[General] background on pair production

Mon Aug 20 14:12:32 PDT 2018

Dear André,

I might have to agree with Chandra in this case. Nevertheless, it is an
interesting experiment and result.

I have not read the paper; so, I can't hear the arguments the authors would
make. While the some model(s) predicts gamma emission from collision of the
electron with the laser beam, the gamma would need to be phased and
energetic enough to interact with the coherent laser beam* E*-field at the
proper angle to divide and transfer the proper momentum to the beam. The
beam would probably need to have enough "cohesiveness" to provide an
effective mass adequate to absorb the necessary momentum. On the one hand,
if the beam could generate the very energetic gamma, then it would require
a large number of laser photons (or a sufficient portion of the coherent
wave energy). Thus, the beam energy (photons) might be able to interact
sufficiently to act as a massive momentum absorbing body. On the other
hand, to have two major events (gamma creation and lepton-pair formation)
within the same small volume of time and space and with the proper phase
and energy relationships, a major statistical miracle would be needed.

With this background, I would agree with Chandra that it is more likely
that a sufficient portion (energetically and spatially) of the laser beam
interacts only once with the electron to produce the lepton pair. The
gamma, if one actually exists in this scenario, might be a virtual
"creature" (in my view an evanescent wave) with properties still needing
definition. To claim that the experiment confirms photon-photon interaction
(which I believe does exist in other contexts) is the same as using the
model of charge being a result of photon exchange to claim that all lepton
creation (and all light interaction with charged particles) is a
photon-photon interaction. I believe that this would be the view (claim?)
of QED where any *E*- or *B*-field is considered to be a photon.

Andrew
_ _ _ _

On Mon, Jul 30, 2018 at 1:16 AM "André Michaud" <srp2 at srpinc.org> wrote:

> Dear Chandra,
>
>  I have no personal interpretation of these experiments carried out at
> SLAC.
>
>
>
Apparently, the electron collision with the beam generated a gamma photon
> in excess of 1.022 MeV that then decoupled into an electron positron pair
> further away in the beam apparently due to interaction between this photon
> and other less energetic photons in the beam, after the initial electron
> had been deflected at an angle coherent with the recoil due to the process
> of emission of the high energy gamma photon.
>
> The process appears to have been successfully recorded a sufficient number
> of times to be accepted as significant to the peer-reviewers.
>
>
>
> The description in the related documentation is there for anyone to do his
> own study and interpretation.
>
>
>
> Given that all EM photons move at c, it seems mandatory that least one
> photon in excess of 1.022 MeV be present in the same highly focused volume
> of space with a sufficient concentration of other less energetic photons
> for the process to be possible. This seems to be what allows the process.
>
>
>
> I am satisfied with the interpretation made by the McDonald team.
>
>
>
> Best Regards
>
>
>
> André
> ---
> André Michaud
> "GSJournal admin" <ntham at gsjournal.net>
> http://www.gsjournal.net/
> https://orcid.org/0000-0003-2740-5684
> http://www.srpinc.org/
>
> On Mon, 30 Jul 2018 01:53:39 +0000, "Roychoudhuri, Chandra" wrote:
>
> Andre:
> The experiment you have cited starts with an electron. The intermediate
> Gamma is a conjecture of current particle theory. To me, this is not pure
> light beam-light beam scattering in pure vacuum.
> The world has several laser fusion labs. Enormous amount of laser energy
> is focussed into about 100 micron size D2/D3 pellet. Only time the labs
> record real particles output when the laser beams successfully hit the
> pellet. Whenever the focussed laser beams miss the pellet, no particles are
> generated.
>      How come there is no photon-Photon interaction to generate particles?
> If my observation is backdated, kindly send me a recent reference.
> Chandra.
>
> Sent from my iPhone
>
> On Jul 29, 2018, at 2:58 PM, André Michaud <srp2 at srpinc.org> wrote:
>
>
>
>
> Dear Andrew,
>
> Just to mention that what seems not to have been covered in the pair
> production historical overview is pair production from photon-photon
> interaction in experiments carried out by McDonald et al. in 1997:
>
> http://www.slac.stanford.edu/exp/e144/
> <https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.researchgate.net%2Fderef%2Fhttp%253A%252F%252Fwww.slac.stanford.edu%252Fexp%252Fe144%252F&data=02%7C01%7Cchandra.roychoudhuri%40uconn.edu%7C969044c478e64c58fa8908d5f585541e%7C17f1a87e2a254eaab9df9d439034b080%7C0%7C0%7C636684875368774034&sdata=FlWkFZa7z2%2FSaBoHykstrQ8cBHP4WSjcmFK6b19LGDU%3D&reserved=0>
>
> Best Regards
>
> André
> ---
>
> André Michaud
> "GSJournal admin" <ntham at gsjournal.net>
> http://www.gsjournal.net/
> https://orcid.org/0000-0003-2740-5684
> http://www.srpinc.org/
>
> On Sun, 29 Jul 2018 06:08:32 -0400, Andrew Meulenberg wrote:
>
> Dear Richard,
>
> Thank you for looking that up. The words you highlighted in the abstract
> are almost exactly like what I remember. I suspect that I read them in "The
> Atomic Nucleus" by Evans (1982), which was often a "Bible" for me in my
> work; but, I may have encountered the info earlier.
>
> The main point is that the curvature of the photon path during its
> "division" in passing by a charge can be quite different for the electron
> interaction compared with that from a nucleus. This puts some light (and
> limits) on the models for conversion of light to matter.
>
> Andrew
>
> On Sun, Jul 29, 2018 at 2:28 AM, <richgauthier at gmail.com> wrote:
>>
>> Hello Andrew (and all),
>>   The below abstract from
>> http://adsabs.harvard.edu/abs/2006RaPC...75..614H
>> <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2006RaPC...75..614H&data=02%7C01%7Cchandra.roychoudhuri%40uconn.edu%7C969044c478e64c58fa8908d5f585541e%7C17f1a87e2a254eaab9df9d439034b080%7C0%7C0%7C636684875368784038&sdata=ZhaaCuAlBx%2BD0QAVLexyQ8kO41MR%2FGmvb7QrpHQOsTQ%3D&reserved=0>
>>  supports your comment about pair production in photon-electron
>> interactions.
>>        Richard
>>
>> *Title:*   Electron positron pair production by photons: A historical
>> overview
>> *Authors:*   Hubbell, J. H.
>> <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fauthor_form%3Fauthor%3DHubbell%2C%2BJ%26fullauthor%3DHubbell%2C%2520J.%2520H.%26charset%3DUTF-8%26db_key%3DPHY&data=02%7C01%7Cchandra.roychoudhuri%40uconn.edu%7C969044c478e64c58fa8908d5f585541e%7C17f1a87e2a254eaab9df9d439034b080%7C0%7C0%7C636684875368784038&sdata=Ic9NsCEom2NNJBBy9Ysg7XQVAO45oVG%2F4jCB%2FKrysrA%3D&reserved=0>
>> *Affiliation:*   AA(National Institute of Standards and Technology, Mail
>> Stop 8463, Gaithersburg, MD 20899-8463, USA.)
>> *Publication:*   Radiation Physics and Chemistry, Volume 75, Issue 6, p.
>> 614-623.
>> *Publication Date:*   06/2006
>> *Origin:*   ELSEVIER
>> <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.elsevier.com%2F&data=02%7C01%7Cchandra.roychoudhuri%40uconn.edu%7C969044c478e64c58fa8908d5f585541e%7C17f1a87e2a254eaab9df9d439034b080%7C0%7C0%7C636684875368794051&sdata=M98OE92Qya3E7RJECgcpIhUT7oNjmf6FbSbgrfr%2ByM0%3D&reserved=0>
>> *Abstract Copyright:*   (c) 2006 Elsevier Science B.V. All rights
>> reserved.
>> *DOI:*   10.1016/j.radphyschem.2005.10.008
>> <https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdoi.org%2F10.1016%2Fj.radphyschem.2005.10.008&data=02%7C01%7Cchandra.roychoudhuri%40uconn.edu%7C969044c478e64c58fa8908d5f585541e%7C17f1a87e2a254eaab9df9d439034b080%7C0%7C0%7C636684875368794051&sdata=EMRxjvup2k1mzhFgHx6NW8VY%2BJCIyGtsehs%2BGjmFaG4%3D&reserved=0>
>> *Bibliographic Code:*   2006RaPC...75..614H
>> <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2006RaPC...75..614H&data=02%7C01%7Cchandra.roychoudhuri%40uconn.edu%7C969044c478e64c58fa8908d5f585541e%7C17f1a87e2a254eaab9df9d439034b080%7C0%7C0%7C636684875368804059&sdata=20rE2Qa4XJnvMhyV4jzHv6ncDFnQ7fqsZYcAlihVjzU%3D&reserved=0>
>> Abstract This account briefly traces the growth of our theoretical and
>> experimental knowledge of electron-positron pair production by photons,
>> from the prediction of the positron by Dirac [1928a. The quantum theory of
>> the electron. Proc. R. Soc. (London) A 117, 610-624; 1928b. The quantum
>> theory of the electron. Part II. Proc. R. Soc. (London) A 118, 1928b,
>> 351-361] and subsequent cloud-chamber observations by Anderson [Energies of
>> cosmic-ray particles. Phys. Rev. 43, 491-494], up to the present time.
>> Photons of energies above 2 mec2 (1.022 MeV) can interact with the
>> Coulomb field of an atomic nucleus to be transformed into an
>> electron-positron pair, the probability increasing with increasing photon
>> energy, up to a plateau at high energies, and increasing with increasing
>> atomic number approximately as the square of the nuclear charge (proton
>> number). *This interaction can also take place in the field of an atomic
>> electron, for photons of energy in excess of 4 m**ec2** (2.044 MeV), in
>> which case the process is called triplet production due to the track of the
>> recoiling atomic electron adding to the tracks of the created
>> electron-positron pair.* The last systematic computations and
>> tabulations of pair and triplet cross sections, which are the predominant
>> contributions to the photon mass attenuation coefficient for photon
>> energies 10 MeV and higher, were those of Hubbell et al. [Pair, triplet,
>> and total atomic cross sections (and mass attenuation coefficients) for 1
>> MeV-100 GeV photons in elements Z=1-100. J. Phys. Chem. Ref. Data 9,
>> 1023-1147], from threshold (1.022 MeV) up to 100 GeV, for all elements
>> Z=1-100. These computations required some ad hoc bridging functions between
>> the available low-energy and high-energy theoretical models. Recently
>> (1979-2001), Sud and collaborators have developed some new approaches
>> including using distorted wave Born approximation (DWBA) theory to compute
>> pair production cross sections in the intermediate energy region (5.0-10.0
>> MeV) on a firmer theoretical basis. These and other recent developments,
>> and their possible implications for improved computations of pair and
>> triplet cross sections, are discussed.
>>
>>
>>
>> On Jul 27, 2018, at 3:28 AM, Andrew Meulenberg <mules333 at gmail.com>
>> wrote:
>>
>> Dear Richard,
>>
>> I realize that I might not have been clear enough in my statement about
>> the scattering charge being a lepton rather than a proton or nucleus. And,
>> my mistake in using the expression for Eγ  certainly did not help the
>> situation. You (and the reference) were focusing on the minimum energy
>> threshold for pair production and the difficulties associated with the low
>> production rates near the threshold. I was looking at the other end of the
>> question where a light scattering center (e.g., an electron) makes energy
>> and momentum conservation have a much greater effect.
>>
>> My memory of photon energy threshold >2 MeV for pair creation from a
>> collision with an electron is consistent with Eγ ≥ 2 mec (1 + me/mr)  =
>> 4 mec = 2.044 MeV. This may only have been based on theoretical
>> calculation. I'm not sure that there was any definitive experimental work
>> to support it. However, the recoiling electron from this interaction  would
>> be energetic enough to give good confirming information. I'm not sure that
>> Compton scattering would not interfere with the experiment.
>>
>> Andrew
>>
>>
>>
>>
>> On Thu, Jul 26, 2018 at 10:04 AM,  <richgauthier at gmail.com> wrote:
>>>
>>> Hi Andrew and all,
>>>   Below is a pdf copy of the article
>>> https://www.researchgate.net/publication/235335367_The_Miracle_of_the_Electron-Positron_Pair_Production_Threshold
>>> <https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.researchgate.net%2Fpublication%2F235335367_The_Miracle_of_the_Electron-Positron_Pair_Production_Threshold&data=02%7C01%7Cchandra.roychoudhuri%40uconn.edu%7C969044c478e64c58fa8908d5f585541e%7C17f1a87e2a254eaab9df9d439034b080%7C0%7C0%7C636684875368804059&sdata=iVz71uH04xB8So0UO7004Le3X9To%2BXATlwlRkOVoggw%3D&reserved=0>
>>>   with the abstract (below) you are quoting from. Definitely the
>>> minimum incoming photon energy is much less than 2 MeV and much nearer to
>>> the quoted value. It turns out that it’s very hard (as explained in the
>>> article) to experimentally confirm the minimum photon energy value for a
>>> particular recoil nuclear mass, given by the formula, so there’s
>>> surprisingly much experimental (and perhaps theoretical also) work still
>>> needed on this relatively straightforward conversion process of a photon to
>>> an electron-positron pair.
>>>     Richard
>>>
>>>
>>>
>>>
>>>
>>>
>>> On Jul 25, 2018, at 9:36 PM, Andrew Meulenberg <mules333 at gmail.com>
>>> wrote:
>>>
>>>
>>> Note that the threshold energy for pair production "...  given by the
>>> relation Eγ ≥ 2 mec (1 + me/mr), where mr is the mass of the recoiling
>>> particle," gives > 1 MeV for an electron or positron. My memory said
>>> that a >2 MeV photon was required. It may be related to the angle of
>>> recoiI. I don't have time to look it up.
>>>
>>> Andrew
>>>
>>> On Tue, Jul 24, 2018 at 1:35 PM, Richard Gauthier <
>>> richgauthier at gmail.com> wrote:
>>>>
>>>> Hi Chip and all,
>>>>   Here's a little background on experimental pair production from the
>>>> abstract to an article on Researchgate.net
>>>> <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fresearchgate.net%2F&data=02%7C01%7Cchandra.roychoudhuri%40uconn.edu%7C969044c478e64c58fa8908d5f585541e%7C17f1a87e2a254eaab9df9d439034b080%7C0%7C0%7C636684875368814063&sdata=j19qTH1qlbtqEx2KUXM1W728mOTtyEZgDNcaFLCLuGk%3D&reserved=0>
>>>>  at
>>>> https://www.researchgate.net/publication/235335367_The_Miracle_of_the_Electron-Positron_Pair_Production_Threshold
>>>> <https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.researchgate.net%2Fpublication%2F235335367_The_Miracle_of_the_Electron-Positron_Pair_Production_Threshold&data=02%7C01%7Cchandra.roychoudhuri%40uconn.edu%7C969044c478e64c58fa8908d5f585541e%7C17f1a87e2a254eaab9df9d439034b080%7C0%7C0%7C636684875368824072&sdata=qj81KsSgsqtZbO3uWGwxjncHFKSQmBobqxxhiEbPWvA%3D&reserved=0>
>>>>
>>>>           Richard
>>>>
>>>>
>>>> Pair production was first observed in 1932, which led to two early
>>>> Nobel prizes in physics, to Carl Anderson for the discovery of positrons
>>>> (1936) and to Paul Dirac for the theory of anti particles (1933). Science
>>>> textbooks state that the production of electron-positron pairs is possible
>>>> at photon energies above 1.022 MeV, which is the sum of the rest masses of
>>>> the particles involved. Measurements at the threshold require a selectable
>>>> photon energy in the range above 1 MeV, high-energy resolution to scan the
>>>> onset, and high intensities. Due to the need of simultaneous energy and
>>>> momentum conservation, pair production needs a recoiling particle, and thus
>>>> it can be observed most easily in solid matter. More exactly, the minimum
>>>> energy required for pair production is given by the relation Eγ ≥ 2 mec (1
>>>> + me/mr), where mr is the mass of the recoiling particle [1]. With the
>>>> particle rest energy of me = 511 keV/c , in heavy atoms we get mr >> me,
>>>> and thus in a good approximation photon energies Eγ ≥ 2·mec = 1.022 keV
>>>> allow the creation of electron-positron pairs. However, for a proton as
>>>> recoil particle the calculated threshold energy is increased by 557 eV, for
>>>> a copper target by 9 eV, and even for the very heavy element 111Roentgenium
>>>> by about 2.1 eV. Thus pair production cannot take place at exactly 2ámec.
>>>>
>>>>
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