# What does “tagging” mean in experimental high energy physics?

Could someone explain in details the meaning of the terminology "tagging" in experimental high energy physics and how is it used in the analysis?

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The term has multiple meaning depending on the context. Are you hearing phases like "B-tagging"? –  dmckee Aug 12 '11 at 15:07
@dmckee Yes exactly, but I thought it refers to some general technique. I did not know that "tagging" has different meanings within experimental HEP –  Revo Aug 12 '11 at 15:18
Well, tagging in general is a reference to metaphorically sticking a tag on an event. What the tag says and how reliable it is are both context dependent. Examples: b-tagging at the collider experiment (this jet probably contains a b quark hadron), the photon tagger at CLAS (the initiating particle of this event is almost certainly a photon of energy $E_\gamma \pm \delta E$), and so on. –  dmckee Aug 12 '11 at 15:26

As @dmckee points out this term is used in multiple contexts, but "b-tagging" is probably the most common usage. As in "the selection requires 2 tagged jets" or something of this sort.

b-tagging is the identification of jets (the spray of color neutral particles originating from a colored quark or gluon) that are likely to originate from a $b$-quark. This can be useful for analyses with a lot of jets, of which some are $b$-jets (it will reduce combinatorial background).

Examples include top-pair production (each top almost exclusively decays into a W boson and a b quark), Higgs (couples strongly to b's), of flavour physics (e.g. B-Physics) A disadvantage is that b-tagging algorithms are not very efficient (common workin points include 50%-70% so out of all true b-jets you will only catch a little more than half)

B-tagging works because the B-mesons, that come early in the hadronization process have a significantly longer lifetime than other mesons. This is due to the $V_{xb}$ matrix elements in the CKM matrix (x being one of u,c,t). A drop down into lower generations via weak decay has a low probability, therfore the lifetime is long.

To find b-jets one possibility is to look at jets coming from secondary secondary vertices macroscopically apart from the primary interactions. Another way is to look at the impact parameters (the distance to the primary vertex) of the tracks of the individual charged particles that constitute the jet.

Aside from b-tagging, there is also c-tagging, since charmed mesons have a significantly larger lifetime as well (compared to so-called "light-flavors" u,d,s), but it doesn't work as well. Collectively the term is "flavor-tagging"

Another usage is when people talk about "tag and probe". This is a method to calibrate and measure the performance of some selection. For example it is crucial to know how well muons are reconstructed. Therefore one will select events from a region in phase space that corresponds to $Z$ boson decays to two muons. One will apply strict requirements for one muon but only loose one to the other. The strictly selected one is the "tagged" muon and the looser one is the muon on which one wants to "probe" the reconstruction efficiency (or something else)

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So I assume there can also be photon-, electron-, up quark-..etc- tagging? Where can one read more about "tag and probe" ? –  Revo Aug 12 '11 at 15:45
@Revo: For something to be taggable (or at least usefully so), you need a reliable signal, and that depends on the physics, the beam, the detector, and so on. –  dmckee Aug 12 '11 at 16:10
try this definitive resource on the ATLAS detector: arxiv.org/abs/0901.0512v4 The Z decay dimuon tag and probe method is explained in section 3.1.1. p216 but if you search for "tag-and-probe" you will find many more examples –  luksen Aug 12 '11 at 16:12
@luksen: In Flavour Physics B-tagging is differently used than the one described by you. E.g. the tagging described in arxiv.org/abs/arXiv:1202.4979 does /not/ determine whether a jet contains a b quark or not. It determines whether a B meson contains a b quark or an anti-b quark (this is what is called b-charge tagging it ATLAS to my knowledge) –  user30790 Oct 9 '13 at 10:41
@Revo, unfortunately the terminology hasn't been unified to the extent you speak of: to the electron community, "tag" more commonly refers to "tag and probe", whereas to the b-tagging community it refers to flavor tagging. Within ATLAS, at least, we say "electron ID" to avoid confusing people. As for other particles, it's hard enough to tag c-quarks, many other particles are even harder. –  Shep May 22 '14 at 11:42