1 <chapter name="Bose-Einstein Effects">
3 <h2>Bose-Einstein Effects</h2>
5 The <code>BoseEinstein</code> class performs shifts of momenta
6 of identical particles to provide a crude estimate of
7 Bose-Einstein effects. The algorithm is the BE_32 one described in
8 <ref>Lon95</ref>, with a Gaussian parametrization of the enhancement.
9 We emphasize that this approach is not based on any first-principles
10 quantum mechanical description of interference phenomena; such
11 approaches anyway have many problems to contend with. Instead a cruder
12 but more robust approach is adopted, wherein BE effects are introduced
13 after the event has already been generated, with the exception of the
14 decays of long-lived particles. The trick is that momenta of identical
15 particles are shifted relative to each other so as to provide an
16 enhancement of pairs closely separated, which is compensated by a
17 depletion of pairs in an intermediate region of separation.
20 More precisely, the intended target form of the BE corrrelations in
23 f_2(Q) = (1 + lambda * exp(-Q^2 R^2))
24 * (1 + alpha * lambda * exp(-Q^2 R^2/9) * (1 - exp(-Q^2 R^2/4)))
26 where <ei>Q^2 = (p_1 + p_2)^2 - (m_1 + m_2)^2</ei>.
27 Here the strength <ei>lambda</ei> and effective radius <ei>R</ei>
28 are the two main parameters. The first factor of the
29 equation is implemented by pulling pairs of identical hadrons closer
30 to each other. This is done in such a way that three-monentum is
31 conserved, but at the price of a small but non-negligible negative
32 shift in the energy of the event. The second factor compensates this
33 by pushing particles apart. The negative <ei>alpha</ei> parameter is
34 determined iteratively, separately for each event, so as to restore
35 energy conservation. The effective radius parameter is here <ei>R/3</ei>,
36 i.e. effects extend further out in <ei>Q</ei>. Without the dampening
37 <ei>(1 - exp(-Q^2 R^2/4))</ei> in the second factor the value at the
38 origin would become <ei>f_2(0) = (1 + lambda) * (1 + alpha * lambda)</ei>,
39 with it the desired value <ei>f_2(0) = (1 + lambda)</ei> is restored.
40 The end result can be viewed as a poor man's rendering of a rapidly
41 dampened oscillatory behaviour in <ei>Q</ei>.
44 Further details can be found in <ref>Lon95</ref>. For instance, the
45 target is implemented under the assumption that the initial distribution
46 in <ei>Q</ei> can be well approximated by pure phase space at small
47 values, and implicitly generates higher-order effects by the way
48 the algorithm is implemented. The algorithm is applied after the decay
49 of short-lived resonances such as the <ei>rho</ei>, but before the decay
50 of longer-lived particles.
53 This algorithm is known to do a reasonable job of describing BE
54 phenomena at LEP. It has not been tested against data for hadron
55 colliders, to the best of our knowledge, so one should exercise some
56 judgement before using it. Therefore by default the master switch
57 <aloc href="MasterSwitches">HadronLevel:BoseEinstein</aloc> is off.
58 Furthermore, the implementation found here is not (yet) as
59 sophisticated as the one used at LEP2, in that no provision is made
60 for particles from separate colour singlet systems, such as
61 <ei>W</ei>'s and <ei>Z</ei>'s, interfering only at a reduced rate.
64 <b>Warning:</b> The algorithm will create a new copy of each particle
65 with shifted momentum by BE effects, with status code 99, while the
66 original particle with the original momentum at the same time will be
67 marked as decayed. This means that if you e.g. search for all
68 <ei>pi+-</ei> in an event you will often obtain the same particle twice.
69 One way to protect yourself from unwanted doublecounting is to
70 use only particles with a positive status code, i.e. ones for which
71 <code>event[i].isFinal()</code> is <code>true</code>.
74 <h3>Main parameters</h3>
76 <flag name="BoseEinstein:Pion" default="on">
77 Include effects or not for identical <ei>pi^+</ei>, <ei>pi^-</ei>
81 <flag name="BoseEinstein:Kaon" default="on">
82 Include effects or not for identical <ei>K^+</ei>, <ei>K^-</ei>,
83 <ei>K_S^0</ei> and <ei>K_L^0</ei>.
86 <flag name="BoseEinstein:Eta" default="on">
87 Include effects or not for identical <ei>eta</ei> and <ei>eta'</ei>.
90 <parm name="BoseEinstein:lambda" default="1." min="0." max="2.">
91 The strength parameter for Bose-Einstein effects. On physical grounds
92 it should not be above unity, but imperfections in the formalism
93 used may require that nevertheless.
96 <parm name="BoseEinstein:QRef" default="0.2" min="0.05" max="1.">
97 The size parameter of the region in <ei>Q</ei> space over which
98 Bose-Einstein effects are significant. Can be thought of as
99 the inverse of an effective distance in normal space,
100 <ei>R = hbar / QRef</ei>, with <ei>R</ei> as used in the above equation.
101 That is, <ei>f_2(Q) = (1 + lambda * exp(-(Q/QRef)^2)) * (...)</ei>.
104 <parm name="BoseEinstein:widthSep" default="0.02" min="0.001" max="1.">
105 Particle species with a width above this value (in GeV) are assumed
106 to be so short-lived that they decay before Bose-Einstein effects
107 are considered, while otherwise they do not. In the former case the
108 decay products thus can obtain shifted momenta, in the latter not.
109 The default has been picked such that both <ei>rho</ei> and
110 <ei>K^*</ei> decay products would be modified.
115 <!-- Copyright (C) 2010 Torbjorn Sjostrand -->