Blogging ICHEP 2010


A collective forum about the 35th edition of
the International Conference on High Energy Physics (Paris, July 2010)

Sunday, May 30, 2010

Up and down

The Standard Model works so well that you wish sometimes that we had not designed such an efficient theory. So where to look for excitement, since everything seems so much and so boringly in control ? Where does New Physics hide, waiting for us to be discovered ?

"LHC" seems the most obvious answer -- test the least-understood sector of the theory (the breaking of electroweak symmetry) by trying to produce the missing piece of the Standard Model (the Higgs boson) and/or hopefully something else, and unexpected. We all hope to be in Rabi's position, who exclaimed "who ordered that ?", when the muon was discovered and did not fit into anybody's plans. So should we just wait for the LHC to give all the answers ?

Fortunately not. Jester's posts show at least one place where things are moving fast, if not always in the direction of your dream -- flavour physics, i.e. the electroweak transitions under which a quark or a lepton changes from one type to another. Obviously, this is not an easy game. Quarks are subjected to strong interaction -- and thus confined into hadrons, so that you never actually observe the decay of a naked quark into another one, but rather that of a meson to one or several others. The computation of such processes is a theorist's nightmare, involving hadronic quantities that are very hard to estimate and possibly hiding faint contributions from New Physics. Leptonic processes are much cleaner, and provide signals of New Physics almost completely free of Standard Model backgrounds, like $\mu\to e\gamma$. But it is a "hit or miss" game : you need New Physics to make such processes frequent enough so that you can detect them... but you have no clue if this will be indeed the case.

The power of flavour physics lies in its sensitivity to virtual effects : by measuring accurately this process, you are sensitive to quantum corrections induced by particles/interactions at much higher energy scales. The past history has proved that it was not only theorists' daydreaming : the non-observation of $K_L\to\mu\mu$ led to the introduction of charm, the violation of the CP symmetry in the kaon system suggested the existence of a third generation of quarks and leptons, and neutrino oscillations were the first hint of physics beyond the Standard Model with the need for right-handed neutrinos. Moreover, despite our difficulties with the strong interactions, we can make (sometimes) quantitative statements : for instance, the upper limits on the difference of masses in the $B\bar{B}$ system showed that the top quark had to be much heavier than previously thought...

The present is a reflection of the past, as the philosophers say. Flavour physics keeps being a stringent constraint on many models of new physics, since they often predict transitions at much higher rates than observed. $\epsilon_K$, $b\to s\gamma$, electric dipole moments and so on have proved stubborn killers for unsuspecting theories for what lies beyond the Standard Model. They are indeed so powerful that for most theories, they push the scale for New Physics much above 1 TeV to make their impact negligible on the processes currently observed... Eh but wait a minute ! This is not what we want -- we want New Physics at the TeV scale !

Fortunately, you can find specific scenarios (such as Minimal Flavour Violation) to accomodate both the data from flavour physics and our LHC expectations. But it would be far more natural (historically speaking) and exciting (scientifically speaking) to see hints of new physics already in some flavour processes, before actually producing new particles at LHC responsible for these deviations. Hence all the recent discussions on the CDF/D0 results, taking back from one hand (CDF $\phi_s$) what they gave us from the other (D0 $A_{SL}$). That is the fundamental problem of flavour physics: since you do not observe new particles, you must spend your time on assessing how much your process has drifted from the Standard Model expectation. And it is a cruel game. Among the many deviations from the Standard Model observed in flavour physics, the majority stays in the grey area between 2 and $3\sigma$ before fading slowly into 1 $\sigma$ oblivion after a more careful scrutiny, both from experimentalists and theorists... Who is still excited by the $b\to s\bar{s}s$ processes, which used to deviate at 3 $\sigma$ from the Standard Model expectations and are now in perfect agreement with it ?

Are the recent results from CDF and D0 two different points in these Russian hills, or will one of these anomalies stay with us to finally be confirmed as a real effect ? Only time -- and experiment ! -- will tell.

2 comments:

  1. Hi Sebastian, thanks for this post ! I have a maybe stupid question (sorry, still studying these things). How can it be that there are particles with mass around 1 TeV but we have not seen them yet ? You talk about Minimal Flavor Violation. What is that, if it can be explained in simple words ? If that case is realized, does it mean that the LHC will discover particles soon ? Thank you.

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  2. Sébastien Descotes-GenonJune 7, 2010 at 11:20 PM

    A simple question... and a complicated answer (as usual !). In the case of the direct detection -- you produce a pair of new partices through collisions at sufficiently high energies -- you need first to have enough energy and then you must be able to disentangle your signal (such as a cascade decay down to your favourite lightest new physics particle) from the huge QCD and/or weak background. In a more simple case (?), the Tevatron might have already produced many Higgses from the Standard Model, but they cannot identify, because the cross section is often low compared to that of the potential background giving the same decay pattern.

    In the case of indirect measurements, as in flavour physics, the problem is the reverse : you want to kill the (generally sizeable) virtual contributions from new physics, which generally scales like a ratio

    product of couplings of new particles to usual ones / masses of new particles

    This a problem in particular for quantities measuring the violation of the CP-symmetry : generic New Physics brings new sources of CP-violation that upset completely the Kobayashi-Maskawa mechanism embedded in the Standard Model. If you want to reduce these contributions, you can think of increasing the masses of the new particles -- something should occur at the TeV scale, but 10 TeV is almost as good as 1 TeV from the theoretical point of view (but not from the experimental one !). The second possibility consists in choosing the couplings in a clever way, similar to that of the Standard Model, in order to suppress loop corrections. Minimal flavour violation is one such clever choice : the flavour symmetry present in the Standard Model is broken only the Yukawa couplings, and this is extended to New Physics models. This allows you to keep the same pattern of CP-violation as in the Standard Model, and thus to fulfill some of the most stringent constraints on New Physics, without having to increase the mass of the new particles by much.

    So there are reasonnably viable scenarios of physics at the TeV scale satisfying the flavour constraints -- but this does not mean that these scenarios are actually realised in Nature... At least, there is some hope !

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