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1 Spin groups
1.1 Description
Spins and spin group mathematics are important subjects both in theoretical physics and mathematics. In physics, the term spin ‘groups’ is often used with the broad meaning of a collection of coupled, or interacting spins, and thus covers the broad ‘spectrum’ of spin clusters ranging from gravitons (as in spin networks and spin foams, for example) to ‘up’ ($u$) and ‘down’ ($d$) quark spins (fermions) coupled by gluons in nuclei (as treated in quantum chromodynamics or theoretical nuclear physics), and electron spin Cooper pairs (regarded as bosons) in lowtemperature superconductivity. On the other hand, in relation to quantum symmetry, spin groups are defined in quantum mechanics and quantum field theories (QFT) in a precise, mathematical (algebraic) sense as properly defined groups, as introduced next. (In a semiclassical approach, the related concept of a spinor has been introduced and studied in depth by É. Cartan, who found that with his definition of spinors the (special) relativistic Lorentz covariance properties were not recovered, or applicable.)
Definition 1.1.
In the mathematical, precise sense of the term, a spin group –as for example the Lie group $Spin(n)$– is defined as a double cover of the special orthogonal (Lie) group $SO(n)$ satisfying the additional condition that there exists the short exact sequence of Lie groups:
$1\to\mathbb{Z}_{2}\to Spin(n)\to SO(n)\to 1$ 
Alternatively one can say that the above exact sequence of Lie groups defines the spin group $Spin(n)$. Furthermore, $Spin(n)$ can also be defined as the proper subgroup (or groupoid) of the invertible elements in the Clifford algebra $\mathbb{C}l(n)$; (when defined as a double cover this should be $Cl_{{p,q}}(R)$, a Clifford algebra built up from an orthonormal basis of $n=p+q$ mutually orthogonal vectors under addition and multiplication, $p$ of which have norm +1 and $q$ of which have norm $1$, as further explained in the spinor definition).
Important examples of $Spin(n)$ and quantum symmetries: there exist the following isomorphisms:
1. $Spin(1)\cong O(1)$
2. $Spin(2)\cong U(1)\cong SO(2)$
3. $Spin(3)\cong Sp(1)\cong SU(2)$
4. $Spin(4)\cong Sp(1)\times Sp(1)$
5. $Spin(5)\cong Sp(2)$
6. $Spin(6)\cong SU(4)$
Thus, the symmetry groups in the Standard Model (SUSY) of current Physics can also be written as : $Spin(2)\times Spin(3)\times SU(3)$.
Remarks

In modern Physics, nonAbelian spin groups are also defined, as for example, spin quantum groups and spin quantum groupoids.

An extension of the concepts of spin group and spinor, is the notion of a ‘twistor’, a mathematical concept introduced by Sir Roger Penrose, generally with distinct symmetry/mathematical properties from those of spin groups, such as those defined above.
1.2 The Fundamental Groups of $Spin(p,q)$
With the usual notation, the fundamental groups $\pi_{1}(Spin(p,q))$ are as follows:
(a) $\left\{0\right\}$ , for $(p,q)=(1,1)$ and $(p,q)=(1,0)$;
(b) $\left\{0\right\}$ , if $p>2$ and $q=0,1$;
(c) $\mathbb{Z}$ for $(p,q)=(2,0)$ and $(p,q)=(2,1)$;
(d) $\mathbb{Z}\times\mathbb{Z}$ for $(p,q)=(2,2)$;
(e) $\mathbb{Z}$ for $p>2,q=2$
(f) $\mathbb{Z}_{2}$ for $p>2,q>2$
References
 1 A. Abragam and B. Bleaney. Electron Paramagnetic Resonance of Transition Ions. 1970. Clarendon Press: Oxford, (dedicated to J. H. Van Vleck), pp. 911.
 2 P.W. Anderson and H. Suhl. 1955. Phys. Rev., 100:17881795.
 3 J.F. Dyson., 1956. General Theory of Spin Wave interactions., Phys. Rev., 102:12171228.
 4 S. Weinberg. 1999. Quantum Theory of Fields, vol. 1, Cambridge University Press: Cambridge, UK.
 5 I.C. Baianu et al. 1980. Ferromagnetic Resonance and Spin Wave Excitations in Metallic Glasses., J. Phys. Chem. Solids., 40: 941950.
 6 I.C. Baianu et al. 1981. Nuclear Magnetic Resonance SpinEcho Responses of Dipolar Coupled Spin 1/2 Triads (Groups in Solids.), J. Magn. Resonance., 43: 101111.
Mathematics Subject Classification
82D30 no label found83C60 no label found81R25 no label found15A66 no label found53C27 no label found Forums
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