Thursday, 19 November 2015

Leptoquarks strike back

Leptoquarks are hypothetical scalar particles that carry both color and electroweak charges. Nothing like that exists in the Standard Model, where the only scalar is the Higgs who is a color singlet. In the particle community, leptoquarks enjoy the similar status as Nickelback in music: everybody's heard of them, but no one likes them.  It is not completely clear why... maybe they are confused with leprechauns, maybe  because they sometimes lead to proton decay, or maybe because they rarely arise in cherished models of new physics.  However,  recently there has been some renewed interest in leptoquarks.  The reason is that these particles seem well equipped to address the hottest topic of this year - the B meson anomalies.

There are at least 3 distinct B-meson anomalies that are currently intriguing:
  1.  A few sigma (2 to 4, depending who you ask) deviation in differential distribution of B → K*μμ decays, 
  2.  2.6 sigma violation of  lepton flavor universality in  B → Kμμ vs B → Kee decays, 
  3.  3.5 sigma violation of lepton flavor universality, but this time in  B → Dτν vs B → Dμν decays. 
Now, leptoquarks with masses in the TeV ballpark can explain either of these anomalies.  How? In analogy to the Higgs, leptoquarks may interact with the Standard Model fermions via Yukawa couplings. Which interactions  are possible is determined by  its color and electroweak charges. For example, this paper proposed a leptoquark transforming as (3,2,1/6) under the Standard Model gauge symmetry (color SU(3) triplet like quarks, weak SU(2) doublet like Higgs,  hypercharge 1/6).  Such particle can have the following Yukawa couplings with b- and s-quarks and muons:
 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

This particle contributes to  b → c τ ν, adding up to the tree-level W boson contribution, and is capable of explaining the apparent excess of semi-leptonic B meson decays into D mesons and tau leptons observed by the BaBar, Belle, and LHCb experiments. The difference to the previous case is that this leptoquark has to be less massive, closer to the TeV scale, because it has to compete with the tree-level contribution in the Standard Model.

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...

Thursday, 12 November 2015

A year at 13 TeV

A week ago the LHC finished the 2015 run of 13 TeV proton collisions.  The counter in ATLAS stopped exactly at 4 inverse femtobarns. CMS reports just 10% less, however it is not clear what fraction of these data is collected with their magnet on (probably about a half). Anyway, it should have been better, it could have been worse...   4 fb-1 is one fifth of what ATLAS and CMS collected in the glorious year 2012.  On the other hand, the higher collision energy in 2015 translates to larger production cross sections, even for particles within the kinematic reach of the 8 TeV collisions.  How this trade off work in practice depends on the studied process.  A few examples are shown in the plot below
We see that, for processes initiated by collisions of a quark inside one proton with an antiquark inside the other proton, the cross section gain is the least favorable. Still, for hypothetical resonances heavier than ~1.7 TeV, more signal events were produced in the 2015 run than in the previous one. For example, for a 2 TeV W-prime resonance, possibly observed by ATLAS in the 8 TeV data, the net gain is 50%, corresponding to roughly 15 events predicted in the 13 TeV data. However, the plot does not tell the whole story, because the backgrounds have increased as well.  Moreover, when the main background originates from gluon-gluon collisions (as is the case for the W-prime search in the hadronic channel),  it grows faster than the signal.  Thus, if the 2 TeV W' is really there, the significance of the signal in the 13 TeV data should be comparable to that in the 8 TeV data in spite of the larger event rate. That will not be enough to fully clarify the situation, but the new data may make the story much more exciting if the excess reappears;  or much less exciting if it does not... When backgrounds are not an issue (for example, for high-mass dilepton resonances) the improvement in this year's data should be more spectacular.

We also see that, for new physics processes initiated by collisions of a gluon in 1 proton with another gluon in the other proton, the 13 TeV run is superior everywhere above the TeV scale, and the signal enhancement is more spectacular. For example, at 2 TeV one gains a factor of 3 in signal rate. Therefore, models where the ATLAS diboson excess is explained via a Higgs-like scalar resonance will be tested very soon. The reach will also be extended for other hypothetical particles pair-produced in gluon collisions, such as  gluinos in the minimal supersymmetric model. The current lower limit on the gluino mass obtained by  the 8 TeV run is m≳1.4 TeV  (for decoupled squarks and massless neutralino). For this mass, the signal gain in the 2015 run is roughly a factor of 6. Hence we can expect the gluino mass limits will be pushed upwards soon, by about 200 GeV or so.  

Summarizing,  we have a right to expect some interesting results during this winter break. The chances for a discovery  in this year's data are non-zero,  and chances for a tantalizing hints of new physics (whether a real thing or a background fluctuation) are considerable. Limits on certain imaginary particles will be somewhat improved. However, contrary to my hopes/fears, this year is not yet the decisive one for particle physics.  The next one will be.