Wednesday, 16 May 2018

Proton's weak charge, and what's it for

In the particle world the LHC still attracts the most attention, but in parallel there is ongoing progress at the low-energy frontier. A new episode in that story is the Qweak experiment in Jefferson Lab in the US, which just published their final results.  Qweak was shooting a beam of 1 GeV electrons on a hydrogen (so basically proton) target to determine how the scattering rate depends on electron's polarization. Electrons and protons interact with each other via the electromagnetic and weak forces. The former is much stronger, but it is parity-invariant, i.e. it does not care about the direction of polarization. On the other hand, since the classic Wu experiment in 1956, the weak force is known to violate parity. Indeed, the Standard Model postulates that the Z boson, who mediates the weak force,  couples with different strength to left- and right-handed particles. The resulting asymmetry between the low-energy electron-proton scattering cross sections of left- and right-handed polarized electrons is predicted to be at the 10^-7 level. That has been experimentally observed many times before, but Qweak was able to measure it with the best precision to date (relative 4%), and at a lower momentum transfer than the previous experiments.   

What is the point of this exercise? Low-energy parity violation experiments are often sold as precision measurements of the so-called Weinberg angle, which is a function of the electroweak gauge couplings - the fundamental parameters of the Standard Model. I don't like too much that perspective because the electroweak couplings, and thus the Weinberg angle, can be more precisely determined from other observables, and Qweak is far from achieving a competing accuracy. The utility of Qweak is better visible in the effective theory picture. At low energies one can parameterize the relevant parity-violating interactions between protons and electrons by the contact term
where v ≈ 246 GeV, and QW is the so-called weak charge of the proton. Such interactions arise thanks to the Z boson in the Standard Model being exchanged between electrons and quarks that make up the proton. At low energies, the exchange diagram is well approximated by the contact term above with QW = 0.0708  (somewhat smaller than the "natural" value QW ~ 1  due to numerical accidents making the Z boson effectively protophobic). The measured polarization asymmetry in electron-proton scattering can be re-interpreted as a determination of the proton weak charge: QW = 0.0719 ± 0.0045, in perfect agreement with the Standard Model prediction.

New physics may affect the magnitude of the proton weak charge in two distinct ways. One is by altering the strength with which the Z boson couples to matter. This happens for example when light quarks mix with their heavier exotic cousins with different quantum numbers, as is often the case in the models from the Randall-Sundrum family. More generally, modified couplings to the Z boson could be a sign of quark compositeness. Another way is by generating new parity-violating contact interactions between electrons and quarks. This can be a result of yet unknown short-range forces which distinguish left- and right-handed electrons. Note that the observation of lepton flavor violation in B-meson decays can be interpreted as a hint for existence of such forces (although for that purpose the new force carriers do not need to couple to 1st generation quarks).  Qweak's measurement puts novel limits on such broad scenarios. Whatever the origin, simple dimensional analysis allows one to estimate  the possible change of the proton weak charge as 
   where M* is the mass scale of new particles beyond the Standard Model, and g* is their coupling strength to matter. Thus, Qweak can constrain new weakly coupled particles with masses up to a few TeV, or even 50 TeV particles if they are strongly coupled to matter (g*~4π).

What is the place of Qweak in the larger landscape of precision experiments? One can illustrate it by considering a simple example where heavy new physics modifies only the vector couplings of the Z boson to up and down quarks. The best existing constraints on such a scenario are displayed in this plot:
From the size of the rotten egg region you see that the Z boson couplings to light quarks are currently known with a per-mille accuracy. Somewhat surprisingly, the LEP collider, which back in the 1990s produced tens of millions of Z boson to precisely study their couplings, is not at all the leader in this field. In fact, better constraints come from precision measurements at very low energies: pion, kaon, and neutron decays,  parity-violating transitions in cesium atoms,  and the latest Qweak results which make a difference too. The importance of Qweak is even more pronounced in more complex scenarios where the parameter space is multi-dimensional.

Qweak is certainly not the last salvo on the low-energy frontier. Similar but more precise experiments are being prepared as we read (I wish the follow up were called SuperQweak, or SQweak in short). Who knows, maybe quarks are made of more fundamental building blocks at the scale of ~100 TeV,  and we'll first find it out thanks to parity violation at very low energies. 

Monday, 7 May 2018

Dark Matter goes sub-GeV

It must have been great to be a particle physicist in the 1990s. Everything was simple and clear then. They knew that, at the most fundamental level, nature was described by one of the five superstring theories which, at low energies, reduced to the Minimal Supersymmetric Standard Model. Dark matter also had a firm place in this narrative, being identified with the lightest neutralino of the MSSM. This simple-minded picture strongly influenced the experimental program of dark matter detection, which was almost entirely focused on the so-called WIMPs in the 1 GeV - 1 TeV mass range. Most of the detectors, including the current leaders XENON and LUX, are blind to sub-GeV dark matter, as slow and light incoming particles are unable to transfer a detectable amount of energy to the target nuclei.

Sometimes progress consists in realizing that you know nothing Jon Snow. The lack of new physics at the LHC invalidates most of the historical motivations for WIMPs. Theoretically, the mass of the dark matter particle could be anywhere between 10^-30 GeV and 10^19 GeV. There are myriads of models positioned anywhere in that range, and it's hard to argue with a straight face that any particular one is favored. We now know that we don't know what dark matter is, and that we should better search in many places. If anything, the small-scale problem of the 𝞚CDM cosmological model can be interpreted as a hint against the boring WIMPS and in favor of light dark matter. For example, if it turns out that dark matter has significant (nuclear size) self-interactions, that can only be realized with sub-GeV particles. 
It takes some time for experiment to catch up with theory, but the process is already well in motion. There is some fascinating progress on the front of ultra-light axion dark matter, which deserves a separate post. Here I want to highlight the ongoing  developments in direct detection of dark matter particles with masses between MeV and GeV. Until recently, the only available constraint in that regime was obtained by recasting data from the XENON10 experiment - the grandfather of the currently operating XENON1T.  In XENON detectors there are two ingredients of the signal generated when a target nucleus is struck:  ionization electrons and scintillation photons. WIMP searches require both to discriminate signal from background. But MeV dark matter interacting with electrons could eject electrons from xenon atoms without producing scintillation. In the standard analysis, such events would be discarded as background. However,  this paper showed that, recycling the available XENON10 data on ionization-only events, one can exclude dark matter in the 100 MeV ballpark with the cross section for scattering on electrons larger than ~0.01 picobarn (10^-38 cm^2). This already has non-trivial consequences for concrete models; for example, a part of the parameter space of milli-charged dark matter is currently best constrained by XENON10.   

It is remarkable that so much useful information can be extracted by basically misusing data collected for another purpose (earlier this year the DarkSide-50 recast their own data in the same manner, excluding another chunk of the parameter space).  Nevertheless, dedicated experiments will soon  be taking over. Recently, two collaborations published first results from their prototype detectors:  one is SENSEI, which uses 0.1 gram of silicon CCDs, and the other is SuperCDMS, which uses 1 gram of silicon semiconductor.  Both are sensitive to eV energy depositions, thanks to which they can extend the search region to lower dark matter mass regions, and set novel limits in the virgin territory between 0.5 and 5 MeV.  A compilation of the existing direct detection limits is shown in the plot. As you can see, above 5 MeV the tiny prototypes cannot yet beat the XENON10 recast. But that will certainly change as soon as full-blown detectors are constructed, after which the XENON10 sensitivity should be improved by several orders of magnitude.
Should we be restless waiting for these results? Well, for any single experiment the chance of finding nothing are immensely larger than that of finding something. Nevertheless, the technical progress and the widening scope of searches offer some hope that the dark matter puzzle may be solved soon.