Light Dark Matter Sector 2017 Light Dark Matter

“Light”? “Dark”? Matter Sector 2017 Light Dark Matter 2017 Searching for Axion-like particles Pb a with heavy-ions Hou Keong (Tim) Lou, Simon Knapen, Tongyan Lin and Tom Melia Phys. Rev. Lett. 118. 171801 *Disclaimer: Not Dark Matter! Sorry! 1

New Physics @LHC Must leave no stone unturned! There may be many surprises! 750 Ge. V (false alarm) lighter? Can we be missing cousins of 750 Ge. V? Possibly: What if resonance is lighter? 2

Axion-like Particle Only couple to EM Decay rate Can we improve limits ~ Ge. V range? (Not DM! sorry!) Existing Constraints J. Jaeckel et. al. 1509. 00476 3

How to look for ALPs Heavy ALPs Light ALPs Beam dumps/fixed targets Decays of mesons Astrophysics Cosmology 4

A. Balantekin et. al. 1985, M. Greiner et. al. 1993… Can observe LBL scattering! D. d’Enterria et. al. 1305. 7142 ATLAS 1702. 01625 5

Ultra-Peripheral Collisions Pb a Pb 6

Photon Luminosity Pb a Pb 7

Proton vs Pb Luminosity p-p Collisions Pb-Pb Collisions ~ 1000 fb-1 ~ 10 -6 fb-1 (1 nb-1) Proton’s wrost nightmare? None Background Regions of sensitivity ~ 1 Te. V (elastic) ~ 170 Ge. V Large pile-up background Clean exclusive events when p and Pb collide… 8

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Main Background Light by Light scattering Pb Pb Fake photons / electron brem. Central Exclusive Production 10

Signal & Background Signal 11

Search Strategy Signal Selection Integrated lumi. ~ nb 12

Results Standard ALP coupling 13

Results Hypercharge coupling 14

ATLAS recast 15

Future Directions ? New production? New decay channel? New kinds of coherent enhancement (baryon number)? Lighter ALPs? (pair production and off-shell rates are too small) Changes in inclusive elastic cross-sections? 16

Conclusion Heavy-ion collisions open a new window into ~ Ge. V ALPs/Hidden sector ATLAS Heavy-ion search places the best constraint for ~ Ge. V ALPs CMS results forthcoming Hidden sector may show up in unexpected places! 17

Where are the ALPs? I think I’ve found it with my lead-flashlight! Thank You! 18

p. T recoil distribution 19

Form Factors Effects M. Klusek-Gawenda et. al. 1601. 07001 20