how well can we control detector effects for precision … · 2016-10-04 · foil makes up for most...

How well can we control detector effects for precisionmeasurements in the HL-LHC period?

Heavy Flavour physics at HL-LHC

Andrea Contuthanks to M.O. Bettler, C. Bozzi, A. Di Canto, F. Dettori, M. Fontana, V. Gligorov, D.Johnson, B. Khanji, U. Langenegger, S. Malde, P. Owen, P. Reznicek, B. Sciascia, F.

Simonetto, M. Whitehead for the input and the useful discussions

CERN

31 Aug 2016 - CERN

Andrea Contu (CERN) Detector effects 31 Aug 2016 1 / 27

Outline

1 Introduction

2 Core physics program

3 Extending our physics reach

4 Conclusions


Introduction

Introduction

Very broad subject, difficult to condense

Concentrate on the few benchmark measurements and look atpossible extensions of the current physics program where we can belimited

I assume the theoretical uncertainty will go down where necessary

I will not discuss computing requirements


Introduction

Luminosity prospects

The LHC is a golden mine for flavour physics

Huge cross sections

σ(pp → B+ + X , 7TeV) ≈ σ(pp → B0 + X , 7TeV) ≈ 40µbσ(pp → B0

s + X , 7TeV) ≈ 10µb [JHEP08(2013)117]σ(pp → cc + X , 13TeV) ≈ 3mb [JHEP03(2016)159]

Unprecedented yields!

LHC era HL-LHC era∫Ldt 2010-12 2015-18 2020-2022 2025-28 2030++

Run I Run II Run III Run IV Run V

ATLAS, CMS 25 fb−1 100 fb−1 300 fb−1 → 3000 fb−1

LHCb 3 fb−1 8 fb−1 23 fb−1 46 fb−1 300 fb−1 (?)

We plan to collect > 100 times the luminosity, statistical uncertainty will shrinkdown..

Will detector performance pose a limit to our hopes?


Core physics program

Benchmark measurements

For now, assume we can keep at least current detector performances

Let’s consider a list of benchmark measurements

Time dependent: φs from b → ccsRare decays: Bs → µµCKM γ from treesMixing and CPV in charm with D0 → KSππ



φs

HFAG 2015

Greig Cowan (Edinbourgh) @ HL-LHC 2015

Systematic uncertainties will notbe a problem even in HL-LHC

New vertex detector in ATLASand CMS with reduce materialbudget and better geometry willbe beneficial for B physics(proper time resolution will getcloser to LHCb)



Bs,d → µµ

No limiting systematics areforeseen

CMS will improve the massresolution thanks to new trackerand focusing on events in thebarrel



CKM γ (tree level)

LHCb already dominates the world average and will get competitionfrom Belle II

σ(γ) ≈ 4◦ is expected and the end of in Run II, < 1◦ after Run III

No limiting systematics are foreseen from the detector side

Detector asymmetries affect some channels but our present (andfuture, see later) knowledge of those is sufficiently precise1

1One of the dominant modes with D → K 0Sππ is insensitive to B production and K

detection asymmetryAndrea Contu (CERN) Detector effects 31 Aug 2016 8 / 27


Charm physics

Golden mode D0 → KSππ will still belimited by statistics

Cleverly constructed low systematicobservables (e.g. ∆ACP) can still beconsidered robust

However we would like to measureasymmetries of O(10−6), do ourassumption make still sense (e.g.no-CPV in our control channels..)?



But can we keep our current performance?

It seems likely that we can keep at least the current performance:

ATLAS and CMS will expand their capabilitiesLHCb will get a significant upgrade of tracking and RICH systemalready in Run IIINew ideas are being investigated concerning the detector requirementsin Run IV (see previous talks)New technologies will probably be needed to cope with radiationdamage and increased occupancy but the direction is clearTrigger strategies will be more software-based to increase S/B andreduce biases (e.g. in lifetime)

The core heavy flavour physics program during HL-LHC looksreasonably safe

But we should not stick to what we can already do



Where are our limits?

Are there areas where we are currently limited and/or where animprovement could expand significantly our physics reach?

Charge-dependent reconstruction asymmetries

Particle Identification (PID)

Low momentum tracks

Flavour Tagging

e, γ and π0 reconstruction



Reconstruction asymmetries

When measuring CP asymmetries, detector induced charge-dependentefficiencies must be kept well under control

In charm (and not only) we often rely on zero CPV on CF modes tocorrect for it

For how long we can make this assumption? Do we have alternatives(that do not rely on any assumption)?

Some asymmetries are dealt with by reversing the magnetic field(which is not clear how easy would be to do in HL-LHC) but thingslike particle interactions with the detector material cannot

MC can be trusted up to a certain level but it seems very unlikely wecan use it, also because “full detector simulation” will probably bedropped in favour of parametric ones

Can we think of a data-driven, assumption-free method to measuredetector asymmetries below 10−5?



Reconstruction asymmetries

We have a method based on the ratio of partial/full reconstruction ofD0 and Λc decays

The decays is kinematically closed even if one of the final stateparticles is missing so a tag and probe approach is used

This method allowed in Run I to measure detection asymmetries ofthe order of 10−4

It is statistically limited so one could envisage reaching O(10−5) inthe HL-LHC

Can we go lower than that?



Reconstruction asymmetries - prospects

Reaching 10−6 with same method looks hard. Also momentumresolution is not great

One could enhance the statistical power by requiring that the probeparticle must be reconstructed as a VELO segment

However, the asymmetry coming from the VELO, which is knownfrom simulation to be lower than the one coming from downstream,has to be taken from MC

One can play tricks by reconstructing the probe in other ways but it isunclear what could be the ultimate precision

Another way could be to reduce the material in the VELO. The RFfoil makes up for most of the material budget in that region, it will belighter in the upgarde, what about removing it completely?



PID prospects

Very large calibration samples, statistic is not a problem

2015

Constant improvement on the kinematic coverage



PID prospects

Uncertainties typically in the range of 1− 0.1% accounting for bothstatistical (mostly from the physics channels) and systematic component

Some systematics are statistical in nature and will be “naturally” reduced(e.g. binning)

Other systematic effects will require the development of specificmethods/tools to be kept under control at the needed level. Typically thiswill also depend on the possibility to generate large statistics MC samples

Muon identification efficiency is less of a priority with respect to the past.More important to keep the mis-ID (particularly π → µ) at the level we havenow but in a much harsher environment. Any departure from this wouldhave serious consequences basically on every rare decay search. However,initial studies (at least in LHCb) look very promising (link)


https://indico.cern.ch/event/481359/contributions/1157528/attachments/1253674/1849731/LHCb_VHlumi_LNF.pdf

Extending our physics reach




PID prospects

Poor PID at low momenta already limits analysis of final states with alarge number of tracks (≥ 6)

PID could be extended at lowmomentum (< 10GeV) in LHCbby measuring the TOF thanksto the TORCH detector

Some benefit also for flavourtagging

Also ATLAS and CMS are considering using some dedicated timingdetector and/or timing information in the tracker to have some K/πseparation (will GPDs be able to join LHCb in some fully hadronicchannel?)



Low momentum tracks

Currently we “throw away” a significant fraction of signal involvinglow momentum particles (few GeV) because it escapes the acceptancein the dipole regionStudies of high multiplicity final states, low mass strange hadrondecays and in general decays with a soft track would greatly benefitfrom additional tracking stations in the magnet regionFrom preliminary studies, a (40± 20)% percent yield increase,depending on the decay channel, is expected



Flavour tagging

FT is crucial in B physics and every bit of performance is welcomesince has a direct impact on the sensitivity

The impact of systematic erros in the FT calibration (includingasymmetries) depends on the channel but it is in general found to bemuch smaller than the statistical uncertainty

Although we are doing a great job at the LHC, even at LHCb we area factor ∼ 6 away from the performance achievable at the B factories

Fortunately, there is still a great potential to be exploited:

By being clever and implement better tagging algorithms. In theLHCb’s B0 → D+D− analysis [arXiv:1608.06620] we achieved a 8%tagging efficiency thanks to a very performant new SS pion taggerBy having a detector more “tagging friendly” → reconstruct verylow momentum tracks. Promising studies are being performed tounderstand how much can be gained here (I do not have numbers but Ibet on a double-digit percentage relative improvement)



Flavour tagging - prospects

What can be done with a FT efficiency > 10%

Scale better than luminosity in our time dependent analyses

Tagging what today is “untaggable”, for example

Bs → φγBs → φµµB0 → µµ...




ECAL invaluable for some of the most exciting results in Run I

In particular LFU is currently one of the ”hot topics“ in flavour physics

Despite all this, analyses using calorimeter objects are relatively rare:

Lower efficiency, only 20% compared to channels with µ instead of e orπ± instead of π0

Poorer mass resolution which also translates into large backgroundsTrigger efficiency difficult to understand




An improved calorimeter in LHCb would basically allow to “double”our physics program:

Hadronic channels with π0sSemileptonic decays with electronsNew radiative channelsBetter performance for low dielectron masses

New technologies are being investigated, particularly for the innerregion

Better energy resolution is unlikelyReducing Moliere radius to fight pile-upPosition resolution could improve with better granularityTiming information?



(Why just) e, γ and π0 reconstruction (?)

LHCb’s unique coverage would allow for some other interestingphysics

Need to increase the dynamic energy range

Feasible but not really compatible with what said previously...



Strange physics

Despite not being optimised for strange physics, LHCb showed a greatpotential in Run I with the best limit on the K 0

S → µµ decay

Acceptance is not optimal but, being at an hadronic machine, strangemeson production is enormous (through simple hadronisation)

Although improved capabilities on low momentum tracks would help,there are not stringent limitations from the detector side

Being produced at very low pT , the efficiency of collecting strangehadrons depend more on the trigger strategy (and availablebandwidth)A fully software trigger will dramatically increase the reconstructionefficiency and open a new area, where we may compete withdedicated experiments in some channels:

K 0S → π0ll

K 0S → π + π−e+e−, K 0

S → π + π−e+e−

K± → π±llStrange baryons



The role of Charm

Besides being interesting per-se,charm physics is a training ground forB physics: the charm yields of todayare the beauty yields of tomorrow

Many challenges we will face in the B case have been alreadyencountered or will be encountered soon in charm physics

In particular, the understanding of detector asymmetries is essentiallydriven by charm physics

We may not have all the answers now, but charm is a powerful tool toget them


Conclusions

Conclusions

Many challenges ahead to keep our current performance in HL-LHC

This seems an achievable goal although we must keep an eye onreconstruction asymmetries

Currently we are limited in some areas (low momentum tracks andcalorimeter objects) which could significantly extend our physics reach

It would be a pity not to exploit our full potential


how well can we control detector effects for precision … · 2016-10-04 · foil makes up for most...

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