# This Week at Physics

## Events at Physics |
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### Events During the Week of August 20th through August 27th, 2017

### Monday, August 21st, 2017

**No events scheduled**### Tuesday, August 22nd, 2017

**No events scheduled**### Wednesday, August 23rd, 2017

**No events scheduled**### Thursday, August 24th, 2017

**Theory Seminar (High Energy/Cosmology)****O(N) invariance of the multi-field bounce****Time:**11:00 am**Place:**5280 Chamberlin Hall**Speaker:**Kfir Blum, Weizmann Institute of Science**Abstract:**In his 1977 paper on vacuum decay in field theory: The Fate of the False Vacuum, Coleman considered the problem of a single scalar field and assumed that the minimum action tunnelling field configuration, the bounce, is invariant under O(4) rotations in Euclidean space. A proof of the O(4) invariance of the bounce was provided later by Coleman, Glaser, and Martin (CGM), who extended the proof to N>2 Euclidean dimensions but, again, restricted non-trivially to a single scalar field. As far as we know a proof of O(N) invariance of the bounce with more than one scalar fields has not been reported in the QFT literature, even though it was assumed in many works since. We make progress towards closing this gap. Following CGM we define the reduced problem of finding a field configuration minimizing the kinetic energy at fixed potential energy. Given a solution of the reduced problem, the minimum action bounce can always be obtained from it by means of a scale transformation. We show that if a solution of the reduced problem exists, then it and the minimum action bounce derived from it are indeed O(N) symmetric. We review complementary results in the mathematical literature that established the existence of a minimizer under specified criteria.### Friday, August 25th, 2017

**NPAC (Nuclear/Particle/Astro/Cosmo) Forum****Cosmic rays, anti-helium, and an old navy spotlight****Time:**11:00 am**Place:**5280 Chamberlin Hall**Speaker:**Kfir Blum, Weizmann Institute of Sciences**Abstract:**Cosmic-ray anti-deuterium and anti-helium have long been suggested as probes of dark matter, as their secondary astrophysical production was thought extremely scarce. But how does one actually predict the secondary flux? Anti-nuclei are dominantly produced in pp collisions, where laboratory cross section data is lacking. We make a new attempt at tackling this problem by appealing to a scaling law of nuclear coalescence with the physical volume of the hadronic emission region. The same volume is probed by Hanbury Brown-Twiss (HBT) two-particle correlations. We demonstrate the consistency of the scaling law with systems ranging from central and off-axis AA collisions to pA collisions, spanning 3 orders of magnitude in coalescence yield. Extending the volume scaling to the pp system, HBT data allows us to make a new estimate of coalescence, that we test against preliminary ALICE pp data. For anti-helium the resulting cross section is 1-2 orders of magnitude higher than earlier estimates. The astrophysical secondary flux of anti-helium could be within reach of a five-year exposure of AMS02.**Host:**Albrecht Karle