[BDXlist] [EXTERNAL] Latest results from PandaX-4T

[Andrea Celentano]

Dear colleagues,
today on the arXiv a paper from PANDAX-4T appeared:

https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_pdf_2308.01540.pdf&d=DwIFaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=Ru7nA6zPBSFajtw1o6aZjchIV9Cs2SBk2RiVldzQhDc&m=-bAdCUCakUv9Lt5GvfSPc2PpXcEdF3ulpDDeY_-7k1QA0on_FGsPpGkCsAV1PkTK&s=VIsXIgqSrHGn743qNZohnLvaVa7fDau46uf4cif8LFM&e= 

PANDAX-4T is a direct-detection experiment in China, based on a dual-phase liquid xenon TPC. In this work, they set very aggressive exclusion limits on DM-nucleus scattering, that are then exported in the y-vs-mChi plane, as seen below.



These limits are really covering a so-far totally unexplored area of the parameters space. I think we must pay attention to this. I gave a first read to the paper, and it seems to me that really no details about the experiment are given, neither on the analysis or on the phenomenology. 

I report below few points that confused me (I am totally not an expert on DM direct detection, so probably my confusion derives from this lack of experience!!!)

In their Eq. 1, the DM velocity “v” appears at the denominator, while their Ref. 24, quoted as the source of this equation, it appears at numerator (see Ref. 24, Eq. 19 and Eq. 20)
In their Eq. 3, they report the total cross section at zero momentum transfer for the chi - nucleon scattering, and they observe that it scales as y / mchi^4. However, I do not understand how this was fed in to Eq. 1; specifically, if I simplify everything and just consider a contact interaction between LDM and nucleus (their Ref. 24, Eq. 16), I get:



This is to be multiplied by the flux of LDM particles and the numerical density of Xe nuclei in the detector to get the interaction rate (assuming that the detection threshold is low enough so that we can integrate the energy distribution of the recoiling nucleus to one). 
The DM flux should bring in a contribution ~ rhoDM / mChi, where rhoDM ~ 0.3 GeV / cm3 is the DM mass density. 
So, bringing in everything the mChi dependency should be 1/mchi^3 (while they talk about 1/mChi^4).

Maybe someone with experience in direct detection should look at this to check how the limits were translated to the y - vs - mChi plane?

Bests,
Andrea
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