4 ms·

What's an fb?

by Roritharr 6y ago

What's an fb?

lippel82 6y ago
A femtobarn, see https://en.m.wikipedia.org/wiki/Barn_(unit) https://en.m.wikipedia.org/wiki/Barn_(unit)
- FeepingCreature 6y ago
  Quoting: > During Manhattan Project research on the atomic bomb during World War II, American physicists at Purdue University needed a secretive unit to describe the approximate cross-sectional area presented by the typical nucleus (10^−28 m2) and decided on "barn". They considered this a large target for particle accelerators that needed to have direct strikes on nuclei, and the American idiom "couldn't hit the broad side of a barn" refers to someone whose aim is very bad. ... > Other related units are the outhouse (1 μb, or 10^−34 m^2) and the shed (10^−24 b (1 yb), or 10^−52 m^2), although these are rarely used in practice.
  - Already__Taken 6y ago
    > needed a secretive unit > American idiom "couldn't hit the broad side of a barn" It's being clever like that what got the Germans in WWII in trouble, I particularly like Wotan -https://en.wikipedia.org/wiki/Code_name#German_code_names https://en.wikipedia.org/wiki/Code_name#German_code_names Were as the brits made up a random code for this purpose - https://en.wikipedia.org/wiki/List_of_Rainbow_Codes https://en.wikipedia.org/wiki/List_of_Rainbow_Codes
    - mhh__ 6y ago
      RV Jones's book is a great read about scientific intelligence of WW2. Also, the NRO have previously given away classified orbits of spy satellites on their mission patches.
cjfd 6y ago
femtobarn
SiempreViernes 6y ago
Strictly speaking, those are inverse femtobarns, the nonsensical unit for accumulated number of collisions that particle physicists use.
semi-extrinsic 6y ago
OK, attempt at ELI5: The people are crashing lots of bunches of pebbles into each other, in two streams that meet. Some of the people are interested in observing what happens when the small blue pebble hits a big gray pebble and the big one cracks. They know the "probability" (cross-section) of this process is around 100 fb. Other people care about what happens when little tiny green pebbles collide three at a time. They know the "probability" of this is 11 fb - not so likely since three things need to line up. The people operating the streams of pebbles keep track of how many inverse fb they produce. Let's say in 2019 they produced 0.7 inverse fb. Then the people who care about blue/gray collisions know they can expect to find around 70 events in all the data from 2019, while the green/green/green people know they can expect about 7-8 events. When the people who collide streams of pebbles are going to build yet another Even Bigger Pebble Crashing Machine, they estimate in advance what the "luminosity" will be - how many inverse fb they will produce per year. Then the people looking for events know, "aha, with the new great crasher I can science even more!"
- choeger 6y ago
  In other words, someone has lost their marbles?
- mcv 6y ago
  While I really appreciate this attempt to explain what /fb means, I'm not sure replacing sub-atomic particles with pebbles really helps. I did onderstand from your explanation that /fb is called inverse fb and it represents an estimation of a number of collisions. So thanks for that.
  - semi-extrinsic 6y ago
    I freely admit the pebbles were an attempt at deadpan humour that would pass the great HN downvote filter - a most arduous challenge.
- jstanley 6y ago
  I appreciate the attempt, but this gives no information about what an fb actually is. The word people need is "femtobarn".
davrosthedalek 6y ago
A femtobarn. Unitwise, it's an area, a measure of the probability of an interaction, the "literal" cross section of the things you try to hit. Luminosity, often measured in inverse (femto) barn, is the other side: The number of tries, given by the number of particles you collided. The product of these is the number of events of this type which occur on average. So if you measured for 10 inverse femtobarn integrated luminosity, a process which has a cross section of 1 femtobarn should have happened 10 times.
SomeoneFromCA 6y ago
Facebook, duh.
pas 6y ago
I found this detailed explanation: https://physics.stackexchange.com/a/12071/246093 https://physics.stackexchange.com/a/12071/246093 See also: https://home.cern/news/news/accelerators/lhc-report-100-inverse-femtobarns-and-counting https://home.cern/news/news/accelerators/lhc-report-100-inve...

What's an fb?

lippel82 6y ago

A femtobarn, see https://en.m.wikipedia.org/wiki/Barn_(unit) https://en.m.wikipedia.org/wiki/Barn_(unit)

FeepingCreature 6y ago
Quoting: > During Manhattan Project research on the atomic bomb during World War II, American physicists at Purdue University needed a secretive unit to describe the approximate cross-sectional area presented by the typical nucleus (10^−28 m2) and decided on "barn". They considered this a large target for particle accelerators that needed to have direct strikes on nuclei, and the American idiom "couldn't hit the broad side of a barn" refers to someone whose aim is very bad. ... > Other related units are the outhouse (1 μb, or 10^−34 m^2) and the shed (10^−24 b (1 yb), or 10^−52 m^2), although these are rarely used in practice.
- Already__Taken 6y ago
  > needed a secretive unit > American idiom "couldn't hit the broad side of a barn" It's being clever like that what got the Germans in WWII in trouble, I particularly like Wotan -https://en.wikipedia.org/wiki/Code_name#German_code_names https://en.wikipedia.org/wiki/Code_name#German_code_names Were as the brits made up a random code for this purpose - https://en.wikipedia.org/wiki/List_of_Rainbow_Codes https://en.wikipedia.org/wiki/List_of_Rainbow_Codes
  - mhh__ 6y ago
    RV Jones's book is a great read about scientific intelligence of WW2. Also, the NRO have previously given away classified orbits of spy satellites on their mission patches.

cjfd 6y ago

femtobarn

SiempreViernes 6y ago

Strictly speaking, those are inverse femtobarns, the nonsensical unit for accumulated number of collisions that particle physicists use.

semi-extrinsic 6y ago

OK, attempt at ELI5: The people are crashing lots of bunches of pebbles into each other, in two streams that meet. Some of the people are interested in observing what happens when the small blue pebble hits a big gray pebble and the big one cracks. They know the "probability" (cross-section) of this process is around 100 fb. Other people care about what happens when little tiny green pebbles collide three at a time. They know the "probability" of this is 11 fb - not so likely since three things need to line up. The people operating the streams of pebbles keep track of how many inverse fb they produce. Let's say in 2019 they produced 0.7 inverse fb. Then the people who care about blue/gray collisions know they can expect to find around 70 events in all the data from 2019, while the green/green/green people know they can expect about 7-8 events. When the people who collide streams of pebbles are going to build yet another Even Bigger Pebble Crashing Machine, they estimate in advance what the "luminosity" will be - how many inverse fb they will produce per year. Then the people looking for events know, "aha, with the new great crasher I can science even more!"

choeger 6y ago
In other words, someone has lost their marbles?
mcv 6y ago
While I really appreciate this attempt to explain what /fb means, I'm not sure replacing sub-atomic particles with pebbles really helps. I did onderstand from your explanation that /fb is called inverse fb and it represents an estimation of a number of collisions. So thanks for that.
- semi-extrinsic 6y ago
  I freely admit the pebbles were an attempt at deadpan humour that would pass the great HN downvote filter - a most arduous challenge.
jstanley 6y ago
I appreciate the attempt, but this gives no information about what an fb actually is. The word people need is "femtobarn".

davrosthedalek 6y ago

A femtobarn. Unitwise, it's an area, a measure of the probability of an interaction, the "literal" cross section of the things you try to hit. Luminosity, often measured in inverse (femto) barn, is the other side: The number of tries, given by the number of particles you collided. The product of these is the number of events of this type which occur on average. So if you measured for 10 inverse femtobarn integrated luminosity, a process which has a cross section of 1 femtobarn should have happened 10 times.

SomeoneFromCA 6y ago

Facebook, duh.

pas 6y ago

I found this detailed explanation: https://physics.stackexchange.com/a/12071/246093 https://physics.stackexchange.com/a/12071/246093 See also: https://home.cern/news/news/accelerators/lhc-report-100-inverse-femtobarns-and-counting https://home.cern/news/news/accelerators/lhc-report-100-inve...