The amount of energy released by a nuclear recoil ionizing the atoms of the active volume of detection appears “quenched” compared to an electron of the same kinetic energy. This different behavior in ionization between electrons and nuclei is described by the Ionization Quenching Factor (IQF) and it plays a crucial role in direct dark matter searches. For low kinetic energies (below  $$50~\textrm{keV}$$  <math> <mrow> <mn>50</mn> <mspace width="3.33333pt"></mspace> <mtext>keV</mtext> </mrow> </math>  ), IQF measurements deviate significantly from common models used for theoretical predictions and simulations. We report measurements of the IQF for proton, an appropriate target for searches of Dark Matter candidates with a mass of approximately  $$1~\textrm{GeV}$$  <math> <mrow> <mn>1</mn> <mspace width="3.33333pt"></mspace> <mtext>GeV</mtext> </mrow> </math>  , with kinetic energies in between  $$2~\textrm{keV}$$  <math> <mrow> <mn>2</mn> <mspace width="3.33333pt"></mspace> <mtext>keV</mtext> </mrow> </math>   and  $$13~\textrm{keV}$$  <math> <mrow> <mn>13</mn> <mspace width="3.33333pt"></mspace> <mtext>keV</mtext> </mrow> </math>   in  $$100~\textrm{mbar}$$  <math> <mrow> <mn>100</mn> <mspace width="3.33333pt"></mspace> <mtext>mbar</mtext> </mrow> </math>   of methane. We used the Comimac facility in order to produce the motion of nuclei and electrons of controlled kinetic energy in the active volume, and a NEWS-G SPC to measure the deposited energy. The Comimac electrons are used as a reference to calibrate the detector with 7 energy points. A detailed study of systematic effects led to the final results well fitted by  $$\textrm{IQF}~(E_K)= E_K^\alpha ~/~(\beta + E_K^\alpha )$$  <math> <mrow> <mtext>IQF</mtext> <mspace width="3.33333pt"></mspace> <mrow> <mo>(</mo> <msub> <mi>E</mi> <mi>K</mi> </msub> <mo>)</mo> </mrow> <mo>=</mo> <msubsup> <mi>E</mi> <mi>K</mi> <mi>α</mi> </msubsup> <mspace width="3.33333pt"></mspace> <mo>/</mo> <mspace width="3.33333pt"></mspace> <mrow> <mo>(</mo> <mi>β</mi> <mo>+</mo> <msubsup> <mi>E</mi> <mi>K</mi> <mi>α</mi> </msubsup> <mo>)</mo> </mrow> </mrow> </math>   with  $$\alpha = 0.70\pm 0.08$$  <math> <mrow> <mi>α</mi> <mo>=</mo> <mn>0.70</mn> <mo>±</mo> <mn>0.08</mn> </mrow> </math>   and  $$\beta = 1.32\pm 0.17$$  <math> <mrow> <mi>β</mi> <mo>=</mo> <mn>1.32</mn> <mo>±</mo> <mn>0.17</mn> </mrow> </math>  . In agreement with some previous works in other gas mixtures, we measured less ionization energy than predicted from SRIM simulations, the difference reaching  $$33\%$$  <math> <mrow> <mn>33</mn> <mo>%</mo> </mrow> </math>   at  $$2~\textrm{keV}$$  <math> <mrow> <mn>2</mn> <mspace width="3.33333pt"></mspace> <mtext>keV</mtext> </mrow> </math>  .

Canada

L. Balogh

France

C. Beaufort

A. Brossard

J.F. Caron

M. Chapellier

J.M. Coquillat

E. Corcoran

S. Crawford

A. Dastgheibi-Fard

Y. Deng

K. Dering

D. Durnford

C. Garrah

G. Gerbier

I. Giomataris

G. Giroux

P. Gorel

M. Gros

P. Gros

O. Guillaudin

United States

E. Hoppe

United Kingdom

I. Katsioulas

F. Kelly

P. Knights

S. Langrock

P. Lautridou

I. Manthos

R. Martin

J. Matthews

J.P. Mols

J.F. Muraz

T. Neep

K. Nikolopoulos

P. O’Brien

M.C. Piro

D. Santos

G. Savvidis

Greece

I. Savvidis

F. Vazquez de Sola Fernandez

M. Vidal

R. Ward

M. Zampaolo

Measurements of the ionization efficiency of protons in methane

Kaluza-Klein axions appear in theories with extra dimensions as higher mass, significantly shorter lifetime, excitations of the Peccei-Quinn axion. When produced in the Sun, they would remain gravitationally trapped in the solar system, and their decay to a pair of photons could provide an explanation of the solar corona heating problem. A low-density detector would discriminate such a signal from the background, by identifying the separation of the interaction point of the two photons. The NEWS-G collaboration uses large volume spherical proportional counters, gas-filled metallic spheres with a spherical anode in their centre. After observation of a single axionlike event in a 42 day long run with the SEDINE detector, a 90% C.L. upper limit of <math><mrow><msub><mrow><mi>g</mi></mrow><mrow><mi>a</mi><mi>γ</mi><mi>γ</mi></mrow></msub><mo>&lt;</mo><mn>8.99</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>13</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>GeV</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math> is set on the axion-photon coupling for the benchmark of a Kaluza-Klein axion density on Earth of <math><mrow><msub><mrow><mi>n</mi></mrow><mrow><mi>a</mi></mrow></msub><mo>=</mo><mn>4.07</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>13</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>m</mi></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup></mrow></math> and two extra dimensions of size <math><mrow><mi>R</mi><mo>=</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>eV</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math>.