There are at least 3 distinct B-meson anomalies that are currently intriguing:
- A few sigma (2 to 4, depending who you ask) deviation in differential distribution of B → K*μμ decays,
- 2.6 sigma violation of lepton flavor universality in B → Kμμ vs B → Kee decays,
- 3.5 sigma violation of lepton flavor universality, but this time in B → Dτν vs B → Dμν decays.
If both λb and λs are non-zero then a tree-level leptoquark exchange can mediate the b-quark decay b → s μ μ. This contribution adds up to the Standard Model amplitudes mediated by loops of W bosons, and thus affects the B-meson observables. It turns out that the first two anomalies listed above can be fit if the leptoquark mass is in the 1-50 TeV range, depending on the magnitude of λb and λs.Also the 3rd anomaly above can be easily explained by leptoquarks. One example from this paper is a leptoquark transforming as (3,1,-1/3) and coupling to matter as
There are more kinds of leptoquarks with different charges that allow for Yukawa couplings to matter. Some of them could also explain the 3 sigma discrepancy of the experimentally measured muon anomalous magnetic moment with the Standard Model prediction. Actually, a recent paper says that the (3,1,-1/3) leptoquark discussed above can explain all B-meson and muon g-2 anomalies simultaneously, through a combination of tree-level and loop effects. In any case, this is something to look out for in this and next year's data. If a leptoquark is indeed the culprit for the B → Dτν excess, it should be within reach of the 13 TeV run (for the 1st two anomalies it may well be too heavy to produce at the LHC). The current reach for leptoquarks is up to 1 TeV mass (strongly depending on model details), see e.g. the recent ATLAS and CMS analyses. So far these searches have provoked little public interest, but that may change soon...
20 comments:
I'll miss my protons.
Obviously, the scalar leptoquark is consistent with the Jester Exclusion principle. This alone weighs in its favor.
But what would be the implications for proton decay?
That of course would be catastrophic for our health :) However, in simple models discussed in this post , baryon number is conserved and proton decay does not occur. Proton decay will happen if a leptoquark has Yukawa couplings with 2 quarks of the 1st generation, but that is not needed to address the B-meson anomalies and it is consistent to assume such couplings are zero.
I see. So it is coupled only to heavier quarks to solve this problem. My mistake. When I read "leptoquark", I immediately started thinking about all our old dreams, like SU(5). Thank you for your explanation.
Thanks Jester: it is the first time I heard about leptoquarks. Now, I wonder if those three B-meson annomalies could be explained only by W bosons (and Z bosons) with different loop orders.
Hi Jester -- Not sure if your response to Phillipe needs a bit of a revision. Even if lepto-quarks in your examples were to couple to first generation, still Baryon number seems to be conserved -- the way the hypercharge and SU(3)_c quantum numbers are chosen for the models you mentioned. I am not sure if any higher scale suppressed non-renormalizable terms are any more dangerous than in std model -- maybe not. An issue is of course having light leptoquark scalars requires finetuning owing to Hierarchy problem.
I meant Yukawa couplings with 2 quarks, like \phi^\dagger u^c d^c . This would violate baryon number. Couplings to the first generation like \phi^\dagger Q_1 L_1 that conserve baryon number are of course no problem.
Nice post, Jester.
You may be interested in 1505.05164 as well. They also propose a leptoquark explanation to the B anomalies. In fact, the authors of this paper claim that with the vector leptoquark V = (3,1,-2/3) one can in principle address all of them (and in this case at tree-level, in contrast to the paper by Bauer and Neubert).
Thx Avelino, I haven't read that yet, I'll do it soon.
Antonio, one cannot explain these anomalies by W and Z bosons alone; if the effect is real, it must be due to a new fairly light particle.
I think the hypercharge assignments of \phi prevent Baryon number violating Yukawa terms like \phi^\dagger u^c d^c -- just like in std model -- may occur at higher powers.
In my notation u^c and d^c are 2-component Weyl fermions corresponding to charge conjugate of the right-handed up and down quarks (sorry everyone else for the Chinese ;) ). So u^c transforms as (3bar,1,-2/3) and d^c as (3bar,1,1/3). With \phi^\dagger transforming as (3bar,1,1/3) \phi^\dagger u^c d^c is allowed by the gauge symmetry. You may be thinking about susy theories where this term would be forbidden by holomorphy.
Thanks Jester. I was taking the Hypercharge (Y) values to be given by Q_em = I_3L + Y/2, so for u^c and d^c I had them as -4/3 and 2/3 respectively, as is the usual convention. So it didn't work. But in your post the Hypercharge values are given by the convention Q_em = I_3L + Y. Else we wont have the term (Q_3 L_3 \phi^\dagger). Yeah so B violating terms are permitted by Gauge symmetry like you say. I guess B would then have to be explicitly introduced as a global symmetry that remains unbroken.
Your analysis is seriously flawed.
Nickelback's first hit, "How You Remind Me", was solid stuff. The problem is that the band then decayed into a long-lived degenerate state that made a large contribution to the background noise.
Jester, how would such a possibility position itself against prospects of SUSY in this run and in general (pro, contra or orthogonal, etc)?
This is mostly orthogonal. Depending on the quantum numbers couplings to matter, some leptoquarks can be interpreted as squarks in R-parity violating SUSY. But, in general, finding leptoquarks does not make the case for SUSY stronger or weaker.
Just wanted to point out that CERN searched their 13 TeV data for narrow resonances and found nothing so far: http://cds.cern.ch/record/2110669
"Proton decay will happen if a leptoquark has Yukawa couplings with 2 quarks of the 1st generation ..." Does that mean that LHC has a good chance of discovering such proton decay if such leptoquarks exist?
Is the following publication likely to be relevant to leptoquarks?
"Intersecting Branes, SUSY Breaking and the 2TeV Excess at the LHC" by Ralph Blumenhagen, 2015
You say leptoquarks aren't theoretically popular. I thought they were a mostly guaranteed particle in most GUTs. Is this not so? I saw a comment languishing SU(5), but how does this look, say, in the popular Pati-Salam model? Is this good news? Is there any reason to suggest a GUT that also requires SUSY to explain this?
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