# iqbal shahin: gluons in the quark soup

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Husky Research Exhibition PresentationTRANSCRIPT

Gluons in the Quark Soup

Peter Arnold1 Shahin Iqbal2

1Department of PhysicsUniversity of Virginia

2Department of PhysicsUniversity of Virginia

Husky Research Exhibition, 2015

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 1 /

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Outline

1 Introduction

2 Motivation

3 The Calculations

4 Summary and Results(so far)

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 2 /

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Background

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 3 /

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Background

An Atom is about Ten thousand times smaller in diameter than ahuman hair.

But atoms have a structure of their own. They are made up ofparticles we call electrons, neutrons and protons.

Electrons are elementary particles. i.e they cannot be subdivided.

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 4 /

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Structure of Proton

Neutrons and protons that make up the nucleus of the atom, are notelementary particles. They are made up of even tinier particles thatwe call QUARKS and GLUONS.

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 5 /

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Background:Quarks, Gluons and Color

Quarks have a very small mass, and are similar to electrons.Gluons onthe other hand, are massless and are similar to photons, the particlesof light.

Quarks and gluons carry a very special kind of charge, which we callcolor.

Unlike the force between electric charges, the attraction betweencolors increases as the distance between two colored particles isincreased.

Therefore, quarks and gluons always pair up and from particles likethe neutron and protons.

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 6 /

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Motivation

When heavy atoms, like Gold smashed together at extremely highspeeds, the structure of the nucleus collapses.

For a brief moment,The temperature reaches several Trillion degreesCelcius, effectively melting the neutrons and protons in the nuclei. Itcreats an extremely hot soup of quarks and gluons that we call quarkgluon plasma.1

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 7 /

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Motivation

This is how our Universe looked like, a millionth of a second after theBig Bang.

QUESTION: How did our Universe, which started out as this hot soup ofquarks and gluons, evolve into stars and galaxies?

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 8 /

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How to Actually Study QGP

Probing QGP is hard: its too small and too short lived.

But its also too hot, so hot in fact that it glows by emittinggluons(and high energy quarks).

They emerge from the plasma carrying a lot of information about itsstructure.

Gluons are similar to light, but are more complicated. These can splitup and produce more gluons as they move through the plasma.

Different splittings interfere with each other, which makes theanalysis much more complicated.

My research is focused on studying the motion of these gluonsthrough the plasma, in particular when they split multiple times whileintereacting and interferring with each other.

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 9 /

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Calculating gluon brem rate

bremsstrahlung: Radiation of a gluon off of a high energygluon/quark after scattering from another particle.

Its the dominant process through which high energy particles in QGPloose their energy.

A Naive Calculation

The brem rate for the following process will be,

nvBut in reality, the rate is smaller than this at high energies and highdensity of particles in the medium. This happens because ofinterference between different possible splittings that produce thesame gluon.

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 10 /

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Complication

To qualitatively understand this problem, imagine a particle scatteringfrom a particle in the medium and radiating a gluon,

A gluon cannot resolve details that are smaller than its wavelength.

When the mean free path of the initial particle is smaller than thewavelenth of the radiated gluon, we cannot know from which particledid the originate, nor can we know if it originated from a singlescattering or many small angle scatterings.

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 11 /

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Complication

Now imagine the medium in which the process is happening wasmoving really fast.The bigger the speed, the more collinear theprocess will become.

The circular region of fuzziness will become elongated like an ellipse.It is called the formation time of the brem gluon.

How do the following two possibilities interfere?

vs

Its interefernce terms like these that make the calculation difficult andcause the reduction in number of radiated gluons.

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 12 /

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What we are computing!

Others have done such calculations for single splittings, but no one(atleast for the full range of energies) has taken this analysis a stepfurther and calcuated the rate for two successvive gluon emissions andtheir interference,

and

We are calculating the rate for two successive emissions to find whendo these terms and their interference become important.

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 13 /

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The Calculation

So we are calculating the rate for the following process!x y

z

The red spring like lines denote interactions with the medium.

There are several possible permutations of the of the final stategluons labled 1,2 and 3 above.

The total rate will then have the form,

Rate Re(xyz)(xyz) + Re(zxy)(zxy) + Re(yzx)(yzx)+ Re(xyz)(zxy) + Re(zxy)(yzx) + Re(yzx)(xyz)

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 14 /

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The Calculation:Classifying diagrams

For calculations like these, it is helpful to think of the interferenceterms as a single process.

We have 4 basic types of interference terms, (xy)(xy), (xy)(yx)

and the ones involving the QCD 4 gluon vertex.

We will call these as, Uncrossed, Crossed and 4 gluon vertex diagramsrespectively.

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 15 /

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Developing Formalism, and Feynman Rules

Since the process is not happening in vaccuum, we cannot apply theusual QCD Feynam rules. In particular, the propagators are verydifferent.

It was a major hurdle, calculating the progation of gluons when 4different gluons were interfering with each other at the same time.We were able to calculate this propagation in the large N limit.

We have developed the formalism that can handle even highernumber of successive gluons emissions, and have also calculated thefeynman rules for these cases.

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 16 /

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Summary and Results(so far)

Using these techniques we have completed the calculation of theCrossed diagrams and have published our results.4

The rate can be reduced to a one dimensional integral which we solvenumerically.

We have also reproduced the results obtained by others in therelevant kinematic limits.2,3

We are working to complete this analysis by calculating thecontribution from the Uncrossed and those with 4 gluon vertices, thatcontribute in this process.

Questions

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 17 /

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References

1 - Nayak, Tapan K. Pramana 79 (2012) 719-735 arXiv:1201.4264[nucl-ex]

2 -B. Wu, Radiative energy loss and radiative p-broadening ofhigh-energy partons in QCD matter, JHEP 1412, 081 (2014)[arXiv:1408.5459 [hep-ph]]

3 -J. P. Blaizot and Y. Mehtar-Tani, Renormalization of thejet-quenching parameter, Nucl. Phys. A 929, 202 (2014)[arXiv:1403.2323 [hep-ph]]

4 -Peter Arnold and Shahin Iqbal, The LPM Effect in SequentialBremsstrahlung[arXiv:1501.04964 [hep-ph]]

Peter Arnold, Shahin Iqbal (University of Virginia) Gluons in the Quark SoupHusky Research Exhibition, 2015 18 /

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IntroductionMotivationThe CalculationsSummary and Results(so far)

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