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    42 Questions For Particle Physicists
    By Tommaso Dorigo | June 29th 2009 05:14 PM | 21 comments | Print | E-mail | Track Comments
    About Tommaso

    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    In a few days, scores of Italian post-doctoral researchers in experimental particle physics will get tested on their knowledge of the matter, without any promise of a position, but just to get one further "stamp" on their curriculum, testifying that they are competent enough to be worth offering a temporary position by INFN, the Italian Institute for (sub)Nuclear Physics. So this is a  national exam, with the sole purpose of giving a green light to be admitted to two-year positions , which are typically paid less than 1400 euros a month, and which are so far not available. Frankly, I feel ashamed, since I myself work for INFN, and I strongly disagree with its current recruitment policies.

    Not surprisingly, this admission exam generated outrage among the several hundred post-docs to which it is addressed, because it completely avoids attacking the real problem of recruitment in particle physics research in Italy: the lack of resources and of positions. Instead, it offers aspiring researchers -most of them already engaged with important tasks within their experiments, but all of them unsure about their near future employment- nothing but a certification -one which was not needed until today. As if a Ph.D. in particle physics plus years of research at physics laboratories was not enough.

    Elsewhere I discussed the reasons for boycotting the exam and the reasons for attending it. Here, I just wish to remind those who will choose to attend the exam the kind of test they will be facing, and provide the rest of you with a flavor of the competences which will be tested. The 42 questions listed below were given in 2005 in the only comparable exam given by INFN in the past. Two hours were given to answer these questions -a really short time, which forced candidates to make tough choices on which questions to address first. Note that the total score of each of the four batches of questions is given, in order to provide guidance on which questions to try and answer first -or to just freak out the candidates further.

    In a few days I will provide my own answers to the questions. For now, here is the list:

    1. Questions on detector physics
    (maximum 15 points in total):

    1.1
    Cathode readout may be used in wire detectors such as multi-wire chambers, TPC, LST, and even in RPC. What is it ? What are its advantages ?

    1.2
    One wants to distinguish pions and kaons of p=2 GeV/c momentum by determining the time of flight, on a L=2m basis. If the instrumenta has a time resolution of 0.2ns, it is asked whether
    a) it is possible to identify each detected particle;
    b) it is possible to measure the fraction of pions and kaons.

    1.3
    A particle at ionization minimum generates on average n electron-positron pairs per cm in a gaseous detector at atmospheric pressure. What is n if the gas is a argon-isobutane mixture (60-40)? Which additional factors besides the statistics of produced electrons determine the standard deviation of the signal ?

    1.4
    The drift velocity of electrons in a given gaseous mixture is v=5cm/us. Which consequences does this value cause in a multi-wire chamber with pitch s=2mm ? And in a drift chamber with TDC having a clock cycle of 500 MHz ?

    1.5
    An electron with medium-high energy releases in a BGO block energy which generates a signal of about 10^6 electrons per GeV, and in a block of lead glass of the same dimensions a signal of about 10^3 electrons per GeV. What is the reason for this difference ?

    1.6
    In a given e.m. calorimeter the statistical contribution to resolution is 0.07/sqrt(E). Can one infer that energy resolution for an electron of 50 GeV energy is 1% ?

    1.7
    Order by decreasing dead-time the following detectors: silicon, plastic scintillator, drift chamber. Which one do you choose for a time measurement with a precision of a few hundred picoseconds ?

    1.8
    A cube of NaI(TI) scintillator read out by a photomultiplier tube measures the line of cesium; estimate the resolution in energy, listing the factors that contribute.

    1.9
    In a measurement where a threshold Vth=0.4V is applied to signals which have a rise time roughly constant and equal to Ts=10ns, but a variation in amplitude between Vmin=0.5V and Vmax=1V, estimate the limit to time resolution due to the variability of height. Is there a technique to reduce this effect ?

    1.10
    A relativistic electron loses energy for ionization and for radiation when it traverses a medium.
    a) How do ionization losses and radiation losses depend on the composition of the traversed medium ?
    b) How do they depend on the electron energy ?
    c) Knowing that the critical energy is defined as the one for which the two losses are equal, would this be smaller for an electron or for a muon ?

    1.11
    In what energy interval does Compton scattering dominates among the processes of interaction betweeh photons and matter ? What prevails at lower and higher energies ? How does all this depend on the absorbing material ?

    1.12
    A completely depleted silicon microstrip detector has a strip pitch of 50 um and woks without charge sharing. What is its spatial resolution ?


    2. Questions on apparata and accelerators
    (maximum 18 points in total):

    2.1
    Compute the average number of interactions per bunch crossing for a luminosity L=2.5E31 cm-2 s-1, if the total cross section is sigma=20mb and bunch crossings occur every T=4 us. What is the probability of having zero interactions in a bunch crossing ?

    2.2
    Justify the luminosity L=10^34 cm-2 s-1 of LHC starting from a accumulated current I=0.5A per beam and assuming that there are B=3000 bunches. How many are the protons per bunch, Np ? What is the cross section of the beam in the interaction region ? (Electron charge is e=1.6E-19C).

    2.3
    A beam of protons of energy E1=20 GeV is made to collide head-on with a beam of protons of energy E2=5 GeV. Determine:
    a) the CM energy;
    b) the boost of the CM in the laboratory;
    c) the angle in the laboratory of a ultra-relativistic particle produced at 90 degrees in the CM
    d) the energy a beam of protons must have to generate the same CM energy in a fixed target collision.

    2.4
    What is the ratio between the radiated power of a proton at LHC and an electron at LEP I ? And between an electron at LEP I and one in Daphne ?

    2.5
    An experiment with continuous beam is endowed with a trigger with efficiency eff=20%. The natural frequency of events to be selected by the trigger is f=5 kHz. The acquisition system generates a dead time T=1 ms per every collected event; during dead time the trigger and detector are ineffective. Determine the average frequency of data collection.

    2.6
    5 kHz of events are analyzed to decide whether the event must be collected or not. The decision takes Tt=20 us and the digitization Td=1 ms. What rate of accepted events can be sustained if the dead time must be kept below D=20% ?

    2.7
    To monitor the luminosity of a collider two scintillation counters A and B are used, which are located before and after the interaction region. The packets cross every T=10 us and for every crossing the two counters detect via digital signals A, B whether there has been at least an event; a logical AND called C signals the coincidence, in the same packet collision, of the A and B signals. Background events of various nature, produced for example by particles belonging to the beam halo or by collisions with residual gas molecules within the vacuum tube, overlap to useful events generating casual coincidences. One measures the following average frequencies for the three signals A,B,C: f_A=7.53E4 Hz, f_B=6.67E4 Hz, f_C=5.43E4 Hz. Compute the average number of collisions per crossing.

    2.8
    How does dP/P depend on the momentum of charged particles tracked in a magnetic field in air ? And in iron ?

    2.9
    A spectrometer for particles with unit charge and momentum of few GeV/c is made by three parallel planes of position detectors with a resolution dx=100 um, distanced a=20 cm and immersed in a uniform magnetic field B=1 T, parallel to the detector planes and orthogonal to the measured coordinate. Particles incide almost perpendicularly to the detector planes. Estimate the transverse momentum resolution at p=2 GeV/c.

    3. Questions on probability and statistics
    (maximum 5 points in total):

    3.1
    An experiment selects signal events with a frequency f and events of background with a frequency b. Calculate the data taking time necessary to observe the signal with a statistical significance of n sigma.

    3.2
    Given events with four b-quarks, knowing that BR(b->xlv)=20%, where l=e or mu, what is the probability to have at least one lepton ?

    3.3
    If a particle with average lifetime tau has not decayed after a time t, what is the probability that it decays in the subsequent interval delta t ?

    3.4
    After collecting an integrated luminosity L=10/fb the analysis of events B->J/psi K_s (J/Psi->ll, Ks->pi pi) selects N=100 candidates. Using N_sim=1000 events it is found that the detection efficiency is eff=37%. What is the measured BR, what is the statistical error and what is the systematical uncertainty due to the fact of having simulated too few events ? (sigma=1 nb).


    4. Questions on subnuclear physics
    (at most 22 points in total):

    4.1
    List synthetically the contributions that K physics has brought to the understanding of subnuclear physics in general.

    4.2
    What is K0s regeneration ? How is it explained ?

    4.3
    Can a neutral pion decay into three gamma ? Can a rho decay into two pizeros ? If not, explain why.

    4.4
    How is it explained, at least qualitatively, that phi decays into three pions about 15% of the time while the remaining BR is basically into KK ?

    4.5
    Weak interactions are responsible besides other things of the decay of muons (tau=10^-6 s), of B mesons (tau=10^-12 s), and neutrons (tau=10^3 s). Say what are the dominant factors which create these differences.

    4.6
    Consider the decays D0->antiK0 pi0 and D0->K0 pi0. Draw the Feynman diagrams of the two decays and estimate the relative size of the two decay amplitudes.

    4.7
    What makes it harder to observe D0-antiD0 oscillations than to B0-anti B0 ones ?

    4.8
    What is the value to first order of the ratio R=sigma(had)/sigma(mumu) in a e+e- machine with 6 GeV in the CM ? What is, still to first order, the BR of a W in a lepton-neutrino pair ?

    4.9
    The CM energy, equal to 270+270 GeV, obtained at the antiproton-proton collider at CERN has certainly been sufficient for the production of W and Z0 bosons, but with a rather small margin. Why ?

    4.10
    Draw qualitatively the structure functions xF3(x), F2(x), G(x) at q^2=10 GeV^2. How do structure functions evolve qualitatively as a function of q^2 of the interaction ?

    4.11
    How do coupling constant alpha_em and alpha_strong change as a function of q^2 ?

    4.12
    Order by decreasing cross-section:
    a) at a hadron collider the processes of production of Z, top, H (150 GeV mass), b, and elastic scattering;
    b) at LHC the following Higgs production processes, for Higgs mass of 150 GeV: qq, gg->Htt, qq->HZ, qq->HW, gg->H, qq->Hqq, qq,gg->Hgg.

    4.13
    What is the ratio between BR(H->bbbar) and BR(H->tau tau), for a Higgs mass of about 120 GeV ?

    4.14
    What is the main background to H->gamma gamma, for a Higgs mass of 120 GeV ?

    4.15
    The decay mu->e gamma and the decay tau-> mu gamma violate the conservation of lepton number.
    a) Draw at least one diagram for such decays
    b) Describe what are the irreducible backgrounds.

    4.16
    A beam of neutrinos of average energy E=20 GeV is created by the decay of pions alone. Estimate:
    a) the energy of the pion beam which generated them;
    b) the divergence of the neutrino beam;
    c) the order of magnitude of the cross section on nucleon;
    d) the mean free path of neutrinos in a detector of density equal to that of water;
    e) the ratio between the cross section on proton and on electron.

    4.17
    A photon may convert into an electron-positron pair next to an electron rather than next to a nucleus. In such case, what is the threshold energy ?

    Comments

    You can order of magnitude or two a lot of these in about 10 seconds. How much accuracy are the test people sensitive too?

    dorigo
    The idea of the exam -the one of 2005- was to hire experimentalists, and so yes, eyeballing was not considered a bad idea. However, when you had to compute, you had to do it right.
    One thing I forgot is the score: to be admitted to the other written test (a dissertation), candidates had to pass this one with over 42 points of the 60 available. The score of the questions is given for the blocks 1,2,3,4.

    Cheers,
    T.
    As a theorist and a future pos-doc candidate in your country, I almost freaked out reading those ("wait, every physics pos-doc in Italy must know experimental stuff now?!"). A clarification (before the question list) about the fact that this was meant for experimentalists only would possibly spare a few hearts from collapsing.

    Hfarmer
    No wonder.  Aside from one course in experimental particle physics, I recognize little of this.  i.e. Knowing the ratio's of various particle accelerators by heart for one question.  How much experience are they looking for here, post doctoral experimental research?  This will certainly weed out those who don't have the practical knowledge.
    Science advances as much by mistakes as by plans.
    1400 euros a month? Is that after tax?

    This is roughly half what I have experienced in other European countries.

    dorigo
    yes vlad, after tax. It is a ridiculous salary for PhDs, but that's how things are here.
    My own salary as a tenured researcher is only 20% higher.
    Cheers,
    T.
    Are the questions from the R5 exam known?

    dorigo
    No, but if you find them please forward them to me too!
    Cheers,
    T.
    Scientists do not realize that there ARE 7 layers to an atom and on each stem, per say, there are 7 more and so on and so forth. Also the calculation for pi is wrong. It is 22.012238584/ 7 = 3.144605512. Keep on trucking though! The first two layers of an atom, by the way, are spiritual in form. Everything that is material is made of of spiritual energy. Seek it, find it, live in truth!

    dorigo
    anon, I guess you wrote "per se" incorrectly. I avoid commenting on the rest of your scribbiling.
    T.
    Hey Tommaso, did you post the answers too ? I cannot find them on your blog, I would be interested ! thanks
    giulia

    dorigo
    Hi Giulia,

    no, I did not post the answers - I found out that they required some work and got entangled in other things. Send me an email.

    Best,
    T.
    Bonny Bonobo alias Brat
    I know I'm just a bonobo but I would love to see the answers too.
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    Bonny Bonobo alias Brat
    Tommaso, do you have a link to the 42 answers to these 42 questions for Particle Physicists please?
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    dorigo
    Hi Helen,
    you are right in reminding this to me. I thought the answers existed somewhere, but I fail to find them. I will try to put together something i I have the time. Is there any one of them in particular you would like me to start with ?
    Cheers,
    T.
    Bonny Bonobo alias Brat
    OK Tommaso, these are the questions I would particularly  like an answer to :-

    2.3
    A beam of protons of energy E1=20 GeV is made to collide head-on with a beam of protons of energy E2=5 GeV. Determine:
    a) the CM energy;
    b) the boost of the CM in the laboratory;
    c) the angle in the laboratory of a ultra-relativistic particle produced at 90 degrees in the CM
    d) the energy a beam of protons must have to generate the same CM energy in a fixed target collision.


    2.5
    An experiment with continuous beam is endowed with a trigger with
    efficiency eff=20%. The natural frequency of events to be selected by
    the trigger is f=5 kHz. The acquisition system generates a dead time T=1
    ms per every collected event; during dead time the trigger and detector
    are ineffective. Determine the average frequency of data collection.


    2.6
    5 kHz of events are analyzed to decide whether the event must be
    collected or not. The decision takes Tt=20 us and the digitization Td=1
    ms. What rate of accepted events can be sustained if the dead time must
    be kept below D=20% ?


    2.7
    To monitor the luminosity of a collider two scintillation
    counters A and B are used, which are located before and after the
    interaction region. The packets cross every T=10 us and for every
    crossing the two counters detect via digital signals A, B whether there
    has been at least an event; a logical AND called C signals the
    coincidence, in the same packet collision, of the A and B signals.
    Background events of various nature, produced for example by particles
    belonging to the beam halo or by collisions with residual gas molecules
    within the vacuum tube, overlap to useful events generating casual
    coincidences. One measures the following average frequencies for the
    three signals A,B,C: f_A=7.53E4 Hz, f_B=6.67E4 Hz, f_C=5.43E4 Hz.
    Compute the average number of collisions per crossing.
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    dorigo
    Dear Helen,

    2.5 and 2.7 are answered in a separate post (http://www.science20.com/quantum_diaries_survivor/answers_selected_questions). I am about to try answering 2.3 in a separate post.

    Cheers,
    T.
    Bonny Bonobo alias Brat
    Oh, so they were here all along, thanks Tommaso.
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    M. reza Tirtgan

    Free moving particle in spatial medium has astrange behavior, According to H particle-paths hypothesis, inHparticles.com/,:

    1)      its trajectory obeys the denser track texture in the medium

    2)      Its type R&L configurations, e.g. spin up&down, is alters duringinfinitesimal time interval deltaT successively

    3)      Its super-symmetric partner is not a independent particle, but, it surrounds the particle in the form of dark matter hallo.

    4)      Thewhole particle and its supersymmetric partner are confined in a spatial patch nominate S-patch. The whole system is correlated to the black hole of host galaxies and clusters by an H hall tunnel (an alternate to wormhole) confining the abstract vacuum.

    5)      During any delta T stay time interval (case 2), the particle beats and emit an expandon of spin (or better to say path-length) of 2 h-bar at its expanded mode propagating at c speed in spatial medium, and at its contracted mode it emits a contracton of spin or path-length -2h-bar within abstract vacuum of H hall tunnel towards the black hole (case 4) at irreversible manner instantaneously.

    6)      Formore information, please refer to Sec. 8(7)2, E5a of Hparticles.com/.

    Tirgan
    Hank
     According to H particle-paths hypothesis, inHparticles.com/
    If 100% of your comments are going to be advertisements for your website - and it seems they are - I can just go ahead and cancel your account right now.
    M. reza Tirtgan
    Dear Hank CampbellThanks you for your comments, in the related website I have used of different references of different authors as you can find it. Thus, I have no aim for advertisement. If this is encountering with your rules I will stopped it.
    Tirgan