forward physics 2014 - unibo.it · organizzazione main physics goals of lhc what is “forward...

Forward Physics at LHCForward Physics at LHC

Corso di Fisica delle Alte Energie

Corsi di Dottorato

Corso di Fisica delle Alte Energie

Per Grafstrom

CERN and University of Bologna

Novembre 2014

OrganizzazioneOrganizzazione

� Main physics goals of LHC

� What is “Forward Physics” and why is it also interesting

� Some “Forward Physics” topics

�The total cross section�The total cross section

�Elastic scattering

�Soft diffraction

�Hard diffraction

�Central Exlusive Production of heavy particles

�Small x physics and saturation

� Some detectors relevant for Forward Physics 2

� Determine what breaks the electroweak symmetry Search for the Higgs boson

� Other phenomena possibly related to symmetry breaking- new particles predicted by SUSYSearch for Dark Matter

� Are there matter field beyond the Standard Model?Search for composite quarks and leptonsAre the force particles beyond the Standard Model ?

Main Physics goals of LHCMain Physics goals of LHC

3

� Are the force particles beyond the Standard Model ?Search for new heavy gauge boson

� Modifications of space-time ?Search for Black holes and extra dimensions

� The unknownFind what we did not look for

In general central collision with high Pt

Understanding the Universe …

Electroweak Transition

Unification ?

•4

•Particle Physics and Philosophy

Maria in der Aue, March 2011, P.

Jenni (CERN)

•Experimental Methods in Particle Physics

A most basic question is why particles (andmatter) have masses (and so different masses)

The mass mystery could be solved with the ‘Higgs me chanism’which predicts the existence of a new elementary pa rticle, the‘Higgs’ particle (theory 1964, P. Higgs, R. Brout a nd F. Englert)

Peter Higgs

The Higgs (H) particle has been searched for since decades at

•5

accelerators, but not yet found…

The LHC has sufficient energy to produce it for sure, if it exists

FrancoisEnglert

•Particle Physics and Philosophy


Jenni (CERN) •Experimental Methods in Particle Physics

Supersymmetry (SUSY)

Establishes a symmetry between fermions (matter)and bosons (forces):

- Each particle p with spin s has a SUSY partner pwith spin s -1/2

- Examples q (s=1/2) � q (s=0) squark

g (s=1) � g (s=1/2) gluino

~

~

~

(Julius Wess and Bruno Zumino, 1974)

•6

Our known world Maybe a new world?

g (s=1) � g (s=1/2) gluino

Motivation:

- Unification (fermions-bosons,matter-forces)

- Solves some deepproblems of the Standard Model

~

•Experimental Methods in Particle Physics•Particle Physics and Philosophy


Jenni (CERN)

This kind of physics is characterized by high pt

and central events in the detectors.

Perturbative theory works

The cross section factorize as a convolution of

parton cross section and PDF

8







�Soft diffraction

�Hard diffraction




•40 % of the total cross section comes from small Pt

diffractive processes like elastic scattering or single- or double-diffractive dissociation

•These are processes without color exchange between the particles

Tuttavia……..

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particles

•Quantum numbers of vacuum exchanged

•Those cross sections are sensitive to strong interaction at large distance

•Processes with a lot of energy in the forward direction

Oggi parliamo di questi processi

Energy go Forward in Nature

11

SomeSome definitionsdefinitions of variablesof variables

Pseudo rapidity

Rapidity

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Forward PhysicsForward Physics

Baseline ATLAS CMS detector covers -5 <η < 5

Tracking -2.5 <η < 2.5

However ηmax ~ ln√ s/mp ~ 9.5

13

However max ~ ln s/mp ~ 9.5

Large region in rapidity not covered

(Angles from 0.8 degree to 0 degrees)

Some difficulties…..Some difficulties…..

� Limited space

� Harsh radiation environment

14

� Harsh radiation environment

� Integration interference with machine

Characteristic of Forward collisionsCharacteristic of Forward collisions

Typical high ptForward Elastic Scattering

(extreme case)

15

No break up

No colour flowBreak up

Colour flow

Examples of Large Rapidity Gap

LRG

16

Why are also low pWhy are also low ptt reactions interesting?reactions interesting?

An interest in its own right :

A first principle description of the total cross section, elastic scattering and diffractive processes does not exist.

e.g. elastic scattering is the most simple low pt -process we can imagine but we don’t know how to write down the amplitude.

Elastic scattering relates to a number of fundamental parameters that we don’t know how to derive from basic principles like σtot or the ρ-parameter .

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� Structure of high energy cosmic rays

� Understanding of underlying event and pile-up events at the LHC

� Understand rapidity gap survival factors

� Luminosity measurement are always intimately connected to σtot measurements

However there are also less fundamental reasons…..

Low pt reactions are important to understand the so called underlying event at LHC.

The underlying event

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The "underlying event" is everything except the two outgoing hard scattered " jets" and consists of the "beam-beam remnants" plus initial and final-state radiation..

There is also the issue of pile-up at high luminosities at the LHC

The “underlying event” is the background we need to subtract to extract new physics

PilePile--up up fromfrom simulationsimulation

19

PilePile--up from real dataup from real data

20







�Soft diffraction

�Hard diffraction




σtot vs √sand fit to (lns)γ γ =1.0

)γ =2.2±σ(best fit)

The total cross sectionThe total cross section

σtotal is a fundamental

parameter to be measured at

any new energy regime

σtotal fixes the normalization

of all other processes

22

of all other processes

The “asymptotic” energy dependence

is still an issue

Different models

wide range of predictions

125 +- 35 mb at the LHC energy

A measurement will narrow

down the different options

General principles:

Froissart-Martin bound

σtotal < (π/mπ )2 ln2s

23

A ln2s behavior does not necessarily

means saturation of the Froissart bound

σtotal < (π/mπ ) ln2s

First results

from the TOTEM

experiment

σtotal = 98.6 +-2.2 mbarn

Now also results from ATLAS

σtotal = 95.4 +-1.4 mbarn

In spite of success of QCD a first principle description not possible !

QCD can not describe but QCD

motivated models exist.

In many models the rise is driven by increasing number of low x gluon-gluon collisions (will violate

unitarity at some point)

The role of the gluons

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unitarity at some point)

Low x gluons get enough energy

to produce jets that contributes

to the total cross section

Gives too steep rise with energy

tempered by soft gluon emission

to give smooth behavior







�Soft diffraction

�Hard diffraction




Elastic scatteringElastic scattering

27

t= (pθ)2

Elastic scattering-from ISR to the Tevatron

28

New results from TOTEM

min at about .5 GeV2 at 7 TeV

Focus on small angle elastic scatteringFocus on small angle elastic scattering

We get access to some fundamental parameters

� σtotal dσ/dt ∝ l fel(θ)|2

29

� ρ = Re fel(t=0)/Im fel (t=0)

dσ/dt ∝ l fel(θ)|

fel(θ) = Re fel(θ) + i Im fel(θ)

σtot = 4π Im fel (0)

Optical Theorem relates the total cross-section

to the forward elastic scattering amplitude

30

The optical theorem is a general law of wave scattering theory derived from

conservation of probability using quantum mechanics. It is a reflection of the fact

that the very existence of scattering requires scattering in the forward direction

in order to interfere with the incident wave, and thereby reduce the probability

current in this direction.

The ρ- parameter and how σtotal is related to elastic scattering

dσ/dt ∝ l fel(θ)|2


32


Im fel(θ) relates to σtotal through the optical theorem

Re fel(θ) relates to σtotal through dispersion relations

Fundamental

and model

independent

Dispersion relations (“old” notion used in optics)

but also used in theory of scattering of elementary particles !

33

Derived from basic notions:

analyticity of scattering amplitude

crossing symmetry

unitarity

causality

Measuring ρ = Re fel/Im fel

α2G4(t)/t2

σtotα(ρ+αϕ)G2(t)ebt/2/tfel=fc+fn

34

(1+ρ2)σ2totebt

σtotα(ρ+αϕ)G (t)e /t

fC=fN for t=-6 10-4 Gev2

fel=fc+fn

Use ρ to predict σ total

Predictions from ISR measurements SPS collider

35

Use ρ to predict σ total

Predictions from Collider and Tevatron measurements

36

Coulomb amplitude ≈ Strong amplitude for –t=0.00065Gev2

This corresponds to 3.5 µrad

The Coulomb region at the SPS collider at 120 µrad

The difficulty at LHC to measure ρ

37

The Coulomb region at the SPS collider at 120 µrad

Two factors make it harder at the LHC

Momentum larger ; t = (p θ) 2⇒ factor 25

Cross section larger ⇒ factor 1.3







�Soft diffraction

�Hard diffraction




DiffractionDiffraction

� Diffraction original an optical phenomena

� Analogy in particle scattering due the wave nature of Quantum Mechanics

� 2 π R/lambda >>1

� Condition full filled in high energy physics once above 1 GeV given hadronic dimensions are order of Fermi

39

GeV given hadronic dimensions are order of Fermi


� Simplest diffractive process is elastic scattering

� However more general: diffraction occurs when there is no exchange of quantum number:

a + b → a* + b*

where a* and b* have the same quantum numbers as a and b

40

No colour flow

(the Pomeron is a colour singlet)

Gluon radiation suppressed

⇒ Rapidity gaps

Impossible to go beyond

phenomenology to describe

soft diffraction

Diffraction (soft) is outside

realm of perturbative QCD


41

Constitutes 20 % of total

inelastic cross section

Important for understanding

of pile-up and underlying

event







�Soft diffraction

�Hard diffraction




Hard diffractionHard diffraction(Diffractive events with hight p(Diffractive events with hight ptt particles)particles)

Striking discovery at HERA

~ 10 % of event in DIS has a leading proton and a large rapidity gap between proton remnant and other hadrons

Combines features of hard and soft scattering

•The electron receives large

momentum transfer-High photon virtuality

43

momentum transfer-High photon virtuality

•The proton hardly change momentum

More complicated in pp scattering

Strong interactions between the protons

Rapidity gap partially filled

Hard diffraction at LHCHard diffraction at LHC

Example: Central Exclusive Production of heavy particles

The full diffractive energy is used to create a hard system in the

central rapidity region and no remnants of the diffractive interactions.

44







�Soft diffraction

�Hard diffraction




Forward energy flow related to low-x physics

Low x physics

46

M goes forward if x2 << x1

i.e one parton must have low-x

Low x physics

47

Strong rise at low-x observed at HERA

What happens at the LHC?

Can saturation be seen??

Low x physics

Saturation when gluons are

numerous enough( low x) & “large”

enough (low-Q2 ) to overlap

48

ρ ~ xG(x,Q2)/πR2

σ ~ αs/Q2

ρσ > 1

Q2s = αs x G(x,Q2s)/π R2







�Soft diffraction

�Hard diffraction




SomeSome detectors relevant for detectors relevant for ForwardForward PhysicsPhysics

1. Roman Pot detectors : ATLAS and TOTEM

50

2. Zero Degree Calorimeters:ATLAS, ALICE and CMS

3.Fast timing detector- possible future detector : ATLAS and CMS

ATLAS Roman PotsATLAS Roman Pots

51

The Roman Pot conceptThe Roman Pot concept

Roman Pot Concept

Coulomb

52

The ALFA detector system

ALFA Test beam Stations Alignment Vacuum Detectors Survey RP movement DCS TDAQ Commissioning Trigger Plans for 2011

ATLAS

Elastic scattering240 m 240 m

Approach the beam

NIELS BOHR INSTITUTEUNIVERSITY OF COPENHAGEN ALFA status and plans for 2011 Sune Jakobsen

2/17ATLAS week 28-02-2011

Beam pipe Beam pipe

Approach the beam

to few mm

Edge less detector

ATLAS use scintillating fibres

Edge reconstruction efficiencyResolution without and with

using non-hit layer

ALFA Test beam Stations Alignment Vacuum Detectors Survey RP movement DCS TDAQ Commissioning Trigger Plans for 2011

NIELS BOHR INSTITUTEUNIVERSITY OF COPENHAGEN ALFA status and plans for 2011 Sune Jakobsen

3/17ATLAS week 28-02-2011

Understanding the

detector rotations

improves resolution

Including information of

fiber layer with no hit

improves resolution

σ = 46 µm

σ = 30 µm

ZDC ZDC ((bothboth ATLAS and CMS)ATLAS and CMS)

55

A Zero Degree Calorimeter (ZDC) is a calorimeter that resides at the junction where the two beam pipes of the LHC become one – at 0° from the pp collisions. It is housed in the shielding unit that protects the S.C magnets from radiation and measures neutralparticle production at 0°.

~10 krad

~1 krad

PMT

150180

PMT PMTTAN slot

PMT

MAPMT

MAPMT

56

~1.8 Grad

~100 Mrad~10 Mrad

~1 Mrad

~100 krad

~10 krad

~100 Mrad~10 Mrad

Beam

800

180290

30 150100 30

1000150 150150 15090

Ioni

zatio

n ch

ambe

r

EMCal Module design(1 module only)

Light collected fromStrips of 1.5 mm quartz

57

Transverse to beam(main energy and timing)

And 1 mm quartz rodsProjective to beam

Latter measure coordinateOf showers.

What can be measured with the ZDCWhat can be measured with the ZDC

� Leading particle and energy flow in the forward direction

� Production cross section for a number of neutral particles in a certain kinematical range

� Measuring the very forward energy flux is essential for understanding the cosmic rays

Interpreting cosmic ray data depends •At LHC pp energy:

58

Interpreting cosmic ray data dependson hadronic simulation programsForward region poorly known/constrainedModels differ by factor 2 or moreNeed forward particle/energy measurementse.g. dE/dη…

The direct measurement of the p production cross section as function of pT is essential to correctly estimate the energy of the primary cosmic rays (LHC: 1017 eV)

•At LHC pp energy:

• 104 cosmic events km-2 year-1

• 107 events at the LHC in one day

�Tagging diffractive events

, MeVγγM

0 200 400 600 800 1000 12001

10

210

310

410

510

, MeVγγM

0 200 400 600 800 1000 12001

10

210

310

410

510 γγ→0π

γγ→η

γγ→’η

, MeV0πnM1000 1200 1400 1600 1800 2000

0

100

200

300

4000π n→ Λ

Mass spectrum in pp collisions using the ZDC

59

, MeV0πnM1000 1200 1400 1600 1800 2000

0

200

400

0π n→ ∆

γγγγ

, MeV0π0πM

0 200 400 600 8000

20

40

60

, MeV0π0πM

0 200 400 600 8000

20

40

600π0π→SK

Possible future fast timing detector

Pileup background rejection/signal confirmation

Ex: Two protons from one interaction and two b-jets from another

WHY?

10 picoseconds is design goal(light travels 3mm in 10 psec!)gives ~x20 fake background rejection;

•6060

Use time difference between protons to measure z-vertex and compare with inner detector vertex.

How?

How Fast?

Detector developed by K. Piotrzkowski (U.C. Louvain).Low index of refraction means little time dispersion, extremely accurate, radiation hard, little material for Multiple scattering

GASTOF

From joint 2010 ATLAS/CMS CERN TBδt(G1-G2)=14 ps implies 10 ps single detector resolution! BUT… only one single measurement, no segmentation, electronics challenging. •61

4x8 array of 5-6 mm2

fused silica bars

QUARTIC Timing Detector UTA, Alberta, Giessen, Stony Brook, FNAL

Only need a 40 psmeasurement if you can do it 16 times: 2 detectors with

proton

it 16 times: 2 detectors with 8 bars each, with about 10 PE’s per bar

•62

Back up

63

AFP Baseline Plan

30 cm

Two types of Cerenkov detector are employed:

GASTOF – a gas Cerenkov detector that makes a single measurement

QUARTIC – two QUARTIC detectors each with 4 rows of 8 fused silica bar will be positioned after the last 3D-Si tracking station because of the multiple scattering effects in the fused silica.

Both detectors employ Microchannel Plate PMTs (MCP-PMTs)

•64

Rapidity gap survival

Suppressed by re-scattering

effects that fill the gap

between the proton and the

central system.

65

The suppression factor (S2)

has to be modeled using soft

interaction cross sections

• Selection rules mean that central system is (to a good approx) 0++ ⇒If you see a new particle produced exclusively with proton tags you know its quantum numbers

•Tagging the protons means excellent mass resolution (~ GeV) irrespective of the decay products of the central system.

Proton tagging may be the discovery channel in

Exclusive Central ProductionExclusive Central Production

66

•Proton tagging may be the discovery channel in certain regions of the MSSM.

Very schematically: exclusive central production is a glue – glue collider where you know the beam energy of the gluons

Very forward detectorsVery forward detectors--upgradeupgrade

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AcceptanceAcceptance

•With nominal LHC beam optics

•@ 1033-34 cm-2s-1:

•220 m: 0.02 < ξ < 0.2

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•ξ1 ξ2 s = M2

•With √s = 14TeV, MH = 120 GeV

•ξ ≈ 0.009 ≈ 1%

•220 m: 0.02 < ξ < 0.2

•420 m: 0.002 < ξ < 0.02

forward physics 2014 - unibo.it · organizzazione main physics goals of lhc what is “forward...

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