Baryon and electric charge stoppings in nuclear collisions and the role of strangeness

Abstract It has been challenging to quantitatively understand the stopping of incoming nucleons in nuclear collisions, and recently it has been proposed that comparing the baryon stopping with electric charge stopping can help address the question. Here we focus on the $$B/Q\times Z/A$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>B</mml:mi> <mml:mo>/</mml:mo> <mml:mi>Q</mml:mi> <mml:mo>×</mml:mo> <mml:mi>Z</mml:mi> <mml:mo>/</mml:mo> <mml:mi>A</mml:mi> </mml:mrow> </mml:math> ratio, which can strongly depend on rapidity although its value is one for the full phase space. We find that this ratio is very sensitive to the difference between strange and anti-strange rapidity distributions (the $$s-\bar{s}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>s</mml:mi> <mml:mo>-</mml:mo> <mml:mover> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> </mml:math> asymmetry), and slightly more anti-strange quarks at mid-rapidity would lead to a ratio well below one. This is the case for Zr + Zr and Ru + Ru isobar collisions at 200 A GeV from a multi-phase transport (AMPT) model. Without the $$s-\bar{s}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>s</mml:mi> <mml:mo>-</mml:mo> <mml:mover> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> </mml:math> asymmetry, the AMPT model would give a mid-rapidity $$B/Q\times Z/A$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>B</mml:mi> <mml:mo>/</mml:mo> <mml:mi>Q</mml:mi> <mml:mo>×</mml:mo> <mml:mi>Z</mml:mi> <mml:mo>/</mml:mo> <mml:mi>A</mml:mi> </mml:mrow> </mml:math> ratio at or above one. In addition, the AMPT model gives $$B/\Delta Q\times \Delta Z/A&lt;1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>B</mml:mi> <mml:mo>/</mml:mo> <mml:mi>Δ</mml:mi> <mml:mi>Q</mml:mi> <mml:mo>×</mml:mo> <mml:mi>Δ</mml:mi> <mml:mi>Z</mml:mi> <mml:mo>/</mml:mo> <mml:mi>A</mml:mi> <mml:mo>&lt;</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> at mid-rapidity for isobar collisions at all centralities, which strongly contradicts the recent data from the STAR Collaboration. We further find that the $$B/\Delta Q\times \Delta Z/A$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>B</mml:mi> <mml:mo>/</mml:mo> <mml:mi>Δ</mml:mi> <mml:mi>Q</mml:mi> <mml:mo>×</mml:mo> <mml:mi>Δ</mml:mi> <mml:mi>Z</mml:mi> <mml:mo>/</mml:mo> <mml:mi>A</mml:mi> </mml:mrow> </mml:math> ratio is very sensitive to the net-light quark ( u , d ) stoppings, but it is less sensitive to the $$s-\bar{s}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>s</mml:mi> <mml:mo>-</mml:mo> <mml:mover> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> </mml:math> asymmetry than the $$B/Q\times Z/A$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>B</mml:mi> <mml:mo>/</mml:mo> <mml:mi>Q</mml:mi> <mml:mo>×</mml:mo> <mml:mi>Z</mml:mi> <mml:mo>/</mml:mo> <mml:mi>A</mml:mi> </mml:mrow> </mml:math> ratio by a factor of 3.