Subnucleondiffraction code

References:

• H. Mäntysaari, B. Schenke, Phys.Rev. D94 (2016) no.3, 034042, arXiv:1607.01711
• H. Mäntysaari, B. Schenke, Phys.Rev.Lett. 117 (2016) no.5, 052301, arXiv:1603.04349

This program calculates scattering amplitude for the process gamma^* + A -> V + A where V is a vector meson (Upsilon, JPsi, rho, phi)

Note This code is constantly developed, and individual commits are not guaranteed to work properly. If you want to use this code in your project, it is probably good idea to first communicate directly with Heikki Mäntysaari heikki.mantysaari@jyu.fi

Compiling

First clone this repository

git clone https://github.com/hejajama/subnucleondiffraction.git

How to compile:

mkdir build
cd build
cmake ..
make

Code requires GSL version 2. CMake takes care of finding the correct compiler flags.

Usage

How to run: See ./build/bin/subnucleondiffraction -help

User has to specify the dipole-target scattering amplitude. Supported dipole amplitudes are

• IPsat with event-by-event fluctuating geometry
• Wilson lines generated using the IPGlasma code (https://github.com/schenke/ipglasma)

Examples

GSL_RNG_SEED=1 ./build/bin/subnucleondiffraction -dipole 1 ipsatproton 3.3 0.7 -real -Q2 0 -W 75 -mcintpoints 1e5

Calculates diffractive scattering amplitude (real part, imaginary part: use -imag instead of -real) Q2 is the photon virtuality in GeV² and W is the center-of-mass energy (again in GeV). This woud use 10⁵ MC integration points points are used in (adaptive) Monte Carlo integration. The MonteCarlo method (Vegas or MISER) can be selected using the -mcint flag (see src/main.cpp)

Using a heavy nucleus instead of proton, replace 1 -> 197 (Au) or any other A. For A>3 Woods-Saxon is used. Deuteron and 3He are handled separately.

Random seed is set by GSL_RNG_SEED enviromental variable. One must use the same RNG_SEED when calculating real and imaginary parts!

Round proton is "ipsatproton 0 4" (first number controls the width of the Gaussian from which the hot spot locations are sampled, and the second number is the width of the hot spots. This code always uses three hotspots, edit src/ipsat_proton.cpp if necessary. If teh center-of-mass should be moved to the origin, add com at the end:

-dipole 1 ipsatproton 4.5 1.0 com

Q_s fluctuations for constituent quarks are set as:

-qsfluct 0.5 -qsfluctshape quarks 

where the first number is the width of the log-normal distribution sigma.

Wilson lines generated using the IPGlasma code can be used instead of the IPsat dipole as follows:

-dipole 1 ipglasma FILENAME step

The step size in fm, and should be L/N (L is the lattice length, N number of lattice points)

Or, it is more efficient use Wilson lines in binary format

-dipole 1 ipglasma_binary FILENAME

In this case there is no need to specify the step size.

The code outputs

t   amplitude(T)  amplitude(L)

So amplitudes for different polarizations separately. All dimensionful units in this code are in GeV unless stated otherwise. Note that the user has to calculate real and imaginary parts separately (but the imaginary part does not affect the coherent cross section).

The coherent cross section is then

dsigma/dt = 1/(16 pi ) <A>^2   [in 1/GeV^4]

Where <A> is the average of the amplitudes.

Similalry the incoherent corss section is computed by replacing <A>^2 by <A^2> - <A>^2

In order to calculate total gamma-p (or gamma-A) cross section, one can calulate F2 as follows

GSL_RNG_SEED=1 ./diffraction -dipole 1 ipsatproton 0 4 -F2 Qsqr xbj

The output is x Q^2 F_2,light F_2,charm F_2,total F_L,light F_L,charm F_L,tot

Additional features

Some branches in this repository include support for additional processes or other improvements. These include

• photon_kt: support finite photon_kt in ultra peripheral collisions
• dvcs_vm_masses: calculate azimuthal correlations between the outgoing lepton and the produced vector meson
• dijet: calculate diffractive dijet production

Further references

The default IPsat fit used is

• H. Mäntysaari, P. Zurita, Phys.Rev.D 98 (2018) 036002, arXiv:1804.05311 [hep-ph]

This program also contains codes to evaluate the dipole amplitude from the IPsat fit

• A. Rezaeian, M. Siddikov, M. Van de Klundert, R. Venugopalan Phys.Rev. D87 (2013) no.3, 034002

The code is directly from their work, but compiled here to the library referred to as libColorDipole. This is not compiled by default, so one has to manually edit CMake files to compile it.

The NRQCD wave function reference is

• T. Lappi, H. Mäntysaari, J. Penttala, Phys.Rev.D 102 (2020) 5, 054020, arXiv:2006.02830 [hep-ph]

The Boosted Gaussia and Gaus-LC wave function parametrizations are from

• H. Kowalski, L. Motyka, G. Watt, Phys.Rev.D 74 (2006) 074016, arXiv:hep-ph/0606272

Deuteron and ³He: see

• H. Mäntysaari, B. Schenke, Phys.Rev.C 101 (2020) 1, 015203, arXiv:1910.03297