R-hadrons are hypothetical particles composed of asupersymmetric particle and at least onequark.

Only a few of the currentsupersymmetry theories predict the existence of R-hadrons, since in most of theparameter space all the supersymmetric particles are so separated in mass that their decays are very fast (with the exception of theLSP, which is stable in all the SUSY theories withR-parity).

R-hadrons are possible when a colored (in the sense ofQCD) supersymmetric particle (e.g., agluino or asquark) has amean lifetime longer than the typicalhadronization time scale, and so QCD bound states are formed with ordinarypartons (quarks andgluons), in analogy with the ordinaryhadrons.

One example of a theory predicting observable R-hadrons isSplit SUSY.Its main feature is, in fact, that all the newbosons are at a very high mass scale, and only the newfermions are at theTeV scale, i.e. accessible by theATLAS andCMS experiments in${\displaystyle pp}$ collisions atLHC.One of such new fermions would be thegluino (spin 1/2, as dictated for thesupersymmetric partner of a spin 1 boson, thegluon).The gluino, being colored, can only decay to other colored particles. ButR-parity prevents a direct decay to quarks and/or gluons, and on the other hand the only other colored supersymmetric particles are thesquarks, 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 avirtual 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 aparticle detector, instead of deducing it by reconstructing itsdecay chain or by themomentum imbalance (as in the case of theLSP).

In other theories belonging to the SUSY family, the same role can be played by the lightestsquark (usually thestop, i.e. the partner of thetop quark).

In the following, for sake of illustration, the R-hadron will be assumed to originate from a gluino created in a${\displaystyle pp}$ collision atLHC, but the observational features are completely general.

• If the lifetime of an R-hadron is of the order of thepicosecond, it decays before reaching the first sensitive layers of atracking detector but can be recognized by thesecondary vertex technique, particularly efficient inATLAS andCMS thanks to their precisevertex detectors (both experiments usepixel detectors). In this case, the signature is acharged particle (from the decay of the R-hadron) whose trajectory is incompatible with the hypothesis of coming from theinteraction vertex.
• If the lifetime is such that the R-hadron can at least partially traverse a detector, more signatures are available:
  • Energy loss: if the hadronization of the gluino has produced a charged R-hadron, it will lose energy byionization when traversing the detector material. The specific energy loss (dE/dx) follows theBethe-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${\displaystyle pp}$ collisions.
  • Time of flight: since the gluino mass is expected to be of the order of theTeV, the same holds for the R-hadrons. Such a high mass makes themnon-relativistic even at these high energies. While ordinary particles, atLHC, have velocities very well approximable with thespeed 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 likeATLAS orCMS can be then measurably longer than for the other particles produced in the same${\displaystyle pp}$ collision.
  • Charge exchange: while the previous two techniques can be applied to any other stable orquasi-stable heavy charged particle, this is specific of R-hadrons, making use of the fact that, being acomposed particle, the R-hadron can change sub-structure throughnuclear interactions with the traversed material. For example, a R-hadron can exchange quarks with the nuclei of the detector, and any trade of anup quark with adown quark or vice versa will result in a variation of 1 in the charge.

Since some of the sub-detectors of a typicalhigh-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 alltracking detectors athigh-energy colliders make use of amagnetic 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 aflipper, i.e. a particle whose charge has flipped.

• Interactions of R-hadrons in ATLAS
• Arkani-Hamed, N.; Dimopoulos, S.; Giudice, G.F.; Romanino, A. (2005). "Aspects of Split Supersymmetry".Nuclear Physics B.709 (1–2):3–46.arXiv:hep-ph/0409232.Bibcode:2005NuPhB.709....3A.doi:10.1016/j.nuclphysb.2004.12.026.S2CID 16632949.

This article incorporates material from theCitizendium article "R-hadron", which is licensed under theCreative Commons Attribution-ShareAlike 3.0 Unported License but not under theGFDL.

• Hypothetical composite particles
• Supersymmetric quantum field theory

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