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Title: Helium $\lambda$-point heat-capacity exponent disagrees with theory
Plain: The heat capacity of liquid helium near its superfluid transition was measured in orbit with high precision. Its critical exponent disagrees with the theoretical value by many error bars.
Precise: The He-4 superfluid transition is in the 3D $O(2)$ universality class, with $\alpha = 2 - 3\nu$. The Space Shuttle measurement gives $\alpha = -0.0127(3)$ (Lipa et al. 2003), while the conformal bootstrap gives $\nu = 0.67175(10)$, i.e. $\alpha = -0.01526(30)$ (Chester et al. 2020), in agreement with Monte Carlo; the tension is about 8 standard deviations. Determine whether the discrepancy comes from the experimental analysis (corrections to scaling, finite size, residual gravity), from the identification with the $O(2)$ fixed point, or from an error in theory.
Settled by: A new He-4 specific-heat measurement with independent systematics, or the identification of a specific error in the 2003 analysis or in the theoretical value.
Kind: phenomenon
Answer type: identification
References: (empty)
Literature status: contested
Status note: No new measurement as of 2025; resummation claims of agreement with experiment have not displaced the bootstrap and Monte Carlo value.
Posed since: (empty)