manfred jeitler the physics of lhc baikal physics school 2011 1 accelerator centers worldwide

Manfred Jeitler The Physics of LHC Baikal Physics School 2011 1

accelerator centers worldwide


the world’s largest accelerators


cross sections vary over many orders of magnitude

• inelastic: 109 Hz• W l: 100 Hz• tt: 10 Hz• Higgs (100 GeV): 0.1 Hz• Higgs (600 GeV): 0.01 Hz

required selectivity 1 : 10 10 - 11

triggertrigger

-

cross sections and rates


projectiles

SPS and Fermilab used proton-antiproton collisions

LHC uses proton-proton collisions

why?


proton-antiproton

proton-proton


how big is a proton?

roughly 1 fm (10-15 m) – “femtometer” or “fermi”

1 barn is the area of a 10 fm × 10 fm square– big unit

– derived from uranium nucleus

– physicists joked: “that cross section is as big as a barn”

proton-proton cross section at LHC energies: 70 mbarn– = 7 fm2

– r ~ 1.5 fm


luminosity

(instant) luminosity is rate per cross section usual units: cm-2 s-1

– e.g., 1030 cm-2 s-1 corresponds, for a reaction cross section of 10-30

cm-2 ( = 1 μbarn), to a rate of 1 event per second

for a collider, the luminosity can be calculated as follows:


integrated luminosity number of events collected divided by the cross section usual units: nb-1 (“inverse nanobarn”),

pb-1 (“inverse picobarn”) etc.

an integrated luminosity of 1 fb-1 means that for a process with a cross section of 1 fb, 1 event (on average) should have been collected or 1000 events for a cross section of 1 nb, etc. so, 1 inverse femtobarn = 1000 inverse picobarns :

1 fb-1 = 1000 pb-1

physicists are now looking for very rare events, so it is vital to reach not only high energies (so that heavy particles can be produced) but also high luminosities handling the resulting data rates is a challenge also for the

detectors, trigger systems, and readout electronics


Instantaneous luminosity

• Nearly all the parameters are variable (and not independent)– Number of bunches per beam kb

– Number of particles per bunch – Normalized emittance n

– Relativistic factor (E/m0) – Beta function at the IP *

– Crossing angle factor F• Full crossing angle c

• Bunch length z

• Transverse beam size at the IP *

FfkN

FfkN

Ln

b

yx

b*

22

44

2

*21/1

zcF

Interaction Region

Energy

Total Intensity

Beam Brightness


LHCproton-protoncircumference: 27 kmbunches: 3564 + 3564 protons / bunch: 1011

beam energy: 2 x 3.5 (7) TeVluminosity: 1033-1034 cm-2s-1

bunch spacing: 25 nscollision rate: 108 - 109 Hzdipole field: 8.4 Tnumber of dipoles: ~ 1200

heavy ions (Pb-Pb)beam energy: 2.8 (5.5) TeV / nucleon pairluminosity: 1027 cm-2s-1


how to hit a proton

p ~ 1 fm beam ~ 10 - 100 μm = 1010 - 1011 fm ratio of area: 1020

– 10-20 chance to hit one proton

1011 protons per beam– typical distance between protons: 10-10 m = 100’000 fm

rate: 1011 × 1011 × 10-20 = 102

– nominal LHC: ~ 20 interactions per bunch crossing (“pileup”)

– achieved now: ~ 8


beam sizes around Atlas


layout of the LHC storage ring(built into the former LEP tunnel)

Manfred Jeitler The Physics of LHC Baikal Physics School 2011 16 16


I don’t want to fall into a black hooooolee... !!!

some (few) physicists believe that at LHC energies we could already produce “mini black holes” – they would disappear very quickly

but what if they don’t ?– could they engulf the Earth?

– eat up Cern, Geneva, Switzerland, Europa ... and then Siberia and Lake Baikal with the nice seals ??

– are those scientists crazy ????

don’t worry, be happy! there are convincing experimental arguments that we are

safe



physicist: those black holes will evaporate much too quickly – we know that from calculations

concerned citizen: and what if those calculations are wrong (as usual)??

physicist: the Earth has been bombarded by cosmic rays of much higher energy for the last 5 billion years and we are still here!

concerned citizen: but maybe then they are so fast they just whiz through the Earth and have no chance to stop and grow?



physicist: at least some of them would be charged and would be slowed down by the Earth

concerned citizen: but maybe due to who knows why they are all neutral? Then they would fly through and we wouldn’t notice

physicist: through Earth, yes – but there are neutron stars and they are so dense that there the black holes would stop! And my astronomer friends tell me there are lots of neutron stars out there, so they (and we) are in no danger!

concerned citizen: you are right, Socrates! – oops ... the last answer must have crept in from one of Platon’s dialogues

manfred jeitler the physics of lhc baikal physics school 2011 1 accelerator centers worldwide

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