Just over a week ago the annual meeting of theoretical particle physicists (RPP 2024) was held at Jussieu, the campus of Sorbonne University where I work. I wrote about the 2020 edition (held just outside Paris) here; in keeping with tradition, this year's version also contained similar political sessions with the heads of the CNRS' relevant physics institutes and members of CNRS committees, although they were perhaps less spicy (despite rumours of big changes in the air).
One of the roles of these meetings is as a shop window for young
researchers looking to be hired in France, and a great way to demonstrate
that they are interested and have a connection to the system. Of course,
this isn't and shouldn't be obligatory by any means; I wasn't really aware
of this prior to entering the CNRS though I had many connections to the
country. But that sort of thing seems especially important after the
problems described by 4gravitons
recently, and
his post about getting a permanent job in France
-- being able to settle in a country is non-trivial, it's a big worry for
both future employers and often not enough for candidates fighting tooth
and nail for the few jobs there are. There was another recent case of
someone getting a (CNRS) job -- to come to my lab, even -- who much more
quickly decided to leave the entire field for personal reasons. Both these
stories saddened me. I can understand -- there is the well-known
Paris syndrome
for one thing -- and the current political anxiety about immigration and
the government's response to the rise of the far right (across the world),
coupled with Brexit, is clearly leading to things getting harder for many.
These stories are especially worrying because we expect to be recruiting
for university positions in my lab this year.
I was obviously very lucky and my experience was vastly different; I love
both the job and the place, and I'm proud to be a naturalised citizen.
Permanent jobs in the CNRS are amazing, especially in terms of the time
and freedom you have, and there are all sorts of connections between the
groups throughout the country such as via the
IRN Terascale or
GdR Intensity Frontier; or IRN Quantum Fields and Strings
and
French Strings meetings
for more formal topics. I'd recommend anyone thinking about working here
to check out these meetings and the communities built around them, as well
as taking the opportunity to find out about life here. For those moving
with family, France also offers a lot of support (healthcare, childcare,
very generous holidays, etc) once you have got into the system.
The other thing to add that was emphasised in the political
sessions at the RPP (reinforcing the message that we're hearing a lot) is
that the CNRS is very keen to encourage people from under-represented
groups to apply and be hired. One of the ways they see to help this is to
put pressure on the committees to hire researchers (even) earlier after
their PhD, in order to reduce the length of the leaky pipeline.
Back to physics
Coming back to the RPP, this year was particularly well attended and had
an excellent
program of
reviews of hot topics, invited and contributed talks, put together very
carefully by my colleagues. It was particularly poignant for me because two former students in my lab who I worked with a lot, one who recently got a permanent job, were talking; and in addition
both a former student of mine and his current PhD student were
giving talks: this made me feel old. (All these talks were fascinating, of course!)
One review that stood out as relevant for this blog was
Bogdan Malaescu's review of progress in understanding the problem with muon g-2. As I
discussed
here, there is currently a lot of confusion in what the Standard Model
prediction should be for that quantity. This is obviously very concerning
for the experiments measuring muon g-2, who
in a paper last year
reduced their uncertainty by a factor of 2 to $$a_\mu (\mathrm{exp}) = 116
592 059(22)\times 10^{−11}. $$
The Lattice calculation (which has been confirmed now by several groups)
disagrees with the prediction using the data-driven R-ratio method
however, and there is a race on to understand why. New data from the
CMD-3 experiment seems to
agree with the lattice result, combining all global data on measurements
of \(e^+ e^- \rightarrow \pi^+ \pi^- \) still gives a discrepancy of more
than \(5\sigma\). There is clearly a significant disagreement within the
data samples used (indeed, CMD-3 significantly disagrees with their own
previous measurement, CMD-2). The confusion is summarised by this plot:
As can be seen, the finger of blame is often pointed at the KLOE data; excluding it but including the others in the plot gives agreement with the lattice result and a significance of non-zero \(\Delta a_\mu\) compared to experiment of \(2.8\sigma\) (or for just the dispersive method without the lattice data \( \Delta a_\mu \equiv a_\mu^{\rm SM} - a_\mu^{\rm exp} = −123 \pm 33 \pm 29 \pm 22 \times 10^{-11} \) , a discrepancy of \(2.5\sigma\)). In Bogdan's talk (see also his recent paper) he discusses these tensions and also the tensions between the data and the evaluation of \(a_\mu^{\rm win}\), which is the contribution coming from a narrow "window" (when the total contribution to the Hadronic Vacuum Polarisation is split into short, medium and long-distance pieces, the medium-range part should be the one most reliable for lattice calculations -- at short distances the lattice spacing may be too small, and at long ones the lattice may not be large enough). There he shows that, if we exclude the KLOE data and just include the BABAR, CMD-3 and Tau data, while the overall result agrees with the BMW lattice result, the window one disagrees by \(2.9 \sigma\) [thanks Bogdan for the correction to the original post]. It's clear that there is still a lot to be understood in the discrepancies of the data, and perhaps, with the added experimental precision on muon g-2, there is even still a hint of new physics ...
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