Beam momentum mismatch between event record and hard process for (SI)DIS
I am trying to generate events for CLAS12, an electron beam with E=10.6 GeV scattering on a fixed proton target, and have been investigating why the proton peak in our missing mass spectrum is off by about 200 MeV. Looking at the event record, I find that the electron beam energy is around 10.4 GeV, rather than 10.6 GeV; the proton also has a momentum of -20 MeV (opposite the beam direction).
I thought there must be some Lorentz transformation here, but I couldn't find one that transforms both the beam and target back to my initial settings (or maybe I got the math wrong...)
Anyway, here is a reproducer, and some output below:
#include <Pythia8/Pythia.h>
#include <cassert>
int const I_BEAM = 1; // beam particle index
int const I_TARGET = 2; // target particle index
int const MIN_EVENT_SIZE = 3;
int const PRECISION = 10; // precision for printout
//////////////////////////////////////////////////////////////////////////////////
void print_particle(Pythia8::Particle const& par, std::string const& name, bool newline=false) {
std::cout << std::setprecision(PRECISION) << name << "pz = " << par.pz() << " E = " << par.e();
if(newline) std::cout << "\n";
}
//////////////////////////////////////////////////////////////////////////////////
int main(int argc, char** argv)
{
Pythia8::Pythia p;
auto& evt = p.event;
auto& proc = p.process;
// pythia parameters ================================================================
p.readString("Beams:frameType = 2");
p.readString("WeakBosonExchange:ff2ff(t:gmZ) = on");
p.readString("PhaseSpace:Q2Min = 0.0");
p.readString("PhaseSpace:mHatMin = 0.0");
p.readString("SpaceShower:dipoleRecoil = off");
p.readString("Beams:idA = 11");
p.readString("Beams:idB = 2212");
p.readString("Beams:eA = 10.6");
p.readString("Beams:eB = 0.0");
p.readString("Random:setSeed = on");
p.readString("Random:seed = 82");
// event loop =======================================================================
p.init();
for(int i=0; i<100; i++) {
if(!p.next()) continue;
if(evt.size() < MIN_EVENT_SIZE) continue;
//// full event printouts
// proc.list(false, false, PRECISION);
// evt.list(false, false, PRECISION);
// info.list();
//// get the beam and target particles
auto const& evt_beam = evt[I_BEAM];
auto const& evt_target = evt[I_TARGET];
auto const& proc_beam = proc[I_BEAM];
auto const& proc_target = proc[I_TARGET];
for(auto const& [beam, target] : std::vector<std::pair<decltype(evt_beam),decltype(evt_beam)>>{{evt_beam, evt_target}, {proc_beam, proc_target}}) {
assert((beam.id() == 11 && target.id() == 2212));
assert((beam.mother1() == 0 && beam.mother2() == 0 && target.mother1() == 0 && target.mother2() == 0));
}
//// print the beam and target particles
print_particle(evt_beam, "event record beam: "); print_particle(proc_beam, " process beam: ", true);
// print_particle(evt_target, "event record target: "); print_particle(proc_target, " process target: ", true);
}
return 0;
}Sample output:
event record beam: pz = 10.3822709852 E = 10.3822709978 process beam: pz = 10.5999999877 E = 10.6000000000
event record beam: pz = 10.3822724221 E = 10.3822724347 process beam: pz = 10.5999999877 E = 10.6000000000
event record beam: pz = 10.3822709677 E = 10.3822709803 process beam: pz = 10.5999999877 E = 10.6000000000
event record beam: pz = 10.3822709061 E = 10.3822709187 process beam: pz = 10.5999999877 E = 10.6000000000
event record beam: pz = 10.3822708718 E = 10.3822708844 process beam: pz = 10.5999999877 E = 10.6000000000And here are 3 full process and event records:
-------- PYTHIA Event Listing (hard process) -----------------------------------------------------------------------------------
no id name status mothers daughters colours p_x p_y p_z e m
0 90 (system) -11 0 0 0 0 0 0 0.0000000000 0.0000000000 10.5999999877 11.5382700000 4.5575952929
1 11 (e-) -12 0 0 3 0 0 0 0.0000000000 0.0000000000 10.5999999877 10.6000000000 0.0005110000
2 2212 (p+) -12 0 0 4 0 0 0 0.0000000000 0.0000000000 0.0000000000 0.9382700000 0.9382700000
3 11 (e-) -21 1 0 5 6 0 0 0.0000000000 0.0000000000 9.8718591674 9.8718591674 0.0000000000
4 2 (u) -21 2 0 5 6 101 0 0.0000000000 0.0000000000 -0.1904087735 0.1904087735 0.0000000000
5 11 e- 23 3 4 0 0 0 0 1.2638931144 0.0403703107 6.8251830019 6.9413384039 0.0005110000
6 2 u 23 3 4 0 0 101 0 -1.2638931144 -0.0403703107 3.0943404419 3.3590025807 0.3300000000
Charge sum: -0.333 Momentum sum: 0.0000000000 0.0000000000 9.9195234439 10.3003409846 2.7749016281
-------- End PYTHIA Event Listing -----------------------------------------------------------------------------------------------
-------- PYTHIA Event Listing (complete event) ---------------------------------------------------------------------------------
no id name status mothers daughters colours p_x p_y p_z e m
0 90 (system) -11 0 0 0 0 0 0 -0.0000000000 -0.0000000000 10.3627962720 11.3207429716 4.5575952929
1 11 (e-) -12 0 0 12 0 0 0 0.0000000000 0.0000000000 10.3822708747 10.3822708873 0.0005110000
2 2212 (p+) -12 0 0 18 0 0 0 -0.0000000000 -0.0000000000 -0.0194746027 0.9384720843 0.9382700000
3 11 (e-) -21 7 0 5 6 0 0 0.0000000000 0.0000000000 9.6690864183 9.6690864183 0.0000000000
4 2 (u) -21 8 8 5 6 101 0 -0.0000000000 -0.0000000000 -0.1944018819 0.1944018819 0.0000000000
5 11 (e-) -23 3 4 9 9 0 0 1.2638931144 0.0403703107 6.6825795454 6.8011708704 0.0005110000
6 2 (u) -23 3 4 10 10 101 0 -1.2638931144 -0.0403703107 3.0252879058 3.2955003383 0.3300000000
7 11 (e-) -41 12 0 11 3 0 0 0.0000000000 -0.0000000000 10.7965569499 10.7965569499 0.0000000000
8 2 (u) -42 13 13 4 4 101 0 -0.0000000000 0.0000000000 -0.1944018819 0.1944018819 0.0000000000
9 11 (e-) -44 5 5 14 14 0 0 1.2681035850 0.0395633888 6.6825142238 6.8018856633 0.0005110000
10 2 (u) -44 6 6 15 15 101 0 -1.2619193688 -0.0407485721 3.0258385520 3.2952541453 0.3300000000
11 22 (gamma) -43 7 0 16 16 0 0 -0.0060385876 0.0011572742 1.1269964150 1.1270131868 0.0000000000
12 11 (e-) -41 1 0 17 7 0 0 0.0000000000 -0.0000000000 10.8417696109 10.8417696109 0.0000000000
13 2 (u) -42 18 18 8 8 101 0 -0.0000000000 -0.0000000000 -0.1944018819 0.1944018819 0.0000000000
14 11 e- 44 9 9 0 0 0 0 1.2694993593 0.0390422422 6.6835369216 6.8031477247 0.0005110000
15 2 (u) -44 10 10 19 19 101 0 -1.2612650706 -0.0409928704 3.0265258732 3.2956378525 0.3300000000
16 22 (gamma) -44 11 11 20 20 0 0 -0.0058052744 0.0010701610 1.1271880289 1.1272034861 0.0000000000
17 22 (gamma) -43 12 0 21 21 0 0 -0.0022351097 0.0008345330 0.0433505210 0.0434161241 0.0000000000
18 2 (u) -61 2 0 13 13 101 0 0.4052381308 -0.3563231857 0.4669423414 0.7135960380 0.0000000000
19 2 (u) -62 15 15 23 23 101 0 -0.7911617353 -0.2312215196 2.5818697078 2.7302658242 0.3300000000
20 22 gamma 62 16 16 0 0 0 0 0.1302403752 -0.1034322955 1.2613503735 1.2722678805 0.0000000000
21 22 gamma 62 17 17 0 0 0 0 0.0029463934 -0.0028184401 0.0467213699 0.0468989471 0.0000000000
22 2101 (ud_0) -63 2 0 24 24 0 101 -0.6115243925 0.2984300130 0.2488166354 0.9276613187 0.5793300000
23 2 (u) -71 19 19 25 26 101 0 -0.7911617353 -0.2312215196 2.5818697078 2.7302658242 0.3300000000
24 2101 (ud_0) -71 22 22 25 26 0 101 -0.6115243925 0.2984300130 0.2488166354 0.9276613187 0.5793300000
25 211 pi+ 82 23 24 0 0 0 0 -1.0192653295 0.0439952829 2.2831431490 2.5046077179 0.1395700000
26 2112 n0 82 23 24 0 0 0 0 -0.3834207984 0.0232132105 0.5475431943 1.1533194251 0.9395700000
Charge sum: 0.000 Momentum sum: 0.0000000000 0.0000000000 10.8222950082 11.7802416953 4.6531736647
-------- End PYTHIA Event Listing -----------------------------------------------------------------------------------------------
-------- PYTHIA Event Listing (hard process) -----------------------------------------------------------------------------------
no id name status mothers daughters colours p_x p_y p_z e m
0 90 (system) -11 0 0 0 0 0 0 0.0000000000 0.0000000000 10.5999999877 11.5382700000 4.5575952929
1 11 (e-) -12 0 0 3 0 0 0 0.0000000000 0.0000000000 10.5999999877 10.6000000000 0.0005110000
2 2212 (p+) -12 0 0 4 0 0 0 0.0000000000 0.0000000000 0.0000000000 0.9382700000 0.9382700000
3 11 (e-) -21 1 0 5 6 0 0 0.0000000000 0.0000000000 10.4450718361 10.4450718361 0.0000000000
4 2 (u) -21 2 0 5 6 101 0 0.0000000000 0.0000000000 -0.1154439997 0.1154439997 0.0000000000
5 11 e- 23 3 4 0 0 0 0 0.2759915322 -1.0549244536 5.4572871242 5.5651612687 0.0005110000
6 2 u 23 3 4 0 0 101 0 -0.2759915322 1.0549244536 5.2140655872 5.3370794332 0.3300000000
Charge sum: -0.333 Momentum sum: 0.0000000000 0.0000000000 10.6713527115 10.9022407019 2.2318341401
-------- End PYTHIA Event Listing -----------------------------------------------------------------------------------------------
-------- PYTHIA Event Listing (complete event) ---------------------------------------------------------------------------------
no id name status mothers daughters colours p_x p_y p_z e m
0 90 (system) -11 0 0 0 0 0 0 0.0000000000 -0.0000000000 10.3627963989 11.3207430878 4.5575952929
1 11 (e-) -12 0 0 7 0 0 0 0.0000000000 -0.0000000000 10.3822709911 10.3822710037 0.0005110000
2 2212 (p+) -12 0 0 12 0 0 0 0.0000000000 0.0000000000 -0.0194745922 0.9384720841 0.9382700000
3 11 (e-) -21 7 0 5 6 0 0 0.0000000000 -0.0000000000 10.2305251371 10.2305251371 0.0000000000
4 2 (u) -21 8 8 5 6 101 0 0.0000000000 0.0000000000 -0.1178650024 0.1178650024 0.0000000000
5 11 (e-) -23 3 4 9 9 0 0 0.2759915322 -1.0549244536 5.3429528546 5.4530892526 0.0005110000
6 2 (u) -23 3 4 10 10 101 0 -0.2759915322 1.0549244536 5.1044129652 5.2300065629 0.3300000000
7 11 (e-) -41 1 0 11 3 0 0 0.0000000000 -0.0000000000 10.8417694791 10.8417694791 0.0000000000
8 2 (u) -42 12 12 4 4 101 0 0.0000000000 -0.0000000000 -0.1178650024 0.1178650024 0.0000000000
9 11 e- 44 5 5 0 0 0 0 0.2815551702 -1.0468262882 5.3507049615 5.4594104478 0.0005110000
10 2 (u) -44 6 6 13 13 101 0 -0.2706657869 1.0626763535 5.1105779889 5.2373130880 0.3300000000
11 22 (gamma) -43 7 0 14 14 0 0 -0.0105444085 -0.0153479365 0.5977632440 0.5980532080 0.0000000000
12 2 (u) -61 2 0 8 8 101 0 0.4946451846 -0.2466203110 0.9531151190 1.1017821320 0.0000000000
13 2 (u) -62 10 10 16 16 101 0 0.1184781357 0.7023563417 4.7886449136 4.8525623755 0.3300000000
14 22 gamma 62 11 11 0 0 0 0 0.0432176470 -0.0532924599 0.6321564127 0.6358691544 0.0000000000
15 2101 (ud_0) -63 2 0 17 17 0 101 -0.4432509529 0.3977624065 0.0507885991 0.8323995855 0.5793300000
16 2 (u) -71 13 13 18 20 101 0 0.1184781357 0.7023563417 4.7886449136 4.8525623755 0.3300000000
17 2101 (ud_0) -71 15 15 18 20 0 101 -0.4432509529 0.3977624065 0.0507885991 0.8323995855 0.5793300000
18 211 pi+ 83 16 17 0 0 0 0 -0.3231886195 0.4875175914 2.4820857230 2.5538898971 0.1395700000
19 223 (omega) -84 16 17 21 23 0 0 0.4475755576 0.4297061367 1.2217535846 1.5794403235 0.7854798981
20 2112 n0 84 16 17 0 0 0 0 -0.4491597554 0.1828950201 1.1355942051 1.5516317404 0.9395700000
21 211 pi+ 91 19 0 0 0 0 0 0.0262061920 0.1164459835 0.0753561338 0.1985063308 0.1395700000
22 -211 pi- 91 19 0 0 0 0 0 -0.0757595888 0.0662590821 0.4399226604 0.4723785699 0.1395700000
23 111 (pi0) -91 19 0 24 25 0 0 0.4971289544 0.2470010711 0.7064747905 0.9085554228 0.1349800000
24 22 gamma 91 23 0 0 0 0 0 0.2558317936 0.0570770205 0.3642231331 0.4487384356 0.0000000000
25 22 gamma 91 23 0 0 0 0 0 0.2412971607 0.1899240506 0.3422516574 0.4598169872 0.0000000000
Charge sum: 0.000 Momentum sum: 0.0000000000 0.0000000000 10.8222948869 11.7802415632 4.6531736125
-------- End PYTHIA Event Listing -----------------------------------------------------------------------------------------------
PYTHIA Warning in PhaseSpace::trialKin123: maximum for cross section violated
-------- PYTHIA Event Listing (hard process) -----------------------------------------------------------------------------------
no id name status mothers daughters colours p_x p_y p_z e m
0 90 (system) -11 0 0 0 0 0 0 0.0000000000 0.0000000000 10.5999999877 11.5382700000 4.5575952929
1 11 (e-) -12 0 0 3 0 0 0 0.0000000000 0.0000000000 10.5999999877 10.6000000000 0.0005110000
2 2212 (p+) -12 0 0 4 0 0 0 0.0000000000 0.0000000000 0.0000000000 0.9382700000 0.9382700000
3 11 (e-) -21 1 0 5 6 0 0 0.0000000000 0.0000000000 10.8666284231 10.8666284231 0.0000000000
4 2 (u) -21 2 0 5 6 101 0 0.0000000000 0.0000000000 -0.2685043718 0.2685043718 0.0000000000
5 11 e- 23 3 4 0 0 0 0 -1.0215122611 -0.0349086912 9.8492738070 9.9021664651 0.0005110000
6 2 u 23 3 4 0 0 101 0 1.0215122611 0.0349086912 0.9493978562 1.4335139363 0.3300000000
Charge sum: -0.333 Momentum sum: 0.0000000000 0.0000000000 10.7986716632 11.3356804014 3.4476572443
-------- End PYTHIA Event Listing -----------------------------------------------------------------------------------------------
-------- PYTHIA Event Listing (complete event) ---------------------------------------------------------------------------------
no id name status mothers daughters colours p_x p_y p_z e m
0 90 (system) -11 0 0 0 0 0 0 -0.0000000000 -0.0000000000 10.3627963864 11.3207430763 4.5575952929
1 11 (e-) -12 0 0 7 0 0 0 -0.0000000000 0.0000000000 10.3822709796 10.3822709922 0.0005110000
2 2212 (p+) -12 0 0 12 0 0 0 -0.0000000000 -0.0000000000 -0.0194745932 0.9384720841 0.9382700000
3 11 (e-) -21 7 0 5 6 0 0 0.0000000000 0.0000000000 10.6434227413 10.6434227413 0.0000000000
4 2 (u) -21 8 8 5 6 101 0 -0.0000000000 0.0000000000 -0.2741352391 0.2741352391 0.0000000000
5 11 (e-) -23 3 4 9 9 0 0 -1.0215122611 -0.0349086912 9.6458672374 9.6998691197 0.0005110000
6 2 (u) -23 3 4 10 10 101 0 1.0215122611 0.0349086912 0.9198485340 1.4141171245 0.3300000000
7 11 (e-) -41 1 0 11 3 0 0 0.0000000000 -0.0000000000 10.8417694921 10.8417694921 0.0000000000
8 2 (u) -42 12 12 4 4 101 0 -0.0000000000 -0.0000000000 -0.2741352391 0.2741352391 0.0000000000
9 11 e- 44 5 5 0 0 0 0 -1.0215345976 -0.0336605363 9.6459540268 9.6999533666 0.0005110000
10 2 (u) -44 6 6 13 13 101 0 1.0215095663 0.0350592748 0.9198599586 1.4141263346 0.3300000000
11 22 (gamma) -43 7 0 14 14 0 0 0.0000245777 -0.0013733921 0.1982502726 0.1982550312 0.0000000000
12 2 (u) -61 2 0 8 8 101 0 -0.2928556426 -0.1066976941 0.0584824339 0.3171261900 0.0000000000
13 2 (u) -62 10 10 16 16 101 0 0.4383452051 -0.0429379214 0.6871317304 0.8803636742 0.3300000000
14 22 gamma 62 11 11 0 0 0 0 -0.0795673330 -0.0266631346 0.2758096915 0.2882930265 0.0000000000
15 2101 (ud_0) -63 2 0 17 17 0 101 0.6627567255 0.1032615923 0.2133994501 0.9116315088 0.5793300000
16 2 (u) -71 13 13 18 19 101 0 0.4383452051 -0.0429379214 0.6871317304 0.8803636742 0.3300000000
17 2101 (ud_0) -71 15 15 18 19 0 101 0.6627567255 0.1032615923 0.2133994501 0.9116315088 0.5793300000
18 211 pi+ 82 16 17 0 0 0 0 0.1720144555 0.0217581169 0.1813208677 0.2870878446 0.1395700000
19 2112 n0 82 16 17 0 0 0 0 0.9290874751 0.0385655540 0.7192103128 1.5049073384 0.9395700000
Charge sum: 0.000 Momentum sum: 0.0000000000 0.0000000000 10.8222948988 11.7802415762 4.6531736176
-------- End PYTHIA Event Listing -----------------------------------------------------------------------------------------------