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high transverse momentum ( @xmath0 ) processes in quantum chromodynamics ( qcd ) have been suggested as a probe to investigate the physics of ultrarelativistic heavy - ion ( a - a ) collisions more than a decade ago and have become a cornerstone of the experimental heavy - ion program both at rhic and lhc . yet , compared with for instance the modelling of the bulk medium evolution using hydrodynamics where an era of precision fits to differential data has begun ( see e.g. @xcite ) , theoretical progress in the physics of hard probes is slow .    partially , this can be attributed to the structure of the problem : hard probes are at the same time sensitive to the microscopical degrees of freedom of the medium and the macroscopical medium density distribution and evolution .
thus high @xmath0 observables have the capability to potentially do both tomography and transport coefficient measurements in the medium , i.e. to constrain its microscopical and macroscopical dynamics .
while the microscopical physics of the medium and hence the basic structure of the parton - medium interaction is currently in detail unknown , the density evolution ( in terms of fluid - dynamical modelling ) is constrained by bulk data , but not unambiguously determined . as a result , the description of any single observable in terms of a parton - medium interaction scenario and a medium evolution model is usually far from unique and does not allow a firm conclusion with regard to either question .
however , systematic multi - observable studies of a matrix of parton - medium interaction and medium evolution models can reduce the allowed model space significantly , resulting in a class of models which shows good consistency across rhic and lhc observables .
in vacuum , at the core of the computation of hard processes in perturbative qcd ( pqcd ) is the factorization of the hard , short - distance interaction part and the long - distance physics of initial state parton distributions and final state parton showers . in a medium ,
qcd factorization is not strictly proven , but remains a plausible working assumption .
a sufficiently large separation of hard and thermal momentum scale then ensures that the hard process itself receives no medium corrections and can serve as a standard - candle , producing high @xmath1 partons at a known and calculable rate .
the partons emerging from the hard process are typically characterized by a large virtuality scale @xmath2 .
if one takes the uncertainty relation to estimate the timescale at which intermediate shower states at lower virtuality can be formed , one finds @xmath3 with @xmath4 the virtual parton energy . comparing this scale with typical medium lifetimes @xmath5 fm
, one finds that for typical rhic and lhc kinematics a significant part of the partonic shower evolution ( i.e. with @xmath6 ) takes place in the medium , with the shower being gradually boosted out of the medium at top lhc energies and on the other hand hadrons with large mass @xmath7 being produced in the medium .
however , over a large kinematical range and for sufficiently light hadrons it can reasonably be assumed that the medium modification largely concerns the evolution of a parton shower with subsequent hadronization as in vacuum .    as a baseline example for in - medium shower modelling , consider a virtuality - ordered parton shower such as implemented in the pyshow algorithm @xcite
this is treated as an interated series of splittings of a parent into two daughter partons @xmath8 where @xmath9 and @xmath10 and where the virtuality of partons in terms of @xmath11 decreases in each branching .
the splitting probability is given by        where @xmath13 are perturbatively calculable objects , the so - called splitting kernels .
the splittings cease when the parton virtuality reaches a non - perturbative lower scale @xmath14 gev at which point a hadronization model needs to be used . in this way
, the original high virtuality of the shower initiator is converted into the transverse momentum distribution of the final shower partons ( and the initial virtuality sets a kinematic bound for observables like jet shapes ) .
an important thing to note is that the splitting kernels @xmath13 are scale invariant , i.e. they do not depend on an absolute momentum scale . as a result ,
the fragmentation functions generated by a series of splittings is self - similar ( the momentum distribution of hadrons in a subjet looks almost the same as the distribution in the whole jet when plotted as a function of @xmath15 ) and up to logarithmic corrections has no strong energy dependence .    estimating the timescale associated with the initial , hard branchings which largely determine jet shape and subjet structure , one finds @xmath16 fm , i.e. well before any medium can be formed .
this gives rise to the expectation that subject structure and jet shape at high @xmath0 should not be very sensitive to medium modifications .
there are two main processes by which this pattern can be modified in a medium : 1 ) direct loss of energy ( by elastic collisions , drag ,  ) into the medium as parametrized by a transport coefficient @xmath17 and 2 ) enhanced soft gluon radiation by medium - induced virtuality as parametrized by a transport coefficient @xmath18 .
the first mechanism is in general incoherent and the energy is lost into the medium , the second mechanism involves a de - coherence time for the radiated gluon and transfers energy from the leading parton into an increased number of subleading shower partons .
note that this distinction can be made on average on the basis of transport coefficients , but not for a single interaction graph with the medium which may involve both direct and radiative energy loss .
a recently suggested additional mechanism is the modification of color flow by the medium , leading to a modification of the hadronic shower evolution @xcite .
models may now be classified based on what part of the dynamics they include , how they treat the medium and how they solve the shower evolution equations .
the most complete approach are models which simulate the whole in - medium shower evolution such as jewel @xcite , yajem @xcite , q - pythia @xcite or resummed higher twist ( ht ) @xcite . however , if one is only interested in observables which probe the leading shower fragments , such as single inclusive high @xmath0 hadron suppression , which probe dominantly showers in which a large fraction of the energy flows through a single parton , then leading parton energy loss modelling in which the medium evolution is cast into an energy loss probability @xmath19 before vacuum fragmentation is applied is sufficient .
models of this type include the asw @xcite , amy @xcite or whdg @xcite frameworks .
finally , in hybrid approaches a vacuum shower is computed down to some scale , then the produced partons are put on - shell and each propagated through the medium using a leading - parton energy loss model .
an example for hybrid modelling is martini @xcite .    of considerable interest
is how the medium is included into the modelling process .
a number of models come with an explicit treatment of medium partons and their interaction with shower partons , typically as a thermal gas of quark and gluon quasiparticles @xcite .
such a description requires that the medium is , at least to some degree , perturbatively tractable , which is not in line with the basic assumption underlying almost ideal hydrodynamical modelling which assumes that the medium is strongly coupled and the mean free path essentially vanishes .
usually an explicit treatment of interactions with medium partons breaks the scale invariance of the fragmentation function around the momentum scale of the medium partons and below , while it largely remains intact above .
in addition , the medium provides additional kinematic phase space for transverse shower broadening .
a different approximation is to cast all medium effects into a modification of the splitting probability , in essence replacing @xmath20 with the detailed form of @xmath21 motivated by a specific physics scenario as done e.g. in q - pythia @xcite .
such a prescription explicitly conserves energy and momentum inside the shower , i.e. there is no additional phase space for transverse broadening .
it also perserves scale invariance of the fragmentation function , but it alters its shape to a different functional form .    finally , the medium can be ( without any explicit reference to its degrees of freedom ) appear via transport coefficients such as @xmath22 and alter the kinematics of propagating partons as implemented in yajem @xcite .
this again breaks scale invariance of the fragmentation function around the medium momentum scale and below and leads to additional phase space for transverse broadening .
there are two main strategies to solve the shower evolution equations  analytical and mote - carlo ( mc ) techniques . in general , analytical techniques
get exact treatment of quantum interference effects while they often rely on kinematic approximations such as eikonal propagation or infinite parent parton energy which violate energy - momentum conservation .
in contrast , mc frameworks usually have exact kinematics , but have to resort to phenomenological prescriptions for interference effects .
it is immediately evident that the combination of all observables cuts the available model space down much more than any single observable .
most of the constraints are provided by the combination of the @xmath0 dependence of @xmath23 at lhc which probes the dynamics of leading partons in combination with a dihadron correlation @xmath24 which is a probe of subleading fragment dynamics .
current jet measurements do not have strong constraining power beyond this , however they do highlight the need to go beyond a purely probabilistic implementation of medium modification and also provide constraints for transverse dynamics of showers .
a number of conclusions with regard to the nature of jet - medium interaction can be drawn from this :    * the combined data is compatible with a fairly standard pqcd picture of medium - induced radiation and a subleading component of elastic interactions leading to energy transfer into the medium * there is no evidence for exotic scenarios , the data neither indicate an @xmath25 dependence of energy loss as suggested by some variants of ads / cft inspired models nor a modification of the hadronization stage . *
data from rhic and lhc consistently indicate that some moderate amount of energy is transferred from the hard parton directly into the non - perturbative medium rather than into subleading jet fragments .
however , the amount of direct energy transfer is constrained by pathlength - dependent observables and found to be significantly smaller than if the medium can be described as a near - ideal quark - gluon gas .
this may imply that the observed degrees of freedom in the medium are massive or correlated quasiparticles .    to make these conclusions more quantitative ,
further high - statistics multi - differential measurements are needed from the experimental side , followed by systematic multi - observable studies from theory .
in particular , the systematics of h - h , jet - h and @xmath26-h correlations ( with decreasing geometry bias ) as a function of the reaction plane angle for various system centralities is expected to be a sensitive probe of both the parton - medium interaction mechanism and the medium density evolution .
once these questions are reliably addressed , the precise nature of the mechanism by which the energy deposited from hard partons is carried by the medium presents itself as the next goal .
high @xmath0 measurements of hard hadrons or jets at rhic and lhc appear contradictory and in some cases counter - intuitive , but upon closer investigation they represent a coherent picture of jet - medium interaction physics which can be established with systematic comparisons of models against a large body of data . 
 this picture is consistent with a perturbative qcd mechanism and does not require exotic assumptions . 
 this overview outlines how several key measurements each partially constrain shower - medium interaction physics and how from the sum of those the outlines of the mechanism of jet quenching can be deduced . 
 most current jet results from lhc can be naturally understood in this picture . 
 a short summary of what can be established about the nature of parton - medium interaction with current data is given in the end .    
 jet quenching