DOCSLIB.ORG

Physics 215B: Particles and Fields Winter 2017

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Physics 215B: Particles and Fields Winter 2017 Physics 215B: Particles and Fields Winter 2017 Lecturer: McGreevy These lecture notes live here and are currently growing. Please email corrections to mcgreevy at physics dot ucsd dot edu. Last updated: 2017/03/15, 23:19:56 1 Contents 0.1 Sources....................................3 0.2 Conventions.................................4 6 To infinity and beyond5 6.1 A parable from quantum mechanics on the breaking of scale invariance5 6.2 A simple example of perturbative renormalization in QFT....... 13 6.3 Classical interlude: Mott formula..................... 15 6.4 Electron self-energy in QED........................ 20 6.5 Big picture interlude............................ 25 6.6 Vertex correction in QED......................... 34 6.7 Vacuum polarization............................ 45 7 Consequences of unitarity 54 7.1 Spectral density............................... 54 7.2 Cutting rules and optical theorem..................... 61 7.3 How to study hadrons with perturbative QCD.............. 67 8 A parable on integrating out degrees of freedom 69 9 The Wilsonian perspective on renormalization 78 9.1 Where do field theories come from?.................... 78 9.2 The continuum version of blocking.................... 86 9.3 An extended example: XY model..................... 89 9.4 Which bits of the beta function are universal?.............. 110 10 Gauge theory 116 10.1 Massive vector fields as gauge fields.................... 116 10.2 Festival of gauge invariance........................ 118 10.3 Lattice gauge theory............................ 122 2 0.1 Sources The material in these notes is collected from many places, among which I should mention in particular the following: Peskin and Schroeder, An introduction to quantum field theory (Wiley) Zee, Quantum Field Theory (Princeton, 2d Edition) Banks, Modern Quantum Field Theory: A Concise Introduction (Cambridge) Schwartz, Quantum field theory and the standard model (Cambridge) David Tong's lecture notes Many other bits of wisdom come from the Berkeley QFT courses of Prof. L. Hall and Prof. M. Halpern. 3 0.2 Conventions Following most QFT books, I am going to use the + − −− signature convention for the Minkowski metric. I am used to the other convention, where time is the weird one, so I'll need your help checking my signs. More explicitly, denoting a small spacetime displacement as dxµ ≡ (dt; d~x)µ, the Lorentz-invariant distance is: 0 1 +1 0 0 0 B 0 −1 0 0 C 2 2 µ ν µν B C ds = +dt − d~x · d~x = ηµνdx dx with η = ηµν = B C : @ 0 0 −1 0 A 0 0 0 −1 µν µ @ ~ µ µν (spacelike is negative). We will also write @µ ≡ @xµ = @t; rx , and @ ≡ η @ν. I'll use µ, ν::: for Lorentz indices, and i; k; ::: for spatial indices. The convention that repeated indices are summed is always in effect unless otherwise indicated. A consequence of the fact that english and math are written from left to right is that time goes to the left. h A useful generalization of the shorthand ~ ≡ 2π is dk ¯dk ≡ : 2π I will also write =δd(q) ≡ (2π)dδ(d)(q). I will try to be consistent about writing Fourier transforms as Z ddk Z eikxf~(k) ≡ ¯ddk eikxf~(k) ≡ f(x): (2π)d IFF ≡ if and only if. RHS ≡ right-hand side. LHS ≡ left-hand side. BHS ≡ both-hand side. IBP ≡ integration by parts. WLOG ≡ without loss of generality. +O(xn) ≡ plus terms which go like xn (and higher powers) when x is small. +h:c: ≡ plus hermitian conjugate. We work in units where ~ and the speed of light, c, are equal to one unless otherwise noted. When I say `Peskin' I usually mean `Peskin & Schroeder'. Please tell me if you find typos or errors or violations of the rules above. 4 6 To infinity and beyond Last quarter we ended at a high point, computing the amplitudes and cross-sections for many processes using QED. More precisely, we studied the leading-order-in-α am- plitudes, using Feynman diagrams which were trees { no loops. The natural next step is to look at the next terms in the perturbation expansion in α, which come from dia- grams with one loop. When we do that we're going to encounter some confusing stuff, in fact some of the same confusing stuff we encountered in thinking about Casimir forces at the beginning of last quarter. We didn't encounter these short-distance issues in studying tree-level diagrams because in a tree-level diagram, the quantum numbers (and in particular the momenta) of the intermediate states are fixed by the external states. In contrast, once there is a loop, there are undetermined momenta which must be summed, and this sum includes, it seems, arbitrarily high momentum modes, about which surely we have no information yet. In order to put ourselves in the right frame of mind to think about that stuff, let's make a brief retreat to systems with finitely many degrees of freedom. Then we'll apply some of these lessons to a toy field theory example (scalar field theory). Then we'll come back to perturbation theory in QED. Reading assignment for this chapter: Zee xIII. 6.1 A parable from quantum mechanics on the breaking of scale invariance Recall that the coupling constant in φ4 theory in D = 3 + 1 spacetime dimensions is dimensionless, and the same is true of the electromagnetic coupling e in QED in D = 3+1 spacetime dimensions. In fact, the mass parameters are the only dimensionful quantities in those theories, at least in their classical avatars. This means that if we ignore the masses, for example because we are interested in physics at much higher energies, then these models seem to possess scale invariance: the physics is unchanged under zooming in. Here we will study a simple quantum mechanical example (that is: an example with a finite number of degrees of freedom)1 with such (classical) scale invariance. It exhibits many interesting features that can happen in strongly interacting quantum field theory { asymptotic freedom, dimensional transmutation. Because the model is simple, we can understand these phenomena without resort to perturbation theory. 1I learned this example from Marty Halpern. 5 They will nevertheless illuminate some ways of thinking which we'll need in examples where perturbating is our only option. Consider the following (`bare') action: Z 1 Z 1 S[q] = dt ~q_2 + g δ(2)(~q) ≡ dt ~q_2 − V (~q) 2 0 2 where ~q = (x; y) are two coordinates of a quantum particle, and the potential involves (2) δ (~q) ≡ δ(x)δ(y), a Dirac delta function. I chose the sign so that g0 > 0 is attractive. (Notice that I have absorbed the inertial mass m in 1 mv2 into a redefinition of the p 2 variable q, q ! mq.) First, let's do dimensional analysis (always a good idea). Since ~ = c = 1, all dimensionful quantites are some power of a length. Let −[X] denote the number of powers of length in the units of the quantity X; that is, if X ∼ (length)ν(X) then we have [X] = −ν(X), a number. We have: [t] = [length=c] = −1 =) [dt] = −1: The action appears in exponents and is therefore dimensionless (it has units of ~), so we had better have: 0 = [S] = [~] and this applies to each term in the action. We begin with the kinetic term: Z 0 = [ dt~q_2] =) 1 1 [~q_2] = +1 =) [~q_] = + =) [~q] = − : 2 2 Since 1 = R dqδ(q), we have 0 = [dq] + [δ(q)] and D [δD(~q)] = −[q]D = ; and in particular [δ2(~q)] = 1: 2 This implies that the naive (\engineering") dimensions of the coupling constant g0 are [g0] = 0 { it is dimensionless. Classically, the theory does not have a special length scale; it is scale invariant. The Hamiltonian associated with the Lagrangian above is 1 H = p2 + p2 + V (~q): 2 x y 6 Now we treat this as a quantum system. Acting in the position basis, the quantum Hamiltonian operator is 2 H = −~ @2 + @2 − g δ(2)(~q) 2 x y 0 2 ~ 2 So in the Schr¨odingerequation H = − 2 r + V (~q) = E , the second term on the LHS is (2) V (~q) (~q) = −g0δ (~q) (0): To make it look more like we are doing QFT, let's solve it in momentum space: Z d2p i~p·~q=~ (~q) ≡ 2 e '(~p) (2π~) The delta function is Z d2p (2) i~p·~q=~ δ (q) = 2 e : (2π~) So the Schr¨odingerequation says 1 − r2 − E (q) = −V (q) (q) 2 Z p2 ¯d2peip·q − E '(p) = +g δ2(q) (0) 2 0 Z 2 ip·q = +g0 ¯d pe (0) (6.1) which (integrating the both-hand side of (6.1) over q: R d2qeip·q ((6.1)) ) says 2 Z 2 0 ~p d p 0 − E '(~p) = +g0 2 '(~p ) 2 (2π~) | {z } = (0) There are two cases to consider: • (~q = 0) = R ¯d2p'(~p) = 0. Then this case is the same as a free theory, with the constraint that (0) = 0, ~p2 − E '(~p) = 0 2 i.e. plane waves which vanish at the origin, e.g. / sin pxx e±ipyy=~. These scat- ~ tering solutions don't see the delta-function potential at all. 7 • (0) ≡ α 6= 0, some constant to be determined. This means ~p2=2 − E 6= 0, so we can divide by it : g Z g '(~p) = 0 ¯d2p0'(~p0) = 0 α: ~p2 ~p2 2 − E 2 − E The integral on the RHS (for (0) = α) is a little problematic if E > 0, since then there is some value of p where p2 = 2E. Avoid this singularity by going to the boundstate region: consider E = −B < 0. So: g '(~p) = 0 α: ~p2 2 + B What happens if we integrate this R ¯d2p to check self-consistency { the LHS should give α again: ! Z Z g 0=! ¯d2p'(~p) 1 − ¯d2p 0 ~p2 + B | {z } 2 = (0)=α6=0 Z g =) ¯d2p 0 = 1 ~p2 2 + B is a condition on the energy B of possible boundstates.
Load more

Recommended publications
  • The Lamb Shift Experiment in Muonic Hydrogen
    The Lamb Shift Experiment in Muonic Hydrogen Dissertation submitted to the Physics Faculty of the Ludwig{Maximilians{University Munich by Aldo Sady Antognini from Bellinzona, Switzerland Munich, November 2005 1st Referee : Prof. Dr. Theodor W. H¨ansch 2nd Referee : Prof. Dr. Dietrich Habs Date of the Oral Examination : December 21, 2005 Even if I don't think, I am. Itsuo Tsuda Je suis ou` je ne pense pas, je pense ou` je ne suis pas. Jacques Lacan A mia mamma e mio papa` con tanto amore Abstract The subject of this thesis is the muonic hydrogen (µ−p) Lamb shift experiment being performed at the Paul Scherrer Institute, Switzerland. Its goal is to measure the 2S 2P − energy difference in µp atoms by laser spectroscopy and to deduce the proton root{mean{ −3 square (rms) charge radius rp with 10 precision, an order of magnitude better than presently known. This would make it possible to test bound{state quantum electrody- namics (QED) in hydrogen at the relative accuracy level of 10−7, and will lead to an improvement in the determination of the Rydberg constant by more than a factor of seven. Moreover it will represent a benchmark for QCD theories. The experiment is based on the measurement of the energy difference between the F=1 F=2 2S1=2 and 2P3=2 levels in µp atoms to a precision of 30 ppm, using a pulsed laser tunable at wavelengths around 6 µm. Negative muons from a unique low{energy muon beam are −1 stopped at a rate of 70 s in 0.6 hPa of H2 gas.
    [Show full text]
  • Radiative Corrections in Curved Spacetime and Physical Implications to the Power Spectrum and Trispectrum for Different Inflationary Models
    Radiative Corrections in Curved Spacetime and Physical Implications to the Power Spectrum and Trispectrum for different Inflationary Models Dissertation zur Erlangung des mathematisch-naturwissenschaftlichen Doktorgrades Doctor rerum naturalium der Georg-August-Universit¨atG¨ottingen Im Promotionsprogramm PROPHYS der Georg-August University School of Science (GAUSS) vorgelegt von Simone Dresti aus Locarno G¨ottingen,2018 Betreuungsausschuss: Prof. Dr. Laura Covi, Institut f¨urTheoretische Physik, Universit¨atG¨ottingen Prof. Dr. Karl-Henning Rehren, Institut f¨urTheoretische Physik, Universit¨atG¨ottingen Prof. Dr. Dorothea Bahns, Mathematisches Institut, Universit¨atG¨ottingen Miglieder der Prufungskommission:¨ Referentin: Prof. Dr. Laura Covi, Institut f¨urTheoretische Physik, Universit¨atG¨ottingen Korreferentin: Prof. Dr. Dorothea Bahns, Mathematisches Institut, Universit¨atG¨ottingen Weitere Mitglieder der Prufungskommission:¨ Prof. Dr. Karl-Henning Rehren, Institut f¨urTheoretische Physik, Universit¨atG¨ottingen Prof. Dr. Stefan Kehrein, Institut f¨urTheoretische Physik, Universit¨atG¨ottingen Prof. Dr. Jens Niemeyer, Institut f¨urAstrophysik, Universit¨atG¨ottingen Prof. Dr. Ariane Frey, II. Physikalisches Institut, Universit¨atG¨ottingen Tag der mundlichen¨ Prufung:¨ Mittwoch, 23. Mai 2018 Ai miei nonni Marina, Palmira e Pierino iv ABSTRACT In a quantum field theory with a time-dependent background, as in an expanding uni- verse, the time-translational symmetry is broken. We therefore expect loop corrections to cosmological observables to be time-dependent after renormalization for interacting fields. In this thesis we compute and discuss such radiative corrections to the primordial spectrum and higher order spectra in simple inflationary models. We investigate both massless and massive virtual fields, and we disentangle the time dependence caused by the background and by the initial state that is set to the Bunch-Davies vacuum at the beginning of inflation.
    [Show full text]
  • Scalar Quantum Electrodynamics
    A complex system that works is invariably found to have evolved from a simple system that works. John Gall 17 Scalar Quantum Electrodynamics In nature, there exist scalar particles which are charged and are therefore coupled to the electromagnetic field. In three spatial dimensions, an important nonrelativistic example is provided by superconductors. The phenomenon of zero resistance at low temperature can be explained by the formation of so-called Cooper pairs of electrons of opposite momentum and spin. These behave like bosons of spin zero and charge q =2e, which are held together in some metals by the electron-phonon interaction. Many important predictions of experimental data can be derived from the Ginzburg- Landau theory of superconductivity [1]. The relativistic generalization of this theory to four spacetime dimensions is of great importance in elementary particle physics. In that form it is known as scalar quantum electrodynamics (scalar QED). 17.1 Action and Generating Functional The Ginzburg-Landau theory is a three-dimensional euclidean quantum field theory containing a complex scalar field ϕ(x)= ϕ1(x)+ iϕ2(x) (17.1) coupled to a magnetic vector potential A. The scalar field describes bound states of pairs of electrons, which arise in a superconductor at low temperatures due to an attraction coming from elastic forces. The detailed mechanism will not be of interest here; we only note that the pairs are bound in an s-wave and a spin singlet state of charge q =2e. Ignoring for a moment the magnetic interactions, the ensemble of these bound states may be described, in the neighborhood of the superconductive 4 transition temperature Tc, by a complex scalar field theory of the ϕ -type, by a euclidean action 2 3 1 m g 2 E = d x ∇ϕ∗∇ϕ + ϕ∗ϕ + (ϕ∗ϕ) .
    [Show full text]
  • Casimir Effect Mechanism of Pairing Between Fermions in the Vicinity of A
    Casimir effect mechanism of pairing between fermions in the vicinity of a magnetic quantum critical point Yaroslav A. Kharkov1 and Oleg P. Sushkov1 1School of Physics, University of New South Wales, Sydney 2052, Australia We consider two immobile spin 1/2 fermions in a two-dimensional magnetic system that is close to the O(3) magnetic quantum critical point (QCP) which separates magnetically ordered and disordered phases. Focusing on the disordered phase in the vicinity of the QCP, we demonstrate that the criticality results in a strong long range attraction between the fermions, with potential V (r) 1/rν , ν 0.75, where r is separation between the fermions. The mechanism of the enhanced∝ − attraction≈ is similar to Casimir effect and corresponds to multi-magnon exchange processes between the fermions. While we consider a model system, the problem is originally motivated by recent establishment of magnetic QCP in hole doped cuprates under the superconducting dome at doping of about 10%. We suggest the mechanism of magnetic critical enhancement of pairing in cuprates. PACS numbers: 74.40.Kb, 75.50.Ee, 74.20.Mn I. INTRODUCTION in the frame of normal Fermi liquid picture (large Fermi surface). Within this approach electrons interact via ex- change of an antiferromagnetic (AF) fluctuation (param- In the present paper we study interaction between 11 fermions mediated by magnetic fluctuations in a vicin- agnon). The lightly doped AF Mott insulator approach, ity of magnetic quantum critical point. To address this instead, necessarily implies small Fermi surface. In this case holes interact/pair via exchange of the Goldstone generic problem we consider a specific model of two holes 12 injected into the bilayer antiferromagnet.
    [Show full text]
  • Asymptotically Flat, Spherical, Self-Interacting Scalar, Dirac and Proca Stars
    S S symmetry Article Asymptotically Flat, Spherical, Self-Interacting Scalar, Dirac and Proca Stars Carlos A. R. Herdeiro and Eugen Radu * Departamento de Matemática da Universidade de Aveiro and Centre for Research and Development in Mathematics and Applications (CIDMA), Campus de Santiago, 3810-183 Aveiro, Portugal; [email protected] * Correspondence: [email protected] Received: 14 November 2020; Accepted: 1 December 2020; Published: 8 December 2020 Abstract: We present a comparative analysis of the self-gravitating solitons that arise in the Einstein–Klein–Gordon, Einstein–Dirac, and Einstein–Proca models, for the particular case of static, spherically symmetric spacetimes. Differently from the previous study by Herdeiro, Pombo and Radu in 2017, the matter fields possess suitable self-interacting terms in the Lagrangians, which allow for the existence of Q-ball-type solutions for these models in the flat spacetime limit. In spite of this important difference, our analysis shows that the high degree of universality that was observed by Herdeiro, Pombo and Radu remains, and various spin-independent common patterns are observed. Keywords: solitons; boson stars; Dirac stars 1. Introduction and Motivation 1.1. General Remarks The (modern) idea of solitons, as extended particle-like configurations, can be traced back (at least) to Lord Kelvin, around one and half centuries ago, who proposed that atoms are made of vortex knots [1]. However, the first explicit example of solitons in a relativistic field theory was found by Skyrme [2,3], (almost) one hundred years later. The latter, dubbed Skyrmions, exist in a model with four (real) scalars that are subject to a constraint.
    [Show full text]
  • A Solvable Tensor Field Theory Arxiv:1903.02907V2 [Math-Ph]
    A Solvable Tensor Field Theory R. Pascalie∗ Universit´ede Bordeaux, LaBRI, CNRS UMR 5800, Talence, France, EU Mathematisches Institut der Westf¨alischen Wilhelms-Universit¨at,M¨unster,Germany, EU August 3, 2020 Abstract We solve the closed Schwinger-Dyson equation for the 2-point function of a tensor field theory with a quartic melonic interaction, in terms of Lambert's W-function, using a perturbative expansion and Lagrange-B¨urmannresummation. Higher-point functions are then obtained recursively. 1 Introduction Tensor models have regained a considerable interest since the discovery of their large N limit (see [1], [2], [3] or the book [4]). Recently, tensor models have been related in [5] and [6], to the Sachdev-Ye-Kitaev model [7], [8], [9], [10], which is a promising toy-model for understanding black holes through holography (see also [11], [12], the lectures [13] and the review [14]). In this paper we study a specific type of tensor field theory (TFT) 1. More precisely, we consider a U(N)-invariant tensor models whose kinetic part is modified to include a Laplacian- like operator (this operator is a discrete Laplacian in the Fourier transformed space of the tensor index space). This type of tensor model has originally been used to implement renormalization techniques for tensor models (see [16], the review [17] or the thesis [18] and references within) and has also been studied as an SYK-like TFT [19]. Recently, the functional Renormalization Group (FRG) as been used in [20] to investigate the existence of a universal continuum limit in tensor models, see also the review [21].
    [Show full text]
  • Proton Polarisability Contribution to the Lamb Shift in Muonic Hydrogen
    Proton polarisability contribution to the Lamb shift in muonic hydrogen Mike Birse University of Manchester Work done in collaboration with Judith McGovern Eur. Phys. J. A 48 (2012) 120 Mike Birse Proton polarisability contribution to the Lamb shift Mainz, June 2014 The Lamb shift in muonic hydrogen Much larger than in electronic hydrogen: DEL = E(2p1) − E(2s1) ' +0:2 eV 2 2 Dominated by vacuum polarisation Much more sensitive to proton structure, in particular, its charge radius th 2 DEL = 206:0668(25) − 5:2275(10)hrEi meV Results of many years of effort by Borie, Pachucki, Indelicato, Jentschura and others; collated in Antognini et al, Ann Phys 331 (2013) 127 Mike Birse Proton polarisability contribution to the Lamb shift Mainz, June 2014 The Lamb shift in muonic hydrogen Much larger than in electronic hydrogen: DEL = E(2p1) − E(2s1) ' +0:2 eV 2 2 Dominated by vacuum polarisation Much more sensitive to proton structure, in particular, its charge radius th 2 DEL = 206:0668(25) − 5:2275(10)hrEi meV Results of many years of effort by Borie, Pachucki, Indelicato, Jentschura and others; collated in Antognini et al, Ann Phys 331 (2013) 127 Includes contribution from two-photon exchange DE2g = 33:2 ± 2:0 µeV Sensitive to polarisabilities of proton by virtual photons Mike Birse Proton polarisability contribution to the Lamb shift Mainz, June 2014 Two-photon exchange Integral over T µn(n;q2) – doubly-virtual Compton amplitude for proton Spin-averaged, forward scattering ! two independent tensor structures Common choice: qµqn 1 p · q p · q
    [Show full text]
  • Arxiv:2003.01034V1 [Hep-Th] 2 Mar 2020 Im Oe.Frta Esntelna Oe a Etogta Ge a Large As the Thought in Be Solved Be Can Can Model Models Linear These the One
    Inhomogeneous states in two dimensional linear sigma model at large N A. Pikalov1,2∗ 1Moscow Institute of Physics and Technology, Dolgoprudny 141700, Russia 2Institute for Theoretical and Experimental Physics, Moscow, Russia (Dated: March 3, 2020) In this note we consider inhomogeneous solutions of two-dimensional linear sigma model in the large N limit. These solutions are similar to the ones found recently in two-dimensional CP N sigma model. The solution exists only for some range of coupling constant. We calculate energy of the solutions as function of parameters of the model and show that at some value of the coupling constant it changes sign signaling a possible phase transition. The case of the nonlinear model at finite temperature is also discussed. The free energy of the inhomogeneous solution is shown to change sign at some critical temperature. I. INTRODUCTION Two-dimensional linear sigma model is a theory of N real scalar fields and quartic O(N) symmetric interaction. The model has two dimensionful parameters: mass of the particles and coupling constant. In the limit of infinite coupling one can obtain the nonlinear O(N) arXiv:2003.01034v1 [hep-th] 2 Mar 2020 sigma model. For that reason the linear model can be thought as a generalization of the nonlinear one. These models can be solved in the large N limit, see [1] for a review. In turn O(N) sigma model is quite similar to the CP N sigma model. Recently the large N CP N sigma model was considered on a finite interval with various boundary conditions [2–8] and on circle [9–12].
    [Show full text]
  • Bhabha-Scattering Consider the Theory of Quantum Electrodynamics for Electrons, Positrons and Photons
    Quantum Field Theory WS 18/19 18.01.2019 and the Standard Model Prof. Dr. Owe Philipsen Exercise sheet 11 of Particle Physics Exercise 1: Bhabha-scattering Consider the theory of Quantum Electrodynamics for electrons, positrons and photons, 1 1 L = ¯ iD= − m − F µνF = ¯ [iγµ (@ + ieA ) − m] − F µνF : (1) QED 4 µν µ µ 4 µν Electron (e−) - Positron (e+) scattering is referred to as Bhabha-scattering + − + 0 − 0 (2) e (p1; r1) + e (p2; r2) ! e (p1; s1) + e (p2; s2): i) Identify all the Feynman diagrams contributing at tree level (O(e2)) and write down the corresponding matrix elements in momentum space using the Feynman rules familiar from the lecture. ii) Compute the spin averaged squared absolut value of the amplitudes (keep in mind that there could be interference terms!) in the high energy limit and in the center of mass frame using Feynman gauge (ξ = 1). iii) Recall from sheet 9 that the dierential cross section of a 2 ! 2 particle process was given as dσ 1 = jM j2; (3) dΩ 64π2s fi in the case of equal masses for incoming and outgoing particles. Use your previous result to show that the dierential cross section in the case of Bhabha-scattering is dσ α2 1 + cos4(θ=2) 1 + cos2(θ) cos4(θ=2) = + − 2 ; (4) dΩ 8E2 sin4(θ=2) 2 sin2(θ=2) where e2 is the ne-structure constant, the scattering angle and E is the respective α = 4π θ energy of electron and positron in the center-of-mass system. Hint: Use Mandelstam variables.
    [Show full text]
  • Published Version
    PUBLISHED VERSION G. Aad ... P. Jackson ... L. Lee ... A. Petridis ... N. Soni ... M.J. White ... et al. (ATLAS Collaboration) Measurement of the total cross section from elastic scattering in pp collisions at √s = 7TeV with the ATLAS detector Nuclear Physics B, 2014; 889:486-548 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license Originally published at: http://doi.org/10.1016/j.nuclphysb.2014.10.019 PERMISSIONS http://creativecommons.org/licenses/by/3.0/ 24 April 2017 http://hdl.handle.net/2440/102041 Available online at www.sciencedirect.com ScienceDirect Nuclear Physics B 889 (2014) 486–548 www.elsevier.com/locate/nuclphysb Measurement of the total cross section√ from elastic scattering in pp collisions at s = 7TeV with the ATLAS detector .ATLAS Collaboration CERN, 1211 Geneva 23, Switzerland Received 25 August 2014; received in revised form 17 October 2014; accepted 21 October 2014 Available online 28 October 2014 Editor: Valerie Gibson Abstract √ A measurement of the total pp cross section at the LHC at s = 7TeVis presented. In a special run with − high-β beam optics, an integrated luminosity of 80 µb 1 was accumulated in order to measure the differ- ential elastic cross section as a function of the Mandelstam momentum transfer variable t. The measurement is performed with the ALFA sub-detector of ATLAS. Using a fit to the differential elastic cross section in 2 2 the |t| range from 0.01 GeV to 0.1GeV to extrapolate to |t| → 0, the total cross section, σtot(pp → X), is measured via the optical theorem to be: σtot(pp → X) = 95.35 ± 0.38 (stat.) ± 1.25 (exp.) ± 0.37 (extr.) mb, where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last is related to uncertainties in the extrapolation to |t| → 0.
    [Show full text]
  • The Concept of the Photon—Revisited
    The concept of the photon—revisited Ashok Muthukrishnan,1 Marlan O. Scully,1,2 and M. Suhail Zubairy1,3 1Institute for Quantum Studies and Department of Physics, Texas A&M University, College Station, TX 77843 2Departments of Chemistry and Aerospace and Mechanical Engineering, Princeton University, Princeton, NJ 08544 3Department of Electronics, Quaid-i-Azam University, Islamabad, Pakistan The photon concept is one of the most debated issues in the history of physical science. Some thirty years ago, we published an article in Physics Today entitled “The Concept of the Photon,”1 in which we described the “photon” as a classical electromagnetic field plus the fluctuations associated with the vacuum. However, subsequent developments required us to envision the photon as an intrinsically quantum mechanical entity, whose basic physics is much deeper than can be explained by the simple ‘classical wave plus vacuum fluctuations’ picture. These ideas and the extensions of our conceptual understanding are discussed in detail in our recent quantum optics book.2 In this article we revisit the photon concept based on examples from these sources and more. © 2003 Optical Society of America OCIS codes: 270.0270, 260.0260. he “photon” is a quintessentially twentieth-century con- on are vacuum fluctuations (as in our earlier article1), and as- Tcept, intimately tied to the birth of quantum mechanics pects of many-particle correlations (as in our recent book2). and quantum electrodynamics. However, the root of the idea Examples of the first are spontaneous emission, Lamb shift, may be said to be much older, as old as the historical debate and the scattering of atoms off the vacuum field at the en- on the nature of light itself – whether it is a wave or a particle trance to a micromaser.
    [Show full text]
  • Measurement of the Total Cross Section from Elastic Scattering in Pp Collisions at √S = 7 Tev with the ATLAS Detector
    Measurement of the total cross section from elastic scattering in pp collisions at √s = 7 TeV with the ATLAS detector The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation “Measurement of the Total Cross Section from Elastic Scattering in Pp Collisions at √s = 7 TeV with the ATLAS Detector.” Nuclear Physics B 889 (December 2014): 486–548. As Published http://dx.doi.org/10.1016/j.nuclphysb.2014.10.019 Publisher Elsevier Version Final published version Citable link http://hdl.handle.net/1721.1/94503 Terms of Use Creative Commons Attribution Detailed Terms http://creativecommons.org/licenses/by/4.0/ Available online at www.sciencedirect.com ScienceDirect Nuclear Physics B 889 (2014) 486–548 www.elsevier.com/locate/nuclphysb Measurement of the total cross section√ from elastic scattering in pp collisions at s = 7TeV with the ATLAS detector .ATLAS Collaboration CERN, 1211 Geneva 23, Switzerland Received 25 August 2014; received in revised form 17 October 2014; accepted 21 October 2014 Available online 28 October 2014 Editor: Valerie Gibson Abstract √ A measurement of the total pp cross section at the LHC at s = 7TeVis presented. In a special run with − high-β beam optics, an integrated luminosity of 80 µb 1 was accumulated in order to measure the differ- ential elastic cross section as a function of the Mandelstam momentum transfer variable t. The measurement is performed with the ALFA sub-detector of ATLAS. Using a fit to the differential elastic cross section in 2 2 the |t| range from 0.01 GeV to 0.1GeV to extrapolate to |t| → 0, the total cross section, σtot(pp → X), is measured via the optical theorem to be: σtot(pp → X) = 95.35 ± 0.38 (stat.) ± 1.25 (exp.) ± 0.37 (extr.) mb, where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last is related to uncertainties in the extrapolation to |t| → 0.
    [Show full text]