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Theory: Reverting Spin edits as per Talk:Spin (particle physics) and Wikipedia talk:WikiProject Physics, replaced: Spin (particle physics) → Spin (physics)

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'''R-hadrons''' are hypothetical particles composed of a [[supersymmetric particle]] and at least one [[quark]].

== Theory ==

Only a few of the current [[supersymmetry]] theories predict the existence of R-hadrons, since in most of the [[parameter space]] all the supersymmetric particles are so separated in mass that their decays are very fast (with the exception of the [[Lightest Supersymmetric Particle|LSP]], which is stable in all the SUSY theories with [[R-parity]]).

R-hadrons are possible when a colored (in the sense of [[Quantum chromodynamics|QCD]]) supersymmetric particle (e.g., a [[gluino]] or a [[squark]]) has a [[mean lifetime]] longer than the typical [[hadronization]] time scale, and so QCD bound states are formed with ordinary [[parton (particle physics)|partons]] ([[quarks]] and [[gluons]]), in analogy with the ordinary [[hadrons]].

One example of a theory predicting observable R-hadrons is [[Split SUSY]].
Its main feature is, in fact, that all the new [[bosons]] are at a very high mass scale, and only the new [[fermions]] are at the [[TeV]] scale, i.e. accessible by the [[ATLAS experiment|ATLAS]] and [[Compact Muon Solenoid|CMS]] experiments in <math>pp</math> collisions at [[LHC]].
One of such new fermions would be the [[gluino]] ([[Spin (physics)|spin]] 1/2, as dictated for the [[supersymmetric partner]] of a spin 1 boson, the [[gluon]]).
The gluino, being colored, can only decay to other colored particles. But [[R-parity]] prevents a direct decay to quarks and/or gluons, and on the other hand the only other colored supersymmetric particles are the [[squarks]], that being bosons (spin 0, being the partners of the spin 1/2 quarks) have a much higher mass in Split SUSY.

All this, together, implies that the decay of the gluino can only go through a [[virtual particle]], a high-mass squark. The mean decay time depends on the mass of the intermediate virtual particle, and in this case can be very long.
This gives a unique opportunity to observe a SUSY particle directly, in a [[particle detector]], instead of deducing it by reconstructing its [[decay chain]] or by the [[momentum conservation|momentum imbalance]] (as in the case of the [[Lightest Supersymmetric Particle|LSP]]).

In other theories belonging to the SUSY family, the same role can be played by the lightest [[squark]] (usually the ''stop'', i.e. the partner of the [[top quark]]).

In the following, for sake of illustration, the R-hadron will be assumed to originate from a gluino created in a <math>pp</math> collision at [[LHC]], but the observational features are completely general.

== Observation techniques ==

*If the lifetime of an R-hadron is of the order of the [[picosecond]], it decays before reaching the first sensitive layers of a [[tracking detector]] but can be recognized by the [[secondary vertex]] technique, particularly efficient in [[ATLAS experiment|ATLAS]] and [[Compact Muon Solenoid|CMS]] thanks to their precise [[vertex detectors]] (both experiments use [[pixel detectors]]). In this case, the signature is a [[charged particle]] (from the decay of the R-hadron) whose trajectory is incompatible with the hypothesis of coming from the [[interaction vertex]].
*If the lifetime is such that the R-hadron can at least partially traverse a detector, more signatures are available:
**'''[[Bethe-Bloch formula|Energy loss]]''': if the hadronization of the gluino has produced a charged R-hadron, it will lose energy by [[ionization]] when traversing the detector material. The specific energy loss (''dE/dx'') follows the [[Bethe-Bloch formula]] and depends on the mass and the charge (as well as the momentum) of the particle, making a striking difference between a R-hadron and the background of ordinary particles produced normally in <math>pp</math> collisions.
**'''[[Time of flight]]''': since the gluino mass is expected to be of the order of the [[TeV]], the same holds for the R-hadrons. Such a high mass makes them [[relativistic particle|non-relativistic]] even at these high energies. While ordinary particles, at [[LHC]], have velocities very well approximable with the [[speed of light]], the velocity of a R-hadron can be significantly less. The time that it takes to reach the outer sub-detectors of a very large detector like [[ATLAS experiment|ATLAS]] or [[Compact Muon Solenoid|CMS]] can be then measurably longer than for the other particles produced in the same <math>pp</math> collision.
**'''Charge exchange''': while the previous two techniques can be applied to any other stable or [[quasi-stable]] heavy charged particle, this is specific of R-hadrons, making use of the fact that, being a [[composed particle]], the R-hadron can change sub-structure through [[nuclear force|nuclear interactions]] with the traversed material. For example, a R-hadron can exchange quarks with the nuclei of the detector, and any trade of an [[up quark]] with a [[down quark]] or vice versa will result in a variation of 1 in the charge.
Since some of the sub-detectors of a typical [[high energy physics|high-energy experiment]] are only sensitive to charged particles, one possible signature is the disappearance of the particle (going from charge +1 or -1 to 0) or vice versa its appearance, while keeping the same trajectory (since most of the momentum is carried by the heaviest component, i.e. the supersymmetric particle inside the R-hadron).
Another signature with very little background would come from the complete inversion of the charge (+1 into -1 or vice versa). Almost all [[tracking detectors]] at [[collider|high-energy colliders]] make use of a [[magnetic field]] and are then able to identify the charge of the particle by its curvature; a change of curvature along the trajectory would be recognized unambiguously as a ''flipper'', i.e. a particle whose charge has flipped.

== References ==

*[http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/Conferences/2005/MoriondQCD05_Kraan.pdf Interactions of R-hadrons in ATLAS]
*{{cite journal |arxiv=hep-ph/0409232|doi=10.1016/j.nuclphysb.2004.12.026|title=Aspects of Split Supersymmetry|year=2005|last1=Arkani-Hamed|first1=N.|last2=Dimopoulos|first2=S.|last3=Giudice|first3=G.F.|last4=Romanino|first4=A.|journal=Nuclear Physics B|volume=709|issue=1–2|pages=3–46|bibcode=2005NuPhB.709....3A|s2cid=16632949}}
{{Citizendium}}

{{Particles}}

[[Category:Hypothetical composite particles]]
[[Category:Supersymmetric quantum field theory]]

R-hadron - Revision history

imported>Dr vulpes: /* Theory */Reverting Spin edits as per Talk:Spin (particle physics) and Wikipedia talk:WikiProject Physics, replaced: Spin (particle physics) → Spin (physics)