Photoproduction of K+K− Pairs in Ultraperipheral Collisions

<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msup><a:mi>K</a:mi><a:mo>+</a:mo></a:msup><a:msup><a:mi>K</a:mi><a:mo>−</a:mo></a:msup></a:math> pairs may be produced in photonuclear collisions, either from the decays of photoproduced <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>ϕ</c:mi><c:mo stretchy="false">(</c:mo><c:mn>1020</c:mn><c:mo stretchy="false">)</c:mo></c:math> mesons or directly as nonresonant <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msup><g:mi>K</g:mi><g:mo>+</g:mo></g:msup><g:msup><g:mi>K</g:mi><g:mo>−</g:mo></g:msup></g:math> pairs. Measurements of <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msup><i:mi>K</i:mi><i:mo>+</i:mo></i:msup><i:msup><i:mi>K</i:mi><i:mo>−</i:mo></i:msup></i:math> photoproduction probe the couplings between the <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>ϕ</k:mi><k:mo stretchy="false">(</k:mo><k:mn>1020</k:mn><k:mo stretchy="false">)</k:mo></k:math> and charged kaons with photons and nuclear targets. The kaon-proton scattering occurs at energies far above those available elsewhere. We present the first measurement of coherent photoproduction of <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:msup><o:mi>K</o:mi><o:mo>+</o:mo></o:msup><o:msup><o:mi>K</o:mi><o:mo>−</o:mo></o:msup></o:math> pairs on lead ions in ultraperipheral collisions using the ALICE detector, including the first investigation of direct <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:msup><q:mi>K</q:mi><q:mo>+</q:mo></q:msup><q:msup><q:mi>K</q:mi><q:mo>−</q:mo></q:msup></q:math> production. There is significant <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:msup><s:mi>K</s:mi><s:mo>+</s:mo></s:msup><s:msup><s:mi>K</s:mi><s:mo>−</s:mo></s:msup></s:math> production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mrow><u:mn>1.1</u:mn><u:mo><</u:mo><u:msub><u:mrow><u:mi>M</u:mi></u:mrow><u:mrow><u:mi>K</u:mi><u:mi>K</u:mi></u:mrow></u:msub><u:mo><</u:mo><u:mn>1.4</u:mn><u:mtext> </u:mtext><u:mtext> </u:mtext><u:mi>GeV</u:mi><u:mo>/</u:mo><u:msup><u:mrow><u:mi>c</u:mi></u:mrow><u:mrow><u:mn>2</u:mn></u:mrow></u:msup></u:mrow></u:math> above the <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mi>ϕ</w:mi><w:mo stretchy="false">(</w:mo><w:mn>1020</w:mn><w:mo stretchy="false">)</w:mo></w:math> resonance, for rapidity <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"><ab:mrow><ab:mo stretchy="false">|</ab:mo><ab:msub><ab:mrow><ab:mi>y</ab:mi></ab:mrow><ab:mrow><ab:mi>K</ab:mi><ab:mi>K</ab:mi></ab:mrow></ab:msub><ab:mo stretchy="false">|</ab:mo><ab:mo><</ab:mo><ab:mn>0.8</ab:mn></ab:mrow></ab:math> and <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML" display="inline"><eb:msub><eb:mi>p</eb:mi><eb:mrow><eb:mi>T</eb:mi><eb:mo>,</eb:mo><eb:mi>K</eb:mi><eb:mi>K</eb:mi></eb:mrow></eb:msub><eb:mo><</eb:mo><eb:mn>0.1</eb:mn><eb:mtext> </eb:mtext><eb:mtext> </eb:mtext><eb:mi>GeV</eb:mi><eb:mo>/</eb:mo><eb:mi>c</eb:mi></eb:math>, the measured coherent photoproduction cross section is <gb:math xmlns:gb="http://www.w3.org/1998/Math/MathML" display="inline"><gb:mrow><gb:mi mathvariant="normal">d</gb:mi><gb:mi>σ</gb:mi><gb:mo>/</gb:mo><gb:mi mathvariant="normal">d</gb:mi><gb:mi>y</gb:mi><gb:mo>=</gb:mo><gb:mn>3.37</gb:mn><gb:mo>±</gb:mo><gb:mn>0.61</gb:mn><gb:mo stretchy="false">(</gb:mo><gb:mtext>stat</gb:mtext><gb:mo stretchy="false">)</gb:mo><gb:mo>±</gb:mo><gb:mn>0.15</gb:mn><gb:mo stretchy="false">(</gb:mo><gb:mi>syst</gb:mi><gb:mo stretchy="false">)</gb:mo><gb:mtext> </gb:mtext><gb:mtext> </gb:mtext><gb:mi>mb</gb:mi></gb:mrow></gb:math>. The center-of-mass energy per nucleon of the photon-nucleus (Pb) system <ob:math xmlns:ob="http://www.w3.org/1998/Math/MathML" display="inline"><ob:mrow><ob:msub><ob:mrow><ob:mi>W</ob:mi></ob:mrow><ob:mrow><ob:mi>γ</ob:mi><ob:mi>Pb</ob:mi><ob:mo>,</ob:mo><ob:mi>n</ob:mi></ob:mrow></ob:msub></ob:mrow></ob:math> ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for <qb:math xmlns:qb="http://www.w3.org/1998/Math/MathML" display="inline"><qb:mi>ϕ</qb:mi><qb:mo stretchy="false">(</qb:mo><qb:mn>1020</qb:mn><qb:mo stretchy="false">)</qb:mo></qb:math> photoproduction alone. The mass spectrum is fit to a cocktail consisting of <ub:math xmlns:ub="http://www.w3.org/1998/Math/MathML" display="inline"><ub:mi>ϕ</ub:mi><ub:mo stretchy="false">(</ub:mo><ub:mn>1020</ub:mn><ub:mo stretchy="false">)</ub:mo></ub:math> decays, direct <yb:math xmlns:yb="http://www.w3.org/1998/Math/MathML" display="inline"><yb:msup><yb:mi>K</yb:mi><yb:mo>+</yb:mo></yb:msup><yb:msup><yb:mi>K</yb:mi><yb:mo>−</yb:mo></yb:msup></yb:math> photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct <ac:math xmlns:ac="http://www.w3.org/1998/Math/MathML" display="inline"><ac:msup><ac:mi>K</ac:mi><ac:mo>+</ac:mo></ac:msup><ac:msup><ac:mi>K</ac:mi><ac:mo>−</ac:mo></ac:msup></ac:math> photoproduction are presented.