85450376
Simple “invariance” of two-body decay kinematics
We study the two-body decay of a mother particle into a massless daughter. We further assume that the mother particle is unpolarized and has a generic boost distribution in the laboratory frame. In this case, we show analytically that the laboratory frame energy distribution of the massless decay product has a peak, whose location is identical to the (xed) energy of that particle in the rest frame of the corresponding mother particle. Given its simplicity and \invariance" under changes in the boost distribution of the mother particle, our nding should be useful for the determination of masses of mother particles. In particular, we anticipate that such a procedure will then not require a full reconstruction of this two-body decay chain (or for that matter, information about the rest of the event). With this eventual goal in mind, we make a proposal for extracting the peak position by tting the data to a well-motivated analytic function describing the shape of such an energy distribution. This tting function is then tested on the theoretical prediction for top quark pair production and its decay, and it is found to be quite successful in this regard. As a proof of principle of the usefulness of our observation, we apply it for measuring the mass of the top quark at the LHC, using simulated data and including experimental eects. PACS numbers: 11.80.Cr It is very well-known that in the rest frame of a mother particle undergoing a two-body decay, the energy of each of the daughter particles is xed in terms of mother and the daughter particle masses. Turning this fact around, we can determine the mass of the mother particle if we can measure these rest-frame energies of the daughter particles. However, often the mother particle is produced in the laboratory with a boost, that too with a magnitude and direction which is (a priori) not known. Moreover, the boost of mother particles produced at hadron colliders is dierent in each event. Such a boost distribution depends on the production mechanism of the particle and on the structure functions of the hadrons in the initial state of the collision, and is thus a complicated function. In turn, the fact that the mother has a dierent boost in each event implies that when we consider the observed energy of the two-body decay product in the laboratory frame, we get a distribution in it. Thus it seems like the information that was encoded in the rest frame energy is lost, and we are prevented from extracting (at least at an easily tractable level) the mass of the mother particle along the lines described above. We show that, remarkably, if one of the daughter particles from the two-body decay is massless and the mother is unpolarized, then such is not the case. Specically, in this case, we demonstrate that the distribution of the daughter particle’s energy in the laboratory frame has a peak precisely at its corresponding rest-frame energy. This result is interesting per se. Furthermore, we expect that it will lead to formulation of new methods for mass measurements. Obviously, for this purpose, we need to be able to determine the location of this peak accurately from the observed energy distribution of the massless daughter. To this end, we propose and motivate an analytic function that can be used to t the data on the energy distribution and thus extract the peak position. We show that this function is a suitable one using the top quark decay, t! W b, as a test case, namely, it ts very well the theory prediction for energy spectrum of the resulting b-jets. Simulating a realistic experimental situation, we then show that we can extract the value of the top mass from the position of the peak in the b-jet energy distribution along with the well-measured mass of the W boson. Let us consider the decay of a heavy particle B of mass mB, i.e., B! Aa where a is a massless visible particle. For the subsequent arguments, the properties of the particle A (other than its mass denoted by mA) are irrelevant. In the rest frame of particle B, the energy of the particle a is simply given by E = m 2 m 2 2mB :
118406159
Distinguishing dark matter stabilization symmetries using multiple kinematic edges and cusps
We emphasize that the stabilizing symmetry for dark matter (DM) particles does not have to be the commonly used parity (Z{sub 2}) symmetry. We therefore examine the potential of the colliders to distinguish models with parity stabilized DM from models in which the DM is stabilized by other symmetries. We often take the latter to be a Z{sub 3} symmetry for illustration. We focus on signatures where a single particle, charged under the DM stabilization symmetry decays into the DM and standard model (SM) particles. Such a Z{sub 3}-charged mother particle can decay into one or two DM particles along with the same SM particles. This can be contrasted with the decay of a Z{sub 2}-charged mother particle, where only one DM particle appears. Thus, if the intermediate particles in these decay chains are off-shell, then the reconstructed invariant mass of the SM particles exhibits two kinematic edges for the Z{sub 3} case but only one for the Z{sub 2} case. For the case of on-shell intermediate particles, distinguishing the two symmetries requires more than the kinematic edges. In this case, we note that certain decay chain topologies of the mother particle which are present for the Z{sub 3} casemore » (but absent for the Z{sub 2} case) generate a cusp in the invariant mass distribution of the SM particles. We demonstrate that this cusp is generally invariant of the various spin configurations. We further apply these techniques within the context of explicit models.« less
118459937
Using energy peaks to count dark matter particles in decays
Abstract We study the determination of the symmetry that stabilizes a dark matter (DM) candidate produced at colliders. Our question is motivated per se , and by several alternative symmetries that appear in models that provide a DM particle. To this end, we devise a strategy to determine whether a heavy mother particle decays into one visible massless particle and one or two DM particles. The counting of DM particles in these decays is relevant to distinguish the minimal choice of Z 2 from a Z 3 stabilization symmetry, under which the heavy particle and the DM are charged and the visible particle is not. Our method is novel in that it chiefly uses the peak of the energy spectrum of the visible particle and only secondarily uses the M T 2 endpoint of events in which the heavy mother particles are pair-produced. We present new theoretical results concerning the energy distribution of the decay products of a three-body decay, which are crucial for our method. To demonstrate the feasibility of our method in investigating the stabilization symmetry, we apply it in distinguishing the decay of a bottom quark partner into a b quark and one or two DM particles. The method can be applied generally to distinguish two- and three-body decays, irrespective of DM.
116971722
Polarized electrons at the Bates Linear Accelerator Center
A beam of polarized electrons have successfully been injected into the MIT Bates Linear Accelerator and accelerated it to 250 MeV. The intense beam was produced by photoemission from a GaAs crystal. The electron polarization at full energy, as measured by a brief test based on Moller scattering from a magnetized foil, was in excess of 30%. The peak intensity for the 15 ..mu..sec long pulses during the first test was about 2mA, representing about a third of the design value. The pulse rate of the accelerator was reduced to 60 Hz to minimize the total beam on the iron target. In a subsequent test, in which the beam hit a thick carbon target, the facility operated successfully at the full rate of 600 Hz. Under this condition, the average current on target was about 10 ..mu..A.
118099621
A measurement of the polarization asymmetry of the tau lepton using the L3 detector at LEP
With a data sample of e e events collected in the L detector at LEP during we did a measurement of the polarization of leptons as a function of the production polar angle with respect to the incident e beam direction We used the following prong decay channels e e and a As the result we obtained the asymmetries A and Ae Combining this with the previous data measurement by L we ob tained A and Ae These asymmetries gave the ratio of vector to axial vector weak neutral coupling con stants for electrons as g V g e A and for taus as g V g A The numbers are consistent with the hypothesis of e lepton universality Assuming the e neutral current universality the e ective electroweak mixing angle is calculated as sin e w Advisor Professor Aihud Pevsner
27444996
Conservative, gravitational self-force for a particle in circular orbit around a Schwarzschild black hole in a radiation gauge
This is the second of two companion papers on computing the self-force in a radiation gauge; more precisely, the method uses a radiation gauge for the radiative part of the metric perturbation, together with an arbitrarily chosen gauge for the parts of the perturbation associated with changes in black-hole mass and spin and with a shift in the center of mass. In a test of the method delineated in the first paper, we compute the conservative part of the self-force for a particle in circular orbit around a Schwarzschild black hole. The gauge vector relating our radiation gauge to a Lorenz gauge is helically symmetric, implying that the quantity h_(αβ)u^(α)u^(β) must have the same value for our radiation gauge as for a Lorenz gauge; and we confirm this numerically to one part in 10^(14). As outlined in the first paper, the perturbed metric is constructed from a Hertz potential that is in a term obtained algebraically from the retarded perturbed spin-2 Weyl scalar, ψ_(0)^(ret). We use a mode-sum renormalization and find the renormalization coefficients by matching a series in L= l + 1/2 to the large-L behavior of the expression for the self-force in terms of the retarded field h_(αβ)^(ret); we similarly find the leading renormalization coefficients of h_(αβ)u^(α)u^(β) and the related change in the angular velocity of the particle due to its self-force. We show numerically that the singular part of the self-force has the form f^(S)_(α) = , the part of ∇_(α)ρ^(-1) that is axisymmetric about a radial line through the particle. This differs only by a constant from its form for a Lorenz gauge. It is because we do not use a radiation gauge to describe the change in black-hole mass that the singular part of the self-force has no singularity along a radial line through the particle and, at least in this example, is spherically symmetric to subleading order in ρ.
118786272
Non SUSY Searches at Tevatron
We present results on physics beyond Standard Model except SUSY with data mostly corresponding to an integrated luminosity of 200 ∼ 300 pb−1 in pp collisions at s = 1.96 TeV at the CDF and D0, Fermilab Tevatron collider experiments. New results on the searches for new phenomena include extra Z′ and W′ bosons, extra dimensions, leptoquarks, excited electrons as well as monopoles. We found no evidence and set the stringent limits on the signature and models.
117312767
Light Higgs Scenario in BMSSM and LEP Precision Data
In this Letter we consider very light Higgs fields in BMSSM(Beyond MSSM). The spectrum below TeV scale is the same as the MSSM but the Higgs potential is modified and is well described in terms of effective dimension five and six operators. A correction from the BMSSM operators allows us to consider new parameter space of Higgs sector which is not allowed in the MSSM. It can be regarded as a constrained version of general 2 Higgs doublet model (2HDM) as long as Higgs sector is concerned. We focus on the possibility that CP odd Higgs (A) mass is about 7 or 8 GeV and charged Higgs mass is comparable to W mass. At the same time one of the CP even Higgs (h) is light enough such that h and A production at the Z pole is kinematically allowed. The tension between forward backward asymmetry of bottom quark Ab_FB measured at LEP and the Standard Model prediction can be ameliorated if bottom quark pair produced from light CP even Higgs is taken into account.