How research is done – two examples

Feynman

• "The first principle is that you must not fool yourself--and you are the easiest person to fool. So you have to be very careful about that. After you've not fooled yourself, it's easy not to fool other scientists. You just have to be honest in a conventional way after that." Richard Feynman

A step by step guide to research• An individual or (more likely) a collaboration perform a measurement

or come up with a theory1. The work is summarised in a research paper and released to the

community as a preprint. 2. Simultaneously the work is sent to an academic journal and one or two

anonymous referees review it and recommend for publication or rejection. 3. If its published (and if its interesting) the work is reproduced by

independent researchers who also have to publish their findings.• It doesn’t matter how sensational the results are, we never believe

anything until it has been reproduced.• Scepticism is good as long one’s views change when thte evidence

becomes unbeatable! – In my career I’ve seen 3 Nobel-prize winning ”discoveries” which ended up as

non-discoveries.

Questions a reviewer or you, as a curious reader, must ask ?

1. What are the basic principles of the measurement and what are the systematic uncertainties ?

2. What does theory say about the claim ? 3. Most important of all - is the measurement in

direct conflict with other measurements or are there special experimental/theoretical circumstances allowing this measurement to be right ?

Some points

• Its usually easy to understand the experimental principles of a measurement

• If you don’t understand then the person explaining is probably doing a bad job.

• Nothing is too complicated such that the basic principles can’t be properly and easily explained.

• The press do a bad job sometimes. Physics blogs are often much better.

Case 1 – The rise of faster-than-light neutrinos !

• Why is the speed of light important ? • What is a neutrino ?• How do Opera make the measurement ? • What are the weak parts of the measurement ?

Theory – the speed of light

• Einsten’s theory of special relativity– if an object can travel faster than light then it

becomes possible to send a message back in time– this leads to a problem, eg the grandfather

paradox. You arrange to kill your grandfather before he met your grandmother. But then how would you have been around to send the message ?

– Our theories are built on the assumption that nature’s speed limit for particles, objects etc. is the speed of light.

neutronproton

e

The energy spectrum of emitted electrons (beta particles)

isn't consistent with the simple deay: .

The decay must be

new particle which is hard to o

n p e

n p e

p e

n p e

E E E

p p p

p p

n p e

n p e X

X

bserve=neutrino

Theory - neutrinos

Neutrinos are emitted from, eg, the sun.50000000000 pass through your thumbnail each seccond.

Neutrino Facts

Neutrino from Enrico Fermifor “Little neutral one”The particle is ~massless

Neutrinos are among the most bizarre of subatomic particles.

Neutrino from sun will pass through 5 LY of solid lead, with 50% chance of interacting

The opera experiment • Basic principles

– Fire a pulse of protons (10.5 ms) into a target (graphite) at CERN

– The interactions with the target lead to short-lived mesons which decay to neutrinos

10.5 mstime

Beam intensity

10.5 ms

50 ms

The opera experiment• The neutrinos are fired from CERN towards

the Gran Sasso lab in Italy ~730km away.

37

8

Travelling time (assuming speed of light)

733 102 10 s=200ns

3 10t

Detecting neutrinos in Gran Sasso

150000 ”bricks” of lead plates (in which neutrinos can interact) and emulsion plates (where the interactions can be measured).

How does the experiment work

A bunch of protons (neutrinos) leaves CERN and is measured at a later time at Gran Sasso. A GPS system is used to synchronise the two measurements.

Comparison of observed neutrino signal with proton time profile

How the measurement is made ?

Define t=Predicted time of flight (assume speed of light) - Measured time of flight

= 1048.5ns

Obs! This measurement was made by "sliding" the expected neutrino pulse shape from

CERN al

ong the x-axis until it matches the observed pulse shape at Gran Sasso.

Final corrections

• Make final corrections for electronic delays etc.

t=60.7 7.4(stat) 6.9 (syst) ns

Discussion

• What do we think the systematic errors should be ?

• Which is likely to be the largest ?

Systematic uncertainties

How were the systematic uncertainties combined ?

• When is it permissible to add them quadratically ?

• What is the systematic error if we add them linearly ?

Interpreting the result• What is the probability of the null hypothesis

i.e. that neutrinos move at ~light speed and that we were just ”unlucky” in this measurement ?

Putting the result in context ?• Has it been reproduced ? What

do other measurements say ? – beta decay

• v=c from most recent results• but maybe different type/energy

v<c

v>c

QuestionConsider supernova 1987a.

Around 167000 light years from earth

when it died. Time difference between

light and neutrinos arriving on earth ~3

hours (understood due to light being impeded

by atmosphere of dying star).

If Opera is correct, when would the neutrinos

have arrived ?

Summary of faster-than-light neutrinos

• Opera reports clear evidence for faster-than-light travel by neutrinos

• No other experiment offers clear evidence– these results are not necessarily in conflict with

the opera result• Repeat/improved measurement by Opera

underway. • We live in interesting times.

Case 2 – the rise and fall of magnetic monopoles

Magnetic monopoles

We observe electric charge.E-field lines ”radiate out” from an electric charge (an electric monopole)A moving electric charge gives a B-field.

We do *not* observe magnetic charge.B-field lines do not ”radiate out” from a magnetic charge (a magnetic monopole)

Magnetic monopoles - theory

• Symmetry – An electric charge exists, why not a magnetic charge ?

• Dirac’s argument– The presence of at least one type of magnetic charge allows

quantum mechanics to explain why electric charge comes in ”packets”, eg 1e,2e,3e and not 1.1232135e, 1.32435e etc. It explains why the electron charge = -1 x proton charge.

• Unified theories– Theories which try to unify the fundamental forces in a single

theory predict monopoles.• They should be there but nobody has seen them!

How to see monopoles

Cabrera’s monopole

• Search in 1981-1982

Examination of everything that could cause a ”jump” of just the right size to match the signal of the expected monopole.

Several possible causes, thermal noise etc. but nothing definitive. More searches were needed.

Systematics

Lots of people tried to reproduce• The early 1980’s suffered from monopole

fever as we’re suffering from neutrino fever now.

Publications on searches as a function of date

”My” search

The best search to date – back to Gran Sasso

Nothing is seen – Cabrera was either very very lucky that a monopole passed his way or he was just wrong.

Summary

• When you see a new exciting result ask yourself – What are the basic principles of the measurement

? – How does it fit into our present understanding of

nature ? – Has the work been reproduced ? Is it in direct

conflict with other measurements ?