internal degrees of freedom.  Then Monte Carlo simulations are
performed.  This method relies on powerful computers, and is very costly.  Moreover,
it is not transparent: human control is essentially lost.

     Shifman, Vainshtein, and Zakharov chose another route.  They observed that 
an intermediate domain of distances exists where, on the one hand, the hadrons are
already well formed and, on the other hand, the gluon ``medium" can be
approximately described by a few average characteristics, the most important being
the gluon and the quark condensates.  If so, the spectral and other properties of the
hadronic states can be approximately related to the condensate parameters.  This
route led to remarkable successes: basic regularities of the hadronic family were
explained, as well as many fine details.  The method goes under an awkward name of
{\em QCD sum rules}.  The original Shifman-Vainshtein-Zakharov publication on the
QCD sum rules is one of the most cited papers in high-energy physics.
 
     The condensate-based description of hadrons was further expanded in the 
1990's, with new elements added and fresh findings incorporated.  One such finding
was a {\em heavy quark symmetry}.  It reveals itself only in the heavy quark sector
that, at that time, became the focus of experimental studies.  One of the early works
establishing the heavy quark symmetry was carried out by Shifman with  Voloshin. 
It provided an impetus for the construction of the heavy quark mass expansion,
which culminated at TPI in the 1990's, in a quantitative theory of the decays of $c$
and $b$ flavored hadrons. 

     The advent of QCD solved, at least at a conceptual level, all problems concerning 
the theory of hadrons.  It brought a new one, however.  CP conservation, which was
believed to be a natural feature of the strong interaction, is lost in QCD due to a
possible presence of the so-called $\theta$ term (an analog of quasimomentum in solid
state physics).  Weinberg and Wilczek suggested a new particle, axion, leading to an
automatic ``vacuum relaxation" and vanishing $\theta$ term.  Their magnificent work
was incompatible with data, however.  Then, an invisible axion was suggested by
Shifman and others --- it preserves positive features of the original axion and,
simultaneously, avoids unwanted contradictions.  The invisible axion became a
standard feature of the present-day theory. 

\vspace{0.3cm}

     Quantum chromodynamics and its applications in hadronic physics is one of 
Shifman's favorites, but not the only one.  Another passion is {\em supersymmetry}. 
The beginnings of supersymmetry also date back to the 1970's.  This is a very deep
geometrical symmetry which connects the spin 1/2 fermions, the building blocks of
matter (such as protons or electrons), with spin-0 or 1 bosons, which mediate the
fundamental forces (e.g., photons or W bosons).  It can be best characterized by the
following quotation from Witten's recent article:

\begin{quote}
     ``... One of the biggest adventures of all is the search for supersymmetry. 
Supersymmetry is the framework in which theoretical physicists have sought to
answer some of the questions left open by the Standard Model of particle physics. 
Supersymmetry, if it holds in nature, is part of the quantum structure of space and
time.  In everyday life, we measure space and time by numbers, ``It's now three
o�clock, the elevation is two hundred meters above see level,�� and so on.  Numbers are
classical concepts, known to humans since long before Quantum Mechanics was
developed in the early twentieth century.  The discovery of Quantum Mechanics
changed our understanding of almost everything in physics, but our basic way of
thinking about space and time has not yet been affected.  Showing that nature is
supersymmetric would change that, by revealing a quantum dimension of space and
time, not measurable by ordinary numbers. ...Discovery of supersymmetry would be
one of the real milestones in physics."
\end {quote}

     In the 1980's Shifman and Vainshtein obtained the first exact results in 
strongly-coupled supersymmetric theories --� $\beta$ functions and the gluino
condensate.  The methods they worked out were precursors to those used in
establishing Seiberg's duality which led to the recent discovery of the string dualities. 

     Till recently, the Standard Model of particle physics and its supergeneralization 
was the only respectable paradigm of the modern theory.  In recent years an
alternative paradigm emerged, based on the idea that our four-dimensional habitat is
nothing but a slice of a higher dimensional world ({\em living on a wall scenarios}).  
The
matter with which our universe is built presents zero modes trapped on the surface of
the wall.  Imagine bugs crawling on the surface of a table unable to leave it.  There is
a critical energy needed to leave the wall and penetrate the bulk.

     In 1982 when this conjecture was put forward (by Rubakov and Shaposhnikov)
it was utterly premature and   was ignored.  The ``living on a wall" idea was
reintroduced in 1996 by Dvali and Shifman in connection with searches for novel
mechanisms of supersymmetry breaking.  This time it took off, and within two years
became the dominant research topic in the community.  Eventually it grew into four
full-blown directions of research.  Thousands of works have been published covering
virtually all aspects of the ``living on a wall" paradigm.  Shifman is currently heavily
involved in the use of 2+1 dimensional walls in four dimensions to study subtle
properties of gauge theories at strong coupling.

     Prof. Shifman was born in 1949 in Latvia.  He got his Ph.D. from the Moscow 
Institute of Theoretical and Experimental Physics in 1976, where he worked till
1989.  In 1990 Shifman moved to Minnesota to join the School of Physics and
Astronomy and TPI.  In 1997 he was elected as a Fellow of the American Physical
Society.  He received the {\em 1999 J. J. Sakurai Prize for Theoretical Particle Physics}
(with Vainshtein and Zakharov).



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