QCD at hadron colliders

Transcription

1 QCD at hadron colliders This will be a brief experimentalist s view, with a concentration on the two hadron-hadron colliders mentioned in the previous talk If you want a good reference book for graduate school (and to get in good with James) you ll want to buy this book

2 The other book I talked about l Note the sub-title

3 A brief review l Simply put, l Mesons consist of a quark and an antiquark. l Baryons consist of three quarks.

4 More flavors of quarks Originally there were three (up, down, and strange) Further research has led to 6

5 So a proton and neutron are why are the quarks painted different colors?

6 So quarks have flavor and color? We could have called it geordie-ness or any other name just as long as it signifies that quarks carry a property that can have 3 different values The theory that describes the strong nuclear force by the exchange of colored particles called gluons is called Quantum Chromodynamics, or QCD antiquarks have anti-color (e.g. red) gluons have color and anti-color (e.g. red-blue)

7 Now back to fundamental forces

8 The Standard Model works amazingly well

9 Looking back in time I work here you are here

10 QCD at hadron colliders By now, QCD is the widely accepted theory describing the interactions between quarks and gluons Basically all interactions at hadron-hadron colliders are governed by or modified by QCD u u both standard model physics as well as possible new physics s new physics has backgrounds from standard model physics s so an understanding of QCD has to precede any searches for new physics

11 Consider jet production at hadron colliders Jet production at the Tevatron currently probes the most violent collisions currently achievable u smallest distance scales (10-17 cm) u some of greatest sensitivity to new physics->quark compositeness New version of Rutherford scattering u u production of jets at high p T indicates that there must be point-like constituents within proton, i.e. quarks If we observe a deviation from the expected jet cross sections at the highest jet p T s (smallest distance scales), this may also be an indication of something inside the quarks

12 QCD at hadron colliders Collisions that produce jets at high transverse momentum take place at small impact parameter (the proton and the anti-proton are hitting head-on) and are more accurately hard collisions between partons (quarks and gluon) inside the two protons D jet p (uud) f x a a d s^ c b x b f (uud) p D jet

13 So what s with the blobs, etc? The proton has 2 valence up quarks and 1 valence down quark (but also lots of sea quarks and gluons) All share the momentum of the proton; the collisions is between 1 parton from the proton carrying a momentum fraction x a and one parton from the anti-proton carrying a momentum fraction x b The distribution of momentum carried by a given parton is given by parton distribution function (pdf s), f parton (x,q 2 ) a measure of the hardness of the collision D p (uud) f x a a d s^ c b x b f (uud) p D jet

14 Blobs, continued So one parton carrying a momentum fraction x a collides with another parton carrying a momentum fraction x b (and usually these are a small fraction of the momenta carried by the proton and anti-proton) The collision is given by the red blob; that s where most of the hard QCD comes in way easy to calculate u to leading order, s a a 2 s been there, done that, for over u to next-to-leading order, s a a 3 s 10 years now u to next-to-next-to-leading order, D errr Nigel will get back to s a a s4 you get the idea you in 2005 or so p (uud) f x a jet D a d s^ c b x b f (uud) p

15 Blobs, continued c and d are the outgoing partons, but problem: they carry color; we don t observe the quarks or gluons themselves (don t see naked color) but instead the partons fragment into jets of hadrons (pions, kaons, and protons) it s the jets of hadrons that we can observe experimentally; D(z,Q 2 ) describes how the partons fragment into hadrons p (uud) f x a jet D a d s^ c b D fraction of momentum carried by hadron x b f Note the hardness of the collision gets involved again (uud) p

16 Original description of fragmentation done by Feynman and Field Who s now working with me on CDF and who has a famous sister

17 Rick s famous sister Rick s wife Rick s sister Rick Trivia question: who is Max Born s famous grand-daughter?

18 Olivia Newton-John John Travola is not related to any famous physicist u as far as I know

19 Jet Fragmentation Functions The parton that gives rise to the jet fragments into a fairly large number of particles (increasing as the jet E T increases) u say 20 particles for a jet with E T of 100 GeV Most of the jet particles have a small fraction (z) of the total jet momentum u and the gluon fragmentation is softer than the quark fragmentation We can t calculate the fragmentation functions perturbatively; have to measure them u mosly at LEP u we can calculate how they change with Q, though D(z) quark gluon 0 z = E hadron /E jet 1

20 Blobs, continued The cross section for jet production to occur depends on a convolution of f parton (x a,q 2 ), f parton (x b,q 2 ) and the hard scattering cross section s^ Now, here s the beauty; the pdf s f can be determined in one process and then used in the calculation of any other u factorization D p (uud) f x a a d s^ c b x b f (uud) p D jet

21 Parton distribution function fits Since all hadron-hadron cross sections depend on pdf s, the determination of them is an important aspect of QCD phenomenology u non-perturbative physics, although evolution of pdf s with Q 2 can be calculated perturbatively Can determine pdf s using data from DIS (deep-inelastic scattering), DY (Drell-Yan pair production), and the production of jets There are two major groups that provide pdf s to the world u MRST (M and S are here; R and T are coming this week) u CTEQ (I am here) CTEQ6 pdf s xf(x,q o ) = A o x A1 (1-x) A2 P(x)

22 Remember that it s not only the two main partons that are interacting with each other; the rest of the proton and anti-proton are interacting as well (but softly) The result is a king of low energy underlying event filling the detector u this underlying event must be subtracted from the jet energy as best as possible A complication

23 Jet production There are many subprocesses that contribute to jet production at hadron colliders u u u u gg scattering gq scattering qq (or qq scattering) qq scattering mostly gg scattering at low E T, which corresponds to low values of parton x (where the gluon dominates)

24 Defining a jet interlude borrowing some transparencies

25 Let s go back to the jet event shown previously Two clear jets or streams of particles coming from the interaction point

26 Jet clearly defined in a cone in h-f space

27 Measuring the inclusive jet cross section Note the lower E T cross section has to be measured by pre-scaled triggers

28 Looking at trigger efficiencies

29 Most of the energy of the jet is in the form of low p T particles Jet Fragmentation

30 Calorimeter measurement of a jet The photons and electrons deposit energy in the electromagnetic calorimeter Hadrons (pions, protons, etc) deposit most of their energy in the hadronic calorimeter (but some in the electromagnetic calorimeter) The response of the 2 calorimeters is different; in addition there are cracks in between detectors Thus, jets of the same energy can give a different response in the calorimeters depending on exactly how they have fragmented

31 Determining the corrected distribution Knowing the response distribution of the detector to jets of given energies, we can determine from the measured jet E T distribution what the corrected or true distribution looks like The corrected distribution can then be compared to theoretical predictions

32 Resulting in, for example, the inclusive jet cross section from Run 2 8 orders of magnitude

33 Data compared to theory could this be sign of something interesting?

34 Highest E T jet event yet observed dijet event: Oct Run: event: Dijet Mass = 1364 GeV/c 2 cos q* = 0.30 z vertex = -25 cm J2 E T = 633 GeV J2 h = J1 E T = 666 GeV J1 h = 0.43

35 One last interesting event no one knows what it is, but it shouldn t be there and we hope that we find a lot more

36 A good screensaver Live events from CDF u b/now/live_events/ind ex.html