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Pile-up in Hard Single Diffraction and Forward Coverage. Mike Albrow. Need for better forward coverage. Pile-up in hard single diffraction …need for rapidity gaps. Instrumenting TAS as “shower counter” Beyond TAM, ZDC Q1 Q2 Q3. Can Hard SD (with p detected) be done with Pile-up?. Answer:

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Pile-up in Hard Single Diffraction and Forward Coverage

Mike Albrow

Need for better forward coverage.

Pile-up in hard single diffraction …need for rapidity gaps.

Instrumenting TAS as “shower counter”



Can Hard SD (with p detected) be done with Pile-up?


No, because you cannot attach the p to the dijet/W/Z

The probability that the p and dijet/W/Z are from

different events is high (>~ 10%) even when ~ 0.1


You must kill pile-up by requiring >~ 1 unit rap-gap within

~ 3 (possibly 4) units of the p. Unfortunately CMS coverage

is v.poor there: need to add detectors |eta| > 6.5 (TAS +)

on at least one side.

Also probably want a -cut at L1 in p-trigger.

I will give some CDF experience.

Simple estimate valid for modest luminosities:

If your p - trigger is for all


Q: Can hard single diffraction be done with pile-up?

CDF experience.

Trigger 1 on central di-jets (2 with ET > 5 (seed))

Trigger 2 on central di-jets + pbar in roman pot

These particles

hardly count

These particles

count a lot!


E, pz cons.

Exercise for student

Most of blue curve (diffractive trigger) has xi’ > 0.1 and is

pile-up. This should be luminosity-dependent. One can only get very small xi

if there are not particles with large –ve eta (close to pbar), and pile-up kills that.

We look at this plot for different bins of instantaneous luminosity

(linear scale)



n (mean no./crossing) 0.36 0.54 0.83

Ratio ND/SD 3.4 5.4 14.5

  • Even when n < 1 most p+JJ (p+W etc) triggers are pile-up.

Only ~ 1% of di-jets are diffractive, while ~ 10% of all interactions are

diffractive (a pbar in pots). One must require at least part of a rapidity gap

(say >= 1 unit) or measure xi’ which needs very forward gap detectors.

CDF diffractive di-jet study uses the MiniPlug calorimeter

-3.6 < eta < -5.2 which makes a major contribution to the xi’ measurement

This shows the xi’ distribution for J5 data (ND) when you include the MP energy

(red) or when you set it to 0 (so it thinks there was a gap there).

It contributes a lot because of the e^-eta term (eta is –ve

on the pbar side). Particles way over on the p-side are not


If you have no detectors within 2-3 units of the beam you cannot

get xi from the detectors to match with xi(p)

single forward gaps sde e g gjj g bjbj gz gw
Single Forward Gaps, SDE, e.g. GJJ, G-BjBj, GZ, GW


Classical probes of diffraction:

Diffractive structure functions

Structure of “pomeron”




Even though the CDF Run II di-jet dataset was only 9 pb^-1

(delivered) and the fraction useful (no-pileup) quite a bit smaller,

we still measured dijets out to masses ~ 150 GeV.

Single Interaction scenario and Effective Luminosity

Rapidity gap physics can only be done when no other interaction.

Optimum scenario is when = 1, then

P(no spoiler) = exp(-) = e^-1 = 0.37.

LHC circ = 27 km, #bunches = 2808 (25ns)/936 (75ns)


~ 90ns (75ns case)

“High cross section


Trigger rate with 2 fwd gaps in CDF

Optimum ~ 20E30 (cf 140E30 at LHC75)

Aim: As complete coverage as possible of veto counters

Exclusive events in low luminosity (single interaction) running


How much effective luminosity with single interactions?

Estimate ~ 400pb-1 with factor 2 uncertainty (200-800)

How much signal remnant in all detectors from previous x-ing?

Can we trigger on 2 EM~5 GeV + forward veto?

Need absolute minimum solid angle not covered.

HCAL + CASTOR  6.5 full azimuth.

Beyond CASTOR? (ZDC small coverage, mainly 0deg neutrals.)

TAS : Copper shielding, instrument somehow?

Beyond: BSC around Q1-Q3?

They exist! Made at LBNL

There is a cavity each side – could insert shower detectors

One goes down in June ... too late. Other in ~ 10 months.

Cavity was foreseen for possible luminosity monitor

Not followed through.

Put detector, near hadronic shower maximum

What is efficiency for particles hitting front face?

Radiation levels to be withstood?

Possible solution: quartz fiber “calorimeter” brick.

Can fibers be at 90deg to shower axis?

Do not need resolution or “calibration”, just shower

detection with reasonable (> 95%?) efficiency.

Would like in L1 trigger in veto (not essential)

Radiation levels and shower acceptance studies

(Nikolai Mokhov & Sergei Striganov)

Details of space for counters. Thickness.

Technique (Radiation hardness, but only to ~ 1 fb-1)

Quartz fibers ?

Quartz plates ?

Diamond pads ?

Liquid? (slow ... unless very thin gaps.)

This is very far forward (140m) and covers small solid angle.

Can this technique be used in TAS?