[1] | 1 | <chapter name="Beam Parameters"> |
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| 2 | |
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| 3 | <h2>Beam Parameters</h2> |
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| 4 | |
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| 5 | The settings on this page relate to the beam identities and energies, |
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| 6 | to a beam momentum spread and to a beam interaction spot. |
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| 7 | As always, momenta and energies are to be given in units of GeV, |
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| 8 | and of space and time in mm. |
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| 9 | |
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| 10 | <h3>Incoming beams</h3> |
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| 11 | |
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| 12 | There are two ways to set the identities and energies of the two incoming |
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| 13 | beam particles. One is to use the <code>init()</code> method with no |
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| 14 | arguments. Then the settings variables below will be read and used. The |
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| 15 | alternative is to call <code><aloc href="ProgramFlow">init(...)</aloc></code> |
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| 16 | with arguments that provide this information. Then you need not use the |
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| 17 | variables below (although it would still be possible). Note that, if nothing |
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| 18 | is done, you will default to LHC at 14 TeV. |
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| 19 | |
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| 20 | <p/> |
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| 21 | Currently the beam particles must be either a hadron pair or a lepton |
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| 22 | pair. In the former category <ei>p p</ei> and <ei>pbar p</ei> |
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| 23 | combinations dominate, but it is also possible to combine with |
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| 24 | <ei>pi^+</ei>, <ei>pi^-</ei> and <ei>pi^0</ei>. In the latter |
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| 25 | <ei>e^+ e^-</ei> and <ei>mu^+ mu^-</ei> would be the most useful |
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| 26 | combinations, but also others should work if combined with an |
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| 27 | appropriate hard process. |
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| 28 | |
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| 29 | <modeopen name="Beams:idA" default="2212"> |
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| 30 | The PDG <code>id</code> code for the first incoming particle. |
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| 31 | Allowed codes include |
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| 32 | <br/><ei>2212 = p</ei>, <ei>-2212 = pbar</ei>, |
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| 33 | <br/><ei>211 = pi^+</ei>, <ei>-211 = pi^-</ei>, <ei>111 = pi^0</ei>, |
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| 34 | <br/><ei>990 = Pomeron</ei> (used in diffractive machinery; |
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| 35 | here mainly for debug purposes), |
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| 36 | <br/><ei>11 = e^-</ei>, <ei>-11 = e^+</ei>, |
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| 37 | <br/><ei>13 = mu^-</ei>, <ei>-13 = mu^+</ei>, |
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| 38 | <br/>and a few more leptons/neutrinos in a few combinations. |
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| 39 | </modeopen> |
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| 40 | |
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| 41 | <modeopen name="Beams:idB" default="2212"> |
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| 42 | The PDG <code>id</code> code for the second incoming particle. |
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| 43 | </modeopen> |
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| 44 | |
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| 45 | <modepick name="Beams:frameType" default="1" min="1" max="5"> |
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| 46 | Choice of frame for the two colliding particles. For options |
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| 47 | 1 - 3 the beam identities are specified above, while they are |
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| 48 | obtained by the Les Houches information for options 4 and 5. |
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| 49 | <option value="1">the beams are colliding in their CM frame, |
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| 50 | and therefore only the CM energy needs to be provided, see |
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| 51 | <code>Beams:eCM</code> below. |
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| 52 | </option> |
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| 53 | <option value="2">the beams are back-to-back, but with different energies, |
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| 54 | see <code>Beams:eA</code> and <code>Beams:eB</code> below. |
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| 55 | This option could also be used for fixed-target configurations. |
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| 56 | </option> |
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| 57 | <option value="3">the beams are not back-to-back, and therefore the |
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| 58 | three-momentum of each incoming particle needs to be specified, see |
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| 59 | <code>Beams:pxA</code> through <code>Beams:pzB</code> below. |
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| 60 | </option> |
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| 61 | <option value="4">the beam and event information is stored in a |
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| 62 | <aloc href="LesHouchesAccord">Les Houches Event File</aloc>, |
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| 63 | see <code>Beams:LHEF</code> below. |
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| 64 | </option> |
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| 65 | <option value="5">the beam and event information is obtained by a |
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| 66 | pointer to an <code><aloc href="LesHouchesAccord">LHAup</aloc></code> |
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| 67 | class instance. |
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| 68 | </option> |
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| 69 | </modepick> |
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| 70 | |
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| 71 | <parm name="Beams:eCM" default="14000." min="10."> |
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| 72 | Collision CM energy, to be set if <code>Beams:frameType</code> = 1. |
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| 73 | </parm> |
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| 74 | |
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| 75 | <parm name="Beams:eA" default="7000." min="0."> |
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| 76 | The energy of the first incoming particle, moving in the |
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| 77 | <ei>+z </ei>direction, to be set if <code>Beams:frameType</code> = 2. |
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| 78 | If the particle energy is smaller than its mass |
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| 79 | it is assumed to be at rest. |
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| 80 | </parm> |
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| 81 | |
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| 82 | <parm name="Beams:eB" default="7000." min="0."> |
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| 83 | The energy of the second incoming particle, moving in the |
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| 84 | <ei>-z</ei> direction, to be set if <code>Beams:frameType</code> = 2. |
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| 85 | If the particle energy is smaller than its mass |
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| 86 | it is assumed to be at rest. |
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| 87 | </parm> |
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| 88 | |
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| 89 | <parm name="Beams:pxA" default="0."> |
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| 90 | The <ei>p_x</ei> component of the first incoming particle, |
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| 91 | to be set if <code>Beams:frameType</code> = 3. |
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| 92 | </parm> |
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| 93 | |
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| 94 | <parm name="Beams:pyA" default="0."> |
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| 95 | The <ei>p_y</ei> component of the first incoming particle, |
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| 96 | to be set if <code>Beams:frameType</code> = 3. |
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| 97 | </parm> |
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| 98 | |
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| 99 | <parm name="Beams:pzA" default="7000."> |
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| 100 | The <ei>p_z</ei> component of the first incoming particle, |
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| 101 | to be set if <code>Beams:frameType</code> = 3. |
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| 102 | </parm> |
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| 103 | |
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| 104 | <parm name="Beams:pxB" default="0."> |
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| 105 | The <ei>p_x</ei> component of the second incoming particle, |
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| 106 | to be set if <code>Beams:frameType</code> = 3. |
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| 107 | </parm> |
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| 108 | |
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| 109 | <parm name="Beams:pyB" default="0."> |
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| 110 | The <ei>p_y</ei> component of the second incoming particle, |
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| 111 | to be set if <code>Beams:frameType</code> = 3. |
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| 112 | </parm> |
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| 113 | |
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| 114 | <parm name="Beams:pzB" default="-7000."> |
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| 115 | The <ei>p_z</ei> component of the second incoming particle, |
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| 116 | to be set if <code>Beams:frameType</code> = 3. |
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| 117 | </parm> |
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| 118 | |
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| 119 | <word name="Beams:LHEF" default="void"> |
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| 120 | The name of a Les Houches Event File, |
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| 121 | to be set if <code>Beams:frameType</code> = 4. |
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| 122 | </word> |
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| 123 | |
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| 124 | <word name="Beams:LHEFheader" default="void"> |
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| 125 | As some information in a Les Houches Event File init block is only known |
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| 126 | at the end of generation, some programs choose to output this as a |
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| 127 | separate file. If <code>Beams:LHEFheader</code> is given, information up |
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| 128 | till the end of the init block is read from this file, with |
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| 129 | the events themselves read as usual from the file given in |
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| 130 | <code>Beams:LHEF</code>. |
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| 131 | </word> |
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| 132 | |
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| 133 | <flag name="Beams:newLHEFsameInit" default="off"> |
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| 134 | Allow to begin reading events from a new LHEF or or a new |
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| 135 | <code>LHAup</code> instance without a completely new initialization. |
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| 136 | Only useful when <code>Beams:frameType</code> = 4 or 5. |
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| 137 | </flag> |
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| 138 | |
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| 139 | <flag name="Beams:readLHEFheaders" default="on"> |
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| 140 | Read in LHEF header blocks and store them in the |
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| 141 | <aloc href="EventInformation">Info</aloc> class. See also |
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| 142 | <aloc href="LesHouchesAccord">LHAupLHEF</aloc> for more information. |
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| 143 | </flag> |
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| 144 | |
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| 145 | <mode name="Beams:nSkipLHEFatInit" default="0"> |
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| 146 | Skip the first <ei>nSkip</ei> events of the input stream |
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| 147 | (cf. the <code>LHAup::skipEvent(nSkip)</code> method). |
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| 148 | Only used when <code>Beams:frameType</code> = 4 or 5. |
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| 149 | </flag> |
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| 150 | |
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| 151 | <h3>Beam momentum spread</h3> |
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| 152 | |
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| 153 | This framework currently is intended for a modest beam spread, such as |
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| 154 | experienced at hadron colliders. Thus it can be safely assumed that the |
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| 155 | physics does not change over the CM energy range probed, so that the |
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| 156 | parameters of the physics initialization at the nominal energy can be |
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| 157 | used as is. Currently it can <b>not</b> be used for the more extensive |
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| 158 | energy spread expected at linear <ei>e^+ e^-</ei> colliders. Also, |
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| 159 | any attempt to combine it with external Les Houches input of |
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| 160 | parton-level events is at own risk. |
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| 161 | |
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| 162 | <p/> |
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| 163 | On this page you can set the momentum spread according to a simple |
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| 164 | Gaussian distribution. If you instead want a more sophisticated |
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| 165 | parametrization, you can write and link your own |
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| 166 | <code><aloc href="BeamShape">BeamShape</aloc></code> class. |
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| 167 | |
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| 168 | <flag name="Beams:allowMomentumSpread" default="off"> |
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| 169 | Allow the beam momenta to be smeared around their initialization |
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| 170 | nominal values. |
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| 171 | </flag> |
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| 172 | |
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| 173 | <parm name="Beams:sigmaPxA" default="0." min="0."> |
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| 174 | The width of a Gaussian distribution of the <ei>p_x</ei> spread of the |
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| 175 | first incoming particle. |
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| 176 | </parm> |
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| 177 | |
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| 178 | <parm name="Beams:sigmaPyA" default="0." min="0."> |
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| 179 | The width of a Gaussian distribution of the <ei>p_y</ei> spread of the |
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| 180 | first incoming particle. |
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| 181 | </parm> |
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| 182 | |
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| 183 | <parm name="Beams:sigmaPzA" default="0." min="0."> |
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| 184 | The width of a Gaussian distribution of the <ei>p_z</ei> spread of the |
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| 185 | first incoming particle. |
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| 186 | </parm> |
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| 187 | |
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| 188 | <parm name="Beams:maxDevA" default="5." min="0."> |
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| 189 | The triply Gaussian distribution <ei>(p_x, p_y, p_z)</ei> is restricted to |
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| 190 | a maximal total deviation from the nominal values <ei>(p_x0, p_y0, p_z0)</ei> |
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| 191 | for the first incoming particle, like |
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| 192 | <eq> |
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| 193 | (p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 + |
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| 194 | (p_z - p_z0)^2/sigma_pz^2 < maxDev^2 |
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| 195 | </eq> |
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| 196 | (Note the absence of a factor 2 in the denominator, unlike the Gaussians |
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| 197 | used to pick <ei>(p_x, p_y, p_z)</ei>.) |
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| 198 | </parm> |
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| 199 | |
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| 200 | <parm name="Beams:sigmaPxB" default="0." min="0."> |
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| 201 | The width of a Gaussian distribution of the <ei>p_x</ei> spread of the |
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| 202 | second incoming particle. |
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| 203 | </parm> |
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| 204 | |
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| 205 | <parm name="Beams:sigmaPyB" default="0." min="0."> |
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| 206 | The width of a Gaussian distribution of the <ei>p_y</ei> spread of the |
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| 207 | second incoming particle. |
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| 208 | </parm> |
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| 209 | |
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| 210 | <parm name="Beams:sigmaPzB" default="0." min="0."> |
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| 211 | The width of a Gaussian distribution of the <ei>p_z</ei> spread of the |
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| 212 | second incoming particle. |
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| 213 | </parm> |
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| 214 | |
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| 215 | <parm name="Beams:maxDevB" default="5." min="0."> |
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| 216 | The triply Gaussian distribution <ei>(p_x, p_y, p_z)</ei> is restricted to |
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| 217 | a maximal total deviation from the nominal values <ei>(p_x0, p_y0, p_z0)</ei>, |
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| 218 | for the second incoming particle, like |
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| 219 | <eq> |
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| 220 | (p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 + |
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| 221 | (p_z - p_z0)^2/sigma_pz^2 < maxDev^2 |
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| 222 | </eq> |
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| 223 | (Note the absence of a factor 2 in the denominator, unlike the Gaussians |
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| 224 | used to pick <ei>(p_x, p_y, p_z)</ei>.) |
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| 225 | </parm> |
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| 226 | |
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| 227 | <h3>Beam interaction vertex</h3> |
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| 228 | |
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| 229 | On this page you can set the spread of the interaction vertex according to |
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| 230 | a simple Gaussian distribution. If you instead want a more sophisticated |
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| 231 | parametrization, you can write and link your own |
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| 232 | <code><aloc href="BeamShape">BeamShape</aloc></code> class. |
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| 233 | |
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| 234 | <flag name="Beams:allowVertexSpread" default="off"> |
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| 235 | Allow the interaction vertex of the two colliding beams to be smeared. |
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| 236 | If off, then the vertex is set to be the origin. |
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| 237 | </flag> |
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| 238 | |
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| 239 | <parm name="Beams:sigmaVertexX" default="0." min="0."> |
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| 240 | The width of a Gaussian distribution of the <ei>x</ei> location of the |
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| 241 | interaction vertex. |
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| 242 | </parm> |
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| 243 | |
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| 244 | <parm name="Beams:sigmaVertexY" default="0." min="0."> |
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| 245 | The width of a Gaussian distribution of the <ei>y</ei> location of the |
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| 246 | interaction vertex. |
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| 247 | </parm> |
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| 248 | |
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| 249 | <parm name="Beams:sigmaVertexZ" default="0." min="0."> |
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| 250 | The width of a Gaussian distribution of the <ei>z</ei> location of the |
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| 251 | interaction vertex. |
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| 252 | </parm> |
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| 253 | |
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| 254 | <parm name="Beams:maxDevVertex" default="5." min="0."> |
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| 255 | The triply Gaussian distribution of interaction vertex position |
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| 256 | <ei>(x, y, z)</ei> is restricted to a maximal total deviation from the |
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| 257 | origin, like |
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| 258 | <eq> |
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| 259 | x^2/sigma_x^2 + y^2/sigma_y^2 + z^2/sigma_z^2 < maxDevVertex^2 |
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| 260 | </eq> |
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| 261 | (Note the absence of a factor 2 in the denominator, unlike the Gaussians |
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| 262 | used to pick <ei>(x, y, z)</ei>.) |
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| 263 | </parm> |
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| 264 | |
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| 265 | <parm name="Beams:sigmaTime" default="0." min="0."> |
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| 266 | The width of a Gaussian distribution of the collision time (in units of |
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| 267 | mm/c). Note that, if the above space parametrization is viewed as the |
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| 268 | effect of two incoming beams along the <ei>+-z</ei> axis, with each beam |
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| 269 | having a Gaussian spread, then the spread of the time would also become |
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| 270 | a Gaussian with the same width as the <ei>z</ei> one (times the |
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| 271 | velocity of the beams, which we expect is close to unity). For flexibility |
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| 272 | we have not enforced any such relation, however. |
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| 273 | </parm> |
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| 274 | |
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| 275 | <parm name="Beams:maxDevTime" default="5." min="0."> |
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| 276 | The collision time is restricted to be in the range |
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| 277 | <ei>|t| < sigma_t * maxDevTime</ei>. |
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| 278 | </parm> |
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| 279 | |
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| 280 | <p/> |
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| 281 | The distributions above are all centered at the origin. It is also |
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| 282 | possible to shift the above distributions to be centered around another |
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| 283 | nominal position. You must have <code>Beams:allowVertexSpread = on</code> |
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| 284 | to use this possibility. |
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| 285 | |
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| 286 | <parm name="Beams:offsetVertexX" default="0."> |
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| 287 | The <ei>x</ei> location of the interaction vertex is centered at this value. |
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| 288 | </parm> |
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| 289 | |
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| 290 | <parm name="Beams:offsetVertexY" default="0."> |
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| 291 | The <ei>y</ei> location of the interaction vertex is centered at this value. |
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| 292 | </parm> |
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| 293 | |
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| 294 | <parm name="Beams:offsetVertexZ" default="0."> |
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| 295 | The <ei>z</ei> location of the interaction vertex is centered at this value. |
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| 296 | </parm> |
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| 297 | |
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| 298 | <parm name="Beams:offsetTime" default="0."> |
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| 299 | The time <ei>t</ei> of the interaction vertex is centered at this value. |
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| 300 | </parm> |
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| 301 | |
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| 302 | </chapter> |
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| 303 | |
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| 304 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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