MSEL = 50
Technicolor (TC) is an alternative way to manifest the Higgs mechanism for giving masses to the and bosons using strong dynamics instead of weakly-coupled fundamental scalars. In TC, the breaking of a chiral symmetry in a new, strongly interacting gauge theory generates the Goldstone bosons necessary for electroweak symmetry breaking. Thus three of the technipions assume the rôle of the longitudinal components of the and bosons, but other states can remain as separate particles depending on the gauge group: technipions ( ), technirhos ( ), techniomegas ( ), etc.
No fully-realistic model of strong electroweak-symmetry breaking has been found so far, and some of the assumptions and simplifications used in model-building may need to be discarded in the future. The processes represented here correspond to several generations of development. Processes 149, 191, 192 and 193 should be considered obsolete and superseded by the other processes 194, 195 and 361-377. The former processes are kept for cross-checks and backward-compatibility. In section it is discussed how processes 71-77 can be used to simulate a scenario with techni- resonances in longitudinal gauge boson scattering.
Process 149 describes the production of a spin-0 techni- meson (particle code KF = 3000331), which is an electroweak singlet and a QCD colour octet. It is one of the possible techni- particles; the name ``techni-'' is not used universally in the literature. The techni- couples to ordinary fermions proportional to fermion mass. The dominant decay mode is therefore , if kinematically allowed. An effective -coupling arises through an anomaly, and is roughly comparable in size with that to . Techni- production at hadron colliders is therefore predominantly through fusion, as implemented in process 149. In topcolor-assisted technicolor (discussed below), particles like the techni- should not have a predominant coupling to quarks. In this sense, the process is considered obsolete.
(The following discussion borrows liberally from the introduction to Ref. [Lan99a] with the author's permission.) Modern technicolor models of dynamical electroweak symmetry breaking require walking technicolor [Hol81] to prevent large flavor-changing neutral currents and the assistance of topcolor (TC2) interactions that are strong near 1 TeV [Nam88,Hil95,Lan95] to provide the large mass of the top quark. Both additions to the basic technicolor scenario [Wei79,Eic80] tend to require a large number of technifermion doublets to make the -function of walking technicolor small. They are needed in TC2 to generate the hard masses of quarks and leptons, to induce the right mixing between heavy and light quarks, and to break topcolor symmetry down to ordinary color. A large number of techni-doublets implies a relatively low technihadron mass scale [Lan89,Eic96], set by the technipion decay constant , where GeV.
The model adopted in PYTHIA is the ``Technicolor
Straw Man Model'' (TCSM) [Lan99a,Lan02a].
The TCSM describes the phenomenology
of color-singlet vector and pseudoscalar technimesons and their
interactions with SM particles.
These technimesons are expected to be the lowest-lying
bound states of the lightest technifermion doublet, ,
with components that
transform under technicolor
fundamentals, but are QCD singlets; they have electric charges and
The vector technimesons form a spin-one isotriplet
. Since techni-isospin is likely to be a good approximate
should be approximately
mass-degenerate. The pseudoscalars,
or technipions, also comprise an isotriplet
and an isosinglet
. However, these are not mass eigenstates. In this
model, they are simple, two-state mixtures of the longitudinal
weak bosons , --the true Goldstone bosons of dynamical
electroweak symmetry breaking in the limit that the
couplings vanish--and mass-eigenstate pseudo-Goldstone technipions
Technipion decays are induced mainly by extended technicolor (ETC) interactions which couple them to quarks and leptons [Eic80]. These couplings are proportional to fermion mass, except for the top quark, which has most of its mass generation through TC2 interactions. Thus, there is no great preference for to decay to top quarks nor for top quarks to decay into them. Also, because of anomaly cancellation, the constituents of the isosinglet technipion may include colored technifermions as well as color-singlets, and it decays into a pair of gluons as well as heavy quarks. The relevant technipion decay modes are , , and ; , , and ; and , , , and . In the numerical evaluation of these widths, the running mass (see PYMRUN) is used, and all fermion pairs are considered as final states. The decay is also included, with the final state kinematics distributed according to phase space (i.e. not weighted by the squared matrix element). The couplings to fermions can be weighted by parameters , , and depending on the heaviest quark involved in the decay.
In the limit of vanishing gauge couplings ,
coupling to technipions are:
Walking technicolor enhancements of technipion masses are assumed to close off the channel (which is not included) and to kinematically suppress the channels and the isospin-violating (which are allowed with appropriate choices of mass parameters). The rates for the isospin-violating decays , , are given by where is the isospin-violating - mixing. Taking the value in analogy with QCD, this decay mode is also dynamically suppressed (but is included). While a light technirho can decay to or through TC dynamics, a light techniomega decays mainly through electroweak dynamics, , , , etc., where and may are transversely polarized. Since , the electroweak decays of to the transverse gauge bosons plus a technipion may be competitive with the open-channel strong decays.
Note, the exact meaning of longitudinal or transverse polarizations only makes sense at high energies, where the Goldstone equivalence theorem can be applied. At the moderate energies considered in the TCSM, the decay products of the and bosons are distributed according to phase space, regardless of their designation as longitudinal or ordinary transverse gauge bosons.
An effective Lagrangian for technivector interactions
can be constructed [Lan99a], exploiting gauge invariance,
chiral symmetry, and angular momentum and parity conservation.
For example, the lowest-dimensional operator
mediating the decay
, where the mass parameter is expected to be
of order several 100 GeV. This leads to the
Final states containing Standard Model particles and/or pseudo-Goldstone bosons (technipions) can be produced at colliders through two mechanisms: technirho and techniomega mixing with gauge bosons through a vector-dominance mechanisms, and anomalies [Lan02] involving no techni-resonances. Processes 191, 192 and 193 are based on -channel production of the respective resonance [Eic96] in the narrow width approximation. All decay modes implemented can be simulated separately or in combination, in the standard fashion. These include pairs of fermions, of gauge bosons, of technipions, and of mixtures of gauge bosons and technipions. Processes 194,195 and 361-377, instead, include interference and a correct treatment of kinematic thresholds, both of which are important effects, but also are limited to specific final states. Therefore, several processes need to be simulated at once to determine the full effect of TC.
Process 194 is intended to more accurately represent the mixing between the , , and particles in the Drell-Yan process [Lan99]. Process 195 is the analogous charged channel process including and mixing. By default, the final state fermions are and , respectively. These can be changed through the parameters KFPR(194,1) and KFPR(195,1), respectively (where the KFPR value should represent a charged fermion).
Processes 361-368 describe the pair production of technipions and gauge bosons through resonances and anomaly contributions. Processes 370-377 describe pair production through the resonance and anomalies. It is important to note that processes 361,362,370,371,372 include final states with only longitudinally-polarized and bosons, whereas the others include final states with only transverse and bosons. Thus, all processes must be simulated to get the full effect of the TC model under investigation.
Cross sections for neutral charged final states
are calculated using the full
propagator matrix, . With
energy-dependent width for
matrix is the inverse of
By default, the TCSM Model has the parameters = 4, = , = , = , = 1, = , = , =0, =1, = 0.05, = GeV, = GeV, = GeV, = GeV. The techniparticle mass parameters are set through the usual PMAS array. Parameters regulating production and decay rates are stored in PARP(137) - PARP(150).
In the original TCSM outlined above, the existence of top-color interactions only affected the coupling of technipions to top quarks, which is a significant effect only for higher masses. In general, however, TC2 requires some new and possibly light colored particles. In most TC2 models, the existence of a large , but not , condensate and mass is due to gauge interactions which are strong near 1 TeV. The interaction is - symmetric while couplings are - asymmetric. There are weaker gauge interactions in which light quarks (and leptons) may [Hil95], or may not [Chi96], participate. The two 's must be broken to weak hypercharge at an energy somewhat higher than 1 TeV by electroweak-singlet condensates. The full phenomenology of even such a simple model can be quite complicated, and many (possibly-unrealistic) simplifications are made to reduce the number of free parameters [Lan02a]. Nonetheless, it is useful to have some benchmark to guide experimental searches.
The two TC2 's can be broken to their diagonal subgroup by using technicolor and interactions, both strong near 1 TeV. This can be explicitly accomplished [Lan95] using two electroweak doublets of technifermions, and , which transform respectively as and under the two color groups and technicolor. The desired pattern of symmetry breaking occurs if and interactions work together to induce electroweak and non-invariant condensates and , . This minimal TC2 scenario leads to a rich spectrum of color-nonsinglet states readily accessible in hadron collisions. The lowest-lying ones include eight ``colorons'', , the massive gauge bosons of broken topcolor ; four isosinglet formed from and the isosinglet pseudo-Goldstone technipions formed from . In this treatment, the isovector technipions are ignored, because they must be pair produced in decays, and such decays are assumed to be kinematically suppressed.
The colorons are new fundamental particles with couplings to quarks. In standard TC2 [Hil95], top and bottom quarks couple to and the four light quarks to . Because the interaction is strong and acts exclusively on the third generation, the residual coupling can be enhanced for and quarks. The coupling for and and for , where is the QCD coupling and is related to the original and couplings. In flavor-universal TC2 [Chi96] all quarks couple to , not , so that colorons couple equally and strongly to all flavors: .
Assuming that techni-isospin is not badly broken by ETC interactions, the are isosinglets labeled by the technifermion content and color index : . The first two of these states, and , mix with and . The topcolor-breaking condensate, , causes them to also mix with and . Technifermion condensation also leads to a number of (pseudo)Goldstone boson technipions. The lightest technipions are expected to be the isosinglet octet and singlet states and .
These technipions can decay into either fermion-antifermion pairs or two gluons; presently, they are assumed to decay only into gluons. As noted, walking technicolor enhancement of technipion masses very likely close off the channels. Then the decay into and . The rate for the former are proportional to the amount of kinetic mixing, set by . Additionally, the decays to .
The colorons are expected to be considerably heavier than the
with mass in the range 0.5-1 TeV. In both the standard and
flavor-universal models, colorons couple strongly to , but with
only strength to . Since relatively light technipions are
states, it is estimated that
. Therefore, these decay
modes are ignored, so that
the decay rate is the
sum over open channels of
The phenomenological effect of this techniparticle structure is
to modify the gluon propagator in ordinary QCD processes, because
between the gluon, and the
The ----- propagator
is the inverse of the symmetric matrix
This extension of the TCSM is still under developmental, and should be used with caution. The main effects are indirect, in that they modify the underlying two-parton QCD processes much like compositeness terms, except that resonances are visible. Similar to compositeness, the effects of these colored technihadrons are simulated by setting MSTP(5)=5. The parameter dependence of the `model' is encoded in (PARP(155)) and a mass parameter (PARP(156)), which determines the decay width analogously to for . For positive (negative) values of PARP(155), the standard (flavor universal) TC2 couplings are used. The mass parameters are set by the PMAS array using the codes: (3100021), (3100111), (3200111), (3100113), (3200113), (3300113), and (3400113). The mixing parameters take on the (arbitrary) values GeV, GeV, GeV and GeV, while the kinetic mixing terms are calculated assuming the technicolor condensates are fully mixed, i.e. .