+++ /dev/null
-<chapter name="Hidden Valley Processes">
-
-<h2>Hidden Valley Processes</h2>
-
-This Hidden Valley (HV) scenario has been developed specifically
-to allow the study of visible consequences of radiation in a
-hidden sector, by recoil effect. A key aspect is therefore that
-the normal timelike showering machinery has been expanded with a
-third kind of radiation, in addition to the QCD and QED ones.
-These three kinds of radiation are fully interleaved, i.e.
-evolution occurs in a common <ei>pT</ei>-ordered sequence.
-The scenario is described in <ref>Car10</ref>. Furthermore
-hadronization in the hidden sector has been implemented.
-Three main scenarios for production into and decay out of the
-hidden sector can be compared, in each case either for an
-Abelian or a non-Abelian gauge group in the HV. For further details
-see <ref>Car11</ref>.
-
-<h3>Particle content and properties</h3>
-
-For simplicity we assume that the HV contains an unbroken <b>SU(N)</b>
-gauge symmetry. This is used in the calculation of production cross
-sections. These could be rescaled by hand for other gauge groups.
-
-<modeopen name="HiddenValley:Ngauge" default="3" min="1">
-is <b>U(1)</b> for <code>Ngauge = 1</code>, is <b>SU(N)</b> if
-<code>Ngauge > 1</code>. Note that pair production cross sections
-contains a factor of <code>Ngauge</code> for new particles
-in the fundamental representation of this group.
-</modeopen>
-
-<p/>
-A minimal HV particle content has been introduced. Firstly, there is
-a set of 12 particles that mirrors the Standard Model flavour
-structure, and is charged under both the SM and the HV symmetry groups.
-Each new particle couples flavour-diagonally to a corresponding SM
-state, and has the same SM charge and colour, but in addition is in
-the fundamental representation of the HV colour, as follows:
-<br/><code>Dv</code>, identity 4900001, partner to the normal
-<code>d</code> quark;
-<br/><code>Uv</code>, identity 4900002, partner to the normal
-<code>u</code> quark;
-<br/><code>Sv</code>, identity 4900003, partner to the normal
-<code>s</code> quark;
-<br/><code>Cv</code>, identity 4900004, partner to the normal
-<code>c</code> quark;
-<br/><code>Bv</code>, identity 4900005, partner to the normal
-<code>b</code> quark;
-<br/><code>Tv</code>, identity 4900006, partner to the normal
-<code>t</code> quark;
-<br/><code>Ev</code>, identity 4900011, partner to the normal
-<code>e</code> lepton;
-<br/><code>nuEv</code>, identity 4900012, partner to the normal
-<code>nue</code> neutrino;
-<br/><code>MUv</code>, identity 4900013, partner to the normal
-<code>mu</code> lepton;
-<br/><code>nuMUv</code>, identity 4900014, partner to the normal
-<code>numu</code> neutrino;
-<br/><code>TAUv</code>, identity 4900015, partner to the normal
-<code>tau</code> lepton;
-<br/><code>nuTAUv</code>, identity 4900016, partner to the normal
-<code>nutau</code> neutrino.
-<br/>Collectively we will refer to these states as <code>Fv</code>;
-note, however, that they need not be fermions themselves.
-
-<p/>
-In addition the model contains the HV gauge particle, either
-a HV-gluon or a HV-photon, but not both; see <code>Ngauge</code>
-above:
-<br/><code>gv</code>, identity 4900021, is the massless
-gauge boson of the HV <b>SU(N)</b> group;
-<br/><code>gammav</code>, identity 4900022, is the massless
-gauge boson of the HV <b>U(1)</b> group.
-
-<p/>
-Finally, for the basic HV scenario, there is a new massive particle
-with only HV charge sitting in the fundamental representation of the
-HV gauge group:
-<br/><code>qv</code>, identity 4900101.
-
-<p/>The typical scenario would be for pair production of one of the
-states presented first above, e.g. <ei>g g -> Dv Dvbar</ei>.
-Such a <ei>Dv</ei> can radiate gluons and photons like an SM quark,
-but in addition HV-gluons or HV-photons in a similar fashion.
-Eventually the <ei>Dv</ei> will decay like <ei>Dv -> d + qv</ei>.
-The strength of this decay is not set as such, but is implicit in
-your choice of width for the <ei>Dv</ei> state. Thereafter the
-<ei>d</ei> and <ei>qv</ei> can radiate further within their
-respective sectors. The <ei>qv</ei>, <ei>gv</ei> or <ei>gammav</ei>
-are invisible, so their fate need not be considered further.
-
-<p/>
-While not part of the standard scenario, as an alternative there is
-also a kind of <ei>Z'</ei> resonance:
-<br/><code>Zv</code>, identity 4900023, a boson that can couple
-both to pairs of Standard Model fermions and to <ei>qv qvbar</ei>
-pairs. Mass, total width and branching ratios can be set as convenient.
-<br/>This opens up for alternative processes
-<ei>l^+l^-, q qbar -> Zv -> qv qvbar</ei>.
-
-<p/>
-The possibility of a leakage back from the hidden sector will be
-considered in the Hadronization section below. For the <b>U(1)</b>
-case the <ei>gammav</ei> acquires a mass and can decay back to a
-Standard-Model fermion pair, while the <ei>qv</ei> remains invisible.
-The <b>SU(N)</b> alternative remains unbroken, so confinement holds
-and the <ei>gv</ei> is massless. A string like
-<ei>qv - gv - ... - gv - qvbar</ei> can break by the production of
-new <ei>qv - qvbar</ei> pairs, which will produce <ei>qv-qvbar</ei>
-mesons. It would be possible to build a rather sophisticated hidden
-sector by trivial extensions of the HV flavour content. For now,
-however, the <ei>qv</ei> can be duplicated in up to eight copies
-with the same properties except for the flavour charge. These are
-assigned codes 4900101 - 4900108. This gives a total of 64 possible
-lowest-lying mesons. We also include a duplication of that, into two
-multiplets, corrsesponding to the pseudoscalar and vector mesons of
-QCD. For now, again, these are assumed to have the same mass and
-other properties. Only the flavour-diagonal ones can decay back into
-the Standard-Model sector, however, while the rest remains in the
-hidden sector. It is therefore only necessary to distinguish a few
-states:
-<br/><code>pivDiag</code>, identity 4900111, a flavour-diagonal
-HV-meson with spin 0 that can decay back into the Standard-Model sector;
-<br/><code>rhovDiag</code>, identity 4900113, a flavour-diagonal
-HV-meson with spin 1 that can decay back into the Standard-Model sector;
-<br/><code>pivUp</code>, identity 4900211, an off-diagonal
-HV-meson with spin 0 that is stable and invisible, with an antiparticle
-<code>pivDn</code> with identity -4900211; the particle is
-the one where the code of the flavour is larger than that of the
-antiflavour;
-<br/><code>rhovUp</code>, identity 4900213, an off-diagonal
-HV-meson with spin 1 that is stable and invisible, with an antiparticle
-<code>rhovDn</code> with identity -4900213; again the particle is
-the one where the code of the flavour is larger than that of the
-antiflavour;
-<br/><code>ggv</code>, identity 4900991, is only rarely used,
-to handle cases where it is kinematically impossible to produce an
-HV-meson on shell, and it therefore is assumed to de-excite by the
-emission of invisible <ei>gv-gv </ei> v-glueball bound states.
-
-<p/>
-Only the spin of the HV-gluon or HV-photon is determined unambiguously
-to be unity, for the others you can make your choice.
-
-<modepick name="HiddenValley:spinFv" default="1" min="0" max="2">
-The spin of the HV partners of the SM fermions, e.g.
-<ei>Dv</ei>, <ei>Uv</ei>, <ei>Ev</ei> and <ei>nuEv</ei>.
-<option value="0">spin 0.</option>
-<option value="1">spin 1/2.</option>
-<option value="2">spin 1.</option>
-</modepick>
-
-<modepick name="HiddenValley:spinqv" default="0" min="0" max="1">
-The spin of <ei>qv</ei> when the <ei>Fv</ei> (the HV partners of
-the SM fermions) have spin 1/2. (While, if they have spin 0 or 1,
-the <ei>qv</ei> spin is fixed at 1/2.)
-<option value="0">spin 0.</option>
-<option value="1">spin 1.</option>
-</modepick>
-
-<parm name="HiddenValley:kappa" default="1.">
-If the <ei>Fv</ei> have spin 1 then their production
-cross section depends on the presence of ananomalous magnetic dipole
-moment, i.e. of a <ei>kappa</ei> different from unity. For other spins
-this parameter is not used.
-</parm>
-
-<flag name="HiddenValley:doKinMix" default="off">
-allow kinemtic mixing or not.
-</flag>
-
-<parm name="HiddenValley:kinMix" default="1.">
-strength of kinetic mixing.
-</parm>
-
-<p/>
-You should set the <ei>Fv</ei> and <ei>qv</ei> masses appropriately,
-with the latter smaller than the former two to allow decays.
-When <b>U(1)</b> hadronization is switched on, you need to set the
-<ei>gammav</ei> mass and decay modes. For <b>SU(N)</b> hadronization
-the HV-meson masses should be set to match the <ei>qv</ei> ones.
-The simplest is to assume that <ei>m_qv</ei> defines a constituent
-mass, so that <ei>m_HVmeson = 2 m_qv</ei>. The <ei>hvMesonDiag</ei>
-decay modes also need to be set.
-
-<h3>Production processes</h3>
-
-<flag name="HiddenValley:all" default="off">
-Common switch for the group of all hard Hidden Valley processes,
-as listed separately in the following.
-</flag>
-
-<flag name="HiddenValley:gg2DvDvbar" default="off">
-Pair production <ei>g g -> Dv Dvbar</ei>.
-Code 4901.
-</flag>
-
-<flag name="HiddenValley:gg2UvUvbar" default="off">
-Pair production <ei>g g -> Uv Uvbar</ei>.
-Code 4902.
-</flag>
-
-<flag name="HiddenValley:gg2SvSvbar" default="off">
-Pair production <ei>g g -> Sv Svbar</ei>.
-Code 4903.
-</flag>
-
-<flag name="HiddenValley:gg2CvCvbar" default="off">
-Pair production <ei>g g -> Cv Cvbar</ei>.
-Code 4904.
-</flag>
-
-<flag name="HiddenValley:gg2BvBvbar" default="off">
-Pair production <ei>g g -> Bv Bvbar</ei>.
-Code 4905.
-</flag>
-
-<flag name="HiddenValley:gg2TvTvbar" default="off">
-Pair production <ei>g g -> Tv Tvbar</ei>.
-Code 4906.
-</flag>
-
-<flag name="HiddenValley:qqbar2DvDvbar" default="off">
-Pair production <ei>q qbar -> Dv Dvbar</ei>
-via intermediate gluon.
-Code 4911.
-</flag>
-
-<flag name="HiddenValley:qqbar2UvUvbar" default="off">
-Pair production <ei>q qbar -> Uv Uvbar</ei>
-via intermediate gluon.
-Code 4912.
-</flag>
-
-<flag name="HiddenValley:qqbar2SvSvbar" default="off">
-Pair production <ei>q qbar -> Sv Svbar</ei>
-via intermediate gluon.
-Code 4913.
-</flag>
-
-<flag name="HiddenValley:qqbar2CvCvbar" default="off">
-Pair production <ei>q qbar -> Cv Cvbar</ei>
-via intermediate gluon.
-Code 4914.
-</flag>
-
-<flag name="HiddenValley:qqbar2BvBvbar" default="off">
-Pair production <ei>q qbar -> Bv Bvbar</ei>
-via intermediate gluon.
-Code 4915.
-</flag>
-
-<flag name="HiddenValley:qqbar2TvTvbar" default="off">
-Pair production <ei>q qbar -> Tv Tvbar</ei>
-via intermediate gluon.
-Code 4916.
-</flag>
-
-<flag name="HiddenValley:ffbar2DvDvbar" default="off">
-Pair production <ei>f fbar -> Dv Dvbar</ei>
-via intermediate <ei>gamma*/Z^*</ei>.
-Code 4921.
-</flag>
-
-<flag name="HiddenValley:ffbar2UvUvbar" default="off">
-Pair production <ei>f fbar -> Uv Uvbar</ei>
-via intermediate <ei>gamma*/Z^*</ei>.
-Code 4922.
-</flag>
-
-<flag name="HiddenValley:ffbar2SvSvbar" default="off">
-Pair production <ei>f fbar -> Sv Svbar</ei>
-via intermediate <ei>gamma*/Z^*</ei>.
-Code 4923.
-</flag>
-
-<flag name="HiddenValley:ffbar2CvCvbar" default="off">
-Pair production <ei>f fbar -> Cv Cvbar</ei>
-via intermediate <ei>gamma*/Z^*</ei>.
-Code 4924.
-</flag>
-
-<flag name="HiddenValley:ffbar2BvBvbar" default="off">
-Pair production <ei>f fbar -> Bv Bvbar</ei>
-via intermediate <ei>gamma*/Z^*</ei>.
-Code 4925.
-</flag>
-
-<flag name="HiddenValley:ffbar2TvTvbar" default="off">
-Pair production <ei>f fbar -> Tv Tvbar</ei>
-via intermediate <ei>gamma*/Z^*</ei>.
-Code 4926.
-</flag>
-
-<flag name="HiddenValley:ffbar2EvEvbar" default="off">
-Pair production <ei>f fbar -> Ev Evbar</ei>
-via intermediate <ei>gamma*/Z^*</ei>.
-Code 4931.
-</flag>
-
-<flag name="HiddenValley:ffbar2nuEvnuEvbar" default="off">
-Pair production <ei>f fbar -> nuEv nuEvbar</ei>
-via intermediate <ei>gamma*/Z^*</ei>.
-Code 4932.
-</flag>
-
-<flag name="HiddenValley:ffbar2MUvMUvbar" default="off">
-Pair production <ei>f fbar -> MUv MUvbar</ei>
-via intermediate <ei>gamma*/Z^*</ei>.
-Code 4933.
-</flag>
-
-<flag name="HiddenValley:ffbar2nuMUvnuMUvbar" default="off">
-Pair production <ei>f fbar -> nuMUv nuMUvbar</ei>
-via intermediate <ei>gamma*/Z^*</ei>.
-Code 4934.
-</flag>
-
-<flag name="HiddenValley:ffbar2TAUvTAUvbar" default="off">
-Pair production <ei>f fbar -> TAUv TAUvbar</ei>
-via intermediate <ei>gamma*/Z^*</ei>.
-Code 4935.
-</flag>
-
-<flag name="HiddenValley:ffbar2nuTAUvnuTAUvbar" default="off">
-Pair production <ei>f fbar -> nuTAUv nuTAUvbar</ei>
-via intermediate <ei>gamma*/Z^*</ei>.
-Code 4936.
-</flag>
-
-<flag name="HiddenValley:ffbar2Zv" default="off">
-Production <ei>f fbar -> Zv</ei> where <ei>Zv</ei> is a generic
-resonace that couples both SM fermion pairs and a <ei>qv qvbar</ei>
-pair. Not part of the framework of the above processes, but as an
-alternative. Code 4941.
-</flag>
-
-<h3>Timelike showers</h3>
-
-One key point of this HV scenario is that radiation off the
-HV-charged particles is allowed. This is done by the standard
-final-state showering machinery. (HV particles are not produced
-in initial-state radiation.) All the (anti)particles <ei>Fv</ei>
-and <ei>qv</ei> have one (negative) unit of HV charge. That is,
-radiation closely mimics the one in QCD. Both QCD, QED and HV
-radiation are interleaved in one common sequence of decreasing
-emission <ei>pT</ei> scales. Each radiation kind defines a set of
-dipoles, usually spanned between a radiating parton and its recoil
-partner, such that the invariant mass of the pair is not changed
-when a radiation occurs. This need not follow from trivial colour
-assignments, but is often obvious. For instance, in a decay
-<ei>Qv -> q + qv</ei> the QCD dipole is between the <ei>q</ei> and
-the hole after <ei>Qv</ei>, but <ei>qv</ei> becomes the recoiler
-should a radiation occur, while the role of <ei>q</ei> and <ei>qv</ei>
-is reversed for HV radiation.
-
-<p/>This also includes matrix-element corrections for a number
-of decay processes, with colour, spin and mass effects included
-<ref>Nor01</ref>. They were calculated within the context of the
-particle content of the MSSM, however, which does not include spin 1
-particles with unit colour charge. In such cases spin 0 is assumed
-instead. By experience, the main effects come from mass and colour
-flow anyway, so this is not a bad approximation. (Furthermore the
-MSSM formulae allow for <ei>gamma_5</ei> factors from wave
-functions or vertices; these are even less important.)
-
-<p/>An emitted <ei>gv</ei> can branch in its turn,
-<ei>gv -> gv + gv</ei>. This radiation may affect momenta
-in the visible sector by recoil effect, but this is a minor
-effect relative to the primary emission of the <ei>gv</ei>.
-
-<flag name="HiddenValley:FSR" default="off">
-switch on final-state shower of <ei>gv</ei> or <ei>gammav</ei>
-in a HV production process.
-</flag>
-
-<parm name="HiddenValley:alphaFSR" default="0.1" min="0.0">
-fixed alpha scale of <ei>gv/gammav</ei> emission; corresponds to
-<ei>alpha_strong</ei> of QCD or <ei>alpha_em</ei> of QED. For
-shower branchings such as <ei>Dv -> Dv + gv</ei> the coupling is
-multiplied by <ei>C_F = (N^2 - 1) / (2 * N)</ei> for an
-<b>SU(N)</b> group and for <ei>gv -> gv + gv</ei> by <ei>N</ei>.
-</parm>
-
-<parm name="HiddenValley:pTminFSR" default="0.4" min="0.1">
-lowest allowed <ei>pT</ei> of emission. Chosen with same default
-as in normal QCD showers.
-</parm>
-
-<h3>Hadronization</h3>
-
-By default the HV particles with no Standard Model couplings
-are not visible. Their presence can only be deduced by the
-observation of missing (transverse) momentum in the event as a
-whole. In the current implementation it is possible to simulate
-two different scenarios where activity can leak back from the
-hidden sector.
-
-<p/>
-The first possibility is relevant for the <b>U(1)</b> scenario.
-The <b>U(1)</b> group may be broken, so that the <ei>gammav</ei>
-acquires a mass. Furthermore, the <ei>gammav</ei> may have a
-small mixing angle with the normal photon, or with some <ei>Z'</ei>
-state or other mediator, and may thus decay back into Standard
-Model particles. The <ei>qv</ei> still escapes undetected;
-recall that there is no confinement in the <b>U(1)</b> option.
-
-<p/>
-In order to enable this machinery two commands are necessary,
-<code>4900022:m0 = ...</code> to set the <ei>gammav</ei> mass
-to the desired value, and <code>4900022:onMode = on</code> to enable
-<ei>gammav</ei> decays. The default <ei>gammav</ei> decay
-table contains all Standard Model fermion-antifermion pairs,
-except top, with branching ratios in proportion to their coupling
-to the photon, whenever the production channel is allowed by
-kinematics. This table could easily be tailored to more specific
-models and needs. For instance, for a mass below 1 - 2 GeV, it
-would make sense to construct a table of exclusive hadronic decay
-channels rather than go the way via a hadronizing quark pair.
-
-<p/>
-The <ei>gammav</ei> are expected to decay so rapidly that no
-secondary vertex will be detectable. However, it is possible to
-set <code>4900022:tau0</code> to a finite lifetime (in mm) that
-will be used to create separated secondary vertices.
-
-<p/>
-The second, more interesting, possibility is relevant for the
-<b>SU(N)</b> scenarios. Here the gauge group remains unbroken, i.e.
-<ei>gv</ei> is massless, and the partons are confined. Like in
-QCD, the HV-partons can therefore be arranged in one single
-HV-colour-ordered chain, with a <ei>qv</ei> in one end, a
-<ei>qvbar</ei> in the other, and a varying number of
-<ei>gv</ei> in between. Each event will only contain (at most)
-one such string, (i) since perturbative branchings
-<ei>gv -> qv qvbar</ei> have been neglected, as is a reasonable
-approximation for QCD, and (ii) since HV-colours are assigned in the
-<ei>N_C -> infinity</ei> limit, just like in the handling of
-string fragmentation in QCD. The HV-string can then fragment by the
-nonperturbative creation of <ei>qv qvbar</ei> pairs, leading to
-the formation of HV-mesons along the string, each with its
-<ei>qv</ei> from one vertex and its <ei>qvbar</ei> from
-the neighbouring one.
-
-<p/>
-Since, so far, we have only assumed there to be one <ei>qv</ei>
-species, all produced <ei>qv qvbar</ei> HV-mesons are of the
-same flavour-diagonal species. Such an HV-meson can decay back to
-the normal sector, typically by whatever mediator particle allowed
-production in the first place. In this framework the full energy put
-into the HV sector will leak back to the normal one. To allow more
-flexibility, an ad hoc possibility of <ei>n_Flav</ei> different
-<ei>qv</ei> species is introduced. For now they are all assumed
-to have the same mass and other properties, but distinguished by
-some flavour-like property. Only the flavour-diagonal ones can decay,
-meaning that only a fraction (approximately) <ei>1/n_Flav</ei> of the
-HV-energy leaks back, while the rest remains in the hidden sector.
-
-<p/>
-This scenario contains more parameters than the first one, for the
-<b>U(1)</b> group. They can be subdivided into two sets. One is
-related to particle properties, both for <ei>qv</ei> and for the
-two different kinds of HV-mesons, here labelled 4900111 and 4900113
-for the diagonal ones, and +-4900211 and +-4900213 for the
-off-diagonal ones. It makes sense to set the HV-meson masses to be
-twice the <ei>qv</ei> one, as in a simple constituent mass context.
-Furthermore the <ei>hvMesonDiag</ei> decay modes need to be set up.
-Like with the
-<ei>gammav</ei> in the <b>U(1)</b> option, the default decay table
-is based on the branching ratios of an off-shell photon.
-
-<p/>
-The second set are fragmentation parameters that extend or replace
-the ones used in normal string fragmentation. Some of them are not
-encoded in the same way as normally, however, but rather scale as
-the <ei>qv</ei> mass is changed, so as to keep a sensible default
-behaviour. This does not mean that deviations from this set should
-not be explored, or that other scaling rules could be prefered
-within alternative scenarios. These parameters are as follows.
-
-<flag name="HiddenValley:fragment" default="off">
-switch on string fragmentation of the HV partonic system.
-Only relevant for <b>SU(N)</b> scenarios.
-</flag>
-
-<modeopen name="HiddenValley:nFlav" default="1" min="1" max="8">
-number of different flavours assumed to exist in the hadronization
-description, leading to approximately <ei>1/n_Flav</ei> of the
-produced HV-mesons being flavour-diagonal and capable to decay back
-to Standard Model particles.
-</modeopen>
-
-<parm name="HiddenValley:probVector" default="0.75" min="0." max="1.">
-fraction of HV-mesons that are assigned spin 1 (vector), with the
-remainder spin 0 (pseudoscalar). Assuming the <ei>qv</ei> have
-spin <ei>1/2</ei> and the mass splitting is small, spin counting
-predicts that <ei>3/4</ei> of the mesons should have spin 1.
-</modeopen>
-
-<parm name="HiddenValley:aLund" default="0.3" min="0.0" max="2.0">
-The <ei>a</ei> parameter of the Lund symmetric fragmentation function.
-See the normal <aloc href="Fragmentation">fragmentation
-function</aloc> description for the shape of this function.</parm>
-
-<parm name="HiddenValley:bmqv2" default="0.8" min="0.2" max="2.0">
-The <ei>b</ei> parameter of the Lund symmetric fragmentation function,
-multiplied by the square of the <ei>qv</ei> mass. This scaling ensures
-that the fragmentation function keeps the same shape when the
-<ei>qv</ei> mass is changed (neglecting transverse momenta).
-</parm>
-
-<parm name="HiddenValley:rFactqv" default="1.0" min="0.0" max="2.0">
-<ei>r_qv</ei>, i.e. the Bowler correction factor to the Lund symmetric
-fragmentation function, which could be made weaker or stronger than
-its natural value.
-</parm>
-
-<parm name="HiddenValley:sigmamqv" default="0.5" min="0.0">
-the width <ei>sigma</ei> of transverse momenta in the HV fragmentation
-process, normalized to the <ei>qv</ei> mass. This ensures that
-<ei>sigma</ei> scales proportionately to <ei>m_qv</ei>.
-See the normal <aloc href="Fragmentation">fragmentation
-<ei>pT</ei></aloc> description for conventions for factors of 2.
-</parm>
-
-</chapter>
-
-<!-- Copyright (C) 2012 Torbjorn Sjostrand -->
-
git://git.uio.no / u / mrichter / AliRoot.git / blobdiff