Research

Large Hadron Collider Mission Summary

The Large Hadron Collider (LHC) experiment housed at the CERN physics laboratory in Geneva, Switzerland was constructed to facilitate a view of unprecedented detail into the deeply sub-microscopic, ultra high-energy world of fundamental particle interactions. Among the investigative targets considered most likely for discovery, two key goals motivated all design decisions: finding evidence for the Higgs boson and Supersymmetry (SUSY). Both had, at that point, the status of purely theoretical suggestions, made to elegantly solve problems in the interpretation of other results.

The Higgs had been introduced to provide mass to particles that must otherwise go without. Specifically, massive particles (which travel at speeds slower than that of light) must allow for the interaction of both left- and right-handed varieties, that being a geometrical description of the particle's spin rotation axis orientation relative to its direction of motion. Said another way, one can always travel faster (in principle) than a massive particle, and by looking "backward" at it, reinterpret a formerly left-handed particle as right-handed, and vice versa. However, matter particles such as the electron strangely behave differently with respect to the weak nuclear force, i.e. radioactive decay, depending on whether they are of the right- or left-handed type; in fact, the right-handed subset do not participate in this interaction at all. This sharp distinction implies that the two "chiralities" are equipped, in some sense, with different interfaces (representation indices), such that they may not directly interact (mix), and thus cannot have a tangible mass. The proposed solution was to allow instead for an indirect interaction via a third party intermediary, i.e. the Higgs boson, which would function as a sort of adapter between the mismatched particles. In a "spontaneous symmetry breaking" procedure, the stable Higgs field configuration would hypothetically shift away from the nominal value of zero, transferring a constant parcel of energy out of the quantum vacuum to the paired left- and right-handed particles, endowing them with newfound mass. Similarly, the W and Z bosons exchanged in interactions of the weak nuclear type also would inherit a mass, rendering the corresponding force short-range and extremely weak, as is observed.

SUSY was proposed to cope with the problem that quantum calculations will always push masses and interaction scales up to some upper bound, highly disfavoring the coexistence of intermediate or widely separated masses such as we observe in Nature, e.g. the lightness of the proton compared to the heaviness of the fundamental "Planck" mass that is inferred out of the gravitational interaction, which is some 1019 times heavier. The mechanism for keeping these scales distinct relies an exact cancellation of all positive quantum corrections by a perfectly matched negative term. It was realized that pairs of particles separated by a half unit of spin angular momentum, but with all other properties identical, would naturally provide this precise effect. Thus, SUSY speculates that all known particles have a yet unseen partner; however, the very fact that the SUSY partners have not been discovered implies that they are substantially heavier, and thus that SUSY is also a broken or imperfect symmetry, allowing instead for only a near cancellation of the quantum shifts, pushing the heavier member of each pairing outside the observational limits of past experiments. Modeled frameworks for systematically anticipating the overarching structure that relates the mass shifts inherited by each SUSY partner are an active area of physics research.

Discovery of SUSY and the Higgs boson thus both involve the production of new heavy particles, which is in fact the special design purpose of the LHC apparatus. An enormous seventeen-mile circumference is employed to allow for a reasonably "gentle" turning of opposing proton beams, which have been accelerated to travel at speeds exceedingly close to that of light itself. At such unbelievable speeds, the kinetic motion energy of each proton is approximately 4,000 times larger than its Einstein "mc2" mass energy. For comparison, a fighter jet travelling at the speed of sound has a kinetic energy that is only about 6×10-10 % of its mass energy. In the resulting impact, this tremendous kinetic energy may be converted into the masses of new particles, which are created in the collision event itself. There are great difficulties, however, that limit how one may observe and interpret the results of such collisions. The particles of interest decay exceedingly rapidly, and are not collected directly. Instead, one can only measure the complicated packets of debris that emerge from the proton-proton collisions, which may have cascaded into dozens or even hundreds of individual particle objects, and attempt to reconstruct the character of the initial event in reverse. Moreover, any weakly interacting particles will escape the detector unseen, and it is instead the lack of an observation (a momentum imbalance) that characterizes such objects. Complicating matters further, components of momentum parallel to the beam cannot be measured, and the most interesting event signatures may feature pairs of escaped particles, such that only their transverse momentum vector sum can be ascertained, and that imperfectly. Consequently, any given observable result may correspond to an innumerably large number of hidden internal processes, many of which may arise as well from only previously known physics; this background competition will, by definition as the easier target, tend to dominate and swamp all indications of new results, and must be radically suppressed by novel event selection discriminants (cuts). No single observation can thus be responsible for a new discovery. On the contrary, out of the untold quadrillions of proton-proton collision events to be ultimately registered, conclusions can emerge only from a cumulative statistical analysis that carefully weighs the observed count of events carrying a certain signature against the reasonably calibrated expectation for that same signature when given only already well understood internal processes.

The recent discovery of the Higgs boson at the LHC, and in particular the isolation of its mass at around 125 GeV, constitutes a vital new constraint on proposed frameworks for unifying the fundamental laws of nature. Taken in conjunction with space telescope observations of the dark matter density, experimental limits on the rates of certain extremely rare interactions, and emerging results for the SUSY search at the LHC, this data strongly disfavors many models that were very recently considered to be leading candidates for describing new physics.

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