MARS vs HARP Goran Skoro

GOAL (general): Comparison between experimental data and Monte Carlo calculations on pion production in proton – tantalum interactions will help to tune all the Neutrino Factory related calculations based on MARS predictions (capture, re-absorbtion, stress, etc.).

GOAL (here): If there is a difference between experiment and MC, what is the consequence related to energy deposition (ie. thermal stress) in the target?

Experiment: HARP beam: protons; momentum range from 3 GeV/c to 12 GeV/c target: tantalum (thin target: 5% interaction length) measured: double differential cross-section for charged pions production; momentum range: [100, 800] MeV/c; angular range: [0.35, 2.15] rad.

NOTE: HARP data are from “Measurement of the production of charged pions by protons on a tantalum target”; HARP Collaboration; accepted for publication on Eur. Phys. J. C.; http://arxiv.org/abs/0706.1600

Monte Carlo code: MARS 15

A few comments before the results Simulations: The target thickness in the simulations is 5%

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The target radius in the simulations is 1%

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The hA interactions of secondary particles can be neglected.

MARS (default) use 2 models for hadron production: The cascade-exciton model (CEM) below 5 GeV; The phenomenological approach (PA) above 5 GeV.

So, the first two sets of HARP data (3 and 5 GeV/c beam momentum) were compared with the cascade-exciton model while the two other sets (8 and 12 GeV/c beam momentum) with the MARS phenomenological approach.

Presentation of the results: All the plots have the same range for y – axis. It looks a bit strange for the low momentum data but allows nice effect if you use PgUp and PgDn in Slide Show.

Short comments about qualitative agreement between experiment and simulation are given on every slide.

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+ RESULTS: Beam momentum = 3 GeV/c

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+ : p > 250 MeV/c -------------- very good p < 250 MeV/c

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< 0.55 rad ---------

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> 0.55 rad -------- good HARP/MARS ~ 2

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: p > 200 MeV/c

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< 1.15 rad --------

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> 1.15 rad ------- p < 200 MeV/c

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< 0.75 rad --------

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> 0.75 rad ------- good very good very good HARP/MARS ~ 2

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+ RESULTS: Beam momentum = 5 GeV/c

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+ : p > 300 MeV/c -------------- very good p < 300 MeV/c

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< 0.75 rad -------- good > 0.75 rad --------- HARP/MARS ~ 5/3

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: p > 200 MeV/c

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< 0.95 rad ------ HARP/MARS ~ 4/5

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> 0.95 rad ----- very good p < 200 MeV/c

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< 1.15 rad ----- good > 1.15 rad ------ HARP/MARS ~ 5/3

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+ RESULTS: Beam momentum = 8 GeV/c

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+ : p > 350 MeV/c -------------- very good p < 350 MeV/c

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< 0.55 rad ---------

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> 0.55 rad -------- good HARP/MARS ~ 2

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: p > 250 MeV/c

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< 0.95 rad ------ HARP/MARS ~ 3/4

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> 0.95 rad ----- very good p < 250 MeV/c

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< 1.15 rad ----- very good > 1.15 rad ------ HARP/MARS ~ 4/3

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+ RESULTS: Beam momentum = 12 GeV/c

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+ : p > 300 MeV/c -------------- very good p < 300 MeV/c

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< 0.95 rad ---------

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> 0.95 rad -------- good HARP/MARS ~ 2

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: p > 200 MeV/c

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< 0.95 rad ------ HARP/MARS ~ 3/5

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> 0.95 rad ----- very good p < 200 MeV/c

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< 1.15 rad ----- very good > 1.15 rad ------ HARP/MARS ~ 5/3

Figure 30. from “Measurement of the production of charged pions by protons on a tantalum target” by the HARP Collaboration

“Production of the pi+ (closed symbols) and pi- (open symbols) yield as a function of incident proton beam momentum for different designs of the neutrino factory focusing stage. Shown are the integrated yields (left) and the integrated yields normalized to the kinetic energy of the proton (right). The circles indicate the integral over the full HARP kinematic range in the forward hemisphere, the squares are integrated over 0.35 rad < theta < 0.95 rad, while the diamonds are calculated for the restricted angular range and 250 MeV/c < p < 500 MeV/c.” N.B. “For the largest region (circles), the yield is expressed as double differential cross section in [mb/(GeV/c sr)]. For the other regions the same normalization is chosen, but scaled with the relative bin size.” “The latter range may be most representative for the neutrino factory.”

How MARS compares with this?

Figure 30. from “Measurement of the production of charged pions by protons on a tantalum target” by the HARP Collaboration

LEGEND: Symbols: Experiment (HARP) closed symbols = pi+ open symbols = pi Lines: Monte Carlo (MARS) full lines = pi+ dotted lines = pi Color code for 3 kinematic ranges: 100 MeV/c < p < 700 MeV/c, 0.35 rad < theta < 1.55 rad; 100 MeV/c < p < 700 MeV/c, 0.35 rad < theta < 0.95 rad; 250 MeV/c < p < 500 MeV/c, 0.35 rad < theta < 0.95 rad; P beam (GeV/c)

MARS generally overestimates the yield of negative pions and underestimates the yield of positive pions. So if we look in the total number of pions…

LEGEND: Symbols: Experiment (HARP) Lines: Monte Carlo (MARS) Color code for 3 kinematic ranges: 100 MeV/c < p < 700 MeV/c, 0.35 rad < theta < 1.55 rad; 100 MeV/c < p < 700 MeV/c, 0.35 rad < theta < 0.95 rad; 250 MeV/c < p < 500 MeV/c, 0.35 rad < theta < 0.95 rad; P beam (GeV/c)

Generally speaking, the agreement between MARS and HARP is good.

Very good agreement for pions with p > 300 MeV/c. (But overestimated number of forward negative pions in case of the PA.) The PA underestimates the production of low momentum backward pions. The CEM overestimates low momentum negative backward pions and underestimates low momentum positive backward pions. We are most interested in results for beam momentum of 12 GeV/c because this is closest value to the ‘standard’ beam energy (10 GeV) we use in the spectrum of our UKNF simulations.

Thermal stress ~ energy deposition ~ number of particles x energy loss The calculations of additional stress because of real life/theory difference in number of particles is not straightforward but keeping in mind all the details * one can make a quick estimate:

HARP

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MARS

 5 %

MARS

This is estimate of the energy deposition difference not the particle yields!

Conclusion: We don’t know material properties at 2000 K with such a precision so we should be very satisfied with MARS predictive power related to energy deposition in tantalum target.

All this is valid for tungsten target too.

*) Following things have been taken in account: 1. The fact that HARP results are given for limited angle range.

2. The fact that low momentum pions (where HARP/MARS agreement is not so good) loose more energy than high momentum pions.

3. Pions contribution to the total energy deposition in the target.